Negative Shift Of Oxidation Potential Research Articles

Staircase voltammetric determination of 2-thiouracil in pharmaceuticals and human biological fluids at polyaniline and polypyrrole film modified sensors

The conducting polymers like polyaniline(PA) and polypyrrole (PP) films modified graphite pencil electrodes (GPE) were used to determine the trace amounts of 2-thiouracil (2TU) in biological samples using cyclic and stair case voltammetric techniques. 0.1mM each of aniline and pyrrole were successfully electropolymerized separately on GPE using cyclic voltammetry. Morphology of electrodes was characterized by AFM. The negative shift of oxidation potential compared to bare GPE, shows that the PA and PP modified electrodes had catalytic activity for the 2TU oxidation. The results showed that the electrochemical oxidation of 2TU on the modified electrodes was diffusion-controlled. The effects of pH, scan rate, concentration of 2TU and modifiers were optimized. Under optimized conditions, the electrodes showed a linear response in the range of 1.0×10−8 − 1.5×10−7M for PP/GPE with a detection limit of 1.6×10−9M and 1.0×10−8 − 1.3×10−7M for PA/GPE with a detection limit of 1.8×10−9M. The sensors were successfully applied to determine the trace amounts of 2TU in human biological fluids and pharmaceutical samples.

A glassy carbon electrode modified with electrochemically reduced graphene for simultaneous determination of guanine and adenine

In this paper, the direct electrochemical oxidation and differential pulse voltammetry (DPV) determination of guanine and adenine are studied by using an electrochemically reduced graphene oxide (er-GO) modified glassy carbon electrode (er-GO/GCE). Compared to the GCE and the GO/GCE, the as-prepared er-GO/GCE shows excellent electrocatalytic activity towards guanine and adenine, with an increase of oxidation current along with a negative shift of oxidation potential. Using the as-prepared er-GO/GCE, the simultaneous determination of guanine and adenine is successfully carried out, and the detection limits (based on S/N of 3) for guanine and adenine are 1.5 × 10−7 mol L−1 and 2.0 × 10−7 mol L−1, respectively. In addition, this er-GO/GCE exhibits a high selectivity, reproducibility and stability, and also can be used for the detection of guanine and adenine in the thermally denatured calf thymus DNA and urine, showing its promising application in the analysis of real samples.

Voltammetric study of polyviologen and the application of polyviologen-modified glassy carbon electrode in amperometric detection of vitamin C

The voltammetric behavior of viologen oligomers prepared from butylviologen dibromide and the factors influencing polyviologen film formation were investigated at a glassy carbon electrode (GCE). Based on the voltammetric observations, phosphoric acid is crucial to the formation of a stable polyviologen film on a GCE. The polyviologen-modified glassy carbon electrode (PVGCE) was employed to determine vitamin C (i.e., ascorbic acid) in order to demonstrate the electroanalytical application of the electropolymerized polyviologen film. The PVGCE was found capable of accumulating vitamin C at electrode surface in a slightly basic solution (pH = 7.8) and induce a negative shift of oxidation potential of vitamin C. Vitamin C was detected by hydrodynamic amperometry at +0.1 V (vs. Ag/AgCl) in a batch-injection cell; no accumulation time is required. The dependence of oxidation current on concentration was linear from 5.00 × 10−7 M to 1.22 × 10−4 M with a regression coefficient of 0.9993. Several real samples were analyzed and the results exhibit good agreement with those determined by iodimetric titration.

Electrocatalytic Oxidation of Glucose on Gold Nanocomposite Electrodes

AbstractThe electrocatalytic oxidation of glucose on gold nanocomposite electrodes was investigated with cyclic voltammetry. The gold nanocomposite electrodes were prepared by precipitating gold nanoparticles of different sizes from the corresponding colloidal solutions onto planar substrates, and the electrodes exhibited higher catalytic activity for the oxidation of glucose in alkaline solution with a negative shift in oxidation potential and a larger current as compared with bare gold electrodes. The high catalytic activity of the gold nanocomposite electrodes also resulted in easy oxidation of gluconolactone produced in the reaction. The modification of the gold nanocomposite electrodes with silver underpotential deposition led to a further negative shift in potential but a drop in current for the glucose oxidation.

