Synthesis of MnO2 sub-microspheres with effective oxidase-mimicking nanozymes for the colorimetric assay of ascorbic acid in orange fruits and juice

Nanozymes, as an alternative to natural enzymes, have the advantages of good catalytic efficiency, high stability, easy storage, low-cost, and the ability to be used under extreme conditions compared with natural enzymes. Improving the sensitivity of nanozyme-based assays is highly required and depends on the nanozyme size, chemical structure and morphology. In the present work, an oxidase-mimicking nanozyme of MnO2 sub-microspheres was synthesized from KMnO4 and MnCl2 center dot 4H(2)O via a one-step hydrothermal method followed by calcination for 2 h at 300 degrees C. The oxidase-like activity of the sub-microspheres was evaluated through the conversion of 3,3 ',5,5 '-tetramethylbenzidine (TMB; colorless) to (ox-TMB (blue color). The kinetic results showed that the catalytic behavior of the MnO2 sub-microspheres follows Michaelis-Menten kinetics with a low constant (K-m = 0.1 mM) compared with the natural enzyme HRP (K-m = 0.434 mM). The catalytic mechanism was deduced, whereby the oxidase-mimicking behavior is dependent on the formation of O-2(-) radicals and partially dependent on OH radicals. An efficient colorimetric platform capable of sensitive determination of ascorbic acid was developed based on fading of the blue color of ox-TMB by ascorbic acid. The linear detection range was from 1.0 to 100 mu M, and the limit of detection was 0.33 mu M. The proposed method showed high selectivity and sensitivity for the detection of ascorbic acid. Further applications in real samples were also successfully demonstrated.

Automated summary

Nanozymes have advantages over natural enzymes such as good catalytic efficiency, high stability, easy storage, and low-cost. In this study, MnO2 sub-microspheres with oxidase-like activity were synthesized and evaluated for their catalytic behavior. The catalytic mechanism of the MnO2 sub-microspheres was deduced, and a colorimetric platform for sensitive determination of ascorbic acid was developed.