Metal Nanoparticles on Polymer Surfaces: 5. Catalytic Activity of Colloidal Platinum Films Incorporated in Polystyrene Surface Layer

The fundamental possibility to enlarge Pt nanoparticles in monolayer ensembles formed on polystyrene surfaces by the adsorption from hydrosol in solution of isopropanol and K₂PtCl₄ is demonstrated for the first time. The enlargement of “seeding” nanoparticles is performed after their preliminary incorporation (partial embedding) into the polymer surface layer by the annealing of a system within the range between “surface” and “bulk” glass transition temperatures of polystyrene. It is shown that a colloidal film of metallic platinum with a thickness up to 200 nm is formed in the course of enlargement and it is mechanically fixed in the polymer surface layer. Such a system exhibits, over a long time, high catalytic activity in the model reaction of methyl viologen reduction with hydrogen.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 3, 2005, pp. 398–403.Original Russian Text Copyright © 2005 by Rudoy, Sukhov, Dement’eva, Abkhalimov, Vereshchagina, Kartseva, Ershov.     

Dynamic fluorescence imaging analysis to investigate the cholesterol recruitment in lipid monolayer during the interaction between β-amyloid (1–40) and lipid monolayers

Extracellular β-amyloid (Aβ) deposit is considered as one of the primary factors in inducing Alzheimer's disease (AD). However, the mechanism of Aβ deposition on the cell membrane and the induced cytotoxicity are still unclear. On the basis of the previous reports and results, we propose the “Recruiting Hypothesis” on the interaction between the plasma membrane and Aβ. Recently, many studies focused on cholesterol, which is considered as an important factor for AD. The most challenging issue in studying the cholesterol is non-ideal mixing behavior and non-dynamic analysis. In the present study, we investigated the cholesterol recruitment in the lipid monolayer during the interaction between β-amyloid peptides Aβ (1–40) and lipid monolayers by dynamic fluorescent imaging analysis. Results from lipid monolayer trough studies showed that the rate of Aβ adsorption onto lipid monolayer is mainly due to the electrostatic effect which is sensitive to the lipid monolayer composition. From the fluorescence imaging analysis, the interaction of Aβ with lipid monolayer containing negative charge lipid and cholesterol brings out the recruiting behavior of the cholesterol and reduces the fluidity of lipid. The present study not only demonstrates the technical application for monitoring the dynamic molecular behaviors at the interface but also reveals the roles to distinguish lipid molecules on the Aβ–membrane interaction.     

Nucleation and growtn of polythiophene films on gold electrodes

The nucleation and growth of polythiophene films on gold electrodes has been studied using potentiostatic steps. The mechanism has been deduced and estimates made of the kinetic parameters. Dissolution of the gold substrate at potentials where thiophene polymerisation occurs is suppressed by the initial rapid formation of a monolayer of polymer. The data indicate that formation of bulk film occurs by the instantaneous nucleation and three-dimensional growth of polymer on top of this monolayer. Rate constants for growth parallel to the surface on the bare gold substrate and the covering polymer layer are surprisingly very similar. Growth perpendicular to the surface is slightly more rapid, typically by a factor of 1.5–3, although it is less dependent on potential. The high density of nuclei results in their overlap at an early stage, after which growth is only possible perpendicular to the surface. Within a narrow potential range, the observation of maxima and minima in current-time transients is interpreted in terms of the “death” and “rebirth” of growing centres.     

Lower Rim Substituted tert-Butyl calix[4]arene (part VII): Ionophoric Properties of Calix[4]arene-crown-6 Derivatives in Plasticized PVC-Membrane Electrodes and in Solution

The synthesis and characterization of p-tert-butylcalix[4]arene-crown-6 derivatives (compounds 2–9) are presented. Their ability to complex alkali, alkaline earth and some “softer” cations was investigated by using these compounds as ionophores in ion-selective membrane electrodes and determining the potentiometric selectivity coefficients and the complex stability constants in the membrane. The selectivities of these compounds depend on the size and nature of the substituents on the distal phenolic oxygens. The complexing properties of the two cone and partial cone conformers of compound 2 in the PVC membrane were compared with those in acetonitrile solution studied by UV absorption spectrophotometry and with alkali metal picrate liquid–liquid extraction.This revised version was published online in July 2005 with a corrected issue number.     

