On the use of electrokinetics for unraveling charging and structure of soft planar polymer films

During the past decade, much attention has been devoted to the use of electrokinetic phenomena for addressing both charging mechanism and structure of multi-responsive soft polymeric layers whose thickness may range from few tens of nanometers to several microns. In particular, major progress was achieved in the quantitative reconstruction of streaming current data collected over a wide range of physico-chemical conditions using recent theories for electrohydrodynamics of soft diffuse planar interphases. In this article, we review the basics of the methodology adopted for deciphering the mechanisms governing the charging of electric double layers at soft planar films in connection with their structure that may vary according to pH, salt concentration or temperature depending on the responsive character of the system. It is demonstrated how the combination of streaming current, surface conductivity and swelling measurements allows for a comprehensive understanding of the interrelated protolytic, hydrodynamic, electrostatic and structural properties of polymer layers. We discuss the benefits and limits of the approach on the basis of studies carried out on uncharged, moderately charged and highly charged soft polymeric films supported by hard charged carriers. In a final part, the basic processes governing the peculiar electrokinetic properties of soft planar polyelectrolyte multilayers under lateral flow conditions are described.     

Grafting of porphyrins on cellulose nanometric films

Ultrathin films of cellulose were functionalized with iron protoporphyrin IX (FePP). Spin-coating allows the production of silylated cellulose films in a controlled way. Cellulose regeneration is achieved through the hydrolyzation of the silane groups, exposing the film to acidic vapors. To enhance the reactivity of the cellulose surface to the protoporphyrin, carbonyldiimidazole (CDI) was used as an activator. The effect of different spacers on the porphyrin grafting such as 1,8-diaminooctane and 1,4-phenylenediamine was studied. The highest level of cellulose functionalization with FePP was achieved when both the cellulose film and FePP were activated by CDI and a diaminoalkane was used as a spacer between the surface and the FePP. ATR/MIR (attenuated total reflection in multiple internal reflections) was performed in situ to follow the kinetics of the different chemical reactions with the cellulose surface. ATR/MIR proved again to be a powerful tool for probing the surface reaction. X-ray photoelectron spectroscopy permitted the elemental analysis of the cellulose surface after the chemical modification.     

Surface analysis of erodible multilayered polyelectrolyte films: nanometer-scale structure and erosion profiles

Atomic force microscopy (AFM) and scanning electron microscopy (SEM) coupled with ellipsometry have been used to characterize the microscale and nanoscale structures of erodible multilayered films fabricated from degradable polyamine 1 and either sodium poly(styrene sulfonate) (SPS) or plasmid DNA. Striking differences were found in the topography, structures, and erosion profiles of these two materials upon incubation in PBS buffer at 37 degrees C. For films fabricated from SPS, AFM data are consistent with an erosion process that occurs uniformly without the generation of holes or pits over large, micrometer-scale areas. By contrast, films fabricated from plasmid DNA undergo structural rearrangements to present surface-bound particles ranging in size from 50 to 400 nm. Additional characterization of these particulate structures by SEM suggested that they are interpenetrated with or fused to underlying polyelectrolyte layers on the silicon surface, providing a potential mechanism to manipulate the adhesive forces with which these particles are bound to the surface. The erosion profile observed for polymer 1/SPS films suggests that it may be possible to design assemblies that release two film components with well-defined release kinetics. In the context of gene delivery, the presentation of condensed DNA as nanoparticles at these surfaces may be advantageous with respect to stimulating the internalization and processing of DNA by cells. A quantitative understanding of the factors influencing the fabrication, structure, and erosion profiles of these materials will be useful for the design of multilayered assemblies for specific applications in which controlled film erosion or the release of therapeutic materials is desired.     

Electronic properties of conjugated polyelectrolyte thin films

The electronic properties of conjugated polyelectrolytes (CPEs) with poly(fluorene-co-phenylene) backbones and different counterions and charges have been investigated using absorption and ultraviolet photoelectron spectroscopy (UPS). The optical energy band gap of CPEs depends mainly on their conjugated backbone and are nearly insensitive to the charges or counterions. UPS measurements reveal that electron injection from Au to polymers with cationic groups is more efficient than for the neutral and anionic counterparts. The vacuum levels of CPEs were also shifted toward higher or lower binding energy, relative to that of Au, depending on the charge and counterion presence, and provide insight into the general alignment of dipoles at the metal/organic interface.     

