Sensitive electrochemical immunosensor for α-fetoprotein based on graphene/SnO2/Au nanocomposite.

A label-free electrochemical immunosensor for sensitive detection of α-fetoprotein (AFP) was developed based on graphene/SnO2/Au nanocomposite. The graphene/SnO2/Au nanocomposite modified glassy carbon electrode was used to immobilize α-fetoprotein antibody (anti-AFP) and to construct the immunosensor. Results demonstrated that the peak currents of [Ru(NH3)6](3+) decreased due to the interaction between antibody and antigen on the modified electrode. Thus, a label-free immunosensor for the detection of AFP was realized by monitoring the peak current change of [Ru(NH3)6](3+). The factors influencing the performance of the immunosensor were investigated in details. Under optimal conditions, the peak currents obtained by DPV decreased linearly with the increasing AFP concentrations in the range from 0.02 to 50 ng mL(-1) with a linear coefficient of 0.9959. This electrochemical immunoassay has a low detection limit of 0.01 ng mL(-1) (S/N=3) and was successfully applied to the determination of AFP in serum samples.

Poly(pyrocatechol-3,5-disodiumsulfonate)/multi-walled carbon nanotubes composite for simultaneous determination of dopamine, ascorbic acid and uric acid.

A novel poly(pyrocatechol-3,5-disodiumsulfonate)/multi-walled carbon nanotubes (PPD/MWCNT) composite film modified glassy carbon electrode (GCE) was fabricated and successfully used to simultaneously determine ascorbic acid (AA), dopamine (DA) and uric acid (UA). Owing to the synergistic effects of PPD and MWCNTs, the electrochemical responses of AA, DA and UA at PPD/MWCNTs/GCE were clearly superior to those at bare GCE. Additionally, the PPD/MWCNTs composite film could reduce the potential overlap of AA, DA and UA so as to favor simultaneous determination of the three substances. Under the optimum conditions, the linear calibration curves of AA, DA and UA were obtained by differential pulse voltammetry in the range of 10-1000, 1-100 and 4-1200 microM, and the detection limits were 5, 0.5 and 0.5 microM (S/N = 3) respectively. The results showed PPD/MWCNTs/GCE possessed good reproducibility and stability, and it could be applied to determine UA in human urine with satisfying recovery by standard addition method.

Highly sensitive and selective colorimetric sensing of antibiotics in milk.

Antibiotics residues in foods are very harmful to human beings. Determination of antibiotics residues relies largely on the availability of adequate analytical techniques. Currently, there is an urgent need for on site and real time detection of antibiotics in food. In this work, a novel one step synthesis of gold nanoparticles (AuNPs) was proposed using pyrocatechol violet (PCV) as a reducer agent. Highly sensitive and selective colorimetric detection of four antibiotics kanamycin mono sulfate (KA), neomycin sulfate (NE), streptomycin sulfate (ST) and bleomycin sulfate (BL) was realized during the formation of AuNPs. PCV has -OH groups and these antibiotics have -OH, -NH2, -NH- groups, so there may be some special hydrogen-bonding interactions between PCV and these antibiotics. Therefore, the presence of KA, NE, ST and BL would influence the synthesis of AuNPs, then the color and state of AuNPs would change, which could be observed with the naked eye or a UV-vis spectrophotometer. Results showed that A670 was linear with the logarithm of KA concentration in the range from 1.0×10(-8) to 5.0×10(-7)M and 5.0×10(-7) to 5.5×10(-5)M. The detection limit of KA was 1.0×10(-9)M (S/N=3). The coexisting substances including 1.0×10(-5)M phenylalanine, alanine, glycerol, glucose, Mg(2+), Ca(2+), Na(+), K(+), CO3(2-), SO4(2-), NO3(-), Cl(-) and Br(-) did not affect the determination of 1.0×10(-7)M antibiotics. In particular, the proposed method could be applied successfully to the detection of antibiotics in the pretreated liquid milk products.

