Time-Resolved Electrochemical Impedance Spectroscopy of Stochastic Nanoparticle Collision: Short Time Fourier Transform versus Continuous Wavelet Transform.

This work demonstrates the utilization of short-time Fourier transform (STFT), and continuous wavelet transform (CWT) electrochemical impedance spectroscopy (EIS) for time-resolved analysis of stochastic collision events of platinum nanoparticles (NPs) onto gold ultramicroelectrode (UME). The enhanced electrocatalytic activity is observed in both chronoamperometry (CA) and EIS. CA provides the impact moment and rough estimation of the size of NPs. The quantitative information such as charge transfer resistance (Rct ) relevant to the exchange current density of a single Pt NP is estimated from EIS. The CWT analysis of the phase angle parameter is better for NP collision detection in terms of time resolution compared to the STFT method.

In Situ Real-Time Monitoring of ITO Film under a Chemical Etching Process Using Fourier Transform Electrochemical Impedance Spectroscopy.

As a novel approach to the in situ real-time investigation of an ITO electrode during the wet etching process, step-excitation Fourier-transform electrochemical impedance spectroscopy (FT-EIS) was implemented. The equivalent circuit parameters (e.g., Rct, Cdl) continuously obtained by the FT-EIS measurements during the entire etching process showed an electrode activation at the initial period as well as the completion of etching. The FT-EIS results were further validated by cyclic voltammograms and impedance measurements of partially etched ITO films using ferri- and ferrocyanide solution in combination with FESEM imaging, EDS, XRD analyses, and COMSOL simulation. We also demonstrated that this technique can be further utilized to obtain intact interdigitated array (IDA) electrodes in a reproducible manner, which is generally considered to be quite tricky due to delicacy of the pattern. Given that the FT-EIS allows for instantaneous snapshots of the electrode at every moment, this work may hold promise for in situ real-time examination of structural, electrokinetic, or mass transfer-related information on electrochemical systems undergoing constantly changing, transient processes including etching, which would be impossible with conventional electroanalytical techniques.

Correlation between Tafel Analysis and Electrochemical Impedance Spectroscopy by Prediction of Amperometric Response from EIS.

Tafel analysis and electrochemical impedance spectroscopy (EIS) have been widely used to characterize many kinds of electrocatalysts. The former provides the kinetic information of an electrochemical reaction with the exchange current while the latter does with the charge transfer resistance closely related to the exchange current. Both techniques, however, suffer from practical troubles which often decrease their reliabilities. In order to circumvent those troubles, an alternative was suggested that Tafel analysis was combined with EIS, even though its theoretical background was not clearly established. Tafel analysis is based on dc measurement, and EIS is on an ac one, respectively. Here, inspired by the second generation of EIS from chronoamperometry, we try to find how those techniques are correlated by investigating an amperometric response from EIS. The first step is Fourier transform of an arbitrary dc potential signal in the time domain to obtain the amplitudes and phases of the Fourier series which are equivalent to ac signals of each frequency. Second, with the Fourier series being applied onto the impedance data, the responding currents of each frequency are calculated by Ohm's law. Third, the current in the frequency domain is transferred back to the time domain by inverse Fourier transform to yield chronoamperometric or Tafel plots depending on the type of the applied dc potential. Finally, we can study Tafel plots based on EIS at different conditions and their correlations which are expected to be a better indicator for characterizing electrocatalysts instead of the slope of the classical Tafel analysis.

Implementation of Second-Generation Fourier Transform Electrochemical Impedance Spectroscopy with Commercial Potentiostat and Application to Time-Resolved Electrochemical Impedance Spectroscopy.

We report the implementation of second-generation Fourier transform electrochemical impedance spectroscopy (2G FT-EIS) with commercial potentiostat. Although 2G FT-EIS based on chronoamperometry has several advantages of short measurement time and ability of time-resolved EIS, a special home-built electrochemical system is essential, which has been an obstacle to the wide application of 2G FT-EIS. Current commercial potentiostat and software, however, has sufficient power thanks to recent state-of-the art electronics and software industry. 2G FT-EIS requires two signals of time versus voltage and time versus current from chronoamperometry with a high sampling rate. In this work, auxiliary input of a commercial potentiostat was used to record voltage signal concomitant with typical chronoamperometry that consisted of time versus current. This simple approach enables the 2G EIS without expensive frequency response analyzer (FRA) and the complex home-built instrument. EISs with various charge transfer kinetics were investigated by the formation of a self-assembled monolayer (SAM) on Au with different chain lengths. More to the point, in situ time-resolved EISs during the SAM were obtained, demonstrating the ability of a commercial potentiostat for time-resolved EIS.

