Simultaneous Determination of 4‐Chlorophenol and Oxalic Acid Using an Expanded Graphite‐Epoxy Composite Electrode

An expanded graphite‐epoxy composite electrode (EG‐Epoxy) was employed for the simultaneous determination of 4‐chlorophenol (4‐CP) and oxalic acid (OA) by using cyclic voltammetry (CV), chronoamperometry (CA), and differential pulse voltammetry (DPV). The results indicated that OA could be determined in the presence of the same concentrations of 4‐CP within the concentration range of 0.1 mM to 0.5 mM with a relative standard deviation (RSD) smaller than 5%. Electrode fouling occurred during CA for 4‐CP concentrations larger than 0.5 mM. The DPV method was used for the simultaneous determination of 4‐CP and OA before and after electrochemical oxidation by chronopotentiometry under galvanostatic conditions (j=0.04 mA cm^?2, t=2 h) of a tap water sample spiked with 0.19 mM 4‐CP and 0.1 M Na₂SO₄.     

Characterization of Kaolin Glassy Carbon Modified Electrodes: Preconcentration of 2‐Chlorophenol

In this paper we describe the use of two kaolin‐type aluminosilicate clays, a commercial ceramic‐grade kaolin (K) and a natural kaolin from mines in Bolívar State Venezuela (K‐Ve), for the preparation of film‐based clay‐modified glassy carbon electrodes. We examine their behavior during the preconcentration and subsequent anodic oxidation of 2‐chlorophenol. Kaolin samples were used as raw materials and modified with cationic surfactant, cetyltrimethylammonium bromide (CTAB) or nonionic surfactant, octylphenoxypolyethoxyethanol (TX100). The electrode polishing was the key step to produce stable films. 2‐Chlorophenol electrooxidation is favored by the presence of the surfactants in the film. The X‐ray patterns show that the kaolin K‐Ve includes quartz as nonclay mineral, while the kaolin K showed only diffraction peaks characteristic of kaolinite phase. This may be why the TX100/K‐Ve/GC electrode adsorbs more 2‐CPh than the TX100/K/GC electrode. On the other hand, analysis of the limiting currents obtained from hydrodynamic techniques indicated that the permeability of TX100/kaolin films is greater than that of CTAB/kaolin films. The TX100/K‐Ve/GC electrode showed excellent stability. A linear response range from 0.01mgL^?1 up to 0.1 mg L^?1 with a detection limit of 0.0016 mg L^?1 was observed in the optimized conditions.     

Electrochemical Behavior and Amperometric Detection of 4‐Chlorophenol on Nano‐Au Thin Films Modified Glassy Carbon Electrode

An amperometric sensor based on nano‐Au thin films was fabricated, by means of which a fast response to 4‐chlorophenol (4‐CP) can be achieved in the range of mM concentrations. The nanostructured Au thin film was prepared on glassy carbon electrodes by a template‐free, double‐potential step electrodeposition technique. Its structural feature can be controlled well by adjusting the deposition time. The amperometric detection of 4‐CP was performed at +0.85 V with a linear detection range from 0.2 to 4.8 mM and a detection limit of 0.11 mM (S/N=3). Besides, the effect of concentrations on the electrochemical behavior of 4‐CP on the Au thin film was investigated by linear sweep voltammetry, differential pulse voltammetry and electrochemical impedance spectroscopy.     

Copper Oxide – Graphite Composite Electrodes: Application to Nitrite Sensing

A simple method for the modification of carbon powder with copper oxides is presented. Carbon powder is impregnated with copper(II) nitrate by stirring carbon powder in copper(II) nitrate solution for 1 hour and subsequently thermally treated at 823 K. The modified carbon powder was characterized using electrochemical and spectroscopic techniques. The existence of both copper(I) and copper(II) oxides have been established. The copper oxide modified carbon powder was used for preparation of composite electrodes, and the electrochemical and electrocatalytic behavior of the resulting composite electrodes was studied. The copper oxide modified carbon powder – epoxy composite electrodes showed a high electrocatalytic activity for the nitrite detection in aqueous media, with the detection limit comparable or lower than detection limits obtained with other electrochemical sensors.     

