Gabriel Loget

Electric field-induced chemical locomotion of conducting objects

Externally triggered motion of small objects has potential in applications ranging from micromachines, to drug delivery, and self-assembly of superstructures. Here we present a new concept for the controlled propulsion of conducting objects with sizes ranging from centimetres to hundreds of micrometres. It is based on their polarization, induced by an electric field, which triggers spatially separated oxidation and reduction reactions involving asymmetric gas bubble formation. This in turn leads to a directional motion of the objects. Depending on the implied redox chemistry and the device design, the speed can be controlled and the motion can be switched from linear to rotational. This type of chemical locomotion is an alternative to existing approaches based on other principles.

Bipolar electrochemistry: from materials science to motion and beyond.

Bipolar electrochemistry, a phenomenon which generates an asymmetric reactivity on the surface of conductive objects in a wireless manner, is an important concept for many purposes, from analysis to materials science as well as for the generation of motion. Chemists have known the basic concept for a long time, but it has recently attracted additional attention, especially in the context of micro- and nanoscience. In this Account, we introduce the fundamentals of bipolar electrochemistry and illustrate its recent applications, with a particular focus on the fields of materials science and dynamic systems. Janus particles, named after the Roman god depicted with two faces, are currently in the heart of many original investigations. These objects exhibit different physicochemical properties on two opposite sides. This makes them a unique class of materials, showing interesting features. They have received increasing attention from the materials science community, since they can be used for a large variety of applications, ranging from sensing to photosplitting of water. So far the great majority of methods developed for the generation of Janus particles breaks the symmetry by using interfaces or surfaces. The consequence is often a low time-space yield, which limits their large scale production. In this context, chemists have successfully used bipolar electrodeposition to break the symmetry. This provides a single-step technique for the bulk production of Janus particles with a high control over the deposit structure and morphology, as well as a significantly improved yield. In this context, researchers have used the bipolar electrodeposition of molecular layers, metals, semiconductors, and insulators at one or both reactive poles of bipolar electrodes to generate a wide range of Janus particles with different size, composition and shape. In using bipolar electrochemistry as a driving force for generating motion, its intrinsic asymmetric reactivity is again the crucial aspect, as there is no directed motion without symmetry breaking. Controlling the motion of objects at the micro- and nanoscale is of primary importance for many potential applications, ranging from medical diagnosis to nanosurgery, and has generated huge interest in the scientific community in recent years. Several original approaches to design micro- and nanomotors have been explored, with propulsion strategies based on chemical fuelling or on external fields. The first strategy is using the asymmetric particles generated by bipolar electrodeposition and employing them directly as micromotors. We have demonstrated this by using the catalytic and magnetic properties of Janus objects. The second strategy is utilizing bipolar electrochemistry as a direct trigger of motion of isotropic particles. We developed mechanisms based on a simultaneous dissolution and deposition, or on a localized asymmetric production of bubbles. We then used these for the translation, the rotation and the levitation of conducting objects. These examples give insight into two interesting fields of applications of the concept of bipolar electrochemistry, and open perspectives for future developments in materials science and for generating motion at different scales.

Propulsion of microobjects by dynamic bipolar self-regeneration.

Dynamic bipolar self-regeneration is a new mechanism that allows controlled motion of metallic microobjects to be induced. This technique is based on the concept of bipolar electrochemistry, in which different redox reactions occur at the two extremities of a substrate under the influence of an external electric field. To create the motion of a metallic object, one end has to be the site of metal deposition and the other the site of metal dissolution. Propulsion of zinc macro- and microswimmers at speeds of up to 80 μm s(-1) has been achieved.

True Bulk Synthesis of Janus Objects by Bipolar Electrochemistry

IO N Referring to the Roman god depicted with two heads, Janus particles are microor nano-objects that exhibit different chemistry or polarity on two opposite sides. [ 1–4 ] This makes them a unique class of materials showing interesting properties. They have received increasing attention from the scientifi c community, since they can be used for a large variety of applications, ranging from catalysis [ 5 ] to medical therapy. [ 6 ] So far the great majority of methods developed for the generation of such objects break the symmetry by using interfaces or surfaces. [ 7–10 ]

Bulk synthesis of Janus objects and asymmetric patchy particles

This review article highlights the most recent advances with respect to the preparation of Janus objects by bulk phase processes. Historically most of the concepts developed for generating asymmetric particles have been based on the use of interfaces or surfaces, which are necessary to break the symmetry. This restricts in many cases the amount of produced particles, due to the two-dimensional nature of the approaches. Therefore the bulk synthesis of such asymmetric micro- and nanoobjects is of primary importance for their production at an industrial scale and helps to open up the field to commercial applications. We summarize here the different alternative concepts, spanning a wide range from sophisticated polymer chemistry to the use of external electromagnetic fields, that have been proposed in recent years in order to break the symmetry in true bulk processes.

