Determination of Zn(II) in rock and vegetable samples after acidic digestion followed by ultrasound-assisted solid-phase extraction with reduced graphene oxide as novel sorbent,in combination with flame atomic absorption spectrometry

Graphene, a novel class of carbon nanostructures, has great promise for use as sorbent materials because of its ultrahigh specific surface area. A new method using reduced graphene oxide (RGO) as sorbent was developed for the preconcentration of trace amounts of zinc (Zn) to its determination by flame atomic absorption spectrometry. Zinc could be adsorbed quantitatively on RGO in the pH range of 1–9, and then eluted completely with 0.5 mL of 0.1 mol L^?1 HCl. Some effective parameters on the extraction were selected and optimized. Under optimum conditions, the calibration graph was linear in the concentration range of 0.2–15 μg L^?1 with a detection limit of 0.14 μg L^?1 with an enrichment factor of 100.12. The relative standard deviation for ten replicate measurements of 10 μg L^?1 of Zn was 0.58 %, respectively. The proposed method was successfully applied in the analysis of rock and vegetable samples. Good spiked recoveries over the range of 99.9–100 % were obtained. This work not only proposes a useful method for sample preconcentration, but also reveals the great potential of graphene as an excellent sorbent material in analytical processes.     

Electrochemical sensor for estriol hormone detection in biological and environmental samples

A stable conducting film for sensing using reduced graphene oxide (RGO), gold nanoparticles (GNPs), and potato starch (PS) is proposed. The characterization of the nanomaterials was obtained by ultraviolet and visible spectroscopy, dynamic light scattering, zeta potential, Fourier transform infrared spectroscopy, atomic force microscopy, and cyclic voltammetry. The voltammetric behavior of the RGO-GNPs-PS/GCE electrodes was studied in the presence of estriol and the results showed a high anodic peak current at 0.64 V. Under optimal conditions, an analytical curve was obtained, in which the anodic peak estriol was linear in the range from 1.5 to 22 μmol L^?1, with a detection limit of 0.48 μmol L^?1. The modified electrodes were applied for determination of estriol in environmental and biological samples. The proposed electrode was used for estriol determination in water and urine samples, which presented a recovery range from 92.1 to 106%, showing that RGO-GNPs-PS/GCE is a viable alternative for the detection of estriol and can be attractive for several electrochemical applications.     

Preparation of Molecularly Imprinted Electrochemical Sensor for Detection of Vincristine Based on Reduced Graphene Oxide/Gold Nanoparticle Composite Film

An innovative molecularly imprinted electrochemical sensor was fabricated based on reduced graphene oxide (RGO) and gold nanocomposite (Au) for rapid detection of vincristine (VCR). The RGO‐Au composite membrane was obtained via direct one‐step electrodeposition technique of graphene oxide (GO) and chloroauric acid (HAuCl₄) on the surface of a glassy carbon electrode (GCE) by means of cyclic voltammetry (CV) in the potential range between ?1.5 and 0.6 V in phosphate buffer solution (PBS) of pH 9.18, which is capable of effectively utilizing its superior electrical conductivity, larger specific surface area due to its synergistic effect between RGO and Au. The molecularly imprinted polymers (MIPs) were synthesized on the RGO‐Au modified glassy carbon electrode surface with VCR as the template molecular, methyl acrylic acid (MAA) as the functional monomer, and ethylene glycol maleic rosinate acrylate (EGMRA) as a cross‐linker. The performance of the sensor was investigated by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS) in detail. Under the optimum conditions, the fabricated sensor exhibited a linear relationship between oxidation peak current and VCR concentration over the range of 5.0×10^?8–5.0×10^?6 mol·L ^minus;1 with a correlation coefficient of 0.9952 and a detection limit (S/N=3) of 2.6×10^minus;8 mol·L^minus;1. The results indicated that the imprinted polymer films exhibited an excellent selectivity for VCR. The imprinted sensor was successfully used to determine VCR in real samples with recoveries of 90% –120% by using the standard addition method.     