Solar Photocatalytic Degradation of 4‐Chlorophenol in Kaolinite Catalysts

This research applies semiconductor photocatalysts, which are formed by metal ion exchange on the surface of kaolinite catalyst with cations, to the study of photocatalytic degradation of 4‐chlorophenol. The analysis results of catalyst properties shows that, after sintering at 400 °C, kaolinite catalyst has a particle size of between 10–100 nm indicating the nano level of synthesized catalysts. Under the same condition, kaolinite‐Ag/Zn catalyst works better in degradation efficiency than single kaolinite‐Ag and kaolinite‐Zn catalysts. Kaolinite‐Zn catalyst declines in degradation efficacy after 150 minutes and performs poorer than the other three types of kaolinite catalysts. In the experiments of different amounts of catalysts, when the concentration exceeds 0.1 wt%, utilization of light energy and degradation efficiency will be reduced due to shielding effect. When at different pH values, the higher the pH value, the more OH‐will be released and that is beneficial for reaction with substances and the increase of reaction rate. Finally multivariate analysis proves that there is one determining factor that influences the photocatalytic degradation of 4‐chlorophenol in kaolinite catalysts, named as “the factor with intermediates competition degree,” the one affecting the 4‐CP degradation at different weight percentages that is referred to as the “shielding effect factor.”     

Fabrication and Electrochemical Behavior of Vanadium‐Substituted Keggin‐Type Polyoxometalates Multilayer Films on 4‐Aminobenzoic Acid Modified Glassy Carbon Electrodes

Two Vanadium‐substituted Keggin‐type polyoxometalates, K₃H₂[α‐SiVW₁₁O₄₀]?6H₂O (SiVW₁₁) and K₄H₂[γ(1, 2)‐SiV₂W₁₀O₄₀]?4H₂O (SiV₂W₁₀) were first successfully immobilized on 4‐aminobenzoic acid modified glass carbon electrodes respectively by layer‐by‐layer assembly with poly (ethylenimine) (PEI) as counterions. The regular growth processes were monitored by cyclic voltammetry (CV), and it was proved that the multilayer films were uniform and stable. The cyclic voltammetry results indicated that the electrochemical behavior of two multilayer films was similar, and their redox couples are pH‐ and scan rate‐dependent. The multilayer films show favorable electrocatalytic active toward the reduction of NO₂^?, IO₃^? and H₂O₂.     

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

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

Graphite Micropowder Modified with 4‐Amino‐2,6‐diphenylphenol Supported on Basal Plane Pyrolytic Graphite Electrodes: Micro Sensing Platforms for the Indirect Electrochemical Detection of Δ9‐Tetrahydrocannabinol in Saliva

Graphite micropowder has been modified with 4‐amino‐2,6‐diphenylphenol immobilized onto a basal plane pyrolytic graphite electrode and explored for the indirect electrochemical sensing of Δ⁹‐tetrahydrocannabinol in artificial saliva. The protocol is based on the electrochemical formation of quinoneimine which specifically reacts with Δ⁹‐tetrahydrocannabinol resulting in the loss of the quinoneimine which can be monitored via voltammetry. It is demonstrated that Δ⁹‐tetrahydrocannabinol can be detected in artificial saliva over the micromolar range. Such a protocol may find application in screening for drug abuse.     

Stripping Voltammetry with Bismuth Modified Graphite‐Epoxy Composite Electrodes

The attractive performance of graphite‐epoxy composite electrodes (GECE) surface‐modified with a bismuth film (Bi‐GECE) for simultaneous and separate stripping determination of trace amounts of heavy metals (lead, cadmium and zinc) is described. Several key parameters have been optimized. Bi‐GECEi electrode shows superior accumulation properties when compared to bare GECE or even to Hg‐GECE. Bi‐GECE exhibits well‐defined, undistorted, reproducible and sharp stripping signals with RSD of 2.99%, 1.56% and 2.19% for lead, cadmium and zinc respectively. Detection limits of 23.1, 2.2 and 600 μg/L for lead, cadmium and zinc were obtained. Sharp peaks with high resolution (of neighboring signals) that permit convenient multi‐elemental measurements resulted. The applicability of the electrodes to the real samples (tap water and soil sample) was also tested and promising results with good recoveries were obtained.     

Amperometric Glucose Biosensors Based on Glassy Carbon and SWCNT‐Modified Glassy Carbon Electrodes

Different carbonaceous materials, such as single‐walled carbon nanotubes (SWCNTs) and glassy carbon submitted to an electrochemical activation at +1.80 V (vs. SCE) for 900 s, have been used with the aim of comparing their performances in the development of enzyme electrodes. Commercial SWCNTs have been pretreated with 2.2 M HNO₃ for 20 h prior to use. The utility of activated GC as promising material for amperometric oxidase‐based biosensors has been confirmed. With glucose oxidase (GOx) as a model enzyme, glucose was efficiently detected up to 1 mM without the use of a mediator. Both electrodes operated in stirred solutions of 0.1 M phosphate buffer (pH 5.5), containing dissolved oxygen, at a potential of ?0.40 V vs. SCE. Although the performances of the two carbonaceous materials were comparable, the biosensors based on activated GC were characterized by a practically unchanged response 40 days after the fabrication, a better signal to noise ratio, and a little worse sensitivity. In addition, the preparation procedure of such biosensors was more simple, rapid and reproducible.     

