Showing papers in "Mikrochimica Acta in 2011"

Electrochemical sensors based on graphene materials

Abstract: Single–layered graphene, emerging as a true two–dimensional nanomaterial, has tremendous potential for electrochemical catalysis and biosensing as a novel electrode material. Considering the excellent properties of graphene, such as large surface–to–volume ratio, high conductivity and electron mobility at room temperature, low energy dynamics of electrons with atomic thickness, robust mechanical and flexibility, we give a general view of recent advances in electrochemical sensors based on graphene. We are highlighting here important applications of graphene and graphene nanocomposites, and the assay strategies in electrochemical sensors for DNA, proteins, neurotransmitters, phytohormones, pollutants, metal ions, gases, hydrogen peroxide, and in medical, enzymatic and immunosensors.

One-pot solvothermal synthesis of a Cu2O/Graphene nanocomposite and its application in an electrochemical sensor for dopamine

Abstract: Nanocomposites composed of cuprous oxide (Cu2O) and graphene were synthesized via reduction of copper(II) in ethylene glycol. This material possesses the specific features of both Cu2O and graphene. Its morphology was characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. Cyclic voltammetry was used to evaluate the electrochemical response of a glass carbon electrode (GCE) modified with the nanocomposite towards dopamine (DA). Compared to the bare GCE, the Cu2O nanoparticles modified electrode and the graphene modified electrode, the nanocomposites modified electrode displays high electrocatalytic activity in giving an oxidation peak current that is proportional to the concentration of DA in the range from 0.1 to 10 μM,with a detection limit of 10 nM (S/N = 3). The modified electrode shows excellent selectivity and sensitivity even in the presence of high concentration of uric acid and can be applied to determine DA in real samples with satisfactory results.

Electrochemistry and voltammetric determination of L-tryptophan and L-tyrosine using a glassy carbon electrode modified with a Nafion/TiO2-graphene composite film

Abstract: We describe a glassy carbon electrode (GCE) modified with a film composed of Nafion and TiO2-graphene (TiO2-GR) nanocomposite, and its voltammetric response to the amino acids L-tryptophane (Trp) and L-tyrosine (Tyr). The incorporation of TiO2 nanoparticles with graphene significantly improves the electrocatalytic activity and voltammetric response compared to electrodes modified with Nafion/graphene only. The Nafion/TiO2-GR modified electrode was used to determine Trp and Tyr with detection limits of 0.7 and 2.3 μM, and a sensitivity of 75.9 and 22.8 μA mM−1 for Trp and Tyr, respectively.

Electrochemical oxidation of p-nitrophenol using graphene-modified electrodes, and a comparison to the performance of MWNT-based electrodes

Abstract: The electrochemical oxidation of p-nitrophenol (p-NP) has been studied comparatively on a graphene modified electrode and a multiwall carbon nanotube (MWNT) electrode by using cyclic and differential pulse voltammetry. The sensors were fabricated by modifying screen-printed electrodes with graphene and MWNT nanomaterials, respectively, both dispersed in Nafion polymer. p-NP is irreversibly oxidized at +0.9 V (vs. the Ag/AgCl) in solutions of pH 7. The height and potential of the peaks depend on pH in the range from 5 to 11. In acidic media, p-NP yields a well-defined oxidation peak at +0.96 V which gradually increases in height with the concentration of the analyte. In case of differential pulse voltammetry in sulfuric acid solution, the sensitivity is practically the same for both electrodes. The modified electrodes display an unusually wide linear response (from 10 μM to 0.62 mM of p-NP), with a detection limit of 0.6 μM in case of the graphene electrode, and of 1.3 μM in case of the MWNT electrode.

Preparation and characterization of nano-sized calcium carbonate as controlled release pesticide carrier for validamycin against Rhizoctonia solani

Abstract: Nano-sized calcium carbonate (nano-CC) was studied in terms of acting as a carrier for a pesticide. Nano-CC was prepared by reaction of calcium chloride and sodium carbonate by the reversed-phase microemulsion method and then loaded with the pesticide validamycin. The resulting material was characterized by X-ray diffraction analysis and scanning electron microscopy. The loading efficiency, sustained-release performance, germicidal efficacy, and stability also were investigated. The size of the loaded nano-CC can be adjusted to between 50 to 200 nm by varying the water/surfactant molar ratio from 30/1 to 10/1, and the loading efficiency can be increased to about 20% by increasing the size of the nano-CC. The material displayed better germicidal efficacy against Rhizoctonia solani compared to conventional technical validamycin after about 7 days, and the time of the release of validamycin was extended to 2 weeks. Given the loading efficiency, stability, sustained-release performance and good environmental compatibility of the material, the method for its preparation may be extended to other hydrophilic pesticide.

