Optical detection and spectroscopy of single molecules and single nanoparticles have been achieved at room temperature with the use of surface-enhanced Raman scattering. Individual silver colloidal nanoparticles were screened from a large heterogeneous population for special size-dependent properties and were then used to amplify the spectroscopic signatures of adsorbed molecules. For single rhodamine 6G molecules adsorbed on the selected nanoparticles, the intrinsic Raman enhancement factors were on the order of 10(14) to 10(15), much larger than the ensemble-averaged values derived from conventional measurements. This enormous enhancement leads to vibrational Raman signals that are more intense and more stable than single-molecule fluorescence.

This paper reports two fundamental observations on the size-dependent optical properties of colloidal gold nanoparticles. First, faceted gold nanocrystals in the size range of 63 ± 3 nm have been found to be highly efficient for surface-enhanced Raman scattering (SERS). These nanocrystals are identified from a heterogeneous population for large optical enhancement at 647-nm laser excitation. Second, spatially isolated single gold particles emit Stokes-shifted Raman photons in an intermittent on-and-off fashion. In contrast to population-averaged studies, blinking surface-enhanced Raman scattering is a signature of single-particle (or even single-molecule) behavior. By directly measuring optical enhancement and time-resolved emission on single nanoparticles, this work opens new possibilities in studying the mechanisms of SERS, in developing metal tips for near-field optical microscopy, and in designing new nanostructured materials.

No abstract

A rapid screening and enrichment method is reported for exploring the size and shape diversities (libraries) of nanometer-scale colloidal particles. With the use of surface-enhanced Raman scattering, a new class of metal nanoparticles has been identified from heterogeneous silver colloids. These particles are relatively large, faceted nanocrystals that are able to enhance the efficiencies of surface optical processes by as much as 14−15 orders of magnitude. The population of these novel nanoparticles is only about 0.1−1% in standard colloid preparations but can be enriched to about 10−15% by size-selective fractionation. This new class of nanoparticles could find potential uses in optoelectronic devices, ultrasensitive chemical sensors, and single-molecule detection.

Single metal nanoparticles and nanoaggregates are known to emit intense bursts of surface-enhanced Raman scattering (SERS) in an intermittent on and off fashion. The characteristic "blinking" timescales range from milliseconds to seconds. Here we report detailed temperature dependence (both heating and cooling) and light-intensity studies to further examine the origins of this intriguing phenomenon. The results indicate that blinking SERS contains both a thermo-activated component and a light-induced component. Several lines of evidence suggest that the observed fluctuations are caused by thermally activated diffusion of individual molecules on the particle surface, coupled with photo-induced electron transfer and structural relaxation of surface active sites or atomic-scale roughness features.