A mercury(II) ion-selective electrode based on neutral salicylaldehyde thiosemicarbazone

A new ion-selective PVC membrane electrode based on salicylaldehyde thiosemicarbazone as an ionophore is developed successfully as sensor for mercury(II) ions. The electrode shows excellent potentiometric response characteristics and displays a linear log[Hg²⁺] versus EMF response over a wide concentration range of 1.778×10⁻⁶-1.0×10⁻¹ M with Nernstian slope of 29 mV per decade with the detection limit of 1.0×10⁻⁶ M. The response time of the electrode is less than 30 s and the membrane electrode operates well in the pH range of 1.0-3.0. The lifetime of the sensor is about 2 months. The electrode shows better selectivity towards Hg²⁺ ions in comparison with the alkali, alkaline and some heavy metal ions; most of these metal ions do not show significant interference (K^Pot_Hg,M values of the order of 10⁻³-10⁻⁴). The present sensor showed comparable or even better performance vis-à-vis similar PVC based ion-selective electrodes reported in literature. The sensor was also applied as an indicator electrode for potentiometric titration of Hg²⁺ions with I⁻ and Cr₂O₇²⁻.     

Preparation and catalytic activity of titanium silicalite-1 zeolite membrane with TPABr as template

The preparation of the supported titanium silicalite-1 (TS-1) zeolite membrane with inexpensive tetrapropylammonium bromide (TPABr)/weak base synthesis system was studied by three methods, and the catalytic activity of the obtained TS-1 zeolite membrane was evaluated with the oxidation of 2-propanol (IPA) under pervaporation condition. It was found that TS-1 zeolite membrane could be successfully prepared with “seeding” or “seeding in situ” method, but could not be achieved with “in situ” method. Adding a little amount of promoter ions of PO₄³⁻ into the synthesis gel was of benefit to the catalytic activity of the prepared TS-1 zeolite membrane, but had no obvious effect on the membrane layer formation on the mullite porous support. For “seeding” method, the membrane prepared with the synthesis gel having molar composition of SiO₂:0.1TPABr:0.9Et₂NH:0.03TiO₂:80H₂O:0.06H₃PO₄ at 150 °C for 48 h showed the highest oxidation conversion of IPA of 72% accompanied by a flux of 0.35 kg/m² h. Further more, much higher IPA oxidation conversion of 76% accompanied by a flux of 0.65 kg/m² h was obtained for the TS-1 zeolite membrane prepared with the same synthesis gel by “seeding in situ” method at 150 °C for 72 h.     

Silver and lead all-plastic sensors—polyaniline vs. poly(3,4-ethyledioxythiophene) solid contact

Silver and lead selective all-plastic ion-selective electrodes were obtained using poly(vinyl chloride)-based membranes and either poly(3,4-ethylenedioxythiophene) or polyaniline dispersion cast on an insulating plastic support as transducer and electrical lead. The effect of interactions of applied conducting polymer with analyte ions on potentiometric responses was evaluated and correlated with changes in elemental composition and element distribution within the ion-selective membrane and the conducting polymer transducer revealed in course of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) experiments. In the case of silver selective electrodes, potentiometric responses obtained are much dependent on the oxidation state of the polymer placed beneath the ion-selective membrane. For semi-oxidized polymer (poly(3,4-ethylenedioxythiophene)-based electrodes, linear responses with detection limit equal to 10^−5.4 M were obtained. For a more oxidized polyaniline (of higher conductivity), although the electrodes were pretreated exactly in the same way and tested in parallel, super Nernstian potential slope was recorded within the AgNO₃ activities range form 10⁻⁶ to 10⁻⁷ M. These responses were consistent with results of LA-ICP-MS, revealing profoundly higher silver signals intensities for poly(3,4-ethylenedioxythiophene) underlying silver selective membrane. It seems highly probable that silver is accumulated in this polymer layer as Ag⁰ due to spontaneous redox reaction leading to oxidation of the polymer; however, this process requires also the presence of silver ions at the interface. In fact, when reduced (deprotonated) polyaniline was used as transducer, the potentiometric responses of the sensor were, within the range of experimental error, the same as obtained for poly(3,4-ethylenedioxythiophene)-based sensor. On the other hand, for lead(II) selective sensors, the difference in responses of electrodes prepared using poly(3,4-ethylenedioxythiophene) or polyaniline was less pronounced, which is in accordance with the elemental composition of these sensors.     

