Theory of the wavelet analysis for electrochemical noise by use of the Laguerre functions

It is shown that the wavelet transform that uses the Laguerre function as a basis function is a useful tool to analyse the stationary electrochemical noise. Knowledge of the variance of the Laguerre wavelet of noise allows the Laplace transform of the correlation function to be found. The Laplace transform of the correlation function may be referred to the spectral density in the Laplace domain as well as to the operational spectral density of noise. It is shown that the operational spectral density of noise can be found not only by averaging over the ensemble of realizations of the noise process but also by averaging over the ensemble of Laguerre wavelets. The results obtained can be useful not only for analysis of electrochemical noise but also for analysis of any stationary random process, in particular for the time series analysis in econometric research.     

Theory of the shot noise in electrochemical bridge contacts

Correlation functions of the current and the noise power for a redox-group-containing electrochemical tunnel contact are calculated under the assumption on a shot character of the electron tunneling. Weak and strong interaction between the redox group and electrodes is considered. Overvoltage dependence of the Fano factor at different bias-potentials is found. The conditions of passing of the calculated dependences into the Schottky law are formulated.     

Multicomponent analysis of electrochemical signals in the wavelet domain

Successful applications of multivariate calibration in the field of electrochemistry have been recently reported, using various approaches such as multilinear regression (MLR), continuum regression, partial least squares regression (PLS) and artificial neural networks (ANN). Despite the good performance of these methods, it is nowadays accepted that they can benefit from data transformations aiming at removing baseline effects, reducing noise and compressing the data. In this context the wavelet transform seems a very promising tool. Here, we propose a methodology, based on the fast wavelet transform, for feature selection prior to calibration. As a benchmark, a data set consisting of lead and thallium mixtures measured by differential pulse anodic stripping voltammetry and giving seriously overlapped responses has been used. Three regression techniques are compared: MLR, PLS and ANN. Good predictive and effective models are obtained. Through inspection of the reconstructed signals, identification and interpretation of significant regions in the voltammograms are possible.     

Detection of SCC of 304 NG stainless steel in an acidic NaCl solution using electrochemical noise based on chaos and wavelet analysis

The stress corrosion crack (SCC) of 304 nuclear grade (NG) stainless steel (SS) in 0.5 mol/L NaCl+1.5 mol/L H₂SO₄ was monitored using electrochemical noise (EN) based on chaos theory, statistics and wavelet analysis. The results indicated that the SCC process was divided into three stages according to the transient features in the EN. In the beginning, compared with the sample without applied stress, the enhanced fluctuation amplitude in the electrochemical current noise (ECN) of the stressed samples was attributed to stress-enhanced pitting corrosion and uniform corrosion; then the fluctuations of ECN for all the samples decreased due to a coverage by the corrosion products; however, the ECN fluctuations of stressed sample were larger than the unstressed sample, suggesting that the stress enhanced the SCC initiation and propagation. Chaos analysis revealed that the correlation dimensions increase from 2.1 to 2.5 during the corrosion process, and the applied stress seems increase the complexity and uncertainty of the ECN signal.     

Analysis of electrochemical current noise data arising from corrosion of Al alloy in a gel electrolyte by wavelet and recurrence methods

Gel electrolytes are preferred to liquid electrolytes due to the slower movement of ions, thereby lower damage. However, the use of gel electrolytes causes an increase in corrosion susceptibility due to the non-flat surfaces of cells. The use of corrosion inhibitors is one of the most practical methods for corrosion protection. The effect of penicillin as an inhibitor in a gel electrolyte containing 3.5% NaCl was investigated on the corrosion of 6061 Al alloy. Both electrochemical impedance spectroscopy (EIS) and electrochemical current noise (ECN) techniques were employed for data collection. The ECN data were analyzed by both the wavelet transform and the recurrence methods. According to EIS data, the inhibition efficiency (IE) increased from 77% to 94% for up to 15 days. The IE values obtained from ECN data increased from 81% to 90% over immersion time. The recurrence plots of the blank sample proved the increase of pit density with increasing the immersion time from 1 day to 15 days. However, the SDPS plots of the blank sample showed an increase of pit size with increasing the immersion time from 1 to 15 days. The recurrence plots proved the inhibition effect of penicillin on the corrosion of Al alloy that is apparent from the significant lowering of vertical black lines.     

An electrochemical cell coupled with disposable screen-printed electrodes for use in flow injection analysis.

An electrochemical cell coupled with disposable screen-printed electrodes (SPEs) that is specifically designed for use in flow injection analysis (FIA) is described in this study. The cell is made of foldable polyoxymethylene (acetal) thick platelets with the bottom portion consisting of a cavity track to drag the SPEs in position and the top portion having predrilled T-like holes to arrange the Ag/AgCl reference electrode and stainless steel inlet & outlet. An "O ring" is suitably fixed on the top of the working electrode to form a thin-layer space where the electrochemical reaction can take place. Hydrodynamic characterization was validated by using a benchmark hexacyanoferrate redox couple. The results of practical analysis of glucose in human plasma clearly demonstrate the characteristics and applicability of the proposed wall-jet electrochemical cell in FIA.     

