Colorimetric and handheld pH meter dual-signal readout platform for E. coli detection based on a cascade reaction

A dual-signal readout has been designed detecting platform based on a cascade reaction for Escherichia coli (E. coli) detection by using colorimetric approach and a handheld pH meter. The immunoreaction was conducted using polydopamine@copper ferrite-Ag nanoparticles (PDA@CuFe₂O₄-Ag NP) and a glucose oxidase (GOD)-conjugated graphene oxide-gold nanosheet composite (GOD-GO/Au NS) to synthesize a sandwich complex mode between targets. Together with the formation of immune complexes, the GOD-GO/Au NS can catalyze glucose to produce gluconic acid and hydrogen peroxide (H₂O₂). The gluconic acid produced altered the pH of the detection solution. Since the PDA@CuFe₂O₄-Ag NP have good peroxidase-like activity, they can catalyze the oxidation of TMB to the blue product oxTMB once H₂O₂ is produced in the reaction system, and the absorbance change of oxTMB at 652 nm can be recorded using ultraviolet-visible (UV-Vis) spectroscopy. Interestingly, the PDA@CuFe₂O₄-Ag NP composites can consume the generated H₂O₂, and can create a reaction cycle that promotes glucose oxidation. Under optimal conditions, the proposed dual-channel signal platform is proportional to the logarithm of the E. coli concentration within a range of 10²-10⁷ cfu mL^-1. Additionally, the devised approach was successfully used to detect E. coli at the required levels in real samples. This dual-mode detection method notably enhances the accuracy and diversity of detection, and curbs the false negative and positive rates.