The electrochemical oxidation of homocysteine at boron-doped diamond electrodes with application to HPLC amperometric detection

The electrochemical oxidation of homocysteine was studied at as-deposited and anodized (oxidized) boron-doped diamond (BDD) thin film electrodes with cyclic voltammetry, flow injection analysis and high-pressure liquid chromatography with amperometric detection. At anodized boron-doped diamond electrodes, highly reproducible, well-defined cyclic voltammograms for homocysteine oxidation were obtained in acidic media, while as-deposited diamond did not provide a detectable signal. In alkaline media, however, the oxidation response was obtained both at as-deposited and anodized diamond electrodes. The potential sweep rate dependence of homocysteine oxidation (peak currents for 1 mM homocysteine linearly proportional to v(1/2), within the range of 0.01 to 0.3 V s(-1)) indicates that the oxidation involves a diffusing species, with negligible adsorption on the BDD surface at this concentration. In the flow system, BDD exhibited a highly reproducible amperometric response, with a peak variation less than 2%. An extremely low detection limit (1 nM) was obtained at 1.6 V vs. Ag/AgCl. In addition, the determination of homocysteine in a standard mixture with aminothiols and disulfide compounds by means of isocratic reverse-phase HPLC with amperometric detection at diamond electrodes has been investigated. The results showed excellent separation, with a detection limit of 1 pmol and a linear range of three orders of magnitude.     

Resistance to Surfactant and Protein Fouling Effects at Conducting Diamond Electrodes

Boron‐doped diamond thin‐film electrodes display negligible fouling effects in the presence of high levels of surface‐active materials, including proteins. Dramatic improvements in the stability of the analyte response (compared to common glassy carbon and carbon paste electrodes) are illustrated using bovine serum albumin (BSA), gelatin, and Triton X‐100 in connection with repetitive square‐wave voltammetric (SWV) measurements. The voltammetric response of ascorbic acid at the diamond electrode exhibits negligible shifts in peak potentials and minimal depressions of current signals over a wide range of surfactant concentrations (0–750 ppm). For example, the diamond electrode exhibited 70, 50 and 60 mV potential shifts for 10 repetitive voltammetric scans in the presence of 100 ppm BSA, gelatin and Triton X‐100, respectively, compared to 120, 190, and 280 mV shifts observed at the glassy carbon electrode. Furthermore, only 4.3 and 6.2% of the initial current decays were observed in the presence of 100 ppm Triton X‐100 and gelatin, respectively (compared to 45.2 and 34.4% diminutions at the glassy carbon electrode). Such improved performance was also confirmed from the SWV measurements of uric acid, dihydroxyphenylacetic acid, and catechol. The greatly improved resistance to surfactant interference reflects the fact that the as‐grown diamond thin film, composed of oxide‐free and hydrogen‐terminated surface, has a relatively lower surface energy and minimal electrostatic attributes, either specific or general, so that little adsorption of surface‐active agents occurs. The topographic AFM images of the diamond electrode surface confirm a negligible BSA fouling effect after repetitive SWV measurements. Such enhanced antifouling features make diamond electrodes very attractive for numerous real‐life electroanalytical applications.     

Electrochemical Determination of Sulphur‐containing Pharmaceuticals Using Boron‐doped Diamond Electrodes

N‐acetylcysteine (NAC) and gentamicin sulfate (GS) are biologically and pharmaceutically relevant thiol‐containing compounds. NAC is well known for its antioxidant properties, whereas GS is an aminoglycoside that is used as a broadband antibiotic. Both pharmaceuticals play a significant role in the treatment of bacterial infections by suppressing the formation of biofilms. According to the European Pharmacopeia protocol, GS is analyzed by high performance liquid chromatography (HPLC) using gold electrodes for electrochemical detection. Here, we report the electrochemical detection of these compounds at NH₂‐terminated boron‐doped diamond electrodes, which show significantly reduced electrode passivation, an issue commonly known for gold electrodes. Cyclic voltammetry experiments performed for a period of 70 minutes showed that the peak current decreased only by 1.6 %/7.4 % for the two peak currents recorded for GS, and 6.6 % for the oxidation peak of NAC, whereas at gold electrodes a decrease in peak current of 14.2 % was observed for GS, and of 64 %/30 % for the two peak currents of NAC. For their quantitative determination, differential pulse voltammetry was performed in a concentration range of 2–49 µg/mL of NAC with a limit of detection (LOD) of 1.527 µg/mL, and a limit of quantification (LOQ) of 3.624 µg/mL, respectively. The quantification of GS in a concentration range of 0.2–50 µg/mL resulted in a LOD of 1.714 µg/mL, and a LOQ of 6.420 µg/mL, respectively.     

