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We demonstrate that optical trapping combined with confocal Raman spectroscopy using a single laser source is a powerful tool for the rapid identification of micrometer-sized particles in an aqueous environment. Optical trapping immobilizes the particle while maintaining it in the center of the laser beam path and within the laser focus, thus maximizing the collection of its Raman signals. The single particle is completely isolated from other particles and substrate surfaces, therefore eliminating any unwanted background signals and ensuring that information is collected only from the selected, individual particle. In this work, an inverted confocal Raman microscope is combined with optical trapping to probe and analyze bacterial spores in solution. Rapid, reagentless detection and identification of bacterial spores with no false positives from a complex mixed sample containing polystyrene and silica beads in aqueous suspension is demonstrated. In addition, the technique is used to analyze the relative concentration of each type of particle in the mixture. Our results show the feasibility for incorporating this technique in combination with a flow cytometric-type scheme in which the intrinsic Raman signatures of the particles are used instead of or in addition to fluorescent labels to identify cells, bacteria, and particles in a wide range of applications.

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Resonance Raman spectra of chlorine dioxide (OClO) dissolved in cyclohexane obtained with excitation throughout the 2 B 1 -2 A 2 electronic transition are presented. Resonance Raman intensity corresponding to all vibrational degrees of freedom (the symmetric stretch, bend, and asymmetric stretch) is observed, demonstrating that excited-state structural evolution along all three coordinates occurs upon photoexcitation. The electronic absorption and absolute resonance Raman cross sections are reproduced employing the time-dependent formalism for Raman scattering using an anharmonic description of the 2 A 2 , excited-state potential-energy surface. Analysis of the resonance Raman cross-sections demonstrates that both homogeneous and inhomogeneous broadening mechanisms are operative in cyclohexane. Comparison of the experimentally determined, gas-phase 2 A 2 surface to that in solution defined by the analysis presented here shows that although displacements along the symmetric stretch and bend are similar in both phases, evolution along the asymmetric stretch is dramatically altered in solution. Specifically, employing the gas-phase potential along this coordinate, the predicted intensity of the overtone transition is an order of magnitude larger than that observed. The analysis presented here demonstrates that the asymmetric stretch overtone intensity is consistent with a reduction in excited-state frequency along this coordinate from 1100 to 750 ( 100 cm -1 . This comparison suggests that differences in evolution along the asymmetric stretch may be responsible for the phase-dependent reactivity of OClO. In particular, the absence of substantial evolution along the asymmetric stretch in solution results in the ground-state symmetry of OClO being maintained in the 2 A 2 excited state. The role of symmetry in defining the reaction coordinate and the nature of the solvent interaction responsible for modulation of the excited-state potential energy surface are discussed.