Reducing the interference from CO2 or organic acids in ion-selective polymer membrane sensors having a field-effect transistor as internal reference element

Changes in sample concentrations of CO₂ or organic acids cause potential instabilities when polymer membranes are directly applied to the surface of ion-selective field-effect transistors (ISFETs). Currently used designs avoid this well-documented effect by placing a layer of aqueous buffer between polymeric membrane and ISFET serving as internal reference element. Here, we propose another solution to the problem. In order to compensate for the effect of pH changes on the ISFET threshold voltage, a double membrane is applied whose inner layer is pH-sensitive, while the outer layer exposed to the sample is a conventional ion-selective membrane. It is shown that this approach strongly reduces the earlier-mentioned interference effects.     

Electrochemical atomic layer epitaxy (ECALE)

Electrodeposition holds promise as a low cost, flexible room temperature technique for the production of II-VI compound semiconductors. Previous studies, however, have resulted in the production of polycrystalline deposits in every case. This paper describes a new method, developed in this laboratory, for depositing these materials epitaxially. The method involves the alternate deposition of the component elements a monolayer at a time. To limit deposition to a monolayer, underpotential deposition (UPD) is employed. UPD occurs because of the enhanced stability provided by bond formation between the II and VI elements, relative to formation of bulk elemental deposits. This method is the electrochemical equivalent of atomic layer epitaxy (ALE), and is thus referred to as “electrochemical atomic layer epitaxy” (ECALE). This paper describes the first example of the ECALE method, involving the thin-layer electrodeposition of CdTe on a Au polycrystalline electrode.     

A solid-state ion-selective electrode with an ionic-electronic transducer for determining chlordiazepoxide

A solid-state ion-selective electrode for determining chlordiazepoxide is described. A characteristic feature of the electrode is the presence of an intermediate conducting polymeric layer (transducer) of poly(o-aminophenol) between the ion-selective membrane with ionic conduction and a Pt conductor. The properties of ion-selective electrodes with transducers are compared with those of ISEs without transducers (coated-wire electrodes, CWEs). The most important distinctive features of ion-selective electrodes with transducers are the high stability of their potentials in time and their small sizes. The main performance characteristics of the electrode and the scope of its application are found.Translated from Zhurnal Analiticheskoi Khimii, Vol. 60, No. 1, 2005, pp. 74–78.Original Russian Text Copyright © 2005 by Gorelov, Ryasenskii, Kartamyshev, Fedorova.     

Mass transfer characteristics for VOC permeation through flat sheet porous and composite membranes: The impact of the different membrane layers on the overall membrane resistance

In this paper, the mass transfer coefficients for trichloroethylene (TCE), toluene (TOL) and dimethyl sulfide (DMS) are experimentally determined for different porous and composite membranes. For polypropylene/polyvinylidenedifluoride porous layer/thin film polydimethylsiloxane dense layer composite membranes, membrane mass transfer coefficients are 2.55E−03, 2.82E−03 and 2.90E−03 m/s for TCE, TOL and DMS in N₂ at 30.0 ± 0.1 °C, respectively. For polyester/polyacrylonitrile porous layer/thin film polydimethylsiloxane dense layer composite membranes, they are higher, namely 4.28E−03, 4.55E−03 and 4.81E−03 m/s for TCE, TOL and DMS in N₂ at 30.0 ± 0.1 °C, respectively. Analysis of the contribution of the dense layer of both composite membranes to the total membrane resistance for mass transfer, showed that this contribution was small for both composite membranes. The higher mass transfer coefficients of the thin film polydimethylsiloxane composite membranes from this study in comparison to others from the literature are primarily due to improvement of the mass transfer characteristics of the porous layer. Analysis of the mass transfer characteristics of the different porous layers of which the total porous layer is composed, showed that the contribution of the porous “backing” layer for mechanical support can be substantial in comparison to the porous layer in contact with the dense layer.     

