Photophoretic trapping-Raman spectroscopy for single pollens and fungal spores trapped in air

Chuji Wang, Yong-Le Pan, Steven C. Hill, Brandon Redding

Photophoretic trapping-Raman spectroscopy (PTRS) is a new technique for measuring Raman spectra of particles that are held in air using photophoretic forces. It was initially demonstrated with Raman spectra of strongly-absorbing carbon nanoparticles (Pan et al. [44] (Opt Express 2012)). In the present paper we report the first demonstration of the use of PTRS to measure Raman spectra of absorbing and weakly-absorbing bioaerosol particles (pollens and spores). Raman spectra of three pollens and one smut spore in a size range of 6.2–41.8 µm illuminated at 488 nm are shown. Quality spectra were obtained in the Raman shift range of 1600–3400 cm−1 in this exploratory study. Distinguishable Raman scattering signals with one or a few clear Raman peaks for all four aerosol particles were observed within the wavenumber region 2940–3030 cm−1. Peaks in this region are consistent with previous reports of Raman peaks in the 1600–3400 cm−1 range for pollens and spores excited at 514 nm measured by a conventional Raman spectrometer. Noise in the spectra, the fluorescence background, and the weak Raman signals in most of the 1600–3400 cm−1 region make some of the spectral features barely discernable or not discernable for these bioaerosols except the strong signal within 2940–3030 cm−1. Up to five bands are identified in the three pollens and only two bands appear in the fungal spore, but this may be because the fungal spore is so much smaller than any of the pollens. The fungal spore signal relative to the air-nitrogen Raman band is approximately 10 times smaller than that ratio for the pollens. The five bands are tentatively assigned to the CH2 symmetric stretch at 2948 cm−1, CH2 Fermi resonance stretch at 2970 cm−1, CH3 symmetric stretch at 2990 cm−1, CH3 out-of-plane end asymmetric stretch at 3010 cm−1, and unsaturated =CH stretch at 3028 cm−1. The two dominant bands of the up-to-five Raman bands in the 2940–3030 cm−1 region have a consistent band spacing of 25 cm−1 in all four aerosols. Finally we discuss improvements to the PTRS that should provide a system which can trap a higher fraction of particle types and obtain Raman spectra over a larger range (e.g., 200–3600 cm−1) than those achieved here.

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