This paper discusses the utilization of water stable, biocompatible CsPbBr₃ perovskite quantum dots (BPQDs) as donors in combination with Rhodamine 6G (R6G) as acceptors, as ratiometric probe, for detection of pathogens and water contaminants via the fluorescence resonance energy transfer (FRET) mechanism. Biocompatibility of BPQDs was attained through phase engineering using succinic acid (SA) ligand which at appropriate pH provides carboxylic environment on the outer surface, facilitating transfer to aqueous phase. Photoluminescence (PL) studies on ratiometric probe reveal quenching of BPQDs’ emission with a simultaneous enhancement of R6G fluorescence, representing a singlet energy transfer. The probe was successfully validated for ratiometric sensing of a model carcinogen (thioacetamide (TAA)) and a model pathogen (Escherichia coli (E. coli)) and the detection limit achieved was 1.8 CFU/mL for E. coli sensing and 1.5 µM for TAA sensing. Dual emission and its quenching mechanism involved in the detection of aforesaid analytes have been elaborately discussed and elucidated, indicating that quenching mechanisms are different for the two analytes. Our work not only demonstrates an important milestone for the integration of perovskite quantum dot in clinical diagnostics but also as an easy, unique, rapid and accurate E. coli detection technique that is complimentary to the rather complicated high throughput and high-sensitivity approaches that are existing.