<p>Despite the potential of AgInGaS₂ (AIGS) quantum dots (QDs) as eco-friendly alternatives to cadmium-based emitters, achieving narrow intrinsic luminescence in aqueous phases remains a tough challenge, due to their surface vulnerability and the prevalence of broad defect-related emissions. This limitation significantly impedes their application in high-precision, multi-channel sensing where spectral resolution is critical. We report AIGS/ZnS QDs synthesized via oil-phase thermal injection, which were successfully transferred into water phase through DSPE-PEG-N₃ self-assembly. The resulting AIGS-PEG nanospheres represent the first aqueous sensing application of intrinsic AIGS QD luminescence, featuring an exceptionally narrow full-width at half maximum (FWHM) of 34&#xa0;nm. These nanospheres exhibit excellent selectivity and sensitivity towards Cu²⁺ and Ag⁺ ions. To realize portable applications, the probe was further integrated into a polyacrylamide hydrogel (AIGS-PAM). This integration lowered the limit of detection (LOD) for Cu²⁺ sensing to 88 nM, and LOD for Ag⁺ remained comparable to that of the AIGS-PEG nanospheres aqueous solution (446 nM). Notably, the system enables multimodal visual differentiation of Cu²⁺ and Ag⁺ through distinct fluorescence quenching and spectral shift patterns. By reconciling high spectral purity with aqueous stability, this work provides a robust surface engineering strategy for non-toxic QDs and establishes a promising, solid-state platform for advanced point-of-care testing (POCT).</p> Graphical Abstract <p></p>