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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. 10.1021/ac101262v

A ribonucleic acid (RNA) sensor based on hybridization of its peptide nucleic acid (PNA) molecule with a target RNA oligomer of the internal ribosome entry site sequence specific to the hepatitis C virus (HCV) and the electrochemical impedance detection is described. This RNA is one of the most conservative molecules of the whole HCV RNA genome. The ammonium ion terminated PNA molecule was immobilized via its host--guest interactions with the diaza crown ring of 3-thiophene-acetamide-diaza-18-crown-6 synthesized by a simple two-step method, which forms a well-defined self-assembled monolayer (SAM) on gold. Hybridization events of the probe PNA with the target RNA were monitored by measuring charge-transfer resistances for the Fe[[(CN).sub.6].sup.3-/4-] redox probe using Fourier transform electrochemical impedance spectroscopy. The ratio of the resistances of the SAM-covered electrode measured before and after hybridization increased linearly with log[RNA] in the rat liver lysate with a detection limit of about 23 pM. 10.1021/ac1019232

In this paper, we report a new electroanalytical technique we call snapshot voltammetry. This method combines the properties of bipolar electrodes with electrogenerated chemiluminescence (ECL) to provide a means for recording optical voltammograms in a single micrograph. In essence, the information in a snapshot voltammogram is contained in the spatial domain rather than in the time domain, which is the case for conventional voltammetry. The use of a triangle-shaped bipolar electrode stabilizes the interracial potential difference along its length. Basic electrochemical parameters extracted from snapshot voltammograms are in good agreement with those obtained by conventional voltammetry. Although not explicitly demonstrated in this paper, this method offers the possibility of using arrays of bipolar electrodes to obtain numerous snapshot voltammograms simultaneously. 10.1021/ac100846v

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 [micro]A/[cm.sup.2], which corresponds to a reporting sensitivity of ~7200 counts/nA in the present experimental system.

A highly selective receptor for serotonin was designed using cages formed by the (R)-lipo-diaza-18-crown-6 self-assembled monolayer (SAM) on gold and experimentally verified by a variety of electrochemical experiments in solutions containing large amounts of dopamine and ascorbic acid, as well as other interferents. The molecular modeling study showed that parameters such as the H-[pi] interaction provided important driving forces for the cage to form a strong inclusion complex with serotonin. The charge-transfer resistance ([R.sub.CT]'s) to/from redox probe ions, Fe[(CN).sub.6.sup.3-/4-], was greatly enhanced because of their electrostatic attractions to ammonium ions of serotonin molecules captured by cages. The changes in [R.sub.CT]-values were shown to be remarkably selective for serotonin in the presence of many interferents.

We report a novel concept of sensing glycated hemoglobin, [HbA.sub.1c], which is now the most important index for a long-term average blood glucose level, by first selectively immobilizing it on the thiophene-3-boronic acid (T3BA) self-assembled monolayer (SAM)-covered gold electrode by a selective chemical reaction with boronic acid. [HbA.sub.1c] thus immobilized is then detected by the label-free electrochemical impedance spectroscopic (EIS) measurements with a redox probe, an equimolar mixture of [K.sub.3]Fe[(CN).sub.6] and [K.sub.4]Fe[(CN).sub.6], present. The rate of charge transfer between the electrode and the redox probe is shown to be modulated by the amount of [HbA.sub.1c] in the matrix hemoglobin solution due to the blocking effect caused by the binding of [HbA.sub.1c] with boronic acid. Both the formation of a well-defined T3BA-SAM on the gold surface and the chemical binding of its boronic acid with [HbA.sub.1c] in solution were confirmed by quartz crystal microbalance, atomic force microscopy, and EIS experiments.

We report a novel comprehensive Fourier transform electrochemical impedance spectroscopic (FIEIS) analysis method of a series of chronoamperometric currents obtained during staircase cyclic voltammetric (SCV) experiments. In our method, FTEIS analysis of a set of chronoamperometric currents recorded upon applying a series of small potential steps during an SCV experiment provides a complete description of an electron-transfer reaction at the electrode/electrolyte interface in forms of equivalent circuit elements. Conversion of the circuit elements thus obtained from the analysis allows electrode kinetic parameters including the electron-transfer rate constant, transfer coefficient, diffusion coefficient, and double layer capacitance as well as thermodynamic parameters such as the half-wave potential and the apparent number of electrons transferred to be determined. Theories for obtaining an ac admittance voltammogram, as well as both the thermodynamic and mass-transfer kinetic parameters thereof, from the SCV data have been developed and verified. A decided advantage of the method is that it provides completely self-contained information regarding an electron-transfer reaction from a single pass of the SCV experiment.

An integrated theory describing both faradaic and nonfaradaic currents obtained upon potential step at an electrified electrode/electrolyte interface has been developed based on equivalent circuits that had been used to explain electrochemical reactions and experimentally verified. The faradaic current is shown to consist of the mass transport-dependent and -independent parts, which is in general agreement with the expression previously derived from the diffusion equations. The decay of the capacitive current is determined by the time constant represented by the product of the resistance obtained from the parallel connection of the solution and polarization resistances and the double layer capacitance; this is not consistent with the current understanding of the capacitive current decay, which takes into account the double layer capacitance and the solution resistance only. Many insights into the electron-transfer reactions are discussed based on the interpretation of impedance representation of the system, which would not have been possible without the present theory.