Disintegration-controllable stimuli-responsive polyelectrolyte multilayer microcapsules via covalent layer-by-layer assembly

The disintegration-controllable stimuli-responsive polyelectrolyte multilayer microcapsules have been fabricated via the covalent layer-by-layer assembly between the amino groups of chitosan (CS) and the aldehyde groups of the oxidized sodium alginate (OSA) onto the sacrificial templates (polystyrene sulfonate, PSS) which was removed by dialysis subsequently. The covalent crosslinking bonds of the multilayer microcapsules were confirmed by FTIR analysis. The TEM analysis showed that the diameter of the multilayer microcapsules was <200nm. The diameter of the multilayer microcapsules decreased with the increasing of the pH values or the ionic strength. The pH and ionic strength dual-responsive multilayer microcapsules were stable in acidic and neutral media while they could disintegrate only at strong basic media.     

Combining hydrogen-bonding complexation in solution and hydrogen-bonding-directed layer-by-layer assembly for the controlled loading of a small organic molecule into multilayer films

We have combined hydrogen-bonding complexation in solution and layer-by-layer assembly for the controlled loading of a water-insoluble small organic molecule, bis-triazine (DTA), an azobenzene derivative containing multiple hydrogen bond donors and acceptors, into layer-by-layer multilayer films of poly(acrylic acid) and diazo-resin. UV-visible spectroscopy indicates that DTA has been loaded into multilayer films, with the loading amount increasing linearly with the number of layers. The loading amount can be well tuned either by changing the concentration of DTA or the solvent composition at the complexation step. Fourier transform infrared spectroscopy has revealed that both the complexation and layer-by-layer assembly are driven by hydrogen bonding. After photo-cross-linking and immersion in dimethyl sulfoxide to release DTA, the film can serve as an absorbent for DTA. This study provides a new unconventional layer-by-layer assembly that combines hydrogen-bonding complexation in solution and hydrogen-bond-driven layer-by-layer assembly at the interface. This method provides a new route to load a variety of water-insoluble functional organic molecules into layer-by-layer films.     

Molecular dynamics simulations of multilayer polyelectrolyte films: effect of electrostatic and short-range interactions

The effect of the strength of electrostatic and short-range interactions on the multilayer assembly of oppositely charged polyelectrolytes at a charged substrate was studied by molecular dynamics simulations. The multilayer buildup was achieved through sequential adsorption of charged polymers in a layer-by-layer fashion from dilute polyelectrolyte solutions. The strong electrostatic attraction between oppositely charged polyelectrolytes at each deposition step is a driving force behind the multilayer growth. Our simulations have shown that a charge reversal after each deposition step is critical for steady multilayer growth and that there is a linear increase in polymer surface coverage after the first few deposition steps. Furthermore, there is substantial intermixing between chains adsorbed during different deposition steps. We show that the polymer surface coverage and multilayer structure are each strongly influenced by the strength of electrostatic and short-range interactions.     

Direct surface force measurements of polyelectrolyte multilayer films containing nanocrystalline cellulose

Polyelectrolyte multilayer films containing nanocrystalline cellulose (NCC) and poly(allylamine hydrochloride) (PAH) make up a new class of nanostructured composite with applications ranging from coatings to biomedical devices. Moreover, these materials are amenable to surface force studies using colloid-probe atomic force microscopy (CP-AFM). For electrostatically assembled films with either NCC or PAH as the outermost layer, surface morphology was investigated by AFM and wettability was examined by contact angle measurements. By varying the surrounding ionic strength and pH, the relative contributions from electrostatic, van der Waals, steric, and polymer bridging interactions were evaluated. The ionic cross-linking in these films rendered them stable under all solution conditions studied although swelling at low pH and high ionic strength was inferred. The underlying polymer layer in the multilayered film was found to dictate the dominant surface forces when polymer migration and chain extension were facilitated. The precontact normal forces between a silica probe and an NCC-capped multilayer film were monotonically repulsive at pH values where the material surfaces were similarly and fully charged. In contrast, at pH 3.5, the anionic surfaces were weakly charged but the underlying layer of cationic PAH was fully charged and attractive forces dominated due to polymer bridging from extended PAH chains. The interaction with an anionic carboxylic acid probe showed similar behavior to the silica probe; however, for a cationic amine probe with an anionic NCC-capped film, electrostatic double-layer attraction at low pH, and electrostatic double-layer repulsion at high pH, were observed. Finally, the effect of the capping layer was studied with an anionic probe, which indicated that NCC-capped films exhibited purely repulsive forces which were larger in magnitude than the combination of electrostatic double-layer attraction and steric repulsion, measured for PAH-capped films. Wherever possible, DLVO theory was used to fit the measured surface forces and apparent surface potentials and surface charge densities were calculated.     

