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 (K_m) of 0.142 mM and maximum initial velocity (V_max) 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 ^[rad]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.