Electrochemical Behavior and Electroanalytical Determination of Indole‐3‐Acetic Acid Phytohormone on a Boron‐Doped Diamond Electrode

In this work, a boron‐doped diamond (BDD) electrode was used for the electroanalytical determination of indole‐3‐acetic acid (IAA) phytohormone by square‐wave voltammetry. IAA yielded a well‐defined voltammetric response at +0.93 V (vs. Ag/AgCl) in Britton–Robinson buffer, pH 2.0. The process could be used to determine IAA in the concentration range of 5.0 to 50.0 µM (n=8, r=0.997), with a detection limit of 1.22 µM. The relative standard deviation of ten measurements was 2.09 % for 20.0 µM IAA. As an example, the practical applicability of BDD electrode was tested with the measurement of IAA in some plant seeds.     

Electrochemical Studies of Inclusion Complex Formed Between Glutathione and β‐cyclodextrin‐modified Carbon Electrodes and its Application for Determination of Glutathione

In this work, the electrochemical determination of glutathione (GSH) using β‐cyclodextrin (β‐CD) modified carbon electrodes was carried out. Different methodologies were used to modify the electrodes. In the first part of this paper, we analyze and compare the ability of the electrodes to determine GSH using the different β‐CD‐modified electrodes and cyclic voltammetry. We found that the carbon paste electrode modified by potential sweeping was the best electrode for GSH determination; in addition, we found that an inclusion complex formed between β‐CD deposited on the electrode surface and GSH. The formation constant for this complex was 2498.54 M^?1 at 25 °C. Furthermore, we have also calculated thermodynamic parameters for the formation of the inclusion complex. In the second part of this paper, we analyze the effect of sweep rate and pH on the determination of GSH. The best results were obtained at a rate of 50 mV s^?1 and a pH of 2.2. The β‐CD‐modified carbon paste electrode exhibits a linear response in a concentration range of 20 to 157 µM with a sensitivity of 1083.65 µA mM^?1cm^?2 and a detection limit of 3.92 µM. Finally, the electrode was used to determine the GSH concentration in Eichhornia crassipes root extract, and the concentration determination accuracy was validated by a well‐known spectroscopic method.     

Gold Nanoparticles‐β‐Cyclodextrin‐Chitosan‐Graphene Modified Glassy Carbon Electrode for Ultrasensitive Detection of Dopamine and Uric Acid

The nanocomposite (denoted as GR‐AuNPs‐CD‐CS) of graphene (GR), gold nanoparticles (AuNPs), chitosan (CS) and β‐cyclodextrin (β‐CD) was prepared to modify a glassy carbon electrode. The as‐modified electrode was explored for the ultrasensitive detection of dopamine (DA) and uric acid (UA). The modified electrode demonstrated linearly increased current response in the concentration range of 0.1–120 µm for DA and 0.05–70 µm for UA, with so far the best detection limit for DA and UA. Good stability and repeatability were further demonstrated for the as‐made sensor.     

Lab‐on‐a‐Chip Sensor with Evaporated Bismuth Film Electrode for Anodic Stripping Voltammetry of Zinc

In this work, we report on the development of a lab‐on‐a‐chip electrochemical sensor that uses an evaporated bismuth electrode to detect zinc using square wave anodic stripping voltammetry. The microscale electrochemical cell consists of a bismuth working electrode, an integrated silver/silver chloride reference electrode, and a gold auxiliary electrode. The sensor exhibits a linear response in 0.1 M acetate buffer at pH 6 with zinc concentrations in the 1–30 μM range and a calculated detection limit of 60 nM. The sensor successfully detected zinc in a bovine serum extract and the results were corfirmed by independent AAS measurements. Our results demonstrate the advantageous qualities of this lab‐on‐a‐chip electrochemical sensor for clinical applications, which include small sample volume (µL scale), reduced cost, short response time and high accuracy at low concentrations of analyte.     