Direct transfer of preformed patterned bio-nanocomposite films on polyelectrolyte multilayer templates

Microarrays containing multiple, nanostructured layers of biological materials would enable high-throughput screening of drug candidates, investigation of protein-mediated cell adhesion, and fabrication of novel biosensors. In this paper, we have examined in detail an approach that allows high-quality microarrays of layered, bionanocomposite films to be deposited on virtually any substrate. The approach uses LBL self-assembly to pre-establish a multilayered structure on an elastomeric stamp, and then uses microCP to transfer the 3-D structure intact to the target surface. For examples, different 3-D patterns containing dendrimers, polyelectrolyte multilayers and two proteins, sADH and sDH, have been fabricated. For the first time, the approach was also extended to create overlaid bionanocomposite patterns and multiple proteins containing patterns. The approach overcomes a problem encountered when using microCP to establish a pattern on the target surface and then building sequential layers on the pattern via LBL self-assembly. Amphiphilic molecules such as proteins and dendrimers tend to adsorb both to the patterned features as well as the underlying substrate, resulting in low-quality patterns. By circumventing this problem, this research significantly extends the range of surfaces and layering constituents that can be used to fabricate 3-D, patterned, bionanocomposite structures. [image in text]     

Photoelectrochemical study of organic–inorganic hybrid thin films via electrostatic layer-by-layer assembly

The stepwise assembly of negatively charged organic molecules (poly(sodium 4-styrenesulfonate) (PSS) or tetrasodium-meso-tetra(4-sulfonatophenyl) porphine (TPPS)) and positively charged TiO₂ colloids on pretreated substrate surfaces utilizing the layer-by-layer (LbL) approach was investigated. The step-by-step formation of these films was studied by UV–vis spectrophotometry and electrochemistry. Photocurrent was generated upon light irradiation of the hybrid thin films assembled on fluorine-doped tin oxide (FTO) conducting glass, which increased linearly as the deposited bilayers increased. In addition, compared to PSS/TiO₂ hybrid thin films, the enhancement of the generated photocurrent and the photocurrent response within the wavelength range from 400 to 450 nm were observed in the TPPS/TiO₂ hybrid thin films. This was attributed to the dye-sensitized effect of the layered TPPS molecules. It was demonstrated that electrostatic LbL films were attractive systems for the photoelectrochemical investigation, and the control of the generated photocurrent could be achieved by the structure of the multilayered films.     

Tunable disintegration of layer-by-layer assembly multilayer films based on hydrolytical-polybetaine at wide-range time

Many efforts have been performed on the poly(vinylidene fluoride), PVDF, due to its piezoelectric, pyroelectric and ferroelectric potentials. In this regard, how to fabricate the PVDF with high content of β-phase, which is also the direct contribution to PVDF's prominent property, becomes a critical issue. In this study, starting with the α-phase dominated sample, the PVDF with extremely high content of β-crystalline phase was obtained by the incorporation of multiwalled carbon nanotubes (MWCNTs) modified by hyperbranched copolymers (HBCs). We proved that, via XRD, DSC as well as the structural characterizations from the polarized optical microscopy and transmission electron microscopy (TEM), the success of this strategy was ascribed to the enhanced dispersibility and stability of MWCNTs endowed by the HBCs, which significantly favors the formation of the β-crystalline phase of PVDF.     

