Multilayer polyelectrolyte films as nanofiltration membranes for separating monovalent and divalent cations

Alternating adsorption of polyanions and polycations on porous supports provides a convenient way to prepare ion-selective nanofiltration membranes. This work examines optimization of ultrathin, multilayer polyelectrolyte films for monovalent/divalent cation separations relevant to water softening. Membranes composed of five bilayers of poly(styrene sulfonate)/poly(allylamine hydrochloride) (PSS/PAH) on porous alumina supports allow a solution flux of 0.85 m³/(m² day) at 4.8 bar, and exhibit 95% rejection of MgCl₂ along with a Na⁺/Mg²⁺ selectivity of 22. Similar results were obtained in Na⁺/Ca²⁺ separations. PSS/poly(diallyl-dimethylammonium chloride) (PDADMAC) films permit higher fluxes than PSS/PAH systems due to the higher swelling of films containing PDADMAC, but the Mg²⁺ rejection by PSS/PDADMAC membranes is less than 45%. However, capping PSS/PDADMAC films with a bilayer of PSS/PAH yields Mg²⁺ rejections and Na⁺/Mg²⁺ selectivities that are typical of pure PSS/PAH membranes. Separation performance can be optimized through control over deposition conditions (pH and supporting electrolyte concentration) and the charge of the outer layer since Donnan exclusion is a major factor in monovalent/divalent cation selectivity. Streaming potential measurements demonstrate that the magnitude of positive surface charge increases with increasing concentrations of Mg²⁺ in solution or when the outer polycation layer is deposited from a solution of high ionic strength.     

Highly sensitive detection of transient absorption in dye-doped ultrathin polymer films by the TiO(2)/K(+) composite optical waveguide method upon pulsed laser excitation

A composite optical waveguide (OWG) composed of a 10–18 nm thick titanium dioxide (TiO₂) film sputtered on a conventional K⁺-doped optical waveguide was first applied to detect transient absorption of organic dyes in ultrathin polymer films upon excitation with ns laser. The thickness of the TiO₂ film considerably affected the relative sensitivity of the composite OWG. The composite OWG with 10 nm thick TiO₂ gave much stronger transient absorption for 30–415 nm thick polymer films containing organic dyes than that with 18 nm TiO₂. Transient absorption of phthalocyanine and spiropyran in 20–135 nm thick polymer films was detected 3–20 times more sensitively by the composite OWG with 10 nm TiO₂ than the conventional K⁺-doped OWG which showed a 150-fold sensitivity as compared with the usual normal incidence method. The relative sensitivity of the composite waveguide was also affected by the thickness and refractive index of polymers.     

Dual-analyte spectroscopic sensing in sol-gel derived polyelectrolyte-silica composite thin films

Ferrozine (3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-p,p′-disulfonic acid, monosodium salt hydrate), an iron indicator, and HTPS (8-hydroxyl-1,3,6-pyrenetrisulfonic acid, trisodium salt), a pH indicator, were immobilized in sol–gel derived PDMDAAC-SiO₂ (where PDMDAAC stands for poly(dimethyldiallylammonium chloride), composite thin films via ion-exchange. The two indicators were immobilized in two adjacent sections of the same PDMDAAC-SiO₂ film which was supported on a glass optical substrate. The spectroscopic response of the film to both Fe²⁺ and H⁺ in solutions was investigated by attenuated total reflection (ATR) spectrometry at two well-separated wavelengths, 562 nm for Fe²⁺ and 460 nm for H⁺. The Ferrozine/HPTS immobilized PDMDAAC-SiO₂ films had the following characteristics: linear range, 2.5×10⁻⁶–5.0×10⁻⁵ M for Fe²⁺, pH 4.1–6.8 for H⁺; sensitivity, 2.2×10⁴ ΔA/M for Fe²⁺, 0.583 ΔA/pH for H⁺.     

