Wireless Multimodal Light-Emitting Arrays Operating on the Principles of LEDs and ECL

Electrochemistry-based light-emitting devices have gained considerable attention in different applications such as sensing and optical imaging. In particular, such systems are an interesting alternative for the development of multimodal light-emitting platforms. Herein we designed a multicolor light-emitting array, based on the electrochemical switch-on of light-emitting diodes (LEDs) with a different intrinsic threshold voltage. Thermodynamically and kinetically favored coupled redox reactions, i. e. the oxidation of Mg and the reduction of protons on Pt, act as driving force to power the diodes. Moreover, this system enables to trigger an additional light emission based on the interfacial reductive-oxidation electrochemiluminescence (ECL) mechanism of the Ru(bpy)3 2+/S2O8 2- system. The synergy between these light-emission pathways offers a multimodal platform for the straightforward optical readout of physico-chemical information based on composition changes of the solution.

Wireless Magnetoelectrochemical Induction of Rotational Motion

Electromagnetically induced rotation is a key process of many technological systems that are used in daily life, especially for energy conversion. In this context, the Lorentz force-induced deviation of charges is a crucial physical phenomenon to generate rotation. Herein, they combine the latter with the concept of bipolar electrochemistry to design a wireless magnetoelectrochemical rotor. Such a device can be considered as a wet analog of a conventional electric motor. The main driving force that propels this actuator is the result of the synergy between the charge-compensating ion flux along a bipolar electrode and an external magnetic field applied orthogonally to the surface of the object. The trajectory of the wirelessly polarized rotor can be controlled by the orientation of the magnetic field relative to the direction of the global electric field, producing a predictable clockwise or anticlockwise motion. Fine-tuning of the applied electric field allows for addressing conducting objects having variable characteristic lengths.

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.

Bulk Electrosynthesis of Patchy Particles with Highly Controlled Asymmetric Features

Asymmetric modification of particles with various patches of different composition and size at predefined positions is an important challenge in contemporary surface chemistry, as such particles have numerous potential applications, ranging from materials science and (photo)catalysis to self-assembly and drug delivery. However, approaches allowing the synthesis of this kind of complex objects in the bulk of a solution in a straightforward way are currently lacking. In this context, bipolar electrochemistry (BE) is a powerful technique for the asymmetric modification of conducting objects. Herein, this approach is used for the highly controlled modification of particles with different metal patches, generated at specific locations of isotropic objects. The synthesis is carried out in the bulk of the solution and leads to predefined patterns of increasing complexity, including even a specific chiral arrangement of the patches.

Spatial Precision Tailoring the Catalytic Activity of Graphene Monolayers for Designing Janus Swimmers

Graphene monolayers have interesting applications in many fields due to their intrinsic physicochemical properties, especially when they can be postmodified with high precision. Herein, we describe the highly site-selective functionalization of freestanding graphene monolayers with platinum (Pt) clusters by bipolar electrochemistry. The deposition of such metal spots leads to catalytically active hybrid two-dimensional (2D) nanomaterials. Their catalytic functionality is illustrated by the spatially controlled decomposition of hydrogen peroxide, inducing motion at the water/air interface due to oxygen bubble evolution. A series of such 2D Janus structures with Pt deposition at predefined positions (corners and edges) is studied with respect to the generation of autonomous motion. The type and speed of motion can be fine-tuned by controlling the deposition time and location of the Pt clusters. These proof-of-principle experiments indicate that this type of hybrid 2D object opens up interesting perspectives in terms of applications, such as environmental detection or remediation.

Flow-through Gas Phase Photocatalysis Using TiO2 Nanotubes on Wirelessly Anodized 3D-Printed TiNb Meshes

In this work, for the first time 3D Ti-Nb meshes of different composition, i.e., Ti, Ti-1Nb, Ti-5Nb, and Ti-10 Nb, were produced by direct ink writing. This additive manufacturing method allows tuning of the mesh composition by simple blending of pure Ti and Nb powders. The 3D meshes are extremely robust with a high compressive strength, giving potential use in photocatalytic flow-through systems. After successful wireless anodization of the 3D meshes toward Nb-doped TiO₂ nanotube (TNT) layers using bipolar electrochemistry, they were employed for the first time for photocatalytic degradation of acetaldehyde in a flow-through reactor built based on ISO standards. Nb-doped TNT layers with low concentrations of Nb show superior photocatalytic performance compared with nondoped TNT layers due to the lower amount of recombination surface centers. High concentrations of Nb lead to an increased number of recombination centers within the TNT layers and reduce the photocatalytic degradation rates.

