Advances in bipolar electrochemiluminescence for the detection of biorelevant molecular targets

Bipolar electrochemistry (BPE) contrasts very much with conventional electrochemistry because it is based on the control of the solution potential instead of the working electrode potential. In a typical setup, a piece of conducting materials is immersed in an electrolyte and submitted to an electric field. Such conditions split the interfacial nature of the materials into cathodic and anodic domains where electrochemical reactions can readily take place. BPE has many potential applications, and the present contribution aims to focus on recent analytical applications that involve electrogenerated chemiluminescence (ECL) detection. ECL is a special case of luminescence where the excited state of the luminophore is populated after a sequence of reaction that is triggered by an initial electron transfer step occurring at the electrode surface. The coupling between BPE and ECL is a powerful approach because it provides a unique opportunity to combine the intrinsic advantages of both techniques. BPE enables the spatial separation of sensing and reporting poles, whereas ECL provides a simple and sensitive visual readout. This opinion article will describe the experimental possibilities and the most recent applications of BPE/ECL coupling for the detection of biorelevant molecular targets.     

The electrical double layer charge and associated dimensional changes of high surface area electrodes as detected by more deflectometry

Due to the high surface to volume ratio, the interfacial tension of porous solids affects their outer dimensions by a minute, but detectable amount. Changes in the interfacial tension of a high surface area graphite electrode could be monitored adopting the moire deflectometry, a new optical technique for ray deflection mapping, by which the resolution in relative length changes of th e electrode was less than 1 microstrain.The strain vs. potential curves are typically pseudo-parabolic. Within the double layer potential range, their slope is a linear function of the electronic charge delivered to the electrode. Unlike the double layer charge, faradaic currents are not recognized as a strain change of the electrode, thus the method may serve to distinguish between faradaic and double layer processes. Hysteresis takes place in curves of charge or strain vs. electrode potential, but not for charge vs. strain curves. The minimum in the strain-potential plot, attributed to the potential of zero charge occurs at about –350 mV vs. SCE, corresponding to a surface group free carbon (or graphite) electrode.     

Metal/insulator/metal junctions for electrochemical surface science

We describe the preparation and characterization of Al-AlO_x-Ag tunnel junctions and calculate the energy distribution of the tunneling hot electrons in the range 0–2.5 eV above the Fermi level of silver. Because the mean free path of the hot electrons is of the order of the thickness of the silver film of the junction, which is at the same time the electrode in contact with an electrolyte, new surface effects can be studied. Hot electrons can be injected into the nonhydrated electron band in water. Hot electrons also cause hydrogen evolution at electrode potentials more positive than the ones needed in common electrochemistry. We observed the emission of hot electrons into silver during transients of hydrogen oxidation at silver and during oxidation of overpotential hydrogen on platinum clusters deposited on the silver electrode. The tunnel current at constant tunnel voltage can be changed by faradaic reactions, but surprisingly also by nonfaradaic reactions; this is assigned to a mesoscopic quantum phenomenon.     

Real-time imaging of surface chemical reactions by electrochemical photothermal reflectance microscopy

Traditional electrochemical measurements based on either current or potential responses only present the average contribution of an entire electrode''s surface. Here, we present an electrochemical photothermal reflectance microscope (EPRM) in which a potential-dependent nonlinear photothermal signal is exploited to map an electrochemical process with sub-micron spatial resolution. By using EPRM, we are able to monitor the photothermal signal of a Pt electrode during the electrochemical reaction at an imaging speed of 0.3 s per frame. The potential-dependent photothermal signal, which is sensitive to the free electron density, clearly revealed the evolution of surface species on the Pt surface. Our results agreed well with the reported spectroelectrochemical techniques under similar conditions but with a much faster imaging speed. We further mapped the potential oscillation during the oxidation of formic acid on the Pt surface. The photothermal images from the Pt electrode well matched the potential change. This technique opens new prospects for real-time imaging of surface chemical reaction to reveal the heterogeneity of electrochemical reactivity, which enables broad applications to the study of catalysis, energy storage, and light harvest systems.

The potential-dependent photothermal signal, which is sensitive to the free electron density, map the evolution of surface species on the electrode in real time.     

