**Chemical Sensors Based on Photonic Structures**

Vittorio M. N. Passaro, Benedetto Troia, Mario La Notte and Francesco De Leonardis *Photonics Research Group, Dipartimento di Elettrotecnica ed Elettronica, Politecnico di Bari, Italy* 

## **1. Introduction**

88 Advances in Chemical Sensors

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Photonic sensors have been the subject of intensive research over the last two decades for use in civil and military environments, especially for detection of a wide variety of biological, chemical and nuclear agents. Photonic sensor technologies involve a lot of application fields like chemical, temperature, strain, biomedical, electrical, magnetic, rotation, pressure, position, acoustic and vibration sensors. Important efforts have been carried out by the international scientific community (academia, industry-R&D and all interested parties), to develop and improve the know-how and the state-of-the-art of photonic sensing. In this context, optical Lab-on-a-chip systems, based on chemical and biochemical sensors, represent the state of the art of photonic sensing, since they are expected to exhibit higher sensitivity and selectivity as well as high stability, immunity to electromagnetic interference, and product improvements such as smaller integration sizes and lower cost (De Leonardis et al., 2007).

In recent years, rapid advancements in photonic technologies have significantly enhanced the photonic biochemical sensor performance, particularly in the areas of light – analyte interaction, device miniaturization and multiplexing, and fluidic design and integration. This has led to drastic improvements in sensor sensitivity, limit of detection, advanced fluidic handling capability, lower sample consumption, faster detection time, and lower overall detection cost per measurement. This trend is not a casual phenomenon, indeed it justifies the economic interest that many industries reveal to photonic chemical and biochemical sensors. With future commercialization of photonic biosensors in lab-on-a-chip systems, next generation biosensors are expected to be reliable and portable, able to be fabricated with mass production techniques to reduce the cost as well as to do multiparameter analysis, enabling fast and real-time measurements of a large amount of biologic parameters within a single, compact sensor chip. For example, needs are expected to be boosted by healthcare, such as the increasing prevalence of diabetes in the population and the growing demand for home and point-of-care testing and monitoring tools.

In conclusion, optical biosensors (Passaro et al., 2007a) have reached a high degree of maturity in crucial areas of application such as environmental monitoring, biotechnology, medical diagnostics, drug screening, food safety (Arshak et al., 2009), and security (Leheny & McCants, 2009).

Chemical Sensors Based on Photonic Structures 91

as an engineering prerogative, because several technical solutions have to be implemented

The remaining blocks labeled as "signal conditioning" and "signal processing", represent the sensor front-end, generally identified by integrated digital/analog CMOS electronics. The electrical signal obtained by the conversion of the photonic one, has to be restored by amplification and filtering, then processed in order to decode the analyte (chemical)

Intrinsic photonic biosensors are generally integrated chips, characterized by small footprints (~ mm2, ~ μm2). On the contrary, in extrinsic photonic biochemical sensors the waveguide does not directly interact with the chemical/biochemical process. In fact, the photonic waveguide cannot represent the transducer because it is designed only to able the optical signal propagation. Extrinsic photonic sensors can be classified as "benchtop" systems. They are expensive and not suitable for mass-scale production. In addition, they

Nowadays, integrated photonic chemical sensors represent the state-of-the-art of photonic sensing. In fact, these sensing systems exhibit ultra high performance and can be realized by standard fabrication processes available in Microelectronics industry (e.g., photolithography and ICP etching). Consequently, additional start-up costs to be invested in clean room equipments updating are not needed, and mass-scale production can represent a concrete

The most popular technological platform adopted for the fabrication of integrated photonic sensors is called silicon-on-insulator (SOI). This one works by placing a thin, insulating layer, such as silicon dioxide or glass, between a thin layer of silicon and the silicon substrate. In Fig. 2 a typical SOI rib optical waveguide designed at 1.55 μm (operative wavelength, λop) is

> (a) (b)

Fig. 2. Optical (a) Ex-field (quasi-TE) and (b) Ey-field (quasi-TM) distributions in a SOI rib

In particular, it is possible to recognize the insulating layer (SiO2) characterized by a

A thin silicon layer (nSi = 3.45 @ 1.55 μm) is properly etched in order to realize the rib waveguide, covered by air (nair = 1). The architecture presented above exhibits a good optical transverse electric (TE) and transverse magnetic (TM) field confinement in the rib

waveguide ( λop = 1.55μm, W = 1μm, H = 1μm, h\_rib = 700nm, h\_box = 1μm).

refractive index nSiO2 = 1.45, and built on the silicon substrate (bulk).

shown. Both quasi-TE and quasi-TM optical field distributions are plotted.

in order to maximize the confinement of the optical field in the sensible area.

information to be recorded and displayed ("record and display").

are characterized by large dimensions and low portability.

business.

## **2. Photonic bio-chemical sensors: Sensing principles and architectures**

A chemical sensor can be defined as an analytical device that coverts chemical or biochemical information (e.g., concentration, composition analysis), into a quantifiable and processable signal.

One of the possible chemical sensor classifications, concerns with the principle of the transducer, i.e. the device that transforms the chemical information about the sample (chemical analytes, molecules, cells or gases), into an analytical signal. To this purpose, electrochemical, electrical, mass sensitive, magnetic, thermometric and optical sensors can be classified as different types of chemical sensors.

A photonic chemical sensor is characterized by an optical-based transduction. This one includes a large number of sensing principles based on absorption, reflection, refraction, dispersion, Raman effect, chemiluminescence, fluorescence and phosphorescence, to name a few. The general scheme of a photonic chemical sensor can be represented as in Fig. 1 below.

Fig. 1. Schematic diagram of a photonic bio-chemical sensor.

Colored blocks named "Sensible area" and "Photonic device", represent the receptor and the transducer of the photonic biochemical sensor, respectively.

The receptor is a fundamental part of the sensor. In fact, it has to be designed in order to catalyze the specific chemical reaction or biochemical process, exhibiting a high selectivity for the analyte or the chemical specie to be detected. Generally, the receptor can be realized by covering the sensor surface (sensible area), with polymeric layers (~ nm-thin). These ones are characterized by selective receptors, able to capture and immobilize only one type of analyte in a complex chemical sample (e.g., glucose in blood, metallic particles in water, antigen/antibody in organic solution). By this way, the sensible area is the direct maker of the sensor selectivity and can be defined as a chemical prerogative.

In an intrinsic photonic bio-chemical sensor, the transducer is generally represented by an integrated optical waveguide (e.g., optical fiber, slot-waveguide, photonic crystal waveguide, photonic wire waveguide). By this way, this photonic device has to satisfy different parallel functions. The first one consists in guiding the photonic signal from the optical source (e.g., led, laser), to the sensible area. Consequently, the photonic waveguide has the rule of transducer, enhancing the interaction between the chemical/biochemical process and the optical signal. By this way, the chemical information can be properly transduced into an optical one and, finally, guided to the photo-detector to be transformed into an analytically useful electrical signal. In this context, the transducer can be considered

A chemical sensor can be defined as an analytical device that coverts chemical or biochemical information (e.g., concentration, composition analysis), into a quantifiable and

One of the possible chemical sensor classifications, concerns with the principle of the transducer, i.e. the device that transforms the chemical information about the sample (chemical analytes, molecules, cells or gases), into an analytical signal. To this purpose, electrochemical, electrical, mass sensitive, magnetic, thermometric and optical sensors can

A photonic chemical sensor is characterized by an optical-based transduction. This one includes a large number of sensing principles based on absorption, reflection, refraction, dispersion, Raman effect, chemiluminescence, fluorescence and phosphorescence, to name a few. The general scheme of a photonic chemical sensor can be represented as in Fig. 1 below.

Colored blocks named "Sensible area" and "Photonic device", represent the receptor and

The receptor is a fundamental part of the sensor. In fact, it has to be designed in order to catalyze the specific chemical reaction or biochemical process, exhibiting a high selectivity for the analyte or the chemical specie to be detected. Generally, the receptor can be realized by covering the sensor surface (sensible area), with polymeric layers (~ nm-thin). These ones are characterized by selective receptors, able to capture and immobilize only one type of analyte in a complex chemical sample (e.g., glucose in blood, metallic particles in water, antigen/antibody in organic solution). By this way, the sensible area is the direct maker of

In an intrinsic photonic bio-chemical sensor, the transducer is generally represented by an integrated optical waveguide (e.g., optical fiber, slot-waveguide, photonic crystal waveguide, photonic wire waveguide). By this way, this photonic device has to satisfy different parallel functions. The first one consists in guiding the photonic signal from the optical source (e.g., led, laser), to the sensible area. Consequently, the photonic waveguide has the rule of transducer, enhancing the interaction between the chemical/biochemical process and the optical signal. By this way, the chemical information can be properly transduced into an optical one and, finally, guided to the photo-detector to be transformed into an analytically useful electrical signal. In this context, the transducer can be considered

**2. Photonic bio-chemical sensors: Sensing principles and architectures** 

processable signal.

be classified as different types of chemical sensors.

Fig. 1. Schematic diagram of a photonic bio-chemical sensor.

the transducer of the photonic biochemical sensor, respectively.

the sensor selectivity and can be defined as a chemical prerogative.

as an engineering prerogative, because several technical solutions have to be implemented in order to maximize the confinement of the optical field in the sensible area.

The remaining blocks labeled as "signal conditioning" and "signal processing", represent the sensor front-end, generally identified by integrated digital/analog CMOS electronics. The electrical signal obtained by the conversion of the photonic one, has to be restored by amplification and filtering, then processed in order to decode the analyte (chemical) information to be recorded and displayed ("record and display").

Intrinsic photonic biosensors are generally integrated chips, characterized by small footprints (~ mm2, ~ μm2). On the contrary, in extrinsic photonic biochemical sensors the waveguide does not directly interact with the chemical/biochemical process. In fact, the photonic waveguide cannot represent the transducer because it is designed only to able the optical signal propagation. Extrinsic photonic sensors can be classified as "benchtop" systems. They are expensive and not suitable for mass-scale production. In addition, they are characterized by large dimensions and low portability.

Nowadays, integrated photonic chemical sensors represent the state-of-the-art of photonic sensing. In fact, these sensing systems exhibit ultra high performance and can be realized by standard fabrication processes available in Microelectronics industry (e.g., photolithography and ICP etching). Consequently, additional start-up costs to be invested in clean room equipments updating are not needed, and mass-scale production can represent a concrete business.

The most popular technological platform adopted for the fabrication of integrated photonic sensors is called silicon-on-insulator (SOI). This one works by placing a thin, insulating layer, such as silicon dioxide or glass, between a thin layer of silicon and the silicon substrate. In Fig. 2 a typical SOI rib optical waveguide designed at 1.55 μm (operative wavelength, λop) is shown. Both quasi-TE and quasi-TM optical field distributions are plotted.

Fig. 2. Optical (a) Ex-field (quasi-TE) and (b) Ey-field (quasi-TM) distributions in a SOI rib waveguide ( λop = 1.55μm, W = 1μm, H = 1μm, h\_rib = 700nm, h\_box = 1μm).

In particular, it is possible to recognize the insulating layer (SiO2) characterized by a refractive index nSiO2 = 1.45, and built on the silicon substrate (bulk).

A thin silicon layer (nSi = 3.45 @ 1.55 μm) is properly etched in order to realize the rib waveguide, covered by air (nair = 1). The architecture presented above exhibits a good optical transverse electric (TE) and transverse magnetic (TM) field confinement in the rib

Chemical Sensors Based on Photonic Structures 93

immobilized on the functionalized waveguide surface. In particular, when the photonic sensor surface is exposed to a complex chemical solution, only target molecules will be recognized by selective receptors and will contribute to the bio-chemical process. By this way, if the thickness of the sensor surface layer was ρ0 before the exposure to the chemical sample, the thickness of the same layer will be greater, for example ρ0 + ρ1, after the selective analyte adsorption. The

In homogenous sensing, the modal effective index change is produced by a change of cover medium refractive index, nc. For example, by assuming that the photonic sensor is initially exposed to air (nair = 1 @ 1.55μm), when a specific gas (e.g., He, CO2, Ar, N2, C2H2) will cover the sensor surface, a sensible cover refractive index change ∆nc will be induced. In addition, the same analysis can be carried out if the sensor is initially covered by water (nwater = 1.33 @ 1.55μm) and subsequently exposed to another liquid solution (e.g., NaCl). In Table 1 it is possible to appreciate the relative effective index changes under different operative

> RI Cover medium RI Gas/Liquid ∆nC (%) nair = 1 nHe = 1.000035 0.0035 nair = 1 nCO2 = 1.000059 0.0059 nair = 1 nAr = 1.000278 0.0278 nair = 1 nN2 = 1.000294 0.0294 nair = 1 nC2H2 = 1.000593 0.0593 nwater = 1.33 nNaCl ≈ 1.33 0.00181

Table 1. Refractive index (RI) changes in homogeneous sensing for different gas and

Integrated silicon wire waveguides define a class of photonic chemical sensors called "photonic wire evanescent field" (PWEF), because of the interaction between the evanescent

Fig. 4. Schematic representation of surface (a) and homogeneous (b) sensing in a SOI-based

In both homogeneous and surface sensing it is possible to define an important performance

1 The refractive index of a NaCl aqueous solution changes 0.0018 RIU per 1% mass concentration (Sun et

parameter of the photonic chemical /biochemical sensor: the sensitivity.

**ZOOM** 

**Sensor top view**

aqueous solution concentrations. All data are referred to λop = 1.55μm.

optical field and the chemical/biochemical process to be detected (see Fig. 4).

thickness change ∆ρ can be transduced in an effective index change ∆neff .

conditions (De Leonardis et al., 2011).

**(a) (b)** 

wire waveguide.

al., 2009).

structure, directly exposed to the cover medium and characterized by the highest refractive index nSi. However, in both quasi-TE and quasi-TM modal distributions, the optical field is quasi entirely confined in the high index region, reducing the interaction with analytes or gases in the cover medium. To this purpose, other SOI-based waveguide architectures, such as the SOI photonic wire waveguide sketched in Fig. 3, can be adopted in order to improve the field confinement in the cover medium, as well as the interaction between the propagating optical field and the chemical/biochemical species, then resulting in an optimized transduction process.

Fig. 3. Optical Ey-field distribution in a silicon-wire waveguide (quasi-TM), λop = 1.55μm.

In this specific waveguide configuration (h\_box = 2μm, W = 450nm, H = 250nm), the optical transverse magnetic (TM) field is concentrated at both interfaces between the high refractive index region (silicon core) and the low refractive index ones (air at the upper side and SiO2 at the bottom one). By this way, photonic wire waveguides represent a better choice if compared with previously analyzed rib ones. For an electromagnetic wave propagating in the z direction (Fig. 3), the major E-field component of the quasi-TM eigenmode undergoes a discontinuity at the horizontal silicon wire interfaces that, according to Maxwell's equations, is determined by the following relation:

$$
\left| \frac{E\_L}{E\_H} \right| = \left( \frac{n\_H}{n\_L} \right)^2 \tag{1}
$$

In Eq. 1 *EL* is the component of the *E*-field, evaluated in the low refractive index (*nL*) region, while *EH* is the same component in the high refractive index (*nH*) region. In particular, the greater the refractive index contrast *∆n = nH - nL*, the higher the *EL*-field confinement. In this context, SOI technology represents a suitable solution for photonic sensing because it is possible to obtain a high refractive index contrast ∆n (nSi – nSiO2 ≈ 2 @ 1.55 μm).

By using one of the two SOI-based waveguides described in the section above, it is possible to realize an intrinsic photonic biosensor. In particular, two different sensing principles characterizing the operating regime of a photonic chemical sensor can be used, i.e. surface and homogeneous sensing (Dell'Olio & Passaro, 2007).

In surface sensing a change of the modal effective index neff of the propagating optical signal is due to a change of thickness of an ultra-thin layer of selective receptor molecules which are

structure, directly exposed to the cover medium and characterized by the highest refractive index nSi. However, in both quasi-TE and quasi-TM modal distributions, the optical field is quasi entirely confined in the high index region, reducing the interaction with analytes or gases in the cover medium. To this purpose, other SOI-based waveguide architectures, such as the SOI photonic wire waveguide sketched in Fig. 3, can be adopted in order to improve the field confinement in the cover medium, as well as the interaction between the propagating optical field and the chemical/biochemical species, then resulting in an

Fig. 3. Optical Ey-field distribution in a silicon-wire waveguide (quasi-TM), λop = 1.55μm.

In this specific waveguide configuration (h\_box = 2μm, W = 450nm, H = 250nm), the optical transverse magnetic (TM) field is concentrated at both interfaces between the high refractive index region (silicon core) and the low refractive index ones (air at the upper side and SiO2 at the bottom one). By this way, photonic wire waveguides represent a better choice if compared with previously analyzed rib ones. For an electromagnetic wave propagating in the z direction (Fig. 3), the major E-field component of the quasi-TM eigenmode undergoes a discontinuity at the horizontal silicon wire interfaces that, according to Maxwell's equations,

> *L H H L E n E n* ⎛ ⎞ <sup>=</sup> ⎜ ⎟ ⎜ ⎟ ⎝ ⎠

In Eq. 1 *EL* is the component of the *E*-field, evaluated in the low refractive index (*nL*) region, while *EH* is the same component in the high refractive index (*nH*) region. In particular, the greater the refractive index contrast *∆n = nH - nL*, the higher the *EL*-field confinement. In this context, SOI technology represents a suitable solution for photonic sensing because it is

By using one of the two SOI-based waveguides described in the section above, it is possible to realize an intrinsic photonic biosensor. In particular, two different sensing principles characterizing the operating regime of a photonic chemical sensor can be used, i.e. surface

In surface sensing a change of the modal effective index neff of the propagating optical signal is due to a change of thickness of an ultra-thin layer of selective receptor molecules which are

possible to obtain a high refractive index contrast ∆n (nSi – nSiO2 ≈ 2 @ 1.55 μm).

and homogeneous sensing (Dell'Olio & Passaro, 2007).

2

(1)

optimized transduction process.

is determined by the following relation:

immobilized on the functionalized waveguide surface. In particular, when the photonic sensor surface is exposed to a complex chemical solution, only target molecules will be recognized by selective receptors and will contribute to the bio-chemical process. By this way, if the thickness of the sensor surface layer was ρ0 before the exposure to the chemical sample, the thickness of the same layer will be greater, for example ρ0 + ρ1, after the selective analyte adsorption. The thickness change ∆ρ can be transduced in an effective index change ∆neff .

In homogenous sensing, the modal effective index change is produced by a change of cover medium refractive index, nc. For example, by assuming that the photonic sensor is initially exposed to air (nair = 1 @ 1.55μm), when a specific gas (e.g., He, CO2, Ar, N2, C2H2) will cover the sensor surface, a sensible cover refractive index change ∆nc will be induced. In addition, the same analysis can be carried out if the sensor is initially covered by water (nwater = 1.33 @ 1.55μm) and subsequently exposed to another liquid solution (e.g., NaCl). In Table 1 it is possible to appreciate the relative effective index changes under different operative conditions (De Leonardis et al., 2011).


Table 1. Refractive index (RI) changes in homogeneous sensing for different gas and aqueous solution concentrations. All data are referred to λop = 1.55μm.

Integrated silicon wire waveguides define a class of photonic chemical sensors called "photonic wire evanescent field" (PWEF), because of the interaction between the evanescent optical field and the chemical/biochemical process to be detected (see Fig. 4).

Fig. 4. Schematic representation of surface (a) and homogeneous (b) sensing in a SOI-based wire waveguide.

In both homogeneous and surface sensing it is possible to define an important performance parameter of the photonic chemical /biochemical sensor: the sensitivity.

 1 The refractive index of a NaCl aqueous solution changes 0.0018 RIU per 1% mass concentration (Sun et al., 2009).

Chemical Sensors Based on Photonic Structures 95

transitions, which, despite of the wide variety of organic molecules, provide unique mid-IR absorption spectra. Therefore, it is possible to reflect the molecularly characteristic arrangement of chemical bonds within the probed molecules via the frequency position of the associated vibrations and mixed rotational-vibrational transitions. In addition to this, a lot of environmental harmful gases like carbon dioxide (CO2), methane (CH4) and sulfur dioxide (SO2), to name a few, are characterized by absorption spectra in mid-IR, as sketched in Fig. 5.

It is possible to detect specific gas species or chemical analytes with high sensitivity and selectivity, by analyzing the transmission spectra of the light intensity in a suitable wavelength range. In particular, the photonic signal intensity is linked to the gas or analyte

> *II L* = − <sup>0</sup> exp( ) α

depends on *C* and *I0* is the light intensity at the initial section of the sensible area of photonic sensor. By this way, the sensor readout is an optical intensity one, thus it will be possible to register steep peaks in the transmission spectra corresponding to specific operative wavelengths, properly selected to be the absorption wavelengths of gas or molecule to be detected. Optical absorption detection allows to design photonic chemical sensors

In typical bio-chemical sensors, detection of specific pathogens or proteins require transduction labeling elements, such as fluorescent dyes or radioactive isotope, in order to generate a physically useful signal from a recognition event. For example, in fluorescencebased detection the intensity of the fluorescence indicates the presence of the target molecules as well as their concentration. Although label detection exhibits high sensitivity and ultra low LOD down to a single molecule, labeling chemistry is expensive and timeconsuming, and may interfere with the function of bio-molecules. In this context, label-free photonic bio-sensing described in this paragraph, allows to preserve the natural form of target molecules and their natural interaction with selective receptors. In conclusion, labelfree photonic sensors nowadays represent a universal platform for biochemical assays.

Slot waveguides represent a very interesting and promising architecture for photonic chemical and bio-chemical ultra-high performance sensing. In fact, by using slot waveguides

where *I* is the light intensity at the end section of the whole path length (*L*),

(6)

α

linearly

Fig. 5. Absorption spectra of several gases and liquid solutions.

characterized by ultra low LOD (e.g., pg/mm2, ng/mL1).

**2.1 Slot-waveguides for sensing applications** 

concentration *C* via the Beer-Lambert law:

$$\delta S\_h = \frac{\partial n\_{\rm eff}}{\partial n\_c} \text{ , } S\_s = \frac{\partial n\_{\rm eff}}{\partial \rho} \tag{2}$$

In homogeneous sensing, according with variational theorem for dielectric waveguides, it is possible to write:

$$\left.S\_{\rm li} = \frac{\partial n\_{\rm eff}}{\partial n\_{\rm c}}\right|\_{n\_{\rm c} = n\_{\rm c}^0} = \frac{2n\_{\rm C}^0}{\eta\_0 P} \iint \left|\vec{E}(\mathbf{x}, y)\right|^2 \,d\mathbf{x} dy = \frac{2n\_{\rm C}^0 \Gamma\_{\rm C}}{\eta\_0 P} \iint \left|\vec{E}(\mathbf{x}, y)\right|^2 \,d\mathbf{x} dy\tag{3}$$

where

$$P = \iint\limits\_{\approx} \left[ \left( \vec{E} \times \vec{H}^\* + \vec{E}^\* \times \vec{H} \right) \bullet \hat{z} \right] dxdy \tag{4}$$

η0 is the free space impedance (377Ω), *E* G and *H* G are the electric and magnetic field vectors, respectively, *nc <sup>0</sup>*is the unperturbed value of cover medium refractive index, *z*ˆ indicates the unit vector along z direction (propagation direction) and ΓC is the confinement factor in the cover medium (Dell'Olio & Passaro, 2007).

In surface sensing, shift of *neff* (*∆neff*) due to a change ∆ρ of ad-layer thickness can be calculated by using a perturbation approach as:

$$
\Delta n\_{\rm eff} = \frac{n\_m^2 - \left(n\_\odot^0\right)^2}{\eta\_0 P} \iint\left|\vec{E}(\mathbf{x}, y)\right|^2 \,d\mathbf{x} dy\tag{5}
$$

where *nm* is the molecular ad-layer refractive index and *M* is the region in which the ad-layer increases. Surface sensitivity Ss, can be calculated by using the definition in Eq. 2.

In conclusion, there is a last consideration concerning with units of measurement of both analyzed sensitivities, Sh and Ss. In the first case, the performance parameter Sh is dimensionless, because it is the ratio of two dimensionless physical quantities. On the contrary, surface sensitivity is generally measured in nm-1, because the ad-layer due to analyte or molecules adsorption is characterized by a nanometer-scale thickness (2-5 nm).

Sensor limit of detection (LOD) represents another important parameter to characterize the sensor performance. It indicates the minimum resolvable signal and can be defined by taking into account the noise in the transduction signal (σ) and the sensor sensitivity (S). Generally, LOD can be calculated as the ratio σ/S.

Photonic chemical sensors based on homogeneous and surface sensing are generally defined RI-based sensors, because the transduction process consists always in a modal effective index change, as previously described. However, there is another class of chemical sensors based on optical absorption detection. In particular, the absorption coefficient, usually indicated with the variable α (cm-1), depends on the operative wavelength of the photonic signal and material (e.g., gas, solid, liquid solutions) electronic and optical properties. For example, fundamental vibrational and rotational modes associated with most inorganic and organic molecules are spectroscopically accessible within the mid-infrared range (mid-IR). By this way, the interaction between mid-IR photons and organic molecules provides particularly sharp

*S*

*n*

0 0 2 2

*<sup>0</sup>*is the unperturbed value of cover medium refractive index, *z*ˆ indicates the

2

(,)

 η

⎡ ⎤ = × +× • ⎢ ⎥ ⎣ ⎦ ∫∫ <sup>G</sup> G GG (4)

<sup>∂</sup> <sup>Γ</sup> == = <sup>∂</sup> ∫∫ ∫∫ <sup>G</sup> <sup>G</sup> (3)

2 2 (,) (,)

ρ

<sup>∂</sup> <sup>=</sup> ∂ (2)

are the electric and magnetic field vectors,

<sup>G</sup> (5)

*eff*

*n*

*c*

In homogeneous sensing, according with variational theorem for dielectric waveguides, it is

0 0

( ) \* \* *P E H E H z dxd* <sup>ˆ</sup> *<sup>y</sup>*

 and *H* G

unit vector along z direction (propagation direction) and ΓC is the confinement factor in the

In surface sensing, shift of *neff* (*∆neff*) due to a change ∆ρ of ad-layer thickness can be

*M*

*n E x y dxdy*

where *nm* is the molecular ad-layer refractive index and *M* is the region in which the ad-layer

In conclusion, there is a last consideration concerning with units of measurement of both analyzed sensitivities, Sh and Ss. In the first case, the performance parameter Sh is dimensionless, because it is the ratio of two dimensionless physical quantities. On the contrary, surface sensitivity is generally measured in nm-1, because the ad-layer due to analyte or molecules adsorption is characterized by a nanometer-scale thickness (2-5 nm). Sensor limit of detection (LOD) represents another important parameter to characterize the sensor performance. It indicates the minimum resolvable signal and can be defined by taking into account the noise in the transduction signal (σ) and the sensor sensitivity (S).

Photonic chemical sensors based on homogeneous and surface sensing are generally defined RI-based sensors, because the transduction process consists always in a modal effective index change, as previously described. However, there is another class of chemical sensors based on optical absorption detection. In particular, the absorption coefficient, usually indicated with the variable α (cm-1), depends on the operative wavelength of the photonic signal and material (e.g., gas, solid, liquid solutions) electronic and optical properties. For example, fundamental vibrational and rotational modes associated with most inorganic and organic molecules are spectroscopically accessible within the mid-infrared range (mid-IR). By this way, the interaction between mid-IR photons and organic molecules provides particularly sharp

( )<sup>2</sup> 2 0

0

η *P* − Δ = ∫∫

increases. Surface sensitivity Ss, can be calculated by using the definition in Eq. 2.

*m C*

*n n*

= ∞

*<sup>n</sup> n n <sup>S</sup> E x <sup>y</sup> dxdy E x <sup>y</sup> dxdy nP P*

*n* <sup>∂</sup> <sup>=</sup> <sup>∂</sup> , *eff s*

*eff C C C*

G

*h*

*S*

0

*c n n C*

η

∞

*eff*

*C C*

possible to write:

where

respectively, *nc*

*h*

η0 is the free space impedance (377Ω), *E*

cover medium (Dell'Olio & Passaro, 2007).

calculated by using a perturbation approach as:

Generally, LOD can be calculated as the ratio σ/S.

transitions, which, despite of the wide variety of organic molecules, provide unique mid-IR absorption spectra. Therefore, it is possible to reflect the molecularly characteristic arrangement of chemical bonds within the probed molecules via the frequency position of the associated vibrations and mixed rotational-vibrational transitions. In addition to this, a lot of environmental harmful gases like carbon dioxide (CO2), methane (CH4) and sulfur dioxide (SO2), to name a few, are characterized by absorption spectra in mid-IR, as sketched in Fig. 5.

