Kirkendall Effect-Mediated Transformation of ZIF-67 to NiCo-LDH Nanocages as Oxidase Mimics for Multicolor Point-of-Care Testing of β-Galactosidase Activity and <i>Escherichia coli</i>

Early and portable detection of pathogenic bacteria is crucial for ensuring food safety, monitoring product quality, and tracing the sources of bacterial infections. Moving beyond traditional plate-culture counting methods, the analysis of active bacterial components offers a rapid means of quantifying bacteria. Here, metal-organic framework (MOF)-derived NiCo-layered double hydroxide nanosheets (LDHs), synthesized via the Kirkendall effect, were employed as highly effective oxidase mimics to generate reactive oxygen species (ROS). These ROS quickly etched gold nanobipyramids (Au NBPs), producing a vivid multicolormetric response. Experimental results and theoretical calculations indicated that the exceptional oxidase-like activity of NiCo-LDHs stemmed from the presence of bimetallic active sites and oxygen vacancies modulating the local electronic structure of LDHs. Additionally, β-galactosidase (β-Gal), a biomarker of <i>Escherichia coli</i>, reacted with <i>p</i>-aminophenyl-β-d-galactopyranoside (PAPG) to form <i>p</i>-aminophenol (PAP), a reducing agent which consumes ROS, thereby inhibiting the etching of Au NBPs. Furthermore, a three-dimensional (3D)-printed point-of-care testing (POCT) shell was designed as a portable device to visually detect β-Gal and <i>E. coli</i> in conjugation with smartphones. This study not only provides a novel approach to the rational design of nanozymes but also establishes a vivid and portably visual biosensing platform for detecting β-Gal activity and pathogenic bacteria.