Spatially-resolved multicolor bipolar electrochemiluminescence

Here we describe a new aspect of multicolor potential-resolved electrochemiluminescence (ECL) based on bipolar electrochemistry (BPE). BPE involves a potential gradient established along a polarized conducting object which thus acts as a bipolar electrode (BE). The resulting driving force can induce electron-transfer reactions, necessary for processes such as ECL occurring at different longitudinal locations along the same BE. In this work, we exploit the entire spatial domain where anodic polarization occurs to demonstrate, for the first time, how the potential gradient along a BE may be used to simultaneously resolve the emissions of ECL-active luminophores with differing oxidation potentials. The control of both size and position of the ECL-emitting domains was achieved by tuning the applied electric field. Multicolor light-emission was analyzed in detail to demonstrate spatial and spectral resolution of a solution containing different emitters.     

Electrochemical deposition of gold at liquid–liquid interfaces studied by thin organic film-modified electrodes

The unique physico-chemical properties of gold nanoparticles portrayed in their chemical stability, the size-dependent electrochemistry, and the unusual optical properties make them suitable modifiers of various surfaces used in the fields of optical devices, electronics, and biosensors. In this work we present two different methods to obtain metallic gold nanoparticles at a liquid–liquid interface, and to control their growth by adjusting the experimental conditions. Decamethylferrocene (DMFC), used as an oxidizable compound dissolved in an organic solvent that is spread as a thin film on the surface of graphite electrode, serves as a redox partner to exchange electrons across the liquid–liquid interface with the other redox counter-partner [AuCl₄]^? present in the conjoined water phase. The interfacial electron transfer between the DMFC and the [AuCl₄]^? ions leads to deposition of metallic gold nanoparticles at the liquid–liquid interface. The structure and features of the deposited Au nanoparticles were studied by means of microscopic and voltammetric techniques. The morphology of the Au deposit depends on the concentration ratio of redox partners and both electrode and liquid–liquid interfacial potential differences. Depending on whether the Au deposit was obtained by ex situ (at open circuit potential) or by “in situ” (by cycling of the electrode potential) approach, we observed quite different effects to the ion transfer reactions probed by the thin-film electrode set-up. The possible reasons for the different behavior of the Au nanoparticles are discussed in terms of the structure and the properties of the obtained Au deposit. In separate experiments, we have demonstrated catalytic effects of the Au nanoparticles towards enhancing the electron transfer between DMFC and two aqueous redox substrates, hexacyanoferrate and hydrogen peroxide.     

Bipolar electrode depletion: membraneless filtration of charged species using an electrogenerated electric field gradient

We report a method for removing ions from aqueous solutions without the use of a membrane. The approach, which we call bipolar electrode depletion (BED), is based on the formation of an asymmetric electric field profile in a microchannel containing a bipolar electrode (BPE). The asymmetric field arises from local increases in conductivity caused by faradaic reactions at the BPE. We show how the asymmetric field can be used to deplete anions from a microchannel via a combination of electrophoresis and electroosmosis. We also apply this approach to filter an anionic species from a mixture of charged and neutral species being transported through a microchannel via electroosmosis. This technique could be utilized for desalination or filtration of any species possessing a net charge (e.g. heavy-metals, bacteria, proteins, or functionalized-nanoparticles).     

Modeling of Current Distribution on Smooth and Columnar Platinum Structures

Studying the growth and stability of anisotropic or isotropic disordered surfaces in electrodeposition is of importance in catalytic electrochemistry. In some cases, the metallic nature of the electrode defines the topography and roughness, which are also controlled by the experimental time and applied external potential. Because of the experimental restrictions in conventional electrochemical techniques and ex situ electron microscopies, a theoretical model of the surface geometry could aid in understanding the electrodeposition process and current distributions. In spite of applying a complex theory such as dynamic scaling method or perturbation theories, the resolution of mixed mass‐/charge‐transfer equations (tertiary distribution) for the electrodeposition process would give reliable information. One of the main problems with this type of distribution is the mathematics when solving the spatial n‐dimensional differential equations. Use of a primary current distribution is proposed here to simplify the differential equations; however it limits wide application of the first assumption. Distributions of concentration profile, current density, and electrode potential are presented here as a function of the distance normal to the surface for the cases of smooth and rough platinum growth. In the particular case of columnar surfaces, cycloid curves are used to model the electrode, from which the concentration profile is presented in a parameterized form after solving a first‐type curvilinear integral. The concentration contour results in a combination of a trigonometric inverse function and a linear distribution leading to a negative concavity curve. The calculation of the current density and electrode potential contours also show trigonometric shapes exhibiting forbidden imaginary values only at the minimal values of the trochoid curve.     

