Preconcentration of atrazine and simazine with multiwalled carbon nanotubes as solid-phase extraction disk

A sensitive and selective preconcentration method using solid-phase extraction (SPE) disk, namely multiwalled carbon nanotubes (MWCNTs) disk, is proposed for the determination of atrazine and simazine in water samples. Atrazine and simazine were extracted on MWCNTs disk and then determined by gas chromatography–mass spectrometry (GC/MS). Several parameters on the enrichment factor of the analytes were investigated. The experimental results showed that it was possible to obtain quantitative analysis when the solution pH was 5 using 200 mL of validation solution containing 0.1 μg of triazines and 5 mL of acetone as an eluent. The maximum enrichment factors for atrazine and simazine were 3900 ± 250 and 4000 ± 110, respectively when 200 mL of sample solution volume was used. Relative standard deviations for seven determinations were 6.9% (atrazine) and 3.0% (simazine) under optimum conditions. The linear range of calibration curves were 0.1 to 1 ng mL^{− 1} for each analyte with good correlation coefficients. The detection limits (3S/N) were 2.5 and 5.0 pg mL^{− 1} for atrazine and simazine, respectively. The proposed method was successfully applied to the determination of atrazine and simazine in environmental water samples with high precision and accuracy.     

Methane preconcentration in a microtrap using multiwalled carbon nanotubes as sorbents

The GC monitoring of green house gases is a challenging task because the concentration of organic species such as methane are relatively low (ppm to ppb) and their analysis requires some level of preconcentration. Since methane is highly volatile, it is not easily retained on conventional sorbents. In this paper we present multiwalled carbon nanotubes (MWNTs) as an effective sorbent for a microtrap designed for methane preconcentration. Its performance was compared to other commercially available carbon based sorbents, and it was found to be the most effective sorbent in terms of breakthrough volume and enthalpy of adsorption.     

Preconcentration procedure trace amounts of palladium using modified multiwalled carbon nanotubes sorbent prior to flame atomic absorption spectrometry: 1st Nano Update

A method for preconcentration of palladium at trace level on modified multiwalled carbon nanotubes columns and determination by flame atomic absorption spectrometry (FAAS) has been developed. Multiwalled carbon nanotubes (MWCNTs) were oxidized with concentrated HNO₃ and the oxidized multiwalled carbon nanotubes were modified with 5-(4′-dimethylamino benzyliden)-rhodanine, and then were used as a solid sorbent for preconcentration of Pd(II) ions. Factors influencing sorption and desorption of Pd(II) ions were investigated. The sorption of Pd(II) ions was quantitative in the pH range of 1.0–4.5, whereas quantitative desorption occurs with 3.0 mL 0.4 mol L^?1 thiourea. The amount of eluted palladium was measured using flame atomic absorption spectrometry. The effects of experimental parameters, including sample flow rate, eluent flow rate, and eluent concentration were investigated. The effect of coexisting ions showed no interference from most ions tested. The proposed method permitted a large enrichment factor (about 200). The relative standard deviation of the method was ±2.73% (for eight replicate determination of 2.0 μg mL^?1 of Pd(II)) and the limit of detection was 0.3 ng mL^?1. The method was applied to the determination of Pd(II) in water, road dust, and standard samples.     

High extraction efficiency for polar aromatic compounds in natural water samples using multiwalled carbon nanotubes/Nafion solid-phase microextraction coating

A novel solid-phase microextraction (SPME) fiber coated with multiwalled carbon nanotubes (MWCNTs)/Nafion was developed and applied for the extraction of polar aromatic compounds (PACs) in natural water samples. The characteristics and the application of this fiber were investigated. Electron microscope photographs indicated that the MWCNTs/Nafion coating with average thickness of 12.5 μm was homogeneous and porous. The MWCNTs/Nafion coated fiber exhibited higher extraction efficiency towards polar aromatic compounds compared to an 85 μm commercial PA fiber. SPME experimental conditions, such as fiber coating, extraction time, stirring rate, desorption temperature and desorption time, were optimized in order to improve the extraction efficiency. The calibration curves were linear from 0.01 to 10 μg mL⁻¹ for five PACs studied except p-nitroaniline (from 0.005 to 10 μg mL⁻¹) and m-cresol (from 0.001 to 10 μg mL⁻¹), and detection limits were within the range of 0.03–0.57 ng mL⁻¹. Single fiber and fiber-to-fiber reproducibility were less than 7.5 (n = 7) and 10.0% (n = 5), respectively. The recovery of the PACs spiked in natural water samples at 1 μg mL⁻¹ ranged from 83.3 to 106.0%.     

