Development of Electrochemical Immunosensors based on Self-Assembled Monolayers

Abstract

The development of an immunosensor for direct electrochemical detection of unlabeled proteins is described. Interferon-gamma is used as a model compound. This immunosensor is based upon antibodies immobilized on a self-assembled monolayer (SAM) of sulfur-containing molecules on a gold electrode. Because a reliable sensor has to be both stable and reproducible, preparation of stable SAMs was of paramount importance. To be able to measure extremely low protein concentrations, a sensing layer with a high capacitance (i.e., a thin layer) is required. Also, the sensor has to be specific and reusability is preferred. By optimization of the experimental conditions, including pretreatment of the gold, the formation of thin SAMs, and the measurement method, a tremendous improvement in the sensitivity of immunosensing has been accomplished. Using SAMs of acetylcysteine, unsurpassed detection limits, as low as 0.02 fg mL-1 (~ 1 aM) have been obtained. An overview of literature on direct electrochemical immunosensing is given, and the effect of hexacyanoferrate, a redox compound used frequently in the characterization of SAMs, on the stability of SAMs is described.