Magnetically tunable catalytic chitosan-agarose hydrogels for green H₂O₂ sensing: A sustainable nanozyme platform

Hydrogen peroxide (H₂O₂) is a key reactive species in biological, environmental, and industrial systems, creating a demand for robust, sustainable, and tunable sensing materials. Here, we report a green, simple in situ strategy to fabricate magnetically responsive catalytic chitosan-agarose hydrogels with tunable nano-microstructure and transport properties. Fe₃O₄ nanoparticles are generated directly within chitosan-agarose hydrogel networks using chloride- and sulfate-based iron precursors, enabling controlled modulation of nanoparticle size distribution, crystallographic plane exposure, and magnetic response without surfactants or organic solvents. We demonstrate that the choice of iron precursor governs nanoparticle nucleation kinetics and interfacial interactions with the biopolymer matrix, leading to distinct microstructural architectures that directly impact mechanical stiffness, swelling behavior, and magnetic properties. Importantly, the embedded Fe₃O₄ nanoparticles exhibit intrinsic peroxidase-like catalytic activity, allowing the hydrogels to function as a solid-state nanozyme platform for colorimetric H₂O₂ detection. Beyond static performance, we show that external magnetic fields dynamically reconfigure hydrogel properties providing reversible, field-controlled regulation of catalytic activity and sensing sensitivity. The optimized hydrogels display linear H₂O₂ detection up to 100 μM with sub-micromolar detection limits (< 1 μM), while offering key materials-level advantages including reusability, structural stability, and facile magnetic manipulation. This work establishes a simple approach for integrating nanozyme catalysis, magnetic actuation, and sustainable biopolymer matrices, offering a broadly applicable platform for magnetically tunable catalytic materials and responsive sensing systems.