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

Abstract

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 ∼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.

Platinum Palladium Bimetallic Nanozymes Stabilized with Vancomycin for the Sensitive Colorimetric Determination of L-cysteine

“…In addition, the catalytic activity of nanozymes is also related to nanoscale factors, such as their size, morphology and surface, which significantly affect their activity. Nayak et al [ 44 ] assembled polyoxometalate (POM) (phosphotungstic acid (PTA)/phosphomolybdic acid (PMA)) nanoclusters and glucose oxidase (GOx) into microsphere structures, which facilitated the better diffusion of the reactants, intermediates and products due to the small size of the microspheres. This resulted in a 3–5-fold increase in the peroxidase-like activity of the PTA nanoclusters in the nanozyme microspheres.…”

Coacervate-Based Plexcitonic Assembly toward Peroxidase-like Activity and Ultrasensitive Glucose Sensing

“…Figure C shows a linear increase in the absorbance at 652 nm over a range of 0–500 μM glucose concentrations with a slope of 1.1 × 10 –3 μmol –1 L. The estimated LOD of glucose is 272 nM, which is superior or comparable to most of the previously reported nanozymatic systems (Table S1). Earlier, Rana and co-workers fabricated a bioinspired coacervate assembly using polyallylamine and citrate ions and demonstrated efficient peroxidase-mimicking activity of surface-anchored polyoxometalate (POM) nanoclusters with a LOD of 0.31 mM for glucose . Our estimated LOD is dependent on the solution pH and the present system shows the highest sensitivity at pH = 6 (Figure D).…”

“…Earlier, Rana and co-workers fabricated a bioinspired coacervate assembly using polyallylamine and citrate ions and demonstrated efficient peroxidase-mimicking activity of surface-anchored polyoxometalate (POM) nanoclusters with a LOD of 0.31 mM for glucose. 57 Our estimated LOD is dependent on the solution pH and the present system shows the highest sensitivity at pH = 6 (Figure 8D). These spectral changes are accompanied by a gradual color change of the solution from colorless to blue as a function of glucose concentrations (Figure 8E, upper panel).…”

Exploring the Reactivity of Polyoxometalates toward Proteins: From Interactions to Mechanistic Insights

“…Understanding the redox activity of POMs toward proteins is of crucial importance since different studies have suggested that the redox properties of POMs are responsible for their antibacterial and antitumor activity. , For example, the observed reduction of K 6 [P 2 W 18 O 62 ]·14H 2 O and K 4 [SiMo 12 O 40 ]·3H 2 O in Gram-positive drug-resistant bacteria is an indication of their ability to penetrate through the cell wall and interfere with the electron transfer system of the cell, making these POMs effective antibacterial agents . Furthermore, the ability of POMs to oxidize biological substrates through activating H 2 O 2 , which is akin to the reactivity of peroxidase enzymes, has enabled their development as biosensors for the detection of H 2 O 2 – such as in colorimetric immunoassays for cancer cells–and as catalysts for the degradation of dye pollutants in water. ,− Therefore, the multiple emerging biological and medical applications of POMs incite studying the redox chemistry of POMs toward biomolecules. ,, …”