Bipolar Verdazyl Radicals for Symmetrical Batteries: Properties and Stability in All States of Charge

Redox flow batteries based on organic electrolytes are promising energy storage devices, but stable long-term cycling is often difficult to achieve. Bipolar organic charge-storage materials allow the construction of symmetrical flow batteries (i. e., with identical electrolyte composition on both sides), which is a strategy to mitigate crossover-induced degradation. One such class of bipolar compounds are verdazyl radicals, but little is known on their stability/reactivity either as the neutral radical, or in the charged states. Here, we study the chemical properties of a Kuhn-type verdazyl radical (1) and the oxidized/reduced form (1^+/- ). Chemical synthesis of the three redox-states provides spectroscopic characterization data, which are used as reference for evaluating the composition of the electrolyte solutions of an H-cell battery during/after cycling. Our data suggest that, rather than the charged states, the decomposition of the parent verdazyl radical is responsible for capacity fade. Kinetic experiments and DFT calculations provide insight in the decomposition mechanism, which is shown to occur by bimolecular disproportionation to form two closed-shell products (leuco-verdazyl 1H and triazole derivative 2).

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.

Efficient Removal of Mercury from Polluted Aqueous Solutions Using the Wireless Bipolar Electrochemistry Technique

Mercury represents one of the major toxic pollutants in water that affect human and ecosystem. Extensive efforts have been globally invested to remove mercury using various chemical and electrochemical approaches. In this study, I propose the use of bipolar electrochemistry for the first time for mercury depollution process. Mercury(II) is removed from aqueous solutions by direct electrodeposition on millimeter scale graphite rods held in a bipolar setup. By adjusting the strength of the applied electric field and the number of the graphite rods the efficiency of the system can be controlled. This wireless technique allows the use of multiple graphite rod arrays within the bulk cell which resulted in high removal efficiency (98 %) of Hg²⁺ ions from the polluted solution. The method is straightforward, green, and efficient. The concept can be adapted to remove other heavy metal ions or electrochemically active contaminants from polluted water as long as their reduction potentials are within the water stability window.

High-Throughput Surface-Enhanced Raman Scattering for Screening Chemical Sensor Candidates Enabled by Bipolar Electrochemistry

A variety of hydrothermal or electrochemical methods have been explored to prepare noble metal nanostructures as surface-enhanced Raman scattering (SERS) substrates. However, most of those metallic nanoarrays are structurally homogeneous, which makes it laborious to select the high-performance substrates for particular Raman sensing purposes. Here, a high-throughput SERS imaging strategy is demonstrated for the first time for screening chemical sensors with sub-nanomolar sensitivities. Bipolar electrochemistry was applied to generate Au or Au-Ag gradient nanoarrays with diverse chemical compositions, morphologies, and particle dimensions ranging from several nanometers to micrometers. The selected "hot-spots" on the Au-Ag alloy nanoarray exhibited a 660-fold enhancement in SERS intensity compared to those on the pure Au gradient nanoarray. The SERS screening of 4-aminothiophenol, 4-nitrothiophenol, and 4-mercaptobenzoic acid was carried out that provided a limit of detection (LOD) between 1 and 5 pM. The distinctive LODs among three thiophenolic Raman probes are ascribed to the differences in the affinity of the probe to the alloy, orientation of the metal-ligand monolayer, or plasmonic environment of the nanoarray surface. As a continuous, rapid, and cost-effective manner to fabricate transitional nanostructures and screen out SERS responsive sites, this method not only facilitates controllable synthesis of noble metal nanoarrays but has the potential to provide an alternative tool for ultrasensitive chemical sensing on a wide range of bimetallic substrates.

Electropolymerization of Pyrrole-Based Ionic Liquids on Selected Wireless Bipolar Electrodes

This paper describes an electropolymerization-based on-chip valving system, accomplished by electrosynthesis of conductive polymeric ionic liquid (CPIL) films at selected points within an array of bipolar electrodes (BPEs), in which each of these wireless electrodes spans an IL-aqueous phase boundary. The low viscosity and high hydrophobicity of the CPIL precursor allow it to be patterned by established microfluidic methods. This advancement has the potential to impact microscale analysis because it allows on-demand creation of solid CPIL microstructures at locations specified by microfluidics, phase boundaries, and electrode potentials. To achieve this outcome, an imidazolium-based IL was functionalized with a pyrrole moiety, and the viscosity was tuned by choosing the appropriate counterion to form a CPIL with the desired viscosity, hydrophobicity, and oxidation potential. This monomer species was then introduced into a microfluidic device, which was prefilled with an aqueous buffer solution. The device comprised many parallel microchannels lined with nanoliter-scale chambers. BPEs interconnected the channels such that the BPE tips were each aligned to a chamber opening. The electrodes contacting the outermost channels were connected directly to a power supply and functioned as driving electrodes. The CPIL displaced the buffer in the channels and established a phase boundary at the opening of each chamber, thereby digitizing the aqueous phase. Finally, an alternating square waveform (under mode 1) was applied for 5 min to yield immobilized polymer films at a location defined by the BPE poles. In total, three modes were developed and three corresponding polymer film patterns were formed. Under mode 2, a DC power supply was used to achieve a dissymmetrical polymer film pattern, and, under mode 3, a regional polymer film pattern was formed under an AC potential with a DC offset. Our preliminary results demonstrate that the generated polymer films are immobile and sufficiently thick to seal the chambers at room temperature over the duration of our observation window (50 min), and this seal is maintained even at elevated temperatures that induce partial evaporation of the chamber contents. A key point is that this method is compatible with a preceding step─dielectrophoretic capture of single melanoma cells within the nanoliter-scale chambers.