Iron(III) Oxide Graphite Composite Electrodes: Application to the Electroanalytical Detection of Hydrazine and Hydrogen Peroxide

Composite electrodes based on iron(III) oxide, Fe₂O₃, carbon powder and epoxy resin have been prepared and characterized using electrochemical methods and X‐ray photoelectron spectroscopy (XPS). Initially composite electrodes were made by mixing micron sized carbon powder surface with iron(III) oxide. However, the voltammetric responses were unsatisfactory. Therefore, a new type of composite electrodes was made using carbon powder modified with iron(III) oxide via a wet impregnation procedure. This technique involves saturation of the carbon powder with iron(III) nitrate followed by thermal treatment at ca. 623 K forming iron(III) oxide on the surface of the carbon powder.     

Impedimetric Aptasensor Based on Disposable Graphite Electrodes Developed for Thrombin Detection

The impedimetric aptasensor for Thrombin (THR) was developed for the first time herein by measuring changes at the charge‐transfer resistance, R_ct upon to protein? aptamer complex formation. After covalent activation of pencil graphite electrode (PGE) surface using covalent agents, amino linked aptamer (APT) was immobilized onto activated PGE surface. Then APT‐THR interaction was explored by electrochemical impedance spectroscopy (EIS). After the optimization of experimental conditions (e.g., APT and THR concentration, immobilization and interaction times), the selectivity of impedimetric aptasensor was tested in the presence of other biomolecules: factor Va and bovine serum albumine (BSA) both in buffer media, or in diluted fetal bovine serum (FBS).     

Poly‐ε‐Lysine Modified Nanocarbon Film Electrodes for LPS Detection

We developed an electrochemical system for detecting lipopolysaccharide (LPS) that uses an ultraflat nanocarbon film electrode modified with poly‐ε‐lysine with a high affinity to LPS. LPS was captured on the modified electrode, and then ferrocene labeled polymyxin B (FcPMB) was captured on the LPS adsorbed electrode via the LPS‐PMB affinity interaction. The adsorbed FcPMB provided an amplified response with Fe²⁺ ions, and the current response was dependent on the amount of captured LPS (LOD=2.0 ng/mL). This was due to the efficient accumulation of the obtained current for LPS and the very low noise made possible by the ultraflat surface.     

Quantitation of Metal Ions in Archaeological Glass by Abrasive Stripping Square‐Wave Voltammetry Using Graphite/Polyester Composite Electrodes

A method for identifying Co, Cu, Sb, Sn, Fe, Zn, and Zr pigments in glasses using square‐wave voltammetry as the detection mode is described. It is based on the abrasive attachment of glass microsamples to graphite/polyester composite electrodes. Sample amounts less than 1 μg are used allowing for the study of archaeological samples. In all cases well‐defined anodic stripping peaks corresponding to the oxidation of metal deposits generated at potentials ranging from ?0.6 to ?1.6 V (vs. SCE) are obtained. This response is in agreement with that displayed by the corresponding metal oxides. Quantitative estimates of the relative population of metal ions in glasses are obtained from peak area measurements with an accuracy comparable to that obtained with SEM/EDX.     

Novel Amperometric Sensor Based on Rigid Near‐Percolation Composite

A novel amperometric sensor based on a rigid graphite‐epoxy composite of which composition is near to the percolation is reported. The electrochemical response of the novel transducer material was evaluated in terms of reproducibility of the fabrication process and reproducibility and repeatability of the analytical signal. The signal to noise ratio was improved. atomic force microscopy (AFM) technique was used to obtain qualitatively information. Amperometric detection of chlorine in water was carried on at a set potential of ?250 mV vs. Ag/AgCl. The developed flow injection analysis (FIA) system responded linearly to chlorine concentration between 0.15 mg L^?1 and 4 mg L^?1with a sensitivity of ?0.20 μA L mg^?1. The proposed system was applied to real samples from swimming‐pool water. No significant difference was observed regarding the standard method.     

