Characterisation and Fabrication of a Multiarray Electrolyte-Insulator-Semiconductor Biosensor

Abstract

Label free detection methods are preferred in biological sensing applications due to their convenience, low reagent usage, and minimal equipment dependence. EIS (Electrolyte-Insulator-Semiconductor) is a type of biosensor that achieves this through the sensing of the intrinsic charges present within a biological entity such as a cell or DNA. The charge generates a field which modulates the width of the depletion region within the semiconductor and can be detected using capacitance measurements. To enable the system to be more functional, a chip containing an array of biosensors allows the detection of multiple biological entities simultaneously, leading to a faster and more convenient system. Furthermore, if the characteristics of the each sensor are identical, a single sensor maybe chosen as a reference (an isolated sensor used to measure background noise and drift) to eliminate problems relating to drift and solution background noise. These issues have been addressed by developing a simple and cost effective EIS multi-array biosensor using standard saw cutting tools and industry standard C-V characterisation methods. A lateral shift in the C-V curve represents a change in the depletion region width which is an indication of charge presence. Polyelectrolytes are the main type of charge species used to characterise the system. Initial tests began with a single sensor in a controlled environment using custom made apparatus and equipment for a high level of control. Results indicate a linear response to the charged species across a range of concentrations which confirmed its usefulness in a biosensing application. The approximate sensitivity of the device was determined to be 20mV voltage shift per 50µM of PE added. Device sensitivity was shown to have improved by 23% when the C-V curve was used to identify substrates with lower doping concentrations (that gave a steeper C-V gradient in the depletion region) compared to a higher doped substrate. The single sensor was improved by fabricating multiple electrodes on a single chip using anodic bonding processes coupled with saw cutting methods to create finger-like structures. It demonstrated similar characteristics to the single sensor system with the advantage of having more than one sensor on a single chip. A good level of consistency was found across all sensors (within 2%) which allow accurate comparisons to be made between sensors with low calibration requirements. As a result, a reference sensor can be integrated into the system for cancelling out noise, solution background and drift. The study also shows that the device responds immediately to charges after a 10 minute polyelectrolyte adsorption period, allowing it to be a relatively fast and rapid system. Future packaging solutions such as the realisation of flow-through (for lab-on-chip) and dip systems are a possible application outcome for this technology. These devices can be deployed in areas including industrial monitoring processes, hospital and clinical care services, environmental control and defence sectors for automated, remote, on-site, real-time and portable analysis of specific analytes.