A Fe-QueNPs@ZIF-67 nanozyme-based colorimetric assay for discriminative detection of Cr(III) and Cr(VI) in Traditional Chinese Medicines

The escalating global consumption of Traditional Chinese Medicine (TCM) necessitates robust quality control, particularly against contaminants like highly toxic hexavalent chromium (Cr(VI)), which must be distinguished from its essential trivalent counterpart (Cr(III)). In this study, a novel nanozyme-based colorimetric assay is presented for chromium detection, utilizing a zeolitic imidazolate framework-67 encapsulated iron-quercetin nanocomposite (Fe-QueNPs@ZIF-67). The core-shell nanostructure was synthesized through a facile one-pot method at room temperature, in which the ZIF-67 shell served as a sacrificial template to prevent agglomeration of the core Fe-QueNPs, thus preserving their intrinsic catalytic activity. In a buffer medium, the ZIF-67 framework disassembled, uniform dispersing Fe-QueNPs uniformly and releasing Co 2+ ions. A bimetallic complex was formed through synergistic coordination of Co 2+ with Fe 3+ and quercetin ligands, significantly enhancing the peroxidase-like activity of nanozyme. This improved catalytic activity promoted the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) by H₂O₂. Importantly, a differential response to chromium species was observed: the catalytic reaction was inhibited by Cr(III), leading to signal reduction, whereas it was promoted by Cr(VI), resulting in signal amplification. This dual-response modulation enabled selective and sensitive quantification of Cr(III) (linear range: 10–1000 μM, LOD: 8.8 μM) and Cr(VI) (linear range: 0.5–70 μM, LOD: 0.36 μM). The method was successfully applied to TCM samples, achieving excellent recoveries (95–105%). This work provides a robust and sensitive platform for chromium speciation, offering a promising tool for quality control of herbal medicines and broader environmental monitoring.