Immunoassay based on urease-encapsulated metal-organic framework for sensitive detection of foodborne pathogen with pH meter as a readout.

The potential of enzyme-encapsulated metal-organic framework (MOF) as an antibody label for the construction of enzyme-linked immunosorbent assay (ELISA) is demonstrated. Zeolitic imidazolate framework-90 (ZIF-90) was employed as a MOF model to load urease and pig immunoglobulin G (IgG) antibody. This leads to the production of U@ZIF-90/IgG composite, in which urease was encapsulated in ZIF-90 to form U@ZIF-90 for amplifying the detection signal, while IgG was anchored on the surface of U@ZIF-90 for specifically recognizing Staphylococcus aureus (S. aureus). Benefiting from the unique porous structure of ZIF-90, the U@ZIF-90 not only allows urease to be encapsulated with an ultrahigh loading efficiency, but also shields the loaded urease against harsh environments. The U@ZIF-90 shows a threefold higher catalytic activity than free urease due to the confinement effect. These findings lead to an ELISA with greatly enhanced sensitivity for S. aureus detection. By using a portable pH meter as a readout, the ELISA has a linear response that covers 10 to 10 CFU/mL S. aureus with a detection limit of 1.96 CFU/mL and exhibits high selectivity over other bacteria. The successful determination of S. aureus in milk samples demonstrates the applicability of the ELISA in a complex biological matrix.