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225(1-2) (2001), 59-66
Polyelectrolyte multilayer (PEM) capsules are introduced as versatile magnetic carrier systems. Superparamagnetic magnetite is mounted to the multilayer shell itself or is a component of the capsule interior. The PEM is formed at different (decomposable) colloidal templates, e.g. melamine formaldehyde resin, glutaraldehyde fixed red blood cells, emulsion oil droplets. The results are illustrated by transmission electron microscopy and confocal laser scanning microscopy. (C) 2001 Elsevier Science B.V. All rights reserved.

[208] Self-Assembly Films from Diazoresin and Carboxy-Containing Polyelectrolytes

H. Luo, J. Y. Chen, G. B. Luo, Y. N. Chen and W. X. Cao

J. Mater. Chem. 11(2) (2001), 419-422

Multilayer films from diazoresin (I) and poly(sodium acrylate) (II) or a hydrolyzed maleic anhydride-styrene copolymer (III) have been fabricated on mica. During the fabrication process the absorbance of the film at 380 epsilon nm, which is the characteristic absorption of the diphenylamine 4-diazonium group of I, increases about 0.033 for each fabrication cycle. This means that the thickness of the film increases regularly. Under irradiation with UV light or heating, following the decomposition of the diazonium group, the ionic bond in the film structure converts to a covalent bond; this conversion was preliminarily verified by FTIR spectrum analysis. The stability of the different films (irradiated or unirradiated) was determined by UV-vis spectroscopy and the results show that the resistance of the film to etching by polar solvents increases significantly after bond conversion.

[209] Precipitation of Multilayered Core-Shell TiO2 Composite Nanoparticles Onto Polymer Layers

M. T. Wang and L. D. Zhang

J. Mater. Res. 16(3) (2001), 765-773

A composite film of titanium dioxide (TiO2) nanoparticles and hydrolyzed styrene-maleic anhydride alternating copolymer (HSMA) was obtained on a substrate when a TiCl4 solution was heated at 80 degreesC with a spin-cast thin HSMA film present in the solution. The composite film was characterized with x- ray photoelectron spectroscopy and transmission electron microscopy. Results revealed that TiO2 nanoparticles discretely dispersed on the poly-mer layer, and they were dominantly rutile phase, of a spherical shape and 18-20 nm in diameter. In contrast, mainly amorphous TiO2 powders were obtained from the identical TiCl4 solution by drying the solution with the absence of the HSMA film. The TiO2 nanoparticles deposited on the polymer layer were regarded to contain polymer chains, rind a multilayered core-shell model was suggested for the formation of these composite nanoparticles. It is regarded that the core of a composite particle consisted of an anatase-phase TiO2 colloidal nanoparticle, while the shell layer was made of rutile-phase TiO2/polymer multilayers; the composite particles formed by a layer-by-layer self-assembly of TiO2 and polymer layers analogous to biomineralization, where the polymer promoted the crystallization of rutile-phase TiO2 when TiO2 deposited from solution.

[210] Fabrication of Polyaniline/Phthalocyanine Hybrid Ultrathin Films via Electrostatic Attraction and Doping Reaction

D. Li, Y. D. Jiang, Y. R. Li, X. J. Yang, L. D. Lu and X. Wang

J. Mater. Sci. Lett. 20(3) (2001), 233-235

[211] Self-Assembled Polyelectrolyte Multilayer Membranes with Highly Improved Pervaporation Separation of Ethanol/Water Mixtures

L. Krasemann, A. Toutianoush and B. Tieke

J. Membrane Sci. 181(2) (2001), 221-228
Ethanol/water pervaporation through ultrathin polyelectrolyte multilayer membranes is described. The membranes were prepared by the layer-by-layer technique, i.e. by alternating sequential adsorption of cationic and anionic polyelectrolytes on a porous support. The separation capability was optimized by variation of the chemical structure of the polyelectrolytes, by variation of pH and ionic strength of the polyelectrolyte solutions used for membrane preparation and by annealing of the polyelectrolyte membranes. It was found that the separation is mainly affected by the charge density of the polyelectrolytes which is controlled by the chemical structure and the degree of ionisation of the polar groups. Selectivity for water was highest, if polyelectrolytes of high charge density such as polyethyleneimine (PEI), polyvinylamine (PVA) and polyvinylsulfate (PVS) were used and if the pH of the polyelectrolyte solutions was equal to the mean of the pK(a) values of the corresponding cationic and anionic polyelectrolyte. Best results were obtained for PVA/PVS and PEI/ PVS membranes which are characterized in detail with regard to their separation behavior. (C) 2001 Elsevier Science B.V. All rights reserved.