Nanometer-scale decomposition of ultrathin multilayered polyelectrolyte films

We report that ultrathin multilayered films fabricated from plasmid DNA and synthetic polyamines undergo nanometer-scale transformations that resemble spinodal decomposition when incubated in aqueous media. The patterns and structures generated by this transformation are similar to those observed for the spinodal dewetting of thin films of conventional polymers. This behavior has not, however, been observed for this class of multilayered assemblies, for which long-range electrostatic interactions play significant roles in governing film structure and stability. We demonstrate that it is possible to promote this behavior, prevent it, or control it by varying polymer structure, film composition, or the conditions to which these materials are exposed. These results suggest the basis of methods that could prove useful for the generation of nanostructure on complex surfaces and contribute to methods for the localized delivery of DNA from surfaces.     

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.     

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.     

Properties of nanometric and micrometric multilayered films made of three conducting polymers

The electrochemical, electrical and morphological properties of multilayered films formed by alternated layers of poly(3,4-ethylenedioxythiophene), polypyrrole, and poly[N-(2-cyanoethyl)pyrrole], which were prepared by electrochemical layer-by-layer deposition, have been investigated and compared with those of multilayered films formed by two conducting polymers. Results indicate that the electrochemical behavior of the films formed by three conducting polymers depends on the micrometric or nanometric thickness of the layers. Thus, the electroactivity increases until the thickness of these films reaches a threshold value (∼2 μm), while the electrostability of the films is very remarkable when their thickness is close to ∼4 μm or higher. On the other hand, comparison between multilayered systems made of two and three conducting polymers indicates that the third component introduces heterogeneity in the interfaces between consecutive layers, reducing the ability to store charge. Among the latter, multilayered films formed by poly(3,4-ethylenedioxythiophene) and polypyrrole have been found to be particularly electroactive and electrostable. The surface morphology and topography of the layers have been used to rationalize the electrochemical properties of the different materials.     

Study of film structure and adsorption kinetics of polyelectrolyte multilayer films: effect of pH and polymer concentration

The alternate adsorption of polycation poly(allylamine hydrochloride)(PAH) and the sodium salt of the polymeric dye poly(1-[ p-(3'-carboxy-4'-hydroxyphenylazo)benzenesulfonamido]-1,2-ethandiyl)(PCBS) on quartz crystals coated with silica was studied to understand the structural properties and adsorption kinetics of these films using a combination of quartz crystal microbalance with dissipation monitoring (QCM-D), absorbance, and ellipsometry measurements. In-situ deposition of the polycation PAH on QCM crystals was monitored, followed by rinsing with water and then deposition of the polyanion PCBS. The effects of polymer concentration and pH on film structure, composition and adsorption kinetics were probed. The polymers were adsorbed at neutral pH conditions and at elevated pH conditions where PAH was essentially uncharged to obtain much thicker films. The change in the resonant frequency, Deltaf, of the QCM-D showed a linear decrease with the number of bilayers, a finding consistent with absorbance and ellipsometric thickness measurements which showed linear growth of film thickness. By using the Delta f ratios of PCBS to PAH, the molar ratios of repeat units of PCBS to PAH in the bilayer films as determined by QCM-D were approximately 1:1 at polyelectrolyte concentrations 5-10 mM repeat unit, indicating complete dissociation of the ionic groups. The frequency and dissipation data from the QCM-D experiments were analyzed with the Voigt model to estimate the thickness of the hydrated films which were then compared with thicknesses of dry films measured by ellipsometry. This led to estimates of the water content of the films to be approximately 45 wt %. In addition to the QCM-D, some films were also characterized by a QCM which measures only the first harmonic without dissipation monitoring. For the deposition conditions studied, the deposited mass values measured by the QCM's first harmonic were similar to the results obtained using higher harmonics from QCM-D, indicating that the self-assembled polyelectrolyte films were rigid.     

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).     

