Acid Multilayers Research Articles

In situ ultraviolet-light-induced TiO2 nanohybrid superhydrophilic membrane for pervaporation dehydration

The membrane surface property is one of the most important factors that influence the solution–diffusion-controlled pervaporation process. In the present study, a superhydrophilic nanohybrid membrane was successfully prepared by in situ ultraviolet irradiation of titanium dioxide (TiO2) nanoparticles imbedded in polyelectrolyte complexes. The TiO2 precursor solution was dynamically filtered through a layer-by-layer-assembled poly(ethyleneimine)/poly(acrylic acid) multilayer under a specific pressure. Subsequently, ultraviolet radiation was used to improve the hydrophilicity of the nanohybrid membranes via photoinduction of the superhydrophilic property of the TiO2 nanoparticles. When the membranes were irradiated, the surface contact angle decreased from 62° to 3°, which is characteristic of a superhydrophilic membrane surface. The pervaporation performance of the membrane for separating alcohol/water mixtures was investigated. The water content could be enriched from 5wt% (in the feed) to 99.89wt% (in the permeate) and the permeate flux was 865g/(m2h) in the pervaporation of the ethanol/water mixture. These results indicated that the superhydrophilic surface was beneficial for improving the pervaporation dehydration performance.

PM‐IRRA Spectroelectrochemistry of Hexacyanoferrate Films in Layer‐by‐Layer Polyelectrolyte Multilayers

AbstractIn situ polarization modulation infrared reflection absorption spectroscopy (PM‐IRRAS) under potential control is used to study self‐assembled layer‐by‐layer (LbL) poly(allylamine) (PAH) and poly(acrylic acid) (PAA) multilayers containing the hexacyanoferrate redox system on conductive substrates. Two systems were examined, which differ in the way the redox complex is entrapped in the LbL polyelectrolyte multilayer: In system I, the LbL polyelectrolyte multilayer, self‐assembled by alternate immersion in PAH and PAA, was exchanged in a 2 mm K3Fe(CN)6 solution of pH 3.5 for 30 min and then thoroughly rinsed with water. In system II, the 2 mm K3Fe(CN)6 solution was complexed with PAA prior to self‐assembly with PAA. PM‐IRRAS studies demonstrate the potential dependence of the vibrational modes of the polyelectrolyte backbones and the CN stretching modes, which are sensitive to the redox charge in the iron center.

Phase-dependent premelting of self-assembled phosphonic acid multilayers

Melting and premelting phenomena in self-organized organic systems have been extensively explored in the literature, exploring distinct behaviors of different molecule lengths and morphologies. Nevertheless, the influence of the supramolecular assembly configuration on the occurrence of premelting remains poorly explored. Here we use phosphonic acids as model systems for self-organized molecular assemblies. These molecules exhibit long-range order on different types of substrates. The balance between chain-to-chain and head-to-head interactions leads to distinct types of stackings. Although their structural configurations are well understood, very little is known about their behavior near the melting transition. We show here that premelting occurs in lamellar structures and that its behavior depends directly on the ordered configuration assumed in the studied multilayers. Two molecules with different chain lengths were investigated: octadecyl phosphonic and octyl phosphonic acids. Although almost no dependence on the molecule length was observed, the occurrence of premelting is strongly influenced by their lamellar packing configuration. For tilted packings premelting is unfavored while in straight configurations, where alkyl chain interactions are weakened with respect to head-to-head interactions, strong premelting is observed. We find that the onset of premelting occurs at the domain boundaries with straight lamellar configurations and the domain sizes exhibit power law temperature dependences.

