Enantioselectivity of redox reaction of DOPA at the gold electrode modified with a self-assembled monolayer of homocysteine

The enantioselectivity of the self-assembled monolayer (SAM) of homocysteine formed on the (111)-oriented gold surface was investigated. We analyzed the redox behavior of 3,4-dihydroxyphenylalanine (DOPA), which is an electrochemically active chiral molecule, by means of cyclic voltammetry at a gold electrode modified with one enantiomeric form of homocysteine. It was demonstrated that the homocysteine SAM of one enantiomeric form blocked the redox reaction of only one enantiomer of DOPA, with cross inversion for the other enantiomer, in acidic solution.     

Electrochemical properties of a gold electrode modified with a mixed monolayer

The electrochemical properties of a gold electrode modified with a mixed thiol monolayer containing both a polar and a non-polar head group have been investigated in aqueous Fe(CN)₆⁴⁻, flavin adenine dinucleotide (FAD) and ubiquinone-0 (2,3-dimethoxy-5-methyl-1,4-benzoquinone, UQ₀) solutions. The cyclic voltammetric current-potential (i-E) response of Fe(CN)₆⁴⁻ was found to be affected considerably by the polarity of the head group contained in the mixed monolayer assembly, as compared with those of FAD and UQ₀. It was also found that in the cases of UQ₀ and FAD the i-E responses for the modified electrode were affected by their own molecular size rather than the polarity of the mixed monolayer head group. Furthermore, compared with Fe(CN)₆⁴⁻ ion, these biologically related molecules are able to permeate readily into the well-organized and hydrophobic alkyl chains of the monolayer assembly. The voltammetric profile of UQ₀ was improved by the modification of aminoethanethiol, as compared with those of bare gold and the electrode modified with other polar thiols. Further, two different permeation paths of the electrode species into the mixed monolayer are suggested from the variation of the i-E response with the cycle of the potential scan.     

Electrochemical detection of copper(II) at a gold electrode modified with a self-assembled monolayer of penicillamine

A penicillamine (PCA) self-assembled monolayer (SAM) was prepared on a gold electrode. It has been found that the modified electrode exhibited a selective response to copper ions. As demonstrated by cyclic voltammetric experiments, the SAM-based electrode showed an attractive ability to preconcentrate efficiently traces of copper(II) from solutions. Under optimum conditions, the anodic peak current was proportional to the concentration of Cu(II) in the range from 8.0 × 10⁻⁷ to 1.0 × 10⁻⁴ M with a detection limit of 4.0 × 10⁻⁷ M. Moreover, this modified gold electrode is also characterized by excellent repeatability, showing a relative standard deviation of 3.2% for nine successive measurements of 1.0 × 10⁻⁵ M Cu(II). The PCA/Au SAM gold electrode was used for the determination of Cu(II) in a tap water sample and the results showed a good agreement with the data obtained by atomic emission spectrometry. The text was submitted by the authors in English.     

Sensitive detection of clozapine using a gold electrode modified with 16-mercaptohexadecanoic acid self-assembled monolayer

Clozapine, an effective antipsychotic drug, was found generating a pair of redox peaks at about 0.33-0.4 V (versus SCE) at 16-mercaptohexadecanoic acid (i.e. MHA) self-assembled monolayer (SAM) modified gold electrode (i.e. MHA/Au) in 0.05 mol L⁻¹ Tris-HCl (pH 8.1) buffer solution. Sensitive and quantitative measurement of clozapine based on anodic peak was established under optimum conditions. The anodic peak current was linear to clozapine concentration in the range from 1 × 10⁻⁶ to 5 × 10⁻⁵ mol L⁻¹ with the detection limit of 7 × 10⁻⁹ mol L⁻¹. This method was successfully applied to the detection of clozapine in drug tablets and proved to be reliable compared with ultraviolet spectrophotometry (UV). The MHA SAM was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), contact angle goniometry, electrochemical impedance spectroscopy (EIS) and electrochemical probe.     

