Valérie Ravaine, Alexander Kuhn, Laurent Bouffier, Dodzi Zigah, Véronique Lapeyre
This part is dedicated to the development of new materials and structures with hierarchical functions, which can be dynamically triggered. This section is divided into 3 sub-sections:
1. Design of functional materials
2. Soft and deformable particles adsorbed at interfaces with tailored packing
3. Bipolar electrochemistry for generating dynamic systems
1.1. Design of functional inorganic nano- and micro-objects by bipolar electrochemistry
Our group has made original contributions concerning the design of inorganic nano- and microobjects, especially dissymmetric, so-called Janus objects. For their synthesis we use an approach 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. We extended the concept in the frame of material science applications, especially by developing direct and indirect bipolar electrodeposition as a straight forward route to particles with a very sophisticated design. The so-obtained objects can carry out, among others, interesting functions such as light emission (see also Micro- and nanoscale Imaging), controlled propulsion and catalysis.
Left: Principle of bipolar electrochemistry: a conductiong object is exposed to an external electric field which leads to a polarization that will allow an oxidation and a reduction reaction to occur at the two opposite sides of the object. Middle: type of objects that can be synthesized. Right: Example of generated Janus particles.
1.2. Design of functional soft and responsive particles
Nano- and microgels are particles made of swollen cross-linked polymers. These soft particles undergo volume phase transitions upon change in their environment such as temperature (see also Micro- and nanoscale Imaging), pH or recognition of a biomolecule (see also Chemical Sensors for Biology). Our group develops various methods for the synthesis of nano-and microgels with controlled internal structure: core-shell particles, hybrid microgels, hydrogel nanocapsules or Janus-type microgels which offer many possibilities for multisensitivity with a hierarchical response.
Transmission electron microscopy images of microgels - Left: plain microgel of thermoresponsive poly(N-isopropylacrylamide); middle: hybrid core-shell microgels with a silica core and a hydrogal shell; right: nanocapsules of hydrogels obtained by dissolution of a sacrificial core.
Colloidal gel particles called microgels have shown their ability to adsorb at an oil-water interface and stabilize emulsions named Pickering emulsions. Such particles are soft, deformable, porous and they can swell or contract under the action of an external stimulus. These specificities make them highly versatile emulsifiers, leading to a large panel of emulsions and materials elaborated thereof. We investigate the mechanisms responsible for emulsion stabilization through studies of microgel conformation and packing at the interface, in relation with the mechanical properties of the interfaces. A better understanding of these features provides powerful tools to develop new complex materials with well-dedicated functionalities.
Examples of emulsions stabilized by microgels, in relation with their organization at the oil-water interface: oil-in-water emulsions with isolated drops are obtained from interfaces covered with densely packed microgels: oil-in-water emulsions with bridged drops are obtained from interfaces covers with heterogeneously packed microgels; water-in-oil emulsions are obtained from oil-swollen microgels forming multilayers at the interface
The fabrication and study of objects that can move in a controlled way and perform tasks at small scales has attracted huge interest across many areas of science ranging from biology to physics. The intrinsic asymmetry provided by bipolar electrochemistry is a very appealing approach for the generation of motion of electrically conducting particles. There are three different ways to generate motion based on bipolar electrochemistry:
3.1. Janus particles generated by bipolar electrodeposition containing a magnetic or a catalytic extremity can be used as synthetic motors. Recently, 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.
3.2. 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 (see video below). This phenomenon can also be considered as a propagating chemical wave.
3.3. The last strategy 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. 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. Translational motion has been induced on millimeter-sized metal beads, and on micrometer-sized carbon beads in polydimethylsiloxane (PDMS) microchannels (right side of the Figure above). We have extended this concept more recently to the levitation of objects and also coupled with simultaneous light emission generated by electrochemiluminescence (see also Micro- and Nanoscale Imaging).