Fig. 5. Absorption spectra of several gases and liquid solutions.

It is possible to detect specific gas species or chemical analytes with high sensitivity and selectivity, by analyzing the transmission spectra of the light intensity in a suitable wavelength range. In particular, the photonic signal intensity is linked to the gas or analyte concentration *C* via the Beer-Lambert law:

$$I = I\_0 \exp\left(-\alpha L\right) \tag{6}$$

where *I* is the light intensity at the end section of the whole path length (*L*), αlinearly depends on *C* and *I0* is the light intensity at the initial section of the sensible area of photonic sensor. By this way, the sensor readout is an optical intensity one, thus it will be possible to register steep peaks in the transmission spectra corresponding to specific operative wavelengths, properly selected to be the absorption wavelengths of gas or molecule to be detected. Optical absorption detection allows to design photonic chemical sensors characterized by ultra low LOD (e.g., pg/mm2, ng/mL1).

In typical bio-chemical sensors, detection of specific pathogens or proteins require transduction labeling elements, such as fluorescent dyes or radioactive isotope, in order to generate a physically useful signal from a recognition event. For example, in fluorescencebased detection the intensity of the fluorescence indicates the presence of the target molecules as well as their concentration. Although label detection exhibits high sensitivity and ultra low LOD down to a single molecule, labeling chemistry is expensive and timeconsuming, and may interfere with the function of bio-molecules. In this context, label-free photonic bio-sensing described in this paragraph, allows to preserve the natural form of target molecules and their natural interaction with selective receptors. In conclusion, labelfree photonic sensors nowadays represent a universal platform for biochemical assays.

#### **2.1 Slot-waveguides for sensing applications**

Slot waveguides represent a very interesting and promising architecture for photonic chemical and bio-chemical ultra-high performance sensing. In fact, by using slot waveguides

Chemical Sensors Based on Photonic Structures 97

To this purpose, interesting results have been presented in literature. For example the slot waveguide optimized for homogeneous sensing and presented in Fig. 6, is characterized by ultra high sensitivity, Sh = 1.0076 for quasi-TE propagation mode. A SOI slot waveguide, (w=135 nm, h=965 nm, g=100 nm) exhibiting ultra-high sensitivity of 1.1, has been also recently demonstrated by the authors. A value Sh > 1 implies that an effective index change

Performance parameters such as sensitivity and confinement factors in cover medium and slot region, depend on waveguide geometrical parameters and materials selected for the fabrication. In fact, in Fig. 7 it is possible to note the sensitivity changes for homogenous sensing as a function of slot height "h", slot region width "g" and wire width "w", in a slot waveguide sensor. By this way, appropriate choice and design of photonic waveguide for chemical or biochemical sensing applications, have to be carried out in order to enhance all

Fig. 7. Slot-waveguide sensitivities for homogeneous sensing as a function of geometrical parameters. (a) Si-on-SiO2 slot waveguide with air cover optimized @ 1.55μm (Sh-MAX = 0.943, with g = 100nm, w = 250nm, h = 700nm). (b) Si-on-SiO2 slot waveguide with air cover designed @ 2.883μm (w = 450nm, h = 650nm) and 3.39μm (w = 520nm, h = 800nm).

Nowadays, ingenious design techniques are needed to extend group IV photonics from near-IR to mid-IR wavelength range. In fact, harmful gases like carbon dioxide (CO2), carbon monoxide (CO), methane (CH4) and sulfure dioxide (SO2) are characterized by absorption spectra in mid-IR, specially in the range 2-8μm, according to Fig. 5. To this purpose, several group IV material systems (e.g., SiGe, SiGeSn, SiGeC, GeC, SiSn) have been investigated (Troia et al., 2011). These alloys have been proposed for the design of slot waveguide sensors for homogeneous sensing, working at 3.39μm and 2.883μm. Interesting theoretical performances have been obtained by optimizing SOI-based slot waveguides designed for optical transparency at working λop and ultra high refractive index contrast ∆n. In particular, Sh > 1 has been demonstrated for two slot waveguides constituted by group IV material system layers, properly designed and stacked to form symmetrical wires, as sketched in Fig. 8 (Passaro et al., 2011). The most important advantages of these mid-IR "slot" sensors with respect to conventional slot sensors optimized at 1.55μm, concern with higher performance, relaxed fabrication tolerances, and presence of a second-order slot mode, able to be used for sensing. Similar investigations have been also proposed about novel optical slot waveguides working at 2.883μm (Si0.15Ge0.85/Si/SiO2 and Si0.15Ge0.85/SiO2).

∆neff > ∆nC is induced by a cover index shift ∆nC.

performance parameters as mentioned above (Passaro, 2009a).

**(a) (b)** 

it is possible to confine an extremely high optical field in the low refractive index region called "slot region", where the chemical solution or gas will be detected (Almeida et al., 2004; Iqbal et al., 2008). In Fig. 6, it is possible to appreciate the optical field distribution in a silicon slot waveguide and a relative geometrical definition of the slab slot architecture.

Fig. 6. 2D-views of the Ex-field spatial distribution (quasi-TE) solved for a Si-wire slot waveguide optimized @ 1.55μm; h = 324nm, g = 100nm, w = 180nm.

The mathematical expression of the *Ex*-field of TM mode in a slab slot waveguide (Fig. 6) is as follows:

$$E\_x(\mathbf{x}) = A \begin{bmatrix} \frac{1}{n\_L^2} \cosh(\mathbf{y}\_S \mathbf{x}) & \vdots \\ \frac{1}{n\_H^2} \cosh(\mathbf{y}\_S a) \cos \left[ k\_H \left( \left| \mathbf{x} \right| - a \right) \right] + \frac{Y\_S}{n\_L^2 k\_H} \sinh \left[ k \left( \left| \mathbf{x} \right| - a \right) \right] & \vdots \\ \frac{1}{n\_L^2} \left\{ \cosh(\mathbf{y}\_S a) \cos \left[ k\_H \left( b \cdot a \right) \right] + \frac{n\_H^2 Y\_S}{n\_L^2 k\_H} \sinh \left( \mathbf{y}\_S a \right) \sin \left[ k\_H \left( b \cdot a \right) \right] \right\} \exp \left[ \cdot r\_C \left( \left| \mathbf{x} \right| - b \right) \right] & \vdots \end{bmatrix} \tag{7}$$

where

$$A = A\_0 \frac{\sqrt{k\_0^2 n\_H^2 - k\_H^2}}{k\_0} \tag{8}$$

A0 is an arbitrary constant, *k0* (2π/λop) is the vacuum wave number, kH is the transverse wave number in the high refractive index (*nH*) region, *γ<sup>C</sup>* is the field decay coefficient in the cover medium and *γS* is the field decay coefficient in the slot region (low RI region, nL).

By using slot waveguides instead of PWEF or rib ones, it is possible to enhance the interaction between the propagating optical field and the chemical test sample. In fact, slotbased photonic sensors can exhibit ultra high performance (e.g., Sh > 1, LOD ≈ 10-4 RIU).

it is possible to confine an extremely high optical field in the low refractive index region called "slot region", where the chemical solution or gas will be detected (Almeida et al., 2004; Iqbal et al., 2008). In Fig. 6, it is possible to appreciate the optical field distribution in a silicon slot waveguide and a relative geometrical definition of the slab slot architecture.

Fig. 6. 2D-views of the Ex-field spatial distribution (quasi-TE) solved for a Si-wire slot

The mathematical expression of the *Ex*-field of TM mode in a slab slot waveguide (Fig. 6) is

<sup>⎪</sup> ⎧ ⎫ <sup>⎪</sup> ⎪ ⎪ ⎡ ⎤ ⎡ ⎤ ⎡ ⎤ <sup>⎪</sup> ⎨ ⎬ ⎣ ⎦ ⎣ ⎦ ⎣ ⎦ <sup>⎩</sup> ⎪ ⎪ ⎩ ⎭

*kn k H H A A k*

A0 is an arbitrary constant, *k0* (2π/λop) is the vacuum wave number, kH is the transverse wave number in the high refractive index (*nH*) region, *γ<sup>C</sup>* is the field decay coefficient in the cover medium and *γS* is the field decay coefficient in the slot region (low RI region, nL).

By using slot waveguides instead of PWEF or rib ones, it is possible to enhance the interaction between the propagating optical field and the chemical test sample. In fact, slotbased photonic sensors can exhibit ultra high performance (e.g., Sh > 1, LOD ≈ 10-4 RIU).

*2 H S 2 2 S H SH C*

( ) ( ) ( ) ( )

<sup>−</sup> <sup>=</sup> (8)

**Schematic of the slab slot waveguide** 

(7)

*<sup>1</sup> <sup>n</sup> <sup>γ</sup> cosh(<sup>γ</sup> a)cos k b - a + sinh <sup>γ</sup> a sin k b - a exp -<sup>γ</sup> x - b ; x > b <sup>n</sup> n k*

22 2 0 0

0

waveguide optimized @ 1.55μm; h = 324nm, g = 100nm, w = 180nm.

( ) ( ) ( )

*H L H*

*L L H*

<sup>⎪</sup> ⎡ ⎤ ⎡⎤ <sup>⎨</sup> ⎣ ⎦ ⎣⎦ <sup>⎪</sup>

*<sup>1</sup> <sup>γ</sup> E x = A cosh(<sup>γ</sup> a)cos k x - a + sinh k x - a ; a < x < b <sup>n</sup> n k*

as follows:

where

⎧ ⎪ ⎪ ⎪

*2 S L*

*<sup>s</sup> <sup>x</sup> 2 2 S H*

*<sup>1</sup> cosh(<sup>γ</sup> x) ; x < a <sup>n</sup>*

To this purpose, interesting results have been presented in literature. For example the slot waveguide optimized for homogeneous sensing and presented in Fig. 6, is characterized by ultra high sensitivity, Sh = 1.0076 for quasi-TE propagation mode. A SOI slot waveguide, (w=135 nm, h=965 nm, g=100 nm) exhibiting ultra-high sensitivity of 1.1, has been also recently demonstrated by the authors. A value Sh > 1 implies that an effective index change ∆neff > ∆nC is induced by a cover index shift ∆nC.

Performance parameters such as sensitivity and confinement factors in cover medium and slot region, depend on waveguide geometrical parameters and materials selected for the fabrication. In fact, in Fig. 7 it is possible to note the sensitivity changes for homogenous sensing as a function of slot height "h", slot region width "g" and wire width "w", in a slot waveguide sensor. By this way, appropriate choice and design of photonic waveguide for chemical or biochemical sensing applications, have to be carried out in order to enhance all performance parameters as mentioned above (Passaro, 2009a).

Fig. 7. Slot-waveguide sensitivities for homogeneous sensing as a function of geometrical parameters. (a) Si-on-SiO2 slot waveguide with air cover optimized @ 1.55μm (Sh-MAX = 0.943, with g = 100nm, w = 250nm, h = 700nm). (b) Si-on-SiO2 slot waveguide with air cover designed @ 2.883μm (w = 450nm, h = 650nm) and 3.39μm (w = 520nm, h = 800nm).

Nowadays, ingenious design techniques are needed to extend group IV photonics from near-IR to mid-IR wavelength range. In fact, harmful gases like carbon dioxide (CO2), carbon monoxide (CO), methane (CH4) and sulfure dioxide (SO2) are characterized by absorption spectra in mid-IR, specially in the range 2-8μm, according to Fig. 5. To this purpose, several group IV material systems (e.g., SiGe, SiGeSn, SiGeC, GeC, SiSn) have been investigated (Troia et al., 2011). These alloys have been proposed for the design of slot waveguide sensors for homogeneous sensing, working at 3.39μm and 2.883μm. Interesting theoretical performances have been obtained by optimizing SOI-based slot waveguides designed for optical transparency at working λop and ultra high refractive index contrast ∆n. In particular, Sh > 1 has been demonstrated for two slot waveguides constituted by group IV material system layers, properly designed and stacked to form symmetrical wires, as sketched in Fig. 8 (Passaro et al., 2011). The most important advantages of these mid-IR "slot" sensors with respect to conventional slot sensors optimized at 1.55μm, concern with higher performance, relaxed fabrication tolerances, and presence of a second-order slot mode, able to be used for sensing. Similar investigations have been also proposed about novel optical slot waveguides working at 2.883μm (Si0.15Ge0.85/Si/SiO2 and Si0.15Ge0.85/SiO2).

Chemical Sensors Based on Photonic Structures 99

Fig. 10. Sensitivity of optimized slot waveguides as a function of the tilting angle θ with slot

All sensitivity values corresponding to θ = 0°, are referred to optimized slot waveguides characterized by vertical sidewalls. By observing Fig. 10a and 10b, it is evident that the greater the tilting angle, the lower the sensitivity. This effect is justified by the fact that increasing values of θ will produce a reduction of the slot region volume. Moreover, the slot region width g is not constant along y-direction. In conclusion, for θ > 4°, the slot mode cannot propagate because the optical field confinement in the slot region approaches zero. Fabrication tolerance analysis represents an important aspect to estimate the real sensor performance. By this way, the solution to the technological problems presented above, is strictly related to the optimization of the etching process for a given combination of material systems. However, standard etching processes can assure tilting angles within 1°-2°. Generally, slot waveguides designed to operate in mid-IR wavelength range, exhibit greater tolerance margins because of their relaxed dimensions (Fig. 10a). To this purpose, θ < 3°-4° represents a good trade-off between technological constraints and device performance.

region width g fixed at 100nm in all cases. (a) Tolerance analysis of group IV-based optimized slot waveguides presented in Fig. 8; (b) Tolerances analysis of conventional SOI slot waveguides (@ 1.55μm, w = 230nm, h = 500nm, @ 3.39 & 2.883 μm, see also Fig. 7b).

λop = 3.39 μm

**(a) (b)** 

**2.1.2 State of the art of slot waveguides designed for chemical sensing** 

2009; Sun et al. 2009).

All theoretical and experimental results demonstrate that slot waveguides are the best suitable photonic devices for sensing applications. In fact, if properly designed, they allow to concentrate a high optical field percentage in the cover medium, improving the interaction between the optical field and the chemical process, thus the limit of detection.

To this purpose, alternative solutions have been proposed in literature with the aim to enhance the efficiency and performance of photonic chemical sensors. For example, slot waveguides based on polymeric materials can ensure high performance, although characterized by low refractive index contrast ∆n (Bettotti et al., 2011). In addition, multipleslot waveguides represent an intriguing solution. By using multiple slot waveguides such as those presented in Fig. 11a-b-c, it is possible to significantly increase the sensitivity for both homogeneous (∆neff/nc ~ 0.2) and surface sensing (∆neff/∆ρ ~ 10-4 RIU/nm) (Kargar & Lee,

Fig. 8. Optimized novel slot photonic waveguides based on group IV alloys and material systems, designed at λop = 3.39μm. (a) Si0.08Ge0.78Sn0.14/Ge0.91Sn0.09/Si/SiO2 with w = 380nm, h = 520nm, t = 20nm, s = 50nm, g = 100nm; (b) Si0.08Ge0.78Sn0.14/Ge0.97C0.03/Si/SiO2 with w = 390nm, h = 560nm, t = 20nm, s = 50nm, g = 100nm.

In addition, these slot waveguides exhibit ultra-high performance (Sh > 1, LOD ~ 4×10-5). The most important limitation that characterizes some of these intriguing alloys is the impossibility to grow the analyzed material systems selectively and directly on silicon dioxide. The effect of this limitation imposes some inevitable technological restrictions in the design of above described novel photonic sensors. For example, GeSn and SiGeSn alloys can be grown either on Si(100) or Ge/Si(100) wafers, using the SnD4, SiGeH6, Ge2H6 and Si3H8 hydride compounds as a source of Sn, Ge and Si constituent atoms, respectively. However, the Si-Ge-Sn class of materials, including all alloys mentioned in this section, constitutes a new paradigm in the integration of Si based electronics with optical components on a single chip. In fact, all fabrication techniques needed for new group IV alloys, are perfectly compatible with CMOS standard processes and facilities.

#### **2.1.1 Fabrication tolerances**

Slot waveguides and PWEF sensors presented until now, have been represented assuming an ideal geometry. In fact, vertical sidewalls (Fig. 4 a-b, Fig. 6, Fig. 8 a-b) are very difficult to be obtained by the state-of-the-art etching processes (e.g., inductively coupled plasma, ICP). Thus, deviations from ideal case have to be considered and one of the most important parameters quantifying this effect is the tilting angle θ, as shown in Fig. 9. By this way, it is possible to distinguish between vertical (θ=0°) and non vertical (θ≠0°) sidewalls.

Fig. 9. Schematic view of slot waveguide. (a) real non vertical architecture for theoretical analysis of fabrication tolerances; (b) ideal slot waveguide with vertical sidewalls.

All well known performance parameters (Sh, ΓC, ΓS) have to be evaluated for different tilting angles θ° in the range 0°-10°. To this purpose, interesting results have been plotted in Fig. 10.

**(a) (b)** 

**(a) (b)** 

Fig. 8. Optimized novel slot photonic waveguides based on group IV alloys and material systems, designed at λop = 3.39μm. (a) Si0.08Ge0.78Sn0.14/Ge0.91Sn0.09/Si/SiO2 with w = 380nm, h = 520nm, t = 20nm, s = 50nm, g = 100nm; (b) Si0.08Ge0.78Sn0.14/Ge0.97C0.03/Si/SiO2 with

In addition, these slot waveguides exhibit ultra-high performance (Sh > 1, LOD ~ 4×10-5). The most important limitation that characterizes some of these intriguing alloys is the impossibility to grow the analyzed material systems selectively and directly on silicon dioxide. The effect of this limitation imposes some inevitable technological restrictions in the design of above described novel photonic sensors. For example, GeSn and SiGeSn alloys can be grown either on Si(100) or Ge/Si(100) wafers, using the SnD4, SiGeH6, Ge2H6 and Si3H8 hydride compounds as a source of Sn, Ge and Si constituent atoms, respectively. However, the Si-Ge-Sn class of materials, including all alloys mentioned in this section, constitutes a new paradigm in the integration of Si based electronics with optical components on a single chip. In fact, all fabrication techniques needed for new group IV alloys, are perfectly

Slot waveguides and PWEF sensors presented until now, have been represented assuming an ideal geometry. In fact, vertical sidewalls (Fig. 4 a-b, Fig. 6, Fig. 8 a-b) are very difficult to be obtained by the state-of-the-art etching processes (e.g., inductively coupled plasma, ICP). Thus, deviations from ideal case have to be considered and one of the most important parameters quantifying this effect is the tilting angle θ, as shown in Fig. 9. By this way, it is

possible to distinguish between vertical (θ=0°) and non vertical (θ≠0°) sidewalls.

 Fig. 9. Schematic view of slot waveguide. (a) real non vertical architecture for theoretical analysis of fabrication tolerances; (b) ideal slot waveguide with vertical sidewalls.

All well known performance parameters (Sh, ΓC, ΓS) have to be evaluated for different tilting angles θ° in the range 0°-10°. To this purpose, interesting results have been plotted in Fig. 10.

w = 390nm, h = 560nm, t = 20nm, s = 50nm, g = 100nm.

compatible with CMOS standard processes and facilities.

**2.1.1 Fabrication tolerances** 

θ

Fig. 10. Sensitivity of optimized slot waveguides as a function of the tilting angle θ with slot region width g fixed at 100nm in all cases. (a) Tolerance analysis of group IV-based optimized slot waveguides presented in Fig. 8; (b) Tolerances analysis of conventional SOI slot waveguides (@ 1.55μm, w = 230nm, h = 500nm, @ 3.39 & 2.883 μm, see also Fig. 7b).

All sensitivity values corresponding to θ = 0°, are referred to optimized slot waveguides characterized by vertical sidewalls. By observing Fig. 10a and 10b, it is evident that the greater the tilting angle, the lower the sensitivity. This effect is justified by the fact that increasing values of θ will produce a reduction of the slot region volume. Moreover, the slot region width g is not constant along y-direction. In conclusion, for θ > 4°, the slot mode cannot propagate because the optical field confinement in the slot region approaches zero.

Fabrication tolerance analysis represents an important aspect to estimate the real sensor performance. By this way, the solution to the technological problems presented above, is strictly related to the optimization of the etching process for a given combination of material systems. However, standard etching processes can assure tilting angles within 1°-2°. Generally, slot waveguides designed to operate in mid-IR wavelength range, exhibit greater tolerance margins because of their relaxed dimensions (Fig. 10a). To this purpose, θ < 3°-4° represents a good trade-off between technological constraints and device performance.

#### **2.1.2 State of the art of slot waveguides designed for chemical sensing**

All theoretical and experimental results demonstrate that slot waveguides are the best suitable photonic devices for sensing applications. In fact, if properly designed, they allow to concentrate a high optical field percentage in the cover medium, improving the interaction between the optical field and the chemical process, thus the limit of detection.

To this purpose, alternative solutions have been proposed in literature with the aim to enhance the efficiency and performance of photonic chemical sensors. For example, slot waveguides based on polymeric materials can ensure high performance, although characterized by low refractive index contrast ∆n (Bettotti et al., 2011). In addition, multipleslot waveguides represent an intriguing solution. By using multiple slot waveguides such as those presented in Fig. 11a-b-c, it is possible to significantly increase the sensitivity for both homogeneous (∆neff/nc ~ 0.2) and surface sensing (∆neff/∆ρ ~ 10-4 RIU/nm) (Kargar & Lee, 2009; Sun et al. 2009).

Chemical Sensors Based on Photonic Structures 101

functionalized cantilever. Consequently, an effective index change ∆neff is caused by strain optic effect. Normally, the stress induced by proteins lies in a range between 3000 and 15000 με (ε is the strain induced on the cantilever). Thus, the sensor response and the sensitivity are quite high. On the contrary, there are some aspects that limit the mass-scale production of these particular photonic sensors. First of all, the mechanical deformation, directly responsible of the sensing principle described above, has to be ensured also by very accurate technological steps, strongly influencing the sensor lifetime. Another important limitation concerns with the reliability of the sensing procedure. In fact, if the cantilever is corrupted by other molecules or particles not involved in the sensing detection, the calibration of the same sensor could be

In conclusion, both vertical and horizontal slot waveguides represent very promising photonic devices for chemical sensing applications. The high refractive index contrast between silicon and its oxide provides excellent optical confinement. By this way, it is possible to guide high optical intensity in the cover medium, preventing non linear effects in

Generally, the choice of the photonic device suitable for label-free sensing applications (rib waveguide, photonic wire and slot waveguide) concerns with the possibility to ensure the best overlapping between optical field and chemical/biochemical sample. As analyzed until now, in both homogeneous and surface sensing the optical transduction leads to the effective index change ∆neff. However, this optical variable has to be transformed into an analytically useful signal. To this purpose, several photonic integrated platforms have been proposed in literature. In particular, it is possible to distinguish between two different types of optical readout: the intensity and wavelength readout (power and wavelength interrogation, respectively). For example, photonic chemical sensors based on optical absorption are characterized by an intrinsic intensity readout, because the change of the absorption coefficient α is directly linked to the output optical intensity, according to the Beer-Lambert law. By this way, it is possible to register the chemical-to-optical transduction

In this context, advanced optical platforms for chemical sensing will be investigated in this

The operation principle of the integrated Mach-Zehnder interferometer (MZI) is illustrated in Fig. 13a. Light is introduced through a photonic waveguide (e.g., slot waveguide) in the Y-branch on the scheme left side. By this way, the input optical power *IIN* is split into two optical beams with an half power *IIN*/2 in the upper and lower arms, called "sensing" and "reference" arm, respectively. If any optical phase delay is applied to the guided mode in the sensing arm (*∆φ* = 0), light will be combined at the output Y-branch exhibiting an output optical power *IOUT* = *IIN*. In all different cases (*∆φ* ≠ 0 in sensing arm), the optical output

> 2 2 cos cos 2

⎛ ⎞ <sup>Δ</sup> ⎛ ⎞ <sup>Δ</sup> = = ⎜ ⎟ ⎜ ⎟ ⎝ ⎠ ⎝ ⎠

ϕ *eff*

λ

π

*n L*

(9)

power will be different from the input one (*IOUT* ≠ *IIN*), according to Eq. 9.

*OUT IN IN*

*II I*

silicon, such as two-photon absorption (TPA), that can lead to high optical losses.

wrong, thus procedure has to be repeated more than one time.

**3. Advanced platforms for chemical photonic sensing** 

by using a photodiode as a transducer (Fig. 1).

**3.1 Mach-Zehnder interferometer** 

Section.

Fig. 11. Cross section of a symmetric (w1 = 133nm, w2 = 33nm, g = 20nm, h = 250nm) (a), and asymmetric (w3 = 200nm, w2 = 33nm, g = 20nm, w4 = 67nm, h = 250nm) (b), silicon quintupleslot waveguides. Cross section of SU-8 (nSU-8 = 1.565 @ 1.55μm) multiple slot waveguide (c).

This improvement is justified by the fact that, in multiple-slot waveguides, the interaction between the optical field and the chemical test sample can be distributed in a larger area, in comparison to single slot waveguides. However, technological problems and reduced mode excitation usually limit the practical number of slots to three.

Another slot waveguide architecture evolution is represented by horizontal slot waveguide, as sketched in Fig. 12a. By this way, the optical propagating slot mode is quasi-TM, because the Ey-field is the only E-field component that undergoes discontinuities.

Fig. 12. Cross sectional view of the horizontal slot chemical sensor (a) with the spatial distribution of the fundamental TM optical mode (b).

A vertical slot fabrication can be realized by ICP etching, for example. However, etching in a very narrow region (slot region width g ~ 20-100nm) can produce large roughness in the vertical interface. By this way, in vertical slot waveguides, where the Ex-field is concentrated at the interface between the high and low refractive index region, propagation losses can be very high, such as 11.6 ± 3.6 dB/cm in a single slot of 50nm (Baehr-Jones et al., 2005). This problem can be partially solved by using horizontal slot waveguides (Fig. 12a-b). In fact, the latter can be fabricated by using thermal oxidation or deposition of different layers. By this way, the quasi TM-mode is not affected by surface roughness and propagation losses can be significantly reduced, down to 6.3 ± 0.2 dB/cm, 7.0 ± 0.2 dB/cm for a single and multiple slot waveguides, respectively (Sun et al., 2007). In Fig. 12a it is possible to see how the adsorption of target molecules on the cantilever surface, causes a small variation in the position of the

**(c)** 

Fig. 11. Cross section of a symmetric (w1 = 133nm, w2 = 33nm, g = 20nm, h = 250nm) (a), and asymmetric (w3 = 200nm, w2 = 33nm, g = 20nm, w4 = 67nm, h = 250nm) (b), silicon quintupleslot waveguides. Cross section of SU-8 (nSU-8 = 1.565 @ 1.55μm) multiple slot waveguide (c).

This improvement is justified by the fact that, in multiple-slot waveguides, the interaction between the optical field and the chemical test sample can be distributed in a larger area, in comparison to single slot waveguides. However, technological problems and reduced mode

Another slot waveguide architecture evolution is represented by horizontal slot waveguide, as sketched in Fig. 12a. By this way, the optical propagating slot mode is quasi-TM, because

Fig. 12. Cross sectional view of the horizontal slot chemical sensor (a) with the spatial

A vertical slot fabrication can be realized by ICP etching, for example. However, etching in a very narrow region (slot region width g ~ 20-100nm) can produce large roughness in the vertical interface. By this way, in vertical slot waveguides, where the Ex-field is concentrated at the interface between the high and low refractive index region, propagation losses can be very high, such as 11.6 ± 3.6 dB/cm in a single slot of 50nm (Baehr-Jones et al., 2005). This problem can be partially solved by using horizontal slot waveguides (Fig. 12a-b). In fact, the latter can be fabricated by using thermal oxidation or deposition of different layers. By this way, the quasi TM-mode is not affected by surface roughness and propagation losses can be significantly reduced, down to 6.3 ± 0.2 dB/cm, 7.0 ± 0.2 dB/cm for a single and multiple slot waveguides, respectively (Sun et al., 2007). In Fig. 12a it is possible to see how the adsorption of target molecules on the cantilever surface, causes a small variation in the position of the

excitation usually limit the practical number of slots to three.

distribution of the fundamental TM optical mode (b).