多巴胺在DTNB自组装膜上的电催化研究

在金电极表面制备了DTNB(5,5′ Di thiobis(2 nitrobenzoicacid))自组装单分子层膜(DTNB/AuSAM)。多巴胺在DTNB自组装膜上有一对可逆性良好的氧化还原峰,其氧化峰电流与多巴胺的浓度在5.0×10-6mol/L～1.0×10-4mol/L的范围内呈线性关系,检出限为1.0×10-6mol/L。在pH3.5的缓冲溶液中,在DTNB自组装膜上多巴胺和抗坏血酸的电化学响应可以明显区分,氧化峰电位分离达276mV。可用于抗坏血酸存在下多巴胺的检测。测定了盐酸多巴胺注射液中多巴胺的含量,其平均回收率为104%。     

Electroanalysis of dopamine at a gold electrode modified with N-acetylcysteine self-assembled monolayer

Voltammetric behavior of dopamine (DA) on a gold electrode modified with the self-assembled monolayer (SAM) of N-acetylcysteine has been investigated, and one pair of well-defined redox peaks of dopamine is obtained at the SAM modified gold electrode. The oxidation peak current increases linearly with the concentration of dopamine in the range of  mol l⁻¹. The detection limit is 8.0×10⁻⁷ mol l⁻¹. This method will be applicable to the determination of dopamine in injection of dopamine hydrochloride, and the good recovery of dopamine is obtained. Furthermore, The SAM modified gold electrode can resolve well the voltammetric responses of dopamine and ascorbic acid (AA), so it can also be applied to the determination of dopamine in the presence of ascorbic acid.     

Pulsed amperometric detection of underivatized amino acids using polypyrrole modified copper electrode in acidic solution

The successful pulsed amperometric detection of underivatized amino acids have been carried out in an acidic media on a polypyrrole (PPy) modified Cu electrode. The formation of PPy film doped with glutamate (glu) on a Cu electrode surface changes the mechanism of Cu dissolution. After application of multistep potential waveform, the PPy film was glu free due to the electro-reduction and overoxidation. High anodic potential polarization treatment yielded partially overoxidized PPy film as long as the Cu surface dissolution and amino acid permeation through the film was well controlled. This overoxidized PPy film acted as a charge and size exclusion barrier in order to improve the selectivity and stability of a Cu electrode. Various process parameters such as film modification time, detection and cleaning potential and pH of solution have been optimized to maximize the beneficial electrocatalytic properties of the electrode surface. At an optimized condition, detection limits for positively charged histidine and arginine are 19 and 22 pg respectively, whereas the neutral amino acids detected in amounts of 0.9–2.3 ng. Furthermore, the PPy coated Cu electrode response was long lived, stable and reproducible.     

Amperometric detection of amino acids with a passivated copper electrode

The amperometric behaviour of a copper electrode towards amino acids is studied by means of a rotating disc electrode. A theoretical model describing the anodic background current in a buffer solution and the increase of the current caused by amino acids is checked experimentally. The influences of the amino acid concentration, the rotation speed of the electrode and the composition of the buffer solution are studied. It is proved that chemical dissolution of a passivating film on the electrode surface, which is enhanced by the complexation reaction between the amino acid and copper(II) ions, is the principle of the phenomena observed. The applicability to flow systems is demonstrated.     

Underpotential deposition of copper on electrodes modified with colloidal gold

This communication is concerned with the electrochemical addressability of gold colloidal particles deposited on a conducting substrate. Cyclic voltammetry of electrodes modified with gold colloid layers indicates that an underpotential deposition of copper onto the gold surface takes place. Analysis of the charge associated with the underpotential deposition permits the electroactive gold area to be calculated. The total gold area may be determined from transmission electron microscopy (TEM) images. Comparison of the geometric and electroactive areas obtained indicates that electrochemically all the gold particles are addressable and the entire colloid surface is accessible.     

Highly sensitive determination of hydroxylamine using fused gold nanoparticles immobilized on sol-gel film modified gold electrode

