Probe beam deflection study of ion exchange in self-assembled redox polyelectrolyte thin films

Probe beam deflection during chronoamperometric oxidation-reduction of osmium complex in layer-by-layer self-assembled redox active polyelectrolyte multilayers has shown that the nature of the charge in the topmost layer determines the ion flux that balances the redox charge.     

Carbon ceramic electrode modified with redox liquid

A carbon ceramic electrode modified with a redox liquid, butylferrocene, exhibiting in aqueous salt solution electrochemical behaviour resulting from the redox process of the modifier and ion transfer across the liquid-liquid interface has been prepared.     

Nanostructured modified electrodes: role of ions and solvent flux in redox active polyelectrolyte multilayer films

Electrodes modified layer-by-layer by self-assembly of redox active polyelectrolytes comprised of osmium bipyridine-pyridine derivatized poly(allyl-amine) and poly(vinyl) sulfonate have been studied by EQCM, ellipsometry, cyclic voltammetry and electrochemical impedance spectroscopy in aqueous solutions of different anions and cations. Redox driven swelling by solvent exchange during oxidation, in excess to the hydration number, occurs by perturbation of the equilibrium between the osmotic and elastic forces as a result of the electrochemical injection of charge into the film. The exchanged mass and volume change during redox switching strongly depends on the nature of the anion under anion Donnan permselectivity conditions.     

Diffusion exchange NMR spectroscopic study of dextran exchange through polyelectrolyte multilayer capsules

Diffusion exchange of dextran with molecular weights 4.4 and 77 kDa through polyelectrolyte multilayer (PEM) hollow capsules consisting of four bilayers of polystyrene sulfonate/polydiallyldimethylammonium chloride has been investigated using two-dimensional nuclear-magnetic-resonance methods: diffusion-diffusion exchange spectroscopy (DEXSY) and diffusion-relaxation correlation spectroscopy (DRCOSY). Results obtained in DRCOSY experiments show that the diffusion process of dextran 77 kDa exhibits an observation time dependence suggesting a diffusion behavior restricted by confinement. We find evidence for both single capsule and capsule aggregate states, with a partitioning of the 77-kDa dextran between the free and capsule states much larger than that suggested by volume fraction alone. Results from DEXSY experiments show that dextran 77 kDa is in diffusive exchange through the capsules with an exchange time of around 1 s. In contrast, the capsules have no detectable influence on the diffusion process of the dextran 4.4 kDa. This quantitative information may be used in designing PEM capsules as drug carriers.     

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.     

Preparation of polyaniline modified electrode in novel ionic liquid and its application in ion chromatography

In this work, a promising electrochemical detector has been fabricated by immersing a glass carbon electrode (GCE) in aniline containing novel ionic liquid and scanning between -1.0 and 1.0 V for 40 cycles, and was used in ion chromatography (IC) system. The morphology of the modified electrode surface was characterized by scanning electron microscope (SEM). The polyaniline (PANI) film showed excellent electrocatalytic activity than bare GCE and provided enhanced selectivity and stability for the detection of ascorbic acid (AA). Separated by IC with phosphate buffer solution (pH = 5.2) as eluent, AA could be determined by the PANI/GCE successfully at the working potential of 0.3 V. The retention time of AA was approximately 5.75 min, and the peak shape of AA was satisfactory. The calibration curve of AA was linear (r > 0.99), in the range between 0.05 mg/L and 1000 mg/L and the detection limit was 23.41 μg/L (S/N = 3). The proposed method was successfully applied in the detection of AA in four beverage samples. The recoveries of AA in these samples were from 92.32% to 110.57%.     

Competing mechanisms in polyelectrolyte multilayer formation and swelling: Polycation–polyanion pairing vs. polyelectrolyte–ion pairing

The competition of interactions between charged groups of polyanions and polycations and their interaction with small counterions strongly affect the formation and stability of polyelectrolyte multilayers. This has consequences for the properties of polyelectrolyte multilayers like mechanics, polymer mobility and swelling in water.     