A novel graphene-DNA biosensor for selective detection of mercury ions.

A novel electrochemical biosensor for sensitive and selective detection of mercury (II) ions (Hg²âº) based on a DNA grafted graphene is proposed. Graphene oxide (GO) was reduced by dopamine, and then the single-strand probe DNA modified at the 5'-end with an alkylamino modifier (NH2-ssDNA) was grafted on the reduced graphene oxide (RGO) surface via Michael addition reaction. In the presence of Hg²âº, the target DNA with four thymine-thymine (T-T) mismatches would hybridize with the probe DNA on the glassy carbon electrode (GCE) through T-Hg²âº-T coordination chemistry. The hybridization of the two oligonucleotides leads to the increase in the peak currents of [Ru(NH3)6]³âº, which could be used for electrochemical sensing of Hg²âº. The difference in the value of the peak currents of [Ru(NH3)6]³âº before and after DNA hybridization was linear with the concentration of Hg²âº in the range from 8.0×10⁻9 to 1.0×10⁻7 M with a linear coefficiency of 0.996. The detection limit was 5.0×10⁻9 M (S/N=3). The proposed electrochemical biosensor is rapid, convenient and low-cost for effective sensing of Hg²âº. Particularly, the proposed method was applied successfully to the determination of Hg²âº in real environmental samples.

A simple one-pot synthesis of graphene nanosheet/SnO2 nanoparticle hybrid nanocomposites and their application for selective and sensitive electrochemical detection of dopamine.

A novel one-pot synthesis of graphene nanosheet/SnO2 nanoparticle hybrid nanocomposites (GN/SnO2) was realized by using graphene oxide nanosheets (GONs) functionalized with sodium dodecyl sulfonate and SnCl2 as the starting materials. The morphology and structure of the synthesized SDS-GN/SnO2 nanocomposites were characterized by Raman spectroscopy, transmission electron microscopy (TEM) and X-ray diffraction analysis. It was found that SnO2 nanoparticles were homogeneously distributed on the graphene nanosheets. The electrochemical behavior of dopamine (DA) at the SDS-GN/SnO2 nanoparticle modified electrode was studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The results showed that the modified electrode exhibited excellent electrocatalytic activity towards the electrochemical oxidation of DA. The separation of the oxidation peak potentials for ascorbic acid (AA)-DA, uric acid (UA)-DA and UA-AA obtained by DPV is about 132 mV, 128 mV and 260 mV, respectively, which allows selective and sensitive detection of DA in the presence of AA and UA. The anodic peak currents were linear with the concentration of DA in the range from 1.0 × 10-7 to 1.0 × 10-5 M with a coefficient of 0.9980. The detection limit was 80 nM (S/N = 3). The proposed method could be applied for the determination of DA in real human urine samples.

Colorimetric determination of melamine by pyridine-3-boronic acid modified gold nanoparticles.

A rapid, simple and sensitive colorimetric detection method for melamine was proposed based on pyridine-3-boronic acid (PBA) modified gold nanoparticles (AuNPs). The formation of supramolecular hydrogen-bonded structure between PBA and melamine resulted in the aggregation of PBA modified AuNPs and the color change from red to blue. Melamine could be detected by colorimetric response of AuNPs that could be detected by naked eyes or a UV-vis spectrophotometer. The detection concentration of melamine ranged from 6.0 x 10(-8) to 1.6 x 10(-6) M, and the detection limit (3sigma) was 3.0 x 10(-8) M (i.e., 3.2 ppb). This provided an effective and facile colorimetric sensor for real-time and on-site determination of melamine. Particularly, the proposed method could be used to detect melamine in pretreated liquid milk products with high sensitivity and low interference, and the recoveries were from 95% to 102%.

Simultaneous determination of dihydroxybenzene positional isomers by capillary electrochromatography using gold nanoparticles as stationary phase.

