Graphene Nanosheets and Quantum Dots: A Smart Material for Electrochemical Applications

A novel material for the electrochemical determination of endocrine disruptors using a composite based on graphene oxide modified with cadmium telluride quantum dots has been evaluated. The morphology, structure and electrochemical performance of the composite electrodes were characterised by transmission electron microscopy, dynamic light scattering, UV‐visible absorption spectra, fluorescence spectra, Raman spectra and cyclic voltammetry. The dynamic light scattering, transmission electronic microscopy and spectrophotometric measurements all showed good distribution of the quantum dots with a small particle size. The electrochemical measurements demonstrated the high performance of the composite response in the presence of a light source. Differential pulse voltammetry allowed the development of a method to determine 17β‐estradiol levels in the range from 0.2 to 4.0 μmol L ^?1 with a detection limit of 2.8 nmol L ^?1 (0.76 μg L ^?1).     

Hydrothermal synthesis of nitrogen-doped carbon dots as a sensitive fluorescent probe for the rapid,selective determination of Hg2+

In the present work, a green synthetic method for producing nitrogen-doped carbon dots (NCDs) by using ammonium citrate and urea is introduced. The obtained NCDs were characterised by transmission electron microscopy, Fourier transform infrared spectra, UV–vis absorption and fluorescence spectra. The results showed that the prepared NCDs were spherical with a size of about 3.5 nm, emitting strong and stable blue fluorescence when excited at 352 nm. It was noting that the NCDs enable sensitive and selective determination of Hg²⁺ in tap water with a linear range of 0.01–5 mg L^?1 based on a possible charge transfer process. The detection limit was 9.4 µg L^?1.     

Variations of thermal-lens spectrophotoetry with helium-cadmium laser excitation for the determination of traces of phosphorus

Single-beam, differential single-beam and dual-beam thermal-lens measurements are compared for the determination of phosphorus traces by means of a helium-cadmium laser. Phosphate is determined as the molybdophosphate/auramine ion-pair which is extracted into 1:2 isobutanol/hexane; acetone is added to dissolve any ion-pair precipitate. In dual-beam thermal lens experiments a low-power HeNe laser is uded as the probe laser. The detection limit, restricted by variable blank measurement, is 0.6 ng cm^?3 phosphorus in all cases. The advantages and drawbacks of each measurement technique are discussed. The dual-beam system of measurement is more sensitive than the single-beam system and is appropriate for measurements on samples of small volume. The dual-beam system is applied for layer-by-layer determination of phosphorus in semiconductor silicon over the concentratin range 5 × 10^?8 ?5 × 10^?4%. The data obtained by the dual-beam method compare well with those obtained by a neutron activation method.     

Determination of cadmium,zinc, and lead by non-dispersive atomic fluorescence spectrometry with a new graphite furnace atomizer

A new graphite furnace atomizer has been developed and applied to the determination of cadmium, zinc, and lead by non-dispersive atomic fluorescence spectrometry. A solar-blind photomultiplier, a lock-in amplifier, and microwave-excited electrodeless discharge lamps are used. The detection limits for cadmium, zinc, and lead in the non-dispersive atomic fluorescence mode are 1·10^?13g, 2·10^?13g, and 2·10^?11g, respectively, which are 20-, 10-, and 2-fold better than those in the atomic absorption mode. The analytical working curves are linear over about three decades of concentration from the detection limits.     

CdS/ZnS core-shell quantum dots capped with mercaptoacetic acid as fluorescent probes for Hg(II) ions

We have synthesised water soluble CdS/ZnS core-shell quantum dots (QDs) capped with mercaptoacetic acid (MAA). They were characterised by UV–vis absorption spectroscopy, fluorescence spectroscopy, FT-IR and transmission electron microscopy. Such QDs can be used as fluorescent probes for the determination of metal ions because they quench the fluorescence of the QDs. The QDs exhibit absorption and emission bands at 345?nm and 475?nm respectively, which is more longer wavelength compared to MAA-capped CdS QDs and obviously is the result of the larger particle size. The fluorescence intensity of CdS-based QDs is strongly enhanced by coating them with a shell of ZnS. In addition, such functionalised QDs are more sensitive to Hg(II) ions. Parameters such as pH, temperature and concentration of the QDs have been optimised. A high selectivity and sensitivity toward Hg(II) ions is obtained at pH 7.4 and a concentration of 12.0?mg of QDs per L. Under optimum conditions, the fluorescence intensity of CdS/ZnS QDs is linearly proportional to the concentration of Hg(II) in the range from 2.5 to 280?nM, with a detection limit of 2.2?nM. The effect of potentially interfering cations was examined and confirmed the high selectivity of this material.

