Characterization and Applications of a Bismuth Bulk Electrode

A bismuth bulk electrode (BiBE), a new solid‐state electrode, is presented. The polycrystalline metal bismuth disk‐shaped electrode was examined for its anodic stripping voltammetry performance, which was found to be well comparable to that achieved with the bismuth or mercury film electrodes. Useful potential windows of the BiBE in aqueous solutions of pH 1 to 13 were found to range from approximately ?1.7 to ?0.1 V, depending on pH, where either hydrogen evolution or anodic dissolution of metallic bismuth limit the electrochemical inertness of the BiBE. Employing cyclic voltammetry (CV), the cathodic behavior of the BiBE was examined by testing inorganic (cadmium(II) ions) and organic (2‐nitrophenol) model compounds; a CV quasi‐reversible behavior was recorded in the case of the Cd(II)‐Cd(0) couple. The characteristics of the BiBE under anodic conditions, i.e., at bismuth surface coated with a thin conductive Bi₂O₃ film, was examined by testing two well‐established redox systems, potassium hexacyanoferate(III) and ruthenium(III) hexaaminechloride; a nearly reversible behavior was recorded in the latter case. Based on the presented preliminary results, BiBE can be considered as an interesting alternative to common solid and (toxic) mercury electrodes for possible use in electrochemical studies and electroanalytical applications.     

Chitosan Incorporating Cetyltrimethylammonium Bromide Modified Glassy Carbon Electrode for Simultaneous Determination of Ascorbic Acid and Dopamine

Simultaneous determination of a neurotransmitter, dopamine (DA), and ascorbic acid (AA) is achieved at neutral pH on a chitosan incorporating cetyltrimethylammonium bromide (CTAB) modified glassy carbon (GC) electrode. Differential pulse voltammetry (DPV) technique was used to investigate the electrochemical response of DA and AA at a glassy carbon electrode modified with chitosan incorporating CTAB. An optimum 6.0 mmol L^?1 of CTAB together with 0.5 wt% of chitosan was used to improve the resolution and the determination sensitivity. In 0.1 mol L^?1 aqueous phosphate buffer solution of pH 6.8, the chitosan‐CTAB modified electrode showed a good electrocatalytic response towards DA and AA. The anodic peak potential of DA shifted positively, while that of AA shifted negatively. Thus, the difference of the anodic peaks of DA and AA reached 0.23 V, which was enough to separate the two anodic peaks very well. The presented method herein could be applied to the direct simultaneous determination of DA and AA without prior treatment. The anodic peak currents (I_pa) of DPV are proportional to DA in the concentration range of 8 μM to 1000 μM, to that of AA 10 μM to 2000 μM, with correlation coefficients of 0.9930 and 0.9945, respectively. The linear range is much wider than previously reported.     

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.     

Simultaneous Voltammetric Determination of Heavy Metals by Use of Crown Ether‐modified Electrodes and Chemometrics

A three‐sensor array consisting of a graphite‐epoxy composite electrode (GEC), 4‐carboxybenzo‐18‐crown‐6‐GEC and 4‐carboxybenzo‐15‐crown‐5‐GEC was employed for the simultaneous determination of Cd(II), Pb(II) and Hg(II) by differential pulse anodic stripping voltammetry (DPASV). Sensors were firstly studied for the determination of Hg(II); secondly, peak current responses confirmed that all sensors showed differentiated response for the three considered metals. A response model was developed to resolve mixtures of Cd(II), Pb(II) and Hg(II) at the µg L^?1 level; Discrete Wavelet Transform was selected as preprocessing tool and artificial neural network used for the modelling of the obtained responses.     

Nucleophilic Addition of Thiaproline to Electrochemically Derived o‐Quinone,Application to the Sensitive Voltammetric Detection of Thiaproline

