Liquid Chromatographic Determination of Glutathione with Electrochemically Pretreated Glassy Carbon Electrode

Abstract

Simple electrochemical pretreatment of a glassy carbon electrode used as a working electrode in an electrochemical detector has been found to enhance the analytical capability of the detector for the determination of glutathione by lowering the required operating potential as well as by increasing the maximum oxidation current. The electrochemical pretreatment of the electrode was made in a 0.2 M phosphate buffer (KH₂PO₄-KOH, pH 6.5) at +1.9 V vs. Ag/AgCl for 2 min. The minimum detectable quantity of glutathione was found to be about 100 pg, when eluate from a reversed-phase column was amperometrically monitored by aid of the detector with the electrochemically pretreated glassy carbon electrode. Amount of glutathione in a rat lens was determined by the chromatographic method developed in this study.     

The effect of electrochemically treated glassy carbon on the activity of supported Pt catalyst in methanol oxidation

Effect of electrochemical oxidation of glassy carbon on deposition of platinum particles and electrocatalytic activity of platinum supported on oxidized glassy carbon (Pt/GC_OX) were studied for methanol oxidation in H₂SO₄ solution. Platinum was potentiostatically deposited from H₂SO₄ + H₂PtCl₆ solution. Glassy carbon was anodically polarised in 0.5 M H₂SO₄ at 2.25 V vs. saturated calomel electrode (SCE) during 35 s. Electrochemical treatment of GC support, affecting not significantly the real Pt surface area, leads to a better distribution of platinum on the substrate and has remarkable effect on the activity. The activity of the Pt/GC_OX electrode for methanol oxidation is larger than polycrystalline Pt and for more than one order of magnitude larger than Pt/GC electrode. This increase in activity indicates the pronounced role of organic residues of GC support on the properties of Pt particles deposited on glassy carbon.     

Analysis of α-Ketocarboxylic Acids by Ion Exchange Hplc With UV and Amperometric Detection

Abstract

Cation exchange high performance liquid chromatography with tandem 200nm spectrophotometric and +1.15V amperometric detection was used for the analysis of severalOt α-ketocarboxylic acids. Derivatization of the α-ketocarboxyiic acids was not necessary. This technique was operable at 2.0 ug/ml concentrations of α-ketocarboxylic acids. The sensitivity limit was not investigated.     

CHEMOSPECIFICITY IN ARYLATIONS OF δ- AND γ-KETOCARBOXYLIC ACIDS WITH P2O5–MsOH,TfOH, AND RELATED ACIDIC MEDIA

ABSTRACT

Remarkable contrast between chemospecificities in acid-mediated arylation of δ- and γ-ketocarboxylic acids was revealed: in the presence of P₂O₅–MsOH, TfOH, PPA, and MsOH, arylation of δ-ketocarboxylic acid 1A with arenes takes place at the carboxycarbonyl carbon, while that of γ-ketocarboxylic acid 1B takes place at the ketone carbonyl carbon, specifically.     

Ion Chromatography Using an Electrochemical Detector: Response to Non-Electroactive Anions

Abstract

The effect of non electroactive anions on the response of glassy carbon electrode used as an electrochemical detector in an ionic chromatographic system with dual column has been investigated and a method to eliminate this anomalous behaviour has been proposed. The results showed that the response of the electrode observed for non electroactive anions is related to the drop in pH of the eluent during the elution of anions.     

Amperometric Biosensors for Glucose Based on Layer‐by‐Layer Assembled Functionalized Carbon Nanotube and Poly (Neutral Red) Multilayer Film

Abstract

Multiwalled carbon nanotubes (MWNTs) were treated with a mixture of concentrated sulfuric and nitric acid to introduce carboxylic acid groups to the nanotubes. Conducting polymer film was prepared by electrochemical polymerization of neutral red (NR). By using a layer‐by‐layer method, homogeneous and stable MWNTs and poly (neutral red) (PNR) multilayer films were alternately assembled on glassy carbon (GC) electrodes. With the introduction of PNR, the MWNTs/PNR multilayer film system showed synergy between the MWNTs and PNR, with a significant improvement of redox activity due to the excellent electron‐transfer ability of carbon nanotubes (CNTs) and PNR. The electropolymerization is advantageous, providing both prolonged long‐term stability and improved catalytic activity of the resulting modified electrodes. The MWNTs/PNR multilayer film modified glassy carbon electrode allows low potential detection of hydrogen peroxide with high sensitivity and fast response time. As compared to MWNTs and PNR‐modified GC electrodes, the magnitude of the amperometric response of the MWNTs/PNR composite‐modified GC electrode is more than three‐fold greater than that of the MWNTs modified GC electrode, and nine‐fold greater than that of the PNR‐modified GC electrode. With the immobilization of glucose oxidase onto the electrode surface using glutaric dialdehyde, a biosensor that responds sensitively to glucose has been constructed. In pH 6.98 phosphate buffer, nearly interference‐free determination of glucose has been realized at ?0.2 V vs. SCE with a linear range from 50 µM to 10 mM and response time <10s. The detection limit was 10 µM glucose (S/N=3).     

