A Microscopic Surface-Enhanced Raman Study of a Single Adsorbate-Covered Colloidal Silver Aggregate

Abstract

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.

Inherent Complexities of Trace Detection by Surface‐Enhanced Raman Scattering

“…These efforts have been aided by the continuous progress in instrumentation which has allowed further refinement, and a better understanding of the intricacies of the technique. [55,[92][93][94][95] Advancements in optics, such as those in near-field scanning optical microscopy (NSOM) have granted unprecedented spatial resolution. [54,96,97] New knowledge has been gained by the coupling of Raman instruments with nontraditional enhancing surfaces, such as scanning probe tips.…”

SERS Excitation Profiles of Phthalazine Adsorbed on Single Colloidal Silver Aggregates as a Function of Cluster Size

“…However, we have shown in a previous study that, by using Raman microspectroscopy, SERS spectra could be obtained from single colloidal aggregates that are gravitationally deposited on a glass surface. 11 In that study we were unable to report the SERS excitation spectrum of a single adsorbate-covered colloid cluster, as had been our original intention, due to the decomposition of the clusters at longer wavelengths. We have since succeeded in overcoming this problem by modifying slightly the method of colloid preparation and began a series of SERS studies using phthalazine-covered single colloid clusters.…”

Monolayer Protected Clusters of Gold and Silver

“…The optical properties of silver may in many ways be more interesting than those of gold due to (1) the more dramatic size and shape dependence of the position and shape of the plasmon absorption band of silver, (2) the absence of d-band transitions, which in the case of gold can overlap with the free-electron contribution, and (3) the comparatively greater Raman enhancement that has been reported for silver particles [61,62]. The synthetic protocols are comparable to those for MPCs of gold.…”