Fast and Highly Sensitive Detection of Pathogens Wreathed with Magnetic Nanoparticles Using Dark-Field Microscopy.

Cryptosporidium parvum ( C. parvum) is a highly potent zoonotic pathogen, which can do significant harm to both human beings and livestock. However, existing technologies or methods are deficient for rapid on-site detection of water contaminated with C. parvum. Better detection approaches are needed to allow water management agencies to stop major breakouts of the pathogen. Herein, we present a novel detection method for cryptosporidium in a tiny drop of sample using a magnetic nanoparticle (MNP) probe combined with dark-field microscopy in 30 min. The designed MNP probes bind with high affinity to C. parvum, resulting in the formation of a golden garland-like structure under dark-field microscopy. This MNP-based dark-field counting strategy yields an amazing PCR-like sensitivity of 8 attomolar (aM) (5 pathogens in 1 μL). Importantly, the assay is very rapid (∼30 min) and is very simple to perform as it involves only one step of mixing and magnetic separation, followed by dropping on a slide for counting under dark-field microscope. By combining the advantages of the specific light-scattering characteristic of MNP probe under dark field and the selective magnetic separation ability of functionalized MNP, the proposed MNP-based dark-field enumeration method offers low cost and significant translational potential.