Single molecule surface‐enhanced resonance Raman scattering on colloidal silver and Langmuir–Blodgett monolayers coated with silver overlayers

Single molecule detection (SMD) of a new perylene dye containing a terminal NH2 group, n‐butylimidoethylenamineperylene (PTCD‐NH2), adsorbed on colloidal silver and silver island films was achieved using surface‐enhanced resonance Raman scattering (SERRS). This work represents the first comparative SERRS study where SMD was achieved using two of the most commonly used SERS substrates, colloidal silver and silver island films. SERRS spectra were obtained in colloidal silver solution with analyte concentrations varying from 10−6 to 10−12 M and also from silver sols dispersed and dried on polysilane‐coated substrates. SERRS on silver islands was obtained by vacuum evaporation of a 6 nm mass thickness silver overlayer on to a Langmuir–Blodgett (LB) monolayer containing 1, 10 and 100 dye molecules per µm2 of surface area. The LB technique is ideally suited to obtain spatially resolved spectra of single molecules dispersed in the matrix of a fatty acid, as is used here to obtain SERRS from a few dye molecules found within the area of illumination. The characteristic fundamental vibrational wavenumbers observed in the single molecule SERRS spectra were unequivocally assigned with the help of their first overtones and combinations.