Reversible pH switchable oxidase-like activities of MnO₂ nanosheets for a visual molecular majority logic gate

Although the enzyme-like activities of MnO2 nanosheets have been investigated, effective strategies for modulating their activities are still lacking. Herein, inspired by the pH-dependent activity of natural enzymes, we demonstrate that the oxidase-like activities of the MnO2 nanosheets are pH switchable and reversible. The MnO2 nanosheets show oxidase-like activities in an acidic medium, which can catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) to form blue oxidized TMB (oxTMB), while the corresponding activities are greatly weakened under alkaline conditions. The reaction mechanism is attributed to the fact that the MnO2 nanosheets can be transformed into hexagonal Na4MnO4 with a low reaction activity in an alkaline environment. Using the pH switchable oxidase-like activities of the MnO2 nanosheets and other reaction processes, we construct a visual three-input molecular majority logic gate with a single-vote veto function. In addition, an OR-INH cascade logic circuit is successfully realized as well.