Electrocatalytic oxidation and determination of norepinephrine in the presence of ascorbic and uric acids at a poly (Evans Blue)—modified glassy carbon electrode

A sensitive and selective electrochemical method for the determination of norepinephrine using a poly (Evans Blue) film-modified glassy carbon electrode was developed. The polymer film-modified electrode shows excellent electrocatalytic activity toward the oxidation of norepinephrine (NE) in phosphate buffer solution (pH 5.0). The linear range of 5.0 × 10⁻⁷–1.8 × 10⁻⁵ M and detection limit of 3.5 × 10⁻⁸ M were observed for the determination of NE in pH 5.0 phosphate buffer solutions. The interference studies showed that the modified electrode had excellent selectivity for the determination of NE in the presence of large excess of ascorbic acid (AA) and uric acid (UA). The differences of the oxidation peak potentials for NE-AA and NE-UA were about 175 and 172 mV, respectively. The resolution is large enough to determine AA, NE and UA individually. This work provides a simple and easy approach to selective detection of NE in the presence of AA and UA in physiological samples. The article is published in the original.     

Electrochemical determination of epinephrine,uric acid and folic acid using a carbon paste electrode modified with novel ferrocene derivative and core–shell magnetic nanoparticles

Research on Chemical Intermediates - The purpose of the current research was to construct an intermediate (2-(4-Ferrocenyl-[1,2,3]triazol-1-yl)-1-(naphthalen-2-yl) ethanone (2FTNE)) and a magnetic...     

Voltammetric determination of uric acid by using gold nanotubule electrode

Gold nanotubule membranes were prepared by using electroless deposition of gold within the pores and surfaces of polycarbonate track-etched membranes.And the gold nanotubule membrane was used as an electrode for determination of uric acid in urine samples for the first time.In Britton-Robinson buffer of pH 4.56,uric acid exhibited well-defined differential pulse voltammograms.And the interference between coexistent ascorbic acid and uric acid was overcome owing to the attractive ability of the gold nanotubule electrode to yield a large anodic peak difference ca.0.404 V(vs.SCE).The proposed method was then applied to the determination of uric acid in urine without any pretreatment.     

Fabrication of poly(orthanilic acid)–multiwalled carbon nanotubes composite film-modified glassy carbon electrode and its use for the simultaneous determination of uric acid and dopamine in the presence of ascorbic acid

A multiwalled carbon nanotubes (MWNT) modified glassy carbon electrode (GCE) coated with poly(orthanilic acid) (PABS) film (PABS–MWNT/GCE) has been fabricated and used for simultaneous determination of dopamine (DA) and uric acid (UA) in the presence of ascorbic acid (AA) by differential pulse voltammetry (DPV). Scanning electron microscopy, Fourier transform infrared spectra, and electrochemical techniques have been used to characterize the surface morphology of the PABS–MWNT composite film and the polymerization of ABS on electrode surface. In comparison with the bare GCE and the MWNT-modified GCE, the PABS–MWNT composite film-modified GCE, which combines the advantages of MWNT and the self-doped PABS, exhibits good selectivity and sensitivity for the simultaneous and selective determination of UA and DA in the presence of AA. Due to the different electrochemical responses of AA, DA, and UA, PABS–MWNT/GCE can resolve the overlapped oxidation peak of DA and UA into two well-defined voltammetric peaks with enhanced current responses using both cyclic voltammetry (CV) and DPV. The peak potential separations between DA and UA are 170 mV using CV and 160 mV using DPV, respectively, which are large enough for the selective and simultaneous determination of these species. In the presence of 0.5 mM AA, the DPV peak currents are linearly dependent on the concentration of UA and DA in the range of 6–55 and 9–48 μM with correlation coefficients of 0.997 and 0.993, respectively. The detection limits (S/N = 3) for detecting UA and DA are 0.44 and 0.21 μM, respectively. The PABS–MWNT/GCE shows good reproducibility and stability and has been used for the simultaneous determination of DA and UA in the presence of AA in samples with satisfactory results.     

