Junfeng Song

Ni(II)-baicalein complex modified multi-wall carbon nanotube paste electrode toward electrocatalytic oxidation of hydrazine.

A modified electrode Ni(II)-BA-MWCNT-PE has been fabricated by electrodepositing nickel(II)-baicalein [Ni(II)-BA] complex on the surface of multi-wall carbon nanotube paste electrode (MWCNT-PE) in alkaline solution. The Ni(II)-BA-MWCNT-PE exhibits the characteristic of improved reversibility and enhanced current responses of the Ni(III)/Ni(II) couple compared with Ni(II)-BA-CPE. It also shows good electrocatalytic activity toward the oxidation of hydrazine. Kinetic parameters such as the electron transfer coefficient alpha, rate constant k(s) of the electrode reaction, the diffusion coefficient D of hydrazine and the catalytic rate constant k(cat) of the catalytic reaction are determined. Moreover, the catalytic currents present linear dependence on the concentration of hydrazine from 2.5 microM to 0.2 mM by amperometry. The detection limit and sensitivity are 0.8 microM and 69.9 microA mM(-1), respectively. The modified electrode for hydrazine determination is of the property of simple preparation, good stability, fast response and high sensitivity.

Voltammetric behavior of urapidil and its determination at multi-wall carbon nanotube paste electrode.

The voltammetric behavior of urapidil was investigated. In pH 6.8 Britton-Robinson buffer, an irreversible oxidation peak of urapidil at 0.62V (versus SCE) at a multi-wall carbon nanotube paste electrode (MWNT-PE) was observed, which was more sensitive with lower potential than that at the carbon paste electrode (CPE). The oxidation of urapidil was a two-electron and two-proton process with adsorption character. A differential pulse voltammetric method was proposed for the determination of urapidil. The peak current of the oxidation peak of urapidil was linearly with its concentration in a range from 5.0x10(-8) to 2.0x10(-6)mol/L at open-circuit accumulation for 60s, with a detection limit of 3.8x10(-8)mol/L. The proposed method was employed to determine urapidil in urapidil tablets.

Flow-injection chemiluminescence determination of chloroquine using peroxynitrous acid as oxidant.

A new flow-injection chemiluminescence (CL) method for determination of chloroquine is proposed based on a stronger chemiluminescence of chloroquine in hydrogen peroxide-nitrite-sulfuric acid medium. The proposed method allows the measurement of chloroquine over the range of 3.0x10(-7) to 1.0x10(-5)moll(-1). The detection limit is 8.6x10(-8)moll(-1), and the relative standard deviation for 1.0x10(-6)moll(-1) chloroquine (n=11) is 1.6%. The CL mechanism is also discussed.

Theoretical and experimental study of the biamperometry for irreversible redox couple in flow system

Abstract The biamperometry for the direct determination of irreversible redox analytes in flow system has been proposed based on coupling two independent and irreversible couples to form the biamperometric detection scheme. In this work, the method is studied both theoretically and experimentally. Equations describing the current–voltage characteristics and the current–concentration relationship are presented. The influence of the applied potential difference (ΔE) and the half-wave potential difference (ΔE1/2) between two irreversible couples on the method are discussed. It shows that small ΔE1/2 is favorable to construct the biamperometric detection system and to achieve high sensitivity and selectivity. Increasing ΔE leads to an increase in sensitivity. This is, however, accompanied by a decrease in selectivity and signal-to-noise ratio. To construct the biamperometric scheme for the irreversible systems with large ΔE1/2, two approaches, adjusting acidity of supporting electroyte or adding new irreversible couple, are proposed by taking uric acid/platinum oxide and phenol/permanganate systems as examples. Uric acid and phenol are, respectively, detected in a flow injection system with a biamperometric detector.

Flow-injection biamperometry of pyrogallol compounds

A biamperometric method for the direct determination of pyrogallol compounds has been designed for flow-injection analysis. The method is based on the electrocatalytic oxidation of pyrogallol compounds at one pretreated platinum electrode and the reduction of platinum oxide at the other pretreated platinum electrode to form a biamperometric detection system with the applied potential difference of 10 mV. Three important compounds, pyrogallol, gallic acid and tannic acid, have been detected by the method. The linear relationships between currents and the concentrations of pyrogallol, gallic acid and tannic acid are obtained over the range 1.0x10(-6)-1.0x10(-4), 1.0x10(-6)-1.0x10(-4) and 1.0x10(-6)-2.0x10(-4) mol l(-1) with the detection limit of 6.0x10(-7), 6.0x10(-7) and 8.0x10(-7) mol l(-1) (S/N=2), respectively. The R.S.D. observed for 30 successive determinations of 5.0x10(-5) mol l(-1) pyrogallol, gallic acid and tannic acid are 1.9, 2.5 and 2.0%, respectively. Most ions and organic compounds tested are found not to cause significant interference in the determinations. The method is simple, selective and efficient (180 h(-1)), performing well as a routine assay, and has been validated by the determination of pyrogallol compounds in tea and Chinese gall.