Property Evaluation of Polylactic Acid Multilayer Braids Used as Bone Scaffolds

Polylactic acid (PLA), which is biodegradable, is largely used as biomaterial such as bone scaffolds. This study twists PLA filaments into PLA plied yarn, during which twist per inch (TPI) varies as 9, 10, 11, 12, and 13. Tensile strength and elongation of the resulting plied yarn are then evaluated. The optimal TPI is 10, which results from an optimal tensile strength of 3.3 g/den and elongation of 42 %. The optimal plied yarn is made into PLA braids on a 16-spindle braiding machine, with a ratio of take-up gear to braid gear of 60:60, 70:60, 80:60, 90:60, or 100:60. The braids with various gear ratios are then made into multilayer braids with a diameter of 4 mm. The resulting multilayer braids are evaluated for surface observation, porosity and water contact angle. The optimal porosity is 55-65% and the optimal water contact angle is below 30°. This optimal condition indicates that the multilayer braid exhibit good hydrophilicity and a good candidate for bone tissue scaffolds.

Thermal Stability and Ordering Study of Long- and Short-Alkyl Chain Phosphonic Acid Multilayers

Long-range order evolution of self-assembled phosphonic acid multilayers as a function of temperature is studied here for two molecules with different alkyl chain length. By using synchrotron conventional diffraction, distinct order configurations are retrieved on phosphonic acid multilayers and their thermodynamic behavior monitored by energy-dispersive diffraction. This later technique allows us to observe the system behavior near order-disorder temperatures, as well as to determine the most stable configurations in the range from room temperature up to 120 °C. Planar order is also addressed by wide-angle X-ray scattering (WAXS) transmission experiments. Order parameter phase diagrams are built based on the experimental results, showing the dominant configuration at each temperature. The multilayer molecular long-range order retrieved from the experiments is corroborated by first principles calculations based on the Density Functional Theory. The bulk configurations depicted in this work are produced by molecule-molecule interactions and allow for future comparisons with the behavior of ordered molecules in few-monolayers configurations, commonly used in organic devices, where the presence of surfaces and interfaces strongly affects the molecule packing.

Formation of Multilayers by Star Polyelectrolytes: Effect of Number of Arms on Chain Interpenetration

We have investigated the influence of number of arms on chain interpenetration in the growth of star poly[2-(dimethylamino)ethyl methacrylate] (PDEM)/star poly(acrylic acid) (PAA) multilayers using a quartz crystal microbalance with dissipation (QCM-D). The oscillations in the changes of dissipation and frequency reflect the chain interpenetration and the variation of the mass of multilayer, respectively. The QCM-D results demonstrate that the growth of multilayers has two different mechanisms in terms of chain interpenetration. That is, the arm chains of star PDEM insert into a predeposited PAA layer to form a swollen multilayer, but the complex of star PAA with predeposited star PDEM is an "octopus-like" structure forming a dense multilayer. The transition between these two penetration modes is controlled by the number of arms in the star polyelectrolytes. As the number of arms of either PAA or PDEM increases, it becomes more difficult for star PDEM to penetrate into the PAA layer, but star PAA can more easily penetrate into the PDEM layer. According to atomic force microscopy and water contact angle measurements, all eight-bilayer multilayer surfaces have similar roughness values, and the surface wettability of the multilayers is dominated by the outermost PDEM layer.

Preparation and characterization of poly(l-histidine)/poly(l-glutamic acid) multilayer on silicon with nanometer-sized surface structures

The specific design and modification of surfaces is of great interest, especially for functional surfaces and medical applications. In order to obtain films on a surface, the layer-by-layer deposition of polyelectrolytes represents a well-established methodology. The alternating deposition of poly(l-histidine) and poly(l-glutamic acid) results in a defined, continuous surface coating that was thoroughly characterized using X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, ellipsometry, X-ray reflectometry, atomic force microscopy, scanning electron microscopy, contact angle, and electrokinetic measurements. Surface charge, film growth, and final thickness were measured and cross-validated. Additionally, the chemical composition and distribution of polyelectrolytes in the layerstack were determined. Finally, the optical parameters were specified and the surface topography was visualized by several methods. These characterizations revealed a coating with embedded spheroids forming from the bottom layers. This rough surface formed by (PLH/PGA)8 was highly reproducible and might provide unique features for the design of tailored surfaces.