Simultaneous determination of nitrophenol isomers at the single-wall carbon nanotube compound conducting polymer film modified electrode

One of the challenging areas of electrochemistry and electroanalytical chemistry is the simultaneous determination of isomers at the same electrode. Con- ventional electrode only possesses a single function of electron transfer; therefore, it is difficult…     

Electrochemical methods for simultaneous determination of trace amounts of dopamine and uric acid using a carbon paste electrode incorporated with multi-wall carbon nanotubes and modified with α-cyclodextrine

In this work, we investigate the electrochemical activity of dopamine (DA) and uric acid (UA) using both a bare and a modified carbon paste electrode as the working electrode, with a platinum wire as the counter electrode and a silver/silver chloride (Ag/AgCl) as the reference electrode. The modified carbon paste electrode consists of multi-walled carbon nanotubes (>95%) treated with α-cyclodextrine, resulting in an electrode that exhibits a significant catalytic effect toward the electro-chemical oxidation of DA in a 0.2-M Britton–Robinson buffer solution (pH 5.0). The peak current increases linearly with the DA concentration within the molar concentration ranges of 2.0 × 10⁻⁶ to 5.0 × 10⁻⁵ M and 5.0 × 10⁻⁵ to 1.9 × 10⁻⁴ M. The detection limit (signal to noise >3) for DA was found to be 1.34 × 10⁻⁷ M, respectively. In this work, voltammetric methods such as cyclic voltammetry, chronoamperometry, chronocuolometry, differential pulse and square wave voltammetry, and linear sweep and hydrodynamic voltammetry were used. Cyclic voltammetry was used to investigate the redox properties of the modified electrode at various scan rates. The diffusion coefficient (D, cm² s⁻¹ = 3.05 × 10⁻⁵) and the kinetic parameters such as the electron transfer coefficient (α = 0.51) and the rate constant (k, cm³ mol⁻¹ s⁻¹ = 1.8 × 10³) for DA were determined using electrochemical approaches. By using differential pulse voltammetry for simultaneous measurements, we obtained two peaks for DA and UA in the same solution, with the peak separation approximately 136 mV. The average recovery was measured at 102.45% for DA injection.     

Platinum nanoparticles–decorated graphene-modified glassy carbon electrode toward the electrochemical determination of ascorbic acid,dopamine, and paracetamol

In the present study, we report the synthesis and characterization of platinum nanoparticles decorated graphene (GPtNPs) nanocomposite toward the electrochemical determination of ascorbic acid (AA), dopamine (DA), and paracetamol (PCT). GPtNPs demonstrated synergistic catalytic activity with enhanced currents in all of the measurements when compared with graphene-modified glassy carbon electrode (G-GCE) and bare GCE. The nanocomposite exhibited low overpotential for AA oxidation and good peak-to-peak separation of 218.0, 218.0, and 436.0 mV for AA–DA, DA–PCT, and AA–PCT, respectively. Cyclic voltammetry (CV) and chronoamperometry (CA) determination of AA, DA, and PCT showed wide linearity ranges. CV determination of AA exhibited linearity range from 300 μM to 20.89 mM and from 22.02 to 39.87 mM. DA determination using CV exhibited linearity range from 5 to 104 μM and from 114 to 684 μM, whereas CA determination of PCT showed a linearity range from 20 μM to 6.43 mM. Differential pulse voltammetry determinations of AA, DA, and PCT exhibited low detection limits of 300, 5, and 5 μM, respectively.     

Carbon nanotubes/alizarin red S–poly(vinylferrocene) modified glassy carbon electrode for selective determination of dopamine in the presence of ascorbic acid

A modified electrode was fabricated by electrochemical formation of poly(vinylferrocene) on the multi-wall carbon nanotube-alizarin red S matrix covered glassy carbon electrode. A higher electrochemical activity was obtained to the electrocatalytic oxidation of dopamine. The electrode surface was characterized electrochemically and spectroscopically. Poly(vinylferrocene) (PVF) in electrode was used as an electron transfer mediator in the electrochemical oxidation of compounds due to its perfect reversible redox properties. Multi-wall carbon nanotubes (MWCNTs) / alizarin red S (ARS)–PVF electrode was used to the determination of dopamine in the presence of ascorbic acid in 0.1 M sulphate buffer solution at pH 7. The performance of the MWCNTs/ARS–PVF electrode was evaluated by DPV and amperometry.     

