MnO nanosheets as oxidase mimics for colorimetric detection of alkaline phosphatase activity.

A sensitive colorimetric method is described for the determination of the activity of alkaline phosphatase (ALP). It is based on the regulation of the oxidase-mimicking activity of MnO nanosheets. In the absence of ALP, MnO nanosheets are capable of catalyzing the oxidation of the colorless substrate 3,3',5,5'-tetramethylbenzidine (TMB) by oxygen to form a blue oxidized product (TMB Ox) with an absorption peak at 652 nm. In the presence of ALP and its substrate ascorbic acid-2-phosphate, the latter is hydrolyzed to form ascorbic acid (AA). AA triggers the decomposition of MnO nanosheets by reducing MnO to Mn, thereby weakening the enzyme mimicking activity of the MnO nanosheets and causing a drop in absorbance. The drop in absorbance at 652 nm is related to the ALP activity in the range from 0.05-10 m-units per mL (mU·mL), and the detection limit is 0.05 mU·mL. The method was applied to the determination of ALP in spiked calf serum samples and gave satisfactory results. Graphical abstract Schematic presentation of a facile and sensitive colorimetric method for detecting the activity of alkaline phosphatase (ALP) based on enzymatic regulation of the oxidase-mimicking activity of MnO nanosheets.