Ac voltammetric and square wave voltammetric study of the self-assembled monolayers of thiol-functionalized viologen derivative

Electroactive self-assembled monolayers (SAMs) containing viologeu group are formed through the adsorption of thiol-functionalized viologen compound CH3(CH2)9V2+(CH2)8SH,where V2+ is N,N'-diaIkylbipyridinium (i.e.a viologen group),onto gold electrodes from methanol/water solution and its electrochemical behavior is investigated by Ac voltammetry and square wave voltam-metry,which have the high sensitivity against background charging.The viologen SAM formed is a sub-monolayer and the normal potentials corresponding to the two successive one-electron transfer processes of the active centers (viologen) are-360 mV and-750 mV (vs.Ag/AgCl) in 0.1mol/L phosphate buffer solutions (pH 6.96) respectively,and the standard electron transfer rate constant is 9.0s-1 The electrochemical behavior of this SAM in various solutions has been preliminarily discussed.     

盐酸克伦特罗在玻碳电极上的伏安行为研究

采用电化学技术研究了盐酸克伦特罗(CLB)在玻碳电极上的电化学还原行为。在pH4.54的Britton Robinson缓冲液中,CLB于 0.38V(vs.Ag AgCl)左右处出现一个明显的方波溶出伏安还原峰,峰电流与CLB的浓度在3.0×10-7～1.0×10-5mol L范围内呈良好的线性关系,检出限为5.1×10-8mol L。该法可应用于模拟兔血清及尿样中CLB的测定。     

径向基神经网络奥斯特杨方波伏安法同时测定铬和锌

径向基函数神经网络(RBFN)和核心偏最小二乘法(KPLS)用于分析重叠的Cr（Ⅲ）和Zn(Ⅱ）的奥斯特杨(Osteryoung)方波伏安图，程序SPRBFN和SPKPLS被设计用于全部计算。在RBFN方法中，普通高斯函数可用作隐藏层非线性转移函数。由于其局部性质，RBFN能被快速训练，避免陷入局部最小。对两个方法预测能力的研究结果显示其所有组分的相对预测标准偏差(RSEP)分别为0．677％和13．0％。因此，RBFN方法较之K眦方法可提供更为精确的结果，而且在解决局部最小，改进收敛速率方面也不失为一个重要的工具。     

Evaluation of a platinum electrode modified with hydroxyapatite in the lead(II) determination in a square wave voltammetric procedure

The behavior of a modified carbon platinum electrode (Pt) for lead(II) determination by square wave voltammetry (SWV) was studied. The modified electrode is obtained by electrodeposition of hydroxyapatite (HAP) on the surface of a bare platinum electrode. The new electrode (HAP/Pt) revealed interesting electroanalytical detection of lead(II) based on the adsorption of this metal onto hydroxyapatite under open circuit conditions. After optimization of the experimental and voltammetric conditions, the best voltammetric responses-current intensity and voltammetric profile were obtained in 0.2 mol L^?1 KNO₃ with: 30 min accumulation time, 5 mV pulse amplitude and 1 mV s^?1 scan rate. The observed detection (DL, 3σ) and quantification (DL, 10σ) limits in pure water were 2.01 × 10^?8 and 6.7 × 10^?7 mol L^?1, respectively. The reproducibility of the proposed method was determined from five different measurements in a solution containing 2.2 × 10^?6 mol L^?1 lead(II) with a coefficients of variation of 2.08%.The electrochemical of hydroxyapatite at platinum surfaces was characterized, after calcinations 900 °C, by X-ray diffraction, infrared spectroscopy, chemical and electrochemical analysis.     

Multiple square wave voltammetry for analytical determination of paraquat in natural water, food, and beverages using microelectrodes

This paper reports on the use of multiple square wave voltammetry (MSWV) for analytical determination of paraquat herbicide at gold microelectrode (Au-ME) in different samples of natural water, food, and beverages. In this work, the MSWV consisted in a sequence of four pairs of potential pulse in the same step and the interval potential evaluated was of the 0.0 V at −1.2 V versus Ag/AgCl 3.0 mol L⁻¹. The paraquat herbicide presented two reduction peaks, in −0.69 V and −0.99 V, with profile of the redox process totally reversible, and the use of multiple pulses allowed a detection of nanomolar levels after the optimization of experimental and voltammetric conditions. Analytical curves were constructed for pulse potential frequency of 250 s⁻¹, pulse amplitude of 50 mV, scan increment of 2 mV and pulse number of 8 pulses in a same step. The two reduction peaks showed that the peak currents were found to be directly proportional to the pesticide concentration in the range comprised between 5.0 × 10⁻⁷ mol L⁻¹ and 1.04 × 10⁻⁵ mol L⁻¹. With this, it was possible to determine detection limits (DL), which resulted in 0.044 μg L⁻¹ (0.044 ppb) and 0.146 μg L⁻¹ (0.146 ppb), respectively, for peak 1 and peak 2. DL results, obtained using MSWV, were 2–3 orders of magnitude lower (10⁻² to 10⁻³) less than those observed for traditional square wave voltammetry or published in literature, clearly pointing to the advantages arising from the possibility of using a MSWV for analytical purposes in contaminated matrices. In addition, the proposed methodology was applied in different samples of natural water, food and beverages without pre-treatment or pre-concentration step, where a recovery measurement indicated that the methodology could be employed to analyze paraquat in such matrices.     

