Electrochemically initiated tagging of thiols using an electrospray ionization based liquid microjunction surface sampling probe two‐electrode cell

This paper reports on the conversion of a liquid microjunction surface sampling probe (LMJ‐SSP) into a two‐electrode electrochemical cell using a conductive sample surface and the probe as the two electrodes with an appropriate battery powered circuit. With this LMJ‐SSP, two‐electrode cell arrangement, tagging of analyte thiol functionalities (in this case peptide cysteine residues) with hydroquinone tags was initiated electrochemically using a hydroquinone‐doped solution when the analyte either was initially in solution or was sampled from a surface. Efficient tagging (～90%), at flow rates of 5–10 µL/min, could be achieved for up to at least two cysteines on a peptide. The high tagging efficiency observed was explained with a simple kinetic model. In general, the incorporation of a two‐electrode electrochemical cell, or other multiple electrode arrangement, into the LMJ‐SSP is expected to add to the versatility of this approach for surface sampling and ionization coupled with mass spectrometric detection. Published in 2009 by John Wiley & Sons, Ltd.     

Electrocatalytic Reduction of Metronidazole on Bismuth Modified Pencil‐lead Electrode

We report a new and simple approach based on an experimental design method for the preparation of pencil‐lead electrode modified with bismuth thin film. The fabrication process consists of reduction of bismuth on the surface of electrode with potentiostate method. Response surface methodology was developed as experimental strategies for modeling and optimization of the influence of some variables on the performance of modified electrode. The electrocatalytic behavior of this modified electrode was exploited as a sensitive detection system for the mercury‐free reduction and determination of metronidazole in pharmaceutical and biological samples by using differential pulse voltammetry and amperometry methods.     

Functionalization of gold electrode surface with heterobifunctional poly(ethylene oxide)s having both mercapto and aldehyde groups

We synthesized heterobifunctional poly(ethylene oxide) (PEO) (α‐formyl‐ω‐mercapto‐PEO; CHO‐PEO₄₀₀‐SH, average molecular weight of PEO part being 400), which had both an aldehyde group as a binding site with amino group of protein and a mercapto group for gold electrode surface. The CHO‐PEO₄₀₀‐SH was adsorbed on a gold electrode surface and cytochrome c (cyt.c) was fixed on this modified electrode. The redox response of covalently immobilized cyt.c was observed on the cyclic voltammetry measurement, showing that CHO‐PEO₄₀₀‐SH can be used as a linker to fix cyt.c on an electrode. Another type of heterobifunctional PEO (α‐formyl‐ω‐(2‐pyridyldithio)‐PEO; CHO‐PEO₃₀₀‐SS‐Py), which had an aldehyde group and a 2‐pyridinethiol (2‐Py) through disulfide bond, was synthesized to form co‐adsorbed monolayer of PEO chain and 2‐Py on an electrode surface. It was expected, due to the spacer with shorter PEO chain and lower surface density, that better redox response of the fixed cyt.c was obtained. However, the redox response of fixed cyt.c was not detected on the CHO‐PEO₃₀₀‐SS‐Py modified gold electrode. Instead, this heterobifunctional PEO was found to function as a good promoter for cyt.c dissolved in phosphate buffer solution. Copyright © 2003 John Wiley & Sons, Ltd.     

Chemical Adsorption onto an ITO Substrate of Single‐Wall Carbon Nanotube Functionalized by Carboxylic Groups as an Efficient Support for Electrocatalyst

A single‐wall carbon nanotube functionalized by carboxylic groups (SWNT‐CA) was found to be adsorbed on an indium tin oxide (ITO) electrode by chemical interaction between carboxylic groups and the ITO surface. The adsorption experiments indicated that the narrow pH conditions (around pH 3.0) exist for its adsorption which is restricted by preparation of stable fluid dispersion (favorable at higher pH) and by the chemical interaction (favorable at lower pH). Atomic force microscopic (AFM) measurements suggest that fragmented SWNT‐CA are adsorbed, primarily lying on the surface. Electrochemical impedance analysis indicated that an electrochemical double layer capacitance of the SWNT‐CA/ITO electrode is considerably higher than that for the ITO electrode, suggesting that the interfacial area between the electrode surface and the electrolyte solution is enlarged by the SWNT‐CA layer. Pt particles were deposited as a catalyst on the bare ITO and SWNT‐CA‐coated ITO (SWNT‐CA/ITO) electrodes to give respective Pt‐modified electrodes (denoted as a Pt/ITO electrode and a Pt/SWNT‐CA/ITO electrode, respectively). The cathodic current for the Pt/SWNT‐CA/ITO electrode was 1.7 times higher than that for the Pt/ITO electrode at 0.0 V, showing that the Pt/SWNT‐CA/ITO electrode works more efficiently for O₂ reduction at 0.0 V due to the SWNT‐CA layer. The enhancement by the SWNT‐CA layer is also effective for electrocatalytic proton reduction. It could be ascribable to the enlarged interfacial area between the electrode surface and the electrolyte solution.     

