Wireless Electrochemical Actuation of Conducting Polymers

Unconventional approaches for chiral resolution

Chirality is a fundamental and ubiquitous property of nature involved in multiple fields of science. In particular, the possible resolution of the enantiomeric forms of a molecule is crucial in the pharmaceutical, food, and agrochemical industries. The search for efficient, broad-spectrum, and yet simple methods for obtaining enantiomerically pure substances is a current challenge. Enantioselective resolution methods rely on an asymmetric environment that allows the two antipodes of a chiral molecule to be distinguished. In addition to enantiomeric separation techniques, such as chromatography and electrophoresis, new promising approaches involving out-of-the-scheme synergistic effects between chiral selectors (CS) and external stimuli are emerging. This Trends article discusses different enantioselective mechanisms triggered by unconventional physicochemical stimuli for the design of avant-garde approaches that could offer novel perspectives in the field of chiral resolution.

Wireless Hollow Miniaturized Objects for Electroassisted Chiral Resolution

Chiral resolution plays a crucial role in the field of drug development, especially for a better understanding of biochemical processes. In such a context, classic separation methods have been used for decades due to their versatility and easy scale-up. Among the many attempts proposed for enantioselective separation, electroassisted methods are presented as an interesting alternative. Herein, we present the use of wirelessly activated hollow tubular systems for the effective, simple, and tunable separation of racemic and enantioenriched mixtures. These double-layered tubular objects consist of an external polypyrrole chassis, a polymer with good electromechanical properties, functionalized in its inner part with an inherently chiral oligomer. The synergy between the electromechanical pumping process of the outer layer and the enantioselective affinity of the inner part induces the system to behave as a miniaturized chiral column. These hybrid objects are able to separate racemic and enantioenriched solutions of chiral model analytes into the corresponding enantiomers in high enantiomeric purity. Finally, these electromechanical systems can resolve mixtures formed by chiral probes with completely uncorrelated molecular structures injected simultaneously into the single antipodes.

Wireless Electrochemical Gel Actuators

Soft actuators generate motion in response to external stimuli and are indispensable for soft robots, particularly future miniature robots with complex structure and motion. Similarly to conventional hard robots, electricity is suitable for the stimulation. However, previous electrochemical soft actuators require a tethered connection to a power supply, limiting their size, structure, and motion. Here, wireless electrochemical soft actuators composed of hydrogels and driven by bipolar electrochemistry are reported. Viologen, which dimerizes by one-electron reduction and dissociates by one-electron oxidation, is incorporated in the side chains of the gel networks and works as a reversible cross-link. Wireless and reversible electrochemical actuation of the hydrogels, i.e., muscle-like shrinking and swelling, is demonstrated at microscopic and even macroscopic scales.

Bi-enzymatic chemo-mechanical feedback loop for continuous self-sustained actuation of conducting polymers

Artificial actuators have been extensively studied due to their wide range of applications from soft robotics to biomedicine. Herein we introduce an autonomous bi-enzymatic system where reversible motion is triggered by the spontaneous oxidation and reduction of glucose and oxygen, respectively. This chemo-mechanical actuation is completely autonomous and does not require any external trigger to induce self-sustained motion. The device takes advantage of the asymmetric uptake and release of ions on the anisotropic surface of a conducting polymer strip, occurring during the operation of the enzymes glucose oxidase and bilirubin oxidase immobilized on its surface. Both enzymes are connected via a redox polymer at each extremity of the strip, but at the opposite faces of the polymer film. The time-asymmetric consumption of both fuels by the enzymatic reactions produces a double break of symmetry of the film, leading to autonomous actuation. An additional break of symmetry, introduced by the irreversible overoxidation of one extremity of the polymer film, leads to a crawling-type motion of the free-standing polymer film. These reactions occur in a virtually unlimited continuous loop, causing long-term autonomous actuation of the device.