Rapid Determination of Free Amino Acids in Milk by Microchip Electrophoresis Coupled with Laser‐induced Fluorescence Detection

A simple and sensitive method for determination of free amino acids in milk by microchip electrophoresis (MCE) coupled with laser‐induced fluorescence (LIF) detection was developed. Seven kinds of standard amino acids were derivated with sulfoindocyanine succinimidyl ester (Cy5) and then perfectly measured by MCE‐LIF within 150 s. The parameters of MCE separation were carefully investigated to obtain the optimal conditions: 100 mmol·L^?1 sodium borate solution (pH 10.0) as running buffer solution, 0.8 kV as injection voltage, 2.2 kV as separation voltage etc. The linear range of the detection of amino acids was from 0.01 µmol·L^?1 to 1.0 µmol·L^?1 and the detection limit was as low as about 1.0 nmol·L^?1. This MCE‐LIF method was applied to the measurements of free amino acids in actual milk samples and satisfactory experimental results were achieved.     

Electrochemical Determination of Pyrogallol at Conducting Poly(3,4‐ethylenedioxythiophene) Film‐Modified Screen‐Printed Carbon Electrodes

The electrochemical oxidation of pyrogallol at electrogenerated poly(3,4‐ethylenedioxythiophene) (PEDOT) film‐modified screen‐printed carbon electrodes (SPCE) was investigated. The voltammetric peak for the oxidation of pyrogallol in a pH 7 buffer solution at the modified electrode occurred at 0.13 V, much lower than the bare SPCE and preanodized SPCE. The experimental parameters, including electropolymerization conditions, solution pH values and applied potentials were optimized to improve the voltammetric responses. A linear calibration plot, based on flow‐injection amperometry, was obtained for 1–1000 µM pyrogallol, and a slope of 0.030 µA/µM was obtained. The detection limit (S/N=3) was 0.63 µM.     

Combination of large‐volume sample stacking with an electroosmotic flow pump with field‐amplified sample injection on cross‐channel chips

A combination of two online sample concentration techniques, large‐volume sample stacking with an electroosmotic flow (EOF) pump (LVSEP) and field‐amplified sample injection (FASI), was investigated in microchip electrophoresis (MCE) to achieve highly sensitive analysis. By applying reversed‐polarity voltages on a cross‐channel microchip, anionic analytes injected throughout a microchannel were first concentrated on the basis of LVSEP, followed by the electrokinetic stacking injection of the analytes from a sample reservoir by the FASI mechanism. As well as the voltage application, a pressure was also applied to the sample reservoir in LVSEP‐FASI. The applied pressure generated a counter‐flow against the EOF to reduce the migration velocity of the stacked analytes, especially around the cross section of the microchannel, which facilitated the FASI concentration. At the hydrodynamic pressure of 15 Pa, 4520‐fold sensitivity increase was obtained in the LVSEP‐FASI analysis of a standard dye, which was 33‐times higher than that obtained with a normal LVSEP. Furthermore, the use of the sharper channel was effective for enhancing the sensitivity, e.g., 29 100‐fold sensitivity increase was achieved with the 75‐μm wide channel. The developed method was applied to the chiral analysis of amino acids in MCE, resulting in the sensitivity enhancement factor of 2920 for the separated d ‐leucine.     

Corrosion and tribocorrosion behaviour of super‐thick diamond‐like carbon films deposited on stainless steel in NaCl solution

Super‐thick diamond‐like carbon (DLC) film is a potential protective coating in corrosive environments. In the present work, three kinds of DLC films whose thickness and modulation periods are 4 µm and 3, 21 µm and 17 and 21 µm and 7, respectively, were fabricated on stainless steel. The effect of different thickness and modulation periods on corrosion and tribocorrosion behaviour of the DLC‐coating stainless steel was investigated in 3.5 wt% NaCl aqueous solution by a ball‐on‐flat tribometer equipped with a three‐electrode electrochemical cell. The DLC‐coating stainless steel served as a working electrode, and its OCP and potentiodynamic polarization were monitored before and during rubbing. The wear–corrosion mechanism of the DLC films was investigated by SEM. The results showed that the increasing thickness can prolong significantly lifetime of DLC films in NaCl aqueous solution. In particular, the modulation period has a significant impact on the tribocorrosion resistance of the DLC super‐thick films. The study suggested that the increasing thickness of compressive stress layer could suppress film damage by reducing crack propagation rate. Thus, the super‐thick DLC film with thickness of 21 µm and 7 periods presented the best tribocorrosion resistance among all studied films. Copyright © 2016 John Wiley & Sons, Ltd.     

