Design and elaboration of original objects by and for electrochemistry
- Porous electrodes
- Chiral materials
- Surface gradients with tailored properties
- Janus particles
- Microswimmers
- Porous electrodes
Electrochemical signals are very often limited in terms of current or power output by the active surface area of the electrode. One way to circumvent such limitations is to employ porous electrodes. The activities within this topic are focused on the development of macro- and mesoporous electrodes with a high structural control. We follow an approach using either beads or supramolecular surfactant assemblies as templates during the electrodeposition of a wide range of materials, from different metals [1] or metal oxides [2] to metal organic frameworks[3] and conducting polymers.[4] This allows a perfect control of the organization of the pores and leads to electrodes with an active surface area that is two or three orders of magnitude higher compared to flat surfaces. The internal surface of these designer electrodes can be subsequently modified with catalysts, redox-mediators and enzymes for a variety of applications ranging from photocatalysis[5] to (bio)sensors[6,7] and (bio)fuel cells.[8,9]
SEM view of a macro/mesoporous electrode
- Chiral materials
Chirality plays an extremely important role in biological systems, such as encountered for antigen-antibody interactions or enzyme-substrate recognition. A lot of effort has been made to mimic such natural systems by designing chemical analogous. We’ve developed an important expertise in the controlled generation of mesoporous metal structures which retain at their internal surface chiral information by imprinting the structure of chiral molecules in the walls of the mesopores. Our experiments show that such electrodes not only can distinguish between two enantiomers when they are used as electroanalytical tools [17, 18], but, most importantly, are also able to generate an enantiomeric excess of over 90%, when they are employed for electrosynthesis [19, 20]. Such a selectivity based on a purely geometric effect holds great promise to impact considerably not only the field of electrochemistry, but also materials science and physical chemistry in general. Our current efforts within this topic are focused on the optimization of the imprinting process, the efficiency of the molecular recognition and the range of materials which can be used as a matrix.
Scheme of the chiral recognition on an imprinted metal surface - Surface gradients with tailored properties
The elaboration of surfaces with specific anisotropic features is a very hot contemporary topic of research. The different possible methods of fabrication need to break the symmetry of the initial isotropic surface. In this context, bipolar electrochemistry (BPE) can be employed for the controlled generation of directional physico-chemical gradients based on the polarization established alongside bipolar electrodes.[21] For that, several metal layers exhibiting a variation of chemical composition, morphology and/or roughness are electrodeposited on conducting substrates with various geometries. Also, the formation of a tunable molecular gradient can be achieved by partial removal of a homogeneous layer under BPE conditions. The combination of electrode surface micro-structuration with such molecular gradients results in original wetting properties [22].
- Janus particles
Asymmetry is a very common feature of many systems, objects and molecules, that we encounter and use in our daily life. Actually, it is in a majority of cases the absolutely crucial ingredient for conferring a certain useful property to a system. In this context so-called Janus particles bearing an intrinsic asymmetric property are interesting model systems for breaking the symmetry at the level of micro- and nanoobjects. Various methods are described in the literature for synthesizing such particles, but most of them need an interface or a surface to break the symmetry. We explore alternative approaches allowing symmetry breaking in the bulk phase of a solution. One straight-forward way to achieve this is based on bipolar electrochemistry. Bipolar electrochemistry is a concept with a quite long history, but has only very recently revealed its virtues in many areas of chemistry, among others for the controlled surface modification at the micro- and nanoscale.[23] We largely extended this concept in the frame of material science applications, especially by developing direct and indirect bipolar electrodeposition, eventually assisted with light, as a simple route to particles with a very sophisticated design [24, 25, 26].
In order to modify insulating objects, such as silica particles, bipolar electrochemistry cannot be used but we are exploring simultaneously alternative approaches. One of them is based on the concept that a removable mask temporarily protects a part of an object in a reactive medium. This is for example possible by carrying out an emulsion polymerization of styrene in the presence of silica particles. Parts of the silica particle will be covered by polymer, thus allowing in a second step the specific functionalization of the uncovered silica surface with a trialkoxysilane derivative. After dissolving the polymer, the unprotected zone of the silica surface can be specifically functionalized in order to introduce the final asymmetry.
Another strategy leading to Janus objects explored by us is based on the use of colloidal microgel particles which can adsorb at an oil-water interface and thus stabilize so-called Pickering emulsions. The microgels which are decorating the surface of the oil droplets intrinsically experience two different chemical environments and thus can be modified selectively on one side. The so-obtained asymmetric objects can carry out, among others, interesting functions such as light emission, catalysis and controlled propulsion.
Scheme of light-assisted bipolar electrodeposition of metals on semiconductor micro- and nanoparticles
- Microswimmers
For controlled propulsion asymmetry is the crucial ingredient and we have explored various concepts leading to so-called microswimmers. In all cases bipolar electrochemistry plays the key role as it is by definition well-suited for symmetry breaking. Essentially there are two different ways of using bipolar electrochemistry for generating controlled motion:
(1) Janus particles generated by bipolar electrodeposition containing a magnetic or a catalytic extremity can be used as synthetic motors [27, 28]. We have shown that carbon microtubes with an electrochemically generated Pt tip on one side can move in H2O2 solutions. The motion was caused by the release of O2 bubbles, due to the catalytic decomposition of H2O2 at the Pt surface.
(2) The asymmetric reactivity, offered by bipolar electrochemistry, can also be used directly to generate motion. The first example of a translational motion was based on a deposition/dissolution mechanism. In this work, a Zn dendrite was placed in capillary that was previously filled with a zinc sulfate solution. When the dendrite is acting as a bipolar electrode, its anodic pole is consumed by the oxidation, while deposition occurs at its cathodic pole, leading to its self-regeneration. This results in an apparent locomotion of the object.[29] Another example is based on asymmetric bubble production, due to water electrolysis at the reactive poles of a spherical bipolar electrode. Because the quantity of produced H2 at the cathodic pole is twice as much as the amount of generated O2 at the anodic pole, the resulting force rolls the bead in a controlled way.[30, 31] A possibility to enhance the propulsion speed and to better control its direction is to quench one of the bubble producing reactions, by adding to the solution a sacrificial molecule (which is easier to oxidize or to reduce than water), thus allowing the bubble production to occur only at one bead pole. A completely different type of motion triggered by bipolar asymmetric reactivity is exemplified with conducting polymer objects. The oxidation and reduction occurring at the two extremities of such objects when they are exposed to the electric field leads to differential swelling and shrinking due to the uptake and release of counter ions. This ultimately results in a deformation of the polymer and can be transformed into controlled motion [32,33].
Deformation and motion of a polypyrrole object triggered by bipolar electrochemistry