Endotoxin detection in a competitive electrochemical assay: Synthesis of a suitable endotoxin conjugate

A biotin–lipopolysaccharide (biotin–LPS) conjugate was synthesized from LPS smooth from Salmonella minnesota, yielding a conjugate with a biotin/LPS ratio equal to 1:1 and endotoxic activity of 0.08 EU ng−1. The conjugate was used in an amperometric competitive assay to determine endotoxins with endotoxin-neutralizing protein (ENP) as the recognition element. The assay is performed on a modified electrode, involving the covalent binding of carboxymethyl dextran (CMDex) to a cystamine-modified gold electrode and then the covalent binding of the recognition protein, ENP, to CMDex. The assay is carried out by incubating the modified electrode in an LPS sample to which biotin–LPS was added. Both species compete for the recognition sites on the modified surface. After the incubation stage and a careful rinsing, the electrode is immersed in a solution containing neutravidin–horseradish peroxidase conjugate (N-HRP), which binds to the sites containing biotin–LPS on the electrode. The system is rinsed and a current signal is generated by the addition of hydrogen peroxide and a redox mediator. The assay is able to detect LPS from Salmonella minnesota at concentrations as low as 0.1 ng ml−1, equivalent to 0.07 EU ml−1.