Biomedical subjects

Wolfgang Knoll

Publications and source records attributed to Wolfgang Knoll.

At least 19 recordsLinked to original sources

PubMed2007

Evaluation of two- and three-dimensional streptavidin binding platforms for surface plasmon resonance spectroscopy studies of DNA hybridization and protein-DNA binding.

Surface plasmon resonance (SPR) spectroscopy has been used to study DNA assembly, DNA hybridization, and protein-DNA interactions on two streptavidin (SA) sensor chips. On one chip, SA molecules are immobilized on a biotin-exposed surface, forming an ordered two-dimensional (2D) SA monolayer. The other chip, BIAcore's SA chip, contains SA molecules immobilized within a three-dimensional (3D) carboxylated dextran matrix. Compared to the 2D chip, the 3D SA matrix allows for a slower immobilization rate of biotinylated DNA due to diffusion limitation in the dextran matrix, but with twice the amount of the immobilized DNA due to the greater number of reactive sites, which in turn enables a higher sensitivity for DNA hybridization detection. Interestingly, having a greater DNA probe dispersion in the 3D matrix does not induce a higher DNA hybridization efficiency. In a study of protein binding to immobilized DNA (estrogen receptor to estrogen response elements), aiming at assessing the DNA sequence dependent protein binding behavior, the 2D and 3D chips produce different binding characteristics. On the 2D chip, the protein binding exhibits a better selectivity to the specific sequences, regardless of binding stringency (e.g. salt concentration), whereas on the 3D chip, the liquid handling system needs to be optimized in order to minimize transport limitations and to detect small affinity differences. Through this study we demonstrate that the physicochemical structure of SPR chips affects the apparent binding behaviors of biomolecules. When interpreting SPR binding curves and selecting a sensor chip, these effects should be taken into account.

Biosensing Techniques↗

PubMed2006

Photophysical properties of hydroxylated amphiphilic poly(p-phenylene)s.

A homologous series of polyhydroxylated poly(p-phenylene)s with different alkoxy groups (C6PPPOH, C12PPPOH, and C18PPPOH) were synthesized with use of the Suzuki polycondensation reaction. Comparative studies of the structure correlation between their photophysical properties and film morphology is described. The absorption and emission spectra of polymers in solution and thin films showed similar features indicating that the electronic properties in solution were retained in the film state. Compared to the polymer with the short alkoxy chains (C6PPPOH), the polymers with long alkoxy groups (C12PPPOH and C18PPPOH) showed improved film forming properties with continuous and smooth film morphology. The absorption properties of the C12PPPOH showed an enhanced effective conjugation length and high quantum yield implying planarization of the backbone through alkoxy chain packing (C12H25O-) and potential hydrogen bonds. No overlap in the absorption and emission spectra was observed, which indicated minimized excimer formation or excitation energy transfer in the films. Time-resolved fluorescence measurements showed that the decay times increased from 43 ps (C6PPPOH) to 78 ps (C12PPPOH) and 99 ps (C18PPPOH). Electrochemical studies were performed for all polymers and the observed oxidation potential for C6PPPOH was higher than that of C12PPPOH and C18PPPOH. In addition, the C12PPPOH has the lowest band gap of DeltaE = 2.59 eV when compared to the 3.1 (C6PPPOH) and 2.61 eV (C18PPPOH) gaps. The optical band gaps estimated from the absorption onset of the polymers are significantly higher than those obtained from electrochemical data. C12PPPOH was chosen for investigating the charge carrier mobility by the time-of-flight (TOF) technique. The observed results also showed negative field dependent values of the drift mobility for the polymer C12PPPOH.

Journal Article↗

PubMed2006

Self-assembled monolayers on mercury probed in a modified surface force apparatus.

