PhotochemicalTransformations of Carbon Dots in AqueousEnvironments
2020
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.
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