Bioelectrocatalysis of Dopamine Using Adsorbed Tyrosinase on Single-Walled Carbon Nanotubes

Abstract

A wealth of information on the reactions of redox-active sites in proteins can be obtained by voltammetric studies in which the protein sample is arranged as a layer on a suitable electrode surface. Here, we describe a method for the performance of a tyrosinase/single-walled carbon nanotubes/glassy carbon (Tyr/SWCNTs/GC) electrode, prepared by the modification of GC electrode surface by SWCNTs and adsorption of tyrosinase on the SWCNT surfaces. SWCNTs were studied with the help of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The dimensions of SWCNTs make them ideal candidates for the adsorption of proteins. The copper-containing enzyme, tyrosinase, exhibited an electrical contact with the electrode, because of the structural alignment of the enzyme on the SWCNT surfaces. The apparent Michaelis–Menten constant (K _m) for dopamine (DA) and the stability of the enzyme electrode were estimated. This method could be suitable for applications to nanofabricated devices.     

Determination of Apomorphine in Tablets Using High Performance Liquid Chromatography with Electrochemical Detection

Abstract

A rapid and selective method using high performance liquid chromatography with electrochemical detection is described for the determination of apomorphine in tablets. Tablet mixes were dissolved in a standard volume of mobile phase containing the internal standard, N-n-propylnorapomorphine. Separation was achieved on a μ-phenyl column using methanol-acetonitrile-0.05M KH₂PO₄ (5:15:80) as mobile phase. The eluted compounds were detected with a sandwich-type electrochemical detector employing a glassy carbon working electrode and operated at 0.5V. Satisfactory accuracy and precision were obtained during analyses of tablets containing apomorphine.     

Highly Sensitive Electrochemical Sensor for Determination of Vitamin D in Mixtures of Water‐Ethanol

This paper describes the electrochemical determination of vitamin D₂ (ergocalciferol) and D₃ (cholecalciferol) in mixed organic/water solvent, using a glassy carbon electrode (GCE). The mixing ratio of organic/water solvent has an important influence on the electrocatalytic response of D vitamins on the surface of the glassy carbon electrode. Well‐defined peaks for Vitamin D₂ and D₃ were observed in a 40 % ethanol/60 % water solution with lithium perchlorate as the support electrolyte. This study demonstrated that the glassy carbon electrode is highly sensitive for the determination of vitamin D₂ and D₃, with a limit of detection of 0.13 and 0.118 µmol L^?1, respectively. No significant interference was seen for vitamins A, E and K in the detection of vitamin D.     

An amperometric biosensor using toluidine blue as an electron transfer mediator intercalated in α-zirconium phosphate-modified horseradish peroxidase immobilization matrix

A novel H₂O₂ biosensor was constructed employing α-zirconium phosphate as a new support substrate to hold an electron shuttle toluidine blue between a glassy carbon electrode and horseradish peroxidase. Toluidine blue was intercalated into α-zirconium phosphate-modified horseradish peroxidase immobilization matrix cross-linked on a glassy carbon electrode surface via bovine serum albumin-glutaraldehyde. This co-immobilization matrix of the mediator and the enzyme was formed from the α-zirconium phosphate (α-ZrP)-toluidine blue (TB) inclusion colloid in which horseradish peroxidase (HRP) was dissolved. Intercalation of TB in layered α-ZrP was investigated by scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and electrochemical measurements. TB immobilized in this way underwent a quasi-reversible electrochemical redox reaction at the electrode. Cyclic voltammetry and amperometric measurements demonstrated good stability and efficiently-shuttled electrons between HRP and the electrode. The sensor responded rapidly to H₂O₂ with a detection limit of 3.0 × 10^–7 mol/L.     

