An Incremental Double-Layer Capacitance of A Planar Nano Gap and Its Application in Biosensors

Surface potential is one of the most important properties at solid-liquid interfaces. It can be modulated by the voltage applied on the electrode or by the surface properties. Hence, surface potential is a good indicator for surface modifications, such as biomolecular bindings. In this dessertation, we proposed to use a planar nano-gap structure to monitor surface-potential difference and biomedical application. Based on the proposed architecture, the variance of surface-potential difference can be determined by electrical double layer capacitance (EDLC) between the nano-gap electrodes. In this work, we used electrochemical impedance spectroscopy to demonstrate the relationship between surface potential and EDLC by chemically modifying surface properties and by physically tuning the gap width. Then, we proposed two pathway equivalent circuit model to explain the relationship. One pathway is away from the surface which is independent of gap surface potential and the other one is near the surface which is dependent of gap surface potential. Further, buffer concentration experiment proved gap surface potential modulated near surface pathway by contracting electrodes debye length. Next, we showed the proposed planar nano-gap device provide the capability for cardiac-troponin T (cTnT) measurements with co-existed 10 µg/ml BSA interference by using cyclic voltammetry. cTnT is one of the most important biomarker of myocardial necrosis. This detection is by monitoring of surface-potential variation and differs from traditional capacitive biosensors. The detection dynamic range of our device is from 10 pg/ml to 1 µg/ml and the detection limit is less than 10 pg/ml in diluted PBS buffer (0.01X PBS). These results demonstrated the planar nano-gap architecture having ability on clinical examination. Moreover, because of the simplicity and fast response of the proposed mechanism, this device has high potential on point-of-care test application. Final part is optimization of the proposed mechanism on bio-detection. We discuss three categories of variables that modulate sensing characteristics. They are response time, electrode structure and antigen features. In response time, we modulated the incubation time of antigen and antibody and showed improvement detection limit. In electrode structure part, we raised the height of gap and the experiment results demonstrated the improvement of sensitivity. In antigen feature part, we compare different charges, sizes and structure of antigen such as catenin, CRP and Hb. Though, the result demonstrates the complexity of protein, there are still some tendency in these features.