[212] Pervaporation Separation of Water/Alcohol Mixtures Using Composite Membranes Based on Polyelectrolyte Multilayer Assemblies

J. Meierhaack, W. Lenk, D. Lehmann and K. Lunkwitz

J. Membrane Sci. 184(2) (2001), 233-243

Composite pervaporation membranes composed of an asymmetric polyamide-6 membrane and an ultrathin self-assembled polyelectrolyte separating layer are described. The supporting membrane was prepared from both an unmodified polyamide-6 and a comb-like polymer with carboxyl terminated polyamide-6 side chains. A high end group concentration was found to be advantageous for sufficient adhesion of the multilayer systems on the supports. Up to 20 layers were deposited onto the membrane surface by dipping the membranes in aqueous solutions containing oppositely charged polyelectrolytes. The polyanions used were poly(acrylic acid), poly(styrene sulfonic acid) and alginic acid. The polycations used were poly(diallyldimethylammoniumchloride), chitosan and poly(ethylenimine). Performance of these membranes depends strongly on the layer number and on the type of polyelectrolytes. In general, membranes modified with two weak polyelectrolytes of high charge density gave the best separation properties while those modified with strong polyelectrolytes of low charge density led to poorer separation properties. However, the highest separation factor (greater than or equal to 10,000) for a water/2-propanol mixture (12/88 w/w) at permeate flux of 300 g/m(2)h was obtained with six double layers consisting of poly(ethylenimine) and alginic acid. These composite membranes were stable over an operating period of at least 400 h. (C) 2001 Elsevier Science B.V. All rights reserved.

[213] Biological Cells as Templates for Hollow Microcapsules

B. Neu, A. Voigt, R. Mitlohner, S. Leporatti, C. Y. Gao, E. Donath, H. Kiesewetter, H. Möhwald, H. J. Meiselman and H. Baumler

J. Microencapsul. 18(3) (2001), 385-395

Microcapsules in the micrometer size range with walls of nanometer thickness are of both scientific and technological interest, since they can be employed as micro- and nano- containers. Liposomes represent one example, yet their general use is hampered due to limited stability and a low permeability for polar molecules. Microcapsules formed from polyelectrolytes offer some improvement, since they are permeable to small polar molecules and resistant to chemical and physical influences. Both types of closed films are, however, limited by their spherical shape which precludes producing capsules with anisotropic properties. Biological cells possess a wide variety of shapes and sizes, and, thus, using them as templates would allow the production of capsules with a wide range of morphologies. In the present study, human red blood cells (RBC) as well as Escherichia coli bacteria were used; these cells were fixed by glutardialdehyde prior to layer-by-layer (LbL) adsorption of polyelectrolytes. The growth of the layers was verified by electrophoresis and flow cytometry, with morphology investigated by atomic force and electron microscopy; the dissolution process of the biological template was followed by confocal laser scanning microscopy. The resulting microcapsules are exact copies of the biological template, exhibit elastic properties, and have permeabilities which can be controlled by experimental parameters; this method for microcapsule fabrication, thus, offers an important new approach for this area of biotechnology.

[214] Sustained-Release Properties of Polyelectrolyte Multilayer Capsules

A. A. Antipov, G. B. Sukhorukov, E. Donath and H. Möhwald

J. Phys. Chem. B 105(12) (2001), 2281-2284

Layer-by-layer (LbL) assembly of oppositely charged polyelectrolytes was used to coat fluorescein particles. These particles, with a size of 4-9 mum, were prepared by precipitation of fluorescein at pH 2. Polystyrensulfonate (PSS) and polyallylamine (PAH) were used to form a polyelectrolyte shell on the fluorescein core. The permeation of fluorescein molecules through the polyelectrolyte shell during core dissolution was monitored at pH 8 by the increasing fluorescence intensity as a result of dequenching. The number of polyelectrolyte layers sufficient to sustain fluorescein release was found to be 8-10. Increasing the number of layers prolonged the core dissolution time for minutes. The permeability of polyelectrolyte multilayers of the thickness of 20 nm for fluorescein is about 10(-9) m/s. The features of the release profile and possible applications of the LbL method for shell formation in order to control release properties for entrapped materials are outlined.

