10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Enhanced Raman Spectroscopy of Individual Aerosol Particles
VASANTHI SIVAPRAKASAM, Matthew B. Hart, Jay D. Eversole, Naval Research Laboratory
Abstract Number: 750 Working Group: Aerosol Physics
Abstract This presentation provides new data from an on-going investigation of the addition of metallic nanoparticles (MNPs) to micron-sized aerosol particles for the purpose of developing and characterizing enhanced Raman spectroscopic signatures that can serve to identify the aerosol material on a particle-by-particle basis. While Raman spectroscopy is a well-established method of chemical identification, typical spontaneous molecular cross sections are too low to permit application to in-situ, on-the-fly interrogation of aerosol particles in the picogram to nanogram mass range. Consequently, the goal of rapidly discriminating among a population of aerosol particles would require some degree of enhanced optical signature. We have investigated enhanced Raman scattering (both its intensity and spectral form) from single particles in which MNPs have been either dispersed within the particle material or deposited on the particle surface. The mechanisms giving rise to Raman enhancement in this type of system are essentially the same as the more familiar arrangement of surface enhanced Raman spectroscopy (SERS).
We recently observed enhancement factors > 105 in single aerosol particles with dilute concentrations of rhodamine 6G (R6G) when these particles were combined with small quantities of gold MNPs (Sivaprakasam et. al. 2017[1]). These experiments were conducted with the use of a linear electrodynamic quadrupole (LEQ) particle trapping chamber so that aerosol particles could be generated as charged droplets with controlled composition and maintained in fixed position in the chamber. Once the solvent liquid evaporated composite residue particles remain suspended in the trap. This experimental system provides a capability to study particles and obtain data over extended periods of time (up to hours). Particles were generated under identical conditions both with and without the presence of MNPs, permitting a direct comparison of spontaneous Raman and MNP-enhanced Raman spectra from particles of the same size and composition. Additionally, droplets of neat ethyl cinnamate served to provide an absolute external reference for calibrating our Raman cross section measurements. Our initial study of R6G-containing particles showed excellent repeatability for both spectral response and enhancement factors, and dependences of the enhanced Raman scattered light on MNP concentration and R6G concentration were determined.
Our most recent results, on micro-droplets of adenine and also of microspheres of polystyrene latex (PSL) and polymethyl methacrylate (PMMA) will be presented and discussed.
[1] V. Sivaprakasam, M. B. Hart, and J. D. Eversole, “Surface Enhanced Raman Spectroscopy of Individual Suspended Aerosol Particles”, J. Phys. Chem. C 2017, 121, 22326−22334.