Role of ion-selective membranes in the carbon balance for CO2 electroreduction via gas diffusion electrode reactor designs

In this work, the effect of ion-selective membranes on the detailed carbon balance was systematically analyzed for high-rate CO₂ reduction in GDE-type flow electrolyzers. By using different ion-selective membranes, we show nearly identical catalytic selectivity for CO₂ reduction, which is primarily due to a similar local reaction environment created at the cathode/electrolyte interface via the introduction of a catholyte layer. In addition, based on a systematic exploration of gases released from electrolytes and the dynamic change of electrolyte speciation, we demonstrate the explicit discrepancy in carbon balance paths for the captured CO₂ at the cathode/catholyte interface via reaction with OH⁻ when using different ion-selective membranes: (i) the captured CO₂ could be transported through an anion exchange membrane in the form of CO₃²⁻, subsequently releasing CO₂ along with O₂ in the anolyte, and (ii) with a cation exchange membrane, the captured CO₂ would be accumulated in the catholyte in the form of CO₃²⁻, while (iii) with the use of a bipolar membrane, the captured CO₂ could be released at the catholyte/membrane interface in the form of gaseous CO₂. The unique carbon balance path for each type of membrane is linked to ion species transported through the membranes.

In this work, the effect of ion-selective membranes on the detailed carbon balance was systematically analyzed for high-rate CO₂ reduction in GDE-type flow electrolyzers.     

Electrochemical Sensors for Sulfur- and Lead-Containing Gases: Effect of Nonstoichiometry of Measuring Electrodes

In accordance with the fact that the region of homogeneity of lead sulfide is two-sided, i.e. it includes compositions with both sulfur excess and lead excess relative to the stoichiometric composition, it is shown that a sensor with a Pb_{1 ±} S measuring electrode (ME) may be used for analyzing both sulfur- and lead-containing media. The general principle of operation of electrochemical sensor Me, Pb/PbCl₂–CaCl₂/Pb_{1 ±} S, is based on measuring EMF of an electrochemical circuit with a lead-conducting electrolyte at a specified sulfur vapor pressure above the reference electrode. At a fixed temperature, the cell's EMF is uniquely defined by the sulfur content on ME. The studies are done in the temperature interval 285 to 300°C where characteristics of the solid electrolyte used are optimum; its upper boundary is limited by mp of the lead reference electrode (327°). The interval of determinable concentrations of sulfur- and lead-containing gases is 10^–1 to 10^–9 vol %. If analysis is performed in a dynamic mode (in a carrier gas flow), the limiting stage is the adsorption of gas molecules on the ME surface and the sensor sensitivity increases after modifying the surface by various techniques and also due to the ME composition altering only in a surface layer. The sensor's working characteristics are affected mostly by the degree of deviation of the ME material from stoichiometry.     

Effect of Inorganic Ions on Oleate Adsorption at a Nigerian Hematite–Water Interface

Using “pure” natural hematite selected from a high silica Nigerian hematitic ore, oleate adsorption densities at the hematite–water interface were determined in the presence of various inorganic ions (anions and cations) of different charges and at varying concentrations. Adsorption density was determined using electrical conductivity measurements. The specific surface area of the hematite particles was determined using the method of adsorption of paranitrophenol in aqueous solution. Inorganic ions in solution depressed oleate adsorption at the aqueous hematite surface. The charge of the ion proved to be the dominant factor determining the depression of oleate adsorption. Ionic strength also was an influence, up to a limiting value at which monolayer oleate coverage of the hematite surface occurred. The inorganic ions in solution are considered to function through nonspecific adsorption in the diffuse region of the electric double layer.     

Adding Plasticizer Dispersible Conducting Polymer to the Ion-selective Membrane Composition – Revitalizing Single Piece Ion-selective Electrodes Concept

Typically, ion-selective electrodes used in current triggered electrochemical sensing apply a conducting polymer layer covered with an ion-selective membrane. In this work we propose an ion-selective membrane containing a dispersed conducting polymer. Thus obtained system allows elimination of the Achilles hell of heterogeneous ion-selective membranes containing solid particulates dispersed within the ion-selective polymeric membrane. The herein proposed system, even for high conducting polymer loading equal to 5 % w/w, is characterized with insensitivity towards redox interferences, as well as potentiometric detection limits, selectivity well comparable with that for other ion-selective electrodes constructions. Under voltammetric conditions, with increasing loading of the conducting polymer in the membrane cathodic peak potentials are shifted towards more negative values, yet the linear dependence of the peak potential on logarithm of concentration of the analyte in the solution is preserved.     

The influence of a self-assembled monolayer on the activity of rough gold for glucose oxidation

Gold surface has been roughened by amalgamation and tested for self-assembled monolayer (SAM) formation and glucose oxidation. The rough gold undergoes structural changes at elevated temperatures, which lead to loss of activity for glucose oxidation and lower affinity for the self-assembly. The transformation of “active” into “inactive” surface can be prevented by SAM formation. The SAM modified gold exhibits high activity for glucose oxidation. These results are important especially for those studies, which use rough gold modified by SAM for enzyme immobilisation and further glucose oxidation, since the gold activity itself was so far neglected.     

