Determination of L-cysteine base on the reversion of fluorescence quenching of calcein by copper(II) ion

We report on a simple and sensitive method for the determination of L-cysteine (Cys). It is based on a redox reaction between the non-fluorescent Cu(II)-calcein complex and Cys which results in fluorescence recovery of calcein. When Cys is added to a solution of the Cu(II)-calcein complex, Cu(II) is reduced to Cu(I), and calcein is released to form a strongly fluorescent complex with Zn(II). The effect was used to develop a fluorescence enhancement method for the determination of Cys. Under the optimum conditions, the increase in signal intensity is linear in the range from 3.0?×?10^?7 to 1.2?×?10^?5?mol?L^?1, with a correlation coefficient (R) of 0.9978. The limit of detection (3σ) is 4.0?×?10^?8?mol?L^?1. The relative standard deviation (RSD) in the determination of 11 samples containing 5.0?×?10^?6?mol?L^?1 of Cys was 3.5%. There is little interference by common ions and other amino acids. The method, which is simple, rapid, and sensitive, was successfully applied to the determination of Cys in human serum samples.

Voltammetric detection of guanosine and adenosine using a carbon paste electrode modified with 1-ethyl-3-methylimidazolium ethylsulfate

A novel kind of carbon paste electrode (CPE) was prepared by mixing graphite powder, liquid paraffin and the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate. The resulting electrode was used for the simultaneous determination of guanosine and adenosine by differential pulse voltammetry. Compared to a conventional CPE, the oxidation peak currents are largely increased, and the oxidation peak potentials are negatively shifted. The electrochemical responses to guanosine and adenosine were investigated. Under optimized conditions, the calibration curves are linear in the concentration range from 1.0?×?10^-6?mol?L^-1 to 1.6?×?10^-4?mol?L^-1 for guanosine, and from 1.0?×?10^-6?mol?L^-1 to 2.7?×?10^-4?mol?L^-1 for adenosine at pH 3.5. Substances potentially interfering in the biological matrix do no interfere. The method was successfully applied to detect adenosine and guanosine in human urine without sample treatments.

Highly sensitive detection of lead(II) ion using multicolor CdTe quantum dots

Multicolor and water-soluble CdTe quantum dots (QDs) were synthesized with thioglycolic acid (TGA) as stabilizer. These QDs have a good size distribution, display high fluorescence quantum yield, and can be applied to the ultrasensitive detection of Pb(II) ion by virtue of their quenching effect. The size of the QDs exerts a strong effect on sensitivity, and quenching of luminescence is most effective for the smallest particles. The quenching mechanism is discussed. Fairly selective detection was accomplished by utilizing QDs with a diameter of 1.6?nm which resulted in a detection limit of 4.7?nmol?L^?1 concentration of Pb(II). The method was successfully applied to the determination of Pb(II) in spinach and citrus leaves, and the results are in good agreement with those obtained with atomic absorption spectrometry.

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

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

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

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

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

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

Electrochemical determination of paraquat using a DNA-modified carbon ionic liquid electrode

Deoxyribonucleic acid (DNA) was electrochemically deposited on a carbon ionic liquid electrode to give a biosensor with excellent redox activity towards paraquat as shown by cyclic voltammetry and differential pulse voltammetry. Experimental conditions were optimized with respect to sensing paraquat by varying the electrochemical parameters, solution pH, and accumulation time of DNA. Under the optimized conditions, a linear relation exists between the reduction peak current and the concentration of paraquat in the range from 5?×?10^?8 mol L^?1 to 7?×?10^?5 mol L^?1, with a detection limit of 3.6?×?10^?9 mol L^?1. The utility of the method is illustrated by successful analysis of paraquat in spiked real water samples.

