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

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 (Fe 3+ ) 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 (Fe 3+ -NGQDs) upon Fe 3+ addition possessed high peroxidase-like activity. Fe 3+ -NGQDs nanocomposite exhibited shuttle-shaped structure (∼30 nm), the lattice structure of NGQDs and electron transfer between Fe 3+ and NGQDs. The Fe 3+ -NGQDs nanocomposite can catalyze the production of superoxide radicals (•O 2 - ) from H 2 O 2 . The Michaelis constant (K m ) of Fe 3+ -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 H 2 O 2 substrate. HQ, unlike its isomers catechol (CC) and resorcinol (RE), could consume •O 2 - generated from the decomposition of H 2 O 2 catalyzed by Fe 3+ -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.