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.

On-Chip Optical Anodic Stripping with Closed Bipolar Cells and Cathodic Electrochemiluminescence Reporting

The aim of this work was to develop a simple, accessible, and point-of-use sensor to measure heavy metal ions in water in low-resource areas that cannot accommodate expensive or technical solutions. This report describes a new bipolar electrochemical sensor platform that reimagines conventional anodic stripping voltammetry in a wireless bipolar format with an optical electrochemiluminescent readout that can be quantified with any simple optical sensor like that found on most modern cell phone cameras. We call this technique as optical anodic stripping. Using a new nonlithographic fabrication process, devices could be produced rapidly and simply at <$1/sensor. The sensing scheme was developed, characterized, and optimized using electrochemical and optical methods. Quantitation of Pb²⁺ in both lab and natural water samples was rapid (2-3 min), accurate, precise, and highly linear in the 25-1000 ppb range and was shown to be sufficiently selective in the presence of other common heavy metal ions such as Cu²⁺, Cd²⁺, and Zn²⁺.

Anodic TiO2 Nanotubes on 3D-Printed Titanium Meshes for Photocatalytic Applications

In this work, large 3D Ti meshes fabricated by direct ink writing were wirelessly anodized for the first time to prepare highly photocatalytically active TiO₂ nanotube (TNT) layers. The use of bipolar electrochemistry enabled the fabrication of TNT layers within the 3D Ti meshes without the establishment of an electrical contact between Ti meshes and the potentiostat, confirming its unique ability and advantage for the synthesis of anodic structures on metallic substrates with a complex geometry. TNT layers with nanotube diameters of up to 110 nm and thicknesses of up to 3.3 μm were formed. The TNT-layer-modified 3D Ti meshes showed a superior performance for the photocatalytic degradation of methylene blue in comparison to TiO₂-nanoparticle-decorated and nonanodized Ti meshes (with a thermal oxide layer), resulting in multiple increases in the dye degradation rate. The results presented here open new horizons for the employment of anodized 3D Ti meshes in various flow-through (photo)catalytic reactors.

Bulk Electrocatalytic NADH Cofactor Regeneration with Bipolar Electrochemistry

Electrochemical regeneration of reduced nicotinamide adenine dinucleotide (NADH) is an extremely important challenge for the electroenzymatic synthesis of many valuable chemicals. Although some important progress has been made with modified electrodes concerning the reduction of NAD⁺ , the scale-up is difficult due to mass transport limitations inherent to large-size electrodes. Here, we propose instead to employ a dispersion of electrocatalytically active modified microparticles in the bulk of a bipolar electrochemical cell. In this way, redox reactions occur simultaneously on all of these individual microelectrodes without the need of a direct electrical connection. The concept is validated by using [Rh(Cp*)(bpy)Cl]⁺ functionalized surfaces, either of carbon felt as a reference material, or carbon microbeads acting as bipolar objects. In the latter case, enzymatically active 1,4-NADH is electroregenerated at the negatively polarized face of the particles. The efficiency of the system can be fine-tuned by controlling the electric field in the reaction compartment and the number of dispersed microelectrodes. This wireless bioelectrocatalytic approach opens up very interesting perspectives for electroenzymatic synthesis in the bulk phase.

Hybrid light-emitting devices for the straightforward readout of chiral information

Bipolar electrochemistry has gained increasing attention in recent years as an attractive transduction concept in analytical chemistry in general and, more specifically, in the frame of chiral recognition. Herein, we use this concept of wireless electrochemistry, based on the combination of the enantioselective oxidation of a chiral probe with the emission of light from a light-emitting diode (LED), as an alternative for an easy and straightforward readout of the presence of chiral molecules in solution. A hybrid polymer-microelectronic device was designed, using an inherently chiral oligomer, that is, oligo-(3,3'-dibenzothiophene) and a polypyrrole strip as the anode and cathode of a miniaturized LED. The wireless induced redox reactions trigger light emission when the probe with the right chirality is present in solution, whereas no light emission is observed for the opposite enantiomer. The average light intensity shows a linear correlation with the analyte concentration, and the concept opens the possibility to quantify the enantiomeric excess in mixtures of the molecular antipodes.

