Voltammetric sensing of thallium at a carbon paste electrode modified with a crown ether

We describe a new method for differential-pulse anodic stripping voltammetric determination of thallium(I) using a carbon paste electrode modified with dicyclohexyl-18-crown-6. The effect of supporting electrolyte (type and pH), accumulation and reduction potential, and of time and amount of modifier were investigated by differential pulse anodic stripping voltammetry. A method was then worked out for the determination of thallium at low levels. Under optimized conditions, the response to Tl(I) is linear in the range from 3.0 to 250 ng mL^?1. The detection limit is 0.86 ng mL^?1. The sensor displays good repeatability (with a relative standard deviation of ±2.70 % for n?=?7) and was applied to the determination of Tl(I) in water, hair samples, and certified reference materials.

Anodic stripping voltammetric determination of silver ion at a carbon paste electrode modified with carbon nanotubes

A carbon paste electrode (CPE) was modified with multi-wall carbon nanotubes and successfully applied to the determination of silver ion by differential pulse anodic stripping voltammetry. Compared to a conventional CPE, a remarkably improved peak current response and sensitivity is observed. The analytical procedure consisted of an open circuit accumulation step for 2?min in ?0.4?V, this followed by an anodic potential scan between +0.2 and?+?0.6?V to obtain the voltammetric peak. The oxidation peak current is proportional to the concentration of silver ion in the range from 1.0?×?10^?8 to 1.0?×?10^?5?mol?L^?1, with a detection limit of 1.8?×?10^?9?mol?L^?1 after an accumulation time of 120?s. The relative standard deviation for 7 successive determinations of Ag(I) at 0.1???M concentration is 1.99%. The procedure was validated by determining Ag(I) in natural waters.

Determination of lead(II) in aqueous solution using carbon nanotubes paste electrodes modified with Amberlite IR-120

A new composite electrode is described for anodic stripping voltammetry determination of Pb(II) at trace level in aqueous solution. The electrode is based on the use of multiwalled carbon nanotubes and Amberlite IR-120. The anodic stripping voltammograms depend, to a large extent, on the composition of the modified electrode and the preconcentration conditions. Under optimum conditions, the anodic peak current at around ?0.57 V is linearly related to the concentration of Pb(II) in the range from 9.6?×?10^?8 to 1.7?×?10^?6 mol L^?1 (R?=?0.998). The detection limit is 2.1?×?10^?8 mol L^?1, and the relative standard deviation (RSD) at 0.24?×?10^?6 mol L^?1 is 1.7% (n?=?6). The modified electrode was applied to the determination of Pb(II) using the standard addition method; the results showed average relative recoveries of 95% for the samples analysed.

Determination of cadmium(II) using carbon paste electrode modified with a Cd-ion imprinted polymer

We have synthesized cadmium(II) ion-imprinted polymers (IIP) and non-imprinted polymers (NIP) using 1-(2-pyridylazo)-2-naphthol as a ligand. The materials were used to prepare a carbon paste electrode for the determination of Cd(II). Polymerization was performed with (a) methacrylic acid as a functional monomer, (b) ethyleneglycol dimethacrylate as the crosslinking monomer, and (c) 2,2′-azobis(isobutyronitrile) as the initiator. Imprinted cadmium ion was removed from the polymeric matrix using nitric acid. The measurements were carried out in an closed circuit after accumulation at ?1.2?V, this followed by electrolysis of the accumulated Cd(II) by voltammetric scanning from ?1.0 to ?0.6?V. The parameters governing the response of the electrode were studied. Under optimized conditions, the response of the electrode is linear in the range from 2.0 to 200?ng?mL^?1. The detection limit is 0.31?ng?mL^?1. The relative standard deviations are ±3.4 and ±2.1?% for 7 successive determinations of 20.0 and 50.0?ng?mL-1 of Cd(II), respectively. The method was applied to the determination of cadmium (II) in water and food samples.

Biotin-avidin-conjugated metal sulfide nanoclusters for simultaneous electrochemical immunoassay of tetracycline and chloramphenicol

We report on a protocol for a simultaneous competitive immunoassay for tetracycline (TC) and chloramphenicol (CAP) on the same sensing interface. Conjugates of TC and of CAP with bovine serum albumin were first co-immobilized on a glassy carbon electrode modified with gold nanoparticles. In parallel, monoclonal anti-TC and anti-CAP antibodies were conjugated onto CdS and PbS nanoclusters, respectively. In a typical assay, the immobilized haptens and the added target analytes competed for binding to the corresponding antibodies on the nanoclusters. Subsequently, Cd(II) and Pb(II) ions are released from the surface of the corresponding nanoclusters by treatment with acid and then were detected by square wave anodic stripping voltammetry. The currents at the peak potentials for Cd(II) and Pb(II) were used as the sensor signal for TC and CAP, respectively. This multiplex immunoassay enables the simultaneous determination of TC and CAP in a single run with dynamic ranges from 0.01 to 50 ng mL^?1 for both analytes. The detection limits for TC and for CAP are 7.5 pg mL^?1 and 5.4 pg mL^?1, respectively. No obvious nonspecific adsorption and cross-reactivity was observed in a series of analyses. Intra-assay and inter-assay coefficients of variation were less than 10 %. The method was evaluated by analyzing TC and CAP in spiked samples of milk and honey. The recoveries range from 88 % to 107 % for TC, and from 91 % to 119 % for CAP.

