Spatial confinement of Mn3O4 in hollow carbon sphere to boost the oxidase-like activity for colorimetric sensing analysis of hydroquinone and hexavalent chromium

It is very meaningful to develop simple and effective methods for detecting hazardous substances. Herein, a newly-designed Mn 3 O 4 -based composite with oxygen vacancy is synthesized and employed as oxidase-like mimic for conducting chromogenic analysis. After loading Mn 3 O 4 in hollow carbon sphere (HCS), the hydrophobicity of Mn 3 O 4 is remarkably enhanced, thereby creating a microscopic three-phase interface to boost the capturing ability towards O 2 . Interestingly, hollow structure of HCS can provide the spatial confinement effect for Mn 3 O 4 , shortening mass transfer process and improving catalytic activity. Thus, HCS/Mn 3 O 4 composite possesses low interfacial resistance and rapid charge transfer rate. More importantly, HCS/Mn 3 O 4 composite can simulate the activity of oxidase-like mimics, which can oxidize colorless 3,3′,5,5′-tetramethyl benzidine (TMB) to blue oxidized TMB (oxTMB) in a short time. Further analysis indicates that O 2 •− plays an important role, and the direct oxidation of TMB over HCS/Mn 3 O 4 composite also plays a part in the color reaction of TMB. Based on the cascade reactions among TMB, HCS/Mn 3 O 4 and target compound (namely hydroquinone and Cr 6+ ), a simple chromogenic sensing method with broad linear range and low LOD for detecting hydroquinone and Cr 6+ is proposed. In short, current study can give a valuable reference for the design of enzyme mimics and the research on chromogenic mechanism.