Poly(Methyl Red) Modified Glassy Carbon Electrodes: Electrosynthesis,Characterization, and Sensor Behavior

Poly(methyl red), PMR, was electropolymerized on glassy carbon electrodes by potential cycling in 50 mM phosphate buffer solution at pH 7.0 and 8.0 and Britton Robinson buffer solution in the pH range 7.0‐11.0. The electrochemical behavior of PMR modified electrodes was investigated by cyclic voltammetry in Britton Robinson buffer solution at different pHs from 5.0 to 10.0 and found that the best PMR film formation was obtained at pH 9.0. Uric acid was quantitatively determined at PMR modified electrodes by cyclic voltammetry and differential pulse voltammetry in Britton Robinson buffer at pH 5.0. Both methods presented a linear dependence between the anodic peak current and the concentration of uric acid in the range of 0.4 to 60 μM and 0.08 to 100 μM with the limits of detection of 0.038 and 0.009 μM for cyclic voltammetry and differential pulse voltammetry, respectively. Poly(methyl red) as redox mediator allowed the determination of uric acid without any interferences from the substances in serum samples.     

Influence of micelles on the electrochemical behaviors of catechol and hydroquinone and their simultaneous determination

A simple and highly selective electrochemical method for the simultaneous determination of CAT and HQ at a glassy carbon electrode in micellar solutions has been developed. The electrochemical behaviors of CAT and HQ in aqueous CPB and SDBS micellar solutions have been studied by cyclic voltammetry. The oxidation peak potentials shift negatively, the reduction peak potentials shift positively, and the peak currents increase in the presence of CPB for both CAT and HQ. However, the oxidation peak potentials shift positively, the reduction peak potentials shift negatively, and the peak currents decrease in the presence of SDBS for both CAT and HQ. The electrochemical kinetic parameters for CAT and HQ in aqueous CPB and SDBS micellar solutions were also determined by chronocoulometry (CC) and chronoamperometry (CA). The cyclic and pulse differential voltammetric behaviors of the system consisting of CAT coexisting with HQ were also investigated in this work. It was found that the oxidation peak potential waves of CAT and HQ were separated by 100 mV in the presence of CPB in 0.10 M PBS (pH 6.8). Therefore, CAT and HQ can be determined simultaneously in such a system. This simple method was applied to the simultaneous determination of HQ and CAT in a household tap water sample and it exhibited high selectivity.     

A graphene-based electrochemical sensor for sensitive and selective determination of hydroquinone

Graphene was prepared by electrochemical reduction of exfoliated graphite oxide at cathodic potentials, and used to fabricate a graphene-modified glassy carbon electrode (GCE) which was applied in a sensor for highly sensitive and selective voltammetric determination of hydroquinone (HQ). Compared to a bare (conventional) GCE, the redox peak current for HQ in pH 5.7 acetate buffer solution is significantly increased, indicating that graphene possesses electrocatalytic activity towards HQ. In addition, the peak-to-peak separation is significantly improved. The modified electrode enables sensing of HQ without interference by catechol or resorcinol. Under optimal conditions, the sensor exhibits excellent performance for detecting HQ with a detection limit of 0.8?μM, a reproducibility of 2.5% (expressed as the RSD), and a recoveries from 98.4 to 101.2%.

Electrochemical Behavior and Ultrasensitive,Simple and Effective Voltammetric Determination of Acetaminophen Using Modified Glassy Carbon Electrode Based on 4-Hydroxyquinoline-3-carboxylic Acid

