Electrochemical Behavior and Determination of Uric Acid at Single-Walled Carbon Nanotube Modified Gold Electrodes

The voltammetric behavior of uric acid was studied at a single-walled carbon nanotube (SWNT) modified gold electrode. Uric acid can effectively accumulate at this electrode and produce an anodic peak at about 0.45 V (vs. SCE) in pH 5.0 sodium acetate buffer solutions (HAc-NaAc). The experimental parameters, such as solution pH, accumulation time, and amount of SWNT, were optimized for determination. Under the optimum conditions, the anodic peak current is linear to the uric acid concentration over the range of 1.0×10⁻⁷ M to 2.5×10⁻⁵ M with a correlation coefficient of 0.998. The detection limit was 5.0×10⁻⁸ M for 60 s accumulation. The electrode could be easily regenerated and exhibited good stability. A 5.0×10⁻⁶ M uric acid solution was measured ten times using the same electrode, and the relative standard deviation of the peak current was 1.3%. This method was successfully applied to the determination of uric acid in human urine samples, and the recovery was 97–99%. The feasibility for simultaneous determination of xanthine, ascorbic acid and uric acid was discussed. These species did not interfere with each other in a certain concentration range. The influence of some surfactants on the anodic peak was also examined.     

Electroanalytical properties of cytochrome c by direct electrochemistry on multi-walled carbon nanotubes incorporated with DNA biocomposite film

A novel conductive biocomposite film (MWCNTs–DNA–cyt c) which contains multi-walled carbon nanotubes (MWCNTs) along with the incorporation of DNA and cytochrome c (cyt c) has been synthesized on glassy carbon electrode (GCE), gold (Au), indium tin oxide (ITO) and screen printed carbon electrode (SPCE) by potentiostatic methods. The presence of both MWCNTs and DNA in the biocomposite film enhances the surface coverage concentration (Γ), increases the electron transfer rate constant (K_s) up to 21% and decreases the degradation of cyt c during the cycling. The biocomposite film also exhibits a promising enhanced electrocatalytic activity towards the reduction of halogen oxyanions and oxidation of biochemical compounds such as ascorbic acid and l-cysteine. The cyclic voltammetry has been used for the measurement of electroanalytical properties of analytes by means of biocomposite film modified GCEs. The sensitivity of MWCNTs–DNA–cyt c modified GCE possess higher values than the values obtained for DNA–cyt c film modified GCE. Further, the reduction potentials of halogen oxyanions E_pc, clearly shows that the activity of the biocomposite is dependent on the electronegativity of halogen oxyanions. Electrochemical quartz crystal microbalance studies revealed the enhancements in the functional properties of MWCNTs, DNA and cyt c. We have studied the surface morphology of the biocomposite films using scanning electron microscopy and atomic force microscopy, which revealed that DNA and cyt c have been incorporated on MWCNTs. Finally, the flow injection analysis has been used for the amperometric detection of analytes at MWCNTs–DNA–cyt c film modified SPCE.     

Electrocatalytic Response of a Glassy-Carbon Electrode Modified with a Polyvinylpyridine Film with Electrodeposited Palladium in the Oxidation of Oxalic Acid

The electrochemical properties of a glassy-carbon electrode modified with a polyvinylpyridine film with electrodeposited palladium were studied. Conditions were selected for preparing a composite film on a glassy-carbon surface. It was found that palladium particles deposited on the polyvinylpyridine film exhibited electrocatalytic activity in the oxidation of H₂C₂O₄. Compared to an unmodified electrode, the oxidation potential of oxalic acid decreased and the current of its oxidation multiply increased. The catalytic current of oxalic acid oxidation was a linear function of its concentration in the range from 1 × 10^–2 to 1 × 10^–6 M.     

SnO2@MWCNT nanocomposite as a high capacity anode material for sodium-ion batteries

We report the synthesis and characterization of SnO₂@multiwalled carbon nanotubes (MWCNTs) nanocomposite as a high capacity anode material for sodium-ion battery. SnO₂@MWCNT nanocomposite was synthesized by a solvothermal method. SEM and TEM analyses show the uniform distribution of SnO₂ nanoparticles on carbon nanotubes. When applied as anode materials in Na-ion batteries, SnO₂@MWCNT nanocomposite exhibited a high sodium storage capacity of 839 mAh g^− 1 in the first cycle. SnO₂@MWCNT nanocomposite also demonstrated much better cycling performance than that of bare SnO₂ nanoparticles and bare MWCNTs. Furthermore, the nanocomposite electrode also showed a good cyclability and an enhanced Coulombic efficiency on cycling.     

