One-step colorimetric immunosensing of foodborne bacteria using static magnet driven magnetophoretic separation and dual noble metal decorated magnetic nanobeads.

Background: Immunomagnetic separation is essential for screening pathogenic bacteria to prevent food poisoning. However, free immunomagnetic nanobeads (IMNBs) coexist with IMNB-bacteria conjugates (IBCs) after traditional immunomagnetic separation resulting in the infeasibility for IMNBs on IBCs to further act as signal label in bacterial detection. Although we have demonstrated that magnetophoretic separation at a high flowrate could separate IBCs from IMNBs, partial IMNBs were still found with IBCs due to chaotic flows and resulted in inevitable interferences. Thus, this study developed a new magnetophoretic separation strategy at a low flowrate under a static ring magnetic field for separating IBCs from IMNBs more thoroughly.

Results: This new magnetophoretic separation strategy was verified for precise sorting of the commercial MNBs with the original wide range of 110-330 nm to obtain the uniform MNBs with a narrow range of 143-193 nm. The decoration of Pd and Pt nanoparticles onto the MNBs was demonstrated with no impact on their capture of target bacteria because they could capture over 90 % target bacteria. Under the optimal conditions, this immunosensor could quantitatively detect Salmonella from 3.4 × 10 to 3.4 × 10 CFU/mL in 40 min with a detection limit of 34 CFU/mL. Besides, this immunosensor had shown a good specificity to Salmonella and a good practicability for detection of Salmonella in the chicken samples.

Significance: This immunosensor only needed one kind of antibody, making it much easier to be extended for other bacterial detection, and only required one-step immunoreaction, making the detection time much shorter, and employed magnetic nanobeads decorated with platinum and palladium nanoparticles, making the detection sensitivity much higher.