A thin poly(acridine orange) film containing reduced graphene oxide for voltammetric simultaneous sensing of ascorbic acid and uric acid

We have fabricated, in a single step, carbon ceramic electrodes modified with a poly(acridine orange) film containing reduced graphene oxide. They display electrocatalytic activity to ascorbic acid (AA) and uric acid (UA) at pH 4.5. The anodic peak potentials of AA and UA are separated by 276 mV so that they can be well resolved in cyclic voltammetry. UA and AA were simultaneously determined in a mixture at working potentials of 170 and 400 mV, respectively. Under optimized conditions, the calibration curves for AA and UA cover the 0.8–5,000 μM and 0.6–900 μM concentration range, respectively, while detection limits are 0.3 μM and 0.2 μM. The electrode was applied to determine AA and UA in urine samples.

Determination of trace amounts of plutonium in low-active liquid wastes from spent nuclear-fuel reprocessing plants by flow injection-based solid-phase extraction/electrochemical detection system

A flow injection-based electrochemical detection system coupled to a solid-phase extraction column was developed for the determination of trace amounts of plutonium in low-active liquid wastes from spent nuclear-fuel reprocessing plants. The oxidation state of plutonium in a sample solution was adjusted to Pu(VI) by the addition of silver(II) oxide. A sample solution was made up in 3 mol L^?1 HNO₃ and loaded onto a column packed with UTEVA^® with 3 mol L^?1 HNO₃ as the carrier. Plutonium(VI) was adsorbed onto the resin, and interfering elements were removed by rinsing the column with 3 mol L^?1 HNO₃. Subsequently, the adsorbed Pu(VI) was eluted with 0.01 mol L^?1 HNO₃, and then introduced directly into the flow-through electrolysis cell with boron-doped diamond electrode. The eluted Pu(VI) was detected by an electrochemical amperometric method at a working potential of 0.1 V (vs. Ag/AgCl). The current produced on reduction of Pu(VI) was continuously monitored and recorded. The plutonium concentration was calculated from the relationship between the peak area and concentration of plutonium. The relative standard deviation of ten analyses was 1.1% for a plutonium solution of 25 μg L^?1 containing 50 ng of Pu. The detection limit calculated from three-times the standard deviation was 0.82 μg L^?1 (1.6 ng of Pu).     

Self-assembled reduced graphene oxide/sulfur composite encapsulated by polyaniline for enhanced electrochemistry performance

Reduced graphene oxide/sulfur/polyaniline (referred to RGO/S/PANI) composite was self-assembled through in situ synthesis and used to investigate the electrochemical properties of lithium/sulfur cells. The RGO/S/PANI composite possessed 809.3/801.9 mAh g^?1 of initial charge/discharge capacities, higher than 588.3/588.2 mAh g^?1 for reduced graphene oxide/sulfur (referred to RGO/S) and 681.4/669.9 mAh g^?1 for sulfur/polyaniline (referred to S/PANI) at similar conditions. The RGO/S/PANI composite obtained 400 mAh g^?1 at 2 C and good reversible capacities of 605.5 and 600.8 mAh g^?1 at 100th charge/discharge cycle at 0.1 C, in comparison with low electrochemical performance of RGO/S and S/PANI. The improved properties could be attributed to the collaboration of RGO and PANI. Co-generation of RGO and sulfur acted as seeds for their depositions, stimulated their uniform distributions, and restricted the agglomeration of sulfur particles in situ synthesis. Polyaniline coated RGO/S and stabilized the nanostructure of RGO/S/PANI in repeated charge/discharge cycles. In addition, RGO and PANI provided many electron channels to enhance sulfur conductivity and sufficient void space for sulfur swelling during charge/discharge cycles.     

