## 2 Towards Optical Gas Sensing: Plasmonic Nanoparticles  
### Ancient stained Glass  
Noble metals such as gold (Au) and silver (Ag) have been used since antiquity for decorative applications, and currently inspired several other practical uses in different scientific and technological areas [31]. Among several examples where noble metal-based colours can be found, the most ancient and fascinating one is the famous fourth century AD Lycurgus Cup displayed in the British Museum, London (Figure 4a). In the 1960s, several chemical analyses were conducted on pieces of this Roman cup that revealed very small concentrations of gold (0.004 at.%) and silver (0.03 at.%) [32]. Later in the 1990s, with the advances in transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX), [PERSON] and [PERSON] were able to explain the unusual optical properties such as a dichroic effect, i.e., the ability to change colour when illuminated from the outside or from within, looking as \"opaque pea-green\" in reflected light, and \"deep wine-red\" in transmitted light [32]. They found that Ag-Au alloy nanoparticles (Figure 4a-iii) with traces of copper (Cu) (Ag 66.2 at.%, Au 31.2 at.%, Cu 2.6 at.%) were responsible for the red transmission colour, while larger Ag nanoparticles were generating the green reflection colour [32]. Similar colours were demonstrated experimentally by [PERSON] et al. using solutions of Au and Ag nanoparticles [33]. Regarding the manufacture of the Lycurgus Cup, it is believed that it was made from a 15 mm-thick blown glass blank (a thick piece of glass with the overall shape of the final piece that undergoes additional decorations), and that the figures were cut, ground, and pressed to their actual shape [34]. Figure 4: **(a)** Lycurgus Cup, a Roman glass from the fourth century AD, depicting a mythological scene, with King [PERSON] being entangled by [PERSON], which was transformed into a vine-shoot by [PERSON] (the Greek god of wine), currently held at the British Museum, London; it looks **(a-i)** “opaque pea-green” with reflected light, and **(a-ii)** “deep wine-red” with transmitted light (copyright 2021 The Trustees of the British Museum), due to **(a-iii)** Ag-Au alloy nanoparticles (adapted from Barber and Freestone [32], copyright 2007 John Wiley and Sons). **(b)** Glass cups coated at CF-UM-UP, University of Minho, with different nanocomposite thin films (1–uncoated glass; 2–Au-TiO\({}_{2}\) with low gold concentration; 3–Au-TiO\({}_{2}\) with higher gold concentration; and 4–Au-Al\({}_{2}\)O\({}_{3}\)) showing different colours with **(b-i)** reflected light and **(b-ii)** transmitted light. Although the Roman glass industry was mature since the first century AD, the production method of the glass blank itself is still unclear. Modern methods of preparing colloidal noble metals were only developed after the 12 th century and reported in the 17 th century, so some theories speculate about successive grinding and melting of the glass, with several metals and metallic oxides added, in several steps. However, the existence of only six other broken pieces worldwide, from the same period, showing a similar but fainter dichroic effect has led some researchers to believe that the achievement of such an optical effect was most probably occasional, rather than the intentional work of a skilful craftsman [34; 35]. With the advancements in nanotechnology, these optical effects can now be easily reproduced using thin film coatings deposited (e.g., by sputtering) directly on common glass, as exemplified in Figure 4b. In this case, the glasses were coated with a nanomaterial containing Au nanoparticles, embedded in an oxide matrix that protects them from mechanical abrasion, allowing to mimic the dichroic effect seen in ancient stained-glass. Cup number 1 is the uncoated reference, cup 2 and 3 were coated with Au nanoparticles embedded in titanium dioxide, TiO\({}_{2}\), (cup 3 has a higher gold amount than cup 2), and cup 4 was coated with Au embedded in aluminium oxide (Al\({}_{2}\)O\({}_{3}\)).