Voltammetric Determination of Leucoindigo Adsorbed on Pretreated Carbon Paste Electrodes: Its Application in a Flow System

The determination of leucoindigo adsorbed on the surface of pretreated carbon paste electrodes has been performed by cyclic (CV) and alternating current voltammetry (ACV). The water‐soluble compound called leucoindigo is obtained through the reduction of water insoluble indigo in an alkaline media and in presence of a dithionite salt (Na₂S₂O₄) as a reducing agent. Cyclic voltammograms of leucoindigo show two reversible electrodic processes, in the aqueous 0.1 M Tris‐HCl pH 7.2, at the formal potential of ?0.4 V and +0.3 V (vs. Ag/AgCl/sat. KCl). In a batch protocol, leucoindigo was quantified by CV with a change of medium between accumulation and detection steps. Reversible voltammetric processes of leucoindigo were greatly enhanced by other voltammetric technique like alternating current voltammetry. Limit of detection in the nanomolar range was achieved for a 2 min accumulation time by ACV in a batch procedure. A flow system was also employed, with adsorptive voltammetric detection of leucoindigo, since this automates the methodology and decreases analysis time. Parameters related to the electrochemical technique are optimized and calibration plots obtained are reported. These data provide useful information about the suitability of using leucoindigo in the detection system of alkaline phosphatase (AP) and horseradish peroxidase (HRP) based affinity devices, in which indigo is generated by the enzymatic hydrolysis of the 3‐indoxyl phosphate substrate.     

Simple and Fast Determination of Warfarin in Pharmaceutical Samples Using Boron‐doped Diamond Electrode in BIA and FIA Systems with Multiple Pulse Amperometric Detection

This paper proposes the use of the boron‐doped diamond electrode (BDDE) in flow and batch injection analysis (FIA and BIA) systems with multiple‐pulse amperometric (MPA) detection for the determination of warfarin (WA) in pharmaceutical formulations. The electrochemical behavior of WA obtained by cyclic voltammetry (CV) in 0.1 mol L⁻¹ phosphate buffer shows an irreversible oxidation process at +1.0 V (vs Ag/AgCl). The MPA was based on the application of two sequential potential pulses as a function of time on BDDE: (1) for WA detection at +1.2 V/100 ms and; (2) for electrode surface cleaning at −0.2 V/200 ms. Both hydrodynamic systems (FIA‐MPA and BIA‐MPA) used for WA determination achieved high precision (with relative standard deviations around 2 %, n =10), wide linear range (2.0−400.0 μmol L⁻¹), low limits of detection (0.5 μmol L⁻¹) and good analytical frequency (94 h⁻¹ for FIA and 130 h⁻¹ for BIA). The WA determination made by the proposed methods was compared to the official spectrophotometric method. The FIA‐MPA and BIA‐MPA methods are simple and fast, being an attractive option for WA routine analysis in pharmaceutical industries.     

Cathodized Gold Nanoparticle‐Modified Graphite Pencil Electrode for Non‐Enzymatic Sensitive Voltammetric Detection of Glucose

A highly sensitive enzymeless electrochemical glucose sensor has been developed based on the simply prepared cathodized gold nanoparticle‐modified graphite pencil electrode (AuNP‐GPE). Cyclic voltammetry (CV) experiments show that AuNP‐GPE is able to oxidize glucose partially at low potential (around −0.27) whereas the bare GPE cannot oxidize glucose in the entire tested potential windows. Besides, fructose and sucrose cannot be oxidized at potential lower than +0.1 V at AuNP‐GPE. As a result, the glucose oxidation peak at around −0.27 V is suitable enough for selective detection of glucose in the presence of fructose and sucrose. Cathodization of AuNP‐GPE under optimum condition (‐1.0 V for 30 s) in the same glucose solution before voltammetric measurement enhanced glucose oxidation peak current around −0.27 V to achieve an efficient electrochemical sensor for glucose with a detection limit of 12 μM and dynamic range between 0.05 to 5.0 mM with a good linearity (R²= 0.999). Almost no interference effect was observed for sensing of glucose in the presence of ascorbic acid, alanine, phenylalanine, fructose, sucrose, and NaCl.     

