Cellular Imaging and Analysis Using SERS-Active Nanoparticles

Raman spectroscopy has long been regarded as a valuable tool for the identification of chemical and biological samples as well as the elucidation of molecular structure, surface processes, and interface reactions (1). Compared to luminescence-based processes, Raman spectroscopy has an inherently small cross section, thus precluding the possibility of analyte detection at low concentration levels without special enhancement processes. Nevertheless, there has been a renewed interest in Raman techniques in the past two decades due to the discovery of the surface-enhanced Raman scattering (SERS) effect, which results from the adsorption of molecules on metallic surfaces having nanostructured morphology. This large enhancement was first reported in 1974 and was initially attributed to a high surface density produced by the roughening of the surface of electrodes [2], and was later determined to be a result of more complex surface enhancement processes, hence the term surface-enhanced Raman scattering (SERS) effect [3,4]. The enhancement factors for the observed Raman scattering signals of adsorbed molecules were found to be more than a millionfold when compared to normal Raman signals expected from gas phase molecules or from nonadsorbed compounds. This giant Raman effect opened a wide spectrum of new possibilities to the Raman technique for trace analysis, chemical analysis, environmen- tal monitoring, and biomedical applications.