Aerosol Fluorescent Labeling via Probe Molecule Volatilization

The physicochemical properties of aerosols, including hygroscopicity, phase state, pH, and viscosity, influence important processes ranging from virus transmission and pulmonary drug delivery to atmospheric light scattering and chemical reactivity. Despite their importance, measurements of these key properties in aerosols remain experimentally challenging due to small particle sizes and low mass densities in air. Fluorescence probe spectroscopy is one of the only analytical techniques that is capable of experimentally determining these properties in situ in a nondestructive and minimally perturbative manner. However, the application of fluorescence probe spectroscopy to important classes of aerosols including exhaled respiratory and ambient atmospheric aerosols has been limited due to a typical reliance on premixing the probe molecule with particle constituents prior to particle generation, which is not always possible. Here, a method for aerosol fluorescent labeling based on probe molecule volatilization is developed. The method is first applied to label model polyethylene glycol (PEG) aerosols with two different polarity-sensitive probes, Nile red and Prodan. The similarity of the relative humidity-dependent fluorescent emission of each probe between prelabeled and volatilized-probe PEG particles validated the methodology. A preliminary application of the technique to indicate the hygroscopicity of artificial saliva respiratory particles and model atmospheric secondary organic aerosol particles is demonstrated. The methodology developed here paves the way for future studies applying powerful fluorescent probe-based analytical techniques to study exhaled or natural aerosols for which fluorescent prelabeling is not possible.