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The detection limit of capacitive affinity sensors based on the gold–alkanethiol system can be improved by optimization of sensor preparation and by signal amplification. The dissociation of the gold–sulfur binding is often a critical point leading to operative errors of such sensors. The stability of self-assembled monolayers prepared with different thiols on gold electrodes in aqueous and organic solvents was studied by the capacitive technique. The results show that monolayers made of 16-mercaptohexadecanoic acid are stable in aqueous solution and can be hardly extracted from a gold surface by ethanol, methanol, or dioxane, while a considerable damage of self-assembled monolayers was observed due to incubation in chloroform or dimethylformamide. In contrast, self-assembled monolayers made from short-chain disulfides or thiols (such as 3,3′-dithio-bis(propionic acid N-hydroxysuccinimide ester) or 11-mercaptoundecanoic acid) displayed a poor stability in aqueous phase. Capacitive affinity sensors based on these short-chain thiols showed a considerable drift of the signal. The use of long-chain thiols resulted in a stable signal; it was applied to compare capacitive effects due to immobilization of different biological molecules and for preparation of different biosensors.
The response of capacitive biosensors can be amplified by formation of a sandwich structure. This principle was illustrated by subsequent adsorption of polyclonal anti-HSA after binding of HSA with a sensor for HSA based on monoclonal antibodies.
The lateral electrical conductivity of gold layers with thickness in the nanometre range was found to decrease due to adsorption of mercury, water, iodine and sulphuric compounds onto the gold surface. A self-assembled monolayer of hexadecanethiol, deposited on the gold surface, blocked the effects of water and volatile sulphuric compounds on the lateral conductivity of the gold layers, but does not block the effects of iodine and mercury. The results indicate a selective permeability of hexadecanethiol monolayers for mercury vapour, thus providing a method for development of ultrathin filters for chemical sensors.
Thin film electrode chips on silicon substrate have been realized and utilized in development of different chemo- and biosensors. These affinity sensors are based on capacitive transducing and their general structure is thin film Au/alkanethiol/receptor. Several applications of thin film gold electrodes in capacitive sensors are presented.