Layer-by-layer self-assembly of polyelectrolyte complexes and their multilayer films for pervaporation dehydration of isopropanol

Two negatively charged polyelectrolyte complex colloidal nanoparticles (PEC⁻) and one positively charged nanoparticle (PEC⁺) were prepared and used as novel layer-by-layer (LbL) building blocks. These PEC nanoparticles include poly(2-methacryloyloxy ethyl trimethylammonium chloride)/sodium carboxymethyl cellulose (PDMC/CMCNa PEC⁻), poly(diallyldimethylammonium chloride)/CMCNa (PDDA/CMCNa PEC⁻) and PDDA/poly(sodium-p-styrenesulfonate) (PDDA/PSS PEC⁺). LbL multilayer films based on (PEC⁺/PEC⁻) were constructed on both quartz slides and modified polyamide (MPA) reverse osmosis support membranes. UV–vis spectroscopy, quartz crystal microbalance (QCM), field emission scanning microscopy (FESEM) and atomic force microscopy (AFM) were utilized to follow the thickness growth and morphology evolution of these multilayer films with increasing bi-layer numbers. LbL multilayer films deposited on MPA support membranes were subjected to pervaporation dehydration of 10 wt% water–isopropanol and effect of bi-layer numbers and feed temperature on pervaporation performance was studied. Generally, PEC⁺/PEC⁻ can be LbL self-assembled successfully on both substrates with a thickness growth rate ca. 200 nm/bi-layer. Moreover, PEC⁺/PEC⁻ multilayer films show high pervaporation performance with film thickness up to several micrometers. For example, performance of the multilayer films in dehydrating 10 wt% water–isopropanol at 50 °C is J = 1.18 kg/m² h, α = 1013 for (PEC⁺/PDMC-CMCNa PEC⁻)₂₄ and J = 1.36 kg/m² h, α = 938 for (PEC⁺/PDMC-CMCNa PEC⁻)₂₅, respectively.     

Layer-by-layer engineered Co-Al hydroxide nanosheets/graphene multilayer films as flexible electrode for supercapacitor

Multilayer films of Co-Al layered double hydroxide nanosheets (Co-Al LDH-NS) and graphene oxide (GO) were fabricated through layer-by-layer (LBL) assembly. By using a three-electrode system, the electrochemical performances of the films were investigated to evaluate their potential as electrode materials to be used in flexible supercapacitor devices. The Co-Al LDH-NS/GO multilayer films exhibited a high specific capacitance of 880 F/g and area capacitance of 70 F/m(2) under the scan rate of 5 mV/s. And the film exhibited good cycle stability over 2000 cycles. After treating the films at 200 °C in H(2) atmosphere, the specific capacitance and area capacitance were largely increased up to 1204 F/g and 90 F/m(2) due to partial reduction of GO. A flexible electrode by depositing Co-Al LDH-NS/GO multilayer film onto PET substrate was prepared to show the potential of Co-Al LDH-NS/GO films for flexible energy storage.     

Hydrophobically modified polyelectrolyte for improved oxygen barrier in nanobrick wall multilayer thin films

The influence of attaching hydrophobic side groups to a polyelectrolyte, used for deposition of a multilayer oxygen gas barrier thin film, was investigated. Polyethyleneimine (PEI) was labeled with pyrene and deposited in “quadlayers” of PEI, poly(acrylic acid), PEI, and sodium montmorillonite clay using layer‐by‐layer assembly. Thin films made of three repeating quadlayers using unmodified PEI had much lower density (1.24 g/cm³) than pyrene‐labeled PEI‐based films (1.45 g/cm³), which is believed to be the result of greater chain coiling from the increased hydrophobicity of pendant pyrene groups. This increased density in pyrene‐labeled PEI layers allowed three quadlayers to match the oxygen transmission rate of a four quadlayer film made with unmodified PEI. This discovery provides an additional tool for tailoring the barrier behavior of clay‐based multilayer thin films that could prove useful for a variety of packaging applications. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1153–1156     

Phospholipid bilayers as biomembrane-like barriers in layer-by-layer polyelectrolyte films