Working Electrodes from Amalgam Paste for Electrochemical Measurements

Paste electrode with paste amalgam as an active electrode material is described here for the first time. Designed electrode from silver paste amalgam (AgA‐PE) is solely metallic and does not contain any organic binder. Mechanical surface regeneration of AgA‐PE is performed in the same way as for classical carbon paste electrodes and reproducibility of such regeneration is about 10%. Electrochemical surface regeneration appeared very efficient for most measurements. In dependence on paste metal content, the electrode surface can be liquid (resembling a film) or rather solid. The hydrogen overvoltage on AgA‐PE is high, and the electrode allows measurements at highly negative potentials. AgA‐PE is well suited for study of reduction or oxidation processes without an accumulation step. Anodic stripping voltammetry of some metals tested on the electrode is influenced by formation of intermetallic compounds. The measurement based on cathodic stripping voltammetry (adenine, cysteine) and on catalytic processes from adsorbed state (complex of osmium tetroxide with 2,2′‐bipyridine) can be performed on AgA‐PE practically under the same conditions as found earlier for HMDE and for silver solid amalgam electrode. The working electrode from paste amalgam combines the advantages of paste and metal electrodes.     

First Report for Electrochemical Determination of Levodopa and Cabergoline: Application for Determination of Levodopa and Cabergoline in Human Serum,Urine and Pharmaceutical Formulations

The electrochemical oxidation of levodopa on the surface of a carbon paste electrode modified with graphene nanosheets, 1‐(4‐bromobenzyl)‐4‐ferrocenyl‐1H‐[1,2,3]‐triazole (1,4‐BBFT) and hydrophilic ionic liquid (n‐hexyl‐3‐methylimidazolium hexafluoro phosphate) as a binder is studied. It has been found that the oxidation of levodopa at the surface of a modified electrode occurs at a potential of about 210 mV less positive than that of an unmodified carbon paste electrode (CPE). The prepared modified electrode exhibits a very good resolution of the voltammetric peaks of levodopa and cabergoline. The electrode has been applied successfully for the determination of levodopa and cabergoline in some real samples.     

Electrochemical Detection of Glutathione Using a Poly(caffeic acid) Nanocarbon Composite Modified Electrode

A modified glassy carbon electrode was prepared through electropolymerization of caffeic acid in the presence of either carbon nanotubes or nano‐carbon drop cast onto the electrode surface. The voltammetric behaviour of the electrode was characterized using the ortho‐quinone moiety on the caffeic acid unit and the surface loading optimized for current response. The nanocomposite mediated electrode was used for the sensitive detection of glutathione at concentrations as low as 500 nM.     

A Competitor-switched Electrochemical Sensor for Detection of DNA

A competitor‐switched electrochemical sensor based on a generic displacement strategy was designed for DNA detection. In this strategy, an unmodified single‐stranded DNA (cDNA) completely complementary to the target DNA served as the molecular recognition element, while a hairpin DNA (hDNA) labeled with a ferrocene (Fc) and a thiol group at its terminals served as both the competitor element and the probe. This electrochemical sensor was fabricated by self‐assembling a dsDNA onto a gold electrode surface. The dsDNA was pre‐formed through the hybridization of Fc‐labeled hDNA and cDNA with their part complementary sequences. Initially, the labeled ferrocene in the dsDNA was far from surface of the electrode, the electrochemical sensor exhibited a "switch‐off" mode due to unfavorable electron transfer of Fc label. However, in the presence of target DNA, cDNA was released from hDNA by target DNA, the hairpin‐open hDNA restored its original hairpin structure and the ferrocene approached onto the electrode surface, thus the electrochemical sensor exhibited a "switch‐on" mode accompanying with a change in the current response. The experimental results showed that as low as 4.4×10⁻¹⁰ mol/L target DNA could be distinguishingly detected, and this method had obvious advantages such as facile operation, low cost and reagentless procedure.     

