We report an elegant small-signal capacitance measurement system for capacitive immunosensors, prototyped around a computer-controlled two-phase lock-in amplifier. Capacitive immunosensors require low-ionic-strength electrolytes for effective antibody-antigen interaction and low dielectric thickness for high sensitivity to phenomena occurring at the insulator-electrolyte interface. The problem of measuring a high intrinsic sensor capacitance in the presence of a modest parasitic cell resistance is overcome by using a potential-sensing electrode close to the electrolyte-dielectric interface for measuring and controlling the stimulus to the intrinsic device. The current phasor through the capacitive sensor is measured after effecting potential control via a software feedback loop and the sensor capacitance calculated. Capacitance can be measured in the range 100 pF to 0.1 µF with an accuracy of ±0.5% at 1 kHz. The technique obviates the need for expensive high-speed potentiostats or impedance analysers for such measurements and can also be used for small-signal admittance measurements in other electrolyte-insulator-semiconductor or electrolyte-insulator-metal systems. The suitability of the proposed measurement system for immunodetection is shown by capacitance measurements on fibronectin-sensitive capacitive immunosensors.
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Prasad et al. (1999) studied this question.
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