Combined QCMD and AFM studies of lysozyme and poly-l-lysine–poly-galacturonic acid multilayers

A quartz crystal microbalance with dissipation monitoring (QCMD) has been used to monitor the adsorption and structure of lysozyme monolayers and multilayers, and poly-l-lysine (PLL)–polygalacturonic acid (PGalA) multilayers at a solid–liquid interface using freshly-cleaved mica as a substrate. QCMD measurements were complemented with atomic force microscopy (AFM). AFM images revealed that lysozyme formed incomplete monolayers and provided a basis for calculation of the thickness of the protein film. Comparative studies of adsorption onto standard and mica-coated quartz crystals showed higher areal mass adsorption and a longer-time adsorption process for mica-coated quartz crystals. Simultaneous AFM images and QCMD data were obtained for lysozyme, linear PLL–PGalA and 7nm PLL dendrimer-PGalA multilayers. The layer-by-layer deposited multilayer films exhibited viscoelastic properties and their growth followed a non-linear regime, associated with the PLL diffusion in and out of the film formation for linear PLL–PGalA films. For the PLL 7nm dendrimer-PGalA films the AFM images revealed marked changes in surface roughness during layer by layer deposition: these changes influence the interpretation of the QCMD data and provide additional information on the growth and structure of the multilayers.

Tunable stimulus-responsive friction mechanisms of polyelectrolyte films and tube forests

The pH-responsive frictional behavior of layer-by-layer assembled poly(allylamine hydrochloride) and poly(acrylic acid) multilayers is quantified in different geometric forms of a continuous planar film and anisotropic tube forests. A mechanistic change from surface adhesion dominated frictional behavior to visco/poroelasticity-governed shear occurs for the planar film upon pH-stimulus. This pH-dependent friction can be further controlled by the discrete anisotropic geometry of the tube forest, which introduces additional friction due to asymmetric deformation of the discrete bending of the tubes during lateral motion. This study provides important insights into the design of polyelectrolyte-based coatings with a wide range of controllable surface frictional properties, tuned via interactions between the inherent stimulus-responsive material behavior and the microgeometry of the anisotropic tube forest.

Multiresponsive Clay-Containing Layer-by-Layer Films

We report on polymer/clay layer-by-layer films responsive to multiple stimuli. Temperature- and salt-responsive films were constructed using assembly of poly(N-isopropylacrylamide) (PNIPAM) and montmorillonite clay nanosheets. An additional pH response was achieved by depositing and cross-linking hybrid, dual-network PNIPAM/clay/PNIPAM/poly(methacrylic acid) (PMAA) multilayers. Both types of films remained stable in a wide pH range and were highly swollen. For example, PNIPAM/clay films swelled up to ~14.5 times their dry film thickness in low-salt solutions at 25 °C, as shown by laser scanning confocal microscopy. At temperatures higher than PNIPAM's lower critical solution temperature (LCST) of 32 °C, or in 0.3 M Na(2)SO(4) solutions at room temperature, both PNIPAM/clay and PNIPAM/clay/PNIPAM/PMAA films reversibly deswelled as a result of collapse of PNIPAM chains. Films of both types showed a decrease in permeability to fluorescein-tagged dextrans of various molecular weights. Importantly, film permeability to dextrans was decreased at temperatures above PNIPAM's LCST, and the effect could be reversed by lowering the temperature. Inclusion of PMAA within multilayers provided an additional pH response to film swelling and permeability. Hybrid PNIPAM/clay/PNIPAM/PMAA films showed drastic deswelling at low pH values due to the onset of hydrogen bonding between PNIPAM and PMAA, and the diffusion of 70 kDa dextran through multilayers at acidic pH was completely blocked. These multiresponse features of clay-containing films make them promising candidates for applications in sensing, actuation, and controlled delivery.