Homogenous redox catalysis competition between electron exchange and proton exchange

Whether proton exchange and nucleophilic substitution reactions can become competitive with electron exchange reactions in homogeneous redox catalysis processes due to the reduction of bromoesters has been evaluated.Electrochemical data have been correlated with the results of the analyses carried out in solution during, and at the end of, the catalytic reduction processes. Thus, it has been possible to obtain the values of γ (γ = c_RX/c_O, where c_RX is the substrate concentration and c_O that of the catalyst corresponding to pure catalysis conditions whereby only the electron-exchange reaction is observed.The hypothesis that the catalyst (9,10-diphenylanthracene) is dihydrogenated by the substrate according to DISP₂ mechanism is in agreement with the experiments.     

Electrochemical behavior of epinephrine at l-cysteine self-assembled monolayers modified gold electrode

The electrochemical behaviors of epinephrine (EP) at the l-cysteine self-assembled monolayers modified gold electrode have been studied. The modified electrode shows an excellent electrocatalytic activity for the oxidation of EP and accelerates electron transfer rate. The diffusion coefficient (D) is 1.48x10(-7) cm(2) s(-1). FTIR has shown that cysteine can bind onto the gold surface through the strong sulfur-gold interaction. The electrocatalytic mechanism to EP has been studied. The catalytic current of EP nu s its concentration has a good linear relation in the range of 1.0x10(-7)-2.0x10(-6) mol l(-1), with the correlation coefficient of 0.9989 by differential pulse voltametric (DPV) response. Detection limit is down to 1.0x10(-8) mol l(-1). At a high EP concentration, the relationship between the catalytic current and its concentration exhibits a Michaelis-Menten kinetic mechanism for the electrocatalytic process and the constant K(m) is about 0.155 mmol l(-1). The highest catalytic current I(m) is 2.72 muA. The modified electrode can be used for the determination of EP in practical injection. The method is simple, quick, sensitive and accurate.     

Photoresponsive ion gating function of an azobenzene polyelectrolyte multilayer spin-self-assembled on a nanoporous support

The fabrication of a polyelectrolyte multilayer (PEM) on a porous membrane was successfully improved by using spin-coating electrostatic self-assembly. Surprisingly, the quality of the PEM film obtained on the nanoporous alumina substrate (i.e., its thickness and surface morphology) was comparable to that of a film deposited on silicon. An optical molecular switch that acts as an ion-gating channel was realized using a PEM membrane deposited layer-by-layer on an alumina support. One of the layer components of this device was a poly(acrylamide) copolymer containing an azobenzene chromophore, which is known to reveal strong voluminous expansion and contraction during light-induced reversible cis/trans isomerizations. The permeability of the bulk SO4(2-) ions was found to be sensitive to the changed channel sizes; for instance, the ion-permeation rate of SO4(2-) increased about 1.6 times after UV irradiation of the PEM, whereas that of the Cl- ion increased only 1.2 times. In the study, it was successfully demonstrated that the ion flow through the PEM membrane could be reversibly switched on and off over several azobenzene isomerization cycles.     

Surface interactions during polyelectrolyte multilayer buildup. 1. Interactions and layer structure in dilute electrolyte solutions

We report the investigation of surface forces between polyelectrolyte multilayers of poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate sodium salt) (PSS) assembled on mica surfaces during film buildup using a surface force apparatus. Up to four polyelectrolyte layers were prepared on each surface ex situ, and the surface interactions were measured in 10(-4) M KBr solutions. The film thickness under high compressive loads (above 2000 microN/m) increased linearly with the number of deposited layers. In all cases, the interaction between identical surfaces at large separations (>100 A from contact) was dominated by electrostatic double-layer repulsion. By fitting DLVO theory to the experimental force curves, the apparent double-layer potential of the interacting surfaces was calculated. At shorter separations, an additional non-DLVO repulsion was present due to polyelectrolyte chains extending some distance from the surface into solution, thus generating an electrosteric type of repulsion. Forces between dissimilar multilayers (i.e., one of the multilayers terminated with PSS and the other with PAH) were attractive at large separations (30-400 A) owing to a combination of electrostatic attraction and polyelectrolyte bridging.     

