A label-free, microfluidics and interdigitated array microelectrode-based impedance biosensor in combination with nanoparticles immunoseparation for detection of Escherichia coli O157:H7 in food samples

A microfluidic flow cell with embedded gold interdigitated array microelectrode (IDAM) was developed and integrated with magnetic nanoparticle-antibody conjugates (MNAC) into an impedance biosensor to rapidly detect pathogenic bacteria in ground beef samples. The flow cell consisting of a detection microchamber and inlet and outlet microchannels was fabricated by bonding an IDAM chip to a poly(dimethylsiloxane) (PDMS) microchannel. The detection microchamber with a dimension of 6 mm × 0.5 mm × 0.02 mm and a volume of 60 nL was used to collect bacterial cells in the active layer above the microelectrode for sensitive impedance change. MNAC were prepared by conjugating streptavidin-coated magnetic nanoparticles with biotin-labeled polyclonal goat anti-E. coli antibodies and were used in the separation and concentration of target bacteria. The cells of E. coli O157:H7 inoculated in a food sample were first captured by the MNAC, separated, and concentrated by applying a magnetic field, washed, and then suspended in mannitol solution and finally injected through the microfluidic flow cell for impedance measurement. This impedance biosensor was able to detect as low as 1.6 × 102 and 1.2 × 103 cells of E. coli O157:H7 cells present in pure culture and ground beef sample, respectively. The total detection time from sampling to measurement was 35 min. Equivalent circuit analysis indicated that the bulk medium resistance, double layer capacitance, and dielectric capacitance were responsible for the impedance change due to the presence of E. coli O157:H7 cells on the surface of IDAM. Sample pre-enrichment, secondary antibodies on the microelectrode surface, and redox probes were not required in this impedance biosensor.