**2. Plasmonic colorimetric sensing**

people since the medieval times. Countless prominent examples of art objects still exist today,

As early as 1857, Michael Faraday published his groundbreaking findings (The Bakerian Lecture of the Royal Society of London [6]) on the experimental interactions of gold and other metals with light [7]. "*Light has a relation to the matter which it meets with in its course, and is affected by it, being reflected, deflected, transmitted, refracted, absorbed, etc. by particles very minute in their dimensions* [6]." He studied the emergence of different colors for *fluids containing gold reduced to diffused particles* and described the *metallic character of the divided gold*: "*Hitherto it may seem that I have assumed the various preparations of gold, whether ruby, green, violet, or blue in color,* 

Between 1900 and 1920, the famous contributions of James Clerk Maxwell Garnett [8], Gustav Mie [9], Richard Gans [10, 11], and Richard Adolf Zsigmondy [12], just to name a few, proved that plasmonic colors were based on optical resonances that occur for particles smaller than the wavelength of light and that can be theoretically described and precisely predicted [13–15]. This birthed the field of colloidal plasmonic nanoscience [16]. During the last few decades, a variety of different nanostructures, both in the form of individual NPs and particle assemblies [17], have been developed with a special focus on their use as colorimetric or SERS

Today's pronounced diversity of available nanostructures demonstrates the high level of interest in these *optically functional materials*. Within the scope of this book chapter, this diversity can, of course, only be covered to a limited extent. For this reason, the focus here is on NPs as defined building blocks for discrete nanostructures by guided self-assembly [20–22]. **Figure 1** provides a rough overview of functional structures which have been found to be particularly suitable for sensor applications. The first area is represented by individual nanocrystals, for example, in spherical, rod-shaped, triangular, or star-shaped morphologies with strong *intrinsic* electromagnetic hot spots. This can be extended by coupling these particles to a metal surface or thin-film, the formation of branched structures by overgrowth, or hollow, core/shell, and nested structures with nanoscale interior gaps. Nanoparticles at short distances form *extrinsic* hot spots by strong electromagnetic interactions [23], referred to as plasmonic coupling [24]. Even in the disordered state, particle aggregates produce strong field enhancements. However, it is the ordered assembly of particles that allows plasmonic hybridization to emerge [25]. Hybridization can result in highly sensitive modes for spectral shift sensing and intense nearfield enhancement for chemical sensing [14]. Again, the diversity ranges from discrete single nanoclusters with uniform coordination numbers to complex superstructures, supercrystals, and patterned structures both in 2D and 3D, which can be fabricated by colloidal engineering. In this chapter, we will first address the field of plasmonic colorimetric sensing. Here, the fundamental concepts for sensing of surface plasmon resonances of metallic thin films are reviewed and then extended to localized surface plasmon resonances [26] of NPs and ordered NP assemblies by guided self-assembly [20–22]. Second, we will review SERS analytics divided into several steps: the first principles of SERS [27] and off-resonance excitation, SERS analytics of dispersed particles with a focus of the tasks of functional shells, SERS analytics using disordered aggregates under controlled conditions, and finally ordered assemblies designed

such as the Lycurgus Cup (4th century AD) and stained-glass windows of cathedrals.

*to consist of that substance in a metallic divided state* [6]."

spectroscopy sensors [18, 19].

152 Plasmonics

for high SERS activity [28, 29].

To begin, we will first briefly review SPR spectroscopy of thin metallic films as a foundation for LSPR spectroscopy. Colorimetric sensing describes the optical determination of chemical or physicochemical properties of a sample. Subsequently, we will discuss concepts in which the plasmonic response is used to obtain information at interfaces or at the near-field environment of metal structures, which would otherwise be inaccessible.

### **2.1. SPR spectroscopy of thin metal films**

Surface plasmons (SPs) are electromagnetic waves, emerging from surface plasmon resonances (SPRs), that propagate along the surface or interface of a conductor, usually a metal/ dielectric interface [30]. Essentially, surface plasmons are light waves *trapped at interfaces* because of their strong interaction with the free electrons of the conductor. Surface structuring can guide this interaction [31]. The response of the free electrons takes place collectively in the form of oscillations in resonance with the light wave. The consequent charge density oscillation at the surface leads to a concentration of light and thus an enhancement of the local electric nearfield. The high sensitivity of this light-matter interaction renders it attractive for sensing applications. SP-based sensing builds on a *simple* resonance condition:

$$k\_{\rm m} = k\_0 \cdot \sqrt{\frac{\varepsilon\_{\rm cl} \cdot \varepsilon\_{\rm m}}{\varepsilon\_{\rm cl} + \varepsilon\_{\rm m}}} \tag{1}$$