Application of multiwalled carbon nanotubes as solid phase extraction sorbent for preconcentration of trace copper in water samples

The adsorption behavior of multiwalled carbon nanotubes (MWNTs) toward copper has been investigated systemically, and a new method has been developed for the determination of trace copper in water samples based on preconcentration with a microcolumn packed with MWNTs prior to its determination by flame atomic absorption spectrometry. The optimum experimental parameters for preconcentration of copper, such as pH of the sample, sample flow rate and volume, elution solution and interfering ions, have been investigated. Copper can be quantitatively retained by MWNTs in the pH range 5-8, and then eluted completely with 0.5 M HNO3. The detection limit of this method for Cu was 0.42 ng/mL, and the RSD was 3.5% at the 10 ng/mL Cu level. The method was validated using a certified reference material, and has been successfully applied for the determination of trace copper in water samples.     

Preconcentration of atrazine and simazine with multiwalled carbon nanotubes as solid-phase extraction disk

A sensitive and selective preconcentration method using solid-phase extraction (SPE) disk, namely multiwalled carbon nanotubes (MWCNTs) disk, is proposed for the determination of atrazine and simazine in water samples. Atrazine and simazine were extracted on MWCNTs disk and then determined by gas chromatography–mass spectrometry (GC/MS). Several parameters on the enrichment factor of the analytes were investigated. The experimental results showed that it was possible to obtain quantitative analysis when the solution pH was 5 using 200 mL of validation solution containing 0.1 μg of triazines and 5 mL of acetone as an eluent. The maximum enrichment factors for atrazine and simazine were 3900 ± 250 and 4000 ± 110, respectively when 200 mL of sample solution volume was used. Relative standard deviations for seven determinations were 6.9% (atrazine) and 3.0% (simazine) under optimum conditions. The linear range of calibration curves were 0.1 to 1 ng mL^− 1 for each analyte with good correlation coefficients. The detection limits (3S/N) were 2.5 and 5.0 pg mL^− 1 for atrazine and simazine, respectively. The proposed method was successfully applied to the determination of atrazine and simazine in environmental water samples with high precision and accuracy.     

Sensitive voltammetric determination of paracetamol by poly (4-vinylpyridine)/multiwalled carbon nanotubes modified glassy carbon electrode

A novel glassy carbon electrode (GCE) modified with a composite film of poly (4-vinylpyridine) (P4VP) and multiwalled carbon nanotubes (P4VP/MWCNT GCE) was used for the voltammetric determination of paracetamol (PCT). This novel electrode displayed a combined effect of P4VP and MWCNT on the electro-oxidation of PCT in a solution of phosphate buffer at pH 7. Hence, conducting properties of P4VP along with the remarkable physical properties of MWCNTs might have combined effects in enhancing the kinetics of PCT oxidation. The P4VP/MWCNT GCE has also demonstrated excellent electrochemical activity toward PCT oxidation compared to that with bare GCE and MWCNT GCE. The anodic peak currents of PCT on the P4VP/MWCNT GCE were about 300 fold higher than that of the non-modified electrodes. By applying differential pulse voltammetry technique under optimized experimental conditions, a good linear ratio of oxidation peak currents and concentrations of PCT over the range of 0.02–450 μM with a limit of detection of 1.69 nM were achieved. This novel electrode was stable for more than 60 days and reproducible responses were obtained at 99% of the initial current of PCT without any influence of physiologically common interferences such as ascorbic acid and uric acid. The application of this electrode to determine PCT in tablets and urine samples was proposed.     