Simultaneous Determination of Dopamine and Uric Acid by Electrocatalytic Oxidation on a Carbon Paste Electrode Using Pyrogallol Red as a Mediator

A sensitive and selective electrochemical method for the simultaneous determination of dopamine (DA) and uric acid (UA) was developed using a pyrogallol red modified carbon paste electrode. Under the optimized conditions, the peak current was linearly dependent on 1.0–700.0 μmol L^?1 DA and 50.0–1000.0 μmol L^?1 UA. The detection limits for DA and UA were 0.78 μmol L^?1 and 35 μmol L^?1, respectively. Finally, this method was also examined for the determination of DA and uric acid in real samples such as drugs and urine.     

A novel biosensor based on ZnO nanoparticle/1,3-dipropylimidazolium bromide ionic liquid-modified carbon paste electrode for square-wave voltammetric determination of epinephrine

The direct electrochemistry of epinephrine (EP) on a modified carbon paste electrode (CPE) was described. The electrode was modified with Zinc oxide (ZnO) nanoparticles and 1,3-dipropylimidazolium bromide as a binder. The oxidation peak potential of EP at the surface of the ionic liquid ZnO nanoparticle CPE (IL/ZnO/NP/CPE) appeared at 350 mV, which was about 80 mV lower than the oxidation peak potential at the surface of the traditional carbon CPE under a similar condition. On other hand, the oxidation peak current was increased for about three times at the surface of IL/ZnO/NP/CPE compared to CPE. The linear response range and detection limit were found to be 0.09–800 μmol L^?1 and 0.06 μmol L^?1, respectively. Other physiological species did not interfere in the determination of EP at the surface of the proposed sensor in the optimum condition. The proposed sensor was successfully applied for the determination of EP in real samples.     

A sensitive electrochemical sensor for lead based on gold nanoparticles/nitrogen-doped graphene composites functionalized with <Emphasis Type="SmallCaps">l</Emphasis>-cysteine-modified electrode

Glassy carbon electrodes (GCEs) modified with l-cysteine (l-cys)/gold nanoparticles (AuNPs)/nitrogen-doped graphene (NG) composite were prepared to fabricate a novel electrochemical sensor for lead. AuNPs were uniformly dispersed into NG and l-cys was successfully decorated on AuNPs through the S–Au bond. The l-cys/AuNPs/NG exhibited a well-distributed nanostructure and high responsivity toward Pb(II). The results indicated that l-cys/AuNPs/NG/GCE exhibited the highest peak current, reflecting that the l-cys/AuNPs/NG composites showed the best response signal toward Pb²⁺. Under optimized conditions, a linear relationship between the current intensity and Pb²⁺ concentration was obtained in a range of 0.5–80 μg L^?1 with a detection limit of 0.056 μg L^?1 (S/N = 3). The analytical interference procedure and practical application were investigated using the prepared electrode, which exhibited an acceptable result.     

Sensitive Analysis of Malondialdehyde in Human Urine by Derivatization with Pentafluorophenylhydrazine then Headspace GC–MS

A sensitive method based on derivatization with pentafluorophenylhydrazine then headspace gas chromatography–mass spectrometry has been used for analysis of malondialdehyde in human urine. Preparation of urine sample by one-step derivatization/evaporation was performed by reaction of malondialdehyde with pentafluorophenylhydrazine in a headspace vial for 10 min; the derivatives were then injected in GC–MS analysis. The reaction was performed at pH 3, and total analysis time was 35 min. The method detection limit was 0.04 μg L^?1. For MDA concentrations of 2.0 and 10.0 μg L^?1 the relative standard deviation was less then 5%. The concentration of MDA in urine was measured to be 0.199 ± 0.252 μmol g^?1 creatinine (0.022 ± 0.028 μmol mmol^?1 creatinine).     