**(a) (b)** 

the Ey-field is the only E-field component that undergoes discontinuities.

**(b)** 

**(a)** 

functionalized cantilever. Consequently, an effective index change ∆neff is caused by strain optic effect. Normally, the stress induced by proteins lies in a range between 3000 and 15000 με (ε is the strain induced on the cantilever). Thus, the sensor response and the sensitivity are quite high. On the contrary, there are some aspects that limit the mass-scale production of these particular photonic sensors. First of all, the mechanical deformation, directly responsible of the sensing principle described above, has to be ensured also by very accurate technological steps, strongly influencing the sensor lifetime. Another important limitation concerns with the reliability of the sensing procedure. In fact, if the cantilever is corrupted by other molecules or particles not involved in the sensing detection, the calibration of the same sensor could be wrong, thus procedure has to be repeated more than one time.

In conclusion, both vertical and horizontal slot waveguides represent very promising photonic devices for chemical sensing applications. The high refractive index contrast between silicon and its oxide provides excellent optical confinement. By this way, it is possible to guide high optical intensity in the cover medium, preventing non linear effects in silicon, such as two-photon absorption (TPA), that can lead to high optical losses.

#### **3. Advanced platforms for chemical photonic sensing**

Generally, the choice of the photonic device suitable for label-free sensing applications (rib waveguide, photonic wire and slot waveguide) concerns with the possibility to ensure the best overlapping between optical field and chemical/biochemical sample. As analyzed until now, in both homogeneous and surface sensing the optical transduction leads to the effective index change ∆neff. However, this optical variable has to be transformed into an analytically useful signal. To this purpose, several photonic integrated platforms have been proposed in literature. In particular, it is possible to distinguish between two different types of optical readout: the intensity and wavelength readout (power and wavelength interrogation, respectively). For example, photonic chemical sensors based on optical absorption are characterized by an intrinsic intensity readout, because the change of the absorption coefficient α is directly linked to the output optical intensity, according to the Beer-Lambert law. By this way, it is possible to register the chemical-to-optical transduction by using a photodiode as a transducer (Fig. 1).

In this context, advanced optical platforms for chemical sensing will be investigated in this Section.

#### **3.1 Mach-Zehnder interferometer**

The operation principle of the integrated Mach-Zehnder interferometer (MZI) is illustrated in Fig. 13a. Light is introduced through a photonic waveguide (e.g., slot waveguide) in the Y-branch on the scheme left side. By this way, the input optical power *IIN* is split into two optical beams with an half power *IIN*/2 in the upper and lower arms, called "sensing" and "reference" arm, respectively. If any optical phase delay is applied to the guided mode in the sensing arm (*∆φ* = 0), light will be combined at the output Y-branch exhibiting an output optical power *IOUT* = *IIN*. In all different cases (*∆φ* ≠ 0 in sensing arm), the optical output power will be different from the input one (*IOUT* ≠ *IIN*), according to Eq. 9.

$$I\_{OUT} = I\_{IN} \cos^2 \left(\frac{\Delta \varphi}{2}\right) = I\_{IN} \cos^2 \left(\frac{\pi \Delta n\_{eff} L}{\lambda}\right) \tag{9}$$

Chemical Sensors Based on Photonic Structures 103

In fact, the signal change is not easily resolvable near the maximum and minimum of the

A surface plasmon is a localized electromagnetic wave that propagates along the metaldielectric interface and exponentially decays into both media. Surface plasmons can be excited due to the resonant transfer of the incident photon energy and momentum to collectively oscillating electrons in a noble metal. The plasmon resonance condition is given

0 0 <sup>2</sup> sin *mr S*

In Eq. 10, *εmr* is the real part of the metal dielectric function, nC is the refractive index of light coupling glass, nS is the refractive index of sensing medium (dielectric), *λ* is the wavelength of incident light and k0 is the wave number (*2π/λ*). The resonance condition is satisfied when *β* (propagation constant of the light beam coupled at the angle θ) is equal to *βSPW* (propagation constant of the surface plasmon polariton). The resonant condition is expressed by Eq. 10 when *θ = θSPR*. At the resonant condition, the reflected light intensity encounters a sharp dip due to optical absorption by surface plasmon wave (SPW). This dip can be easily detected by the photo-detector, as reported in the scheme proposed in Fig. 14.

Fig. 14. Schematic view of a SPR biosensor characterized by prism coupling configuration.

Bio-molecular recognition elements at the metal surface selectively capture target analytes in a chemical liquid test sample, the reflectance dip makes a shift due to the change of local refractive index near the surface. In particular, plasmonic related sensing procedure has been used for label-free detection of biological analytes at pico/femto-molar level, pesticides, immunoassays, DNA with a refractive index resolution of 1.4×10-7 (Le et al. 2011), RNA, allergens and human blood-group identification (Jha & Sharma, 2010). For example, a SPR mid-infrared immunosensor used in a Fourier-transform infrared (FTIR) spectroscopy platform, exhibits a sensitivity of 3022 nm/RIU and a limit of detection of ~70 pg/mm2 (DiPippo et al., 2010). Generally, the detection of the SPR change due to the adsorption of target molecules on the sensing surface, can be quantified by monitoring the resonant intensity, wavelength or angle change. Although SPR chemical sensors exhibit high sensing performance, they are characterized by some operative constrains. In particular,

*<sup>n</sup> kn k*

θ

*C*

1 2 2

(10)

*mr S*

ε

⎛ ⎞ <sup>=</sup> ⎜ ⎟ + ⎝ ⎠

ε

*n*

cosine function.

**3.2 Surface plasmon resonance biosensors** 

by the following expression (Jha, R. & Sharma, 2010):

( )

In Eq. 9, L is the interaction length, thus the guiding path in which the overlapping between the optical field and the chemical test sample occurs, and *λ* is the operative wavelength.

Fig. 13. (a) Schematic view of Mach-Zehnder interferometer; (b) Normalized output power as a function of different values of L (black = 3mm, red = 1.5mm, green = 750nm, λop = 1.55μm); (c) Image of a balanced MZI characterized by spiral arms.

In Fig. 13b, it is possible to see the effect of different values of L on sensing efficiency. In fact, sensor resolution is the capability to detect the minimum effective index perturbation. By this way, it is possible to conclude that the longer the interaction length, the higher the sensor resolution. Graphically, this concept can be observed by analyzing the steepness and width of each cos2 lobe. In conclusion, the closer the transmission spectra lobes, the smaller the detectable effective index change. Obviously, it is not convenient to realize cm-long arms because photonic chemical sensors have to be characterized by very small footprints. To this purpose, interesting technological solutions have been presented in literature. In Fig. 13c, a spiral path configuration is presented. By this way, it is possible to improve sensor performance and efficiency, without compromising the sensor dimensions. In fact, mm-long arms (~ 2-3mm) can be realized and concentrated in spiral architecture characterized by μmlong diameter.

Very high integration with spiral-path configurations allows to design biosensor arrays for multiplexed real-time and label-free molecular detection. Ultra high performance can be achieved in terms of sensitivity and LOD. For example, the measured sensitivity of the 2.1 mm-long interferometer fabricated on SOI wafer with 2μm silica under 260nm of silicon, is dφ/dnC = 4930 rad/RIU, while the calculated change of polydimethylsiloxane (PDMS) refractive index upon xylene is 8.7×10-7 RIU/ppm (Saunders et al., 2010). PDMS has been used as sensor cladding in order to enable sensing of BTEX (Benzene, Toluene, Ethylbenzene, Xylenes) volatile organic compound. In addition, a surface mass coverage of 0.3 pg/mm2 can be detected with a silicon photonic wire MZI sensor array, with integrated SU-8 micro-fluidic channel (Densmore et al., 2009). In conclusion, the principal penalty of using MZI photonic sensors is represented by the cosine-dependent intensity function.

In Eq. 9, L is the interaction length, thus the guiding path in which the overlapping between the optical field and the chemical test sample occurs, and *λ* is the operative wavelength.

**(b)** 

Fig. 13. (a) Schematic view of Mach-Zehnder interferometer; (b) Normalized output power as a function of different values of L (black = 3mm, red = 1.5mm, green = 750nm, λop =

In Fig. 13b, it is possible to see the effect of different values of L on sensing efficiency. In fact, sensor resolution is the capability to detect the minimum effective index perturbation. By this way, it is possible to conclude that the longer the interaction length, the higher the sensor resolution. Graphically, this concept can be observed by analyzing the steepness and width of each cos2 lobe. In conclusion, the closer the transmission spectra lobes, the smaller the detectable effective index change. Obviously, it is not convenient to realize cm-long arms because photonic chemical sensors have to be characterized by very small footprints. To this purpose, interesting technological solutions have been presented in literature. In Fig. 13c, a spiral path configuration is presented. By this way, it is possible to improve sensor performance and efficiency, without compromising the sensor dimensions. In fact, mm-long arms (~ 2-3mm) can be realized and concentrated in spiral architecture characterized by μm-

Very high integration with spiral-path configurations allows to design biosensor arrays for multiplexed real-time and label-free molecular detection. Ultra high performance can be achieved in terms of sensitivity and LOD. For example, the measured sensitivity of the 2.1 mm-long interferometer fabricated on SOI wafer with 2μm silica under 260nm of silicon, is dφ/dnC = 4930 rad/RIU, while the calculated change of polydimethylsiloxane (PDMS) refractive index upon xylene is 8.7×10-7 RIU/ppm (Saunders et al., 2010). PDMS has been used as sensor cladding in order to enable sensing of BTEX (Benzene, Toluene, Ethylbenzene, Xylenes) volatile organic compound. In addition, a surface mass coverage of 0.3 pg/mm2 can be detected with a silicon photonic wire MZI sensor array, with integrated SU-8 micro-fluidic channel (Densmore et al., 2009). In conclusion, the principal penalty of using MZI photonic sensors is represented by the cosine-dependent intensity

1.55μm); (c) Image of a balanced MZI characterized by spiral arms.

**(a)** 

**(c)** 

long diameter.

function.

In fact, the signal change is not easily resolvable near the maximum and minimum of the cosine function.

#### **3.2 Surface plasmon resonance biosensors**

A surface plasmon is a localized electromagnetic wave that propagates along the metaldielectric interface and exponentially decays into both media. Surface plasmons can be excited due to the resonant transfer of the incident photon energy and momentum to collectively oscillating electrons in a noble metal. The plasmon resonance condition is given by the following expression (Jha, R. & Sharma, 2010):

$$k\_0 n\_\mathbb{C} \sin \left(\theta \right) = k\_0 \left(\frac{\varepsilon\_{mr} n\_S^2}{\varepsilon\_{mr} + n\_S^2} \right)^{\frac{1}{2}} \tag{10}$$

In Eq. 10, *εmr* is the real part of the metal dielectric function, nC is the refractive index of light coupling glass, nS is the refractive index of sensing medium (dielectric), *λ* is the wavelength of incident light and k0 is the wave number (*2π/λ*). The resonance condition is satisfied when *β* (propagation constant of the light beam coupled at the angle θ) is equal to *βSPW* (propagation constant of the surface plasmon polariton). The resonant condition is expressed by Eq. 10 when *θ = θSPR*. At the resonant condition, the reflected light intensity encounters a sharp dip due to optical absorption by surface plasmon wave (SPW). This dip can be easily detected by the photo-detector, as reported in the scheme proposed in Fig. 14.

Fig. 14. Schematic view of a SPR biosensor characterized by prism coupling configuration.

Bio-molecular recognition elements at the metal surface selectively capture target analytes in a chemical liquid test sample, the reflectance dip makes a shift due to the change of local refractive index near the surface. In particular, plasmonic related sensing procedure has been used for label-free detection of biological analytes at pico/femto-molar level, pesticides, immunoassays, DNA with a refractive index resolution of 1.4×10-7 (Le et al. 2011), RNA, allergens and human blood-group identification (Jha & Sharma, 2010). For example, a SPR mid-infrared immunosensor used in a Fourier-transform infrared (FTIR) spectroscopy platform, exhibits a sensitivity of 3022 nm/RIU and a limit of detection of ~70 pg/mm2 (DiPippo et al., 2010). Generally, the detection of the SPR change due to the adsorption of target molecules on the sensing surface, can be quantified by monitoring the resonant intensity, wavelength or angle change. Although SPR chemical sensors exhibit high sensing performance, they are characterized by some operative constrains. In particular,

Chemical Sensors Based on Photonic Structures 105

the Bragg grating spectral response (Fig. 15b), following the relation as in Eq. 12. In particular, m is the grating order, *neff* is the effective index of the propagating mode and *λC* is

> *eff m n* Λ =

> > 2 2 22 2 tanh tanh *k L*

δ

where *k* is the coupling coefficient between backward and forward propagating optical

By this way, it is possible to explain the sensor device operative regime. In particular, all geometrical parameters (h, H, D, W, d, Λ) and the index contrast ∆n, have to be designed in order to determinate the central operative wavelength *λC*. In Fig. 15b, it corresponds to 1550 nm. By observing Eq. 12, it is possible to see that *λC* only depends on the effective index of the mode propagating, because the grating period Λ is fixed after the device fabrication. Consequently, any effective index change *∆neff* induced by homogeneous or surface sensing,

γ= − *k*

 π

Fig. 15. (a) Schematic view of a grating–based sensor device. (b) Spectral response of the first-order grating for different values of cover refractive index (H = 1μm, d = 30nm, W =

In the graph reported in Fig. 15b, the sensor operative regime is characterized by *λ<sup>C</sup>* = 1550nm with air cover. When the sensor is exposed to aqueous solution (e.g., water), *λ<sup>C</sup>* = 1552.7nm. By this way, if the selected operative wavelength *λop* is 1550nm, than it is possible to detect a change in the solution composition by measuring the new center wavelength shift, *∆λC* = 2.69nm. By using third-order grating based sensor devices (m = 3), it

<sup>=</sup> <sup>+</sup>

In conclusion, the grating reflectivity can be expressed by the following expression:

γ λ

( )

γ

γ

*L*

2 2

δ

(12)

(13)

(14)

the central wavelength, previously defined.

causes a central wavelength shift, *∆λC*.

0.56μm, D = 0.62μm, Λ = 240nm, nair = 1, nwater = 1.33).

2 *<sup>C</sup>*

( ) ( )

*R*

δ

modes, *L* is the grating length, *γ* and *δ* are two optical parameters defined as:

2 *neff* π

λ= − <sup>Λ</sup> ;

**(a) (b)** 

δ

surface plasmon evanescent field penetrates in the sensing layer only for ~100nm. In this way, it is not possible to detect large target molecules like cells and bacteria. Another limitation concerns with the impossibility of the SPR photonic sensor, to distinguish between the refractive index surface change and the bulk solution refractive index change. Then, if the sensor is covered by a complex chemical solution (e.g., blood, bacteria-solution), it is very difficult to ensure high detection resolution. To this purpose, interesting architectures have been proposed in literature such as a novel optical structure, in which the metal layer is sandwiched by two dielectric layers with similar refractive index. In this way, both long (LSPR) and short (SSPR) surface plasmon modes can be generated at both metaldielectric interfaces, allowing to differentiate the background and surface bound refractive index change (Li et al., 2010). In conclusion, all commercialized SPR biosensors can be characterized by different coupling methods. In addition to prism coupling configuration sketched in Fig. 14, there are guided-wave, fiber optic and grating coupling configurations (Marrocco et al., 2010). Slot plasmonic waveguides have been also proposed as an integrated chemical sensor with high sensitivity (Hu et al., 2010).

#### **3.3 Chemical sensors based on photonic crystals**

Photonic crystals (PhCs) are periodic systems characterized by symmetrical separation between high dielectric and low dielectric regions. Generally, it is possible to distinguish between three different geometrical configurations as a function of the spatial periodicity. In fact, there are one-dimensional PhCs where the dielectric function periodicity is distributed only to one dimension (1D), thus bi-dimensional (2D) and three-dimensional (3D) PhCs. By using photonic crystals it is possible to guide and trap the light. In addition, photonic crystals exhibit a very large selective photonic band gap that prevents photons in the band gap from propagating in the material. By this way, it is possible to design a photonic waveguide by creating a line defect in the structure. The basic element of a photonic crystal is called "elemental" or "primary cell" and it can be represented by a line, circle, hexagonal or tetrahedral geometry, to name a few. For example, the periodical repetition of the hexagonal geometry produces a photonic crystal with a particular structure defined as "honeycomb". Photonic crystals exhibit interesting and promising features, suitable for high performance chemical sensing.

#### **3.3.1 Integrated optical Bragg-grating-based chemical sensors**

Photonic Bragg gratings are characterized by a periodic spatial distribution of the dielectric constant, thus of the refractive index, along the propagation direction. Consequently they are photonic crystals and exhibit a selective range of propagating wavelengths.

It is possible to realize an integrated optical sensor based on Bragg gratings in SOI technology, as sketched in Fig. 15 (Passaro et al., 2008). A Bragg grating is characterized by a periodic perturbation of the waveguide refractive index along the propagating direction z:

$$n(\mathbf{x}, y, z) = n\_0(\mathbf{x}, y) + \Delta n(\mathbf{x}, y, z) \tag{11}$$

In particular, *n0(x,y*) is the waveguide refractive index distribution, *∆n(x,y,z)* represents the periodic refractive index change characterized by a period Λ (Fig. 15a). Λ represents a physical parameter that has to be designed in order to ensure a precise center wavelength of

surface plasmon evanescent field penetrates in the sensing layer only for ~100nm. In this way, it is not possible to detect large target molecules like cells and bacteria. Another limitation concerns with the impossibility of the SPR photonic sensor, to distinguish between the refractive index surface change and the bulk solution refractive index change. Then, if the sensor is covered by a complex chemical solution (e.g., blood, bacteria-solution), it is very difficult to ensure high detection resolution. To this purpose, interesting architectures have been proposed in literature such as a novel optical structure, in which the metal layer is sandwiched by two dielectric layers with similar refractive index. In this way, both long (LSPR) and short (SSPR) surface plasmon modes can be generated at both metaldielectric interfaces, allowing to differentiate the background and surface bound refractive index change (Li et al., 2010). In conclusion, all commercialized SPR biosensors can be characterized by different coupling methods. In addition to prism coupling configuration sketched in Fig. 14, there are guided-wave, fiber optic and grating coupling configurations (Marrocco et al., 2010). Slot plasmonic waveguides have been also proposed as an integrated

Photonic crystals (PhCs) are periodic systems characterized by symmetrical separation between high dielectric and low dielectric regions. Generally, it is possible to distinguish between three different geometrical configurations as a function of the spatial periodicity. In fact, there are one-dimensional PhCs where the dielectric function periodicity is distributed only to one dimension (1D), thus bi-dimensional (2D) and three-dimensional (3D) PhCs. By using photonic crystals it is possible to guide and trap the light. In addition, photonic crystals exhibit a very large selective photonic band gap that prevents photons in the band gap from propagating in the material. By this way, it is possible to design a photonic waveguide by creating a line defect in the structure. The basic element of a photonic crystal is called "elemental" or "primary cell" and it can be represented by a line, circle, hexagonal or tetrahedral geometry, to name a few. For example, the periodical repetition of the hexagonal geometry produces a photonic crystal with a particular structure defined as "honeycomb". Photonic crystals exhibit interesting and promising features, suitable for high

Photonic Bragg gratings are characterized by a periodic spatial distribution of the dielectric constant, thus of the refractive index, along the propagation direction. Consequently they

It is possible to realize an integrated optical sensor based on Bragg gratings in SOI technology, as sketched in Fig. 15 (Passaro et al., 2008). A Bragg grating is characterized by a periodic perturbation of the waveguide refractive index along the propagating direction z:

( ) () ( ) <sup>0</sup> *nxyz n xy nxyz* ,, , ,, = +Δ (11)

In particular, *n0(x,y*) is the waveguide refractive index distribution, *∆n(x,y,z)* represents the periodic refractive index change characterized by a period Λ (Fig. 15a). Λ represents a physical parameter that has to be designed in order to ensure a precise center wavelength of

chemical sensor with high sensitivity (Hu et al., 2010).

**3.3 Chemical sensors based on photonic crystals** 

**3.3.1 Integrated optical Bragg-grating-based chemical sensors** 

are photonic crystals and exhibit a selective range of propagating wavelengths.

performance chemical sensing.

the Bragg grating spectral response (Fig. 15b), following the relation as in Eq. 12. In particular, m is the grating order, *neff* is the effective index of the propagating mode and *λC* is the central wavelength, previously defined.

$$
\Lambda = \frac{m}{2n\_{\text{eff}}} \mathcal{A}\_{\text{C}} \tag{12}
$$

In conclusion, the grating reflectivity can be expressed by the following expression:

$$R\left(\mathcal{S}\right) = \frac{\left|k\right|^2 \tanh^2\left(\mathcal{VL}\right)}{\mathcal{r}^2 + \delta^2 \tanh^2\left(\mathcal{VL}\right)}\tag{13}$$

where *k* is the coupling coefficient between backward and forward propagating optical modes, *L* is the grating length, *γ* and *δ* are two optical parameters defined as:

$$\delta = \frac{2\pi}{\lambda} n\_{eff} - \frac{\pi}{\Lambda} \; ; \; \gamma = \sqrt{\left| k \right|^2 - \delta^2} \tag{14}$$

By this way, it is possible to explain the sensor device operative regime. In particular, all geometrical parameters (h, H, D, W, d, Λ) and the index contrast ∆n, have to be designed in order to determinate the central operative wavelength *λC*. In Fig. 15b, it corresponds to 1550 nm. By observing Eq. 12, it is possible to see that *λC* only depends on the effective index of the mode propagating, because the grating period Λ is fixed after the device fabrication. Consequently, any effective index change *∆neff* induced by homogeneous or surface sensing, causes a central wavelength shift, *∆λC*.

Fig. 15. (a) Schematic view of a grating–based sensor device. (b) Spectral response of the first-order grating for different values of cover refractive index (H = 1μm, d = 30nm, W = 0.56μm, D = 0.62μm, Λ = 240nm, nair = 1, nwater = 1.33).

In the graph reported in Fig. 15b, the sensor operative regime is characterized by *λ<sup>C</sup>* = 1550nm with air cover. When the sensor is exposed to aqueous solution (e.g., water), *λ<sup>C</sup>* = 1552.7nm. By this way, if the selected operative wavelength *λop* is 1550nm, than it is possible to detect a change in the solution composition by measuring the new center wavelength shift, *∆λC* = 2.69nm. By using third-order grating based sensor devices (m = 3), it

Chemical Sensors Based on Photonic Structures 107

principle of IR absorption spectroscopy is based on the Beer-Lambert law, described in the

The photonic device shown in Fig. 17a, has been realized by using a circular geometry as

Fig. 17. (a) Photonic crystal slot waveguide built on SOI platform. (b) Photonic crystal

Consequently, a periodic 3D spatial distribution of the refractive index is realized with a lattice constant a. In fact, it is possible to observe that circular holes are characterized by low refractive index (nSiO2), while the reciprocal space represents the high refractive index region (nSi). A slot waveguide has been realized by introducing a line defect. By this way, a high electric field intensity is localized in a low-index (nair) 90nm wide slot. The silicon photonic crystal slot waveguide device is characterized by a total length of 300μm in order to increase the optical absorption path length. A methane concentration of 100ppm in nitrogen has been

**(b)** 

In Fig. 17b a photonic crystal air-slot cavity has been presented (Jagerska et al., 2010). The structure is similar to the previously analyzed slot waveguide. In fact, the proposed air-slot cavity has been processed on a 220nm thick silicon-on-insulator wafer with a 2μm buried oxide layer. The cavity has been realized by reducing the line defect width by 20nm, thus unchanging the photonic crystal lattice constant. Reduced-slit width results in the formation of reflective barriers for the cavity mode. By this way, only resonant wavelengths can propagate inside the photonic cavity. The sensor has been tested by exposing the chip to gases of different refractive index such as N2 (RI = 1.00270), He (1.00032) and CO2 (1.00407) at the operative wavelength, λop = 1570nm. The refractive index change (∆n) due to the sensor exposition to different gases, causes a resonant wavelength shift (∆λ). Consequently, it is possible to detect different gas concentrations by monitoring the cavity transmission

*eff*

Δ (17)

 λ

*n n*

= =Γ

*S*

Δλ

resonant cavity based on slot waveguide for sensing application.

previous section (Lai et al., 2011).

primary cell.

**(a)** 

measured.

spectrum.

The sensor sensitivity can be defined as follows:

is possible to maximize the device efficiency and employ shorter length chips. In conclusion, grating sensitivity can be defined as following:

$$\Delta S = \frac{\partial \mathcal{J}\_{\text{C}}}{\partial n\_{\text{clad}}} = \frac{\partial \mathcal{J}\_{\text{C}}}{\partial n\_{\text{eff}}} \frac{\partial n\_{\text{eff}}}{\partial n\_{\text{clad}}} = \frac{\Lambda}{m} S\_{\text{av}} = \frac{\mathcal{J}\_{\text{C}}}{2n\_{\text{eff}}} S\_{\text{av}} \tag{15}$$

In Eq. 15, *nclad* is the cladding refractive index and *Sw* is the waveguide sensitivity, whose definition depends on sensing mechanism (homogeneous or surface sensing). A device sensitivity *S* around 120nm/RIU is practically obtainable. In conclusion, Bragg gratings analyzed above can be adopted in different waveguide architectures such fiber optics (fiber Bragg gratings, FBG). Sensitivity of 92 nm/RIU has been demonstrated for refractive index change of water in a FBG chemical sensor (Lee et al., 2010).

The sensing principle described above is based on the wavelength interrogation of the Bragg grating. However, another type of interrogation can be used, based on the incident angle θ (°) that the propagating light forms with the Bragg grating surface. The device operation can be expressed by the following relation:

$$m\mathcal{X} = 2\Lambda \sin \theta \tag{16}$$

Coupling wavelength *λ* depends on the grating period and interrogation angle. By this way, by monitoring changes in coupling angle, molecular binding at the waveguide surface can be detected and measured by using SPR sensors, as in Fig 16a-b. In particular, the reflected intensity spectrum exhibits a sharp resonance peak at the coupling wavelength.

Fig. 16. (a) Schematic view of a photonic silicon waveguide sensor based on surface grating interrogation. (b) Chemical grating–coupled surface plasmon resonance sensor.

The sensing principle based on angle interrogation can be used also in surface plasmon resonance sensor characterized by a grating coupling. In particular, a refractive index resolution of 1.5×10-6 RIU has been demonstrated by using a sensor as that sketched in Fig. 16b (Kuo & Chang, 2010).

#### **3.3.2 State of the art of integrated chemical sensors based on photonic crystals**

Nowadays, one and two dimensional photonic crystals represent an interesting and promising research field for sensing applications, because of their compactness and high resolution in the detection process. Recently, a photonic crystal slot waveguide has been fabricated and used as infrared (IR) absorption spectrometer for methane monitoring. The

is possible to maximize the device efficiency and employ shorter length chips. In conclusion,

*n S S S n nn m n*

∂ Λ

In Eq. 15, *nclad* is the cladding refractive index and *Sw* is the waveguide sensitivity, whose definition depends on sensing mechanism (homogeneous or surface sensing). A device sensitivity *S* around 120nm/RIU is practically obtainable. In conclusion, Bragg gratings analyzed above can be adopted in different waveguide architectures such fiber optics (fiber Bragg gratings, FBG). Sensitivity of 92 nm/RIU has been demonstrated for refractive index

The sensing principle described above is based on the wavelength interrogation of the Bragg grating. However, another type of interrogation can be used, based on the incident angle θ (°) that the propagating light forms with the Bragg grating surface. The device operation can

Coupling wavelength *λ* depends on the grating period and interrogation angle. By this way, by monitoring changes in coupling angle, molecular binding at the waveguide surface can be detected and measured by using SPR sensors, as in Fig 16a-b. In particular, the reflected

Fig. 16. (a) Schematic view of a photonic silicon waveguide sensor based on surface grating

The sensing principle based on angle interrogation can be used also in surface plasmon resonance sensor characterized by a grating coupling. In particular, a refractive index resolution of 1.5×10-6 RIU has been demonstrated by using a sensor as that sketched in Fig.