Electrochemical characterization of 2, 2′-[1, 2-ethanediylbis (nitriloethylidyne)]-bis-hydroquinone-carbon nanotube paste electrode and its application to simultaneous voltammetric determination of ascorbic acid and uric acid

The preparation and electrochemical characterization of a carbon nanotube paste electrode modified with 2,2′-[1,2-ethanediylbis (nitriloethylidyne)]-bis-hydroquinone, referred to as EBNBH, was investigated. The EBNBH carbon nanotube paste electrode (EBNBHCNPE) displayed one pair of reversible peaks at E _pa = 0.18 V and E _pc = 0.115 V vs Ag/AgCl. Half wave potential (E _1/2) and ΔE _p were 0.148 and 0.065 V vs Ag/AgCl, respectively. The electrocatalytic oxidation of ascorbic acid (AA) has been studied on EBNBHCNPE, using cyclic voltammetry, differential pulse voltammetry and chronoamperometry techniques. It has been shown that the oxidation of AA occurs at a potential where oxidation is not observed at the unmodified carbon paste electrode. The heterogeneous rate constant for oxidation of AA at the EBNBHCNPE was also determined and found to be about 1.07 × 10⁻³ cm s⁻¹. The diffusion coefficient of AA was also estimated as 5.66 × 10⁻⁶ cm² s⁻¹ for the experimental conditions, using chronoamperometry. Also, this modified electrode presented the property of electrocatalysing the oxidation of AA and uric acid (UA) at 0.18 and 0.35 V vs Ag/AgCl, respectively. The separations of anodic peak potentials of AA and UA reached 0.17 V. Using differential pulse voltammetry, the calibration curves for AA and UA were obtained over the range of 0.1–800 μM and 20–700 μM, respectively. With good selectivity and sensitivity, the present method provides a simple method for selective detection of AA and UA in biological samples.     

Potentiometric determination of ascorbic acid in water–acetonitrile solution using pyrite and chalcopyrite electrodes

Journal of Solid State Electrochemistry - Performance characteristics of the solid-state potentiometric sensors, based on natural sulfide minerals pyrite and chalcopyrite, are assayed in aqueous...     

Non-enzymatic sensing of uric acid using a carbon nanotube ionic-liquid paste electrode modified with poly(β-cyclodextrin)

We describe a nonenzymatic electrochemical sensor for uric acid. It is based on a carbon nanotube ionic-liquid paste electrode modified with poly(β-cyclodextrin) that was prepared in-situ by electropolymerization. The functionalized multi-walled carbon nanotubes and the surface morphology of the modified electrodes were characterized by transmission electronic microscopy and scanning electron microscopy. The electrochemical response of uric acid was studied by cyclic voltammetry and linear sweep voltammetry. The effects of scan rate, pH value, electropolymerization cycles and accumulation time were also studied. Under optimized experimental conditions and at a working voltage of 500 mV vs. Ag/AgCl (3 M KCl), response to uric acid is linear in the 0.6 to 400 μΜ and in the 0.4 to 1 mΜ concentration ranges, and the detection limit is 0.3 μΜ (at an S/N of 3). The electrode was successfully applied to the detection of uric acid in (spiked) human urine samples.

SEM images of (a) carbon ionic liquid electrode (CILE) (b) MWNT-CILE (c) β-CD/CILE (d) β-CD/ MWNT-CILE. The surfaces of carbon ionic liquid electrode (CILE) (a) and MWNT-CILE (b) were homogenous and no separated carbon layers can be observed; After β- cyclodextrin (CD) was modified on CILE and MWNT-CILE, the surfaces of β-CD modified electrodes (c and d) exhibited loose and porous morphologies.