Luminescent probes for detection and imaging of hydrogen peroxide

Abstract: The relevance of hydrogen peroxide (H2O2) in biological processes has been underestimated for a long time. In recent years, various reports showed that H2O2 not only acts as a cytotoxic compound appearing in the course of oxidative stress, but also functions as an important signaling molecule. Fluorescent probes (or indicators) and nanoparticles that respond selectively to hydrogen peroxide can be applied for intracellular measurements or in vivo imaging, and are superior to electrochemical methods, e.g. in terms of spatial resolution. In contrast to previous reviews that concentrated on the adoption of different probes for certain applications, this survey highlights the basic principles of different probes in terms of their chemical design, structures and functionalities. Thus, the probes are classified according to the underlying reaction mechanism: oxidation, hydrolysis, photoinduced electron transfer, and lanthanide complexation. Other assays are based on fluorescent proteins and nanoparticles, and chemi- or bioluminescent reagents. We confine this review to probes that display a more or less distinct selectivity to hydrogen peroxide. Indicators responding to reactive oxygen species (ROS) in general, or to particular other ROS, are not covered. Finally, we briefly discuss future trends and perspectives of these luminescent reporters in biomedical research and imaging.

Determination of acetamiprid by a colorimetric method based on the aggregation of gold nanoparticles

Abstract: A method was developed for the detection of the insecticide acetamiprid based on the strong interaction of the cyano group of acetamiprid with gold nanoparticles (AuNPs). The interaction results in the aggregation of gold nanoparticles and is accompanied by a color change from red to purple. The concentration of acetamiprid can be determined qualitatively and quantitatively by visually monitoring the color change or by using a spectrometer. Transmittance electron microscopy and UV-vis spectroscopy have been used to characterize the process. The experimental parameters were optimized with regard to the size of the AuNPs, pH, and incubation time. Under optimal experimental conditions, linear relationships between the logarithm of the concentration of acetamiprid and the absorbance were found over the range of 0.66 to 6.6 μM for AuNPs with diameters of 22.0 ± 1.0 nm and of 6.6–66 μM for AuNPs with diameters of 15.0 ± 1.0 nm. This method was successfully applied to detect acetamiprid in vegetables.

Erratum to: Flame atomic absorption spectrometric determination of trace amounts of Pb(II) and Cr(III) in biological, food and environmental samples after preconcentration by modified nano alumina

Abstract: A new solid-phase extraction sorbent was used for the preconcentration of Pb(II) and Cr(III) ions prior to their determination by flame atomic absorption spectrometry. It was prepared by immobilization of 2,4-dinitrophenylhydrazine on nano-alumina coated with sodium dodecyl sulfate. The sorbent was characterized by scanning electron microscopy, N2 adsorption and Fourier transform infrared spectrometry, and used for preconcentration and separation of Pb(II) and Cr(III) from aqueous solutions. The ions on the sorbent were eluted with a mixture of nitric acid and methanol. The effects of sample pH, flow rates of samples and eluent, type of eluent, breakthrough volume and potentially interfering ions were studied. Linearity is maintained between 1.2 and 350 μg L-1 of Pb(II), and between 2.4 and 520 μg L-1 of Cr(III) for an 800-mL sample. The detection limit (3 s, N = 10) for Pb(II) and Cr(III) ions is 0.43 and 0.55 μg L-1, respectively, and the maximum preconcentration factor is 267. The method was successfully applied to the evaluation of these trace and toxic metals in various water, food, industrial effluent and urine samples.