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.     

Electrochemical Sensor Based on Casein and Carbon Black for Bisphenol A Detection

Bisphenol A (BPA) is an environmental endocrine disrupting chemical, which can lead to various adverse health effects. Aiming to develop effective tools for the detection of BPA, this work reports a low cost and stable film based on casein (CAS) and Carbon Black (CB). The proposed material (CAS‐CB) showed structures of CAS surrounded by CB agglomerates observed by Scanning Electron Microscopy while Fourier Transform Infrared Spectroscopy analysis illustrated characteristic bands of casein. Cyclic Voltammetry (CV) and linear sweep voltammetry (LSV) were used to investigate the electrochemical behavior of BPA using the CAS‐CB. Under optimal conditions, LSV detection presented a limit of detection of 0.25 μmol L^?1 in a linear range from 0.49 to 24 μmol L^?1. Additionally, the working electrode (GC) modified by the proposed film (CAS‐CB) was applied for BPA sensing in environmental and milk samples. The results showed recoveries between 95.4 to 114 % attesting the efficiency of this new material, which has simplicity in the preparation, high conductivity, and adsorption capability.     

Gold and Silver Micro‐Wire Electrodes for Trace Analysis of Metals

Micro‐wire electrodes were made from gold and silver wires (diameter: 25 μm; length: 3–21 mm) and sealed in a polyethylene holder; micro‐disk electrodes were made from the same wires and polished. The gold electrodes were electrochemically coated with mercury before use; the silver wires were used without coating. Comparative measurements demonstrated that the micro‐wire electrodes had much higher sensitivity, and a much (10–100×) lower limit of detection, than micro‐disk electrodes, and the sensitivity increased linearly with the area and length of the electrodes. Using a gold micro‐wire electrode of 21 mm and a deposition time of 300 s the limit of detection was 0.07 nM Pb in seawater of natural pH, compared to a limit of detection of 10 nM Pb (more than 100×greater) using a gold micro‐disk electrode of the same diameter. Using the silver micro‐wire electrode the limit of detection of lead was improved by a factor of 10 to 0.2 nM in acidified seawater. It is expected that the improved sensitivity of micro‐wire electrodes will lead to successful in situ detection of metals in natural waters.     

Effect of pressure on the growth of boron and nitrogen doped HFCVD diamond films on WC–Co substrate

Boron and nitrogen compounds are added in the acetone/hydrogen gas mixture to deposit hot filament chemical vapor deposition (HFCVD) diamond films on the cobalt cemented tungsten carbide (WC–Co) substrate under the pressure of 1–4 kPa. The as‐deposited diamond films are characterized by field emission scanning electron microscope (FESEM), atomic force microscopy (AFM), X‐ray diffraction (XRD) spectroscopy and Raman spectroscopy. The results reveal that the surface morphology, growth rate, structure and quality of the diamond films vary with the pressure and the type of the impurity addition. The diamond grains tend to develop into the nanometer scale with the decrease of the pressure. However, adding of boron or nitrogen impurities in the gas mixture will weaken the nanocrystallization effect by reducing the carbon supersaturation. Density functional theory (DFT) calculations indicate that co‐adsorption of B and N containing radicals can favor the adsorption of CH₃ on diamond (100) surface. Thus, at low pressure of 1 kPa, large grained cubic (100) facet diamond rather than typical nanometer diamond is produced for B–N co‐addition gas mixture. The present results appear to be useful to efficiently synthesize high quality doped diamonds with desirable properties for mechanical application. Copyright © 2015 John Wiley & Sons, Ltd.     

Determination of Mercury Using a Novel Sonogel‐Carbon 3‐Methylthiophene Electrode

Abstract

A solid Sonogel‐Carbon electrode was modified for the determination of Hg (II) in industrial waste water. 3‐Methylthiophene (3MT), pyrrol, poly‐3‐methylthiophene (P3MT), polypyrrol, and C18 were used for the chemical modification. The obtained composite electrodes were tested for their response to Hg(II); the best results were observed for the 3MT monomer modification with a detection limit of 10^?2 mg · l^?1 and 3 weeks lifetime of use. A linear relationship between anodic peak height and concentration inside the range of 0.07–0.42 mg · l^?1 was obtained. A study of interferences due to other heavy metal is also included.     