Nowadays, one and two dimensional photonic crystals represent an interesting and promising research field for sensing applications, because of their compactness and high resolution in the detection process. Recently, a photonic crystal slot waveguide has been fabricated and used as infrared (IR) absorption spectrometer for methane monitoring. The

**3.3.2 State of the art of integrated chemical sensors based on photonic crystals** 

interrogation. (b) Chemical grating–coupled surface plasmon resonance sensor.

**(a) (b)** 

 θ

λ

intensity spectrum exhibits a sharp resonance peak at the coupling wavelength.

∂ ∂ λ

change of water in a FBG chemical sensor (Lee et al., 2010).

2 sin *m*

 λ

*C C eff <sup>C</sup> w w clad eff clad eff*

2

λ= = == ∂ ∂∂ (15)

= Λ (16)

grating sensitivity can be defined as following:

be expressed by the following relation:

16b (Kuo & Chang, 2010).

principle of IR absorption spectroscopy is based on the Beer-Lambert law, described in the previous section (Lai et al., 2011).

The photonic device shown in Fig. 17a, has been realized by using a circular geometry as primary cell.

Fig. 17. (a) Photonic crystal slot waveguide built on SOI platform. (b) Photonic crystal resonant cavity based on slot waveguide for sensing application.

Consequently, a periodic 3D spatial distribution of the refractive index is realized with a lattice constant a. In fact, it is possible to observe that circular holes are characterized by low refractive index (nSiO2), while the reciprocal space represents the high refractive index region (nSi). A slot waveguide has been realized by introducing a line defect. By this way, a high electric field intensity is localized in a low-index (nair) 90nm wide slot. The silicon photonic crystal slot waveguide device is characterized by a total length of 300μm in order to increase the optical absorption path length. A methane concentration of 100ppm in nitrogen has been measured.

In Fig. 17b a photonic crystal air-slot cavity has been presented (Jagerska et al., 2010). The structure is similar to the previously analyzed slot waveguide. In fact, the proposed air-slot cavity has been processed on a 220nm thick silicon-on-insulator wafer with a 2μm buried oxide layer. The cavity has been realized by reducing the line defect width by 20nm, thus unchanging the photonic crystal lattice constant. Reduced-slit width results in the formation of reflective barriers for the cavity mode. By this way, only resonant wavelengths can propagate inside the photonic cavity. The sensor has been tested by exposing the chip to gases of different refractive index such as N2 (RI = 1.00270), He (1.00032) and CO2 (1.00407) at the operative wavelength, λop = 1570nm. The refractive index change (∆n) due to the sensor exposition to different gases, causes a resonant wavelength shift (∆λ). Consequently, it is possible to detect different gas concentrations by monitoring the cavity transmission spectrum.

The sensor sensitivity can be defined as follows:

$$S = \frac{\Delta \mathcal{X}}{\Delta n} = \Gamma \frac{\mathcal{X}}{n\_{\text{eff}}} \tag{17}$$

Chemical Sensors Based on Photonic Structures 109

where k is the coupling coefficient and depends on the overlap between the two propagating modal fields, and L is the device length. In synchronism condition (*∆β* = 0), complete power transfer from the waveguide 1 to the waveguide 2 is achieved when kL = π/2, 3π/2, 5π/2, etc. In this way, it will results *P2* = *P0* and *P1* = 0, at the end section x = L. When a chemical test sample, gas or liquid solution, covers the waveguide 1, the refractive index of the cover medium will change, causing a modal effective index change of the propagating mode neff1. Consequently, the synchronous coupling condition will not be ensured because β1 ≠ β2, thus ∆β ≠ 0. According to Eq. 18, P1 will be different to zero because a complete optical power transfer now cannot occur. By monitoring the change of coupler outputs P1 and P2, it is possible to detect a particular chemical target and estimate its concentration with high limit of detection. The sensitivity S can be calculated as follows:

( )

*clad REF*

quasi-TE mode with a coupler length L = 400μm (Passaro et al., 2009b).

300nm, w = 180nm, g = 100nm, s = 500nm).

**(a) (b)** 

*<sup>T</sup> S n*

( )

<sup>∂</sup> <sup>=</sup> ∂ (19)

*clad nclad nclad REF*

*n* <sup>=</sup>

where T is the transmittance registered at the output of the reference waveguides, nclad is the chemical analyte refractive index and *nclad(REF)* is the reference analyte refractive index value for which the derivative is calculated. The highest sensitivity can be achieved at the point of maximum slope of the main peak of the transmittance curve. Consequently, this condition indicates the operating point of the sensor. Obviously, the overall sensitivity depends on the waveguide sensitivity, *SW*. In this context, the use of slot waveguides represents the best solution. To this purpose, interesting results have been obtained by using coupled slot SOI waveguides sketched in Fig. 19a. In particular, a sensitivity S = 215.29 was achieved for

Fig. 19. (a) Architecture of coupled SOI slot waveguides for chemical sensing (h = 324nm, w = 180nm, g = 100nm). (b) Multi-channel directional coupler with slot waveguides (h =

The sensor sketched in Fig. 19a allows to estimate an analyte concentration in the aqueous solution. In particular, the photonic sensor integrated with a footprint of ~ 1mm2, can detect a minimum refractive index change of ~10-5 and it can be adopted to estimate the glucose

In Fig. 19b, a multi-channel directional coupler with slot waveguides is shown (McCosker & Town, 2010a). The sensing principle adopted in this device is different with respect to the

concentration in aqueous solution with a theoretical resolution of 0.1 g/L.

In Eq. 17, *λ* is the resonance wavelength of the cavity, Γ is the mode field overlap and *neff* the modal effective index. The high quality factor of the cavity (Q ~ 2.6×104) allows to achieve an experimental sensitivity *S* = 510 nm/RIU and a detection limit below 1×10-5 RIU.

In conclusion, other intriguing solutions have been proposed such as a single-defect silicon 2D photonic crystal nano-cavity for strain sensing (Tung et al., 2011). The sensing principle is the same as the gas sensor described above, but the resonant wavelength shift is induced by geometrical longitudinal and transverse strain. The theoretical minimum detectable strain for the photonic crystal cavity has been estimated to be 8.5×10-9 ε (8.5nε). By this way, advanced chemical sensing applications can be contemplated, based on cantilever sensing architectures.

#### **3.4 Integrated optical directional coupler biosensors**

The use of optical directional couplers for the measurement of interactions between biological molecules and the detection of target chemical and biochemical species, is an interesting research field for sensing applications. In fact, these photonic sensors are characterized by high performance and real-time monitoring of multiple outputs. The scheme of a directional coupler is presented in Fig. 18.

Fig. 18. Schematic representation of an integrated direction coupler.

The sensing principle is based on the super-mode theory. In Fig. 18 it is possible to see that only one waveguide (in this case the waveguide 1), is directly exposed to the chemical test sample. The other one has to be isolated by using appropriate coatings (e.g., polymers, Teflon). Generally, both waveguides have to be designed as symmetrical (e.g., identical geometries and contrast index ∆n), in order to exhibit the same propagation features. In this way, β1 and β2, the propagation constants of the optical modes propagating in the waveguide 1 and 2, respectively, can be considered equal (β1 = β2) in a "synchronous" regime, where the propagation constant mismatch is equal to zero (∆β = 0). By assuming that an optical power P0 is introduced in the waveguide 1 at the section x = 0, optical powers (P1, P2) at the output section (x = L) can be calculated by the following expressions:

$$P\_2 = \frac{\sin^2\left[\left(kL\right)^2 + \left(\frac{\Delta\beta L}{2}\right)^2\right]\right^{\frac{1}{2}}}{1 + \left(\frac{\Delta\beta}{2k}\right)^2} P\_0 \; P\_1 = P\_0 - P\_2 \tag{18}$$

In Eq. 17, *λ* is the resonance wavelength of the cavity, Γ is the mode field overlap and *neff* the modal effective index. The high quality factor of the cavity (Q ~ 2.6×104) allows to achieve

In conclusion, other intriguing solutions have been proposed such as a single-defect silicon 2D photonic crystal nano-cavity for strain sensing (Tung et al., 2011). The sensing principle is the same as the gas sensor described above, but the resonant wavelength shift is induced by geometrical longitudinal and transverse strain. The theoretical minimum detectable strain for the photonic crystal cavity has been estimated to be 8.5×10-9 ε (8.5nε). By this way, advanced chemical sensing applications can be contemplated, based on cantilever sensing

The use of optical directional couplers for the measurement of interactions between biological molecules and the detection of target chemical and biochemical species, is an interesting research field for sensing applications. In fact, these photonic sensors are characterized by high performance and real-time monitoring of multiple outputs. The

The sensing principle is based on the super-mode theory. In Fig. 18 it is possible to see that only one waveguide (in this case the waveguide 1), is directly exposed to the chemical test sample. The other one has to be isolated by using appropriate coatings (e.g., polymers, Teflon). Generally, both waveguides have to be designed as symmetrical (e.g., identical geometries and contrast index ∆n), in order to exhibit the same propagation features. In this way, β1 and β2, the propagation constants of the optical modes propagating in the waveguide 1 and 2, respectively, can be considered equal (β1 = β2) in a "synchronous" regime, where the propagation constant mismatch is equal to zero (∆β = 0). By assuming that an optical power P0 is introduced in the waveguide 1 at the section x = 0, optical powers

(P1, P2) at the output section (x = L) can be calculated by the following expressions:

*<sup>L</sup> kL*

2 0 2

*P P*

⎧ ⎫ ⎡ ⎤ ⎪ ⎪ ⎛ ⎞ <sup>Δ</sup> ⎨ ⎬ ⎢ ⎥ <sup>+</sup> ⎜ ⎟ ⎪ ⎪ ⎢ ⎥ ⎝ ⎠ ⎩ ⎭ ⎣ ⎦ <sup>=</sup>

2

*k*

β

⎛ ⎞ Δ + ⎜ ⎟ ⎝ ⎠

2

β

1 2 2

, *PPP* <sup>102</sup> = − (18)

( )

1

2 2

sin

an experimental sensitivity *S* = 510 nm/RIU and a detection limit below 1×10-5 RIU.

**3.4 Integrated optical directional coupler biosensors** 

scheme of a directional coupler is presented in Fig. 18.

Fig. 18. Schematic representation of an integrated direction coupler.

architectures.

where k is the coupling coefficient and depends on the overlap between the two propagating modal fields, and L is the device length. In synchronism condition (*∆β* = 0), complete power transfer from the waveguide 1 to the waveguide 2 is achieved when kL = π/2, 3π/2, 5π/2, etc. In this way, it will results *P2* = *P0* and *P1* = 0, at the end section x = L.

When a chemical test sample, gas or liquid solution, covers the waveguide 1, the refractive index of the cover medium will change, causing a modal effective index change of the propagating mode neff1. Consequently, the synchronous coupling condition will not be ensured because β1 ≠ β2, thus ∆β ≠ 0. According to Eq. 18, P1 will be different to zero because a complete optical power transfer now cannot occur. By monitoring the change of coupler outputs P1 and P2, it is possible to detect a particular chemical target and estimate its concentration with high limit of detection. The sensitivity S can be calculated as follows:

$$S = n\_{\text{clad}(REF)} \left. \frac{\partial T}{\partial n\_{\text{clad}}} \right|\_{\text{nclad}=\text{nclad}(REF)}\tag{19}$$

where T is the transmittance registered at the output of the reference waveguides, nclad is the chemical analyte refractive index and *nclad(REF)* is the reference analyte refractive index value for which the derivative is calculated. The highest sensitivity can be achieved at the point of maximum slope of the main peak of the transmittance curve. Consequently, this condition indicates the operating point of the sensor. Obviously, the overall sensitivity depends on the waveguide sensitivity, *SW*. In this context, the use of slot waveguides represents the best solution. To this purpose, interesting results have been obtained by using coupled slot SOI waveguides sketched in Fig. 19a. In particular, a sensitivity S = 215.29 was achieved for quasi-TE mode with a coupler length L = 400μm (Passaro et al., 2009b).

Fig. 19. (a) Architecture of coupled SOI slot waveguides for chemical sensing (h = 324nm, w = 180nm, g = 100nm). (b) Multi-channel directional coupler with slot waveguides (h = 300nm, w = 180nm, g = 100nm, s = 500nm).

The sensor sketched in Fig. 19a allows to estimate an analyte concentration in the aqueous solution. In particular, the photonic sensor integrated with a footprint of ~ 1mm2, can detect a minimum refractive index change of ~10-5 and it can be adopted to estimate the glucose concentration in aqueous solution with a theoretical resolution of 0.1 g/L.

In Fig. 19b, a multi-channel directional coupler with slot waveguides is shown (McCosker & Town, 2010a). The sensing principle adopted in this device is different with respect to the

Chemical Sensors Based on Photonic Structures 111

Fig. 20. (a) Schematic top view diagram of a silicon photonic ring resonator with a single bus waveguide. (b) Transmission spectrum of the ring resonator before (neff = 2.5333) and after

Fig. 21. (a) Top view of the spiral-path folded resonant cavity coupled to a bus waveguide. (b) Parametric analysis of the transmission spectrum (Lorentzian), windowed to a single resonant wavelength (λop = 1.294μm), as a function of three different interaction lengths.

The transmission spectrum shown in Fig. 20b has been calculated for a 250μm-long cavity characterized by α = 0.8875, t = 0.9198. In particular, it is possible to appreciate that a modal effective index change ∆n ~ 10-4 theoretically induces a resonant wavelength shift ∆λ of about 0.1nm. As analyzed about the MZI configuration (see. Para. 3.1), in chemical sensors based on single ring resonator as sketched in Fig. 20a, the resonant cavity length assumes a key role in the photonic sensor optimization. In fact, in Fig. 21b it is possible to see that the longer the interaction length "L" between the optical field and the chemical analyte, the

The most important parameter that allows to appreciate the ring-based sensor resolution is the linewidth of its transmission spectrum. In fact, in Fig. 21b it is possible to see that the longer the interaction length, the sharper the resonant peak. By this way, it is possible to see that a small linewidth corresponds to a high slope of the resonant peak, resulting in a signal

(neff = 2.5335) the exposure to the chemical analyte.

**(a) (b)** 

**(a) (b)** 

higher the detectable resolution.

previous one. In particular, it is possible to observe that there is not an isolated waveguide, because all silicon rails are simultaneously covered with the chemical solution. In this way, it is impossible to register a propagation constant mismatch change, because it will be constant. Thus, the sensing principle consists in the coupling coefficient change ∆k, as a function of the cover refractive index change ∆nclad. In particular, the coupling coefficient k has an exponential dependence on analyte refractive index (McCosker & Town, 2010b). Interesting results have been theoretically demonstrated with a 1,607μm-long device. For example, a concentration of glucose or ethanole dispersed in deionised water could be detected with sensitivities of -172 (negative T slope in Eq.19) and +155 (positive T slope), respectively.

Photonic sensors based on directional couplers, exhibit very high performance. However, it is necessary to achieve a good trade-off between sensitivity and device length. In fact, in both sensing principles as analyzed above, mm-long interaction lengths allow to appreciate transmittance changes at the device output.

#### **3.5 Chemical sensors based on integrated optical resonant microcavities**

Photonic devices exhibiting best performance for chemical sensing are those based on resonant microcavities (Passaro et al., 2007b). In fact, they allow to achieve ultra low detection limits and ultra high sensitivities due to their wavelength interrogation, poorly influenced by optical noise. Optical resonant microcavities can be designed with different paths, for example ring, race-track or spiral-path ring. Generally, a resonance structure consists in a waveguide closed into a loop and coupled with one or two input/output bus waveguides (Fig. 20a). Resonance theory imposes the propagation of precise guided stationary modes inside the resonance cavity, in particular those modes whose wavelengths satisfy the well known resonance condition:

$$
\lambda = \frac{2\pi n\_{\rm eff} R}{m} \tag{20}
$$

where neff is the modal effective index, *R* is the ring resonator radius and m is an integer (m = 1, 2, 3,…, n), that represents the resonant order. This result has been obtained by imposing that the total round trip shift of the guided mode inside the ring resonator, must be an integer multiple of *2π*. The analytical expression of the transmission spectrum of a ring resonator with a single bus (Fig. 20a), is given by the following equation:

$$T\_{ring}\left(\lambda\right) = \frac{\alpha^2 + t^2 - 2\alpha t \cos\left(\frac{2\pi Ln\_{eff}}{\lambda}\right)}{1 + \alpha^2 t^2 - 2\alpha t \cos\left(\frac{2\pi Ln\_{eff}}{\lambda}\right)}\tag{21}$$

In Eq. 21, α is the loss factor that gives the field attenuation after one round trip through the ring cavity waveguide (i.e., α = 1 in lossless case). The cosine argument represents the round trip phase, *L* = *2πR* is the cavity length depending on the ring radius, and t is the coupling ratio between the bus waveguide and the ring cavity.

In this context, the sensing principle consists in a resonant wavelength shift *∆λ* due to the modal effective index change *∆neff* caused by the presence of the chemical analyte in the sensing area (Fig. 20b).

previous one. In particular, it is possible to observe that there is not an isolated waveguide, because all silicon rails are simultaneously covered with the chemical solution. In this way, it is impossible to register a propagation constant mismatch change, because it will be constant. Thus, the sensing principle consists in the coupling coefficient change ∆k, as a function of the cover refractive index change ∆nclad. In particular, the coupling coefficient k has an exponential dependence on analyte refractive index (McCosker & Town, 2010b). Interesting results have been theoretically demonstrated with a 1,607μm-long device. For example, a concentration of glucose or ethanole dispersed in deionised water could be detected with sensitivities of -172

Photonic sensors based on directional couplers, exhibit very high performance. However, it is necessary to achieve a good trade-off between sensitivity and device length. In fact, in both sensing principles as analyzed above, mm-long interaction lengths allow to appreciate

Photonic devices exhibiting best performance for chemical sensing are those based on resonant microcavities (Passaro et al., 2007b). In fact, they allow to achieve ultra low detection limits and ultra high sensitivities due to their wavelength interrogation, poorly influenced by optical noise. Optical resonant microcavities can be designed with different paths, for example ring, race-track or spiral-path ring. Generally, a resonance structure consists in a waveguide closed into a loop and coupled with one or two input/output bus waveguides (Fig. 20a). Resonance theory imposes the propagation of precise guided stationary modes inside the resonance cavity, in particular those modes whose wavelengths

> *m* π

> > 2

π

*eff*

*Ln*

⎛ ⎞

λ

*Ln*

λ

⎝ ⎠

*eff*

(21)

2

π

2 cos

+ − ⎜ ⎟ ⎝ ⎠ <sup>=</sup> ⎛ ⎞ + − ⎜ ⎟

where neff is the modal effective index, *R* is the ring resonator radius and m is an integer (m = 1, 2, 3,…, n), that represents the resonant order. This result has been obtained by imposing that the total round trip shift of the guided mode inside the ring resonator, must be an integer multiple of *2π*. The analytical expression of the transmission spectrum of a ring

= (20)

λ

2 2

α

α

2 2

1 2 cos

In Eq. 21, α is the loss factor that gives the field attenuation after one round trip through the ring cavity waveguide (i.e., α = 1 in lossless case). The cosine argument represents the round trip phase, *L* = *2πR* is the cavity length depending on the ring radius, and t is the coupling

In this context, the sensing principle consists in a resonant wavelength shift *∆λ* due to the modal effective index change *∆neff* caused by the presence of the chemical analyte in the

*t t*

 α

*t t*

 α

resonator with a single bus (Fig. 20a), is given by the following equation:

( )

λ

*ring*

*T*

ratio between the bus waveguide and the ring cavity.

sensing area (Fig. 20b).

**3.5 Chemical sensors based on integrated optical resonant microcavities** 

(negative T slope in Eq.19) and +155 (positive T slope), respectively.

transmittance changes at the device output.

satisfy the well known resonance condition:

<sup>2</sup> *n R eff*

Fig. 20. (a) Schematic top view diagram of a silicon photonic ring resonator with a single bus waveguide. (b) Transmission spectrum of the ring resonator before (neff = 2.5333) and after (neff = 2.5335) the exposure to the chemical analyte.

Fig. 21. (a) Top view of the spiral-path folded resonant cavity coupled to a bus waveguide. (b) Parametric analysis of the transmission spectrum (Lorentzian), windowed to a single resonant wavelength (λop = 1.294μm), as a function of three different interaction lengths.

The transmission spectrum shown in Fig. 20b has been calculated for a 250μm-long cavity characterized by α = 0.8875, t = 0.9198. In particular, it is possible to appreciate that a modal effective index change ∆n ~ 10-4 theoretically induces a resonant wavelength shift ∆λ of about 0.1nm. As analyzed about the MZI configuration (see. Para. 3.1), in chemical sensors based on single ring resonator as sketched in Fig. 20a, the resonant cavity length assumes a key role in the photonic sensor optimization. In fact, in Fig. 21b it is possible to see that the longer the interaction length "L" between the optical field and the chemical analyte, the higher the detectable resolution.

The most important parameter that allows to appreciate the ring-based sensor resolution is the linewidth of its transmission spectrum. In fact, in Fig. 21b it is possible to see that the longer the interaction length, the sharper the resonant peak. By this way, it is possible to see that a small linewidth corresponds to a high slope of the resonant peak, resulting in a signal

Chemical Sensors Based on Photonic Structures 113

index variation can be read out as a resonant wavelength shift of the disk resonator, obtaining interesting results. In particular, a deflection sensitivity of 33 nm-1, a detection limit of the slot waveguide disk resonator of 3×10-5nm, an estimated cantilever stress sensitivity of 1.76 nm/(mJ/m2), that means that the minimum detectable surface stress is

One of the most important aspects for photonic bio-sensing concerns with temperature effects affecting final measurements. The temperature effect is directly linked to very small refractive index changes of the adopted material system. Consequently, the global modal

Δ =Δ ±Δ *nn n eff eff SENSING eff TEMP* , , . (25)

By this way, the resonance wavelength of the ring resonator can change not only due to sensing principles (e.g., homogenous or surface sensing), but also due to temperature changes. The final result is a corrupted measure of biological analytes and molecules to be detected. Three approaches have been used for thermal noise reduction: active temperature control, a-thermal waveguide design, and temperature drift compensation by on chip

In this context, an on-chip temperature compensation in an integrated slot waveguide ring resonator refractive index sensor array has been investigated (Gylfanson et al., 2010). Experimental study has demonstrated a low temperature dependence of -16.6 pm/K while, at the same time, a large refractive index sensitivity of 240±10 nm/RIU. Furthermore, by using on chip temperature referencing, a differential temperature sensitivity of only 0.3 pm/K has been obtained, without any individual sensor calibration. This low value indicates good sensor-to-sensor repeatability, thus enabling use in highly parallel chemical assays. The detection limit has been demonstrated to be 8.8×10-6 RIU in a 7K temperature operating windows. Another interesting solution to the problem of temperature effects in SOI wire waveguide ring resonator label-free biosensor arrays, has been proposed (Xu et al., 2010). The device is constituted by four ring resonators with a reference ring for tracking sensor temperature changes. The reference ring is protected by a 2μm-thick upper cladding layer and optically isolated from the sensing medium. By this way, real-time measurements have shown that the reference resonator resonances are linked to the temperature changes without any noticeable time delay, enabling an effective cancellation of temperature-induced shifts. A concentration of 20pM has been demonstrated by monitoring the binding between complementary IgG protein pairs. In particular, the sensor is able to detect a fluid refractive index fluctuation of ± 4×10-6. Better results have been achieved by using an array of 32 silicon ring resonator sensors

With a bulk refractive index sensitivity of 7.6×10-7 it is possible to detect a concentration of ~ 60fM, by using immobilized biotin to capture streptavidin diluted in bovine serum albumin

In conclusion, a folded cavity SOI micro-ring sensor characterized by a folded spiral path geometry with a 1.2mm long ring waveguide, enclosed in a 150μm diameter sensor area, has been designed (Xu et al., 2008). The spiral cavity resonator is used to monitor the streptavidin protein binding with a detection limit of ~3 pg/mm2, or a total mass of ~5fg.

∆σmin= 1.7×10-5 mJ/m2, have been achieved.

effective index perturbation can be written as:

referencing.

integrated on the same platform.

solution (Iqbal et al., 2010).

change easily resolvable near the maximum and minimum of the transmission function. However, very long interaction lengths will result in large device areas that are not suitable for high scale integration. To this purpose, intriguing folded spiral–path cavities have been proposed (Fig. 21a). In fact, they allow to achieve mm-long interaction length concentrated in very small footprint (~μm2). The sharpness of the resonant peak in a ring resonator transmission spectrum is usually expressed in terms of its quality factor Q:

$$Q = \frac{\lambda}{\Delta \lambda\_{\rm FWHM}} = \frac{\pi n\_g L}{\lambda} \left[ \arccos \left( \frac{2at}{1 + \alpha^2 t^2} \right) \right]^{-1} \tag{22}$$

In Eq. 22, *λ* is the resonant wavelength, *∆λFWHM* is the resonance full-width-at-half-maximum (*FWHM*) and ng is the ring resonator group index. It is evident that the smaller the linewidth, the higher the resonant cavity quality factor.

In conclusion, it is possible to revisit the concept of sensitivity and limit of detection (LOD):

$$S = S\_W \frac{\lambda}{n\_{\text{eff}}} \, \text{LOD} = \frac{\Delta \lambda}{S} \tag{23}$$

The sensitivity *S* (nm/RIU) depends on the waveguide sensitivity SW and other well known parameters (resonant operative wavelength λ and modal effective index *neff*). The limit of detection *LOD* (RIU), depends on the minimum detectable resonant wavelength shift *∆λ*. By this way, ultra high performance for chemical, biochemical and gas sensing (*S* ~ 2000 nm/RIU, *LOD* ~ 3.8×10-5 RIU) can be achieved by using slot waveguides (Passaro et al., 2011).

Particular research efforts are still doing today in this field. In fact, the state-of-the-art of photonic sensors for bio-sensing applications is mainly dedicated to the optimization and characterization of novel resonant microcavities based on slot waveguides. In particular, a novel integrated SOI optic racetrack resonator has been proposed for bio-sensing applications (Malathi et al., 2010). The device is capable of distinguishing compressive and tensile stresses on a cantilever due to conformational changes of protein, Bovine Serum Albumin (BSA) and Immunoglobulin G (IgG). The change of surface stress upon adsorption of IgG is compressive, while for BSA it is tensile. The sensing principle of this specific photonic sensor configuration can be expressed by the following equation:

$$\frac{\Delta \mathcal{L}}{\mathcal{N}} = \frac{\Delta L}{L} + \frac{\Delta n\_{eff}}{n\_{eff}} \tag{24}$$

When the bio-molecules adhere to the device surface, both effective refractive index and racetrack length change due to photo elastic effect. When the cantilever bends, one arm of the racetrack experiences a small variation in *L* by *∆L* and *neff* changes by *∆neff*, due to the strain optic effect. Bio-molecules stress on the cantilever has been simulated and the wavelength shift from the resonance has been found to be 0.3196×103 nm/με, where ε is the strain induced on the cantilever. Normally, the stress induced by proteins lies in a range between 3000με and 15000με, thus the sensor response and sensitivity obtained are quite high.