An electrochemical oxidation of acetaminophen (ACOP) has been successfully performed by using glassy carbon electrode covered with 4-hydroxyquinoline-3-carboxylic acid (4HQ3CA) to reinforce electrode's feature. To characterize the modified electrode (4HQ3CA/GC), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and Fourier transform infrared spectroscopy (FT-IR) techniques were used. The finding optimum conditions (supporting electrolyte, pH) and the electrochemical determination studies were performed with differential pulse voltammetry (DPV). It was decided that the supporting electrolyte medium suitable for ACOP determination was Britton-Robinson (BR) buffer and the effect of pH change on the oxidation peak of ACOP in this media was investigated. The effect of changing scan rate on the oxidation peak of ACOP was examined and this study showed that the oxidation process of ACOP on the 4HQ3CA/GC modified electrode surface was diffusion and adsorption controlled process. A wide concentration range from 0.0025 μM to 141 μM with a limit of detection (LOD) of 5.98×10⁻¹⁰ M (3 s/m) was obtained. This prepared sensor was carried out for the determination of ACOP in pharmaceutical sample.     

Covalent modification of a glassy carbon electrode with penicillamine for simultaneous determination of hydroquinone and catechol

A simple and highly selective electrochemical method has been developed for the simultaneous determination of hydroquinone (HQ) and catechol (CC) at a glassy carbon electrode covalently modified with penicillamine (Pen). The electrode is used for the simultaneous electrochemical determination of HQ and CC and shows an excellent electrocatalytical effect on the oxidation of HQ and CC upon cyclic voltammetry in acetate buffer solution of pH 5.0. In differential pulse voltammetric measurements, the modified electrode was able to separate the oxidation peak potentials of HQ and CC present in binary mixtures by about 103 mV although the bare electrode gave a single broad response. The determination limit of HQ in the presence of 0.1 mmol L⁻¹ CC was 1.0 × 10⁻⁶ mol L⁻¹, and the determination limit of CC in the presence of 0.1 mmol L⁻¹ HQ was 6.0 × 10⁻⁷ mol L⁻¹. The method was applied to the simultaneous determination of HQ and CC in a water sample. It is simple and highly selective.     

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.     

Simultaneous determination of hydroquinone and catechol based on glassy carbon electrode modified with gold-graphene nanocomposite

We have synthesized a virtually monodisperse gold-graphene (Au-G) nanocomposite by a single-step chemical reduction method in aqueous dimethylformamide solution. The nanoparticles are homogenously distributed over graphene nanosheets. A glassy carbon electrode was modified with this nanocomposite and displayed high electrocatalytic activity and extraordinary electronic transport properties due to its large surface area. It enabled the simultaneous determination of hydroquinone (HQ) and catechol (CC) in acetate buffer solution of pH?4.5. Two pairs of well-defined, quasi-reversible redox peaks are obtained, one for HQ and its oxidized form, with a 43 mV separation of peak potentials (ΔE_p), the other for CC and its oxidized form, with a ΔE_p of 39 mV. Due to the large separation of oxidation peak potentials (102 mV), the concentrations of HQ and CC can be easily determined simultaneously. The oxidation peak currents for both HQ and CC increase linearly with the respective concentrations in the 1.0 μM to 0.1 mM concentration range, with the detection limits of 0.2 and 0.15 μM (S/N?=?3), respectively. The modified electrode was successfully applied to the simultaneous determination of HQ and CC in spiked tap water, demonstrating that the Au-G nanocomposite may act as a high-performance sensing material in the selective detection of some environmental pollutants.

Simultaneous Determination of Hydroquinone and Catechol at an Activated Glassy Carbon Electrode

A selective and very simple electrochemical method, based on anodization of a glassy carbon electrode (GCE), was developed for the simultaneous detection of hydroquinone (HQ) and catechol (CT). It was found that the activated GCE showed an excellent catalytic behavior and enhanced reversibility towards the oxidation of both HQ and CT. The redox responses from the mixture of HQ and CT were easily resolved at an activated GCE. The detection limits for HQ and CT were calculated to be 0.16 and 0.11 μM, respectively. The activated GCE was successfully examined for real sample analysis with tap water and it showed a stable and reliable recovery data.     