Adsorption-parallel catalytic waves of cinnamic acid in hydrogen peroxide-tetra-n-butylammonium bromide- acetate system

The mechanism of the adsorption-parallel catalytic wave of cinnamic acid (C6H5-CH = CH-COOH) in acetate buffer (pH = 4.0)-H2O2-tetra-n-butylammonium bromide (Bu4N · Br) solution was studied by the linear-sweep polarography, cyclic voltammetry and digital simulation approach. Experimental results indicate that the reduction mechanism of cinnamic acid isECdimE’ process, in which the C = C double bond of cinnamic acid first undergoes 1e, 1H+ reduction to produce an intermediate free radical C6H5-CH-CH2-COOH(E), then the further reduction of the free radical in 1e, 1H+ addition (E’) occurs simultaneously with a dimerization reaction between two free radicals (Cdim). Bu4N· Br enhances the polarographic current of cinnamic acid and shifts the peak potential to positive direction. The enhancement action of Bu4N· Br is due to the adsorption of cinnamic acid induced by Bu4N+ species. In addition, H2O2 causes the parallel catalytic wave of cinnamic acid. The mechanism of the catalytic wave isEC’ process because H2O2 oxidizes the free radical of cinnamic acid to regenerate the original C =C bond(C’), preventing both the further reduction and the dimerization of the free radicals. The apparent rate constantkf of the oxidation reaction is 1.35×102 mol· L-1· s-1. A new class of catalytic waves for organic compounds, the adsorption-parallel catalytic waves upon the dual enhancement action of both the surfactant and oxidant, has been presented.

Voltammetric determination of glucose based on reduction of copper(II)–glucose complex at lanthanum hydroxide nanowire modified carbon paste electrodes

A novel voltammetric method for the determination of beta-d-glucose (GO) is proposed based on the reduction of Cu(II) ion in Cu(II)(NH(3))(4)(2+)-GO complex at lanthanum(III) hydroxide nanowires (LNWs) modified carbon paste electrode (LNWs/CPE). In 0.1mol L(-1) NH(3).H(2)O-NH(4)Cl (pH 9.8) buffer containing 5.0x10(-5)mol L(-1) Cu(II) ion, the sensitive reduction peak of Cu(II)(NH(3))(4)(2+)-GO complex was observed at -0.17V (versus, SCE), which was mainly ascribed to both the increase of efficient electrode surface and the selective coordination of La(III) in LNW to GO. The increment of peak current obtained by deducting the reduction peak current of the Cu(II) ion from that of the Cu(II)(NH(3))(4)(2+)-GO complex was rectilinear with GO concentration in the range of 8.0x10(-7) to 2.0x10(-5)mol L(-1), with a detection limit of 3.5x10(-7)molL(-1). A 500-fold of sucrose and amylam, 100-fold of ascorbic acid, 120-fold of uric acid as well as gluconic acid did not interfere with 1.0x10(-5)mol L(-1) GO determination.

A new type of polarographic catalytic wave of organic compound

The polamgraphic behavior and catalytic wave mechanism of medroprogestemne acetate (MPA) were studied in both aqueous and DMF media. In 0.2 mol/L acetic acid-sodium acetate (pH 5.0) buffer solution, then bond of MPA first undergoes le, lH+ reduction to form protonated free radical HMPA, the further reduction of HMPA in le,1H+ process is simultaneous with the dimerization reaction between HMPA and neutral molecular MPA. In DMF media containing 0.1 mol/L tetrabutylammonium tetrafluoborate (TBA.BF4), then bond of MPA shows two le, 1H+reduction waves, which are ascribed to the reduction of MPA and free radical MPA.-, respectively. Here, no dimerization reaction occurs. These processes produce the reduction wave of MPA. In the presence of oxidant KIO3,a polamgraphic catalytic wave of MPA is observable due to a chemical reaction between HMPA. or MPA.- and KIO3 as well as its intermediate species to regenerate MPA. The catalytic wave, which is caused by the reduction of organic compound itself and the chemical reaction between oxidant and organic intermediate free radical to regenerate original organic compound, is a new-type wave of organic compound. Under optimum experimental conditions, the sensitivity of MPA catalytic wave in the presence of KIO3 is an order of magnitude higher than that of its reduction wave. The catalytic wave can be used for analytical purpose. The calculated rate constant of catalytic reaction is 1.7 × 103 mol·L-1·s-1.

Voltammetric determination of sericin based on its interaction with carmine.

A simple yet sensitive method is developed for the determination of sericin using voltammetry based on the interaction between sericin and carmine for the first time. In the absence of sericin, carmine has a pair of well-defined redox peaks in a pH 1.81 Britton-Robinson buffer solution. Although no new redox peaks appear upon the addition of sericin into a carmine solution, the peak currents of the old peaks reduce while the peak potentials shift positively. This observation is attributed to the decrease in the diffusion coefficient and electrode reaction rate constant of carmine in the presence of sericin. A binding mechanism is proposed and discussed, and the binding constant and binding ratio are calculated as 2.32 x 10(6) L mol(-1) and 1:1, respectively. Furthermore, the decrease in the peak currents is found proportional to the sericin concentration in the range of 32.0-800.0 microg mL(-1) with a detection limit of 13.52 microg mL(-1). The method is further applied to the determination of sericin in degumming wastewater with satisfied average recoveries from 96.7 to 103.3%. The results are in good agreement with those obtained by the conventional Coomassie brilliant blue G-250 spectrophotometric method.