Modeling of electrocatalytic processes at conducting polymer modified electrodes

A mathematical model of electrocatalytic processes taking place at conducting polymer modified electrodes has been developed. The model takes into account the diffusion of solution species into a polymer film, diffusion of charge carriers within the film, and a chemical redox reaction within the film. The space- and time-resolved profiles for reactant and charge carrier concentration within the film, as well as dependencies of electric current on the concentration of solute species, reaction rate constant and thickness of a polymer layer have been obtained and discussed. It has been shown that, even at a relatively fast diffusion of charge carriers within the conducting polymer film, exceeding the diffusion rate of reactant by two orders of magnitude, electrocatalysis of solute species at conducting polymer modified electrodes proceeds within the polymer film rather than at the outer polymer/solution interface, i.e., electrocatalytic conversion follows a redox-mechanism rather than metal-like one. Based on the results obtained, optimization of reaction system parameters could be made for any particular case to get an optimum efficiency or reactant to product conversion.      

A glass capillary ultramicroelectrode with an electrokinetic sampling ability.

A glass capillary ultramicroelectrode (tip diameter approximately 1.2 microm) having an electrokinetic sampling ability is described. It is composed of a pulled glass capillary filled with an inner solution and three internal electrodes (Pt working and counter electrodes and an Ag/AgCl reference electrode). The voltammetric response of the capillary electrode is based on electrokinetic transport of analyte ions from the sample solution into the inner solution across the conical tip. It was found that the electrophoretic migration of analytes at the conical tip is faster than electroosmotic flow, enabling electrokinetic transport of analyte ions into the inner solution of the electrode. By using [Fe(CN)6]4- and (ferrocenylmethyl)trimethylammonium (FcTMA+) ions as model analytes, differential pulse voltammetric responses of the capillary electrode were investigated in terms of tip diameter of the capillary, sampling voltage, sampling time, detection limit and selectivity. The magnitude of the response depends on the size and charge of analyte ions. With a capillary electrode having a approximately 1.2-microm tip diameter, which minimizes non-selective diffusional entry of analytes, the response after 1 h sampling at +1.7 V is linearly related to [Fe(CN)6]4- concentration in the range of 0.50-5.0 mM with the detection limit of 30 microM. Application of a potential of the same sign as that of the analyte ion forces the analyte to move out from the electrode to the solution, enabling reuse of the same capillary electrode. The charge-selective detection of analytes with the capillary electrode is demonstrated for [Fe(CN)6]4- in the presence of FcTMA+.     

基于二级结构TiO_2纳米晶光阳极的高效染料敏化太阳能电池

本文报道了一种新型的二级结构TiO2纳米晶(nano-TiO2)光阳极的简单制备方法及其在高效染料敏化太阳能电池中的应用.通过添加适量TiCl4异丙醇溶液到传统nano-TiO2浆料中,可生成微米级nano-TiO2聚集体.该二级结构能有效提高光阳极光谱吸收和散射性能及电子传输和收集效率.基于这种结构光阳极的染料敏化太阳能电池光电性能有显著提高.在光阳极中将6μm厚传统nano-TiO2膜用相同厚度nano-TiO2聚集体替换,电池光电转换效率由5.03%提高到7.30%.进一步增加nano-TiO2聚集体的厚度能制备出更高光电转换效率的电池.     

Hydrodynamic chronocoulometry

The technique which we call hydrodynamic chronocoulometry was developed to fulfill our need to know the diffusion coefficient and concentration of oxygen in aqueous buffer solutions which we use for experiments with polymer-film modified electrodes. The utility of the technique is most apparent in its application to this type of analysis, a dissolved electroactive gas in solution. Standard solutions for these analyses are difficult to prepare and to store. Techniques for the simultaneous determination of diffusion coefficients and concentration exist, but some are difficult to use because the techniques themselves are complex or require changes of electrode or instrumentation. Hydrodynamic chronocoulometry promises to solve these problems through use of a single measurement at a single electrode, with a single analyte solution. The results are simple in form and straightforward analysis is possible.     

