AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
Abstract View
Measurement of the pH of Individual Aerosol Droplets by Surface-Enhanced Raman Spectroscopy
HAORAN WEI, Linsey Marr, Peter Vikesland, Virginia Tech
Abstract Number: 63 Working Group: Aerosol Chemistry
Abstract Suspended aqueous aerosol droplets (<50 µm) are micro-reactors for many important atmospheric reactions. In droplets, and other aquatic environments, pH is arguably the key parameter dictating aqueous chemistry. The dynamic air/water interface of a droplet has the potential to significantly alter droplet pH from bulk water values. However, it is challenging to measure the pH inside single droplets due to their inaccessibility when using conventional pH meters. In this study, droplets containing ≈75 nm gold nanoparticle (AuNP) based pH nanoprobes were collected onto a superhydrophobic substrate and scanned by laser confocal Raman microscopy. The Raman signal of the pH reporter, 4-mercaptobenzoic acid (4-MBA), was enhanced substantially through surface-enhanced Raman scattering (SERS) enabled by the AuNP clusters. By using these nanometer-sized pH probes the pH inside individual aerosol droplets was for the first time acquired. We show that the pH in a droplet is higher than that of bulk water by ≈3.3 pH units, which we attribute to the accumulation of protons at the air/water interface. The existence of this unexpected pH shift was corroborated by the observation that two catalytic reactions that only occur under basic conditions (i.e., the decarboxylation of 4-MBA and dimercaptoazobenzene (DMAB) formation from 4-aminothiophenol (4-ATP)) occur within droplets, but not bulk solution.