Switching “on and off” faradaic electrochemistry at an otherwise passivated electrode using gold-coated magnetic nanoparticles

Herein we combined the use of self-assembled monolayer (SAM) modified electrodes and gold coated magnetic nanoparticles (Au@MNPs) to modulate faradaic electrochemistry when nanoparticles are absorbed on and removed from, a passivated electrode using an external magnetic field. Substantial faradaic electrochemistry of potassium ferricyanide in solution is first prevented by modifying gold electrodes with 11-mercaptoundecanoic acid to form a passivating SAM. Restoration of the faradaic electrochemistry is then achieved by introducing Au@MNPs which are brought to the surface using an external magnetic field. The faradaic electrochemistry can again be suppressed by removing Au@MNPs from the SAM using another external magnetic field.     

Pressure-driven bipolar electrochemistry

Here we report that pressure-driven flow alone (no external electrical energy) can be used to drive faradaic electrochemical reactions in microchannels with charged walls. Specifically, we show that solution flow can generate streaming potentials on the order of volts and that this is sufficient to carry out reactions on the anodic and cathodic poles of a bipolar electrode (BPE). The existence of faradaic reactions is proven by electrodissolution of Ag from the anodic end of the BPE.     

The electrochemistry of particles, droplets, and vesicles �C the present situation and future tasks

Presently, a plethora of techniques is available to study the electrochemical properties of solid inorganic and organic micro- and nano-particles immobilized on electrode surfaces, provided that they possess a faradaic electroactivity. Similarily, immobilized droplets of liquids and solutions, which are immiscible with the electrolyte solution, give access to the three-phase electrochemistry of redox centers in the droplets, allowing determinations of free energies of ion transfer between the immiscible liquid phases. Possible and necessary future activities in the field of immobilized particles and droplets will be discussed here. The electrochemistry of suspended micro- and nano-particles possessing faradaic electroactivity is much more complex and needs special attention in future research. Finally, the electrochemistry of liposomes and biological vesicles, which do not possess faradaic activity, but the ability to produce capacitive signals upon attachment to electrodes, will be discussed focusing on possible future developments.     

Quasi-reversible faradaic depolarization processes in the electrokinetics of the metal/solution interface

Bipolar faradaic depolarization of the metal/solution interface is quantitatively analyzed for the case where the solution is subject to lateral flow and contains a quasi-reversible redox couple. Transversal convective diffusion of the electroactive species and a position-dependent degree of reversibility of the interfacial electron-transfer (e.t.) reaction are among the primary features that govern depolarization. The spatial distributions of species concentrations and electric potential are numerically simulated. The system is characterized by nonlinear coupling between the transport (diffusion and flow) and the electric potential distribution under conditions of finite local currents. The resulting picture is that the reversibility of the e.t. reaction varies with position on the surface, with the highest reversibility downstream. This, in itself, generally leads to strongly asymmetric profiles of the faradaic current density along the surface. The impact on the electrokinetic properties of the interface is huge. For example, the steady-state streaming potential is depressed by the contribution from the bipolar faradaic process to the back current to an extent that varies from insignificant to complete, depending on the e.t. rate constant and concentrations of the electroactive species.     

Electrokinetics of the amphifunctional metal/electrolyte solution interface in the presence of a redox couple

Double layers (DL) at amphifunctionally electrified interfaces, such as that of an oxidized metal in an aqueous electrolyte solution, arise from coupling between ionic and electronic surface-charging processes. The electronic component enters the double-layer formation in the well-known situation where a potential is externally applied. In that case, the DL is fully or partly polarized depending on the possibility of interfacial electron transfer, that is, a faradaic process. This paper reports on the conjunction of the chemical/electrochemical processes at the interface in the case where the solution contains a redox-active couple. This makes it possible to polarize/depolarize a DL without invoking any external circuit. Streaming potential data obtained for the gold/(Fe(CN)6(3-)/Fe(CN)6(4-), KNO3) electrolyte interface are analyzed in terms of a recently developed theory which takes into account reversible bipolar faradaic depolarization, the inherent nonlinearity of the lateral field, and the effects of flow on the rate of the faradaic reactions. It appears that the theory largely overestimates the bipolar currents, leading to physically unrealistic zeta-potentials. A careful analysis of monopolar voltammetric data reveals quasi-reversible behavior of the redox couple under the typical convective conditions and electrolyte compositions met in electrokinetic experiments. Inclusion of reduced reversibility (the extent of which is position-dependent under the streaming-potential measurement conditions) leads to a consistent set of zeta-potentials which compare well to the values for the background electrolyte.     