A polyelectrolyte bearing metal ion receptors and electrostatic functionality for layer-by-layer self-assembly

A polyelectrolyte (BiPE) containing bipyridine ligands as metal ion receptors and quaternary ammonium groups is described, which can be assembled via electrostatic interactions or metal ion coordination. Electrostatic layer-by-layer self-assembly of BiPE with sodium poly(styrene sulfonate) (PSS) as oppositely charged component results in striated multilayers. The BiPE/PSS multilayers can reversibly bind and release transition metal ions including Fe(II), Ni(II), and Zn(II). Formation of 2-D arrays of metallo-units is achieved by μ-contact stamping transition metal salts onto the BiPE/PSS interface. Also, multilayers of BiPE are readily assembled through metal ion coordination. Due to the reversible nature of metal ion coordination, exposure of the multilayers to EDTA causes instant disassembly of the layer, a property needed to implement stimulus triggered release functions. The importance of metal ion coordination for multilayer formation is demonstrated by force-distance curves measured with AFM.     

Metal ion-enriched polyelectrolyte complexes and their utilization in multilayer assembly and catalytic nanocomposite films

The mixing of Ag ion-doped poly(ethyleneimine) (PEI) and poly(acrylic acid) (PAA) produced Ag ion-doped polyelectrolyte complex particles (PECs) in solution. Positively charged Ag ion-doped PECs (Ag ion PECs) with a spherical shape were deposited alternatively with PAA to form a multilayer assembly. The multilayered film containing Ag ion PECs was reduced to generate a composite nanostructure. Metal nanoparticle (NP)-enriched nanocomposite films were formed by an additional process of the postadsorption of precursors on PECs within the nanocomposite films, which resulted in the enhancement of the catalytic and electrical properties of the composite films. Because the films contain PECs that are responsive to changes in pH and most of the NPs are embedded in the PECs, interesting catalytic properties, which are unexpected in a particle-type catalyst, were observed upon pH changes. As a result of the reversible structural changes of the films and the immobilization of the NPs within the films, the film-type catalysts showed enhanced performance and stability during catalytic reactions under various pH conditions, compared to particle-type catalysts.     

Electrochemical study of methylene blue/titanate nanotubes nanocomposite and its layer-by-layer assembly multilayer films

Titanate nanotubes (TNT) were proven to be efficient support for the immobilization of methylene blue (MB). UV–vis absorption and Fourier transform infrared spectra showed that the effect of MB absorbed on TNT was better than nanocrystalline anatase TiO₂ (TNP). The quantity of MB absorbed onto TNT was found to be greater than that of TNP and the electrode modified with the MB–TNT film was more stable due to the strong interaction between TNT and MB as well. The absorption of MB on TNT was impacted by the pH value of the reaction solution for the change of surface charge. Electrochemical oxidation of dopamine (DA) at different electrodes was studied. The result showed that the MB–TNT composite film exhibited excellent catalytic activities to DA compared to those of pure TNT, which is a result of the great promotion of the electron-transfer rate between DA and the electrode surface by the MB–TNT film. Furthermore, the layer-by-layer self-assembly behavior of the electrochemically functional MB–TNT nanocomposite was also discussed after obtaining the stable colloid suspension of MB–TNT. The excellent electrochemical ability and the easy fabrication of layered nanocomposite make the MB–TNT nanocomposite very promising in electrochemistry study and new nanotube-based devices.     

