Voltammetric Method for the Simultaneous Determination of Melatonin and Pyridoxine in Dietary Supplements Using a Cathodically Pretreated Boron‐doped Diamond Electrode

The simultaneous voltammetric determination of melatonin (MT) and pyridoxine (PY) has been carried out at a cathodically pretreated boron‐doped diamond electrode. By using cyclic voltammetry, a separation of the oxidation peak potentials of both compounds present in mixture was about 0.47 V in Britton‐Robinson buffer, pH 2. The results obtained by square‐wave voltammetry allowed a method to be developed for determination of MT and PY simultaneously in the ranges 1–100 μg mL⁻¹ (4.3×10⁻⁶–4.3×10⁻⁴ mol L⁻¹) and 10–175 μg mL⁻¹ (4.9×10⁻⁵–8.5×10⁻⁴ mol L⁻¹), with detection limits of 0.14 μg mL⁻¹ (6.0×10⁻⁷ mol L⁻¹) and 1.35 μg mL⁻¹ (6.6×10⁻⁶ mol L⁻¹), respectively. The proposed method was successfully to the dietary supplements samples containing these compounds for health‐caring purposes.     

Electrodetermination of Gallic Acid Using Multi-walled Carbon Nanotube Paste Electrodes and N-Octylpyridinium Hexafluorophosphate

In this work, the determination of gallic acid was performed using surface-renewable carbon paste electrodes fabricated with multi-walled carbon nanotubes (MWCNT) and a mixture of N-octylpyridinium hexafluorophosphate (OPyPF₆) ionic liquid with mineral oil (MO) as binder. This system shows remarkable amperometric sensor characteristics and promotes a better electronic transfer. An electroanalytical study of gallic acid shows a linear range from 4.98±0.25 to 74.1±2.2 μmol L⁻¹, with R²=0.9958 and an experiment a limit of detection of 2.70±0.08 μmol L⁻¹ (S/N=3), and a sensitivity of 0.029 μA μmol⁻¹ L.     

A comparison study of adsorptive transfer voltammetry and solution phase voltammetry for the determination of caffeic acid

This study reports a comparison of adsorptive transfer and solution phase voltammetric methods for the study of caffeic acid. For this purpose, a platform was prepared by the modification of glassy carbon electrodes (GCEs) with MWCNTs and samarium nanoparticles (SmNPs) by means of an ultrasonic bath. The surface morphology of the platform was characterized using SEM, EDX and XRD. The adsorptive transfer voltammetric method was based on the adsorption of caffeic acid (CFA) at the surface of the modified electrode by keeping it into a solution of CFA. Afterwards, the modified electrode was transferred with the adsorbed species in a cell containing only 0.1 mol L⁻¹ phosphate buffer solution (PBS) for the analysis. The current response of CFA was found to be linear over a concentration from 5.0 × 10⁻¹⁰ mol L⁻¹ to 1.0 × 10⁻⁷ mol L⁻¹. The values of the limit of detection (LOD) and limit of quantification (LOQ) were 2.0 × 10⁻¹⁰ mol L⁻¹ and 6.67 × 10⁻¹⁰ mol L⁻¹, respectively. The adsorptive transfer method using the modified electrode (SmNPs/MWCNTs/GCE) has successfully been applied to food samples for determining CFA. The solution phase voltammetry was carried out by dipping the electrode into a voltammetric cell containing CFA. The plot of peak currents was linear over the concentration range of 5.0 × 10⁻⁹ mol L⁻¹ –8.0 × 10⁻⁸ mol L⁻¹. The values of LOD and LOQ were 2.0 × 10⁻⁹ mol L⁻¹ and 6.67 × 10⁻⁹ mol L⁻¹ for CFA using a classical solution phase voltammetry at the proposed platform. It was shown that the LOD obtained at adsorptive transfer voltammetry was 10-fold lower when compared to classical solution phase voltammetry.     

