1,721,411 research outputs found
Dual-focus fluorescence correlation spectroscopy of colloidal solutions: Influence of particle size
No full textFluorescence correlation spectroscopy (FCS) is a powerful technique for measuring diffusion coefficients of small fluorescent molecules at pico- to nanomolar concentrations. Recently, a modified version of FCS, dual-focus FCS (2fFCS), was introduced that significantly improves the reliability and accuracy of FCS measurements and allows for obtaining absolute values of diffusion coefficients without the need of referencing again a known standard. It was shown that 2fFCS gives excellent results for measuring the diffusion of small molecules. However, when measuring colloids or macromolecules, the size of these objects can no longer be neglected with respect to the excitation laser focus. Here, we analyze how 2fFCS data evaluation has to be modified for correctly taking into a count these finite size effects. We exemplify the new method of measuring the absolute size of polymeric particles with simple and complex fluorophore distributions
Remote temperature measurements in femto-liter volumes using dual-focus-Fluorescence Correlation Spectroscopy
No full textRemote temperature measurements in microfluidic devices with micrometer spatial resolution are important for many applications in biology, biochemistry and chemistry. The most popular methods use the temperature-dependent fluorescence lifetime of Rhodamine B, or the temperature-dependent size of thermosensitive materials such as microgel particles. Here, we use the recently developed method of dual-focus fluorescence correlation spectroscopy (2fFCS) for measuring the absolute diffusion coefficient of small fluorescent molecules at nanomolar concentrations and show how these data can be used for remote temperature measurements on a micrometer scale. We perform comparative temperature measurements using all three methods and show that the accuracy of 2fFCS is comparable or even better than that achievable with Rhodamine B fluorescence lifetime measurements. The temperature dependent microgel swelling leads to an enhanced accuracy within a narrow temperature range around the volume phase transition temperature, but requires the availability of specific microgels, whereas 2fFCS is applicable under very general conditions
Dual-focus fluorescence correlation spectroscopy: a robust tool for studying molecular crowding
No full textConventional single-focus fluorescence correlation spectroscopy (FCS) is often used for studying molecular diffusion in crowded environments. However, these measurements usually deal with concentrations of the crowding agent far beyond the overlap-concentration, resulting in a crowding effect which slows down the diffusion coefficient by several orders of magnitude. In the present paper, we would like to study the transition range from free diffusion to crowding. Therefore, high accuracy of the determination of the diffusion coefficient is needed. In the majority of cases, the local refractive index in a sample is different from the refractive index of the immersion medium of the used objective. To achieve a high accuracy during experiments it is necessary to account for the refractive index mismatch in single-focus FCS calculations. In this work, we study theoretically and experimentally the influence of the refractive index mismatch on performance of single-focus FCS as well as the recently developed dual-focus FCS (2fFCS). By looking at the transition from free tracer diffusion to crowding it is shown that, in contrast to conventional FCS, 2fFCS allows measuring absolute values of the diffusion coefficient and its change in the range of half an order of magnitude. Even under conditions of strong refractive index mismatch between sample and immersion medium, without the need of additional calibration. This is demonstrated on a system of fluorescently labeled 70 kDa dextrane in an unlabeled 70 kDa dextrane matrix. Therefore, 2fFCS is a perfect tool for investigating molecular dynamics in crowded environments
Calibrating differential interference contrast microscopy with dual-focus fluorescence correlation spectroscopy
No full textWe present a novel calibration technique for determining the shear distance of a Nomarski Differential Interference Contrast prism, which is used in Differential Interference Contrast microscopy as well as for the recently developed dual-focus fluorescence correlation spectroscopy. In both applications, an exact knowledge of the shear distance induced by the Nomarski prism is important for a quantitative data evaluation. In Differential Interference Contrast microscopy, the shear distance determines the spatial resolution of imaging, in dual-focus fluorescence correlation spectroscopy, it represents the extrinsic length scale for determining diffusion coefficients. The presented calibration technique is itself based on a combination of fluorescence correlation spectroscopy and dynamic light scattering. The method is easy to implement and allows for determining the shear distance with nanometer accuracy
Rheologie komplexer Flüssigkeiten
No full textInvestigated complex fluids comprised polymer-surfactant-mixtures (PTMs) and oil-in-water (o/w)-emulsions. The mechanical properties were probed by oscillatory and continuous flow rheometry, microstructures of emulsions were evaluated by optical microscopy while droplet size and droplet size distribution were studied by flow particle image analysis (FPIA). 0.3 - 4.0 wt% of hydrophobically modified (hm)-(acrylic acid)copolymers and hm-(poly)urethans were combined with 0.0 - 2.0 wt% low molecular surfactant (sodium dodecylsulfate (SDS) - anionic, cetyltrimethylammonium-bromide (CTAB) - cationic, ethoxylated fatty alcohol (Arylpon F, Dehydol LS 9.5) - nonionic). The resulting PTMs revealed different polymer-surfactant interactions, due to surfactant / polymer charge and structure. In case of nonionic surfactants and SDS, polymer-surfactant interactions resulted in a more or less pronounced "thin-to-thick"-effect of PTMs depending on surfactant concentration and structure, whereas in CTAB-PTMs thickening as well as phase separation were observed depending on CTAB concentration. For PTMs from 1.0 wt% crosslinked polymer and surfactants an uncommon second shear thinning region, which is not mentioned in the literature so far, region was observed in the flow curves, which could be related to shear induced structural changes. The investigations on PTMs gave basic understanding of a possible applicability of polymeric thickeners in emulsions. Because low molecular surfactants showed some disadvantages in emulsions, only polymeric stabilized emulsions