Rapid and specific quantitation of proteins is desirable for assays used at the point-of-care, as well as in clinical and research laboratories. The advantages of such devices include short turnaround times for acquiring critical data, low cost, and improved patient compliance with diagnosis and therapeutic regimens. Electrochemical detection is currently employed in a wide range of biosensors because of its inherent signal stability, high sensitivity, and ease of calibration. Our laboratories have developed the electrochemical proximity assay (ECPA) for protein detection, a sensitive and selective assay based on the electrochemical detection of surface-confined affinity complexes. ECPA demonstrates extremely high sensitivity (e.g., 10 fM for insulin).

Bipolar electrodes (BPEs) have been used for decades in industrial applications ranging from organic electrosynthesis to catalysis. Recently, sensor platforms have been reported in which electrochemiluminescence (ECL) is used to readout the state of a bipolar sensor operating under steady state conditions. In these measurements ECL intensity is directly proportional to the analyte concentration. In contrast to these conventional amperometric sensors, the ECPA signal is based on a surface limited reaction, and therefore does not lend itself to such a straightforward readout strategy. In this paper, we describe the development of an ECPA based on a bipolar platform in which a square wave voltage program is employed both to interrogate and reset the redox state of the surface confined affinity complex. Signal averaging over multiple cycles allows us to detect proteins at low concentration levels.