CuFe2O4 magnetic particles assisted construction of a label-free fluorescent sensor for detection of Escherichia coli

An economical and label-free fluorescent sensor was developed for the selective detection of Escherichia coli (E. coli) based on the competitive reduction of Cu2+ in CuFe2O4 magnetic particles (MPs) by E. coli and o-phenylenediamine (OPD). Cu2+ in CuFe2O4 MPs can oxidize OPD to generate 2,3-diaminophenazine (OPDox) with fluorescent property. The presence of E. coli can weaken the oxidative ability of CuFe2O4 MPs toward OPD owing to the copper reductase activity of NADH dehydrogenase-2 (NDH-2) in E. coli. NDH-2 can reduce Cu2+ to Cu+, thereby inhibiting the OPD oxidation and resulting in the decrease of the fluorescence of the system. Under the optimum conditions, the fluorescent sensor exhibited a wide linear response to E. coli in the concentration range of 103 – 106 cfu mL−1, and the detection limit was calculated to be 5.8 × 102 cfu mL−1. Furthermore, the feasibility of the sensor was verified by successful detection of E. coli in popsicle and swimming pool water samples. More importantly, the sensor utilized the magnetic property of CuFe2O4 MPs to realize the phase separation, thus averting the interference of other impurities in the sample matrix.