Probing photoreactions of individual suspended carbonaceous aerosols by multi-wavelength OT-SERS†

This work reports the first application of multi-wavelength optical trapping surface-enhanced Raman spectroscopy (OT-SERS) for in situ probing of photochemical reactions on individual suspended aerosol particles. A composite aerosol model (4-MBA/silver nanoparticles/activated carbon) with a size ranging from 2 to 10 μm, comparable to the particle sizes of PM_2.5 and PM₁₀, was designed to validate the ability of OT-SERS to probe interfacial photoreactions on aerosols. By integrating non-contact optical trapping with SERS detection, we directly monitored the interfacial photoreactions occurring on suspended composite particles under laser irradiation at wavelengths of 473, 589 and 671 nm. Under 473 nm irradiation, the temporal OT-SERS spectra showed dynamic intermediate formation and aromatic ring cleavage, which were attributed to the photooxidations induced by activated carbon. In the experiments involving 589 nm irradiation, relatively weak photooxidations were observed compared to those under 473 nm irradiation. Among the three wavelengths (473, 589 and 671 nm), the irradiation at 671 nm resulted in the weakest photooxidation. OT-SERS was successfully employed for in situ detection of the photooxidation process on the composite particles, which confirmed the formation of hydroxylated intermediates and the cleavage of benzene rings caused by photooxidation.