A dual-electrode approach for highly selective detection of glucose based on diffusion layer theory: experiments and simulation

A dual-electrode configuration for the highly selective detection of glucose in the diffusion layer of the substrate electrode is presented. In this approach, a glassy carbon electrode (GCE, substrate) modified with a conductive layer of glucose oxidase/Nafion/graphite (GNG) was used to create an interference-free region in its diffusion layer by electrochemical depletion of interfering electroactive species. A Pt microelectrode (tip, 5 microm in radius) was located in the diffusion layer of the GNG-modified GCE (GNG-G) with the help of scanning electrochemical microscopy. Consequently, the tip of the electrode could sense glucose selectively by detecting the amount of hydrogen peroxide (H2O2) formed from the oxidization of glucose on the glucose oxidase layer. The influences of parameters, including tip-substrate distance, substrate potential, and electrolyzing time, on the interference-removing efficiency of this dual-electrode approach have been investigated systematically. When the electrolyzing time was 30 s, the tip-substrate distance was 1.8 a (9.0 microm) (where a is the radius of the tip electrode), the potentials of the tip and substrate electrodes were 0.7 V and 0.4 V, respectively, and a mixture of ascorbic acid (0.3 mM), uric acid (0.3 mM), and 4-acetaminophen (0.3 mM) had no influence on the glucose detection. In addition, the current-time responses of the tip electrode at different tip-substrate distances in a solution containing interfering species were numerically simulated. The results from the simulation are in good agreement with the experimental data. This research provides a concept of detection in the diffusion layer of a substrate electrode, as an interference-free region, for developing novel microelectrochemical devices.     

极性和非极性溶剂在聚乙烯中的无限稀释扩散系数的测定

 气相法研究小分子溶剂与高分子聚合物材料之间的相互作用是一个快速、准确、方便的方法 ,可以测量多种小分子溶剂在聚合物中的溶解、扩散参数。通过气相法测定了 5种小分子溶剂 (正己烷、正庚烷、正癸烷、乙醇和水 )在固定液聚乙烯中的保留时间和半峰宽 ,运用vanDeemter模型进行数据处理 ,得到上述 5种小分子在聚乙烯中的无限稀释扩散系数 ,获得了十分有意义的结果。     

Diffusion in dilute aqueous acetic acid solutions at 25°C

A conductimetric technique has been used to measure diffusion coefficients for aqueous solutions of acetic acid at concentrations from 0.002 to 0.02 mol-dm^–3 at 25°C. The acetic acid component diffuses more rapidly at lower concentrations where a higher proportion of the slower acid molecules are converted by dissociation to acetate ions and highly mobile hydrogen ions. The observed concentration dependence of the diffusion coefficient verifies the limiting law for weak electrolyte diffusion. A new type of conductimetric diffusion cell with several practical advantages over earlier designs is described together with an improved procedure for the conductimetric determination of accurate diffusion coefficients for weak electrolyte systems.     

Electron Transport Properties of Ions in Aqueous Solutions of Sodium Selenite

Ion diffusion kinetics has been studied using the data of conductivity measurements for aqueous solutions of sodium selenite with different concentrations and at different temperatures. Molecular and ionic self-diffusion coefficients have been determined for infinitely dilute solutions in the temperature range 288 K-313 K. The limiting values of ion mobility and changes in the energies of translation of water molecules from ions’ hydration shell have been found. At elevated temperatures, ΔE _tr ⁰ increases for both ions in direct proportion to the crystallographic radius of the latter. Ion hydration numbers at 298 K have been calculated. The results of this study are interpreted in the light of Samoilov’s theory on positive and negative hydration of ions.Original Russian Text Copyright © 2004 by L. T. Vlaev and S. D. Genieva__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 5, pp. 870–876, September–October, 2004.     

The theory of non-Cottrellian diffusion on the surface of a sphere or truncated sphere.

A method is developed whereby spherical and other particles can be derivatised with electroactive species on their surface and then immobilised on the surface of an electrode. The chronoamperometric and voltammetric responses in the limit of reversible electrode kinetics are modelled using a theory of charge movement over the surface of the spheres where this movement is considered as a diffusional process. The model is extended to include different distributions of sphere radii and to model the scenario of truncated spheres resting on the electrode surface. It is found that a good estimation of the truncation angle can be found by fitting the experimental data with theoretical predictions.     