Development of a Carbon Paste Electrode Modified with Reduced Graphene Oxide and an Imidazole Derivative for Simultaneous Determination of Biological Species of N‐acetyl‐L‐cysteine,Uric Acid and Dopamine

In this work, the modified carbon paste electrode (CPE) with an imidazole derivative 2‐(2,3 dihydroxy phenyl) 4‐methyl benzimidazole (DHPMB) and reduced graphene oxide (RGO) was used as an electrochemical sensor for electrocatalytic oxidation of N‐acetyl‐L‐cysteine (NAC). The electrocatalytic oxidation of N‐acetyl‐L‐cysteine on the modified electrode surface was then investigated, indicating a reduction in oxidative over voltage and an intensive increase in the current of analyte. The scan rate potential, the percentages of DHPMB and RGO, and the pH solution were optimized. Under the optimum conditions, some parameters such as the electron transfer coefficient (α) between electrode and modifier, and the electron transfer rate constant) k_s) in a 0.1 M phosphate buffer solution (pH=7.0) were obtained by cyclic voltammetry method. The diffusion coefficient of species (D) 3.96×10^?5 cm² s^?1 was calculated by chronoamperometeric technique and the Tafel plot was used to calculate α (0.46) for N‐ acetyl‐L‐cysteine. Also, by using differential pulse voltammetric (DPV) technique, two linear dynamic ranges of 2–18 µM and 18–1000 µM with the detection limit of 61.0 nM for N‐acetyl‐L‐cysteine (NAC) were achieved. In the co‐existence system of N‐acetyl‐L‐cysteine (NAC), uric acid (UA) and dopamine (DA), the linear response ranges for NAC, UA, and DA are 6.0–400.0 µM, 5.0–50.0 µM and 2.0–20.0 µM, respectively and the detection limits based on (C=3s_b/m) are 0.067 µM, 0.246 µM and 0.136 µM, respectively. The obtained results indicated that DHPMB/RGO/CPE is applicable to separate NAC, uric acid (UA) and dopamine (DA) oxidative peaks, simultaneously. For analytic performance, the mentioned modified electrode was used for determination of NAC in the drug samples with acceptable results, and the simultaneous determination of NAC, UA and DA oxidative peaks was investigated in the serum solutions, too.     

A Novel Three‐Electrode System Fabricated on Polymethyl Methacrylate for On‐Chip Electrochemical Detection

A new strategy of three‐electrode system fabrication in polymer‐based microfluidic systems is described here. Standard lithography, hot embossing and UV‐assisted thermal bonding were employed for fabrication and assembly of the microfluidic chip. For the electrode design the gold working (WE) and counter electrodes (CE) are placed inside a main channel through which the sample solution passes. A silver reference electrode (RE) is embedded in a small side channel containing KCl solution that is continuously pushed into the main channel. In the present work, the overall electrochemical set up and its microfabrication is described. Conditions including silver ion concentration, cyclic voltammetry (CV) settings, and the flow rate of KCl solution in the RE channel were optimized. The electrochemical performance of the three‐electrode system was evaluated by CV and also by amperometric oxidation of ferro hexacyanide ([Fe(CN)₆]^4?) and ruthenium bipyridyl ([Ru(bipy)₃]²⁺) at 400 mV and 1200 mV, respectively. CV analysis using ferri/ferro hexacyanide showed a stable, quasi‐reversible redox reaction at the electrodes with 96 mV peak separation and an anodic/cathodic peak ratio of 1. Amperometric analysis of the electrochemical species resulted in linear correlation between analyte concentration and current response in the range of 0.5–15 µM for [Fe(CN)₆]^4?, and 0–1000 µM for [Ru(bipy)₃]²⁺. Upon the given experimental conditions, the limit of detection was found to be 3.15 µM and 24.83 µM for [Fe(CN)₆]^4? and [Ru(bipy)₃]²⁺, respectively. As a fully integrated three‐electrode system that is fabricated on polymer substrates, it has great applications in microfluidic‐based systems requiring stable electrochemical detection.     