Determination of hydrazine in polyvinylpyrrolidone by derivatization and square wave voltammetry

A rapid and precise method has been developed for the determination of hydrazine in polyvinylpyrrolidone (PVP). Hydrazine is converted to salicylaldazine by reaction with salicylaldehyde. After extraction, the azine is reduced at –1.56 V versus the SCE at the hanging drop electrode. Detection is accomplished using square wave voltammetry and comparison to an external salicylaldazine standard. The described procedure can detect as little as 70 ng/g hydrazine in PVP.     

Determination of ziram in vegetable samples by square wave voltammetry

The electrochemical behaviour and electrode reaction mechanism of ziram （zinc-dimethyl dithiocarbamate） on a hanging mercury drop electrode were investigated in Britton-Robinson （B-R） buffer by using cyclic and square wave voltammetry （SWV）. Based on these studies a voltammetric method for determination of ziram wasdeveloped and applied to determine the ziram in synthetic and spiked vegetable samples, satisfactory results were obtained in both cases.     

The surface composition of spray-dried protein—lactose powders

The surface composition of spray-dried mixtures of lactose-protein and lactose-glycine were estimated by means of electron spectroscopy for chemical analysis (ESCA). The results show that even with a low concentration of protein (0.01 wt.%) in the solution to be dried, protein starts to appear on the surface of the powder. At a protein/lactose ratio of 1/99 the protein starts to dominate the powder surface. At a protein/lactose ratio of 20/80, about 70% of the surface is covered by protein. The results are similar for the proteins sodium caseinate and bovine albumin.

The spray drying of mixtures of lactose and glycine gives a different result. In this case, the surface composition of the powder reflects the composition of the mixture to be dried.

The surface tensions of the solutions show that the proteins have a higher surface activity than lactose, since even a small amount of protein added to a lactose solution lowers the surface tension considerably. Glycine affects the surface tension only to a minor extent.

These results show that the composition of the air-water interface of the drying droplets is reflected in the surface composition of the dried powder. In addition, scanning electron micrographs show that the changes in the powder structure when protein is added to the solution are associated with the presence of protein on the surface. When the surface coverage of protein increases, dents start to appear in the particles. The powders made from lactose-glycine solutions are highly agglomerated regardless of the glycine concentration.     

Selective square wave voltammetric determination of (+)-catechin in commercial tea samples using beta-cyclodextrin modified carbon paste electrode

Osteryoung square wave cathodic or anodic voltammetry has been used for the determination of (+)-catechin (CAT) at physiological pH in the presence of 0.05 mol · L⁻¹ phosphate/borate buffer using beta-cylcodextrin modified carbon paste electrode. A linear range up to 70.0 μg · mL⁻¹ CAT in cathodic voltammetry and up to 45.0 μg · mL⁻¹ CAT in anodic voltammetry was achieved. The method gave reproducible and reliable results with 1.35 μg · mL⁻¹ CAT as a detection limit with R.S.D. less than 2%. The proposed electrode was applied successfully for the determination of CAT in commercial tea samples with acceptable recovery range (98–102%). The extraction of CAT from commercial tea samples was rather simple, making it suitable for studies with a large number of commercial tea samples giving high accuracy of the proposed method. Correspondence: Deia El-Hady, Department of Chemistry, Faculty of Science, Assiut University, 71516 Assiut, Egypt     

Determination of BHT in the presence of surfactants using a cellulose acetate coated electrode and square wave voltammetry

A direct voltammetric method for the rapid determination of butylated hydroxytoluene has been developed. The procedure utilizes a hydrolyzed cellulose acetate film in intimate contact with a glassy carbon electrode and square wave voltammetry. The coated electrode prevents passivation due to adsorption of polymeric oxidation products by way of a size exclusion principle. BHT solutions containing lipid-like surfactants can be analyzed without the need for frequent resurfacing of the electrode.     