The paper describes the enhanced separation of o-, m-, p-dihydroxybenzene by capillary electrochromatography (CEC) using gold nanoparticles (AuNPs) as stationary phase. The effect of the AuNPs concentration upon separation was investigated. The experimental parameters, including separation voltage, pH, and concentration of running buffer, were optimized. Under the optimum conditions, a good resolution of three dihydroxybenzene isomers was obtained within 15 min in a 50 cm effective length capillary modified with 0.02 nmol/L AuNPs at a separation voltage of 16 kV in a 50 mmol/L acetate buffer (pH 5.0). The linear ranges were from 10(-6) to 10(-4) mol/L and the detection limits were as low as 10(-7) mol/L. This method was successfully used to analysis two kinds of hair coloring agent sample with recoveries in the range of 90-105% and relative standard deviations (RSD) less than 5.0%.

Colorimetric sensing of clenbuterol using gold nanoparticles in the presence of melamine.

A highly sensitive method for the detection of trace amount of clenbuterol based on gold nanoparticles (AuNPs) in the presence of melamine was described in this paper. Hydrogen-bonding interaction between clenbuterol and melamine resulted in the aggregation of AuNPs and a consequent color change of AuNPs from wine red to blue. The concentration of clenbuterol could be determined with naked eye or a UV-vis spectrometer. Results showed that the absorption ratio (A(670)/A(520)) was liner with the logarithm of clenbuterol concentration in the range of 2.8×10(-10) to 2.8×10(-7)M and 2.8×10(-7) to 1.4×10(-6)M with linear coefficients of 0.996 and 0.993, respectively. The detection limit was 2.8×10(-11)M (S/N=3), which was much lower than most existing methods. The coexisting substances including dl-epinephrine, phenylalamine, tryptohan, alamine, uric acid, glycine, glycerol, glucose, MgCl(2), CaCl(2) and NaCl did not affect the determination of clenbuterol. The proposed method could be successfully applied to the determination of clenbuterol in human urine.

The comparison of different gold nanoparticles/graphene nanosheets hybrid nanocomposites in electrochemical performance and the construction of a sensitive uric acid electrochemical sensor with novel hybrid nanocomposites.

In this paper, water soluble poly(diallyldimethylammonium chloride)-graphene nanosheets (PDDA-GNs) were synthesized and characterized by UV-visible absorption spectroscopy, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). On the basis of PDDA-GNs, three different types of gold nanoparticles/graphene nanosheets (AuNPs/GNs) hybrid nanocomposites were obtained by one-pot synthesis, in situ reduction and adsorption methods, respectively. These nanocomposites were used as electrode materials for electrochemical determination of uric acid (UA). The results indicated adsorption to be the best method to synthesize hybrid nanocomposites from the electrochemical point of view. Given the fact positively charged PDDA-AuNPs could interact with negatively charged UA molecules, we then synthesized PDDA-protected gold nanoparticles/graphene nanosheets (PDDA-AuNPs/GNs) hybrid nanocomposites by adsorption method, for the first time. As were expected, PDDA-AuNPs/GNs gave better performance for UA than AuNPs/GNs obtained by adsorption, and the anodic peak current of UA obtained by cyclic voltammetry (CV) increased 102.1-fold in comparison to bare GCE under optimizing conditions. Differential pulse voltammetry (DPV) was used to quantitatively determine UA. The linear range of UA was from 0.5µM to 20µM and the detection limit was 0.1µM (S/N=3) with a high sensitivity of 103.08µAµM(-1)cm(-2). The assay results of urine sample provided satisfying recoveries by standard addition method. In addition, the anodic peaks of adrenaline (AD) and UA were well resolved at PDDA-AuNPs/GNs modified electrode, while they were too overlapped to separate at bare electrode, as a result of that UA was successfully detected in the presence of AD. In conclusion, rapid synthesis of PDDA-AuNPs/GNs were realized and applied as an advanced hybrid electrode material for UA determination.

