Development of a contactless capacitive immunosensor

Abstract

In the present work, a label-free, contactless and capacitive immunosensor is developed using impedance spectroscopy, in the aim to perform low-cost immunoassays. Chapter 1 puts this work in perspective with some existing techniques, while a presentation of impedance theory used in this work is carried out in chapter 2. In Chapter 3, numerical simulations using a commercial finite element method software is carried out. The response of coplanar and face-to-face designs using an insulating layer is studied with respect to a frequency ranging from 100 Hz to 10 MHz . Two levels of capacitance were observed across the frequency range : the low frequency capacitance created by the insulating layer and the high frequency capacitance created by the solution. These capacitances depend on parameters like the solution conductivity, the distance between the electrodes, the electrodes width or the insulating layer thickness. A dimensionless parameter is defined to evaluate the quality of the geometry at high frequencies. Microchips using a coplanar design are developed in Chapter 4. They are composed of two silver electrodes drilled in a PET sheet by laser photoablation. The design of both holder and of the microchips is optimized to increase as much as possible the signal-to-noise ratio. Bovine Serum Albumin is detected by a variation of the channel conductivity. Chapter 5 introduces the design of a sensor using electrodes made of a mass market aluminium foil. The study of the frequency response of the electrodes led to the creation of a discrete analytical model. The electrodes are then mounted into a holder using a face-to-face or coplanar design. The system is characterized through the variation of several geometrical parameters (height of fluid in the reservoir, electrode surface area, solution conductivity, ...). The coplanar design is also optimized to be able to work in a holder equipped with a fluidic channel. Finally, the ability of the aluminium electrodes based sensor to monitor an adsorption is studied in Chapter 6. The resonance is used to detect the adsorption of proteins like BSA on the electrodes using coplanar and face-to-face designs. The adsorption is found to follow a Langmuir isotherm and an adsorption equilibrium constant is extracted. The second adsorbate layer is detected using a coplanar design, enabling the achievement of a immunoassay.