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This book introduces the recent development in Japan of diamond electrodes, which has attracted much attention in the world. For example, electrochemical sensors using diamond electrodes are now being utilized commercially. Newly developing applications such as electrochemical organic synthesis including CO2 reduction are also expected to form an important future technology. Those emerging applications to various fields which are receiving increasing attention are described in detail here. This book is useful not only for students who would like to begin their study of diamond electrodes but also for industries that are exploring novel electrochemical applications.

BDD electrode was utilized as the cathode for the electrochemical CO2 reduction due to its high hydrogen overpotential and high stability. Electroreduction of CO2 is one of the ways to convert CO2 into useful compounds which can be used in industrial fields and can be used as fuels. The efficiency or selectivity for the production of CO2 reduction products can be affected by electrode materials and electrolysis conditions. BDD electrode is inert, so formic acid production was predominant. However, we could also produce various kinds of products from CO2 by optimizing the electrode itself or electrolysis conditions. Many kinds of BDD electrodes were used such as BDD electrodes with various boron doping level, BDD electrodes with sp2 carbon impurities and BDD electrodes modified with metal particles electrochemically. Electrolysis conditions were also optimized such as the applied potential, applied current, catholyte and anolyte. In this chapter, the electrochemical CO2 reduction using BDD electrodes or using metal-modified BDD electrodes was described.

Boron-doped diamond (BDD) has attracted much attention as a promising electrode material, because it has excellent electrochemical properties such as a wide potential window and low background current. It is known that the electrochemical properties of BDD electrodes are very sensitive to the surface termination such as to whether it is hydrogen- or oxygen-terminated. Especially for electrochemical sensor application, pretreating BDD electrodes by cathodic reduction (CR) to hydrogenate the surface has been widely used to achieve high sensitivity. However, little is known about the effects of the CR treatment conditions on surface hydrogenation. In this chapter, at first, a systematic study of CR treatments in order that we can achieve effective surface hydrogenation is discussed. Also, direct observation of surface hydroxylation by anodic oxidation was reported. We have developed in situ spectroscopic measurement systems on BDD electrodes, i.e., in situ attenuated total reflection infrared spectroscopy (ATR-IR) and electrochemical X-ray photoelectron spectroscopy (EC-XPS). Furthermore, surface modification by functional molecules to introduce specific functions is also discussed. As examples, photochemical modification method via UV irradiation and electrochemical modification method are introduced. These surface control and modification should be important not only for better understanding of BDD’s fundamentals but also for a variety of applications.

Electrogenerated chemiluminescence (also known as electrochemiluminescence and abbreviated ECL) is a complex phenomenon of luminescence triggered by electrochemical reactions where the heterogeneous electron transfer, then ECL as consequence, is affected by the electrode materials which is crucial in the signal development and intensity. Among electrode materials, doped diamond electrodes are still underdeveloped for ECL application especially compared to the more commons noble metals and other carbon-based. After a brief and general introduction on electrochemiluminescence, this chapter will focus on several studies and developments of doped diamond electrodes by taking advantage on its own unique properties as the electrode for ECL.

Drugs play a key role in the treatment of patients with various diseases. A compound, when administered systemically, shows differential spatial and temporal distribution patterns not only in the body but also within each organ. In response to an increase or decrease in local concentrations in the organ, the activity of the cell population expressing the drug’s target protein(s) changes over time. Therefore, real time, simultaneous detection of kinetics of the drug and its pharmacological effects in in vivo microenvironments is essential for evaluating the efficacy of medicines. Although such challenging dual-mode measurement has not yet been addressed by any conventional methods, it has been successfully achieved via a microsensing system that we recently developed. The system consists of two different sensors: A needle-type boron-doped diamond microelectrode for monitoring the drug and a glass microelectrode for tracking electrophysiological activity of the target cells. This state-of-the-art approach is applicable to various drugs in terms of “local” pharmacokinetic and pharmacodynamic assays in vivo and may contribute to the development of next-generation therapeutic interventions.

Monitoring free chlorine concentration is an important issue in various situations. Residual chlorine that does not react with organic compounds, and metals dissolve in water to produce HClO (hypochlorous acid) and ClO- (hypochlorite ion). In 2008, we reported that free chlorine, which has a strong disinfecting effect, can be monitored with high sensitivity using a boron-doped diamond (BDD) electrode. However, the problem of pH fluctuation of the solution and the detailed mechanism of redox have not been completely clarified. Then, in 2016, the electrochemical redox mechanism of this free chlorine was carefully studied. Based on these results, we designed a new system using BDD electrodes that can monitor the free chlorine concentration in real time.

A first example of electrochemical measurement using diamond as an electrode is a study by Iwaki et al. (RIKEN) in 1983. This is a preview of subscription content, access via your institution.

Boron-doped diamond (BDD) has been established as a superior electrode among other conventional solid electrodes due to its unique properties, such as very low background current, wide potential window, and high physical and chemical stability. However, in compare to metal electrodes, BDD has much lower kinetic activity in some important chemical reactions, which causes limitation for sensor and biosensor applications. Modification of BDD surface with redox-active particles/compounds was reported to facilitate electron transfer between the BDD substrate and analytes with a significant reduction in activation overpotential; therefore, the catalytic activity and sensitivity are improved. In addition, the modification can increase the selectivity in some reactions. On the other hand, to have a stable modified surface of BDD electrode is not easy, since the main composition of BDD is carbon atoms with sp3 configuration, which is very compact and stable, and actually one of the advance characteristics of BDD electrodes. However, the stability of the BDD surface causes the modified surface to be easily detached or dissolved after several usages. In this chapter, preparations and applications of the modified BDD for electrochemical sensors and biosensors are described and compared.