Hydrolysis-Induced Cu₂O Networks and the Triggered Peroxidase-Mimic Activity by Cr⁶⁺ under Neutral Conditions

The current small-scale synthesis and relatively large size of Cu₂O have limited its practical applications. Herein, we developed a hydrolysis strategy to prepare phase-pure Cu₂O networks composed of small granules (ca. 25 nm) on a gram scale. The preparation involves in situ hydrolyzing the H[CuCl₂] complexes prereduced in N,N'-dimethylformamide (DMF). The DMF-soluble H[CuCl₂] complexes are critical for the homogeneous nucleation of CuCl seeds and subsequent hydrolysis, allowing for separate control over the nucleation and growth stages to regulate the formation of Cu₂O networks. The novel Cu₂O networks possess numerous exposed active sites and hierarchical porosities, conferring high catalytic activity and fast mass transfer capability. The inherent peroxidase-mimic activity of Cu₂O is severely inhibited under neutral conditions but can be triggered by Cr⁶⁺, enabling the colorimetric assay of Cr⁶⁺ with the assistance of the oxidation-induced color change of 3,3',5,5'-tetramethylbenzidine. Through density functional theory calculation, we confirmed that the attachment of Cr⁶⁺ on the Cu₂O surface reduced the dissociation energy of H₂O₂, enhancing the enzyme-mimic activity. The colorimetric detection method demonstrated a sensitive and specific assay capability for Cr⁶⁺ (LOD = 0.095 μM). Our work offers a straightforward protocol for novel design of metal or metal-based nanomaterials for nanozymes or other applications.