## 3 Metal Oxides as Robust Platforms to Host Plasmonic Nanoparticles  
To mechanically protect the Au nanoparticles and use environment-friendly deposition processes to obtain it, transparent metal oxides, or even metal nitrides, are often used as the \"host\" for the nanoparticles. The host matrix also allows for tuning the LSPR band since, on the one hand, it changes the refractive index surrounding the nanoparticles and, on the other hand, it may limit the size distribution [36]. Moreover, the host matrix micro- and nanostructure is of paramount importance to tailor the sensitivity to gas molecules. In fact, as mentioned in Section 1, oxide materials are recognized worldwide as being convenient platforms for gas sensing using electrical signals as transduction mechanisms. Two examples of metal oxides that could work as potentially suitable host matrices for gold nanoparticles are titanium dioxide (TiO\({}_{2}\)) and copper oxide (CuO). Pure TiO\({}_{2}\) is a transparent n-type semiconductor material, with a large bandgap varying between 3.0 and 3.4 eV [87; 88; 89; 90; 91].ENREF_4. TiO\({}_{2}\) is known for its biocompatibility, nontoxicity, chemical stability, high hardness, and high optical transmittance combined with a high refractive index that ranges between 2.4 and 2.9 [92; 93; 94; 95].ENREF_10. TiO\({}_{2}\) exists in both amorphous and crystalline forms, with the most important phases anatase and rutile, both showing a tetragonal lattice structure [96]. Among these phases, anatase is known for its excellent photocatalytic activity [96; 97; 98], for antifogging and self-cleaning coating material for glasses [99; 100], and it is kinetically stable at low temperatures. In the rutile form, TiO\({}_{2}\) has good structural stability at high temperatures, together with a higher refractive index [101]. Conversely, in its amorphous form, TiO\({}_{2}\) has high blood compatibility and thus it is often used in several types of biomedical applications [102]. In addition, TiO\({}_{2}\) is widely used as a (chemical) gas sensor (Figure 9a) [103; 104; 105; 106; 107; 108; 109] and this is also a major reason why it is being explored for plasmonic sensing. Figure 8: (**a**) Illustration of strategies used to develop plasmonic nanosensors (adapted from [PERSON] et al. [64], copyright 2015 Elsevier). (**b**) Schemes for LSPR biosensors through extinction measurement, total internal reflection measurement, and darkfield measurement microscopy (adapted from [PERSON] et al. [70], copyright 2016 De Gruyter). (**c**) Schematic of an ordered gold nanorod array chip for plasmon-enhanced fluorescence (PEF) DNA detection based on a molecular beacon (adapted from [PERSON] and [PERSON] [72], copyright 2017 American Chemical Society). (**d**) Schematic of a localized surface plasmon amplification by stimulated emission of radiation (spaser) as a multimodal cellular nanoprobe (adapted from [PERSON] et al. [83], copyright 2017 Springer Nature). (**e**) PET/CT coregistered images of mice intravenously administrated with either saline or Au nanocages, followed by laser treatment. A—saline-injected mouse prior to laser irradiation; B—nanocage-injected mouse prior to laser irradiation; C—saline-injected mouse after laser irradiation; and D—nanocage-injected mouse after laser irradiation. (adapted from [PERSON] et al. [86], copyright 2010 John Wiley and Sons). Pure CuO is a p-type semiconductor material with a band gap in the range 1.2-2.1 eV and a monoclinic crystal structure [87, 111, 112]. In recent years, it has attracted much interest due to its potential applications in several technological fields, such as catalysis [113, 114, 115], solar cells [116, 117, 118], electronics [119, 120], and sensors [121, 122, 123]. In addition, CuO films have several unique features, such as low cost, nontoxicity, and abundant availability of constituents [124].