A proton-coupled electron transfer process from functionalized carbon dots to molecular substrates: the role of pH

Multiple electron and proton transfers in nanomaterials pose significant demands and challenges across the various fields such as renewable energy, chemical processes, biological applications, and photophysics. In this context, pH-responsive functional group-enriched carbon dots (C-Dots) emerge as superior proton-coupled electron transfer (PCET) agents owing to the presence of multiple functional groups (-COOH, -NH₂, and -OH) on the surface and redox-active sites in the core. Here, we elucidate the 2e^-/2H⁺ transfer ability of carboxyl-enriched C-Dots (C-Dot-COOH) and amine-enriched C-Dots (C-Dot-NH₂) with molecular 2e^-/2H⁺ acceptor (benzoquinone, BQ) as a function of pK_a, facilitated by the formation of new O-H bonds. The ground state and excited state pK_a values of different functional groups on the surface of C-Dots are determined using steady-state absorbance and photoluminescence (PL) spectroscopy. The optical spectroscopy and electrochemical studies are employed to comprehend the influence of the surface and core of C-Dots on the proton and electron transfer processes as a function of pH. The cyclic voltammetry analysis reveals a standard Nernstian shift in E^1/2 per pH unit of 30 mV, indicating that the functionalized C-Dots hold promise as candidates for the 2e^-/2H⁺ transfer process. The calculated bond dissociation free energy (BDFE) of the electroactive O-H/N-H bonds provides a more nuanced and detailed understanding of PCET thermodynamic landscapes. These findings underscore the potential of nanoscale functionalized C-Dots for facilitating multiple PCET reactions in future energy technologies.