Electrooxidation of catecholamines at carbon nanotube-modified indium tin oxide electrodes

In this study, we prepared carbon nanotube (CNT)/Nafion-modified ITO electrodes and investigated their electrochemical behavior. The CNTs were dissolved in a solution of the ionic polymer Nafion and then CNT/Nafion composite films were deposited onto ITO electrodes through spin-coating of this homogeneous solution. We studied the effects of chemical pretreatment of the CNTs and the pH of the buffer on the electroanalytical behavior of the CNT/Nafion-modified ITO electrodes toward catecholamines. The modified electrodes enhanced the peak current and lowered the overpotentials. We observed high electrooxidative performance for the modified ITO electrodes: the oxidative currents of the catecholamines were up to 125-fold higher than those obtained using bare ITO electrodes.     

Electrocatalytic detection of insulin at RuOx/carbon nanotube-modified carbon electrodes

A bilayer surface coating, prepared by electrodepositing ruthenium oxide (RuOx) onto a carbon nanotube (CNT) layer, offers dramatic improvements in the stability and sensitivity of voltammetric and amperometric measurements of insulin compared to the individual (CNT or RuOx) coated electrodes. The enhanced electrocatalytic activity towards insulin is indicated from lowering the potential of the oxidation process (starting around 0.35 versus Ag/AgCl) and the substantially higher sensitivity over the entire potential range. A wide linear dynamic range (10-800 nM) was achieved with a detection limit of 1 nM. The marked electrocatalytic activity of the RuOx/CNT coating towards insulin is coupled with a greatly enhanced stability. For example, the insulin amperometric response of the RuOx/CNT-coated electrodes is highly stable, with 97% of the initial activity remaining after 60 min stirring of 2 × 10⁻⁶ M solution (compared to significantly faster current diminutions at the RuOx- or CNT-coated surfaces). The results suggest great promise for miniaturized sensors and detectors for monitoring insulin.     

Determination of benorilate in pharmaceutical formulations and its metabolite in urine at carbon paste electrode modified by silver nanoparticles

Benorilate was determined by the differential pulse voltammetry (DPV) using a carbon paste electrode modified by silver nanoparticles in 1.25 × 10⁻³ mol l⁻¹ KH₂PO₄ and Na₂HPO₄ buffer solution (pH = 6.88, 25 °C) .The anodic peak potential was +0.970 V (versus SCE). A good linear relationship was realized between the anodic peak currents and benorilate concentrations in the range of 1.0 × 10⁻⁷ to 2.5 × 10⁻⁴ mol l⁻¹ with the detection limit of 1.0 × 10⁻⁸ mol l⁻¹. The recovery was 95.2-103.6% with the relative standard deviation of 3.6% (n = 9). The pharmaceutical preparations, benorilate tablets samples and its metabolite (salicylic acid) in urine were determined with the desirable results.     

Direct electrochemical conversion of bilirubin oxidase at carbon nanotube-modified glassy carbon electrodes

We report on direct electron transfer reactions of bilirubin oxidase at multi-walled carbon nanotube (MWCNT) modified glassy carbon electrodes (GCE). The bioelectrocatalytic oxygen reduction was recorded using linear sweep voltammetry (LSV) with BOD in solution, adsorbed and covalently linked to the nanotubes. The MWCNT modification of GC electrodes strongly enhances the oxygen reduction compared to the signals at unmodified GCE. Under anaerobic conditions with a high protein concentration in solution a pair of redox peaks with a formal potential of 450 ± 15 mV vs Ag/AgCl, 1 M KCl (pH 7.4) was found with cyclic voltammetry. The redox conversion is indicated to be surface-controlled and pH-dependent (54.5 mV/pH). The quasi-reversible redox reaction might be attributed to the trinuclear T2/T3 cluster of BOD.     