The determination of uranium in aqueous samples by means of a pulsed-source fluorescence spectrometer

Luminescence from aqueous uranyl ions is examined by means of a fluorescence spectrometer with a pulsed xenon light source. Background fluorescence is reduced by using time-based discrimination of the uranyl emission, but interference can still occur from quenchers such as iron(III). Such interferences are reduced by extraction of the uranyl ion into hexane containing trin-n-butyl phosphate, with back-extraction into dilute phosphoric acid before measurement. A detection limit of 5 ng ml^?1 is found with a linear calibration range of 0–10 μg ml^?1.     

Atomic absorption spectroscopy in the v.u.v.-range: determination of bromine

The conditions for the determination of elements by atomic absorption spectroscopy (AAS) in the vacuum-ultraviolet range are discussed. The requirements for the absorption vessel are considered and the equilibrium concentrations of possible dissociation products of different chemical compounds are calculated. In this way it is possible to estimate the detection limit attained with such an AAS arrangement. When losses by diffusion of sample vapour are disregarded a theoretical value of 5 × 10^?10 g Br results. AAS measurements are described using a special graphite oven in conjunction with a vacuum-ultraviolet monochromator. For KBr as a representative substance we obtained the experimental detection limit of 1.5 × 10^?9 g Br with Br I 148.86 nm.     

Gold Nanoparticle-Based Fluorescence Resonance Energy Transfer Aptasensor for Ochratoxin A Detection

In this paper, a sensitive and specific fluorescence resonance energy transfer (FRET) aptasensor for the detection of Ochratoxin A (OTA) was developed based on a dye-tagged ssDNA hybridized with aptamer-conjugated Au nanoparticles (Au NPs). The binding between the aptamer-Au NPs conjugate and the dye-labeled ssDNA leads to the fluorescence quenching of FAM due to its close proximity. The addition of OTA results in fluorescence recovery, attributed to the formation of a quadruplex-OTA complex, which detaches from the surface of Au NPs. Under optimal conditions, the relative fluorescence intensity (ΔI) is proportional to the concentration of the OTA in the range of 5 × 10^?12 to 5 × 10^?9 g/mL, with a detection limit of 2 × 10^?12 g/mL. The proposed method was successfully applied to measure the concentration of OTA in naturally contaminated maize samples and validated using a commercially available enzyme-linked immunosorbent assay (ELISA) method. This work demonstrates that the combination of an aptamer that has a high binding affinity for the analyte with highly sensitive Au NPs that undergo FRET is a promising approach for the detection of small molecule toxins.     

Optimization of a sensitive method for the “switch-on” determination of mercury(II) in waters using Rhodamine B capped gold nanoparticles as a fluorescence sensor

Monodisperse and “naked” gold nanoparticles (GNPs) were modified with thioglycolic acid (TGA). The fluorescence of rhodamine B (RB) is quenched completely by the gold NPs surface with negative charge mainly as a result of fluorescence resonance energy transition (FRET) and collision. The quenching mechanism can be described by a Langmuir isotherm, which was systematically investigated by steady-state fluorescence spectrometry and absorption spectrometry. Hg(II) ion disrupts the GNPs–RB pair, producing a large “switch-on” fluorescence. A low background, highly sensitive and reproducible fluorescence assay for Hg(II) is presented. Under the optimum conditions, the restoration fluorescence intensity is proportional to the concentration of Hg(II). The calibration graphs are linear over the range of 1.0?×?10^?9 to 3.1?×?10^?8 mol L^?1 with a detection limit of 4.0?×?10^?10 mol L^?1. The relative standard deviation was 1.2% for a 5.0?×?10^?9 mol L^?1 Hg(II) solution (N?=?6). This method was applied to the analysis of Hg(II) in environmental water samples, and the results were consistent with those of atomic absorption spectroscopy (AAS).     

