Controlling the rate and direction of electroosmotic flow in template-prepared carbon nanotube membranes

We describe here carbon nanotube membranes (CNMs) prepared by doing chemical vapor deposition of graphitic carbon into the pores of microporous alumina template membranes. This approach yields a freestanding membrane containing a parallel array of carbon nanotubes (outside diameter ～200 nm, wall thickness ～40 nm) that spans the complete thickness of the membrane (60 μm). The electroosmotic flow (EOF) can be driven across these CNMs by allowing the membrane to separate two electrolyte solutions and using an electrode in each solution to pass a constant ionic current through the nanotubes. The as-synthesized CNM has anionic surface charge and as a result, the EOF is in the direction of cation migration across the membrane. In addition to this result, the key findings of this correspondence are: (1) that by using an electrochemical derivatization method, carboxylate groups can be covalently attached to the carbon nanotube walls; this enhances the anionic surface charge density, resulting in a corresponding increase in the EOF rate; (2) that electrochemical derivatization can also be used to attach cationic ammonium sites to the nanotube walls to yield CNMs that show EOF in the opposite direction of the as-synthesized or carboxylated membranes.     

Counterion layering at high surface charge in an electric double layer. Effect of local concentration approximation

A systematic study of the counterion layering phenomenon at high surface charge in a planar electric double layer is reported. Extensive canonical Monte Carlo simulations are performed on a 1:1 electrolyte to investigate the effect on layering due to a variation of electrode charge density, electrolyte concentration, and ionic size. For a 1 mol dm⁻³ salt at room temperature and ionic diameter 400 pm, the second layer begins to be formed at around a surface charge density of ∼0.4 C m⁻². It is seen that the variation of ionic size (at constant surface charge and salt concentration) leads to the most pronounced changes in the counterion layering. At an ionic diameter of 600 pm a third layer of counterions is observed. The simulations are extended to asymmetric valency 2:1 and 1:2 salts where layering effects are observed for monovalent counterions only. The calculated double layer capacitance is found to decrease in the presence of layering. A mean-field Poisson–Boltzmann equation coupled with a recently derived ionic exclusion volume term is found to predict this layering in qualitative agreement with the simulations. The use of a local concentration approximation improves the agreement in most cases.     

Equilibrium potential of potentiometric ion sensors under steady-state current by using current-reversal chronopotentiometry

Potentiometric ion sensors based on a solvent polymeric ion-sensing membrane and a conducting polymer as the solid contact were investigated by current-reversal chronopotentiometry. The low-frequency electrochemical behavior of the electrodes resembles a series RC equivalent circuit. The resistance R was attributed primarily to the bulk resistance of the ion-sensing membrane and the capacitance C was attributed to the bulk redox capacitance of the conducting polymer layer acting as an ion-to-electron transducer. It is shown that the equilibrium potential of the ion sensors can be obtained in the presence of a steady-state current by using current-reversal chronopotentiometry (double current step chronopotentiometry).     

Silicon surface states and subsurface hydrogen

The n-Si|fluoride electrolyte interface exhibits a small interface-state density, as shown by the presence of a weak capacitance peak in the capacitance–potential curves. After in situ chemical dissolution of an anodically formed oxide, a quasi ideal interface with a very low interface-state density was obtained. At open-circuit potential, a slow increase in the interface-state density was observed. These interface states result from the penetration of hydrogen into the silicon subsurface region. The inward diffusion of hydrogen produced by electrochemical etching of silicon accounts for the large time constant observed.     

Charge–discharge kinetics of electric-paint displays

The charge–discharge kinetics of electrochromic displays, based on porous nanostructured viologen-derivatized TiO₂ films, were investigated by standard electrochemical techniques. Various display compositions and geometries were studied. Chronocoulometry data were analyzed in terms of a simple approximate model for the display charging in optical switches. It was found that the charging speed of “standard displays” is limited mainly by the electrical resistance in the transparent conducting substrates and the capacitance due to the electrochemically active porous films. Support for the assumed voltage-dependence of the display resistance and capacitance in the charging model comes from cyclic voltammetry and impedance spectroscopy measurements. Based on observations of rapid current pulses following voltage steps, a modified charging model is suggested. In this model, the display capacitance is decomposed in contributions from the porous films and the transparent conductive substrates.     

