## 3 Etching Technique
### Hydrogen Peroxide (H\({}_{2}\)O\({}_{2}\))
Because the standard reduction potential of AuCl\({}_{4}\)-/Au is higher than that of Ag+/Ag, Au can be replaced by Ag following Equation (4):  
\[3 Ag(s)+AuCl_{4}\cdot(l)=Au(s)+3 AgCl(s)+Cl\cdot(l) \tag{4}\]  
Based on the chemical equation, silver nanoboxes can be transformed into gold nanoboxes. The synthesis procedure is to form Au-Ag alloy nanoboxes by titrating Ag nanocubes with aqueous HAuCl\({}_{4}\). By controlling the amount of HAuCl\({}_{4}\) aqueous, they can precisely control the thickness of nanoboxes via titrating HAuCl\({}_{4}\) aqueous solution as presented in Figure 9a-d, causing the redshift of LSPR peaks. Then Ag atoms are removed from the nanoboxes and alloy walls via H\({}_{2}\)O\({}_{2}\) etching. Figure 9e-h show the corresponding Au-based nanocages after the addition of excess H\({}_{2}\)O\({}_{2}\). [PERSON] _et al_. [74] proposed a similar pathway to synthesize well-defined gold nanoframes from decahedral silver nanoparticles, but unlike [PERSON] _et al_. they controlled the thickness of frames by adjusting the amount of deposited gold. Furthermore, they attempted to make similar nanoframes using other silver nanostructures. Results indicated that only nanostructures with (111) facets such as nanorods and icosahedra can be used for the formation of nanoframes. They argued that gold prefers to deposit on the (111) facets, whereas (100) facets are more reactive to cause nanocages or nanoshells. We believe that the etching technique is a facile method to prepare hollow nanostructures with well-controlled shapes and sizes. Figure 8: Growth mechanisms of ag nanostructures prepared from mixtures of spherical nanoparticles and prisms and nanorods, cubes, and bipyramids (reprinted with permission from [66]. Copyright (2012) American Chemical Society). It is well known that nanostructures with sharp edges or gaps can enhance SERS intensity [75, 76, 77, 78], therefore the etching technique can also be applied to prepare flower-like silver nanostructures or make the surface of nanocrystals rough. For example, [PERSON] _et al_. [79] etched silver nanooctahedra by H\({}_{2}\)O\({}_{2}\) and NH\({}_{4}\)OH mixtures. From Figure 10 we can see that the intensity of extinction significantly increases along with the etching progress and the octopod structures have LSPR with greater intensity in the near-infrared regions (red line in Figure 10). They presented a novel etching technique for synthesis of complex nanostructures which have broad range of resonance band and strong enhancement of SERS  
Figure 9: TEM images of Au -Ag nanoboxes before and after etching with an excess amount of H\({}_{2}\)O\({}_{2}\): (**A–D**) the Au-Ag nanoboxes with their LSPR peaks being tuned to 460, 520, 675, and 745 nm, respectively, prior to the addition of H\({}_{2}\)O\({}_{2}\); and (**E–H**) the corresponding Au-based nanocages after the addition of excess H\({}_{2}\)O\({}_{2}\) (reprinted with permission from [73]. Copyright (2010) American Chemical Society). signals. Another method increasing the intensity of SERS is to fabricate nanostructures with hot spots. The definition of hot spots is the junctions or gaps between two or more closely spaced nanoparticles in which enormous electromagnetic enhancements are generated in contrast to individual particles [80]. The formation of hot spots has attracted much attention, including the theoretical aspects of electromagnetic enhancement factors in SERS [81] and experimental fabrication of nanostructures with hot spots [82; 83; 84]. In theory, Ag nanowire is an ideal nanostructure with large surface to be a SERS substrate, however, the smooth surface limits the ability to form localized surface plasmons. To solve the problem, [PERSON] _et al._[85] increased the number of SERS hot spots on the surface of silver nanowires by etching in H\({}_{2}\)O\({}_{2}\) and NH\({}_{4}\)OH mixture resulting in a 10\({}^{4}\)-fold enhancement of SERS compared with that obtained by pre-etching.