Preparation of an amperometric sensor for norfloxacin based on molecularly imprinted grafting photopolymerization

A sensitive amperometric sensor for norfloxacin (NF) was introduced. The receptor layer was prepared by molecularly imprinted photopolymerization of acrylamide and trimethylolpropane trimethacrylate on the surface of a gold electrode. The binding mechanism of the molecularly imprinted polymer was explored by ultraviolet (UV) and infrared (IR) spectroscopy. The chemosensor was characterized by cyclic voltammetry (CV), differential pulse voltammetry (DPV), electrochemical impedance (EI), and scanning electron microscopy (SEM). The electrode prepared by photopolymerization has a better recognition ability to template molecules than that of electropolymerization and NIP. Some parameters affecting sensor response were optimized. Norfloxacin was detected by measurements of an amperometric i–t curve. The linear relationships between current and logarithmic concentration are obtained from 1.0?×?10^?9 to 1.0?×?10^?3?mol?L^?1. The detection limit of the sensor was 1.0?×?10^?10?mol?L^?1. The proposed method is sensitive, simple, and cheap, and is applied to detect NF in human urine successfully.

Detection of DNA hybridization by various spectroscopic methods using the copper tetraphenylporphyrin complex as a probe

We are presenting new and highly sensitive hybridization assays. They are based on various spectroscopic methods including resonance light scattering, circular dichroism, ultraviolet spectra and fluorescence spectra, as well as atomic force microscopy, and relies on the interaction of the Cu(II), Ni(II), Mg(II), Co(II), Cd(II), and Zn(II) complexes, respectively, of tetraphenylporphyrin (TPP) with double-strand DNA (dsDNA) and single strand DNA (ssDNA). The interaction results in amplified resonance light scattering (RLS) signals and enables the detection of hybridization without the need for labeling DNA. The RLS signals are strongest in case of the Cu (II)-TPP complex which therefore was selected as the probe. The technique is simple, robust, accurate, and can be completed in less than one hour.

Determination of mercury(II) by surface-enhanced Raman scattering spectroscopy based on thiol-functionalized silver nanoparticles

Silver nanoparticles (Ag NPs) modified with sodium 2-mercaptoethanesulfonate (mesna) exhibit strong surface-enhanced Raman scattering (SERS). Their specific and strong interaction with heavy metal ions led to a label-free assay for Hg(II). The covalent bond formed between mercury and sulfur is stronger than the one between silver and sulfur and thus prevents the adsorption of mesna on the surface of Ag NPs. This results in a decrease of the intensity of SERS in the presence of Hg(II) ions. The Raman peak at 795?cm^?1 can be used for quantification. The effect of the concentration of mesna, the concentration of sodium chloride, incubation time and pH value on SERS were optimized. Under the optimal conditions, the intensity of SERS decreases with increasing concentration of Hg(II). The decrease is linear in the 0.01 and 2?μmol L^?1 concentration range, with a correlation coefficient (R²) of 0.996 and detection limit (S/N?=?3) is 0.0024?μmol L^?1. The method was successfully applied to the determination of the Hg(II) in spiked water samples.

Method for determination of microcystin-leucine-arginine in water samples based on the quenching of the fluorescence of bioconjugates between CdSe/CdS quantum dots and microcystin-leucine-arginine antibody

We have developed a method for the determination of microcystin-leucine-arginine (MC-LR) in water samples that is based on the quenching of the fluorescence of bioconjugates between CdSe/CdS quantum dots (QDs) and the respective antibody after binding of MC-LR. The core-shell CdSe/CdS QDs were modified with 2-mercaptoacetic acid to improve water solubility while their high quantum yields were preserved. Monoclonal MC-LR antibody was then covalently bioconjugated to the QDs. It was found that the fluorescence intensity of the bioconjugates was quenched in the presence of MC-LR. A linear relationship exists between the extent of quenching and the concentration of MC-LR. Parameters affecting the quenching were investigated and optimized. The limit of detection is 6.9?×?10^?11 mol L^?1 (3σ). The method was successfully applied to the determination of MC-LR in water samples.

Magnetic nanoparticles grafted with β-cyclodextrin for solid-phase extraction of 5-hydroxy-3-indole acetic acid

