Diffusion of laser polarized gases in MRI

The use of Magnetic Resonance Imaging (MRI) as a diagnostic tool is increasingly employing functional contrast agents to study or contrast entire mechanisms. Contrast agents in MRI can be classified in two categories. One type of contrast agents alters the NMR signal of the protons in its surrounding, e.g. lowers the T1 relaxation time. The other type enhances the Nuclear Magnetic Resonance (NMR) signal of specific nuclei. For hyperpolarized gases the NMR signal is improved up to several orders of magnitude. However, gases have a high diffusivity which strongly influences the NMR signal strength, hence the resolution and appearance of the images. The most interesting question in spatially resolved experiments is of course the achievable resolution and contrast by controlling the diffusivity of the gas. The influence of such diffusive processes scales with the diffusion coefficient, the strength of the magnetic field gradients and the timings used in the experiment. Diffusion may not only limit the MRI resolution, but also distort the line shape of MR images for samples, which contain boundaries or diffusion barriers within the sampled space. In addition, due to the large polarization in gaseous 3He and 129Xe, spin diffusion (different from particle diffusion) could play a role in MRI experiments. It is demonstrated that for low temperatures some corrections to the NMR measured diffusion coefficient have to be done, which depend on quantum exchange effects for indistinguishable particles. Physically, if these effects can not change the spin current, they can do it indirectly by modifying the velocity distribution of the different spin states separately, so that the subsequent collisions between atoms and therefore the diffusion coefficient can eventually be affected. A detailed study of the hyperpolarized gas diffusion coefficient is presented, demonstrating the absence of spin diffusion (different from particle diffusion) influence in MRI at clinical conditions. A novel procedure is proposed to control the diffusion coefficient of gases in MRI by admixture of inert buffer gases. The experimental measured diffusion agrees with theoretical simulations. Therefore, the molecular mass and concentration enter as additional parameters into the equations that describe structural contrast. This allows for setting a structural threshold up to which structures contribute to the image. For MRI of the lung this allows for images of very small structural elements (alveoli) only, or in the other extreme, all airways can be displayed with minimal signal loss due to diffusion.

Describing and explaining the foreign expansion of network managing electronic business companies (NM-EBCs)

This study deals with the internationalization behavior of a new and specific type of e-business company, namely the network managing e-business company (NM-EBC). The business model of such e-business companies is based on providing a platform and applications for users to connect and interact, on gathering and channeling the inputs provided by the users, and on organizing and managing the cross-relationships of the various participants. Examples are online communities, matching platforms, and portals. Since NM-EBCs internationalize by replicating their business model in a foreign market and by building up and managing a network of users, who provide input themselves and interact with each other, they have to convince users in foreign markets to join the network and hence to adopt their platform. We draw upon Rogers’ Diffusion of Innovations Theory and Network Theory to explain the internationalization behavior of NM-EBCs. These two theories originate from neighboring disciplines and have not yet been used to explain the internationalization of firms. We combine both theories and formulate hypotheses about which strategies NM-EBCs may choose to expand abroad. To test the applicability of our theory and to gain rich data about the internationalization behavior of these firms, we carried out multiple case studies with internationally active Germany-based NM-EBCs.

Genesis of metasomatic sapphirine-corundum-spinel-bearing granulites in Sri Lanka