Sensitive determination of thiamethoxam, imidacloprid and acetamiprid in environmental water samples with solid-phase extraction packed with multiwalled carbon nanotubes prior to high-performance liquid chromatography

This paper describes a novel method for sensitive determination of thiamethoxam, imidacloprid and acetamiprid based on solid-phase extraction with multiwalled carbon nanotubes as the packed materials. Factors that maybe influence the enrichment efficiency, such as sample flow rate, sample pH, and sample volume, were investigated in detail. Under the optimized conditions, the detection limits of thiamethoxam, imidacloprid and acetamiprid were 6.1, 5.4 and 6.7 ng L⁻¹, respectively. The experimental results indicated that there was good linearity (R ²>0.9993) over the range of 0.08~100 ng mL⁻¹ and good reproducibility with the relative standard deviations over the range of 0.7~1.1% (n=6). The proposed method has been applied to the analysis of real-world water samples, and satisfactory achievements were obtained. The average spiked recoveries were in the range of 87.5~109.8%. All the results indicated that the proposed method could be used for the simultaneous determination of the three pesticides in environmental water samples at trace levels.     

Determination of atrazine and simazine in environmental water samples using multiwalled carbon nanotubes as the adsorbents for preconcentration prior to high performance liquid chromatography with diode array detector

Multiwalled carbon nanotubes, a new nanoscale material, has been gained many interests for use in various fields, and has exhibited exceptional merit as SPE absorbents for enrichment of environmental pollutants. This paper focused on the enriching power of atrazine and simazine, two important widely used triazine herbicides and described a novel and sensitive method for determination of these two herbicides based on SPE using multiwalled carbon nanotubes as solid phase absorbents followed by high performance liquid chromatography with diode array detector. Factors that maybe affect the enrichment efficiency of multiwalled carbon nanotubes such as the volume of eluent, sample flow rate, sample pH, and volume of the water samples were optimized. Under the optimal procedures, multiwalled carbon nanotubes as the absorbents have obtained excellent enrichment efficiency for atrazine and simazine. The detection limits of the atrazine and simazine were 33 and 9 ng l⁻¹, respectively. The spiked recoveries of the two analytes were over the range of 82.6-103.7% in most cases. Good analytical performance was achieved from real-world water samples such as river water, reservoir water, tap water and wastewater after primary pretreatment with proposed method. All these experimental results indicated that the developed method could be used as an alternative for the routine analysis of atrazine and simazine in many real water samples.     

Electropolymerized multiwalled carbon nanotubes/polypyrrole fiber for solid-phase microextraction and its applications in the determination of pyrethroids

A novel solid-phase microextraction (SPME) fiber coated with multiwalled carbon nanotubes/polypyrrole (MWCNTs/Ppy) was prepared with an electrochemical method and used for the extraction of pyrethroids in natural water samples. The results showed that the MWCNTs/Ppy coated fiber had high organic stability, and remarkable acid and alkali resistance. In addition, the MWCNTs/Ppy coated fiber was more effective and superior to commercial PDMS and PDMS/DVD fibers in extracting pyrethroids in natural water samples. Under optimized conditions, the calibration curves were found to be linear from 0.001 to 10 μg mL⁻¹ for five of the six pyrethroids studied, the exception being fenvalerate (which was from 0.005 to 10 μg mL⁻¹), and detection limits were within the range 0.12-0.43 ng mL⁻¹. The recoveries of the pyrethroids spiked in water samples at 10 ng mL⁻¹ ranged from 83 to 112%.     

Quick and selective extraction of Z‐ligustilide from Angelica sinensis using magnetic multiwalled carbon nanotubes

A facile and highly efficient magnetic solid‐phase extraction method has been developed for Z‐ligustilide, the major therapeutic agent in Angelica sinensis. The solid‐phase adsorbent material used was prepared by conjugating carbon nanotubes with magnetic Fe₃O₄ nanoparticles via a hydrothermal reaction. The magnetic material showed a high affinity toward Z‐ligustilide due to the π–π stacking interaction between the carbon nanotubes and Z‐ligustilide, allowing a quick and selective exaction of Z‐ligustilide from complex sample matrices. Factors influencing the magnetic solid‐phase extraction such as the amount of the added adsorbent, adsorption and desorption time, and desorption solvent, were investigated. Due to its high extraction efficiency, this method was proved highly useful for sample cleanup/enrichment in quantitative high‐performance liquid chromatography analysis. The proposed method had a linear calibration curve (R² = 0.9983) over the concentration between 4 ng/mL and 200 μg/mL Z‐ligustilide. The accuracy of the method was determined by the recovery, which was from 92.07 to 104.02%, with the relative standard deviations >4.51%.     