Electrochemically Stimulated Insulin Release from a Modified Graphene‐functionalized Carbon Fiber Electrode

A graphene‐functionalized carbon fiber electrode was modified with adsorbed polyethylenimine to introduce amino functionalities and then with trigonelline and 4‐carboxyphenylboronic acid covalently bound to the amino groups. The trigonelline species containing quarterized pyridine groups produced positive charge on the electrode surface regardless of the pH value, while the phenylboronic acid species were neutral below pH 8 and negatively charged above pH 9 (note that their pK_a=8.4). The total charge on the monolayer‐modified electrode was positive at the neutral pH and negative at pH > 9. Note that 4‐carboxyphenylboronic acid was attached to the electrode surface in molar excess to trigonelline, thus allowing the negative charge to dominate on the electrode surface at basic pH. Negatively charged fluorescent dye‐labeled insulin (insulin‐FITC) was loaded on the modified electrode surface at pH 7.0 due to its electrostatic attraction to the positively charged interface. The local pH in close vicinity to the electrode surface was increased to ca. 9–10 due to consumption of H⁺ ions upon electrochemical reduction of oxygen proceeding at the potential of −1.0 V (vs. Ag/AgCl) applied on the modified electrode. The process resulted in recharging of the electrode surface to the negative value due to the formation of the negative charge on the phenylboronic acid groups, thus resulting in the electrostatic repulsion of insulin‐FITC and stimulating its release from the electrode surface. The insulin release was characterized by fluorescence spectroscopy (using the FITC‐labeled insulin), by electrochemical measurements on an iridium oxide, IrO_x, electrode and by mass spectrometry. The graphene‐functionalized carbon fiber electrode demonstrated significant advantages in the signal‐stimulated insulin release comparing with the carbon fiber electrode without the graphene species.     

Square‐wave Voltammetry of Two‐step Surface Electrode Mechanisms Coupled with Chemical Reactions – A Theoretical Overview

Square‐wave voltammetry (SWV) of so‐called “surface redox reactions” is seen as a simple and efficient tool to quantify large number of drugs, physiologically active substances and other important chemicals. It also provides elegant methods to get access to relevant kinetic and thermodynamic parameters related to many lipophilic compounds. Moreover, with this technique we can study activity of various enzymes by exploring the “protein‐film voltammetry” set up. In this work, we focus on theoretical SWV features of four complex surface electrode mechanisms, in which the electron exchange between the working electrode and the studied redox substrate takes place in two successive steps. While we present large number of calculated square‐wave voltammograms, we give hints to recognize particular two‐step surface mechanism, but also to distinguish it from other similar mechanisms. We present plenty of relevant aspects of surface two‐step surface EE, two‐step surface ECE and surface catalytic EEC’ mechanisms. Moreover, we present for the first time a series of theoretical results related to two‐step surface EECrev mechanism (i. e. two‐step surface reaction coupled to follow‐up reversible chemical step). The simulated voltammetric patterns presented in this work can bring relevant aspects to resolve some experimental situations met in voltammetry of many redox enzymes and other important substances whose electrochemical transformation occurs in two‐steps.     

Development and Characterization of Nickel‐NTA‐Polyaniline Modified Electrodes

The engineered addition of hexa‐histidine sequences to biomolecules such as antibody fragments has been found to be an excellent means of purifying these materials. This tagging methodology has also been extended to its use as a tool for immobilization and orientation of antibodies on transducer surfaces. Polyvinyl sulfonate‐doped polyanilne (PANI/PVS) can be used as a mediator in amperometric biosensors. This short communication looks at the effect of nickel chelate materials and nickel chelation on this conducting polymer and evaluates it as a potential surface for the immobilization of his‐tagged biomolecules. N‐nitrilotriacetic acid (NTA) was doped into the electropolymerized PANI/PVS at a screen‐printed carbon paste electrode. The resulting NTA‐PANI/PVS film was shown to have comparable electrochemical properties of polymer without the chelating agent. When Ni²⁺ was applied to the electrode, the incorporated NTA was found to efficiently chelate the metal ions at the electrode surface.     