Insights into analyte electrolysis in an electrospray emitter from chronopotentiometry experiments and mass transport calculations

Insights into the electrolysis of analytes at the electrode surface of an electrospray (ES) emitter capillary are realized through an examination of the results from off-line chronopotentiometry experiments and from mass transport calculations for flow through tubular electrodes. The expected magnitudes and trends in the interfacial potential in an ES emitter under different solution conditions and current densities, using different metal electrodes, are revealed by the chronopotentiometry data. The mass transport calculations reveal the electrode area required for complete analyte electrolysis at a given volumetric flow rate. On the basis of these two pieces of information, the design of ES emitters that may maximize and those that may minimize analyte electrolysis during ES mass spectrometry are discussed.     

Dynamic response of gold nanoparticle chemiresistors to organic analytes in aqueous solution

We investigate the response dynamics of 1-hexanethiol-functionalized gold nanoparticle chemiresistors exposed to the analyte octane in aqueous solution. The dynamic response is studied as a function of the analyte-water flow velocity, the thickness of the gold nanoparticle film and the analyte concentration. A theoretical model for analyte limited mass-transport is used to model the analyte diffusion into the film, the partitioning of the analyte into the 1-hexanethiol capping layers and the subsequent swelling of the film. The degree of swelling is then used to calculate the increase of the electron tunnel resistance between adjacent nanoparticles which determines the resistance change of the film. In particular, the effect of the nonlinear relationship between resistance and swelling on the dynamic response is investigated at high analyte concentration. Good agreement between experiment and the theoretical model is achieved.     

Stripping voltammetry with the electrode material acting as a ‘built-in' internal standard

The oxidation peak of the bismuth layer of bismuth-film electrodes is used as an internal standard in anodic stripping voltammetry. Such use of the electrode material as a ‘built-in' internal standard simplifies the analytical protocol (as it obviates the need for an external standard or long calibration and standard additions procedures) while correcting for run-to-run variations due to uncertainties (e.g., changes in mass transport, surface area, or other variables). The new internal standard strategy relies on common in situ plated film electrodes, where the deposition of both the analyte and electrode material is subject to the same variations. Such one-step quantitation and improvements are illustrated for trace measurements of lead. The use of the intrinsic signal of the system for obtaining the concentration of the target analyte is unique not only for electroanalysis, but for analytical chemistry in general.     

Insights into electrodeposition of an inhibitor film and its inhibitive effects on calcium carbonate deposition

Polycarboxylic acid (PAA), a common scale inhibitor has demonstrated adsorption properties on stainless steel surfaces. An electrochemically based technique has been used to assess the extent of film formation. The presence of calcium and magnesium ions in the solution and the cathodic electrochemical activity at the metal surface have been shown to enhance the inhibitor film formation by promoting the transport of the inhibitor from the solution to the metal surface. The effect of the inhibitor film in retarding scale deposition is assessed using measurement of the deposition onto metal electrodes immersed in a supersaturated solution of CaCO(3). The practical implications of these findings are discussed.     

Theory and practice of electrochemical titrations with dual microband electrodes

Dual parallel microband electrodes, operated as a generator–collector pair and made by a simple, inexpensive mass-production method, have been used to implement a ‘titration’ method. The solution contains the electro-inactive analyte and a reagent from which the titrant can be generated electrochemically. The galvanostatically generated titrant is detected at the collector amperometrically. The collection efficiency is affected by the reaction between the titrant and the analyte. Determination of ascorbic acid by titration against ferricyanide is given as an example. The measurement is performed as follows: After application of the collector potential, the boundary conditions between measurements are renewed by a quick pulsed motion of the electrode assembly. Then the generator current is applied. Following an initial delay, the collector current increases as t^1/2, with slope and collection efficiency dependent on analyte concentration. This results in a fast and effective method for implementing some standard titration methods without the need for accurate volume measurement and reagent preparation. The accuracy is determined by the reproducibility of electrodes. The present work shows concentration measurements in the mM scale to ≈±10%, in a time of a few seconds. The highly stable extrapolation method used for numerical simulation of the generator–collector experiment, which takes into account the non-uniform current distribution over a microband electrode with a galvanostatic boundary condition, is developed using a conformal map. A good agreement between simulations and experimental results was obtained in voltammetric, potential step and generator–collector measurements. It is shown that a useful approximate calculation can be made rather easily by representing the problem in terms of a reaction layer in the conformal space.     

