Noble metallic nanosurface exhibits both plasmonic and catalytic functions. Surface-enhanced Raman scattering of para-aminothiophenol (p-ATP) was measured on a highly uniform two-dimensional silver nanoparticle array at different intensities of an excitation laser (532 nm) ranging from 4 to 4000 W/mm 2 . It was observed that p-ATP was chemically transformed to 4,4′-dimercaptoazobenzene (DMAB) with the laser intensities of ≥40 W/mm 2 during the Raman measurement. At 4 W/mm 2 , the Raman peaks of DMAB disappeared, which indicates that the laser intensity was insufficient for the chemical transformation although it was sufficient for the Raman measurement. The highly uniform silver nanoparticle array allowed quantitative analysis on the Raman peak intensity. The threshold of the chemical transformation from p-ATP to DMAB was estimated to be ∼18.8 W/mm 2 on the silver nanoparticle array whose enhancement factor is ∼10 4 .
Noble metallic nanostructures provide a platform for high-sensitivity spectroscopic sensing with significantly enhanced electromagnetic fields due to surface plasmon polaritons. However, target molecules can be transformed into other molecules under irradiation with an excitation laser during the surface-enhanced measurement, which thus disturbs detection of unknown samples. In this paper, we perform Raman measurements of p-aminothiophenol on gold nanosurfaces with and without deposition of SiO 2 thin films at the surface. The Raman signals are enhanced on both substrates, but the deposition of the glass thin film clearly prevents the chemical transformation. This indicates that hot electrons are effective for chemical transformation and that thin glass films are sufficient to prevent this while still benefiting from surface plasmons.
Gold/silver nanoparticles were trapped at the oil/water interface of oil droplets dispersed in water. The metallic nanoparticles were self-assembled into a uniform two-dimensional large array structure through the aggregation and coalescence of the nanoparticle-covered oil droplets. The plasmon resonance of the array structure was tunable and a surface-enhanced Raman scattering measurement was performed with the silver nanoparticle array. The enhancement factor was ∼105 and enhanced Raman signals were observed over the whole array (
) with high reproducibility, which is an advantage of a self-assembly method using a liquid/liquid interface.
The transformation of para-aminothiophenol (PATP) to dimercaptoazobenzene (DMAB) is widely believed to be due to the emission of hot electrons from the plasmonic nanostructures,1 which are generated during the decay of the surface plasmons (LSPPs). Our aim here is to separate the catalytic activity of plasmonic nanostructures from their SERS activity by using a 5 nm thick silicon dioxide (SiO2) layer. This layer blocks hot electrons from reaching the PATP molecules but lets the electromagnetic field penetrate, allowing us to measure the SERS of the monomer without triggering a chemical reaction. The SERS measurement was performed at 633 nm on two-dimensional gold nanoparticle (2D AuNP) arrays covered with/without thin SiO2.
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