Adsorption of charged macromolecules on oppositely charged porous column materials

A family of cationic polyelectrolytes possessing defined chain lengths, narrow chain length distributions, uniform charge density, but substituents of different hydrophilicity at the quaternary ammonium group served as model compounds for adsorption studies. These studies quantitatively revealed that polymer characteristics and electrostatic parameters affect the adsorption behavior on oppositely charged porous column materials. The presence of electrostatic exclusion, in addition to size exclusion, was proved comparing molecular, electrostatic and geometrical parameters. The dominance of electrostatic effects could be concluded evaluating the relation between molecular and electrostatic dimensions. The results provide a contribution how to estimate the threshold for electrostatic exclusion from pores as a function of dimensions and experimental conditions.     

聚腺嘌呤核苷酸在金电极上的吸附氧化

采用微电极循环伏安法和吸附转移溶出伏安法研究了单链多聚腺嘌呤核苷酸Poly(A)在金电极上的伏安行为。重点了考察了Poly(A)的吸附和氧化过程。发现Poly(A)分子在金电极表面能够形成多种状态的特性吸附。影响吸附态和吸附强度的主要因素是吸附电势, 吸附时间和溶液浓度。在负电位区, 随着吸附时间(t~s)的变化, Poly(A)分子在金电极表面发生不同状态的以腺嘌呤碱基为吸附位点的强吸附, 这种以腺嘌呤碱基吸附的Poly(A)分子能够在零伏附近给出很强的氧化电流峰和对应的还原峰。     

Electrochemical determination of homocysteine at a gold nanoparticle-modified electrode

The construction of a colloidal gold-cysteamine-carbon paste electrode, Au_coll-Cyst-CPE, for the electrochemical determination of homocysteine is reported. The improved voltammetric behaviour of homocysteine at Au_coll-Cyst-CPE with respect to that observed at a gold disk electrode is attributed to an enhanced electron transfer kinetics as a consequence of the array distribution of gold nanoparticles immobilized onto the Cyst SAM. Cyclic voltammtery of homocysteine showed an adsorption-controlled current for scan rates between 500 and 5000 mV s⁻¹. The hydrodynamic voltammogram constructed for homocysteine allowed the selection of a potential value of +600 mV, where the background current is negligible, for the amperometric detection of the analyte at the Au_coll-Cyst-CPE. Using a flow rate of 0.8 ml min⁻¹, the R.S.D. value for i_p after 25 repetitive injections of homocysteine was of 4.3%, and one single electrode could be used for more than 15 days without any treatment or regeneration procedure of the modified electrode surface. An HPLC method for the separation and quantification of homocysteine and related thiols, using amperometric detection at the modified electrode has been developed. A mobile phase consisting of 2:98% (v/v) acetonitrile:0.05 mol l⁻¹ buffer solution of pH 2.0, and a detection potential of +0.80 V were selected. Separation with baseline resolution and retention times of 3.00, 3.60, 4.52, 5.71 and 7.79 min were obtained for cysteine, homocysteine, glutathion, penicillamine and N-acetyl-cysteine, respectively. Calibration graphs were constructed for all the separated compounds. Detection limits ranged between 20 nM for cysteine and 120 nM for penilcillamine, with a value for homocysteine of 30 nM. These values compare advantageously with those achieved with previously reported HPLC methods using electrochemical, UV, fluorescence and MS detection modes. The developed method was applied to the determination of cysteine and homocysteine serum samples with good results.     

Flow-injection amperometric determination of chlorine at a gold electrode

The applied potential is +0.2 V vs. SCE, flow rate is 1 ml min^?1 and sample volume is 30 μ1. The background electrolyte is 0.05 M phosphate, pH 7.4. Electrode pretreatment is +1.3 V vs. SCE for 40 s, followed by a pre-injection delay of 20 s. Peak current over a receding baseline is used. Linear range extends down to 0.4 μM for chloramine and 0.2 μM for hypochlorite. Sensitivities are 70 and 95 nA μM^? respectively. Time per determination is less than 1.5 min. Monochlorinated glycine is active whereas chlorinated cyanuratesshow no response. Chlorine and monochloramine in river water were determined.     

