Rapid, visual and autocatalytic quantifying Ag(I) and Fe(Ⅲ) by ratiometric fluorescence sensor of N, Si, S-GQDs/OPD

N, Si, S co-doped graphene quantum dots (N, Si, S-GQDs) were synthesized from waste toner and glutathione (GSH) via a one-pot hydrothermal method. Combined with o-phenylenediamine (OPD), a rapid, visual, and autocatalytic ratiometric fluorescence sensor of N, Si, S-GQDs/OPD was fabricated for detecting Ag(I) and Fe(III) ions by producing 2,3-diaminophenazine (oxOPD) under pH 8, which emits yellow fluorescence at 560 nm while quenching the blue fluorescence of N, Si, S-GQDs at 440 nm due to an inner filter effect. With the increase of Ag(I)/Fe(III), the blue fluorescence of N, Si, S-GQDs at 440 nm was gradually weakened, along with the enhancement of yellow fluorescence at 560 nm. Hence, this color change from blue to yellow under UV light enables semi-quantitative visual detection. The sensor demonstrates high sensitivity with detection limits of 0.016 µg mL^-1 for Ag(I) and 0.010 µg mL^-1 for Fe(III), and it successfully detects these ions in lake and tap water without pretreatment. The autocatalytic mechanism involves Ag(I) and Fe(III) reduction to Ag nanoparticles and Fe(II), respectively, which further catalyze the reaction, enhancing selectivity and efficiency. The method is cost-effective, simple, and suitable for on-site environmental monitoring.