A Bioinspired Assembly to Simultaneously Heterogenize Polyoxometalates as Nanozymes and Encapsulate Enzymes in a Microstructure Endowing Efficient Peroxidase-Mimicking Activity

A polyamine-mediated bioinspired strategy to assemble polyoxometalate (POM) [phosphotungstic acid (PTA)/phosphomolybdic acid (PMA)] nanoclusters and glucose oxidase (GOx) generating microsphere structures under very mild reaction conditions is demonstrated. The noncovalent interactions between multivalent anions (citrate ions) and polyamine chains [poly-(allylaminehydrochloride), PAH] are key to the assembly process, which results in positively charged coacervates, allowing the assembly of GOx and POM nanoclusters into microsphere structures. As observed in the process, in addition to having an impact on the size, composition, and textural properties of the spheres, the pH of the reaction medium has a pivotal role in safeguarding the enzyme. The enzyme assay shows that the activity of GOx is totally preserved during the assembly process, and the enzyme loading is slightly improved with the increase in the pH of the medium. In contrast, a physical mixture of GOx and POM leads to the inactivation of the enzyme retaining only similar to 50% GOx activity. The peroxidase-like activity of PTA nanoclusters assembled in enzyme-nanozyme microspheres, as indicated by the maximal velocity (V-max) and catalytic constant (k(cat)) values, is improved by 3- to5-fold with the increase in the pH, which in turn is attributed to the smaller size of the microspheres, facilitating better diffusion of reactants, intermediates, and products. Importantly, the confinement and proximity of GOx and POM in the microspheres not only play a crucial role in defining the peroxidase-like activity of POM but also endow high selectivity (by virtue of GOx) in detecting glucose at physiological pH conditions. Thus, the microsphere assembly, while enabling effective heterogenization of the POM as a nanozyme, simultaneously addresses the issues related to high cost and instability associated with natural peroxidases.

Automated summary

A bioinspired strategy using polyamines to assemble POM nanoclusters and GOx into microsphere structures under mild reaction conditions is demonstrated. Noncovalent interactions and pH of the reaction medium play key roles in the assembly process and enzyme protection.