Determination of Sudan I Using Electrochemically Reduced Graphene Oxide

Electrochemically reduced graphene oxide (ER-GO) was prepared by reducing exfoliated graphene oxide sheets on a glassy carbon electrode (GCE). The voltammetric responses of Sudan I-IV were studied at the ER-GO modified GCE (ER-GO/GCE). Compared with chemically reduced graphene oxide (CR-GO) modified electrode (CR-GO/GCE), ER-GO/GCE showed higher voltammetric responses to Sudan I. The electrode had a linear response to Sudan I in the range of 0.04–8.0 µmol L^?1 and a detection limit of 0.01 µmol L^?1. The real sample determination indicated that the proposed method was reliable, effective, and sufficient.     

A stir foam composed of graphene oxide,poly(ethylene glycol) and natural latex for the extraction of preservatives and antioxidant

A stir foam composed of graphene oxide, poly(ethylene glycol) and natural latex (GO-PEG-NL) was prepared for use in micro-solid phase extraction sorbent of preservative agents and antioxidants from cosmetic products. The extracted analytes were quantified by GC-MS. Under the optimized conditions, the calibration plots are linear in the concentration ranges between 5.0 μg·L^?1 to 1.0 mg·L^?1 for benzoic acid, of 10.0 μg·L^?1 to 1.0 mg·L^?1 for 2-methyl-3-isothiazolinone (MI), and between 1.0 μg·L^?1 and 1.0 mg·L^?1 for both 3-tert-butyl-4-hydroxyanisole (BHA) and 2,6-di-tert-butyl-p-hydroxytoluene (BHT). The LODs are 1.0 μg·L^?1 for benzoic acid, 5.0 μg·L^?1 for MI and 0.5 μg·L^?1 for both BHA and BHT. The stir-foam can be easily prepared, is inexpensive and well reproducible (RSDs <3%, for n?=?6). It can be re-used for up to 12 times after which extraction efficiency has dropped to 90%. The method was successfully applied to the determination of preservatives and antioxidants in cosmetic samples. Recoveries from spiked samples ranged between 94.5?±?2.1% and 99.8?±?1.8%.

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Graphical abstract A stir foam was prepared from graphene oxide, poly(ethylene glycol) and natural latex (GO-PEG-NL) and is shown to be a most viable sorbent for the microextraction of trace amounts of preservative agents and antioxidants from cosmetic products.

     

Novel potentiometric sensor for the trace-level determination of Zn2+ based on a new nanographene/ion imprinted polymer composite

In this research, a new strategy for construction of a development potentiometric carbon paste Zn²⁺-ion selective electrode based on a novel nano-sensing layer is suggested. The proposed nano-sensing layer was prepared with the addition of a synthesised Zn²⁺-ion imprinted polymer nanoparticles ‘as an efficient sensing agent’ into the carbon paste matrix consisting of graphite powder, nanographene-composite ‘graphene nanosheets decorated with silver nanoparticles’ and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ‘ an ionic liquid ’, as the conducting binder. Under the optimised experimental conditions, the suggested nanographene-composite potentiometric sensor presented a low detection limit of 1.93 × 10^?1 μg L^?1 and a linear analytical range from 2.62 × 10^?1 to 6.54 × 10⁵ μg L^?1 with an excellent Nernstian slope of 29.80 mV decade^?1. The proposed zinc selective sensor was successfully applied for the highly sensitive determination of trace amounts of Zn²⁺ in environmental and biological samples with satisfactory results.     

Rapid and simple electrochemical detection of morphine on graphene–palladium-hybrid-modified glassy carbon electrode

A hybrid of reduced graphene oxide–palladium (RGO–Pd) nano- to submicron-scale particles was simultaneously chemically prepared using microwave irradiation. The electrochemical investigation of the resulting hybrid was achieved using cyclic voltammetry and differential pulse voltammetry. RGO–Pd had a higher current response than unmodified RGO toward the oxidation of morphine. Several factors that can affect the electrochemical response were studied, including accumulation time and potential, Pd loading, scan rate, and pH of electrolyte. At the optimum conditions, the concentration of morphine was determined using differential pulse voltammetry in a linear range from 0.34 to 12 μmol L^?1 and from 14 to 100 μmol L^?1, with detection limits of 12.95 nmol L^?1 for the first range. The electrode had high sensitivity toward morphine oxidation in the presence of dopamine (DA) and of the interference compounds ascorbic acid (AA) and uric acid (UA). Electrochemical determination of morphine in a spiked urine sample was performed, and a low detection limit was obtained. Validation conditions including reproducibility, sensitivity, and recovery were evaluated successfully in the determination of morphine in diluted human urine.

