Going Deep into the Surface Chemistry of Carbon Dots: Influence of Functional Groups on the Redox Abilities

Carbon dots (CDs) are emerging nanomaterials with low-cost synthesis and highly tunable surface chemistry, which have propelled their widespread use in catalysis and photocatalysis. Yet, unraveling their chemical structure remains essential to unlocking their full potential. Here, we systematically investigated the redox behavior of three amine-rich CD batches synthesized from either pure L-arginine or mixtures of L-arginine with alkyl diamines (ethylenediamine or putrescine), yielding different surface amine densities. Strikingly, the density and accessibility of surface amines were found to directly govern CD (photo)redox activity, evaluated through the reduction of resazurin to resorufin. CDs enriched in surface amines exhibited markedly superior performance under both dark and illuminated conditions. Electrochemical studies confirmed electron transfer feasibility in both scenarios. Control experiments with molecular amines showed no activity, highlighting the indispensable role of supramolecular CD-substrate interactions, likely involving at least two vicinal amines, in enabling electron transfer. The presence of these interactions was confirmed by both 1H-NMR spectroscopy and isothermal titration calorimetry. Furthermore, absolute CD molecular weights determined by multi-detection gel permeation chromatography enabled precise stoichiometric analysis. Together, these findings establish a framework for tailoring CD surface chemistry to achieve controlled electron transfer, opening new opportunities across (nano)materials science, organic synthesis, and chemical biology.