Electrochemistry and electrocatalysis with myoglobin in biomembrane-like surfactant-polymer 2C12N+PA- composite films

Biomembrane-like polyionic complex, 2C₁₂N⁺PA⁻, was prepared by reacting sodium polyacrylate (Na⁺PA⁻) with didodecyldimethylammonium bromide (2C₁₂N⁺Br⁻). Stable thin films made from 2C₁₂N⁺PA⁻, with incorporated myoglobin (Mb), on pyrolytic graphite (PG) electrodes were then characterized by electrochemistry and other techniques. Cyclic voltammetry of Mb-2C₁₂N⁺PA⁻ films showed a pair of well-defined quasi reversible peaks for MbFe(III)/Fe(II) couple at about −0.19 V versus SCE in pH 5.5 buffers. The electron transfer rate between Mb and PG electrodes was greatly facilitated in the microenvironment of 2C₁₂N⁺PA⁻ films. Square wave voltammetry data were used to estimate the apparent heterogeneous electron transfer rate constants by nonlinear regression analysis using a model featuring dispersion of formal potentials. Positions of Soret absorption bands suggested that Mb keeps its secondary structure similar to its native state in 2C₁₂N⁺PA⁻ films at the medium pH. The results of differential scanning calorimetry and X-ray diffraction suggest that synthesized 2C₁₂N⁺PA⁻ lipid films have an ordered multibilayer structure and the incorporated Mb does not disturb this structure. Oxygen was catalytically reduced by Mb-2C₁₂N⁺PA⁻ films with a significant decrease in the electrode potential. MbFe(I), a highly reduced form of Mb, was also produced in Mb-2C₁₂N⁺PA⁻ films at about –1.09 V, and could be used to catalytically reduce organohalide pollutants such as perchloroethylene (PCE) and trichloroethylene (TCE). The catalytic reduction peak currents had linear relationships with concentrations of PCE and TCE in a range of 10–100 μM. The potential applications of the film electrode as a sensor for detecting organohalides are discussed.     

Fabrication of solid thin film electrolyte and LiCoO2 by aerosol flame pyrolysis deposition

Aerosol flame pyrolysis deposition method was applied to deposit the oxide glass electrolyte film and LiCoO₂ cathode for thin film type Li-ion secondary battery. The thicknesses of as-deposited porous LiCoO₂ and Li₂O–B₂O₃–P₂O₅ electrolyte film were about 6 μm and 15 μm, respectively. The deposited LiCoO₂ was sintered for 2 min at 700 °C to make partially densified cathode layer, and the deposited Li₂O–P₂O₅–B₂O₃ glass film completely densified by the sintering at 700 °C for 1 h. After solid state sintering process the thicknesses were reduced to approximately 4 μm and 6 μm, respectively. The cathode and electrolyte layers were deposited by continuous deposition process and integrated into a layer by co-sintering. It was demonstrated that Aerosol flame deposition is one of the good candidates for the fabrication of thin film battery.     

Alginate/gelatin blend films and their properties for drug controlled release

Films of alginate and gelatin, cross-linked with Ca²⁺, with ciprofloxacin hydrochloride as model drug incorporated in different concentrations, were obtained by a casting/solvent evaporation method. Chemical, morphological and mechanical properties characterization was carried out, as well as the studies of the factors that influence the drug releasing from alginate and gelatin films. These factors included the component ratio of alginate and gelatin, the loaded amount of ciprofloxacin hydrochloride, the pH and ionic strength of the release solution, the thickness of the drug loaded films and the cross-linking time with Ca²⁺ and others. The best values of the tensile strength at 101.5 MPa and breaking elongation at 19.4% of blend films were obtained when the gelatin content was 50 wt.%. The results of controlled release tests showed that the amount of ciprofloxacin hydrochloride released decreased with an increase in the proportion of gelatin present in the film. Moreover, the release rate of drug decreased as the amount of drug loaded in the film increased. The alginate/gelatin films were also sensitive to pH and ionic strength. For pH 7.4 the drug release was faster compared to pH 3.6, being simultaneously accelerated by a higher ionic strength. It was observed that in simulated intestinal fluid, the thickness of the film increased from 30 μm to 55 μm with a concomitant reduction of the ciprofloxacin hydrochloride concentration from 100% to 83.5%. When the cross-linking time of these films in the Ca²⁺ solution were 0 min, 5 min, 15 min and 30 min, the drug release rate attained 100%, 100%, 77.6% and 52.4%, respectively, within 24 h. All the results indicated that the alginate/gelatin film was potentially useful in drug delivery systems.     

