In Situ pH Measurement of Water Droplets Using Flash-Freeze Surface-Enhanced Raman Spectroscopy

Characterization of single droplet pH is required to better understand differences in droplet and bulk water chemistries and the variability of individual atmospheric droplets. In this study, we demonstrate the use of flash-freeze surface-enhanced Raman spectroscopy (SERS) as a means to characterize the pH of micrometer-sized droplets generated from mixtures of low solute concentration environmental water samples and a pH nanoprobe. These samples included stream waters, rain, and snowmelt from areas surrounding Blacksburg, Virginia. Compared to room-temperature SERS, flash-freeze SERS preserves the initial droplet properties and minimizes water loss due to localized heat generation in the vicinity of SERS hot spots. Moreover, the flash-freeze SERS approach provides greater sensitivity relative to pH paper for low solute concentration droplets. We observed that pH values measured by flash-freeze SERS were generally comparable with those obtained at room temperature, but did not result in heating-induced droplet destabilization. Using flash-freeze SERS, we observed that droplet and bulk pH were comparable for most environmental samples. One rainwater sample showed a higher droplet pH compared to its bulk sample. Upon the basis of ICP-MS results, the ionic composition of this rainwater was significantly different from the other collected samples, thus suggesting the effect of droplet composition on droplet pH.