Over the past decade, bipolar electrochemistry has emerged from relative obscurity to provide a promising new means for integrating electrochemistry into lab-ona-chip systems. This article describes the fundamental operating principles of bipolar electrodes, as well as several interesting applications.A bipolar electrode (BPE) is an electronic conductor in contact with an ionically conductive phase. When a sufficiently high electric field is applied across the ionic phase, faradaic reactions occur at the ends of the BPE even though there is no direct electrical connection between it and an external power supply. In this article, we describe the fundamental principles and some electroanalytical applications of BPEs for array-based sensing, separations, and concentration enrichment in microelectrochemical systems. Specifically, we show how the latter three operations, which are normally thought of as arising from different phenomena, are linked by processes occurring on and near BPEs confined within a convenient, miniaturized microfluidic format. The results presented here demonstrate that under a particular set of conditions, up to 1000 well-defined BPEs can be simultaneously activated and interrogated using just a single pair of driving electrodes. Furthermore, a slight change to the resistance of the buffer solution within the microfluidic channel leads to the separation and concentration enrichment of charged analytes. OVERVIEW OF BIPOLAR ELECTROCHEMISTRYA traditional three-electrode electrochemical cell, which consists of a working electrode, an auxiliary electrode, and a reference electrode, is illustrated in Scheme 1a. In this configuration, the potential of the working electrode, which is related to the energy of the electrons in the electrode, is controlled (versus a reference electrode) using a potentiostat. The potential of the solution is not directly controlled; in other words, it is at a floating potential that (in the absence of an externally applied electric field) depends on the composition of the solution. When the potential of the working electrode is set to a value more negative than that of an electroactive molecule in the solution, electrons may (depending upon kinetics) transfer from the electrode to reduce species in solution (Scheme 1b; note that positive potentials are up in this diagram to make it consistent with Scheme 1c). Similarly, oxidation reactions occur when the electron transfer is in the opposite direction. The faradaic current measured in the circuit connecting the working and auxiliary electrodes is a direct FRANÇ OIS MAVRÉ
We report a microelectrochemical array composed of 1000 individual bipolar electrodes that are controlled with just two driving electrodes and a simple power supply. The system is configured so that faradaic processes occurring at the cathode end of each electrode are correlated to light emission via electrogenerated chemiluminescence (ECL) at the anode end. This makes it possible to read out the state of each electrode simultaneously. The significant advance is that the electrode array is fabricated on a glass microscope slide and is operated in a simple electrochemical cell. This eliminates the need for microfluidic channels, provides a fabrication route to arbitrarily large electrode arrays, and will make it possible to place sensing chemistries onto each electrode using a robotic spotter.
Bipolar electrodes are potentially useful for a variety of sensing applications, but their implementation has been hampered by an inability to easily monitor the current through such electrodes. However, current can be indirectly determined using electrogenerated chemiluminescence (ECL) as a reporting mechanism. This paper provides a detailed theoretical analysis of ECL reporting at bipolar electrodes. In addition, experiments are described that confirm the theory. Finally, we correlate ECL intensity directly to current through the use of split bipolar electrodes. The results indicate that the lowest current that can be indirectly detected through ECL reporting is ∼32 µA/cm 2 , which corresponds to a reporting sensitivity of ∼7200 counts/nA in the present experimental system.In this report, we show how the magnitude of the electrogenerated chemiluminescence (ECL) emission at a floating, bipolar electrode relates to the current flowing through the electrode. This correlation presents an experimental challenge, because bipolar electrodes lack external connections. We address this problem using electrode configurations that mimic the behavior of bipolar electrodes, and a rigorous, quantitative analysis of the results makes it possible to determine the emission efficiency of bipolar electrodes. This detailed level of understanding is essential for future analytical applications of bipolar electrode arrays. [1][2][3][4][5] We and others previously demonstrated that an isolated, conductive wire placed within a microfluidic channel can act as a bipolar electrode when a sufficiently high potential difference is applied across the solution that contacts the electrode. 1-14 Scheme 1 illustrates this principle. It is important to note that we assume a linear electric field, because, as will be discussed later, ∼99% of the total current passes through the electrolyte solution, rather than through the bipolar electrode. Therefore, distortion of the electric field by faradaic current is negligible.
Here we report an electrochemical DNA microarray sensor whose function is controlled using just two wires regardless of the number of individual sensing electrodes. This advance is enabled by confining bipolar sensing electrodes within a microfluidic channel (part a of Scheme 1) and exerting potential control over the electrolyte solution rather than individual electrodes. In this configuration, the two driving electrodes control the potential difference between the sensing electrodes and the solution, and the current at the sensing electrodes is indirectly measured by taking advantage of electrogenerated chemiluminescence (ECL) present at the anode end of each bipolar electrode. In this communication, we show that this approach can be used to sense hybridization of DNA oligonucleotides.Electrochemistry is normally carried out by controlling the electrode potential. However, because the potential difference between the electrode and the solution drives the electron-transfer reaction, it is equally effective to control the potential of the solution. Our approach for using this principle is illustrated in part b of Scheme 1. 1,2 An external potential (E tot ) is applied to the two ends of a microchannel, and the resistance of the electrolyte solution results in a linear potential gradient (dE/dx) along the channel. The difference in potential between the two ends of the bipolar electrode (∆E elec ) is the fraction of E tot dropped across the length of this electrode (L elec ). If ∆E elec is sufficiently large, then faradaic electrochemical processes will occur simultaneously at both ends of the bipolar electrode. Because of the requirement for charge balance, the rate of electron transfer (i.e., the current) at both ends of the electrode must be the same.A significant deficiency of bipolar electrochemistry is that there is no means for directly measuring current flowing within an electrode. 1,2 The experimental configuration illustrated in part c of Scheme 1 addresses this problem. Here, E tot is held at a sufficiently high value that the ECL reaction resulting from the oxidation of Ru(bpy) 3 2+ and tri-n-propylamine (TPrA), 3-5 indicated at the anode end of the bipolar electrode, is activated upon electrocatalytic reduction of O 2 at the cathode end of the electrode. In the sensing experiments discussed next, the oxygen reduction reaction (ORR) is catalyzed by hybridization of target DNA labeled with Pt nanoparticles (Pt-NPs) to previously immobilized capture DNA. 6 Accordingly, in the presence of DNA hybridization at the cathode, light is emitted from the anode.Part a of Figure 1 shows that the sensor consists of a microfluidic device and three Au electrodes. The poly(dimethylsiloxane) (PDMS) microfluidic device was prepared by a standard micromolding method. 7 The microchannel was 12 mm long, 1.75 mm wide, and 26 µm high. The three Au electrodes (1.00 × 0.25 mm) were microfabricated on a glass slide and configured parallel to one another at the center of the channel. Two Pt wires were placed in reservoirs at ei...
Here we report a new type of sensing platform that is based on electrodissolution of a metallic bipolar electrode (BPE). When the target DNA binds to the capture probe at the cathodic pole of the BPE, it triggers the oxidation and dissolution of Ag metal present at the anodic pole. The loss of Ag is easily detectable with the naked eye or a magnifying glass and provides a permanent record of the electrochemical history of the electrode. More importantly, the decrease in the length of the BPE can be directly correlated to the number of electrons passing through the BPE and hence to the sensing reaction at the cathode.
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