NaCl interaction with interfacially polymerized polyamide films of reverse osmosis membranes: A solid-state Na NMR study

²³Na nuclear magnetic resonance (NMR) spectroscopy of NaCl-exchanged polyamide (PA) films comparable to those of the active skin layer of many reverse osmosis (RO) membranes provides novel insight into the structural environments and dynamical behavior of Na⁺ in such films. Unsupported PA films were synthesized via interfacial polymerization of trimesoyl chloride in hexane and m-phenylenediamine in aqueous solution, and SEM, FT-IR, and ¹³C NMR data demonstrate successful thin film polymerization. Compositional data confirm this conclusion and demonstrate equal Na and Cl incorporation during NaCl exchange from aqueous solution. The ²³Na NMR spectra for freshly made polymer samples exchanged in 1 M NaCl solution show significant relative humidity (RH) dependence. At near 0% RH, there are resonances for crystalline NaCl and rigidly held Na⁺ in the PA. With increasing RH, a resonance for solution-like dynamically averaged Na⁺ appears and above 51% RH is the only signal observed. The slightly negative chemical shift of this resonance suggests a dominantly hydrous environment with some atomic-scale coordination by atoms of the polymer. The greatly reduced ²³Na T₁ relaxation rates for this resonance relative to bulk solution and crystalline NaCl confirm close association with the polymer. Variable temperature ²³Na NMR spectra for a sample equilibrated at 97% RH obtained from −80 to 20 °C show the presence of rigidly held Na⁺ in a hydrated environment at low temperatures and replacement of this resonance by the dynamically averaged signal at temperatures above about −20 °C. The results provide support for the solution–diffusion model for RO membranes transport and demonstrate the capabilities of multi-nuclear NMR methods to investigate molecular-scale structure and dynamics of the interactions between dissolved species and RO membranes.     

Dye-sensitized solar cells made from BaTiO3-coated TiO2 nanoporous electrodes

We reported on the preparation of a thin BaTiO₃-coated layer (2.27 nm) on the surface of TiO₂ and its further application in the dye-sensitized solar cells (DSCs). The as-prepared BaTiO₃–TiO₂ films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscope (TEM). The performances of the DSCs with and without BaTiO₃ coating were analyzed by cyclic voltammograms (CVs), electrochemical impedance spectroscopy (EIS), and current–voltage measurements. It was found that the BaTiO₃–TiO₂ films with about 12 μm thickness increased the dye adsorption, resulting in increased J_sc. In the meantime, the BaTiO₃ modification on the TiO₂ surface is beneficial to the formation of an energy barrier against the electron transfer from TiO₂ to I₃⁻, providing the increase of V_oc due to the increased electron density in the TiO₂ that is caused by the increased electron lifetime.     

Development of low-cost metal oxide pH electrodes based on the polymeric precursor method

In this work, the polymeric precursor method was used to prepare low-cost solid-state sensors for pH determination based on iridium oxide as the main pH sensitive material. The iridium content was reduced with addition of TiO₂, forming the binary system IrO_x–TiO₂, whose electroanalytical properties were evaluated in comparison with a commercial glass pH electrode. The minimum iridium content which gave suitable results was 30 mol%, and the electrode presented Nernstian and fast response in the pH range from 1 to 13, with no hysteresis effect observed. Besides, the electrode showed high selectivity in the presence of alkali ions as Li⁺, Na⁺ or K⁺. The amount of iridium in the prepared electrodes was very small (<0.1 mg), supporting the efficiency of this method on the simple preparation of functional low-cost pH electrodes.     