TiCl4 assisted formation of nano-TiO2 secondary structure in photoactive electrodes for high efficiency dye-sensitized solar cells

A new kind of photoactive electrodes with nanocrystalline TiO2(nano-TiO2)secondary structure is successfully prepared via a simple method of adding a small amount of TiCl4 2-propanol solution in conventional nano-TiO2 paste to form micro-sized nano-TiO2 aggregates.The benefits of this special structure include improved optical absorption,increased light scattering ability,and enhanced electron transport and collection efficiency.Dye-sensitized solar cells(DSCs)based on these photoactive electrodes show improved performance.The power conversion efficiency of the cells can be increased from 5.03%to 7.30%by substituting 6μm conventional nano-TiO2 thin film with the same thickness of as-prepared nano-TiO2 aggregates film in the photoactive electrodes.A higher power conversion efficiency of the cells can be obtained by further increasing the thickness of the nano-TiO2 aggregates film.     

强度调制光电流谱研究纳晶薄膜电极过程

用强度调制光电流谱研究半导体纳晶薄膜电极光生电荷的界面转移和输运动力学过程.从测量不同外加电压和不同硫化钠溶液浓度下CdSe纳晶薄膜电极的光电流响应得到了参数:归一化稳态光电流和表面态寿命,分析界面空穴的直接转移和通过表面态的间接转移过程.通过测量不同背景光强下TiO2纳晶薄膜电极的电子扩散系数研究电子输运过程.应用HCl化学处理方法明显增大了电子扩散系数,改善了电子在TiO2纳晶薄膜电极中的输运性能.     

Bismuth film electrode for anodic stripping voltammetric determination of tin

The bismuth film electrode (BiFE), in combination with anodic stripping voltammetry, offers convenient measurement of low concentrations of tin. The procedure involves simultaneous in situ formation of the bismuth film electrode on a glassy carbon substrate electrode, together with electrochemical deposition of tin, in a non-deaerated model solution containing bismuth ions, catechol as complexing agent and the metal analyte, followed by an anodic stripping scan. The BiFE is characterized by an attractive electroanalytical performance, with two distinct voltammetric stripping signals corresponding to tin, accompanied with low background contributions. Several experimental parameters were optimized, such as concentration of bismuth ions and catechol, deposition potential, deposition time and pH of the model solution. In addition, a critical comparison is given with bare glassy carbon and mercury film electrodes, revealing the superior characteristics of BiFE for measurement of tin. BiFE exhibited highly linear behavior in the examined concentration range from 1 to 100 μg L⁻¹ of tin (R² = 0.997), an LoD of 0.26 μg L⁻¹ tin, and good reproducibility with a calculated R.S.D. of 7.3% for 10 μg L⁻¹ tin (n = 10). As an example, the practical applicability of BiFE was tested with the measurement of tin in a real sample of seawater.     