Voltammetric Determination of 4‐Nitrophenol and 5‐Nitrobenzimidazole Using Different Types of Silver Solid Amalgam Electrodes – A Comparative Study

The voltammetric behavior of two genotoxic nitro compounds (4‐nitrophenol and 5‐nitrobenzimidazole) has been investigated using direct current voltammetry (DCV) and differential pulse voltammetry (DPV) at a polished silver solid amalgam electrode (p‐AgSAE), a mercury meniscus modified silver solid amalgam electrode (m‐AgSAE), and a mercury film modified silver solid amalgam electrode (MF‐AgSAE). The optimum conditions have been evaluated for their determination in Britton‐Robinson buffer solutions. The limit of quantification (L_Q) for 5‐nitrobenzimidazole at p‐AgSAE was 0.77 µmol L^?1 (DCV) and 0.47 µmol L^?1 (DPV), at m‐AgSAE it was 0.32 µmol L^?1 (DCV) and 0.16 µmol L^?1 (DPV), and at MF‐AgSAE it was 0.97 µmol L^?1 (DCV) and 0.70 µmol L^?1 (DPV). For 4‐nitrophenol at p‐AgSAE, L_Q was 0.37 µmol L^?1 (DCV) and 0.32 µmol L^?1 (DPV), at m‐AgSAE it was 0.14 µmol L^?1 (DCV) and 0.1 µmol L^?1 (DPV), and at MF‐AgSAE, it was 0.87 µmol L^?1 (DCV) and 0.37 µmol L^?1 (DPV). Thorough comparative studies have shown that m‐AgSAE is the best sensor for voltammetric determination of the two model genotoxic compounds because it gives the lowest L_Q, is easier to prepare, and its surface can be easily renewed both chemically (by new amalgamation) and/or electrochemically (by imposition of cleaning pulses). The practical applicability of the newly developed methods was verified on model samples of drinking water.     

Renewable Silver‐Amalgam Film Electrode for Rapid Square‐Wave Voltammetric Determination of Thiamethoxam Insecticide in Selected Samples

The paper presents the use of a renewable silver‐amalgam film electrode (Hg(Ag)FE) for the determination of the insecticide thiamethoxam (TMO) in Britton‐Robinson buffer pH 7.0 (LOD=0.25 µg mL^?1, LOQ=0.70 µg mL^?1) by direct cathodic square‐wave voltammetry (SWV). The voltammetric response for TMO obtained at this electrode was the same as that obtained with a hanging mercury drop electrode, represented by two distinct reduction peaks. Since the electron transfer processes are coupled with chemical reactions involving protons, the SWV signals strongly depend on the pH of the supporting electrolyte. The developed Hg(Ag)FE‐SWV method was tested for the determination of TMO in spiked honey and river water samples, as well as for the determination of its content in the commercial formulation Actara 25 WG.     

Toxicological screening of human plasma by on‐line SPE‐HPLC‐DAD: identification and quantification of basic drugs and metabolites