Shaping and exploring the micro- and nanoworld using bipolar electrochemistry

AbstractBipolar electrochemistry is a technique with a rather young history in the field of analytical chemistry. Being based on the polarization of a conducting object which is exposed to an external electric field, it allowed recently the development of new methods for controlled surface modification at the micro- and nanoscale and very original analytical applications. Using bipolar electrodes, analyte separation and detection becomes possible based on miniaturized systems. Moreover, the modified objects that can be created with bipolar electrochemistry could find applications as key components for detection systems. In this contribution, the principles of bipolar electrochemistry will be reviewed, as well as recent developments that focus on the modification of objects at the nano- and microscale and their potential application in miniaturized analytical systems. FigureThe polarization of an object in an external electric field leads to bipolar electrochemical reactions. The advantages of bipolar electrochemistry as an emerging original tool in the field of analytical and bioanalytical chemistry are reviewed, with a special focus on the latest developments.

Light-Emitting Electrochemical “Swimmers”†

Swimmer in the dark: propulsion of a conducting object is intrinsically coupled with light emission using bipolar electrochemistry. Asymmetric redox activity on the surface of the swimmer (black bead) causes production of gas bubbles to propel the swimmer in a glass tube with simultaneous electrochemiluminescence (ECL) emission to monitor the progress of the swimmer.

Indirect Bipolar Electrodeposition

Based on the principles of bipolar electrochemistry, localized pH gradients are generated at the surface of conducting particles in solution. This allows the toposelective deposition of inorganic and organic polymer layers via a pH-triggered precipitation mechanism. Due to the intrinsic symmetry breaking of the process, the concept can be used to generate in a straightforward way Janus particles, with one section consisting of deposits obtained from non-electroactive precursors. These indirect electrodeposits, such as SiO(2), TiO(2), or electrophoretic paints, can be further used as an immobilization matrix for other species like dyes or nanoparticles, thus opening promising perspectives for the synthesis of a variety of bifunctional objects with a controlled shape.

Bipolar anodization enables the fabrication of controlled arrays of TiO2 nanotube gradients

We report here a new concept, the use of bipolar electrochemistry, which allows the rapid and wireless growth of self-assembled TiO2 NT layers that consist of highly defined and controllable gradients in NT length and diameter. The gradient height and slope can be easily tailored with the time of electrolysis and the applied electric field, respectively. As this technique allows obtaining in one run a wide range of self-ordered TiO2 NT dimensions, it provides the basis for rapid screening of TiO2 NT properties. In two examples, we show how these gradient arrays can be used to screen for an optimized photocurrent response from TiO2 NT based devices such as dye-sensitized solar cells.

Wireless Electrografting of Molecular Layers for Janus Particle Synthesis

The chemical functionalization of surfaces with biological, redox-active or photosensitive molecules has been shown to be useful for applications ranging from molecular electronics, catalysis and energy conversion to chemical or biochemical sensors. Among several methods proposed to tailor the properties of conducting surfaces, electrografting of diazonium salts is one of the most popular strategies for tuning the chemical nature of substrate surfaces without losing its bulk properties. This electrochemical reduction of diazonium salts allows a covalent attachment of various functional groups to a wide range of substrates, and especially to carbon surfaces. These grafted organic layers are frequently used to fine-tune the surface properties of such substrates. The grafting is usually carried out by normal electrochemical reduction, meaning that the substrate has to be physically connected to an electrode. However in some situations it might be interesting and even mandatory to modify objects that are suspended in a solution, and thus not being in direct contact with an electrode. This is for example the case for the bulk production of Janus particles. It has been shown recently that this can be achieved by bipolar electrochemistry for the asymmetric deposition of metal layers. Herein we present an original method in which this attractive concept is for the first time used to generate a grafted organic layer, localized on one half sphere of carbon beads, thus leading to Janus particles bearing organic functional groups. The concept of bipolar electrochemistry applied to microspheres has been described by Fleischmann et al. Briefly, when a high electric field polarizes an object with sufficient electrical conductivity suspended in a solution, redox reactions can be carried out at the opposite side of the object, namely reductions at the cathodically polarized side and oxidations at the anodically polarized side (Figure 1). The polarization (DV) of the object is directly proportional to the effective length of the particle. This concept has recently become the driving force for the detec-