Enhanced oxidase mimic activity of raspberry-like N-doped Mn3O4 with oxygen vacancies for efficient colorimetric detection of gallic acid coupled with smartphone

The content of gallic acid (GA) is positively correlated with the quality grade of tea. Here, we developed a colorimetric method based on raspberry-like N-doped Mn3O4 nanospheres (N-Mn3O4 NSs) with oxidase-like activity for GA assay. Modulating the electronic structure of Mn3O4 by N doping could promote the catalysis ability, and the produced oxygen vacancies (OVs) can provide high surface energy and abundant active sites. The N-Mn3O4 NSs presented low Michaelis-Menten constant (Km) of 0.142 mM and maximum initial velocity (Vmax) of 9.8 × 10−6 M s−1. The sensor exhibited excellent analytical performance towards GA detection, including low LOD (0.028 μM) and promising linear range (5 ∼ 30 μM). It is attributed that OVs and O2− participated in TMB oxidation. Based on the reaction color changes, a visualized semi-quantitative GA detection could be realized via a smartphone-based system. It could be applied for evaluating GA quality in market-purchased black tea and green tea.