Enzyme–Nanozyme Cascade Reaction-Mediated Etching of Gold Nanorods for the Detection of Escherichia coli

Species-specific enzyme detection refers to an important modality in the field of monitoring foodborne pathogens. Particularly, β-galactosidase (β-gal) has been extensively employed for ascertaining the concentration of Escherichia coli (E. coli). The major challenges facing the recent works include improving the accessibility, reducing the analysis time, and increasing the sensitivity of the detection techniques. In the current study, a multicolor colorimetric platform involving enzyme–nanozyme cascade reaction-triggered etching of gold nanorods (Au NRs) was devised for ultrafast and ultrasensitive detection of E. coli. In an acidic solution, MnO2 nanosheets with the oxidase-mimicking activity catalyzed the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) to TMB2+. This was followed by rapid etching of Au NRs, resulting in a notable blue shift of the longitudinal local surface plasmon resonance peaks and polychromatic changes of Au NRs. In the presence of E. coli, a cascade reaction was initiated through β-gal hydrolysis of p-aminophenyl β-d-galactopyranoside (PAPG) to generate p-aminophenol (PAP). Subsequently, PAP mediated the reduction of the MnO2 nanosheets, destroying their oxidase-mimicking activity, thereby affecting the amount of TMB2+ generated. Consequently, with different concentrations of E. coli, the sensing system exhibiting different colors can be easily observed. The total assay time was 37 min, which demonstrates the efficiency of the described approach. Moreover, the improvement in the sensitivity was realized through the enzyme–nanozyme cascade reaction, providing a highly promising method for monitoring E. coli bacteria.