**3. Angular interrogation approach and performance parameters of the sensor**

Due to its outstanding performance characteristics, commercial standardization, and ease of manufacturing technology, the angular interrogation method using ATR has become more popular today among various SPR based sensors. When light is directly coupled to the metal-dielectric interface, due to a mismatch of momentum, the SPs are not sufficiently excited to generate SPWs [70]. Researchers have suggested several special arrangements called Otto configuration [71], Kretschmann configuration [72, 73] as visualized in **Figure 2** to alter the momentum of the photon to couple with the SPPs leading to propagation of SPW. In prism

*Hybrid Heterostructures for SPR Biosensor DOI: http://dx.doi.org/10.5772/intechopen.94932*

**Figure 2.**

*Special Arrangements [74] e.g. (a) Kretschmann configuration, and (b) Otto configuration to match the momentum of incident photon and SPW.*

based Otto configuration, there is a distance where a dielectric layer with a smaller RI is used between the prism and metal sheet on which the light is employed. On contrary, Kretschmann configuration the metallic layer is in direct contact with the prism. Among them, the Kretschmann configuration is the most popular solution to ensure the coupling of the strongest evanescent wave passing through the metal and generate SPW [53, 74–76]. In the Kretschmann configuration, the light is incident at the metal-dielectric interface through a high index prism [77].

Usually, the incident light bounces back from the interface while the evanescent field is induced by a portion of light penetrating through the metal. For a particular sensor configuration and light frequency, the momentum of the evanescent field is aligned with the wave vector of SPW at a specific angle called resonance angle [76]. Maximum light is coupled to the oscillating electrons at this resonance condition, leading to minimum reflection. If the reflected light is plotted concerning the incident angle, then a resonance dip of reflection spectrum is observed called SPR point which is highly responsive to the RI of the sensing medium. By interrogating this SPR point the analyte can be detected easily. The performance measuring parameters e.g. sensitivity, detection accuracy, FOM, and QF should be as high as possible to eliminate false positive detection. The sensitivity of the sensor operating on the angular interrogation approach depends on the change in the SPR point or resonance angle with a change in RI of the sensing medium. **Figure 3** illustrates the

**Figure 3.** *Illustration of the SPR curve variation due to change in sensing medium RI.*

SPR curve variation due to change in sensing medium RI where the resonance point is found at *<sup>a</sup>* θ *res* and + ∇ *a a* θ θ *res res* for sensing medium RI of *n*a and *n n a a* + ∇ . Due to change in RI of ∇*na* the shift in SPR is observed as ∇ *<sup>a</sup>* θ *res* . Thus, the sensitivity *(S*a) of the sensor with the angular interrogation approach can be measured as [78]:

$$\mathbf{S}\_a = \frac{\nabla \theta\_{rs}^a}{\nabla n\_a} \tag{4}$$

A sensor's detection accuracy, which depends on the width of the SPR curve, determines how quickly and accurately the SPR point can be measured by the sensor. It is inversely proportional to the width of SPR. If ∇θ0.5 is the width of the

SPR curve corresponding to 50% reflection then the detection accuracy (D.A.), FOM, and QF can be defined as [15, 79, 80]:

$$D.A. = \frac{1}{\nabla \theta\_{0.5}}\tag{5}$$

$$FOM = \frac{\nabla \, \theta\_{rs}^{\ast} \int \nabla n\_{\bullet}}{\nabla \, \theta\_{0.5}} = \mathbb{S}\_{\omega} \times D.A. \tag{6}$$

$$QF = \frac{\nabla \, \theta\_{\text{res}}^{a}}{\nabla \, \theta\_{0.5}} \times \mathbb{S}\_{a} \tag{7}$$

## **4. Recent trends to enhance the performance of the SPR sensors**

Nowadays, the prime concern of scientists, researchers, and academicians are to enhance the performance of the SPR based sensor. To date, several attempts have

**Figure 4.**

*Schematic Illustration of SPR biosensor employing hybridization of 2D materials with Ag/Au [86].*

