Highly sensitive oxidation of MBTH/DMAB by MnFe2O4 nanoparticles as a promising method for nanozyme-based sensor development

Nanozymes are human-made nanomaterials with an intrinsic enzyme-like activity that offer a versatile alternative to natural enzymes for disease treatment, environmental remediation, and sensing. In this work, we proposed a novel spectrophotometric assay for determining the oxidase-like activity of manganese ferrite nanoparticles (MnFe2O4 NPs) based on the oxidative coupling of 3-methyl-2-benzothiazolinone-hydrazone (MBTH) and 3-(dimethylamino) benzoic acid (DMAB). The effects of pH (2−8), temperature (20–50 °C), and kinetics parameters as the effect of substrate and nanoparticle concentration on the oxidase-like activity were evaluated. The MnFe2O4 NPs (~3.19 ± 0.62 nm size) were synthesized by a co-precipitation method and characterized by TEM, SEM/EDS, VSM-SQUID, PXRD, and FTIR-ATR analyses. The optimum condition for MBTH/DMAB oxidation catalyzed by MnFe2O4 NPs was observed at pH = 3.9 and 30 °C. The MBTH/DMAB oxidation was adjusted to the Michaelis Menten kinetics model (R2 = 0.96) and showed a Km = 13.59 µM MBTH, kcat = 5.25 × 107 s−1 and kcat/Km = 3.86 × 1012 M−1 s−1. These high kcat and kcat/Km values indicate that the MBTH/DMAB couple is more sensitive than previously reported oxidase substrates by about three orders of magnitude. Hence, our findings open up a wide range of new potential applications on the development of more sensitive sensors based on the MBTH/DMAB reaction catalyzed by MnFe2O4 NPs for detecting several analytes (including alkaline phosphatase, heavy metals, dissolved oxygen in water, among others) in the medical and environmental field.