How far can we probe by SERS?

Abstract

Surface-enhanced Raman spectroscopy (SERS) has gained paramount importance in the recent past due to its widespread applications in biodetection, monitoring chemical reactions, small molecule protein interactions, etc. It is believed that SERS is a distance-dependent phenomenon and is effective within 1 nm from the nanoparticle surface. In this work, we have investigated this distance dependence of SERS as a function of nanoparticle size. Earlier attempts have made use of flexible separators, like DNA and chemical molecules, between nanoparticle and analyte to vary the distance. We have used silica coating to vary the distance, without ambiguity, of the analyte from the silver nanoparticle surface. Our results suggest that SERS is observed up to a distance of 1 nm for 20 nm silver nanoparticles juxtaposed to 5 nm in the case of 90 nm silver nanoparticles. This is due to large scattering cross sections and increased radiative damping in the case of the larger nanoparticles. This study gives direct correlation between the size of nanoparticles and distance probed through SERS which would aid in designing nanoparticle system for various applications and analytes in the future.