A colorimetric immunoassay for determination of Escherichia coli O157:H7 based on oxidase-like activity of cobalt-based zeolitic imidazolate framework.

Cobalt-based zeolitic imidazolate framework nanosheets (ZIF-67) with oxidase-like catalytic activities as an immunoprobe were employed to enhance the sensitivity of an immunoassay. ZIF-67 was synthesized via the solvothermal method using 2-methylimidazole and cobalt dichloride as substrates. A colorimetric immunoassay for Escherichia coli (E. coli) O157:H7 was designed. Preparation of the immunoprobe involved self-polymerized dopamine being applied for the surface modification of ZIF-67 nanosheets in order to bind to the antibody, which was used to identify E. coli O157:H7. ZIF-67 catalyze the oxidation of 3,3',5,5'-tetramethylbiphenyl (TMB) and produced a color change from colorless to blue. Upon reaction termination, the absorbance was measured at 450 nm. By combining ZIF-67@PDA catalyzed chromogenic reaction with antibody recognition and magnetic separation, the limit of determination is 12 CFU mL and the linear range is 30 to 3.0 × 10 CFU mL. The proposed colorimetric immunoassay was successfully utilized to detect E. coli O157:H7 of spiked food samples. Graphical abstract.