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 H2O2, a rapid colourimetric assay for ascorbic acid (AA) detection was developed with a wide range of 1-120 mu M and a detection limit of 0.435 mu M (3 sigma/k). The peroxidase-like activity of Fe-CQDs has been proved to originate from their ability to catalyze H2O2 to produce center dot 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.