Detection of silver(I) ion based on mixed surfactant-adsorbed CdS quantum dots

Mixed cationic and anionic surfactants were adsorbed on cadmium sulfide quantum dots (CdS QDs) capped with mercaptoacetic acid. The CdS QDs can be extracted into acetonitrile with 98 % efficiency in a single step. Phase separation only occurs at a molar ratio of 1:1.5 between cationic and anionic surfactants. The surfactant-adsorbed QDs in acetonitrile solution display stronger and more stable photoluminescence than in water solution. The method was applied for determination of silver(I) ion based on its luminescence enhancement of the QDs. Under the optimum conditions, the relative fluorescence intensity is linearly proportional to the concentration of silver(I) ion in the range between 50 pmol L^?1and 4 μmol L^?1, with a 20 pmol L^?1 detection limit. The relative standard deviation was 1.93 % for 9 replicate measurements of a 0.2 μmol L^?1 solution of Ag(I).

Determination of trace silver in environmental samples by room temperature ionic liquid-based preconcentration and flame atomic absorption spectrometry

We report on a new method for preconcentration of silver ion at trace level in environmental samples, and its subsequent determination by flame atomic absorption spectrometry (FAAS). The room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium hexafuorophosphate and the chelator 5-(4-dimethylaminobenzylidene)-rhodanine were used for extraction. Ag(I) was back-extracted from the organic phase into thiosulfate solution and then determined via FAAS. The effects of pH, concentration of chelating agent, extraction time and temperature, amounts of ionic liquid, ionic strength and potentially interfering ions were studied. Under optimized conditions, the enhancement factor is 30 was achieved. The detection limit (3???) is 0.28?ng?mL^?1, and the relative standard deviation is 4.1% for 7 replicate determinations at 5?ng?mL^?1 of Ag(I). The method was validated by analysis of certified reference materials and applied to the determination of Ag(I) in environmental samples with satisfactory results.

Determination of thiourea using a carbon paste electrode decorated with copper oxide nanoparticles

We report on a novel sensor for the electrochemical determination of thiourea (TU). It is based on an active carbon paste electrode modified with copper oxide nanoparticles. The modified electrode and the electrochemical properties of thiourea on its surface were investigated using cyclic voltammetry and differential pulse voltammetry. Under optimized conditions, the detection limit is 20 μg?L^?1 of TU. The method was applied to the determination of thiourea in fruit juice, orange peel and industrial waste water.

Sputtering deposition of Pt nanoparticles on vertically aligned multiwalled carbon nanotubes for sensing L-cysteine

A highly sensitive electrochemical sensor for determination of L-cysteine (CySH) is presented. It is based on vertically aligned multiwalled carbon nanotubes modified with Pt nanoparticles by magnetron sputtering deposition. The morphology of the nanocomposite was characterized by scanning electron microscopy, transmission electron microscopy and energy-dispersive. The electrochemistry of CySH was investigated by cyclic voltammetry, differential pulse voltammetry and chronoamperometry. The mechanism for the electrochemical reaction of CySH at the modified electrode at different pH values is discussed. The electrode exhibits a higher electrocatalytic activity towards the oxidation of CySH than comparable other electrodes. It displays a linear dependence (R ²?=?0.9980) on the concentration of CySH in the range between 1 and 500 μM and at an applied potential of +0.45 V, a remarkably low detection limit of 0.5 μM (S/N?=?3), and an outstandingly high sensitivity of 1.42?×?10³ μA?mM^?1?cm^?2, which is the highest value ever reported. The electrode also is highly inert towards other amino acids, creatinine and urea. The sensor was applied to the determination of CySH in urine with satisfactory recovery, thus demonstrating its potential for practical applications.

Visual detection of silver(I) ions by a chromogenic reaction catalyzed by gold nanoparticles

We report on a novel method for visual detection silver(I) ion. It is based on the finding that Ag(I) ions are rapidly reduced by hydroquinone to form a shell of silver on the surface of gold nanoparticles (AuNPs) which act as catalysts for this reaction. This leads to a color change from red to yellow which can be seen with bare eyes. This scheme is sensitive and highly specific for Ag(I) ions. The detection limits are 5 μM for visual inspection and 1 μM for photometric readout, respectively. The method was successfully applied to the determination of Ag(I) ions in spiked lake water and soil.

