Aerosol Fluorescent\nLabeling via Probe Molecule Volatilization

The physicochemical properties of\naerosols, including hygroscopicity,\nphase state, pH, and viscosity, influence important processes ranging\nfrom virus transmission and pulmonary drug delivery to atmospheric\nlight scattering and chemical reactivity. Despite their importance,\nmeasurements of these key properties in aerosols remain experimentally\nchallenging due to small particle sizes and low mass densities in\nair. Fluorescence probe spectroscopy is one of the only analytical\ntechniques that is capable of experimentally determining these properties\nin situ in a nondestructive and minimally perturbative manner. However,\nthe application of fluorescence probe spectroscopy to important classes\nof aerosols including exhaled respiratory and ambient atmospheric\naerosols has been limited due to a typical reliance on premixing the\nprobe molecule with particle constituents prior to particle generation,\nwhich is not always possible. Here, a method for aerosol fluorescent\nlabeling based on probe molecule volatilization is developed. The\nmethod is first applied to label model polyethylene glycol (PEG) aerosols\nwith two different polarity-sensitive probes, Nile red and Prodan.\nThe similarity of the relative humidity-dependent fluorescent emission\nof each probe between prelabeled and volatilized-probe PEG particles\nvalidated the methodology. A preliminary application of the technique\nto indicate the hygroscopicity of artificial saliva respiratory particles\nand model atmospheric secondary organic aerosol particles is demonstrated.\nThe methodology developed here paves the way for future studies applying\npowerful fluorescent probe-based analytical techniques to study exhaled\nor natural aerosols for which fluorescent prelabeling is not possible.