     

Application of a novel 1,1′-dimethylferricinimum dye for the determination of uric acid in urine

Water-soluble 2-hydroxypropyl-β-cyclodextrin (Hp-β-CyD), a cyclic and nonreducing oligosaccharide was used to enclose a hydrophobic guest molecule 1,1′-dimethylferrocene (DMF) to form a water-soluble yellow complex. At high concentrations (300 mM), Hp-β-CyD enclosed up to 100 mM DMF. The yellow complex was electrochemically oxidized (platinum vs Ag/AgCl poised at +450 mV) to form a blue dye, 1,1′-dimethylferricinium (DMF⁺). This is a one-electron transfer process and the ferricinium cation formed exhibited an absorption peak at 650 nm. The concentrated DMF⁺ was stable for at least 4 mo at 4°C and insensitive to a wide pH variation (pH 2–11). Application of the novel DMF⁺ complex as a colorimetric dye for the determination of uric acid in urine was successfully demonstrated. The reaction between the dye and uric acid is almost instantaneous and decrease in absorbance caused by the reduction of 1,1′-dimethylferricinium to 1,1′-dimethylferrocene can be followed at 650 nm. The results obtained agreed well with those of the reference reversed-phase HPLC method.     

Attapulgite with poly(methylene blue) composite film–Electrocatalytic determination of ascorbic acid

A conductive composite film consisted of natural nanostructure attapulgite (AT) with poly(methylene blue) (PMB) was constructed on glassy carbon (GC) electrode. The electrode exhibited an effective electrocatalytic activity towards the oxidation of ascorbic acid (AA) and well-defined oxidation peaks were observed in 0.1 M phosphate buffer solutions (PBS, pH 7.0) via cyclic voltammetry. Linear calibration plot was obtained over the range of 1.0 × 10⁻⁵ to 5.0 × 10⁻² M for ascorbic acid with the detection limit value of 1.0 × 10⁻⁶ M. The main interfering factor in biological samples was experimentally excluded. In addition, UV–Vis spectra were applied to reveal the formation of the nanocomposite film of PMB-AT.     

Study of photoelectrocatalytic degradation behavior of p-nitrophenol with nano-TiO2 modified film at a rotating ring–disk electrode

The photoelectrocatalytic degradation behavior of p-nitrophenol (pNP) was investigated by using hydrodynamic differential pulse voltammetry (HDPV) technique. The method was applied on a nano-TiO₂ modified platinum rotating ring–disk electrode (RRDE) as versatile working electrode. The voltammetric response of the intermediate product was recorded instantly at the rotating platinum-ring electrode under hydrodynamic conditions via compulsive transport during the photoelectrocatalytic degradation process of pNP. A distinct anodic peak at about 0.55 V is mainly attributed to the result of the formation of electroactive intermediate product, namely hydroquinone (HQ), through the direct reaction between photo-generated powerful oxidant (hydroxyl radicals, OH) and pNP. The present work has demonstrated that HDPV can be effectively used to in situ monitor the formation of intermediate product at nano-TiO₂ modified RRDE, providing a promising approach to investigate the photoelectrocatalytic degradation mechanism of organic pollutants like toxic nitrophenols.     

Automated determination of the pK a values of 4-hydroxybenzoic acid in cosolvent–water mixtures and related solvent effects using a modified HPLC system

An automated spectrophotometric method based on an HPLC system with a diode array detector was used to determine the pK _a values of compounds with low water solubility in a universal buffer containing acetonitrile as cosolvent. The column of the system was replaced with a capillary connecting the injection system and the diode array detector. Specific solvent effects were corrected for using the dielectric constants of the mixed solvent and pure water. The method was tested using 4-hydroxybenzoic acid and the results were compared with those obtained with a spectrophotometer. Linear regression lines with different slopes were obtained from spectrophotometric measurements of different cosolvent–water mixtures. These effects were shown to depend upon the polarity of the solventwater mixture, and they were explained by the solvatochromic behavior of the 4-hydroxybenzoic acid in the solvent–water mixture.     

