Engineered Mn/Co oxides nanocomposites by cobalt doping of Mn-BTC - New oxidase mimetic for colorimetric sensing of acid phosphatase

We report that the Co/Mn oxides nanocomposite, obtained by directly pyrolying cobalt doped Mn-BTC precursor, is a new oxidase mimetic for colorimetric sensing of acid phosphatase (ACP). By tuning the molar ratios of cobalt ions to Mn-BTC (BTC = 1,3,5-benzenetricarboxylate) precursor, four bimetal Co/Mn-BTCs and their derivatives were obtained. These Co/Mn oxides showed 1.4-fold to 2.6-fold higher oxidase-like activity than that of Mn2O3 derived from Mn-BTC, indicating that Co-doping is an effective way for improving and tuning oxidase-like activity of manganese oxides. The rambutan-like Co/Mn oxides with Co/Mn molar ratio of 0.33 (denoted CMO-0.33) in the MOFs precursors exhibited the highest oxidase-like activity. The estimated Michaelis constant for 3,3′,5,5′-tetramethylbenzidine (TMB) was 2.26-fold lower than that of Mn2O3, indicating its stronger affinity to TMB. Based on the inhibitory effect of ascorbic acid (AA) on the TMB-CMO-0.33 system, a new colorimetric strategy was paved for screening ACP activity when coupled with L-ascorbic acid 2-phosphate trisodium salt (AAP) because ACP could catalyze hydrolysis of AAP to generate AA. The proposed colorimetric biosensor allows detection of ACP in the range 0.02–1.0 U/L with a limit of detection of 8.2 mU/L. Its potential utilization in ACP assay in human serum samples is successfully demonstrated.