Electrochemical Sensor for Caffeine Based on a Glassy Carbon Electrode Modified with an Attapulgite/Nafion Film

In the present work, a simple and economic analytical method based on attapulgite/nafion coated glassy carbon electrode (AT/Naf/GCE) has been developped for the electrochemical determination of caffeine. Prior to its use, the ionic exchange properties and conductivity of AT/Naf/GCE were studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Caffeine gave an irreversible oxidation peak around +1.41 V (vs Ag/AgCl reference electrode) in 0.1 M H₂SO₄ at pH 1.5. The peak current varied linearly with the square root of the scan rate, showing that the transfer process is controlled by diffusion. The heterogeneous rate constant, the transfer coefficient and the number of electrons involved were calculated. Upon optimization of key analytical parameters involved in the electroanalysis of caffeine by DPV, the recorded oxidation peak current varied linearly with caffeine concentration in the range from 0.1 to 4 μm, leading to a detection limit of 4.57×10^?8 M (S/N=3). The developed electrode exhibited good stability and was easily regenerated. The effect of some important potential interfering compounds (ascorbic acid, dopamine, uric acid, sulphite ions and glucose) on the signal of caffeine was also examined. The obtained electrode was successfully employed in the determination of caffeine content in a commercial drug.     

Immobilization of CoII-(N,N′-bis(salicylidene)-2-aminobenzylamine) on Poly(pyrrole-co-o-anisidine)/Chitosan Composite Films: Application to Electrocatalytic Oxidation of Catechol

In this study, poly (pyrrole-co-o-anisidine)/chitosan composite (Cs) films were prepared by cyclic voltammetry technique on platinum electrode using different pyrrole and o-anisidine mole ratios. Immobilization process was accomplished in Co^II-(N,N′-bis(salicylidene)-2-aminobenzylamine)(CoL) dissolved 0.15 M acetonitrile-LiClO₄ solution by cyclic voltammetry technique at 0.2–2.0 V potential range. Three electrode methods were applied in all electrochemical studies. After immobilization process, the characterizations of the electro catalytic surfaces (Cs−CoL−Pt) were carried out by cyclic voltammetry and SEM images. The SEM images clearly indicated that the [CoL] complex is immobilized onto composite films. The electrocatalytic activity of the modified electrodes on the catechol was investigated using buffer solutions of different pH values. The results of catalytic studies revealed that, pH=10 buffer solution was the optimal solution and 1 : 1 Cs−CoL−Pt electrode was the best electrode for catechol oxidation. In square wave voltammetry measurements using this electrode, two linear working ranges were determined. The linear response ranges for catechol determination were found as 3.0 μM–6.0 μM and 16 μM–80 μM for the first and the second linear working ranges, respectively, with 1.1 μM detection limit.     

Comparison of the stem-loop and linear probe-based electrochemical DNA sensors by alternating current voltammetry and cyclic voltammetry

Here we systematically characterized the sensor performance of the stem-loop probe (SLP) and linear probe (LP) electrochemical DNA sensors using alternating current voltammetry (ACV) and cyclic voltammetry (CV), with the goal of generating the set of operational criteria that best suits each sensor architecture, in addition to elucidating the signaling mechanism behind these sensors. Although the LP sensor shows slightly better % signal suppression (SS) upon hybridization with the perfect match target at 10 Hz, our frequency-dependent study suggests that it shows optimal % SS only in a very limited AC frequency range. Similar results are observed in CV studies in which the LP sensor, when compared to the SLP sensor, displays a narrower range of voltammetric scan rates where the optimal % SS can be achieved. More importantly, the difference between the two sensors' performance is particularly pronounced if the change in integrated charge (Q) upon target hybridization, rather than the peak current (I), is measured in CV. The temperature-dependent study further highlights the differences between the two sensors, where the LP sensor, owing to the flexible linear probe architecture, is more readily perturbed by temperature changes. Both SLP and LP sensors, however, show a loss of % SS when operated at elevated temperatures, despite the significant improvement in the hybridization kinetics. In conjunction with the ACV, CV, and temperature-dependent studies, the electron-transfer kinetics study provides further evidence in support of the proposed signaling mechanism of these two sensors, in which the SLP sensor's signaling efficiency and sensor performance is directly linked to the hybridization-induced conformational change in the redox-labeled probe, whereas the performance of the LP sensor relies on the hybridization-induced change in probe dynamics.     

