pH-responsive compartmentalized alginate beads enable spatial control of sequential nanozyme reactions

Abstract

Controlling two-step sequential catalytic reactions through external stimuli is a powerful approach for developing responsive chemical systems with potential applications in, for example, logic-gated sensing, process-sequence checking and programmable pollutant remediation. Here, a pH-responsive compartmentalized hydrogel bead system was fabricated via coaxial microfluidic electrospray, in which gold (Au) and iron oxide (Fe<inf>3</inf>O<inf>4</inf>) nanozymes were spatially segregated into distinct domains. We systematically assessed the pH-dependent reactivity of Au and Fe<inf>3</inf>O<inf>4</inf> nanozymes between pH 2 and 9 to evaluate individual catalytic activities. Au nanoparticles (AuNPs) exhibited glucose oxidase (GOx)-like activity at pH 8–9, quantified by a cobalt‑carbonate (Co/CO₃) UV–vis assay, while Fe<inf>3</inf>O<inf>4</inf> NPs showed strong peroxidase (POD)-like activity at pH 2–3, quantified by 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) oxidation. Leveraging this pH-selective behaviour, the Au-rich domain catalyses glucose oxidation to generate hydrogen peroxide (H<inf>2</inf>O<inf>2</inf>), which then diffuses into the Fe<inf>3</inf>O<inf>4</inf>-rich domain for decomposition. Compared to free and single hydrogel system, the compartmentalized system enhances the reaction efficiency by minimizing interference between nanozymes through spatial separation.