Voltammetric behavior of gold nanotrench electrodes

We report the fabrication and electrochemical response of a gold nanoband electrode located at the bottom of a glass/epoxy nanotrench, hereafter referred to as a gold nanotrench electrode. Gold nanotrench electrodes of 12.5 and 40 nm in width with various depths from a few tens of nanometers to approximately 4 μm are fabricated and further characterized by cyclic voltammetry. The fabrication of a Au nanotrench electrode follows a simple electrochemical etching process in which a small AC signal is applied to an inlaid Au nanoband electrode submersed in a NaCl solution. The voltammetric behavior of a Au nanotrench electrode is characterized by a quasi-steady-state response at lower scan rates (e.g., <1 V/s for a 12.5-nm-wide electrode). We present an analytical expression for the quasi-steady-state diffusion-limited current of the nanotrench electrode based upon the analysis of the mass-transport resistance. Finite-element simulation of steady-state and transient voltammetric responses of the nanotrench electrodes provides additional insights for the analytical model. Peak-shaped transient voltammetric responses were observed at scan rates as low as 5 V/s for both inlaid and nanotrench electrodes. This result may suggest that the exposed area of the nanoband electrode is much greater than that expected from the fabrication of the inlaid bands. However, the extent to which this is seen is greatly decreased in the nanotrench electrode by a smoothing effect during etching. Our results confirm previous reports of excess overhanging metal and delamination crack contributing significantly to the shape and magnitude of the voltammetric response.     

Voltammetric study and electrochemical detection of hexavalent chromium at gold nanoparticle-electrodeposited indium tinoxide (ITO) electrodes in acidic media

The voltammetric behavior of hexavalent chromium species (Cr(VI)) was respectively studied at ITO, bulk Au, and Au-electrodeposited electrodes in 0.01 M NaCl aqueous solutions containing 0.01 M HCl. It was found that performance degradation of the ITO electrodes toward the reduction of Cr(VI) can be suppressed by modifying the electrode surface with gold nanoparticles (AuNPs), which were formed on ITO electrodes by potential-sweeping or potential-step electrodeposition in a 0.01 M Na(2)SO(4) solution containing 1 mM HAuCl(4) x 3 H(2)O and 0.01 M H(2)SO(4). After the modification, the surface of ITO electrodes turned to the characteristically red or blue color exhibited by AuNPs. The gold nanoparticle-electrodeposited indium-tinoxide electrode (AuNP-ITO) demonstrates unique catalytic behavior, higher sensitivity and stability in the reduction of Cr(VI). Cr(VI) species was detected by either cyclic voltammetry or hydrodynamic amperometry. By cyclic voltammetry, the dependence of cathodic peak current on concentration was linear from 5 to 100 microM with a detection limit of 2 microM (sigma=3), and linearity was obtained from 0.5 to 50 microM by hydrodynamic amperometry where a constant potential of +0.2V (vs. Ag/AgCl) was applied and a batch-injection cell was employed. For hydrodynamic amperometry, the detection limit was 0.1 microM (sigma=3).     

Voltammetric trace determination of ubiquinones at mercury electrodes

Summary Voltammetric methods were developed for the determination of ubiquinones on the basis of their adsorption and redox behaviour at mercury electrodes. Linear calibration plots were obtained in the concentration range between 10^–4 mol/l and 2.3·10^–5 mol/l from differential pulse voltammetric measurements under experimental conditions avoiding significant adsorption of the analyte. A remarkable decrease of the detection limit down to 10^–7 mol/l was achieved with the help of adsorptive accumulation of ubiquinone molecules at the mercury electrode followed by differential pulse detection. The resulting non-linear calibration plot was explained with a model, which takes into account both adsorption and diffusion of the analyte.     

Voltammetric determination of ubiquinone adsorption at mercury electrodes

Adsorption isotherms of the ubiquinones UQ₄, UQ₆, UQ₉ and UQ₁₀ have been determined by two independent voltammetric techniques at the stationary mercury/solution interface. While the redox behavior is essentially the same for all homologues investigated, the molar free enthalpy of adsorption Δ_AG increases steadily with the number of isoprenic units from UQ₄ to UQ₁₀. In case of UQ₁₀, the experiments have been carried out with three different potentiostatic systems. This allows one to estimate the accuracy of the methods which are direct ones and do not require calibration.     