OH functionalized Multi-Walled Carbon Nanotube modified electrode as electrochemical sensor for the detection of Aceclofenac

ABSTRACT

The rapid electrochemical determination of Aceclofenac (ACF) has been employed by cyclic voltammetry (CV), differential pulse voltammetry (DPV) using developed OH-functionalised multiwalled carbon nanotube carbon paste electrode (OH-MWCNT/CPE). Modified electrode was characterised by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDAX), X-ray diffraction spectroscopy (XRD), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The ACF exhibits two oxidation peaks at +0.4 V, +0.66 V and one reduction peak at +0.3 V. The active surface area of the bare carbon paste electrode (BCPE) and modified electrode have been characterised by using K₃[Fe(CN)₆] solution containing 0.1 M KCl. In DPV mode, variation of ACF gave the limit of detection (LOD = 3s/m) 0.246 μM over the concentration range 1.0 to 190.0 μM (R² = 0.9994). The developed electrode has good stability, reproducibility and could be successfully validated for the detection of ACF in pharmaceutical samples and biological fluids.     

Hydrothermal Synthesis of Three Dimensional Graphene‐Multiwalled Carbon Nanotube Nanocomposite for Enhanced Electro Catalytic Oxidation of Caffeic Acid

Three dimensional graphene‐multiwalled carbon nanotube nano composite (3DG/MWCNTs−Nc) was synthesized by simple hydrothermal method for the amperometric determination of caffeic acid (CA). The prepared nanocomposite was characterized by scanning electron microscopic technique (SEM), ultraviolet‐visible spectroscopy (UV), Raman spectroscopy and infrared spectroscopy (IR). Moreover, the interfacial electron transfer properties of the modified electrode were carried out by the electro chemical impedance spectroscopy (EIS). Besides, the electro chemical performance of the modified electrode was carried out by the cyclic voltammetry (CV) and amperometric (i‐t ) technique. The proposed electrode was exhibited an enhanced electrocatalytic activity towards the detection of CA. Under the optimal condition, the 3DG/MWCNTs−Nc modified electrode displayed a linear range from 0.2 to 174 μM, detection limit (LOD) 17.8 nM and sensitivity of 5.8308 μA μM⁻¹ cm⁻² and on applied potential + 0.2 V. These result showed, 3DG/MWCNTs−Nc modified electrodes showed good repeatability, reproducibility, and higher stability. In addition, the fabricated electrode was then successfully used to determine the CA in real samples with satisfactory recoveries. Which suggests that the 3DG/MWCNTs−Nc as a robust sensing materials for the electrochemical detection of CA.     

Firstly Electrochemical Examination of Myosmine at Glassy Carbon Electrode: Sensitive Determination in Tobacco Leaves by Differential Pulse Voltammetry

In this study, the electrochemical properties of myosmine, one of the tobacco alkaloids, were investigated for the first time using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques on glassy carbon electrode (GCE). Using GCE with CV technique, it gave an irreversible reduction peak with diffusion control at about −1.38 V in Britton-Robinson (BR, pH 12.0) medium. A good linear relationship between concentration and current in the range of 0.5 μM–3.5 μM in BR (pH 12.0) medium on GCE with DPV technique [ I p(μA)=0.847 C(μM)+0.114, r=0.995, n=7] has been observed. The proposed method has been successfully applied to tobacco leaves.     