Colorimetric detection of Cd2+ using gold nanoparticles cofunctionalized with 6-mercaptonicotinic acid and L-cysteine.

We have developed a colorimetric assay for the highly sensitive and selective detection of Cd(2+) using gold nanoparticles (AuNPs) cofunctionalized with 6-mercaptonicotinic acid (MNA) and L-Cysteine (L-Cys) through the formation of an Au-S bond. In the presence of Cd(2+), the aggregation of functionalized AuNPs occurred by means of a metal-ligand interaction that led to visible color changes. Most importantly, cofunctionalized AuNPs had better responses for Cd(2+) than that functionalized by either MNA or L-Cys. Cd(2+) could be detected by the colorimetric response of AuNPs that could be detected by the naked eye or a UV-vis spectrophotometer. The absorbance ratio (A(620)/A(523)) was linear with the Cd(2+) concentration in the range of 2.0 × 10(-7) to 1.7 × 10(-6) M. Under optimum conditions (2.0 × 10(-5) M MNA, 2.0 × 10(-6) M L-Cys and 0.020 M NaCl at pH 10.0), the detection limit (3σ) of Cd(2+) could be as low as 1.0 × 10(-7) M. Interference experiments showed that Pb(2+) and Cu(2+) caused a slight interference for Cd(2+) determination while other metal ions caused no interference. The proposed method was successfully applied to determine the concentration of Cd(2+) in environmental samples (lake water).

Determination of Cu2+ using poly(2-aminothiazole)/ multi-walled carbon nanotubes composite film modified glassy carbon electrodes.

2-Aminothiazole was electropolymerized by cyclic voltammetry (CV) on the multi-walled carbon nanotubes (MWCNTs) modified glassy carbon electrode (GCE) surface. Poly(2-aminothiazole)/MWCNTs/GCE was used for determination of copper ions. The anodic peak currents of copper ions evaluated by differential pulse stripping voltammetry (DPSV) are linear with the concentrations in the range from 1.0 x 10(-7) M to 2.0 x 10(-5) M with a linear coefficiency of 0.9985. The detection limit is 2.0 x 10(-9) M calculated for a signal-to-noise ratio of 3 (S/N = 3). The proposed method was applied successfully to the determination of copper ions in drinking water, and the recovery was 96%.

Determination of fumaric and maleic acids with stacking analytes by transient moving chemical reaction boundary method in capillary electrophoresis.

The paper presents an on-line transient moving chemical reaction boundary (MCRB) method for simply but efficiently stacking analytes in capillary electrophoresis (CE). The CE technique was developed for a rapid determination of fumaric and maleic acid. Based on the theory of MCRB, Effects of several important factors such as the pH and concentration of running buffer and the conditions of stacking analytes were investigated to acquire the optimum conditions. The optimized separations were carried out in a 20 mmol/L sulphate neutralized with ethylenediamine to pH 6.0 electrolytes using a capillary coated with poly (diallyldimethylammonium chloride) and direct UV detection at 214 nm. The optimized preconcentrations were carried out in 50 mmol/L borax (pH 9.0). The calibration curves were linear in the concentration range of 1.0×10⁻7-1.0×10⁻4 mol/L and 5.0×10⁻7-1.0×10⁻4 mol/L for fumaric and maleic acid with correlation coefficients higher than 0.9991. The detection limits were 5.34×10⁻8 mol/L for fumaric acid and 1.92×10⁻7 mol/L for maleic acid. This method was applied for determination of fumaric acid in apple juice and of fumaric and maleic acid in dl-malic, the recovery tests established for real samples were within the range 95-105%. This work provided a valid and simple approach to detect fumaric and maleic acid.

Simultaneous determination of 3,4-dihydroxyphenylacetic acid, uric acid and ascorbic acid by poly(L-arginine)/multi-walled carbon nanotubes composite film.