Zinc oxide nanoparticles incorporated in ultrathin layer silicate films and their photocatalytic properties

This study has been conducted to determine whether the nanosized semiconductor crystals qualify as photocatalysts also in that case when they are self-assembled to form ultrathin films in the thickness range of 10–500 nm. For this purpose, multilayer films of Zn(OH)₂ and ZnO nanoparticles were prepared by the layer-by-layer self-assembly method on glass surface. A transparent layer silicate, synthetic hectorite was used as sticking material. The quality of multilayer formation has been investigated in detail by absorption spectrophotometry, XRD and atomic force microscopy (AFM). Evidence was found for the uniform deposition of the different components. The increment of nanofilm thickness was constant and independent from the number of layers deposited previously. Photocatalytic measurements were made with model organic materials β-naphtol and industrial kerosene in a home-made loop-type batch reactor. Decomposition has been continously monitored by UV spectrometry. Significant photodegradation of the organic molecules were only found in the presence of the nanofilms.     

Investigations of the optical properties of thin, highly absorbing films under attenuated total reflection conditions: Leaky waveguide mode distortions

Spectra of thin highly absorbing Nafion films doped with Ru(bpy)₃²⁺ on SF11 glass substrates were studied by internal reflection spectroscopy using a single reflection configuration. For the system under study, two modes of light interaction with the film are available: attenuation due to evanescent wave penetration and light propagation within the absorbing film. Unlike evanescent wave spectroscopy, light propagation within the film causes distortions in the measured spectra due to leaky waveguide propagation modes. Upon light propagation in a film doped with Ru(bpy)₃²⁺ spectral shifts up to 50 nm to longer wavelengths can occur and additional absorbance peaks can appear in the spectra. These film-based distortions depend on the complex refractive index, the thickness of the film and the angle of incidence. These effects become significant for an extinction coefficient above 0.01 and a film thickness above 200 nm. It is shown that spectral distortions can lead to quite complex dynamics in the internal reflection spectra upon analyte preconcentration in the film. Ru(bpy)₃²⁺ partitioning into the Nafion film causes significant refractive index changes that in turn alter leaky waveguide mode conditions in the film and, can even lead to a reduction of measured absorbance despite the increase in the extinction coefficient of the film.     

Electrochemical behavior of methyl viologen at graphite electrodes modified with Nafion sol–gel composite

Electrodes were prepared by spin-coating spectroscopic graphite rods with a Nafion doped sol. Coating solutions consisting of Nafion:TEOS (tetraethoxysilane) ratios of 3:1 and 4:1 gave smooth films on the electrode surface. These modified electrodes were evaluated and compared with Nafion modified and bare spectroscopic graphite electrodes using methyl viologen (MV²⁺) as a representative cationic electroactive probe. Substantial partitioning of MV²⁺ into the Nafion:sol–gel matrix to the electrode surface was observed by cyclic voltammetry and square wave voltammetry. Cyclic voltammograms of MV²⁺ in 0.1 M NaCl at Nafion:sol–gel 4:1 modified electrodes showed a reversible reduction to MV⁺ with E^0′=−0.695 V vs. Ag/AgCl. Results of scan rate variation showed the wave to be characterized by semi-infinite diffusion for scan rates in the range 50–500 mV/s. Slowing the scan rate below 50 mV/s resulted in a transition to thin-layer behavior. MV²⁺ partitioned much more quickly into the sol–gel-Nafion modified electrodes compared to pure Nafion modified electrodes. Reversibility of the MV²⁺-loaded modified Nafion-doped sol–gel coatings on electrodes was obtained by soaking in 1 M NaCl solution. Concentration calibration plots for MV²⁺ at the sol–gel-Nafion modified electrodes were nonlinear. Substantial enhancement of current signal at low concentrations was observed by square wave voltammetry.     

Thermal modification of layer-by-layer assembled gold nanoparticle films

Gold nanoparticle films are assembled on glass and quartz substrates by a simple and highly efficient layer-by-layer deposition procedure that uses only commercially available cationic polymers. The film samples are then modified by heat curing in the temperature range of 25–1100 °C. The changes in the film conductance and colour with the curing temperature are related to the respective changes in micro-morphology of films on quartz as observed by scanning electron microscopy. In addition, we have demonstrated that the heat curing can embed the gold nanoparticle layer in the glass substrates. Because of the preparation simplicity and peculiar properties of these films, they could be used in various practical applications.     