On the formation and structure of self-assembling monolayers. I. A comparative atr-wettability study of Langmuir—Blodgett and adsorbed films on flat substrates and glass microbeads

Organized oleophobic monolayers of several long chain compounds and steroid derivatives produced on flat solid substrates by spontaneous adsorption from organic solutions are compared with Langmuir—Blodgett (LB) monolayers transferred on identical substrates from the water-air interface. Quantitative infrared ATR and polarized ATR spectroscopy, and wettability measurements are used to correlate the various films and to determine their molecular density and orientation, mode of film-to-surface binding, and other structural characteristics. Formation of oleophobic adsorbed monolayers on a model powder substrate—smooth glass microbeads—is also investigated. It is concluded that, irrespective of the mode of film-to-surface binding (ionic, covalent, or hydrogen bonding), and the nature of the substrate (Ge, Si, ZnSe, glass slides, glass microbeads), saturation of the adsorption leads in all studied systems to the formation of tightly packed and highly oriented monolayers, structurally equivalent to LB monolayers of same or similar compounds deposited on the bare surfaces of the respective substrates. These findings are interpreted in terms of a cooperative surface process leading to aggregation of molecules into a characteristic “monolayer phase.” Significant structural differences may develop in LB built-up films thicker than one monolayer. A mechanism for the formation of covalently bonded silane monolayers is proposed.     

All-plastic, disposable, low detection limit ion-selective electrodes

A novel concept for all-plastic and all-solid-state ion-selective electrodes (ISEs) is introduced. Planar, flexible ion-selective electrodes, comprising only polymeric materials, with no internal solution, were obtained. The cast conducting polymer layer (obtained from aqueous suspension) was covered with a solvent polymeric based membrane to obtain a planar all-plastic sensor. The conducting polymer layer served both as electrical contact and as ion-to-electron transducer. To illustrate this concept, the conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) ions (PEDOT-PSS, Baytron P) was chosen. Due to interaction, analyte cations-poly(4-styrenesulfonate) anions, an extended linear range of potentiometric responses was obtained, with lowered detection limit.As example, Ca²⁺-selective and K⁺-selective all-plastic electrodes were fabricated and yielded with high selectivity, near Nernstian slopes and fast responses. The detection limits obtained for Ca²⁺- and K⁺-selective sensors were 5 × 10⁻⁹ M CaCl₂ and 4.4 × 10⁻⁷ M KCl, respectively.The possibilities of modifying the conducting polymer-phase composition is highlighted. This method is extremely useful to tune the desired type of responses, and cannot be directly applied for electrochemically deposited conducting polymers.     

Monolayer properties of fatty acids : II. Surface vapor pressure and the free energy of compression

The properties of monolayers spread at the air-water interface were measured for saturated, unsaturated, and hydroxy fatty acids, differing in the type and degree of unsaturation, geometric isomerism, and position of unsaturated and hydroxy groups. Surface vapor pressures, reflecting the equilibrium between “gaseous” and “liquid” monolayer states were determined, as were the free energies of compression, ΔF_c, from essentially infinite dilution (100,000 Ų/molecule) to the area per molecule, A_e at the equilibrium spreading pressure, π_e. Surface vapor pressures and free energies of compression for saturated and unsaturated fatty acids with a double bond, or bonds, change in a manner expected because of chain-chain interactions. Hydroxy and acetylenic acids produce relatively high surface vapor pressures, despite their tendency for strong chain-chain interaction. It is concluded that chain-water interactions are very significant for the acetylenic and hydroxy acids and less so for the saturated and ethylenic acids.     

Modellberechnung der EMK und der Ionenselektivität von Membranelektroden-Messketten

Based upon a set of clearly specified assumptions, a universal integral equation is derived which describes the steady-state EMF of an electrochemical cell containing any type of ion-selective membrane electrode. The various modes of delineating the phenomenon ion-selectivity by an explicit function are shown. For all types of analytically important ion-sensitive electrodes, the selectivity between monovalent cations and anions, respectively, can be neatly and comprehensively rationalized. These theoretically derivable expressions are compiled.