Sensitive and Facile Determination of Catechol and Hydroquinone Simultaneously Under Coexistence of Resorcinol with a Zn/Al Layered Double Hydroxide Film Modified Glassy Carbon Electrode

A stable dihydroxybenzene sensor was fabricated by electrochemical deposition of Zn/Al layered double hydroxide film on glassy carbon electrode (LDHf/GCE). The sensitive and facile electrochemical method for the simultaneous determination of catechol (CA) and hydroquinone (HQ) under coexistence of resorcinol (RE) has been achieved at the LDHf/GCE in phosphate buffer solution (pH 6.5). Under the optimized conditions, the differential pulse voltammetry response of the modified electrode to CA (or HQ) shows a linear concentration range of 0.6 μM to 6.0 mM (or 3.2 μM to 2.4 mM) with a correlation coefficient of 0.9987 (or 0.9992) and the calculated limit of detection is 0.1 μM (or 1.0 μM) at a signal‐to‐noise ratio of 3. In the presence of 50 μM isomer, the linear concentration ranges for CA and HQ are 3.0 μM to 1.5 mM and 12.0 μM to 0.8 mM, respectively. The detection limits are 1.2 μM and 9.0 μM. Further, the proposed method has been performed to successfully detect dihydroxybenzene isomers in analysis of real samples, such as water and tea.     

Selective determination of cysteine in the presence of tryptophan by carbon paste electrode modified with quinizarine

A sensitive and selective electrochemical method for the determination of L-cysteine was developed using a modified carbon paste electrode (MCPE) with quinizarine. Cyclic voltammetry was used to investigate the redox properties of this modified electrode at various solution pH values and at various scan rates. The apparent charge transfer rate constant, ks and transfer coefficient for electron transfer between quinizarine and carbon paste electrode (CPE) were calculated as 2.76 s^?1 and 0.6, respectively. This modified carbon paste electrode shows excellent electrocatalytic activity toward the oxidation of L-cysteine in a phosphate buffer solution (pH 7.0). The linear range of 1.0 × 10^?6 to 1.0 × 10^?3 M and a detection limit (3s) of 2.2 × 10^?7 M were observed in pH 7.0 phosphate buffer solutions. In differential pulse voltammetry, the quinizarine modified carbon paste electrode (QMCPE) could separate the oxidation peak potentials of L-cysteine and tryptophan present in the same solution, though at the unmodified CPE the peak potentials were indistinguishable. This work introduces a simple and easy approach to selective detection of L-cysteine in the presence of tryptophan. Also, the modified electrode was employed for the determination of L-cysteine in the real samples such as serum of blood and acetylcysteine tablet.     

Covalent modification of glassy carbon electrode with aspartic acid for simultaneous determination of hydroquinone and catechol

Glassy carbon electrode (GCE) is covalently modified with aspartic acid (Asp). The modified electrode is used for the simultaneous electrochemical determination of hydroquinone (HQ) and catechol (CC) and shows an excellent electrocatalytical effect on the oxidation of HQ and CC by cyclic voltammetry (CV) in 0.1 mol/L acetate buffer solution (pH 4.5). In differential pulse voltammetric (DPV) measurements, the modified electrode could separate the oxidation peak potentials of HQ and CC present in binary mixtures by about 101 mV though the bare electrode gave a single broad response. A successful elimination of the fouling effect by the oxidized product of HQ on the response of CC has been achieved at the modified electrode. The determination limit of HQ in the presence of 0.1 mmol/L CC was 9.0 x 10(-7) mol/L and the determination limit of CC in the presence of 0.1 mmol/L HQ was 5.0 x 10(-7) mol/L. The proposed method has been applied to the simultaneous determination of HQ and CC in a water sample with simplicity and high selectivity.     