Energy dispersive X-ray reflectivity applied to the study of thermal stability of self-assembled organic multilayers: Results on phosphonic acids

The temperature evolution of self-assembled phosphonic acid multilayers was investigated by energy dispersive X-ray reflectivity and angular-resolved reflectivity. Energy dispersive measurements were performed in an experimental setup specially designed for the X-ray fluorescence beamline of the Brazilian Synchrotron Light Laboratory. It allows the precise monitoring of phase transitions observed in organic thin film and multilayer systems. The studied multilayers – obtained from dip coating of a solution of octadecylphosphonic acid – present different bilayer periodicities of 50Å (straight bilayer) and 34Å (tilted bilayer). Energy dispersive and angular-resolved data evidence re-organization of the lamellar ordering of octadecylphosphonic acid multilayers as a function of temperature. The energy dispersive technique presents many advantages over conventional methods such as short acquisition time, possibility to vary external parameters and high flux, making it suitable for light scatterers as polymers and other organic molecules.

Effect of Chain Length on the Assembly of Mercaptoalkanoic Acid Multilayer Films Ligated through Divalent Cu Ions

The structural stability of alkenthiolate monolayers assembled on gold surfaces is a result of the well-defined organization of the individual molecules within the film. The formation of three-dimensional films assembled by stacking multiple molecular monolayers is substantially more challenging because the correct organization of the molecular components is required not only within the individual monolayers but also between the monolayers of the film. In this paper we examine the structure of multilayer films based on mercaptoalkanoic acid monolayers in which ligation between adjacent monolayers is achieved using the interaction of carboxylic acid and thiol groups with a divalent Cu ion. Using contact angle analysis and atomic force microscopy, we show that the use of Cu(2+) has profound implications on the properties and structure of the multilayer film. In particular, the divalent ions effectively prohibit the complete assembly of the next monolayer. For multilayer SAMs assembled from short alkane chains with six methylene groups, we find that molecules in the incomplete adlayer organize themselves randomly over the underlying monolayer. However, as the number of methylene groups increases (11 and 16 methylene groups), the upper layer tends to fracture into discrete islands which cover around 50% of the surface. The height of these islands is found to be equal to that expected for a complete, well-ordered monolayer assembled from the equivalent mercaptoalkanoic acid molecules. This relationship between chain length and island growth results from the migration of molecules into ordered aggregates driven by the reduction of free energy associated with maximizing intermolecular interactions.

Tailored design of mechanically sensitive biocatalytic assemblies based on polyelectrolyte multilayers

Mechanically sensitive surfaces responding to mechanical forces constitute an attractive emerging field of research. This requires the engineering of complex surfaces with finely controlled properties, especially regarding the permeability behaviour towards specific molecules. Here we designed such surfaces using polyelectrolyte multilayer nanostructures. Polylysine/hyaluronic acid multilayer films were used as a micro-container of enzymes and denser multilayers deposited on top of the reservoir were tailored to control their permeability. We find that permeability towards fluorescein diphosphate (FDP) not only depends on the number of bilayers constituting the barrier but more surprisingly on the deposition time of the polyelectrolytes during the barrier buildup, a long contact time (10 min) leading to porous barriers. This effect is explained by diffusion and exchange processes taking place in the reservoir during the buildup process. For films composed of a non-permeable barrier towards enzymatic substrate FDP, we tested the enzymatic activity when mechanical stretching was applied to the architecture. Under stretch and in the presence of FDP on top of the film, the catalytic activity was switched on. These biologically inspired surfaces constitute a first step to the development of novel platforms able to trigger and to modulate chemical reactions under a mechanical stimulus.