Adsorptive stripping square wave voltammetry (Ad-SSWV) accomplished with second-order multivariate calibration determination of fenitrothion and its metabolites in river water samples

A method, using stripping square wave voltammetry (Ad-SSWV), for the simultaneous determination of fenitrothion (FEN) and its metabolites: fenitrooxon (OXON) and 3-methyl-4-nitrophenol (3-MET) in environmental samples is reported. All three compounds produce, at mercury electrode (HMDE), an electrochemical signal due to an adsorptive-reductive process. The electrochemical approach shows a very high overlap degree for FEN and OXON voltammograms, however the adsorption kinetic profile could be used as an additional differential variable between both analytes. Second-order multivariate calibration has been tested to solve the mixture of the three compounds. The second-order assayed methods were parallel factor analysis (PARAFAC), unfolded partial least squares (U-PLS), multidimensional partial least squares (N-PLS) and the latest ones were used in combination with the residual bilinearization procedure RBL. U-PLS/RBL model was stated as the best second-order algorithm for the simultaneous determination of these three compounds up to 50 ng mL⁻¹ for each analyte. The detection limits and recovery values were 1.6 ng mL⁻¹ and 92 ± 7% for FEN; 3.7 ng mL⁻¹ and 101 ± 9% for OXON and 0.6 ng mL⁻¹ and 97 ± 8% for 3-MET.     

Simultaneous Determination of Adenosine and Adenosine‐5′‐triphosphate at Nanogold Modified Indium Tin Oxide Electrode by Osteryoung Square‐Wave Voltammetry

Simultaneous determination of adenosine and adenosine‐5′‐triphosphate has been described using nanogold modified indium tin oxide electrode. Gold nanoparticles catalyze adenosine oxidation which results in increasing separation of oxidation peaks of adenosine and ATP, making it possible to determine adenosine and adenosine‐5′‐triphosphate simultaneously. The detection limits for adenosine and ATP were found as 0.07 μM and 0.10 μM respectively with sensitivity 22.9 nA μM^?1 and 20.9 nA μM^?1. The proposed method was also used for sensing the compounds in biological samples. Influence of various square‐wave parameters and different pH conditions on peak current has also been reported.     

Simultaneous square-wave voltammetric determination of riboflavin and folic acid in pharmaceutical preparations

The square-wave voltammetric (SWV) behaviour of riboflavin and folic acid was studied at a static mercury drop electrode by square wave voltammetry. In 0.05M KCl (pH 5.89) a cathodic scan gave peaks at — 0.56 and — 0.87 V vs. Ag/AgCl for riboflavin and folic acid, respectively. The reduction peak currents are linearly dependent on the concentration of vitamins. Both vitamins can be simultaneously determined from the same voltammogram. The method proposed for the determination of riboflavin and folic acid in multivitamin tablets is very simple, rapid and does not involve time-consuming separation steps. The average contents of riboflavin and folic acid were found to be 14.8 ± 1.26% and 1.46 ± 2.66%, for tablet A and 9.86 ±1.40% and 1.47 ± 2.0% for tablet B, respectively.     

Development of a simple extraction procedure for chlorpyrifos determination in food samples by immunoassay

The suitability of immunoassay methodology for rapid and accurate determination of chlorpyrifos in vegetables was tested. The optimised ELISA detection limit was 0.32 ng/ml, with a working range from 0.69 to 6.21 ng/ml and an immunoassay test-mid point (IC₅₀) of 2.08 ng/ml. A rapid sample preparation procedure considering different parameters such as the amount of sample, volume of extractant, extraction time and dilution factor was optimised. The developed direct extraction (DE) and multiresidue (ME) standard procedures were performed in different fortified fresh and processed vegetable samples (tomato, bonnet pepper, bean, pea, asparagus, broccoli, watermelon, melon, lettuce, cucumber, celery and red pepper). Recoveries were in all cases in the whole range 85.2-108.9% for both DE and ME extracts. Also, the comparison of the results obtained by both immunochemical and chromatographic methods for spiked fruits and vegetables were good with a correlation coefficient (r) of 0.97.     

Applications of stripping voltammetric techniques in food analysis

Stripping voltammetric techniques are powerful analytical tools that are becoming widely used in various chemical analysis fields. Hence, the objective of this survey is to give a general overview on the scope of the applicability of stripping voltammetric methods in food industries. The applications discussed include recent studies on the utilization of these electroanalytical methods in determination of food contaminants (toxic metals, pesticide, fertilizers and veterinary drugs residuals), trace essential elements, food additive dyes and other organic compounds of biological significance. Tables that give method summaries referenced to the original work are provided.     