In this context, an ultrasensitive nano-mechanical photonic sensor based on horizontal slot waveguide resonator, has been also proposed on SOI platform (Barrios, 2006). The effective

change easily resolvable near the maximum and minimum of the transmission function. However, very long interaction lengths will result in large device areas that are not suitable for high scale integration. To this purpose, intriguing folded spiral–path cavities have been proposed (Fig. 21a). In fact, they allow to achieve mm-long interaction length concentrated in very small footprint (~μm2). The sharpness of the resonant peak in a ring resonator

arccos

*g*

<sup>−</sup> <sup>⎡</sup> ⎛ ⎞⎤ = = <sup>⎢</sup> ⎜ ⎟⎥ <sup>Δ</sup> <sup>⎣</sup> ⎝ ⎠ <sup>+</sup> <sup>⎦</sup>

In Eq. 22, *λ* is the resonant wavelength, *∆λFWHM* is the resonance full-width-at-half-maximum (*FWHM*) and ng is the ring resonator group index. It is evident that the smaller the

In conclusion, it is possible to revisit the concept of sensitivity and limit of detection (LOD):

*eff*

The sensitivity *S* (nm/RIU) depends on the waveguide sensitivity SW and other well known parameters (resonant operative wavelength λ and modal effective index *neff*). The limit of detection *LOD* (RIU), depends on the minimum detectable resonant wavelength shift *∆λ*. By this way, ultra high performance for chemical, biochemical and gas sensing (*S* ~ 2000 nm/RIU, *LOD* ~ 3.8×10-5 RIU) can be achieved by using slot waveguides (Passaro et al.,

Particular research efforts are still doing today in this field. In fact, the state-of-the-art of photonic sensors for bio-sensing applications is mainly dedicated to the optimization and characterization of novel resonant microcavities based on slot waveguides. In particular, a novel integrated SOI optic racetrack resonator has been proposed for bio-sensing applications (Malathi et al., 2010). The device is capable of distinguishing compressive and tensile stresses on a cantilever due to conformational changes of protein, Bovine Serum Albumin (BSA) and Immunoglobulin G (IgG). The change of surface stress upon adsorption of IgG is compressive, while for BSA it is tensile. The sensing principle of this specific

*eff*

= + (24)

*L n L n*

When the bio-molecules adhere to the device surface, both effective refractive index and racetrack length change due to photo elastic effect. When the cantilever bends, one arm of the racetrack experiences a small variation in *L* by *∆L* and *neff* changes by *∆neff*, due to the strain optic effect. Bio-molecules stress on the cantilever has been simulated and the wavelength shift from the resonance has been found to be 0.3196×103 nm/με, where ε is the strain induced on the cantilever. Normally, the stress induced by proteins lies in a range between 3000με and

In this context, an ultrasensitive nano-mechanical photonic sensor based on horizontal slot waveguide resonator, has been also proposed on SOI platform (Barrios, 2006). The effective

*n* λ= , *LOD*

*n L <sup>t</sup> <sup>Q</sup>*

 λ

π

1

*S* Δλ 1

= (23)

(22)

2 2 2

 α

α

*t*

transmission spectrum is usually expressed in terms of its quality factor Q:

*S S*

photonic sensor configuration can be expressed by the following equation:

15000με, thus the sensor response and sensitivity obtained are quite high.

λ

Δ Δ Δ

λ

*eff*

*FWHM*

λ

λ

linewidth, the higher the resonant cavity quality factor.

*<sup>W</sup>*

2011).

index variation can be read out as a resonant wavelength shift of the disk resonator, obtaining interesting results. In particular, a deflection sensitivity of 33 nm-1, a detection limit of the slot waveguide disk resonator of 3×10-5nm, an estimated cantilever stress sensitivity of 1.76 nm/(mJ/m2), that means that the minimum detectable surface stress is ∆σmin= 1.7×10-5 mJ/m2, have been achieved.

One of the most important aspects for photonic bio-sensing concerns with temperature effects affecting final measurements. The temperature effect is directly linked to very small refractive index changes of the adopted material system. Consequently, the global modal effective index perturbation can be written as:

$$
\Delta n\_{\rm eff} = \Delta n\_{\rm eff, SENNGIG} \pm \Delta n\_{\rm eff, TEMP.} \tag{25}
$$

By this way, the resonance wavelength of the ring resonator can change not only due to sensing principles (e.g., homogenous or surface sensing), but also due to temperature changes. The final result is a corrupted measure of biological analytes and molecules to be detected. Three approaches have been used for thermal noise reduction: active temperature control, a-thermal waveguide design, and temperature drift compensation by on chip referencing.

In this context, an on-chip temperature compensation in an integrated slot waveguide ring resonator refractive index sensor array has been investigated (Gylfanson et al., 2010). Experimental study has demonstrated a low temperature dependence of -16.6 pm/K while, at the same time, a large refractive index sensitivity of 240±10 nm/RIU. Furthermore, by using on chip temperature referencing, a differential temperature sensitivity of only 0.3 pm/K has been obtained, without any individual sensor calibration. This low value indicates good sensor-to-sensor repeatability, thus enabling use in highly parallel chemical assays. The detection limit has been demonstrated to be 8.8×10-6 RIU in a 7K temperature operating windows. Another interesting solution to the problem of temperature effects in SOI wire waveguide ring resonator label-free biosensor arrays, has been proposed (Xu et al., 2010). The device is constituted by four ring resonators with a reference ring for tracking sensor temperature changes. The reference ring is protected by a 2μm-thick upper cladding layer and optically isolated from the sensing medium. By this way, real-time measurements have shown that the reference resonator resonances are linked to the temperature changes without any noticeable time delay, enabling an effective cancellation of temperature-induced shifts. A concentration of 20pM has been demonstrated by monitoring the binding between complementary IgG protein pairs. In particular, the sensor is able to detect a fluid refractive index fluctuation of ± 4×10-6. Better results have been achieved by using an array of 32 silicon ring resonator sensors integrated on the same platform.

With a bulk refractive index sensitivity of 7.6×10-7 it is possible to detect a concentration of ~ 60fM, by using immobilized biotin to capture streptavidin diluted in bovine serum albumin solution (Iqbal et al., 2010).

In conclusion, a folded cavity SOI micro-ring sensor characterized by a folded spiral path geometry with a 1.2mm long ring waveguide, enclosed in a 150μm diameter sensor area, has been designed (Xu et al., 2008). The spiral cavity resonator is used to monitor the streptavidin protein binding with a detection limit of ~3 pg/mm2, or a total mass of ~5fg.

Chemical Sensors Based on Photonic Structures 115

The second approach allowing to ensure an ultra-sensitivity and the reduction of the detector complexity, consists in the integration of two cascaded ring resonators in the same chip. The principle used in this configuration is called "Vernier-scale" or "Vernier-effect". It consists of two wavelength scales with different periods, of which one slides the other one.

In Fig. 23, two resonators with different optical roundtrip lengths (R2 < R1) are cascaded, so

Fig. 23. Schematic view of a cascaded double ring photonic sensor based on Vernier effect.

minimum detectable wavelength shift ∆λ is 18pm and the LOD is 8.3×10-6 RIU.

slot waveguides in order to further improve their performance.

In conclusion, both integrated architectures analyzed above, can be designed by adopting

Each individual ring resonator has a comb-like transmission spectrum with peaks at its resonance wavelengths, as already seen. The transmission spectrum of the cascade of two ring resonators is the product of the transmission spectra of the isolated individual resonators. Consequently, it will only exhibit peaks at wavelengths for which two resonance peaks of the respective ring resonators overlap, and the height of each of these peaks will be determined by the amount of this overlap. The sensor ring resonator will act as the sliding part of the Vernier-scale, as its evanescent field can interact with the refractive index in the environment of the sensor, where a change will cause a shift of the resonance wavelengths. The filter ring resonator is shielded from these refractive index changes by the cladding and will act as the constant part of the Vernier-scale. By this way, the whole architecture can be designed such that a small shift of the resonant wavelengths of the sensor ring resonator will result in a much larger shift of the transmission spectrum of the cascade. To this purpose, very interesting results have been demonstrated by using a silicon photonic biosensor whom architecture is the same as that sketched in Fig. 23 (Claes et al., 2010). The sensor has been tested with three aqueous solutions of NaCl with different concentrations and the operative wavelength range is from 1.52μm to 1.54μm. A sensitivity of 2169 nm/RIU has been experimentally determined in aqueous environment. In addition, the

The overlap between lines of two scales is used to perform the measurements.

that drop signal of the first ring resonator serves as the input for the second one.

The sensor presented above is characterized by Q-factor of 90,650 and an extinction ratio of 8.60 dB.

#### **3.5.1 Resonant architectures for high performance photonic chemical sensing**

Due to the increasing demand of high sensitivity and high limit of detection in biological and chemical detections, many kinds of sophisticated photonic sensors have been proposed. In particular, the minimum detectable wavelength shift for a traditional ring sensor is given by the resonance linewidth and resolution of the optical spectra analysis (OSA). In order to achieve ultra high limit of detection, very high resolution optical spectrum analysis and very stable tunable lasers have to be usually used for the whole measurement setup, usually not convenient and very expensive. An innovative approach consists in the joint use of two resonance cavities, for example a ring resonator or a racetrack integrated inside a Fabry-Perot cavity or two ring resonators integrated in the same photonic chip (Fig. 22a). By this way, interesting ultra high performance have been demonstrated.

Fig. 22. (a) Micro ring resonator integrated inside a Fabry-Perot resonance cavity. (b) Illustration of the asymmetric Fano line-shape.

The Fano resonance is due to complex interference present in the structure, as formed by the Fabry-Perot cavity between the reflecting elements and the ring resonator. In Fig. 22b, a typical Fano line-shape has been calculated by considering a silicon ring resonator characterized by a 30μm-long radius, integrated between two reflecting Bragg gratings with a reflection coefficient equal to 0.75. In addition, the central resonant wavelength has been imposed to be λ0 = 1.535μm. Fano line-shape is characterized by a resonant peak steeper than the Lorentzian's one, that can be obtained with a standard ring resonator (Fig. 21b). By this way, the resolution of the photonic device and the limit of detection are improved. The improvement in terms of sensor performance is obtained because in Fano resonance the slope of the line-shape is greater than that obtained with Lorentzian resonance. To this purpose, a highly sensitive silicon micro-ring sensor with sharp asymmetrical resonance has been presented in literature (Yi et al., 2010). Coupled waveguides and micro-ring resonator have been fabricated using a SOI wafer which has a 1μm buffered oxide layer topped with 230nm of Si. Using a Fabry-Perot cavity characterized by a cavity length of 10mm, a quality factor Q = 3.8×104 and a limit of detection of ~10-8 have been measured.

The sensor presented above is characterized by Q-factor of 90,650 and an extinction ratio of

Due to the increasing demand of high sensitivity and high limit of detection in biological and chemical detections, many kinds of sophisticated photonic sensors have been proposed. In particular, the minimum detectable wavelength shift for a traditional ring sensor is given by the resonance linewidth and resolution of the optical spectra analysis (OSA). In order to achieve ultra high limit of detection, very high resolution optical spectrum analysis and very stable tunable lasers have to be usually used for the whole measurement setup, usually not convenient and very expensive. An innovative approach consists in the joint use of two resonance cavities, for example a ring resonator or a racetrack integrated inside a Fabry-Perot cavity or two ring resonators integrated in the same photonic chip (Fig. 22a). By this

The Fano resonance is due to complex interference present in the structure, as formed by the Fabry-Perot cavity between the reflecting elements and the ring resonator. In Fig. 22b, a typical Fano line-shape has been calculated by considering a silicon ring resonator characterized by a 30μm-long radius, integrated between two reflecting Bragg gratings with a reflection coefficient equal to 0.75. In addition, the central resonant wavelength has been imposed to be λ0 = 1.535μm. Fano line-shape is characterized by a resonant peak steeper than the Lorentzian's one, that can be obtained with a standard ring resonator (Fig. 21b). By this way, the resolution of the photonic device and the limit of detection are improved. The improvement in terms of sensor performance is obtained because in Fano resonance the slope of the line-shape is greater than that obtained with Lorentzian resonance. To this purpose, a highly sensitive silicon micro-ring sensor with sharp asymmetrical resonance has been presented in literature (Yi et al., 2010). Coupled waveguides and micro-ring resonator have been fabricated using a SOI wafer which has a 1μm buffered oxide layer topped with 230nm of Si. Using a Fabry-Perot cavity characterized by a cavity length of 10mm, a quality factor Q = 3.8×104 and a limit of

Fig. 22. (a) Micro ring resonator integrated inside a Fabry-Perot resonance cavity.

**3.5.1 Resonant architectures for high performance photonic chemical sensing** 

way, interesting ultra high performance have been demonstrated.

(b) Illustration of the asymmetric Fano line-shape.

**(a) (b)** 

detection of ~10-8 have been measured.

8.60 dB.

The second approach allowing to ensure an ultra-sensitivity and the reduction of the detector complexity, consists in the integration of two cascaded ring resonators in the same chip. The principle used in this configuration is called "Vernier-scale" or "Vernier-effect". It consists of two wavelength scales with different periods, of which one slides the other one. The overlap between lines of two scales is used to perform the measurements.

In Fig. 23, two resonators with different optical roundtrip lengths (R2 < R1) are cascaded, so that drop signal of the first ring resonator serves as the input for the second one.

Fig. 23. Schematic view of a cascaded double ring photonic sensor based on Vernier effect.

Each individual ring resonator has a comb-like transmission spectrum with peaks at its resonance wavelengths, as already seen. The transmission spectrum of the cascade of two ring resonators is the product of the transmission spectra of the isolated individual resonators. Consequently, it will only exhibit peaks at wavelengths for which two resonance peaks of the respective ring resonators overlap, and the height of each of these peaks will be determined by the amount of this overlap. The sensor ring resonator will act as the sliding part of the Vernier-scale, as its evanescent field can interact with the refractive index in the environment of the sensor, where a change will cause a shift of the resonance wavelengths. The filter ring resonator is shielded from these refractive index changes by the cladding and will act as the constant part of the Vernier-scale. By this way, the whole architecture can be designed such that a small shift of the resonant wavelengths of the sensor ring resonator will result in a much larger shift of the transmission spectrum of the cascade. To this purpose, very interesting results have been demonstrated by using a silicon photonic biosensor whom architecture is the same as that sketched in Fig. 23 (Claes et al., 2010). The sensor has been tested with three aqueous solutions of NaCl with different concentrations and the operative wavelength range is from 1.52μm to 1.54μm. A sensitivity of 2169 nm/RIU has been experimentally determined in aqueous environment. In addition, the minimum detectable wavelength shift ∆λ is 18pm and the LOD is 8.3×10-6 RIU.

In conclusion, both integrated architectures analyzed above, can be designed by adopting slot waveguides in order to further improve their performance.

Chemical Sensors Based on Photonic Structures 117

This work has been supported by Fondazione della Cassa di Risparmio di Puglia, Bari, Italy.

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**6. References** 

## **4. Conclusions**

Photonic sensors represent a real revolution in chemical sensing technologies. In fact, their most important features are:


In addition, the silicon-on-insulator technology platform allows to integrate photonics (laser, sensing architecture, photo-detector) and electronics (CMOS data processing) on the same chip in partially monolithic form to constitute the so called Lab-on-a-chip photonic system. Moreover, the refractive index contrast of the Si/SiO2 material system, enables record reduction in photonic device footprint with ultra-high performance and portability. Undoubtedly, photonic Lab-on-a-chip represents the innovative approach to the main mission of the modern age: the improvement of the human life quality.

Finally, a comparative Table is presented below in order to appreciate all features and performance of actual different photonic sensor configurations.


Table 2. Comparative analysis of several silicon photonic platforms optimized for chemical and biochemical sensing.

## **5. Acknowledgments**

This work has been supported by Fondazione della Cassa di Risparmio di Puglia, Bari, Italy.

## **6. References**

116 Advances in Chemical Sensors

Photonic sensors represent a real revolution in chemical sensing technologies. In fact, their

• Low-cost and high integration with front-end and support electronic systems (silicon,

In addition, the silicon-on-insulator technology platform allows to integrate photonics (laser, sensing architecture, photo-detector) and electronics (CMOS data processing) on the same chip in partially monolithic form to constitute the so called Lab-on-a-chip photonic system. Moreover, the refractive index contrast of the Si/SiO2 material system, enables record reduction in photonic device footprint with ultra-high performance and portability. Undoubtedly, photonic Lab-on-a-chip represents the innovative approach to the main

Finally, a comparative Table is presented below in order to appreciate all features and

Architecture Technology Performance Size Analyte Author

PhC-slot SOI 100ppm 300μm-long Methane Lai et al.,

compatible 0.3 pg/mm2 1.8mm-long

3022nm/RIU

1×10-5RIU

MMI SOI +152, -172 1.607μm-long Glucose,

3.8×10-5RIU

8.3×10-6RIU

Table 2. Comparative analysis of several silicon photonic platforms optimized for chemical

RIU/ppm 2.1mm-long BTEX Saunders et

70pg/mm2 ~ 800μm2 Molecules DiPippo et

IgG goat ,rabbit

reactions

Gases N2, He, CO2

(footprint) Glucose Passaro et al.,

etanole

Molecules, Gases

(×32- array) DNA Iqbal et al.,

NaCl, molecules

(×9 - array)

~ 10-4RIU 173μm-long Biological

2μm-cavity length

~ 1mm2 (footprint)

200×70μm2 (2x- array) al., 2010

Densmore et al., 2009

al., 2010

Passaro et al., 2008

2011

Jagerska et al., 2010

2009

McCosker & Town, 2010b

Passaro et al., 2011

2010

Claes et al., 2010

**4. Conclusions** 

most important features are:

• Real-time processing.

• Extremely high selectivity and sensitivity; • Multi-variable and parallel processing in a chip;

CMOS-compatible processing);

• Wavelength readout (noise and interference immunity);

mission of the modern age: the improvement of the human life quality.

performance of actual different photonic sensor configurations.

MZI SOI 8.7×10-7

compatible

Grating SOI ~ 120nm

PhC-slot SOI 510nm/RIU

resonator SOI 2000nm/RIU

resonators SOI 2169nm/RIU

coupler SOI 0.1 g/L ~ 1mm2

resonator SOI 60fM 175×500μm2

MZI CMOS-

SPR CMOS-

Directional

Slot-ring

Ring

Cascaded

and biochemical sensing.


Chemical Sensors Based on Photonic Structures 119

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**6** 

*China* 

Hongqi Li, Li Cai and Zhen Chen

 *Chemical Engineering and Biotechnology,* 

 *Donghua University, Shanghai,* 

*College of Chemistry,* 

**Coumarin-Derived Fluorescent Chemosensors** 

Fluorescent chemosensors are highly valuable in a variety of fields including environmental chemistry, analytical chemistry, and bio-medicinal science. They have provided accurate, on-line, and low-cost detection of toxic heavy metal ions, anions, and enzymes with high selectivity and sensitivity. Coumarins, with the structure of benzopyrone, have many advantages including high fluorescence quantum yield, large Stokes shift, excellent light stability, and less toxicity. Therefore coumarins have been widely used in the fields of biology, medicine, perfumes, cosmetics, and fluorescent dyes. By far coumarin derivatives have been used as fluorescent probes of pH, for detection of nitric oxide, nitroxide, and hydrogen peroxide. Moreover, coumarin derivatives have served as good chemosensors of anions including cyanide, fluoride, pyrophosphate, acetate, benzoate, and dihydrogenphosphate as well as various metal ions comprised of Hg(II), Cu(II), Zn(II), Ni(II), Ca(II), Pb(II), Mg(II), Fe(III), Al(III), Cr(III), and Ag(I). Several systems containing coumarin exhibited simultaneous sensitivity toward two or more different metal ions, e.g. Ca(II) and Mg(II), Ni(II) and Co(II), Cu(II) and Hg(II), Na(I) and K(I), Cu(II) and Ni(II), Hg(II) and Ag(I), Cu(II)/Ni(II)/Cd(II), Zn(II)/Cd(II)/Pb(II), or Ni(II)/Pd(II)/Ag(I). Herein a brief review of fluorescent chemical sensors derived from

The fusion of a pyrone ring with a benzene ring gives rise to a class of heterocyclic compounds known as benzopyrones, of which two distinct types are recognized, namely benzo-α-pyrones, commonly called coumarins, and benzo-γ-pyrones, called chromones, the latter differing from the former only in the position of the carbonyl group in the heterocyclic ring as shown in Fig. 1 (Sethna & Shah, 1945). It is well known that stilbene with a *trans* conformation is highly fluorescent. From the viewpoint of molecular structure, coumarins bear a carbon-carbon double bond which is fixed as *trans* conformation as in *trans*-stilbene through a lactone structure. This can help to avoid the *trans*-*cis* transformation of the double bond under ultraviolet (UV) irradiation as observed in stilbene compounds and results in strong fluorescence and high fluorescence quantum yield and photostability in most of

**1. Introduction** 

coumarins is presented.

coumarin derivatives.

**2. Structural characteristics of coumarin** 

biomolecular binding. *Optics Express*, Vol.16, No.19, (September 2008), pp. 15137- 15148


## **Coumarin-Derived Fluorescent Chemosensors**

Hongqi Li, Li Cai and Zhen Chen

*College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai, China* 

#### **1. Introduction**

120 Advances in Chemical Sensors

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Yi, H.; Citrin, D.S. & Zhou, Z. (2010). Highly sensitive silicon microring sensor with sharp

15148

22879

2972

biomolecular binding. *Optics Express*, Vol.16, No.19, (September 2008), pp. 15137-

Schmid, J.H.; Post, E.; Messaoudene, S. & Fedeli, J.-M. (2010). Real-time cancellation of temperature induced resonance shifts in SOI wire waveguide ring resonator label-free biosensor arrays. *Optics Express*, Vol.18, No.22, (October 2010), pp. 22867-

asymmetrical resonance. *Optics Express*, Vol.18, No.3, (February 2010), pp. 2967-

Fluorescent chemosensors are highly valuable in a variety of fields including environmental chemistry, analytical chemistry, and bio-medicinal science. They have provided accurate, on-line, and low-cost detection of toxic heavy metal ions, anions, and enzymes with high selectivity and sensitivity. Coumarins, with the structure of benzopyrone, have many advantages including high fluorescence quantum yield, large Stokes shift, excellent light stability, and less toxicity. Therefore coumarins have been widely used in the fields of biology, medicine, perfumes, cosmetics, and fluorescent dyes. By far coumarin derivatives have been used as fluorescent probes of pH, for detection of nitric oxide, nitroxide, and hydrogen peroxide. Moreover, coumarin derivatives have served as good chemosensors of anions including cyanide, fluoride, pyrophosphate, acetate, benzoate, and dihydrogenphosphate as well as various metal ions comprised of Hg(II), Cu(II), Zn(II), Ni(II), Ca(II), Pb(II), Mg(II), Fe(III), Al(III), Cr(III), and Ag(I). Several systems containing coumarin exhibited simultaneous sensitivity toward two or more different metal ions, e.g. Ca(II) and Mg(II), Ni(II) and Co(II), Cu(II) and Hg(II), Na(I) and K(I), Cu(II) and Ni(II), Hg(II) and Ag(I), Cu(II)/Ni(II)/Cd(II), Zn(II)/Cd(II)/Pb(II), or Ni(II)/Pd(II)/Ag(I). Herein a brief review of fluorescent chemical sensors derived from coumarins is presented.

#### **2. Structural characteristics of coumarin**

The fusion of a pyrone ring with a benzene ring gives rise to a class of heterocyclic compounds known as benzopyrones, of which two distinct types are recognized, namely benzo-α-pyrones, commonly called coumarins, and benzo-γ-pyrones, called chromones, the latter differing from the former only in the position of the carbonyl group in the heterocyclic ring as shown in Fig. 1 (Sethna & Shah, 1945). It is well known that stilbene with a *trans* conformation is highly fluorescent. From the viewpoint of molecular structure, coumarins bear a carbon-carbon double bond which is fixed as *trans* conformation as in *trans*-stilbene through a lactone structure. This can help to avoid the *trans*-*cis* transformation of the double bond under ultraviolet (UV) irradiation as observed in stilbene compounds and results in strong fluorescence and high fluorescence quantum yield and photostability in most of coumarin derivatives.

Coumarin-Derived Fluorescent Chemosensors 123

Structure of sensor Reference Structure of sensor Reference

Shiraishi et al., 2010

S. H. Lee et al., 2010

Wang et al., 2006

Ryu et al., 2010

H. J. Kim et al., 2009a

Q.-J. Ma et al., 2010

R. Sheng et al., 2008

Table 1. Structures and references of coumarin-derived chemosensors for Hg(II) ions

coumarin fluorophore resulting in blue-shift in absorption and quenching of the fluorescence (S. H. Lee et al., 2010). The fluorescent ratiometric Hg2+ ion sensor **4**, based on a coumarin platform coupled with a tetraamide receptor, can specifically detect Hg2+ ions through the ICT mechanism (Wang et al., 2006). Fluorescein-coumarin chemodosimeter **5** for

Et2N O O **9**

**10**: R = acetylene

**11**: R = 1,3-dithiane-2-yl

N H NH O

R

S S

N S

N

NH

O

OMe OH SPr O Jiang &

> OMe OMe

O

O

O

Et2N O

O

**13**

O O O

S

Et2N O OMe **15**

Et2N **16**

O

O O

NH

**14**

**12**

Et2N S

Et2N O

O N

<sup>H</sup> <sup>S</sup> <sup>N</sup> W. Ma et

al., 2010

D.-N. Lee et al., 2009 J. H. Kim et al., 2009

M. G. Choi et al., 2009

H. J. Kim et al., 2010

Voutsadak i et al., 2010

Wang, 2009

Cho & Ahn, 2010

O O

O N O

O O N

O O

HO O OH **5**

O

Et2N <sup>O</sup> <sup>O</sup> **<sup>8</sup>**

NEt Et2N <sup>2</sup>

N S N

O N O

O

MeO MeO

N S

<sup>S</sup> **<sup>3</sup>**

**2** N H

S

H <sup>N</sup> <sup>H</sup> N S

O O

O O

HN H N

N H O HN O

> O O

> > NEt2

O O NEt2

**7**

OH

OH

OMe OMe

> OMe OMe

OH OH

N

N

HO O OH

Cl Cl

N N N N

O

O

**6**

<sup>O</sup> <sup>O</sup>

N N

O O O O

**4**

N

Fig. 1. Structures and numbering scheme of coumarin and related compounds

It was showed in the late 1950s that substitutions on the coumarin structure shifted the fluorescence band. For instance, adding a methyl group to the 4-position of 7-hydroxy- or 7 methoxycoumarin red shifts the fluorescence spectra. Addition of electron-repelling groups in the 4-, 6-, or 7-position or electron-attracting groups in the 3-position all shifts the fluorescence band to longer wavelengths. When the carbonyl is substituted with a thione, the absorbance was red shifted and the fluorescence was quenched (Trenor et al., 2004). The reduction of the acceptor strength of the substituent at the 3-position, as in a 7 diethylaminocoumarin dye **1** (the structure of which is shown in Fig. 1) does not always result in sustained fluorescence in polar, aprotic solvents. Thus 7-diethylamino-3 styrylcoumarin dyes are not technically important as laser dyes; however, the extreme sensitivity of coumarin **1** to the medium polarity could provide an opportunity to probe the microenvironment experienced by the molecule (Bangar Raju & Varadarajan, 1995).

Changing the solvent or the solution pH also affected the fluorescence spectra. Study on the effect of solution pH on 7-hydroxy-4-methylcoumarin showed that increasing the solution pH raised the fluorescence intensity. Studies on the effect of changing the solvent polarity on 13 coumarin derivatives revealed that increasing solvent polarity red shifted the absorbance as well as red shifted and broadened the emission of the coumarins due to increased hydrogen bonding. Studies on the excited-state properties of 4- and 7-substituted coumarin derivatives revealed that solvent polarity shifted both the emission and absorption peaks, with a greater shift observed in the emission spectra, indicating that the excited- state dipole moment of the solute molecule was greater than the ground-state dipole moment (Trenor et al., 2004).

#### **3. Coumarin-derived fluorescent chemosensors**

#### **3.1 Coumarin-derived fluorescent chemosensors for metal ions**

Considering the threat of mercury to the environment and human health, a great effort has been devoted to the utilization of fluorescent methods for detection of Hg2+ ions. More than ten coumarin-derived fluorescent chemosensors for Hg2+ ions have been reported. The structures and references of these chemical sensors are listed in Table 1.

The recognition mechanisms of these chemosensors mainly involve photoinduced electron transfer (PET), intramolecular charge transfer (ICT), fluorescence resonance energy transfer (FRET), coordination, and desulfurization. For instance, the fluorescence detection of **2** upon Hg2+ addition is promoted by a Hg2+-induced desulfurization of the thiourea moiety, leading to a decrease in an ICT character of the excited-state coumarin moiety (Shiraishi et al., 2010). Coumarin-thiazolobenzo-crown ether based chemosensor **3** has been developed for Hg(II) ions that utilizes the strong coordination of Hg(II) ions on the crown oxygen and thiazole nitrogen. The complexation of Hg(II) disrupts the ICT from the oxygen donor to the

O

Fig. 1. Structures and numbering scheme of coumarin and related compounds

microenvironment experienced by the molecule (Bangar Raju & Varadarajan, 1995).