#### *Hybrid Heterostructures for SPR Biosensor DOI: http://dx.doi.org/10.5772/intechopen.94932*

been reported to attain highly sensitive sensors where the use of bimetallic coating and hybridization of numerous 2D materials along with plasmonic materials are the most popular approach to accommodate the angular interrogation approach. Benaziez S. et al. [81] reported a sensor where Ag is considered as an SPR active material. They showed that the addition of mostly used 2D material graphene on Ag surface enables to reduce the oxidation problem as well as increase the sensitivity up to 9.3%. Yet, the detection accuracy of the sensor is slightly reduced. Also, Rouf H. K. and Haque A. [82] proposed a hybrid structure using InP and Ti with the Ag-Au bimetallic configuration. Their sensor shows maximum sensitivity of 70.90 deg/ RIU. Similarly, Mishra S. K. and their team [83] have demonstrated a configuration with excellent sensor sensitivity of 229 deg/RIU. They used a rarely used material

#### **Figure 5.**

*Sensitivity variation due to change in the thickness of PtSe2, and number of (a) Graphene layer (b) MoS2 layer, (c) WS2 layer for BK7/Ag (50 nm)/PtSe2/2D materials (Graphene/MoS2/WS2) hybrid structure; and number of (d) Graphene layer, (e) MoS2 layer, and (f) WS2 layer for BK7/Au (50 nm)/PtSe2/2D materials (Graphene/MoS2/WS2) hybrid structure [86].*

Rhodium (Rh) with Ag to realize bimetallic configuration. Also, they used a silicon layer on the bimetallic layer to lessen the limitations of Ag. Likewise, N. Mudgal et al. [3] proposed a four-layer hybrid structure that consists of Au, molybdenum disulfide (MoS2), h-BN (hexagonal boron nitride), and graphene to detect urine glucose. The structure can enhance the sensor sensitivity up to 194.12 deg/RIU with the detection accuracy of 16.04/RIU. In the same way, Hailin Xu et al. [84] proposed an optical sensor with the graphene-Al-graphene sandwich structure where graphene prevents the oxidation issue of Al as well as enhances the sensor sensitivity 3.4 times more than only Al-based sensor. Besides, Wang M. et al. [85] suggested a sensor consisting of graphene, Tungsten disulfide (WS2), and Au-Ag bimetallic film. They observed that hybridization of single layer graphene and WS2 with Au-Ag bimetallic nanostructure leads to sensitivity up to 182.5 deg/RIU which is superior to Au-only based sensor. Incorporating the advantages of hybrid structure and bimetallic configuration, very recently Rahman M. et al. [86] also proposed a new configuration of SPR biosensors utilizing the newly emerged TMDC (PtSe2) embedded 2D materials as illustrated in **Figure 4**.

In this configuration, a heterostructure of PtSe2/2D material (e.g., graphene, MoS2, WS2) has been employed to realize the hybrid configuration whereas BK7 prism is used as a coupler that increases the momentum of the evanescent wave to match with the wave vector of the SPW. The sensor comprises a thin layer (50 nm) of Au or Ag as an SPR active material between the prism coupler and PtSe2/2D material heterostructure. A monochromatic He-Ne laser source having a wavelength of 633 nm have been incorporated to excite the SPPs. The sensor parameters are altered and optimized varying the thickness of PtSe2 and number 2D material's layer to get better performance where the results are revealed in **Figure 5**.

The effects of alteration of different parameters of PtSe2, and 2D materials have been analyzed comprehensively and two new sensors have been introduced with excellent performance characteristics. The details of optimized design parameters and performances are listed in **Table 1**. As well, **Table 2** shows the performance comparison of different SPR biosensors based on Kretschmann configuration with a hybrid structure.


**Table 1.**

*Details of optimized design parameters and results of the proposed SPR biosensors [86].*

*Hybrid Heterostructures for SPR Biosensor DOI: http://dx.doi.org/10.5772/intechopen.94932*


**Table 2.**

*Sensitivity comparison of Kretschmann configuration based SPR biosensors comprising hybrid structures.*