Spontaneous wrinkling of layer-by-layer assembled polyelectrolyte films for humidity-responsive superhydrophobicity

The fabrication of smart films with reversible wettability enabled by the stimulus-induced morphology changes has attracted growing interest but remains a challenge. Here we report a smart film that can reversibly changes its wettability between transparent hydrophobicity to translucent superhydrophobicity through the humidity-induced wrinkling/de-wrinkling process. The film was fabricated by depositing hydrophobic SiO₂ nanoparticles (NPs) on poly(acrylic acid) (PAA)/poly(allylamine hydrochloride) (PAH) films, followed by partially exfoliating the films from the underlying substrates. The partially exfoliated PAA/PAH film can reversibly wrinkle and de-wrinkle when being alternately subjected to humid and dry environments. The deposition of hydrophobic SiO₂ NPs on the wrinkling PAA/PAH film does not hinder the humidity-enabled wrin-kling/de-wrinkling ability of the composite film. The hydrophobic SiO₂ NPs and the underlying humidity-wrinkling PAA/PAH film enable the composite film to spontaneously change from hydrophobic and transparent to superhydrophobic and translucent with the rise of environmental humidity.     

Correlation of antithrombogenicity and heat treatment for layer-by-layer self-assembled polyelectrolyte films

Antithrombogenic films with high durability were fabricated in a wet process. Antithrombogenicity was achieved with polyelectrolyte multilayer thin film prepared from poly(vinyl alcohol)-poly(acrylic acid) (PVA-PAA) blends, deposited in alternate layers with poly(allylamine hydrochloride) (PAH). Film durability, assessed by abrasion resistance and water resistance, was enhanced by forming cross-links via amide bonds induced by heat treatment of the film. The film was found to be resistant to protein adsorption, as measured by the amount of fibrinogen adsorbed from an aqueous solution. The antithrombogenic efficacy was assessed in ex vivo experiments by the ability of stainless steel mesh, coated with the polyelectrolyte and inserted into a pig blood vessel, to inhibit thrombus formation. Mesh coated with the polyelectrolyte did not reduce blood flow over a period of 15 min, whereas with uncoated mesh blood flow stopped within 6 min because of blood vessel blockage by thrombus formation.     

Strong polyelectrolyte quantum dot surface for stable bioconjugation and layer-by-layer assembly applications

A series of quantum dot (QD) ligands are reported that can make strong polyelectrolyte QD surfaces with sulfonates or quaternary ammoniums, which can endow QDs with excellent colloidal stability independent of the pH and ionic strength, minimal hydrodynamic size, and can be exploited to achieve stable and flexible bioconjugations and layer-by-layer assembly.     

Peculiarities of polyelectrolyte multilayer assembly on patterned surfaces

The layer-by-layer assembly of poly(diallyldimethylammonium chloride) and poly(sodium 4-styrenesulfonate) is studied on templates with imprinted arrays of microwells ranging from 2 to 25 μm and different aspect ratios. The thickness and microstructure of polyelectrolyte multilayers (PEMs) are measured using scanning electron microscopy. At 0.2 M ionic strength, the PEM film evenly coats the template both inside and outside the microwells. If the film is thinner than the critical value of about 400 nm, PEM microstructures collapse upon dissolving the template. Euler's model of critical stress is used to describe the collapse. At 2 M ionic strength, a substantially thinner PEM film is assembled inside the 25 μm wells than outside. If the well diameter is reduced to 7 and 2 μm, a much thicker PEM film is formed inside the microwells. These observations have been attributed to the changing of polyelectrolyte conformation in the solutions.     

Spontaneous surface flattening via layer-by-layer assembly of interdiffusing polyelectrolyte multilayers

We report a facile means to achieve planarization of nonflat or patterned surfaces by utilizing the layer-by-layer (LbL) assembly of highly diffusive polyelectrolytes. The polyelectrolyte pair of linear polyethylenimine (LPEI) and poly(acrylic acid) (PAA) is known to maintain intrinsic diffusive mobility atop or even inside ionically complexed films prepared by LbL deposition. Under highly hydrated and swollen conditions during the sequential film buildup process, the LbL-assembled film of LPEI/PAA undergoes a topological self-deformation for minimizing surface area to satisfy the minimum-energy state of the surface, which eventually induces surface planarization along with spontaneous filling of surface textures or nonflat structures. This result is clearly different from other cases of applying nondiffusive polyelectrolytes onto patterned surfaces or confined structures, wherein surface roughening or incomplete filling is developed with the LbL assembly. Therefore, the approach proposed in this study can readily allow for surface planarization with the deposition of a relatively thin layer of polyelectrolyte multilayers. In addition, this strategy of planarization was extended to the surface modification of an indium tin oxide (ITO) substrate, where surface smoothing and enhanced optical transmittance were obtained without sacrificing the electronic conductivity. Furthermore, we investigated the potential applicability of surface-treated ITO substrates as photoelectrodes of dye-sensitized solar cells prepared at room temperature. As a result, an enhanced photoconversion efficiency and improved device characteristics were obtained because of the synergistic role of polyelectrolyte deposition in improving the optical properties and acting as a blocking layer to prevent electron recombination with the electrolytes.     