Changing the solvent or the solution pH also affected the fluorescence spectra. Study on the effect of solution pH on 7-hydroxy-4-methylcoumarin showed that increasing the solution pH raised the fluorescence intensity. Studies on the effect of changing the solvent polarity on 13 coumarin derivatives revealed that increasing solvent polarity red shifted the absorbance as well as red shifted and broadened the emission of the coumarins due to increased hydrogen bonding. Studies on the excited-state properties of 4- and 7-substituted coumarin derivatives revealed that solvent polarity shifted both the emission and absorption peaks, with a greater shift observed in the emission spectra, indicating that the excited- state dipole moment of the solute molecule was greater than the ground-state

Considering the threat of mercury to the environment and human health, a great effort has been devoted to the utilization of fluorescent methods for detection of Hg2+ ions. More than ten coumarin-derived fluorescent chemosensors for Hg2+ ions have been reported. The

The recognition mechanisms of these chemosensors mainly involve photoinduced electron transfer (PET), intramolecular charge transfer (ICT), fluorescence resonance energy transfer (FRET), coordination, and desulfurization. For instance, the fluorescence detection of **2** upon Hg2+ addition is promoted by a Hg2+-induced desulfurization of the thiourea moiety, leading to a decrease in an ICT character of the excited-state coumarin moiety (Shiraishi et al., 2010). Coumarin-thiazolobenzo-crown ether based chemosensor **3** has been developed for Hg(II) ions that utilizes the strong coordination of Hg(II) ions on the crown oxygen and thiazole nitrogen. The complexation of Hg(II) disrupts the ICT from the oxygen donor to the

1

6 7

8

2 3 4

dipole moment (Trenor et al., 2004).

**3. Coumarin-derived fluorescent chemosensors** 

**3.1 Coumarin-derived fluorescent chemosensors for metal ions** 

structures and references of these chemical sensors are listed in Table 1.

<sup>O</sup> **<sup>1</sup>** <sup>O</sup> <sup>O</sup> coumarin <sup>O</sup> chromone

It was showed in the late 1950s that substitutions on the coumarin structure shifted the fluorescence band. For instance, adding a methyl group to the 4-position of 7-hydroxy- or 7 methoxycoumarin red shifts the fluorescence spectra. Addition of electron-repelling groups in the 4-, 6-, or 7-position or electron-attracting groups in the 3-position all shifts the fluorescence band to longer wavelengths. When the carbonyl is substituted with a thione, the absorbance was red shifted and the fluorescence was quenched (Trenor et al., 2004). The reduction of the acceptor strength of the substituent at the 3-position, as in a 7 diethylaminocoumarin dye **1** (the structure of which is shown in Fig. 1) does not always result in sustained fluorescence in polar, aprotic solvents. Thus 7-diethylamino-3 styrylcoumarin dyes are not technically important as laser dyes; however, the extreme sensitivity of coumarin **1** to the medium polarity could provide an opportunity to probe the

5 N

Et2N O

N

H

Table 1. Structures and references of coumarin-derived chemosensors for Hg(II) ions

coumarin fluorophore resulting in blue-shift in absorption and quenching of the fluorescence (S. H. Lee et al., 2010). The fluorescent ratiometric Hg2+ ion sensor **4**, based on a coumarin platform coupled with a tetraamide receptor, can specifically detect Hg2+ ions through the ICT mechanism (Wang et al., 2006). Fluorescein-coumarin chemodosimeter **5** for

Coumarin-Derived Fluorescent Chemosensors 125

Detection of trace amount of Cu2+ is important not only for environmental applications, but also for toxicity determination in living organs. Following the report that a new cavitand bearing four coumarin groups acts as fluorescent chemosensor for Cu2+ (Jang et al., 2006), over ten more coumarin-derived fluorescent chemosensors for Cu(II) ions have been envisaged. The structures and references of these chemical sensors are listed in Table 2.

Structure of sensor Reference Structure of sensor Reference

G. He et al., 2010a

M. H. Kim et al., 2009

Chandrasekha r et al., 2009

Chandrasekha r et al., 2009

J. R. Sheng et al., 2008

Zhao et al., 2010

Table 2. Structures and references of coumarin-derived chemosensors for Cu(II) ions

A turn-on fluorescent probe **18** for Cu2+ is presented by incorporating coumarin fluorophores within the benzyl dihydrazone moiety. It is among the brightest Cu2+ binding sensors in aqueous media reported to date (G. He et al., 2010a). Coumarin **19** is a highly effective turn-on fluorescent sensor that is catalytically hydrolyzed by Cu2+ leading to a

**24**

N

**25**

**27**

N

O HN HN O

S N HO

> N H O

**<sup>29</sup>** <sup>N</sup>

N **<sup>26</sup>** OH

<sup>N</sup> <sup>N</sup>

O

N N O

O

Et2N O O

Et2N O O

O O NH O

Et2N <sup>O</sup> <sup>O</sup> **28**

Et2N O O

Et2N

F3C O

O CF3

<sup>N</sup> HS Ko et al.,

NO2

<sup>O</sup> X. Chen et

al., 2011

2011

N. Li et al., 2010

Ciesienski et al., 2010

Helal et al., 2011

Jung et al., 2009

N

**18** O

HN

N

O

OMe

P <sup>O</sup> <sup>N</sup> <sup>N</sup> <sup>N</sup> <sup>N</sup> <sup>O</sup> <sup>O</sup> <sup>O</sup> <sup>O</sup> Et2N NEt2

S P <sup>N</sup> <sup>N</sup> <sup>N</sup> <sup>N</sup> <sup>N</sup> <sup>O</sup> <sup>O</sup> <sup>O</sup> <sup>O</sup> Et2N NEt2

N

O O

NEt2

OH

**21**

HO

O O

<sup>N</sup> <sup>N</sup>

N

**22**

O

N H

**23**

OH

O

NEt2

**20**

N

N

O O **19**

<sup>N</sup> <sup>N</sup>

O

Et2N O O

HO

O

Et2N O

signaling Hg2+ ions is designed based on FRET arising from the interaction between a pair of fluorophores (Ryu et al., 2010). Rhodamine-coumarin conjugate **7** was developed as a probe for Hg(II) ions. The fluorescence response to Hg(II) ions is attributed to the 1:1 complex formation between probe **7** and Hg2+ (Q.-J. Ma et al., 2010). Chemosensor **9** based on the coumarin thiosemicarbazide displays a selective fluorescence enhancement for Hg2+, which is attributed to the transformation of thiosemicarbazide unit to 1,3,4-oxadiazoles via Hg2+ induced desulfurization reaction in aqueous media (W. Ma et al., 2010). Coumarinyldithiane **11** and thiocoumarin **12** selectively sense Hg2+ also due to the Hg2+-induced desulfurization reaction. Probe **14** belongs to the turn-on class of sensors, functioning via a PET process (Voutsadaki et al., 2010).

Chemosensors **15** and **16** are not coumarin derivatives but in the presence of Hg(II) ions the weakly fluorescent precursor **15** can be transformed to strongly fluorescent coumarin **17** via a desulfurization-lactonization cascade reaction as shown in Fig. 2 (Jiang & Wang, 2009). Similarly, **16** selectively senses inorganic mercury in the turn-on mode through a Hg(II) ionpromoted hydrolysis-cyclization reaction that leads to coumarin **17** as shown in Fig. 3 (Cho & Ahn, 2010).

Fig. 2. Transformation of sensor **14** to fluorescent coumarin **16** via Hg(II)-induced desulfurization-lactonization

Fig. 3. The reaction of probe **16** with HgCl2

signaling Hg2+ ions is designed based on FRET arising from the interaction between a pair of fluorophores (Ryu et al., 2010). Rhodamine-coumarin conjugate **7** was developed as a probe for Hg(II) ions. The fluorescence response to Hg(II) ions is attributed to the 1:1 complex formation between probe **7** and Hg2+ (Q.-J. Ma et al., 2010). Chemosensor **9** based on the coumarin thiosemicarbazide displays a selective fluorescence enhancement for Hg2+, which is attributed to the transformation of thiosemicarbazide unit to 1,3,4-oxadiazoles via Hg2+ induced desulfurization reaction in aqueous media (W. Ma et al., 2010). Coumarinyldithiane **11** and thiocoumarin **12** selectively sense Hg2+ also due to the Hg2+-induced desulfurization reaction. Probe **14** belongs to the turn-on class of sensors, functioning via a PET process

Chemosensors **15** and **16** are not coumarin derivatives but in the presence of Hg(II) ions the weakly fluorescent precursor **15** can be transformed to strongly fluorescent coumarin **17** via a desulfurization-lactonization cascade reaction as shown in Fig. 2 (Jiang & Wang, 2009). Similarly, **16** selectively senses inorganic mercury in the turn-on mode through a Hg(II) ionpromoted hydrolysis-cyclization reaction that leads to coumarin **17** as shown in Fig. 3 (Cho

Hg2+

(Voutsadaki et al., 2010).

Et2N O OMe

**15**

almost non-fluoresent

Et2N O OMe

OMe

Fig. 3. The reaction of probe **16** with HgCl2

HgCl2 H2O

OMe

desulfurization-lactonization

**16**

O

O O

Et2N

Hg2+, H2O

OH SPr O

Hg2+

OH SPr O

OMe

OMe

Fig. 2. Transformation of sensor **14** to fluorescent coumarin **16** via Hg(II)-induced

Et2N

desulfurization Hg(SPr)2

OH O

O

Et2N

OMe

OMe

OH

Et2N O O

O

**17** strongly fluoresent

> OMe CO2Me

fast Et2N <sup>O</sup> <sup>O</sup>

**17**

OMe

O

spontaneous lactonization

OMe

O

<sup>H</sup> OH2

& Ahn, 2010).

Detection of trace amount of Cu2+ is important not only for environmental applications, but also for toxicity determination in living organs. Following the report that a new cavitand bearing four coumarin groups acts as fluorescent chemosensor for Cu2+ (Jang et al., 2006), over ten more coumarin-derived fluorescent chemosensors for Cu(II) ions have been envisaged. The structures and references of these chemical sensors are listed in Table 2.

Table 2. Structures and references of coumarin-derived chemosensors for Cu(II) ions

A turn-on fluorescent probe **18** for Cu2+ is presented by incorporating coumarin fluorophores within the benzyl dihydrazone moiety. It is among the brightest Cu2+ binding sensors in aqueous media reported to date (G. He et al., 2010a). Coumarin **19** is a highly effective turn-on fluorescent sensor that is catalytically hydrolyzed by Cu2+ leading to a

Coumarin-Derived Fluorescent Chemosensors 127

Cl

HO O O

N

**38**

HO O O

**40**

O

HO O O

N

HO O O

N NH

H2N O

**44**

N

MeO MeO N

N

**39**

N

HN N H N <sup>H</sup> <sup>O</sup> <sup>O</sup> <sup>N</sup> .4 HCl

> N N N N

> > **41**: R = H **42**: R = CH2CO2Et

R

R

R

O

**43**

O OEt N

N

Mizukami et al., 2009

N. C. Lim et al., 2005

N. C. Lim et al., 2005

N. C. Lim et al., 2005

Yan et al., 2011

Su et al., 2010

Structure of sensor Reference Structure of sensor Reference

Brückner, 2004

N. C. Lim & Brückner, 2004

H. Li et al., 2009 Ray et al., 2010

Jung et al., 2010

Mizukami et al., 2009

Mizukami et al., 2009

Table 3. Structures and references of coumarin-derived chemosensors for Zn(II) ions

be competent for detecting zinc pools in cultured rat pituitary (GH3) and hepatoma (H4IIE) cell lines (N. C. Lim et al., 2005). Coumarin **44** is a fluorescent sensor for Zn2+ and exhibits lower background fluorescence due to intramolecular PET but upon mixing with Zn2+ in aqueous ethanol, a turn-on fluorescence emission is observed (Su et al., 2010). Recently it was reported that a biscoumarin linked by bi-thiazole acted as a colorimetric receptor

MeO O O **30**

N N

O O

**31**

N

O O

**34**

**35**

**36**: R = H **37**: R = Cl

selectively for Zn2+ (Upadhyay & Mishra, 2010).

<sup>N</sup> <sup>N</sup>

N

N

<sup>N</sup> <sup>N</sup>

S N

O

HO O O

N H

HO O O

N H

R O

N

OH

N

N

**32**: R = OH **33**: R = NEt2

SMe

N

<sup>N</sup> N. C. Lim &

MeO

N

N

Cl

R

large increase in the fluorescence intensity (M. H. Kim et al., 2009). Studies on interaction of phosphorus-supported multidentate coumarin-containing fluorescent sensors **20** and **21** with various transition metal ions reveal substantial fluorescence enhancement upon interaction with Cu2+ enabling a selective detection mechanism for Cu2+ (Chandrasekhar et al., 2009). Biscoumarin **22** linked by a C=N double bond is highly sensitive and selective to Cu2+ and the fluorescent sensing mechanism is based on C=N isomerization (J. R. Sheng et al., 2008). Coumarin probe **23** is highly selective for Cu2+ over biologically relevant alkali metals, alkaline earth matals and the first row transition metals due to the formation of a 1:2 complex between Cu2+ and **23** (Zhao et al., 2010). Fluorescent biscoumarin **24** linked by a piperazine unit shows high selectivity towards Cu2+ (Chen et al., 2011). Rationally designed iminocoumarin fluorescent sensor **25** displays high selectivity for Cu2+ over a variety of competing metal ions in aqueous solution with a significant fluorescence increase (Ko et al., 2011). Nonfluorescent coumarin derivative **26** is synthesized as an efficient turn-on fluorescent chemodosimeter for Cu2+ in water. Mechanism studies suggest that **26** forms a complex with Cu2+ in a 1:2 metal-to-ligand ratio, and a 50-fold fluorescence enhancement is observed when the complex simultaneously undergoes Cu2+-promoted hydrolysis (N. Li et al., 2010). Sensor **27** relies on a coumarin-tagged ligand that selectively binds Cu2+ over other biometals to induce fluorescence quenching, which is subsequently relieved upon UV irradiation to provide the turn-on response (Ciesienski et al., 2010). For chemosensor **28** the mechanism of fluorescence is based on ICT, which is modified by the introduction of an electron-donating diethylamino group making it chromogenic and increasing the binding affinity (Helal et al., 2011). Coumarin **29** appending 2-picolylamide enables efficient tridentate complexation for Cu(II) in preference to a variety of other common heavy and toxic metal ions (Jung et al., 2009).

Owing to the important role of zinc, the second most abundant transition metal in the human body, more and more attention has been paid to development of Zn2+-specific chemosensors including coumarin-derived fluorescent chemosensors for Zn(II) ions, the structures and references of which are listed in Table 3.

Study on the sensory capabilities of two novel di(2-picolyl)amine (DPA)-substituted coumarins **30** and **31** shows that the variation of the point of attachment of the DPA group to the coumarin framework controls their sensing behavior: the 4-subsituted system **30** is a chelation-enhanced fluorescence (CHEF)-type sensor which shows a significant increase in fluorescence intensity upon Zn2+ binding, whereas the 3-substituted coumarin **31** is a ratiometric sensor (N. C. Lim & Brückner, 2004). Coumarin Schiff-base **32** acts as a turn-on fluorescent chemosensor for Zn(II) ions (H. Li et al., 2009) and **33** does not show any twophoton activity in the wavelength range 760-860 nm but in the presence of Zn(II) **33** exhibits large two-photon absorption as well as emission in the same wavelength range (Ray et al., 2010). Another coumarin Schiff-base **43** is a highly sensitive and selective fluorescent probe for Zn2+ in tetrahydrofuran (THF) (Yan et al., 2011). In coumarin Schiff-base **34** the fluorescence quenching is dominant because of the nitrogen lone pair orbital contribution to the excitation. Upon Zn2+ coordination **34** shows a significant fluorescence enhancement due to the blocking of the nitrogen lone pair orbital by metal coordination (Jung et al., 2010). A series of coumarin-based fluorescent probes **35**-**38** for detecting Zn2+ with high affinities show the ratiometric fluorescent properties (Mizukami et al., 2009). Another series of coumarin-derived chemosensors **39**-**42** belong to the CHEF-type and have been showed to

large increase in the fluorescence intensity (M. H. Kim et al., 2009). Studies on interaction of phosphorus-supported multidentate coumarin-containing fluorescent sensors **20** and **21** with various transition metal ions reveal substantial fluorescence enhancement upon interaction with Cu2+ enabling a selective detection mechanism for Cu2+ (Chandrasekhar et al., 2009). Biscoumarin **22** linked by a C=N double bond is highly sensitive and selective to Cu2+ and the fluorescent sensing mechanism is based on C=N isomerization (J. R. Sheng et al., 2008). Coumarin probe **23** is highly selective for Cu2+ over biologically relevant alkali metals, alkaline earth matals and the first row transition metals due to the formation of a 1:2 complex between Cu2+ and **23** (Zhao et al., 2010). Fluorescent biscoumarin **24** linked by a piperazine unit shows high selectivity towards Cu2+ (Chen et al., 2011). Rationally designed iminocoumarin fluorescent sensor **25** displays high selectivity for Cu2+ over a variety of competing metal ions in aqueous solution with a significant fluorescence increase (Ko et al., 2011). Nonfluorescent coumarin derivative **26** is synthesized as an efficient turn-on fluorescent chemodosimeter for Cu2+ in water. Mechanism studies suggest that **26** forms a complex with Cu2+ in a 1:2 metal-to-ligand ratio, and a 50-fold fluorescence enhancement is observed when the complex simultaneously undergoes Cu2+-promoted hydrolysis (N. Li et al., 2010). Sensor **27** relies on a coumarin-tagged ligand that selectively binds Cu2+ over other biometals to induce fluorescence quenching, which is subsequently relieved upon UV irradiation to provide the turn-on response (Ciesienski et al., 2010). For chemosensor **28** the mechanism of fluorescence is based on ICT, which is modified by the introduction of an electron-donating diethylamino group making it chromogenic and increasing the binding affinity (Helal et al., 2011). Coumarin **29** appending 2-picolylamide enables efficient tridentate complexation for Cu(II) in preference to a variety of other common heavy and

Owing to the important role of zinc, the second most abundant transition metal in the human body, more and more attention has been paid to development of Zn2+-specific chemosensors including coumarin-derived fluorescent chemosensors for Zn(II) ions, the

Study on the sensory capabilities of two novel di(2-picolyl)amine (DPA)-substituted coumarins **30** and **31** shows that the variation of the point of attachment of the DPA group to the coumarin framework controls their sensing behavior: the 4-subsituted system **30** is a chelation-enhanced fluorescence (CHEF)-type sensor which shows a significant increase in fluorescence intensity upon Zn2+ binding, whereas the 3-substituted coumarin **31** is a ratiometric sensor (N. C. Lim & Brückner, 2004). Coumarin Schiff-base **32** acts as a turn-on fluorescent chemosensor for Zn(II) ions (H. Li et al., 2009) and **33** does not show any twophoton activity in the wavelength range 760-860 nm but in the presence of Zn(II) **33** exhibits large two-photon absorption as well as emission in the same wavelength range (Ray et al., 2010). Another coumarin Schiff-base **43** is a highly sensitive and selective fluorescent probe for Zn2+ in tetrahydrofuran (THF) (Yan et al., 2011). In coumarin Schiff-base **34** the fluorescence quenching is dominant because of the nitrogen lone pair orbital contribution to the excitation. Upon Zn2+ coordination **34** shows a significant fluorescence enhancement due to the blocking of the nitrogen lone pair orbital by metal coordination (Jung et al., 2010). A series of coumarin-based fluorescent probes **35**-**38** for detecting Zn2+ with high affinities show the ratiometric fluorescent properties (Mizukami et al., 2009). Another series of coumarin-derived chemosensors **39**-**42** belong to the CHEF-type and have been showed to

toxic metal ions (Jung et al., 2009).

structures and references of which are listed in Table 3.

Table 3. Structures and references of coumarin-derived chemosensors for Zn(II) ions

be competent for detecting zinc pools in cultured rat pituitary (GH3) and hepatoma (H4IIE) cell lines (N. C. Lim et al., 2005). Coumarin **44** is a fluorescent sensor for Zn2+ and exhibits lower background fluorescence due to intramolecular PET but upon mixing with Zn2+ in aqueous ethanol, a turn-on fluorescence emission is observed (Su et al., 2010). Recently it was reported that a biscoumarin linked by bi-thiazole acted as a colorimetric receptor selectively for Zn2+ (Upadhyay & Mishra, 2010).

Coumarin-Derived Fluorescent Chemosensors 129

Fig. 6 shows the structures of coumarin-derived fluorescent chemosensors for Ag(I). A highly sensitive and selective fluorescent chemosensor **55** for Ag+ based on a coumarin-Se2N chelating conjugate is developed. Due to inhibiting a PET quenching pathway, a fluorescent enhancement of 4-fold is observed under the binding of the Ag+ cation to **55** with a detection limit down to the 10–8 M range (S. Huang et al., 2011). Coumarin **56** and **57** are highly silver ion selective fluorescence ionophores (Sakamoto et al., 2010). By a microwave-assisted dual click reaction, fluorogenic 3-azidocoumarin can be rapidly introduced onto a 3,4-dipropargylglucoor galactosyl scaffold with restored fluorescence. Subsequent desilylation leads to water soluble sugar-bis-triazolocoumarin conjugates which are applicable toward selective Ag+

**57**

OH

O

X

**60**: X = S **61**: X = NH

N O O

KO2C **59**: X = O

MeO <sup>N</sup>

N

S

S

S O S

detection in aqueous media via fluorescence spectroscopy (X.-P. He et al., 2011).

O

O

Fig. 6. Structures of coumarin-derived fluorescent chemosensors for silver(I) ion

O

Fig. 7. Structures of coumarin-derived fluorescent chemosensors for lead(II) ion

O

**56**

Structures of coumarin-derived fluorescent chemosensors for lead(II) are shown in Fig. 7. Fluorescent chemosensor **58** based on a coumarin-crown ether conjugate exhibits a high affinity and selectivity for Pb2+ (C.-T. Chen & W.-P. Huang, 2002). Coumarin dyes **59**-**61** seem to fulfill most of the criteria required for intracellular lead indicators, as they exhibit high selectivity for Pb2+ (Roussakis et al., 2008). Ion competition studies and fluorescence experiments show that a fullerene-coumarin dyad is selective for Pb2+ complexation (Pagona

KO2C

Apart from the above-mentioned metal ions, coumarin-derived fluorescent chemosensors have been used for detection of other metal ions including Cd2+ (Taki et al., 2008), Al3+ (Maity & Govindaraju, 2010), and Cr3+ (Hu et al., 2011). Some coumarin-derived fluorescent chemosensors exhibit simultaneous sensitivity toward two or more different metal ions, e.g. Cu(II) and Hg(II) based on FRET mechanism (G. He et al., 2010b) or via selective anioninduced demetallation (Lau et al., 2011), Cu2+ and Ni2+ (H. Li et al., 2011), Ni2+ and Zn2+ (Chattopadhyay et al., 2006), Ni2+ and Co2+ (Lin et al., 2009), Ca2+ and Mg2+ (Suresh & Das, 2009), Na+ and K+ (Ast et al., 2011), Hg2+ and Ag+ (Tsukamoto et al., 2011), Cu2+/Ni2+/Cd2+ (Lin et al., 2008), Zn2+/Cd2+/Pb2+ (Kulatilleke et al., 2006), or Ni2+/Pd2+/Ag+ (Santos et al.,

OH

N S

S

HO

**55**

O

<sup>N</sup> Se Se

et al., 2010).

2009).

Et2N O O

O

O

**58** <sup>O</sup>

N

O

Structures of several coumarin-derived fluorescent chemosensors for iron(III) are shown in Fig. 4. Squarate hydroxamate-coumarin conjugate **45** is designed as a CHEF-type sensor for Fe(III). Due to a PET process, **45** possesses a low fluorescence yield but upon exposure of **45** to Fe(III), an irreversible 9-fold fluorescence intensity increase is observed as the result of an oxidation/hydrolysis reaction (N. C. Lim et al., 2009). Coumarin derivative **46** exhibits high selectivity for Fe3+ and the selectivity is not affected by the presence of representative alkali metals, alkali earth metals and other transition metal salts (Yao t al., 2009). Mugineic acidcoumarin derivative **47** synthesized by click chemistry acts as a fluorescent probe for Fe3+ (Namba et al., 2010). Coumarin-based hexadentate fluorescent probes for selective quantification of iron(III) have also been designed and synthesized (Y. M. Ma & Hider, 2009).

Fig. 4. Structures of coumarin-derived fluorescent chemosensors for iron(III) ion

Structures of coumarin-derived fluorescent chemosensors for Mg(II) are shown in Fig. 5. Coumarin-based two-photon probe **48** is developed for the detection of free Mg2+ ions in living cells and living tissues. The probe can be excited by 880 nm laser photons, emits strong twophoton excited fluorescence in response to Mg2+ ions (H. M. Kim et al., 2007). Coumarin Schiffbase **49**, without two-photon activity in the wavelength range 760-860 nm, exhibits large twophoton absorption as well as emission in the presence of Mg2+ (Ray et al., 2010). Coumarinbased chromoionophore **50** implemented in a transparent membrane can be used as an optical one-shot sensor for Mg2+ (Capitán-Vallvey, 2006). Two coumarin salen-based sensors **51** and **52** exhibit a pronounced fluorescence enhancement response toward Mg2+ in the presence of Na+ as a synergic trigger (Dong et al., 2011). Coumarin-derived fluorescent molecular probes **53** and **54** can be used for highly selective detection of Mg2+ versus Ca2+ by means of monitoring the absorption and fluorescence spectral change (Suzuki et al., 2002).

Fig. 5. Structures of coumarin-derived fluorescent chemosensors for Mg(II) ion

Structures of several coumarin-derived fluorescent chemosensors for iron(III) are shown in Fig. 4. Squarate hydroxamate-coumarin conjugate **45** is designed as a CHEF-type sensor for Fe(III). Due to a PET process, **45** possesses a low fluorescence yield but upon exposure of **45** to Fe(III), an irreversible 9-fold fluorescence intensity increase is observed as the result of an oxidation/hydrolysis reaction (N. C. Lim et al., 2009). Coumarin derivative **46** exhibits high selectivity for Fe3+ and the selectivity is not affected by the presence of representative alkali metals, alkali earth metals and other transition metal salts (Yao t al., 2009). Mugineic acidcoumarin derivative **47** synthesized by click chemistry acts as a fluorescent probe for Fe3+ (Namba et al., 2010). Coumarin-based hexadentate fluorescent probes for selective quantification of iron(III) have also been designed and synthesized (Y. M. Ma & Hider, 2009).

O

OH OH

Structures of coumarin-derived fluorescent chemosensors for Mg(II) are shown in Fig. 5. Coumarin-based two-photon probe **48** is developed for the detection of free Mg2+ ions in living cells and living tissues. The probe can be excited by 880 nm laser photons, emits strong twophoton excited fluorescence in response to Mg2+ ions (H. M. Kim et al., 2007). Coumarin Schiffbase **49**, without two-photon activity in the wavelength range 760-860 nm, exhibits large twophoton absorption as well as emission in the presence of Mg2+ (Ray et al., 2010). Coumarinbased chromoionophore **50** implemented in a transparent membrane can be used as an optical one-shot sensor for Mg2+ (Capitán-Vallvey, 2006). Two coumarin salen-based sensors **51** and **52** exhibit a pronounced fluorescence enhancement response toward Mg2+ in the presence of Na+ as a synergic trigger (Dong et al., 2011). Coumarin-derived fluorescent molecular probes **53** and **54** can be used for highly selective detection of Mg2+ versus Ca2+ by means of monitoring

**52**

Fig. 5. Structures of coumarin-derived fluorescent chemosensors for Mg(II) ion

O N N O O O

N

**49**

O O

OH HO

OH

MeO O O

N N N

**47**

O N

HN

HO

O O

**53**: R = H **54**: R = CH3

R R

N

R R

NEt2

**50**

O

Et2N O O

O

O O O

O

O

CO2H

CO2H

CO2H

O O

HN

Fig. 4. Structures of coumarin-derived fluorescent chemosensors for iron(III) ion

**45 46**

the absorption and fluorescence spectral change (Suzuki et al., 2002).

O

O

OMe

O

O

OMe

**51**

O N N O O O

OH HO

**48**

Me2N

O O

OH

Et2N

O

NH

N OH

O

O

Fig. 6 shows the structures of coumarin-derived fluorescent chemosensors for Ag(I). A highly sensitive and selective fluorescent chemosensor **55** for Ag+ based on a coumarin-Se2N chelating conjugate is developed. Due to inhibiting a PET quenching pathway, a fluorescent enhancement of 4-fold is observed under the binding of the Ag+ cation to **55** with a detection limit down to the 10–8 M range (S. Huang et al., 2011). Coumarin **56** and **57** are highly silver ion selective fluorescence ionophores (Sakamoto et al., 2010). By a microwave-assisted dual click reaction, fluorogenic 3-azidocoumarin can be rapidly introduced onto a 3,4-dipropargylglucoor galactosyl scaffold with restored fluorescence. Subsequent desilylation leads to water soluble sugar-bis-triazolocoumarin conjugates which are applicable toward selective Ag+ detection in aqueous media via fluorescence spectroscopy (X.-P. He et al., 2011).