Conformal Surface Coatings to Enable High Volume Expansion Li‐Ion Anode Materials

An alumina surface coating is demonstrated to improve electrochemical performance of MoO₃ nanoparticles as high capacity/high‐volume expansion anodes for Li‐ion batteries. Thin, conformal surface coatings were grown using atomic layer deposition (ALD) that relies on self‐limiting surface reactions. ALD coatings were tested on both individual nanoparticles and prefabricated electrodes containing conductive additive and binder. The coated and non‐coated materials were characterized using transmission electron microscopy, energy‐dispersive X‐ray spectroscopy, electrochemical impedance spectroscopy, and galvanostatic charge/discharge cycling. Importantly, increased stability and capacity retention was only observed when the fully fabricated electrode was coated. The alumina layer both improves the adhesion of the entire electrode, during volume expansion/contraction and protects the nanoparticle surfaces. Coating the entire electrode also allows for an important carbothermal reduction process that occurs during electrode pre‐heat treatment. ALD is thus demonstrated as a novel and necessary method that may be employed to coat the tortuous network of a battery electrode.     

New Aspects of Protein‐film Voltammetry of Redox Enzymes Coupled to Follow‐up Reversible Chemical Reaction in Square‐wave Voltammetry

Protein‐film square‐wave voltammetry of uniformly adsorbed molecules of redox lipophilic enzymes is applied to study their electrochemical properties, when a reversible follow‐up chemical reaction is coupled to the electrochemically generated product of enzyme's electrode reaction. Theoretical consideration of this so‐called “surface ECrev mechanism” under conditions of square‐wave voltammetry has revealed several new aspects, especially by enzymatic electrode reactions featuring fast electron transfer. We show that the rate of chemical removal/resupply of electrochemically generated Red(ads) enzymatic species, shows quite specific features to all current components of calculated square‐wave voltammograms and affects the electrode kinetics. The effects observed are specific for this particular redox mechanism (surface ECrev mechanism), and they got more pronounced at high electrode kinetics of enzymatic reaction. The features of phenomena of “split net‐SWV peak” and “quasireversible maximum”, which are typical for surface redox reactions studied in square‐wave voltammetry, are strongly affected by kinetics and thermodynamics of follow‐up chemical reaction. While we present plenty of relevant voltammetric situations useful for recognizing this particular mechanism in square‐wave voltammetry, we also propose a new approach to get access to kinetics and thermodynamics of follow‐up chemical reaction. Most of the results in this work throw new insight into the features of protein‐film systems that are coupled with chemical reactions.     

Sensitive Voltammetric Determination of Cadmium(II) with 8‐Hydroxyquinoline and Ionic Liquid Modified Carbon Paste Electrode

In this paper 8‐hydroxyquinoline (HQ) and ionic liquid (IL) modified carbon paste electrode was fabricated and used for the sensitive determination of cadmium(II) with differential pulse anodic stripping voltammetry (DPASV). The modified electrode was prepared by the addition of HQ and IL 1‐ethyl‐3‐methylimidazoliam ethylsulphate as the modifiers into the traditional carbon paste mixture. Cd(II) was preconcentrated and reduced on the surface of the modified electrode at the potential of ‐1.0 V (vs. SCE) by the co‐contributions from the formation of HQ‐Cd(II) complex and the accumulation effect of IL. Then the reduced Cd on the electrode surface was reoxidized by DPASV with a sensitive oxidation peak appeared at ‐0.79 V (vs. SCE). Under the optimal conditions the oxidation peak current was proportional to the Cd(II) concentration in the range from 0.03 to 2.0 mol/L with the detection limit as 5.0 nmol/L (3σ). The proposed method was successfully applied to the water samples detection with the recovery in the range from 95.6% to 96.6%.     

Electrochemical Properties of Lanthanum Hexacyanoferrate Particles Immobilized onto Electrode Surface by Au‐Codeposition Method

Lanthanum hexacyanoferrate (LaHCF) was immobilized onto a substrate surface as an electroactive material by Au‐codeposition method. The LaHCF particles were attached to the electrode surface as the result of occlusion within the gold film deposited. This deposition method was first introduced for the preparation of hexacyanoferrate‐based modified electrodes. It was demonstrated that this deposition method provides a higher stability of the electroactive film in comparison with available methods for the mechanical attachment of electroactive films. On the other hand, electrochemical properties of the LaHCF film modified electrode were studied for the first time. The results showed that LaHCF film has excellent electrochemical activity as well as other analogues of Prussian blue. The modified electrode was successfully used as an electrocatalyst for the oxidation of ascorbic acid.     