Voltammetric determination of bisphenol A in food package by a glassy carbon electrode modified with carboxylated multi-walled carbon nanotubes

Abstract: A highly sensitive and mercury-free method for determination of bisphenol A (BPA) was established using a glassy carbon electrode that was modified with carboxylated multi-walled carbon nanotubes. A sensitive oxidation peak is found at 550 mV in linear sweep voltammograms at pH 7. Based on this finding, trace levels of bisphenol A can be determined over a concentration range that is linear from 10 nM to 104 nM, the correlation coefficient being 0.9983, and the detection limit (S/N = 3) being 5.0 nM. The method was successfully applied to the determination of BPA in food package.

Dispersive liquid-liquid microextraction for the analysis of three organophosphorus pesticides in real samples by high performance liquid chromatography-ultraviolet detection and its optimization by experimental design

Abstract: A method was developed for analyzing the organophosphorus pesticides Fenitrothion, Diazinon and Ethion in water samples via dispersive liquid-liquid microextraction and high-performance liquid chromatography. Chloroform and methanol were selected as extraction and dispersive solvents, respectively. Extraction and dispersive solvents (and their volumes), the pH of the sample, and salting out effect were optimized to give a calibration plot that is linear in the range from 10 to 4000 ng.mL−1. Detection limits are 2 ng.mL−1 for Fenitrothion and 3 ng.mL−1 for Diazinon and Ethion. The relative standard deviations for six replicate measurements of 500 ng.mL−1 of the three pesticides are 3.3, 2.2 and 4.1%, respectively.

Sensitive simultaneous determination of catechol and hydroquinone using a gold electrode modified with carbon nanofibers and gold nanoparticles

Abstract: A highly sensitive electrochemical sensor for the simultaneous determination of catechol (CC) and hydroquinone (HQ) was fabricated by electrodeposition of gold nanoparticles onto carbon nanofiber film pre-cast on an Au electrode. Both CC and HQ cause a pair of quasi-reversible and well-defined redox peaks at the modified electrode in pH 7.0 solution. Simultaneously, the oxidation peak potentials of CC and HQ become separated by 112 mV. When simultaneously changing the concentrations of both CC and HQ, the response is linear between 9.0 μM and 1.50 mM. In the presence of 0.15 mM of the respective isomer, the electrode gives a linear response in the range from 5.0 to 350 μM, and from 9.0 to 500 μM for CC and HQ, respectively, and detection limits are 0.36 and 0.86 μM. The method was successfully examined for real sample analysis with high selectivity and sensitivity.

Voltammetric sensing of paracetamol, dopamine and 4-aminophenol at a glassy carbon electrode coated with gold nanoparticles and an organophillic layered double hydroxide

Abstract: A differential pulse voltammetric method was developed for the simultaneous determination of paracetamol, 4-aminophenol and dopamine at pH 7.0 using a glassy carbon electrode (GCE) coated with gold nanoparticles (AuNPs) and a layered double hydroxide sodium modified with dodecyl sulfate (SDS-LDH). The modified electrode displays excellent redox activity towards paracetamol, and the redox current is increased (and the corresponding over-potential decreased) compared to those of the bare GCE, the AuNPs-modified GCE, and the SDS-LDH-modified GCE. The modified electrode enables the determination of paracetamol in the concentration range from 0.5 to 400 μM, with a detection limit of 0.13 μM (at an S/N of 3). The sensor was successfully applied to the stimultaneous determination of paracetamol and dopamine, and of paracetamol and 4-aminophenol, respectively, in pharmaceutical tablets and in spiked human serum samples.

Fast and selective magnetic solid phase extraction of trace Cd, Mn and Pb in environmental and biological samples and their determination by ICP-MS

Abstract: We have immobilized iminodiacetic acid on mesoporous Fe3O4@SiO2 microspheres and used this material for efficient and cost effective method of magnetic solid phase extraction (SPE) of trace levels of Cd, Mn and Pb. The microspheres were characterized by infrared spectroscopy, scanning electron microscopy and transmission electron microscopy. The loaded microspheres can be easily separated from the aqueous sample solution by applying an external magnetic field. The effects of pH, sample volume, concentration and volume of eluent, and of interfering ions were investigated in detail. The method has detection limit of 0.16, 0.26 and 0.26 ng L−1 for the ions of Cd, Mn and Pb, respectively, and the relative standard deviations (RSDs, c = 1 μg L−1, n = 7) are 4.8%, 4.6% and 7.4%. The method was successfully applied to the determination of these metals in biological and environmental samples using ICP-MS. Two certified reference materials were analyzed, and the results coincided well with the certified values.