Application of NaClO-treated multiwalled carbon nanotubes as solid phase extraction sorbents for preconcentration of trace 2,4-dichlorophenoxyacetic acid in aqueous samples

Multiwalled carbon nanotubes functionalized by oxidation of original multiwalled carbon nanotubes with NaClO were prepared and their application as solid phase extraction sorbent for 2,4-dichlorophenoxyacetic acid (2,4-D) was investigated systemically, and a new method was developed for the determination of trace 2,4-D in water samples based on extraction and preconcentration of 2,4-D with solid phase extraction columns packed with NaClO-treated multiwalled carbon nanotubes prior to its determination by HPLC. The optimum experimental parameters for preconcentration of 2,4-D, including the column activating conditions, the amount of the sorbent, pH of the sample, elution composition, and elution volume, were investigated. The results indicated 2,4-D could be quantitatively retained by 100 mg NaClO-treated multiwalled carbon nanotubes at pH 5, and then eluted completely with 10 mL 3:1 (v/v) methanol-ammonium acetate solution (0.3 mol/L). The detection limit of this method for 2,4-D was 0.15 μg/L, and the relative standard deviation was 2.3% for fortified tap water samples and 2.5% for fortified riverine water sample at the 10 μg/L level. The method was validated using fortified tap water and riverine water samples with known amount of 2,4-D at the 0.4, 10, and 30 μg/L levels, respectively.     

Multiwalled carbon nanotubes as solid-phase extraction materials for the gas chromatographic determination of organophosphorus pesticides in waters

In the present work, a GC method with nitrogen-phosphorus detection (NPD) was developed for the simultaneous determination of eight organophosphorus pesticide (OPP) residues (i.e., ethoprofos, diazinon, chlorpyrifos-methyl, fenitrothion, malathion, chlorpyrifos, fenamiphos, and buprofezin) in water samples. Preconcentration of the water samples was carried out using an SPE procedure with multiwalled carbon nanotubes (MWCNTs) of 10-15 nm od, 2-6 nm id, and 0.1-10 microm length as stationary phase. Extraction parameters, such as the amount of MWCNTs, sample volume, pH, and type and amount of the eluent were optimized. The most favorable conditions were as follows: 40 mg MWCNTs, 800 mL water, pH 6.0, and 20 mL dichloromethane, respectively. The MWCNTs-SPE-GC-NPD method was applied to the determination of these pesticides in real water samples: mineral and ground water as well as run-off water from an agricultural area collected shortly before opening out onto the sea. A recovery study was developed with five consecutive extractions of the three types of water spiked at three concentration levels (n = 15). Mean recovery values were in the range of 75-116% for mineral water (RSD < 6.3%), 67-119% for ground water (RSD < 5.8%), and 57-81% for run-off waters (RSDs < 6.9%), except for fenamiphos (mean recovery values between 40 and 84% for the three types of waters, RSDs < 8.9%). LODs were in the low ng/L level (i.e., levels below the maximum residue limits (MRLs) established by the European Union (EU) legislation for these compounds in waters). The proposed method was also applied to the analysis of six water samples (two of each type: mineral, ground, and run-off waters) in which no residues of the selected pesticides were found. Results show that the MWCNTs used in this work have a high adsorbability of the pesticides under study. The main advantage of the use of these MWCNTs is their low cost when compared with those MWCNTs previously used in the literature and with conventional SPE cartridges.     

Fabrication of a highly sensitive electrochemiluminescence lactate biosensor using ZnO nanoparticles decorated multiwalled carbon nanotubes

ZnO nanoparticles (nanoZnO) were decorated on multiwalled carbon nanotubes (MWCNTs) and then the prepared nano-hybrids, nanoZnO-MWCNTs, were immobilized on the surface of a glassy carbon electrode (GCE) to fabricate nanoZnO-MWCNTs modified GCE. The prepared electrode, GCE/nanoZnO-MWCNTs, showed excellent electrocatalytic activity towards luminol electrochemiluminescence (ECL) reaction. The electrode was then further modified with lactate oxidase and Nafion to fabricate a highly sensitive ECL lactate biosensor. Two linear dynamic ranges of 0.01-10 μmol L⁻¹ and 10-200 μmol L⁻¹ were obtained for lactate with the correlation coefficient better than 0.9996. The detection limit (S/N = 3) was 4 nmol L⁻¹ lactate. The relative standard deviation for repetitive measurements (n = 6) of 10 μmol L⁻¹ lactate was 1.5%. The fabrication reproducibility for five biosensors prepared and used in different days was 7.4%. The proposed ECL lactate biosensor was used for determination of lactate in human blood plasma samples with satisfactory results.     

