Polyelectrolyte multilayers prepared from water-soluble poly(alkoxythiophene) derivatives.

Electronically conducting polyanion and polycation based on poly(alkoxythiophene) derivatives, poly-3-(3'-thienyloxy)propanesulfonate (P3TOPS) and poly-3-(3'-thienyloxy)propyltriethylammonium (P3TOPA) have been synthesized. Both polymers are water-soluble and exhibit high conjugation length in solution and in the solid state. These polyelectrolytes were used to prepare conducting and electroactive polyelectrolyte multilayers by the sequential layer-by-layer adsorption technique. In aqueous solutions multilayers of P3TOPS with inactive polyelectrolytes (e.g., poly(diallyldimethylammonium chloride), PDADMA) displayed electrochemical and optical behavior similar to polythiophene films prepared in organic media. Their in-plane conductivity was low (ca. 1.6 x 10(-)(5) S cm(-)(1)). The conductivity could, however, be increased by a factor of ca. 40 in "all-thiophene" films, in which P3TOPA was substituted for the inactive polycation (PDADMA). The interpenetration of layers is of prime importance in films containing conducting components. The interpenetration of P3TOPS was studied by measuring the charge-transfer rate across an insulating polyelectrolyte multilayer between the substrate and the P3TOPS layer with modulated electroreflectance. The extent of interpenetration was 8-9 polyelectrolyte layers, the length scale (7-15 nm) depending on the nature of the insulating layer and, especially, on the ionic strength of the solution used for the adsorption of P3TOPS.     

Au nanoparticle micropatterns prepared from self-assembled films

A kind of hybrid multilayer film based on mercaptobenzoic acid-capped Au nanoparticles (MBA-Au-NPs) and photoreactive nitrodiazoresin (NDR) has been fabricated via electrostatic self-assembly. Upon exposure to UV light, the initial ionic bonds between the layers of the film convert into covalent bonds and the film stability toward polar solvents, salt, or surfactant solutions increases significantly. The micropatterned NDR/MBA-Au-NP film with the covalently linked architecture was formed by selecting exposure of the film through a photomask and later developed in sodium dodecyl sulfate (SDS) aqueous solution. The metallic Au-NP micropatterns, furthermore, are produced by sintering the micropatterned NDR/MBA-Au-NP film at 550 degrees C, at which the organic components are removed completely. The well-defined micropatterns were characterized with atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), microscope with a charge-coupled device (CCD) camera, and X-ray photoelectron spectroscopy (XPS).     

Resonant Raman spectroscopy of PAH--Os self-assembled multilayers

We present a resonant Raman scattering study of (PAH--Os/PVS)n and (PAH--Os/GOx)m self-assembled multilayers (n=1-11 and m=1-3). These Os polymer multilayers can be used in electrodes as efficient molecular wires for biomolecular recognition. The Raman intensity dependence on the number of self-assembly cycles provides information on the deposition process. The spectra are identical to that observed for PAH--Os in aqueous solution, indicating that the PAH--Os metal complex structure is conserved in the multilayers. We observe at approximately 500 nm incoming and outgoing Raman resonances of osmium and bipyridine vibrational modes. These resonances are associated to the metal-to-ligand charge transfer (MLCT) transition. We study the evolution of these Raman modes as a function of the Os oxidation state during in situ electrochemistry. During the oxidation process, Os(II)-->Os(III), the Raman resonance related to the MLCT disappears and the bipyridine related modes harden by approximately 10 cm(-1). These results are correlated with optical transmission measurements which show the disappearance of the visible region absorption when the Os complex is oxidized. We also find partial quenching of the Raman mode intensity after in situ voltamperometric cycles which demonstrates the existence of photo-electro-chemical processes.     

Polyoxometalates as pH-sensitive probes in self-assembled multilayers

The polyoxometalate cluster [CoII4(H2O)2P4W30O112]16- (Co-POM) embedded in a self-assembled polyelectrolyte matrix shows a remarkable pH dependence of its electrochemical response, opening a route to use Co-POM as a molecular probe or to fabricate pH microelectrodes.     