The mechanism of electrochemical behavior of catechol in the presence of thiaproline is investigated by cyclic voltammetry, controlled‐potential coulometry and spectrophotometric tracing of the reaction coordinate. The results indicate that thiaproline participate in with an ECEC mechanism in a nucleophilic (Michael) addition to o‐quinone. Effect of pH of buffer solution on reaction pathway is studied and showed that addition of thiaproline to the o‐quinone is performed only in solutions with pHs higher than 5. These results indicate that the addition of thiaproline is occurred first from amine functional group. In the second step, the addition of carboxylate group of thiaproline to C‐5 of catechol results the final product with a lactone ring in its structure. Observation of two isosbestic point in absorption spectrum during the progress of the electrolysis together with the FT‐IR results for final product can be presented as evidence for two step addition of thiaproline to catechol. Final product, due to the electron donor property of thiaproline, more easily oxidized respect to the former catechol and as a result, a new redox couple is obtained for this compound in lower potentials. The easier anodic oxidation of addition product (relative to catechol) caused to an increase in anodic current for catechol, which is proportional to the thiaproline concentration. The differential pulse voltammetry (DPV) is applied as a sensitive voltammetric method for the detection of thiaproline. A linear range from 5×10^?8 to 5×10^?6 M with a detection limit of 1×10^?8 M is resulted for thiaproline. With respect to the reversibility of the electrochemical reactions in the mechanism, and also more facile oxidation of the addition products, the square‐wave voltammetry is presented as a method with considerably more sensitivity for the detection of sub‐micromolar amounts of thiaproline. The advantageous properties of the voltammetric method for thiaproline detection lie in its excellent catalytic activity, sensitivity and simplicity.     

A Simple Route of Modifying Copper Electrodes for the Determination of Methanol and Ethylene Glycol

Copper foil electrodes were modified in a stirred NaOH solution at room temperature. Measurements with scanning electron microscopy, energy‐dispersive X‐ray spectroscopy and X‐ray photoelectron spectroscopy confirmed the simultaneous structural transformation and chemical modification of the foil surface. The as‐prepared electrodes produced isolated anodic peaks in the cyclic voltammogram of methanol or ethylene glycol in alkaline solution. The highest anodic peak current was achieved at the electrode that was modified for 5 hours. Amperometric study illustrates that the modified electrode can detect lower than 100 µM methanol with a sensitivity of 15.8 µA/mM or 10 µM ethylene glycol with a sensitivity of 193 µA/mM. Importantly, the method is robust and the prepared electrodes show great reproducibility.     

A Simple and Sensitive Method for the Detection of Trace Pb(II) and Cd(II) based on Nafion‐coated Antimony Film Electrode

Nafion‐coated antimony film electrode (NCAFE) was prepared in situ by simultaneously plated antimony with analytes, and applied to the determination of trace Pb(II) and Cd(II) in non‐deaerated solutions by differential pulse anodic stripping voltammetry (DPASV). Various experimental parameters, which influenced the response of the NCAFE to those metals, were thoroughly optimized and discussed. The results indicated that the sensitivity and resistance to surfactants at the NCAFE were remarkably improved with relative to the antimony film electrode (AFE). In the presence of 5 mg·L^?1 gelatin, the peak heights at the NCAFE showed 4‐fold enhancement for Pb and a 9‐fold enhancement for Cd over a bare AFE. Reproducibility of the sensor was satisfactory, and the relative standard deviations were 4.8% for 20 μg·L^?1 Pb and 3.2% for 25 μg·L^?1 Cd (n=15) with preconcentration time of 180 s. The determination limits (S/N=3) of this sensor were determined to be 0.15 μg·L^?1 for Pb and 0.30 μg·L^?1 for Cd with accumulation time of 300 s. The NCAFE was successfully applied to determining Pb(II) and Cd(II) in vegetable and water samples with satisfactory results.     

Silver Amalgam Tubular Detector Combined with Platinum Auxiliary Electrode for Electrochemical Measurements in Flow Systems

A new type of tubular detector with platinum auxiliary electrode inside the silver amalgam tube (TD+AuxE) was proposed, fabricated, tested and compared with a typical silver amalgam tubular detector developed earlier. Non‐stop‐flow differential pulse voltammetric anodic stripping method (AS‐DPV) and amperometric method in a glucose oxidase biosensor arrangement were tested. Both detectors were applied for AS‐DPV detection in flow systems for the first time. Solutions of zinc and cadmium cations were used as the testing species for voltammetry, and detection of oxygen concentration was used for amperometry. All these experiments require application of highly negative potentials, which is possible to realize with detectors made of silver solid amalgam. The proposed combination of TD+AuxE provides a much greater current response than the arrangement with three individual electrodes. All relevant parameters were optimized for the developed TD+AuxE. The simple and fast measuring protocol for the determination of the zinc content in commercial food supplement tablets has been developed.     