Self-assembled monolayers (SAMs) of three thiol compounds formed on mercury are investigated by a combination of cyclic voltammetry, electrocapillary curves, and a novel method of measuring electrical double-layer properties. The last method involves a modified surface force apparatus in which a flat mica surface is pressed down toward a fixed mercury drop held beneath it, while both are immersed in aqueous electrolyte solution. Optical interference measurements are made of the mica-mercury separation as a function of electrical potential applied to the mercury, which yields information on the double-layer interaction between the two surfaces. Mercury is decorated by SAMs of 11-mercapto-1-undecanoic acid, which is shown to bring negative charge to the mercury/aqueous interface due to dissociation of the carboxylic acid groups; 11-mercapto-1-undecanol, which although it is uncharged changes the dipole potential of the interface; and 1-undecanethiol, which likewise changes the dipole potential, but by a different amount. The difference between the changes in dipole potential (90 mV) can be related to the different terminal groups of these two SAMs, -CH3 compared to -OH, that are in contact with the aqueous phase.

Electrochemistry↗

PubMed2006

Preparation of multilayered trimodal colloid crystals and binary inverse opals.

We report for the first time the preparation of multilayered trimodal colloid crystals (tCC) and their corresponding binary inverse opals (bIO), which present complex hierarchical structures that may have significant potential in photonics, phononics, separations, and catalysis, among others. A trimodal colloidal mixture of 465 nm polystyrene (PS), 84 nm poly(methyl methacrylate) (PMMA), and 6 nm silica particles in suspension was transferred onto a glass substrate and self-assembled into highly ordered trimodal crystal structures during vertical lifting deposition. Pyrolysis of the organic particles in the tCC resulted in silica bIO with interconnected meso- and macropores. Vis-NIR spectra of all structures were analyzed to reveal the internal architecture with each PS sphere correlating to 21-23 PMMA particles (a LI21-23 stoichiometry), which corresponded well with computer models.

Journal Article↗

PubMed2006

Simultaneous excitation of propagating and localized surface plasmon resonance in nanoporous gold membranes.

Materials multifunctionality for optical sensing of adsorbates has obvious advantages-in addition to the potential for greater sensitivity, the different length scales associated with a variety of optical phenomena allow a greater variety of adsorption characteristics to be examined. Here, we show that ultrathin (approximately 100 nm) nanoporous gold membranes possess features of both planar metal films that exhibit propagating SPR excitations and nanofeatured metals that exhibit localized SPR excitations. This is the first report of such multifunctionality in an optically active metal. We give illustrative examples of using this material to probe biorecognition reactions and to probe the structure evolution of a layer-by-layer deposition of charged dendrimers. Our results are consistent with the very different lengths of the tail of the evanescent field decays associated with each of these plasmon excitation modes.

Journal Article↗

PubMed2006

Fabrication and characterization of multilayer films from amphiphilic poly(p-phenylene)s.

Over the past two decades, considerable efforts have been devoted to the development of conjugated polymeric materials for electronic applications due to the tunability of their properties through variation of their chemical structure. The LB technique is one of the most effective and precise methods for controlling the organization and thereby the properties of polymer films at the nanoscale for device fabrication. A detailed study was performed on the Langmuir-Schaefer (LS) monolayer and Langmuir-Blodgett-Kuhn (LBK) multilayer formation of newly designed conjugated poly(p-phenylene)s (C(n)PPPOH), incorporated with alkoxy groups with different chain lengths (C(6)H(13)O-, C(12)H(25)O-, and C(18)H(37)O-) and hydroxyl groups on the polymer backbone. The monolayer formed at the air-water interface was characterized using surface pressure-area isotherms, including hysteresis measurements. The films were then transferred to different hydrophilic solid substrates and analyzed using surface plasmon resonance spectroscopy, UV-vis spectroscopy, fluorescence spectroscopy, and AFM measurements. The results showed that the polymer with a short alkoxy chain (C(6)PPPOH) forms uniform monolayers at the air-water interface and can be transferred as multilayer films compared to C(12)PPPOH and C(18)PPPOH. The observed film thicknesses measured by SPR and AFM studies were similar to the theoretical value obtained in the case of C(6)PPPOH, whereas this was not the case with the other two polymers. The present study shows that the polymer C(n)PPPOH with short alkoxy chain can be transferred onto different solid substrates for device fabrication with molecular level control.

Journal Article↗

PubMed2006

Estimation of dielectric function of biotin-capped gold nanoparticles via signal enhancement on surface plasmon resonance.