Dipalmitoylphosphatidylcholine (DPPC) bilayer was created on the surface of an exponentially growing poly(glutamic acid)/poly(lysine) (PGA/PLL) layer-by-layer polyelectrolyte film. The lipid bilayer decreased the surface roughness of the polyelectrolyte film. The layer-by-layer construction of the polyelectrolyte film could be continued on the top of the DPPC layer. The lipid bilayer, however, formed a barrier in the interior of the polyelectrolyte film, which blocked the diffusion (a prerequisite for exponential growth) of the polyelectrolytes. Thus, a new growth regime started in the upper part of the polyelectrolyte film, which was added to embed the DPPC bilayer. The structure and the dynamics of the DPPC bilayer on the polyelectrolyte film surface remained similar to that of its hydrated multibilayers, except that the phase transition became wider. In the case of embedded DPPC bilayers, in addition, the phase-transition temperature also decreased. This is the result of interactions with the nonconcerted movements of the barrier-separated lower and higher parts of the polyelectrolyte film. Gramicidin A (GRA) as a model of lipid-soluble peptides and proteins was successfully incorporated into such DPPC films. The DPPC films, either with or without GRA, were remarkably stable; as many heating-cooling cycles to measure phase transition could be carried out without visible alterations as wanted.     

Layer-by-layer deposition of polyelectrolyte macroinitiators for enhanced initiator density in surface-initiated ATRP

The layer-by-layer (L-b-L) deposition of oppositely charged polyelectrolytic macroinitiators has been demonstrated on planar silica substrates. The build-up of the macroinitiator multilayers was monitored by ellipsometry (up to 21 layers) and dual polarization interferometry (up to 17 layers) and good agreement was found between these techniques. The increase in L-b-L thickness was approximately linear, with an average thickness of 2.3 A per layer of deposited macroinitiator. Surface-initiated ATRP of a model nonionic methacrylic monomer, 2-hydroxyethyl methacrylate (HEMA) in a 1:1 methanol/water mixture was conducted at ambient temperature. Increasing the number of macroinitiator layers led to a significant increase in PHEMA brush thickness up to 110 nm, which is attributed to the greater surface grafting density. PHEMA brush thicknesses obtained after 22 h showed a linear dependence on the number of layers of deposited macro-initiator, with all layers exhibiting near-identical growth kinetics. X-ray photoelectron spectroscopy was used to monitor L-b-L assembly and also to confirm PHEMA growth. This technique indicated the loss of small counterions from the multilayers during L-b-L deposition and confirmed an increase in the surface density of bromoester initiator groups as the number of deposited macroinitiator layers was increased. For 17 macroinitiator layers, the bromoester initiator density is estimated to be approximately 4.9 +/- 0.2 nm (-2) from the DPI data. This is comparable to that calculated for ATRP initiator monolayers obtained by either thiol or silane chemistry. Ellipsometry suggested that the macroinitiator multilayers were weakly hydrated prior to the in situ HEMA polymerization. AFM studies indicated that the PHEMA brushes had appreciable surface roughness, but this roughness became negligible compared to the brush thickness with increasing macroinitiator layers.     

Freestanding polyelectrolyte films as sensors for osmotic pressure.

Freestanding ultrathin polyelectrolyte-multilayer membranes, transferred to topographically structured polydimethylsiloxane (PDMS), are used as mechanical sensors. Due to the membranes' semipermeability, high-molecular-weight molecules can be either entrapped inside them or excluded, thus generating an osmotic pressure. This leads to a deformation. We investigate the deformation as a function of the osmotic pressure and present an analytical theory that fully describes the data. Thus, osmotic pressures can be determined quantitatively. The individual osmotic-sensitive elements have only microscopic dimensions, and arrays can be easily produced.     

Layer-by-layer self-assembly preparation of layered double hydroxide/polyelectrolyte nanofilms monitored by surface plasmon resonance spectroscopy

Layer-by-layer self-assembly was used to prepare nanofilms of (2:1) MgAl-layered double hydroxide (LDH) nanoparticles and polyacrylic acid or sodium polystyrene sulfonate. The multilayers were attached to ~50-nm thick gold films on microscopy glass slides prepared by vacuum evaporation. The contact between the gold film and the multilayered films was mediated via surface modification with thiols, adsorption of poly(diallyl dimethyl ammonium) chloride (PDDA) or direct binding of the LDH particles. Surface plasmon resonance (SPR) spectra of the multilayered films were analyzed by fitting the Fresnel equations. The shifts in the SPR angle (_SPR) due to the adsorption/deposition on the gold surface were used to evaluate the process of building up the multilayers. Strong surface/multilayer contact formed when electrostatic attraction and hydrophobic interaction were combined as in the case of mercaptopropanoic acid or PDDA sticking layers. The LDH suspension concentration strongly influenced the number of deposited layers. The multilayer films were investigated by reflection FT-IR spectroscopy.     

