Capacitive monitoring of protein immobilization and antigen-antibody reactions on monomolecular alkylthiol films on gold electrodes.

Self-assembled monolayers of omega-mercaptohexadecanoic acid and omega-mercaptohexadecylamine on gold electrodes are stable at neutral pH and display pure capacitive behavior at frequencies around 20 Hz. Different methods of covalent immobilization of proteins on these monolayers are compared. Various reagents including succinimides, thionylchloride, p-nitrophenol and carbodiimides were used to activate the carboxy groups of the adsorbed monolayer of omega-mercaptohexadecanoic acid. Glutaraldehyde, cyanuric chloride and phenylene diisocyanate were used to activate the amino groups of the monolayer of omega-mercaptohexadecylamine. The immobilization of albumin on the activated surface was studied by capacitive measurements. The N-hydroxysuccinimide and carbodiimide methods were identified as most suitable for protein immobilization in that they did not compromise the insulating properties of the alkylthiol layer and led to maximal increase of its dielectric thickness. These approaches were used for a layer-by-layer preparation of a capacitive immunosensor. Specifically, antibodies to human serum albumin were immobilized on the alkylthiol mono-layer. Binding of the antigen led to a decrease of the electrode capacitance. The detection limit of the immunosensor is as low as 15 nM (1 mg/l).