Adsorption of adenine derivatives on a gold electrode studied by specular reflectivity measurement

The adsorption of adenine, deoxyadenosine, deoxyadenosine-5′-monophosphate,-diphosphate and-triphosphate on a gold electrode has been studied by specular reflectivity measurement in 0.1 M NaClO₄ solution. In the presence of these compounds, a marked decrease in reflectivity was found on reflectivity-potential curves in the potential region more positive than ?0.8 V vs. Ag/AgCl, the decrease being ascribed to the adsorption of them. The magnitude of change in reflectivity was dependent on both the concentration and the electrode potential. The reflectivity change observed in the negative potential region was analyzed quantitatively according to the procedure previously described. The results were elucidated on the basis of the same isotherm as used by Green and Dahms in their adsorption study of aromatic hydrocarbons, and the number of solvent molecules being replaced through the adsorption of one organic molecule and the free energy change of adsorption were obtained. The former is suggestive of a flat orientation of the adsorbed molecule in contact with its adenine moiety on the electrode surface. It is also suggested from the latter that the presence of phosphate groups leads to a decrease in ΔG_ad⁰ resulting from their hydrophilic properties and a repulsive interaction between these groups and the negative charges on the surface.     

Adsorption of sodium phenylanthranylate on gold electrode from aqueous solution

Adsorption of sodium phenylanthranylate on a gold electrode from a borate buffer solution is studied using the methods of ellipsometry and piezo-quartz nanobalance. Statistical evaluation of adequacy between the obtained experimental isotherms and theoretical models is performed using the F-test. It is shown that they are adequately described using the Frumkin and Temkin equation.     

Adsorption phenomena on polycrystalline gold electrode modified by Zn adatoms

Summary In this paper, experimental results obtained by the in-situ radiotracer and voltammetric studies of the competitive adsorption of the HSO₄^-/SO₄^2- (labeled with ³⁵S) and Cl^- (labeled with ³⁶Cl) anions on bare Au_poly substrate and Au_poly surfaces modified by the Zn adatoms in the lower pH region (pH≤4.5) are presented and discussed. In addition, some data on the formation of Zn adlayers (labeled with ⁶⁵Zn) are reported to demonstrate the complex features of the underpotential deposition (UPD) phenomenon. It has been revealed that (1) the relative adsorption strength of Cl^- ions on bare Au_poly is higher than that of H₂PO₄^-/PO₄^3- and HSO₄^-/SO₄^2- ions in the entire pH region studied; (2) anomalous tendency of the specific adsorption of anions on polycrystalline gold modified by Zn adatoms occurs at pH 4.5 as follows: PO₄^3->SO₄^2->Cl^->>ClO₄^- and (3) the UPD of Zn²⁺ ions on polycrystalline gold is measurable even from solutions containing 5 ^. 10^-8M Zn²⁺ over the entire potential range where the hydrogen evolution takes place.It is, however, plausible to assume that the coverage of the gold surface by Zn adatoms measured even at solution concentrations of c≥5 ^. 10^-4M does not exceed one monolayer.     

Adsorption of methylamine at a polycrystalline gold/solution interface

Adsorptive behaviour of the methylamine molecules has been investigated by measuring changes in the differential capacitance of the double layer at the gold/solution interface by the tensammetric method. The differences in adsorption parameters at Θ < 0.8 and >0.8 have been explained by changes in arrangement of the adsorbate.     

Sensing phenothiazine drugs at a gold electrode co-modified with DNA and gold nanoparticles.

DNA and gold nanoparticles are co-immobilized at a gold electrode through elaborate self-assembly processes. This configuration has proven to be useful as a sensor for phenothiazine drugs, taking advantage of the well-known, relatively large surface area of gold nanoparticles and the strong intercalation between dsDNA and phenothiazine drugs. This modified electrode has demonstrated good sensitivity and stability towards the oxidation of two model phenothiazine drugs: promethazine and chlorpromazine. A linear dependence between the concentration of phenothiazine drugs and the peak current is observed, with a concentration range of 2.0 x 10(-5)-1.6 x 10(-4) M and 1.0 x 10(-5)-1.2 x 10(-4) M, and a detection limit of 1.0 x 10(-5) M and 7.0 x 10(-6) M, for promethazine and chlorpromazine, respectively.     

Electrocatalytic response of dopamine at a metallothioneins self-assembled gold electrode.