We describe the synthesis of ß-cyclodextrin modified magnetic nanoparticles (CD-mNPs) as a material for solid-phase extraction of the cancer biomarker 5-hydroxy-indole-3-acetic acid (5-HIAA) from urine. The CD-mNPs were characterized by TEM, FTIR, and XRD, and the kinetics and adsorption isotherms were studied. The strong interaction between the CD-mNPs and 5-HIAA is the main driving force for recognition and extraction, while the magnetic core of the NPs allows their separation from the sample matrix. Recovery of 5-HIAA from the adsorbent using an adequate solvent regenerated the adsorbent for further use. 5-HIAA was then quantified by fluorometry of its complex with ß-CD. The method works in the 1?×?10^?7 to 1?×?10^?5 mol L^?1 (R² 0.9982–0.9996) concentration range, and the limits of detection (3σ) and quantification (10 σ) of the method are 1.2?×?10^?8 mol L^?1 and 4.01?×?10^?8 mol L^?1 5-HIAA, respectively. The recovery of 5-HIAA from urine samples spiked with 5-HIAA in three concentrations (1.4?×?10^?6, 4.50?×?10^?6 and 1.0?×?10^?5 mol L^?1) are within 63?±?3 %.

Non-enzymatic amperometric sensor for hydrogen peroxide based on a biocomposite made from chitosan, hemoglobin, and silver nanoparticles

We report on a novel non-enzymatic sensor for hydrogen peroxide (HP) that is based on a biocomposite made up from chitosan (CS), hemoglobin (Hb), and silver nanoparticles (AgNPs). The AgNPs were prepared in the presence of CS and glucose in an ultrasonic bath, and CS is found to act as a stabilizing agent. They were then combined with Hb and CS to construct a carbon paste biosensor. The resulting electrode gave a well-defined redox couple for Hb, with a formal potential of about ?0.17?V (vs. SCE) at pH?6.86 and exhibited a remarkable electrocatalytic activity for the reduction of HP. The sensor was used to detect HP by flow injection analysis, and a linear response is obtained in the 0.08 to 250?μM concentration range. The detection limit is 0.05?μM (at S/N?=?3). These characteristics, along with its long-term stability make the sensor highly promising for the amperometric determination of HP.

Determination of cadmium(II) using glassy carbon electrodes modified with cupferron, ß-naphthol, and multiwalled carbon nanotubes

We report on a simple and reliable method for the determination of trace cadmium ion using a glassy carbon electrode (GCE) modified with cupferron, ß-naphthol and MWCNTs. The operational mechanism consists of several steps: first, the ligand cupferron on the modified electrode reacts with Cd²⁺ ion to form a chelate compound. Next, this chelate is adsorbed by the carrier ß-naphthol following the principle of organic co-precipitation. Finally, the coprecipitated complex is detected by the GCE. This scheme is interesting because it combines preconcentration and electrochemical detection. Two linear responses are obtained, one in the concentration range of 5.0?×?10^?11 to 1.6?×?10^?8 M, the other in the range of 1.6?×?10^?8 to 1.42?×?10^?6 M, with a lower detection limit of 1.6?×?10^?11 M. This modified GCE does not suffer from significant interferences by Cu(II), Hg(II), Ag(I), Fe(III), Pb(II), Cr(III), Zn(II), NO^3?, Cl^?, SO ₄ ^2? ions and EDTA. The response of the electrode remained constant for at least 3 weeks of successive operation. The method presented here provides a new way for the simultaneous separation, enrichment, and electrochemical detection of trace cadmium ion.

Carbon paste electrode modified with a binuclear manganese complex as a sensitive voltammetric sensor for tryptophan

We report on a carbon paste electrode that was modified with a binuclear manganese(II) complex by the drop-coating method. A study on the mechanism of the electro-oxidation of tryptophan (Trp) at this electrode indicated that it enables Trp to be determined with good sensitivity and selectivity. Second-order derivative linear sweep voltammetry at pH 4.1 revealed that a sensitive anodic peak appears at 812?mV (vs. SCE) whose current is proportional to the concentration of Trp in the concentration range from 0.1 to 1.0???mol?L^?1 and 1.0 to 80???mol?L^?1, with a detection limit (S/N?=?3) of 0.08???mol?L^?1 (60?s of accumulation). The method was applied to the determination of Trp in amino acid injection solutions with satisfactory results.

Glassy carbon electrode modified with organic�Cinorganic pillared montmorillonites for voltammetric detection of mercury

A glassy carbon electrode modified with organic?Cinorganic pillared montmorillonite was used for voltammetric detection of mercury(II) in water. High sensitivity is obtained due to the use of the montmorillonites which displays outstanding capability in terms of adsorbing mercury ion due to its high specific surface and the presence of multiple binding sites. The experimental parameters and the effect of a chelating agent were optimized to further enhance sensitivity and selectivity. Linear calibration curves were obtained over the Hg(II) concentration range from 10 to 800???g?L^?1 for 5?min accumulation, with a detection limit of 1???g?L^?1. Simultaneous determination of Hg(II) and Cu(II) was also studied, and no interference was observed.