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Comparison of uranium extraction from model fresh water on TiO–PAN and NaTiO–PAN composite absorbers

Possibility of uranium extraction on titanium oxide (TiO)–polyacrylonitrile (PAN) and sodium titanate (NaTiO)–PAN composite absorbers prepared from hydrated TiO and NaTiO embedded into a matrix of PAN binder was studied. Both TiO–PAN and NaTiO–PAN absorbers might be advantageous for repeated extraction of uranium from fresh water. Appropriate flow rates were found up to 60 BV h^?1 for both the absorbers. Elution of uranium and regeneration of both the absorbers may be accomplished by hydrochloric acid solutions of concentration 0.1 mol L^?1 and more. Practical sorption capacity (up to 10 % break-through) from tap water containing 2.3 μg U mL^?1 was ~4.6 and ~1.5 mg of uranium per 1 mL of swollen TiO–PAN and NaTiO–PAN absorber, respectively.     

Carbon paste electrode modified with Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles and BMI.PF<Subscript>6</Subscript> ionic liquid for determination of estrone by square-wave voltammetry

A carbon paste electrode (CPE) modified with Fe₃O₄ nanoparticles (Fe₃O₄ NP) and the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (IL BMI.PF₆) was employed for the electroanalytical determination of estrone (E1) by square-wave voltammetry (SWV). At the modified electrode, cyclic voltammograms of E1 in B–R buffer (pH 12.0) showed an adsorption-controlled irreversible oxidation peak at around +0.365 V. The anodic current increased by a factor of five times and the peak potential shifted 65 mV to less positive values compared with the unmodified CPE. Under optimized conditions, the calibration curve obtained showed two linear ranges: from 4.0 to 9.0 μmol L^?1 and from 9.0 to 100.0 μmol L^?1. The limits of detection (LOD) and quantification (LOQ) attained were 0.47 and 4.0 μmol L^?1, respectively. The proposed modified electrode was applied to the determination of E1 in pork meat samples. Data provided by the proposed modified electrode were compared with data obtained by UV–vis spectroscopy. The outstanding performance of the electrochemical device indicates that Fe₃O₄ NP and the IL BMI.PF₆ are promising materials for the preparation of chemically modified electrodes for the determination of E1.     

Solid phase extraction and preconcentration of trace mercury(II) from aqueous solution using magnetic nanoparticles doped with 1,5-diphenylcarbazide

A new method for solid-phase extraction and preconcentration of trace mercury(II) from aqueous solution was developed using 1,5-diphenylcarbazide doped magnetic Fe₃O₄ nanoparticles as extractant. The surface treatment did not result in the phase change of Fe₃O₄. Various factors which influenced the recovery of the analyte were investigated using model solutions and batch equilibrium technique. The maximum adsorption occurred at pH?>?6, and equilibrium was achieved within 5 min. Without filtration or centrifugation, these mercury loaded nanoparticles could be separated easily from the aqueous solution by simply applying an external magnetic field. At optimal conditions, the maximum adsorption capacity was 220 μmol g^?1. The mercury ions can be eluted from the composite magnetic particles using 0.5 mol L^?1 HNO₃ as a desorption reagent. The detection limit of the method (3σ) was 0.16 μg L^?1 for cold vapor atomic absorption spectrometry, and the relative standard deviation was 2.2%. The method was validated by the analysis of a certified reference material with the results being in agreement with those quoted by manufactures. The method was applied to the preconcentration and determination of trace inorganic mercury(II) in natural water and plant samples with satisfactory results.     

