MOF-derived porous ZnO-Co₃O₄ nanocages as peroxidase mimics for colorimetric detection of copper(ii) ions in serum

Sensitive detection of copper ions (Cu²⁺) in biological samples is extremely important since an abnormal level of Cu²⁺ is linked with many diseases. Herein, we demonstrated a novel turn-on colorimetric sensor for selective detection of Cu²⁺ both in buffered solution and serum samples based on porous bimetallic transition metal oxide nanocages (ZnO-Co₃O₄ NCs) as peroxidase mimics. The ZnO-Co₃O₄ NCs were prepared by using ZnCo-zeolitic-imidazolate-framework (ZnCo-ZIF) as precursors via direct calcination. With the high peroxidase-like activity, the obtained ZnO-Co₃O₄ NCs can catalyze the oxidation of tetramethylbenzidine (TMB) in the presence of H₂O₂ to form a blue colored product. The inhibition effect of cysteine (Cys) on the catalytic activity of ZnO-Co₃O₄ NCs and its strong binding ability toward Cu²⁺ enabled the ZnO-Co₃O₄ NCs/Cys system to be utilized for sensitive detection of Cu²⁺, in which the catalytic activity of ZnO-Co₃O₄ NCs/Cys can be recovered by the introduction of Cu²⁺ with an obvious color change of the solution. The linear range for Cu²⁺ determination was 2 to 100 nM with a detection limit of 1.08 nM. More importantly, this colorimetric sensor has been successfully applied to detect Cu²⁺ in serum without pretreatment. Our findings are expected to expand the scope of application of nanozyme and shed light on early disease diagnosis.