Analyte-driven switching of DNA charge transport: de novo creation of electronic sensors for an early lung cancer biomarker

A general approach is described for the de novo design and construction of aptamer-based electrochemical biosensors, for potentially any analyte of interest (ranging from small ligands to biological macromolecules). As a demonstration of the approach, we report the rapid development of a made-to-order electronic sensor for a newly reported early biomarker for lung cancer (CTAP III/NAP2). The steps include the in vitro selection and characterization of DNA aptamer sequences, design and biochemical testing of wholly DNA sensor constructs, and translation to a functional electrode-bound sensor format. The working principle of this distinct class of electronic biosensors is the enhancement of DNA-mediated charge transport in response to analyte binding. We first verify such analyte-responsive charge transport switching in solution, using biochemical methods; successful sensor variants were then immobilized on gold electrodes. We show that using these sensor-modified electrodes, CTAP III/NAP2 can be detected with both high specificity and sensitivity (K(d) ～1 nM) through a direct electrochemical reading. To investigate the underlying basis of analyte binding-induced conductivity switching, we carried out F?rster Resonance Energy Transfer (FRET) experiments. The FRET data establish that analyte binding-induced conductivity switching in these sensors results from very subtle structural/conformational changes, rather than large scale, global folding events. The implications of this finding are discussed with respect to possible charge transport switching mechanisms in electrode-bound sensors. Overall, the approach we describe here represents a unique design principle for aptamer-based electrochemical sensors; its application should enable rapid, on-demand access to a class of portable biosensors that offer robust, inexpensive, and operationally simplified alternatives to conventional antibody-based immunoassays.     

Detection of Explosive Vapors: The Roles of Exciton and Molecular Diffusion in Real‐Time Sensing

Time‐resolved quartz crystal microbalance with in situ fluorescence measurements are used to monitor the sorption of the nitroaromatic (explosive) vapor, 2,4‐dinitrotoluene (DNT) into a porous pentiptycene‐containing poly(phenyleneethynylene) sensing film. Correlation of the nitroaromatic mass uptake with fluorescence quenching shows that the analyte diffusion follows the Case‐II transport model, a film‐swelling‐limited process, in which a sharp diffusional front propagates at a constant velocity through the film. At a low vapor pressure of DNT of ≈16 ppb, the analyte concentration in the front is sufficiently high to give an average fluorophore–analyte separation of ≈1.5 nm. Hence, a long exciton diffusion length is not required for real‐time sensing in the solid state. Rather the diffusion behavior of the analyte and the strength of the binding interaction between the analyte and the polymer play first‐order roles in the fluorescence quenching process.     

Extending the dynamic range of electrochemical sensors using multiple modified electrodes

Multiple electrodes, combined with a chemometric strategy to calibrate the measurement response, have been used for the determination of an analyte across a broader dynamic range than is possible with a single electrode. The model system used for the detection of copper comprised electrodes modified with a self-assembled monolayer. The electrodes were modified with the copper-complexing species (3-mercaptopropionic acid, thioctic acid, and the peptides cysteine and Gly-Gly-His) and copper was determined over concentrations ranging from nanomolar to millimolar using voltammetric analysis. We have demonstrated that by combining the calibration functions from the four electrodes a better estimate (i.e. with smaller variance) of the concentration of the analyte is obtained. Measurement uncertainty is expressed for independently prepared electrodes, which allows the possibility of commercial production and factory calibration. The principles of using multiple electrodes modified with recognition elements with different affinities for the target analyte to extend the dynamic range of sensors is a general one that could be applied to other analytes.     

Electrokinetic trapping and concentration enrichment of DNA in a microfluidic channel

We report a simple and efficient method for enriching the concentration of charged analytes within microfluidic channels. The method relies on exerting spatial control over the electrokinetic velocity of an analyte. Specifically, the electroosmotic (eo) velocity of the buffer solution in one region of the microfluidic system opposes the electrophoretic (ep) velocity of the analyte in the other region. This results in ep transport of DNA to the location where the ep and eo velocities are equal and opposite. Accumulation of the analyte occurs at this location. This enrichment method is conceptually distinct from field-amplification stacking, isotachophoresis, micelle sweeping, size exclusion, and other methods that have been previously reported. The method requires no complex microfabricated structures, no special manipulation of the solvent, and the enriched analyte remains in solution rather than being captured on a solid support. A concentration enrichment factor of 800 can be achieved for 20mer DNA in a fluidic channel having dimensions of 100 mum x 25 mum x 5 mm. The time required to achieve this level of enrichment is 300 s, and the enriched zone has a minimum width of 100 mum.     

