Functionalized polymeric magnetic nanoparticle assisted SERS immunosensor for the sensitive detection of S. typhimurium.

The quest for detecting bacteria has gained momentum in food and beverage industry for preventing spoilage of products to maintain requisite quality. The present paper describes the development of a SERS immunosensor for the detection of model pathogen, S. typhimurium using strategically synthesized functionalized polymeric magnetic nanoparticles (FPMNPs) as effective capture probe and immunomagnetic separator. The synthesized probe contains surface diketonic functionalities which covalently link with amino groups of antibodies against Salmonella common structural antigen (CSA-1-Ab) and hence specifically captured the target bacteria. Magnetic core of nanoparticles facilitated easy separation of target bacteria from the milieu of non-specific molecules. Gold nanoparticles (GNPs) modified with CSA-1-Ab and external Raman reporter molecules (RRM) were used as signal probes. We compared the signalling attributes of 4-mercapto benzoic acid (MBA) and 5,5'-dithiobis(succinimidyl-2-nitrobenzoate) (DSNB) as RRMs. Capture and signal probes sandwich the target bacteria upon its addition, generating Raman signal from the 'hot-spots' created by signal probe. Under optimal conditions, the SERS intensities of MBA and DSNB at 1588 and 1336 cm respectively were used to measure the concentration of the pathogen in the range of 10-10 cells mL. Limit of detection (LOD) of MBA and DSNB based immunosensor was measured as 100 cells mL, and 10 cells mL respectively. Moreover, appreciable recovery (82-114%) was recorded for sensing method for different spiked food products. Thus, the developed magnetically assisted SERS immunosensor is sensitive, specific and has strong potential to be used for detecting contamination in food samples in field conditions.