Determination of Mercury by Anodic Stripping Voltammetry with a Gold Nanoparticle‐Modified Glassy Carbon Electrode

The aim of this work is the development of a procedure for the determination of aqueous Hg(II) by anodic stripping voltammetry at a gold nanoparticle‐modified glassy carbon electrode (AuNPs‐GCE). The signal of aqueous Hg(II) was measured in the square wave mode; the effect of potential scan parameters, deposition potential and deposition time on the analytical signal was examined. The supporting electrolyte was 0.06 M HCl. The repeatability, the linearity, the accuracy, the detection limit of the procedure and the interferences of other cations and of anions were evaluated. The performance of the AuNPs‐GCE was compared with those of a solid (SGE) and a film (FGE) gold electrode: the AuNPs‐GCE showed to provide lower detection limits and higher repeatability. The renewable surface permits to eliminate memory effects, to maintain a stable baseline and response, and to avoid frequent mechanical cleaning steps. The applicability of the AuNPs‐GCE for Hg(II) determination in drinking waters, sediments and pharmaceuticals was demonstrated.     

Edge Plane Pyrolytic Graphite Electrodes for Stripping Voltammetry: a Comparison with Other Carbon Based Electrodes

The first examples of using edge plane pyrolytic graphite electrodes for anodic and cathodic stripping voltammetry (ASV and CSV) are presented, notably the ASV of silver and the CSV of manganese. In the former example, detection limits for silver (based on 3σ) of 8.1 nM and 0.185 nM for 120 s and 300 s accumulation time, respectively, were achievable using the edge plane electrode, which were superior to those observed on glassy carbon, basal plane pyrolytic graphite and boron‐doped diamond electrodes. In the second example, a detection limit for manganese of 0.3 μM was possible which was comparable with that achievable with a boron‐doped diamond electrode but with an increased sensitivity. Comparison of the edge plane pyrolytic graphite electrode with boron‐doped diamond electrodes reveals that the edge plane electrode has comparable detection limits and sensitivities whilst exhibiting a lower signal‐to‐noise ratio and large potential window for use in trace analysis suggesting boron‐doped diamond can be conveniently replaced by edge plane pyrolytic graphite as an electrode material in many applications.     

Tellurium Film Electrodes Deposited on Carbon and Mesoporous Carbon Screen‐printed Substrates for Anodic Stripping Voltammetric Determination of Copper

The performance of screen‐printed electrodes modified in situ with tellurium film for the anodic stripping voltammetric (ASV) determination of Cu(II) is reported. It was found that two types of screen‐printed substrates, namely carbon and mesoporous carbon, were optimal for this application. The selected in situ tellurium film modified electrodes were applied for the square wave ASV determination of copper at μg L⁻¹ concentration levels. Well‐defined and reproducible Cu oxidation stripping peaks were produced at a potential more negative than the anodic dissolution of tellurium. The highest sensitivity of Cu determination was achieved in 0.05 M HCl containing 50 μg L⁻¹ Te(IV) after 300 s of accumulation at −0.5 V. Using the optimized procedure, a linear range from 2 to 35 μg L⁻¹ of Cu(II) was obtained with a detection limit of 0.5 μg L⁻¹ Cu(II) (S/N=3) for 300 s of deposition time. Both sensors, carbon TeF‐SPE and mesoporous carbon TeF‐SPE, were successfully applied for the quantification of Cu in a certified reference surface water sample.     

Siloxane‐Crown Ether Polyamide Based Electrode for Electrochemical Determination of Lead(II) in Aqueous Solution

A siloxane‐crown ether polyamide copolymer (PDMS‐PA‐DB18C6) was electrochemically investigated for fabrication of lead‐sensitive electrodes for trace analysis in aqueous solutions. The PDMS‐PA‐DB18C6 electrodes were successfully evaluated for anodic stripping voltammetric determination of lead(II) as a promising alternative for the detection of lead at ppb levels. By a judicious choice of the deposition time, electrolyte concentration and pulse amplitude, good analytical performance of the developed sensor could be achieved, with a linear response in the range of 20–700 ppb, when LOD of 3.5 ppb could be attained. This method showed a good degree of selectivity and sensitivity for lead, suitable for the determination of Pb²⁺ in wastewater sample.     

Recent Advances in Anodic Stripping Voltammetry with Bismuth‐Modified Carbon Paste Electrodes