Photopatterned nanoporosity in polyelectrolyte multilayer films

We report on spatial control of nanoporosity in polyelectrolyte multilayer (PEM) films using photopatterning and its effects on film optical and adsorption properties. Multilayers assembled from poly(acrylic acid-ran-vinylbenzyl acrylate) (PAArVBA), a photo-cross-linking polymer, and poly(allylamine hydrochloric acid) (PAH) were patterned using ultraviolet light followed by immersion in low pH and then neutral pH solutions to induce nanoporosity in unexposed regions. Model charged small molecules rhodamine B, fluorescein, and propidium iodide and the model protein albumin exhibit increased adsorption to nanoporous regions of patterned PEM films as shown by fluorescence microscopy and radiolabeling experiments. Films assembled with alternating stacks of PAH/poly(sodium-4-styrene sulfonate) (SPS), which do not become nanoporous, and stacks of PAH/PAArVBA were patterned to create nanoporous capillary channels. Interdigitated channels demonstrated simultaneous, separate wicking of dimethyl sulfoxide-solvated fluorescein and rhodamine B. In addition, these heterostack structures exhibited patternable Bragg reflectivity of greater than 25% due to refractive index differences between the nanoporous and nonporous stacks. Finally, the PEM assembly process coupled with photo-cross-linking was used to create films with two separate stacked reflective patterns with a doubling in reflectivity where patterns overlapped. The combined adsorptive and reflective properties of these films hold promise for applications in diagnostic arrays and therapeutics delivery.     

Layer-by-layer assembled thin films composed of carboxyl-terminated poly(amidoamine) dendrimer as a pH-sensitive nano-device

Dendrimer-containing multilayer thin films have successfully been prepared by a layer-by-layer deposition of carboxyl-terminated poly(amidoamine) dendrimer (PAMAM–COOH) and poly(methacrylic acid) (PMA) on a solid surface at pH 4.0, while the multilayer film did not form at pH 7.0. The PMA/PAMAM–COOH multilayer films prepared at pH 4.0 are decomposed at neutral pH due to electrostatic repulsion between negatively-charged carboxylate residues. The results suggest that the primary force for the successful deposition of PAMAM–COOH and PMA at pH 4.0 is hydrogen bonding between COOH residues on the surface of the dendrimer and PMA. The multilayer films are decomposed also at strongly acidic pH, suggesting an electrostatic force of attraction between the protonated tertiary amino groups in PAMAM–COOH and a small fraction of COO⁻ residues in PMA contributes in part to the multilayer formation at pH 4.0. The PMA/PAMAM–COOH thin films can accommodate model dyes, Rose Bengal and 5,10,15,20-tetraphenyl-21H,23H-porphinetetrasulfonate, and the release can be controlled by changing pH.     

Layer-by-layer assembled protein/polymer hybrid films: nanoconstruction via specific recognition

In the present study it is shown that streptavidin-containing multilayer films with varying numbers of polyelectrolyte spacer layers can be fabricated reproducibly using optimized deposition conditions. Direct alternation of streptavidin and PLB leads to multilayer systems with an average streptavidin thickness of 5.3 nm which is in good agreement with the dimensions of the protein. When the streptavidin layers are spacered by more polyelectrolyte layers the distance between the protein sheets is increased up to e.g. 6.5 nm in the case of (PLB/PSS/PAH/PSS/PLB) as spacer layer. X-ray reflectivity reveals that streptavidin increases the surface roughness of the films probably due to the rigid three-dimensional structure of the protein. The control of surface roughness seems to be essential for a successful multilayer build-up. The property of PLB to provide for multilayer construction by two different interactions (electrostatic and specific) allowed to probe the interpenetration depth of adjacent layers. For the [PLB/(PSS/PL)₂/streptavidin] system an interpenetration depth of about 4 polymer layers corresponding to approximately 3.4 nm has been derived. These data are in agreement with a model for pure polyelectrolyte films obtained from neutron and X-ray reflectivity data.     

