## 5 Application
Similar to Hg\({}^{2+}\), Cu\({}^{2+}\) has paramagnetic properties  
Figure 14: SERS spectra of R6G (1 \(\mu\)L, 80 \(\mu\) M) deposited with NR-0 (1), NR-1 (2), and NR-3 (3) rods at a Au concentration of 14 g/L (reprinted with permission from [115]. Copyright (2013) American Chemical Society). which can transfer electrons or energy resulting in photoluminescence (PL) quenching of Au@Ag core-shell nanoparticles. [PERSON] _et al_. [122] first employed Au@Ag core-shell nanoparticles in the development of a PL sensor for the detection of Cu\({}^{2+}\). As the concentration of Cu\({}^{2+}\) increased, the PL spectra of Au@Ag core-shell nanoparticles dropped dramatically, so we can analyze Cu\({}^{2+}\) according to the intensity of PL spectra. In addition to metallic ions analyses, biomolecules and organic molecules have attracted much attention. [PERSON] _et al_. [123] first studied the application of multicolor Au@Ag core-shell nanoparticles in a multiplexed solid-phase dot immunoassay. During the transformation process from silver nanocubes to Au@Ag core-shell nanoparticles, the color varied from yellow to blue with the shift of the SPR peak from 450 to 700 nm. Then different color nanoparticles were conjugated with different immuno gamma globulin molecular probes. As a result, when these colorless anti-rat rabbit antibodies were stained by functional molecular probes, they showed the corresponding colors, which can be regarded as detection basis of immunoassay. [PERSON] _et al_. [124] reported the shell thickness-dependent Raman enhancement of Au@Ag core-shell nanoparticles for the identification and detection of pesticide residues at various fruit peels, where it was first proposed that the huge Raman enhancement was contributed by individual Au@Ag NPs rather than aggregated Au@Ag core-shell nanoparticles with hotspots among the neighboring NPs. Figure 16 illustrates a schematic drawing (A) for the direct detection of pesticide residues at fruit peels by SERS spectroscopy using Au@Ag NPs as Raman amplifier, and the SERS spectra of (B) apple and (C) mango peels spiked with thiram by the enhancement of Au@Ag NPs colloid that was cast onto the surfaces of the fruit peels. Figure 15: **(A)** Schematic drawing for direct detection of Hg\({}^{2+}\) with the SERS chip fabricated by the assembly of Au@Ag NPs on a silicon wafer; **(B)** Evolution of SERS spectra of Dpy with the addition of anaqueous Hg\({}^{2+}\) droplet. The inset is the linear correlation of Raman intensity (at 1614 cm\({}^{-1}\)) with the logarithm of Hg\({}^{2+}\) concentrations from 1.0 \(\times\)10\({}^{-14}\) to 1.0 \(\times\)10\({}^{-10}\) M (reprinted with permission from [121]. Copyright (2013) American Chemical Society). In the research of [PERSON] _et al_. [125], Au@Ag core-shell bipyramids was used for a fast and reproducible determination of thiram, which provided satisfactory results for the dectection of thiram in environmental water samples. Furthermore, bifunctional core-shell particles have multiple functions. For example, FePt@Ag core-shell nanostructures not only have excellent SERS properties, but also high saturation magnetization [126]. Although Fe and Pt belong to the transition-metals which have rather weak SERS signals [127], they can achieve high SERS activity by borrowing SERS activity from silver or gold nanostructures which can create long-range effect of the very strong electromagnetic field to enhance the Raman scattering of adsorbents. The magnetic-plasmonic FePt@Ag NPs have potential applications in biomedicine. Figure 16: **(A)** Schematic drawing for the direct detection of pesticide residues at fruit peels by the SERS spectroscopy using Au@Ag NPs as Raman amplifier, and the SERS spectra of **(B)** apple and **(C)** mango peels spiked with thiram by the enhancement of Au@Ag NPs colloid that was cast onto the surfaces of the fruit peels. All Raman spectra were recorded with a 532 nm laser with 5 mW power and 10\(\times\) objectives (reprinted with permission from [124]. Copyright (2012) American Chemical Society).