Ionic Organic Film Sensor for Determination of Phenolic Compounds

This paper describes the development of a new sensor based on an ionic organic film. The amphiphilic molecule, 4‐[(4‐decyloxyphenyl)‐ethynyl]‐1‐methylpyridinium iodide (10PyI), which has liquid‐crystalline properties, was synthesized and applied in the construction of a GCE/10PyI sensor. Analytical parameters for caffeic acid, repeatability (4.8 %), reproducibility (2.8 %), linearity (two ranges: 9.9×10^?7 to 3.8×10^?5 mol L^?1 and 4.7×10^?5 to 9.9×10^?5 mol L^?1) and detection limits (9.0×10^?7 mol L^?1 and 8.7×10^?6 mol L^?1), were determined. The method was successfully applied in the determination of total phenolic compounds (TPC) in mate herb extracts.     

Determination of Gallic Acid by Flow Injection with Electrochemiluminescent Detection

ABSTRACT

Potential dependent inhibition and enhancement effects of gallic acid on the electrochemiluminescence (ECL) of luminol in alkaline solutions were found. On the basis of the enhancement effect, a flow injection method was developed for the determination of gallic acid. The method was simple, convenient and sensitive. A detection limit of 1.8×10^?8 mol/L, linear range of 3.0×10^?8 - 1.0×10^?4 mol/L, relative standard deviation of 1.0% for eleven measurements of 5.0×10^?6 mol/L of gallic acid was found. This method was successfully applied to determine the content of gallic acid in Chinese gall (moshizi).     

Simultaneous Differential Pulse Voltammetric Determination of Ascorbic Acid and Caffeine in Pharmaceutical Formulations Using a Boron‐Doped Diamond Electrode

A cathodically pretreated boron‐doped diamond electrode was used for the simultaneous anodic determination of ascorbic acid (AA) and caffeine (CAF) by differential pulse voltammetry. Linear calibration curves (r=0.999) were obtained from 1.9×10^?5 to 2.1×10^?4 mol L^?1 for AA and from 9.7×10^?6 to 1.1×10^?4 mol L^?1 for CAF, with detection limits of 19 μmol L^?1 and 7.0 μmol L^?1, respectively. This method was successfully applied for the determination of AA and CAF in pharmaceutical formulations, with results equal to those obtained using a HPLC reference method.     

MDMA Electrochemical Determination and Behavior at Carbon Screen-printed Electrodes: Cheap Tools for Forensic Applications

This article describes a method that uses Carbon Screen-printed Electrodes (C-SPEs) to detect 3,4 – methylenedioxyamphetamine (MDMA) by Linear Sweep Voltammetry in aqueous medium. Major parameters of this technique were evaluated aiming improve the method sensibility. Amines interference were conducted in order to verify disturbs at the MDMA response. The method obtained a linear response from 1×10⁻⁵ mol L⁻¹ to 1×10⁻⁴ mol L⁻¹ with linear correlation coefficient of 0.996, Amperometric Sensitivity (AS) of 0.025 A×mol⁻¹ L, Limit of Detection (LOD) of 1,83×10⁻⁶ mol L⁻¹, and Limit of Quantification (LOQ) of 6,11×10⁻⁶ mol L⁻¹. The method applicability, reproducibility and reproducibility were carried over inter/intra days tests and its application on seized samples.     

Flow—injection Chemiluminescence Sensor for the Determination of Gallic Acid by Immobilizing Luminol and Periodate on Anionexchange Resin

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Comparative Study on Electroanalysis of Fenthion Using Silver Amalgam Film Electrode and Glassy Carbon Electrode Modified with Reduced Graphene Oxide

Oxidation and reduction processes of the insecticide fenthion was comparatively investigated at a reduced graphene oxide modified glassy carbon electrode (RGO‐GCE) and a cyclic renewable silver amalgam film electrode (Hg(Ag)FE) using square wave stripping voltammetry (SWSV). The influence of pH and SW parameters was investigated. The linear concentration ranges were found to be 1 × 10⁻⁶ – 2 × 10⁻⁵ and 1 × 10⁻⁷ – 2 × 10⁻⁵ mol L⁻¹ for Hg(Ag)FE and RGO‐GCE, respectively. The detection and quantification limits were calculated as 1.3 × 10⁻⁷ and 4.5 × 10⁻⁷ mol L⁻¹ for Hg(Ag)FE and 7.6 × 10⁻⁹ and 2.5 × 10⁻⁸ mol L⁻¹ for RGO‐GCE. Both of the developed electroanalytical methods offer rapid and simple detection of fenthion and were used on spiked tap and river water and apple juice samples. Scanning electron microscopy was used for RGO‐GCE surface characterization.     

