Bifunctional Oxidase-Peroxidase Inorganic Nanozyme Catalytic Cascade for Wastewater Remediation

Abstract Nanozymes are inorganic nanoparticles with enzyme-like features. Noble metals and metal oxide-based nanomaterials can present enzyme-mimicking behavior mediating catalytic reactions including oxidase-, peroxidase-, catalase-, and superoxide dismutase-like activities with important applications in environmental and biomedical grounds. Thus, in this study, it was designed and developed a novel nanosystem that exploits the oxidase-like (OD) behavior of gold nanoparticles (AuNPs, GOLDzyme) and the peroxidase-like (POD) characteristics of cobalt-doped magnetic iron oxide nanoparticles (MIONs, Co-MIONzyme) amalgamated in an inorganic-inorganic bi-nanozyme cascade for the degradation of dye pollutants. GOLDzyme and Co-MIONzyme nanomaterials were synthesized and stabilized by citrate and carboxymethylcellulose (CMC) ligands, respectively, using a green aqueous colloidal process at mild conditions. The physicochemical characterization results indicated that water-dispersible colloidal supramolecular nanostructures were effectively produced, with core-shell morphologies (i.e., inorganic nanoparticle core/organic-shell). The AuNPs (GOLDzyme) presented crystalline nanostructure, with a uniform spherical shape, zeta potential (ZP) = - 46 ± 3 mV, hydrodynamic diameter (DH) = 11 ± 3 nm. Analogously, the Co-MIONzyme stabilized by CMC ligand evidenced the formation of nanocrystalline substituted magnetite (CoxFe3-xO4) with uniform spherical morphology, average size = 7.0 ± 2 nm, ZP = - 47 ± 3 mV, DH = 46 ± 3 nm, and superparamagnetic behavior. When tested separately using a colorimetric assay based on a chromogenic molecule (3,3',5,5’ tetramethylbenzidine, TMB), they demonstrated catalytic activity upon the injection of each specific substrate in the medium, i.e., glucose for GOLDzyme that behaved predominantly as oxidase-like nanomaterial, and H2O2 for Co-MIONzyme, which showed a peroxidase-like or catalase-like behavior. In addition, these nanozymes demonstrated pH-dependent and temperature-dependent catalytic activities. As a proof of concept, when combined into a single-pot reaction system, these bifunctional nanozymes confirmed integrated catalytic cascade as oxidase and peroxidase-like nanocatalysts towards the oxidation of TMB and the degradation of methylene blue (MB, ~18%) as the model dye pollutant. Based on the preliminary encouraging results of this research, it can be foreseen that the combination of nanozymes paves the way for the development of new nanoplatforms for prospective applications in water treatment and environmental remediation.