Application of modified multiwalled carbon nanotubes as solid sorbent for separation and preconcentration of trace amounts of manganese ions

In this work, the potential of modified multiwalled carbon nanotubes for separation and preconcentration of trace amounts of manganese ion is studied. Multiwalled carbon nanotubes were oxidized with concentrated HNO₃ and then modified with loading 1-(2-pyridylazo)-2-naphtol. Mn(II) ions could be quantitatively retained by modified multiwalled carbon nanotubes in the pH range of 8–9.5. Elution of the adsorbed manganese was carried out with 5.0 mL of 0.1 mol L^?1 HNO₃. Detection limit is 0.058 ng mL^?1 and analytical curve is linear in the range of 0.1 ng mL^?1–5.0 μg mL^?1 in the initial solution with a correlation coefficient 0.9977 and the preconcentration factor is 100. Relative standard deviation for eight replicate determination of 0.5 μg mL^?1 of manganese in the final solution is 0.41%. The effects of the experimental parameters, including the sample pH, flow rates of sample and eluent solution, eluent type, breakthrough volume and interference ions, were studied for preconcentration of Mn(II) ions in detail to optimize the conditions. The method was successfully applied for separation, preconcentration and determination of manganese in different samples.     

Evaluation of multi-walled carbon nanotubes as solid-phase extraction adsorbents of pesticides from agricultural, ornamental and forestal soils

A new, simple and cost-effective method based on the use of multi-walled carbon nanotubes (MWCNTs) as solid-phase extraction stationary phases is proposed for the determination of a group of seven organophosphorus pesticides (i.e. ethoprophos, diazinon, chlorpyriphos-methyl, fenitrothion, malathion, chlorpyriphos and phosmet) and one thiadiazine (buprofezin) in different kinds of soil samples (forestal, ornamental and agricultural) using gas chromatography with nitrogen phosphorus detection. Soils were first ultrasound extracted with 10 mL 1:1 methanol/acetonitrile (v/v) and the evaporated extract redissolved in 20 mL water (pH 6.0) was passed through 100 mg of MWCNTs of 10-15 nm o.d., 2-6 nm i.d. and 0.1-10 μm length. Elution was carried out with 20 mL dichloromethane. The method was validated in terms of linearity, precision, recovery, accuracy and selectivity. Matrix-matched calibration was carried out for each type of soil since statistical differences between the calibration curves constructed in pure solvent and in the reconstituted soil extract were found for most of the pesticides under study. Recovery values of spiked samples ranged between 54 and 91% for the three types of soils (limits of detection (LODs) between 2.97 and 9.49 ng g⁻¹), except for chlorpyrifos, chlorpyrifos-methyl and buprofezin which ranged between 12 and 54% (LODs between 3.14 and 72.4 ng g⁻¹), which are the pesticides with the highest soil organic carbon sorption coefficient (K_OC) values. Using a one-sample test (Student's t-test) with fortified samples at two concentration levels in each type of soil, no significant differences were observed between the real and the experimental values (accuracy percentages ranged between 87 and 117%). It is the first time that the adsorptive potential of MWCNTs for the extraction of organophosphorus pesticides from soils is investigated.     