(2)

155

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where *n* denotes the refractive indices of the dielectric prism (p) and the sensing medium (s), λ is the wavelength in free space, and *θ* is the incident angle of light [34]. Thus, changes of the refractive index (Δ*n*) of the sensing medium will shift the resonance dip by altering the resonance angle (Δ*θ*) and/or the resonance wavelength (Δλ). The sensing of this resonant spectral response can be realized in different micro- and nanostructured sensor configurations (e.g., prism-, waveguide-, channel-, grating-based setups) [35]. Oates et al. demonstrated that the established methods of SPR spectroscopy for chemical and biological sensing can be enhanced by using the ellipsometric

The transition from SPR to localized surface plasmon resonance (LSPR) sensing is accompanied by the step from sensors using metallic thin-films to nanosensors in the form of particulate matter [36–38]. The plasmon generated on a small nanoparticle, for example, a sphere, experiences strong spatial confinement because of its hindered and limited propagation [39]. This confinement, also known as localization, results in discrete charge density oscillations [9, 40], which manifest themselves by intensive colors [41]. The excitation frequency of localized plasmons (absorbance band) is highly sensitive for the size, shape, composition, and refractive index environment of the NP. Though LSPRs were capitalized for various nanophotonic applications covering many fields [16, 26, 42], this chapter is limited to their use as sensor elements. For this purpose, we briefly survey the most commonly used and prominent concepts

**Figure 2** summarizes the fundamental colorimetric sensing concepts using single NPs and disordered NP assemblies. The first example shows the working principle of a pregnancy test for which AuNPs (conjugated to anti-hCG antibodies, blue) serve as an inert dye to detect the presence of hCG antigens (green, **Figure 2B**) [43]. The test is basically a lateral flow sandwich immunoassay consisting of a test line with anti-hCG antibodies (violet), a control line with immunoglobulin G (IgG, red) antibodies, and a mixing with immobilized anti hCGconjugated AuNPs. By application of a urine sample, the NPs bind to available hCG antigens (which are indicative for a pregnancy). This is followed by the selective binding of AuNPs to the control and test line, while the latter only happens in the presence of hCG antibodies. In this example, the AuNPs serve as an inert dye which does not interfere with the antibody–

**Figure 2C** highlights the refractive index sensitivity of an AgNP-based optical sensor in various solvent environments (left) [44]. Van Duyne et al. found a linear relationship between the refractive index environment and the LSPR position. This enabled to detect the adsorption of fewer than 60,000 1-hexadecanethiol molecules on single AgNPs which corresponded to a 41 nm shift followed by dark-field spectroscopy. However, for ultralow concentrations, the variations in the physical property (e.g., LSPR shift) become increasingly smaller, and

phase information [31]. Next, we focus on colorimetric sensing using plasmonic NPs.

**2.2. LSPR spectroscopy of single NPs and disordered NP assemblies**

antigen binding by biorecognition and possesses high chemical stability.

for colorimetric sensing.

The resonance condition requires the SP mode (with frequency-dependent wave-vector *k*sp) to be greater than that of a free-space photon of the same frequency (free-space wave-vector *k*0 ). In addition, for SPRs, the frequency-dependent permittivity of metal (*ε*m) and dielectric (*ε*d) need to be of opposite signs. As a consequence, the SP resonance phenomenon has been employed for biochemical sensing [32] and clinical diagnosis [33]. Under appropriate conditions, the reflectivity of a thin metal film is extremely sensitive to changes in the local refractive index environment. **Figure 2A** (left) shows an exemplary SPR sensor in a fluidic channel in Kretschmann configuration using a prism for coupling p-polarized light into the metallic film interface [34]. The resulting evanescently decaying field reaches beyond the metallic interface into the sensing medium. When the SPR condition is satisfied, the reflection spectrum for monochromic light shows a characteristic resonance dip (**Figure 2A**, right). Here, the resonance condition is

**Figure 2.** Fundamental concepts of colorimetric plasmonic sensing. (A) SPR sensor in a fluidic channel. Copyright 2011 MDPI, adapted with permission [34]. (B) Pregnancy test using AuNPs as inert dye. (C) Refractive index sensitivity of an AgNP-based optical sensor. Copyright 2003 ACS, adapted with permission [44]. (D) LSPR sensor concept for enzyme-guided inverse sensitivity [45]. (E) Dual-responsive hydrogel/AuNP hybrid particles. Copyright 2016 NPG, adapted with permission [47]. (F) Protease-triggered dispersion of AuNP assemblies. Copyright 2007 ACS, adapted with permission [48].

