Analysis of singlet oxygen transport from air to surface by phosphorescence—slope inflection angle (SIA) approach

Singlet oxygen (1O2) is a reactive species that plays a role in environmental and biological surface chemistry; however, the mechanisms of the association of airborne 1O2 at the air/surface interface are poorly known. Here, we help resolve this problem using 1O2's near-infrared (NIR) phosphorescence and geometric analysis based on the slope inflection angle (θ) of air-to-particle transfer. This offers insight into 1O2-surface binding as opposed to conventional kinetic analysis. Two 9,10-disubstituted anthracene quenchers were adsorbed to the particle surface, producing θ ranging from ~91° (greater quenching) to ~99° (less quenching) due to the reduction of airborne 1O2 lifetime (τairborne) by 43% to 95%. A more efficient (lower θ) 1O2 quenching is observed in the order dimethylanthracene-coated particle > anthracene dianion-coated particle > native silica. The anthracene dianion charges and surface silanols did not enhance the 1O2 surface quenching. Indeed, the quenching of airborne 1O2 by native silica was minimal, in which a slight reduction in its surface lifetime (τsurf) was observed (0-5%). This θ approach opens up opportunities in fields such as surface oxidation processes in nanoplastics, which is an emerging concern.