Oxide cathodes for electrochemical devices made with the use of a nanostructured composition material

The possibility of using the LnO _x mischmetal (Ln = Ce, La, Nd, Pr, Sm) for preparation of cathodes for solid-oxide fuel cells with the supported YSZ electrolyte is studied. The electrical and electrochemical characteristics of Ln-Mn-O electrodes with the ratio of all lanthanides contained in the mischmetal except for cerium to manganese Ln: Mn = 1: 1 and also of a material comprised of Ln-Mn-O and La_0.8Sr_0.2MnO₃ are studied. The latter electrode material that contains 35?C40 wt % of Ln-Mn-O and was sintered at 1200°C has the specific ohmic resistance of 0.1 ?? cm at 800°C. The polarization conductivity is compared for electrodes made of 100% Ln-Mn-O, 40 wt % Ln-Mn-O + 60 wt % La_0.8Sr_0.2MnO₃, and 100% La_0.8Sr_0.2MnO₃ in the initial state and after their modification through the introduction of an electrocatalyst (PrO_{2 ? x} ). The highest polarization conductivity is typical of Ln-Mn-O + (La, Sr)MnO₃ electrodes containing 40 wt % Ln-Mn-O and PrO_{2 ? x} . The polarization conductivity of these electrodes is found to be 25 S/cm² at 800°C.     

Wide frequency band electrochemical noise measurement and analysis of a Li/SOCl2 primary battery

Journal of Solid State Electrochemistry - Electrochemical noise of a primary LiSOCl2 battery was measured in a wide frequency band during discharge at a constant value resistor. Power spectral...     

Shot noise sets the limit of quantification in electrochemical measurements

Detection of single molecules, particles, and rapid redox events is a challenge of electrochemical investigations and requires either an amplification strategy or significant averaging for the electrochemical current to exceed the noise level. We consider the minimum number of electrons required to reach the limit of quantification in these electrochemical measurements. A survey of the literature indicates that the state-of-the-art limit in current detection for different types of measurements (e.g. voltammetry, single-molecule redox cycling, ion channel recordings of single molecules, metal nanoparticle collision, and phase nucleation) is independent of the nature of the measurement and increases linearly with reciprocal response time, Δt^?1, over ～5 orders of magnitude (from ～10 to ～10⁶ s^?1). We demonstrate that the practical limit of quantification requires cumulative measurement of ～2100 electrons during Δt and is determined by statistics of counting electrons, that is, the shot noise in the current.     

Evaluation of electrochemical charge-transfer rates by using (real) laplace space analysis: Single potential-step chronoamperometry of hexacyanoferrate(III)/(II) and Europium(III)/(II) couples

The one-electron hexacyanoferrate(III)/(II) and europium(III)/(II) redox couples were evaluated by using single-pulse chronoamperometry at a planar glassy carbon electrode (and also a mercury-pool electrode for europium) to test Wijnen's method for calculating heterogeneous electron-transfer rate constants by using (real-axis) Laplace space analysis. The k⁰ values obtained for the former couple in potassium or lithium chloride supporting electrolytes agreed well with published constants obtained by diverse real-time techniques. Transfer coefficients (α⁰) obtained from (? in k^a/?η)_η=0 for LiCl electrolyte were 0.4–0.5, rather than 0.22 previously reported. The cathodic rate values calculated for europium(III) reduciton (in NaClO₄/HClO₄) on both glassy carbon and mercury agreed very well with each other and with published values obtained by d.c. polarography and faradaic impedance measurements. Owing to several factors, including its ability to utilize virtually any set of recorded i(t) data points, Wijnen's Laplace technique offers an attractive alternative to conventional single-pulse analysis in real-time.     

Recent advances in the use of ionic liquids for electrochemical sensing

Ionic Liquids are salts that are liquid at (or just above) room temperature. They possess several advantageous properties (e.g. high intrinsic conductivity, wide electrochemical windows, low volatility, high thermal stability and good solvating ability), which make them ideal as non-volatile electrolytes in electrochemical sensors. This mini-review article describes the recent uses of ionic liquids in electrochemical sensing applications (covering the last 3 years) in the context of voltammetric sensing at solid/liquid, liquid/liquid interfaces and carbon paste electrodes, as well as their use in gas sensing, ion-selective electrodes, and for detecting biological molecules, explosives and chemical warfare agents. A comment on the future direction and challenges in this field is also presented.     