Uric Acid Determination by Adsorptive Stripping Voltammetry on Multiwall Carbon Nanotubes Based Screen‐Printed Electrodes

A sensitive electroanalytical methodology for the determination of uric acid in real samples using adsorptive voltammetry at a multiwalled carbon nanotubes (MWCNT) modified screen printed electrode (SPCE) is presented. Adsorption of uric acid takes place at open circuit potential at an optimized pH 5.0. Studies about the effect of accumulation time and scan rate on the analytical signal were developed and confirm the adsorption nature of the electrodic process. Quantitative analysis of uric acid by using its oxidation process at +0.18 V (vs. an Ag pseudoreference electrode) was carried out with an accumulation time of 5 min. Thus, a linear voltammetric based reproducible determination of uric acid (RSD 5 %) in the range 1–100 µM was obtained. The method was then successfully used for the determination of uric acid in real clinical samples of urine without detection of interferences. The proposed methodology only requires a dilution of the real sample and present advantages as low cost and easy handling for non specialized technicians.     

RNA Modified Electrodes for Simultaneous Determination of Dopamine and Uric Acid in the Presence of High Amounts of Ascorbic Acid

A promising electrochemical biosensor was fabricated by electrochemical grafting of ribonucleic acid (RNA) at 1.8 V (vs. SCE) on glassy carbon electrode (GCE) (denoted as RNA/GCE), for simultaneous detection of dopamine (DA) and uric acid (UA) with coexistence of excess amount of ascorbic acid (AA). The electrode was characterized by X‐ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques. The RNA modified layer on GCE exhibited superior catalytic ability and anionic exclusive ability in comparison with the DNA modified electrode. Three separated anodic DPV peaks were obtained at 0.312, 0.168 and ?0.016 V for UA, DA and AA, respectively, at the RNA/GCE in pH 7.0 PBS. In the presence of 2.0 mM AA, a linear range of 0.37 to 36 μM with a detection limit of 0.2 μM for DA, and in the range of 0.74 to 73 μM with a detection limit of 0.36 μM for UA were obtained. The co‐existence of 5000 fold AA did not interfere with the detection of DA or UA. The modified electrode shows excellent selectivity, good sensitivity and good stability.     

Voltammetric determination of dopamine in the presence of ascorbic acid at a chemically modified electrode

A Nafion/ruthenium oxide pyrochlore chemically modified electrode (CME) was used for the selective determination of dopamine (DA) in the presence of a high concentration of ascorbic acid by square-wave voltammetry. Compared to a bare glassy carbon electrode, the CME exhibits an apparent shift of the oxidation potentials in cathodic direction and a marked enhancement of the current response. The selective sensing of DA is achieved by combining the electrocatalytic function of the ruthenium oxide pyrochlore catalyst with the charge-exclusion and preconcentration features of Nation. With a preconcentration time of 60 s at a potential of −0.3 V (vs. Ag/AgCl), linear calibration plots are obtained for dopamine in 0.1 M, phosphate buffer (pH 7.4) over 0–20 μM with a detection limit (3σ) of 0.1 μM.     

电活化玻碳电极差分脉冲伏安法测定水杨酸的研

采用循环伏安法和差分脉冲伏安法对水杨酸在电活化玻碳电极上的电化学行为进行研究.在pH7.0的PBS溶液中,将玻碳电极用恒电位法在+1.7V电位阳极氧化400 s.在0.2 mol·L- NaOH溶液中,水杨酸在0.602 V处有一良好的氧化峰,其氧化峰电流与扫描速率在0.02～0.2 V·s-1范围内呈良好线性关系,表...     

Electroanalysis of Carbendazim using MWCNT/Ca‐ZnO Modified Electrode

The present research involves the report on electrochemical deportment of Carbendazim (MBC) at multiwalled carbon nanotubes and calcium‐doped zinc oxide nanoparticles altered nanocomposite based carbon paste electrode (MWCNTs/Ca‐ZnO‐CPE). The modified carbon paste evidenced manifest electrocatalytic behavior for MBC in 0.2 M phosphate buffer (PB) solutions. Cyclic voltammetry (CV), linear sweep voltammetry (LSV), and square wave voltammetry (SWV) techniques were used for the analysis. The working electrode assembly exhibits faster electron transfer of MBC with increase in the peak current. At bare CPE, MBC showed maximum peak current of 1.098 μA at potential 0.7568 V whereas at MWCNT/Ca‐ZnO/CPE peak current of 5.203 μA was observed at potential 0.7541 V in 0.2 M PBS of pH 7.0 at the sweep rate of 50 mV s^?1. The synthesized 5 % Ca‐ZnO nanoparticles (NPs) were characterized by X‐ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive X‐ray analysis (EDX), and Transmission electron microscopy (TEM) analysis. Various factors influencing the voltammetry of MBC such as pre‐concentration time, pH, sweep rate, and amount of MBC were studied and from the studies we observed that the response was found to be diffusion‐controlled. The concentration variation studies for MBC was watched in the linear working range of 0.01 μM to 0.45 μM and the detection limit was found by SWV technique.     

