Exploring Solvation and Aggregation Effects in Microdroplets Using Time-Resolved Fluorescence

MIN-HSIEN (TONY) KAO, Conlan Broderick, Jim Walker, Thomas A. A. Oliver, Andrew J. Orr-Ewing, Bryan R. Bzdek, University of Bristol

     Abstract Number: 48
     Working Group: Aerosol Chemistry

Abstract
Surface effects can significantly change chemical behavior. To investigate such changes at the molecular scale, we developed a linear-quadrupole droplet trap coupled with time-correlated single photon counting (TCSPC) to measure fluorescence emission lifetimes in microdroplets with diameters of ~40 µm or smaller. This setup provides time resolution of fluorescence decay ranging from 100 ps to 100 ns and was tested by using coumarin 480, a common laser dye known for its sensitivity to solvation environments. Coumarin derivatives have been widely employed to study solvation dynamics in bulk solvents and micellar systems.

We measured the fluorescence lifetimes of coumarin 480 in a 9:1 (v:v) water/acetone mixture both in cuvettes and in trapped droplets. In bulk solution, the fluorescence lifetime was ~6.1 ns and remained consistent across concentrations from 2 µM to 25 µM. In contrast, the emission decay in microdroplets exhibited a bi-exponential profile, with time constants varying between ~2 ns and ~11 ns depending on concentration, as did the associated amplitude weights. In addition to the fluorescence lifetime, the fluorescence emission spectra in cuvettes and droplets were measured. The observed Stokes shifts differed between the cuvette and droplet environments, suggesting a change in solvation environment at the microscale.

We attribute these differences to environmental heterogeneity: while the bulk solution is homogeneous, acetone rapidly evaporates from droplets upon generation, leaving behind water droplets supersaturated with coumarin 480. This leads to dye aggregation and surface accumulation, creating confined interfacial environments that alter fluorescence behavior. These results demonstrate the capability of our setup to probe photochemical processes in microdroplets and aerosols, providing insight into surface-induced effects at the microscale.