## 2 Towards Optical Gas Sensing: Plasmonic Nanoparticles
### Gold Nanoparticles: LSPR Band Curvature and Applications  
The plasmonic resonances are broadened because of several natural reasons, which include shape and size dispersion in nanoparticle ensembles, and the electron scattering in each individual particle. Bulk plasmons are damped mostly because of the electron-phonon interaction and in small particles there is an additional source of damping called surface scattering, for which it has been shown that it increases linearly with the inverse of the radius in the small size regime (less than 25 nm) [56]. All these factors result in a broadened LSPR band. Thus, the curvature of the band depends basically on the nature of the nanoparticles and their distribution, and on the surrounding dielectric function [51]. Although it may be counterintuitive, the awareness of the resonant properties of plasmonic metal nanoparticles is readily apparent to the naked eye. Since the nanoparticles absorb and scatter visible light, they can generate a wide palette of colours, depending on their concentration, geometries, and dimensions, and on their surroundings [57; 58; 59; 60]. As can be seen in Figure 7a, by increasing the size of the Au nanoparticles (observed using scanning electron microscopy--SEM), the LSPR band shape changes smoothly, thus altering the colour [61]. On the other hand, if the \"same\" nanoparticles are embedded in different materials, the LSPR band resonance will be shifted to higher or lower wavelengths, whether the surrounding refractive index is higher or lower, thus changing the colour of the material. An example of this colour change is shown in Figure 7b when only the material surrounding the Au nanoparticles is changed, from a refractive index of 1.33 (water) to 1.7 (poly(vinyl alcohol), PVA) [62]. Moreover, by changing the shape of gold nanoparticles (Figure 7c) [58], from nanospheres to nanorods or nanostars (the shape is observed in the transmission electron microscopy, TEM, images), the LSPR band on the optical absorption spectra also changes, and hence the colour, from deep-red to sky-blue. Furthermore, the adjustment of the aspect ratio (AR) of gold nanoparticles from 1.1 to 2.8 [59] (Figure 7d) redshifts appreciably the longitudinal LSPR band (\(\omega_{p}^{\prime}\)), thus changing considerably the colour. Beyond the already mentioned decorative field, several other applications based on the LSPR phenomenon have shown very promising impacts. As a matter of fact, due to their high sensitivity to the surrounding dielectric environment, plasmonic metal nanoparticles have received considerable attention over the past decade for sensing applications. Among them are the detection of biomolecules by plasmonic sensing [63; 64; 65; 66; 67; 68; 69; 70] using several methods, as depicted in Figure 8a,b. To analyse the LSPR band curvature changes in sensing experiments, [PERSON] et al. [71] developed a software that calculates several parameters of the LSPR band, such as the (i) wavelength and transmittance coordinates of the optical transmittance spectrum minimum, (ii) several transmittance values at discrete wavelengths, and (iii) the (statistic) first central moments of the normalized spectral distribution. Other examples that are still related to biomolecular detection are plasmon-enhanced fluorescence (PEF, Figure 8c) [72; 73] and SERS [74; 75; 76]. Furthermore, plasmonic nanoparticles are currently used in several other fields, from the enhancement of absorbed light in solar cells [77; 78; 79; 80] to biological imaging (Figure 8d) [81; 82; 83; 84] and phototherapy of tumours (Figure 8e) [82; 85; 86]. Figure 7: Influence of Au nanoparticles size, shape, and surrounding dielectrics on the LSPR band curvature and colour. (**a**) SEM images of colloidal Au nanoparticles with increasing size, corresponding colours, and LSPR band by UV–Vis spectroscopy (adapted from [PERSON] et al.