Conductive diamond: A unique electrode material for analytical applications

Akira Fujishima

Department of Applied Chemistry, School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan

e-mail: akira-fu@fchem.chem.t.u-tokyo.ac.jp, fax: +81-3-3812-6227, Phone: +81-3-5841-7245

Abstract

Electrochemical analysis has assumed an important place in the arsenal of analytical techniques for the detection of trace amounts of many organic and inorganic molecules of biological significance. Various electroanalytical techniques including differential-pulse voltammetry (DPV), square-wave voltammetry (SWV), stripping voltammetry, electrochemical detection coupled with flow-injection analysis (FIA-ED) and high-performance liquid chromatography (HPLC-ED) have been developed for electroanalysis. The selection of electrode material is very crucial for the selective analysis of specific group of compounds. Although wide range of organic species are electroactive at glassy carbon and pyrolytic graphite electrodes, these electrodes suffer from practical problems such as electrode fouling due to binding of reaction products and non-electrochemically active biochemical species. Boron-doped diamond, while acting as conducting electrode, has overcome many of these deactivation problems.

Diamond exhibits several unique properties, which make it superior to conventional electrode materials, such as glassy carbon. First, the wide electrochemical potential window of the diamond electrode allows the sensitive electroanalytical detection of chemical species that react at relatively high potentials. We have recently demonstrated the detection of histamine at a potential of 1.28 V vs. Ag/AgCl, with an experimental detection limit of 500 nM (S/N = 13). Secondly, the low background current (low double-layer capacitance) of the diamond electrode has facilitated the detection of several electroactive species with sensitivity at least one order of magnitude greater than that of the GC electrode even at moderate potentials. We have achieved amperometric detection limits as low as 10 nM for NADH and serotonin at operating potentials of 0.58 V and 0.43 V vs. Ag/AgCl, respectively. We could not observe any analytically useful signal at GC for these concentrations, due to large background current and noise. The third promising feature of this electrode is its long-term stability due to lack of adsorption of chemical species on the inert electrode surface. An additional advantage with diamond for electroanalysis is that it is relatively insensitive to dissolved oxygen over a wide range of potentials in both alkaline and acidic aqueous electrolytes. This property enables the electroanalysis without deaeration of the electrolyte.

Recent developments in the electroanalytical use of diamond electrodes will be discussed with a special emphasis on the detection of chlorophenols and glutathione. Diamond has been proved to be excellent material for chlorophenol detection due to its high resistance to fouling at low phenol concentrations. After long-term use, this electrode can be cleaned and reactivated by applying high anodic potentials. Glassy carbon undergoes rapid deactivation and electrochemical cleaning damages the electrode. In case of glutathione, diamond is especially useful for the detection of reduced as well as oxidized forms of glutathione due to its wide potential window. Glassy carbon is not suitable for the detection of oxidized form of glutathione. Apart from these studies, we have also demonstrated the high suitability of diamond as the inert support for the metal particals. We have modified diamond electrode with various metals such as Pt, Ni, Cu and Ir. Detection glucose using these electrodes will also be discussed.

Key words: Diamond electrode; Electrochemical detection; High sensitivity; Metal-modified diamond; Sensors