An automated multi‐analyte screening method for the identification and quantification of 92 drugs and metabolites based on on‐line solid‐phase extraction–high‐performance liquid chromatography–diode array detection technique was developed and successfully validated. In addition, a database with 870 entries including UV‐spectra, relative/retention times and response factors of toxicologically relevant compounds was created. Plasma samples (0.2 mL) were treated with methanol, diluted with buffer and on‐line extracted (Strata X, 20 ×2 mm, 25 µm) at pH 9. Analytical separation was carried out on a Gemini NX column (150 ×4.6 mm, 3 µm) using gradient elution with acetonitrile–water (90:10,v/v) and 0.05 m potassium dihydrogen phosphate buffer (pH 2.3). Linear calibration curves with correlation coefficients ≥0.9950 were obtained for 78 analytes. As an additional benefit, the newly developed method allows the quantification of 42 analytes (e.g. antidepressants, neuroleptics and anticonvulsants) in a concentration range suitable for therapeutic drug monitoring. Limits of quantitation ranged from 0.02 mg/L (chlordiazepoxide) to 3.4 mg/L (mexiletine). Inter‐ and intra‐day precisions of quality control samples (low/high) were better than 15% (zolpidem) and accuracy (bias) ranged from ?11% (opipramol, venlafaxine) to 11% (venlafaxine, trazodone). Tests for carry‐over and sample stability under different storage conditions were also performed and stability was adequate. Four cases of poisoning analysis are presented. Copyright © 2014 John Wiley & Sons, Ltd.     

Quantitative analysis of a ubiquitin‐dependent substrate using capillary electrophoresis with dual laser‐induced fluorescence

Protein degradation by the ubiquitin‐proteasome system (UPS) affects many biological processes. Inhibition of the proteasome has emerged as a potential therapeutic target for cancer treatment. In this study, we developed a method for monitoring the degradation and accumulation of UPS‐dependent substrates in cells using CE with dual LIF. We used a green fluorescent protein (GFP)‐fusion of the ubiquitin substrate ribophorin 1 (GFP‐RPN1) along with red fluorescent protein (RFP) as an internal control to normalize transfection efficiency. Determination of GFP‐RPN1 and RFP in cell lysates were performed in an untreated capillary (75 μm × 50 cm) and 100 mM Tris‐CHES buffer (pH 9.0) containing 10 mM SDS. GFP‐RPN1 and RFP fluorescence were detected at excitation wavelengths of 488 and 635 nm, and emission wavelengths of 520 and 675 nm, respectively, without any interference or crosstalk. The intensity of GFP‐RPN1 fluorescence was normalized to that of RFP. Additionally, the proposed approach was used successfully to detect the degradation of GFP‐RPN1 and evaluate proteasome inhibitors. These results show that the developed method is effective and promising for rapid and quantitative monitoring of UPS‐dependent substrates compared to the current common methods, such as immunoblotting and pulse chase assays.     

Quasi‐simultaneous Mapping of Local Current Density,pH and Dissolved O2

For the first time we report on quasi‐simultaneous mapping of local current density, pH and concentration of dissolved O₂. The three micro‐probes were positioned 50 micron apart from each other and 50 to 100 µm above the cut‐edge metal coated steel sample. The vibrating probe recorded the local current density and glass‐capillary micro‐electrode measured local pH. Additionally, two different micro‐probes were used for DO measurements: either an amperometric micro‐probe or micro‐optode was coupled with the other two sensors using the triple head stage. The data on local distributions were acquired quasi‐simultaneously (within ≤ 2.8 s) and, hence, are correlated in space and time.     

Preparation of a Xanthine Sensor Based on the Immobilization of Xanthine Oxidase on a Chitosan Modified Electrode by Cross‐linking

Here in this paper, xanthine oxidase (XOD) was immobilized onto the chitosan (CHT) modified electrode by a simple way of cross‐linking with glutaraldehyde (GTD) and 3‐aminopropyltriethoxysilane (KH). The electrode displayed a sharp peak to the oxidation of xanthine at a potential about 0.67 V and the optimum of pH for determination was investigated. Under the optimum conditions, the biosensor fabricated on the KH/GTD/XOD/CHT modified electrode showed excellent response to the oxidation of xanthine within the range of 0.5 to 18 μmol/L with a low detection limit of 0.0215 µmol/L, a good stability and a high selectivity. The sensor can also be used for the determination of hypoxanthine. The electrochemical results indicated that the immobilized enzyme still retained its biological activity and this provided a new way for the construction of biosensors and determination of xanthine.     