Speciation of inorganic arsenic in environmental waters using magnetic solid phase extraction and preconcentration followed by ICP-MS

Abstract: A new method was developed for the speciation of inorganic arsenic in environmental water by using selective magnetic solid-phase extraction followed by inductively coupled plasma mass spectrometry. It is found that As(V) selectively adsorbed on amino-modified silica-coated magnetic nanoparticles (MNPs) in the pH range from 3 to 8, while As(III) is not be retained. The As(V)-loaded MNPs can be separated easily from the aqueous sample solution by simply applying an external magnetic field. The adsorbed As(V) was quantitatively recovered from the MNPs using using 1 M nitric acid. Total inorganic As was extracted after the permanganate oxidation of As(III) to As(V). Parameters affecting the separation were investigated systematically, and the optimal separation conditions were established. Under the optimal conditions, the limit of detection is 0.21 ng L−1, and the precision is 6.8% (at 10 ng L−1, for n = 7). The method was applied to the speciation of inorganic arsenic in environmental water of tobacco growing area.

Simultaneous determination of four different antibiotic residues in honey by chemiluminescence multianalyte chip immunoassays

Abstract: We are presenting the first method for identification and quantification of antibiotic derivatives in honey samples using regenerable antigen microarrays in combination with an automated flow injection system The scheme is based on an indirect competitive immunoassay format using monoclonal antibodies bound to the surface of the microarray The surface of glass slides was coated with epoxy-activated poly(ethylene glycol) and enables direct immobilization of the antibiotic derivatives The antigen/antibody interaction on the surface of the chip can be detected by chemiluminescence (CL) read-out via CCD camera The method allows for fast analysis of the four analytes simultaneously and without purification or extraction An effective data evaluation method also was developed to warrant unambiguous identification of the spots and to establish grey levels of CL intensities The software developed enables fast and automated processing of the CL images Dose–response curves were obtained for the derivatives of enrofloxacin, sulfadiazine, sulfamethazine and streptomycin Spiking experiments revealed adequate recoveries within the dynamic ranges of the calibration curves of enrofloxacin (92% ± 6%), sulfamethazine (130% ± 21%), sulfadiazine (89% ± 20%) and streptomycin (93% ± 4%)

Hydrogen peroxide sensor based on glassy carbon electrode modified with β-manganese dioxide nanorods

Abstract: A novel sensor for hydrogen peroxide (H2O2) was fabricated using β-MnO2 nanorods on a glassy carbon electrode (GCE). The nanorods were obtained by a hydrothermal method and characterized by scanning electron microscopy and X-ray diffraction. Cyclic voltammetry was used to evaluate the electrochemical performance of the modified GCE. The sensor exhibits excellent catalytic activity toward the oxidation of H2O2 and displays a rather wide linear range (from 2.5 μM to 42.9 mM), high sensitivity (21.74 μA·mM−1), a low detection limit (2.45 μM at an S/N of 3), and a response time of <5 s.

Voltammetric sensor for caffeine based on a glassy carbon electrode modified with Nafion and graphene oxide

Abstract: We report on a voltammetric sensor for caffeine that is based on a glassy carbon electrode modified with Nafion and graphene oxide (GO). It exhibits a good affinity for caffeine (resulting from the presence of Nafion), and excellent electrochemical response (resulting from the pressence of GO) for the oxidation of caffeine. The electrode enables the determination of caffeine in the range from 4.0 × 10−7 to 8.0 × 10−5 mol L−1, with a detection limit of 2.0 × 10−7 mol L−1. The sensor displays good stability, reproducibility, and high sensitivity. It was successfully applied to the quantitative determination of caffeine in beverages.