Internal structure of polyelectrolyte multilayer assemblies

The paper deals with the correlation between the internal structure and dynamics of polyelectrolyte multilayers on one hand and their functional properties on the other hand. It considers different concepts of multilayer formation like driving forces, adsorption kinetics, mode of growth and stability aspects. A further focus is the control of internal structure and dynamics which is of high impact with respect to the design of stimuli-responsive material.     

Modulation of hemocompatibility of polysulfone by polyelectrolyte multilayer films

Polyelectrolyte multilayer (PEM) films have been recently applied to surface modification of biomaterials. Cellular interactions with PEM films consisted of weak polyelectrolytes are greatly affected by the conditions of polyelectrolyte deposition, such as pH of polyelectrolyte solution. Previous studies indicated that the adhesion of several types of mammalian cells to PAH/PAA multilayer films was hindered by low pH and high layer numbers. The objective of this study is to evaluate whether the hemocompatibility of polysulfone can be modulated by deposition of poly(allylamine hydrochloride) (PAH)/poly(acrylic acid) (PAA) multilayer films. PAH/PAA multilayer films with different layer numbers were assembled onto polysulfone at either pH 2.0 or pH 6.5. The number of platelet adhesion and the morphology of adherent platelets were determined to evaluate hemocompatibility of modified substrates. Compared to non-treat polysulfone, the PEM films developed at pH 2.0 decreased platelet adhesion, while those built at pH 6.5 enhanced platelet deposition. Platelet adhesion was found positively correlated to polyclonal antibodies binding to surface-bound fibrinogen. The extent of platelet spreading was increased with layer numbers of PEM films, suggesting that the adherent platelets on thick PEM films were prone to activation. In conclusion, PAH/PAA films with few layers developed at pH 2.0 possessed better hemocompatibility compared to other substrates.     

Interrelations between soil respiration and its thermal stability

Biological transformation of organic matter in soil is a crucial factor affecting the global carbon cycle. In order to understand these complex processes, soils must be investigated by a combination of various methods. This study compares the dynamics of biological mineralization of soil organic matter (SOM) determined via CO₂ evolution during an 80-day laboratory incubation with their thermo-oxidative stability determined by thermogravimetry (TG). Thirty-three soil samples, originating from a wide range of geological and vegetation conditions from various German national parks were studied. The results showed a correlation between the amount and rate of respired CO₂ and thermal mass losses of air-dried, conditioned soils occurring around 100?°C with linear coefficients of determination up to R ²?=?0.85. Further, correlation of soil respiration with thermal mass losses around 260?°C confirmed previous observations. The comparison of TG profiles from incubated and non-incubated soils underlined the importance of thermal mass losses in these two temperature intervals. Incubated soils had reduced thermal mass losses above 240?°C and conversely an increased mass loss at 100?C120?°C. Furthermore, the accurate determination of soil properties by TG such as soil organic carbon content was confirmed, and it was shown that it can be applied to a wider range of carbon contents as was previously thought. It was concluded that results of thermal analysis could be a helpful starting point for estimation of soil respiration and for development of methods revealing processes in soils.     

Strong response of multilayer polyelectrolyte films to cationic surfactants

The influence of common cationic surfactants on the physical properties of differently composed polyelectrolyte films prepared by the layer-by-layer (LbL) technology was investigated. Free-standing polyelectrolyte films as microcapsules showed a fast, strong response to the addition of less than 1 mM cationic surfactant cetyltrimethylammonium bromide (CeTAB). As a function of the polyelectrolyte composition, the behavior of the capsules varied from negligible changes to complete disintegration via strong swelling. The response of microcapsules consisting of (poly(allylamine hydrochloride)(PAH)/poly(styrene sulfonate)(PSS))(4) was associated with a 5-fold volume increase, a fast switch of permeability, and in the case of fluorescently labeled films a 4-fold increase in fluorescence intensity. The kinetics and strengths of the interaction process were investigated by confocal laser scanning microscopy (CLSM) and fluorescence spectroscopy. Also, the relative stabilities of the polycation/polyanion and surfactant/polyanion complexes were determined. A mechanism was suggested to explain the interactions between the cationic surfactants and polyelectrolyte capsules. The strong response can be exploited in potential applications such as the triggered release of drugs or other encapsulated materials, the fluorescence-based detection of cationic detergents, and a switchable stopper in microchannels. However, the high sensitivity of LbL films to traces of cationic surfactants can also limit their applicability to the encapsulation of drugs or other materials because pharmaceutical or technical formulations often contain cationic surfactants as preservatives such as benzalkonium salts (BAC). It was demonstrated that undesired capsule opening can be effectively prevented by cross-linking the polyelectrolyte multilayers.     

