Size selection of colloidal gold aggregates by filtration: Effect on surface-enhanced raman scattering intensities

The effect of colloidal Au particle aggregation on surface-enhanced Raman scattering (SERS) spectra was probed by SERS filtration experiments. In this approach, SERS and optical spectra were recorded for trans-1,2-bis(4-pyridyl)ethylene (BPE)-aggregated solutions of colloidal Au filtered through straight-channel membranes with successively smaller diameters. This allowed the overall SERS intensity to be factored into aggregate size-dependent contributions. Experiments were carried out as a function of adsorbate concentration (0.5-2.5 μM BPE) and initial particle size (12-50 nm diameter colloidal Au). The key findings are as follows: (i) under conditions of minimal aggregation, appreciable SERS intensity derives from aggregates with effective diameters less than 200 nm; (ii) the amount of aggregant clearly controls the average aggregate size; and (iii) similarly aggregated solutions based on different diameter colloidal Au particles give different distributions of aggregates. These studies provide an insight into the dynamics of colloidal Au aggregation, suggest a procedure for signal optimization in colloid SERS experiments, and set the stage for controlled surface confinement of SERS-active particle clusters.