A Simple and Rapid Estimation of Totals Polyphenols Based On Carbon Paste Electrode Modified with Ruthenium Oxo‐Complex

A simple and fast method based on electrochemical measurements was proposed for estimation of total polyphenols using a carbon paste modified electrode. The method was based on catalytic response exhibited by a ruthenium complex [(bpy)₂(NH₃)Ru^III(µ‐O)Ru^III(NH₃)(bpy)₂]⁴⁺ in presence of gallic acid (GA). Calibration plots using chronoamperometry (CA) showed a linear response for GA concentrations ranging from 6.6×10^?6 to 1.9×10^?4 mol L^?1 with a detection limit of 4.9×10^?7 mol L^?1. Comparative studies using the official method revealed a good agreement between methods suggesting that the proposed method can be applied for polyphenol estimation without any sample treatment.     

Laccase-Nafion Based Biosensor for the Determination of Polyphenolic Secondary Metabolites

A laccase-based biosensor was developed by specific enzyme adsorption on screen-printed working electrodes of DROPSENS cells, and stabilized with Nafion 0.1% membrane. The electrode was characterized with respect to response time, sensitivity, linear range, detection limit, pH dependence, interferences, and long-term stability. The tested substrates were catechol, rosmarinic acid, caffeic acid, chlorogenic acid, and gallic acid. The optimized biosensor proved the following characteristic performances: the apparent Michaelis Menten calculated considering rosmarinic acid substrate 8.3 × 10^?6 mol L^?1 (r = 0.995, n = 6); the dynamic range of biosensor response for rosmarinic acid 7 × 10^?7 ? 1.5 × 10^?6 mol L^?1; the detection limit for rosmarinic acid 1.19 × 10^?7 mol L^?1 (RSD = 1.08%, n = 3). It was noticed that the biosensor reaches systematically 90% to 94.3% from the response obtained by LC-DAD-ESI-MS for real samples.     

Fast analysis of domoic acid using microchip electrophoresis with laser‐induced fluorescence detection

A fast and effective method was developed to detect domoic acid based upon microchip electrophoresis combined with laser‐induced fluorescence detection. Through study of the gated injection process on the cross channel of the microchip, the low‐voltage mode with relatively longer sample loading time was adopted to reduce the sample discrimination and improve the signal sensitivity. Fluorescein isothiocyanate was used as the derivatizing reagent for domoic acid. Under the optimized conditions, domoic acid was completely separated in 60 s with separation efficiency of 1.35 × 10⁵ m⁻¹. The calibration curve was obtained in the range of 1.0 × 10⁻⁹ to 1.0 × 10⁻⁷ mol/L, and the detection limit reached 2.8 × 10⁻¹⁰ mol/L. This developed method was successfully applied to analyze domoic acid in real samples.     

Simultaneous voltammetric measurement of ascorbic acid and dopamine on poly(caffeic acid)-modified glassy carbon electrode

A poly(caffeic acid) thin film was deposited on the surface of a glassy carbon electrode by potentiostatic technique in an aqueous solution containing caffeic acid. The poly(caffeic acid)-modified electrode was used for the determination of ascorbic acid (AA), dopamine (DA), and their mixture by cyclic voltammetry. This modified electrode exhibited a potent and persistent electron-mediating behavior followed by well-separated oxidation peaks toward AA and DA at a scan rate of 10 mV s⁻¹ with a potential difference of 135 mV, which was large enough to determine AA and DA individually and simultaneously. The catalytic peak current obtained was linearly dependent on the AA and DA concentrations in the range of 2.0 × 10⁻⁵−1.2 × 10⁻³ and 1.0 × 10⁻⁶−4.0 × 10⁻⁵ mol L⁻¹ in 0.15 mol L⁻¹ phosphate buffer (pH 6.64). The detection limits for AA and DA were 9.0 × 10⁻⁶ and 4.0 × 10⁻⁷ mol L⁻¹, respectively. The modified electrode shows good sensitivity, selectivity, and stability and has been applied to the determination of DA and AA in real samples with satisfactory results.     