Hollow capsules prepared from all block copolymer micelle multilayers

We introduce a novel and versatile approach for preparing hollow multilayer capsules containing functional hydrophobic components. Protonated polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) and anionic polystyrene-block-poly(acrylic acid) (PS-b-PAA) block copolymer micelles (BCM) were used as building blocks for the layer-by-layer assembly of BCM multilayer films onto polystyrene (PS) colloids. After removing the PS colloids, the stabilities of the formed BCM hollow capsules were found to be strongly dependent on the charge density of the hydrophilic corona segments (i.e., P4VP and PAA block segments) as well as the relative molecular weight ratio of hydrophobic core (i.e., PS segments) blocks and hydrophilic corona shells. Furthermore, in the case of incorporating hydrophobic fluorescent dyes into the PS core blocks of micelles, the hairy/hairy BCM multilayers showed well-defined fluorescent images after colloidal template removal process. These phenomena are mainly caused by the relatively high degree of electrostatic interdigitation between the protonated and anionic corona block shells.     

The structure of self-assembled multilayers with polyoxometalate nanoclusters

Using electrostatic layer-by-layer self-assembly (ELSA), the formation of multilayers with polyelectrolytes and nanoscopic polyoxometalate (POM) clusters of different sizes and charges is investigated. The multilayers are characterized by UV-vis absorption spectroscopy, optical ellipsometry, cyclic voltammetry, and atomic force microscopy. In all cases, it is possible to find experimental conditions to achieve irreversible adsorption and regular multilayer deposition. Most importantly, the surface coverage is directly related to the total charge of the POM anion and can be controlled from submonolayer to multilayer coverage by adjusting the ionic strength of the dipping solutions. Imaging the interfaces after POM deposition by atomic force microscopy reveals a granular surface texture with nanometer-sized features. The average interfacial roughness amounts to approximately 1 nm. Cyclic voltammetry indicates that the electrochemical properties of the POM clusters are fully maintained in the polyelectrolyte matrix, which opens a route toward practical applications such as sensors or heterogeneous catalysts. Moreover, the permeability toward electrochemically active probe molecules can be tailored through the multilayer architecture and deposition conditions. Finally, we note that despite the low total charge and comparably small size of the discrete POM anions, the multilayers are remarkably stable. This work provides basic guidelines for the assembly of POM-containing ELSA multilayers and provides detailed insight into characteristic surface coverage, permeability, and electrochemical properties.     

Preparation of water-soluble sodium deoxycellulosesulfonate from homogeneously prepared tosylcellulose

Water-soluble sodium deoxycellulosesulfonate (DCS-Na) was prepared by nucleophilic substitution of the p-toluenesulfonyl (tosyl) groups of cellulose p-toluenesulfonate (tosylcellulose) by a sulfonate group in aqueous Na2SO3 solution. In the substitution, the yield and degree of substitution (DS) by the sulfonate group (DSsul) were found to increase with increasing reaction temperature and with reaction time, and reached up to 80% and 0.28, respectively, at 100 °C for 72 h. Although the DS of the tosyl group (DStosyl) decreased with increasing reaction temperature and with reaction time, a respectable amount of tosyl group still remained even at 100 °C for 72 h. Furthermore, the intrinsic viscosity, [], of the DCS-Na obtained decreased considerably with increasing reaction temperature and with reaction time. The decreases in DStosyl and [] were very similar to each other in that increasing DSsul was independent of the reaction temperature and the reaction time. The similarity of the decreases suggests that the mechanisms of scission of the cellulose backbone and the leaving of the tosyl groups (from tosylcellulose) in the nucleophilic substitution were closely related to each other. The partial conversion of the tosyl group in the tosylcellulose to the sulfonate group, by the nucleophilic substitution, was also confirmed by the change in the IR absorption spectrum. The product could be considered to be a ter-polymer from the point of view of the AGU (anhydro glucose unit). Thus, we have AGU-co-DAGUS-Na-tosyl AGU as possible options     