[215] Optical-Properties of Thin-Films of Au-at-SiO2 Particles

T. Ung, L. M. Lizmarzan and P. Mulvaney

J. Phys. Chem. B 105(17) (2001), 3441-3452

Homogeneous films of Au@SiO2 particles have been deposited on glass as a prototype 3D ''artificial solid'' using the LBL method. The film thickness is controlled by the number of dipping cycles and is measured by AFM. Each cycle results in approximately one monolayer of particles being deposited. The particle films are dense, but disordered. The optical properties of the resulting thin films have been analyzed as a function of the particle volume fraction, which is controlled through the silica shell thickness. We find that the surface plasmon peak position in films with volume fractions up to phi > 0.5 is accurately predicted by the Maxwell-Garnett model. The films exhibit remarkably uniform, transmitted colors and display metallic reflection at low angles of incidence, even at low volume fractions. The films can be annealed at T > 500 K to provide extremely stable, optical films.

[216] Metal-Ion Sensing Using Ultrathin Organic Films Prepared by the Layer-by-Layer Adsorption Technique

C. Pearson, J. Nagel and M. C. Petty

J. Phys. D, Appl. Phys. 34(3) (2001), 285-291

Bilayer films of poly(ethyleneimine) and poly(ethylene-co- maleic acid) have been prepared using the layer-by-layer assembly technique. These films have been characterized using surface plasmon resonance (SPR). Exposure to aqueous solutions of metal acetate (metal = copper, nickel) resulted in a shift in the position of the SPR curve. This shift could be monitored by observing the intensity of the reflected laser beam at fixed angle and concentrations of copper acetate down to at least 10(- 6) M could be detected.

[217] Protein Adsorption Onto Auto-Assembled Polyelectrolyte Films

G. Ladam, P. Schaaf, F. J. G. Cuisinier, G. Decher and J. C. Voegel

Langmuir 17(3) (2001), 878-882

We investigate the adsorption processes of a series of positively and negatively charged proteins onto the surface of polyelectrolyte multilayers. We find that proteins strongly interact with the polyelectrolyte film whatever the sign of the charge of both the multilayer and the protein. When the charges of the multilayer and the protein are similar, one usually observes the formation of protein monolayers which can become dense. We also show that when the protein and the multilayer become oppositely charged, the adsorbed amounts are usually larger and the formation of thick protein layers extending up to several times the largest dimension of the protein can be observed. Finally, we find that proteins are mainly adsorbed in a strong way on polyelectrolyte multilayers and protein surface diffusion is strongly suggested. Our results confirm that electrostatic interactions play an important role in polyelectrolyte multilayer/protein interactions.

[218] Controlling the Permeability of Multilayered Polyelectrolyte Films Through Derivatization, Cross-Linking, and Hydrolysis

J. H. Dai, A. W. Jensen, D. K. Mohanty, J. Erndt and M. L. Bruening

Langmuir 17(3) (2001), 931-937

Partial Fischer esterification of poly(acrylic acid) allows tailoring of the hydrophobicity and charge density of multilayered films containing poly(allylamine hydrochloride) (PAH) and derivatized poly(acrylic acid) (d-PAA). As hydrophobicity and charge density strongly affect film permeability, control over these properties is vital for possible applications of PAH/d-PAA films as ion-separation membranes and sensors. The hydrophobicity of these films depends on both the extent of esterification and the nature of the derivatizing alcohol. Even though PAH/d-PAA films are composed of polyelectrolytes, the presence of hydrophobic ester groups results in advancing water contact angles as high as 101 degrees. The hydrophobicity of these coatings allows them to effectively passivate underlying electrodes as shown by minimal peak currents in cyclic voltammograms (CVs) of RU(NH3)(6)(3+) and Fe(CN)(6)(3-). Cross-linking of hydrophobic PAH/ d-PAA films via heat-induced amidation stabilizes coatings over a wide pH range but does not significantly decrease the already low film permeability to Ru(NH3)(6)(3+) and Fe(CN)(6)(3-). Stabilization due to cross-linking does, however, allow base- promoted hydrolysis of the ester groups of PAH/d-PAA coatings. After hydrolysis, films are extremely hydrophilic and selectively permeable to Ru(NH3)(6)(3+) over Fe(CN)(6)(3-) due to,the high density of newly formed -COO- groups. In the case of some hydrolyzed films, the presence of small concentrations of Ca2+ results in dramatic current decreases in CVs of RU(NH3)(6)(3+), suggesting possible use of these films in sensing applications.