Wireless electrochemical actuation of soft materials towards chiral stimuli

Different areas of modern chemistry, require wireless systems able to transfer chirality from the molecular to the macroscopic event. The ability to recognize the enantiomers of a chiral analyte is highly desired, since in the majority of cases such molecules present different physico-chemical properties that could lead, eventually, to dangerous or harmful interactions with the environment or the human body. From an electrochemical point of view, enantiomers have the same electrochemical behavior except when they interact in a chiral environment. In this Feature Article, different approaches for the electrochemical recognition of chiral information based on the actuation of conducting polymers are described. Such a dynamic behavior of π-conjugated materials is based on an electrochemically induced shrinking/swelling transition of the polymeric matrix. Since all the systems, described so far in the literature, are achiral and require a direct connection to a power supply, new strategies will be presented in the manuscript, concerning the implementation of chirality in electrochemical actuators and their use in a wireless manner through bipolar electrochemistry. Herein, the synergy between the wireless unconventional actuation and the outstanding enantiorecognition of inherent chiral oligomers is presented as an easy and straightforward read out of chiral information in solution. This approach presents different advantages in comparison to classic electrochemical systems such as its wireless nature and the possible real-time data acquisition.

Wireless electromechanical enantio-responsive valves

Microfluidic valves based on chemically responsive materials have gained considerable attention in recent years. Herein, a wireless enantio-responsive valve triggered by bipolar electrochemistry combined with chiral recognition is reported. A conducting polymer actuator functionalized with the enantiomers of an inherently chiral oligomer was used as bipolar valve to cover a tube loaded with a dye and immersed in a solution containing chiral analytes. When an electric field is applied, the designed actuator shows a reversible cantilever-type deflection, allowing the release of the dye from the reservoir. The tube can be opened and closed by simply switching the polarity of the system. Qualitative results show the successful release of the colorant, driven by chirality and redox reactions occurring at the bipolar valve. The device works well even in the presence of chemically different chiral analytes in the same solution. These systems open up new possibilities in the field of microfluidics, including also controlled drug delivery applications.

Monitoring Bipolar Electrochemistry and Hydrogen Evolution Reaction of a Single Gold Microparticle under Sub-Micropipette Confinement

Herein, an approach to track the process of autorepeating bipolar reactions and hydrogen evolution reaction (HER) on a micro gold bipolar electrode (BPE) is established. Once blocking the channel of the sub-micropipette tip, the formed gold microparticle is polarized into the wireless BPE, which induces the dissolution of the gold at the anode and the HER at the cathode. The current response shows a periodic behavior with three regions: the bubble generation region (I), the bubble rupture/generation region (II), and the channel opening region (III). After a stable low baseline current of region I, a series of positive spike signals caused by single H2 nanobubbles rupture/generation are recorded standing for the beginning of region II. Meanwhile, the dissolution of the gold blocking at the orifice will create a new channel, increasing the baseline current for region III, where the synthesis of gold occurs again, resulting in another periodic response. Finite element simulations are applied to unveil the mechanism thermodynamically. In addition, the integral charge of the H2 nanobubbles in region II corresponds to the consumption of the anode gold. It simultaneously monitors autorepeating bipolar reactions of a single gold microparticle and HER of a single H2 nanobubble electrochemically, which reveals an insightful physicochemical mechanism in nanoscale confinement and makes the glass nanopore an ideal candidate to further reveal the heterogeneity of catalytic capability at the single particle level.

Bipolar Electrochemical Measurement of Enantiomeric Excess with Inherently Chiral Polymer Actuators

Straightforward enantioselective analytical methods are very important for drug safety, considering that in certain cases one of the two enantiomers of a chiral molecule might be harmful for humans. In this work, we propose a simple system for the direct and easy read-out of the enantiomeric excess of 3,4-dihydroxyphenylalanine (DOPA) as a model analyte. A conducting oligomer, i.e. oligo-(3,3'-dibenzothiophene), bearing inherently chiral features, is electrogenerated on a polypyrrole film. The resulting freestanding hybrid material is used as a wireless enantioselective actuator in a bipolar electrochemical cell. Combining in a single setup two individual actuators with opposite chiral features allows a direct visual read-out of enantiomeric excess, as the bending amplitude of each of the two actuators is directly correlated with the concentration of the corresponding stereoisomer of the analyte. Optimization of the experimental parameters results in efficient bending, giving access to the percentage values of the enantiomeric excess in mixtures containing different ratios of the antipodes, thus opening the way to potential applications for chiral in situ analysis.

