Stefan Köstler

Wettability and surface composition of partly and fully regenerated cellulose thin films from trimethylsilyl cellulose.

The wettability and surface free energy (SFE) of partly and fully regenerated cellulose model surfaces from spin coated trimethylsilyl cellulose were determined by static contact angle (SCA) measurements. In order to gain detailed insight into the desilylation reaction of the surfaces the results from SCA measurements were compared with data from other surface analytical methods, namely thickness measurements, X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance infrared spectroscopy (ATR-IR). Additionally, the influence of ultra high vacuum treatment (UHV) during XPS measurements on the water wettability and surface morphology of regenerated cellulose thin films was investigated. The wetting of polar and non-polar liquids increased with prolonged regeneration time, which is reflected in the higher SFE values and polarities of the films. After UHV treatment the water SCA of partly regenerated films decreases, whereas fully regenerated cellulose shows a higher water SCA. Therefore it is assumed that volatile desilylation products tend to adsorb on partly regenerated films, which strongly influences their wettability.

Integrated organic electronic based optochemical sensors using polarization filters

A compact, integrated photoluminescence based oxygen and pH sensor, utilizing an organic light emitting device (OLED) as the light source and an organic photodiode (OPD) as the detection unit, is described. The main challenge in such an integrated sensor is the suppression of the excitation light at the detector, which is typically by many orders of magnitude higher in intensity than the emitted fluorescence. In our approach, we refrain from utilizing edge filters which require narrow band excitation sources and dyes with an adequate large Stokes shift. We rather developed an integrated sensor concept relying on two polarizers to separate the emission and excitation light. One polarizer is located right after the OLED, while the other one, oriented at 90° to the first, is placed in front of the OPD. The main advantage of this solution is that any combination of excitation and emission light is acceptable, even if the two signals overlap spectrally. This is especially important for the use of OLEDs as the ...

Optical oxygen sensors based on Pt(II) porphyrin dye immobilized on S-layer protein matrices

This paper describes the development of planar and fiber optic oxygen sensors utilizing surface layer (S-layer) proteins as immobilization matrix for oxygen sensitive dyes. S-layer proteins have the intrinsic capability to reassemble into two-dimensional arrays in suspension and at interfaces. Due to their crystalline character the distribution of functional groups, such as carboxylic groups, is repeated with the periodicity of the lattice and thus allows the reproducible and geometrically distinct binding of functional molecules. For the development of oxygen sensors an oxygen sensitive Pt(II) porphyrin dye was covalently bound to the S-layer matrix. Measurement of the oxygen concentration was performed by phase modulation fluorimetry. Setups comprising low cost optoelectronic components like LEDs and silicon photodiodes were constructed. For both sensor setups (planar and fiber optic) variations in the oxygen concentrations resulted in distinct and reproducible changes in luminescence lifetime and intensity. The luminescence quenching efficiency of these sensors was found to be 1.5-1.9 (expressed as the ratio of signal under nitrogen and air) which compares well to other sensor systems using luminophores embedded in polymer matrices. These results demonstrated the application potential of S-layers as immobilization matrices in the development of (bio-)sensors.

A planar waveguide optical sensor employing simple light coupling

The novel optical sensor concept utilizes the sensing layer as light propagating layer and employs a new method to couple light into a planar waveguide.

Triggering protein adsorption on tailored cationic cellulose surfaces.

The equipment of cellulose ultrathin films with BSA (bovine serum albumin) via cationization of the surface by tailor-made cationic celluloses is described. In this way, matrices for controlled protein deposition are created, whereas the extent of protein affinity to these surfaces is controlled by the charge density and solubility of the tailored cationic cellulose derivative. In order to understand the impact of the cationic cellulose derivatives on the protein affinity, their interaction capacity with fluorescently labeled BSA is investigated at different concentrations and pH values. The amount of deposited material is quantified using QCM-D (quartz crystal microbalance with dissipation monitoring, wet mass) and MP-SPR (multi-parameter surface plasmon resonance, dry mass), and the mass of coupled water is evaluated by combination of QCM-D and SPR data. It turns out that adsorption can be tuned over a wide range (0.6-3.9 mg dry mass m(-2)) depending on the used conditions for adsorption and the type of employed cationic cellulose. After evaluation of protein adsorption, patterned cellulose thin films have been prepared and the cationic celluloses were adsorbed in a similar fashion as in the QCM-D and SPR experiments. Onto these cationic surfaces, fluorescently labeled BSA in different concentrations is deposited by an automatized spotting apparatus and a correlation between the amount of the deposited protein and the fluorescence intensity is established.

Functional Polysaccharide Conjugates for the Preparation of Microarrays

A method for the immobilization of functional molecules on cellulose surfaces was developed. The irreversible deposition of the water-soluble polyelectrolyte carboxymethyl cellulose (CMC) on solid cellulose surfaces was used as a basis for this immobilization. CMC was modified using aminofluorescein (AMF) as a model compound for a functional molecule. The carbodiimide mediated coupling efficiency of AMF to CMC was studied in detail, and the functional conjugates were isolated. A quartz crystal microbalance with dissipation was employed to study the immobilization of the functionalized CMC onto cellulose model films in situ. The influence of the carbodiimide concentration, the degree of substitution, and the molecular weight of CMC on the immobilization process was investigated. Atomic force microscopy was used to characterize the changes in the surface morphology of the modified cellulose films. Finally, microspotted arrays of AMF-CMC conjugates were prepared with the knowledge obtained from the basic interaction studies. The successful deposition of AMF-CMC conjugates onto cellulose surfaces was proven by fluorescence microscopy. The conjugation of functional molecules to CMC and the subsequent deposition of these products on cellulose can be seen as a versatile method to immobilize these molecules for applications in the field of microarrays and other sensor surfaces. It offers the possibility to introduce new properties on a variety of cellulosic materials.