Electric Field Induced Biomimetic Transmembrane Electron Transport Using Carbon Nanotube Porins

Cells modulate their homeostasis through the control of redox reactions via transmembrane electron transport systems. These are largely mediated via oxidoreductase enzymes. Their use in biology has been linked to a host of systems including reprogramming for energy requirements in cancer. Consequently, the ability to modulate membrane redox systems may give rise to opportunities to modulate underlying biology. The current work aims to develop a wireless bipolar electrochemical approach to form on-demand electron transfer across biological membranes. To achieve this goal, it is shown that by using membrane inserted carbon nanotube porins (CNTPs) that can act as bipolar nanoelectrodes, one can control electron flow with externally applied electric fields across membranes. Before this work, bipolar electrochemistry has been thought to require high applied voltages not compatible with biological systems. It is shown that bipolar electrochemical reaction via gold reduction at the nanotubes can be modulated at low cell-friendly voltages, providing an opportunity to use bipolar electrodes to control electron flux across membranes. The authors provide new mechanistic insight into this newly describe phenomena at the nanoscale. The results presented give rise to a new method using CNTPs to modulate cell behavior via wireless control of membrane electron transfer.

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.

Fabrication of Gradient and Patterned Organic Thin Films by Bipolar Electrolytic Micelle Disruption Using Redox-Active Surfactants

Bipolar electrochemistry could be regarded as a powerful approach for selective surface modification due to the beneficial feature that a wirelessly controllable potential distribution on bipolar electrodes (BPEs). Herein we report a bipolar electrolytic micelle disruption (BEMD) system for the preparation of shaped organic films. A U-shaped bipolar electrolytic system with a sigmoidal potential gradient on the BPE gave gradient-thin films including various interesting organic compounds, such as a polymerizable monomer, an organic pigment and aggregation induced emission (AIE) molecules. The gradient feature was characterized by UV-Vis absorption, thickness measurements and surface morphology analysis. Corresponding patterned films were also fabricated using a cylindrical bipolar electrolytic setup that enables site-selective application of the potential on the BPE. Such a facile BEMD approach will open a long-term perspective with respect to organic film preparation.

On the mechanistic pathways of exfoliation-and-deposition of graphene by bipolar electrochemistry

Amongst the different graphene fabrication techniques, bipolar electrochemistry (BPE) has been recently reported as a simple, controllable, low cost, eco-friendly, and scalable method. It consists of a wirelessly placed carbon source between two feeding electrodes subjected to direct current (DC) voltage in a deionized water bath. Although the physicochemical characteristics of produced graphene have been evaluated, the exfoliation and deposition mechanisms are still unclear. In this study, a novel modified BPE system with an electrically-connected graphite-platinum couple acting as the bipolar electrode has been designed in order to decouple and investigate the contribution of anodic/cathodic exfoliation and deposition of graphene in the BPE process. Electron microscopy and Fourier transform infrared spectroscopy results indicate that both anodic and cathodic exfoliation of graphene could take place regardless of the type of polarization; however, the morphology and deposition rate highly depend on the polarization. Furthermore, the graphene fabricated by anodic exfoliation was found to show higher levels of oxidation compared to the graphene produced by cathodic exfoliation.

Bipolar Electrochemiluminescence Imaging: A Way to Investigate the Passivation of Silicon Surfaces

This work depicts the original combination of electrochemiluminescence (ECL) and bipolar electrochemistry (BPE) to map in real-time the oxidation of silicon in microchannels. We fabricated model silicon-PDMS microfluidic chips, optionally containing a restriction, and monitored the evolution of the surface reactivity using ECL. BPE was used to remotely promote ECL at the silicon surface inside microfluidic channels. The effects of the fluidic design, the applied potential and the resistance of the channel (controlled by the fluidic configuration) on the silicon polarization and oxide formation were investigated. A potential difference down to 6 V was sufficient to induce ECL, which is two orders of magnitude less than in classical BPE configurations. Increasing the resistance of the channel led to an increase in the current passing through the silicon and boosted the intensity of ECL signals. Finally, the possibility of achieving electrochemical reactions at predetermined locations on the microfluidic chip was investigated using a patterning of the silicon oxide surface by etched micrometric squares. This ECL imaging approach opens exciting perspectives for the precise understanding and implementation of electrochemical functionalization on passivating materials. In addition, it may help the development and the design of fully integrated microfluidic biochips paving the way for development of original bioanalytical applications.

Nanoconfined Electrochemical Sensing of Single Silver Nanoparticles with a Wireless Nanopore Electrode

Single entity electrochemistry (SEE) has emerged as a promising method for precise measurement and fundamental understanding of the heterogeneity of single entities. Herein, we propose the dual responsive SEE sensing of the silver nanoparticles (AgNPs) collisions through a wireless nanopore electrode (WNE). Given the high temporal resolution and low background noise features, the Faradaic and capacitive currents provide the AgNPs' collision response. The electron transfer between the AgNPs and the electrode surface is identified under a bipolar electrochemical mechanism. Compared to the ultramicroelectrode, multistep oxidation of 30 nm AgNPs is observed due to the decreased interaction of the nanoparticles to the electrode. Moreover, the nanoconfinement of WNE plays a vital role in the repeated capturing of nanoparticles from the nontunneling region into the tunneling region until a complete oxidation. As a comparison, the collision of 5 nm AgNPs with higher interaction at the electrode surface shows great decrease in the multistep events. Thus, we propose a nanoconfined interaction based SEE method which could be used for simultaneously capturing the Faradaic and capacitive response. The nanoconfined interaction based SEE method holds great promise in the better understanding of heterogeneity of single particles.

