Surface-Enhanced Raman Spectroscopy with High Spatial Resolution

Abstract

The identification of individual molecules and the determination of how these interact with their local environment are critical steps toward a better understanding of complex organic systems. Optical detection techniques have always played a key role in the nondestructive and noninvasive analysis of complex materials. Until recently, however, optical microscopy has lacked the sensitivity to study processes on the molecular scale. This has changed with the recent development of new schemes that limit the optical detection volume, and the advent of new, highly quantum-efficient photon detectors. These inventions have enabled researchers to optically probe biomolecular processes at the single molecule level by observing the fluorescence of specific marker molecules. The requirement to specifically label biomolecules and the fact, that fluorescence emission is prone to photodecomposition of the marker molecules, however, have limited this approach to a few, well-characterized case studies. Raman scattering, is one of few optical techniques that can identify atomic species and in addition determine their chemical bonds by observing their distinct vibrational fingerprints; but it is orders of magnitude weaker than fluorescence. In this project, we have developed new optical probes that allow for the non-destructive characterization and identification of organic and inorganic matter at the singlemore » molecule level by surface-enhanced Raman spectroscopy. Our approach combines confocal Raman microscopy with surface-enhanced Raman spectroscopy (SERS) generated by coating scanning probe microscope (SPM) tips with thin (30-40 nm) gold and silver films. The scanning SERS probe generates an image of the physical structure of a sample together with detailed chemical information about its composition. In a complementary approach, we have also used gold or silver nanoparticles to generate SER spectra from single molecules adsorbed to these particles. This project has led to the development of a new capability at LLNL, i.e. the field of optical single molecule detection.« less