Optimization of capacitive affinity sensors: drift suppression and signal amplification

The detection limit of capacitive affinity sensors based on the gold-alkane thiol system can be improved by optimization of sensor preparation and by s ignal 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 electro des 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-asse mbled monolayers was observed due to incubation in chloroform or dimethylfo rmamide. In contrast, self-assembled monolayers made from short-chain disul fides or thiols (such as 3,3'-dithio-bis(propionic acid N-hydroxysuccinimid e ester) or 11-mercaptoundecanoic acid) displayed a poor stability in aqueo us phase. Capacitive affinity sensors based on these short-chain thiols sho wed a considerable drift of the signal. The use of long-chain thiols result ed in a stable signal; it was applied to compare capacitive effects due to immobilization of different biological molecules and for preparation of dif ferent biosensors. The response of capacitive biosensors can be amplified by formation of a sa ndwich structure. This principle was illustrated by subsequent adsorption o f polyclonal anti-HSA after binding of HSA with a sensor for HSA based on m onoclonal antibodies, (C) 1999 Elsevier Science B.V. All rights reserved.