Asymmetric Modification of Carbon Nanotube Arrays with Thermoresponsive Hydrogel for Controlled Delivery

In this work, bipolar electrochemistry is used to perform wireless indirect electrodeposition of two different polymer coatings on both sides of carbon nanotube arrays. Using a thermoresponsive hydrogel on one side and an inert insoluble polymer on the other side, it is possible to generate, in a single step, a nanoporous reservoir with Janus character closed on one side by a thermoresponsive membrane. The thermoresponsive polymer, poly(N-isopropylacrylamide) (pNIPAM), is generated by the local reduction of persulfate ions, which initiates radical polymerization of NIPAM. Electrophoretic paint (EP) is chosen as an inert polymer. It is deposited by precipitation because of a local decrease in pH during water oxidation. Both polymers can be deposited simultaneously on opposite sides of the bipolar electrode during the application of the electric field, yielding a double-modified Janus object. Moreover, the length and thickness of the polymer layers can be controlled by varying the electric field and the deposition time. This concept is applied to vertically aligned carbon nanotube arrays (VACNTs), trapped inside an anodic aluminum oxide membrane, which can further be used as a smart reservoir for chemical storage and release. A fluorescent dye is loaded in the VACNTs and its release is studied as a function of temperature. Low temperature, when the hydrogel layer is in the swollen state, allows diffusion of the molecule. Dye release occurs on the hydrogel-modified side of the VACNTs. At high temperatures, when the hydrogel layer is in the collapsed state, dye release is blocked because of the impermeability of the pNIPAM layer. This concept paves the way toward the design of advanced devices in the fields of drug storage and directed delivery.

Nanostructured Cu2O Synthesized via Bipolar Electrochemistry

Cuprous oxide (Cu2O) was synthesized for the first time via an open bipolar electrochemistry (BPE) approach and characterized in parallel with the commercially available material. As compared to the reference, Cu2O formed through a BPE reaction demonstrated a decrease in particle size; an increase in photocurrent; more efficient light scavenging; and structure-correlated changes in the flat band potential and charge carrier concentration. More importantly, as-synthesized oxides were all phase-pure, defect-free, and had an average crystallite size of 20 nm. Ultimately, this study demonstrates the impact of reaction conditions (e.g., applied potential, reaction time) on structure, morphology, surface chemistry, and photo-electrochemical activity of semiconducting oxides, and at the same time, the ability to maintain a green synthetic protocol and potentially create a scalable product. In the proposed BPE synthesis, we introduced a common food supplement (potassium gluconate) as a reducing and complexing agent, and as an electrolyte, allowing us to replace the more harmful reactants that are conventionally used in Cu2O production. In addition, in the BPE process very corrosive reactants, such as hydroxides and metal precursors (required for synthesis of oxides), are generated in situ in stoichiometric quantity, providing an alternative methodology to generate various nanostructured materials in high yields under mild conditions.

Electrochemiluminescence Imaging for Bioanalysis

Electrochemiluminescence (ECL) is a widely used analytical technique with the advantages of high sensitivity and low background signal. The recent and rapid development of electrochemical materials, luminophores, and optical elements significantly increases the ECL signals and, thus, ECL imaging with enhanced spatial and temporal resolutions is realized. Currently, ECL imaging is successfully applied to high-throughput bioanalysis and to visualize the distribution of molecules at single cells. Compared with other optical bioassays, no optical excitation is involved in imaging, so the approach avoids a background signal from illumination and increases the detection sensitivity. This review highlights some of the most exciting developments in this field, including the mechanisms, electrode designs, and the applications of ECL imaging in bioanalysis and at single cells and particles.

Exploring the Kinetic and Thermodynamic Relationship of Charge Transfer Reactions Used in Localized Electrodeposition and Patterning in a Scanning Bipolar Cell

Bipolar electrochemistry involves spatial separation of charge balanced reduction and oxidation reactions on an electrically floating electrode, a result of intricate coupling of the work piece with the ohmic drop in the electrochemical cell and to the thermodynamics and kinetics of the respective bipolar reactions. When paired with a rastering microjet electrode, in a scanning bipolar cell (SBC), local electrodeposition and patterning of metals beneath the microjet can be realized without direct electrical connections to the workpiece. Here, we expand on prior research detailing electrolyte design guidelines for electrodeposition and patterning with the SBC, focusing on the relationship between kinetics and thermodynamics of the respective bipolar reactions. The kinetic reversibility or irreversibility of the desired deposition reaction influences the range of possible effective bipolar counter reactions. For kinetically irreversible deposition systems (i.e., nickel), a wider thermodynamic window is available for selection of the counter reaction. For kinetically reversible systems (i.e., copper or silver) that can be easily etched, tight thermodynamic windows with a small downhill driving force for spontaneous reduction are required to prevent metal patterns from electrochemical dissolution. Furthermore, additives used for the bipolar counter reaction can influence not only the kinetics of deposition, but also the morphology and microstructure of the deposit. Cyclic voltammetry measurements help elucidate secondary parasitic reduction reactions occurring during bipolar nickel deposition and describe the thermodynamic relationship of both irreversible and reversible bipolar couples. Finally, finite element method simulations explore the influence of bipolar electrode area on current efficiency and connect experimental observations of pattern etching to thermodynamic and kinetic relationships.

