Highly active nanozyme based on nitrogen-doped graphene quantum dots and iron ion nanocomposite for selective...

Inspired by the iron porphyrin structure of natural horseradish peroxidase (HRP), an efficient carbon-based nanozyme was fabricated using nitrogen-doped graphene quantum dots (NGQDs) and iron ion (Fe³⁺) nanocomposite, enabling selective distinguishment of hydroquinone (HQ) from its isomers. NGQDs with good dispersibility and uniform size were synthesized via a one-step hydrothermal process. NGQDs lacked peroxidase-like activity but the formed nanocomposite (Fe³⁺-NGQDs) upon Fe³⁺ addition possessed high peroxidase-like activity. Fe³⁺-NGQDs nanocomposite exhibited shuttle-shaped structure (∼30 nm), the lattice structure of NGQDs and electron transfer between Fe³⁺ and NGQDs. The Fe³⁺-NGQDs nanocomposite can catalyze the production of superoxide radicals (•O₂^-) from H₂O₂. The Michaelis constant (K_m) of Fe³⁺-NGQDs (0.115 mM) was lower than that of natural HRP (0.434 mM) with 3,3',5,5'-tetramethylbenzidine (TMB) as the substrate and the maximum initial reaction rate (V_max, 16.47 × 10^-8 M/s) was nearly 4 times higher than that of HRP using H₂O₂ substrate. HQ, unlike its isomers catechol (CC) and resorcinol (RE), could consume •O₂^- generated from the decomposition of H₂O₂ catalyzed by Fe³⁺-NGQDs nanocomposite, reducing the oxidation of TMB. This principle enabled selective colorimetric determination of HQ ranged from 1 μM to 70 μM and a limit of detection (LOD) of 0.2 μM. Successful determination of HQ in pond water was also realized.