The metallothioneins (MT) self-assembled monolayer modified gold electrode (MT/Au) is demonstrated to catalyze the electrochemical response of dopamine (DA) by cyclic voltammetry. A pair of well-defined redox waves was obtained and the calculated standard rate constant (k(s)) is 6.97 x 10(-3) cm s(-1) (20 degrees C) at the self-assembled electrode. The electrode reaction is a quasi-reversible process. The oxidation peak of DA can be used to determine the concentration of DA. The peak current and the concentration of DA follow a linear relationship in the range of 2.0 x 10(-5) M to 8.0 x 10(-4) M. The detection limit is 6.0 x 10-6 M. By ac impedance spectroscopy, the apparent electron transfer rate constant (k(app)) of Fe(CN)6(3-)/Fe(CN)6(4-) at the MT/Au electrode was obtained as 2.0 x 10(-5) cm s(-1). The MT/Au was characterized with grazing angle FT-IR spectroscopy and contact angle goniometry.     

The direct electrochemistry of myoglobin at a DL-homocysteine self-assembled gold electrode

A pair of well-defined redox waves of myoglobin at a DL-homocysteine self-assembled gold electrode was achieved. Myoglobin can strongly bind to the homocysteine self-assembled gold electrode, so the Mb-Hcy self-assembled gold electrode was prepared and the standard rate constant (ks) of Mb was calculated as 9.3 x 10(-1) s(-1). X-ray photoelectron spectroscopy (XPS) measurements demonstrated that Hcy monolayer was formed and Mb was bound to Hcy monolayer.     

Interactive adsorption behavior of NAD+ at a gold electrode surface

The adsorption of an oxidized form of nicotinamide adenine dinucleotide, NAD+, on a polycrystalline gold electrode surface and the subsequent surface conformation of the molecule were investigated over a wide temperature and potential range, using electrochemical differential capacitance and PM-IRRAS techniques. The adsorption process was described by the Langmuir adsorption isotherm. The corresponding thermodynamic parameters were determined: the Gibbs energy, enthalpy, and entropy of adsorption. The large negative Gibbs energy of adsorption (-43 +/- 4 kJ mol-1 and -39 +/- 2 kJ mol-1 on a positively and negatively charged surface, respectively) confirms that the NAD+ adsorption process is highly spontaneous, while the large entropy gain (285 J K-1 mol-1 and 127 J K-1 mol-1 on a positively and negatively charged surface, respectively) was found to represent the adsorption driving force. It was demonstrated that the energetics of the adsorption process is surface-charge controlled, while its kinetics is both mass-transport and surface-charge controlled. A surface-charge dependent conformation model for the adsorbed NAD+ molecule is proposed. These findings suggest that the origin of the NAD+ reduction overpotential is related to the surface conformation of the adsorbed NAD+ molecule, rather than to the electrode Fermi level position.     

Electrochemical behavior of uric acid at a penicillamine self-assembled gold electrode

The fabrication and electrochemical characteristics of a penicillamine (PCA) self-assembled monolayer modified gold electrode were investigated. The electrode can enhance the electrochemical response of uric acid (UA), and the electrochemical reaction of UA on the PCA electrode has been studied by cyclic voltammetry and differential pulse voltammetry. Some electrochemical parameters, such as diffusion coefficient, standard rate constant, electron transfer coefficient and proton transfer number have been determined for the electrochemical behavior on the PCA self-assembled monolayer electrode. The electrode reaction of UA is an irreversible process, which is controlled by the diffusion of UA with two electrons and two protons transfer at the PCA/Au electrode. In phosphate buffer (pH 5.0), the peak current is proportional to the concentration of UA in the range of 6.0 × 10⁻⁵–7.0 × 10⁻⁴ mol L⁻¹ and 2.0 × 10⁻⁵–7.0 × 10⁻⁴ mol L⁻¹ for the cyclic voltammetry and differential pulse voltammetry methods with the detection limits of 5.0 × 10⁻⁶ and 3.0 × 10⁻⁶ mol L⁻¹, respectively. The method can be applied to determine UA concentration in real samples.     