On the impedance of a lipid-modified Hg | electrolyte interface

The impedance of a lipid-modified mercury | electrolyte interface was investigated, concentrating on the adsorption and desorption of a dioleoyl phosphatidylcholine (DOPC) monolayer adsorbed from the gas | solution (G | S) interface as well as that formed from liposome adsorption. The impedance measurements showed two remarkable effects. First, when compared to the electrolyte in the absence of lipid, the high frequency resistance increased when DOPC was adsorbed as well as when it was desorbed. Second, the double layer capacitance exhibited a marked frequency dependence for higher frequencies. Both effects can be explained on the basis of a new model, which is able to fit the frequency dependence of the impedance accurately. At the adsorption potential, the increase of the high frequency resistance can be related to the formation of a coherent condensed film of the lipid, which strongly decreases the mobility of electrolyte ions near the interface. At desorption potentials, the lipid film, held by adhesive forces near to the electrode surface, remains stable and shows the same decrease in electrolyte ion mobility. The frequency dependence of the double layer capacitance at desorption potentials can be explained through a decrease in the exchange rate of the adsorbed water molecules due to the presence of the desorbed lipid film. In the equivalent circuit representation, the double layer capacitance has then to be replaced by a high frequency capacitance in parallel with a series connection of an adsorption resistance and an adsorption capacitance. The product of adsorption capacitance and adsorption resistance gives the adsorption relaxation time. The adsorption relaxation time of water was estimated to be 10^?4 s for the interfacial region modified by the desorbed lipid. The influence of Ca²⁺ ions added to the electrolyte on the high frequency resistance and capacitance is described. The implications of these results on the classical picture of organic adsorption/desorption introduced by Frumkin are also discussed.     

Electrosorption capacitance of nanostructured carbon aerogel obtained by cyclic voltammetry

Cyclic voltammetry experiments at various electrolyte solution concentrations (0.001–0.1 M) and scan rates (1 to 5 mV s⁻¹) have been performed to study the electrical double layer (edl) formation in nanostructured carbon aerogel. The results show that carbon aerogel is a good edl capacitor and can be further divided into mesoporous and microporous capacitors. According to the experiments, the mesoporous capacitor shows a fast charging/discharging response and is only minimally affected by the electrolyte concentration and scan rate. Therefore, the specific capacitance of the mesoporous capacitor is found to be constant over a wide range of applied electrical potentials. On the other hand, the microporous capacitor shows a slow charging/discharging response and its capacitance strongly depends on the electrolyte concentration and potential. Unlike previous experiments, in which only a flat minimum was observed at the point of zero charge (pzc), in the current study, a deep minimum is observed near the pzc at low electrolyte concentration if a slow scan rate is used. This unique feature is a result of edl overlapping in the micropores and is consistent with the predictions by the Gouy–Chapman model employed in this study. Based on this behavior, a new approach is suggested for pzc measurements of solid porous materials for which a large portion of the surface area is in the micropore region.     

The capacitance of the electric double layer of electrodes in molten salts

High exchange current electrodes are reversible electrodes with an exchange current density greater than 0.1 A cm^?2. Both in the time and frequency domains, their behaviour is predominantly inductive. Using the inductive model of high exchange current electrodes developed earlier, the double layer capacitance of a liquid Mg electrode in a melt composed of 20 mol% of MgCl₂ and 80 mol% of NaCl or KCl, is evaluated. As in aqueous electrolytes, the double layer capacitance is a sum of the compact and the diffuse layer capacitances. A model of the electric double layer of electrodes with high exchange current density is proposed, which allows for the calculation of the compact layer capacitance. It has been assumed in this calculation that the compact layer of the electric double layer is composed of a primary ionic shell, formed by specifically adsorbed chloride anions. The capacitance of such a layer is calculated as the Helmholtz capacitance, with the dielectric permittivity equal to 1. This capacitance is then corrected for the exchange current density. The corresponding amount of counterions to the specifically adsorbed anions in the primary ionic shell, is transferred to the neighbouring holes, creating the diffuse layer. The capacitance of the diffuse layer is thus calculated from the experimentally evaluated double layer capacitance and calculated by the present model compact layer capacitance.     