Electrochemical detection of phenolic estrogenic compounds at carbon nanotube-modified electrodes

The use of a carbon nanotube-modified glassy carbon electrode (CNT-GCE) for the LC-EC detection of phenolic compounds with estrogenic activity is reported. Cyclic voltammograms for phenolic endocrine disruptors and estrogenic hormones showed, in general, an enhancement of their electrochemical oxidation responses at CNT-GCE attributable to the electrocatalytic effect caused by CNTs. Hydrodynamic voltammograms obtained under flow injection conditions lead to the selection of +700 mV as the potential value to be applied for the amperometric detection of the phenolic estrogenic compounds, this value being remarkably less positive than those reported in the literature using other electrode materials. Successive injections of these compounds demonstrated that no electrode surface fouling occurred. A mobile phase consisting of a 50:50 (v/v) acetonitrile:0.05 mol l⁻¹ phosphate buffer of pH 7.0 was selected for the chromatographic separation of mixtures of these compounds, with detection limits ranging between 98 and 340 nmol l⁻¹. Good recoveries were obtained in the analysis of underground well water and tap water samples spiked with some phenolic estrogenic compounds at a 14 nmol l⁻¹ concentration level.     

Electrochemical sensitive determination of acetaminophen in pharmaceutical formulations at iron oxide/graphene composite modified electrode

Acetaminophen (AC) is one of the most commonly prescribed analgesic and antipyretic drug, which is considered to be safe as well as effective. Rational use of AC does not pose any toxicity or adverse effects, however, an overdose or prolonged use could lead to nephrotoxicity and severe hepatoxicity. Thus, monitoring of AC is essential for drug safety. In this work, a facile Fe₂O₃/reduced graphene oxide (Fe₂O₃/RGO) nanocomposite was synthesized for improved electrochemical detection of AC. The material was synthesized through a simple one-step process. For characterization of synthesized Fe₂O₃/RGO composite, energy-dispersive X-ray spectroscopy (EDX), field emission-scanning electron microscopy (FE-SEM) and electrochemical impedance spectroscopy (EIS) were employed. To verify the electrochemical performance of Fe₂O₃/RGO nanocomposite, GCE was modified with this nanocomposite and utilized for quantification of AC. The detection limit of AC was 21 nM in a linear range from 1.0 × 10⁻⁷ to 74 × 10⁻⁶ M. Furthermore, the sensor also unveiled good stability, promising sensitivity and selectivity. Hence, Fe₂O₃/RGO could be applied as a sensing material for electrochemical detection of AC. Finally, the analytical utility of the method was also verified in human urine and drug samples with some preliminary treatments.     

KCl-assisted,chemically reduced graphene oxide for high-performance supercapacitor electrodes

Journal of Solid State Electrochemistry - Graphene with highly flaky state has been successfully prepared through chemical reduction process with the assistance of potassium chloride (KCl) to...     

Simultaneous determination of paracetamol and caffeine in pharmaceutical formulations and synthetic urine using cork-modified graphite electrodes

Journal of Solid State Electrochemistry - Cork-modified graphite electrodes were used to develop a simple and selective sensor for the simultaneous determination of paracetamol (PAR) and caffeine...     

Self-assembled electrodes based on polyaniline grafted with reduced graphene oxide and polystyrene sulfonate

Journal of Solid State Electrochemistry - In this work, composites based on polyaniline (PAni) grafted with reduced graphene oxide (rGO) were obtained by the in situ chemical polymerization of...     

Adsorptive stripping voltammetric determination of ketoconazole in pharmaceutical preparations and urine using carbon paste electrodes

The oxidation of ketoconazole on a bare carbon paste electrode was studied voltammetrically. The results indicated that the process is irreversible and controlled by an adsorption-extraction process which allows the accumulation of the drug at the electrode surface. After the optimization of solution pH, accumulation variables and instrumental parameters, sensitive differential pulse and linear sweep voltammetric peaks were obtained whose peak currents were linearly proportional to the ketoconazole concentration over the ranges 2.4 x 10(-8)-4.8 x 10(-7) and 9.1 x 10(-7)-1.0 x 10(-5) M, respectively. Based on these findings, a simple procedure was developed for the determination of ketoconazole in human urine and formulations.     

Determination of phenothiazines in pharmaceutical formulations and human urine using capillary electrophoresis with chemiluminescence detection

A CE instrument coupled with chemiluminescence (CL) detection was designed for the determination of promethazine hydrochloride (PTH) and promazine hydrochloride (PMH) in real samples. An important enhancement of the CL emission of luminol with potassium ferricyanide was observed in the presence of these phenothiazines; so this system was selected for their detection after CE separation. Parameters affecting the electrophoretic separation were optimized in a univariate way, while those affecting CL detection were optimized by means of a multivariate approach based on the use of experimental designs. Chemometrics was also employed for the study of the robustness of the factors influencing the postcolumn CL detection. The method allows the separation of the phenothiazines in less than 4 min, achieving LODs of 80 ng/mL for PMH and 334 ng/mL for PTH, using sample injection by gravity. Electrokinetic injection was used to obtain lower LODs for the determination of the compounds in biological samples. The applicability of the CE-CL method was illustrated in the determination of PTH in pharmaceutical formulations and in the analysis of PMH in human urine, using a previous SPE procedure, achieving an LOD of 1 ng/mL and recoveries higher than 85%.     

