Rapid detection of Salmonella using a redox cycling-based electrochemical method

An electrochemical method based on redox cycling combined with immunomagnetic separation and pre-concentration was developed for rapid and sensitive detection of Salmonella. Electrochemical methods for the detection of bacteria offer the advantages of instant quantification with minimal equipment. Unfortunately, the limits of detection are often poor compare to other transduction methods such as fluorescence and chemiliuminescence. We demonstrated an electrochemical method which is both rapid and has a low limit of detection. A two-step strategy, which included immunomagentic pre-concentration and redox cycling was used to amplify the signal. Magnetic beads modified with anti-Salmonella antibodies were used for separation and pre-concentration of Salmonella from phosphate buffered saline (PBS) and agricultural water. Then anti-Salmonella antibodies conjugated with alkaline phosphatase were employed for labeling the Salmonella which had been captured by magnetic beads. Alkaline phosphatase (ALP) catalyzed the substrate l-ascorbic acid 2-phosphate (AAP) to electroactive species l-ascorbic acid (AA) while tris(2-carboxyethyl)phosphine (TCEP) facilitated the regeneration of AA on the gold electrode to form redox cycling resulting in an amplified signal. Under the optimal conditions, the Salmonella in PBS buffer as well as in agricultural water were detected. The limit of detection of this approach was approximately 7.6 × 102 CFU/mL and 6.0 × 102 CFU/mL in PBS buffer and agricultural water, respectively, without pre-enrichment in 3 h. When the agricultural water has been pre-enriched for 4 h, the limit of detection was approximately 10 CFU/mL.