In this article, the results of some recent investigations on two types of bismuth‐modified carbon paste electrodes are presented. In the first study, the bismuth‐film carbon paste electrode (BiF‐CPE) operated in situ and employed in anodic stripping voltammetry of Cd(II) and Pb(II) at the low μg L^?1 level was of interest in view of choosing the proper Bi(III)‐to‐Me(II) concentration ratios (where Me: Pb or Cd). Such optimization has resulted in significant improvement of detection limits down to 1.0 μg L^?1 Cd and 0.8 μg L^?1 for Pb, which allowed us to apply the BiF‐CPE for analysis of selected real samples of tap and sea water. The BiF‐CPE was also further investigated for its application in highly alkaline media. In this case, attention was focused on the complex‐forming capabilities of the OH ^– ions and their effect on the anodic stripping characteristics of some heavy metals (i.e. Cd, Pb, Tl) as well as upon the formation of the bismuth film itself. The last example deals with the continuing characterization of the recently introduced carbon paste electrodes modified with bismuth powder (Bi‐CPEs) which combine the advantageous properties of carbon paste material with the favorable electrochemical properties of bismuth. Three series of electrodes, differing either in the content of metallic bismuth (from 8 to 50% w/w) or in the type of the carbon powder used (two spectroscopic types of graphite and powdered glassy carbon), were compared and the respective relations to the optimal carbon paste composition evaluated. Attractive electroanalytical performance of the Bi‐CPE in anodic stripping voltammetry is demonstrated for selected model mixtures of heavy metals (Mn, Zn, Cd, Pb, Tl, and In).     

Voltammetric Determination of Hemoglobin Using a Pencil Lead Electrode

In this paper the electrochemical behavior of hemoglobin (Hb) immobilized on a pencil lead electrode (PLE) was investigated. Immobilization of Hb on the pencil lead electrode was performed by nonelectrochemical and electrochemical methods. In phosphate buffer solution with pH 7.0 Hb showed a pair of well‐defined and nearly reversible redox waves (the anodic and cathodic peak potentials are located at ?0.18 V and ?0.22 V, respectively). The dependence of the anodic peak potential (E_pa) on the pH of the buffer solution indicated that the conversion of Hb? Fe(III)/Hb? Fe(II) is a one‐electron‐transfer reaction process coupled with one‐proton‐transfer. In addition the effect of scan rate on peak currents and peak separation potential was investigated and electrochemical parameters such as α and k_s were calculated. In the second part of this work, the ability of the electrode for determination of Hb concentration was investigated. The results showed a linear dynamic range from 0.15 to 2 µM and a detection limit of 0.11 µM. The relative standard deviation is 4.1 % for 4 successive determinations of a 1 µM Hb solution.     

Determination of Rutin by a Graphene-Modified Glassy Carbon Electrode

A reduced graphene oxide-modified glassy carbon electrode for sensitive detection of rutin is reported. The modified electrode was obtained by one-step electrochemical reduction of graphene oxide on the bare glassy carbon electrode. In the presence of graphene, an enhanced electrochemical response for rutin appeared with a pair of well-defined anodic and cathodic peaks in pH 3.0 phosphate buffer. Under the optimized conditions, the anodic peak currents exhibited a linear relationship with rutin concentration from 0.1 to 2.0 µM with a detection limit of 23.2 nM. The modified electrode was employed to the analysis of tablets (with satisfactory recovery of 19.96 mg/per tablet) and Flos Sophorae. The graphene-modified electrode exhibited high sensitivity, good stability, and selectivity for the determination of rutin.     

Voltammetric Determination of Mercury(II) at Poly(3‐hexylthiophene) Film Electrode. Effect of Halide Ions

The well‐known method for the determination of mercury(II), which is based on the anodic stripping voltammetry of mercury(II), has been adapted for applications at the thin film poly(3‐hexylthiophene) polymer electrode. Halide ions have been found to increase the sensitivity of the mercury response and shift it more positive potentials. This behavior is explained by formation of mercuric halide which can be easily deposited and stripped from the polymer electrode surface. The procedure was optimized for mercury determination. For 120 s accumulation time, detection limit of 5 ng mL^?1 mercury(II) has been observed. The relative standard deviation is 1.3% at 40 ng mL^?1 mercury(II). The performance of the polymer film studied in this work was evaluated in the presence of surfactants and some potential interfering metal ions such as cadmium, lead, copper and nickel.     

The Performance of Boron‐Doped Diamond Electrode for the Determination of Ramipril and its Association with Hydrochlorothiazide

The voltammetric behavior of ramipril (RMP), an angiotensin‐converting enzyme inhibitor, was investigated for the first time in the literature on a boron‐doped diamond electrode (BDDE) using cyclic (CV) and square‐wave voltammetry (SWV). Its behavior was also considered in the presence of a thiazide diuretic, hydrochlorothiazide (HCTZ), for the simultaneous determination. The performance of BDDE and glassy carbon electrode (GCE) was compared, since RMP provides a high oxidation potential. It was observed that the anodic peak potentials for HCTZ and RMP at the BDDE were 1.23 and 1.67 V (vs . Ag/AgCl (3.0 mol L^‐1 KCl)) by CV in BR buffer solution (pH 2.0), respectively. The influence of supporting electrolyte, pH and scan rate in the current response of these both drugs was examined to select optimum experimental conditions. By this way, the determination of RMP and its association with HCTZ using SWV and a BDDE was successfully applied in real samples (single and combined dosage forms), with results in close agreement at 95% confidence level with those obtained using high‐performance liquid chromatography.     