Electrocatalytic applications of a sol–gel derived cobalt phthalocyanine–dispersed carbon–ceramic electrode

The preparation and characterization of a surface renewable carbon–ceramic electrode, SiO₂/SnO₂/C-graphite/(SiPy⁺)₄CoPcTs⁻⁴, is reported. Cobalt(II) tetrasulfonated phthalocyanine (CoPcTs⁻⁴) absorbed on a 3-n-propylpyridinium chloride silsesquioxane polymer was dispersed in a stannic-silica C-graphite sol–gel matrix. The performance of SiO₂/SnO₂/C-graphite/(SiPy⁺)₄CoPcTs⁻⁴ as electrode material was investigated by cyclic voltammetry in the electrocatalytic oxidation of oxalic acid and nitrite. The modified carbon–ceramic material was characterized by X-ray fluorescence spectroscopy, BET specific surface area, thermogravimetric analysis, X-ray photoelectron spectroscopy, and scanning electron microscopy.     

Photoelectrochemical behavior of coupled SnO2|CdSe nanocrystalline semiconductor films

A photoelectrochemical cell with a coupled SnO₂|CdSe nanocrystalline semiconductor electrode has been prepared by sequential deposition of SnO₂ and CdSe films onto an optically transparent electrode (OTE), and its photoelectrochemical behavior has been studied. The results show that the coupling of CdSe with SnO₂ leads to an improvement in the performance of OTE|SnO₂|CdSe over OTE|CdSe cells in terms of increased incident photon-to-current conversion efficiency, increased stability and smaller reversal of current. The favorable positioning of the energy bands of SnO₂ and CdSe is responsible for the above observations. Various photoelectrochemical parameters of the OTE|SnO₂|CdSe cell obtained for an incident light power of 0.31 mW cm⁻² at 470nm, are as follows: I_sc ≈ 25–30 μA cm⁻², V_oc ≈ 0.5–0.6 V, ƒƒ = 0.47 and a power conversion efficiency of about 2.25%.     

A novel nonenzymatic hydrogen peroxide sensor based on multi-wall carbon nanotube/silver nanoparticle nanohybrids modified gold electrode

A novel strategy to fabricate hydrogen peroxide (H₂O₂) sensor was developed based on multi-wall carbon nanotube/silver nanoparticle nanohybrids (MWCNT/Ag nanohybrids) modified gold electrode. The process to synthesize MWCNT/Ag nanohybrids was facile and efficient. In the presence of carboxyl groups functionalized multi-wall carbon nanotubes (MWCNTs), silver nanoparticles (Ag NPs) were in situ generated from AgNO₃ aqueous solution and readily attached to the MWCNTs convex surfaces at room temperature, without any additional reducing reagent or irradiation treatment. The formation of MWCNT/Ag nanohybrids product was observed by transmission electron microscope (TEM), and the electrochemical properties of MWCNT/Ag nanohybrids modified gold electrode were characterized by electrochemical measurements. The results showed that this sensor had a favorable catalytic ability for the reduction of H₂O₂. The resulted sensor could detect H₂O₂ in a linear range of 0.05-17 mM with a detection limit of 5 × 10⁻⁷ M at a signal-to-noise ratio of 3. The sensitivity was calculated as 1.42 μA/mM at a potential of −0.2 V. Additionally, it exhibited good reproducibility, long-term stability and negligible interference of ascorbic acid (AA), uric acid (UA), and acetaminophen (AP).     