Theory and mechanism of electron transfer in a condensed medium at high temperatures with allowance for change in vibrational frequencies

An expression for electron transfer rate has been obtained through the solution of a time wave equation by the variational method by defining the wave function as a linear combination of functions corresponding to electron localization on the donor and on the acceptor. A dependence of electron transfer on temperature, on the electronic and vibrational characteristics of the system has been derived. An activation energy temperature-variation effect has been obtained. It has been proved that many-electron transfers are impossible.     

Development and optimization of a method for the analysis of Brazilein by HPLC with electrochemical detection

The aim of this study was to establish an easy and accurate method for the determination of Brazilein in plant samples due to its potential pharmacological activities. High-performance liquid chromatography (HPLC) with electrochemical detection (ED) was used for the assay of Brazilein in this study for the first time. Crucial influence parameters including concentration of dodecane-1-sulfonic acid sodium salt (DSASS), inorganic modifier, tetrabutyl-ammonium hydroxide solution (TBAOH), and applied potential of proposed method were investigated. The proposed method is simple, rapid (analysis time: approximately 10 min), sensitive [(detection limit: 0.6 ng per injection (20 microl) at a signal-noise ratio 3:1)], highly selective and precise (intra- and inter-day precisions were within 5%, n = 7). The calibration graph of Brazilein was linear in the range 0.6-150 ng per injection 20 microl. Recovery of Brazilein was over 92% by standard addition method.     

Synthesis and electrochemical characterization of carbon nanotubes decorated with nickel nanoparticles for use as an electrochemical capacitor

Attachment of nickel nanoparticles on multiwalled carbon nanotubes (MWCNTs) was conducted to explore the influence of Ni loading on the electrochemical capacitance of MWCNT electrodes. A chemical impregnation leaded to homogeneously disperse Ni particles onto the surface of MWCNTs, and the Ni particles were found to be an average size of 30–50 nm. The capacitive behavior of the MWCNT electrodes was investigated in 6 M KOH, by using cyclic voltammetry (CV), charge–discharge cycling, and ac electrochemical impedance spectroscopy. CV measurements showed that the Faradaic current was found to increase with the Ni coverage, indicating that the presence of Ni would enhance the pseudocapacitance through the redox process. Equivalent circuit analysis indicated that both of electrical connection and charge transfer resistances accounted for the major proportion of the overall resistance and were found to decrease with the amount of nickel. A linearity relationship between the total capacitance and the Ni population reflected that each Ni particle exhibits an identical electrochemical activity in enhancing the electrochemical capacitance. The overall electrochemical capacitance (including double layer capacitance and pseudocapacitance) of Ni-MWCNT electrode can reach a maximum of 210 F/g over 500 cycles.     

Reviewing the use of chitosan and polydopamine for electrochemical sensing

Biopolymers possess highly favorable properties for electrochemical biosensing such as their inherent biocompatibility, inexpensive nature, and strong interfacial adhesion. In this mini-review, we will focus on chitosan and polydopamine, two of the most commonly used biopolymers, for electrochemical sensing applications. Chitosan is a polysaccharide that exhibits high chemical resistance, offers straightforward modification and cross-linking, and possesses antibacterial properties and mucoadhesion. Polydopamine has the benefit of universal adhesion, in addition to the ability to form self-assembled structures. We will demonstrate how the unique structural and electrochemical features of these biopolymers can be used in a range of electrochemical biosensing platforms.     

Microchip capillary electrophoresis with a boron-doped diamond electrochemical detector for analysis of aromatic amines

The attractive features of a boron-doped diamond (BDD) thin-film detector for microchip capillary electrophoretic (CE) separations of dye-related amino-substituted aromatic compounds are described. The diamond electrode was employed in the end-column amperometric detection of 4-aminophenol (4-AP), 1,2-phenylenediamine (1,2-PDA), 2-aminonaphthalene (2-AN), 2-chloroaniline (2-CA), and o-aminobenzoic acid (o-ABA), and its attractive behavior was compared to commonly used screen-printed carbon and glassy-carbon electrodes. These conventional electrode materials exhibit a significant degree of passivation and low sensitivity to the above-mentioned environmental pollutants. The diamond-based electrochemical detection system displayed a favorable analytical performance, including lower noise levels, higher peak resolution with enhanced sensitivity, and improved resistance against electrode passivation. Factors influencing the on-chip analysis were assessed and optimized. The diamond detector displayed detection limits of 2.0 and 1.3 microM for 4-AP and 2-AN, respectively, and a wide linear response for these compounds over the 2-50 microM range. The enhanced stability was demonstrated by relative standard deviation (RSD) values of 1.4% and 4.7% for 100 microM 1,2-PDA and 200 microM 2-CA, respectively, for repetitive detections (n = 7). Besides, the simultaneously observed current decrease was 2.4 and 9.1% for 1,2-PDA and 2-CA, respectively (compared to 21.8 and 41.0% at the screen-printed carbon electrode and 28.3 and 34.1% at the glassy carbon electrode, respectively). The favorable properties of the diamond electrode indicate great promise for environmental applications in CE and other microchip devices.