Electrocatalytic Oxidation of Estrogenic Phenolic Compounds at a Nickel‐Modified Glassy Carbon Electrode

This paper demonstrates for the first time, successful electrocatalytic oxidation of electroactive estrogenic phenolic compounds (EPCs) at a nickel‐modified glassy carbon electrode (Ni‐GCE). The electrode was evaluated in terms of electrocatalytic activity, sensitivity, linear dynamic range, limit of detection, and response stability. In comparison to bare glassy carbon electrode, current amplification was observed for EPCs at Ni‐GCE, for example, for a 40 µM estrone at Ni‐GCE was amplified by a factor of 1224. The Ni‐GCE gave good figures of merit with no evidence of electrode fouling. As an example, the limit of detection (S/N=3) for 17β‐estradiol was 100 nM and the response precision (n=5) was 3.4 %.     

The Analysis of Chlorinated Phenol Solutions by Capillary Electrophoresis Coupled with Direct and Indirect Amperometric Detection Using a Boron‐Doped Diamond Microelectrode

Analysis of aqueous solutions containing chlorinated phenol pollutants was accomplished by capillary electrophoresis with direct and indirect amperometric detection using a boron‐doped diamond microelectrode. The microelectrode was prepared by (i) coating a thin film of boron‐doped polycrystalline diamond on a sharpened platinum wire (76‐μm diameter) and (ii) sealing the coated wire in a polypropylene pipet tip. The diamond microelectrode, used in end‐column detection, exhibited a low and stable background current with low peak‐to‐peak noise and good electrochemical activity for the pollutants without any conventional pretreatment. The electrode performance was evaluated in terms of the linear dynamic range, sensitivity, limit of quantitation, and response precision for the detection of several priority pollutants (2‐chlorophenol, 3‐chlorophenol, 4‐chlorophenol, 2,4‐dichlorophenol, 2,4,6‐trichlorophenol, and pentachlorophenol). The diamond microelectrode gave good detection figures of merit for these contaminants in the direct amperometric mode with no evidence of any electrode fouling. As an example, the concentration limit of quantitation for 2‐chlorophenol was 100 nM or 13 ppb (S/N=3) and the relative standard deviation of the peak height for 9 injections was 4.7±0.5% (est. 1.1 nL inj.). The separation efficiency was greater than 100 000 plates/m for all seven solutes. The microelectrode was also employed for the indirect detection of the chlorinated phenols. In this approach, which is useful for detecting electroinactive solutes, ferrocene carboxylic acid was added to the run buffer as the electrophore. Good detection figures of merit were also achieved for the separation and detection of 2‐chlorophenol, 3‐chlorophenol, and 2,4‐dichlorophenol in this mode, although the linear dynamic range was not as wide and the limit of quantitation was not as low as in direct amperometry. For example, the concentration limit of quantitation for these pollutants was in the mid micromolar range (1–10 ppm) with excellent response reproducibility of 3.2±0.8%, or less.     

Whole cell environmental biosensor on diamond

A whole-cell environmental biosensor was fabricated on a diamond electrode. Unicellular microalgae Chlorella vulgaris was entrapped in the bovine serum albumin (BSA) membrane and immobilized directly onto the surface of a diamond electrode for heavy metal detection. We found that the unique surface properties of diamond reduce the electrode fouling problem commonly encountered with metal electrodes. The cell-based diamond biosensor can attain a detection limit of 0.1 ppb for Zn(2+) and Cd(2+), and exhibits higher detection sensitivity and stability compared to platinum electrodes.     

Preparation of grain size controlled boron-doped diamond thin films and their applications in selective detection of glucose in basic solutions

Boron-doped diamond (BDD) thin films with different crystal grain sizes were prepared by controlling the reacting gas pressure using hot filament chemical vapor deposition (HFCVD). The morphologies and structures of the prepared diamond thin films were characterized by scanning electron microscopy (SEM) and Raman spectroscopy. The electrochemical responses of K₄Fe(CN)₆ on different BDD electrodes were investigated. The results suggested that electron transfer was faster at the boron-doped nanocrystalline diamond (BDND) thin film electrodes in comparison with that at other BDD thin film electrodes. The prepared BDD thin film electrodes without any modification were used to directly detect glucose in the basic solution. The results showed that the as-prepared BDD thin film electrodes exhibited good selectivity for detecting glucose in the presence of ascorbic acid (AA) and uric acid (UA). The higher sensitivity was observed on the BDND thin film grown on the boron-doped microcrystalline diamond (BDMD) thin film surface, and the linear response range, sensitivity and the low detection limit were 0.25–10 mM, 189.1 μA mmo^?1 cm^?2 and 25 μM (S/N=3) for glucose in the presence of AA and UA, respectively.     