A copper ion-selective electrode with high selectivity prepared by sol-gel and coated wire techniques

A sol-gel electrode and a coated wire ion-selective poly(vinyl chloride) membrane, based on thiosemicarbazone as a neutral carrier, were successfully developed for the detection of Cu (II) in aqueous solutions. The sol-gel electrode and coated electrode exhibited linear response with Nernstian slopes of 29.2 and 28.1 mV per decade respectively, within the copper ion concentration ranges 1.0×10^–5–1.0×10^–1 M and 6.0×10^–6–1.0×10^–1 M for coated and sol-gel sensors. The coated and sol-gel electrodes show detection limits of 3.0×10^–6 and 6.0×10^–6 M respectively. The electrodes exhibited good selectivities for a number of alkali, alkaline earth, transition and heavy metal ions. The proposed electrodes have response times ranging from 10–50 s to achieve a 95% steady potential for Cu²⁺ concentration. The electrodes are suitable for use in aqueous solutions over a wide pH range (4–7.5). Applications of these electrodes for the determination of copper in real samples, and as an indicator electrode for potentiometric titration of Cu²⁺ ion using EDTA, are reported. The lifetimes of the electrodes were tested over a period of six months to investigate their stability. No significant change in the performance of the sol-gel electrode was observed over this period, but after two months the coated wire copper-selective electrode exhibited a gradual decrease in the slope. The selectivity of the sol-gel electrode was found to be better than that of the coated wire copper-selective electrode. Based on these results, a novel sol-gel copper-selective electrode is proposed for the determination of copper, and applied to real sample assays.     

Flow injection analysis of gallic acid with inhibited electrochemiluminescence detection

A flow injection (FI)–electrochemiluminescent (ECL) method has been developed for the determination of gallic acid, based on an inhibition effect on the Ru(bpy)₃²⁺/tri-n-propylamine (TPrA) ECL system in pH 8.0 phosphate buffer solution. The method is simple and convenient with a determination limit of 9.0×10^–9 mol/L and a dynamic concentration range of 2×10^–8–2×10^–5 mol/L. The relative standard deviation (RSD) was 1.0% for 1.0×10^–6 mol/L gallic acid (n=11). It was successfully applied to the determination of gallic acid in Chinese proprietary medicine—Jianming Yanhou Pian. The inhibition mechanism proposed for the quenching effect of the gallic acid on the Ru(bpy)₃²⁺/TPrA ECL system was the interaction of electrogenerated Ru(bpy)₃^2+* and o-benzoquinone derivative at the electrode surface. The ECL emission spectra and UV-visible absorption spectra were applied to confirm the mechanism.     

Investigation on properties of V<Subscript>2</Subscript>O<Subscript>5</Subscript>–MWCNTs composites as cathode materials

An effective method to prepare the composites of multi-wall carbon nanotubes (MWCNTs) and vanadium pentoxide (V₂O₅) was presented. Vanadyl-triisopropoxide (VO(OC₃H₇)₃) was used as the starting material, MWCNTs pretreated with acids by a two-step process was used as the conductive ingredient. V₂O₅–MWCNTs composites were synthesized via a sol–gel method with solvent exchange and an ambient pressure drying technique. The samples were characterized by SEM, TEM, XRD, BET, Raman spectra and electrical resistivity measurement respectively. The experimental results indicate that the V₂O₅–MWCNTs nanocomposite has a fiber-like and tri-dimensional network structure. Its surface area is up to 189.7 m²/g when the MWCNTs’ content is 15 wt%. And MWCNTs are dispersed homogeneously in the composites. The electrical resistivity of the composites decreases from 1,239 to 765 Ω·cm when the MWCNTs’ content increases from 0 to 10 wt%. Thus MWCNTs can improve properties of V₂O₅ aerogels as the cathode material in lithium batteries.     

Potentiometric study using chemically modified silica gel with pyridinium as membrane for ClO 4 − ions

A potentiometric sensor for the perchlorate anion was developed by mixing chemically modified silicagel with pyridinium perchlorate, with an epoxy polymer and graphite. The electrode showed Nernstian response between 1.0 × 10^–2 and 1.0 × 10^–3 M perchlorate concentrations. The electrode showed high selectivity to this ion at solutions pH between 5.5 and 8.0. The presence of IO ₄ ^– , NO ₃ ^– ,Br^–, IO ₃ ^– , Cl^– and SO ₄ ^2– ions in the solutions, had only small interference in the electrode response in the range mentioned.     