Determination of nitrite in environmental waters by flow injection catalytic spectrophotometry

ABSTRACT

The flow injection catalytic spectrophotometry is proposed for the determination of nitrite based on the catalytic effect on the redox reaction between methylene blue and potassium bromate in acidic medium. The reaction is monitored spectrophotometrically by measuring the decrease in the absorbance of methylene blue at the maximum absorption wavelength of 664 nm. The method is characterised by low solvent consumption and easy automatic continuous analysis. It has higher sensitivity and lower detection limit. Experimental analysis conditions of the flow injection-catalytic photometry are optimised, and the best analysis conditions are: the concentration of the potassium bromate is 0.068 mol L^?1; the concentration of the phosphoric acid in oxidation liquid is 0.045 mol L^?1; the concentration of the methylene blue in colour-substrate solution is 2.4 mg L^?1, the volume of sample ring is 200 μL; the reaction coil is around 7 m in length; the inject time is 50 s and analysis time is 70 s. Under the optimal conditions, the linear range is from 10 to 500 μg L^?1 and the detection limit is 1 μg L^?1. The nitrate standard solution is continuously determined with a mass concentration of 300 μg L^?1. The RSD is determined to be 1.41% (n = 10). The nitrite in water samples, which were from the Half Acre pond, the Ink River and the Small West lake in a campus, was determined respectively by this method. A satisfactory standard addition recovery of 96.7%–103.9% was obtained.     

Fabrication of a nanostructured TiO2/carbon nanotube composite electrode for voltammetric and impedimetric determination of NTO explosive in the water and soil samples

An electrochemical oxidation route was developed for sensitive and selective assay of nitrotriazolone (NTO) explosive in some environmental samples on a multi-walled carbon nanotube (MWCNTs)/TiO₂ nanocomposite paste electrode, for prevention of the analytical interference of conventional reducible energetic compounds. Detailed evaluations were made for the electrochemical behaviour of NTO on the modified electrode by adsorptive stripping voltammetry, electrochemical impedance spectroscopy (EIS) and chronoamperometry techniques in the pH range of 2.0–10.0. Parameters such as diffusion coefficient constant of NTO were calculated, and various experimental conditions were also optimised. Under optimal conditions the calibration curve had two linear dynamic ranges of 130.0–3251.5 μg L^?1 and 6.5–26.0 mg L^?1 with a detection limit of 26.0 μg L^?1 (0.2 μmol L^?1) and precision of <3%. This electrochemical sensor was further applied to determine NTO in real soil and water samples with satisfactory results.     

Electroanalytical detection of heavy metals using metallophthalocyanine and silica-coated iron oxide composites

The monitoring of heavy metal ions particularly in water is important in safeguarding the environment and humans from the toxic effects these metal ions pose. This work describes the synthesis, characterization and electrocatalytic properties of silica-coated iron oxide nanoparticles (Si-NP) in the presence of cobalt or iron phthalocyanines (MPc) for heavy metal (HM) detection. TEM, XRD, XPS and VSM confirmed the successful synthesis of Si-NP with an average diameter of 12.07 nm. The electrochemical sensing properties of MPc/Si-NP-modified glassy carbon electrodes (GCE) were assessed for HM detection. Differential pulse anodic stripping voltammetry (DPASV) studies indicated detection limits that compared positively with the literature. The FePc/Si-NP composite showed the lowest detection limits (S/N?=?3) of 3.66, 11.56, 2.28, 4.54 μg L^?1 for arsenic (As), cadmium (Cd), mercury (Hg) and lead (Pb), respectively. A linear working range of 10–100 μg L^?1 was obtained for As³⁺, Hg²⁺ and Pb²⁺ ions while it was between 20 and 100 μg L^?1 for Cd²⁺ ions. Both composites displayed reproducible signals for the simultaneous detection of the HMs for ten consecutive scans. These composites offer a cheap and simplistic sensing device for HM analysis.