Biotin-capped gold nanoparticles assembled on flat gold with volume fraction f are studied by surface plasmon resonance (SPR) spectroscopy and atomic force microscopy (AFM) in order to estimate the dielectric function of the gold nanoparticles based on the Maxwell-Garnett (MG) theory. The complex dielectric function (epsilon',epsilon'') of the spherical nanoparticles at three representative wavelengths in the vis-near-IR region, i.e., lambda = 543, 632.8, and 1152 nm, is estimated for a surface homogeneously covered with nanoparticles in order to discuss the wavelength dependence of the dielectric function. The SPR response of a surface covered with particles in 2D aggregates is also analyzed. The experimental SPR curve of the particle aggregates deviates from the theoretical predictions, suggesting dipole interactions between particles.

Adsorption↗

PubMed2006

Morphologies in solvent-annealed thin films of symmetric diblock copolymer.

We have systematically studied the thin film morphologies of symmetric poly(styrene)-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer after annealing to solvents with varying selectivity. Upon neutral solvent vapor annealing, terraced morphology is observed without any lateral structures on the surfaces. When using PS-selective solvent annealing, the film exhibits macroscopically flat with a disordered micellar structure. While PMMA-selective solvent annealing leads to the dewetting of the film with fractal-like holes, with highly ordered nanoscale depressions in the region of undewetted films. In addition, when decreasing the swelling degree of the film in the case of PMMA-selective solvent annealing, hills and valleys are observed with the coexistence of highly ordered nanoscale spheres and stripes on the surface, in contrast to the case of higher swelling degree. The differences are explained qualitatively on the basis of polymer-solvent interaction parameters of the different components.

Journal Article↗

PubMed2006

An optical waveguide study on the nanopore formation in block copolymer/homopolymer thin films by selective solvent swelling.

Thin films of mixtures of asymmetric poly(styrene-block-methyl methacrylate) (PS-b-PMMA) diblock copolymers and PMMA homopolymers with cylindrical PMMA microdomains oriented normal to the substrate surface were used to couple optical modes in the Kretschmann configuration, and their optical properties were investigated by optical waveguide spectroscopy (OWS). The nanopore formation in the block copolymer (BCP) waveguide layer via selective solvent swelling and subsequent reannealing was monitored in terms of shifts in the coupling mode angles. The sequential swelling/reannealing of the initial mixture film resulted in a number of discrete or partially interconnected pores instead of cylindrical pores with a high aspect ratio. The simultaneous processes occurring inside and on top of the BCP waveguide layer were discerned independently with high selectivity for p- and s-polarization.

Membranes, Artificial↗

PubMed2006

Binding of small mono- and oligomeric integrin ligands to membrane-embedded integrins monitored by surface plasmon-enhanced fluorescence spectroscopy.

We recently developed a binding assay format by incorporating native transmembrane receptors into artificial phospholipid bilayers on biosensor devices for surface plasmon resonance spectroscopy. By extending the method to surface plasmon-enhanced fluorescence spectroscopy (SPFS), sensitive recording of the association of even very small ligands is enabled. Herewith, we monitored binding of synthetic mono- and oligomeric RGD-based peptides and peptidomimetics to integrins alphavbeta3 and alphavbeta5, after having confirmed correct orientation and functionality of membrane-embedded integrins. We evaluated integrin binding of RGD multimers linked together via aminohexanoic acid (Ahx) spacers and showed that the dimer revealed higher binding activity than the tetramer, followed by the RGD monomers. The peptidomimetic was also found to be highly active with a slightly higher selectivity toward alphavbeta3. The different compounds were also evaluated in in vitro cell adhesion tests for their capacity to interfere with alphavbeta3-mediated cell attachment to vitronectin. We hereby demonstrated that the different RGD monomers were similarly effective; the RGD dimer and tetramer showed comparable IC50 values, which were, however, significantly higher than those of the monomers. Best cell detachment from vitronectin was achieved by the peptidomimetic. The novel SPFS-binding assay platform proves to be a suitable, reliable, and sensitive method to monitor the binding capacity of small ligands to native transmembrane receptors, here demonstrated for integrins.

Amino Acid Sequence↗

PubMed2006

Stabilization of plasma-polymerized allylamine films by ethanol extraction.