Long-lived photoinduced charge separation in carbazole–pyrene–viologen system incorporated in Langmuir–Blodgett films of substituted diazacrown ethers

Diaza-18-crown-6 ethers appending two pyrenyl (Py) or two carbazolyl (Cz) groups were synthesized. These macrocyclic compounds form 1:1 host–guest complexes with methyl viologen chloride (MV²⁺), and these complexes were assembled into monolayers by Langmuir–Blodgett technique. The generated assembly involves the general structure of donor–sensitizer–acceptor (Cz–Py–MV²⁺) in space, although any of the photo- and redox-active components are not covalently bonded. Photoirradiation of the pyrenyl group resulted in the charge-separated pair Cz√⁺–Py–MV√⁺ which survived up to hours in a well anaerobic atmosphere. An electrode was fabricated by transferring the L–B film on an ITO glass. The photoinduced voltage of this electrode was measured with a saturated calomel reference electrode in hydroquinone (H₂Q) solution to be ca. 168 mV when the light intensity was 218 mW/cm². This electrode was also used as the light electrode to construct a photogalvanic cell with a platinum electrode as the dark electrode. Irradiation of the light electrode with visible light results in anodic photocurrent, and there is no net chemical change associated with the function of the cell which converts light to electricity.     

Electron beam irradiated polyamide-6,6 films—II: mechanical and dynamic mechanical properties and water absorption behavior

Electron beam irradiation of poly(iminohexamethylene-iminoadipoyl) (Polyamide-6,6) films was carried out over a range of irradiation doses (20–500 kGy) in air. The mechanical properties were studied and the optimum radiation dose was 200 kGy, where the ultimate tensile stress (UTS), 10% modulus, elongation at break (EB) and toughness showed significant improvement over the unirradiated film. At a dose of 200 kGy, the UTS was improved by 19%, the 10% modulus by 9% and the EB by 200% over the control. The dynamic mechanical properties of the films were studied in the temperature region 303–473 K to observe the changes in the glass transition temperature (T_g) and loss tangent (tan δ) with radiation dose. The storage modulus of the film receiving a radiation dose of 200 kGy was higher than the unirradiated film. The water uptake characteristics of the Polyamide-6,6 films were investigated. The water uptake was less for the films that received a radiation dose of 200 and 500 kGy than the unirradiated film. The role of crystallinity, crosslinking and chain scission in affecting the tensile, dynamic mechanical and water absorption properties was discussed.     

1.54 and 1.75 μm infrared luminescence of Y2O3:Er

A new 1.75 μm infrared emission transition of Y₂O₃:Er³⁺ is assigned to the ⁴S_3/2 → ⁴I_9/2 transition of Er³⁺ ions situated at the C₂ sites of cubic RE₂O₃ (RE = Y, Gd, Lu). The intensities of features in the 1.54 μm ⁴I_15/2–⁴I_13/2 absorption transition due to Er³⁺ at S₆ and C₂ sites are consistent with the site occupation ratio and the relative magnetic dipole–electric dipole intensity contributions of Er³⁺ at the different sites. The 1.54 μm emission lines are predominantly from Er³⁺ ions at C₂ sites. The different behaviours of the emission intensities 1.75 and 1.54 μm groups with change in Er³⁺ dopant ion concentration, preparation technique, Yb³⁺ co-doping, temperature change and different excitation line are rationalized.     

Electrochemiluminescence sensor using tris(2,2′-bipyridyl)ruthenium(II) immobilized in Eastman-AQ55D–silica composite thin-films

An electrochemiluminescence (ECL) sensor with good long-term stability and fast response time has been developed. The sensor was based on the immobilization of tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy)₃²⁺) into the Eastman-AQ55D–silica composite thin films on a glassy carbon electrode. The ECL and electrochemistry of Ru(bpy)₃²⁺ immobilized in the composite thin films have been investigated, and the modified electrode was used for the ECL detection of oxalate, tripropylamine (TPA) and chlorpromazine (CPZ) in a flow injection analysis system and showed high sensitivity. Because of the strong electrostatic interaction and low hydrophobicity of Eastman-AQ55D, the sensor showed no loss of response over 2 months of dry storage. In use, the electrode showed only a 5% decrease in response over 100 potential cycles. The detection limit was 1 μmol l⁻¹ for oxalate and 0.1 μmol l⁻¹ for both TPA and CPZ (S/N=3), respectively. The linear range extended from 50 μmol l⁻¹ to 5 mmol l⁻¹ for oxalate, from 20 μmol l⁻¹ to 1 mmol l⁻¹ for TPA, and from 1 μmol l⁻¹ to 200 μmol l⁻¹ for CPZ.     