Differential Pulse Anodic Stripping Voltammetric Determination of Lead (II) Using Poly Xylenol Orange Modified Electrode

In this work a method for sensitive anodic stripping voltammetric determination of Pb(II) ions using a poly xylenol orange film modified electrode (PXOFME) has been proposed. Poly xylenol orange film (PXOF) was formed on a paraffin impegrenated graphite electrode (PIGE) using electro polymerization method by scanning the potential between −0.5 V to 1.3 V, at a scan rate of 50 mV/s for 30 segments in 0.1 M phosphate buffer solution (PBS) of pH 7. The PXOFME was characterized by scanning electron microscopy (SEM), ATR‐IR spectroscopy, cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). The modified electrode has been used to develop a sensitive method for the determination of Pb(II) ions by anodic stripping voltmmetry (ASV). The PXOFME was used to preconcentrate Pb(II) ions through complexation, the complexed metal ions were reduced electrochemically and then stripped anodically from the surface of the electrode. A linear range of 5 μg/L to 413 μg/L with a limit of detection (S/N=3) of 1.6 μg/L was observed for the determination of Pb(II) ions. The method was applied to lead determination in sea water and tap water samples and the results were found to be satisfactory.     

Selective Voltammetric Detection of Uric Acid in the Presence of Ascorbic Acid at Well‐Aligned Carbon Nanotube Electrode

The voltammetric behaviors of uric acid (UA) and L ‐ascorbic acid (L ‐AA) were studied at well‐aligned carbon nanotube electrode. Compared to glassy carbon, carbon nanotube electrode catalyzes oxidation of UA and L ‐AA, reducing the overpotentials by about 0.028 V and 0.416 V, respectively. Based on its differential catalytic function toward the oxidation of UA and L ‐AA, the carbon nanotube electrode resolved the overlapping voltammetric response of UA and L ‐AA into two well‐defined voltammetric peaks in applying both cyclic voltammetry (CV) and differential pulse voltammetry (DPV), which can be used for a selective determination of UA in the presence of L ‐AA. The peak current obtained from DPV was linearly dependent on the UA concentration in the range of 0.2 μM to 80 μM with a correlation coefficient of 0.997. The detection limit (3δ) for UA was found to be 0.1 μM. Finally, the carbon nanotube electrode was successfully demonstrated as a electrochemical sensor to the determination of UA in human urine samples by simple dilution without further pretreatment.     

Adsorptive Stripping Voltammetric Detection of Single‐Stranded DNA at Electrochemically Modified Glassy Carbon Electrode

Electrochemically modified glassy carbon electrode (GCE) was used to study the electrochemical oxidation and detection of denatured single‐stranded (ss) DNA by means of adsorptive stripping voltammetry. The modification of GCE, by electrochemical oxidation at +1.75 V (vs.SCE) for 10 min and cyclic sweep between +0.3 V and ?1.3 V for 20 cycles in pH 5.0 phosphate buffer, results in 100‐fold improvement in sensitivity for ssDNA detection. We speculated that the modified GCE has a high affinity to single‐stranded DNA through hydrogen bond (specific static adsorption). Single‐stranded DNA can accumulate at the GCE surface at open circuit and produce a well‐defined oxidation peak corresponding to the guanine residues at about +0.80 V in pH 5.0 phosphate buffer, while the native DNA gives no signal under the same condition. The peak currents are proportional to the ssDNA concentration in the range of 0–18.0 μg mL^?1. The detection limit of denatured ssDNA is ca. 0.2 μg mL^?1 when the accumulation time is 8 min at open circuit. The accumulation mechanism of ssDNA on the modified GCE was discussed.     