The poly(L-Arginine)(PArg)-multiwalled carbon nanotubes (MWCNTs) composite film was used to modify glassy carbon electrode (GCE) to fabricate PArg/MWCNTs/GCE through electropolymerization of L-Arginine on MWCNTs/GCE. The PArg/MWCNTs/GCE exhibited high electro-catalytic activities towards the oxidation of 3,4-dihydroxyphenylacetic acid (DOPAC), uric acid (UA), and ascorbic acid (AA), and could be sensitively used for simultaneous determination of DOPAC, AA, and UA in pH 7.4 phosphate-buffered solution (PBS).The linear ranges were 7 microM to 2.7 mM for DOPAC, 3 microM to 1.2 mM for UA, and 70 microM to 1.4 mM for AA. The detection limits were 1.3 microM for DOPAC, 0.7 microM for UA and 20 microM for AA.

Colorimetric detection of melamine during the formation of gold nanoparticles.

A sensitive and simple colorimetric method for determination of melamine was reported based on the seedless production of gold nanoparticles (Au-NPs). Au-NPs were synthesized by using pyrocatechol-3,5-disodiumsulfonate (PD) as reducer without adding nanoparticles seeds and stabilizing agent. PD can form intramolecular hydrogen-bonding in solution by adjacent sulfurnate and α-phenolic hydroxyl group, resulting in its weak reducing capacity and thus the synthesis of Au-NPs was slow. While in the presence of melamine, PD reacted with melamine through hydrogen-bonding. Therefore, the intramolecular hydrogen-bonding of PD was interrupted by melamine, and the α-phenolic hydroxyl group was free to reduce Au(3+), hence, the synthesis of Au-NPs was accelerated. Especially, the presence of melamine led to a shift in the surface plasmon bond and a color change of Au-NPs from green to yellow. Results showed that the absorbance ratio (A(436)/A(600)) was linear with the logarithm of melamine concentration in the range of 4.8×10(-9) to 1.6×10(-6) M with a correlation coefficient of 0.9949. The detection limit (3σ) obtained by UV-vis spectrum was 6.4×10(-10) M (i.e., 0.08 ppb). The proposed method was applied successfully to the determination of melamine in pretreated liquid milk products, and the recoveries were from 93% to 107%.

Direct electrochemistry and electrocatalysis of hemoglobin immobilized in a magnetic nanoparticles-chitosan film.

Magnetic nanoparticles (Fe(3)O(4)) were synthesized by a chemical coprecipitation method. X-ray diffraction (XRD) and transmission electron microscope (TEM) were used to confirm the crystallite structure and the particle's radius. The Fe(3)O(4) nanoparticles and chitosan (CS) were mixed to form a matrix in which haemoglobin (Hb) can be immobilized for the fabrication of H(2)O(2) biosensor. The Fe(3)O(4)-CS-Hb film exhibited a pair of well-defined and quasi-reversible cyclic voltammetric peaks due to the redox of Hb-heme Fe (III)/Fe (II) in a pH 7.0 phosphate buffer. The formal potential of Hb-heme Fe(III)/Fe(II) couple varied linearly with the increase of pH in the range of 4.0-10.0 with a slope of 46.5 mV pH(-1), indicating that electron transfer was accompanied with single proton transportation in the electrochemical reaction. The surface coverage of Hb immobilized on Fe(3)O(4)-CS film glassy carbon electrode was about 1.13 x 10(-10)mol cm(-2). The heterogeneous electron transfer rate constant (k(s)) was 1.04 s(-1), indicating great facilitation of the electron transfer between Hb and magnetic nanoparticles-chitosan modified electrode. The modified electrode showed excellent electrocatalytic activity toward oxygen and hydrogen peroxide reduction. The apparent Michaelis-Menten constant K(M)(app) for H(2)O(2) was estimated to be 38.1 micromol L(-1).

Determination of dopamine in the presence of ascorbic acid using poly(3,5-dihydroxy benzoic acid) film modified electrode.