Fig. 6. Structures of coumarin-derived fluorescent chemosensors for silver(I) ion

Structures of coumarin-derived fluorescent chemosensors for lead(II) are shown in Fig. 7. Fluorescent chemosensor **58** based on a coumarin-crown ether conjugate exhibits a high affinity and selectivity for Pb2+ (C.-T. Chen & W.-P. Huang, 2002). Coumarin dyes **59**-**61** seem to fulfill most of the criteria required for intracellular lead indicators, as they exhibit high selectivity for Pb2+ (Roussakis et al., 2008). Ion competition studies and fluorescence experiments show that a fullerene-coumarin dyad is selective for Pb2+ complexation (Pagona et al., 2010).

Fig. 7. Structures of coumarin-derived fluorescent chemosensors for lead(II) ion

Apart from the above-mentioned metal ions, coumarin-derived fluorescent chemosensors have been used for detection of other metal ions including Cd2+ (Taki et al., 2008), Al3+ (Maity & Govindaraju, 2010), and Cr3+ (Hu et al., 2011). Some coumarin-derived fluorescent chemosensors exhibit simultaneous sensitivity toward two or more different metal ions, e.g. Cu(II) and Hg(II) based on FRET mechanism (G. He et al., 2010b) or via selective anioninduced demetallation (Lau et al., 2011), Cu2+ and Ni2+ (H. Li et al., 2011), Ni2+ and Zn2+ (Chattopadhyay et al., 2006), Ni2+ and Co2+ (Lin et al., 2009), Ca2+ and Mg2+ (Suresh & Das, 2009), Na+ and K+ (Ast et al., 2011), Hg2+ and Ag+ (Tsukamoto et al., 2011), Cu2+/Ni2+/Cd2+ (Lin et al., 2008), Zn2+/Cd2+/Pb2+ (Kulatilleke et al., 2006), or Ni2+/Pd2+/Ag+ (Santos et al., 2009).

Coumarin-Derived Fluorescent Chemosensors 131

CF3

N N

CF3

N

N

Fig. 10. Structures of coumarin-derived fluorescent chemosensors for anions other than CN–

Structures of coumarin-derived fluorescent chemosensors for detection of anions other than CN– and F– have been shown in Fig. 10. Coumarin-based fluorescent probes **73**-**76** with salicylaldehyde functinoality as recognition unit have been developed for selective detection of bisulfite anions in water (K. Chen et al., 2010). Coumarin Schiff-base **77** is a highly selective and sensitive turn-on fluorogenic probe for detection of HSO4– ions in aqueous solution (H. J. Kim et al., 2009b). Coumarin-based hydrazone **78** acts as an ICT probe for detection of acetate, benzoate, and dihydrogenphosphate (Upadhyay et al., 2010b). Another coumarin-based hydrazone **79** has been utilized as both a colorimetric and ratiometric chemosensor for acetate and a selective fluorescence turn-on probe for iodide (Mahapatra et al., 2011). Compound **80** is a colorimetric and fluorescence anion sensor with the urea group as binding site and the coumarin moiety as signal unit. The sensor displays significant fluorescence enhancement response to anions such as acetate, because of complex formation (Shao, 2010). Coumarin-derived fluorescent chemosensors have also been used for detection of pyrophosphate (S. K. Kim et al., 2009), H2PO4– and PhPO3H– (K. Choi & Hamilton, 2001), and multiple anions including pyrophosphate, citrate, ATP and ADP (Mizukami et al., 2002). A new ZnII-2,2':6',2''-terpyridine complex, derivatized with a coumarin moiety, acts as a fluorescent chemosensor for different biologically important phosphates like PPi, AMP and ADP in mixed aqueous media (Das et al., 2011). A strapped calix[4]pyrrole-coumarin conjugate has been developed as a fluorogenic anions (Cl–, Br– and AcO–) receptor

**69**

O2N NO2 O O

**77**

N O O

Et2N O O

**72**

**80**

O

O H N N H

O

N H

O **78**

N N H

O O O NO2

**70**

H N

N H

O O

O

N H N

NO2

O2N NO2

NO2

**68**

Et2N O O

CHO

HO

and F–

**73**: X = H **74**: X = F **75**: X = Cl **76**: X = Br

O O

OH

X CO2Et

**79**

N N H

O2N NO2

modulated by cation and anion binding (Miyaji et al., 2005).

O

O

**71**

O

N H N O

Si

Fig. 9. Structures of coumarin-derived fluorescent chemosensors for F– ion

OH

O

O O

Si

#### **3.2 Coumarin-derived fluorescent chemosensors for anions**

Development of highly efficient chemosensors for cyanide is of extreme significance due to the detrimental aspect of cyanide. Much attention has been paid to the utilization of fluorescent methods for the detection of cyanide. Several coumarin-derived fluorescent chemosensors for CN– have been reported, the structures of which are shown in Fig. 8.

Fig. 8. Structures of coumarin-derived fluorescent chemosensors for cyanide

Cobalt(II)-coumarinylsalen complex **62** exhibits selective and tight binding to a cyanide anion and displays a significant fluorescence enhancement upon the addition of cyanide owing to the interruption of PET from the coumarin fluorophore to the cobalt(II)-salen moiety (J. H. Lee et al., 2010). Coumarin-spiropyran conjugate **63** is a highly sensitive chemosensor for CN– and shows a CN–-selective fluorescence enhancement under UV irradiation (Shiraishi et al., 2011). An indole conjugated coumarin **64** for KCN chemodosimeter displays considerable dual changes in both absorption (blue-shift) and emission (turn-on) bands exclusively for KCN. The fluorescence enhancement of the **64**-KCN is mainly due to blocking of the ICT process (H. J. Kim et al., 2011). Doubly activatived coumarin **65** acts as a colorimetric and fluorescent chemodosimeter for cyanide (G.-J. Kim & H.-J. Kim, 2010a). A simple aldehyde-functionalized coumarin **66** has been utilized as a doubly activated Michael acceptor for cyanide (G.-J. Kim & H.-J. Kim, 2010b). Coumarin-based fluorescent chemodosimeter **67** with a salicylaldehyde functionality as a binding site has shown selectivity for cyanide anions over other anions in water at biological pH (K.-S. Lee et al., 2008a).

Recognition and detection of fluoride, the smallest anion with unique chemical properties is of growing interest. Several coumarin-derived fluorescent chemosensors for F– have been developed, the structures of which are shown in Fig. 9.

Coumarin derivative **68** has been developed as a fluorescent probe for detection of F– ion in water and bioimaging in A549 human lung carcinoma cells (S. Y. Kim et al., 2009). Coumarin **69** is a simple, highly selective, and sensitive chemosensor for fluoride anions in organic and aqueous media based on the specific affinity of fluoride anion to silicon (Sokkalingam & Lee, 2011). Coumarin-derived chemosensor **70** shows an obvious color change from yellow to blue upon addition of F– ion with a large red shift of 145 nm in acetonitrile (Zhuang et al., 2011). Coumarin-based hydrazone **71** is an ICT probe for fluoride in aqueous medium (Upadhyay et al., 2010a). Coumarin-based system **72** has been developed as a novel turn-on fluorescent and colorimetric sensor for fluoride anions (J. Li et al., 2009).

Development of highly efficient chemosensors for cyanide is of extreme significance due to the detrimental aspect of cyanide. Much attention has been paid to the utilization of fluorescent methods for the detection of cyanide. Several coumarin-derived fluorescent chemosensors for CN– have been reported, the structures of which are shown in Fig. 8.

O

O

O O

N

**64**

N

**67**

HO O O

CHO

**66**

Cobalt(II)-coumarinylsalen complex **62** exhibits selective and tight binding to a cyanide anion and displays a significant fluorescence enhancement upon the addition of cyanide owing to the interruption of PET from the coumarin fluorophore to the cobalt(II)-salen moiety (J. H. Lee et al., 2010). Coumarin-spiropyran conjugate **63** is a highly sensitive chemosensor for CN– and shows a CN–-selective fluorescence enhancement under UV irradiation (Shiraishi et al., 2011). An indole conjugated coumarin **64** for KCN chemodosimeter displays considerable dual changes in both absorption (blue-shift) and emission (turn-on) bands exclusively for KCN. The fluorescence enhancement of the **64**-KCN is mainly due to blocking of the ICT process (H. J. Kim et al., 2011). Doubly activatived coumarin **65** acts as a colorimetric and fluorescent chemodosimeter for cyanide (G.-J. Kim & H.-J. Kim, 2010a). A simple aldehyde-functionalized coumarin **66** has been utilized as a doubly activated Michael acceptor for cyanide (G.-J. Kim & H.-J. Kim, 2010b). Coumarin-based fluorescent chemodosimeter **67** with a salicylaldehyde functionality as a binding site has shown selectivity for cyanide anions over other anions in

Recognition and detection of fluoride, the smallest anion with unique chemical properties is of growing interest. Several coumarin-derived fluorescent chemosensors for F– have been

Coumarin derivative **68** has been developed as a fluorescent probe for detection of F– ion in water and bioimaging in A549 human lung carcinoma cells (S. Y. Kim et al., 2009). Coumarin **69** is a simple, highly selective, and sensitive chemosensor for fluoride anions in organic and aqueous media based on the specific affinity of fluoride anion to silicon (Sokkalingam & Lee, 2011). Coumarin-derived chemosensor **70** shows an obvious color change from yellow to blue upon addition of F– ion with a large red shift of 145 nm in acetonitrile (Zhuang et al., 2011). Coumarin-based hydrazone **71** is an ICT probe for fluoride in aqueous medium (Upadhyay et al., 2010a). Coumarin-based system **72** has been developed as a novel turn-on fluorescent and

**63**

O O

Fig. 8. Structures of coumarin-derived fluorescent chemosensors for cyanide

N

CHO

Et2N O O

**3.2 Coumarin-derived fluorescent chemosensors for anions** 

**62**

N Co N

O

NO2

O O **65**

water at biological pH (K.-S. Lee et al., 2008a).

developed, the structures of which are shown in Fig. 9.

colorimetric sensor for fluoride anions (J. Li et al., 2009).

Et2N O O

Et2N

Et2N O O

Fig. 9. Structures of coumarin-derived fluorescent chemosensors for F– ion

Fig. 10. Structures of coumarin-derived fluorescent chemosensors for anions other than CN– and F–

Structures of coumarin-derived fluorescent chemosensors for detection of anions other than CN– and F– have been shown in Fig. 10. Coumarin-based fluorescent probes **73**-**76** with salicylaldehyde functinoality as recognition unit have been developed for selective detection of bisulfite anions in water (K. Chen et al., 2010). Coumarin Schiff-base **77** is a highly selective and sensitive turn-on fluorogenic probe for detection of HSO4– ions in aqueous solution (H. J. Kim et al., 2009b). Coumarin-based hydrazone **78** acts as an ICT probe for detection of acetate, benzoate, and dihydrogenphosphate (Upadhyay et al., 2010b). Another coumarin-based hydrazone **79** has been utilized as both a colorimetric and ratiometric chemosensor for acetate and a selective fluorescence turn-on probe for iodide (Mahapatra et al., 2011). Compound **80** is a colorimetric and fluorescence anion sensor with the urea group as binding site and the coumarin moiety as signal unit. The sensor displays significant fluorescence enhancement response to anions such as acetate, because of complex formation (Shao, 2010). Coumarin-derived fluorescent chemosensors have also been used for detection of pyrophosphate (S. K. Kim et al., 2009), H2PO4– and PhPO3H– (K. Choi & Hamilton, 2001), and multiple anions including pyrophosphate, citrate, ATP and ADP (Mizukami et al., 2002). A new ZnII-2,2':6',2''-terpyridine complex, derivatized with a coumarin moiety, acts as a fluorescent chemosensor for different biologically important phosphates like PPi, AMP and ADP in mixed aqueous media (Das et al., 2011). A strapped calix[4]pyrrole-coumarin conjugate has been developed as a fluorogenic anions (Cl–, Br– and AcO–) receptor modulated by cation and anion binding (Miyaji et al., 2005).

Coumarin-Derived Fluorescent Chemosensors 133

cancer and AIDS. Study on fluorescent and colorimetric probes for detection of thiols has received much attention and many coumarin-derived fluorescent chemosensors for detection of thiols have been reported. The structures of these chemical sensors are shown in

CN <sup>O</sup> <sup>O</sup>

Cl Et <sup>O</sup> <sup>O</sup> **98** 2N

Generally detection of thiols by optical probes is based on two features of thiols, their strong nucleophilicity and high binding affinity toward metal ions. Accordingly, most of the fluorescent probes for thiols are in fact chemodosimeters, which involve specific reactions between probes and thiols, such as Michael addition, cyclization with aldehyde (or ketone), cleavage of disulfide by thiols, metal complexes-displace coordination, demetalization from Cu-complex, thiolysis of dinitrophenyl ether, and Staudinger ligation. For instance, detection of thiols by chemosensors **87**-**91** and **93** involves Michael addition between the probes and thiols. For sensors **67** (Fig. 8), **86** and **98**, cyclization reactions occur between the sensors and thiols in the detection process. Coumarin **86** is a ratiometric fluorescent probe for specific detection of Cys over Hcy and GSH based on the drstic distinction in the kinetic profiles (Yuan et al., 2011). Nonfluorescent coumarin-malonitrile conjugate **87** can be transformed into a strongly fluorescent molecule through the Michael addition and thus exhibits a highly selective fluorescence response toward biothiols including Cys, Hcy and GSH with micromolar sensitivity (Kwon et al., 2011). Similarly, nonfluorescent **88** displays a highly selective fluorescence enhancement with thiols and has been successfully applied to thiols determination in intracellular, in human urine and blood samples (Zuo et al., 2010). Coumarin **89** has been developed as a water-soluble, fast-response, highly sensitive and selective fluorescence thiol quantification probe (Yi et al., 2009). Compound **90** (G.-J. Kim et al., 2011) and **91** (S. Y. Lim et al., 2011) with a hydrogen bond act as highly selective ratiometric fluorescence turn-on probes for GSH. Structure **92** has been judiciously designed and synthesized as a new type of selective benzenethiol fluorescent probe based on the thiolysis of dinitrophenyl ether (Lin et al., 2010a). Coumarin-based chemodosimeter **93** effectively and selectively recognizes thiols based on a Michael type reaction, showing a preference for Cys over other biological materials including Hcy and GSH (Jung et al.,

N

O OH

**92**

O O O

H N

CO2Me

N S

**89**

HO O O

Br

O

CO2Et

**<sup>88</sup>** Et2N CO2Et <sup>O</sup>

O

**95**

P

O

O2N NO2

HS O O

**96**

**87**

O O **94**

<sup>N</sup> Cu

S N H N NH2

<sup>O</sup> <sup>O</sup> **<sup>91</sup>**

N O ClO4

<sup>O</sup> <sup>O</sup> NEt2 Cl

Fig. 12. Structures of coumarin-derived chemosensors for detection of thiols

CN

Et2N O O

Fig. 12.

Et2N

Et2N

N

Et2N

O O

**90**

O O **86**

**93**

CO2Et

O O **97**

S NH2 Hg Cl Hg Cl

Et2N <sup>O</sup> <sup>O</sup>

O OH

CO2Et

N H N

CHO

#### **3.3 Coumarin derivatives as fluorescent probes of pH**

Structures of coumarin-derived fluorescent chemosensors for sensing pH are shown in Fig. 11. Several iminocoumarin (**81**) derivatives **82**-**84** have been synthesized as a new type of fluorescent pH indicator. The indicators possess moderate to high brightness, excellent photostability and compatibility with light-emitting diodes. The indicators can be covalently immobilized on the surface of amino-modified polymer microbeads which in turn are incorporated into a hydrogel matrix to afford novel pH-sensitive materials. When a mixture of two different microbeads is used, the membranes are capable of optical pH sensing over a very wide range comparable to the dynamic range of the glass electrode (pH 1-11) (Vasylevska et al., 2007). Coumarin derivative **85** containing piperazine and imidazole moieties has been developed as a fluorophore for hydrogen ions sensing. The fluorescence enhancement of the sensor with an increase in hydrogen ions concentration is based on the hindering of PET from the piperazinyl amine and the imidazolyl amine to the coumarin fluorophore by protonation. The sensor **85** has a novel molecular structure design of fluorophore-spacer-receptor(1) receptor(2) format and therefore is proposed to sense two range of pH from 2.5 to 5.5 and from 10 to 12 instead of sensing one pH range (Saleh et al., 2008). By using rational molecular design, two molecular functions, the transport by vesicular monoamine transporter (VMAT) and ratiometric optical pH sensing, have been integrated to develop ratiometric pH-responsive fluorescent false neurotansmitter (FFN) probes (M. Lee et al., 2010). A FRET sensor with a donor and an acceptor attached to each end of pH-sensitive polysulfoamides exhibits an instantaneous conformation change from coil to globule at a specific pH, which results in the drastic on-and-off FRET efficiency. To detect a specific pH region, sulfadimethoxine and sulfamethizole are selected among various sulfonamides since their p*K*a values are in the physiological pH. For tuning the emission color arising from FRET, 7-hydroxy-4 bromomethylcoumarin and coumarin 343 are used as a FRET donor and an acceptor, respectively, for a blue-to-green FRET sensor (Hong & Jo, 2008).

Fig. 11. Structures of coumarin-derived fluorescent chemosensors for sensing pH

#### **3.4 Coumarin-derived fluorescent chemosensors for thiols**

Biothiols such as cysteine (Cys), homocysteine (Hcy), and glutathione (GSH) are of great significance because they are involved in myriad vital cellular processes including redox homeostasis and cellular growth. Alteration of the cellular biothiols is also implicated in

Structures of coumarin-derived fluorescent chemosensors for sensing pH are shown in Fig. 11. Several iminocoumarin (**81**) derivatives **82**-**84** have been synthesized as a new type of fluorescent pH indicator. The indicators possess moderate to high brightness, excellent photostability and compatibility with light-emitting diodes. The indicators can be covalently immobilized on the surface of amino-modified polymer microbeads which in turn are incorporated into a hydrogel matrix to afford novel pH-sensitive materials. When a mixture of two different microbeads is used, the membranes are capable of optical pH sensing over a very wide range comparable to the dynamic range of the glass electrode (pH 1-11) (Vasylevska et al., 2007). Coumarin derivative **85** containing piperazine and imidazole moieties has been developed as a fluorophore for hydrogen ions sensing. The fluorescence enhancement of the sensor with an increase in hydrogen ions concentration is based on the hindering of PET from the piperazinyl amine and the imidazolyl amine to the coumarin fluorophore by protonation. The sensor **85** has a novel molecular structure design of fluorophore-spacer-receptor(1) receptor(2) format and therefore is proposed to sense two range of pH from 2.5 to 5.5 and from 10 to 12 instead of sensing one pH range (Saleh et al., 2008). By using rational molecular design, two molecular functions, the transport by vesicular monoamine transporter (VMAT) and ratiometric optical pH sensing, have been integrated to develop ratiometric pH-responsive fluorescent false neurotansmitter (FFN) probes (M. Lee et al., 2010). A FRET sensor with a donor and an acceptor attached to each end of pH-sensitive polysulfoamides exhibits an instantaneous conformation change from coil to globule at a specific pH, which results in the drastic on-and-off FRET efficiency. To detect a specific pH region, sulfadimethoxine and sulfamethizole are selected among various sulfonamides since their p*K*a values are in the physiological pH. For tuning the emission color arising from FRET, 7-hydroxy-4 bromomethylcoumarin and coumarin 343 are used as a FRET donor and an acceptor,

MeO O O **85**

Biothiols such as cysteine (Cys), homocysteine (Hcy), and glutathione (GSH) are of great significance because they are involved in myriad vital cellular processes including redox homeostasis and cellular growth. Alteration of the cellular biothiols is also implicated in

N

HO OH

N N

O2N

N

Et2N O

**83** N

N N

OH

CO2H

**3.3 Coumarin derivatives as fluorescent probes of pH** 

respectively, for a blue-to-green FRET sensor (Hong & Jo, 2008).

Et2N O

OH

**3.4 Coumarin-derived fluorescent chemosensors for thiols** 

OMe

NO2

**82**

Fig. 11. Structures of coumarin-derived fluorescent chemosensors for sensing pH

N

N N

Et2N O NH **81**

Et2N O

N H

N

**84**

N

N N

cancer and AIDS. Study on fluorescent and colorimetric probes for detection of thiols has received much attention and many coumarin-derived fluorescent chemosensors for detection of thiols have been reported. The structures of these chemical sensors are shown in Fig. 12.

Fig. 12. Structures of coumarin-derived chemosensors for detection of thiols

Generally detection of thiols by optical probes is based on two features of thiols, their strong nucleophilicity and high binding affinity toward metal ions. Accordingly, most of the fluorescent probes for thiols are in fact chemodosimeters, which involve specific reactions between probes and thiols, such as Michael addition, cyclization with aldehyde (or ketone), cleavage of disulfide by thiols, metal complexes-displace coordination, demetalization from Cu-complex, thiolysis of dinitrophenyl ether, and Staudinger ligation. For instance, detection of thiols by chemosensors **87**-**91** and **93** involves Michael addition between the probes and thiols. For sensors **67** (Fig. 8), **86** and **98**, cyclization reactions occur between the sensors and thiols in the detection process. Coumarin **86** is a ratiometric fluorescent probe for specific detection of Cys over Hcy and GSH based on the drstic distinction in the kinetic profiles (Yuan et al., 2011). Nonfluorescent coumarin-malonitrile conjugate **87** can be transformed into a strongly fluorescent molecule through the Michael addition and thus exhibits a highly selective fluorescence response toward biothiols including Cys, Hcy and GSH with micromolar sensitivity (Kwon et al., 2011). Similarly, nonfluorescent **88** displays a highly selective fluorescence enhancement with thiols and has been successfully applied to thiols determination in intracellular, in human urine and blood samples (Zuo et al., 2010). Coumarin **89** has been developed as a water-soluble, fast-response, highly sensitive and selective fluorescence thiol quantification probe (Yi et al., 2009). Compound **90** (G.-J. Kim et al., 2011) and **91** (S. Y. Lim et al., 2011) with a hydrogen bond act as highly selective ratiometric fluorescence turn-on probes for GSH. Structure **92** has been judiciously designed and synthesized as a new type of selective benzenethiol fluorescent probe based on the thiolysis of dinitrophenyl ether (Lin et al., 2010a). Coumarin-based chemodosimeter **93** effectively and selectively recognizes thiols based on a Michael type reaction, showing a preference for Cys over other biological materials including Hcy and GSH (Jung et al.,

Coumarin-Derived Fluorescent Chemosensors 135

The current rise in international concern over criminal terorist attacks using chemical warfare agents has brought about the need for reliable and affordable detection methods of toxic gases. One of the applicable technologies is the design of fluorogenic chemosensors for the specific detection of nerve agents (Royo et al., 2007). A coumarin oximate **103** has been developed for detection of chemical warfare simulants based on the PET mechanism that gives an "off-on" fluorescent response with a half-time of approximately 50 ms upon phosphorylation of a reactive oximate functionality (Wallace et al., 2006). Coumarin-derived hydroxy oxime **104** serves as a nerve agent sensor based on the reaction of β-hydroxy oxime with organophosphorus agent mimics (Dale & Rebek, 2009). A FRET approach towards potential detection of phosgene has been developed as shown in Fig. 14. When both coumarins **105** and **106** are mixed together with triphosgene in the presence of Et3N in CHCl3, hybrid urea **107** forms in a statistical yield. Significant fluorescence enhancement is detected which is particularly important since the acceptor unit alone does not emit under the same condition. The fluorescence increase is obviously due to the formation of urea **107**. Simultaneously, the fluorescence from the donor unit decreases due to the quenching, indicating that efficient energy transfer takes place from the donor to the acceptor. This system is selective, since other gases/agents rarely can serve for cross-linking (Zhang &

B O O

Et2N

HN

O

O O

N

Fig. 14. Coumarins **105** and **106** react with phosgene to form urea **107**

O

N

O

N

**100**

Fig. 13. Structures of coumarin-derived fluorescent sensors for H2O2, O2, hydroxyl radicals

O O **103**

NOH

N N N

O

O O **104**

N

Et2N

HO HON

O

O

O NH

> O NH

O

O

HN

O

**COCl2** Et3N

HN

**107**

NH

O

NH2

+

H2N **<sup>105</sup> <sup>106</sup>**

N

Pt OO N S

O **101**

Rudkevich, 2007).

O O

B O O

Et2N O O **102**

or chemical warfare agents

O

O

**99**

2011a). Iminocoumarin-Cu(II) ensemble-based chemodosimeter **94** sensitively senses thiols followed by hydrolysis to give a marked fluorescence enhancement over other amino acids based on demetalization from Cu-complex (Jung et al., 2011b). Nonfluorescent coumarinphosphine dye **95** reacts with S-nitrosothiols (RSNOs) to form a fluorescent coumarin derivative and thus may be used as a tool in the detection of RSNOs. The reaction mechanism is similar to the well-known Staudinger ligation (Pan et al., 2009). 7-Mercapto-4 methylcoumarin **96** is a reporter of thiol binding to the CdSe quantum dot surface (González-Béjar et al., 2009). Coumarin-derived complex **97** has been developed as a reversible fluorescent probe for highly selective and sensitive detection of mercapto biomolecules such as Cys, Hcy and GSH (J. Wu et al., 2011). A simple coumarin derivative **98** is the first fluorescence turn-on probe for thioureas by the double functional group transformation strategy. The probe exhibits high sensitivity and selectivity for thioureas over other structurally and chemically related species including urea and thiophenol (Lin et al., 2010b). The simple coumarin sensor **67** (Fig. 8) has shown fluorescence selectivity for not only cyanide anions but also Hcy and Cys in water (K.-S. Lee et al., 2008b). A new coumarincontaining zinc complex has been developed as a colorimetric turn-on and fluorescence turn-off sensor which shows high selectivity for hydrogen sulfide in the presence of additional thiols like Cys or GSH (Galardon et al., 2009).

#### **3.5 Coumarin-derived fluorescent chemosensors for H2O2, O2, hydroxyl radicals or chemical warfare agents**

Structures of coumarin-derived fluorescent chemosensors for hydrogen peroxide, oxygen, hydroxyl radicals or chemical warfare agents are shown in Fig. 13. Water-soluble umbelliferone-based fluorescent probe **99** shows very large increase (up to 100-fold) in fluorescent intensity upon reaction with hydrogen peroxide, and good selectivity over other reactive oxygen species (Du et al., 2008). Another water-soluble fluorescent hydrogen peroxide probe **100** based on a 'click' modified coumarin fluorophore shows significant intensity increases (up to fivefold) in near-green fluorescence upon reaction with H2O2, and good selectivity over other reactive oxygen species (Du Ý et al., 2010). More recently a simple and highly sensitive fluorometric method was proposed for the determination of H2O2 in milk samples. In this method, nonfluorescent coumarin was oxidized to highly fluorescent 7-hydroxycoumarin by hydroxyl radicals generated in a Fenton reaction, and the oxidation product had strong fluorescence with a maximum intensity at 456 nm and could be used as a fluorescent probe for H2O2 (Abbas et al., 2010). Thiazo-coumarin ligand directly cyclometallated Pt(II) complex **101** has been used for luminescent O2 sensing (W. Wu et al., 2011). A hybrid coumarin-cyanine platform **102** has been developed as the first ratiometric fluorescent probe for detection of intracellular hydroxyl radicals (Yuan et al., 2010). More recently a coumarin-neutral red (CONER) nanoprobe was developed for detection of hydroxyl radical based on the ratiometric fluorescence signal between 7-hydroxy coumarin 3-carboxylic acid and neutral red dyes. Biocompatible poly lactide-*co*-glycolide nanoparticles containing encapsulated neutral red were produced using a coumarin 3 carboxylic acid conjugated poly(sodium *N*-undecylenyl-N*ε*-lysinate) as moiety reactive to hydroxyl radicals. The response of the CONER nanoprobe was dependent on various parameters such as reaction time and nanoparticle concentration. The probe was selective for hydroxyl radicals as compared with other reactive oxygen species including O2• –, H2O2, 1O2 and OCl– (Ganea et al., 2011).