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.

Remotely Controlled in Situ Growth of Silver Microwires Forming Bioelectronic Interfaces

There is a pressing need to advance our ability to construct three-dimensional (3D) functional bioelectronic interfaces. Additionally, to ease the transition to building cellular electronic systems, a remote approach to merge electrical components with biology is desirable. By combining 3D digital inkjet printing with bipolar electrochemistry, we remotely control and fabricate conductive wires, forming a first of its kind contactless bionic manufacturing procedure. It enables controlled fabrication of conductive wires in a three-dimensional configuration. Moreover, we demonstrate that this technology could be used to grow and interface conductive conduits in situ with mammalian cells, offering a new strategy to engineering bioelectronic interfaces. This represents a step change in the production of functional complex circuitry and considerably increases the manufacturing capabilities of merging cells with electronics. This approach provides a platform to construct bioelectronics in situ offering a potential paradigm shift in the methods for building bioelectronics with potential applications in biosensing and bioelectronic medicine.

ZnO Synthesized Using Bipolar Electrochemistry: Structure and Activity

The photoactive materials broadly applied in catalysis and energy conversion are generally composed of metal oxides. Among these oxides, ZnO showed a promising photocatalytic activity; however, traditional synthetic routes generated by-products and large amounts of secondary waste. Herein, we report the use of bipolar electrochemistry to generate ZnO nanoparticles using deionized water and a zinc metal to conform to green chemistry practices. TEM imaging demonstrated that the sizes of the bipolar-made ZnO particles were smaller than the commercial sample. The presence of structural defects in ZnO was correlated with the chemical shifts analyzed by X-ray photoelectron spectroscopy (XPS) and by different concentrations of O2- ions in stoichiometric and defected lattice. Further, the diffuse reflectance UV⁻Vis studies demonstrated a blue-shift in the reflectance spectrum for the bipolar-made oxide. This was also an indication of defects in the ZnO lattice, which related to the formation of shallow levels in the bandgap of the material. The structural and morphological differences influenced the photocatalytic characteristics, revealing a higher photocurrent for the bipolar-made ZnO when compared to the reference sample. This was further manifested in lower total resistivity for all anodes made from the non-stoichiometric ZnO, and also in their shorter diffusion length for charge exchange and electron lifetimes.

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.

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.

High-Throughput Screening of Rat Mesenchymal Stem Cell Behavior on Gradient TiO2 Nanotubes

The dimension of TiO₂ nanotubes (TNTs) ranges from several nanometers to hundreds of nanometers. This variety raises the difficulty of screening suitable nanotube dimension for biomedical applications. Herein, we report the use of a simple one-step bipolar anodization method for fabrication of TNT gradients with diameter range from 30 to 100 nm. The gradient TNTs were successfully applied for high-throughput screening of TNT size effect on cell responses, including cell adhesion, proliferation, and differentiation. Results reveal that no significant difference in adherent cell number could be found within the range of 30-87 nm in both the presence and absence of serum proteins. On the contrary, large nanotubes (with outer diameter >87 nm) profoundly reduce cell adhesion in both the presence and absence of serum proteins, indicating TNT size could affect cell adhesion directly without the adsorbed proteins. The size effect on cell behavior becomes prominent with time that cell proliferation and differentiation decrease with increasing nanotube size. This size effect can be comprehended by protein adsorption and the formation of focal adhesion. Another two sample applications of gradient TNTs demonstrate gradient TNTs are promising for high-throughput screening.

MoS2 Nanoparticles as Electrocatalytic Labels in Magneto-Immunoassays

Ability to detect biomolecules with a simple and cost-effective approach has been very demanding in today's medicine. The nanoparticles and two-dimensional materials have been extensively used within this field in devices with high selectivity and sensitivity. Here, we report the use of MoS₂ nanoparticles (MoS₂ NPs) as a signal-enhancing label in a standard immunoassay test. MoS₂ NPs were prepared by a bipolar electrochemistry method. The current response during the hydrogen evolution reaction catalyzed by MoS₂ was measured. This current was directly proportional to the amount of the MoS₂ NPs and thus also to the concentration of desired protein. The immunoassay containing the MoS₂ NPs displays extraordinary low limit of detection (1.94 pg mL^-1), good selectivity, and reproducibility. This MoS₂ NP detection system could have profound implication for analytical applications.

