High-Throughput Electrochemistry - a new approach to the rapid development of modified carbon electrodes

Electrochemistry is widely used in the world around us from batteries of different types both large and small, through the industrial processes used to make chlorine and sodium hydroxide and methods to deposit metals for decorative effects and to make microchips, to the portable devices used several times a day by diabetics to measure their blood glucose. Electrochemical reactions occur at surfaces and one of their great advantages is that the voltage applied to the electrode is used directly to drive the chemical reaction and the current that flows is a direct measure of the speed of the reaction. In many cases the challenge is to design the surface of the electrode to carry out a particular chemical reaction so that we can exploit these advantages. At bare metal, or carbon, surfaces reactions occur by the transfer of electrons one at a time. As a result in many reactions that we would like to carry out unstable intermediates are formed which then undergo further reactions that lead to fouling of the electrode surface and the production of undesirable side products. A way to overcome this problem is to modify the electrode surface by attaching molecules which act as intermediates or mediators in the overall reaction. The reaction at the electrode surface then occurs by first transferring the electrons one at a time to (or from) the mediator attached to the electrode surface. Then, in a second step these mediators react with molecules in solution, thus catalysing the reaction that we wish to carry out at the electrode. The big advantage of this approach is that, in principle, we can select the molecules we choose to attach to the surface of the electrode so that they exchange electrons rapidly with the electrode and react selectively with the molecules in solution - we can design the electrode surface for the reaction we want. The challenge is to find the right molecules and the right way to attach them to the electrode surface. For the last 20 years or so efforts to do this have used inspired guesswork to pick one or two molecules to try and then prepared electrode surfaces with these molecules attached. In this project we will tackle this problem in a much more effective way. We will synthesise hundreds or thousands of related, but each slightly different, molecules on electrode surfaces and then screen these to find the best for the particular reactions we are interested in. To do this we will develop new ways of preparing the electrode surfaces and new ways to screen the surfaces for activity. We have chosen three particular reactions for our study. The first is the oxidation of NADH, a common coenzyme. There are hundreds of enzymes in nature which use NADH. If we can find good electrodes for the oxidation of NADH we can then use these different enzymes to make sensors and in fuel cells. In particular a good modified electrode for NADH oxidation could be important in developing better sensors to allow diabetics to measure their blood glucose. The second reaction is the oxidation of ascorbate (vitamin C). Ascorbate is an important possible interference when trying to oxidise NADH because ascorbate is present in blood and many biological samples. Therefore for the NADH electrodes we want to find modified surfaces at which NADH reacts much better than ascorbate. On the other hand ascorbate is also important in its own right as we need to be able to measure its concentration in drinks and foodstuffs so we will also be looking for modified electrodes which are very good for ascorbate oxidation. The final target is dopamine, a molecule involved in signalling between neurones in the brain. Many of the molecules which catalyse the reaction of NADH also catalyse the oxidation of dopamine. We will screen the different molecules we produce to see if any are especially good for the detection of dopamine so that we can produce minute electrodes that can be used to measure dopamine in studies of the brain.