Pulsed coulometric detection of carbohydrates at a constant detection potential at gold electrodes in alkaline media

A significant increase in the signal-to-noise ratio for the pulsed amperometric detection (PAD) of carbohydrates at gold electrodes is obtained by increasing the length of the current integration period (t_i) from the traditional value of 16.7 ms (i.e., $\text{1}\text{60}$ Hz). For t_i > 16.7 ms, the integrated response (q, coulombs) is plotted as the signal. This pulsed coulometric detection (PCD) is applied in a flow-injection system. For t_i = 500 ms, the detection limit with the instrumentation used is 1 μM (S/N = 2) for glucose which is a significant improvement on the value 35 μM found with PAD. The absolute detection limits for glucose and sucrose are ca. 50 pmol and 125 pmol, respectively, in 50-μl samples. Calibration plots (q_p vs. C^b) for PCD are linear over significantly larger dynamic ranges than those observed for PAD because of the lower detection limits.     

Accumulation and stripping behavior of silver ions at dl-dithiothreitol self-assembled monolayer modified gold electrodes

Silver ions can be entrapped at the dl-dithiothreitol (HSCH₂(CHOH)₂CH₂SH, DTT) self-assembled monolayer films modified gold electrodes. When the potential was made moving, an anodic peak was observed at about 0.23 V (vs. SCE). When the electrode Au/DTT was modified with dodecyl mercaptan further, more Ag (I) can be accumulated and the peak grows. Conditions, such as solution pH and supporting electrolyte, were optimized for Ag (I) determination. Under the selected conditions, i.e. 0.010 M pH 4.3 potassium hydrogen phthalate, preconcentration time of 5 min at open circuit, the anodic peak height is linear to the concentration of Ag (I) in the small range of 0.6-2.4 μM. The influence of some ions on the determination of Ag (I) was examined. The Br⁻ ion makes the peak decrease and NCS⁻ makes the peak increase. But the determination is not interfered by 1000-fold Pb²⁺, Cd²⁺, Hg²⁺, Fe³⁺, Ni²⁺, Co²⁺, Cu²⁺ and Sn²⁺ when EDTA was added into the solution. The mechanism involved was discussed.     

Voltammetric behavior of selenocystine at modified gold substrates

The voltammetric response of seleno-l-cystine has been studied at gold substrates under physiological conditions. The reactivity of diselenides is utilized to generate a modified electrode surface, which in turn allows for electroanalysis of solution-based selenium species. Stable and reproducible voltammetry is observed for selenocystine reduction at pH 7 on selenium-modified gold substrates (E_mid = − 486 mV vs. Ag/AgCl) and is consistent with a diffusionally controlled process. Modification of the gold surface is readily achieved via electrochemical cycling in the presence of a diselenide source at conventional scan rates. These studies afford voltammetric characterization of the selenocystine/selenocysteine redox couple under physiologically relevant conditions and highlight the potential utility of selenium-modified substrates for electroanalysis of chalcogen-containing species.     

Voltammetric behaviour of immunoglobulin G at stationary mercury electrodes

A.c. voltammetric measurements have shown that bovine and human immunoglobulin G are adsorbed at a mercury electrode over a broad potential range and at open circuit. A current signal at ca. -0.55 V was identified as being due to an interfacial process connected with reversible protein re-orientations in the adsorption layer and, possibly, with fast faradaic reactions of both adsorbed redox states of the immunoglobulin.     

Voltammetric determination of Rutin at Screen-Printed carbon disposable electrodes

Rutin is a flavonoid commonly employed for many therapeutic purposes. Although the electroactive phenolic groups of rutin might be oxidized at low applied potential, the adsorption of oxidized species changes the electrode surface. As a consequence, the repeatability and reproducibility of the method decreases, which limits electroanalytical applications. This paper describes the use of disposable screen-printed electrodes as an alternative to improve the electrochemical quantification of rutin in commercial and standard samples. The electrochemical behavior was consistent to what is observed using other carbon electrodes: an adsorption-involved step and a pH-dependent oxidation process. The replacement of the electrodes between the analyses ensured rapid analysis, good intermediate precision and repeatability. The proposed method was successfully applied to rutin determination in pharmaceutical samples of capsules, with the limit of quantification being 0.30 μM.      

Voltammetric measurement of haloperidol following adsorptive accumulation at glassy-carbon electrodes

A sensitive stripping voltammetric method is reported for trace measurement of the psychotherapeutic drug haloperidol. The method is based on adsorptive preconcentration of haloperidol on the glassy-carbon electrode in an open circuit, followed by medium exchange and voltammetric determination of surface species. Cyclic voltammetry was used to explore the adsorptive behaviour and the results obtained suggest that the adsorption of haloperidol corresponds to the Frumkin-type isotherm. The adsorptive stripping response was evaluated with respect to stripping mode, electrolyte. pH, preconcentration time, concentration dependence, possible interference and other variables. The detection limit was 1.3 x 10(-9)M (10 min preconcentration) and the response was linear. The relative standard deviation (at the 1.3 x 10(-6)M level) was 2.3%. Applicability to a patient's urine sample is illustrated.     