Influence of Nafion Coatings and Surfactant on the Stripping Voltammetry of Heavy Metals at Bismuth‐Film Modified Carbon Film Electrodes

Nafion polymer coated bismuth‐film‐modified carbon film electrodes have been investigated for reducing the influence of contaminants such as surfactants in the anodic stripping voltammetry of trace metal ions. The influence of the coating on electrode response has been tested with both ex situ and in situ bismuth film deposition, with and without the polymer coating. The electrode assemblies and interfacial characteristics in the presence of the non‐ionic surfactant Triton‐X‐100 have been probed with electrochemical impedance spectroscopy. The Nafion coating successfully decreases the adsorption of Triton on the bismuth film surface, and demonstrates that this strategy allows measurement of these trace metals in environmental samples containing surfactants.     

Effect of Ionic Liquid Structure on the Electrochemical Response of Dopamine at Room Temperature Ionic Liquid‐modified Carbon Paste Electrodes (IL–CPE)

In this work 12 different ionic liquids (ILs) have been used added as co‐binders in the preparation of modified carbon paste electrodes (IL–CPEs) used for the voltammetric analysis of dopamine in Britton‐Robinson buffer. The ionic liquids studied were selected based on three main criteria: (1) increasing chain length of alkyl substituents (studying 1‐ethylimidazolium and ethyl, propyl, butyl, hexyl and decylmethylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids); (2) nature of the counter ion (dicyanamide, bis(trifluoromethylsulfonyl)imide and hexafluorophosphate) in 1‐butyl‐3‐methylimidazolium ionic liquids; and (3) cation ring structures (1‐butyl‐3‐methylimidazolium, 1‐butyl‐1‐methylpiperidinium, 1‐butyl‐1‐methylpyrrolidinium and 1‐butyl‐3‐methylpyridinium) in bis(trifluoromethylsulfonyl)imide or hexafluorophosphate (1‐butyl‐3‐methylimidazolium or 1‐butyl‐3‐methylpyridinium as cations) ionic liquids. The use of IL as co‐binders in IL–CPE results in a general enhancement of both the sensitivity and the reversibility of dopamine oxidation. In square wave voltammetry experiments, the peak current increased up to a 400 % when 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide was used as co‐binder, as compared to the response found with the unmodified CPE. Experimental data provide evidence that electrostatic and steric effects are the most important ones vis‐à‐vis these electrocatalytic effects on the anodic oxidation of dopamine on IL–CPE. The relative hydrophilicity of dicyanamide anions reduced the electrocatalytic effects of the corresponding ionic liquids, while the use of 1‐ethyl‐3‐methylimidazolium hexafluorophosphate or 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide (two relatively small and highly hydrophobic ionic liquids) as co‐binders in IL–CPE resulted in the highest electrocatalytic activity among all of the IL–CPE studied.     

Mechanistic Pathways and Identification of the Electrochemically Generated Oxidation Products of Flavonoid Eriodictyol in the Presence of Glutathione

The metabolic oxidation pathways of dietary flavonoid eriodictyol (Er) are not very well‐probed. In the present work, the electrochemical oxidation behavior of Er was studied in aqueous Britton‐Robinson (B‐R) buffer solution using cyclic voltammetry (CV), chronoamperometry (CA), and bulk‐electrolysis (BE). The oxidation products and reaction pathways of Er in the absence and the presence of glutathione (GSH) were proposed and identified in view of the results obtained by ultra‐high‐performance liquid chromatography coupled with mass spectrometry (UPLC‐MS). In the absence of GSH, eriodictyol shows one quasi‐reversible oxidation process at E_1/2=0.305 V, followed by a totally irreversible anodic peak at a more positive potential (E_p^a=1.05 V vs. Ag/AgCl, 3 M KCl). Putatively, the first process corresponds to the oxidation of the catechol moiety on the B ring of Er while the second one is attributed to the oxidation of the resorcinol moiety on the A ring. In the presence of GSH, however, the anodic oxidation of Er was proposed to be an ECEC‐type mechanism. The Er molecule first underwent a two‐electron oxidation coupled with loss of two‐proton to generate the corresponding quinone, which was either reduced to the original Er molecule by GSH, or further interacted with GSH to produce mono‐ and bi‐ glutathione conjugates of Er. The proposed mechanism was confirmed by digital simulation of the cyclic voltammograms.