The effect of ethanol extraction on plasma-polymerized allylamine (PPAA) films was investigated by measuring their thickness change using surface plasmon resonance (SPR) spectroscopy and optical waveguide spectroscopy (OWS). It was found that much of the freshly deposited PPAA films is lost upon ethanol treatment. The decrease in PPAA thickness is related to the plasma input power, the monomer vapor pressure, and the thickness of the deposited films. Despite the relatively high loss in film thickness, the densities of the amine groups in the extracted PPAA are comparable to those of fresh films, as seen by Fourier transform infrared (FT-IR) spectroscopy. The results of this study are of specific importance with respect to the biomedical application of plasma-polymerized functional thin films, in which the stability of a plasma polymer in contact with aqueous media is essential.

Coated Materials, Biocompatible↗

PubMed2006

New method to measure packing densities of self-assembled thiolipid monolayers.

For a monolayer of 2,3-di-phytanyl-sn-glycerol-1-tetraethylene glycol-D,L-a-lipoic acid ester lipid (DPTL) self-assembled (SAM) at a gold electrode surface we propose a new method to determine the charge number per adsorbed molecule and the packing density (area per molecule) in the monolayer. The method relies on chronocoulometry to measure the charge density at the SAM covered gold electrode surface. Two series of measurements have to be performed. In the first series, charge densities are measured for a monolayer transferred from the air-solution to the metal-solution interface using the Langmuir-Blodgett (LB) technique. This series of measurements allows one to determine charge numbers per adsorbed DPTL molecule. The second series is performed using a gold electrode covered with a self-assembled monolayer. The charge densities obtained in this series are then used to calculate the packing density with the help of charge numbers per adsorbed DPTL determined in the first series. The area per adsorbed molecule determined by the new method was compared to the area per molecule determined by the popular reductive desorption method. The molecular area determined with the new method is about 20% larger than the area calculated from the van der Waals model, which is a physically reasonable result. In contrast, the popular reductive desorption method gives an area per molecule 20% lower than the minimum estimated based on a van der Waals model. This is a physically unreasonable result. It is also shown that the charge numbers per adsorbed molecule depend on the electrode potential and may assume values smaller than the number of electrons participating in the reductive desorption step. An explanation of the origin of the "partial charge numbers" is provided. We recommend the new method be used in future studies of thiol adsorption at metal surfaces.

Journal Article↗

PubMed2006

Immobilization and AFM of single 4 x 6-mer tarantula hemocyanin molecules.

The high molecular mass respiratory protein of the tarantula Eurypelma californicum, a 4 x 6-mer hemocyanin, was investigated by atomic force microscopy (AFM). Various substrates and methods were evaluated for immobilization of individual hemocyanin molecules on a solid surface. Samples were imaged after physisorption on mica and self-assembled monolayers, and after chemisorption on Au(111) and N-hydroxy-succinimide (NHS) functionalized surfaces. AFM measurements were carried out preferable in solution and contact mode, but also in Tapping mode and on air-dried samples. Adsorption of the protein on mica followed by drying and carrying out the measurements in Tapping mode gave the best results. In the AFM images the four hexamers of the native 4 x 6-mer hemocyanin have been defined. The results were compared with independent available structural data and represent a validation case for this technique applied for the first time on such giant and complex molecules. As observable in images taken by transmission electron microscopy and also proposed from SAXS data, 4 x 6-mers could be found where the half-molecules are tilted against each other. This study is a step in resolving conformational heterogeneities, involved in oxygen binding of hemocyanins, at the single-molecule level by AFM.

Air↗

PubMed2006

Surface plasmon optical studies of carboxymethyl dextran brushes versus networks.

The conformational changes of carboxymethyl dextran (CMD) substrates induced by variations in pH and ionic strength were studied using surface plasmon field-enhanced fluorescence spectroscopy (SPFS). A typical response was an increase in swelling upon increasing the pH and decreasing the ionic strength. Furthermore, the effects of the surface charge and cross-link density of the CMD on the degree of stimulus responses were investigated. The swelling/collapse ratio decreased with decreasing carboxyl group surface concentration and increasing cross-link density.

Dextrans↗

PubMed2006

Pulsed plasma polymerized maleic anhydride films in humid air and in aqueous solutions studied with optical waveguide spectroscopy.