Development of multiwalled carbon nanotubes based micro-solid-phase extraction for the determination of trace levels of sixteen polycyclic aromatic hydrocarbons in environmental water samples

Micro-solid-phase extraction (μ-SPE) was developed for the determination of trace level of 16 United States Environmental Protection Agency priority polycyclic aromatic hydrocarbons (PAHs) in river water samples with gas chromatography-mass spectrometry (GC-MS). In the μ-SPE device, multiwalled carbon nanotubes was employed as sorbent and was packed inside an porous polypropylene membrane "envelope" whose edges were heat-sealed to secure the contents. The μ-SPE device was placed in a stirred sample solution to extract the analytes. The porous polypropylene membrane envelope in μ-SPE device acts as a filter to exclude potential interferences, such as eliminating or reducing the influence of particles that are bigger than the pore size. After extraction, analyte desorption was carried out with a suitable organic solvent under ultrasonication. Important extraction parameters were optimized in detail, including the selection and amount of sorbent materials, the extraction temperature and extraction time, desorption solvent and desorption time, amount of organic modifier, agitation speed and sample ionic strength. Under the developed extraction conditions, the proposed method provided good linearity in the range of 0.1-50 μg/L, low limits of detection (4.2-46.5 ng/L), and good repeatability of the extractions (relative standard deviations, <12%, n=5). The developed μ-SPE method was successfully applied to the extraction of PAHs in river water samples. The μ-SPE method was demonstrated to be a fast and efficient method for the determination of PAHs from environmental water samples.     

Preparation of cyano-functionalized multiwalled carbon nanotubes as solid-phase extraction sorbent for preconcentration of phenolic compounds in environmental water

In the present work, we showed a novel method to synthesize cyano-functionalized multiwalled carbon nanotubes (MWCNTs-CN) and utilize it as a solid-phase extraction sorbent for preconcentration of phenolic compounds in environmental water samples. MWCNTs-CN was synthesized through surface functionalization of multiwalled carbon nanotubes (MWCNTs). The functional groups on the surface of modified MWCNTs were characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis and scanning electron microscopy. The analytical procedure was based on a conventional solid-phase extraction step for which 100 mg of MWCNTs-CN were packed in a 3 mL polypropylene cartridge. Analytes were thus isolated and preconcentrated from the pretreated samples and subsequently detected on high-performance liquid chromatography-ultraviolet detection. The results showed the proposed method exhibited good sensitivity and precision for the extraction and elution of analytes. The limit of detections (S/N = 3) of the method were 0.45, 0.09, 0.08, and 3.00 ng mL(-1) for p-chlorophenol, 1-naphthol, 2-naphthol, and 2,4-dichlorophenol, respectively. The mean relative recoveries (n = 3) were between 80.28 and 103.13%, and the repeatability (RSD ≤ 5.10%) and reproducibility (RSD ≤ 7.68%) were accepted. This developed method was applied to determine phenolic compounds in environmental water samples. There is a positive result only for 2-naphthol with concentration of 0.38 ng mL(-1) in seawater sample.     

Development of magnetic multiwalled carbon nanotubes as solid-phase extraction technique for the determination of p-hydroxybenzoates in beverage

In this work, magnetic multiwalled carbon nanotubes were synthesized through a facile hydrothermal process, and then successfully used as magnetic solid-phase extraction sorbents for the determination of p-hydroxybenzoates in beverage. The prepared magnetic multiwalled carbon nanotubes presented both satisfactory superparamagnetism and strong capacity of absorption, with magnetic Fe(3)O(4) beads of 200 nm average diameters decorated at either ends of the tubes. The hybrid nanocomposites showed a high efficiency in the extraction and enrichment of p-hydroxybenzoates via π-π stacking of targeted molecules onto the polyaromatic composed surface of multiwalled carbon nanotubes, which entitled them promising magnetic solid-phase extraction sorbents for p-hydroxybenzoates at trace level from complex drink samples. By using an external magnetic field, p-hydroxybenzoates adsorbed on magnetic multiwalled carbon nanotubes could be rapidly isolated in only 30 s, and subsequently analyzed by liquid chromatography-diode array detector after elution with organic solvents. Extraction conditions such as eluting solvent, the amounts of magnetic sorbents added, pH values, adsorption and desorption time were investigated and optimized to achieve the best effect. Method validations including linearity, detection limit, and precision were also studied. The linearities were in the wide range of 0.05-500 μg/mL with correlation coefficients higher than 0.9983 for all p-hydroxybenzoates. The limits of detection were less than 20 ng/mL. Acceptable RSDs were achieved within 5-8% for all analytes. The results indicated that the proposed method based on magnetic multiwalled carbon nanotubes as magnetic solid-phase extraction absorbents was rapid, efficient, and convenient for the analysis of the targeted compounds of p-hydroxybenzoates in beverage sample.