Plasmonics in Sensing: From Colorimetry to SERS Analytics http://dx.doi.org/10.5772/intechopen.79055 155

$$\frac{2}{\lambda} \frac{\pi}{\lambda} \cdot n\_{\text{p}} \cdot \sin \theta - \text{Re}\left(\frac{\omega}{c} \cdot \sqrt{\frac{n\_{\text{a}}^2 \cdot \varepsilon\_{\text{m}}}{n\_{\text{a}}^2 + \varepsilon\_{\text{m}}}}\right) \tag{2}$$

where *n* denotes the refractive indices of the dielectric prism (p) and the sensing medium (s), λ is the wavelength in free space, and *θ* is the incident angle of light [34]. Thus, changes of the refractive index (Δ*n*) of the sensing medium will shift the resonance dip by altering the resonance angle (Δ*θ*) and/or the resonance wavelength (Δλ). The sensing of this resonant spectral response can be realized in different micro- and nanostructured sensor configurations (e.g., prism-, waveguide-, channel-, grating-based setups) [35]. Oates et al. demonstrated that the established methods of SPR spectroscopy for chemical and biological sensing can be enhanced by using the ellipsometric phase information [31]. Next, we focus on colorimetric sensing using plasmonic NPs.

### **2.2. LSPR spectroscopy of single NPs and disordered NP assemblies**

(1)

The resonance condition requires the SP mode (with frequency-dependent wave-vector *k*sp) to be greater than that of a free-space photon of the same frequency (free-space wave-vector

). In addition, for SPRs, the frequency-dependent permittivity of metal (*ε*m) and dielectric (*ε*d) need to be of opposite signs. As a consequence, the SP resonance phenomenon has been employed for biochemical sensing [32] and clinical diagnosis [33]. Under appropriate conditions, the reflectivity of a thin metal film is extremely sensitive to changes in the local refractive index environment. **Figure 2A** (left) shows an exemplary SPR sensor in a fluidic channel in Kretschmann configuration using a prism for coupling p-polarized light into the metallic film interface [34]. The resulting evanescently decaying field reaches beyond the metallic interface into the sensing medium. When the SPR condition is satisfied, the reflection spectrum for monochromic light shows a characteristic resonance dip (**Figure 2A**, right). Here, the

**Figure 2.** Fundamental concepts of colorimetric plasmonic sensing. (A) SPR sensor in a fluidic channel. Copyright 2011 MDPI, adapted with permission [34]. (B) Pregnancy test using AuNPs as inert dye. (C) Refractive index sensitivity of an AgNP-based optical sensor. Copyright 2003 ACS, adapted with permission [44]. (D) LSPR sensor concept for enzyme-guided inverse sensitivity [45]. (E) Dual-responsive hydrogel/AuNP hybrid particles. Copyright 2016 NPG, adapted with permission [47]. (F) Protease-triggered dispersion of AuNP assemblies. Copyright 2007 ACS, adapted with

*k*0

154 Plasmonics

resonance condition is

permission [48].

The transition from SPR to localized surface plasmon resonance (LSPR) sensing is accompanied by the step from sensors using metallic thin-films to nanosensors in the form of particulate matter [36–38]. The plasmon generated on a small nanoparticle, for example, a sphere, experiences strong spatial confinement because of its hindered and limited propagation [39]. This confinement, also known as localization, results in discrete charge density oscillations [9, 40], which manifest themselves by intensive colors [41]. The excitation frequency of localized plasmons (absorbance band) is highly sensitive for the size, shape, composition, and refractive index environment of the NP. Though LSPRs were capitalized for various nanophotonic applications covering many fields [16, 26, 42], this chapter is limited to their use as sensor elements. For this purpose, we briefly survey the most commonly used and prominent concepts for colorimetric sensing.