A carbon nitride electrode for highly selective and sensitive determination of lead(II)

We have constructed a carbon nitride electrode modified with a bismuth film and show that it can be used for the electroanalysis of lead(II) by differential pulse anodic stripping voltammetry. The combination of such materials is shown to greatly improve the sensing capability of the electrode. Under the optimal conditions, the electrochemical response of the modified electrode is linearly related to the concentration of Pb(II) in the 6 – 1,000 nM concentration range (R = 0.9983). The detection limit is 2.0 pM (at an SNR of 3), and the sensitivity is 8 times better of that of respective graphite electrodes. The sensor enables rapid, highly sensitive, continuous, and environmentally friendly determination of trace levels of Pb(II) at affordable costs.

Detection of lead(II) using an glassy carbon electrode modified with Nafion, carbon nanotubes and benzo-18-crown-6

A glassy carbon electrode was modified with Nafion, carbon nanotubes and benzo-18-crown-6 to give an electrode for the selective determination of lead(II) via square wave anodic stripping voltammetry. The use of carbon nanotubes with their extraordinary electrical conductivity and strong adsorption ability warrants high sensitivity. Benzo-18-crown-6 is employed as a “molecular scavenger” because of its excellent selectivity for lead(II). The modified electrode shows enhanced sensitivity, reproducibility and selectivity for lead(II) even without applying an electrical potential during the accumulation time. It responds linearly to lead(II) in the 1 to 30 nM concentration range (with a correlation coefficient of 0.9992) after a 10-min accumulation time. The detection limit is 1 nM. The sensor exhibits excellent selectivity over other heavy metal ions such as Cd(II), Cu(II), Zn(II), and Hg(II).

Sensitive immunoassay for porcine pseudorabies antibody based on fluorescence signal amplification induced by cation exchange in CdSe nanocrystals

We report on a novel immunoassay for porcine pseudorabies virus (PRV) antibody that is based on fluorescence signal amplification induced by silver(I) ion exchange in CdSe nanocrystals. An antigen-antibody-secondary antibody sandwich structure was first formed from PRV, PRV antibody, and CdSe-labeled rabbit anti-pig antibody. Then, the Cd(II) ions in the CdSe labels were released by a cation exchange reaction with Ag(I). Released Cd(II) was finally quantified using the sensitive fluorescent probe Rhodamine 5 N. Due to this signal amplification, the sensitivity and linear range of the immunoassay were largely improved (compared to the traditional ELISA) in having a limit of detection as low as 1.2 ng?mL^?1 of PRV antibody and a linear range from 2.44 to 312 ng?mL^?1. The successful determination of PRV antibody in pig serum samples is proof for the utility of the method.

Micro-solid phase extraction of ochratoxin A, and its determination in urine using capillary electrophoresis

We describe a simple, environmentally friendly and selective technique for the determination of ochratoxin A (OTA) in urine. It involves (a) the use of a molecularly imprinted polymer as a sorbent in micro-solid-phase extraction in which the sorbent is contained in a propylene membrane envelope, and (b) separation and detection by capillary electrophoresis (CE). Under optimized conditions, response is linear in the range between 50 and 300 ng mL^?1 (with a correlation coefficient of 0.9989), relative standard deviations range from 4 to 8 %, the detection limit for OTA in urine is 11.2 ng mL^?1 (with a quantification limits of 32.5 ng mL^?1) which is lower than those of previously reported methods for solid-phase extraction combined with CE. The recoveries of OTA from urine spiked at levels of 50, 150 and 300 ng mL^?1 ranged from 93 to 97 %.

A glassy carbon electrode modified with antimony and poly(p-aminobenzene sulfonic acid) for sensing lead(II) by square wave anodic stripping voltammetry

We have developed a sensor for the square wave anodic stripping voltammetric determination of Pb(II). A glassy carbon electrode was modified with a thin film of an antimony/poly(p-aminobenzene sulfonic acid) composite in air-saturated aqueous solution of pH 2.0. Compared to a conventional antimony film electrode, the new one yields a larger stripping signal for Pb(II). The conditions of polymerization, the concentration of Sb(III), the pH value of the sample solution, the deposition potential and time, frequency, potential amplitude, and step increment potential were optimized. Under the optimum conditions, a linear response was observed for Pb(II) in the range of 0.5 to 150.0 μg?L^?1. The detection limit for Pb(II) is 0.1 μg?L^?1.

