5. A brief summary

response (320 ms) and recovery (11 s) compared with pure SnO2 counterpart (Figure 12b). This could be associated with both electronic sensitization of Au metal and the ultrathin wrapping layer in such a core/shell structure. Further work is needed to understand and reveal the origin

Figure 12. (a) Responses to H2S gas at room temperature for the Au@SnO2 NPs' film and the pure SnO2 NPs' film. (b) The

Quantitative detection makes sensing more reliable and scientific. The response of Au@SnO2 NPs to H2S here shows obvious concentration dependence. With the concentration increasing from 1 to 10 ppm, the sensitivity increases linearly, as shown in Figure 13a. This provides an

Figure 13. (a) The responses of the Au@SnO2 NPs' film to different concentrations of H2S gas. (b) The selectivity of the

of this phenomenon.

216 Plasmonics

substrates to a variety of gases.

4.2.2. Quantifiable sensing and selectivity

response and recover part of the plot of Au@SnO2 NPs' film to 1 ppm H2S gas.

We have introduced a facile strategy for fabrication of ultrathin oxide layer-wrapped plasmonic metal NPs based on colloidal electrostatic attraction and self-assembly. In this approach, hydrolysis-induced small positively charged hydroxide colloids are wrapped on negatively charged plasmonic metal NPs via the electrostatic self-assembly. After dehydration process by annealing, the shell will be transformed to oxide, resulting in oxides wrapped metal NPs. Based on this strategy, one-step laser ablation of metal targets in the hydrolysis-induced hydroxide sol solutions have been conducted to fabricate the Au@oxides (Fe2O3, Al2O3, Al2O3, CuO, and ZnO) as well as Pt@TiO2 and Pd@TiO2 NPs. Furthermore, the thickness of these oxide layers are as thin as 13 nm and homogenous. And it also shows independence on the plasmonic metal NPs' size. Additionally, such a strategy shows excellent controllability to the shell in the fabrication. Typically, a secondary irradiation can homogenize the NPs' size. Prolonging the ablation duration can improve the shell's crystallinity hugely. And the shell thickness also could be tuned by the temperature, concentration, and pH value, simply by adjusting the hydrolysis of the metal ion. Finally, enhanced SERS and gas-sensing performances of such oxide layer-wrapped plasmonic metal NPs also have been demonstrated. It demonstrates that ultrathin TiO2-wrapped Au NPs can achieve a much stronger SERS performances in the detection of nitrates due to its positively charged composite NPs. In addition, such a SERS substrate can be recycled by irradiating the used substrate to photodegradate the target organic molecules. Also, significantly better gas-sensing performance of Au@SnO2 NPs has been studied, which demonstrates quickly and linearly respond to H2S gas at room temperature with excellent selectivity.
