Martin Müller

Selective Interaction Between Proteins and the Outermost Surface of Polyelectrolyte Multilayers: Influence of the Polyanion Type, pH and Salt

Protein adsorption was studied by in-situ ATR-FT-IR spectroscopy of consecutively deposited polyelectrolyte multilayer systems terminated either with poly(ethyleneimine) (PEI) or polyanions, such as poly-(acrylic acid) (PAC), poly(maleic acid-co-propylene) (PMA-P) or poly(vinyl sulfate) (PVS). The influence of the polyanion type, pH and ionic strength was investigated. Negatively charged human serum albumin (HSA) was strongly repelled by multilayers terminated with weak polyanions (PAC, PMA-P), whereas moderate attraction was observed for those terminated with the strong polyanion PVS. Changing the pH from 7.4 to 5 resulted in enhanced HSA adsorption onto PAC-terminated multilayers. An increase in ionic strength diminished the attractive HSA adsorption onto PEI-terminated multilayers. For the PEI/PAC system, the biomedically relevant adsorption of human fibrinogen (FGN) is determined via its isoelectric point in accordance with three other proteins.

An in‐situ ATR‐FTIR study on polyelectrolyte multilayer assemblies on solid surfaces and their susceptibility to fouling

In-situ attenuated total reflection (ATR)-FTIR spectroscopy was used to monitor the consecutively alternating adsorption of polyethylenimine (PEI) and poly(acrylic acid) (PAC) onto both Si crystals (SiO) and CO2 plasma-treated polypropylene (PP) films. The vibration band vas(COO−) and v(CO) of PAC are diagnostic for the polyelectrolyte layer build-up and sensitive to protonation changes. Human serum albumine (HSA) adsorption experiments revealed a strong decrease of fouling for the PP films, which were modified with polyelectrolyte multilayers, in comparison to the unmodified ones.

Charging and structure of zwitterionic supported bilayer lipid membranes studied by streaming current measurements, fluorescence microscopy, and attenuated total reflection Fourier transform infrared spectroscopy.

The authors report on the characterization of the charge formation at supported bilayer lipid membranes (sBLMs) prepared from the zwitterionic lipid 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine on planar silicon dioxide substrates. The charging of the sBLMs was studied in KCl solutions of different ionic strengths between 0.1 and 10 mM by streaming current measurements. In addition, attenuated total reflection Fourier transform infrared spectroscopy and fluorescence microscopy were applied to determine the lipid concentration in the membrane and to study the influence of the harsh conditions (pH 9-2, shear forces) during the electrokinetic measurements on the membrane stability and the lipid diffusion coefficient. The sBLMs were found to be extremely stable. Isoelectric points of about 4 revealed that unsymmetrical adsorption of hydroxide and hydronium ions determined the charging of the outer leaflet of the membrane in the investigated pH range. The diffusion coefficients were found to be rather independent on the ionic strength at neutral and alkaline pH. However, significantly decreased lipid diffusion at pH<4 indicated a charge-induced transition of the fluidic bilayer into a gel/ordered-phase bilayer.

Deposition of polylelectrolyte complex nano-particles at silica surfaces characterized by ATR-FTIR and SEM

Abstract We report on the adsorption of preformed polyelectrolyte complex (PEC) particles at silica surfaces. PEC dispersions were prepared by mixing poly(diallyldimethylammonium chloride) (PDADMAC) optionally with two maleic acid copolymers: sodium poly(maleic acid-co-propylen) (PMA-P) or sodium poly(maleic acid-co-α-methylstyrene) (PMA-MS). Stable colloid nanoparticles were formed as it was characterized by PCS and turbidity. ATR-FTIR spectroscopy was used to study the adsorbed amount of the PECs and SEM and AFM were applied for the lateral characterization of the PEC layers. The influence of surface pretreatment (i), polyanion (ii), pH (iii) and PEC centrifugation (iv) on the adsorption behavior was studied. Different pretreatments of the silicon substrate, i.e. cleaning in sulfuric acid, ‘piranha’ solution or multilayer deposition caused different adsorbed amounts, whereby additionally a linear dependence of the adsorbed amount on the bulk PEC concentration was found. Deposition of PEC-PDADMAC/PMA-P resulted in flat highly deformed adsorbed particles, whereas the adsorbed PEC-PDADMAC/PMA-MS formed spherical caps on the surface. The adsorbed amount obtained at pH=9 was significantly higher compared to that at pH=6. Centrifugation of the PEC dispersion resulted in a monodisperse coacervate fraction for PMA-MS containing PECs showing also a low polydispersity in the adsorbed state. Using silicon surfaces modified by polyanion terminating polyelectrolyte multilayers, the highest adsorbed amount of PECs was found, whereby additionally no deformation of the adsorbed PECs was observed. Furthermore consecutive deposition of oppositely charged PEC particles resulted in a complete surface coverage. From these findings we outline the potential of PEC nanoparticles, which might be compared in properties and applications to latex systems.