Fullerene Black Modified Screen Printed Electrodes for the Quantification of Acetaminophen and Guanine

Fullerene Black (FB) and Extracted Fullerene Black (EFB) were used in modified screen‐printed electrodes producing electrochemical transducers (FB‐SPEs and EFB‐SPEs). A complete electrochemical study was performed and the best results are obtained working with FB‐SPEs, especially in terms of: 1. improved electron‐transfer kinetic mechanisms and 2. sensitivity and selectivity toward Acetaminophen (Ac) and Guanine (G). These latter represent two important electro‐active targets to quantify in medicine field application, because: Ac is a preferred alternative (as analgesic‐antipyretic agent) to aspirin, particularly for patients who cannot tolerate aspirin; the oxidation signal of G is useful for the fabrication of emerging analytical tools, such as DNA chipsand user‐friendly diagnostic devices. Ac and G are quantify by using FB‐SPEs electrochemical devices, with an extended linearity (1–300 μM for Ac; 0.1–300 μM for G), an excellent sensitivity (2.82 μA μM⁻¹ cm⁻² in the case of Ac; and 0.183 μA μM⁻¹ cm⁻² in the case of G), a low detection limit (0.01 μM for Ac; 0.005 μM for G), a very good reproducibility (both: intra‐; inter‐electrodes reproducibility RSD % ranging from 0.3–0.5 for Ac; and 0.50–0.85 for G) and a very fast response time (6 s for Ac; 5 s in the case of G). In addition, high selectivity is obtained at FB‐SPEs, meaning that the FB‐SPEs electrochemical transducers are suitable to simultaneously quantify Ac and G in real samples, having several different (highly concentrated) interference.     

Electrochemical Detection of Catechol Based on As‐Grown and Nanograss Array Boron‐Doped Diamond Electrodes

The electrochemical behavior of different redox systems and detection of catechol were performed on the as‐grown boron‐doped diamond (BDD) electrodes and the nanograss array BDD. Compared with as‐grown BDD, the electron transfer on the nanograss array BDD surface became slower toward the negatively charged Fe(CN)₆^3?, whereas changed little toward the positively charged Ru(NH₃)₆³⁺. The nanograss array BDD showed higher electrocatalytic activity toward the catechol detection than did the as‐grown BDD. Good linearity was observed for a concentration range from 5 to 100 μM with a sensitivity of 719.71 mA M^?1 cm^?2 and a detection limit of 1.3 μM on the nanograss array BDD.     

Selective amperometric detection of dopamine using OPPy-modified diamond microsensor system

Highly boron-doped diamond microfiber electrodes (BDDMF) were fabricated and characterized by the use of Scanning Electron Microscopy (SEM), Raman spectroscopy, and cyclic voltammetry. Amperometric detection of dopamine (DA), a neurotransmitter was achieved at pH 7.0, using BDDMF electrodes. The interferences from ascorbic acid (AA) and DOPAC were efficiently eliminated by using overoxidized polypyrrole-modified BDDMF electrodes, which also increased the sensitivity for the detection of dopamine. The limit of detection (S/N = 3) for dopamine was 0.1 nM, which is one order lower than that observed for carbon microfiber electrodes (CMFE), and the linear dynamic range was obtained from 0.5 nM to 100 microM (r2 = 0.997). The amperometric response for 0.5 nM dopamine has shown high stability with an RSD of 5.4% (n = 5). Highly reproducible results were obtained with an RSD of 6.2% for 10 measurements of 1 nM DA taken during 10 h and also remained the same, during measurements for 7 days, with no variation in efficiency for rejection of AA and DOPAC.     

Prostate Cancer Biomarker Detection with Carbon Nanotubes Modified Screen Printed Electrodes

DNA hypermethylation is an epigenetic alteration and a promising biomarker for early prostate cancer detection. Simple, sensitive, easy to handle and rapid detection methodologies are imperative for point of care diagnostics especially for cancer. Herein, we describe for the first time a regenerable and compatible electrochemical biosensor for detection of Glutathione S‐Transferase P‐1 (GSTP‐1) gene hypermethylation related to prostate cancer via DNA hybridization onto the disposable Carbon and Multi Walled Carbon Nanotubes (MWCNT) Screen Printed Electrodes (SPEs). In the study, capture probes were adsorbed onto the SPEs by simple passive adsorption and then hybridization was achieved by sending the complementary target onto the probe‐modified electrodes. The selectivity of the biosensor was proved by control studies. Differential Pulse Voltammetry (DPV) technique was used to detect hybridization via guanine oxidation signals changes. The total time of the optimized method was nearly 1h, measurements took for less than 1 min, and the biosensor response was stable up to 40 days of storage period at 4 °C. The main advantages of the biosensor are very low detection limit (picomolar range) and capability of reusing the biosensor for at least 3 times after very simple regeneration process that is a unique property to reduce the cost of the assay. In addition, this is the first study that demonstrates the detection of GSTP‐1 hypermethylation electrochemically by using SPEs in order to create point of care diagnostics. The optimum parameters for the biosensor, as well as its future prospects to enhance the performance of DNA biosensors were also presented.     