SERS in PAH-Os and gold nanoparticle self-assembled multilayers

We present a detailed structural and surface-enhanced Raman scattering (SERS) study of poly(allylamine) modified with Os(byp)2ClPyCHO (PAH-Os) and gold nanoparticles self-assembled multilayers [PAH-Os+(Au-nanoparticlesPAH-Os)n, n=1 and 5]. Atomic force microscopy and variable-angle spectroscopic ellipsometry measurements indicate that the first nanoparticle layer grows homogenously by partially covering the substrate without clustering. Analyzing the sample thickness and roughness we infer that the growth process advances thereafter by filling with nanoparticles the interstitial spaces between the previously adsorbed nanoparticles. After five immersion steps the multilayers reach a more compact structure. The interaction between plasmons of near-gold nanoparticles provides a new optical absorption around 650 nm which, in addition, allows a more effective SERS process in that spectral region than at the single-plasmon resonance (approximately 530 nm). We compare the electronic resonance Raman and SERS amplification mechanisms in these self-assembled multilayers analyzing Raman resonance scans and Raman intensity micromaps. As a function of nanoparticle coverage we observe large changes in the Raman intensity scans, with maxima that shift from the electronic transitions, to the plasmon resonance, and finally to the coupled-plasmon absorption. The Raman micromaps, on the other hand, evidence huge intensity inhomogeneities which we relate to "hot spots." Numerical discrete dipole approximation calculations including the interaction between gold nanoparticles are presented, providing a qualitative model for the coupled-plasmon absorption and redshifted Raman hot spots in these samples.     

Direct visualization of layer-by-layer self-assembled multilayers of organometallic polymers

Direct visualization of organometallic-organic and novel all-organometallic multilayer superlattices prepared by layer-by-layer assembly of cationic/anionic polyferrocenylsilane and anionic polystyrene sulfonate polyelectrolytes using a gold coating/transmission electron microscopy (TEM) technique is reported.     

Polyelectrolyte multilayers prepared on hydrogel surfaces

Poly(vinyl alcohol) hydrogels were alternately immersed in aqueous solutions of oppositely charged polymers. The adsorption of the cationic dye methylene blue to the immersed hydrogels suggested the presence of a coating on the hydrogel surfaces. Static contact angles with an air bubble in water showed layer‐by‐layer growth of the films. The films could be transferred onto solid substrates for mechanical strength after the hydrogels were placed on the solid substrates, and this resulted in an estimation of the film thickness. The number of assembly steps could regulate the film thickness. We present here coatings of hydrogels with thin polymer films prepared by layer‐by‐layer assembly. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1062–1067, 2005     

Layer-by-layer self-assembled multilayers of redox polyelectrolytes and gold nanoparticles

Construction and characterization of structural and charge transport properties of electrostatically LbL self-assembled multilayers of gold nanoparticles and a viologen-based redox-active polyelectrolyte is reported.     

Surface modification by self-assembled polycation/azo dye multilayers

The formation and stability of some polycation/azo dye multilayers by the alternate adsorption of one polycation of integral type having 95 mol% of N,N-dimethyl-2-hydroxypropyleneammonium chloride repeat units (PCA₅) and four multicharged azo dyes: Crocein Scralet MOO (CSMOO), Ponceau SS (PSS), Direct Blue 1 (DB 1) and Direct Red 80 (DR 80) are discussed in this paper. The dyes were different either by the number of sulfonic groups (PSS, DB 1 and DR 80) or by their position (CSMOO and PSS). The multilayer growth, when PCA₅ was adsorbed from 0.2 M Na₂SO₄ aqueous solution, was followed by UV-Vis spectroscopy and by electrokinetic measurements. Solid state polycation/azo dye complexes were also prepared to get qualitative information on the charge compensation in the multilayers.     

Defect-free polymer multilayers prepared via chemoselective immobilization

In this paper, we investigated electrochemical properties of polymer multilayers on gold substrates using impedance spectroscopy. The multilayer was prepared by chemoselective ligation between aldehyde- and oxyamine-functionalized polymers via a layer-by-layer approach. The impedance spectra in a buffer solution in the absence of redox species revealed the formation of highly impermeable and defect-free films. The dielectric thickness of the polymer film, which is proportional to the reciprocal of capacitance, linearly increased as the number of deposition layer increased. The defect area of the polymer multilayer was obtained using the faradaic impedance with redox species. The surface coverage of eight polymer layers was determined to be 99.99%. Thus, the layer-by-layer deposition via chemoselective ligation offers a new way to prepare a highly insulating and defect-free polymer layer with finely tunable capacitance as a function of the number of deposition layers.     

