Synthesis of high oxidase-like activity palladium-graphene quantum dot-N-doped porous carbon nanozymes for colorimetric detection of nitrite

Abstract

The synthesis of palladium-based nanozymes with high catalytic activity for nitrite detection remains challenging. Herein, we report a novel palladium-tryptophan-functionalized graphene quantum dot-N-doped porous carbon composite (Pd-Try-GQD-NPC). The composite was synthesized by coordinating Try-GQDs with palladium ions, followed by mixing with chitosan, freeze-drying, and thermal reduction. The resulting Pd-Try-GQD-NPC exhibits a three-dimensional porous network structure, with small-sized Pd nanoparticles uniformly dispersed on carbon sheets. This unique structure contributes to the enhanced oxidase-like activity of Pd-Try-GQD-NPC, with K_m, V_max and K_cat/K_m values of 0.28 mM, 7.96 × 10⁻⁸ M s⁻¹ and 57.1 M⁻¹ s⁻¹. Furthermore, a colorimetric sensor was constructed based on Pd-Try-GQD-NPC for the determination of nitrite. Pd-Try-GQD-NPC catalyzes the oxidation of TMB to form a blue TMB complex, which shows a strong absorption peak at 652 nm. In the presence of nitrite, the TMB complex undergoes further oxidation to generate yellow diazotized oxTMB, resulting in a decrease in absorbance at 652 nm accompanied by an enhancement of the absorbance at 445 nm. The A₆₅₂/A₄₄₅ ratio decreases with increasing nitrite concentration in the range of 0.5–500 μM, with a detection limit of 0.11 μM. The developed colorimetric method has been successfully applied to the determination of nitrite in cabbage and water samples.