Surface Enhanced Raman Spectroscopy of Individual Suspended Aerosol Particles

We report the observation and measurement of surface enhanced Raman spectroscopy (SERS) signatures from individual suspended aerosol particles. To our knowledge, this is the first report of SERS from single suspended particles formed from droplets containing analyte molecules and metallic nanoparticles (MNPs). We describe our experimental setup to generate, charge, and trap microdroplets and an associated spectroscopic measurement arrangement configured to easily switch between trapped, suspended particles and bulk liquid samples of the same materials. Trapped droplets quickly dry to form micron-sized particles mainly composed of inert material, NaCl (>99%) with trace amounts of analyte molecules, and MNPs. As an initial investigation and demonstration of our method, surface enhanced resonance Raman spectroscopy (SERRS) is conducted on 3–5 μm diameter suspended solid particles containing R6G dye molecules and silver nanoparticles (NPs). SERRS signal intensities are linear with particle dye concentrations up to a saturation point. A detection limit of 105 molecules (80 attograms in mass, which would be a 50 nm equivalent sphere) is established in particles containing approximately 300 Ag NPs. Comparison with an external standard with a known spontaneous Raman cross-section establishes a SERRS analytical enhancement factor of 105 for these particles. Comparable SERRS enhancement factors are obtained from similar samples as bulk suspensions, and SERRS spectra show good agreement between solid aerosol particles and bulk liquid suspensions. We plan to extend our study to quantify SERS response from atmospheric and threat aerosols to evaluate the feasibility of implementing this technique for ambient aerosol evaluations.