Rapid detection of Listeria monocytogenes using quantum dots and nanobeads-based optical biosensor

A rapid, specific method was developed to detect Listeria monocytogenes using magnetic nanobeads to separate and concentrate the target bacteria and quantum dots (QDs) as fluorescent markers. QDs are semiconductor nanocrystals which measure from a few nanometers to a few hundred nanometers, and are particularly significant in optical detections due to their high quantum yield. Both streptavidin conjugated QDs 605 and magnetic nanobeads were separately coated with specific biotin-conjugated anti-L. monocytogenes antibody. The conjugated magnetic nanobeads were then mixed with L. monocytogenes culture dilutions. After immunomagnetic separation, the magnetic nanobeads-L. monocytogenes conjugates were mixed with conjugated QDs. Unattached conjugated QDs were removed using immunomagnetic separation. A fluorescence spectrometer was used to measure the fluorescence of the complexes of magnetic beads-L. monocytogene-QDs. Results indicated that this method could detect L. monocytogenes at a concentration of 2-3 cfu/mL in a pure culture. A linear co-relationship was found between the fluorescence intensity and L. monocytogenes in a range of 10(0)-10(7) cfu/mL. The total detection time was 1.5 h.