Colloidal Metal Films as a New Substrate for Enhanced Spectroscopy of Biomolecules

Abstract

Recently we introduced a new substrate which provides an enhancement of both Raman scattering and fluorescence from adsorbed molecules [1]. Enhancement was tested with several nucleotides, as well as with an avidin/biotin complex. This system consists of silver or gold particles covalently attached to a glass substrate. Other substrates, such as silicon oxide, aluminum oxide, tin oxide and germanium, have been shown to form colloidal metal films. These films are formed by self-assembly of metal particles from a colloidal suspension onto substrates derivatized with mercaptosilanes. The films were characterized by UV-Vis spectroscopy, electron microscopy and atomic force microscopy. The advantages of these films relative to other substrates commonly used for enhanced spectroscopies (roughened metal surfaces, colloidal suspensions, island films, etc.) are their stability, reproducibility and greater enhancement. For example, the enhancement of the Raman spectrum of adenine on a silver colloidal film was almost an order of magnitude larger than in an aggregated citrate silver colloid. Moreover, the aggregation state of these films can be controlled prior to their preparation. Because these films are very stable in different environments, they provide a unique possibility for studying fundamental properties of plasmon resonances in small metal particles as a function of the dielectric medium. From a practical point of view, these highly reproducible and stable films offer the potential for development of quantitative analytical methods based on surface-enhanced spectroscopy.