Bipolar Electrochemistry

A bipolar electrode (BPE) is an electrically conductive material that promotes electrochemical reactions at its extremities (poles) even in the absence of a direct ohmic contact. More specifically, when sufficient voltage is applied to an electrolyte solution in which a BPE is immersed, the potential difference between the BPE and the solution drives oxidation and reduction reactions. Because no direct electrical connection is required to activate redox reactions, large arrays of electrodes can be controlled with just a single DC power supply or even a battery. The wireless aspect of BPEs also makes it possible to electrosynthesize and screen novel materials for a wide variety of applications. Finally, bipolar electrochemistry enables mobile electrodes, dubbed microswimmers, that are able to move freely in solution.     

Direct route to well-defined, chemically diverse electrode arrays

The selective placement of molecules of interest at specific locations on surfaces is a keystone for the bridge between interfacial science and technology. One approach to this problem is the use of electrochemistry to direct interfacial reactions that immobilize species from solution onto surfaces. In this study, sets of individually functionalized gold electrodes were formed by the selective formation of monolayers from four different alkyl thiosulfates. Analysis of the arrays using spatially resolved X-ray photoelectron spectroscopy (XPS) revealed each type of functionality exclusively on the electrode to which it was directed. The wetting behavior of these surfaces was also consistent with homogeneous monolayers placed selectively on each electrode. The flexibility of this method provides the ability to produce a wide variety of chemical patterns at interfaces of interest for a range of technological applications.     

Fabrication of zinc oxide nanowires/polymer composites by two‐photon polymerization

We present an approach to fabricate ZnO nanowires/polymer composite into three‐dimensional microstructures, based on two‐photon polymerization direct laser writing, a fabrication method that allows submicrometric spatial resolution. The structural integrity of the structures was inferred by scanning electron microscopy, while the presence and distribution of ZnO nanowires was investigated by energy dispersive X‐ray, Raman spectroscopy, and X‐ray diffraction. The optical properties of the produced composite microstructures were verified by imaging the characteristic ZnO emission using a fluorescence microscope. Hence, such approach can be used to develop composite microstructures containing ZnO nanowires aiming at technological applications. © 2013 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014 , 52, 333–337     

Gradient doping of conducting polymer films by means of bipolar electrochemistry

In this paper, we report a novel electrochemical doping method for conducting polymer films based on bipolar electrochemistry. The electrochemical doping of conducting polymers such as poly(3-methylthiophene) (PMT), poly(3,4-ethylenedioxythiophene) (PEDOT), and poly(aniline) (PANI) on a bipolar electrode having a potential gradient on its surface successfully created gradually doped materials. In the case of PEDOT film, the color change at the anodic side was also observed to be gradually transparent. PANI film treated by the bipolar doping gave a multicolored gradation across the film. The results of UV-vis and energy dispersive X-ray analyses for the doped films supported the distribution of dopants in the polymer films reflecting the potential gradient on the bipolar electrode. Furthermore, the reversibility of the bipolar doping of the PMT film was demonstrated by a spectroelectrochemical investigation.     

The principles of bipolar electrochemistry and its electroanalysis applications

The present study reports the wireless technique that generates asymmetric reactivity on the surface of the conducting substrate without any direct electrical connection in the electrolyte solution by inducing external power. In recent years, bipolar electrochemical systems have received special attention that they are used for new kinds of electrochemical applications ranging from electrodeposition to electroanalytical chemistry. Bipolar electrochemistry is a unique technique because of the lack of direct electrical connection to the bipolar electrode. In this perspective article, we first illustrate the concept and history of the bipolar electrochemistry as well as their application based on the open and closed bipolar configuration in different fields.