Shengshui Hu

Simultaneous determination of dopamine and serotonin on a glassy carbon electrode coated with a film of carbon nanotubes

A chemically modified electrode based on the carbon nanotube film-coated glassy carbon electrode (GCE) is described for the simultaneous determination of dopamine (DA) and serotonin (5-HT). The multiwall carbon nanotube (MWNT) film-coated GCE exhibits a marked enhancement effect on the current response of DA and 5-HT and lowers oxidation overpotentials. The responses of DA and 5-HT merge into a large peak at a bare GCE, but they yield two well-defined oxidation peaks at the MWNT film-coated GCE. The experimental parameters were optimized, and a direct electrochemical method for the simultaneous determination of DA and 5-HT was proposed. The interference of ascorbic acid (AA) was investigated, and the results showed that a large excess of AA did not interfere with the voltammetric responses of DA and 5-HT. The modified electrode has been successfully applied for the assay of 5-HT and DA in human blood serum.

Mercury-free simultaneous determination of cadmium and lead at a glassy carbon electrode modified with multi-wall carbon nanotubes

Abstract A multi-wall carbon nanotube (MWNT) modified glassy carbon electrode (GCE) was described for the simultaneous determination of trace levels of cadmium and lead by anodic stripping voltammetry (ASV). In pH 4.5 NaAc–HAc buffer containing 0.02xa0mol/l KI, Cd2+ and Pb2+ first adsorb onto the surface of a MWNT film coated GCE and then reduce at −1.20xa0V. During the positive potential sweep, reduced cadmium and lead were oxidized, and two well-defined stripping peaks appeared at −0.88 and −0.62xa0V. Compared with a bare GCE, a MWNT film coated GCE greatly improves the sensitivity of determining cadmium and lead. Low concentration of I− significantly enhances the stripping peak currents since it induces Cd2+ and Pb2+ to adsorb at the electrode surface. The striping peak currents change linearly with the concentration of Cd2+ from 2.5×10−8 to 1×10−5xa0mol/l and with that of Pb2+ from 2×10−8 to 1×10−5xa0mol/l. The lowest detectable concentrations of Cd2+ and Pb2+ are estimated to be 6×10−9 and 4×10−9xa0mol/l, respectively. The high sensitivity, selectivity, and stability of this MWNT film coated electrode demonstrated its practical application for a simple, rapid and economical determination of trace levels of Cd2+ and Pb2+ in water samples.

Voltammetric determination of 4-nitrophenol at a sodium montmorillonite-anthraquinone chemically modified glassy carbon electrode.

A new method for the determination of 4-nitrophenol(4-NP) by differential pulse voltammetry (DPV) based on adsorptive stripping technique was described. Cyclic voltammetry (CV) and linear scan voltammetry (LSV) were used in a comparative investigation into the electrochemical reduction of 4-NP at a Na-montmorillonite(SWy-2) and anthraquione (AQ) modified glassy carbon electrode. With this chemically modified electrode, 4-NP was first irreversibly reduced from phiNO(2) to phiNHOH at -0.78 V. A couple of well-defined redox peaks at +0.22 V (vs. SCE) were responsible for a two-electron redox peak between phiNHOH and phiNO. Studies on the effect of pH on the peak height and peak potential were carried out over the pH range 2.0-9.0 with the phosphate buffer solution. A pH of 3.4 was chosen as the optimum pH. The other experimental parameters, such as film thickness, accumulation time and potential etc. were optimized. Anodic peak currents were found to be linearly related to concentration of 4-NP over the range 0.3-45 mg l(-1), with a detection limit of 0.02 mg l(-1). The interference of organic and inorganic species on the voltammetric response have been studied. This modified electrode can be used to the determination of 4-NP in water samples.

Simultaneous determination of lead(II) and cadmium(II) at a diacetyldioxime modified carbon paste electrode by differential pulse stripping voltammetry

A simple and effective chemically modified carbon paste electrode (CMCPE) for the simultaneous determination of lead(II) and cadmium(II) was developed in this work. The electrode was prepared by the addition of diacetyldioxime into a carbon paste mixture. Pb(2+) and Cd(2+) were preconcentrated on the surface of the modified electrode by complexing with diacetyldioxime and reduced at a negative potential (-1.10 V). Then the reduced products were oxidized by differential pulse stripping. The fact that two stripping peaks appeared on the voltammograms at the potentials of -0.65 V (Cd(2+)) and -0.91 V (Pb(2+)) demonstrates the possibility of simultaneous determination of Pb(2+) and Cd(2+). Under the optimized working conditions, calibration graphs were linear in the concentration ranges of 1.0x10(-7)-1.5x10(-5) mol l(-1) (Pb(2+)) and 2.5x10(-7)-2.5x10(-5) mol l(-1) (Cd(2+)), respectively. For 5 min preconcentration, detection limits of 1x10(-8) mol l(-1) (Pb(2+)) and 4x10(-8) mol l(-1) (Cd(2+)) were obtained at the signal noise ratio (SNR) of 3. To evaluate the reproducibility of the newly developed electrode, the measurements of 5x10(-7) mol l(-1) Pb(2+) and Cd(2+) were parallel carried out for six times at different electrodes and the relative standard deviations were 2.9% (Pb(2+)) and 3.2% (Cd(2+)), respectively. Interferences by some metals were investigated. Only Ni(2+) and Hg(2+) apparently affected the peak currents of Pb(2+) and Cd(2+). The diacetyldioxime modified carbon paste electrode was applied to the determination of Pb(2+) and Cd(2+) in water samples. The results indicate that this electrode is sensitive and effective for the simultaneous determination of Pb(2+) and Cd(2+).