Deposition and properties of polyelectrolyte multilayers studied by ATR-FTIR spectroscopy

Abstract In-situ ATR-FTIR spectroscopy was adopted to study multilayer assemblies of oppositely charged polyelectrolytes consecutively deposited on plasma treated SiO surface. Two different polyelectrolyte systems were selected for this series of studies in order to address (i) conformational changes in polyelectrolyte multilayers as a response to the external stimuli and (ii) protein adsorption on preformed polyelectrolyte multilayers. The former phenomenon was examined for the polyelectrolyte pair of poly(acrylic acid) (PAA) and poly( l -lysine) (PLL), where PLL provides an advantage of detecting the early adsorption events due to its intense amide I band. Protein adsorption was followed on the multilayer assembly consisting of sodium alginate (SA), cellulose sulfate (CS) and poly(methylene-co-guanidine) (PMCG). These multilayers were used as a model system to simulate protein interactions with the membranes formed by polyelectrolyte complexation.

Examination of poly(butadiene epoxide)-coatings on inorganic surfaces☆

Abstract The reactive coating of different inorganic model surfaces (dispersed SiO2-particles and planar Si/SiO2-, Ge/GeO2-, Al2O3- and TiO2-substrates) with poly(butadiene epoxide) (PBDE) from organic solution is reported on. Furthermore, selected synthetic modification reactions at the coating surface are described. PBDE with different epoxidation degrees was synthesized by a polymer-analogous reaction using poly(butadiene) and peracetic acid. As surface sensitive methods FTIR-spectroscopy (Transmission, ATR, DRIFT), 13C CPMAS NMR spectroscopy and wetting measurements were chosen. For the adsorption of PBDE onto dispersed SiO2-particles, a correlation was found between the total surface concentration and the degree of PBDE-epoxidation. As hints for chemisorption the formation of covalent SiO–C-bonds on SiO2-particles was detected by 13C CPMAS NMR and the loss of silanol groups was proven by DRIFT-spectroscopy. Evidence for covalent PBDE binding on planar inorganic surfaces was obtained by means of ATR-FTIR-spectroscopy using a specific ν(CH) component at 2970 cm−1, due to the epoxide group. Its intensity significantly decreased due to a chemical reaction with the inorganic surface, whereby the decrease was dependent on the kind of surface and the surface pretreatment. Obviously, after the chemisorption unreacted epoxy groups of the polymer coating remained, offering synthetic potential for further surface modification. Among a variety of modification reactions on PBDE coated surfaces, the following were chosen: (1) hydrolysis to form diol functionalities; (2) further reaction with a diisocyanate; and (3) grafting of nucleophilic poly(ethylene glycol) (NH2-PEG) derivatives (3).

Cast adhesive polyelectrolyte complex particle films of unmodified or maltose-modified poly(ethyleneimine) and cellulose sulphate: fabrication, film stability and retarded release of zoledronate

The bone therapeutic drug zoledronate (ZOL) was loaded at and released by polyelectrolyte complex (PEC) particle films composed of either pure poly(ethyleneimine) (PEI) or maltose-modified poly(ethyleneimine) (PEI-M) and oppositely charged cellulose sulfate attached to model germanium (Ge) substrates by solution casting. Dispersions of colloidally stable polyelectrolyte complex (PEC) particles in the size range 11–141xa0nm were obtained by mixing PEI or PEI-M, CS and ZOL in defined stoichiometric ratios. TRANS-FTIR spectroscopy was used to determine the stability of the PEC films against detachment, in-situ-ATR-FTIR spectroscopy for the ZOL loss in the PEC film and UV–VIS spectroscopy for the ZOL enrichment of the release medium. Films of casted ZOL/CS/PEI-M or ZOL/CS/PEI particles were stable in contact to water, while films of the pure drug (ZOL) and of the binary systems ZOL/PEI-M or ZOL/PEI were not stable against detachment. Retarded releases of ZOL from various PEC films compared to the pure drug film were observed. The molecular weight of PEI showed a considerable effect on the initial burst (IB) of ZOL. No significant effect of the maltose modification of PEI-25xa0K on IB could be found. Generally, after one day the ZOL release process was finished for all measured ZOL/PEC samples and residual amounts of 0-30% were obtained. Surface adhesive drug loaded PEC particles are promising drug delivery systems to supply and release a defined amount of bone therapeutics and to functionalize bone substitution materials.

