Photochemical Transformations of Carbon Dots in Aqueous Environments

The unique physicochemical and luminescent properties of carbon dots (CDs) have motivated research efforts toward their incorporation into commercial products. Increased use of CDs will inevitably lead to their release into the environment where their fate and persistence will be influenced by photochemical transformations, the nature of which is poorly understood. This knowledge gap motivated the present investigation of the effects of direct and indirect photolysis on citric and malic acid-based CDs. Our results indicate that natural sunlight will rapidly and non-destructively photobleach CDs into optically inactive carbon nanoparticles. We demonstrate that after photobleaching, ^•OH exposure degrades CDs in a two-step process that will span several decades in natural waters. The first step, occurring over several years of ^•OH exposure, involves depolymerization of the CD structure, characterized by volatilization of over 60% of nascent carbon atoms and the oxidation of nitrogen atoms into nitro groups. This is followed by a slower oxidation of residual carbon atoms first into carboxylic acids and then volatile carbon species, while nitrogen atoms are oxidized into nitrate ions. Considered alongside related CD studies, our findings suggest that the environmental behavior of CDs will be strongly influenced by the molecular precursors used in their synthesis.