Voltammetric determination of selenosulfate ions at a mercury-film electrode

A procedure is proposed for the voltammetric determination of selenium as selenosulfate (SO₃Se^2?) ions at a mercury-film electrode (MFE). Selenosulfate ions are determined in the range from 2 × 10^?4 to 1.0 × 10^?3 M without analyte accumulation, using peak current at ?0.92 ± 0.02 V and in the range from 1 × 10^?7 to 2 × 10^?4 M after analyte accumulation with the open circuit, using peak current at ?1.18 ± 0.03 V as the analytical signal. The mechanisms of SO₃Se^2? reduction at an MFE under the conditions of direct voltammetry and stripping voltammetry with accumulation are proposed and discussed.     

Indirect detection method for flow systems based on perturbation of oxyen reduction at gold and gold chloride electrodes

A simple scheme for potential indirect electrochemical detection in flow analyses (liquid chromatography and ion chromatography) with experimental examples is presented. The proposed scheme exploits the influence of the analyte on the reduction current of an intrinsic mobile phase additive-oxygen-at the detector electrode, and utilizes the resulting changes in background current as an analytical signal. Several analytes: chloropromazine, ascorbic acid, nitrate and sulfate, cause a shift in the oxygen reduction peak, particularly at the AuCl electrode, that results in a cathodic or anodic shift in the current at a fixed potential.     

Voltammetric detection of synthetic water-soluble phenolic antioxidants using carbon nanotube based electrodes

Glassy carbon electrodes (GCE) modified with carbon nanotubes (CNT) have been created for detection of phenolic compounds—one of the important group of antioxidants in life sciences. The surface of electrode has been characterized by atomic force microscopy. The presence of CNT leads to an at least 20-fold increase in the surface roughness of the electrode. The CNT layer displays closely intertwined vermicular structures with high degree of homogeneity at CNT suspension concentration of 0.2–0.5 mg L⁻¹. Synthetic water-soluble antioxidants (hydroquinone, catechol, pyrogallol, and their derivatives) are electrochemically active on bare GCE and CNT-modified GCE in phosphate buffer solution pH 7.4. Effect of substitutes in molecular structure of phenolic antioxidants has been evaluated. In several cases, oxidation at CNT-modified GCE occurs at potentials that are less positive by 100–200 mV in comparison to bare GCE. The electrodes were studied with respect to their capability of phenols voltammetric sensing. CNT-modified GCE display an enlarged linear range in the calibration graphs and lower detection limits. Voltammetric method for determination of hydroquinone, catechol, pyrogallol, and their derivatives has been developed.     

Voltammetric behavior of tertiary butyl bromide at mercury electrodes in dimethylformamide

Polarograms for t-butyl bromide in dimethylformamide containing tetramethylammonium perchlorate exhibit two waves which signal stepwise reduction of the starting material to the t-butyl radical and the t-butyl carbanion. Cyclic voltammograms obtained with a hanging mercury drop and at a scan rate of 50 mV s^?1 confirm that the alkyl bromide undergoes stepwise reduction; but the height of the second peak is abnormally small, indicating that t-butyl radicals engage in combination and disproportionation. At a faster scan rate (500 mV s^?1), the first wave nearly merges with the second wave. At all scan rates, two pairs of anodic-cathodic waves, associated with reversible bromide-assisted oxidation of mercury, appear at positive potentials. In addition, another set of anodic-cathodic waves is seen if the potential is scanned rapidly to a value at which the t-butyl carbanion is generated; this set of waves might be related to the formation of trimethylammonium methylide (produced when the tetramethylammonium cation donates a proton to the t-butyl carbanion). Although this latter set of waves is eliminated by addition of a proton donor (diethyl malonate) to the system, the anion of diethyl malonate is itself involved in the appearance of yet another pair of anodic-cathodic waves.     

