Electrochemical Immunoassay for Salmonella Typhimurium Based on an Immuno‐magnetic Redox Label

We have described a sensitive voltammetric immunoassay for Salmonella Typhimurium based on the employment of a dual‐function label. Methylene blue was adsorbed onto multi‐walled carbon nanotubes (MWCNTs) at a loading of 6.5×10 −3  mol g −1 , and these modified MWCNTs were then adsorbed onto carboxylic acid‐modified Fe 2 O 3 magnetic particles of approx. diameter 1 μm. It was estimated from voltammetry that each magnetic particle delivered 2.3×10 8 molecules of methylene blue. Following adsorption of monoclonal anti‐ Salmonella antibodies onto the particles, the particles were used for the immunomagnetic separation of Salmonella cells, which were then detected in a sandwich assay using methylene blue reduction at screen printed electrodes modified with polyclonal anti‐ Salmonella antibodies. Calibration in buffer and in pasteurized milk (10 to 10 6  CFU mL −1 ) resulted in LODs of 7.9 CFU mL −1 and 17.3 CFU mL −1 respectively, without pre‐enrichment. The assay platform showed good discrimination against E. coli in both buffer and milk, and the label fabrication process was shown to be reproducible at a 95 % confidence limit as examined by analysis of variance. When stored at 4 °C the labels gave stable assay responses for a period of 60 days.