A fluorescence quenching study of the interaction of Suwannee River fulvic acid with iron oxide nanoparticles

The fluorescence quenching behaviour of a manufactured nanoparticle (NP, iron oxide, 7 nm) on the standard Suwannee River fulvic acid (SRFA) was investigated for the first time. Size, aggregation and fluorescence was examined as a function of NP:SRFA ratio and of pH. Aggregation state varied as both a function of pH and NP:SRFA ratio, with maximum aggregation at near neutral pH values (6–8). SRFA fluorescence quenching increased non-linearly with increasing NP concentrations (>0.22 × 10−3 M iron nanoparticles), indicating the complex nature of NP:SRFA interactions. Aggregates of iron oxide present at pH 7–8 appeared to have a much larger effect on quenching compared with dispersed NPs or dissolved phase iron. Fluorescence quenching is demonstrated to indicate different mechanisms of NP:SRFA binding with pH.