Adsorption of pyridine on a polycrystalline gold electrode surface studied by infrared reflection absorption spectroscopy

The adsorption behavior of pyridine on a smooth polycrystalline gold electrode surface was investigated over a wide wavenumber region (2000–500 cm⁻¹) by in situ infrared reflection absorption spectroscopy (IRAS). The reversible adsorption/desorption of pyridine was observed upon the change in applied electrode potential, and the adsorption state at positive potentials was found to depend strongly on the kind of halide ion used as a supporting electrolyte. Symmetry analysis of absorption bands observed revealed that pyridine molecules adsorb with the molecular axis (C₂ axis) perpendicular to the electrode surface (vertical configuration) at positive potentials in 0.5 M KF, KCl and KBr solutions. A band due to the out-of-plane bending mode of the adsorbed pyridine molecule was observed at potentials more negative than ca. 0 V for 0.5 M KF solution containing 100 mM pyridine. We concluded that even in the 100 mM pyridine solution, adsorbed pyridine forms a monolayer and that the molecules reorient from a flat (parallel) to the vertical configuration as the potential becomes less negative. No bands due to adsorbed pyridine were detected for 0.5 M KI solution. The amount of adsorbed pyridine was found to depend strongly on the strength of specific adsorption of halide ions.     

Electrocatalytic response of norepinephrine at a thiolactic acid self-assembled gold electrode

The thiolactic acid (TLA) self-assembled monolayer modified gold electrode (TLA/Au) is demonstrated to catalyze the electrochemical response of norepinephrine (NE) by cyclic voltammetry. A pair of well-defined redox waves were obtained and the calculated standard rate constant (k_s) is 5.11×10⁻³ cm s⁻¹ at the self-assembled electrode. The electrode reaction is a pseudo-reversible process. The peak current and the concentration of NE are a linear relationship in the range of 4.0×10⁻⁵–2.0×10⁻³ mol l⁻¹. The detection limit is 2.0×10⁻⁶ mol l⁻¹. By ac impedance spectroscopy the apparent electron transfer rate constant (k_app) of Fe(CN)³⁻/Fe(CN)⁴⁻ at the TLA/Au electrode was obtained as 2.5×10⁻⁵ cm s⁻¹.     

Effects of solution pH on sulfite oxidation at a gold electrode

Journal of Solid State Electrochemistry - The oxidation of sulfite at a renewable gold electrode in solutions with pH values ranging from 2 to 14 was studied using voltammetry. The voltammogram...     

Controlled-potential electrosynthesis of glucosaminic acid from glucosamine at a gold electrode

The electrocatalytic oxidation of d-glucosamine (2-amino-2-deoxy-d-glucose) in alkaline and neutral solutions was examined using a carbon felt electrode modified with 2 nm core sized gold nanoparticles (Au_2 nm nanoparticles) and a gold plate electrode. The electrocatalytic voltammetric oxidation curves of d-glucosamine were obtained in both solutions. The voltammetric responses for the electrocatalytic oxidation at a Au_2 nm nanoparticle-modified electrode in both alkaline and neutral solutions were almost the same to those at a gold plate electrode. The oxidized product was identified to be d-glucosaminic acid (2-amino-2-deoxy- d-gluconic acid) generated by the 2-electron oxidation product of d-glucosamine by electrospray ionization time-of-flight mass spectra (ESI TOF-MS). The HPLC results also indicated that the oxidation product was d-glucosaminic acid.The controlled-potential electrolysis of d-glucosamine was performed at the Au_2 nm nanoparticle-modified carbon felt electrodes in both alkaline and neutral solutions. In the alkaline solution, at a potential of −0.2 V, d-glucosaminic acid was formed with a current efficiency of 100%. In the neutral solution, electrolysis at 0.4 V on d-glucosaminic acid was obtained with current efficiencies of 70%.     

Electrochemical response of dopamine at a penicillamine self-assembled gold electrode.

The penicillamine (Pen) self-assembled monolayer (SAM) modified gold electrode (Pen/Au) is demonstrated to catalyze the electrochemical response of dopamine (DA) by cyclic voltammetry. A pair of well-defined redox waves was obtained and the calculated standard rate constant (k(s)) is 3.88 x 10(-3) cm/s at the self-assembled electrode. The electrode reaction is a quasi-reversible process. The oxidation peak of DA can be used to determine the concentration of DA. The peak current and the concentration of DA are a linear relationship in the range of 2.0 x 10(-5) M to 8.0 x 10(-4) M. The detection limit is 4.0 x 10(-6) M. By ac impedance spectroscopy the apparent electron transfer rate constant (k(app)) of Fe(CN)(3-)/Fe(CN)(4-) at the Pen/Au electrode was obtained as 2.08 x 10(-5) cm/s. The Pen SAM was characterized with X-ray photoelectron spectroscopy (XPS), grazing angle FT-IR spectroscopy and contact angle goniometer.