2011a). Iminocoumarin-Cu(II) ensemble-based chemodosimeter **94** sensitively senses thiols followed by hydrolysis to give a marked fluorescence enhancement over other amino acids based on demetalization from Cu-complex (Jung et al., 2011b). Nonfluorescent coumarinphosphine dye **95** reacts with S-nitrosothiols (RSNOs) to form a fluorescent coumarin derivative and thus may be used as a tool in the detection of RSNOs. The reaction mechanism is similar to the well-known Staudinger ligation (Pan et al., 2009). 7-Mercapto-4 methylcoumarin **96** is a reporter of thiol binding to the CdSe quantum dot surface (González-Béjar et al., 2009). Coumarin-derived complex **97** has been developed as a reversible fluorescent probe for highly selective and sensitive detection of mercapto biomolecules such as Cys, Hcy and GSH (J. Wu et al., 2011). A simple coumarin derivative **98** is the first fluorescence turn-on probe for thioureas by the double functional group transformation strategy. The probe exhibits high sensitivity and selectivity for thioureas over other structurally and chemically related species including urea and thiophenol (Lin et al., 2010b). The simple coumarin sensor **67** (Fig. 8) has shown fluorescence selectivity for not only cyanide anions but also Hcy and Cys in water (K.-S. Lee et al., 2008b). A new coumarincontaining zinc complex has been developed as a colorimetric turn-on and fluorescence turn-off sensor which shows high selectivity for hydrogen sulfide in the presence of

**3.5 Coumarin-derived fluorescent chemosensors for H2O2, O2, hydroxyl radicals or** 

Structures of coumarin-derived fluorescent chemosensors for hydrogen peroxide, oxygen, hydroxyl radicals or chemical warfare agents are shown in Fig. 13. Water-soluble umbelliferone-based fluorescent probe **99** shows very large increase (up to 100-fold) in fluorescent intensity upon reaction with hydrogen peroxide, and good selectivity over other reactive oxygen species (Du et al., 2008). Another water-soluble fluorescent hydrogen peroxide probe **100** based on a 'click' modified coumarin fluorophore shows significant intensity increases (up to fivefold) in near-green fluorescence upon reaction with H2O2, and good selectivity over other reactive oxygen species (Du Ý et al., 2010). More recently a simple and highly sensitive fluorometric method was proposed for the determination of H2O2 in milk samples. In this method, nonfluorescent coumarin was oxidized to highly fluorescent 7-hydroxycoumarin by hydroxyl radicals generated in a Fenton reaction, and the oxidation product had strong fluorescence with a maximum intensity at 456 nm and could be used as a fluorescent probe for H2O2 (Abbas et al., 2010). Thiazo-coumarin ligand directly cyclometallated Pt(II) complex **101** has been used for luminescent O2 sensing (W. Wu et al., 2011). A hybrid coumarin-cyanine platform **102** has been developed as the first ratiometric fluorescent probe for detection of intracellular hydroxyl radicals (Yuan et al., 2010). More recently a coumarin-neutral red (CONER) nanoprobe was developed for detection of hydroxyl radical based on the ratiometric fluorescence signal between 7-hydroxy coumarin 3-carboxylic acid and neutral red dyes. Biocompatible poly lactide-*co*-glycolide nanoparticles containing encapsulated neutral red were produced using a coumarin 3 carboxylic acid conjugated poly(sodium *N*-undecylenyl-N*ε*-lysinate) as moiety reactive to hydroxyl radicals. The response of the CONER nanoprobe was dependent on various parameters such as reaction time and nanoparticle concentration. The probe was selective for hydroxyl radicals as compared with other reactive oxygen species including O2• –, H2O2,

additional thiols like Cys or GSH (Galardon et al., 2009).

**chemical warfare agents** 

1O2 and OCl– (Ganea et al., 2011).

The current rise in international concern over criminal terorist attacks using chemical warfare agents has brought about the need for reliable and affordable detection methods of toxic gases. One of the applicable technologies is the design of fluorogenic chemosensors for the specific detection of nerve agents (Royo et al., 2007). A coumarin oximate **103** has been developed for detection of chemical warfare simulants based on the PET mechanism that gives an "off-on" fluorescent response with a half-time of approximately 50 ms upon phosphorylation of a reactive oximate functionality (Wallace et al., 2006). Coumarin-derived hydroxy oxime **104** serves as a nerve agent sensor based on the reaction of β-hydroxy oxime with organophosphorus agent mimics (Dale & Rebek, 2009). A FRET approach towards potential detection of phosgene has been developed as shown in Fig. 14. When both coumarins **105** and **106** are mixed together with triphosgene in the presence of Et3N in CHCl3, hybrid urea **107** forms in a statistical yield. Significant fluorescence enhancement is detected which is particularly important since the acceptor unit alone does not emit under the same condition. The fluorescence increase is obviously due to the formation of urea **107**. Simultaneously, the fluorescence from the donor unit decreases due to the quenching, indicating that efficient energy transfer takes place from the donor to the acceptor. This system is selective, since other gases/agents rarely can serve for cross-linking (Zhang & Rudkevich, 2007).

Fig. 13. Structures of coumarin-derived fluorescent sensors for H2O2, O2, hydroxyl radicals or chemical warfare agents

Fig. 14. Coumarins **105** and **106** react with phosgene to form urea **107**

Coumarin-Derived Fluorescent Chemosensors 137

The fluorescence quenching of these copolymers in solution can be attributed to the collisional quenching. The response of these polymeric sensors is promising and can easily detect DNT and TNT at few parts per billion levels (Kumar et al., 2010). A novel kind of luminescent vesicular chemosensors for the recognition of biologically important ions and molecules such as imidazoles has been developed by the self-assembly of lipids, amphiphilic binding sites, and fluorescent coumarin reporter dyes that are sensitive to their environment (Gruber et al., 2010). Two hybrid compounds **114** and **115**, linked via an ester-bond between the 7-hydroxyl residue of an umbelliferone and a carboxylic acid residue of two nitroxide radicals, act as fluorescence and spin-label probes. The ESR intensities of **114** and **115** are proportionally reduced after the addition of ascorbic acid sodium salt, and their fluorescence intensities are

**3.7 Coumarin-derived fluorescent chemosensors for TiO2, monolayer, polymerization** 

Structures of coumarin-derived fluorescent chemosensors for detection of TiO2, monolayer, or photopolymerization are shown in Fig. 16. A novel acac-coumarin chromophore linker **116** for robust sensitization of TiO2 has been developed to find molecular chromophores with suitable properties for solar energy conversion. The synthesis and spectroscopic characterization confirms that **116** yields improved sensitization to solar light and provides robust attachment to TiO2 even in aqueous conditions (Xiao et al., 2011). A new amphiphilic coumarin dye, 7-aminocoumarin-4-acetic acid octadecylamide (**117**) forms a stable monolayer at the air-water interface and may be utilized as an efficient fluorescent probe for monolayer studies (Kele et al., 2001). Performance of amidocoumarins **118**-**120** as probes for monitoring of cationic photopolymerization of monomers by fluorescence probe technology has been investigated. 7-Diethylamino-4-methylcoumarin **118** can be used for monitoring cationic photopolymerization of monomers using the fluorescence intensity ratio as an indicator of the polymerization process. The replacement of diethylamino group in **118** with benzamido or acetamido groups eliminates the effect of the probe protonation on kinetics of cationic photopolymerization. 7-Benzamido-4-methylcoumarin **119** and 7-acetamido-4 methylcoumarin **120** can be used as fluorescent probes for monitoring progress of cationic polymerization of vinyl ethers under stationary measurement conditions, using normalized fluorescence intensity as an indicator of the polymerization progress (Ortyl et al., 2010). Coumarin 153 has been used as a fluorescent probe molecule to monitor the possible

increased maximally by eight- and nine-fold, respectively (Sato et al., 2008).

micellization of several amphiphilic block copolymers (Basu et al., 2009).

O

N H

<sup>O</sup> <sup>O</sup> <sup>O</sup> **<sup>120</sup>** <sup>N</sup>

Fig. 16. Structures of coumarin-derived fluorescent sensors for detection of TiO2, monolayer,

O

H

O O H2N O O **117**

H

Et2N <sup>O</sup> <sup>O</sup> **<sup>119</sup>** <sup>N</sup>

**or polymeric micelles** 

**116**

or photopolymerization

O O

**118**

OH O

#### **3.6 Coumarin-derived fluorescent chemosensors for amines, amino acids or other organic compounds**

Structures of coumarin-derived fluorescent chemosensors for amines, amino acids or other organic compounds are shown in Fig. 15. Butyl-subsituted coumarin aldehyde **108** is an excellent chemosensor for detection of amines and unprotected amino acids in aqueous conditions by formation of highly fluorescent iminium ions (Feuster & Glass, 2003). Boronic acid-containing coumarin aldehyde **109** binds to primary catecholamines with good affinity and acts as an effective colorimetric sensor for dopamine and norepinephrine with excellent selectivity over epinephrine, amino acids, and glucose. In the fluorescence manifold, sensor **109** responds differentially to catechol amines over simple amines, giving a fluorescence decrease in response to catechol-containing compounds and a fluorescence increase with other amines (Secor & Glass, 2004). Coumarin-based fluorescent functional monomers containing a carboxylic acid functionality, **110** and **111** have been synthesized, which allow for the preparation of fluorescent imprinted polymer sensors for chiral amines (Nguyen & Ansell, 2009). Coumarin aldehyde **66** (Fig. 8) can be utilized as not only a doubly activated Michael acceptor for cyanide but also a highly selective and sensitive fluorescence turn-on probe for proline (G.-J. Kim & H.-J. Kim, 2010c). Coumarin-azacrown ether conjugate **112** has been developed as a fluorescent probe for identifying melamine (Xiong et al., 2010). Thiocoumarin **113** can be efficiently desulfurized to its corresponding coumarin by the reaction with mCPBA, and results in a pronounced fluorescence turn-on type signaling. The conversion also provides a significant change in absorption behavior which allows a ratiometric analysis, providing a convient detection method for mCPBA in aqueous environment (Cha et al., 2010). Polymers containing 4,8-dimethylcoumarin have been developed for detection of 2,4-dinitrotoluene (DNT) and 2,4,6-trinitrotoluene (TNT).

Fig. 15. Structures of coumarin-derived fluorescent chemosensors for amines, amino acids or other organic compounds

Structures of coumarin-derived fluorescent chemosensors for amines, amino acids or other organic compounds are shown in Fig. 15. Butyl-subsituted coumarin aldehyde **108** is an excellent chemosensor for detection of amines and unprotected amino acids in aqueous conditions by formation of highly fluorescent iminium ions (Feuster & Glass, 2003). Boronic acid-containing coumarin aldehyde **109** binds to primary catecholamines with good affinity and acts as an effective colorimetric sensor for dopamine and norepinephrine with excellent selectivity over epinephrine, amino acids, and glucose. In the fluorescence manifold, sensor **109** responds differentially to catechol amines over simple amines, giving a fluorescence decrease in response to catechol-containing compounds and a fluorescence increase with other amines (Secor & Glass, 2004). Coumarin-based fluorescent functional monomers containing a carboxylic acid functionality, **110** and **111** have been synthesized, which allow for the preparation of fluorescent imprinted polymer sensors for chiral amines (Nguyen & Ansell, 2009). Coumarin aldehyde **66** (Fig. 8) can be utilized as not only a doubly activated Michael acceptor for cyanide but also a highly selective and sensitive fluorescence turn-on probe for proline (G.-J. Kim & H.-J. Kim, 2010c). Coumarin-azacrown ether conjugate **112** has been developed as a fluorescent probe for identifying melamine (Xiong et al., 2010). Thiocoumarin **113** can be efficiently desulfurized to its corresponding coumarin by the reaction with mCPBA, and results in a pronounced fluorescence turn-on type signaling. The conversion also provides a significant change in absorption behavior which allows a ratiometric analysis, providing a convient detection method for mCPBA in aqueous environment (Cha et al., 2010). Polymers containing 4,8-dimethylcoumarin have been developed for detection of 2,4-dinitrotoluene (DNT) and 2,4,6-trinitrotoluene (TNT).

O O O

Fig. 15. Structures of coumarin-derived fluorescent chemosensors for amines, amino acids or

CO2H N N

O O

O

O O

O O

Et2N **109**

O

N

**112**

N O

B(OH)2

O O O O

O O

CO2H

**110**

O O O

N N O O O

**115**

O

**3.6 Coumarin-derived fluorescent chemosensors for amines, amino acids or other** 

**organic compounds** 

O O S

other organic compounds

**111**

O O

**108**

Et2N

O

**113 114**

N

O

O

O O

The fluorescence quenching of these copolymers in solution can be attributed to the collisional quenching. The response of these polymeric sensors is promising and can easily detect DNT and TNT at few parts per billion levels (Kumar et al., 2010). A novel kind of luminescent vesicular chemosensors for the recognition of biologically important ions and molecules such as imidazoles has been developed by the self-assembly of lipids, amphiphilic binding sites, and fluorescent coumarin reporter dyes that are sensitive to their environment (Gruber et al., 2010). Two hybrid compounds **114** and **115**, linked via an ester-bond between the 7-hydroxyl residue of an umbelliferone and a carboxylic acid residue of two nitroxide radicals, act as fluorescence and spin-label probes. The ESR intensities of **114** and **115** are proportionally reduced after the addition of ascorbic acid sodium salt, and their fluorescence intensities are increased maximally by eight- and nine-fold, respectively (Sato et al., 2008).

#### **3.7 Coumarin-derived fluorescent chemosensors for TiO2, monolayer, polymerization or polymeric micelles**

Structures of coumarin-derived fluorescent chemosensors for detection of TiO2, monolayer, or photopolymerization are shown in Fig. 16. A novel acac-coumarin chromophore linker **116** for robust sensitization of TiO2 has been developed to find molecular chromophores with suitable properties for solar energy conversion. The synthesis and spectroscopic characterization confirms that **116** yields improved sensitization to solar light and provides robust attachment to TiO2 even in aqueous conditions (Xiao et al., 2011). A new amphiphilic coumarin dye, 7-aminocoumarin-4-acetic acid octadecylamide (**117**) forms a stable monolayer at the air-water interface and may be utilized as an efficient fluorescent probe for monolayer studies (Kele et al., 2001). Performance of amidocoumarins **118**-**120** as probes for monitoring of cationic photopolymerization of monomers by fluorescence probe technology has been investigated. 7-Diethylamino-4-methylcoumarin **118** can be used for monitoring cationic photopolymerization of monomers using the fluorescence intensity ratio as an indicator of the polymerization process. The replacement of diethylamino group in **118** with benzamido or acetamido groups eliminates the effect of the probe protonation on kinetics of cationic photopolymerization. 7-Benzamido-4-methylcoumarin **119** and 7-acetamido-4 methylcoumarin **120** can be used as fluorescent probes for monitoring progress of cationic polymerization of vinyl ethers under stationary measurement conditions, using normalized fluorescence intensity as an indicator of the polymerization progress (Ortyl et al., 2010). Coumarin 153 has been used as a fluorescent probe molecule to monitor the possible micellization of several amphiphilic block copolymers (Basu et al., 2009).

Fig. 16. Structures of coumarin-derived fluorescent sensors for detection of TiO2, monolayer, or photopolymerization

Coumarin-Derived Fluorescent Chemosensors 139

Apart from the above-mentioned coumarin-derived fluorescent chemosensors for enzymes, clikable biocompatible nanoparticles have been prepared in a one-pot process by microemulsion polymerization, which are then readily modified by the Huisgen Cu(I) catalyzed azide-alkyne cycloaddition reaction to afford a coumarin-containing subtilisin responsive nanosensor (Welser et al., 2009). A coumarin-containing time-resolved fluorescence probe for dipeptidyl peptidase 4 has also been reported (Kawaguchi et al., 2010). A coumarin-derived triple-signaling fluorescent probe has been successfully applied for intracellular measurement of different enzyme activity (Y. Li et al., 2011). As shown in Fig. 18, a sensitive, selective, and fluorogenic probe **131** has been developed for monoamine oxidases (MAO A and B). Nonfluorescent aminocoumarin **131** can be converted to fluorescent pyrrolocoumarin **132** in the presence of MAO A and B (G. Chen et al., 2005). A new fluorogenic transformation based on a quinone reduction/lactonization sequence as shown in Fig. 19 has been developed and evaluated as a tool for probing redox phenomena

MAO A & B

Fig. 18. Systematic mapping of aminocoumarin **131** and the corresponding pyrrolocoumarin

**3.9 Coumarin-derived fluorescent chemosensors for proteins, DNA, RNA and other** 

Structures of coumarin-derived fluorescent chemosensors for DNA, RNA, nitroxyl and proteins are shown in Fig. 20. A novel coumarin C-riboside **133** is designed and synthesized based on the well-known photoprobe Coumarin 102. The coumarin C-glycoside **133** has been incorporated synthetically into DNA oligomers, and has been used to probe ultrafast dynamics of duplex DNA using time-resolved Stokes shift methods (Coleman et al., 2007). Coumarin-triazole **134** reacts with CuCl2 to form a chelated Cu(II)-**134** complex which shows highly selective turn-on type fluorogenic behavior upon addition of Angeli's salt (Na2N2O3) and can be used for detection of nitroxyl in living cells (Zhou et al., 2011). A simple coumarin derivative 7-diethylaminocoumarin-3- carboxylic acid **135** has been used as an acceptor to construct a useful and effective FRET system for detection of RNA-small molecule binding (Xie et al., 2009). Coumarin dye bearing an indolenine substituent **136** displays high emission and bright fluorescence and offers promise as an fluorescent chemosensor for protein detection (Kovalska et al., 2010). Coumarin-containing trifunctional probe **137**, assembled using a cleavable linker, is useful for efficient enrichment and detection of glycoproteins (Tsai et al., 2010). Coumarin 6 has been used as a fluorescent probe to monitor protein aggregation and can distinguish between both amorphors and fibrillar aggregates (Makwana et al., 2011). An SNAr reaction-triggered fluorescence probe is developed using a new fluorogenic compound derivatized from 7-aminocoumarin for oligonucleotides detection (Shibata et al., 2009). Coumarin C343 has been conjugated to silica nanoparticles and entrapped in a sol-gel matrix to produce a nanosensor capable of monitoring lipid peroxidation (Baker et al., 2007). Coumarin-containing dual-emission chemosensors for nucleoside polyphosphates have been developed based on a new

H N

O O **132**

in a biochemical context.

**132**

**uses** 

H2N

O O

H2N **131**

#### **3.8 Coumarin-derived fluorescent chemosensors for enzymes**

Structures of coumarin-derived fluorescent chemosensors for enzymes are shown in Fig. 17. Hemicyanine-coumarin hybrid **121** represents a new class of far-red emitting fluorogenic dyes whose fluorescence is unveiled through an enzyme-initiated domino reaction and thus acts as a fluorogenic probe for penicillin G acylase (Richard et al., 2008). Similarly novel selfimmolative spacer systems **122** and **123** have been developed and are utilized as fluorogenic probes for sensing penicillin amidase (Meyer et al., 2008). A library of 6-arylcoumarins has been developed as candidate fluorescent sensors of which **124** has the strongest fluorescence intensity, whose quantum yield is similar to that of ethyl 7-diethylaminocoumarin-3 carboxylate, a well-known fluorophore as labeling or sensing biomolecule. The transormation of the methoxy group (**125**) to a hydroxyl group (**126**) induces a change of fluorescence intensity, which suggests that **125** may be useful as a fluorescent sensor for dealkylating enzymes such as glycosidase. Coumarin **127** shows 50% decrease of the fluorescence intensity at pH 8.0 compared with that at pH 6.0 and this decrease may be derived from the deprotonation of the triazole ring. Thus **127** may be used as a fluorescent sensor for nitric oxide (Hirano et al., 2007). Histone deacetylases are intimately involved in epigenetic regulation and, thus, are one of the key therapeutic targets for cancer. Coumarinsuberoylanilide hydroxamic acid **128** is a fluorescent probe for determining binding affinities and off-rates of histone deacetylase inhibitors (Singh et al., 2011). A quinonemethide-rearrangement reaction as the off-on optical switch has been successfully implemented into the design of the first long-wavelength latent fluorogenic substrate **129** which is a sensitive fluorimetric indicator for analyte determination in salicylate hydroxylase-coupled dehydrogenase assay (S.-T. Huang et al., 2010). Another switch-on long-wavelength latent fluorogenic substrate **130** is a fluorescent probe for nitroreductase (H.-C. Huang et al., 2011).

Fig. 17. Structures of coumarin-derived fluorescent chemosensors for enzymes

Structures of coumarin-derived fluorescent chemosensors for enzymes are shown in Fig. 17. Hemicyanine-coumarin hybrid **121** represents a new class of far-red emitting fluorogenic dyes whose fluorescence is unveiled through an enzyme-initiated domino reaction and thus acts as a fluorogenic probe for penicillin G acylase (Richard et al., 2008). Similarly novel selfimmolative spacer systems **122** and **123** have been developed and are utilized as fluorogenic probes for sensing penicillin amidase (Meyer et al., 2008). A library of 6-arylcoumarins has been developed as candidate fluorescent sensors of which **124** has the strongest fluorescence intensity, whose quantum yield is similar to that of ethyl 7-diethylaminocoumarin-3 carboxylate, a well-known fluorophore as labeling or sensing biomolecule. The transormation of the methoxy group (**125**) to a hydroxyl group (**126**) induces a change of fluorescence intensity, which suggests that **125** may be useful as a fluorescent sensor for dealkylating enzymes such as glycosidase. Coumarin **127** shows 50% decrease of the fluorescence intensity at pH 8.0 compared with that at pH 6.0 and this decrease may be derived from the deprotonation of the triazole ring. Thus **127** may be used as a fluorescent sensor for nitric oxide (Hirano et al., 2007). Histone deacetylases are intimately involved in epigenetic regulation and, thus, are one of the key therapeutic targets for cancer. Coumarinsuberoylanilide hydroxamic acid **128** is a fluorescent probe for determining binding affinities and off-rates of histone deacetylase inhibitors (Singh et al., 2011). A quinonemethide-rearrangement reaction as the off-on optical switch has been successfully implemented into the design of the first long-wavelength latent fluorogenic substrate **129** which is a sensitive fluorimetric indicator for analyte determination in salicylate hydroxylase-coupled dehydrogenase assay (S.-T. Huang et al., 2010). Another switch-on long-wavelength latent fluorogenic substrate **130** is a fluorescent probe for nitroreductase

> O O O **129**

CN

N S

**122**: n = 1 **123**: n = 2

n

<sup>S</sup> <sup>H</sup> N

CO2H

N S

HO

CO2H

O

O O

O

O O

O O

O O O

CN

**128**

N

<sup>N</sup> <sup>O</sup> <sup>N</sup>

O

Fig. 17. Structures of coumarin-derived fluorescent chemosensors for enzymes

N H

SO3

SO3H

**130**

O

O

**3.8 Coumarin-derived fluorescent chemosensors for enzymes** 

(H.-C. Huang et al., 2011).

O O

O

MeO

H N O

MeO

HO

H N N N

Et2N

**124**: Ar =

**125**: Ar =

**126**: Ar =

**127**: Ar =

Ar

OEt

O

H <sup>N</sup> HO

O O O **121**

O2N

O

Apart from the above-mentioned coumarin-derived fluorescent chemosensors for enzymes, clikable biocompatible nanoparticles have been prepared in a one-pot process by microemulsion polymerization, which are then readily modified by the Huisgen Cu(I) catalyzed azide-alkyne cycloaddition reaction to afford a coumarin-containing subtilisin responsive nanosensor (Welser et al., 2009). A coumarin-containing time-resolved fluorescence probe for dipeptidyl peptidase 4 has also been reported (Kawaguchi et al., 2010). A coumarin-derived triple-signaling fluorescent probe has been successfully applied for intracellular measurement of different enzyme activity (Y. Li et al., 2011). As shown in Fig. 18, a sensitive, selective, and fluorogenic probe **131** has been developed for monoamine oxidases (MAO A and B). Nonfluorescent aminocoumarin **131** can be converted to fluorescent pyrrolocoumarin **132** in the presence of MAO A and B (G. Chen et al., 2005). A new fluorogenic transformation based on a quinone reduction/lactonization sequence as shown in Fig. 19 has been developed and evaluated as a tool for probing redox phenomena in a biochemical context.

Fig. 18. Systematic mapping of aminocoumarin **131** and the corresponding pyrrolocoumarin **132**

#### **3.9 Coumarin-derived fluorescent chemosensors for proteins, DNA, RNA and other uses**

Structures of coumarin-derived fluorescent chemosensors for DNA, RNA, nitroxyl and proteins are shown in Fig. 20. A novel coumarin C-riboside **133** is designed and synthesized based on the well-known photoprobe Coumarin 102. The coumarin C-glycoside **133** has been incorporated synthetically into DNA oligomers, and has been used to probe ultrafast dynamics of duplex DNA using time-resolved Stokes shift methods (Coleman et al., 2007). Coumarin-triazole **134** reacts with CuCl2 to form a chelated Cu(II)-**134** complex which shows highly selective turn-on type fluorogenic behavior upon addition of Angeli's salt (Na2N2O3) and can be used for detection of nitroxyl in living cells (Zhou et al., 2011). A simple coumarin derivative 7-diethylaminocoumarin-3- carboxylic acid **135** has been used as an acceptor to construct a useful and effective FRET system for detection of RNA-small molecule binding (Xie et al., 2009). Coumarin dye bearing an indolenine substituent **136** displays high emission and bright fluorescence and offers promise as an fluorescent chemosensor for protein detection (Kovalska et al., 2010). Coumarin-containing trifunctional probe **137**, assembled using a cleavable linker, is useful for efficient enrichment and detection of glycoproteins (Tsai et al., 2010). Coumarin 6 has been used as a fluorescent probe to monitor protein aggregation and can distinguish between both amorphors and fibrillar aggregates (Makwana et al., 2011). An SNAr reaction-triggered fluorescence probe is developed using a new fluorogenic compound derivatized from 7-aminocoumarin for oligonucleotides detection (Shibata et al., 2009). Coumarin C343 has been conjugated to silica nanoparticles and entrapped in a sol-gel matrix to produce a nanosensor capable of monitoring lipid peroxidation (Baker et al., 2007). Coumarin-containing dual-emission chemosensors for nucleoside polyphosphates have been developed based on a new

Coumarin-Derived Fluorescent Chemosensors 141

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Fig. 19. Structures of coumarin-derived fluorescent chemosensors for DNA, RNA, nitroxyl and proteins

## **4. Conclusions**

Coumarin-derived fluorescent chemosensors have been extensively applied in a variety of fields. Though these sensors are effective for detection of many species, their performance toward different species might decrease in the following order: metal ions, anions, biothiols, enzymes, pH, amines and amino acids, chemical warfare agents, proteins, hydrogen peroxide, hydroxyl radicals, polymerization and polymeric micelles, DNA and RNA, oxygen, titania. Continuous efforts will be devoted to development of fluorescent chemosensors with higher selectivity and sensitivity for more single target or simultaneously for multiple targets, thus providing practical fluorescent chemosensors for application in environmental chemistry, analytical chemistry, and bio-medicinal science.

#### **5. References**


mechanism involving binding-induced recovery of FRET. These sensors demonstrate that binding-induced modulation of spectral overlap is a powerful strategy for the rational

O O

N3

N

**136**

H N

O

O O

O

OH N

S S

N H

O

Fig. 19. Structures of coumarin-derived fluorescent chemosensors for DNA, RNA, nitroxyl

Coumarin-derived fluorescent chemosensors have been extensively applied in a variety of fields. Though these sensors are effective for detection of many species, their performance toward different species might decrease in the following order: metal ions, anions, biothiols, enzymes, pH, amines and amino acids, chemical warfare agents, proteins, hydrogen peroxide, hydroxyl radicals, polymerization and polymeric micelles, DNA and RNA, oxygen, titania. Continuous efforts will be devoted to development of fluorescent chemosensors with higher selectivity and sensitivity for more single target or simultaneously for multiple targets, thus providing practical fluorescent chemosensors for application in environmental chemistry, analytical chemistry, and bio-medicinal science.

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N

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OH

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and proteins

**4. Conclusions** 

**5. References** 

N

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S

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**Part 2** 

**Chemical Sensor with Nanostructure** 