Analysis of bacterial fatty acids by flow modulated comprehensive two-dimensional gas chromatography with parallel flame ionization detector/mass spectrometry

Comprehensive two-dimensional gas chromatography (GC × GC) offers an interesting tool for profiling bacterial fatty acids. Flow modulated GC × GC using a commercially available system was evaluated, different parameters such as column flows and modulation time were optimized. The method was tested on bacterial fatty acid methyl esters (BAMEs) from Stenotrophomonas maltophilia LMG 958^T by using parallel flame ionization detector (FID)/mass spectrometry (MS). The results are compared to data obtained using a thermal modulated GC × GC system. The data show that flow modulated GC × GC-FID/MS method can be applied in a routine environment and offers interesting perspectives for chemotaxonomy of bacteria.     

Identification of Abnormal Urinary Organic Acids in Inherited Metabolic Diseases by Gas Chromatography-Mass Spectrometry

A gas chromatography/mass spectrometry (GC/MS) coupled system has been established for the confirmatory identification of abnormal urinary organic acids in inherited metabolic diseases. Samples of patient urines were extracted with an organic solvent and trimethylsilylated (TMS). A mass spectra of gas chromatographically separated TMS derivatives can be obtained using the GC/MS coupled system with a single analytical run. Those compounds with close methylene units (e.g., 4-hydroxyphenylacetaic acid and phenylpyruvic acid) in the gas chromatograph can be identified by their specific mass spectra. The results indicate that this GC/MS system is a powerful method for identifying abnormal urinary organic acids. These acids can be identified by comparison with authentic mass spectra established in our laboratories or with mass spectra files from other sources or they can be directly identified by analysis of the mass spectrum. By using this system, we were able to make positive identification of several inherited metabolic diseases found in Chinese patients, including phenylketonuria, propionic acidemia, and methylmalonic aciduria. This GC/MS system is a powerful tool for the diagnosis of inherited metabolic diseases.     

Sensitive Hydrazine Electrochemical Biosensor Based on a Porous Chitosan–Carbon Nanofiber Nanocomposite Modified Electrode

A novel electrochemically-based biosensor was developed for the determination of hydrazine by modifying a glassy carbon electrode with an aqueous dispersion of carboxylic group-functionalized carbon nanofiber/chitosan solution, and then absorbing hemoglobin on the surface of chitosan-carbon nanofibers. Nafion was used to coat the hemoglobin membrane. The interactions of hemoglobin and the nafion/chitosan-carbon nanofibers were investigated by ultraviolet-visible absorption, infrared, and circular dichroism spectroscopies. The results indicated that the native structure of hemoglobin was retained post-immobilization. The circular dichroism results showed that the α-helical structure of hemoglobin was preserved though a small change was observed in the presence of the nafion/chitosan-carbon nanofibers. The modified nanofibers were further characterized by scanning electron microscopy, electron impendence spectroscopy, and cyclic voltammetry. The electrocatalytic mechanism of hemoglobin to the oxidation of hydrazine was investigated and an irreversible diffusion-controlled electrode process was obtained. The electron transfer rate constant (k_s), transfer coefficient (α), and Michaelis–Menten constant (K_m) were also evaluated. The peak current of the catalytic oxidation was linear with hydrazine concentration from 3.722 × 10^?5 to 1.601 × 10^?3 molar with a correlation coefficient of 0.995. The detection limit was estimated to be 2.7 micromoles per liter. The sensitivity, stability, and reproducibility of the nafion/hemoglobin/chitosan-carbon nanofiber/glassy carbon electrode for the oxidation of hydrazine were also investigated.     

Temperature-Responsive Electrocatalysis Based on Poly(N-Isopropylacrylamide)-Modified Graphene Oxide (PNIPAm-GO)

Poly(N-isopropylacrylamide)-modified graphene oxide (PNIPAm-GO), which is a type of thermally responsive GO, was designed and synthesized through a covalent “grafting-from” strategy. The as-prepared modified nanosheets integrated the individual advantages of two components, such as the thermal sensitivity of the PNIPAm terminal as well as the conductivity and the open 2D structure of the GO substrate. PNIPAm-GO was able to perform the reversible regulation of hydrophilicity/hydrophobicity in aqueous solution upon variations in the temperature. Such a unique property might also lead to the utilization of PNIPAm-GO as an intelligent electrode material to achieve a switchable electrochemical response toward a [Fe(CN)₆]^3−/4− probe. The PNIPAm-GO modified glassy carbon electrode (PNIPAm-GO/GC electrode) was able to exhibit better electrochemical performance in an ON/OFF switching effect than the PNIPAm-modified glassy carbon electrode (PNIPAm/GC electrode) without GO owing to the intrinsic properties and large surface area of the introduced GO. Moreover, it was found that the PNIPAm-GO/GC electrode also displayed excellent thermally responsive electrocatalysis toward the detection of 1,4-dihydro-β-nicotinamide adenine dinucleotide (NADH) and dopamine (DA), which resulted in two different catalytic statuses on the same electrode. This kind of switchable catalytic performance of the PNIPAm-GO/GC electrode might greatly enhance the flexibility of its application, and thus it is expected to have wide potential for applications in the fields of biosensors and biocatalysis.     

