SELF-ASSEMBLED MONOLAYERS AND ENZYME ELECTRODES - PROGRESS, PROBLEMS AND PROSPECTS

Initial results on the combined use of self-assembled monolayers and r edox enzymes on electrodes to prepare electrochemical sensors are pres ented. Specifically, electrodes coated with self-assembled monolayers of 6-mercaptohexanol and 11-mercaptoundecanol are shown to exhibit dra matically 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 presen ce of a soluble redox mediator) while still retaining the diminished b ackground currents. It is shown that oxidation of ascorbate; urate, 4- acetamidophenol and hydrogen peroxide, and reduction of oxygen, are st rongly suppressed at monolayer-coated gold electrodes relative to unco ated gold electrodes. This suppression is the source of the reduced ba ckground currents at the monolayer-coated electrodes, however, it also dictates that sensor strategies based on detection of hydrogen peroxi de produced by enzyme-catalyzed reactions will not work with these ele ctrodes. It is furthermore shown that oxidation of selected redox medi ators, e.g, hydroxymethylferrocene, can proceed at monolayer-coated go ld electrodes at which other redox reactions are suppressed. This sugg ests that an enzyme-based sensor could operate at a monolayer-coated g old electrode provided that an appropriate redox mediator was used to shuttle charge between the enzyme and the electrode. Data on the respo nse of 6-mercaptohexanol-glucose oxidase-modified electrodes to change s in glucose concentration, and data which address the stability of th e self-assembled monolayers on continuous contact with a bioactive med ium (a yeast fermentation), the effect of homogeneous redox reactions between oxidized mediators and ascorbate, interference by molecular ox ygen, and the effect of local hydrodynamics, are presented. Strategies for preparing improved sensors that overcome some of the problems wit h the present configuration are discussed.