Molecular Origins of the Optical Response of Nile Red in Model Aerosol Aqueous Ionic Environments

Aerosol particles are ubiquitous and their physicochemical properties such as hygroscopicity, phase state, pH, and viscosity influence processes ranging from virus transmission to atmospheric light scattering and chemical reactivity. While direct in situ measurements of these properties have been experimentally challenging, fluorescence probe spectroscopy has emerged as a powerful technique for in situ aerosol analysis. The fluorophore Nile red (NR) has seen recent use in aerosol studies to indicate phase state in chemically complex environments, though open questions remain surrounding the molecular origins and influence of common aerosol chemical environments, notably high ionic strengths, on NR optical properties. Here, time-dependent density functional theory quantum mechanics/molecular mechanics calculations were employed to calculate the excitation energies of NR in aqueous environments of ionic strengths up to 1 M NaCl. The results revealed both a blueshift in the average excitation energy and a broadening of the distribution of energies with increasing ionic concentration. The relative roles of ion-induced changes in NR conformation versus solvation effects for a given NR conformation were investigated and both were found to play a substantial role in the total blueshift. Overall, this study advances molecular-level understanding of the solvatochromic behavior of NR and structurally similar fluorophores in aqueous ionic environments, supporting effective selection and application of fluorescent probes for characterizing aerosol physicochemical properties.