We are reporting the highly sensitive determination of hydroxylamine (HA) using 2-mercapto-4-methyl-5-thiazoleacetic acid (TAA) capped fused spherical gold nanoparticles (AuNPs) modified Au electrode. The fused TAA-AuNPs were immobilized on (3-mercaptopropyl)-trimethoxysilane (MPTS) sol-gel film, which was pre-assembled on Au electrode. The immobilization of fused TAA-AuNPs on MPTS sol-gel film was confirmed by UV-vis absorption spectroscopy and atomic force microscopy (AFM). The AFM image showed that the AuNPs retained the fused spherical morphology after immobilized on sol-gel film. The fused TAA-AuNPs on MPTS modified Au electrode were used for the determination of HA in phosphate buffer (PB) solution (pH = 7.2). When compared to bare Au electrode, the fused AuNPs modified electrode not only shifted the oxidation potential of HA towards less positive potential but also enhanced its oxidation peak current. Further, the oxidation of HA was highly stable at fused AuNPs modified electrode. Using amperometric method, determination of 17.5 nM HA was achieved for the first time. Further, the current response of HA increases linearly while increasing its concentration from 17.5 nM to 22 mM and a detection limit was found to be 0.39 nM (S/N = 3). The present modified electrode was also successfully used for the determination of 17.5 nM HA in the presence of 200-fold excess of common interferents such as urea, NO₂⁻, NH₄⁺, oxalate, Mn²⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺ and Cu²⁺. The practical application of the present modified electrode was demonstrated by measuring the concentration of HA in ground water samples.     

Potential-dependent chemiluminescence of luminol on the gold electrode modified with ferrocenylalkanethiol self-assembled monolayer

Self-assembled monolayer of ferrocenylundecanethiol (FcC11SH) on gold electrode was used for the potential-dependent catalyst for chemiluminescence of luminol. Ferrocene head groups adsorbed on gold were oxidized to ferricinium cation species electrochemically and catalyzed the chemiluminescence of luminol. As the redox state of ferrocene group can be regulated by electrode potential, chemiluminescence response can also be controlled electrochemically. The presented system was adopted for detection of glucose in the presence of glucose oxidase since the light emission was detected even in the neutral and weak acid solution.     

Label-free immunosensing of microcystin-LR using a gold electrode modified with gold nanoparticles

The hepatotoxic microcystins, especially microcystin?CLR (MC?CLR), are causing serious problems to public health and fisheries. We describe here a label-free amperometric immunosensor for rapid determination of MC?CLR in water sample. The sensor was prepared by immobilizing antibody on a gold electrode coated with L-cysteine-modified gold nanoparticles. The stepwise self-assembly of the immunosensor was monitored and characterized by means of electrochemical impedance spectroscopy and differential pulse voltammetry. A 0.60?mmol L^?1 solution of hydroquinone was used as the electron mediator. The immunosensor was incubated with MC?CLR at 25?°C for 20?min, upon which the differential pulse voltammetric current changed linearly over the concentration range from 0.05 to 15.00???g L^?1, with a detection limit of 20?ng L^?1. The developed biosensor was used to determine MC?CLR in spiked crude algae samples. The recovery was in the range from 95.6 to 105%. This method is simple, economical and efficient, this making it potentially suitable for field analysis of MC-LR in crude algae and water samples.

2，3-二巯基乙二酸自组装膜修饰电极测定对苯二酚

研究了DMSA自组装膜的组装过程及电化学行为.DMSA自组装膜对于对苯二酚的氧化还原能够起到明显的电催化作用.在pH 6.4的H3PO4缓冲溶液中,氧化、还原峰电流与对苯二酚的浓度在6.0×105～1.0×10-3 mol/L范围内呈良好的线性关系,检出限达8.0×10-7mol/L.该电极可用于模拟废水样的测定.     

Determination of electroactive organic acids by anion-exchange chromatography using a copper modified electrode

An ion-chromatographic method combined with electrochemical detection at a copper-based chemically modified glassy carbon electrode (Cu-GC) has been shown to provide a simple analytical approach for the determination of some common organic acids in alkaline medium. Under the optimized isocratic chromatographic conditions (i.e. 0.1 M NaOH plus 80 mM CH3COONa), organic acids such as gallic, ascorbic, gluconic, lactobionic, galacturonic and glucuronic acid could be separated in less than 20 min. Under constant potential amperometric detection (i.e. 0.55 V vs. Ag-AgCl) the Cu-GC modified electrode allowed detection limits between 2 and 5 pmol for all investigated organic acids while the linear dynamic range spanned generally over three orders of magnitude. Examples of applications included the separation and quantitation of some common organic acids in vinegar, honey and tea samples, are given.     

