Organic carbon at air-water interfaces drives singlet oxygen enrichment, accelerating redox processes

Singlet oxygen (1O2) is a short-lived, highly reactive oxidant crucial for driving natural element cycles, yet its precise spatial distribution and reactivity in complex multiphase systems remain poorly understood. Current models often oversimplify 1O2 dynamics, failing to account for microheterogeneities that could significantly impact its environmental role. Understanding these localized concentrations is vital for accurately predicting atmospheric chemistry, ocean-atmosphere interactions, and potential oxidative stress in biological systems exposed to such interfaces.

Researchers investigated microheterogeneous singlet oxygen (1O2) generation by irradiating organic carbon (OC)-containing aqueous microdroplets. They combined advanced fluorescence imaging techniques with a reaction-diffusion kinetic model to resolve and map 1O2 gradients across the air-water boundary. To directly quantify the impact on molecular transformation rates, they employed tailored peptide probes, allowing for direct measurement of 1O2-mediated changes in molecules residing at the air-water interface versus those in the bulk aqueous phase. The kinetic model specifically analyzed a 1 µm-radius droplet to characterize these microenvironments.

The study demonstrated pronounced interfacial enrichment of 1O2, primarily driven by the surface accumulation of photosensitizing organic carbon. Their combined fluorescence imaging and kinetic modeling approach successfully resolved steep 1O2 gradients across the air-water boundary. Specifically, in a 1 µm-radius droplet, 1O2 levels dropped by 90% within 10 µm from the interface into the gaseous phase. Even more dramatically, within the aqueous phase, 1O2 levels dropped by 90% within just 230 nm from the interface. Crucially, tailored peptide probes confirmed that: > Molecules residing at the interface undergo substantially faster 1O2-mediated transformation than their bulk counterparts.