Influence of Electrode Design and Contacting Layers on Performance of Electrolyte Supported SOFC/SOEC Single Cells

The solid oxide cell is a basis for highly efficient and reversible electrochemical energy conversion. A single cell based on a planar electrolyte substrate as support (ESC) is often utilized for SOFC/SOEC stack manufacturing and fulfills necessary requirements for application in small, medium and large scale fuel cell and electrolysis systems. Thickness of the electrolyte substrate, and its ionic conductivity limits the power density of the ESC. To improve the performance of this cell type in SOFC/SOEC mode, alternative fuel electrodes, on the basis of Ni/CGO as well as electrolytes with reduced thickness, have been applied. Furthermore, different interlayers on the air side have been tested to avoid the electrode delamination and to reduce the cell degradation in electrolysis mode. Finally, the influence of the contacting layer on cell performance, especially for cells with an ultrathin electrolyte and thin electrode layers, has been investigated. It has been found that Ni/CGO outperform traditional Ni/8YSZ electrodes and the introduction of a ScSZ interlayer substantially reduces the degradation rate of ESC in electrolysis mode. Furthermore, it was demonstrated that, for thin electrodes, the application of contacting layers with good conductivity and adhesion to current collectors improves performance significantly.

Growth Factor-Bearing Polymer Brushes - Versatile Bioactive Substrates Influencing Cell Response

In this study we present the development of responsive nanoscale substrates exhibiting cell-guiding properties based on incorporated bioactive signaling cues. The investigative approach considered the effect of two different surface-bound growth factors (GFs) on cell behavior and response: hepatocyte growth factor (HGF) and basic fibroblast growth factor (bFGF). Two surface biofunctionalization strategies were explored in order to conceive versatile, bioactive thin polymer brush films. Polymer brushes made of tethered poly(acrylic)acid (PAA) polymer layers with a high grafting density of polymer chains were biofunctionalized with GFs either by physisorption or chemisorption. Both GFs showed high binding efficiencies to PAA brushes based on their initial loading concentrations. The GF release kinetics can be distinguished depending on the applied biofunctionalization method. Specifically, a high initial burst followed by a constant slow release was observed in the case of both physisorbed HGF and bFGF. In contrast, the release kinetics of chemisorbed GFs were quite different. Remarkably, chemisorbed HGF remained bound to the brush surface for over 1 week, whereas 50% of chemisorbed bFGF was released slowly. Furthermore, the effect of these GF-biofunctionalized PAA brushes on different cells was investigated. A human hepatoma cell line (HepG2) was used to analyze the bioactivity of HGF-modified PAA brushes by measuring cell growth inhibition and scattering effects. Additionally, the differentiation of mouse embryonic stem cells (mESCs) toward endoderm was studied on bFGF-modified PAA brush surfaces. Finally, the results illustrate that PAA brushes, particularly those biofunctionalized with chemisorbed GFs, produce an expected measurable effect on both cell types. Therefore, PAA polymer brushes biofunctionalized with GFs can be used as bioactive cell culture substrates with tuned efficiency.

In-Situ-ATR-FTIR Detection of Protein Sorption at Polyelectrolyte Multilayers: Variation of the Thickness

In-situ ATR FTIR spectroscopy was used to study protein adsorption at polyelectrolyte multilayers (PEM). PEM were fabricated by consecutively adsorbing poly(ethyleneimine) (PEI) and poly(acrylic acid) (PAC) analogously to the technique introduced by Decher at silicon internal reflection elements in an in-situ ATR cell. The thickness of PEM of PEI/PAC could be varied by pH value, concentration and number of adsorption steps. At this PEM system adsorption of human serum albumin was studied under electrostatically attractive conditions in dependence of the thickness. Evidence was found, that HSA was predominantly bound in the uncompensated outermost layer region rather than in the bulk phase of the PEM.