Direct dynamic read-out of molecular chirality with autonomous enzyme-driven swimmers

A key approach for designing bioinspired machines is to transfer concepts from nature to man-made structures by integrating biomolecules into artificial mechanical systems. This strategy allows the conversion of molecular information into macroscopic action. Here, we describe the design and dynamic behaviour of hybrid bioelectrochemical swimmers that move spontaneously at the air-water interface. Their motion is governed by the diastereomeric interactions between immobilized enantiopure oligomers and the enantiomers of a chiral probe molecule present in solution. These dynamic bipolar systems are able to convert chiral information present at the molecular level into enantiospecific macroscopic trajectories. Depending on the enantiomer in solution, the swimmers will move clockwise or anticlockwise; the concept can also be used for the direct visualization of the degree of enantiomeric excess by analysing the curvature of the trajectories. Deciphering in such a straightforward way the enantiomeric ratio could be useful for biomedical applications, for the read-out of food quality or as a more general analogue of polarimetric measurements.

Bipolar (Bio)electroanalysis

This contribution reviews a selection of the most recent studies on the use of bipolar electrochemistry in the framework of analytical chemistry. Despite the fact that the concept is not new, with several important studies dating back to the middle of the last century, completely novel and very original approaches have emerged over the last decade. This current revival illustrates that scientists still (re)discover some exciting virtues of this approach, which are useful in many different areas, especially for tackling analytical challenges in an unconventional way. In several cases, this "wireless" electrochemistry strategy enables carrying out measurements that are simply not possible with classic electrochemical approaches. This review will hopefully stimulate new ideas and trigger scientists to integrate some aspects of bipolar electrochemistry in their work in order to drive the topic into yet unexplored and eventually completely unexpected directions.

Designing tubular conducting polymer actuators for wireless electropumping

Rational design and shaping of soft smart materials offer potential applications that cannot be addressed with rigid systems. In particular, electroresponsive elastic materials are well-suited for developing original active devices, such as pumps and actuators. However, applying the electric stimulus requires usually a physical connection between the active part and a power supply. Here we report about the design of an electromechanical system based on conducting polymers, enabling the actuation of a wireless microfluidic pump. Using the electric field-induced asymmetric polarization of miniaturized polypyrrole tubes, it is possible to trigger simultaneously site-specific chemical reactions, leading to shrinking and swelling in aqueous solution without any physical connection to a power source. The complementary electrochemical reactions occurring at the opposite extremities of the tube result in a differential change of its diameter. In turn, this electromechanical deformation allows inducing highly controlled fluid dynamics. The performance of such a remotely triggered electrochemically active soft pump can be fine-tuned by optimizing the wall thickness, length and inner diameter of the material. The efficient and fast actuation of the polymer pump opens up new opportunities for actuators in the field of fluidic or microfluidic devices, such as controlled drug release, artificial organs and bioinspired actuators.

Tubular conducting polymer actuators are used for developing a wireless electropumping device. Bipolar electrochemistry, allowing symmetry breaking in terms of polarization and electrochemical reactions, is the key ingredient for efficient pumping.

Light and electric field induced unusual large-scale charge separation in hybrid semiconductor objects

Separation of electric charges is the most crucial phenomenon in natural photosynthesis, and is also extremely important for many artificial energy conversion systems based on semiconductors. The usual roadblock in this context is the fast recombination of electrons and holes. Here we demonstrate that the synergy of light and electric fields allows separating very efficiently electric charges over an unusually large distance in TiO2. The generated internal electric field can also be used to shuttle electrons simultaneously to the two opposite sides of a hybrid TiO2-polyaniline object. This counterintuitive behavior is based on the combination of the principles of bipolar electrochemistry and semi-conductor physics.

Tailoring the nanostructures of electrochemical actuators for fast response and large deformation

Electrochemical actuators (EAs) can effectively convert electric energy to mechanical energy through chemical reactions. However, the response rate and deformability, two of the crucial and antithetic factors in EA studies, can both hardly be improved just by developing or hybridizing different kinds of materials. In this work, this challenge is overcome through tailoring the nanostructures of EAs. A 3D nanoporous structure formed by aggregating spherical MoS₂ nanoparticles (NPs) is reported. The NP-aggregated nanoporous structure not only provides a fast ion-migration process but also ensures strong mechanical strength. Experiments show that the voltage-dependent response rate and curvature amplitude respectively approach 0.015 mm^-1·s^-1·V^-1 and 0.244 mm^-1·V^-1, which simultaneously exceed those of most EAs. A continuous energy density of 14 kJ·m^-3, almost double that of mammalian muscle, enables the EA to rotate a stainless-steel weight which is over 550 times heavier than itself. By opening a new way to improve EAs' comprehensive performance, this research propels their potential applications in microrobotics and mini-medical devices.