Determination of triclosan and triclocarban in environmental water samples with ionic liquid/ionic liquid dispersive liquid-liquid microextraction prior to HPLC-ESI-MS/MS

Abstract: A hydrophobic ionic liquid was finely dispersed in aqueous solution along with a hydrophilic ionic liquid. Following centrifugation, the two phases aggregate to form relatively large droplets. Based on this phenomenon, a method termed ionic liquid/ionic liquid dispersive liquid-liquid microextraction was developed. It was applied to the enrichment of triclosan (TCS) and triclocarban (TCC) from water samples prior to HPLC with electrospray tandem MS detection. The type and volume of the hydrophobic ionic liquid (the extraction solvent) and the hydrophilic ionic liquid (the disperser solvent), salt content, and extraction time were optimized. Under optimum conditions, the method gives a linear response in the concentration ranges from 0.5 to 100 μg L−1 for TCC and from 2.5 to 500 μg L−1 for TCS, respectively. The limits of detection are 0.23 and 0.35 μg L−1, and the repeatability is 5.4 and 6.4% for TCC and TCS, respectively. The method was validated with four environmental water samples, and average recoveries of spiked samples were in the range from 88% to 111%. The results indicate that the method is a promising new approach for the rapid enrichment and determination of organic pollutants.

Modification of multi-walled carbon nanotubes with tannic acid for the adsorption of La, Tb and Lu ions

Abstract: We have prepared an environmental friendly sorbent by modifying multi-walled carbon nanotubes with tannic acid. The adsorption of La (III), Tb (III) and Lu (III) as a function of contact time, initial solution pH, and quantity of adsorbent was studied using a batch technique. Both Langmuir and Freundlich isotherms can be used to describe the process. The major adsorption mechanisms were attributed to ion exchange and surface complexation. The kinetics of the adsorption follows a pseudo-second-order model. The thermodynamic functions ΔH, ΔG, and ΔS indicate that the sorption is endothermically driven. The adsorbed ions can be readily desorbed from the surface with 1 M hydrochloric acid.

Simultaneous determination of multi-class antibiotic residues in water using carrier-mediated hollow-fiber liquid-phase microextraction coupled with ultra-high performance liquid chromatography tandem mass spectrometry

Abstract: A method has been developed for carrier-mediated hollow-fiber liquid-phase microextraction (HF-LPME) and enrichment of multiple classes of antibiotics in water samples. Eleven compounds (erythromycin, spiramycin, tilmicosin, sulfathiazole, sulfamethazine, sulfamerazine, oxytetracycline, tetracycline, ciprofloxacin, danofloxacin and enrofloxacin) from four important classes of antibiotics (of the macrolide, sulfonamide, tetracycline and quinolone type) have been simultaneously preconcentrated with one set of HF-LPME conditions, followed by determination by ultra-HPLC combined with electrospray ionization tandem mass spectrometry (UHPLC-MS/MS). Antibiotics can be determined at ng L−1 levels using this highly sensitive and selective method. Parameters including immersion time, liquid membrane composition, sample pH, acceptor composition and extraction time were optimized to finally give detection limits in the 10–250 ng L−1 range. Good linearity was achieved, with up to 156 times enrichment over the four classes of antibiotics. This multi-residue method enabled the simultaneous enrichment of all 11 multi-class antibiotics from spiked river water samples, with relative recovery between 79 and 118%.

In-situ ionic liquid-dispersive liquid-liquid microextraction method to determine endocrine disrupting phenols in seawaters and industrial effluents

Abstract: We have evaluated an in-situ ionic liquid-dispersive liquid-liquid microextraction procedure for the determination of six endocrine disrupting phenols in seawaters and industrial effluents using HPLC. The optimized method requires 38 μL of the water-soluble ionic liquid 1-butyl-3-methylimidazolium chloride, and 5 mL of seawater or industrial effluent. After appropriate work-up, a drop (~10 μL) of an ionic liquid is formed that contains the analytes of interest. It is diluted with acetonitrile and injected into the HPLC system. This procedure is accomplished without heating or cooling the solutions. The method is characterized by (a) average relative recoveries of 90.2%, (b) enrichment factors ranging from 140 to 989, and (c) precisions (expressed as relative standard deviations) of less than 11% when using a spiking level of 10 ng mL−1. The limits of detection range from 0.8 ng mL−1 for 4-cumylphenol to 4.8 ng mL−1 for bisphenol-A.

