Spatially resolved analysis of small particles by confocal Raman microscopy: depth profiling and optical trapping

Raman microscopy is a powerful method to provide spatially resolved information about the chemical composition of materials. With confocal collection optics, the method is well suited to the analysis of small particles, either resting on a surface or optically trapped at a laser focus, where the confocal collection volume optimizes the signal from the particle. In this work, the sensitivity and spatial selectivity of detecting Raman scattering from single particles was determined as a function of particle size. An inverted confocal Raman microscope was used to acquire spectra of individual surface-bound and optically trapped polystyrene particles with sizes ranging between 200 nm and 10 microm. The particles are in contact with aqueous solution containing perchlorate ion that served as a solution-phase Raman-active probe to detect interferences from the surrounding medium. The collection volume is scanned through single particles that are attached to the surface of the coverslip, and the sensitivity and selectivity of detection are measured versus particle size. The results compare favorably with a theoretical analysis of the excitation profile and confocal collection efficiency integrated over the volumes of the spherical particles and the surrounding solution. This analysis was also applied to the detection of particles that are optically trapped and levitated above the surface of the coverslip. The results are consistent with the optical trapping of particles at or near the excitation beam focus, which optimizes excitation and selective collection of Raman scattering from the particle.