PVP-Assisted Fe-Doped Carbon Dots as Robust Nanozymes for Visual H₂O₂ Sensing

Iron-doped carbon dots (Fe-CDs) exhibit exceptional peroxidase-like activity and are utilized for colorimetric sensing of hydrogen peroxide (H2O2). However, the Fe–H2O2 system falls short of generating reactive oxygen species for oxidizing the TMB (3,3′,5,5′-tetramethylbenzidine) due to Fe aggregation and uneven dispersion, resulting in less sensitive sensing of H2O2. This study synthesized polymer-assisted Fe-doped CDs with remarkable peroxidase-mimicking activity through the hydrothermal carbonization of waste orange peel and iron chloride salt. Different types of polymers, such as PAN, PVP, PVA, PS, and cellulose acetate, were used to engineer the surface of CD for the successful uniformity of Fe doping. Among the polymers, PVP with the appropriate molecular weight acts as an efficient dispersant to prevent Fe aggregation and regulate the size of Fe. On the other hand, CDs act as Lewis acid sites for chromogen substrate TMB (hard base) adsorption. The intense interaction between ultrafine Fe and CDs/PVP matrix shows excellent peroxidase activity in activating H2O2 and oxidizing TMB. Notably, nanozyme Fe-CD/PVP demonstrates high selectivity of H2O2 with concentrated interferences, with a low detection limit (52 nM). Moreover, Fe-CD/PVP was successfully utilized to sense H2O2 in milk.