Substrate-induced modulation of the Raman scattering signals from self-assembled organic nanometric films

Raman scattering signals recorded by microscopy from organic self-assembled monolayers (thin nanometric films of calibrated thickness) on silica substrates were found to be much stronger than those obtained from identical films assembled on bulk silicon substrates. This effect, observed in the backscattering geometry, is shown to result from interferences between the direct and reflected beams (including both the excitation and scattered radiation) in front of a smooth reflecting surface. Strong dependence of the effect on the distance between the sampled monolayer and the bulk silicon substrate allows enhancement of the Raman signals of organic monolayer films on silicon by factors up to approximately 70 by using appropriate silica spacers. The dependence of the Raman signal intensity on film thickness was also studied for thicker nanometric films comprising a series of self-assembled organosilane multilayers on bulk silicon and fused silica substrates, and the predicted deviation from linearity in the case of the silicon substrate is experimentally confirmed.     

Absorption of light and heavy water vapours in polyelectrolyte multilayer films

We studied the swelling and the uptake of water (H₂O or D₂O) vapours in polyelectrolyte (PE) multilayer (PEM) samples deposited on solid support (Si wafers) as a function of the isotope nature of the vapour and the charge of the last polymer layer. The samples were prepared with deuterated poly(sodium 4-styrenesulfonate) (dPSS) and poly(allylamine hydrochloride) (PAH). Two types of samples were studied. The sample with a structure Si/PEI/(dPSS/PAH)₆/dPSS was negatively charged. A positively charged sample was PAH terminated and had the structure Si/PEI/(dPSS/PAH)₆. The film thickness and scattering length density were estimated from neutron reflectometry (NR) experiments and the results were complemented with in-situ QCM measurements.We demonstrate that the swelling of PEM in H₂O and D₂O vapours is similar. However, the amount of adsorbed D₂O is around 10% more than the adsorbed H₂O. Such isotope effect correlates well with the rough estimation that the isotope effect usually scales with the difference in the mass density of the different isotope forms of the substances. For precise analysis of the NR data we assumed existence of empty voids in the structure of the PEM. These voids might be filled with “condensed” water when the samples are exposed to water vapors. We show that the layers we studied consist of up to 25% of such voids.We showed that the amount of sorbed water depends on the nature of the last layer which builds the PEM thus confirming the “odd-even effect” already shown in the literature.     

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.     

pH-induced interconversion between J-aggregates and H-aggregates of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin in polyelectrolyte multilayer films

We fabricated a layer-by-layer (LbL) film composed of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) and poly(allylamine) (PAA) and investigated its pH response by UV-visible spectrometry. When the (PAA/TPPS)5PAA film was immersed in a pH 1.5 solution, J-aggregate bands were observed at 484 and 691 nm. Above pH 3.0, the J-aggregates were completely dissociated and an H-aggregate band was observed at 405 nm. The interconversion between the J-aggregates and H-aggregates in the LbL film was repeatable and controllable by changing the pH of the solutions.     

Similarities between polyelectrolyte gels and biopolymer solutions

Small angle neutron scattering (SANS) measurements and osmotic swelling pressure measurements are reported for polyelectrolyte gels and solutions under nearly physiological conditions. A synthetic polymer (sodium-polyacrylate) and three biopolymers (DNA, hyaluronic acid, and polyaspartic acid) are studied. The neutron scattering response of these anionic polyelectrolytes is closely similar, indicating that at larger length scales the organization of the polymer molecules is not significantly affected by the fine details of the molecular architecture (e.g., size and chemical structure of the monomer unit, type of polymer backbone). The results suggest that specific interactions between the polyelectrolyte chains and the surrounding monovalent cations are negligible. It is found that the osmotic compression modulus of these biopolymer solutions determined from the analysis of the SANS response decreases with increasing chain persistence length. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3679–3686, 2006