Polyhedral MgMn₂O₄ oxide nanozyme based for visual detection of epinephrine

Polyhedral MgMn₂O₄ nanozymes were synthesized through a solvothermal route followed by high-temperature calcination. Their particle size and morphology were precisely tailored by varying the precursor concentration. Decreasing the precursor dosage reduced the particle size from approximately 3 μm to 1 μm while simultaneously transforming the morphology from bispherical aggregates into well-defined polyhedra, accompanied by a gradual decrease in crystallinity. The optimized polyhedral MgMn₂O₄ nanozymes exhibited pronounced intrinsic oxidase-like activity, directly catalyzing the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) into blue oxidized TMB (TMBox) without the addition of exogenous hydrogen peroxide (H₂O₂). Exploiting this intrinsic catalytic activity, an intelligent colorimetric sensing platform was developed for the sensitive determination of epinephrine in human urine. The proposed platform exhibited a linear detection range of 10-100 μM with limits of detection of 0.382 μM (UV-vis), 0.365 μM (smartphone RGB), 0.431 μM (smartphone grayscale), and 0.289 μM (SERS). Owing to its excellent stability, high sensitivity, and outstanding selectivity, the integrated sensing platform demonstrates considerable potential for practical point-of-care biosensing applications.