Immunomagnetic separation and size-based detection of O157 at the meniscus of a membrane strip.

We developed a facile method for the detection of pathogenic bacteria using gold-coated magnetic nanoparticle clusters (Au@MNCs) and porous nitrocellulose strips. Au@MNCs were synthesized and functionalized with half-fragments of O157 antibodies. After the nanoparticles were used to capture O157 in milk and dispersed in a buffer solution, one end of a test strip was dipped into the solution. Due to the size difference between the -Au@MNC complexes (approximately 1 μm) and free Au@MNCs (approximately 180 nm), only -Au@MNC complexes accumulated at the meniscus of the test strip and induced a color change. The color intensity of the meniscus was proportional to the concentration, and the detection limit for in milk was 10 CFU mL by the naked eye. The presence of -Au@MNC complexes at the meniscus was confirmed using a real-time PCR assay. The developed method was highly selective for when compared with , , and .