The goal of the present study is to understand the mechanism of mass transfer, the composition and the role of fluids during crustal metasomatism in high-temperature metamorphic terranes. A well constrained case study, a locality at Rupaha, Sri Lanka was selected. It is located in the Highland Complex of Sri Lanka, which represents a small, but important fragment of the super-continent Gondwana. Excellent exposures of ultramafic rocks, which are embedded in granulites, were found at 10 localities. These provide a unique background for understanding the metasomatic processes. The boundary between the ultramafic and the granulite rocks are lined with metasomatic reaction zones up to 50cm in width. Progressing from the ultramafics to the granulite host rock, three distinct zones with the following mineral assemblages can be distinguished: (1). phlogopite + spinel + sapphirine, (2). spinel + sapphirine and (3). corundum + biotite + plagioclase. In order to assess the P-T-t path, the peak metamorphism and the exhumation history were constrained using different thermobarometers, as well as a diffusion model of garnet zoning. A maximum temperature of 875 ± 20oC (Opx-Cpx thermometer) and at the peak pressure of 9.0 ± 0.1 kbar (Grt-Cpx-Pl-Qtz) was calculated for the silicic granulite. The ultramafic rocks recorded a peak temperature of 840 ± 70oC (Opx-Cpx thermometer) at 9 kbar. Coexisting spinel and sapphirine from the reaction zone yield a temperature of 820 ± 40oC. This is in agreement with the peak-temperatures recorded in the adjacent granulites and ultramafics rocks. The structural concordance of the ultramafic rocks with the siliceous granulite host rock further support the suggestion, that all units have experienced the same peak metamorphism. Diffusion modeling of retrograde zoning in garnets from mafic granulites suggests a three-step cooling history. A maximum cooling rate of 1oC/Ma is estimated during the initial stage of cooling, followed by a cooling rate of ~30oC/Ma. The outermost rims of garnet indicate a slightly slower cooling rate at about 10-15oC/Ma. The sequences of mineral zones, containing a variety of Al-rich, silica undersaturated minerals in the reaction zones separating the ultramafic rocks from the silica-rich rocks can be explained by a diffusion model. This involves the diffusion of Mg from ultramafic rocks across the layers, and K and Si diffuse in opposite direction. Chemical potential of Mg and Si generated continuous monotonic gradient, allowing steady state diffusional transport across the profile. The strong enrichment in Al, and the considerable loss of Si, during the formation of reaction bands can be inferred from isocon diagrams. Some Al was probably added to the reaction zones, while Si was lost. This is most likely due to fluids percolating parallel to the zones at the boundary of the rock units. This study has shown that not only pressure and temperature conditions but most importantly PH2O and the concentration of the chlorine and fluorine in aqueous fluids also control the mass transport in different geological environments.

Thermodynamics of aqueous biopolymer solutions and fractionation of Dextran

Flory-Huggins interaction parameters and thermal diffusion coefficients were measured for aqueous biopolymer solutions. Dextran (a water soluble polysaccharide) and bovine serum albumin (BSA, a water soluble protein) were used for this study. The former polymer is representative for chain macromolecules and the latter is for globular macromolecules. The interaction parameters for the systems water/dextran and water/BSA were determined as a function of composition by means of vapor pressure measurements, using a combination of headspace sampling and gas chromatography (HS-GC). A new theoretical approach, accounting for chain connectivity and conformational variability, describes the observed dependencies quantitatively for the system water/dextran and qualitatively for the system water/BSA. The phase diagrams of the ternary systems water/methanol/dextran and water/dextran/BSA were determined via cloud point measurements and modeled by means of the direct minimization of the Gibbs energy using the information on the binary subsystems as input parameters. The thermal diffusion of dextran was studied for aqueous solutions in the temperature range 15 < T < 55 oC. The effects of the addition of urea were also studied. In the absence of urea, the Soret coefficient ST changes its sign as T is varied; it is positive for T > 45.0 oC, but negative for T < 45.0 oC. The positive sign of ST means that the dextran molecules migrate towards the cold side of the fluid; this behavior is typical for polymer solutions. While a negative sign indicates the macromolecules move toward the hot side; this behavior has so far not been observed with any other binary aqueous polymer solutions. The addition of urea to the aqueous solution of dextran increases ST and reduces the inversion temperature. For 2 M urea, the change in the sign of ST is observed at T = 29.7 oC. At higher temperature ST is always positive in the studied temperature range. To rationalize these observations it is assumed that the addition of urea opens hydrogen bonds, similar to that induced by an increase in temperature. For a future extension of the thermodynamic studies to the effects of poly-dispersity, dextran was fractionated by means of a recently developed technique called Continuous Spin Fractionation (CSF). The solvent/precipitant/polymer system used for the thermodynamic studies served as the basis for the fractionation of dextran The starting polymer had a weight average molar mass Mw = 11.1 kg/mol and a molecular non-uniformity U= Mw / Mn -1= 1.0. Seventy grams of dextran were fractionated using water as the solvent and methanol as the precipitant. Five fractionation steps yielded four samples with Mw values between 4.36 and 18.2 kg/mol and U values ranging from 0.28 to 0.48.