Sensitive simultaneous determination of catechol and hydroquinone using a gold electrode modified with carbon nanofibers and gold nanoparticles

A highly sensitive electrochemical sensor for the simultaneous determination of catechol (CC) and hydroquinone (HQ) was fabricated by electrodeposition of gold nanoparticles onto carbon nanofiber film pre-cast on an Au electrode. Both CC and HQ cause a pair of quasi-reversible and well-defined redox peaks at the modified electrode in pH?7.0 solution. Simultaneously, the oxidation peak potentials of CC and HQ become separated by 112?mV. When simultaneously changing the concentrations of both CC and HQ, the response is linear between 9.0???M and 1.50?mM. In the presence of 0.15?mM of the respective isomer, the electrode gives a linear response in the range from 5.0 to 350???M, and from 9.0 to 500???M for CC and HQ, respectively, and detection limits are 0.36 and 0.86???M. The method was successfully examined for real sample analysis with high selectivity and sensitivity.

Electrochemical studies on lawsone and its determination in henna (Lawsonia inermis) extract using glassy carbon electrode

The electrochemical behaviour of lawsone at glassy carbon electrode (GCE) was investigated by using cyclic and differential pulse anodic stripping voltammetric (DPASV) techniques. Cyclic voltammetry was used to study the influence of pH on the peak current and peak potential. The Mcllavnie’s buffer of pH 3.0 was selected as a suitable analytical medium in which lawsone exhibited sensitive diffusion controlled redox peaks (vs. Ag/AgCl). The peak current varied linearly with lawsone concentration in the range between 0.60 and 1.40 μM with a detection limit of 6 nM. The applicability of the proposed method was illustrated by the determination of lawsone present in real samples. A mean recovery of lawsone in the leaf of Lawsonia inermis was 99.5% with a relative standard deviation of 1.15%.     

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.     

Measurement of Dissolved Oxygen in Biological Fluids by Using a Modified Carbon Paste Electrode

A modified carbon paste electrode was constructed for the determination of dissolved oxygen using diamino‐o‐benzoquinone (DABQ) as the modifier. The electrochemical behavior of the electrode in citrate buffer (pH 2.0) was studied. In the presence of dissolved oxygen (DO) both cathodic and anodic peak currents decreased, indicating a chemical reaction between modifier and O₂. The decrease in peak current was linearly proportional to the amount of dissolved oxygen in the concentration range of 252–1260 μM of DO. The electrode was utilized in the determination of DO in urine samples. The relative error and RSD of the method were 1.6% and 4.1%, respectively. The electrode was applied more than two months for the determination of DO without any significant divergence in its voltammetric response.     

Simultaneous determination of dopamine, ascorbic acid and uric acid at poly (Evans Blue) modified glassy carbon electrode

A sensitive and selective electrochemical method for the determination of dopamine using an Evans Blue polymer film modified on glassy carbon electrode was developed. The Evans blue polymer film modified electrode shows excellent electrocatalytic activity toward the oxidation of dopamine in phosphate buffer solution (pH 4.5). The linear range of 1.0 x 10(-6)-3.0 x 10(-5) M and detection limit of 2.5 x 10(-7) M were observed in pH 4.5 phosphate buffer solutions. The interference studies showed that the modified electrode exhibits excellent selectivity in the presence of large excess of ascorbic acid and uric acid. The separation of the oxidation peak potentials for dopamine-ascorbic acid and dopamine-uric acid were about 182 mV and 180 mV, respectively. The differences are large enough to determine AA, DA and UA individually and simultaneously. This work provides a simple and easy approach to selectively detect dopamine in the presence of ascorbic acid and uric acid in physiological samples.     