Simultaneous Voltammetric Detection of Salsolinol and Uric Acid in the Presence of High Concentration of Ascorbic Acid with Gold Nanoparticles/Functionalized Multiwalled Carbon Nanotubes Composite Film Modified Electrode

This work demonstrates gold nanoparticles (AuNPs)/functionalized multiwalled carbon nanotubes (f‐MWCNT) composite film modified gold electrode via covalent‐bonding interaction self‐assembly technique for simultaneous determination of salsolinol (Sal) and uric Acid (UA) in the presence of high concentration of ascorbic acid (AA). In pH 7.0 PBS, the composite film modified electrode exhibits excellent voltammetric response for Sal and UA, while AA shows no voltammetric response. The oxidation peak current is linearly increased with concentrations of Sal from 0.24–11.76 μmol L^?1 and of UA from 3.36–96.36 μmol L^?1, respectively. The detection limits of Sal and UA is 3.2×10^?8 mol L^?1 and 1.7×10^?7 mol L^?1 , respectively.     

Amperometric detection of indapamide on glassy carbon electrode

This work describes the development of a fast, precise and reliable voltammetric method for the quantification of indapamide, an orally active diuretic sulfonamide used for hypertensive treatment. This compound acts inhibiting sodium reabsorption and increasing the elimination of water. This characteristic was responsible for its banishment by the International Olympic Committee since 1999. The study begins by finding an adequate potential range (−0.20 to 0.30 V) to avoid poisoning the working glassy carbon electrode (GCE) in phosphate buffer 0.10 mol L⁻¹ (pH=12.0). Utilizing flow injection analysis, linear responses between 2.0 × 10⁻⁶ mol L⁻¹ to 2.5 × 10⁻⁵ mol L⁻¹ of indapamide (R²=0.995), and detection limit (LOD) 3.0 × 10⁻⁷ mol L⁻¹ were obtained. This method was applied for the quantification of indapamide in tablets and in synthetic urine. The same flow system was used for the analysis of commercial drugs and the response obtained corresponded to 98 % of the concentration indicated on the drug label. These tablets were also analyzed by high performance liquid chromatography (HPLC), obtaining a recovery of 103 % and LOD 4.0 × 10⁻⁷ mol L⁻¹. The velocity of analysis using flow methods compares advantageously to the classical chromatographic methods. For synthetic urine, linear responses were obtained in samples spiked in the region from 5.0 × 10⁻⁶ mol L⁻¹ to 30 × 10⁻⁶ mol L⁻¹ (R²=0.991) and LOD 3.0 × 10⁻⁷ mol L⁻¹.     

Self‐powered Photoelectrochemical Sensor for Gallic Acid Exploiting a CdSe/ZnS Core‐shell Quantum Dot Sensitized TiO2 as Photoanode

Herein is described the development of a self‐powered sensor for gallic acid (GA) determination exploiting CdSe/ZnS quantum dot sensitized TiO₂ nanoparticles (CdSe/ZnS/TiO₂/FTO) as photoanode and an all copper oxide photocathode (CuO/Cu₂O/FTO) to reduce water. A two‐chamber self‐powered photoelectrochemical cell was employed in order to maintain separated the photoelectrodes. The self‐powered photoelectrochemical cell is based on water reduction in the cathodic chamber while gallic acid acts as a hole scavenger in the anodic chamber to generate the necessary cell output to drive GA oxidation in the anodic compartment. Electrochemical impedance measurements were performed to evaluate the electronic characteristics of CdSe/ZnS/TiO₂/FTO photoanode and CuO/Cu₂O/FTO photocathode in terms of flat band potential, carrier density, and nature of semiconductor. Under optimized conditions, the self‐powered photoelectrochemical cell presented a wide linear response range for GA from 1 μmol L⁻¹ up to 200 μmol L⁻¹.     

Catalytic Adsorptive Stripping Voltammetry of Germanium(IV) in the Presence of Gallic Acid and Vanadium(IV)‐EDTA

A highly sensitive and selective catalytic adsorptive cathodic striping procedure for the determination of trace germanium is presented. The method is based on adsorptive accumulation of the Ge(IV)‐gallic acid (GA) complex onto a hanging mercury drop electrode, followed by reduction of the adsorbed species. The reduction current is enhanced catalytically by addition of vanadium(IV)‐EDTA. The optimal experimental conditions include the use of 0.03 mol/L HClO₄ (pH1.6), 6.0×10^?3 mol/L GA, 3.0×10^?3 mol/L V(IV), 4.0×10^?3 mol/L EDTA, an accumulation potential of ?0.10 V(vs. Ag/AgCl), an accumulation time of 120 s and a differential pulse potential scan mode. The peak current is proportional to the concentration of Ge(IV) over the range of 3.0×10^?11 to 1.0×10^?8 mol/L and the detection limit is 2×10^?11 mol/L for a 120 s adsorption time. The relative standard deviation at 5.0×10^?10 mol/L level is 3.1%. No serious interferences were found. The method was applied to the determination of germanium in ore, mineral water and vegetable samples with satisfactory results.     