Organic Profiles and Pattern Recognition Analysis for Clinical Applications

Efficient gas chromatographic (GC) and capillary electrophoretic (CE) profiling methods were combined with simple pattern recognition methods for the correlation between urinary organic profiles (organic acids,polyamines and nucleosides) and uterine cervical cancer.For the urinary organic acid profiles, free organic acids were recovered by dual solid-phase extraction (SPE) in anion-exchange and normal-phase partition modes after methoximation of keto acids in alkalinized urine samples, followed by conversion to tert.-butyldimethylsilyl derivatives for the direct analysis by dual-capillary column GC.     

Electrochemical sandwich immunoassay for the peptide hormone prolactin using an electrode modified with graphene,single walled carbon nanotubes and antibody-coated gold nanoparticles

We describe a new kind of electrochemical immunoassay for the peptide hormone prolactin. A glassy carbon electrode (GCE) was modified with a hybrid material consisting of graphene, single walled carbon nanotubes and gold nanoparticles (AuNPs) in a chitosan (CS) matrix. The graphene and the single wall carbon nanotubes were first placed on the GCE, and the AuNPs were then electrodeposited on the surface by cyclic voltammetry. This structure results in a comparably large surface for immobilization of the capturing antibody (Ab₁). The modified electrode was used in a standard sandwich-type of immunoassay. The secondary antibody (Ab₂) consisted of AuNPs with immobilized Ab₂ and modified with biotinylated DNA as signal tags. Finally, alkaline phosphatase was bound to the biotinylated DNA-AuNPs-Ab₂ conjugate via streptavidin chemistry. The enzyme catalyzes the hydrolysis of the α-naphthyl phosphate to form α-naphthol which is highly electroactive at an operating voltage as low as 180 mV (vs. Ag/AgCl). The resulting immunoassay exhibits high sensitivity, wide linear range (50 to 3200 pg∙mL‾¹), low detection limit (47 pg∙mL‾¹), acceptable selectivity and reproducibility. The assay provides a pragmatic platform for signal amplification and has a great potential for the sensitive determination of antigens other than prolactine.

The immunoassay for prolactin is based on a glassy carbon electrode modified with SWCNTs, graphene and antibody-coated gold nanoparticles, and a secondary antibody conjugated to other gold nanoparticles via a biotinylated DNA linker

     

Immobilization of [Cu(bpy)2]Br2 complex onto a glassy carbon electrode modified with α-SiMo12O40 and single walled carbon nanotubes: Application to nanomolar detection of hydrogen peroxide and bromate

A simple procedure has been used for preparation of modified glassy carbon electrode with carbon nanotubes and copper complex. Copper complex [Cu(bpy)₂]Br₂ was immobilized onto glassy carbon (GC) electrode modified with silicomolybdate, α-SiMo₁₂O₄₀⁴⁻ and single walled carbon nanotubes (SWCNTs)_. Copper complex and silicomolybdate irreversibly and strongly adsorbed onto GC electrode modified with CNTs. Electrostatic interactions between polyoxometalates (POMs) anions and Cu-complex, cations mentioned as an effective method for fabrication of three-dimensional structures. The modified electrode shows three reversible redox couples for polyoxometalate and one redox couple for Cu-complex at wide range of pH values. The electrochemical behavior, stability and electron transfer kinetics of the adsorbed redox couples were investigated using cyclic voltammetry. Due to electrostatic interaction, copper complex immobilized onto GC/CNTs/α-SiMo₁₂O₄₀⁴⁻ electrode shows more stable voltammetric response compared to GC/CNTs/Cu-complex modified electrode. In comparison to GC/CNTs/Cu-complex the GC/CNTs/α-SiMo₁₂O₄₀⁴⁻ modified electrodes shows excellent electrocatalytic activity toward reduction H₂O₂ and BrO₃⁻ at more reduced overpotential. The catalytic rate constants for catalytic reduction hydrogen peroxide and bromate were 4.5(±0.2) × 10³ M⁻¹ s⁻¹ and 3.0(±0.10) × 10³ M⁻¹ s⁻¹, respectively. The hydrodynamic amperommetry technique at 0.08 V was used for detection of nanomolar concentration of hydrogen peroxide and bromate. Detection limit, sensitivity and linear concentration range proposed sensor for bromate and hydrogen peroxide detection were 1.1 nM and 6.7 nA nM⁻¹, 10 nM-20 μM, 1 nM, 5.5 nA nM⁻¹ and 10 nM-18 μM, respectively.     