Platinum Nanoparticles Decorated Multiwalled Carbon Nanotubes – Ionic Liquid Composite Film Coated Glassy Carbon Electrodes for Sensitive Determination of Theophylline

Platinum nanoparticles (Pt_nano) decorated multiwalled carbon nanotubes (MWCNTs)–1‐octyl‐3‐methylimidazolium hexafluorophosphate ([omim][PF₆]) composite material (MWCNTs‐Pt_nano‐[omim][PF₆]) was fabricated and characterized for the first time. In the presence of [omim][PF₆], more Pt_nano could deposit on MWCNTs. The average diameter of the deposited Pt_nano was about 5 nm. The composite material film coated glassy carbon electrode (GCE) exhibited sensitive voltammetric response to theophylline (TP). Under the optimized conditions (i.e., preconcentration for 2 minutes on open circuit in 0.10 M pH 3.0 phosphate buffer), the anodic peak current of TP at about 1.1 V (vs. SCE) was linear to TP concentration over the range of 1.0×10^?8–1.0×10^?5 M. The detection limit was estimated to be 8.0×10^?9 M. The modified electrode was successfully applied to the determination of TP in medicine tablet and green tea. In addition, the voltammetric responses of hypoxanthine (HX), xanthine (Xan) and uric acid (UA) on the MWCNTs‐Pt_nano‐[omim][PF₆]/GCE were also discussed.     

Changes in Pt(111) Two-dimensional Long-range Order Induced by Slow Mercury Adsorption and Alloying

A long-term cyclic voltammetry study of Pt(111) electrode in dilute solutions of mercury sulfate (5 × 10^–8–5 × 10^–7 M Hg₂SO₄ + 0.5 M H₂SO₄) has shown that a slow transformation of Pt(111) surface takes place. This transformation leads to a decrease in the bi-dimensional long-range order of the surface. The interpretation of the process involves the increase in mobility of Pt atoms and surface alloying in the presence of mercury. Similar processes of Pt(111) surface disordering take place in acid solution of copper sulfate with the addition of Hg₂SO₄. The penetration of Hg atoms beneath the Pt(111) topmost layer proceeds when only a fraction of the mercury monolayer is deposited on the electrode surface.     

Determination of Selenium(IV) by Stripping Voltammetry at a Mercury-Film Electrode

A procedure was proposed for the determination of selenium(IV) by stripping voltammetry on a mercury-film electrode at an electrolysis potential of +0.4 V versus the saturated silver–silver chloride reference electrode in a 1 M H₂SO₄ solution. The current of the cathodic peak is a linear function of the selenium(IV) concentration in the range from 5 × 10^–3 to 3 × 10^–1 mg/L (6.3 × 10^–8 to 3.8 × 10^–6 M) at a time of electrolysis of 30 s (t _el). The detection limit for selenium is 1 × 10^–4 mg/L (1.3 × 10^–9 M) at t _el = 300 s. It was shown that selenium(IV) can be determined in the presence of 10 mg/L Zn(II), 1 mg/L Cd(II), 0.5 mg/L Pb(II), and 0.2 mg/L Cu(II). A procedure for the determination of selenium in natural, mineral, and potable water was proposed.     

Cobalt-Selective Electrode Based on Titanium Ditelluride Intercalated with Cobalt

A cobalt-selective solid-contact electrode was prepared on the basis of titanium ditelluride. The electrode exhibits the slope of the electrode function –(29 ± 1) mV/pC and provides the determination of 1 × 10^–5–1 × 10^–1 M cobalt(II) in the pH range 4.5–6.5. The electrode was used for the potentiometric indication of the titration end point in the determination of cobalt(II) in lanthanum–strontium cobaltite La_0.5Sr_0.5CoO₃. The intercalant concentration range providing optimal ion-selective properties was determined.     

Electrochemical characteristics of a platinum electrode modified with a matrix of polyvinyl butyral and colloidal Ag containing immobilized horseradish peroxidase

A new hydrogen peroxide biosensor was constructed, which consisted of a platinum electrode modified by a matrix of polyvinyl butyral (PVB) and nanometer-sized Ag colloid containing immobilized horseradish peroxidase (HRP), and using Co(bpy)₃³⁺ as mediator in the hydrogen peroxide solution. The electrochemical characteristics of the biosensor were studied by cyclic voltammetry and chronoamperometry. The modified process was characterized by electrochemical impedance spectroscopy and cyclic voltammetry. The HRP immobilized on colloidal Ag was stable and retained its biological activity. The sensor displays excellent electrocatalytic response to the reduction of H₂O₂. Analytical parameters such as pH and temperature were also studied. Linear calibration for H₂O₂ was obtained in the range of 1×10^–5 to 1×10^–2 M under optimized conditions. The sensor was highly sensitive to H₂O₂, with a detection limit of 2×10^–6 M, and the sensor achieved 95% of steady-state current within 10 s. The sensor exhibited high sensitivity, selectivity and stability.     