Layer by layer self-assembled polyelectrolyte multilayers with embedded phospholipid vesicles

We describe a method to embed phospholipid vesicles into polyelectrolyte multilayers built up by the alternate deposition of polyanions and polycations. Before deposition, the vesicles are rigidified by polycation adsorption onto their surface avoiding their fusion once deposited on the multilayer surface. The vesicles adsorb to form a compact and "hard" monolayer as imaged by atomic force microscopy. The thickness of the adsorbed vesicle layer, of the order of 250 nm, is very close to the diameter of the vesicles in solution. This work should open the route to the buildup of multilayer films containing phospholipid vesicles that could act as "reservoirs" for drugs or enzymatic nanoreactors.     

Orange-red luminescence from Cu doped CdS nanophosphor prepared using mixed Langmuir-Blodgett multilayers

Cu doped CdS nanophosphors were fabricated through Langmuir-Blodgett route for the first time. Precursors mixed Langmuir-Blodgett multilayers of cadmium arachidate-copper arachidate were used to grow doped sulfide nanoparticles within the organic matrix through postdeposition treatment with H(2)S gas. Changes in composition and layered structure of precursor multilayers were studied using Fourier transform infrared and x-ray reflection. Uptake of Cu in the multilayers was analyzed by inductively coupled plasma atomic emission spectroscopy measurements. Unannealed H(2)S exposed multilayers containing CdS nanoparticles show strong surface state emission centered at approximately 570 nm, whereas Cu doped CdS nanoparticles show orange-red luminescence. Photoluminescence (PL) spectra of annealed-Cu doped CdS nanoparticles show distinct Cu-related emission compared to annealed-undoped CdS nanoparticles. Power dependent PL measurements of annealed samples show that an efficient carrier recombination takes place at T(2) level of Cu(++). The carrier relaxation from the excitonic states to T(2) level results in the strong orange-red luminescence.     

Organized multilayers of polydiacetylenes prepared by electrostatic self-assembly

New types of polydiacetylene multilayer are presented. The first type is based on electrostatic self-organization of diacetylene bolaamphiphiles and polyelectrolytes on a charged substrate followed by subsequent ultraviolet (UV) polymerization. The second type is prepared by direct adsorption of a water soluble polydiacetylene and a polyelectrolyte in alternating sequence. The monomeric diacetylenes are of general formula X(CH₂)₉CCC C(CH₂)₉X, with X being a sulfate (1a), phosphate (2) or pyridinium (3) head group. The polydiacetylene (1b) chosen for the multilayer is obtained by γ irradiation of the corresponding diacetylene monomer 1a. It is found that all diacetylene derivatives are well suited for building up self-assembled multilayers and that two of the monomers (1a, 2) can be polymerized on the substrate, while 3 is photo-inactive. The morphology of the multilayers is studied by scanning force microscopy and discussed. The smoothest surface topology is found for multilayers built up from the polydiacetylene 1b and a cationic polyelectrolyte in alternating sequence, while the largest unevenness is found when the anionic diacetylene 1a is alternatingly adsorbed with the cationic bolaamphiphile 3 followed by subsequent UV polymerization on the substrate.     

Design of fluorescent self-assembled multilayers and interfacial sensing for organophosphorus pesticides

This paper details the fabrication of indole (ID) self-assembled multilayers (SAMs) and fluorescence interfacial sensing for organophosphorus (OP) pesticides. Quartz/APES/AuNP/l-Cys/ID film was constructed on l-cysteine modified Quartz/APES/AuNP surface via electrostatic attraction between ID and l-cysteine. Cyclic voltammetry indicates that ID is immobilized successfully on the gold surface. Fluorescence of the Quartz/APES/AuNP/l-Cys/ID film shows sensitive response toward OPs. The fluorescent sensing conditions of the SAMs are optimized that allow linear fluorescence response for methylparathion and monocrotophos over 5.97 × 10⁻⁷ to 3.51 × 10⁻⁶ g L⁻¹ and 3.98 × 10⁻⁶ to 3.47 × 10⁻⁵ g L⁻¹, with detection limit of 6.1 × 10⁻⁸ gL⁻¹ and 3.28 × 10⁻⁶ gL⁻¹, respectively. Compared to bulk phase detection, interfacial fluorescence sensing based on the SAMs technology shows higher sensitivity by at least 2 order of magnitude.     

