Experimental demonstration of the electromagnetic mechanism underlying surface enhanced Raman scattering using single nanoparticle spectroscopy

Nanostructures and nanoparticles of Ag and Au offer large enhancements of optical responses such as optical absorption, Rayleigh and Raman scatterings and their nonlinear counterparts to adsorbed molecules. Such enhancements originate from the strong plasma (plasmon) resonance of Ag and Au and have been widely applied for the sensitive detection of various organic and biomolecules under the terminology of surface enhanced spectroscopy, the most common example being surface enhanced Raman scattering (SERS). Although SERS has turned out to be one of the most sensitive analytical techniques, fundamental issues such as lack of conclusive experimental evidence for validating the mechanism underlying SERS and difficulty to establish potential applications of SERS in daily life limit its recognition. Single nanoparticle spectroscopy due to its ability to correlate enhanced Raman scattering with other optical properties of metal nanoparticles is a powerful method for clarifying the mechanism underlying SERS. In this article, we review our recent advances in the experimental evaluation of SERS mechanism. In particular, using single nanoparticle spectroscopy, we attempted to quantitatively evaluate the enhancement mechanism underlying SERS at the onset of electromagnetic (EM) theory. Also, we briefly comment on the advantages of SERS over fluorescence spectroscopy for label-free detection of biomolecules by taking examples from our own investigations and recent reports by others.