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 Fe₂O₃ magnetic particles of approx. diameter 1 μm. It was estimated from voltammetry that each magnetic particle delivered 2.3×10⁸ 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 10⁶ 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.