Composition-Graded MoWSx Hybrids with Tailored Catalytic Activity by Bipolar Electrochemistry

Among transition metal dichalcogenide (TMD)-based composites, TMD/graphene-related material and bichalcogen TMD composites have been widely studied for application toward energy production via the hydrogen evolution reaction (HER). However, scarcely any literature explored the possibility of bimetallic TMD hybrids as HER electrocatalysts. The use of harmful chemicals and harsh preparation conditions in conventional syntheses also detracts from the objective of sustainable energy production. Herein, we present the conservational alternative synthesis of MoWS_x via one-step bipolar electrochemical deposition. Through bipolar electrochemistry, the simultaneous fabrication of composition-graded MoWS_x hybrids, i.e., sulfur-deficient Mo_xW_(1-x)S₂ and Mo_xW_(1-x)S₃ (MoWS_x/BPE_cathodic and MoWS_x/BPE_anodic, respectively) under cathodic and anodic overpotentials, was achieved. The best-performing MoWS_x/BPE_cathodic and MoWS_x/BPE_anodic materials exhibited Tafel slopes of 45.7 and 50.5 mV dec^-1, together with corresponding HER overpotentials of 315 and 278 mV at -10 mA cm^-2. The remarkable HER activities of the composite materials were attributed to their small particle sizes, as well as the near-unity value of their surface Mo/W ratios, which resulted in increased exposed HER-active sites and differing active sites for the concurrent adsorption of protons and desorption of hydrogen gas. The excellent electrocatalytic performances achieved via the novel methodology adopted here encourage the empowerment of electrochemical deposition as the foremost fabrication approach toward functional electrocatalysts for sustainable energy generation.

Field-Controlled Electrical Switch with Liquid Metal

When immersed in an electrolyte, droplets of Ga-based liquid metal (LM) alloy can be manipulated in ways not possible with conventional electrocapillarity or electrowetting. This study demonstrates how LM electrochemistry can be exploited to coalesce and separate droplets under moderate voltages of ~1-10 V. This novel approach to droplet interaction can be explained with a theory that accounts for oxidation and reduction as well as fluidic instabilities. Based on simulations and experimental analysis, this study finds that droplet separation is governed by a unique limit-point instability that arises from gradients in bipolar electrochemical reactions that lead to gradients in interfacial tension. The LM coalescence and separation are used to create a field-programmable electrical switch. As with conventional relays or flip-flop latch circuits, the system can transition between bistable (separated or coalesced) states, making it useful for memory storage, logic, and shape-programmable circuitry using entirely liquids instead of solid-state materials.

Bipolar Electrode Array Embedded in a Polymer Light-Emitting Electrochemical Cell

A linear array of aluminum discs is deposited between the driving electrodes of an extremely large planar polymer light-emitting electrochemical cell (PLEC). The planar PLEC is then operated at a constant bias voltage of 100 V. This promotes in situ electrochemical doping of the luminescent polymer from both the driving electrodes and the aluminum discs. These aluminum discs function as discrete bipolar electrodes (BPEs) that can drive redox reactions at their extremities. Time-lapse fluorescence imaging reveals that p- and n-doping that originated from neighboring BPEs can interact to form multiple light-emitting p-n junctions in series. This provides direct evidence of the working principle of bulk homojunction PLECs. The propagation of p-doping is faster from the BPEs than from the positive driving electrode due to electric field enhancement at the extremities of BPEs. The effect of field enhancement and the fact that the doping fronts only need to travel the distance between the neighboring BPEs to form a light-emitting junction greatly reduce the response time for electroluminescence in the region containing the BPE array. The near simultaneous formation of multiple light-emitting p-n junctions in series causes a measurable increase in cell current. This indicates that the region containing a BPE is much more conductive than the rest of the planar cell despite the latter's greater width. The p- and n-doping originating from the BPEs is initially highly confined. Significant expansion and divergence of doping occurred when the region containing the BPE array became more conductive. The shape and direction of expanded doping strongly suggest that the multiple light-emitting p-n junctions, formed between and connected by the array of metal BPEs, have functioned as a single rod-shaped BPE. This represents a new type of BPE that is formed in situ and as a combination of metal, doped polymers, and forward-biased p-n junctions connected in series.

2H → 1T Phase Change in Direct Synthesis of WS2 Nanosheets via Solution-Based Electrochemical Exfoliation and Their Catalytic Properties

Metallic 1T-WS₂ has various interesting properties such as increased density of catalytically active sites on both the basal planes and edges as well as metallic conductivity which allows it to be used in applications such as biosensing and energy devices. Hence, it is highly beneficial to develop a simple, efficient, and low-cost synthesis method of 1T-WS₂ nanosheets from commercially available bulk 2H-WS₂. In this study, we reported WS₂ nanosheets synthesized directly from bulk WS₂ via solution-based electrochemical exfoliation with bipolar electrodes and investigated their electrocatalytic performances toward hydrogen evolution and oxygen reduction reactions. We successfully synthesized WS₂ nanosheets of regular hexagonal symmetry with a 2H → 1T phase transition. This represents a novel method of producing 1T-WS₂ nanosheets from bulk 2H-WS₂ without compromising on its electrocatalytic properties.