Isatin Analysis Using Flow Injection Analysis with Amperometric Detection – Comparison of Tetrahedral Amorphous Carbon and Diamond Electrode Performance

Isatin is an endogenous indole compound in humans and rodents that has a wide range of biological activity. In rat models, isatin concentrations have been shown to increase in the heart, brain, blood plasma, and urine with stress. Studies on patients suffering from Parkinson's disease have indicated a correlation between progress of the disease and urinary output of the molecule. Isatin is electrochemically active and can therefore be detected with electrochemical techniques. In this work, we compared the performance of a nitrogen‐incorporated tetrahedral amorphous carbon (ta‐C:N) and a boron‐doped nanocrystalline diamond thin‐film electrode for the oxidative detection of this biomolecule using flow injection analysis with amperometric detection. The measurements were performed in 0.1 phosphate buffer pH 7.2. The ta‐C:N electrode, like boron‐doped nanocrystalline diamond, exhibits some excellent properties for electroanalytical measurements including (i) low background current and noise, (ii) microstructural stability at positive detection potentials, and (iii) good activity for a wide range of bioanalytes without conventional surface pretreatment. The results reveal that both electrodes exhibit a linear dynamic range from 100 to 0.1 μmol L⁻¹, a short‐term response variability 3–4 % RSD (30 injections), a sensitivity of 18 mA M^‐1, and a limit of detection (S/N=3) of 1.0×10⁻⁷ mol L⁻¹ (14 ng mL⁻¹ or 2.5 fmol).     

Computerized flow constant-current stripping analysis for nickel(II) and cobalt(II) with carbon fibre and modified carbon fibre electrodes

Nickel (II) and cobalt (II) are determined by constant-current stripping analysis, with mercury- coated carbon fibre electrodes, in non-deoxygenated solutions after potentiostatic adsorptive accumulation of their dimethylglyoxime complexes. By adding several stripping scans, each obtained after a short period of potentiostatic deposition, instead of a single scan obtained after the same total time of potentiostatic adsorption, the linear range between the analytical signal and the analyte concentration can be extended. By using pulsed-potential procedures for adsorptive accumulation, cobalt (II) can be determined in the presence of a larger amount of nickel (II) and vice versa. Carbon fibre electrodes were modified by sucking ethanolic solutions of either dimethylglyoxime (of Nafion and dimethylglyoxime) through the electrodes; Nafion did not affect the general behaviour of this electrode. The background of the modified electrodes was lower than that of the unmodified electrodes. The accuracy of the method was confirmed by analysis of sea- water reference samples. The detection limit for nickel (II) was about 40 ng l^?1 for a total potentiostatic deposition time of 100 s (5×20 s); the reagent blank was about 150 ng l^?1 under these conditions.     

Electroanalysis of Norepinephrine at Bare Gold Electrode Pure and Modified with Gold Nanoparticles and S‐Functionalized Self‐Assembled Layers in Aqueous Solution. Part II

Gold electrodes modified with S‐containing compounds and gold were used for determination of norepinephrine (NEP) in aqueous solution. A linear relationship between norepinephrine concentration and current response was obtained in the range of 0.1 µM to 600 µM with the detection limit ≤0.090 µM for the electrodes modified at 2D template and in the range of 0.1 µM to 700 µM with the detection limit ≤0.075 µM for the electrodes modified at 3D template. The results have shown that modified electrodes could clearly resolve the oxidation peaks of norepinephrine, ascorbic (AA) and uric acid (UA) with peak‐to‐peak separation enabling determination of NEP, AA and UA in the presence of each other.     

Polyethylenimine Functionalized Multi‐walled Carbon Nanotubes for Electrochemical Detection of Chromium(VI)

Multi‐walled carbon nanotubes (MWCNTs) functionalized with polyethylenimine (PEI) were synthesized and characterized by dispersibility, field‐emission scanning electron microscope (FE‐SEM), FT‐IR and thermogravimetric Analyzer (TGA). The glassy carbon electrodes modified by MWCNT‐PEI composite were used for sensitive and selective detection of chromium (VI). A linear response was obtained over a wide range of Cr(VI) concentrations (0.002–20 µmol L^?1) with the detection limit of 0.0006 µmol L^?1 (S/N=3). The proposed electrodes were used successfully for Cr(VI) detection in three real water samples.