Core-Corona Functionalization of Diblock Copolymer Micelles by Heterogeneous Metal Nanoparticles for Dual Modality in Chemical Reactions.

Nanoscale assemblies composed of different types of nanoparticles (NPs) can reveal interesting aspects about material properties beyond the functions of individual constituent NPs. This research direction may also represent current challenges in nanoscience toward practical applications. With respect to the assembling method, synthetic or biological nanostructures can be utilized to organize heterogeneous NPs in specific sites via chemical or physical interactions. However, those assembling methods often encounter uncontrollable particle aggregation or phase separation. In this study, we anticipated that the self-segregating properties of block copolymer micelles could be particularly useful for organizing heterogeneous NPs, because the presence of chemically distinct domains such as the core and the corona can facilitate the selective placement of constituent NPs in separate domains. Here, we simultaneously functionalized the core and the corona of micelles by Au NPs and Ag NPs, which exhibited plasmonic and catalytic functions, respectively. Our primary question is whether these plasmonic and catalytic functions can be combined in the assembled structures to engineer the kinetics of a model chemical reaction. To test this hypothesis, the catalytic reduction of 4-nitrophenol was selected to evaluate the collective properties of the micellar assemblies in a chemical reaction.

Development of the smartphone-based colorimetry for multi-analyte sensing arrays.

Here we report development of a smartphone app (application) that digitizes the colours of a colorimetric sensor array. A conventional colorimetric sensor array consists of multiple paper-based sensors, and reports the detection results in terms of colour change. Evaluation of the colour changes is normally done by the naked eye, which may cause uncertainties due to personal subjectivity and the surrounding conditions. Solutions have been particularly sought in smartphones as they are capable of spectrometric functions. Our report specifically focuses on development of a practical app for immediate point-of-care (POC) multi-analyte sensing without additional devices. First, the individual positions of the sensors are automatically identified by the smartphone; second, the colours measured at each sensor are digitized based on a correction algorithm; and third, the corrected colours are converted to concentration values by pre-loaded calibration curves. All through these sequential processes, the sensor array taken in a smartphone snapshot undergoes laboratory-level spectrometry. The advantages of inexpensive and convenient paper-based colorimetry and the ubiquitous smartphone are tied to achieve a ready-to-go POC diagnosis.

Development of galvanostatic Fourier transform electrochemical impedance spectroscopy.

Here, we report development of the galvanostatic Fourier transform electrochemical impedance spectroscopy (FTEIS), which monitors impedance of electrochemical reactions activated by current steps. We first derive relevant relations for potential change upon application of a step current, obtain impedances theoretically from the relations by simulation, and verify them with experimental results. The validity of the galvanostatic FTEIS technique is demonstrated by measuring impedances of a semiconductive silicon wafer using the conventional frequency response analysis (FRA), the potentiostatic FTEIS, and the galvanostatic FTEIS methods, and the results are in excellent agreement with each other. This work is significant in that the galvanostatic FTEIS would allow one to record impedance changes during charge/discharge cycles of secondary batteries and fuel cells as well as electrochemically irreversible systems which may produce noise level chronoamperometric currents by potentiostatic techniques.

Two-channel microelectrochemical bipolar electrode sensor array.

We report a two-channel microelectrochemical sensor that communicates between separate sensing and reporting microchannels via one or more bipolar electrodes (BPEs). Depending on the contents of each microchannel and the voltage applied across the BPE, faradaic reactions may be activated simultaneously in both channels. As presently configured, one end of the BPE is designated as the sensing pole and the other as the reporting pole. When the sensing pole is activated by a target, electrogenerated chemiluminescence (ECL) is emitted at the reporting pole. Compared to previously reported single-channel BPE sensors, the key advantage of the multichannel architecture reported here is physical separation of the ECL reporting cocktail and the solution containing the target. This prevents chemical interference between the two channels.

