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.

Platinum dendrimer-encapsulated nanoparticles (DENs) containing an average of 55, 100, 147, 200, and 240 atoms were prepared within sixth-generation, hydroxyl-terminated, poly(amidoamine) dendrimers. These DENs were immobilized on glassy carbon electrodes, and the effect of particle size on the kinetics of the oxygen reduction reaction (ORR) was quantitatively evaluated using rotating disk voltammetry. The total areas of the Pt DENs were determined by electrochemical CO stripping and hydrogen desorption, and the results were found to be in reasonable agreement with calculated values. The largest particles exhibited the highest specific activities for the ORR.

d Cardiolipin (CL) is an anionic phospholipid with a characteristically large curvature and is of growing interest for two primary reasons: (i) it binds to and regulates many peripheral membrane proteins in bacteria and mitochondria, and (ii) it is distributed asymmetrically in rod-shaped cells and is concentrated at the poles and division septum. Despite the growing number of studies of CL, its function in bacteria remains unknown. 10-N-Nonyl acridine orange (NAO) is widely used to image CL in bacteria and mitochondria, as its interaction with CL is reported to produce a characteristic red-shifted fluorescence emission. Using a suite of biophysical techniques, we quantitatively studied the interaction of NAO with anionic phospholipids under physiologically relevant conditions. We found that NAO is promiscuous in its binding and has photophysical properties that are largely insensitive to the structure of diverse anionic phospholipids to which it binds. Being unable to rely solely on NAO to characterize the localization of CL in Escherichia coli cells, we instead used quantitative fluorescence microscopy, mass spectrometry, and mutants deficient in specific classes of anionic phospholipids. We found CL and phosphatidylglycerol (PG) concentrated in the polar regions of E. coli cell membranes; depletion of CL by genetic approaches increased the concentration of PG at the poles. Previous studies suggested that some CL-binding proteins also have a high affinity for PG and display a pattern of cellular localization that is not influenced by depletion of CL. Framed within the context of these previous experiments, our results suggest that PG may play an essential role in bacterial physiology by maintaining the anionic character of polar membranes.