铜卟啉-L-半胱氨酸自组装复合膜修饰电极的制备及电化学性能研究

利用电化学扫描法在L 半胱氨酸(Cys)自组装单分子膜修饰金电极表面现场制备了金属卟啉复合膜,对其进行SEM和ATR FTIR表征。修饰电极的支持电解质以及pH值对膜的稳定性和灵敏度有很大影响。铜卟啉 L Cys膜对H2O2具有良好的电催化还原特性,催化电流与H2O2浓度在1 0×10 6到3 0×10 5mol·L 1范围内线性关系,相关系数0 9995,检测限达1 0×10 7mol·L 1。     

Lithium ion phase-transfer reaction at the interface between the lithium manganese oxide electrode and the nonaqueous electrolyte

The lithium ion phase-transfer reaction between the spinel lithium manganese oxide electrode and a nonaqueous electrolyte was investigated by the ac impedance spectroscopic method. The dependence of the impedance spectra on the electrochemical potential of the lithium ion in the electrode, the lithium salt concentration in the electrolyte, the kind of solvent, and the measured temperature were examined. Nyquist plots, obtained from the impedance measurements, consist of two semicircles for high and medium frequency and warburg impedance for low frequency, indicating that the reaction process of two main steps for high and medium frequency obey the Butler-Volmer type equation and could be related to the charge-transfer reaction process accompanied with lithium ion phase-transfer at the interface. The dependency on the solvent suggests that both steps in the lithium ion phase-transfer at the electrode/electrolyte interface include the desolvation process and have high activation barriers.     

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%。     

Preparation of ammonium intercalated vanadyl phosphate by redox intercalation and ion exchange

Redox intercalation of NH₄⁺ into vanadyl phosphate dihydrate (VOPO₄·2H₂O) leads to a two-phase (NH₄)_xVOPO₄·H₂O (x=0.2−0.9) compound with interlayer distances of 6.7 and 6.4 Å. Ammonium ions can be incorporated into the interlayer space of VOPO₄ also by an ion exchange, starting from alkali-metal redox-intercalated vanadyl phosphate Me_xVOPO₄·yH₂O (Me=Li, Na, K, Rb). Several phases are formed during the ion exchange, one of them with the interlayer distance corresponding to the original Me_xVOPO₄·yH₂O phase, the second one corresponding to formed (NH₄)_xVOPO₄·H₂O. In addition, a third phase is formed by the ion exchange when Li_0.98VOPO₄·1.98H₂O or Rb_0.60VOPO₄·1.3H₂O are used as starting compounds. An opposite ion exchange of NH₄⁺ for Me⁺ starting from (NH₄)_xVOPO₄·H₂O does not proceed.     

Enzyme electrode studies of glucose oxidase modified with a redox mediator

Glucose oxidase modified by the covalent attachment of ferrocenecarboxylic acid or ferrocene-acetic acid groups undergoes direct oxidation at metal electrodes. Studies of the comparative stability of the two modified enzymes on storage and on electrochemical cycling show that the material modified with ferroceneacetic acid is the more stable. Amperometric studies of enzyme electrodes based on these modified forms of glucose oxidase show that their application in practical biosensors is severely limited by the poor stability of the oxidized form of the covalently attached ferrocene mediator. A comparison of the results obtained with the native enzyme and with that modified with ferroceneacetic acid, for the oxidation of glucose, d-mannose, 2-deoxy-d-glucose, d-xylose and d-galactose, suggests that the modification procedure has little effect on the selectivity of the enzyme.     

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.     

Production of an electrode material modified by a lithium-conducting solid electrolyte

The process for producing the electrode material LiCoPO₄ modified by the lithium-conducting solid electrolyte Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) was studied. To create a composite consisting of an electrochemically active substance and an electrically conductive additive distributed uniformly between LiCoPO₄ particles, a peroxide solution of a LATP precursor was used. After annealing at 700°C, the two-phase composite LiCoPO₄/LATP was obtained, the conductivity of which was two orders of magnitude higher than that of binary lithium cobalt phosphate at room temperature.     

Effect of self-assembled surfactant structures on ion transport across the liquid|liquid interface

In this paper, the effect of a coadsorbed polyanion–cationic surfactant system on the transport of tetraethylammonium ion across the water|1,2-dichloroethane interface is studied. It is shown that the change in double-layer structure due to the presence of adsorbed or coadsorbed surfactant can explain the experimental observations, thus concluding that no other effects on ion transfer (e.g., steric hindrance) are relevant under these experimental conditions. The implications of these results are discussed.