Biosensors utilizing monolayers on electrode surfaces

The applications of monolayer technologies to the fabrication and use of biosensors are described. The introduction of accessible cysteine residues and that of lipid tags by genetic engineering techniques were demonstrated to be viable methods to immobilize protein molecules in controlled manner using the self-assembly and Langmuir–Blodgett (LB) methods, respectively. A self-assembled monolayer of a thiol-derivatized adenine on a gold electrode could recognize thymidine, a complementary base derivative, in an aqueous solution via the hydrogen bond pairing at the monolayer/solution interface. Monolayers of redox-active species, such as hydroquinone and ferrocene derivatives, were used for transducing the signal of enzyme-catalyzed reactions. For example, the pH-dependent benzoquinone/hydroquinone-redox system was applied to detect the pH change at the immobilized urease surface with the enzymatic hydrolysis of urea. A two-dimensional cross-linked polysiloxane LB film has been proved to be useful as a permselective layers in amperometric hydrogen peroxide detecting biosensors. The formation/desorption process of thiol monolayers could be applied to the construction of a high-sensitive immunoassay system for a cardiac marker, BNP. Thus, the monolayer-based systems are of interest not only from the fundamental aspect as shown in the bottom-up approach to the construction of protein or nucleic acid base-based sensors but also from the viewpoint of possible practical use as shown in the immunosensor for BNP.