Spectroelectrochemistry and investigation of charge transport mechanisms of iron poly(pyridyl) redox polymers

In this work, we describe the characterization of the complex [Fe(tpy-NH₂)₂](PF₆)₂ (tpy-NH₂ = bis[4′-(3-aminophenyl)-2, 2′:6′,2″-terpyridine]. The complex was oxidatively electropolymerized on glassy-carbon electrodes in CH₃CN/0.1 M tetraethylammonium perchlorate (TEAP) to generate polymer films that exhibit reversible oxidative electrochemical behavior in a wide potential range (0.0–1.6 V), as well as high conductivity and stability/durability. In situ spectrocyclic voltammetry of this modified electrode was carried out on a photodiode array spectrophotometer attached to a potentiostat, which provided UV–Vis absorption spectra of the redox species during the potential sweep. We determined charge transport parameters as a function of time and thickness of the modified electrode, and the results showed that poly-[[Fe(tpy-NH₂)₂]²⁺]_n can be made to exhibit three regimes of charge transport behavior by manipulation of the film thickness and the experimental time-scale. Morphological characterization of the film was provided by atomic force microscopy.     

Infrared studies of pyroelectric Langmuir–Blodgett films of arachidic acid and 1,2-bis(dodecyloxy)-4,5-diaminobenzene

Fourier Transform Infrared (FTIR), p-polarized grazing angle (GAIR) and Horizontal Attenuated Total Reflectance (HATR) spectra have been recorded of arachidic acid (AA)/1,2-bis(dodecyloxy)-4,5-diaminobenzene (DADB) Y-type alternate LB films deposited on an aluminium plate with 31 layers. It is well known that the frequencies of CH₂ stretching bands of a hydrocarbon chain are sensitive to the conformational ordering of the chain. Changes in frequency and intensity can be used to characterize film ordering and preferential molecular packing. The observed peak frequencies and intensities of these bands indicate that the alkyl chains are present in a mostly trans conformation and tilted from the normal direction with respect to the substrate in LB films. The FTIR–GAIR and HATR spectra of 31 layers alternate film show significant changes in the region 1700–1400 cm⁻¹ due to the partial proton transfer between acid and amine head groups. According to the HATR spectrum, the peak at 1731 cm⁻¹ is observed due to a proportion of the carboxylic acid groups forming sideways dimers indicating that if the carboxylic acid groups form sideways dimers, they are less likely to undergo proton transfer with the amino groups.     

Optical spectra of Yb and Yb–Nd interaction in Cs2NaNd0,4Yb0,6Cl6

Interaction between octahedrally coordinated Nd³⁺ and Yd³⁺ in Cs₂NaNd_0,4Yb_0,6Cl₆ reduces the Nd³⁺ luminescence lifetime by roughly two orders of magnitude with respect to that found in Cs₂NaNdCl_{6– · –} Analysis of low temperature absorption and emission spectra reveals that the nonradiative Nd³⁺–Yb³⁺ energy transfer has to be assisted by lattice phonon emission, nevertheless the rate of the transfer is high in the 4–300 K temperature region and attains 5.8×10⁵s^-1 at room temperature. A phase transition of Cs₂NaNd_0,4Yb_0,6Cl₆ between 12 and 13 K is evidenced by abrupt change of both the spectra and lifetimes of Yb³⁺. Reduction of Yb³⁺ lifetime from 5.3 ms to 150 μs is at the transition from low symmetry phase to high symmetry phase is supposed to be associated with a three ion interaction which occurs in ordered lattice and disappears in low temperature disordered structure.