**Figure 2** summarizes the fundamental colorimetric sensing concepts using single NPs and disordered NP assemblies. The first example shows the working principle of a pregnancy test for which AuNPs (conjugated to anti-hCG antibodies, blue) serve as an inert dye to detect the presence of hCG antigens (green, **Figure 2B**) [43]. The test is basically a lateral flow sandwich immunoassay consisting of a test line with anti-hCG antibodies (violet), a control line with immunoglobulin G (IgG, red) antibodies, and a mixing with immobilized anti hCGconjugated AuNPs. By application of a urine sample, the NPs bind to available hCG antigens (which are indicative for a pregnancy). This is followed by the selective binding of AuNPs to the control and test line, while the latter only happens in the presence of hCG antibodies. In this example, the AuNPs serve as an inert dye which does not interfere with the antibody– antigen binding by biorecognition and possesses high chemical stability.

**Figure 2C** highlights the refractive index sensitivity of an AgNP-based optical sensor in various solvent environments (left) [44]. Van Duyne et al. found a linear relationship between the refractive index environment and the LSPR position. This enabled to detect the adsorption of fewer than 60,000 1-hexadecanethiol molecules on single AgNPs which corresponded to a 41 nm shift followed by dark-field spectroscopy. However, for ultralow concentrations, the variations in the physical property (e.g., LSPR shift) become increasingly smaller, and thus, harder to detect with confidence. Contrary to conventional transducers which generate a signal that is directly proportional to the concentration of the target molecule, Stevens and coworkers proposed an LSPR sensor with inverse sensitivity (**Figure 2D**) [45]. The key for this inverse sensitivity is the enzymatic control over the rate of nucleation of Ag on Au nanostars (top: overgrowth; bottom: nucleation), accompanied by a blueshift of the LSPR. Different biosensing strategies have been proposed building on enzymatic reactions and NPs [46].

Apart from dispersed NPs, the plasmonic coupling between NPs, which is dependent on their spatial interspacings, can be utilized for colorimetry. Song and Cho reported dual-responsive architectures by mixing hydrogel and AuNP-decorated hydrogel particles (**Figure 2E**) [47]. This hybrid ensemble responds to both temperature and ions by means of a volume and color change in aqueous systems. Both stimuli can be used to reversibly trigger the transition of uncoupled well-separated AuNPs (red tint) to a state that allows for plasmonic coupling (blue tint), mediated by the hydrogel matrix. Ulijn and Stevens et al. demonstrated the bioresponsive transition from aggregated to dispersed state [48]. **Figure 2F** shows the protease-triggered dispersion of AuNP assemblies using thermolysin for the removal of attractive self-assembly groups and revelation of repulsive charged groups. The consequent blueshift of the LSPR allowed for simple and highly sensitive detection of the presence of thermolysin, which could be tailored for different proteases.

Another approach builds on measuring the orientation of a sample. For this, it is necessary to align the ensemble of NPs macroscopically. This was achieved for anisotropic NPs homogenously dispersed in an elastic polymer matrix [49, 50]. By uniaxial stretching of the material, the NPs are oriented along the direction of elongation. As a result, the material exhibits uniform plasmonic response, which enables for optical detection of the orientation of the material. Continuing on, we will examine assemblies containing ordered NPs and patterns.

### **2.3. LSPR spectroscopy of ordered NP assemblies and defined patterned superstructures**

**Figure 3** highlights colorimetric sensing examples of ordered NP assemblies and defined patterned superstructures. Because the LSPR depends on the local dielectric environment at the NP surface, the LSPR shift can be evaluated to detect changes in effective refractive index. The first example is a macroscopic *plasmonic library* consisting of well-separated non-coupling AuNPs with a gradient in size, induced by Au overgrowth [51]. Along the array, the size increase goes along with a color change from colorless to pink (**Figure 3A**, left). Aided by electromagnetic simulations, it was possible to evaluate the effective refractive index and thus changes in the local density of the hydrogel shell around the substrate-supported particles (right). The initial increase of the refractive index indicated a densification of the hydrogel network upon particle growth from 10 to 30 nm. The subsequent decrease above 30 nm might result from internal breakup/rupture of the network.

satellite assemblies to optimize the spectral shift due to satellite attachment or release. They provided clear strategies for improving the sensitivity and signal-to-noise ratio for molecular detection, enabling simple colorimetric assays [53]. **Figure 3C** depicts the disassembly of substrate-supported core/satellite assemblies for biomolecular detection [54]. By addition of trypsin, the cysteine/biotin-streptavidin peptide tethers were proteolytically cleaved to

