Peroxide-Free Hexavalent Chromium Detection via Surface Electron Transfer in a Redox-Active Bismuth Metal-Organic Framework Nanozyme

Designing selective nanozymes with well-defined catalytic mechanisms remains a significant challenge, particularly for oxidoreductase mimics targeting environmental contaminants. In this study, we present SU-101, a bismuth-based metal-organic framework (SU-101 Bi MOF), as a robust oxidoreductase-like nanozyme for the selective reduction of hexavalent chromium (Cr⁶⁺) without the need for hydrogen peroxide (H₂O₂). SU-101 Bi MOF catalyzes the conversion of Cr⁶⁺ to Cr³⁺ while simultaneously oxidizing 3,3′,5,5′-tetramethylbenzidine (TMB), resulting in a visually distinct bluish-green product. Density Functional Theory (DFT) calculations reveal a favorable Cr⁶⁺ reduction pathway involving intermediates such as Cr₂O₆^2- and CrO₃^-, culminating in the formation of Cr(OH)₃. Mechanistic insights are further validated through active-site inhibition studies using cysteine, a thiol-containing amino acid, which markedly suppresses catalytic activity by forming stable Bi-S coordination bonds with the MOF’s exposed Bi³⁺ centers. Spectroscopic evidence confirms Bi-S bond formation, highlighting the critical role of accessible metal sites in enabling selective electron transfer. Kinetic analyses demonstrate the strong intrinsic affinity of SU-101 Bi MOF for Cr⁶⁺ (K_m = 0.182 mM) and negligible peroxidase-like activity (K_m = 212 mM for H₂O₂), underscoring its Cr⁶⁺-selective reactivity. The system enables detection within 30 min across a wide concentration range (0.5-50 μM), achieving a low detection limit of 0.36 μM. It exhibits excellent operational stability in acidic conditions, outstanding selectivity against 24 competing ions, and reusability in real environmental matrices. Collectively, these findings establish SU-101 Bi MOF as a mechanistically defined, selective oxidoreductase-mimicking nanozyme platform with significant promise for Cr⁶⁺ detection, cysteine sensing, and broader applications in environmental monitoring and biosensing.