Electrochemical behavior of a covalently modified glassy carbon electrode with aspartic acid and its use for voltammetric differentiation of dopamine and ascorbic acid

Aspartic acid was covalently grafted on to a glassy carbon electrode (GCE) by amine cation radical formation in the electrooxidation of the amino-containing compound. X-ray photoelectron spectroscopic (XPS) measurement and cyclic voltammetric experiments proved the aspartic acid was immobilized as a monolayer on the GCE. Electron transfer to Fe(CN)₆^4– in solution of different pH was studied by cyclic voltammetry. Changes in solution pH resulted in the variation of the charge state of the terminal group; surface pK_a values were estimated on the basis of these results. Because of electrostatic interactions between the negatively charged groups on the electrode surface and dopamine (DA) and ascorbic acid (AA), the modified electrode was used for electrochemical differentiation between DA and AA. The peak current for DA at the modified electrode was greatly enhanced and that for AA was significantly reduced, which enabled determination of DA in the presence of AA. The differential pulse voltammetric (DPV) peak current was linearly dependent on DA concentration over the range 1.8×10^–6–4.6×10^–4 mol L^–1 with slope (nA mol^–1 L) and intercept (nA) of 47.6 and 49.2, respectively. The detection limit (3) was 1.2×10^–6 mol L^–1. The high selectivity and sensitivity for dopamine was attributed to charge discrimination and analyte accumulation. The modified electrode has been used for determination of DA in samples, in the presence of AA, with satisfactory results.     

Activity coefficients of aqueous perchloric acid

Mean ionic activity coefficients for aqueous perchloric acid at 25°C were obtained from the emf of a HClO₄ concentration cell (m=1×10^–3–1.5) containing a ClO ₄ ^– ion solution electrode and a glass electrode. The results of an application of the Pitzer equations are compared with those previously reported from several different measuring methods. Perchloric acid activity coefficients have been calculated for the mixture HClO₄–NaClO₄. In both cases, good agreement has been obtained between our results and those derived from the isopiestic method.Deceased November 15, 1984.     

A highly sensitive PVC membrane iodide electrode based on complexes of mercury(II) as neutral carrier

A novel solvent polymeric membrane electrode based on bis(1,3,4-thiadiazole) complexes of Hg(II) is described which has excellent selectivity and sensitivity toward iodide ion. The electrode, containing 1,4-bis(5-methyl-1,3,4-thiadiazole-2-yl-thio)butanemercury(II) [Hg(II)BMTB(NO₃)₄], has a Nernstian potentiometric response from 2.0×10^–8 to 2.0×10^–2 mol L^–1 with a detection limit of 8.0×10^–9 mol L^–1 and a slope of –59.0±0.5 mV/decade in 0.01 mol L^–1 phosphate buffer solution (pH 3.0, 20°C). The selectivity sequence observed is iodide>bromide>thiocyanate>nitrite>nitrate>chloride>perchlorate>acetate>sulfate. The selectivity behavior is discussed in terms of the UV–Vis spectrum, and the process of transfer of iodide across the membrane interface is investigated by use of the AC impedance technique. The electrode was successfully applied to the determination of iodide in Jialing River and Spring in Jinyun Mountains, with satisfactory results.     

Fabrication of a novel Li<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MoO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> film modified electrode and its application as an electrochemical sensor of iodate

A novel organic gel film modified electrode was simply and conveniently fabricated by casting Li_xMoO_y and polypropylene carbonate (PPC) onto the surface of a gold electrode. The cyclic voltammetry and amperometry studies demonstrated that the Li_xMoO_y film modified electrode has a high stability and a good electrocatalytic activity for the reduction of iodate. In amperometry, a good linear relationship between the steady current and the concentration of iodate was obtained in the range from 3×10^–7 to 1×10^–4 mol L^–1 with a correlation coefficient of 0.9997 and a detection limit of 1×10^–7 mol L^–1.