Electrochemical determination of acetaminophen at a carbon electrode modified in the presence of β-cyclodextrin: role of the activated glassy carbon and the electropolymerised β-cyclodextrin

Acetaminophen is a well-known drug commonly used to provide pain relief, but it can also lead to acute liver failure at high concentrations. Therefore, there is considerable interest in monitoring its concentrations. Sensitive and selective acetaminophen electrochemical sensors were designed by cycling a glassy carbon electrode (GCE) to high potentials in the presence of β-CD in a phosphate electrolyte, or by simply activating the GCE electrode in the phosphate solution. Using cyclic voltammetry, adsorption-like voltammograms were recorded. The acetaminophen oxidation product, N-acetyl benzoquinone imine, was protected from hydrolysis, and this was attributed to the adsorption of acetaminophen at the modified GCE. The rate constants for the oxidation of acetaminophen were estimated as 4.3 × 10^–3 cm² s^–1 and 3.4 × 10^–3 cm² s^–1 for the β-CD-modified and -activated electrodes, respectively. Using differential pulse voltammetry, the limit of detection was calculated as 9.7 × 10^–8 M with a linear concentration range extending from 0.1 to 80 μM. Furthermore, good selectivity was achieved in the presence of caffeine, ascorbic acid and aspirin, enabling the determination of acetaminophen in a commercial tablet. Similar electrochemical data were obtained for both the β-CD-modified and activated GCE surfaces, suggesting that the enhanced detection of acetaminophen is connected mainly to the activation and oxidation of the GCE. Using SEM, EDX and FTIR, no evidence was obtained to indicate that the β-CD was electropolymerised at the GCE.

     

Sensitive electrochemical determination of calcium dobesilate on the carbon–iron nanoparticle modified glassy carbon electrode

A carbon–iron nanoparticle modified glassy carbon electrode (CIN-GCE) has been developed for the determination of calcium dobesilate (CD) in pharmaceutical formulations. The CINs were characterized by Transmission electron microscopy and X-ray diffraction. It was found that the CIN has strong electrocatalytic effect for CD and leads to a greatly improved anodic detection of CD including higher sensitivity and better reproducibility. A detection limit of 2.0 × 10⁻⁷ M (S/N = 3) was obtained. The proposed CIN-GCE was applied to detect CD in pharmaceutical formulations with satisfactory results. The proposed CIN electrochemical sensing platform holds great promise for simple, rapid and accurate detection of CD in pharmaceutical formulations.     

Simultaneous determination of dl-lactic acid and dl-3-hydroxybutyric acid enantiomers in saliva of diabetes mellitus patients by high-throughput LC–ESI-MS/MS

A simultaneous determination method for the enantiomers of chiral carboxylic acids by the combination of ultraperformance liquid chromatography and mass spectrometry (UPLC-MS/MS) has been developed. (S)(+)-1-(2-Pyrrolidinylmethyl)-pyrrolidine (S-PMP) was used as the derivatization reagent for the high-throughput determination of biological chiral carboxylic acids, i.e., lactic acid (LA) and 3-hydroxybutyric acid (HA). The S-PMP efficiently reacted with the carboxylic acids under mild conditions at room temperature in the presence of 2,2'-dipyridyl disulfide and triphenylphosphine. The resulting S-PMP derivatives were highly responsive in the electrospray ionization (ESI)-MS operating in the positive-ion mode and gave characteristic product ions during the MS/MS, which enabled the sensitive detection using selected reaction monitoring. The derivatization was effective for the enantiomeric separation of the chiral carboxylic acids, and the resolution values of DL-LA and DL-HA were 4.91 and 9.37, respectively. Furthermore, a rapid separation of the derivatives of DL-LA and DL-HA within 7?min was performed using the UPLC system. The limits of detection on the column were in the low femtogram range (5-12?fg). The proposed procedure was successfully applied for the determination of the D- and L-isomers of LA and HA in the saliva of diabetes mellitus (DM) patients and healthy volunteers. The D-LA in DM patients was clearly higher than that in normal subjects. The derivatization followed by UPLC-ESI-MS/MS enabled the enantiomeric separation and detection of trace amounts of LA and HA in human saliva with a simple pretreatment and small sample volume.