Mapping single-molecule SERRS from Langmuir-Blodgett monolayers, on nanostructured silver island films

To explore the breakdown of ensemble averaging as the single molecule regime is approached, spatial mapping of surface-enhanced resonance Raman scattering (SERRS) intensities was employed in the study of mixed dye-fatty acid Langmuir-Blodgett (LB) monolayers deposited on nanostructured Ag island films. By variation of the ratio of the two components in the films, the effects of dye concentration, on both SERRS spectra and LB monolayer architecture, were explored down to the single-molecule level. Insight was gained into the nature of areas of intense local electromagnetic field strength (i.e. 'hot spots') that provide enormous enhancement of Raman signals, and their wavenumber of occurrence on nanostructured Ag island films. These enhancing films were further characterized by UV-visible surface plasmon absorbance and atomic force microscopy. The target analyte employed in this work, n-pentyl-5-salicylimidoperylene, was dispersed in monolayers of arachidic acid on Ag nanostructured films for single point and 2D mapping SERRS experiments. The optical characterization of this dye was completed with solution-phase molecular absorption and fluorescence, in addition to monolayer resonance Raman scattering (RRS). Single-molecule SERRS mapping results suggest that electromagnetic hot spots on Ag island films are in fact highly localized on the nanoscale, corresponding to relatively few molecular sites, and hence also stress the rarity of coincidence between hot spots and single isolated molecules as a key consideration in ultrasensitive SERRS measurements of this type. Copyright (c) 2005 John Wiley & Sons, Ltd.