Label-Free Colorimetric Detection of Acid Phosphatase and Screening of Its Inhibitors Based on Biomimetic Oxidase Activity of MnO2 Nanosheets

In this research, we attempted to develop a sensitive colorimetric sensing strategy for the detection of acid phosphatase (ACP) based on MnO₂ nanosheets and explored its applications in screening and evaluating inhibitors of ACP. The MnO₂ nanosheets exhibit intrinsic biomimetic oxidase activity, which can catalyze the oxidation of the colorless 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonate) diammonium salt (ABTS) into green oxidized ABTS (oxABTS). Upon the introduction of ACP, l-ascorbic acid-2-phosphate can be dephosphorylated to ascorbic acid, which arouses the disintegration of MnO₂ nanosheets into Mn²⁺ ions. This disintegration weakens the enzyme mimicking activity of the MnO₂ nanosheets, leading to the impediment of the oxidation of ABTS. Conversely, in the absence of ACP, the ABTS is rapidly oxidized by MnO₂, leading to a significant colorimetric signal change. The absorbance difference at 420 nm displayed a linear relationship with the concentration of ACP ranging from 0.075 to 0.45 mU·mL^-1, generating a detection limit of 0.046 mU·mL^-1. In the inhibition assays, this sensing platform provided simple detection for parathion-methyl (PM), a representative inhibitor of ACP. The facile evaluation of the inhibitory effect of PM, including its IC50 toward ACP, was also realized.