Polymer-modified glassy carbon electrode for the electrochemical detection of quinine in human urine and pharmaceutical formulations

Glassy carbon electrode was modified by electropolymerization of 4-amino-3-hydroxynaphthalene sulfonic acid. Cyclic voltammetric study of quinine showed higher current response at the modified electrode compared to the bare and activated glassy carbon electrodes in pH 7.0 phosphate buffer solution. Under optimized conditions, a calibration curve was obtained by square wave voltammetry at the modified electrode. The linear relationship between the peak current and the concentration of quinine in the range of 1.0?×?10^?7 to 1.0?×?10^?5 M was I _pa (in microamperes)?=?6.26C (in micromolars)?+?0.2997 (R ²?=?0.999). The detection limit calculated (S/N?=?3) was 1.42?×?10^?8 M, which is much lower than similar reports. The method was successfully applied for the determination of quinine in spiked human urine, and pharmaceutical formulations and recovery values >90 % were obtained.     

Poly(brilliant green)/carbon nanotube-modified carbon film electrodes and application as sensors

Poly(brilliant green) (PBG) films were formed on carbon film electrodes (CFE) by electropolymerisation of brilliant green monomer using potential cycling or at fixed potential from different pH solutions. The modified electrodes, PBG/CFE, were characterised by cyclic voltammetry (CV) in electrolytes of different pH by electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). In order to increase the stability of the polymer film and enhance the response, multi-walled carbon nanotubes (MWCNTs) were first deposited on CFE and then PBG was formed on top, PBG/CNT/CFE. The modified electrodes were applied to the amperometric determination of ascorbic acid (AA) in phosphate buffer pH?7.0 at 0.0 V vs. saturated calomel electrode (SCE) and the results were compared, the presence of CNT leading to a significant increase in sensitivity. An interference study was carried out and good separation between AA and dopamine (DA) peaks was achieved that led to the successful determination of DA without interferences. Other interferents: aspirin, acetaminophen, salicylic acid and uric acid exhibited no response on the PBG/CNT/CFE. Determination of AA in pharmaceutical samples was successfully performed.     

Preparation of reduced graphene oxide electrodes treated by electron beam irradiation and their electrochemical behaviors

Research on Chemical Intermediates - We have demonstrated the preparation of reduced graphene oxide (RGO) using a two-step synthesis by a modified Hummer’s method and borohydride reduction....     

Electrochemical study and flow injection analysis of paracetamol in pharmaceutical formulations based on screen-printed electrodes and carbon nanotubes

Acetaminophenol or paracetamol is one of the most commonly used analgesics in pharmaceutical formulations. Acetaminophen is electroactive and voltammetric mechanistic studies for the electrode processes of the acetaminophenol/N-acetyl-p-quinoneimine redox system are presented. Carbon nanotubes modified screen-printed electrodes with enhanced electron transfer properties are used for the study of the electrochemical-chemical oxidation mechanism of paracetamol at pH 2.0.Quantitative analysis of paracetamol by using its oxidation process (in a Britton-Robinson buffer solution pH 10.0) at +0.20 V (vs. an Ag pseudoreference electrode) on an untreated screen-printed carbon electrode (SPCE) was carried out. Thus, a cyclic voltammetric based reproducible determination of acetaminophen (R.S.D., 2.2%) in the range 2.5 × 10⁻⁶ M to 1 × 10⁻³ M, was obtained. However, when SPCEs are used as amperometric detectors coupled to a flow injection analysis (FIA) system, the detection limit achieved for paracetamol was 1 × 10⁻⁷ M, one order of magnitude lower than that obtained by voltammetric analysis. The repeatability of the amperometric detection with the same SPCE is 2% for 15 successive injections of 10⁻⁵ M acetaminophen and do not present any memory effect.Finally, the applicability of using screen-printed carbon electrodes for the electrochemical detection of paracetamol (i.e. for quality control analysis) was demonstrated by using two commercial pharmaceutical products.