The selective and sensitive detection of transition and coinage metal ions remains a formidable challenge in environmental and biomedical analysis. We introduce 2-(2-(allyloxy)benzoyl)--ethylhydrazine-1-carbothioamide (S3), a novel thiosemicarbazone-based soft-donor ligand, as a dual-mode UV-Vis chemosensor that discriminates between Fe and Ag under pH control. The chemical structure of the newly synthesized ligand S3 was confirmed by elemental analysis, FT-IR, UV-Vis, and NMR spectroscopy. Upon titration with Fe in NHCl/NH buffer (pH 10), a broad ligand-to-metal charge-transfer envelope develops across 450-650 nm with a bathochromic shift of from ∼540 to ∼620 nm. Hill analysis ( = 1.16, = (2.1 ± 0.3) × 10 M) confirms predominantly 1 : 1 (L : M) binding with slight positive cooperativity. The sensor exhibits excellent linearity ( = 0.9997) in the low-ppm range, with a limit of detection (LOD) of 0.12 ppm and limit of quantification (LOQ) of 0.35 ppm. In contrast, under acidic conditions (acetate buffer, pH 4), Ag addition triggers a sharp new charge-transfer band at ∼400 nm. The sigmoidal response reflects stronger cooperativity (Hill coefficient = 2.35), consistent with 1 : 1 binding, enabling quantification over 0.2-12 ppm with a LOD/LOQ of 0.07/0.23 ppm. solution adducts give clean CT-dominated spectra without resolved d → d transitions. S3 thus serves as a versatile, high-performance, pH-orthogonal probe for Fe (alkaline) and Ag (acidic), rivaling or exceeding recent thiosemicarbazone sensors. Overall, S3 stands out as a versatile, pH-switchable optical probe that achieves selective, high-sensitivity detection of Fe in alkaline media and Ag in acidic media, performance that matches or surpasses recently reported thiosemicarbazone-based sensors.