Ligand Synergy Enhances Oxygen Vacancies: A Heterostructured Nanozyme for Ultra-Sensitive Dual-Mode Cr(VI) Speciation

The catalytic efficiency of nanozymes is governed by their surface electronic structure and defect chemistry, yet the synergistic regulation of oxygen vacancies (OVs) and mixed-valence states remains challenging. Herein, we report a ternary core-shell heterostructure, Cu/Cu2O/CuVO3@MOF(Cu-His-BDC)(CCVO@@MOF(Cu-His-BDC)), constructed via an in situ dual-ligand coordination strategy using histidine and terephthalic acid. This dual-ligand architecture enriches surface oxygen vacancies (33.86% → 59.87%) and reshuffles valence equilibria (V4+: 10.75% → 29.21%; Cu+: +10.21%), while creating a hierarchical porous network. Spectroscopic and electrochemical characterizations reveal that the enhanced oxidase-like activity stems from coupled ROS generation and interfacial electron transfer. Interestingly, Cr(VI) acts as a bifunctional regulator, triggering metal valence cycling (Cu+/V4+ ⇌ Cu2+/V5+) to amplify colorimetric signals while suppressing electrochemical responses. Leveraging this orthogonal promotion-inhibition effect, a dual-mode platform covering pM to μM ranges is established with recoveries of 94-107% in complex matrices. Combined with EDTA masking and alkaline persulfate oxidation, the platform enables reliable Cr(VI)/Cr(III) speciation, validated by ICP-MS, while smartphone-based RGB readout offers field-deployable capability. This work establishes a design paradigm for intelligent nanozymes through synergistic ligand and defect modulation.