A capacitive immunosensor measurement system with a lock-in amplifier and potentiostatic control by software

We report an elegant small-signal capacitance measurement system for capaci tive immunosensors, prototyped around a computer-controlled two-phase lock- in amplifier. Capacitive immunosensors require low-ionic-strength electroly tes for effective antibody-antigen interaction and low dielectric thickness for high sensitivity to phenomena occurring at the insulator-electrolyte i nterface. The problem of measuring a high intrinsic sensor capacitance in t he presence of a modest parasitic cell resistance is overcome by using a po tential-sensing electrode close to the electrolyte-dielectric interface for measuring and controlling the stimulus to the intrinsic device. The curren t phasor through the capacitive sensor is measured after effecting potentia l control via a software feedback loop and the sensor capacitance calculate d. Capacitance can be measured in the range 100 pF to 0.1 mu F with an accu racy of +/-0.5% at 1 kHz. The technique obviates the need for expensive hig h-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 suitabi lity of the proposed measurement system for immunodetection is shown by cap acitance measurements on fibronectin-sensitive capacitive immunosensors.