Spectroscopy studies of sandwich-type complex of silver(I) co-ordinated to nuclear fast red and adenine and its analytical applications

A novel fluorometric method for the determination of adenine was proposed, based on the fluorescence quenching of a nuclear fast red-silver complex as a small molecule optical probe in the presence of adenine. The characteristic spectra of absorption, resonance Rayleigh scattering (RRS) and the fluorescence of the system were analysed. Adenine, Ag⁺ and nuclear fast red formed a sandwich-type complex. The composition of the compound and the mechanisms of fluorescence quenching were discussed. The optimal conditions for the reaction and the effects of co-existing substances were also investigated. The quenched fluorescence intensity was proportional to the concentration of adenine in the range of 0.03–0.90 µg mL^?1, with the limit of detection of 0.023 µg mL^?1. The method was applied satisfactorily to the determination of adenine in synthetic samples, herring sperm DNA, human sera and vitamin B₄ tablets with recoveries from 95.3 % to 105 %.     

Determination of sulphur in fuel oils by absorption spectrometry of electrothermally generated carbon sulphide molecules

Molecular absorption spectra of CS are observed during the vaporization of crude and fuel oils in an electrothermal atomizer. The CS absorbance at 257.6 nm is used to determine the sulphur content of the oil, based on measurements in a conventional electrothermal atomic absorption spectrometer. The results for various fuel oils generally agree with those obtained by x-ray fluorescence spectrometry (ASTM D2622). The detection limit referred to the undiluted oil is 50 mg kg^?1, and the repeatability is 3% at the 250 mg kg^?1 level. Some oils exhibit uneven vaporization of sulphur species.     

Electrochemical detection of blood lead based on the enhancement effect of copper

The anodic stripping peak current of lead on the glassy carbon electrode surface was greatly increased in the presence of high concentration of copper ion. The effects of supporting electrolyte, concentration of Cu²⁺, accumulation potential and accumulation time were studied on the stripping peak current of Pb²⁺. As a result, a sensitive, simple and rapid electrochemical method was developed for the detection of lead. In 0.01 M HNO₃ solution containing 800 g,g L^?1 Cu²⁺, the stripping peak current of Pb²⁺ increases linearly with its concentration over the range from 2 to 100 μg L^?1. The detection limit is 1 μg L^?1 after 4-min accumulation at ?0.8 V. It was used to detect the concentration of lead in blood samples, and the results consisted with the values that obtained by atomic absorption spectrometry.     

Evaluation of x-ray fluorescence spectrometry for the determination of platinum in catalysts and comparison with other spectrometric techniques

An x-ray fluorescence (x.r.f.) method is described for the determination of platinum contents (0.3–0.6%) in some special types of platinum catalysts. Calibration graphs were linear in the range 30–700 μg g^?1 platinum with a 3σ detection limit of 10 μg g^?1 for a 2000-s counting time. Because of the lack of certified standard platinum catalysts for checking accuracy, the results obtained were compared with results from visible spectrophotometry (tin (II) chloride method), optical emission spectrometry and atomic absorption spectrometry. Statistical evaluation showed no significant errors at the 95% confidence interval.     

Spectrofluorimetric determination of diquat by manual and flow-injection methods

A reliable, highly sensitive and selective method is described for the determination of the herbicide diquat in different materials. It involves the formation of a stable radical obtained by reduction of diquat with sodium dithionite. The radical is a very fluorescent species (φ_F=0.37), and there is a linear relationship between diquat concentration and fluorescence intensity over the range 3–900 μg l^?1. The limit of detection is 0.4 μg l^?1. The method can be successfully adapted as a flow system using a two-channel manifold, the peak height being proportional to the diquat concentration over the range 18–4000 μg l^?1. Manual and flow-injection methods were successfully applied to the determination of diquat in commercial herbicides, waters, potatoes, flowers and soils. Both procedures were also been applied to the determination of diquat in serum and urine without prior preparation of the samples and with good results.     

A Fluorescence Quenching Method for the Determination of Bromate Ion with 4,5-Dibromophenylfluorone and Cetyltrimethylammonium Bromide

Abstract

A highly sensitive and selective fluorescence quenching method was developed to determine the bromate ion (BrO₃ ^?) with 4,5-dibromophenylfluorone (DB-PF) as fluorogenic reagent. BrO₃ ^? reacts with potassium bromide in sulfuric acid solution (0.6M) giving bromine (Br₂) which is estimated with fluorescence quenching method using DB-PF as fluorogenic reagent. Bromine reacts with DB-PF to produce a compound, whose maximum excitation wavelength and emission wavelength are 543nm, 560nm respectively. The linear calibration range is 0.05–0.5 μg/25ml. The detection limit is 0.05μg/25ml. The method may be used to determine microamounts of BrO₃ ^? in potassium chlorate with satisfactory results. The method offers the advantages of simplicity, rapidity and sensitivity.     