Optical waveguide spectroscopy (OWS) was employed to monitor the swelling behavior of pulsed plasma polymerized maleic anhydride (PPPMA) films in humid air and in aqueous solutions by measuring the film thicknesses and refractive indices. With the relative humidity of air increasing, both the thickness and the refractive index of the PPPMA films increased, indicating water penetration into and uptake by the films. The swelling of the hydrated PPPMA films in humid air is reversible. In aqueous media, the thickness and the refractive index of the washed PPPMA film increased with an increase of pH and ionic strength, respectively. On the basis of the present data, a hypothesis concerning the structure of the PPPMA film is proposed. Our model suggests that the unique structure of the PPPMA films originates from the cyclic structure of maleic anhydride and depends on parameters of the plasma deposition process, and the interaction between H(2)O and the carboxylic groups.

Journal Article↗

PubMed2006

Synthesis and photoluminescence of titania nanoparticle arrays templated by block-copolymer thin films.

High-density arrays of titania nanoparticles were prepared using a polystyrene-b-poly(ethylene oxide) block copolymer (PS-b-PEO) as a template and a titanium tetraisopropoxide based sol-gel precursor as titania source via a spin-coating method. The hydrophilic titania sol-gel precursor was selectively incorporated into hydrophilic PEO domains of PS-b-PEO and form titania nanoparticle arrays, due to a microphase separation between the PS block and the sol-gel/PEO phase. Field emission scanning electron microscopy (FESEM) and scanning probe microscopy (SPM) images showed that the uniformity and long-range order of the titania/PEO domains improved with increasing sol-gel precursor amount. Grazing incidence small-angle X-ray scattering (GISAXS) results indicate that the ordered structures exist over large length scales. Titania nanocrystal arrays of anatase modification were obtained by calcination at 600 degrees C for 4 h. After calcination, separated particles were observed for low and medium amounts of sol-gel precursors. Films with higher precursor amounts showed wormlike structures due to the aggregation between neighboring particles. Removal of the polymer matrix via UV treatment leads to highly ordered arrays of amorphous titania while retaining the domain size and interparticle distance initially present in the hybrid films. Photoluminescence (PL) properties were investigated for samples before and after calcination. The PL intensity increases with the increasing amount of sol-gel precursor. Bands at 412 nm were ascribed to self-trapped exitons and bands at 461 and 502 nm to oxygen vacancies, respectively. Uncalcined or UV-treated samples also showed PL properties similar to calcined samples, indicating that the local environment of the titanium atoms is similar to the environment of the crystalline anatase modification.

Journal Article↗

PubMed2006

PNA-DNA hybridization study using labeled streptavidin by voltammetry and surface plasmon fluorescence spectroscopy.

Using ferrocene-streptavidin conjugates as amplifiers, we recently have demonstrated the simultaneous detection of DNA hybridization to peptide nucleic acid (PNA)-modified gold surfaces at the femtomole level by electrochemical and surface plasmon resonance techniques (Liu, J.; Tian, S.; Tiefenauer, L.; Nielsen, P. E.; Knoll, W. Anal. Chem. 2005, 77, 2756-2761). In this paper, a detailed study of the binding behavior of PNA-DNA is presented by square wave voltammetry and surface plasmon field-enhanced fluorescence spectroscopy (SPFS). The different binding constants for fully matched and single-mismatched DNA were obtained. The effect of the buffer concentration on the PNA-DNA hybrids was investigated using labeled streptavidin by cyclic voltammetry (CV) and SPFS. At high ionic strength, both the CV and SPFS signals were restrained dramatically, which is most probably due to a conformational change of the short-strand PNA-DNA helices on the surface. We conclude that the combination of electrochemical techniques with SPFS is very useful for the study of short DNA structure transformation.

DNA↗

PubMed2006

Synthesis of dumbbell-shaped manganese oxide nanocrystals.

Dumbbell-like homogeneous MnO nanocrystals are obtained for the first time via the pyrolysis of manganese formate in trioctylamine/oleic acid media and the orientation aggregation mechanism is proposed for this 1D growth route.

Manganese Compounds↗