Voltammetric behavior and determination of estrogens at Nafion-modified glassy carbon electrode in the presence of cetyltrimethylammonium bromide

Abstract A method is described for the determination of estrogens (estradiol, estrone and estriol) by stripping voltammetry. These estrogens yield one well-defined oxidation peak at the Nafion-modified glassy carbon electrode in the presence of cetyltrimethylammonium bromide (CTAB). The peak current is proportional to the concentration of estradiol in the range from 2.5×10 −8 to 1.5×10 −6 xa0mol/l, and the detection limit is 1×10 −9 xa0mol/l after accumulation of 6xa0min. The total amounts of estrogens in the blood serums were determined using the voltammetric method, and the average recovery was 106.04%. The mechanism of the oxidation of estradiol was investigated by electrochemical techniques and UV spectra.

Adsorption stripping voltammetry of phenol at Nafion-modified glassy carbon electrode in the presence of surfactants

In this paper, a new voltammetric method for the determination of phenol is described. In pH 8.00 phosphate buffer and in the presence of long-chain cationic surfactant-cetyltrimethylammonium bromide-phenol has a very sensitive oxidation peak at 0.47 V (vs. SCE) on the Nafion-modified glassy carbon electrode (GCE). The experimental parameters, such as supporting electrolyte and pH values, amounts of Nafion, varieties and concentration of surfactants, accumulation potential and time, as well as scan rate were optimized. The peak current is linear with the concentration of phenol in the range from 8x10(-9) to 1x10(-5) M, and the detection limit is 1x10(-9) M after being accumulated at -0.50 V (vs. SCE) for 3 min. Trace levels of phenol in water samples were determined by using this voltammetric method, the average recovery was calculated to be 99.56%.

Voltammetric determination of diethylstilbestrol at carbon paste electrode using cetylpyridine bromide as medium

In this paper, the voltammetric properties of diethylstilbestrol (DES) at the carbon paste electrode were described. The oxidation peak currents of DES increase significantly in the presence of surfactant cetylpyridine bromide (CPB), compared with that in the absence of CPB. Based on this fact, a voltammetric technique for determining DES is proposed. The accumulation potential has no effects on the peak current of the DES. An open-circuit accumulation is carried out. The experimental parameters, such as pH value of buffer, scan rate, and accumulation time were optimized. The interferences of some metal ions and organic compounds have also been studied, and some metal ions almost do not interfere with the determination of DES. Using this voltammetric method, DES in the injection sample was measured. The results show that this voltammetric method is reliable for the practice determination of DES.

Simultaneous determination of cadmium (II) and lead (II) with clay nanoparticles and anthraquinone complexly modified glassy carbon electrode

An anthraquinone (AQ) improved Na-montmorillonite nanoparticles (nano-SWy-2) chemically modified electrode (CME) has been developed for the simultaneous determination of trace levels of cadmium (II) and lead (II) by differential pulse anodic stripping voltammetry (DPASV). This method is based on a non-electrolytic preconcentration via ion exchange model, followed by an accumulation period via the complex formation in the reduction stage at -1.2V, and then by an anodic stripping process. The mechanism of this design was proposed and the analytical performance was evaluated with several variables. Under the optimized working conditions, the detection limit was 3 and 1nM for Cd(2+) and Pb(2+), respectively. The calibration graphs were linear in the concentration ranges of 8x10(-9) to 1x10(-6)molL(-1) (Cd(2+)) and of 2x10(-9) to 1x10(-6)molL(-1) (Pb(2+)). Many inorganic species did not interfere with the assay significantly; the high sensitivity, selectivity, and stability of this nano-SWy-2-AQ CME were demonstrated. The applications for the detection of trace levels of Cd(2+) and Pb(2+) in milk powder and lake water samples indicate that it is an economical and potent method.

Electro-oxidation of glucose at self-assembled monolayers incorporated by copper particles.

A novel copper incorporated self-assembled monolayers (SAMs) modified gold electrode (Cu/SAMs) for determination of glucose was developed by electrodepositing Cu particles on the SAMs of hexanethiol. The scanning electron microscopic (SEM) images showed that copper formed orbicular particles of nanosizes on the SAMs, which was much different from the fractal-like particles of copper formed at gold electrode. The Cu/SAMs film electrode exhibited high sensitivity to glucose oxidation and depressed responses towards some interferents of glucose in blood like uric acid and ascorbic acid. Under optimal working conditions, the oxidation current of glucose was proportional to the concentration of glucose in the range from 3.0muM to 10mM by amperometry with a low detection limit of 0.7muM glucose (S/N=3). This electrode was successfully applied to the determination of glucose in rat blood and the results were satisfactory.

A hydrogen peroxide biosensor based on the direct electrochemistry of hemoglobin modified with quantum dots

Direct electron transfer of hemoglobin modified with quantum dots (QDs) (CdS) has been performed at a normal graphite electrode. The response current is linearly dependent on the scan rate, indicating the direct electrochemistry of hemoglobin in that case is a surface-controlled electrode process. UV–vis spectra suggest that the conformation of hemoglobin modified with CdS is little different from that of hemoglobin alone, and the conformation changes reversibly in the pH range 3.0–10.0. The hemoglobin in a QD film can retain its bioactivity and the modified electrode can work as a hydrogen peroxide biosensor because of its peroxidase-like activity. This biosensor shows an excellent response to the reduction of H2O2 without the aid of an electron mediator. The catalytic current shows a linear dependence on the concentration of H2O2 in the range 5xa0×xa010−7–3xa0×xa010−4xa0M with a detection limit of 6xa0×xa010−8xa0M. The response shows Michaelis–Menten behavior at higher H2O2 concentrations and the apparent Michaelis–Menten constant is estimated to be 112xa0μM.