Direct electron transfer and voltammetric determination of roxithromycin at a single-wall carbon nanotube coated glassy carbon electrode

The electrochemical behavior of roxithromycin (RM) at a single-wall carbon nanotube (SWNT) coated glassy carbon (GC) electrode was studied. It was found that RM could produce an irreversible anodic peak at the electrode. When the pH of supporting electrolyte (i.e. phosphate buffer solution) was 7 the peak potential was 0.86V (vs. SCE). The electrochemical reaction contained electron and proton transfer, and the electron-transfer coefficient (α) was ca. 0.87. The anodic peak depended on the adsorption of RM, the maximum adsorption amount was about 3.99×10(-10)molcm(-2). The adsorbed RM could be removed by cycling between 0.1 and 1.1V in a blank solution for about two minutes, and the electrode thus could be regenerated. Under the optional conditions, the anodic peak current was linear to RM concentration over the range of 5.0×10(-6) to 1.0×10(-4)M. The limit of detection was 5.0×10(-7)M (S/N=3) for 180s accumulation at -0.8V. The modified electrode had good stability and repeatability, and it was successfully applied to the determination of RM in medicine samples.     

Use of DPPH⋅|DPPH Redox Couple for Biamperometric Determination of Antioxidant Activity

A novel electrochemical method for selective determination of antioxidant activity based on 2,2‐diphenyl‐1‐picrylhydrazyl | 2,2‐diphenyl‐1‐picrylhydrazin (DPPH?|DPPH) redox couple and biamperometric technique is proposed. Two identical glassy carbon disk electrodes were mounted in a classic electrochemical cell and the tested working potential difference was between 50 and 200 mV. The DPPH?|DPPH redox couple exhibited a high degree of reversibility. Selectivity of detector was tested by different antioxidant compounds in ethanol‐phosphate buffer solution, pH 7.40. The results of antioxidant activity of pure antioxidant compounds and of actual samples of beverages, expressed as Trolox equivalents and determined by proposed biamperometric and spectroscopic measurements, were in good correlation (R=0.9959). The detection limit for Trolox established by the applied biamperometry was 0.05 μM while the resulting current was linear up to 25 μM of Trolox.     

Simultaneous determination of hydroquinone and catechol based on three-dimensionally ordered macroporous polycysteine film modified electrode

A three-dimensionally ordered macroporous (3DOM) polycysteine (PCE) film was electropolymerized on the glassy carbon electrode (GCE) using polystyrene spheres as template. The electrochemical behaviors of hydroquinone (HQ) and catechol (CC) were studied, and two independent oxidation peaks were observed. Compared with the bare GCE and GCE modified with PCE without using template, this electrode displays larger peak currents which may be attributed to the structure of PCE and the large surface area of the nanopore array structure. As a result, a novel electrochemical method was developed for the simultaneous determination of HQ and CC. Under the optimized conditions, the peak currents were linear to concentrations in the wider ranges of 9 to 700 μM for HQ and from 3 to 700 μM for CC. The method was successfully applied to the simultaneous determination of HQ and CC in spiked water samples, and the results are satisfactory.     

Electrochemical behavior and determination of clozapine on a glassy carbon electrode modified by electrochemical oxidation.

The adsorptive and electrochemical behaviors of clozapine (CLZ) were investigated on a glassy carbon electrode that was electrochemically treated by anodic oxidation at +1.8 V, following potential cycling in the potential range from -0.8 to 1.0 V vs. Ag/AgCl reference electrode. Based on the obtained electrochemical results, an electrochemical-chemical (EC) mechanism was proposed to explain the electrochemical oxidation of CLZ. The resulting electrochemically pretreated glassy carbon electrode (EPGCE) showed good activity to improve the electrochemical response of the drug. CLZ was accumulated in a phosphate buffer (pH 6) at a certain time, and then determined by differential pulse voltammetry. The anodic and cathodic peak currents showed a linear function in the concentration ranges of 0.1 - 1, 1 - 10 and 10 - 100 microM with various accumulation times. The proposed method was successfully used for the determination of CLZ in pharmaceutical preparations. The preconcentration medium-exchange approach was utilized for the selective determination of the drug in spiked urine samples with satisfactory results. The recovery levels of the method reached 96% (RSD, 1.8%) and 90% (RSD, 2.8%) for urine and plasma samples, respectively.