QCM immunosensor detection of Escherichia coli O157:H7 based on beacon immunomagnetic nanoparticles and catalytic growth of colloidal gold

In this paper, we describe a novel method for detecting Escherichia coli (E. coli) O157:H7 by using a quartz crystal microbalance (QCM) immunosensor based on beacon immunomagnetic nanoparticles (BIMPs), streptavidin-gold, and growth solution. E. coli O157-BIMPs were magnetic nanoparticles loaded with polyclonal anti-E. coli O157:H7 antibody (target antibody, T-Ab) and biotin-IgG (beacon antibody, B-Ab) at an optimized ratio of 1:60 (T-Ab:B-Ab). E. coli O157:H7 was captured and separated by E. coli O157-BIMPs in a sample, and the streptavidin-gold was subsequently conjugated to E. coli O157-BIMPs by using a biotin-avidin system. Finally, the gold particles on E. coli O157-BIMPs were enlarged in growth solution, and the compounds containing E. coli O157:H7, E. coli O157-BIMPs, and enlarged gold particles were collected using a magnetic plate. The QCM immunosensor was fabricated with protein A from Staphylococcus aureus and monoclonal anti-E. coli O157:H7 antibody. The compounds decreased the immunosensor's resonant frequency. E. coli O157-BIMPs and enlarged gold particles were used as "mass enhancers" to amplify the frequency change. The frequency shift was correlated to the bacterial concentration. The detection limit was 23 CFU/ml in phosphate-buffered saline and 53 CFU/ml in milk. This method could successfully detect E. coli O157:H7 with high specificity and stability. The entire procedure for the detection of E. coli O157:H7 took only 4 h.