Multiplexed label-free electronic biosensors for clinical diagnostics

The development of a highly sensitive, label-free, multiplexed biosensor platform for point-of-care diagnostics is presented. The sensor surface of a non-faradaic electrochemical impedance spectroscopy (EIS) immuno-sensor platform was developed and fully characterised. Optimisation of the binding of monoclonal antibodies (mAb) towards the model target human chorionic gonadotropin (hCG) to the OEG self-assembled monolayers (SAMs) was carried out. Optimal conditions for immobilisation were found for buffer pH approximately one unit below the pI of the antibody. The same condition resulted in both higher antibody density on the sensor surface as well as higher response to the antigen. At the same time the surface showed good resistance to non-specific adsorption of proteins. Based on these principles, a biosensor to detect hCG in full serum was demonstrated. By using the phase of the impedance at 100 mHz as the sensor response, a linear relationship of the phase shift vs the logarithm of hCG concentration was established between 2.6 x 10�14 M and 2.6 x 10�10 M with a sensitivity of 0.6 degree per decade, which is a significant improvement over current state-of-the-art biosensor systems. Finally, The dielectric properties of COOH-terminated hexa(ethylene glycol)undecanethiol (OEG) and 11-mercaptoundecanol (MUD) and mixed MUD:OEG SAMs, at different ratios, were studied by means of EIS. The study demonstrates that small amounts of MUD in the mixed MUD:OEG SAMs lead to a considerable decrease of the phase of the impedance as well as a significant increase in the resistivity of the SAM at low frequencies, indicating a significant improvement of the dielectric properties. Furthermore, a considerable change in the formation of clusters of OEG molecules for mixed MUD:OEG SAMs with increasing MUD content was shown by AFM imaging.