KRSR多肽／Ti02纳米管复合涂层对骨髓间充质干细胞粘附与成骨的影响

目的：本研究在二氧化钛纳米管层表面固定了KRSR序列短链多肽,对此材料的粘附和诱导成骨的能力进行了研究。方法：于二氧化钛纳米管层上共价连接固定多肽后,对材料进行表征,并在试件上接种大鼠骨髓间充质干细胞,使用各种技术对细胞的粘附和分化进行评价。结果：固定KRSR的二氧化钛纳米管层对细胞的粘附率无明显影响,但铺展、骨向分化程度均优于对照组。结论：固定KRSR的二氧化钛纳米管层能够较好地诱导细胞骨向分化,有一定的临床应用价值。     

Capacitance and photocurrent study of electronic properties of anodic oxide films on Nb and Ta. Evaluation of the ionized donor concentration profile in Nb2O5 film

Electronic properties of electrochemically formed oxide films on Nb were studied by photocurrent and differential capacitance measurements in 0.025 M KH₂PO₄+0.025 M Na₂HPO₄ electrolyte, pH 6.9. Oxide films of n-type conductivity were formed galvanostatically for final potentials ranging from 4 to 230 V. Measurements were performed in two potential regions, which correspond to formation of a depleted layer of variable thickness at relatively low potentials, and to complete depletion of oxide films of electronic charge carriers at higher potentials. In the first potential region the behavior of both capacitance and photocurrent, was governed by a build up of a depleted layer of potential dependent thickness. In the second, high potential, region, which extends up to the oxide film formation potential, the photocurrent and capacitance of oxide films in most features followed the trends typical of dielectric films containing defects and traps. The photocurrent and capacitance measurements on presumably dielectric oxide films formed on Ta were staged for comparison. The capacitance–potential measurements performed in the first potential region enabled us to construct the ionized donor concentration profile across the Nb₂O₅ film width. The limitations on the use of the C–E profiling method for electrochemically formed oxide layers are considered.     

A comparative study of the surface electrochemical oxidation reactions of α-, β-, and γ-aminobutyric acid at platinum

The electrochemical oxidation reactions of α-, β- and γ-aminobutyric acid at a Pt electrode were investigated in aqueous solutions at pH 1 and pH 13 using steady-state current—potential measurements, cyclic voltammetry and open-circuit potential decay. The capacitance behaviour and the high Tafel slopes suggest the production of free radicals at the surface of the electrode accompanied by a second reaction involving loss of CO₂ which is the rate-determining step. The adsorbed intermediate species is either hydrolysed anodically to propionaldehyde or produces the aldehyde by the formation of a carbonium ion which is subsequently hydrolysed in solution. The mechanisms are compared with those observed in our earlier investigations of the electrochemical oxidation reactions of glycine and α- and β-alanine. Formation of propionaldehyde and other aldehyde products was confirmed by polarographic measurements, and no dimerized products were detected by gas chromatography. This behaviour differs from the dimerization process typical of the radical reactions associated with the Kolbe mechanism.     

Modification of TiO2 nanotube surfaces by electro-spray deposition of amoxicillin combined with PLGA for bactericidal effects at surgical implantation sites