Nanomechanical properties of polyaniline and azo polyelectrolyte multilayer films

<正>Nanomechanical properties of multilayer films constructed of polyaniline(PANI) and azobeneze-containing polyelectrolytes(PNACN and PPAPE) were studied by using nanoindentation method.The multilayer films were prepared by the electrostatic layer-by-layer self-assembly through alternately dipping in the polymer solutions.The multilayer films deposited onto the glass slides after proper dry were used for the nanomechanical property testing.The nanomechanical measurement indicated that the PANI/PNACN and PANI/PPAPE multilayers possessed the mean elastic modulus of 5.42 GPa and 4.35 GPa,and hardness of 0.26 GPa and 0.18 GPa,respectively.The nanoscratch properties of the PANI/PNACN and PANI/PPAPE multilayer films were also measured.The critical loads of PANI/PNACN and PANI/PPAPE films were 103.52 mN and 100.59 mN.The degree of electrostatic cross-linking in the multilayers could be altered by exposing the films to aqueous solutions with different pH values.As a result,the modulus and hardness of the multilayer films were changed through the solvent treatment.Both modulus and hardness of the PANI/PNACN films obviously increased after dipping the multilayer films in solutions with pH in a range from 9 to 11.     

Direct evidence of layer-by-layer assembly of polyelectrolyte multilayers on soft and porous temperature-sensitive PNiPAM microgel using fluorescence correlation spectroscopy

We describe the layer-by-layer assembly of polyelectrolyte multilayers on soft and porous temperature-sensitive poly(N-isopropylacrylamide) (PNiPAM) microgel. Microgels are not hard and rigid but rather are soft and porous particles, and polyelectrolytes not only interdigitate with each other during multilayer formation but also with the microgel. Because of this difference, there could be concerns about the feasibility of the layer-by-layer technique on these systems. The argument is that the layer being deposited is stripping the underlying layer instead of anchoring to the latter, and common methods of characterizing film growth on particles such as zeta-potentials will still show "successful" charge reversal. To address this issue, we used two differently labeled polyelectrolytes during the deposition. Because of the small size of the microgel (400 nm) studied, we cannot distinguish between polyelectrolytes adsorbed on or in the microgel. However, with fluorescence correlation spectroscopy, we can clearly distinguish between free labeled polyelectrolytes and those that are bound to the microgel. Dual-color correlation confirms the presence of both polyelectrolytes bound to the same particle while fluorescence imaging (on a dry sample) provides the visual proof.     

Fabrication of polyelectrolyte multilayer films comprising nanoblended layers

Polyelectrolyte multilayer thin films were prepared via the alternate deposition of poly(allylamine hydrochloride) (PAH) and a blend of poly(acrylic acid) (PAA) and poly(styrenesulfonate) (PSS). When the pH of the blend solution was 3.5, the presence of PAA in this solution significantly increased the total film thickness. With only 10 wt % PAA in the blend adsorption solution, a large increase in film thickness was observed (92 nm cf. 18 nm). It was also demonstrated that the total amount of PSS adsorbed was enhanced by the presence of PAA in the blend solution, showing that the blend solution composition influenced that of the multilayer films. Thin films prepared with nanoblended layers also showed improved pH stability, because they exhibited reduced film rearrangement upon exposure to acidic conditions (pH = 2.5).