Ratiometric Optical Sensing of Aerosol Phase State with Excited-State Intramolecular Proton Transfer Probes

Phase transitions in respiratory and environmental aerosols impact critical processes ranging from virus transmission to atmospheric light scattering. Yet, small particle sizes and low mass densities in air make experimental measurements of aerosol phase state challenging. Fluorescence probe spectroscopy is one of the only analytical techniques capable of determining aerosol phase state in situ at the submicron sizes that are implicated in long-range virus transmission and that dominate the size distribution in the atmosphere. However, previous fluorescent probe-based measurements of aerosol phase state have relied on solvatochromic probe molecules and their associated relatively small shifts in emission wavelength, necessitating relatively high-resolution spectral measurements and greatly limiting optical throughput and therefore sensitivity. Here, measurements of aerosol phase state are demonstrated using a different class of molecules, excited-state intramolecular proton transfer (ESIPT) probes, that exhibit two emission peaks with an intensity ratio that is highly dependent on the surrounding chemical environment. The ESIPT probe 2-(2-benzofuranyl)-3-hydroxychromone is shown to be sensitive to phase state, including both solid-liquid and liquid-liquid phase transitions, in mixed organic/inorganic aerosols. The origin of the sensitivity was investigated by varying the chemical identity of the aerosol constituents and the results indicate that the probe is particularly sensitive to the presence of Na⁺ and Cl^- ions, which are involved in key phase transitions in respiratory particles as well as sea-spray aerosols. These findings highlight the potential of ESIPT-based fluorescent sensing as a powerful technique for real time analysis of aerosol phase state in submicron particles combining unprecedented sensitivity and experimental simplicity.