Abstract

Objective. To fabricate the antibiotic-releasing coatings on TiO₂ nanotube surfaces for wide applications of implant and bone plate in medical and dental surgery, the optimal deposition time of amoxicillin/PLGA solution simultaneously performing non-toxicity and a high bactericidal effect for preventing early implant failures was found. Materials and methods. FE-SEM, ESD and FT-IR were used for confirming deposition of amoxicillin/PLGA on the TiO₂ surface. Also, the elution of amoxicillin/PLGA in a TiO₂ nanotube surface was measured by a UV-VIS spectrophotometer. The bactericidal effect of amoxicillin on the TiO₂ nanotube surface was evaluated by using Staphylococcus aureus (S. aureus). The cytotoxicity and cell proliferation were observed by WST assay using MC3T3-E1 osteoblast cells. Results. The results indicated that the TiO₂ nanotube surface controlled by electro-spray deposition time with amoxicillin/PLGA solution could provide a high bactericidal effect against S. aureus by the bactericidal effect of amoxicillin, as well as good osteoblast cell proliferation at the TiO₂ nanotube surface without toxicity. Conclusions. This study used electro-spray deposition (ESD) methodology to obtain amoxicillin deposition in nanotube structures of TiO₂ and found the optimal deposition time of amoxicillin/PLGA solution simultaneously performing non-toxicity and a high bactericidal effect for preventing early implant failures.     

Theoretical prediction of state of charge of lithium ion cells

The prediction of the state of charge of batteries is a classic topic of considerable interest. The present work aims to predict the state of charge of lithium ion cells using a simple diffusion model. The extent of lithium ion diffusion into the cathode is given as a measure of the state of charge. Similarly an attempt has been made to correlate the impedance response of the cell to its state of charge. Coin cells were assembled. Charge–discharge studies and impedance measurements at various states of charge were carried out and the results are compared with the values predicted theoretically.     

Electrochemical impedance analysis of the redox switching hysteresis of poly(3,4-ethylenedioxythiophene) films

The effect of the redox hysteresis of poly(3,4-ethylenedioxythiophene) films on the electrochemical mechanisms analyzed by impedance has been studied. Impedance spectra were recorded at fixed steady-state potentials under both negative and positive sweeps. The effects of film thickness were also investigated by varying the polymerization charge used. It has been shown that not only do quasi-equilibrium circuit elements (low-frequency capacitance) vary with the potential sweep direction but also kinetics elements (interfacial charge transfer resistance) depend on it, both exhibiting hysteretic effects. The onset of the charge transfer resistance increase is shifted approximately ?300 mV during the negative polarization.     

Weak adsorption of anions on gold: measurement of partial charge transfer using Fast Fourier Transform electrochemical impedance spectroscopy

Using electrochemical impedance spectroscopy (EIS) based on the Fast Fourier Transform (FFT) method, we have investigated the adsorption properties of F^?, NO₃^? and CH₃COO^? on Au in aqueous electrolytes. The results show that these anions adsorb on Au in considerably small amounts. The adsorption reactions are associated with partial charge transfer across the interface. We present a theoretical framework to analyze these data. Certain previously reported impedance features of simple adsorption reactions are found as special cases of the present analysis. By combining theory and experiment, we determine the double layer capacitance, adsorption capacitance, and the amount of partial charge transfer as functions of the applied voltage. The theoretical framework of the present work can be applied to both cases of weak and strong adsorption, and can be extended to describe coadsorption of different anions.     

Potentiodynamic electrochemical impedance spectroscopy: lead underpotential deposition on tellurium

Lead underpotential deposition (upd) on tellurium and the effect of halides (iodide, bromide and chloride) in the upd and the monolayer anodic oxidation have been investigated with potentiodynamic electrochemical impedance spectroscopy (PDEIS). PDEIS gives, with a virtual spectrometer, the ac and dc responses of the electrochemical interface along the trajectory of a potential scan. The built-in analyser decomposes the total response into the constituents related to the elements of the equivalent electric circuit and gives the dependences on the potential of the double layer capacitance C_dl and the parameters of the Faraday impedance. No adsorption capacitance was found in the equivalent circuit of Pb upd on tellurium. Pb upd manifests itself in the increase of the double layer capacitance in the negative scan, additional C_dl increase in the positive scan below the oxidation potential of Pb adatoms and the decrease in C_dl observed during monolayer anodic stripping from the tellurium surface. The most significant halide effect has been found for iodide. Iodide stabilises the monolayer against the anodic oxidation.     

