# Towards a Glass New World: The Role of Ion-Exchange in Modern Technology
## 2 Glass Strengthening: The Ion-Exchange Contribution
### Ion-Exchanged Noble Metal Nanoparticles in Glasses for Sers Applications
#### 3.2.5 Metal Thin Film Deposition for Noble Metal Ions Reduction in Ion Exchange Process  
The possibility of realizing SERS active metal nanostructures on high quality, free from impurities, commercial glasses (such as, Corning 0211 borosilicate glass) by means of ion-exchange process can be favoured by applying on the sample surface a metal thin film working as a mask or electrode. In both cases, the presence of the film plays the role of reducing element for the silver ions introduced into the glass following the ion-exchange, as well investigated in the works of [PERSON] and co-workers [103; 104; 159]. In the masked ion-exchange process in silver molten salts, the reduction mechanism of silver ions in metallic form is strictly related to the presence of electrical potential  
Figure 6: (**a**) Growth process of metal silver nanoparticles on the ion-exchanged sample surface at the edge of an exposed zone by high power laser irradiation; (**b**) a comparison between the distribution of silver nanoparticles in the exposed and not-exposed region, respectively (adapted with permission from [165] © The Optical Society); (**c**) a SEM image of a quasi-periodic arrangement between silver NPs; (**d**) an ordered silver nanoparticle array, with its surface profile plot, obtained by laser beam irradiation technique (reprinted with permission from [166] © The Optical Society). differences among the molten salt, the glass and the metal mask. This produces an ionic current flow: the silver ions migrate from the molten salt into the glass through the mask openings, concentrating in the region under the mask edges. Here, by capturing an electron made available by the metal mask, the silver ions are progressively reduced to metallic state [103; 104]. Figure 7 shows the effect of a metal mask on the reduction of silver ions following the ion-exchange process. The main advantage of this photolithographic approach is to provide the formation of well-defined patterns of silver nanoparticles, aligned with ion-exchange channel waveguides, without any thermal post-processing of the sample. Furthermore, this method represents a valid alternative to that of laser irradiation to realize integrated sensors, based on optical waveguides and SERS. In the second approach, the deposed metal thin film works as an electrode in a double ion-exchange process. The glass sample is loaded with silver ions through a simple ion-exchange in molten salt. Subsequently, a thin metal film of aluminium is deposited on one of the two sample faces. The sample is then immersed in a potassium molten salt bath. In this way, the overall system represented by the metal thin film, the ion-exchanged glass and the molten salt acts as a real galvanic cell. In fact, two electric potentials are soon established on the two faces of the glass. The first, positive type, is located near the glass-salt interface and takes place from the different inter-diffusivity of the ionic species involved in the process. In this region, the Ag\({}^{+}\) and Na\({}^{+}\) ions diffuse faster in the molten salt than the K\({}^{+}\) ions in the glass. The second, negative type, is generated near the other thin film-molten-salt interface. Here the thin aluminium film tends to release electrons to the Ag\({}^{+}\) ions in the glass while Al\({}^{3+}\) ions migrate to the molten potassium salt. This electrical potential difference produces an ionic current inside the glass which forces the silver ions to move to the surface of the specimen, in an area immediately below the metal electrode. In this region, the silver ions are reduced in metallic form through the acquisition of an electron coming from the metallic film. The formation of silver nanoparticles is also promoted by the continuous flow of silver ions coming from the deeper regions of the glass towards the interface with the aluminium thin film [159].