: We report a fluorescence biomimetic sensor that integrates carbon dots (CDs) and CdTe quantum dots (QDs) for the rapid, antibody-free, and aptamer-free detection of Escherichia coli O157:H7 (E. coli O157:H7) in water samples. This biosensor operates on a ratiometric principle, leveraging green-emitting CDs (GCDs) as a dynamic signal with the red-emitting QDs (RQDs) as a stable reference that significantly enhances the sensor's sensitivity, enabling straightforward visual detection of bacterial contamination. Selectivity was achieved by conjugating mannosea natural biomimetic receptor to GCDs, allowing specific recognition of FimH proteins on E. coli O157:H7. Upon bacterial binding, the green emission at 508 nm intensifies proportionally to bacterial concentra tion, while the red fluorescence at 694 nm remains unchanged. This ratiometric biomimetic sensor detects bacterial concentrations ranging from 101 to 108 CFU/mL within 30 min. The biosensor provides a simple, on-site, naked-eye detection under a portable blue-LED light, with color changes indicating water safety status: green for high bacterial loads, yellow/ orange for moderate contamination, and red for safe water. It was successfully applied to tap water, bottled mineral water, surface and groundwater, as well as to samples collected at multiple stages of water treatment plants (12 sources; five replicates). Comprehensive characterization of the nanomaterials was performed using Fourier-transform infrared spectrosc
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