A fluorescent, biocompatible natural product, quinine (commercially available at low-cost), has been employed for ratiometric, dual-mode sensing of diethyl chlorophosphate (DCP, a sarin surrogate) in the aqueous medium (using pH 7.0 buffer). The addition of DCP induces a rapid (< 30s) change in solution color from colorless to yellow, accompanied by an appearance of intense blue-colored fluorescence (detection limit: 18 nM). Mechanistic investigation indicates phosphorylation of the hydroxyl unit of quinine in the presence of DCP, followed by protonation at the quinoline nitrogen end. The chemodosimetric adduct, so formed, possesses a conformationally-locked structure with a significantly different charge-transfer state as compared to the parent molecule. Such a bifunctional labeling reaction is found to be highly selective towards DCP, no other warfare chemicals, including diethyl cyanophosphate (DCNP, a tabun surrogate) could render any detectable interference. Further, a solid-supported sensing device has been designed as a sustainable strategy for on-site vapor phase detection of DCP.