Electrocatalytic oxidation of L-tryptophan using copper hexacyanoferrate film modified gold nanoparticle graphite-wax electrode

A novel copper hexacyanoferrate (CuHCF) film modification on cysteamine (Cys)-gold nanoparticle (AuNp) graphite-wax (GW) composite electrode was achieved for the quantitative determination of L-Tryptophan (L-Trp) at a reduced overpotential of 400mV in comparison with the bare Cys-AuNp-GW composite electrode. This modified electrode exhibited a well resolved pair of redox peaks corresponding to the hexacyanoferrate (II/III) reactions of CuHCF film at a formal potential of 0.65 V at a scan rate of 20 mV s(-1). Electrochemical impedance spectroscopy (EIS) studies with the modified electrode showed a very low charge transfer resistance to the electron transfer kinetics of Fe(II)/Fe(III) reactions. A linear range of 8.5×10(-7) M to 1.2×10(-4) M with a detection limit of 1.85×10(-8) M was achieved for the determination of L-Trp with a sensitivity of 0.1198 μA/μM. The influence of ultrasonication on the stability of the CuHCF film modified electrode was investigated. In addition, the CuHCF film modified electrode displayed an excellent reproducibility towards the real time analysis of L-Trp in commercial milk samples.     

Halogen-substituted phenylalanines as enantioselective selectors for enantioselective discrimination of amino acids: effect of halogen

Halogen-substituted phenylalanines with a halogen X (X = F, Cl, Br or I) in the para position in the aromatic ring of L-phenylalanine are used as enantioselective selectors to explore the effect of the halogen substituent on the enantioselective discrimination of amino acids. Enantioselective discrimination is achieved by investigating the collision-induced dissociation spectra of the trimeric complex ion, [CuII(ref)2(A)-H]+, generated by electrospraying a solution of a mixture of D- or L-analyte amino acid (A), enantioselective reference ligand (ref) and CuCl2. The relative abundances of fragment ions resulting from the competitive loss of reference and analyte amino acids are considered for measuring the degree of enantioselective discrimination by applying the kinetic method. The enantioselectivity of the p-halogenated derivatives of L-Phe increases from fluorine to iodine for the studied amino acids (except for acidic amino acids). The validity of the present method has also been checked by cross enantioselective experiments using p-iodo-D-phenylalanine as the reference in place of p-iodo-L-phenylalanine. The enantioselectivity of fluoro-substituted L-phenylalanine is less than that obtained with L-phenylalanine. The high inductive effect of the fluorine atom decreases the strength of the pi-pi stacking interaction. The presence of halogen affects the enantioselectivity by inductive and steric effects.     

Electrochemical determination of riboflavin using a synthesized ethyl[(methythio)carbonothioyl] glycinate monolayer modified gold electrode

The surface of a gold disk electrode, for the first time, was modified with a self-assembled monolayer of a synthesized compound, ethyl [(methythio)carbonothioyl] glycinate (ECTG), for construction of an electrode sensitive to riboflavin (vitamin B₂). The electrochemical properties of the monolayer assembled on the gold disk were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. Under the optimized conditions, the voltammetric peak currents resulting from vitamin B₂ (VB₂) species were linear for VB₂ concentrations in the range from 10^–6 to 10^–2 M. The effect of pH, type of buffer solution and scan rate on the response of the modified electrode was studied. The constructed electrochemical sensor responses very well to VB₂ in the presence of most common vitamins. Finally, the performance of the Au–ECTG modified electrode was successfully tested for electrochemical detection of VB₂ in a pharmaceutical sample.     

Sheath-flow electrochemical detection of amino acids with a copper wire electrode in capillary electrophoresis

Here, we report the detection of native amino acids using a sheath-flow electrochemical detector with a working electrode made of copper wire. A separation capillary that was inserted into a platinum tube in the detector acted as a grounded electrode for electrophoresis and as a flow channel for sheath liquid. Sheath liquid flowed outside the capillary to support the transport of the separated analytes to the working electrode for electrochemical detection. The copper wire electrode was aligned at the outlet of the capillary in a wall-jet configuration. Amino acids injected into the capillary were separated following elution from the end of the capillary and detection by the copper electrode. Three kinds of copper electrodes with different diameters-50, 125, and 300 μm-were examined to investigate the effect of the electrode diameter on sensitivity. The peak widths of the analytes were independent of the diameter of the working electrode, while the 300-μm electrode led to a decrease in the signal-to-noise ratio compared with the 50- and 125-μm electrodes, which showed no significant difference. The flow rate of the sheath liquid was also varied to optimize the detection conditions. The limits of detection for amino acids ranged from 4.4 to 27 μM under optimal conditions.