Gold nanoparticle-based dynamic light scattering immunoassay for ultrasensitive detection of Listeria monocytogenes in lettuces

Gold nanoparticle (GNP)-based dynamic light scattering (DLS) assay has been widely used for sensitive detection of small analytes based on analyte binding-induced GNP aggregation. However, the use of this new method to detect large biological objectives, such as pathogenic bacteria, has not been reported. This study is the first to describe a homogeneous GNP-based DLS immunoassay for ultrasensitive detection of Listeria monocytogenes. Compared with small analytes, L. monocytogenes has a larger surface and a higher number of antigen epitopes, which serve as carriers that bind to GNP probes to form “GNP-coated bacteria” complexes. To achieve better analytical performance, various parameters including GNP diameter and concentration, amount of labeled antibodies, and immunoreaction time were systematically investigated and optimized. Under the developed optimum conditions, limit of detection (LOD) for L. monocytogenes reached as low as 3.5×101 CFU mL−1 in 0.01 M phosphate-buffered saline. Coupled with a large-volume immunomagnetic separation method, LOD for spiked lettuce samples reached 2.2×101 CFU g−1, which was one order of magnitude lower than the maximum limit imposed in Canada (100 CFU g−1). The proposed method also exhibited excellent discrimination against 17 common pathogenic bacteria in lettuces. The developed GNP-based DLS immunoassay is highly promising as an approach for detecting large biological objectives.