Voltammetric properties at pH 14 of electrodes modified by azobenzene polymers

Azobenzene polymers were prepared by condensation of p-phenylazobenzoyl chloride and poly(ethylenimine). Their loadings in electroactive sites range from 5 to 95%. They were adsorbed on a glassy carbon electrode or on a hanging mercury drop electrode (HMDE) whose area could be expanded after the adsorption. The voltammetric behavior of the polymeric films is described at pH 14. The azobenzene sites which are in the vicinity of the electrode surface are electroactive, but the electrochemical reaction does not propagate to the bulk of the coating. When the loading of the polymer is not too high, the expansion of the HMDE causes an increase in the number N of azobenzene double bonds which are reduced, N remaining proportional to the drop area A, because-the film is sufficiently flexible to cover the new surface which appears (soap bubble effect). For the highly loaded polymers (loading larger than about 50%), conversely, N remains nearly constant, owing to the rigidity of the film, which causes it to break up when the drop is expanded. The reversibility of the electrochemical reaction depends both on the loading and on the expansion, which could be due to changes in the orientation of the azobenzene molecules at the surface of the electrode.     

Voltammetric study of nifuroxazide at unmodified and Sephadex-modified carbon paste electrodes

The electrooxidation of nifuroxazide was investigated by cyclic and differential-pulse voltammetry at carbon paste and Sephadex-modified carbon paste electrodes. Nifuroxazide is irreversibly oxidized at all pH values and gives rise to a well-defined oxidation peak. The modification of the carbon paste surface with Sephadex allowed a preconcentration process to take place for nifuroxazide such that higher sensitivity was achieved compared with the bare surface. The influence of the scan rate, time of accumulation, modifier loading, solution conditions and pH on the adsorptive step at the modified carbon paste electrodes was investigated. The direct determination of the drug in urine is also discussed. Received: 2 June 1998 / Revised: 29 January 1999 / Accepted: 2 February 1999     

Voltammetric study of nifuroxazide at unmodified and Sephadex-modified carbon paste electrodes

The electrooxidation of nifuroxazide was investigated by cyclic and differential-pulse voltammetry at carbon paste and Sephadex-modified carbon paste electrodes. Nifuroxazide is irreversibly oxidized at all pH values and gives rise to a well-defined oxidation peak. The modification of the carbon paste surface with Sephadex allowed a preconcentration process to take place for nifuroxazide such that higher sensitivity was achieved compared with the bare surface. The influence of the scan rate, time of accumulation, modifier loading, solution conditions and pH on the adsorptive step at the modified carbon paste electrodes was investigated. The direct determination of the drug in urine is also discussed.     

Tetralactam-modified gold electrodes for amperometric detection of acrylic acid

The presence of toxic acrylamide in a wide range of food products such as potato crisps, French fries or bread has been confirmed by Swedish scientists from Stockholm University. The neurotoxicity and possible carcinogenicity of this compound and its metabolites compel us to control them by quantitative and qualitative assays. Exposing acrylamide to pH extremes results in its hydrolysis to acrylic acid or its salt. In this work, we present the use of gold electrodes coated with self-assembled monolayers (SAMs) containing tetralactam molecules and its precursor as active elements for voltammetric detection of acrylic acid in water solution. The host molecules have been immobilised on the electrode surface by covalent Au–S bond or by embedment method into the thiol layer via hydrophobic and van der Waals interactions. Interactions with analytes were confirmed by Osteryoung square-wave voltammetry.     

Voltammetric determination of DNA hybridization using methylene blue and self-assembled alkanethiol monolayer on gold electrodes

An electrochemical DNA biosensor based on the recognition of single stranded DNA (ssDNA) by hybridization detection with immobilized complementary DNA oligonucleotides is presented. DNA and oligonucleotides were covalently attached through free amines on the DNA bases using N-hydroxysulfosuccinimide (NHS) and N-(3-dimethylamino)propyl-N′-ethylcarbodiimide hydrochloride (EDC) onto a carboxylate terminated alkanethiol self-assembled monolayers (SAM) preformed on a gold electrode (AuE). Differential pulse voltammetry (DPV) was used to investigate the surface coverage and molecular orientation of the immobilized DNA molecules. The covalently immobilized probe could selectively hybridize with the target DNA to form a hybrid on the surface despite the bases being attached to the SAM. The changes in the peak currents of methylene blue (MB), an electroactive label, were observed upon hybridization of probe with the target. Peak currents were found to increase in the following order: hybrid-modified AuE, mismatched hybrid-modified AuE, and the probe-modified AuE which indicates the MB signal is determined by the extent of exposed bases. Control experiments were performed using a non-complementary DNA sequence. The effect of the DNA target concentration on the hybridization signal was also studied. The interaction of MB with inosine substituted probes was investigated. Performance characteristics of the sensor are described.