Disposable amperometric sensor for neurotransmitters based on screen-printed electrodes modified with a thin iridium oxide film.

Potential cycling in the range from -0.2 to +1.2 V is used for the electrodeposition of hydrous iridium oxide films onto a screen-printed electrode from a saturated solution of alkaline iridium(III) solution. The iridium oxide redox couple shows a stable and obvious reversible redox, with the formal potential being pH dependent in the range 1-14. The properties, stability and electrochemical properties of iridium oxide films were investigated by cyclic voltammetry. A modified electrode showed excellent catalytic activity toward the oxidation of neurotransmitters (catecholamines) over a wide pH range (2-8). The electrocatalytic behavior is further exploited as a sensitive detection scheme for adrenaline and dopamine by hydrodynamic amperometry. Under the optimized conditions, the calibration curves are linear in the concentration range 0.1-70 and 0.1-15 microM for dopamine and adrenaline determination, respectively. The detection limit and sensitivity are 30 nM and 30 nA/microM for adrenaline and 15 nM and 80 nA/microM for dopamine. Finally, the analytical performance of the modified electrode was demonstrated for the elimination of interference by uric acid in catecholamines determination when present in a 1000-fold concentration excess.     

The determination of ethanol in wine by voltammetry with an internal standard

A rapid method for the electroanalysis of ethanol is presented that incorporates flow extraction at room temperature, with voltammetric detection and potassium ferrocyanide [K₄Fe(CN)₆] as internal standard. In 0.1 M NaOH electrolyte, ethanol was oxidised at a platinum comb-shaped working electrode at −300 mV (vs. a Ag/AgCl reference electrode) and K₄Fe(CN)₆ was oxidized at +180 mV. The ratio of the anodic peak currents was linear with ethanol concentration in the range of 0.1 to 8.0% (v./v.), and the detection limit (calculated as 3 σ background) was 0.012 % (v./v.) for Osteryoung square wave voltammetry (OSWV) and 0.023 %(v./v.) for cyclic voltammetry (CV). The average extraction efficiency of ethanol from aqueous solutions, at 20 ± 1°C, was 8.5%. The repeatability was in the range of 2.5 to 3.3% RSD (n = 8), and accuracy was in the range of 95.2 to 104.7% for the determination of wine samples. Application to wines compared well with GC and HPLC methods and the nominal ethanol concentration determined by gravimetry. Analytical parameters in CV and OSWV are optimized, and the dependence of the extraction efficiency with temperature and nitrogen gas flow is presented.     

Indirect cathodic reduction of dispersed indigo by 1,2-dihydroxy-9,10-anthraquinone-3-sulphonate (Alizarin Red S)

The indirect cathodic reduction of dispersed indigo (Vat Blue 1) with 1,2-dihydroxy-9,10-anthraquinone-3-sulphonate (Alizarin Red S) as soluble mediator system was studied in 0.1 M NaOH by cyclic voltammetry, voltammetry in a flow cell and in galvanostatic reduction experiments. In cyclic voltammetry, the presence of 17.1 mM indigo led to an increase in the diffusion-controlled cathodic peak current (I _p)_d by a factor of 2. During the reverse scan of the voltammograms the oxidation of reduced indigo could be observed at −650 mV (vs. Ag/AgCl, 3 M KCl). In voltrammograms of 4.0 mM ALS in 0.1 M NaOH, recorded in a flow cell, a current density of 0.40–0.46 mA cm⁻² was determined for the diffusion-controlled cathodic current plateau, which appeared in the potential range of −850 to −1,050 mV. In galvanostatic batch electrolysis, solutions containing 2.5–3.8 mM reduced indigo were prepared and analysed by spectrophotometry and tested in dyeing experiments. The dyeing behaviour of the reduced indigo was independent of the reduction technique used. Energy consumption for electrochemical reduction of 1 kg of indigo could be estimated to 6.5 kWh kg⁻¹.