Underestimated Color Centers: Defects as Useful Reducing Agents in Lanthanide-Activated Luminescent Materials

Inorganic hosts, such as SrB4 O7 or certain nitrides, intrinsically stabilize Eu2+ even when the dopant is an Eu3+ -based precursor and reducing conditions are not employed in the synthesis. Although this concept is well known in the synthesis of phosphorescent materials, the mechanistic details are scarcely understood. Herein, we demonstrate that trapped charge carriers, such as color centers, can also act as redox partners to stabilize certain oxidation states of activators. Eu-activated CsMgCl3 and CsMgBr3 are used as examples. Upon doping with EuCl3 and in the absence of reducing conditions during the synthesis, dominant cyan or green luminescence from Eu2+ ions was observed. Photoluminescence spectroscopy at 10 K revealed that the reduction is correlated to color centers localized at defects. Although defects are typically undesired in phosphors, we have shown that their role may be underestimated and they could be used on purpose in the preparation of selected inorganic phosphors.

Organic Electropolymerized Multilayers for Light-Emitting Diodes and Displays

In electrochemistry, the carbazole is generally coupled to dimer but not to polymer. This work has reported that organic electropolymerization (OEP) of 4,4',4″-tri(N-carbazolyl)triphenylamine (TCTA) would form a high cross-linked carbazole polymer by its high activity/reversibility and a synchronous viscosity control. It has significantly improved the OEP film quality of both hole-transporting and electroluminescent layers in organic light-emitting diodes. As a result, the conductivity and power efficiency of the organic light-emitting diodes with TCTA are eight and four times of that without TCTA. A prototype display device with a 1.7 in. monochrome passive matrix of 58 ppi under the driving chip is successfully fabricated with accurate pixel size and uniform electroluminescence, which shows a great potential of OEP in the electroluminescent application.

Physical Chemistry, a Discipline in Its Golden Age

Alexander Kuhn is Full Professor at the Institute of Molecular Science (University Bordeaux, CNRS, Bordeaux INP) as well as Adjunct Professor at VISTEC (Thailand) and Henan University (China). He currently serves as a chair of the ChemPhysChem Editorial Board.

Spatially resolved electrochemical reversibility of a conducting polymer thin film imaged by oblique-incidence reflectivity difference

The spatially resolved electrochemical reversibility of a polyaniline (PANI) thin film is successfully imaged by an oblique-incidence reflectivity difference (OIRD) technique.

Chiral platinum-polypyrrole hybrid films as efficient enantioselective actuators

We report the synthesis of a hybrid bilayer, being composed of a free-standing conducting polymer film and a layer of mesoporous metal, encoded with chiral features. The resulting structure constitutes an enantioselective actuator, which can be electrochemically addressed in a wireless way. The controlled discriminatory deformation of the film allows an easy readout of chiral information.

Wireless Coupling of Conducting Polymer Actuators with Light Emission

Combining the actuation of conducting polymers with additional functionalities is an interesting fundamental scientific challenge and increases their application potential. Herein we demonstrate the possibility of direct integration of a miniaturized light emitting diode (LED) in a polypyrrole (PPy) matrix in order to achieve simultaneous wireless actuation and light emission. A light emitting diode is used as a part of an electroactive surface on which electrochemical polymerization allows direct incorporation of the electronic device into the polymer. The resulting free-standing polymer/LED hybrid can be addressed by bipolar electrochemistry to trigger simultaneously oxidation and reduction reactions at its opposite extremities, leading to a controlled deformation and an electron flow through the integrated LED. Such a dual response in the form of actuation and light emission opens up interesting perspectives in the field of microrobotics.

Rational Design of Silver Gradient for Studying Size Effect of Silver Nanoparticles on Contact Killing

The cellular mechanism underlying bacteria responses to silver nanoparticles (AgNPs) has not been fully elucidated. Especially, it is difficult to distinguish the contact killing from release killing as Ag⁺ releases from AgNPs. In this paper, AgNPs gradient was designed for evaluating the size effect of AgNPs on contact killing. A size gradient of AgNPs (5-45 nm) was achieved on TiO₂ nanotubes (TNTs) by rational design of bipolar electrochemical reaction, including applied voltage, electrolyte concentration, and sample size. High-throughput investigation of cellular responses showed that the smallest AgNPs were the most efficient in suppressing bacteria whereas the largest AgNPs were more favorable for MC3T3-E1 cell adhesion and proliferation. As Ag⁺ concentration was the same for the entire gradient, the difference in cellular responses was dominated by the contact effect (rather than difference in released Ag⁺) which was tuned by AgNPs size. This method offers new prospect for efficient evaluation of the contact effect of nanoparticles, such as Ag, Au, and Cu.

