pH-gated core-shell hydrogel beads for selective adsorption and spatially localized metal-free catalysis.

We report a pH-gated core-shell hydrogel bead that transitions from electrostatic molecular partitioning under low ionic strength to confined catalytic oxidation under buffered conditions. The system consists of a chitosan core and an alginate shell within which N-doped reduced graphene oxide (N-rGO) is selectively immobilized. Under low-ionic-strength media, the distinct pH-dependent ionisation of chitosan and alginate generates a charge contrast between core and shell, enabling pH-controlled partitioning of oppositely charged species. Upon transfer to buffered peroxymonosulfate (PMS) solution, ionic shielding substantially diminishes this pH dependence. Nevertheless, confinement of N-rGO within the alginate shell localizes PMS activation and shifts the optimal oxidation performance from acidic to near-neutral pH. Combined electron paramagnetic resonance (EPR) and reactive oxygen species (ROS) quenching experiments indicate that PMS activation proceeds predominantly through a 1O2-mediated pathway, while radical species coexist as secondary contributors. These findings highlight how ionic-strength-dependent charge screening and spatial confinement regulate coupled transport and catalysis in core-shell hydrogels.