Electrocatalytic oxidation of ascorbic acid by [Fe(CN) 6] 3−/4− redox couple electrostatically trapped in cationic N, N-dimethylaniline polymer film electropolymerized on diamond electrode

Multinegatively charged metal complex, hexacyanoferrate ([Fe(CN) 6] 4−), was electrostatically trapped in the cationic polymer film of N, N-dimethylaniline (PDMA) which was electrochemically deposited on the boron-doped diamond (BDD) electrode by controlled-potential electro-oxidation of the monomer. This ferrocyanide-trapped PDMA film was used to catalyze the oxidation of ascorbic acid (AA). Increase in the oxidation current response with a negative shift of the anodic peak potential was observed at the cationic PDMA film-coated BDD (PDMA|BDD) electrode, compared with that at the bare BDD electrode. A more drastic enhancement in the oxidation peak current as well as more negative shift of oxidation potential was found at the ferrocyanide-trapped PDMA film-coated BDD ([Fe(CN) 6] 3−/4−|PDMA|BDD) electrode. This [Fe(CN) 6] 3−/4−|PDMA|BDD electrode can be used as an amperometric sensor of AA. Ferrocyanide, electrostatically trapped in the polymer film shows more electrocatalytic activity than that coordinatively attached to the polymer film or dissolved in the solution phase. The electrocatalytic current depends on the surface coverage of ferricyanide, Γ Fe, within the polymer film. Diffusion coefficient ( D) of AA in the solution was estimated by rotating disk electrode voltammetry: D = (5.8 ± 0.3) × 10 −6 cm 2 s −1. The second-order rate constant for the catalytic oxidation of AA by ferricyanide was also estimated to be 9.0 × 10 4 M −1 s −1. In the hydrodynamic amperometry using the [Fe(CN) 6] 3−/4−|PDMA|BDD electrode, a successive addition of 1 μM AA caused the successive increase in current response with equal amplitude and the sensitivity was calculated as 0.233 μA cm −2 μM −1.

Surface modification and electrocatalytic properties of Pt(100), Pt(110), Pt(320) and Pt(331) electrodes with Sb towards HCOOH oxidation

Behaviors of irreversibly adsorbed Sb adatoms on Pt(100), Pt(110), Pt(320) and Pt(331) single crystal surfaces and electrocatalytic properties of the modified electrodes towards formic acid oxidation were investigated. It was determined that Sb adatoms are stable at potentials below 0.45 V (SCE) on Pt(100) and Pt(110), below 0.40 V on Pt(320), and below 0.35 V on Pt(331). Different coverage of Sb ad was obtained conveniently by partially stripping Sb ad from saturation coverage of Sb ad. It has demonstrated that the redox behaviors of Sb adatoms and the coadsorption properties of Sb ad with H ad depend strongly on the orientations of the Pt single crystal electrode. Significant catalytic effects towards HCOOH oxidation were observed on Pt single crystal electrodes modified with Sb adatoms, which consist of (1) the inhibition of dissociative adsorption of HCOOH, (2) the enhancement of oxidation current, and (3) the negative shift of oxidation potential that was measured about 220 mV on Pt(110)/Sb, 110 mV on (110) sites of Pt(320)/Sb, and 100 mV on Pt(331)/Sb electrode. Neither enhancement of oxidation current nor negative shift of oxidation potential can be observed on Pt(100)/Sb electrode. The results suggested that electronic effect is the main effect presented on Pt(110), Pt(320) and Pt(331) surface upon Sb modification, while geometric effect is considered to the major effect on Pt(100) electrode.

Synthesis and Properties of Bismerocyanines Linked by a 1,8-Naphthylene Skeleton. Novel Solvatochromism Based on Change of Intramolecular Excitonic Coupling Mode

A newly prepared bismerocyanine linked by a 1,8-naphthylene skeleton has two rotational isomers, syn and anti conformers, with respect to orientation of the two merocyanine chromophores. The NMR studies revealed that a polar solvent such as acetonitrile enhanced a preference for the syn conformer to bring about much larger bathochromic shift of the UV/vis absorption band than that of a 1-naphthyl-substituted merocyanine. The INDO/S-CI calculation indicated that the UV/vis absorption bands of the syn and anti conformers are hypsochromically and bathochromically shifted, respectively, compared to that of the 1-naphthylmerocyanine. In accordance with the prediction based on an exciton coupling theory, the anti conformer fluoresces while the syn conformer does not. A polarographic analysis indicated a positive shift of reduction potential and a negative shift of oxidation potential compared to the corresponding value of the 1-naphthylmerocyanine. The shift in the reduction potential of the bismerocyanine rela...