Electrochemistry at carbon nanotubes (CNTs) is a large and growing field, but one in which there is still uncertainty about the fundamental activity of CNTs as electrode materials. On the one hand, there are many reports which focus on the favourable electrochemical properties of CNT electrodes, such as enhanced detection sensitivity, electrocatalytic effects and reduced fouling. On the other hand, other studies suggest that CNTs may be no more electroactive than graphitic powder. Furthermore, it has been proposed that the catalytic nanoparticles from which CNTs are formed may dominate the electrochemical characteristics in some instances. A considerable body of the literature presumes that the CNT sidewall is inert and that edge-plane-graphite-like open ends and defect sites are responsible for the electron transfer activity observed. In contrast, studies of well characterised single-walled nanotube (SWNT) electrodes, either as individual tubes or as two-dimensional networks, suggest sidewall activity. This review highlights how the various discrepancies in CNT electrochemistry may have arisen, by taking a historical view of the field and identifying crucial issues that still need to be solved. When assessing the behaviour of CNT electrodes, it is vitally important that careful consideration is given to the type of CNT used (SWNT or multi-walled), the quality of the material (presence of impurities), the effect of chemical processing steps in the fabrication of electrodes and the experimental arrangements adopted. Understanding these key features is an essential requirement to develop a fundamental understanding of CNT electrochemistry, to allow a wide range of electroanalytical applications, and to move the field forward rationally. As part of this process, high resolution electrochemical and electrical imaging techniques are expected to play a significant role in the future, as well as theoretical developments which examine the fundamentals of electron transfer at different types of CNTs and their characteristic surface sites.

Ultramicroelectrodes (UMEs) fabricated from networks of chemical vapor deposited single-walled carbon nanotubes (SWNTs) on insulating silicon oxide surfaces are shown to offer superior qualities over solid UMEs of the same size and dimensions. Disk shaped UMEs, comprising two-dimensional "metallic" networks of SWNTs, have been fabricated lithographically, with a surface coverage of <1% of the underlying insulating surface. The electrodes are long lasting and give highly reproducible responses (either for repeat runs with the same electrode or when comparing several electrodes with the same size). For redox concentrations

Chemical functionalization is an important aspect of single-walled carbon nanotube (SWNT) research, of interest to many proposed applications of SWNTs, including electrical and electrochemical sensing. In this study, the effects of two common in situ treatments on the electrochemical and solution conductance properties of SWNTs are assessed. The first is acid reflux, used for the purification of SWNTs and a common first step toward chemical functionalization of SWNTs. The second is an air plasma treatment, compatible with microfabrication processing. Rather than studying bulk quantities and using bulk analysis techniques, we investigate two-dimensional networks of individual SWNTs grown on an insulating substrate, enabling the effects of the treatments to be investigated at the level of individual SWNTs, as well as ensemble average behavior. The SWNTs are grown using catalyzed chemical vapor deposition, and electrical, electrochemical, atomic force microscopy, field emission scanning electron microscopy, and micro-Raman analysis are performed before and after applying the treatments. It is found that the major effect of the acid treatment is cutting of the SWNTs followed by gradual etching at the cut ends. Micro-Raman spectroscopy indicates preferential oxidative attack at the metallic SWNTs and minimal damage to the sidewalls. In contrast, plasma treatment does not affect the morphology of the SWNTs. Raman microscopy indicates a dramatic change in SWNT electronic structure, with a possible increase in sp 3 -hybridized carbon. Both treatments have a negligible effect on the voltammetric response of a simple outer-sphere electron-transfer redox process, Ru(NH 3 ) 6 3+/2+ . However, both acid reflux and air plasma treatment enhance the electron-transfer kinetics for the oxidation of inner-sphere dopamine. In both cases this is likely due to the creation of defect sites. A key result of these studies is the strong correlation between increasing functionalization (with a view to increasing chemical sensitivity) and decreasing conductivity, which is an important consideration for electrical and electrochemical applications. It is clear that a balance must be struck between the two to enhance the performance of a SWNT device.

An important open question on the electrochemistry of single-walled carbon nanotube (SWNT) electrodes concerns the sites at which electron transfer (ET) occurs. This issue is addressed herein for the case of a simple outer sphere redox couple, (ferrocenymethyl)trimethylammonium (FcTMA+). Using relatively sparse networks (<1% surface coverage) of electrically connected SWNTs, coupled to a scanning electrochemical microscopy (SECM) substrate generation−tip collection setup, we show that high rates of mass transport can be generated to SWNTs, allowing kinetic effects to be observed in the voltammetric waveshape. By developing a numerical model that faithfully represents all aspects of the experimental geometry, highly accurate kinetic data can be obtained. Assuming that all SWNTs are active, a minimum average ET rate constant k 0 SWNT > 1.0 ± 0.6 cm s−1 is assigned, which is of similar size to other electrode materials and suggests that the sidewall of SWNTs has considerable ET activity.