Fluorescent and Magnetic Relaxation Switch Immunosensor for the Detecting Foodborne Pathogen Salmonella enterica in Water Samples

Fluoroimmunoassay based on quantum dots(QDs) and magnetic relaxation switch (MRS) immunoassay based on superparamagnetic nanoparticles(SMN) were constructed to detect Salmonella enterica(S. enterica) in water samples. In fluoroimmunoassay, magnetic beads was conjugated with S. enterica capture antibody (MB-Ab2) to enrich S. enterica from sample solution, then the QDs was conjugated with the S. enterica detection antibody (QDs-Abl) to detect S. enterica based on sandwich immunoassay format. And the fluorescence intensity is positive related to the bacteria concentration of the sample. Results showed that the limit of detection (LOD) of this method was 10(2) cfu . mL(-1). and analysis time was 2 h. In MRS assay, magnetic nanoparticle-antibody conjugate (MN-Ab1) can switch their dispersed and aggregated state in the presence of the target. This state of change can modulate the spin-spin relaxation time (T-2) of the neighboring water molecule. The change in T-2 (Delta T-2) positively correlates with the amount of the target in the sample. Thus, Delta T-2 can be used as a detection signal in MRS immunosensors. Results showed that LOD of MRS sensor for S. enterica was 10(8) cfu . mL(-1) and analysis time was 0.5 h. Two methods were compared in terms of advantages and disadvantages in detecting S. enterica.