Photocatalytic self-assembled solid porphyrin microcrystals from water-soluble porphyrins: Synthesis,characterization and application

In this article, we report on the formation of a photocatalytic porphyrin crystalline structure using two oppositely charged commercially available low cost porphyrins, [meso-tetra(N-methy-4-pyridyl)porphyrin tetratosylate (TMPyP) and zinc-tetrakis(4-sulfonatophenyl)porphyrin (Zn-TPPS)], by self-assembly at room temperature without acidification. Using optical microscopy, the crystals were determined to have a length of 30–55 μm and width of 2–50 μm, depending on the molar ratio of the porphyrins in the starting solution. The porphyrin crystals were characterized by means of powder X-ray diffraction and UV–Vis, fluorescence, and optical microscopic techniques. The UV–Vis absorbance spectrum of the crystalline structure is different than those of the monomer solutions, involving a broadened Soret band that is split into two blue-shifted and red-shifted peaks and broadened red-shifted Q-bands. The crystals exhibit a different emission spectrum from those of the porphyrin solutions in that they are red-shifted, split, and show a dramatic decrease in intensities. A hypothetical model for the crystal structure of the porphyrin crystals is developed. Illumination of the crystals in a 2,4,6-trinitrotoluene solution with a tungsten lamp results in TNT reduction to 1,3,5-trinitrobenzene and 4-amino-2,6-dinitrotoluene.     

Oriented thin films from soluble polythiophenes

The degree of orientation of thin films of eight different polymeric structures, belonging to the class of soluble polyalkylthiophenes (PATs) has been studied. Thin films of the polymers, obtained by spin coating onto glass substrates, were oriented by means of the rubbing technique. The degree of orientation is related to the regioregularity of the system: highly regioregular polymers can be oriented, while regiorandom materials are not oriented. The degree of orientation can be improved by thermal annealing of the oriented films. As a result of annealing, which was performed at different temperatures according to the thermal behavior of the polymers, it was possible to increase the polarization ratio for poly(3-decylthiophene) up to 12–13 as detected from the UV-vis spectra in polarized light. Moreover in the investigated PATs, both the key role of molecular weight and its distribution for achieving a high orientation degree has been assessed. Copyright © 2003 John Wiley & Sons, Ltd.     

Electrochemical characterization of polyelectrolyte/gold nanoparticle multilayers self-assembled on gold electrodes

Polyelectrolyte/gold nanoparticle multilayers composed of poly(l-lysine) (pLys) and mercaptosuccinic acid (MSA) stabilized gold nanoparticles (Au NPs) were built up using the electrostatic layer-by-layer self-assembly technique upon a gold electrode modified with a first layer of MSA. The assemblies were characterized using UV-vis absorption spectroscopy, cyclic and square-wave voltammetry, electrochemical impedance spectroscopy, and atomic force microscopy. Charge transport through the multilayer was studied experimentally as well as theoretically by using two different redox pairs [Fe(CN)(6)](3-/4-) and [Ru(NH(3))(6)](3+/2+). This paper reports a large sensitivity to the charge of the outermost layer for the permeability of these assemblies to the probe ions. With the former redox pair, dramatic changes in the impedance response were obtained for thin multilayers each time a new layer was deposited. In the latter case, the multilayer behaves as a conductor exhibiting a strikingly lower impedance response, the electric current being enhanced as more layers are added for Au NP terminated multilayers. These results are interpreted quite satisfactorily by means of a capillary membrane model that encompasses the wide variety of behaviors observed. It is concluded that nonlinear slow diffusion through defects (pinholes) in the multilayer is the governing mechanism for the [Fe(CN)(6)](3-/4-) species, whereas electron transfer through the Au NPs is the dominant mechanism in the case of the [Ru(NH(3))(6)](3+/2+) pair.