Layer‑resolved core-shell hydrogel for in situ fluorescence monitoring of Hg(II) adsorption and removal.

Monitoring adsorption in bifunctional materials, especially via visual readouts, can provide immediate feedback during contaminant removal. However, most bifunctional systems have performed sensing or removal in isolation within a single operation, limiting real-time information. Herein, we reported a layer-resolved core-shell hydrogel that integrated Hg(II) detection and removal and enabled real-time fluorescence (FL) monitoring of Hg(II) adsorption in water. Specifically, the highly sensitive ZIF-8-confined gold nanoclusters (gAuNCs/ZIF-8) probe with green emitting was embedded into the shell layer of hydrogel to preferentially recognize and adsorb the low concentration of Hg(II). Meanwhile, the red-emitting rAuNCs was dispersed in the inner core layer of hydrogel to detect and remove the high concentration of Hg(II). As Hg(II) diffused inward, the shell and core emissions were quenched sequentially, yielding a direct visual record of diffusion and adsorption. Coupled with a self-developed portable FL imaging device, these multicolor changes were captured throughout adsorption removal and were converted into quantitative data for accurate Hg(II) sensing (the limit of detection = 5 μg/L). In irrigation water, the hydrogel achieved > 95.85% Hg(II) removal, and the removal-efficiency feedback and quantitative detection results were in good agreement with national standard methods. This hierarchical hydrogel architecture provided a pathway to unify detection and removal in a single process and to enable in situ monitoring of adsorption.