Preparation of PDMS ultrathin films and patterned surface modification with cellulose

In this investigation, polydimethylsiloxane (PDMS) ultrathin films are prepared on a variety of solid surfaces by a simple and fast spin coating method, and patterned with the natural biopolymer cellulose via lithographic methods. Two surface patterning methods are developed to create coatings of hydrophilic cellulose, regenerated from trimethylsilyl cellulose (TMSC) on the PDMS thin films. In method 1, spin coated TMSC films on PDMS are covered with a lithographic mask and exposed to vapors of hydrochloric acid, which results in spatially separated cellulose pads surrounded by TMSC. Subsequent selective dissolution of TMSC with organic solvents results in a direct anchoring of cellulose pads on the PDMS. In method 2, PDMS thin films covered with a lithographic mask are exposed to UV/ozone, spray coated with TMSC and regenerated to give cellulose. The conversion of hydrophobic TMSC into hydrophilic cellulose coatings is confirmed by wettability and fluorescence measurements. The developed structures are highly transparent and stable in aqueous solutions (pH 3–9) and organic solvents. The surface properties of the polymer films are characterized using a quartz crystal microbalance with dissipation (QCM-D), ellipsometry, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), contact angle and streaming potential measurements.

Gold nanoparticles in the engineering of antibacterial and anticoagulant surfaces.

Simultaneous antibacterial and anticoagulant surfaces have been prepared by immobilization of engineered gold nanoparticles onto different kinds of surfaces. The gold nanoparticle core is surrounded by a hemocompatible, anticoagulant polysaccharide, 6-O chitosan sulfate, which serves as reduction and stabilizing agent for the generation of gold nanoparticles in a microwave mediated reaction. The particle suspension shows anticoagulant activity, which is investigated by aPTT and PT testing on citrated blood samples of three patients suffering from congenital or acquired bleeding disorders. The amount of nanoparticles deposited on the surfaces is quantified by a quartz crystal microbalance with dissipation unit. All gold containing surfaces exhibit excellent antimicrobial properties against the chosen model organism, Escherichia coli MG 1655 [R1-16]. Moreover, blood plasma coagulation times of the surfaces are increased after deposition of the engineered nanoparticles as demonstrated by QCM-D.

Oscillating streaming potential measurement system for macroscopic surfaces

A method and instrumentation is described capable of streaming potential measurements of various macroscopic surfaces. It differs from other approaches due to the creation of an oscillatory flow of electrolyte solutions through or alongside the sample. This technique offers a wide range of applied flow frequency and amplitude resulting in a fast and highly accurate measurement. This enables the streaming potential detection at rather high ionic strength and in a short time regime, which allows the monitoring of adsorption processes. Streaming potential and applied pressure are measured simultaneously, together with the specific conductivity of the bulk solution, pH value, and temperature. Combining these data, the zeta potential (zeta) for many different material types (fibers, films, foils, granules, and particles) can be calculated. The apparatus comprises reliable and robust measurements, simple handling, a high degree of automation, and advanced software control. With this setup, automated pH and concentration dependent zeta-potential measurements are possible for a variety of analytes and adsorbing species (e.g., ionic strength, surfactants, polyelectrolytes, and proteins); time-resolved measurements are facilitated down to the seconds time scale. The device allows the necessary sample preparation and equilibration outside the instrument using exchangeable sample holders. This offers the opportunity of high sample throughput.

2D crystalline protein layers as immobilization matrices for the development of DNA microarrays

There is a growing demand for functional layers for the immobilization of (bio)molecules on different kinds of substrates in the field of biosensors, microarrays, and lab-on-a-chip development. These functional coatings should have the ability to specifically bind (bio)molecules with a high binding efficiency, while showing low unspecific binding during the following assay. In this paper we present rSbpA surface layer proteins (S-layer proteins) as a versatile immobilization layer for the development of DNA microarrays. S-layer proteins show the ability to reassemble into two-dimensional arrays on solid surfaces and their functional groups, such as carboxylic groups, are repeated with the periodicity of the lattice, allowing for immobilization of other (bio)molecules. Different fluorescently labeled amino functionalized DNA oligomers were covalently linked to the S-layer matrices to allow the characterization of DNA binding on S-layers. Hybridization and dissociation of DNA-oligomers were studied on S-layer coated slides, revealing low levels of unspecific adsorption of DNA on S-layer based immobilization matrices. In the following the principle was transferred to a DNA microarray design showing successful spotting and hybridization on whole microarray slides. Besides common laser scanning for fluorescence detection, S-layer based microarrays were evaluated with a compact, low cost platform for direct fluorescence imaging based on surface plasmon enhanced fluorescence excitation. It could be shown that S-layer protein layers are promising as immobilization matrices for the development of biosensors and microarrays.