Development of a green chromatographic method for determination of colorants in food samples

A green chromatographic analytical method for determination of Tartrazine, Brilliant Blue and Sunset Yellow in food samples is proposed. The method is based on the modification of a C18 column with a 0.25% (v/v) Triton X-100 aqueous solution at pH 7 and in the usage of the same surfactant solution as mobile phase without the presence of any organic solvent modifier. After the separation process on the chromatographic column, the colorants are detected at 430, 630 and 480 nm, respectively. The chromatographic procedure yielded precise results and is able to run one sample in only 8 min, consuming 15.0 mg of Triton X-100 and 38.8 mg of phosphate. When the flow rate of the mobile phase is 1 ml min⁻¹ the retention times are 2.1, 3.6 and 7.0 min for Tartrazine, Brilliant Blue and Sunset Yellow, respectively; and all peak resolutions are ca. 2. The analytical curves present the following linear equations: area = 7.44 10⁵ + 2.71 10⁵ [Tartrazine] (R = 0.998, n = 7); area = 1.09 10⁵ + 3.75 10⁵ [Brilliant] (R = 0.9995, n = 7) and area = −7.34 10⁴ + 2.33 10⁵ [Sunset] (R = 0.998), n = 7) and, the limits of detection for Tartrazine, Brilliant Blue and Sunset Yellow were estimated as 0.125, 0.080 and 0.143 mg l⁻¹. When the proposed method is applied to food samples analysis, precise results are obtained (R.S.D. < 5%, n = 3) and in agreement with those obtained by using the classical spectrophotometric method. The traditional usage of organic solvent as mobile phase in HPLC is not used here, which permits to classify the present method as green.     

Square wave adsorptive stripping voltammetric determination of famotidine in urine

Electrochemical studies of famotidine were carried out using voltammetric techniques: cyclic voltammetry, linear sweep and square wave adsorptive stripping voltammetry. The dependence of the current on pH, buffer concentration, nature of the buffer, and scan rate was investigated. The best results for the determination of famotidine were obtained in MOPS buffer solution at pH 6.7. This electroanalytical procedure enabled to determine famotidine in the concentration range 1 × 10⁻⁹–4 × 10⁻⁸ mol L⁻¹ by linear sweep adsorptive stripping voltammetry (LS AdSV) and 5 × 10⁻¹⁰–6 × 10⁻⁸ mol L⁻¹ by square wave adsorptive stripping voltammetry (SW AdSV). Repeatability, precision and accuracy of the developed methods were checked. The detection and quantification limits were found to be 1.8 × 10⁻¹⁰ and 6.2 × 10⁻¹⁰ mol L⁻¹ for LS AdSV and 4.9 × 10⁻¹¹ and 1.6 × 10⁻¹⁰ mol L⁻¹ for SW AdSV, respectively. The method was applied for the determination of famotidine in urine.     

A voltammetric method for diuron determination in river water

A sensitive, simple, fast, and inexpensive method for determining diuron in water using square wave voltammetry and a glassy carbon electrode is presented. This method was developed to evaluate diuron contamination in river waters close to sugarcane cultivation, located in the metropolitan region of João Pessoa, in the state of Paraíba, Brazil. An analytical curve with a linear response ranging from 38.5–115.0 nmol L⁻¹ (r² = 0.993), a LOD equal 0.2 nmol L⁻¹, and recovery rates from 88.0 to 108.0 %. The proposed method does not require any previous chemical treatment of the sample, thus contributing to green analytical chemistry.     

Simultaneous spectrophotometric determination of three food dyes by using the first derivative of ratio spectra

Ternary mixtures of colorants E-123 (Amaranth), E-124 (Ponceau 4R) and E-120 (Carminic acid) are resolved by using the first derivative ratio spectrum-zero crossing method without the need for any separation step. Calibration graphs were linear up to 64 mg l⁻¹ of carminic acid, 32 mg l⁻¹ of Amaranth and 32 mg l⁻¹ of Ponceau 4R. The method was applied to different commercial food products and results concordant with high performance liquid chromatography were obtained.     

Direct determination of sulfite in food samples by a biosensor based on plant tissue homogenate

In the work described here, a biosensor was developed for the determination of sulfite in food. Malva vulgaris tissue homogenate containing sulfite oxidase enzyme was used as the biological material. M. vulgaris tissue homogenate was crosslinked with gelatin using glutaraldehyde and fixed on a pretreated Teflon membrane. Sulfite was enzymatically converted to sulfate in the presence of the dissolved oxygen, which was monitored amperometrically. Sulfite determination was carried out by standard curves, which were obtained by the measurement of consumed oxygen level related to sulfite concentration. Several operational parameters had been investigated: the amounts of plant tissue homogenate and gelatin, percentage of glutaraldehyde, optimum pH and temperature. Also, some characterization studies were done. There was linearity in the range between 0.2 and 1.8 mM at 35 °C and pH 7.5. The results of real sample analysis obtained with the biosensor agreed well with the enzymatic reference method using spectrophotometric detection.