Electrochemical behavior of methyl parathion and its sensitive determination at a glassy carbon electrode modified with ordered mesoporous carbon

Abstract: Ordered mesoporous carbon (OMC) was synthesized and used to modify the surface of a glassy carbon (GC) electrode Due to the unique properties of OMC, a decrease in the overvoltage of the reduction potential of methyl parathion (MP) (to ca 219 mV) and a 76-fold increase in the peak current are observed (compared with a bare GC electrode) The absorption capacity of the surface of the electrode for MP was determined by chronocoulometry The results show that the Г value of the modified electrode (234 × 10–9 mol cm–2) is 95 times as large as that of the GC electrode (247 × 10–10 mol cm–2) The new electrode exhibits synergistic electrocatalytic and accumulative effects on MP MP can be determined by linear sweep voltammetry (LSV) which displays a linear relationship between peak current and MP concentration in the range from 009 to 61 μM, with a detection limit as low as 76 nM (at an S/N of 3) and after an accumulation at 0 V for 5 min The electrode was successfully applied to the determination of MP in spiked lake water samples

Fluorescent probe for copper(II) ion based on a rhodamine spirolactame derivative, and its application to fluorescent imaging in living cells

Abstract: A fluorescent probe for Cu(II) ion is presented. It is based on the rhodamine fluorophore and exhibits high selectivity and sensitivity for Cu(II) ion in aqueous methanol (2:8, v/v) at pH 7.0. The response is based on a ring opening reaction and formation of a strongly fluorescent 1:1 complex. The response is reversible and linear in the range between 50 nM and 900 nM, with a detection limit of 7.0 nM. The probe was successfully applied to fluorescent imaging of Cu(II) ions in HeLa cells.

Gold nanoparticle-coated multiwall carbon nanotube-modified electrode for electrochemical determination of methyl parathion

Abstract: We report on an electrochemical sensor for the determination of methyl parathion. It is based on an electrode modified with multi-walled carbon nanotubes that were covered with gold nanoparticles (Au-NPs). The vertically aligned array of MWCNTs on a tantalum substrate was coated with Au-NPs by overhead magnetron sputtering deposition. Scanning and transmission electron microscopy and XRD were used to characterize the Au-NP-MWCNTs composite. Cyclic voltammetry and differential pulse voltammetry were employed to evaluate the suitability of the new electrode for the determination of methyl parathion. Under the optimal conditions, the current response of the electrode to methyl parathion is linear in the range from 0.50 to 16.0 mg mL-1, with a detection limit of 50 μg mL-1 (signal/noise = 3), and the sensitivity is 4.5 times better than that of the plain MWCNTs electrode. We conclude that this method represents a simple, rapid, effective and sensitive approach for the detection of methyl parathion.

Using ICP-OES and SEM-EDX in biosorption studies.

Abstract: We have compared the analytical results obtained by inductively coupled plasma optical emission spectroscopy (ICP-OES) and by scanning electron microscopy with an energy dispersive X-ray analytical system (SEM-EDX) in order to explore the mechanism of metal ions biosorption by biomass using two independent methods. The marine macroalga Enteromorpha sp. was enriched with Cu(II), Mn(II), Zn(II), and Co(II) ions via biosorption, and the biosorption capacity of alga determined from the solution and biomass composition before and after biosorption process was compared. The first technique was used to analyze the composition of the natural and metal-loaded biomass, and additionally the composition of the solution before and after biosorption. The second technique was used to obtain a picture of the surface of natural and metal ion-loaded macroalgae, to map the elements on the cell wall of dry biomass, and to determine their concentration before and after biosorption. ICP-OES showed a better precision and lower detection limit than EDX, but SEM-EDX gave more information regarding the sample composition of Enteromorpha sp. Both techniques confirmed that biosorption is a surface phenomenon, in which alkali and alkaline earth metal ions were exchanged by metal ions from aqueous solution.

Ultrasensitive detection of mercury(II) ion using CdTe quantum dots in sol-gel-derived silica spheres coated with calix[6]arene as fluorescent probes

Abstract: We have developed a simple method for the preparation of highly fluorescent and stable, water-soluble CdTe quantum dots in sol-gel-derived composite silica spheres that were coated with calix[6]arene. The resulting nanoparticles (NP) were characterized in terms of UV, fluorescence and FT-IR spectroscopy and by TEM. The results show that the new NPs display more intense fluorescence intensity and are more stable than its precursors of the type SiO2/CdTe. In addition, the new NPs exhibit a higher selectivity for the determination of Hg2+ than for other metal ions. Under the optimum conditions, the relative fluorescence intensity decreases with the concentration of Hg2+ in the range from 2.0 to 14.0 nmol L−1 and the detection limit is 1.55 nmol L−1. The method is based on the quenching of fluorescence by Hg2+ and expected to serve as a practical fluorescence test for rapid detection of Hg2+. A mechanism is suggested to explain the inclusion process by a Langmuir binding isotherm, and fluorescence quenching is best described by the Stern-Volmer equation.

