Nitrogen-rich negative-charged porous carbon mimicking peroxidase and oxidase for total antioxidant capacity sensing

Abstract

Metal-free nitrogen-doped carbons represent promising nanozymes due to their inherent enzyme mimicking and high structural stability. Achieving precise control over the nitrogen species, surface charge, porosities, and hydrophilicity of them promises to boost enzyme mimicking but remains a rare endeavour. Herein, we employed N-rich melamine formaldehyde networks as precursors to yield N-rich negative-charged porous carbon (NNC) through molten salt pyrolysis. By varying the pyrolysis temperature, we successfully fine-tuned the peroxidase- and oxidase-mimic (POD-/OD-mimic) activities of NNC. Among them, NNC-900 prepared at 900 °C exhibited optimal POD-/OD-mimic activities for selectively catalysing oxidation of positive 3,3′,5,5′-tetramethylbenzidine owing to its high content of pyridinic N and graphitic N species (13.2 wt%), negative charge (–22 mV of Zeta potential), hierarchical porosities (328 m2 g−1 of SBET and 0.23 cm3 g−1 of micropore/mesopore volumes), and hydrophilicity (65° of contact angle). Oxidation-state intermediates connected to active N sites in NNC were revealed as the dominant active species, accompanied by the partial generation of free 1O2 during POD-mimicking. Colorimetric detection platforms based on the POD- and OD-mimic properties of NNC-900 were developed for sensing total antioxidant capacity (TAC), with limit of detection values as low as 0.07 and 0.16 μM, respectively. The detection platforms successfully assessed the TAC levels in fruits, demonstrating significant potential in evaluating the quality of antioxidant food for healthy diets. Our contribution offers opportunities to advance the tunning of multi-enzyme-mimic nitrogen-doped carbon nanozymes.