Self-assembly of ultra-small gold nanoparticles on an indium tin oxide electrode for the enzyme-free detection of hydrogen peroxide

A novel enzyme-free electrochemical sensor for H₂O₂ was fabricated by modifying an indium tin oxide (ITO) support with (3-aminopropyl) trimethoxysilane to yield an interface for the assembly of colloidal gold. Gold nanoparticles (AuNPs) were then immobilized on the substrate via self-assembly. Atomic force microscopy showed the presence of a monolayer of well-dispersed AuNPs with an average size of ~4 nm. The electrochemical behavior of the resultant AuNP/ITO-modified electrode and its response to hydrogen peroxide were studied by cyclic voltammetry. This non-enzymatic and mediator-free electrode exhibits a linear response in the range from 3.0?×?10^?5 M to 1.0?×?10^?3 M (M?=?mol?·?L^?1) with a correlation coefficient of 0.999. The limit of detection is as low as 10 nM (for S/N?=?3). The sensor is stable, gives well reproducible results, and is deemed to represent a promising tool for electrochemical sensing.

Determination of aminoglycoside antibiotics using an on-chip microfluidic device with chemiluminescence detection

We describe an on-chip microflow injection (μFI) approach for the determination of aminoglycoside antibiotics using chemiluminescence (CL) detection. The method is based on the inhibition of the Cu(II)-catalyzed CL reaction of luminol and hydrogen peroxide by the aminoglycosides due to the formation of a complex between the antibiotic and Cu(II). The main features of the method include small sample volumes and a fast response. Syringe pumps were used to insert the sample and the reagents into the microfluidic device. CL was collected using a fiber optic bundle connected to a luminescence detector. All instrumental, hydrodynamic and chemical variables involved in the system were optimized using neomycin as the aminoglycoside model. Inhibition is proportional to the concentration of the antibiotics. The dynamic ranges of the calibration graphs obtained for neomycin, streptomycin and amikacin are 0.3–3.3, 0.9–13.7, and 0.8–8.5?μmol?L^?1, and the detection limits are 0.09, 0.28 and 0.24?μmol?L^?1, respectively. The precision of the methods, expressed as relative standard deviation, is in the range from 0.8 to 5.0?%. The method was successfully applied to the determination of neomycin in water samples, with recoveries ranging from 80 to 120?%.

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.

Selective fluorescence response and magnetic separation probe for 2,4,6-trinitrotoluene based on iron oxide magnetic nanoparticles

Despite the rapid development of nanomaterials and nanotechnology, it is still desirable to develop novel nanoparticle-based techniques which are cost-effective, timesaving, and environment-friendly, and with ease of operation and procedural simplicity, for assay of target analytes. In the work discussed in this paper, the dye fluorescein isothiocyanate (FITC) was conjugated to 1,6-hexanediamine (HDA)-capped iron oxide magnetic nanoparticles (FITC–HDA Fe₃O₄ MNPs), and the product was characterized. HDA ligands on the surface of Fe₃O₄ MNPs can bind 2,4,6-trinitrotoluene (TNT) to form TNT anions by acid–base pairing interaction. Formation of TNT anions, and captured TNT substantially affect the emission of FITC on the surface of the Fe₃O₄ MNPs, resulting in quenching of the fluorescence at 519 nm. A novel FITC–HDA Fe₃O₄ MNPs-based probe featuring chemosensing and magnetic separation has therefore been constructed. i.e. FITC–HDA Fe₃O₄ MNPs had a highly selective fluorescence response and enabled magnetic separation of TNT from other nitroaromatic compounds by quenching of the emission of FITC and capture of TNT in aqueous solution. Very good linearity was observed for TNT concentrations in the range 0.05–1.5 μmol?L^?1, with a detection limit of 37.2 nmol?L^?1 and RSD of 4.7 % (n?=?7). Approximately 12 % of the total amount of TNT was captured. The proposed methods are well-suited to trace detection and capture of TNT in aqueous solution.