Electrochemical sensor for lead(II) ion using a carbon ionic-liquid electrode modified with a composite consisting of mesoporous carbon, an ionic liquid, and chitosan

A novel electrode was prepared that enables sensing of lead(II) ion. A suspension composed of ordered mesoporous carbon (OMC), an ionic liquid (IL), and chitosan was deposited on the highly conductive surface of a carbon ionic-liquid electrode (CILE). The surface of the sensing electrode was characterized by scanning electron microscopy and cyclic voltammetry. The new electrode can be used to determine lead(II) ion because the hydrophobic ionic liquid of the CILE can extract Pb(II), while the OMC accelerates the electron transfer rate between the electrode and Pb(II) and also strongly adsorbs Pb(II). The resulting electrode displays excellent and synergistic response to Pb(II) which is linear in the range from 0.05 to 1.4?μM, with a correlation coefficient of 0.997 and a detection limit of 25 nM.

Square wave anodic stripping voltammetric determination of lead(II) using a glassy carbon electrode modified with a lead ionophore and multiwalled carbon nanotubes

We report on a glassy carbon electrode (GCE) modified with a lead ionophore and multiwalled carbon nanotubes. It can be applied to square wave anodic stripping voltammetric determination of Pb(II) ion after preconcentration of Pb(II) at ?1.0?V (vs. SCE) for 300?s in pH?4.5 acetate buffer containing 400?μg?L^?1 of Bi(III). The ionophore-MWCNTs film on the GCE possesses strong and highly selective affinity for Pb(II) as confirmed by quartz crystal microbalance experiments. Under the optimum conditions, a linear response was observed for Pb(II) ion in the range from 0.3 to 50?μg?L^?1. The limit of detection (at S/N?=?3) is 0.1?μg?L^?1. The method was applied to the determination of Pb(II) in water samples with acceptable recovery.

Analysis of dextromethorphan and pseudoephedrine in human plasma and urine samples using hollow fiber-based liquid–liquid–liquid microextraction and corona discharge ion mobility spectrometry

We report on the use of hollow fiber liquid-liquid-liquid microextraction (HF-LLLME) followed by corona discharge ion mobility spectrometry for the determination of dextromethorphan and pseudoephedrine in urine and plasma samples. The effects of pH of the donor phase, stirring rate, ionic strength and extraction time on HF-LLLME were optimized. Under the optimized conditions, the linear range of the calibration curves for dextromethorphan in plasma and urine, respectively, are from 1.5 to 150 and from 1 to 100 ng mL^?1. The ranges for pseudoephedrine, in turn, are from 30 to 300 and from 20 to 200 ng mL^?1. Correlation coefficients are better than 0.9903. The limits of detection are 0.6 and 0.3 ng mL^?1 for dextromethorphan, and 8.6 and 4.2 ng mL^?1 for pseudoephedrine in plasma and urine samples, respectively. The relative standard deviations range from 6 to 8%.

The electrochemistry of uric acid at a gold electrode modified with L-cysteine, and its application to sensing uric urine

The electrochemistry of uric acid at a gold electrode modified with a self-assembled film of L-cysteine was studied by cyclic voltammetry and differential pulse voltammetry. Compared to the bare gold electrode, uric acid showed better electrochemical response in that the anodic peak current is stronger and the peak potential is negatively shifted by about 100 mV. The effects of experimental conditions on the oxidation of uric acid were tested and a calibration plot was established. The differential pulse response to uric acid is linear in the concentration range from 1.0?×?10^?6 to ~?1.0?×?10^?4 mol?L^?1 (r?=?0.9995) and from 1.0?×?10^?4 to ~?5.0?×?10^?4 mol?L^?1 (r?=?0.9990), the detection limit being 1.0?×?10^?7 mol?L^?1 (at S/N?=?3). The high sensitivity and good selectivity of the electrode was demonstrated by its practical application to the determination of uric acid in urine samples.

Iodide-induced adsorption of lead(II) ion on a glassy carbon electrode modified with ferroferric oxide nanoparticles

Spherical Fe₃O₄ nanoparticles (NPs) were prepared by hydrothermal synthesis and characterized by scanning electron microscopy and X-ray diffraction. A glassy carbon electrode was modified with such NPs to result in a sensor for Pb(II) that is based on the strong inducing adsorption ability of iodide. The electrode gives a pair of well-defined redox peaks for Pb(II) in pH 5.0 buffer containing 10 mM concentrations of potassium iodide, with anodic and cathodic peak potentials at ?487 mV and ?622 mV (vs. Ag/AgCl), respectively. The amperometric response to Pb(II) is linear in the range from 0.10 to 44 nM, and the detection limit is 40 pM at an SNR of 3. The sensor exhibits high selectivity and reproducibility.