Nanocomposite with Polypyrrole Encapsulated within SBA‐15 Mesoporous Silica: Preparation and Its Electrochemical Application

The nanocomposite with polypyrrole (PPy) confined in ordered mesoporous silica SBA‐15 channels was synthesized by in situ electropolymerization. X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, N₂ adsorption/desorption, and FT‐IR studies indicated that the nanocomposite has the well‐ordered hexagonal structures and PPy was in situ polymerized into the channels instead of the outer surface of SBA‐15. Furthermore, the PPy/SBA‐15 nanocomposite was used as an electrode modifier. We found that the nanocomposite‐modified electrode exhibited good electrocatalytic activities for hydroquinone oxidation where PPy chains could facilitate the electron transfer between molecular sieves and electrode surface. Three dihydroxybenzene isomers (hydroquinone, catechol and resorcinol) have been successfully detected at PPy/SBA‐15 modified electrode by preconcentration of the analyte.     

Electrode Modification Using Alkynyl Substituted Fe(II) Phthalocyanine via Electrografting and Click Chemistry for Electrocatalysis

In this work, tetrakis(5‐hexyn‐oxy)Fe(II) phthalocyanine was synthesised in order to perform a click reaction between the terminal alkyne groups and an azide group on a glassy carbon electrode (GCE) surface. An azide group was formed on the electrode surface following electrografting using 4‐azidobenzene diazonium tetrafluoroborate by electrochemical reduction. The Cu(I) catalyzed alkyne‐azide Huisgen cycloaddition reaction was then employed in order to react the terminal alkyne groups on the phthalocyanine with the azide groups on the GCE surface. The modified electrode was employed to catalyse the oxidation of hydrazine. The electrode showed good electrocatalytic ability towards the detection of hydrazine with a sensitivity of 15.38 µA mM^?1 and a limit of detection of 1.09 µM.     

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.     

Chlorpromazine Electro‐oxidation at BDD Electrode Modified with nZVI Nanoparticles Impregnated NiAl LDH

Nanocomposite of nanoscale zero‐valent iron (nZVI) and layered double hydroxide (LDH) was used as modifier for boron‐doped diamond electrode in determination of anti‐psychotic drug chlorpromazine (CPZ). nZVI nanoparticles were prepared by liquid phase reduction of ferric chloride with sodium borohydride on the surface of NiAl LDH matrix owing to the strong exchange and confinement efficiency of LDH. The structure, binding and surface properties of the nZVI@LDH nanocomposite were monitored using powder X‐ray diffractometry, FT‐IR spectroscopy, scanning and transmission microscopy and BET techniques. The electrochemical properties of the modified electrode were investigated by CV and EIS, performed in a phosphate buffer containing ferro/ferricyanide as redox probe. The modified electrode exhibited excellent electrochemical performance compared with unmodified electrode. As regard potential application of the nanocomposite surface to the CPZ detection, square‐wave voltammetric signals showed a good linear correlation over CPZ concentrations in a broad range from 0.1 to 8.0 μM with low detection limit of 0.005 μM. Nevertheless, these results suggest that the proposed nanocomposite modifier surface provides exceptional synergy and significant enhancement effect to the voltammetric response of CPZ and thus could be applied as highly efficient and stable platform of sensors in clinical analysis.     

Multiwall Carbon Nanotube (MWCNT) Based Electrochemical Biosensors for Mediatorless Detection of Putrescine

γ‐Aminopropyltriethoxysilane (APTES)‐induced solubilization of multi‐wall carbon nanotube CNTs allowed for the modification of electrode surfaces. APTES also served as an immobilization matrix for putrescine oxidase (POx) to construct an amperometric biosensor. Although CNTs modified by APTES acted as semiconductors to reduce the exposed sensing surface, we reasoned that nanoscale “dendrites” of CNTS modified by APTES formed a network and projected outwards from the electrode surface and acted like bundled ultra‐microelectrodes that allowed access to the active site and facilitated direct electron transfer to the immobilized enzyme. Our biosensor was able to efficiently monitor direct electroactivity of POx at the electrode surface. The putrescine biosensor prepared using the modified glassy carbon electrode exhibited current response within 10 s with a detection limit of 500 nM.