were investigated which contain silicon-based polymeric emulsifiers (dimethicones). Rheological properties and microstructures of polymer stabilized emulsions were compared to properties of classic emulsions with low molecular emulsifiers as stabilizing components. The investigations concerning rheological properties and droplet size of emulsions, presented here, dealt with o/w-emulsions containing polymeric thickener Pemulen TR-1 (TR-1) and silicone based emulsifier (DC 193). By varying thickener and emulsifier content rheological properties and droplet size of emulsions changed significantly. Experimental results and a statistical analysis showed that the physical network, build up by TR-1 in a concentration range from 0.1 - 1.0 wt%, was the dominating factor for rheological properties and increased moduli and viscosity of emulsions. The development of droplet diameters revealed that a systematic control of droplet parameters was possible by increasing the DC 193 concentration from 0.0 - 5.0 wt%. In contrast, increasing TR-1 concentration led to either large or small droplets. The influence of larger droplets in the emulsions was revealed when the arithmetic diameter and the Sauter diameter were compared and displayed huge differences. These differences resulted from a rather small amount of big droplets with diameters above 40 micrometer, which could be seen in the droplet size distributions of emulsions. An influence of oil droplets on emulsion elasticity was only observed for emulsions with low TR-1 concentration (~ 0.1 wt%), because at higher concentrations the influence of oil droplets was superimposed by thickening properties of TR-1
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Polymer induced colloidal interactions : measured by direct and indirect methods
No full textIn this thesis we study interaction potentials between colloidal particles and a wall and between colloids in bulk using direct methods, such as Total Internal Reflection Microscopy (TIRM), and indirect techniques, such as Static Light Scattering (SLS). The work was motivated by the observation that the physical properties of colloidal suspensions, e.g. solution structure and phase behaviour, differ from the bulk behaviour in the ultimate vicinity of an interface. The structural properties of bulk suspensions can often be quantitatively described, knowing the pair interaction potential between the colloids. We show this for example in chapter 5 of this thesis for the case of associating colloids. It is reasonable to conjecture that the interaction of the colloidal particles with an interface is one of the reasons for the deviating behaviour of suspension as compared to the bulk. Therefore it was the task of this work to study this type of interaction potential experimentally, with the long time goal to provide input information for the treatment of near wall properties with theoretical techniques and/or computer simulation. The focus of this work is on polymer-induced interactions in colloidal systems. We have directly studied bridging attraction and steric repulsion due to attached polymer layers as well as depletion attraction due to non-adsorbing polymer chains. A detailed discussion of various other types of forces and interactions such as, van der Waals attraction, electrostatic interactions, structural forces, capillary forces, etc. is beyond the scope of this work and these forces will be mentioned here only briefly. On the basis of the experimentally obtained interaction potentials information about the colloidal near wall properties, i.e. surface phase behaviour can be obtained and compared with microscopic observations. To perform the microscopic observations in a wide range of colloidal concentrations and to avoid multiple scattering we needed particles which are easy to index-match with the solvent that was water in all our studies. Moreover, to enable a precise image analysis these colloids needed to have core-shell morphology with a fluorescent core and a non-fluorescent shell. Therefore, we introduced a new type of colloidal particles: fluorinated fluorescent latex with core-shell morphology which has a refractive index close to that of water. In the future these particles will be used to study the colloidal phase behaviour at the surface in solutions of biological depletants such as fd-viruses. This thesis begins with an introductory chapter (chapter 1) about polymer-induced forces which is intended to give the reader an overview of possible interactions in colloid-polymer mixtures. In this chapter we review some theoretical approaches and collect experimental data of polymer-induced forces, which were obtained using various techniques. Our main experimental technique, TIRM, is described in detail in chapter 2, where we also compare it with other techniques which enable direct measurements of interactions in colloidal systems. In chapters 3 and 4 we present directly measured interaction potentials between a colloidal sphere and a solid wall immersed in polymer solutions. Two different types of interactions were found depending on the nature of the polymer. Thus, in chapter 3 we show that dextran (a biopolymer) does not adsorb onto the glass and particles’ surfaces and this leads to an attractive depletion interaction. The polymer size polydispersity is shown to significantly influence the depletion potential. On the other hand, polyethylene oxide was found (chapter 4) to adsorb onto the surfaces of the colloidal sphere and the glass wall, leading to a steric repulsion between adsorbed polymer layers. In chapter 5 we present an indirect method to study interactions in colloidal systems. Thus, aqueous solutions of m-oxyethylene-n-ether (CnEm) non-ionic surfactants have been studied by static light scattering. We propose semi-phenomenological expressions for the pair interaction potential in aqueous CmEn-solutions, which enable the quantitative description of the scattering behaviour and the phase diagrams for five different surfactant systems. In chapter 6 we present a new model system to study the colloidal phase behaviour at the surface: fluorinated fluorescent latex spheres, which have a low refractive index and are highly charged and are therefore, almost transparent and very stable in water. These qualities make the particles very useful in studies with biological materials. Moreover, the morphology of these colloids, consisting of a fluorescent core and a non-fluorescent shell, makes them especially suited for studies using confocal microscopy. In future these particles will be used to directly determine many-body interaction potentials using confocal scanning microscopy
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