Self-assembled monolayers and enzyme electrodes: Progress, problems and prospects

Abstract

Initial results on the combined use of self-assembled monolayers and redox enzymes on electrodes to prepare electrochemical sensors are presented. Specifically, electrodes coated with self-assembled monolayers of 6-mercaptohexanol and 11-mercaptoundecanol are shown to exhibit dramatically reduced background currents relative to uncoated electrodes, and addition of a glucose oxidase layer on top of the self-assembled monolayer yielded electrodes which responded to glucose (in the presence of a soluble redox mediator) while still retaining the diminished background currents. It is shown that oxidation of ascorbate, urate, 4-acetamidophenol and hydrogen peroxide, and reduction of oxygen, are strongly suppressed at monolayer-coated gold electrodes relative to uncoated gold electrodes. This suppression is the source of the reduced background currents at the monolayer-coated electrodes, however, it also dictates that sensor strategies based on detection of hydrogen peroxide produced by enzyme-catalyzed reactions will not work with these electrodes. It is furthermore shown that oxidation of selected redox mediators, e.g. hydroxymethylferrocene, can proceed at monolayer-coated gold electrodes at which other redox reactions are suppressed. This suggests that an enzyme-based sensor could operate at a monolayer-coated gold electrode provided that an appropriate redox mediator was used to shuttle charge between the enzyme and the electrode. Data on the response of 6-mercaptohexanol-glucose oxidase-modified electrodes to changes in glucose concentration, and data which address the stability of the self-assembled monolayers on continuous contact with a bioactive medium (a yeast fermentation), the effect of homogeneous redox reactions between oxidized mediators and ascorbate, interference by molecular oxygen, and the effect of local hydrodynamics, are presented. Strategies for preparing improved sensors that overcome some of the problems with the present configuration are discussed.