Intensity Fluctuations in Single Molecule Surface-Enhanced Raman Scattering

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Surface-enhanced Raman scattering (SERS) is a highly sensitive vibrational spectroscopy technique capable of providing unique molecular fingerprinting information. SERS is observed for molecules adsorbed on certain areas of nanostructured metallic surfaces known as SERS hotspots. The tightly focussed light at the hotspots allows even the detection of single individual molecules by Raman scattering. Experimentally, the single molecule signature consists of strong time- (or space-) dependent SERS intensity fluctuations (SIFs). Although the phenomenon of single-molecule SERS (SM-SERS) has been demonstrated more than 20 years ago, there were very few examples of actual uses of SM-SERS to either acquire fundamental information about the hotspot environment or for chemical applications, such as in analytical chemistry. The long term goal of our Discovery program is to lead the SERS field to a new direction that takes into consideration the statistics of single particle, single molecule and even single photon events. This new approach will lead to several interesting new insights and potential new applications. In order to achieve this long term goal, we propose the application of a new technology for the study of SIFs with high spatial and time resolution. In the short term, this new method will be used to follow dynamic processes at the hotspot that have remained hidden due to the notorious low acquisition rate (~ 1 Hz) typically used in a regular SERS measurements. Experiments will be realized at different temperatures, allowing, for instance, the quantification of the energetics of fundamental processes, such as metallic cluster reconstruction at the hotspot area. A systematic investigation of the role of several factors, such as the nature of the metal, the geometric characteristics of the nanometric environment at the hotspot region, and the molecular affinity to the metallic surface, will be investigated. The overall goal in the short term will be to extract fundamental information about the metal-molecule interaction at the hotspot that was not accessible before. The analysis of the frequencies of the fast fluctuations will be used to generate an alternative approach in SERS quantification. Although SERS is a very sensitive method, its application in analytical chemistry has been hampered by intrinsic intensity fluctuations that translate into large errors in analytical calibration curves. We suggest that, for very low concentrations, SIF frequencies might correlate well (less variations) with the concentration of a given analyte in solution. Finally, we will be able to combine SM-SERS measurements with high-speed acquisition to investigate single photons emitted by single molecules.