Electrocatalytic Oxidation and Determination of Hydrazine at an AuCu Nanoparticles – Graphene – Ionic Liquid Composite Film Coated Glassy Carbon Electrode

Gold‐copper alloy nanoparticles (AuCu NPs) were electrodeposited on a graphene – ionic liquid composite film (EGN‐IL). The AuCu NPs showed high electrocatalysis to the oxidation of hydrazine with a catalytic reaction rate constant of about 5.0×10⁴ mol/Ls. In phosphate buffer solutions (pH 6.8) the oxidation current of hydrazine at 0.15 V (vs. SCE) at the resulting electrode (AuCu? EGN‐IL/GCE) was linear to its concentration in the range of 0.2–110 µM with a sensitivity of 56.7 µA/mM, and the detection limit was 0.1 µM (S/N=3). The electrode was successfully applied to the determination of waste water.     

Anti‐cancer Herbal Drug Berberine – Sensitive Determination Using Boron‐doped Diamond Electrode

Determination of berberine, an isoquinoline plant alkaloid, with antibacterial, antiparasitic, antifungal, hypotensive and antitumoral effects, was proposed by introducing square wave voltammetry on boron‐doped diamond electrode. At optimized experimental parameters, in Britton‐Robinson buffer solution pH 5 berberine provides 3 oxidation peaks (+0.63; +1.14 and +1.34 V) and one reduction (+0.15 V) (vs. Ag/AgCl electrode), with good repeatability (relative standard deviation of 2.6 % and 1.9 % for 8 measurements at 0.5 and 10 µM concentration level, respectively). Calibration curve was linear in wade linear range from 0.1 to 50 µM with limit of detection of 0.04 µM. The proposed procedure was successfully applied for the determination of berberine in seed extract from Argemone mexicana with satisfactory recovery (102–102.6 %). The developed method may represent a sensitive alternative to highly toxic mercury electrodes, modified electrodes and chromatographic methods.     

Determination of Tetracycline at a UV‐Irradiated DNA Film Modified Glassy Carbon Electrode

In this study, interaction of tetracycline (TC) and DNA in the Britton? Robinson buffer solution (BR) was studied by cyclic voltammetry. The experimental results reveal that TC can bind strongly to DNA and the association constant and binding number between TC and DNA was obtained. Then DNA was immobilized on a glassy carbon electrode by UV‐irradiation. Through this process, water‐soluble DNA was converted into insoluble materials, and a stable DNA film was formed on the electrode. The electrochemical oxidation behavior of TC was studied at UV‐irradiated DNA film modified glassy carbon electrode (UV‐DNA‐GCE). The response of modified electrode was optimized with respect to pH, accumulation time, ionic strength, drug concentration and other variables. TC at the surface of modified electrode showed a linear dynamic range of 0.30–90.00 µM and a detection limit of 0.27 µM. To demonstrate the applicability of the modified electrode, it was used for the analysis of real samples such as pharmaceutical formulations and milk.     

Microfluidic preparative free‐flow isoelectric focusing in a triangular channel: System development and characterization

A preparative scale free‐flow IEF device is developed and characterized with the aim of addressing needs of molecular biologists working with protein samples on the milligrams and milliliters scale. A triangular‐shape separation channel facilitates the establishment of the pH gradient with a corresponding increase in separation efficiency and decrease in focusing time compared with that in a regular rectangular channel. Functionalized, ion‐permeable poly(acrylamide) gel membranes are sandwiched between PDMS and glass layers to both isolate the electrode buffers from the central separation channel and also to selectively adjust the voltage efficiency across the separation channel to achieve high electric field separation. The 50×70 mm device is fabricated by soft lithography and has 24 outlets evenly spaced across a pH gradient between pH 4 and 10. This preparative free‐flow IEF system is investigated and optimized for both aqueous and denaturing conditions with respect to the electric field and potential efficiency and with consideration of Joule‐heating removal. Energy distribution across the functionalized polyacrylamide gel is investigated and controlled to adjust the potential efficiency between 15 and 80% across the triangular separation channel. The device is able to achieve constant electric fields high as 370±20 V/cm through the entire triangular channel given the separation voltage of 1800 V, enabling separation of five fluorescent pI markers as a demonstration example.