**Figure 3.** Colorimetric sensing concepts of ordered NP assemblies and defined patterned superstructures: (A) Effective refractive index of a substrate-supported array of particles with a gradient in sizes. Copyright 2014 ACS, adapted with permission [51]. (B) Core/satellite assemblies for highly sensitive refractive index sensing. Copyright 2015 ACS, adapted with permission [52]. (C) Biomolecular detection by disassembly of core/satellite assemblies. Copyright 2011 ACS, adapted with permission [54]. (D) Stress memory sensor based on disassembly of AuNP chains. Copyright 2014 ACS, adapted with permission [56]. (E) Mechanochromic strain sensor based on AuNP-decorated microparticles dispersed in a polymer matrix. Copyright 2017 Wiley, adapted with permission [59]. (F) Reversible strain-induced fragmentation of quasi-infinite linear assemblies to defined plasmonic oligomers. Copyright 2017 ACS, adapted with permission [62].

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157

Different concepts have been developed for opto-mechanic sensitivity and control [55]. Yin et al. reported a stress-responsive colorimetric film that can memorize the stress it has experienced (**Figure 3D**, left) [56]. This stress memory sensor is based on the LSPR shift associated with the disassembly of chains of AuNPs embedded in a polymer matrix (middle). By plastic deformation, the LSPR experiences a blueshift by irreversible breaking of the linear AuNP assemblies, initially formed in colloidal suspension [57]. The sensitivity of the optical change to stress could be tuned by doping with different amounts of PEG as plasticizer (right) [56]. Instead of mixing NPs with an elastic matrix, AuNPs can also be grown at the surface of a flexible substrate [58]. This enables mechanical control of the plasmonic coupling and electromagnetic fields at the surface. Dreyfus and coworkers designed mechanochromic AuNP-decorated microparticles as strain sensor (**Figure 3E**) [59]. After dispersion in an

release the satellites into solution enabling colorimetric detection of the protease.

Sönnichsen et al. proposed the use of core/satellite assemblies for highly sensitive refractive index sensing [52]. **Figure 3B** (left) shows 60 nm AuNPs as cores linked to 20 nm AuNPs as satellites. The average number of satellites allowed tuning the LSPR from 543 to 575 nm. The core/satellite nanostructures showed about twofold higher colorimetric sensitivity (Δλ/Δ*n*) than similar sized gold NPs (right). Lee et al. developed a theory-based design of such core/

thus, harder to detect with confidence. Contrary to conventional transducers which generate a signal that is directly proportional to the concentration of the target molecule, Stevens and coworkers proposed an LSPR sensor with inverse sensitivity (**Figure 2D**) [45]. The key for this inverse sensitivity is the enzymatic control over the rate of nucleation of Ag on Au nanostars (top: overgrowth; bottom: nucleation), accompanied by a blueshift of the LSPR. Different bio-

Apart from dispersed NPs, the plasmonic coupling between NPs, which is dependent on their spatial interspacings, can be utilized for colorimetry. Song and Cho reported dual-responsive architectures by mixing hydrogel and AuNP-decorated hydrogel particles (**Figure 2E**) [47]. This hybrid ensemble responds to both temperature and ions by means of a volume and color change in aqueous systems. Both stimuli can be used to reversibly trigger the transition of uncoupled well-separated AuNPs (red tint) to a state that allows for plasmonic coupling (blue tint), mediated by the hydrogel matrix. Ulijn and Stevens et al. demonstrated the bioresponsive transition from aggregated to dispersed state [48]. **Figure 2F** shows the protease-triggered dispersion of AuNP assemblies using thermolysin for the removal of attractive self-assembly groups and revelation of repulsive charged groups. The consequent blueshift of the LSPR allowed for simple and highly sensitive detection of the presence of thermolysin, which could

Another approach builds on measuring the orientation of a sample. For this, it is necessary to align the ensemble of NPs macroscopically. This was achieved for anisotropic NPs homogenously dispersed in an elastic polymer matrix [49, 50]. By uniaxial stretching of the material, the NPs are oriented along the direction of elongation. As a result, the material exhibits uniform plasmonic response, which enables for optical detection of the orientation of the material. Continuing on, we will examine assemblies containing ordered NPs and patterns.