Diffusion in polymer systems studied by fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) is a powerful technique to determine the diffusion of fluorescence molecules in various environments. The technique is based on detecting and analyzing the fluctuation of fluorescence light emitted by fluorescence species diffusing through a small and fixed observation volume, formed by a laser focused into the sample. Because of its great potential and high versatility in addressing the diffusion and transport properties in complex systems, FCS has been successfully applied to a great variety of systems. In my thesis, I focused on the application of FCS to study the diffusion of fluorescence molecules in organic environments, especially in polymer melts. In order to examine our FCS setup and a developed measurement protocol, I first utilized FCS to measure tracer diffusion in polystyrene (PS) solutions, for which abundance data exist in the literature. I studied molecular and polymeric tracer diffusion in polystyrene solutions over a broad range of concentrations and different tracer and matrix molecular weights (Mw). Then FCS was further established to study tracer dynamics in polymer melts. In this part I investigated the diffusion of molecular tracers in linear flexible polymer melts [polydimethylsiloxane (PDMS), polyisoprene (PI)], a miscible polymer blend [PI and poly vinyl ethylene (PVE)], and star-shaped polymer [3-arm star polyisoprene (SPI)]. The effects of tracer sizes, polymer Mw, polymer types, and temperature on the diffusion coefficients of small tracers were discussed. The distinct topology of the host polymer, i.e. star polymer melt, revealed the notably different motion of the small tracer, as compared to its linear counterpart. Finally, I emphasized the advantage of the small observation volume which allowed FCS to investigate the tracer diffusions in heterogeneous systems; a swollen cross-linked PS bead and silica inverse opals, where high spatial resolution technique was required.

Characterization of structure and dynamics of submicrometric systems via fluorescence correlation spectroscopy

The presented thesis revolves around the study of thermally-responsive PNIPAAm-based hydrogels in water/based environments, as studied by Fluorescence Correlation Spectroscopy (FCS).rnThe goal of the project was the engineering of PNIPAAm gels into biosensors. Specifically, a gamma of such gels were both investigated concerning their dynamics and structure at the nanometer scale, and their performance in retaining bound bodies upon thermal collapse (which PNIPAAm undergoes upon heating above 32 ºC).rnFCS’s requirements, as a technique, match the limitations imposed by the system. Namely, the need to intimately probe a system in a solvent, which was also fragile and easy to alter. FCS, on the other hand, both requires a fluid environment to work, and is based on the observation of diffusion of fluorescents at nanomolar concentrations. FCS was applied to probe the hydrogels on the nanometer size with minimal invasivity.rnVariables in the gels were addressed in the project including crosslinking degree; structural changes during thermal collapse; behavior in different buffers; the possibility of decreasing the degree of inhomogeneity; behavior of differently sized probes; and the effectiveness of antibody functionalization upon thermal collapse.rnThe evidenced results included the heightening of structural inhomogeneities during thermal collapse and under different buffer conditions; the use of annealing to decrease the inhomogeneity degree; the use of differently sized probes to address different length scale of the gel; and the successful functionalization before and after collapse.rnThe thesis also addresses two side projects, also carried forward via FCS. One, diffusion in inverse opals, produced a predictive simulation model for diffusion of bodies in confined systems as dependent on the bodies’ size versus the characteristic sizes of the system. The other was the observation of interaction of bodies of opposite charge in a water solution, resulting in a phenomenological theory and an evaluation method for both the average residence time of the different bodies together, and their attachment likelihood.