Polymer-metal complexes in polyelectrolyte multilayer films as catalysts for oxidation of toluene

We report on the binding of metal ions (Me(2+); Co(2+) and Cu(2+)) with weak polyelectrolyte multilayers (PEMs), as well as on catalytic activity of PEM-Me(2+) films for oxidation of toluene. Using several types of PEM films constructed using branched polyethyleneimine (BPEI) or quaterinized poly-4-vinylpyridines (QPVPs) as polycations and poly(acrylic acid) (PAA) or poly(styrene sulfonate) (PSS) as polyanions, we found that binding of Co(2+) and Cu(2+) ions with a PEM matrix can occur both through coordination to polycationic amino groups and/or ionic binding to polyacid groups. The amount of metal ions loaded within the film increased linearly with film thickness and was strongly dependent on polyelectrolyte type, film assembly pH, and fraction of permanent charge in polymer chains. Among various PEM-Me(2+) systems, BPEI/PAA-Co(2+) films assembled at pH 8.5 show the best catalytic performance, probably because of the preservation of high mobility of Co(2+) ions coordinated to amino groups of BPEI in these films. With BPEI/PAA-Co(2+) films, we demonstrated that films were highly permeable to reagents and reaction products within hundreds of nanometers of the film bulk; i.e., film catalytic activity increased linearly with layer number up to 30 bilayers and slowed for thicker films.     

Silver nanocomposite layer-by-layer films based on assembled polyelectrolyte/dendrimer

Silver nanocomposite multilayer films were prepared through the in situ method. Multilayer thin films, prepared through the sequential electrostatic deposition of a positively charged third-generation poly(amidoamine) dendrimer (PAMAM) and negatively charged poly(styrenesulfonate) (PSS) and poly(acrylic acid) (PAA), were utilized as nanoreactors for the formation of silver nanoparticles. The silver ions were preorganized in layer-by-layer (LBL) films composed of PAMAM dendrimers and subsequently reduced with hydrogen to prepare the silver nanoparticles. The UV-vis spectrum and profilometer were used to characterize the regular growth of bilayers. UV-vis absorption from plasmon resonance at 435 nm and TEM images indicated the formation of the silver nanoparticles in the multilayer films. The silver nanocomposite LBL films were also constructed on the indium tin oxide-glass and investigated using cyclic voltammetry. The silver nanoparticles in the multilayer films have a stronger negative redox potential. The silver nanocomposite LBL films may have a potential application in the catalysis of reduction of 4-nitrophenol with sodium borohydride.     

Single-component layer-by-layer weak polyelectrolyte films and capsules: Loading and release of functional molecules

Poly(carboxylic acid) hydrogel films and hollow capsules undergo reversible size changes in response to variations in pH and/or ionic strength. The films and capsules were obtained from hydrogenbonded poly-N-vinylpyrrolidone/poly(carboxylic acid) layer-by-layer films by chemical crosslinking of the polyacid, followed by pH-induced removal of poly-N-vinylpyrrolidone. Surface-attached hydrogel films present attractive matrices for reversible pH-stimulated loading and/or controlled release of large amounts of synthetic or natural macromolecules including proteins. By varying acidity of poly(carboxylic acids), the hydrogel swelling and the corresponding values of pH for encapsulation/release of functional molecules could be tuned in a wide range from pH 5 to 10. In addition, the capsules are capable of entrapping macromolecules by “locking” the capsule wall with an electrostatically associating polycation, followed by the release of the encapsulated macromolecules at high salt concentrations. The text was submitted by the authors in English.     

Layer-by-layer deposited organic/inorganic hybrid multilayer films containing noncentrosymmetrically orientated azobenzene chromophores

Organic/inorganic hybrid multilayer films with noncentrosymmetrically orientated azobenzene chromophores were fabricated by the sequential deposition of ZrO2 layers by a surface sol-gel process and subsequent layer-by-layer (LbL) adsorption of the nonlinear optical (NLO)-active azobenzene-containing polyanion PAC-azoBNS and poly(diallyldimethylammonium chloride) (PDDA). Noncentrosymmetric orientation of the NLO-active azobenzene chromophores was achieved because of the strong repulsion between the negatively charged ZrO(2) and the sulfonate groups of the azobenzene chromophore in PAC-azoBNS. Regular deposition of ZrO(2)/PAC-azoBNS/PDDA multilayer films was verified by UV-vis absorption spectroscopy and quartz crystal microbalance measurements. Both UV-vis absorption spectroscopy and transmission second harmonic generation (SHG) measurements confirmed the noncentrosymmetric orientation of the azobenzene chromophores in the as-prepared ZrO2/PAC-azoBNS/PDDA multilayer films. The square root of the SHG signal (I(2omega)(1/2)) increases with the increase of the azobenzene graft ratio in PAC-azoBNS as the number of deposition cycles of the ZrO(2)/PAC-azoBNS/PDDA films remains the same, while the second-order susceptibility chi(zzz)(2) of the film decreases with the increase of the azobenzene graft ratio. Furthermore, the present method was successfully extended to realize the noncentrosymmetric orientation of azobenzene chromophores in multilayer films when small organic azobenzene compounds with carboxylic acid and/or hydroxyl groups at one end and sulfonate groups at the other end were used. The present method was characterized by its simplicity and flexibility in film preparation, and it is anticipated to be a facile way to fabricate second-order nonlinear optical film materials.     

