Microcapsule Structure with a Tunable Textured Surface via the Assembly of Polyoxomolybdate Clusters: A Bioinspired Strategy and Enhanced Activities in Alkene Oxidation

Abstract

A polyamine-mediated bioinspired strategy to assemble Keggin-type phosphomolybdic acid (PMA) clusters is demonstrated for the fabrication of microcapsule (MC) structures with unique surface textures. It involves supramolecular aggregation of polyamines with multivalent anions, which then allows the assembly of negatively charged PMA into MCs in an aqueous medium under ambient conditions. Resembling the role of polyamines in biosilicification of diatoms, the polyamine-anion interaction is shown to be the key for the assembly process. It not only provides structural stability but also facilitates an interesting transition from a smooth to a wrinkled surface alongside a change in the Keggin form to its lacunary form depending on the pH of the medium. Moreover, the presence of isolated PMA units in the hybrid structure enables them to be active in catalyzing the aerobic oxidation of alkenes under solvent-free conditions with better selectivity and reusability. Hence, the assembly approach represents an effective way for heterogenization of PMA-based materials and is expected to find considerable application in the wider hybrid-cluster field.

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

“…42 In addition, as demonstrated earlier, the super-chaotropic property of POM plays an important role in the assembly process, according to which POMs have a tendency for both electrostatic and hydrophobic interactions with the PAH-citrate aggregates. 22,36 This interaction of POM not only leads to the formation of a thin layer on the surface of the polyamine-citrate aggregates but also generates surface wrinkling as a result of mechanical instability at the interface similar to that is generally seen in other spheroidal systems. Because the delocalized charge on POM anions is responsible for the abovementioned property, there is a dependency of this interaction on the charge or valency on the POM clusters.…”

“…The electrostatic interactions between PAH and the multivalent anion (citrate) resulted in the microscopic phase separation (turbidity) because of the formation of polymer-anion aggregates . The assembly of PAH with citrate was carried out at pH 6.0, as it is known that pH plays a crucial role in the formation of polymer-anion aggregates, and pH ≥ 4 was necessary for electrostatic interactions between PAH and citrate. , The negatively charged GOx (note: at neutral pH, GOx is negatively charged as it has an isoelectric point of 4.2) and PTA nanoclusters were then assembled on the cationic polymer-anion aggregates in a stepwise manner to form enzyme (GOx)-nanozyme (PTA)-assembled microspheres (GOx-PTA@MS-6.5) (Scheme ). The pH of the PTA solution was brought up to 7.2 by adding NaOH, which when added to PAH-citrate aggregates (pH 6.0) resulted in the formation of GOx-PTA-assembled microspheres with the pH of the final mixture to be 6.5.…”

“…The surface wrinkling, as demonstrated earlier, is due to interfacial instability and in turn depends upon the type of the anion (used for the formation PAH-anion aggregates) and the pH of the medium. 22,36 The Xray diffraction (XRD) pattern of pure PTA revealed sharp peaks in the 2θ range of 10−70 degrees, indicating the crystalline structure of the Keggin units (Figure 1c). In contrast, no characteristic XRD pattern was observed in the case of GOx-PTA@MS-6.5, which is due to the separation of the Keggin units from one another, forming highly dispersed amorphous-like material.…”

“…It is well known that POMs can catalyze the oxidation of organic substrates in the presence of O 2 and H 2 O 2 via multistep electron-transfer processes. Therefore, these nanoclusters can act as peroxidase mimics in biochemical assays by catalyzing the oxidation of chromogenic substrates in the presence of H 2 O 2 . , In addition to this, the high solubility of POMs in water leads to difficulties in their separation from the reaction medium or leaching is another major issue, which needs to be addressed via efficient heterogenization of the POMs . Moreover, in the case of a bioreactor or biosensor, wherein an enzyme is needed to be coupled with the nanozyme, the acidic nature of POMs could be highly detrimental to the enzymatic stability and activity.…”

“…20,21 In addition to this, the high solubility of POMs in water leads to difficulties in their separation from the reaction medium or leaching is another major issue, which needs to be addressed via efficient heterogenization of the POMs. 22 Moreover, in the case of a bioreactor or biosensor, wherein an enzyme is needed to be coupled with the nanozyme, the acidic nature of POMs could be highly detrimental to the enzymatic stability and activity.…”