Tracer diffusion at water-oil interfaces studied by fluorescence correlation spectroscopy

The adsorption of particles and surfactants at water-oil interfaces has attracted continuous attention because of its emulsion stabilizing effect and the possibility to form two-dimensional materials. Herein, I studied the interfacial diffusion of single molecules and nanoparticles at water-oil interfaces using fluorescence correlation spectroscopy. rnrnFluorescence correlation spectroscopy (FCS) is a promising technique to study diffusion of fluorescent tracers in diverse conditions. This technique monitors and analyzes the fluorescence fluctuation caused by single fluorescent tracers coming in and out of a diffraction-limited observation volume “one at a time”. Thus, this technique allows a combination of high precision, high spatial resolution and low tracer concentration. rnrnIn chapter 1, I discussed some controversial questions regarding the properties of water-hydrophobic interfaces and also introduced the current progress on the stability and dynamic of single nanoparticles at water-oil interfaces. The materials and setups I used in this thesis were summarized in chapter 2. rnrnIn chapter 3, I presented a new strategy to study the properties of water-oil interfaces. The two-dimensional diffusion of isolated molecular tracers at water/n-alkane interfaces was measured using fluorescence correlation spectroscopy. The diffusion coefficients of larger tracers with a hydrodynamic radius of 4.0 nm agreed well with the values calculated from the macroscopic viscosities of the two bulk phases. However, for small molecule tracers with hydrodynamic radii of only 1.0 and 0.6 nm, notable deviations were observed, indicating the existence of an interfacial region with a reduced effective viscosity. rnrnIn chapter 4, the interfacial diffusion of nanoparticles at water-oil interfaces was investigated using FCS. In stark contrast to the interfacial diffusion of molecular tracers, that of nanoparticles at any conditions is slower than the values calculated in accordance to the surrounding viscosity. The diffusion of nanoparticles at water-oil interfaces depended on the interfacial tension of liquid-liquid interfaces, the surface properties of nanoparticles, the particle sizes and the viscosities of surrounding liquid phases. In addition, the interfacial diffusion of nanoparticles with Janus motif is even slower than that of their symmetric counterparts. Based on the experimental results I obtained, I drew some possibilities to describe the origin of nanoparticle slowdown at water-oil interfaces.

Diffusion in heterogeneous systems studied by laser scanning confocal microscopy and fluorescence correlation spectroscopy

Understanding and controlling the mechanism of the diffusion of small molecules, macromolecules and nanoparticles in heterogeneous environments is of paramount fundamental and technological importance. The aim of the thesis is to show, how by studying the tracer diffusion in complex systems, one can obtain information about the tracer itself, and the system where the tracer is diffusing. rnIn the first part of my thesis I will introduce the Fluorescence Correlation Spectroscopy (FCS) which is a powerful tool to investigate the diffusion of fluorescent species in various environments. By using the main advantage of FCS namely the very small probing volume (<1µm3) I was able to track the kinetics of phase separation in polymer blends at late stages by looking on the molecular tracer diffusion in individual domains of the heterogeneous structure of the blend. The phase separation process at intermediate stages was monitored with laser scanning confocal microscopy (LSCM) in real time providing images of droplet coalescence and growth. rnIn a further project described in my thesis I will show that even when the length scale of the heterogeneities becomes smaller than the FCS probing volume one can still obtain important microscopic information by studying small tracer diffusion. To do so, I will introduce a system of star shaped polymer solutions and will demonstrate that the mobility of small molecular tracers on microscopic level is nearly not affected by the transition of the polymer system to a “glassy” macroscopic state. rnIn the last part of the thesis I will introduce and describe a new stimuli responsive system which I have developed, that combines two levels of nanoporosity. The system is based on poly-N-isopropylacrylamide (PNIPAM) and silica inverse opals (iOpals), and allows controlling the diffusion of tracer molecules. rn