Electrochemical Characterization of Pyrocatechol Sulfonephthalein-Modified Glassy Carbon Electrode and Separation of Electrocatalytic Responses for Ascorbic Acid and Dopamine Oxidation

A pyrocatechol sulfonephthalein- (PS-) modified glassy carbon (PS/GC) electrode has been prepared by adsorption of PS on a glassy carbon electrode surface. Cyclic voltammograms of the PS/GC electrode indicate the presence of a couple of well-defined redox peaks, and the formal potential shifts in the negative direction with increasing solution pH. The relation between formal potential,E^0′, and solution pH can be fit to the equationE^0′(mV) = −51.4 pH + 538.7. The PS/GC electrode shows high electrocatalytic activity toward ascorbic acid oxidation, with an overpotential ca. 380 mV less than that of the bare electrode and a drastic enhancement of the anodic currents. The electrocatalytic reaction rate constant (k), which was decreased with increasing concentration of H₂A, was determined using rotating disk electrode measurements. The values ofkwas also affected by the solution pH. The electrode can also separate the electrochemical responses of ascorbic acid and dopamine. The separation between the anodic peak potentials of ascorbic acid and dopamine is more than 50 mV by the differential pulse voltammetry.     

Fabrication of Chitosan‐Multiwall Carbon Nanotube Nanocomposite Containing Ferri/Ferrocyanide: Application for Simultaneous Detection of D‐Penicillamine and Tryptophan

We report a simple and effective strategy for fabrication of the nanocomposite containing chitosan (CS) and multiwall carbon nanotube (MWNT) coated on a glassy carbon electrode (GCE). The characterization of the modified electrode (CS‐MWNT/GC) was carried out using scanning electron microscopy (SEM) and UV–vis absorption spectroscopy. The electrochemical behavior of CS‐MWNT/GC electrode was investigated and compared with the electrochemical behavior of chitosan modified GC (CS/GC), multiwalled carbon nanotube modified GC (MWNT/GC) and unmodified GC using cyclic voltammetry (CV) and electron impedance spectroscopy (EIS). The chitosan films are electrochemically inactive; similar background charging currents are observed at bare GC. The chitosan films are permeable to anionic Fe(CN)₆^3?/4? (FC) redox couple. Electrochemical parameters, including apparent diffusion coefficient for the Fe(CN)₆^3?/4? redox probe at FC/CS‐MWNT/GC electrode is comparable to values reported for cast chitosan films. This modified electrode also showed electrocatalytic effect for the simultaneous determination of D‐penicillamine (D‐PA) and tryptophan (Trp). The detection limit of 0.9 μM and 4.0 μM for D‐PA and Trp, respectively, makes this nanocomposite very suitable for determination of them with good sensitivity.     

Electrochemical Oxidation of 8‐Oxo‐2′‐Deoxyguanosine on Glassy Carbon,Gold, Platinum and Tin(IV) Oxide Electrodes

Cyclic voltammetry was used to investigate the oxidation of 8‐oxo‐2′‐deoxyguanosine (8‐oxo‐dG) on the glassy carbon (GC), platinum, gold and SnO₂ electrodes over a range of the sweep rate, 8‐oxo‐dG concentration and the solution pH. Reaction mechanism that is common to all these electrodes involves the two‐electron two‐proton charge transfer step followed by the irreversible chemical reaction(s). Rate of the charge transfer reaction decreases with the increasing solution pH (GC, Pt, Au), and depends on the nature of the electrode material following the sequence GC>Pt, Au>>SnO₂. These effects can be related to the degree of oxidation of the electrode surface (Pt, Au, SnO₂), or to the density of the active surface sites (GC). Any of these electrodes can be used for the fabrication of an amperometric detector for 8‐oxo‐dG .