Be(II) Graphite Coated Membrane Sensor Based on a Recently Synthesized Benzo‐9‐Crown‐3 Derivative

A highly sensitive and selective membrane electrode with 9‐crown‐3 derivative (CD) as ionophore, potassium tetrakis‐(p‐chlorophenyl) borate as anionic additive (KTB), acetophenone (AP) as solvent mediator was prepared and investigated as a Be(II) sensor. The best performance was observed with the membrane having the percent ratio 30% PVC: 8% CD: 6% KTB: 56% Acetophenone. The poly(vinyl chloride) PVC membrane containing 9‐crown‐3 derivative (CD) directly coated on a graphite electrode, shows a Nernstian response for Be(II) ions over a very wide concentration range (1.0×10^?1?1.0×10^?7 M) with a detection limit of 8.0×10^?8 M (ca. 0.72 ng/mL). It has a fast response time of ca. 20 s and can be used for at least 10 weeks without any major deviation in potential. The proposed sensor exhibits very good selectivity with respect to common alkali, alkaline earth, transition and heavy metal ions. The proposed sensor was used as end point indicator electrode in the titration of Be(II) ions with EDTA. It was also applied to determination of Be(II) in real sample.     

Incorporation of Disposable Screen‐Printed Electrodes for Use in Capillary Electrophoresis End‐Column Amperometric Detection System

The development and performance of an end‐column amperometric detection system integrated with disposable screen‐printed electrodes for capillary electrophoresis is presented. In this system, the electrode and capillary can be easily replaced and the capillary/electrode alignment procedure is straightforward. The use of easily replaceable screen‐printed electrodes offers a tremendous benefit for capillary electrophoresis applications requiring frequent replacement of the working electrode due to fouling. This simple and convenient system is very attractive for routine analyses by capillary electrophoresis with electrochemical detection. The separation and determination of uric acid in human urine is presented.     

基于两种复合纳米材料修饰的电极对多巴胺的催化氧化

本文先在Au电极表面自组装硫辛酰胺(T-NH_2),再利用电化学还原的方法将还原氧化石墨烯(ERGO)和纳米金(AuNPs)电沉积到T-NH_2表面,采用循环伏安法考察了电极的电化学性能。实验表明,该修饰电极对多巴胺(DA)具有良好的电催化作用,优化条件下,DA的氧化峰电流与其浓度在6.49×10~(-6)~7.62×10~(-3) mol/L范围内呈良好的线性关系(R=0.996),检出限为2.0×10~(-6) mol/L。     

Electrochemical Oxidative Determination and Electrochemical Behavior of 4‐Nitrophenol Based on an Au Electrode Modified with Electro‐polymerized 3,5‐Diamino‐1,2,4‐triazole Film

A simple and fast electrochemical method was described and evaluated to determine the hazardous compound, 4‐nitrophenol (4NP). In this work, concentration of 4NP was determined by differential wave voltammetry (DPV). A gold electrode (Au) was modified with 3,5‐diamino‐1,2,4‐triazole (35DT). The modified electrode (35DT‐Au) was characterized by using electrochemical impedance spectroscopy (EIS), fouirer transform infrared spektrofotometre (FTIR), cyclic voltammetry (CV) and DPV. The modified electrode showed more sensitivity towards 4NP compared to unmodified one. A wide linear concentration range from 0.24 to 130.6 μM was obtained for 4NP with a detection limit of 0.09 μM. In the reproducibility and repeatability studies, the relative standard deviation (RSD%) values of the method were obtained as 3.72 % and 2.56 %, respectively, which are acceptable values. This proposed method was successfully used for the analysis of 4NP in lake and tap water samples. Simplicity, sensitivity, selectivity and high efficiency of the proposed method can be used in routine analysis of trace amounts of 4NP in polluted waters.     

Pyrolyzed Photoresist Carbon Electrodes for Microchip Electrophoresis with Dual‐Electrode Amperometric Detection

The fabrication and evaluation of pyrolyzed photoresist films (PPF) for microchip capillary electrophoresis (CE) with dual‐electrode electrochemical (EC) detection is described. The sensitivity, linearity, and reproducibility were evaluated using catecholamines and related compounds, including dopamine (DA), 5‐hydroxyindole‐3‐acetic acid (5‐HIAA), ascorbic acid (AA), and catechol. Initial studies with DA show the response of the PPF electrodes to be linear between 25 and 500 μM (r²=0.999) with a limit of detection (LOD) of 5 μM (S/N=3) and sensitivity of 5.8 pA/μM. Selectivity was further enhanced by employing dual‐electrode detection in the series configuration for detection of species exhibiting chemically reversible redox reactions.