Transient electrical response of ion-exchange membranes with fixed-charge due to ion adsorption. A network simulation approach

The transient electrical properties of ion-exchange membranes with inhomogeneous fixed-charge distributions due to ion adsorption processes, have been investigated using the network simulation method. A network model is proposed for the Nernst–Planck and Poisson equations describing the transport of a binary electrolyte through an ion-exchange membrane and the two diffusion boundary layers on both sides of the membrane. An electric circuit simulation program is used to solve the network model in order to obtain the transient electrical properties of a system where the fixed-charge concentration of the membrane is due to an anion adsorption process. The chronoamperometric and chronopotentiometric responses of the whole membrane system have been simulated for the case of a membrane with surface charge described by a Langmuir kinetics.     

二氧化钛纳米管的制备及其对成骨细胞增殖和碱性磷酸酶活性的影响

目的 探讨阳极氧化过程中不同工艺条件对TiO_2纳米管形貌的影响,并对TiO_2纳米管对成骨细胞增殖和碱性磷酸酶相对活性的影响进行初步评定.方法 采用阳极氧化法在钛基底表面制备TiO_2纳米管,通过控制阳极氧化电压(5、15、20和25 V)、作用时间(1.5和3 h)以及阳极化后冲洗工艺(是否行超声处理),得到不同结构的TiO_2纳米管,扫描电镜观察TiO_2纳米管管径和管长.以未行阳极氧化的钛试件为对照组,以行阳极氧化(阳极氧化电压为15 V,作用时间为3 h,反应后行超声清洗)的钛试件为纳米管组;在两组钛试件表面培养成骨细胞,用扫描电镜观察接种2 h后的细胞形态;用甲基噻唑基四唑(MTT)检测培养24、48、96 h后细胞的增殖情况;用碱性磷酸酶半定量测试培养7、14、21 d后细胞的碱性磷酸酶活性.结果 阳极氧化的电压可以影响TiO_2纳米管的管径和管长.细胞在纳米管表面的铺展范围大于对照组.培养96 h后纳米管组吸光度值为(0.62±0.02).对照组为(0.55±0.03),两组差异有统计学意义(P＜0.05).21 d后纳米管组碱性磷酸酶相对活性[(130.8±5.1)A405/mg]与对照组[(109.6±4.5)A_(405)/mg]的差异有统计学意义(P＜0.05).结论 阳极氧化的工艺条件,特别是电压可以控制TiO_2纳米管形貌,TiO_2纳米管结构可促进成骨细胞的早期黏附、增殖和碱性磷酸酶的活性.     

Ion-channel-mimetic sensor for trivalent cations based on self-assembled monolayers of thiol-derivatized 4-acyl-5-pyrazolones on gold

Self-assembled monolayers (SAMs) of thiol-derivatized 4-acyl-5-pyrazolone on gold electrodes were used for ion-channel-mimetic sensing of inorganic cations. The SAMs on gold electrodes were characterized with reductive-desorption and capacitance measurements. The pH dependence of the cyclic voltammograms (CVs) of [Fe(CN)₆]^3? and [Ru(NH₃)₆]³⁺ as electroactive markers suggested the protonation not only of the β-diketone part but also of the nitrogen moiety in the pyrazolone. With regard to the voltammetric responses to metal cations, an increase and a decrease in redox current of [Fe(CN)₆]^3?/4? and [Ru(NH₃)₆]^3+/2+, respectively, were observed with increasing concentrations of the trivalent cations examined (La³⁺, Gd³⁺, Yb³⁺ or Al³⁺) from 10^?6–10^?4 up to 10^?2 M at pH 5.5, at which the chelating group is present as its deprotonated form. In contrast, such responses were negligible in the presence of up to 10^?2 M divalent cation (Mg²⁺, Ca²⁺, Sr²⁺ or Ba²⁺), Li⁺ or Na⁺. The order of the magnitudes of the responses was Al³⁺>Yb³⁺≈Gd³⁺>La³⁺?divalent cations, which is quite similar to that of the stability of 1:1 and 1:2 complexes between a β-diketonate-type chelate and the metal ions. The highly selective responses to trivalent cations seem to reflect the selectivity of the chelating group as well as the large change in the surface charge induced by the complexation.