Optical trapping-surface enhanced Raman spectroscopy (OT-SERS) for sensing single bioaerosol particles in air

A rapid and accurate method for the characterization and identification of airborne bioaerosol particles is highly desired. Surface-enhanced Raman spectroscopy (SERS) is an attractive technique because it combines the molecular fingerprints of Raman spectroscopy with the enhanced spectral sensitivity resulting from the interactions of metallic nanoparticles with aerosols. In this study, we integrated optical trapping (OT) with SERS to characterize single bioaerosol particles in air. This work demonstrates the first OT-SERS to measure Raman spectra of common airborne bioaerosols (pollens and fungal spores). We used commercial Ag-doped multiwall carbon nanotubes (MWNT) to validate the OT-SERS system. Experimentally, we showed that the Raman spectra intensity of single MWNT particles can be significantly enhanced by incorporating Ag nanoparticles. Five airborne bioaerosols were measured: three pollens (English oak, ragweed, and Arizona ash) and two fungal spores (Bermuda grass smut and Johnson grass smut). The enhanced Raman spectra from optically trapped particles achieved ∼100 times enhancement and are fluorescence-free. The enhanced OT-SERS are consistent with the regular Raman spectra, except for two bands at 754 cm−1 and 1372 cm−1, attributed to interactions between Ag nanoparticles and functional groups in thymine, adenine, or carboxylates on the particle surfaces. The intensity of the enhanced Raman signal, with an integration time of 3 s, is as strong as the regular Raman spectrum with an integration time of 60 s. Raman bands in the region of 500 - 1000 cm−1 which were barely resolvable or overwhelmed by fluorescence in regular Raman spectroscopy, are clearly resolved with OT-SERS.