GOLD NANOPARTICLE-INCORPORATED POLYELECTROLYTE MULTILAYER FOR SENSITIVE ELECTROCHEMICAL IMMUNOSENSING

Reported was a novel amperometric immunosensor based on layer-by-layer (LbL) assembled polyethylenimine/gold nanoparticles/poly (acrylic acid) (PEI/AuNPs/PAA) multilayer. The assembly process was in situ monitored by surface plasmon resonance (SPR) technique. Anti-goat IgG as a capture antibody was covalently immobilized on the outermost PAA layer of the multilayer to construct an immunosensor. A target protein, goat IgG was electrochemically detected with alkaline phosphatase-conjugated anti-goat IgG (ALP-anti-goat IgG) as a recognition antibody. Electrochemical investigations suggest that the incorporation of AuNPs facilitates the electron transfer between the underlying electrode and the redox species in solution, and thus enhances the electrochemical signals and in turn improves the immunosensing performance. A detection limit of 100 pg mL-1 with a dynamic range of five orders of magnitude was achieved. Due to the protein-friendly environment and the protein resistance of the polyelectrolyte multilayer, the resulting immunosensor demonstrates excellent storage stability, satisfying assay specificity.

Mechanism of Formation and Construction of Self-Assembled Myoglobin/Hyaluronic Acid Multilayer Films: An Electrochemical QCM, Impedance, and AFM Study

Self-assembled multilayer films of hyaluronic acid (HA) and the protein myoglobin (Mb) were built up layer by layer on Au covered quartz crystal microbalance (AuQCM) electrode substrates. Film formation and growth were monitored by an electrochemical quartz crystal microbalance (EQCM), and the step-by-step construction was investigated through quantification of the mass variation corresponding to adsorption of each monolayer together with cyclic voltammetry. A decrease of friction at the liquid/electrode interface was observed, indicating that the electrode surface becomes less rough after deposition of several monolayers. The properties of the {HA/Mb}(n) films were evaluated by electrochemical impedance spectroscopy (EIS). Modeling of the impedance spectra shows smoothing of the modified electrode surface with reorganization of the film structure after few monolayers, and the contribution of each layer to the electron transfer process was analyzed. The smoothing of the surfaces and the structural differences between successive bilayers was confirmed by morphological observations by using atomic force microscopy.

Iron sulphide formation in the ferric stearate Langmuir–Blodgett films

Ferric stearate (FeSt) Langmuir–Blodgett (LB) films have been reacted chemically with H 2S gas for making iron sulphide within the organic matrix. Films, before and after the reaction with H 2S, have been analyzed with the X-ray reflectivity (XRR), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) studies. After sulphidation, more ‘pinhole’ defects form which changes the film morphology and the number of layers increases due to the rearrangement of the molecules. Formation of less ordered iron sulphide within the stearic acid multilayers after sulphidation increases the interfacial roughness that decreases the reflectivity. XPS analysis shows that polysulphide forms within the microenvironment of the FeSt LB films after reaction with H 2S whereas both mono and polysulphide are produced when the reaction occurs with FeSt in bulk.

Tunable Synthesis of Prussian Blue in Exponentially Growing Polyelectrolyte Multilayer Films

Polyelectrolyte multilayer (PEM) films have become very popular for surface functionalization and the design of functional architectures such as hollow polyelectrolyte capsules. It is known that properties such as permeability to small ionic solutes are strongly dependent on the buildup regime of the PEM films. This permeability can be modified by tuning the ionization degree of the polycations or polyanions, provided the film is made from weak polyelectrolytes. In most previous investigations, this was achieved by playing on the solution pH either during the film buildup or by a postbuildup pH modification. Herein we investigate the functionalization of poly(allylamine hydrochloride)/poly(glutamic acid) (PAH/PGA) multilayers by ferrocyanide and Prussian Blue (PB). We demonstrate that dynamic exchange processes between the film and polyelectrolyte solutions containing one of the component polyelectrolyte allow one to modify its Donnan potential and, as a consequence, the amount of ferrocyanide anions able to be retained in the PAH/PGA film. This ability of the film to be a tunable reservoir of ferrocyanide anions is then used to produce a composite film containing PB particles obtained by a single precipitation reaction with a solution containing Fe(3+) cations in contact with the film. The presence of PB in the PEM films then provides magnetic as well as electrochemical properties to the whole architecture.

