Bipolar Electrochemistry: An Emerging Tool in Nanoscience
- Fig.1: Scheme of a spherical bipolar electrode in solution exposed to an electric field. An opposite polarization occurs at the two sides of the object, which can be used to trigger an oxidation reaction (left side) and a reduction reaction (right side) simultaneously, leading in fine to the dissymmetric modification of the object or its propulsion [11, 12].
- Fig.2: Scheme of the experimental set-up for a typical Capillary Assisted Bipolar ElectroDeposition (CABED) experiment . The unmodified particles are injected together with metal salts at the anodic side, travel through the capillary due to the electroosmotic flow and are collected at the cathodic side.
- Fig.3: Single point modification of carbon tubes with metal clusters. The left side shows a TEM picture of a short carbon nanotube piece with a gold cluster on one side obtained by the CABED process . In the middle is the corresponding computer model. The right side illustrates a carbon microtube modified with a platinum cluster .
- Fig.4: Bipolar electrochemistry applied to the modification of an isotropic object, in this case a carbon microparticle (left side); dissymmetric deposition of metal and conducting polymer on a carbon microtube (middle); site selective deposition of an insulating material, illustrated by a layer of electrophoretic paint on a nickel particle (right side) .
- Alexander Kuhn, Professor, Université de Bordeaux, France
- Gabriel Loget, PhD student, Université de Bordeaux, France
Bipolar electrochemistry is a concept with a quite long history, but has only very recently revealed its virtues in the field nanoscience. It allows controlled surface modification at the micro- and nanoscale, with very original applications ranging from analytical chemistry to material science. Here we review some of the latest achievements in the field and explain why this straight forward concept leads to a very attractive tool for shaping the micro- and nanoworld.
The concept of bipolar electrochemistry is known for quite a long time, but it has never crossed the frontier of a few industrial applications, that have been developed in the 1960ies. Traditional examples of bipolar electrochemistry can be found under the name of fluidized bed electrodes for applications such as electrosynthesis, water splitting and also for increasing fuel cell performances [1,2]. The very simple concept behind these processes is based on the fact that a bipolar redox behavior is induced on a substrate under the influence of an external electric field .
Let's consider a conducting object immersed in a solution that is placed in a strong electric field between two electrodes (Figure 1). Between the two sides of the object a polarization ΔV occurs, that is proportional to the electric field E/L and the characteristic dimensions of the object r.
ΔV= 2 E r/L (1)
When this polarization is strong enough, redox reactions can be carried out at the opposite ends of the object. The negatively polarized side can be for example the site of metal ion reduction, leading to the formation of a metal cluster, whereas at the positively polarized side an electropolymerization can lead to the formation of a layer of conducting polymer.
Different advantages become immediately clear when looking at this concept: first of all it allows the controlled modification of an object using an electrochemical process without any physical connection of the object to an electrode. This means that in principle the same processes can be triggered on thousands or millions of objects simultaneously, which makes the approach very attractive in highly parallelized devices like integrated microelectrode arrays .
A second obvious argument in favor of this technique is the fact that it is one of the rare approaches that allows breaking the symmetry in a modification process without using a surface or an interface. It is therefore a straightforward synthetic way to dissymmetric particles obtained via a true bulk process. These dissymmetric objects, also named Janus particles, after the roman God who had two different faces, are of great importance for studying fundamental and applied issues in micro- and nanotechnology, like directional self-assembly, nanoelectronics, photosplitting of water and electronic paper.
In principle the initial objects can be made out of any kind of conductive material with any characteristic dimensions and geometry. However, as shown by equation (1), a smaller object will require a higher external electric field in order to be polarized enough to induce redox reactions, and this is an intrinsic problem when modifications of nanometer sized objects should be carried out. One can easily calculate that in this case typical external electric fields of the order of MV/m are needed in order to achieve a dissymmetric modification, values that seem barely compatible with normal laboratory conditions. Therefore most of the work in the past using bipolar electrochemistry has been carried out with rather big objects, in the best case with dimensions in the submicrometer range .
In order to circumvent the problems associated with the above mentioned high voltages, like bubble formation due to solvent decomposition and rotation of the objects during the modification, it is possible to use a set-up that separates the anodic and cathodic compartment via a capillary (Figure 2). While the objects are driven through the capillary due to the intrinsic electroosmotic flow, they get at the same time modified if the necessary ingredients like metal salts or monomers are present in the solution. At the outlet of the capillary the Janus particles can be easily collected and then characterized. This Capillary Assisted Bipolar ElectroDeposition (CABED process) allows indeed the selective modification of nanometer sized objects, because overall voltages of up to 50 kV, corresponding to electric fields almost in the MV/m range, can be applied, analogue to what is routinely achieved with a classic capillary electrophoresis equipment .
This set-up led for the first time to the site selective generation of metal clusters on carbon nanotubes (Figure 3).
The CABED process can also be used for the localized modification of other objects such as micrometer sized carbon tubes, as illustrated on the right side of Figure 3 for the toposelective deposition of platinum . Other metals that have been successfully tested include copper and nickel . Because nickel has a quite negative deposition potential, one can conclude that a large variety of metals is accessible for modifying this kind of objects using the same approach. As the condition for successful modification is that the object needs to be a good conductor in order to allow electron transfer from one side to the other, one can for example imagine to use nickel modified carbon nanotubes to collect only the tubes which are good conductors with the help of a magnet, as the semiconducting or insulating tubes will not be modified.
The CABED procedure has definitely many advantages, but suffers from the fact, despite of being a bulk procedure, that only quite small volumes can be treated and therefore the quantity of produced Janus objects is rather small. This problem can be avoided by redesigning the bipolar cell, thus allowing the production of much larger quantities . The process can be also adapted to the deposition of others materials than metals, as long as a redox process is involved at one point of the material synthesis. This is the case for example for the generation of conducting polymers  or the precipitation of semiconducting or insulating metal oxides triggered by electrochemical induced pH changes. The modification of isotropic particles such as spheres or other, less uniform objects is also a possibility (Figure 4). The obtained Janus objects with a sophisticated design could be used as attractive ingredients for many applications, such as switching pixels in electronic paper or electrophoretic screens.
The recent increase of interest in bipolar electrochemistry has led in a short time to several very original new concepts that are of direct importance for nanoscience and nanotechnology. Bipolar electrochemistry is a very powerful tool for solving material science problems, since its toposelectivity allows the synthesis of complex structures with a highly controlled composition and design, such as dissymmetric micro- and nanoparticles, functionalized pores or molecular surface gradients. It is important to keep in mind that this new area of bipolar electrochemistry is still at an early stage of development, but already now one can expect many specific applications to emerge in the near future , which sometimes can go way beyond the field of material science .
 Kazdobin K. et al.: Chem Eng J 79, 203 (2000)
 Matsuno Y. et al.: J Hydrogen Energy 22, 615 (1997)
 Fleischmann M. et al.: J Phys Chem 90, 6392 (1986)
 Chow K.F. et al.: J Am Chem Soc 131, 8364 (2009)
 Mavré F. et al.: Anal Chem 82, 8766 (2010)
 Warakulwit C. et al.: Nano Lett 8, 500 (2008)
 Fattah Z. et al.: Electrochim Acta in press (2011) doi:10.1016/j.electacta.2011.01.048
 Loget G. et al.: Electrochim Acta 55, 8116 (2010)
 Kuhn A. et al.: French patent application, No 1061031 (2010)
 Loget G. et al.: submitted (2011)
 Loget G. et al.: Anal Bioanal Chem in press (2011)
 Loget G. et al.: J Am Chem Soc 132, 15918 (2010)