Nanoparticle

Aqueous solution

Chemistry

Materials science

Chromatography

Engineering

Chemical physics

Aerosol

Physics

Nanotechnology

Organic chemistry

Optics

Catalysis

Raman scattering

Raman spectroscopy

Microreactor

Analytical Chemistry (journal)

Chemical engineering

Proceedings of the National Academy of Sciences

Significance Aerosols with high water content (aerosol droplets) are ubiquitous and play a significant role in atmospheric chemistry and meteorology. However, directly measuring the pH of an individual aerosol droplet remains challenging due to its inaccessibility to pH electrodes. In this study, nanometer-sized pH probes were dispersed in droplets to report pH via surface-enhanced Raman spectroscopy. The droplet core exhibits higher pH than the bulk solution, suggesting the presence of a stable pH gradient. This in situ technique extends pH characterization to confined water environments and deepens our understanding of aerosol chemistry and the air/water interface.

Aerosol microdroplets exhibit a stable pH gradient