Closed Bipolar Electrodes for Spatial Separation of H2 and O2 Evolution during Water Electrolysis and the Development of High-Voltage Fuel Cells

Electrolytic water splitting could potentially provide clean H₂ for a future "hydrogen economy". However, as H₂ and O₂ are produced in close proximity to each other in water electrolyzers, mixing of the gases can occur during electrolysis, with potentially dangerous consequences. Herein, we describe an electrochemical water-splitting cell, in which mixing of the electrogenerated gases is impossible. In our cell, separate H₂- and O₂-evolving cells are connected electrically by a bipolar electrode in contact with an inexpensive dissolved redox couple (K₃Fe(CN)₆/K₄Fe(CN)₆). Electrolytic water splitting occurs in tandem with oxidation/reduction of the K₃Fe(CN)₆/K₄Fe(CN) redox couples in the separate compartments, affording completely spatially separated H₂ and O₂ evolution. We demonstrate operation of our prototype cell using conventional Pt electrodes for each gas-evolving reaction, as well as using earth-abundant Ni₂P electrocatalysts for H₂ evolution. Furthermore, we show that our cell can be run in reverse and operate as a H₂ fuel cell, releasing the energy stored in the electrogenerated H₂ and O₂. We also describe how the absence of an ionically conducting electrolyte bridging the H₂- and O₂-electrode compartments makes it possible to develop H₂ fuel cells in which the anode and cathode are at different pH values, thereby increasing the voltage above that of conventional fuel cells. The use of our cell design in electrolyzers could result in dramatically improved safety during operation and the generation of higher-purity H₂ than available from conventional electrolysis systems. Our cell could also be readily modified for the electrosynthesis of other chemicals, where mixing of the electrochemical products is undesirable.

Salt Pumping by Voltage-Gated Nanochannels

This Letter investigates voltage-gated nanochannels, where both the potential applied to the conductive membrane containing the channel (membrane potential) and the potential difference between the solutions at both sides of the membrane (transmembrane potential) are independently controlled. The predicted conductance characteristics of these fixed-potential channels dramatically differ from those of the widely studied fixed-charge nanochannels, in which the membrane is insulating and has a fixed surface charge density. The difference arises because the transmembrane potential induces an inhomogeneous charge distribution on the surface of fixed-potential nanochannels. This behavior, related to bipolar electrochemistry, has some interesting and unexpected consequences for ion transport. For example, continuously oscillating the transmembrane potential, while holding the membrane potential at the potential for which it has zero charge in equilibrium, creates fluxes of neutral salt (fluxes of anions and cations in the same direction and number) through the channel, which is an interesting phenomenon for desalination applications.

Bipolar electrochemical method for dynamic in situ control of single metal nanowire growth

Fabrication plays a key role in determining the unique electrical, optical, and catalytic properties of metal nanowires. Here we present a bipolar electrochemical method for dynamically monitoring and controlling the rate of single metal nanowire growth in situ without a direct electrical connection. Solutions of a metal precursor and a reducing agent are placed on either side of a silica nanochannel, and a pair of electrodes is used to apply a tunable electric potential across the channel. Metal nanowire growth is initiated by chemical reduction when the two solutions meet and continues until the nanochannel is blocked by the formation of a short metal wire segment. Further growth is driven by a bipolar electrochemical mechanism which enables the reduction of metal precursor ions at one end of the nanowire and the oxidation of the reducing agent at the other. The growth rate is monitored in real time by simultaneously recording both the faradaic current and optical microscope video and can be adjusted accordingly by changing the applied electric potential. The resulting nanowire is solid, electrically insulated, and can be used as a bipolar nanoelectrode. This technique can be extended to other electrochemical systems, as well, and provides a confined reaction space for studying the dynamics of any process that can be optically or electrically monitored.

Parallel- and serial-contact electrochemical metallization of monolayer nanopatterns: A versatile synthetic tool en route to bottom-up assembly of electric nanocircuits

Contact electrochemical transfer of silver from a metal-film stamp (parallel process) or a metal-coated scanning probe (serial process) is demonstrated to allow site-selective metallization of monolayer template patterns of any desired shape and size created by constructive nanolithography. The precise nanoscale control of metal delivery to predefined surface sites, achieved as a result of the selective affinity of the monolayer template for electrochemically generated metal ions, provides a versatile synthetic tool en route to the bottom-up assembly of electric nanocircuits. These findings offer direct experimental support to the view that, in electrochemical metal deposition, charge is carried across the electrode-solution interface by ion migration to the electrode rather than by electron transfer to hydrated ions in solution.