Poly (3-dodecylthiophene) Langmuir–Blodgett films: Preparation and characterization

Poly(3-dodecylthiophene) (P3DDT)/stearic acid (SA) monolayer and multilayers have been successfully deposited onto indium tin oxide (ITO) substrates through Langmuir–Blodgett (LB) technique. Surface pressure–mean molecular area ( π– A) isotherms, X-ray diffraction (XRD) pattern, absorption spectrum, fluorescence spectrum, atomic force microscope (AFM), and photo-assistant conducting AFM (PC-AFM) have been used for characterization. The AFM topographic images of P3DDT/SA LB film showed high coverage morphology with some aggregated islands dispersed uniformly in the film when deposited at surface pressure of 40 mN/m. Compared with the absorption and fluorescence spectra of P3DDT in toluene, the spectra of P3DDT/SA LB films showed an obvious red-shift phenomena, which were due to the formation of P3DDT aggregates in LB films. It was confirmed by XRD measurement that P3DDT/SA LB films formed an ordered structure with an average thickness of 2.9 nm. The results from PC-AFM measurements showed that the P3DDT/SA LB films had much higher conductivity under UV light irradiation, which indicated that P3DDT molecules had a good photocurrent response in such ordered structure.

Dynamics of Poly(l-lysine) in Hyaluronic Acid/Poly(l-lysine) Multilayer Films Studied by Fluorescence Recovery after Pattern Photobleaching

Poly( L-lysine) (PLL)/hyaluronic acid (HA) multilayers are films whose thickness increases exponentially with the number of deposition steps. Such a growth process was attributed to the diffusion, in and out of the whole film, of at least one of the polyelectrolytes constituting the film. In the case of PLL/HA, PLL is known to be the diffusing species. In order to better understand the growth mechanism of such films, it is of primary importance to well characterize the diffusion process of the polyelectrolytes in the multilayer. This process is studied here by fluorescence recovery after pattern photobleaching. We show that the diffusion behavior is different when we consider either PLL chains that are deposited on top of the film or PLL chains embedded in the film, even below only one HA layer. For chains that are embedded, we find two populations: a mobile one with a diffusion coefficient, D, of the order of 0.1 microm(2) x s(-1) and a population that appears immobile ( D < 0.001 microm(2) x s(-1)). For chains deposited on top of the multilayer, a third population appears which is rapidly diffusing ( D congruent with 1 microm(2) x s(-1)). These results confirm the validity of the model generally accepted for the exponential growth process and in particular the existence of up to three subgroups of PLL chains from the point of view of their diffusion coefficient.

Characterization of self-cleaning glasses using Langmuir–Blodgett technique to control thickness of stearic acid multilayers: Importance of spectral emission to define standard test

The photocatalytic properties of commercialized self-cleaning glasses have been evaluated by the degradation of stearic acid C 17H 35COOH (SA), selected as a model molecule representative of fatty dirt. The relative amount of SA has been determined by measuring the FTIR peak area in the range 2752 and 2992 cm −1 corresponding to the C–H stretching bands. In order to estimate the thickness of one layer, the deposit of SA has been performed using the Langmuir–Blodgett (LB) method, which allows obtaining organized monomolecular layers of amphiphilic organics on a substrate. The thickness of one SA layer, obtained by Langmuir–Blodgett deposition, has been correlated to the FTIR signal. The photocatalytic degradation of a SA layer of a controlled thickness was carried out in a double jacket Pyrex reactor equipped with a UV Philips HPK 125 lamp. The photocatalytic degradation rate of the SA layer with different thicknesses was determined. The SA degradation rate was investigated under UV-A and UV-B irradiations to determine the influence of the photon wavelength range on efficiency.