Electroanalytical applications of carbon electrodes using novel hydrodynamic flow devices

Abstract

Since the first reported use of carbon nanotubes (CNTs) as an electrode material in 1996 the use of CNTs within electrochemistry has grown rapidly. Single walled carbon nanotubes offer bio-compatibility combined with nano-scale dimensions and low background currents in the pristine state. Over the past decade the quantity of SWNTs synthesised globally has greatly increased making the material available for a variety of studies and potentially a feasible material for commercial electrodes. Despite this rise in popularity there is still an on going debate about the sites of electron transfer (ET) at a carbon nanotube. Some reports claim that the sidewall of the carbon nanotube exhibits sluggish ET rates with the majority of the ET occurring at defect sites and the end of the CNT. In contrast there is also evidence that suggests that ET at the sidewall is facile and not sluggish. The origin of ET is investigated using both theoretical and experimental data to probe the developing diffusion profiles to active ET sites. This is achieved on the timescale of a typical voltammetric experiment by significantly reducing the rate of diffusion to the electroactive sites using a NafionTM film. The reduced rate of diffusion allows the developing diffusion profiles to the individual sites to be decoupled. The use of convection and diffusion is a proven electrochemical technique to increase the sensitivity of analytical measurements and to probe reaction rates and mechanisms. The well-defined mass transport within a channel flow cell or an impinging jet electrode, combined with the continual replacement of solution, makes this geometry amenable to online studies, e.g. bedside or industrial monitoring, or a combination with chromatography. One draw back of conventional channel flow and impinging jet electrode set-ups is the need for specialist equipment or calibration steps each time the system is assembled. The use of microstereo lithography (MSL) to construct custom designed cells for use with a variety of planar electrodes is investigated. The hydrodynamics within the proposed designs are theoretically tested and verified experimentally. The devices constructed are easily assembled using a wide range of electrode materials and the computer aided manufacture provides flexibility in critical dimensions. Importantly, the devices only require a one-off determination of the height prior to assembly, removing the need for an electrochemical calibration step as the cells do not distort during assembly. Of particular interest for analytical studies is the greatly reduced background currents provided by a carbon nanotube network compared to an equivalent size carbon macroelectrode. The lower background signal allows small Faradaic currents to be observed experimentally, allowing lower concentrations to be distinguished. The enhanced sensitivity is combined with the increased mass transport of channel flow and impinging jet convective systems to determine the limit of detection for particular channel and impinging jet geometries under constant flow rates. This approach allows the successful detection of nano-molar concentrations under hydrodynamic control using standard voltammetric techniques