[219] Molecular-Weight Dependence of Alternate Adsorption Through Charge-Transfer Interaction

Y. Shimazaki, R. Nakamura, S. Ito and M. Yamamoto

Langmuir 17(3) (2001), 953-956

[220] Layered Polyelectrolyte Films on Au Electrodes - Characterization of Electron-Transfer Features at the Charged Polymer Interface and Application for Selective Redox Reactions

V. Pardoyissar, E. Katz, O. Lioubashevski and I. Willner

Langmuir 17(4) (2001), 1110-1118
Oppositely charged polyelectrolyte assemblies of poly(acrylic acid) and poly-L-lysine are deposited as alternate layers on cystamine-functionalized Au surfaces. Microgravimetric, quartz- crystal-microbalance, measurements and ellipsometric studies reveal a nonlinear increase in the polymer assembly thickness upon the buildup of the polymer layers. This is attributed to the swelling of the polymer upon the stepwise assembly of the layered film. The interfacial and intra-assembly properties of the polyelectrolyte systems were characterized by Faradaic impedance spectroscopy. In the presence of Fe(CN)(6)(3-)/ Fe(CN)(6)(4-) as the redox label, all assemblies that terminate with a negatively charged interface are characterized by a high interfacial electron-transfer resistance, originating from the electrostatic repulsion of the redox label from the interface. The interfacial electron-transfer resistance also increases as the number of layers, or assembly thickness, increases. For assemblies terminated with a positively charged polyelectrolyte, a very low interfacial electron-transfer resistance for the redox process of the negatively charged redox probe is detected. This is attributed to a neutralized, porous structure of the polymer assembly. For the positively charged redox label, protonated N,N-dimethylaminomethyl- ferrocene, similar results are observed for the assemblies with the opposite dependence on the charge of the terminal layer. The electrodes functionalized with the polymers were used for the selective oxidation of a mixture consisting of Fe(CN)(6)(4- ) and protonated N,N-dimethylaminomethyl-ferrocene.

[221] Ion-Transport and Equilibria in Polyelectrolyte Multilayers

T. R. Farhat and J. B. Schlenoff

Langmuir 17(4) (2001), 1184-1192

Transport of redox-active probe ions through multilayers made from highly charged polyelectrolytes is described. Because of intrinsic charge balance between polyelectrolyte segment ion pairs, there are no sites for the exchange of small ions within the bulk of as-made multilayers. In the presence of salt in the external bathing solution, sites for small ion exchange are forced into these ''reluctant'' amphoteric exchangers. Equilibrium considerations lead to the conclusion that, under conditions of high salt concentration and strong polymer ion pairing, the exchanger site concentration is proportional to the solution salt concentration, while the population of redox ions within the multilayer remains constant. Rotating disk electrodes coated with multilayers yield cyclic voltammograms which retain their classical sigmoidal shape, indicating well- behaved membrane transport. Limiting currents are strongly attenuated by membranes of thickness on the order of a few hundred angstroms. Layer-by-layer buildup reveals enhanced transport of negative ions through multilayers bearing a positive surface charge, attributed to Donnan-enhanced membrane inclusion of negative ions. Good matching of internal charge, efficiently excluding ions, is a distinctic feature of membranes made from polyelectrolyte multilayers. The flux through membranes is a strongly nonlinear function of salt concentration for the multiply charged ferro- and ferricyanide ions. This leads to significant transport selectivity between ions, favoring species with lower charge. With the membrane concentration of redox ion, charge n, constant, membrane flux is proportional to membrane diffusion coefficient, (D) over bar, which depends on solution salt concentration in the manner (D) over bar similar to k [salt](n). The implications of this for membrane transport control of multiply charged biomolecules are discussed.

[222] Fuzzy Assembly and 2nd-Harmonic Generation of Clay/Polymer/Dye Monolayer Films

B. Vanduffel, T. Verbiest, S. Vanelshocht, A. Persoons, F. C. Deschryver and R. A. Schoonheydt

Langmuir 17(4) (2001), 1243-1249

Fuzzy assembled multicompound films consisting of natural and synthetic colloidal clay particles, poly(diallyldimethylammonium chloride) (PDDA) and the dyes methylene blue (MB+) and 4-(4-[N-allyl-N-methylamino] phenylazo)benzenesulfonic acid, sodium salt (NAMO(-)) have been prepared. The organization of the clay particles was imaged by means of atomic force microscopy. Fuzzy assembled glass/clay/ methylene blue films have been investigated by visible spectroscopy, and the spectra were explained in terms of the substrate-clay interaction and clay particle overlap. Glass/ clay/PDDA/NAMO films were investigated by means of absorption spectroscopy, polarized absorption spectroscopy, and second harmonic generation. The nonlinear optical properties of the films were found to be determined by the adsorbed amount of NAMO and its noncentrosymmetric organization. These factors are in turn governed by the substrate type, the PDDA concentration, and the clay type. Optimized second harmonic generation for the glass/laponite/ PDDA/NAMO films was found in which the clay particles are deposited-onto a (3-aminopropyl)-trimethoxysilane modified glass surface and PDDA chains are adsorbed from a 0.1 M solution.

