Autogenous iron-based peroxidase-like nanozyme from paper mill sludge for ascorbic acid detection

The high-value conversion of waste resources to carbon materials offers an opportunity to alleviate the resource crisis. Owing to their low cost and easy preservation, iron-based nanoenzymes are receiving increasing attention in the fields of catalysis, detection and biomedicine. In this work, for the first time, paper mill sludge (PMS), which contains iron ions from a flocculant used in paper mill wastewater treatment, is used as a precursor to produce iron-doped carbon quantum dots (Fe-CQDs) by oil bath pyrolysis process. The synthesized Fe-CQDs with carboxyl groups and abundant carbon-deficient structure, exhibit higher peroxidase-like activity. Compared to horseradish peroxidase (HRP), Fe-CQDs have a better affinity for the substrate and better stability. Based on the good affinity of Fe-CQDs for H 2 O 2 , a rapid colourimetric assay for ascorbic acid (AA) detection was developed with a wide range of 1–120 μM and a detection limit of 0.435 μM (3σ/k). The peroxidase-like activity of Fe-CQDs has been proved to originate from their ability to catalyze H 2 O 2 to produce·OH, and successfully applied to determine AA content in the roots and leaves of sweet potato. This work indicates the reliability of the method in practical application and provides a greening strategy for the value-added utilization of solid sludge.