Dynamic Bipolar Electrode Array for Visualized Screening of Electrode Materials in Light-Emitting Electrochemical Cells

Charge injection at a metal/semiconductor interface is of paramount importance for many chemical and physical processes. The dual injection of electrons and holes, for example, is necessary for electroluminescence in organic light-emitting devices. In an electrochemical cell, charge transfer across the electrode interface is responsible for redox reactions and Faradic current flow. In this work, we use polymer light-emitting electrochemical cells (PLECs) to visually assess the ability of metals to inject electronic charges into a luminescent polymer. Silver, aluminum, and gold microdisks are deposited between the two driving electrodes of the PLEC in the form of a horizontal array. When the PLEC is polarized, the individual disks functioned as bipolar electrodes (BPEs) to induce redox p- and n-doping reactions at their extremities, which are visualized as strongly photoluminescence-quenched growth in the luminescent polymer. The three metals initially generate highly distinct doping patterns that are consistent with differences in their work function. Over time, the doped regions continue to grow in size. Quantitative analysis of the n/p area ratio reveals an amazing convergence to a single value for all 39 BPEs, regardless of their metal type and large variation in the size of individual doped areas. We introduce the concept of a dynamic BPE, which transforms from an initial metal disk of a fixed size to one that is a composite of p- and n-doped polymer joined by the initial metallic BPE. The internal structure of the dynamic BPE, as measured by the n/p area ratio, reflects the properties of only the mixed conductor of the PLEC active layer itself when the area ratio converges.

Wireless Electromechanical Readout of Chemical Information

Collecting electrochemical information concerning the presence of molecules in a solution is usually achieved by measuring current, potential, resistance, or impedance via connection to a power supply. Here, we suggest wireless electromechanical actuation as a straightforward readout of chemical information. This can be achieved based on the concept of bipolar electrochemistry, which allows measuring the presence of different model species in a quantitative way. We validate the concept by using a free-standing polypyrrole film. Its positively polarized extremity participates in an oxidation of the analyte and delivers electrons to the opposite extremity for the reduction of the polymer. This reduction is accompanied by the insertion of counterions and thus leads to partial swelling of the film, inducing its bending. The resulting actuation is found to be a linear function of the analyte concentration, and also a Michaelis-Menten type correlation is obtained for biochemical analytes. This electromechanical transduction allows an easy optical readout and opens up very interesting perspectives not only in the field of sensing but also far beyond, such as for the elaboration of self-regulating biomimetic systems.

Controlling Nerve Growth with an Electric Field Induced Indirectly in Transparent Conductive Substrate Materials

Innovative neurostimulation therapies require improved electrode materials, such as poly(3,4-ethylenedioxythiophene) (PEDOT) polymers or IrO_x mixed ionic-electronic conductors and better understanding of how their electrochemistry influences nerve growth. Amphibian neurons growing on transparent films of electronic (metal) conductors and electronic-ionic conductors (polymers and semiconducting oxides) are monitored. Materials are not connected directly to the power supply, but a dipole is created wirelessly within them by electrodes connected to the culture medium in which they are immersed. Without electrical stimulation neurons grow on gold, platinum, PEDOT-polystyrene sulfonate (PEDOT-PSS), IrO_x , and mixed oxide (Ir-Ti)O_x , but growth is not related to surface texture or hydrophilicity. Stimulation induces a dipole in all conductive materials, but neurons grow differently on electronic conductors and mixed-valence mixed-ionic conductors. Stimulation slows, but steers neurite extension on gold but not on platinum. The rate and direction of neurite growth on PEDOT-PSS resemble that on glass, but on IrO_x and (Ir-Ti)O_x neurites grow faster and in random directions. This suggests electrochemical changes induced in these materials control growth speed and direction selectively. Evidence that the electric dipole induced in conductive material controls nerve growth will impact electrotherapies exploiting wireless stimulation of implanted material arrays, even where transparency is required.