Smartphone-assisted colorimetric sensing platform based on manganese phosphonate MOF nanozyme for total antioxidant capacity evaluation in fruits and cosmetics.

Accurate evaluation of total antioxidant capacity (TAC) is crucial for assessing the functional benefits of antioxidant-rich foods and cosmetic products. Although colorimetric sensing offers a simple and low-cost strategy for TAC determination, its practical performance depends strongly on the stability and catalytic efficiency of the chromogenic system. Herein, we report a manganese phosphonate metal-organic framework (MnP MOF), constructed from Mn2+ nodes and a rigid biphenyl-4,4'-diphosphonic acid ligand, as an oxidase-mimetic nanozyme for H2O2-free TAC determination. In contrast to the more commonly reported carboxylate- or imidazole-based MOF nanozymes, this phosphonate-based Mn MOF features strong Mn-O-P coordination and a rigid aromatic framework, which are associated with improved structural robustness and accessible catalytic sites. The MnP MOF exhibits pronounced oxidase-like activity toward 3,3',5,5'-tetramethylbenzidine (TMB) without requiring H2O2, thereby reducing peroxide-related instability and potential matrix interference. The material showed a low apparent Km value of 0.32 mM and enabled sensitive detection of ascorbic acid (AA), L-cysteine (L-Cys), and glutathione (GSH), with limits of detection of 0.37, 1.02, and 1.78 μM, respectively. Combined with smartphone-based readout, the resulting platform allows rapid and portable TAC determination in fruit and cosmetic samples, demonstrating the promise of phosphonate-based MOFs for practical antioxidant sensing.