A redox-coupled carbon dots-MnO2 nanosheets based sensory platform for label-free and sensitive detection of E. coli

[Display omitted] •CDs-MnO2 FRET-conjugate used for the first time for label-free detection of E. coli.•A unique enzymatic redox-relay reaction ensures selective sensing of E. coli.•The LOD of 50 CFU/mL of E. coli is the lowest among fluorescent sensors.•The sensing tool is validated for E. coli detection in rainwater, food samples.•Smartphone based image capturing and visual detection of E. coli on test-strips. An enzymatic redox reaction was exploited in developing a novel carbon dots (CDs)-manganese dioxide (MnO2) nanosheets based fluorescent sensing platform for sensitive, selective, and rapid detection of Escherichia coli (E. coli) in aqueous media. The detection of E. coli is achieved by rationally placing a relay of redox reactions mediated by enzyme and a redox couple of p-benzoquinone (BQ) and hydroquinone (HQ). Initially, CDs-MnO2 forms a FRET conjugate and the inherent fluorescence of CDs remains in the turn-off state. Presumably, the enzymes present in E. coli respiratory pathway (such as NADH-quinone reductase), converts BQ to HQ by two-electron reduction, which in turn reacts with MnO2 nanosheets to reduce it to free Mn2+ ions. As MnO2 nanosheets disintegrate, the CDs are released from electrostatic interaction and go to a disperse phase in the aqueous solution to emit blue fluorescence. Thus, it devises a label-free sensing assay for the detection of live E. coli. The LOD was determined as 50 CFU/mL for E. coli. The utility of this cost-effective sensing tool was demonstrated in real-sample analysis and quantitation of E. coli was done in rainwater and food samples. Further, the fluorescent images of bacterial solutions and agar-polymer based test-strips were captured on a smartphone and analyzed using ImageJ for a practical demonstration of on-site detection of E. coli. The present method with merits like cost-effective synthesis from biomass, rapid detection, low detection limit, high selectivity offers a viable alternative to existing detection tools for on-site detection and quantitation of E. coli.