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In this study, a fast and sensitive sandwich assay has been developed with the combination of immunomagnetic separation (IMS) and fluorescence techniques to enumerate Escherichia coli (E.coli). Iron oxide core gold shell (Fe₃O₄@Au) magnetic nanoparticles were prepared and modified with biotinylated antibodies specific to E.coli. Fluorescence labels have been constructed by preparing chitosan coated CdTe quantum dots (CdTe QDs) with a diameter of 3 ± 1 nm. The amounts of magnetic nanoparticles and CdTe QDs are optimized to get the best sensitivity. The calibration graph fluorescence intensity versus E.coli concentration was linear in the range of 10²–10⁸ cfu mL⁻¹ with a coefficient of determination (R²) of 0.9905. The limit of detection (LOD) and limit of quantification (LOQ) values are calculated as 30 and 100 cfu mL⁻¹, respectively. Selectivity of the method is examined by applying the same …