Ratiometric Biomimetic Sensor Based on Quantum Dots–Enhanced Glycosylated Carbon Dots for Visual Detection of <i>Escherichia coli</i>

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 <i>Escherichia coli</i> <i>O157:H7</i> (<i>E. coli</i> <i>O157:H7</i>) 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 mannose─a natural biomimetic receptor─ to GCDs, allowing specific recognition of FimH proteins on <i>E. coli</i> <i>O157:H7</i>. Upon bacterial binding, the green emission at 508 nm intensifies proportionally to bacterial concentration, 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 spectroscopy (FTIR), UV-vis absorption, fluorescence spectroscopy, transmission electron microscopy (TEM), dynamic light scattering (DLS), and zeta potential analysis. This portable, cost-effective biomimetic sensor provides a rapid, reliable alternative to conventional microbiological methods for monitoring drinking water.