[223] Release Behavior of Thin-Walled Microcapsules Composed of Polyelectrolyte Multilayers

X. Y. Shi and F. Caruso

Langmuir 17(6) (2001), 2036-2042

The release properties of the fluorescent probe pyrene, PYR, encapsulated in polyelectrolyte multilayer capsules were examined by fluorescence spectroscopy. PYR was encapsulated by the layer-by-layer deposition of oppositely charged polyelectrolytes onto PYR microcrystal templates, and its release was accomplished by exposing the polyelectrolyte-coated microcrystals to ethanol solutions. Solubilization of the hydrophobic probe by ethanol resulted in its expulsion from the core through th permeable polyelectrolyte: multilayer shells. The PYR release rate, quantified by following the monomer fluorescence of the solubilized PYR as a function of time, decreased with increasing number of polyelectrolyte layers deposited onto the microcrystal cores. In addition, two amphiphiles (sodium dodecyl sulfate and dipalmitoyl-(DL)-alpha - phosphatidylcholine) used to disperse the microcrystals in aqueous solution prior to coating with polyelectrolytes significantly influenced the PYR release rate. The maximum release time, defined as the time required to achieve saturation release, could be varied by a factor of 2-5 depending on the nature of the first adsorbed layer and the number of polyelectrolyte layers composing the shell wall. The influence of flow (i.e., centrifugation vs magnetic stirring) as well as solvent on the PYR release rate is also discussed.

[224] Formation of Multilayer Composite-Particles Comprised of Silica/ Vesicle/Silica Particles by Heterocoagulation

B. Yang, H. Matsumura, K. Katoh, H. Kise and K. Furusawa

Langmuir 17(8) (2001), 2283-2286

The multilayer composite particles comprised of silica/PC (phosphatidylcholine) vesicle/silica particles have been prepared by using the heterocoagulation technique. The formation process is as follows: at first, large silica particles (2r = 1.5 mum) and PC vesicles (2r = 0.2 mum) were mixed at a definite particle number ratio in 10(-4) M LaCl3 aqueous solution. The PC vesicle/silica composites were spontaneously formed by mutual electrostatic attraction. After removal of the excess PC vesicles from the system, the small silica particle (2r = 0.5 mum) suspension was added into the PC vesicle/silica composite dispersions to form the multilayer composite particles composed of small silica, PC vesicle, and core silica particles. The formation of multilayer composite particles was confirmed by the electrophoresis, dynamic light scattering (DLS), and adsorption measurements of PC molecules. Finally, the existence of multilayers was confirmed directly using a special optical microscope.

[225] Influence of Thickness on Catalytic Efficiency of Cobalt Corrin- Polyion Scaffolds on Electrodes in Microemulsions

C. J. Campbell, C. K. Njue, B. Nuthakki and J. F. Rusling

Langmuir 17(11) (2001), 3447-3453

Catalytic electrodes were prepared by covalently binding poly-L- lysine (PLL) onto oxidized carbon electrodes and then linking the cobalt corrin vitamin B-12 hexacarboxylic acid [B- 12(COOH)(6)] to this surface. Additional layers of PLL-B- 12(COOH)(6) were attached in a similar way. Quartz crystal microbalance studies showed regular and reproducible layer formation. Electrochemical and catalytic properties of the (CoL)-L-II/ (CoL)-L-I redox couple in these films were investigated using voltammetry and preparative electrolysis in an sodium dodecyl sulfate microemulsion. These films obeyed theoretical predictions of a maximum in voltammetric catalytic efficiency as thickness increased for the reduction of 1,2- dibromocyclohexane (DBCH) in a microemulsion. In films with less' than optimum thickness, kinetic control of the chemical reaction between (CoL)-L-I and DBCH predominated. As film thickness was increased beyond that found for maximum efficiency, electron and reactant mass transport within the films became limiting factors. Under synthetic electrolysis conditions, optimal turnover numbers were found for very thin films on porous electrodes, and best yields and current efficiencies were obtained with the relatively small catalyst coverage of about 2 nmol cm(-2).

[226] Stability and Mechanical-Properties of Polyelectrolyte Capsules Obtained by Stepwise Assembly of Poly(Styrenesulfonate Sodium- Salt) and Poly(Diallyldimethyl Ammonium) Chloride Onto Melamine Resin Particles

C. Y. Gao, S. Leporatti, S. Moya, E. Donath and H. Möhwald

Langmuir