Design and operation of microelectrochemical gates and integrated circuits.

Here we report a simple design philosophy, based on the principles of bipolar electrochemistry, for the operation of microelectrochemical integrated circuits. The inputs for these systems are simple voltage sources, but because they do not require much power they could be activated by chemical or biological reactions. Device output is an optical signal arising from electrogenerated chemiluminescence. Individual microelectrochemical logic gates are described first, and then multiple logic circuits are integrated into a single microfluidic channel to yield an integrated circuit that can perform parallel logic functions. AND, OR, NOR, and NAND gates are described. Eventually, systems such as those described here could provide on-chip data processing functions for lab-on-a-chip devices.

Bipolar electrodes: a useful tool for concentration, separation, and detection of analytes in microelectrochemical systems.

Over the past decade, bipolar electrochemistry has emerged from relative obscurity to provide a promising new means for integrating electrochemistry into lab-on-a-chip systems. This article describes the fundamental operating principles of bipolar electrodes, as well as several interesting applications.

Label-free impedimetric sensor for a ribonucleic acid oligomer specific to hepatitis C virus at a self-assembled monolayer-covered electrode.

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)(6)(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.

Electrochemical impedance spectroscopy.

This review describes recent advances in electrochemical impedance spectroscopy (EIS) with an emphasis on its novel applications to various electrochemistry-related problems. Section 1 discusses the development of new EIS techniques to reduce measurement time. For this purpose, various forms of multisine EIS techniques were first developed via a noise signal synthesized by mixing ac waves of various frequencies, followed by fast Fourier transform of the signal and the resulting current. Subsequently, an entirely new concept was introduced in which true white noise was used as an excitation source, followed by Fourier transform of both excitation and response signals. Section 2 describes novel applications of the newly developed techniques to time-resolved impedance measurements as well as to impedance imaging. Section 3 is devoted to recent applications of EIS techniques, specifically traditional measurements in various fields with a special emphasis on biosensor detections.

Two-dimensional bipolar electrochemistry.

This paper introduces the concept of two-dimensional bipolar electrochemistry and discusses its principle of operation. The interesting new result is that electrochemical reactions can be localized at particular locations on the perimeter of a two-dimensional bipolar electrode (2D-BPE), configured at the intersection of two orthogonal microfluidic channels, by controlling the electric field within the contacting electrolyte solution. Experimentally determined maps of the electric field in the vicinity of the 2D-BPEs are in semiquantitative agreement with finite element simulations.

A sensing platform based on electrodissolution of a Ag bipolar electrode.

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.

Snapshot voltammetry using a triangular bipolar microelectrode.

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 interfacial 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.

A large-scale, wireless electrochemical bipolar electrode microarray.

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.

(R)-lipo-diaza-18-crown-6 self-assembled monolayer as a selective serotonin receptor.

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(CT)'s) to/from redox probe ions, Fe(CN)(6)(3-/4-), was greatly enhanced because of their electrostatic attractions to ammonium ions of serotonin molecules captured by cages. The changes in R(CT)-values were shown to be remarkably selective for serotonin in the presence of many interferents.

Selective electrochemical sensing of glycated hemoglobin (HbA1c) on thiophene-3-boronic acid self-assembled monolayer covered gold electrodes.

We report a novel concept of sensing glycated hemoglobin, HbA 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 1c thus immobilized is then detected by the label-free electrochemical impedance spectroscopic (EIS) measurements with a redox probe, an equimolar mixture of K 3Fe(CN) 6 and K 4Fe(CN) 6, present. The rate of charge transfer between the electrode and the redox probe is shown to be modulated by the amount of HbA 1c in the matrix hemoglobin solution due to the blocking effect caused by the binding of HbA 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 1c in solution were confirmed by quartz crystal microbalance, atomic force microscopy, and EIS experiments.

Highly cooperative ion-pair recognition of potassium cyanide using a heteroditopic ferrocene-based crown ether-trifluoroacetylcarboxanilide receptor.

A heteroditopic receptor having crown ether and trifluoroacetylcarboxanilide groups selectively recognizes both potassium and cyanide ions in acetonitrile with an association constant of as high as Ka = 1.9 x 10(7) M(-1) through a highly cooperative ion-pair interaction, resulting in two orders of magnitude enhancement in the binding affinity.