## 5 Application
Au@Ag core-shell nanostructures have attracted considerable attention because of their improved properties compared with silver nanostructures or gold nanostructures, especially optical properties, which allows them to be used as SERS substrates. Generally, SERS substrates should be sensitive, stable  
Figure 13: UV-vis-NIR spectra of the samples that correspond to the Au-Ag alloy nanoboxes and nanocages shown in Figure 5. The spectra in black are for the starting Au-Ag nanoboxes and the spectra in red are for the resulting Au-based nanocages after a reaction with an excess amount of H\({}_{2}\)O\({}_{2}\). For the nanoboxes, the LSPR peaks were at (**A**) 460; (**B**) 520; (**C**) 675; and (**D**) 745 nm. The spectra were normalized to unity. For the nanocages, the LSPR peak was red-shifted to the near-infrared region for the first two samples of nanocages due to their extremely thin walls. The spectra were normalized by dividing the same factor to the corresponding spectra of the nanobox (reprinted with permission from [73]. Copyright (2010) American Chemical Society). and rapid. [PERSON] _et al_. [115] investigated the SERS spectra of rhodamine 6G (R6G) by using Au@Ag core-shell nanorods as substrate. NR-0, NR-1 and NR-2 represent gold nanorods, Au@Ag core-shell nanorods with 1.05 nm and 2 nm silver shells, respectively. It can be seen from Figure 14 that the Raman signal of R6G with NR-1 was much higher than NR-0, and with the increase of thickness of silver shells, the intensity enhanced. They assumed that the SERS enhancement of Au@Ag core-shell nanorods was related to the electromagnetic field distribution. To confirm their assumption, they resorted to three-dimensional finite-difference time-domain (FDTD) simulations, which indicated that the hotspots near the end-to-end and end-to-side nanoparticle contacts caused the SERS enhancement of Au@Ag core-shell nanorods. We previously reported that gold nanoparticle thin films can be used as sensitive SERS substrates, because hot spots formed in the narrow gaps regions between neighboring nanoparticles can enhance the Roman signal [11]. This opinion was also proved by [PERSON] _et al_. [116]. They prepared Au@Ag core-shell nanotubes with hotspots in the surface, which exhibited more excellent SERS properties than silver nanowires. It is well known that silver nanoparticles are easy to oxidize, so another challenge is how to keep the SERS substrate stable. [PERSON] _et al_. [117] found that when silver dendrite was coated by gold and was used as SERS substrate, the stability was improved with a slight decrease of intensity of the SERS enhancement. Therefore, Au@Ag core-shell nanostructures can serve as ideal SERS substrates for chemical and biological detection processes. Metal ions do not have Raman signals, which have to be detected by indirect methods leading to inefficiency. To solve the problem, many efforts have been devoted to find ways to detect metal ions (such as Hg\({}^{2+}\), Cu\({}^{2+}\)) in a simple, rapid and ultrasensitive way [118, 119, 120]. However, we found that the detection limitation of Hg\({}^{2+}\) was 10\({}^{-12}\) M by using a single-metallic nanostructures as sensor. To further decrease the detection limitation, [PERSON] _et al_. [121] applied a SERS chip constituted of Au@Ag core-shell nanoparticles with an organic ligand (Dpy) in the detection of Hg\({}^{2+}\) as shown in Figure 15a. When the water droplet containing Hg\({}^{2+}\) was added onto the chip, the Raman signals of Dpy obviously quenched due to the formation of Hg\({}^{2+}\)-Dpy complex. The results in Figure 15b show that even 10\({}^{-14}\) M Hg\({}^{2+}\) can been detected due to the more excellent SERS properties of Au@Ag core-shell nanostructures. In addition, this approach just needs 4 min and 20 \(\mu\)L of sample. Therefore, Au@Ag core-shell nanostructures can be used for the ultrasensitive detection of trace Hg\({}^{2+}\).