Magnetite Nanoparticles Bonded Carbon Quantum Dots Magnetically Confined onto Screen Printed Carbon Electrodes and their Performance as Electrochemical Sensor for NADH

Hybrid magnetite/carbon quantum dots (MagNP/C‐dots) were prepared and their characterization performed by high resolution transmission electron microscopy (HR‐TEM), X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS). Because of their suitable magnetization and electrochemical properties, they were used as versatile electrode modifiers after magnetically confining onto screen printed carbon electrodes (SPE), with the aid of a miniature external magnet. The reported strategy introduces a convenient procedure for assembling modified electrodes, since the nanoparticles can be easily released by removing the magnet. The non‐enzymatic magnetic biosensor showed excellent performance in the determination of NADH at the concentration range 2×10⁻⁷ to 5×10⁻⁶ mol L⁻¹, exhibiting a sensitivity of 0.15 μmol L⁻¹ and detection limit of 20 nmol L⁻¹. The MagNP/C‐dots/SPE sensor was also successfully applied for the determination of NADH in serum samples. The interference of typical biological molecules has also been investigated.     

Preparation of Organophilic Nanohybrid Kaolinite and Application to the Electrochemical Detection of Organic Pesticide

Novel organophilic nanohybrid materials (K‐TDD) were obtained by the grafting of 1,2‐tetradecanediol (TDD) onto the surface of kaolinite (K). XRD, IR, TGA‐DTG, and SEM characterization showed that TDD grafting results in a partial exfoliation of kaolinite layers. This material was used to modify a glassy carbon electrode (GCE/K‐TDD) and applied for the trace analysis of methyl parathion (MP). The signal of MP recorded on GCE/K‐TDD was more intense compared to the unmodified GCE or to one modified with a film of natural kaolinite. Several parameters that can affect the stripping response were systematically investigated to optimize the sensitivity of the organokaolinite‐modified electrode. A linear calibration curve for MP was obtained in the concentration range from 2×10⁻⁶ to 14×10⁻⁶ mol .L⁻¹ in acetate buffer (pH 6), giving a detection limit of 9×10⁻⁸ mol .L⁻¹. The sensitivity of the method was found to be 2.42 μA/μM for the range of concentrations that gives a linear calibration curve. The electrode was shown to be very stable, with the electrochemical response of MP decreasing by only 1.5 % after a series of nine measurements. The interference of various inorganic ions and organic compounds likely to influence the stripping determination of the MP were also examined. The results showed that the GCE/K‐TDD electrode was effective in solutions containing interfering species and could be applied for the quantification of MP pesticide in natural water.     

Electropolymerization of Methylene Blue on Carbon Ionic Liquid Electrode and Its Electrocatalysis to 3,4‐Dihydroxybenzoic Acid

In this paper an ionic liquid modified carbon paste electrode (CILE) was prepared and methylene blue (MB) was electropolymerized on the CILE by using the cyclic voltammetric technique in the potential range from −1.0 V to 0.8 V (vs. SCE). A stable polymer film was obtained and exhibited a pair of redox peaks. The morphology and characteristics of poly(methylene blue) (PMB) film was studied by the techniques such as scanning electron microscopy and electrochemical impedance spectroscopy. This PMB modified CILE (PMB/CILE) showed excellent electrocatalytic response to 3,4‐dihydroxybenzoic acid with the increase of the electrochemical responses. The oxidation peak current had a linear relationship with 3,4‐dihydroxybenzoic acid concentration in the range of 5.0 × 10⁻⁴ ∼ 3.0 × 10⁻² mol L⁻¹ and the detection limit was 1.72 × 10⁻⁴ Mol L⁻¹ (3 σ).