Determination of mercury(II) in water samples using dispersive liquid-liquid microextraction and back extraction along with capillary zone electrophoresis

Abstract: We have developed a method for the determination of mercury in water samples that combines dispersive liquid-liquid microextraction (DLLME) with back-extraction (BE) and detection by capillary zone electrophoresis. DLLME is found to be a simple, cost-effective and rapid method for extraction and preconcentration. The BE procedure is based on the fact that the stability constant of the hydrophilic chelate of Hg(II) with L-cysteine is much larger than that of the respective complex with 1-(2-pyridylazo)-2-naphthol. Factors affecting complex formation and extraction efficiency (such as pH value, concentration of the chelating agent, time of ultrasonication and extraction, and type and quantity of disperser solvent) were optimized. Under the optimal conditions, the enrichment factor is 625, and the limit of detection is 0.62 mu g L(-1). The calibration plot is linear in the range between 1 and 1,000 mu g L(-1) (R (2) = 0.9991), and the relative standard deviation (RSD, for n = 6) is 4.1%. Recoveries were determined with tap water and seawater spiked at levels of 10 and 100 mu g L(-1), respectively, and ranged from 86.6% to 95.1%, with corresponding RSDs of 3.95-5.90%.

Chemically modified alumina nanoparticles for selective solid phase extraction and preconcentration of trace amounts of Cd(II)

Abstract: We have developed a solid phase extraction method for the determination of cadmium ions in aqueous samples. It is based on the adsorption of Cd(II) on alumina nanoparticles coated with sodium dodecyl sulfate and modified with a newly synthesized Schiff base. Analytical parameters such as pH value, amount of adsorbent, type and concentration of eluent, flow rates of the sample and eluent, sample volume and matrix effects were optimized. Desorption is accomplished with 2 mol L−1 nitric acid. Cd(II) was then determined by flame atomic absorption spectrometry. The maximum enrichment factor is 75. Under the optimum experimental conditions, the detection limit is 0.14 μg L−1 in original solution. The adsorption capacity of the modified sorbent is 4.90 mg g−1 for cadmium ions. The method was applied to the determination of trace quantities of Cd(II) in water, wastewater, and biological and food samples with satisfactory results.

A surface ion-imprinted mesoporous sorbent for separation and determination of Pb(II) ion by flame atomic absorption spectrometry

Abstract: A surface-imprinted mesoporous sorbent for Pb(II) ion was synthesized by the post-synthesis method. The material was characterized by transmission electron microscopy and nitrogen adsorption-desorption isotherms. The adsorption by the material was studied by batch experiments with respect to effects of pH value, contact time, kinetics, and adsorption isotherms. Both the pseudo-second-order kinetic model and the Langmuir model fit the experimental data well. Compared to other imprints for Pb(II), to the traditional sorbents and to the non-imprinted polymer, the new sorbent displays fast kinetics and higher selectivity. Pb(II) ion can be desorbed from the imprint with 2 M hydrochloric acid with high efficiency. The sorbent was applied to the selective separation and determination of Pb(II) in water and sediment samples with satisfactory results.

Electrodeposition of cobalt oxide nanoparticles on carbon nanotubes, and their electrocatalytic properties for nitrite electrooxidation

Abstract: We describe a modified glassy carbon electrode (GCE) for the sensitive determination of nitrite in waste water samples. The GCE was modified by electrodeposition of cobalt oxide nanoparticles on multi-walled carbon nanotubes (MWCNTs) deposited on a conventional GCE. Scanning electron microscopy and electrochemical techniques were used for the characterization of the composite material which is very uniform and forms a kind of nanoporous structure. Electrochemical experiments showed that the modified electrode exhibited excellent electrocatalytic properties for nitrite. Amperometry revealed a good linear relationship between peak current and nitrate concentration in the 0.5 to 250 μM range with a detection limit of 0.3 μM (S/N = 3). The method has been applied to the amperometric detection of nitrite. The modified electrode displays good storage stability, reproducibility, and selectivity for a promising practical application.