Design Considerations of Liquid Gated High Field Modulated FET Biosensor for Enhanced Sensing Characteristics

Abstract

Medical diagnostics are rapidly evolving to meet the requirements of personalized medicine and home-care based in-vitro diagnostics are in very high demand. Translating the clinical blood based assays into home-care tests pose several challenges. Primarily, we need to develop a compact, portable system that can enable the end user to perform the assay with minimal protocols that can be carried out without prior training, cost-effective and quick response time. More importantly, the home-care tests should retain the sensitivity and selectivity of the laboratory based assays. We previously developed high field modulated field effect transistor (FET) biosensors for the direct detection of protein biomarkers in whole blood. In this work, we investigate the sensor structure in detail to assess the parameters that govern the sensing characteristics. The basic sensor structure includes a functionalized reference electrode to which gate voltage is applied, placed at a narrow gap from the extended gate metal, which is connected to the FET’s gate terminal. The test solution containing the target analyte is placed across the gap such that it is sandwiched between the reference electrode and extended gate metal, together forming the sensing region. It was revealed from our previous studies that there are two ways in which the field generated across the test solution can be modulated: by applying higher amplitude of gate voltage and by decreasing the gap between the electrodes. It was found that at smaller gap distances, the potential drop in the solution is linearly dependent on the gap between the electrodes, and hence higher sensitivity can be observed for smaller gap configurations. In this work we initially explore the gap dependence of sensor response for micron scale and nano scale gap distances. The results indicate that noise characteristics and sensitivity are improved when the gap between the reference electrode and extended gate metal is decreased. The geometry of the sensing areas are also studied to evaluate the optimal electric field distribution across the test solution to generate enhanced sensing characteristics. Finally the surface functionalization is investigated to analyse the effect of surface coverage and number of receptor binding sites on the sensor performance. Through the integration of sensor design, geometry and optimal surface functionalization, we can improve the sensing characteristics, while satisfying the practical considerations of implementation of a home-care whole blood assay.