A microscopic surface-enhanced Raman study of a single adsorbate-covered colloidal silver aggregate

Surface-enhanced Raman scattering (SERS) spectra have been successfully recorded from individual, selected clusters of colloidal silver particles on which phthalazine, dopamine, or 2,2'-bipyridine was adsorbed. The method involved the controlled deposition of silver colloid aggregates onto a Pyrex microscope cover slide and the selection of an individual cluster by the optical microscope of a micro-Raman spectrometer followed by Raman measurement. SERS spectra of clusters of various size were recorded. With phthalazine, good quality SERS spectra were obtained with 476.2- and 530.9-nm laser excitation. Poor spectra were obtained with 568.2- and 590-nm excitation, probably due to the photofragmentation of the cluster. The wavelength at which photofragmentation occurred was cluster size and structure dependent. Smaller clusters tended to fragment at shorter wavelengths. A micropreparative technique was developed for preparing single adsorbate-covered silver colloid aggregates. Using this technique in conjunction with Raman microscopy, good quality SERS spectra were obtained from 5 pmol of phthalazine and dopamine. SERS spectra of 2,2'-bipyridine deposited out of a dichloromethane solution also were obtained, illustrating the possibility of using this single-cluster technique for recording SERS spectra of water-insoluble adsorbates or of adsorbates dissolved in nonaqueous solvents.