Ternary mutual diffusion coefficients of ZnCl2−KCl−H2O at 25°C by Rayleigh interferometry

Mutual diffusion coefficients and densities were measured for aqueous ZnCl₂–KCl mixtures at 25° by using free-diffusion Rayleigh interferometry and pycnometry, respectively. The ZnCl₂ concentrations were fixed at 1.5 mol-dm^–3, whereas those of KCl were 0.5, 1.25, 2.0, or 4.0 mol-dm^–3. This corresponds to a half charged zinc-chlorine storage battery at various suporting electrolyte concentrations. The main-term coefficient of ZnCl₂ only varies by 10% with KCl concentration, whereas that of KCl varies by about 22%. The ZnCl₂ cross-term coefficient remains small and positive; in contrast the KCl cross-term coefficient goes through a maximum and is negative at high and low KCl concentrations. At KCl concentrations of 0.5 and 4.0 mol-dm^–3, solutions with the KCl c0 are statically and dynamically (diffusively) unstable at the top and bottom of the boundary. Evaluation of the parameters of the non-linear least-squares solution to the diffusion equation is difficult for the 1.25 mol-dm^–3 KCl case, since this system has nearly equal eigenvalues in its diffusion coefficient matrix.     

Efficient synthesis of magnetic nanoparticle-Musa acuminata peel composite for the adsorption of anionic dye

The impregnation of magnetite (Mt) nanoparticle (NPs) onto Musa acuminata peel (MApe), to form a novel magnetic combo (MApe-Mt) for the adsorption of anionic bromophenol blue (BPB) was studied. The SEM, EDX, BET, XRD, FTIR and TGA were used to characterize the adsorbents. The FTIR showed that the OH and CO groups were the major sites for BPB uptake onto the adsorbent materials. The average Mt crystalline size on MApe-Mt was 21.13 nm. SEM analysis revealed that Mt NPs were agglomerated on the surface of the MApe biosorbent, with an average Mt diameter of 25.97 nm. After Mt impregnation, a decrease in BET surface area (14.89 to 3.80 m²/g) and an increase in pore diameter (2.25–3.11 nm), pore volume (0.0052–0.01418 cm³/g) and pH point of zero charge (6.4–7.2) was obtained. The presence of Pb(II) ions in solution significantly decreased the uptake of BPB onto both MApe (66.1–43.8%) and MApe-Mt (80.3–59.1%), compared to other competing ions (Zn(II), Cd(II), Ni(II)) in the solution. Isotherm modeling showed that the Freundlich model best fitted the adsorption data (R² > 0.994 and SSE < 0.0013). In addition, maximum monolayer uptake was enhanced from 6.04 to 8.12 mg/g after Mt impregnation. Kinetics were well described by the pseudo-first order and liquid film diffusion models. Thermodynamics revealed a physical, endothermic adsorption of BPB onto the adsorbents, with ΔH^o values of 15.87–16.49 kJ/mol, corroborated by high desorption (over 90%) of BPB from the loaded materials. The viability of the prepared adsorbents was also revealed in its reusability for BPB uptake.     

Proton transfer in nafion membrane by quantum chemistry calculation

We estimated the energy barriers of proton transfers in the systems of (CF₃SO₃/H/SO₃CF₃)⁻ and (CF₃SO₃/H/H₂O/SO₃CF₃)⁻ as models of a water-swollen Nafion membrane by an ab initio density functional quantum calculation method with consideration of the hydration effect. As a result, the proton transfer between the SO sites, which is accompanied by one water molecule, was found to be one of the proton-transfer mechanisms in the water-swollen Nafion membrane; that is, the surface diffusion mechanism was found to be important for the proton transfer in that membrane. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1905–1914, 2004     

Flow-injection determination of inorganic forms of nitrogen by gas diffusion and conductimetry

A flow-injection—conductimetric method was applied to the determination of ammonia, nitrate and nitrite at concentrations down to 5, 20 and 20 ng ml^?1, respectively. Ammonia was determined by merging the injected sample with an alkaline solution (NaOHEDTA) and passing the mixture through a diffusion cell. The ammonia released was collected by a flowing stream of deionized water that passed through a conductance flow cell. Nitrate and nitrite concentrations were determined after reduction to ammonia in alkaline medium using a column filled with metallic zinc. The ammonia produced was then measured as described above. About 60 samples per hour can be processed with a relative standard deviation of about 1%. Satisfactory agreement was observed between results for ammonia in samples of natural water and nitrate in tap and mineral water determined by the proposed method and by standard spectrophotometric procedures. Speciation can be achieved by adding sulphanilic acid to remove nitrite from the sample and determining the ammonia without the use of the column.     

Oxygen transport in La<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>Co<Subscript>1−<Emphasis Type="Italic">y</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>O<Subscript>3−δ</Subscript>

The surface exchange coefficient and chemical diffusion coefficient of oxygen for the perovskites La_0.6Sr_0.4Co_1–yFe_yO_3– (y=0.2, 0.5 and 0.8) were measured using the conductivity relaxation technique. Measurements were performed between 600 and 800 °C in an oxygen partial pressure range between 10^–4 and 1 bar. Both transport coefficients decrease markedly with decreasing oxygen partial pressure below about 10^–2 bar at all temperatures. This is attributed to ordering of oxygen vacancies. Implications for using La_0.6Sr_0.4Co_1–yFe_yO_3– as an oxygen separation membrane are discussed.Presented at the OSSEP Workshop Ionic and Mixed Conductors: Methods and Processes, Aveiro, Portugal, 10-12 April 2003