Polarity of layer-by-layer deposited polyelectrolyte films as determined by pyrene fluorescence.

The polarity of polyelectrolyte (PE) multilayer films is investigated with pyrene as a polarity-sensitive probe. Multilayer films of poly(styrene sulfonate) (PSS) and various polycations were prepared by the layer-by-layer self-assembly technique. Pyrene (PY) molecules were inserted into the films by exposing the multilayers to pyrene solutions. By this method a homogeneous distribution of pyrene molecules at low concentration within the film was obtained. The ratio of the fluorescence intensities of the first (I) to the third (III) vibronic band (Py-value) of the pyrene emission spectrum is employed here to determine the polarity of the PE films. PSS and poly(allylamine hydrochloride) (PAH) multilayer films yielded a pyrene value close to the solvent polarity of acetone, while multilayers of PSS and poly(diallyldimethylammonium chloride) (PDADMAC) displayed a value higher than the one corresponding to water. The pyrene values of the polyelectrolyte films were independent from the solvent employed for probe dissolving. Although no direct relationship between solvent polarity and dielectric constant (epsilon) is available, an estimate of the static dielectric constant of the films can be provided by comparing the Py-values of the films with those of various solvents. Changes in the humidity conditions of the film environment in a closed cell did not affect the film polarity. However, a drastic and irreversible reduction of polarity could be induced by actively drying the samples by a nitrogen flow.     

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.     

Redox and acid-base coupling in ultrathin polyelectrolyte films

A single layer of poly(allylamine) with a covalently attached osmium pyridine-bipyridine complex adsorbed onto a Au surface modified by mercaptopropanesulfonate has been studied theoretically with a molecular approach and experimentally by cyclic voltammetry. These investigations have been carried out at different pHs and ionic strengths of the electrolyte solution in contact with the redox polyelectrolyte modified electrode. The theory predicts strong coupling between the acid-base and redox equilibria, particularly for low ionic strength, pH close to the pKa, and high concentration of redox sites. The coupling leads to a decrease in the peak potential at pH values above the apparent pKa of the weak polyelectrolyte, in good agreement with the experimental pH dependence at 4 mM NaNO3. Theoretical calculations suggest that the inflection point in the peak position versus pH curves can be used to estimate the apparent pKa of the amino groups in the polymer. Comparison of the apparent pKa for PAH-Os in the film with that of poly(allylamine) reported in the literature shows that the underlying charged thiol strongly influences charge regulation in the film. A systematic study of the film thickness and the degree of protonation in sulfonate and amino groups for solutions of different pH and ionic strength shows the coupling between the different interactions. It is found that the variation of the film properties has a non-monotonic dependence on bulk pH and salt concentration. For example, the film thickness shows a maximum with electrolyte ionic strength, whose origin is attributed to the balance between electrostatic amino-amino repulsions and amino-sulfonate attractions.     

Cellulose-based functional polycarbonates as degradable enzyme carriers

Cellulose-based matrices are expected to be ideal enzyme carriers due to their sustainability and biocompatibility. However, the linkages between immobilized enzymes and celluloses often suffer from low-density and non-biodegradability, leading to inefficient loading of enzymes as well as persistent generation of solid wastes after reuse. In the present study, cellulose-based functional materials with degradable polycarbonates brushes have been successfully synthesized as enzyme carriers via ring-opening polymerization of 5-methyl-5-allyloxycarbonyl-1,3-dioxan-2-one (MAC) following with ally epoxidation of MAC units. After covalent bonding with laccase, the resulting HPC-PMAC-Laccase could assemble in aqueous solution to form spherical nanoparticles with an enzyme immobilization efficiency of 88%. The immobilized laccase showed more tolerance towards pH and high temperature compared with free laccase. Moreover, the immobilized laccase demonstrated effective removal efficiency of bisphenol A and reached 83% in 3 h. After repeated usage for 8 times, the HPC-PMAC-Laccase still maintained relatively high enzyme activity. Especially, the polycarbonates brushes in the enzyme carriers could be totally hydrolyzed in 12 h to achieve its degradable property.

Graphical abstract

Cellulose-based functional polycarbonates as degradable enzyme carriers.