Duplex detection of foodborne pathogens using a SERS optofluidic sensor coupled with immunoassay

Herein, we developed a surface-enhanced Raman spectroscopy (SERS) optofluidic sensor coupled with immunoprobes to simultaneously separate and detect the foodborne pathogens, Escherichia coli O157:H7, and Salmonella in lettuce and packed salad. The method consists of three steps of (i) enrichment to enhance detection sensitivity, (ii) selective separation and labelling of target bacteria by their specific antibody-bearing SERS-nanotags and (iii) detection of tagged bacterial cells using SERS within a hydrodynamic flow-focusing SERS optofluidic device, where even low counts of bacterial cells were detectable in the very thin-film-like sample stream. SERS-nanotags consisted of different Raman reporter molecules, representing each species, i.e., the detection of Raman reporter confirms the presence of the target pathogen. The anti-E. coli antibody used in this study functions against all strains of E. coli O157:H7 and the anti-Salmonella antibody used in this work acts on a wide range of Salmonella enterica strains. Bacterial counts of 1000, 100, and 10 CFU/ 200 g sample were successfully detected after only 15 min enrichment. Our method showed a very low detection limit value of 10 CFU/ 200 g sample for the bacterial mixture in both lettuce and packed salad, proving the efficiency and high sensitivity of our method to detect multiple pathogens in the food samples. The total analysis time, including sample preparation for simultaneous detection of multiple bacteria, was estimated to be 2 h, which is much less than the time required in conventional methods. Hence, our proposed protocol is considered a promising rapid and efficient approach for pathogen screening of food samples.