Formation and surface exchange of nitrous acid

Die salpetrige Säure (HONO) ist eine der reaktiven Stickstoffkomponenten der Atmosphäre und Pedosphäre. Die genauen Bildungswege von HONO, sowie der gegenseitige Austausch von HONO zwischen Atmosphäre und Pedosphäre sind noch nicht vollständig aufgedeckt. Bei der HONO-Photolyse entsteht das Hydroxylradikal (OH) und Stickstoffmonooxid (NO), was die Bedeutsamkeit von HONO für die atmosphärische Photochemie widerspiegelt.rnUm die genannte Bildung von HONO im Boden und dessen anschließenden Austausch mit der Atmosphäre zu untersuchen, wurden Messungen von Bodenproben mit dynamischen Kammern durchgeführt. Im Labor gemessene Emissionsflüsse von Wasser, NO und HONO zeigen, dass die Emission von HONO in vergleichbarem Umfang und im gleichen Bodenfeuchtebereich wie die für NO (von 6.5 bis 56.0 % WHC) stattfindet. Die Höhe der HONO-Emissionsflüsse bei neutralen bis basischen pH-Werten und die Aktivierungsenergie der HONO-Emissionsflüsse führen zu der Annahme, dass die mikrobielle Nitrifikation die Hauptquelle für die HONO-Emission darstellt. Inhibierungsexperimente mit einer Bodenprobe und die Messung einer Reinkultur von Nitrosomonas europaea bestärkten diese Theorie. Als Schlussfolgerung wurde das konzeptionelle Model der Bodenemission verschiedener Stickstoffkomponenten in Abhängigkeit von dem Wasserhaushalt des Bodens für HONO erweitert.rnIn einem weiteren Versuch wurde zum Spülen der dynamischen Kammer Luft mit erhöhtem Mischungsverhältnis von HONO verwendet. Die Messung einer hervorragend charakterisierten Bodenprobe zeigte bidirektionale Flüsse von HONO. Somit können Böden nicht nur als HONO-Quelle, sondern auch je nach Bedingungen als effektive Senke dienen. rnAußerdem konnte gezeigt werden, dass das Verhältnis von HONO- zu NO-Emissionen mit dem pH-Wert des Bodens korreliert. Grund könnte die erhöhte Reaktivität von HONO bei niedrigem pH-Wert und die längere Aufenthaltsdauer von HONO verursacht durch reduzierte Gasdiffusion im Bodenporenraum sein, da ein niedriger pH-Wert mit erhöhter Bodenfeuchte am Maximum der Emission einhergeht. Es konnte gezeigt werden, dass die effektive Diffusion von Gasen im Bodenporenraum und die effektive Diffusion von Ionen in der Bodenlösung die HONO-Produktion und den Austausch von HONO mit der Atmosphäre begrenzen. rnErgänzend zu den Messungen im Labor wurde HONO während der Messkampagne HUMPPA-COPEC 2010 im borealen Nadelwald simultan in der Höhe von 1 m über dem Boden und 2 bis 3 m über dem Blätterdach gemessen. Die Budgetberechnungen für HONO zeigen, dass für HONO sämtliche bekannte Quellen und Senken in Bezug auf die übermächtige HONO-Photolyserate tagsüber vernachlässigbar sind (< 20%). Weder Bodenemissionen von HONO, noch die Photolyse von an Oberflächen adsorbierter Salpetersäure können die fehlende Quelle erklären. Die lichtinduzierte Reduktion von Stickstoffdioxid (NO2) an Oberflächen konnte nicht ausgeschlossen werden. Es zeigte sich jedoch, dass die fehlende Quelle stärker mit der HONO-Photolyserate korreliert als mit der entsprechenden Photolysefrequenz, die proportional zur Photolysefrequenz von NO2 ist. Somit lässt sich schlussfolgern, dass entweder die Photolyserate von HONO überschätzt wird oder dass immer noch eine unbekannte, HONO-Quelle existiert, die mit der Photolyserate sehr stark korreliert. rn rn