A polymerized film of 3,5-dihydroxy benzoic acid (DBA) was prepared on the surface of a glassy carbon electrode (GCE) in neutral solution by cyclic voltammetry (CV). The poly(DBA) film-coated GCE exhibited excellent electrocatalytic activity toward the oxidation of dopamine (DA). A linear range of 1.0 x 10(-7) to 1.0 x 10(-4)M and a detection limit of 6.0 x 10(-8)M were observed in pH 7.4 phosphate buffer solutions. Moreover, the interference of ascorbic acid (AA) was effectively eliminated. This work provides a simple and easy approach to selective detection of DA in the presence of AA.

[Study on solid phase extraction and spectrophotometric determination of uranium in water with 2-(2-quinolylazo)-5-dimthylaminophenol].

A new chromogenic reagent 2-(2-quinolylazo)-5-Dimthylaminophenol (QADMAP) was synthesized, and its structure was verified by elemental analysis, infrared spectrum, 1H nuclear magnetic resonance spectrum, mass spectrumand UV-spectrum. The color reaction of QADMAP with uranium was studied. In the presence of pH 7.8 buffer solution, when fluorin ion and TritonX-100 medium exist, QADMAP can react with uranium and fluorin to form a stable 1 : 1 : 1 stable complex [F- :QADMAP : U(VI)]. The molar absorptivity is 1.05 x 10(5) L x mol(-1) x cm(-1) at 590 nm. Beer's law is obeyed in range of 0-20 microg/10 mL. The uranium in samples can be enriched and separated by solid phase extraction with TBP resin cartridge. This method is applied to the determination of uranium in water sample. The relative standard deviations are 2.2%-3.6%, and the recoveries are 94%-105%.

Comparative study on the chiral separation of phenyl alcohols by capillary electrophoresis and liquid chromatography.

Structurally related phenyl alcohols were separated by capillary electrophoresis and liquid chromatography. A statistical experimental design was used in order to optimize the main electrophoretic parameters such as pH, concentration of selector and separation voltage in capillary electrophoresis (CE). Response surfaces were derived using the mathematical model and used for a selection of the optimal experimental conditions. Concentration of the chiral selector, the distance between the aromatic group and asymmetric center of the analytes, were identified as the factors influencing the complexation, selectivity and resolution. Experiments were also performed by high-performance liquid chromatography (HPLC) and the results of CE and HPLC were compared.

Direct electrochemistry of xanthine oxidase at a gold electrode modified with single-wall carbon nanotubes.

The direct electrochemistry of xanthine oxidase (XOD) was accomplished at a gold electrode modified with single-wall carbon nanotubes (SWNTs). A pair of well-defined redox peaks was obtained for XOD with the reduction peak potential at -0.478 V and a peak potential separation of 28 mV at pH 7.0. Both FT-IR spectra and the dependence of the reduction peak current on the scan rate revealed that XOD adsorbed onto the SWNT surfaces. The redox wave corresponds to the redox center of the flavin adenine dinucleotide (FAD) of the XOD adsorbate. Compared to other types of carbonaceous electrode materials, the electron transfer rate of XOD redox reaction was greatly enhanced at the SWNT-modified electrode. The peak potential was shown to be pH dependent. Spectral methods verified that the attachment of XOD onto SWNTs does not perturb the XOD conformations drastically.

Combination of orthogonal array design and triangle overlapping resolution mapping scheme for resolution optimization in micellar electrokinetic chromatography.

This paper describes the application of a combined orthogonal array design and overlapping resolution mapping to the optimization of miceller electrokinetic chromatography for the separation of 10 substituted benzenes. The most important factors were first determined according to an OA16(2(15)) through 16 pre-designed experiments; a second set of experiments was carried out according to a triangle overlapping resolution mapping scheme, in which 7 pre-planed experiments were executed and global optimum conditions for the separation were obtained.