Selective Kinetic Spectrophotometric Determination of Nitrite in Food and Water

Abstract

A highly selective and sensitive method for the kinetic spectrophothometric determination of sub-microgram amounts of nitrite has been development based on its reaction with Nile blue 2B in acidic medium. The reaction is monitored spectrophotometrically at 595 nm at a fixed time of 4.5 min. The change in absorbance at 595 nm is related to the concentration of nitrite in the range 0.005 - 1.100 μg.ml^?1 The detection limit is 0.001 μg.ml^?1. The relation standard deviation is 1% for 0.020 μg.ml^?1 of nitrite for ten replicate measurements. Most common anions and cations do not interfere. The procedure was applied to the determination of trace amounts of nitrite in sausage and water.     

Entwicklung einer flieβinjectionsmethode zur bestimmung von chlorid im spurenbereich durch leitfähighkeitsdiffernzenA flow-injection method for the determination of trace amounts of chloride

The indirect determination of chloride in water is based on measurement of the difference in conductivity after the sample has passed through ion-exchange columns in the hydrogen form and silver form. The linear response range is about 0.5–10 μg g^?1 chloride (with 3 μg g^?1 nitrate and 5 μg g^?1 sulfate); the detection limit is about 50 ng g^?1 chloride but depends strongly on the concentrations of other anions.     

CdS Quantum Dots as Fluorescence Probes for the Detection of Selenite

Abstract

Water-soluble cadmium sulfide (CdS) quantum dots (QD) capped by mercaptoacetic acid were synthesized by aqueous-phase arrested precipitation and characterized by transmission electron microscopy, a spectrofluorometer, and an ultraviolet visible (UV-Vis) spectrophotometer. Based on the fluorescence quenching of CdS QD by selenite in the presence of glutathione (GSH), a simple, rapid, sensitive, and selective detection method for selenite was proposed. Under the optimum conditions, the calibration graph was linear in the range of 0.05 µmol L^?1 to 11.2 µmol L^?1. The limit of detection is 0.03 µmol L^?1. The usefulness of the proposed method was evaluated for the determination of selenite in sodium selenite tablet and sodium selenite and vitamin E injection, and the results agreed with the labeled values. In addition, the effect of foreign ions (common anions and biologically relevant cations) on the fluorescence of the CdS QD was examined to evaluate the selectivity. The quenching mechanism is also described.     

Determination of trace selenium in biological material by preconcentration and x-ray emission spectrometry

Selenium is determined in the ng g^?1 to μg g^?1 range in biological and environmental samples. A wet digestion procedure was optimized with respect to volatility losses and recovery yields, by using ⁷⁵Se metabolically incorporated into rat organs. Selenium is preconcentrated from the digestion liquid by a two-step reduction with 4 M HCl and ascorbic acid. The colloidal selenium formed is adsorbed on activated carbon and filtered on a Nucleopore membrane for measurement by energy-dispersive x-ray fluorescence. Almost complete recovery was obtained, and the detection limit was 20 ng, corresponding to 10 ng g^?1 for a 2-g sample. Biological reference materials were analyzed with satisfactory results, and the accuracy of the method was good.     

On-line preconcentration of trace copper for flame atomic absorption spectrometry using a spherical cellulose sorbent with chemically bound quinolin-8-ol

Preconcentration was effected using a 50 mm × 2 mm i.d. minicolumn packed with a spherical cellulose sorbent with chemically bound quinolin-8-ol function groups (Ostorb Oxin). The column was connected to the nebulizer of the atomic absorption spectrometer and the sample solution and eluent (2 M hydrochloric acid) were sucked through it at a flow-rate of 2–3 ml min^?1 by utilizing the negative pressure of the nebulizer. The experimental design was tested with the determination of traces of copper. Peak-area and peak-height measurements were compared. Owing to the simple calibration, the former method was used for quantification. The dynamic range was from 0.3 ng ml^?1 (detection limit) to 5 μg ml^?1 (breakthrough). The reproducibility in the concentration range 25 ng ml^?1-5 μg ml^?1 was better than 5%. Water-soluble inorganic salts, ammonia and sodium hydroxide were analysed. The accuracy of the results was checked by anodic-stripping voltammetry and by electrothermal AAS.