Affinity biosensors based on a capacitive transducer

Abstract: Capacitive biosensors were developed for several analytes such as hormones, interleukines, viruses and heavy metal ions. The sensing surfaces were prepared by immobilizing the biorecognition element to self-assembled monolayers (SAMs) on gold. SAMs form spontaneously on gold by adsorption of thiols, sulfides or disulfides. After immobilization of the biorecognition element a long chain hydrocarbon thiol was used to block any uncovered spots on the surface. The electrode was mounted in a flow-system and served as working electrode in a potentiostatically controlled system. Between measurements the working electrode was held at a rest potential of 0 mv vs. an Ag/AgCl reference electrode. To measure the capacitance at the sensing layer/electrolyte interface a potentiostatic step of 50 mV was applied and the resulting current response was recorded. By assuming that the current response followed that for a model consisting of a resistor and a capacitor in series the capacitance could be calculated by a linear least-squares fitting. Immunosensors specific for hormones and interleukines were developed with detection limits around 15 femtomolar for non-labeled antigens. DNA biosensors were developed by immobilizing a short oligonucleotide probe in two different ways; either by direct self-assembling of an SH- modified oligonucleotide or by coupling an oligonucleotide to a SAM. In both cases 0.2 attomolar of a DNA virus fragment could be detected. Also heavy metal ion specific biosensors were developed by immobilizing heavy metal specific proteins on an electrode surface by the self- assembling process. The heavy metal specific proteins are believed to change conformation when heavy metal ions bind thereby resulting in a change in capacitance. The heavy metal ions Cu2+, Zn2+, Cd2+ and Hg2+ could all be detected down to femtomolar concentrations.