A quantitative foam immunoassay for detection of Escherichia coli O157:H7 based on bimetallic nanocatalyst‑gold platinum

Inspired by the very popular demonstration, Elephant's Toothpaste, we developed a cost-effective, portable foam immunoassay for quantitative detection of Escherichia coli O157:H7 (E. coli O157:H7) by measuring foam volume changes using a low-cost ruler. This quantitative immunoassay for E. coli O157:H7 is built by integrating a catalyzed foam-generation reaction with pathogen recognition component. E. coli O157:H7 was enriched and captured by monoclonal antibody to E. coli O157:H7 (mAb1) modified Fe3O4 magnetic nanoparticles (MNP). The peroxidase-like nanozymes gold and platinum nanoparticles (Au@Pt NP) were loaded on silica nanoparticle (SiO2 NP), forming Au@Pt/SiO2 NP. The Au@Pt/SiO2 NP were functionalized by antibody against E. coli O157:H7 (mAb2-Au@Pt/SiO2 NP) and used as signal labels. In the final sandwich immune complex, Au@Pt NP catalyzes the hydrolysis of hydrogen peroxide (H2O2) to produce oxygen (O2). In the presence of sodium dodecyl sulfate (SDS), O2 is trapped by SDS forming thick foam. The foam rises in an acrylic tube and produces sensitive height readout. There is a linear relationship between the foam height and the logarithm of E. coli O157:H7 concentration. Under the optimum conditions, the detection limit was 2.16 × 102 CFU·mL−1 (3σ) in the range of 1.19 × 103–1.19 × 107 CFU·mL−1. This method demonstrates good practical application potential during detection of E. coli O157:H7 in milk samples. This new signaling strategy opens up a new way for portable, simple and sensitive on-site analysis in various environments.