Optically Transparent Electrodes Modified with Sulfide Ion‐Covered CdS Quantum Dots for Sensitive Photoelectrochemical Detection of Sulfhydryl‐Containing Compounds

A sensitive and selective photoelectrochemical sensor has been developed. Negatively charged, citrate‐capped CdS quantum dots (QDs) are assembled layer‐by‐layer onto indium tin oxide (ITO) electrodes precoated with positively charged poly(diallyldimethylammonium chloride). By exposing the modified photoelectrochemical electrodes to a Na₂S solution, QDs can be covered with excess S^2? ions. The weakly bound S^2? ions are easily replaceable with a sulphydryl‐containing analyte, and the photocurrent decrease is proportional to the analyte concentration. The detection limit is 0.4 nM for cysteine with a linear range of 1.0–100.0 nM. The level of cysteine in human serum samples has been quantified.     

Electrochemical analysis of d-penicillamine using a boron-doped diamond thin film electrode applied to flow injection system

The electroanalysis of d-penicillamine in 0.1 phosphate buffer (pH 7) was studied at a boron-doped diamond thin film (BDD) electrode using cyclic voltammetry as a function of concentration of analyte and pH of analyte solution. Comparison experiments were performing using a glassy carbon (GC) electrode. The BDD electrode exhibited a well-resolved and irreversible oxidation voltammogram, but the GC electrode provided only an ill-defined response. The BDD electrode provided a linear dynamic range from 0.5 to 10 mM and a detection limit of 25 muM (S/B>/=3) in voltammetric measurement. It was also found that the peak potentials were decreased when the pH of the analyte solution was increased. In addition, penicillamine has been studied by hydrodynamic voltammetry and flow injection analysis with amperometric detection using the BDD electrode. The flow injection analysis results at the diamond electrode indicated a linear dynamic range from 0.5 to 50 muM and a detection limit of 10 nM (S/N approximately 4). The proposed method was applied to determine d-penicillamine in dosage form (capsules), the results obtained in the recovery study (255+/-2.50 mg per tablet) were comparable to those labeled (250 mg per tablet).     

脉冲溶出伏安法中汞膜碳纤维电极性能的研究

采用脉冲阳极溶出伏安法,研究了汞膜碳纤维电极测定痕量金属离子的行为,并与汞膜玻碳电极和悬汞电极进行了系统的对比,显示了碳纤维电极在静止态溶液、极稀溶液、有机溶液和双电极体系中具有特殊的优越性。本文对该电极的制作,预处理和实际测试条件进行了探讨。     

Controlling the charge of pH-responsive redox hydrogels by means of redox-silent biocatalytic processes. A biocatalytic off/on switch

Coupling of redox-silent biocatalytic processes for analyte detection with enzyme-catalyzed redox reactions for signal generation is proposed by the modulation of electrostatic interactions between a pH-responsive polymer and a redox enzyme to control the off–on transition for electrochemical signal generation. Glassy carbon electrodes are modified with a poly(vinyl)imidazole Os(bipyridine)₂Cl redox hydrogel film entrapping urease and PQQ-dependent glucose dehydrogenase, while glucose is present in the solution. The off–on transition is based on the detection of urea as model analyte which is hydrolyzed to ammonia by urease within the hydrogel film concomitantly increasing the local pH value thus invoking deprotonation of the imidazole groups at the polymer backbone. The decrease of positive charges at the polymer decreases electrostatic repulsion between the polymer and the positively charged PQQ-dependent glucose dehydrogenase. Hence, electron transfer rates between polymer-bound Os complexes and PQQ inside the enzyme are enhanced activating electrocatalytic oxidation of glucose. This process generates the electrochemical signal for urea detection.