**Figure 3** highlights colorimetric sensing examples of ordered NP assemblies and defined patterned superstructures. Because the LSPR depends on the local dielectric environment at the NP surface, the LSPR shift can be evaluated to detect changes in effective refractive index. The first example is a macroscopic *plasmonic library* consisting of well-separated non-coupling AuNPs with a gradient in size, induced by Au overgrowth [51]. Along the array, the size increase goes along with a color change from colorless to pink (**Figure 3A**, left). Aided by electromagnetic simulations, it was possible to evaluate the effective refractive index and thus changes in the local density of the hydrogel shell around the substrate-supported particles (right). The initial increase of the refractive index indicated a densification of the hydrogel network upon particle growth from 10 to 30 nm. The subsequent decrease above 30 nm might

Sönnichsen et al. proposed the use of core/satellite assemblies for highly sensitive refractive index sensing [52]. **Figure 3B** (left) shows 60 nm AuNPs as cores linked to 20 nm AuNPs as satellites. The average number of satellites allowed tuning the LSPR from 543 to 575 nm. The core/satellite nanostructures showed about twofold higher colorimetric sensitivity (Δλ/Δ*n*) than similar sized gold NPs (right). Lee et al. developed a theory-based design of such core/

**2.3. LSPR spectroscopy of ordered NP assemblies and defined patterned** 

result from internal breakup/rupture of the network.

sensing strategies have been proposed building on enzymatic reactions and NPs [46].

be tailored for different proteases.

**superstructures**

156 Plasmonics

**Figure 3.** Colorimetric sensing concepts of ordered NP assemblies and defined patterned superstructures: (A) Effective refractive index of a substrate-supported array of particles with a gradient in sizes. Copyright 2014 ACS, adapted with permission [51]. (B) Core/satellite assemblies for highly sensitive refractive index sensing. Copyright 2015 ACS, adapted with permission [52]. (C) Biomolecular detection by disassembly of core/satellite assemblies. Copyright 2011 ACS, adapted with permission [54]. (D) Stress memory sensor based on disassembly of AuNP chains. Copyright 2014 ACS, adapted with permission [56]. (E) Mechanochromic strain sensor based on AuNP-decorated microparticles dispersed in a polymer matrix. Copyright 2017 Wiley, adapted with permission [59]. (F) Reversible strain-induced fragmentation of quasi-infinite linear assemblies to defined plasmonic oligomers. Copyright 2017 ACS, adapted with permission [62].

satellite assemblies to optimize the spectral shift due to satellite attachment or release. They provided clear strategies for improving the sensitivity and signal-to-noise ratio for molecular detection, enabling simple colorimetric assays [53]. **Figure 3C** depicts the disassembly of substrate-supported core/satellite assemblies for biomolecular detection [54]. By addition of trypsin, the cysteine/biotin-streptavidin peptide tethers were proteolytically cleaved to release the satellites into solution enabling colorimetric detection of the protease.

Different concepts have been developed for opto-mechanic sensitivity and control [55]. Yin et al. reported a stress-responsive colorimetric film that can memorize the stress it has experienced (**Figure 3D**, left) [56]. This stress memory sensor is based on the LSPR shift associated with the disassembly of chains of AuNPs embedded in a polymer matrix (middle). By plastic deformation, the LSPR experiences a blueshift by irreversible breaking of the linear AuNP assemblies, initially formed in colloidal suspension [57]. The sensitivity of the optical change to stress could be tuned by doping with different amounts of PEG as plasticizer (right) [56]. Instead of mixing NPs with an elastic matrix, AuNPs can also be grown at the surface of a flexible substrate [58]. This enables mechanical control of the plasmonic coupling and electromagnetic fields at the surface. Dreyfus and coworkers designed mechanochromic AuNP-decorated microparticles as strain sensor (**Figure 3E**) [59]. After dispersion in an elastic polymer matrix of PVA, the capsules change in color upon elongation. When the film is stretched, the capsules are deformed into elongated ellipsoidal shapes and the distance between the AuNPs, embedded in their shells, concomitantly increases. Another mechanoplasmonic approach has been proposed for substrate-supported chains of AuNPs. **Figure 3F** (left) shows oriented linear assemblies, above the so-called infinite chain limit [60, 61], in a periodic pattern over cm2 areas on an elastic support [62]. Upon external strain, the assemblies experience a transition from long to short chains by reversible strain-induced fragmentation. The transition from plasmonic polymers to oligomers was accompanied by a pronounced spectral shift (right). A similar strain sensing approach was reported by Minati and coworkers using 1D arrays of broader line widths [63]. These multiparticle arrays showed a blueshift of the reflectance, lineally scaling with the external strain. Here, we will leave the field of colorimetric sensing and turn our attention to the enhancement of the electric field and the concomitant SERS activity.
