A microfluidic impedance biosensor based on immunomagnetic separation and urease catalysis for continuous-flow detection of E. coli O157:H7

•This biosensor could accurately measure the impedance in continuous-flow condition.•The impedance normalization could reduce the impact from microelectrode variation.•This biosensor was able to detect E. coli O157:H7 as low as 12 CFU/mL within 2 h.•Urease catalysis was used to greatly amplify the detection signal. Early screening of foodborne pathogens is a key to control the outbreaks of foodborne illnesses. In this study, a microfluidic impedance biosensor combined with the immune magnetic nanoparticles (MNPs) for bacteria separation, the urease for biological signal amplification and the microfluidic chip for impedance measurement was developed for rapid, sensitive and continuous-flow detection of E. coli O157:H7. After the streptavidin modified MNPs were conjugated with the biotinylated polyclonal antibodies (PAbs) to form the immune MNPs, the target bacteria was first separated by the MNPs from the background to form the MNP-bacteria complexes. Then, the gold nanoparticles (GNPs) modified with the urease and the aptamers against E. coli O157:H7 were conjugated with the MNP-bacteria to form the MNP-bacteria-GNP-urease complexes. Finally, the complexes were used to catalyze the hydrolysis of urea into ammonium carbonate, leading to the decrease in the impedance. The impedance was online measured by this proposed biosensor and analyzed using the impedance normalization to determine the concentration of E. coli O157:H7. A good linear relationship between the impedance relative change rate of the catalysate and the concentration of the bacteria was obtained with low detection limit of 12 CFU/mL.