Joachim Kohlbrecher

SASfit: a tool for small-angle scattering data analysis using a library of analytical expressions

A computer program to perform small-angle X-ray and neutron scattering data evaluation is presented.

Electric field control of the skyrmion lattice in Cu2OSeO3

Small-angle neutron scattering has been employed to study the influence of applied electric (E-)fields on the skyrmion lattice in the chiral lattice magnetoelectric Cu(2)OSeO(3). Using an experimental geometry with the E-field parallel to the [111] axis, and the magnetic field parallel to the [11(-)0] axis, we demonstrate that the effect of applying an E-field is to controllably rotate the skyrmion lattice around the magnetic field axis. Our results are an important first demonstration for a microscopic coupling between applied E-fields and the skyrmions in an insulator, and show that the general emergent properties of skyrmions may be tailored according to the properties of the host system.

Nanostructure surveys of macroscopic specimens by small-angle scattering tensor tomography

The mechanical properties of many materials are based on the macroscopic arrangement and orientation of their nanostructure. This nanostructure can be ordered over a range of length scales. In biology, the principle of hierarchical ordering is often used to maximize functionality, such as strength and robustness of the material, while minimizing weight and energy cost. Methods for nanoscale imaging provide direct visual access to the ultrastructure (nanoscale structure that is too small to be imaged using light microscopy), but the field of view is limited and does not easily allow a full correlative study of changes in the ultrastructure over a macroscopic sample. Other methods of probing ultrastructure ordering, such as small-angle scattering of X-rays or neutrons, can be applied to macroscopic samples; however, these scattering methods remain constrained to two-dimensional specimens or to isotropically oriented ultrastructures. These constraints limit the use of these methods for studying nanostructures with more complex orientation patterns, which are abundant in nature and materials science. Here, we introduce an imaging method that combines small-angle scattering with tensor tomography to probe nanoscale structures in three-dimensional macroscopic samples in a non-destructive way. We demonstrate the method by measuring the main orientation and the degree of orientation of nanoscale mineralized collagen fibrils in a human trabecula bone sample with a spatial resolution of 25 micrometres. Symmetries within the sample, such as the cylindrical symmetry commonly observed for mineralized collagen fibrils in bone, allow for tractable sampling requirements and numerical efficiency. Small-angle scattering tensor tomography is applicable to both biological and materials science specimens, and may be useful for understanding and characterizing smart or bio-inspired materials. Moreover, because the method is non-destructive, it is appropriate for in situ measurements and allows, for example, the role of ultrastructure in the mechanical response of a biological tissue or manufactured material to be studied.

Size-Dependent Interaction of Silica Nanoparticles with Different Surfactants in Aqueous Solution

The size-dependent interaction of anionic silica nanoparticles with ionic (anionic and cationic) and nonionic surfactants has been studied using small-angle neutron scattering (SANS). The surfactants used are anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethyl ammonium bromide (DTAB), and nonionic decaoxyethylene n-dodecylether (C(12)E(10)). The measurements have been carried out for three different sizes of silica nanoparticles (8, 16, and 26 nm) at fixed concentrations (1 wt % each) of nanoparticles and surfactants. It is found that irrespective of the size of the nanoparticles there is no significant interaction evolved between like-charged nanoparticles and the SDS micelles leading to any structural changes. However, the strong attraction of oppositely charged DTAB micelles with silica nanoparticles results in the aggregation of nanoparticles. The number of micelles mediating the nanoparticle aggregation increases with the size of the nanoparticle. The aggregates are characterized by fractal structure where the fractal dimension is found to be constant (D ≈ 2.3) independent of the size of the nanoparticles and consistent with diffusion-limited-aggregation-type fractal morphology in these systems. In the case of nonionic surfactant C(12)E(10), micelles interact with the individual silica nanoparticles. The number of adsorbed micelles per nanoparticle increases drastically whereas the percentage of adsorbed micelles on nanoparticles decreases with the increase in the size of the nanoparticles.

Synthesis and characterization of high concentration block copolymer-mediated gold nanoparticles.

The formation of high concentration gold nanoparticles at room temperature is reported in block copolymer-mediated synthesis where the nanoparticles have been synthesized from hydrogen tetrachloroaureate(III) hydrate (HAuCl(4)·3H(2)O) using block copolymer P85 (EO(26)PO(39)EO(26)) in aqueous solution. The formation of gold nanoparticles in these systems has been characterized using UV-visible spectroscopy and small-angle neutron scattering (SANS). We show that the presence of additional reductant (trisodium citrate) can enhance nanoparticle concentration by manyfold, which does not work in the absence of either of these (additional reductant and block copolymer). The stability of gold nanoparticles with increasing concentration has also been examined.

SANS study of salt induced micellization in PEO–PPO–PEO block copolymers

Abstract Small-angle neutron scattering (SANS) measurements have been carried out from the aqueous solutions of polyethylene oxide (PEO)–polypropylene oxide (PPO)–PEO block copolymers of pluronics F88 and P84 in presence of salt KCl. It is seen that the effect of addition of salt is similar to that of increasing temperature. The pluronics exist as unimers at low temperatures and their micellization takes place when the salt is added or the temperature is increased. The measurements for different salts of the lyotropic series on the above pluronic systems show that micellar structure is sensitive to the sizes of the hydrated ions of these salts.

Size and shape of micelles studied by means of SANS, PCS, and FCS.

The hexaethylene glycol monododecyl ether (C(12)E(6)) micelles at concentrations up to 10% have been studied in their isotropic phase (10-48 degrees C) by means of small angle neutron scattering (SANS) and photon correlation spectroscopy (PCS). The SANS data obtained at low temperatures could be unequivocally interpreted as a result of scattering from a suspension of compact globular micelles with the shape of a triaxial ellipsoid or a short end-capped elliptical rod. Different models have been applied to analyze the SANS data obtained at higher temperatures: (i) elongated rod-like micelles with purely sterical interactions, (ii) compact globular micelles with a weak attractive potential, and (iii) globular micelles influenced by the critical phenomena in the whole temperature range studied. The good quality of the experimental data indicated model (i) as the best fit for our data. The diffusion coefficients obtained from the PCS measurements have been compared to the diffusion coefficients calculated for the rod-like micelles--results of the SANS data analysis. A good agreement was achieved using the solvent viscosity, in agreement with the theoretical predictions for sterically interacting globular colloidal particles. Finally, the SANS results obtained at 24 degrees C were compared to the micelle self-diffusion coefficients previously measured by means of fluorescence correlation spectroscopy (FCS) at this temperature. The good agreement obtained after scaling the data with solution viscosity supports the validity of the generalized Stokes-Einstein relation in sterically interacting systems: the product of the colloidal particle self-diffusion coefficient and the macroscopic viscosity remains constant in a broad range of concentrations. It has been concluded that the FCS technique in combination with simple viscosity measurements might serve as a tool for estimating the micellar size and shape.

Mechanistic aspects of the horseradish peroxidase-catalysed polymerisation of aniline in the presence of AOT vesicles as templates

The mechanism of the horseradish peroxidase (HRP)–H2O2-catalysed polymerisation of aniline in the presence of AOT vesicles was investigated. AOT (= bis-(2-ethylhexyl)sulfosuccinate) served as vesicle-forming surfactant and dopant for obtaining at pH = 4.3 and room temperature within 24 h under optimal reaction conditions the green emeraldine salt form of polyaniline in 90–95% yield. Based on UV/VIS/NIR and EPR measurements carried out during the polymerisation reaction, and based on changes in aniline and H2O2 concentrations and HRP activity, a mechanism is proposed. According to this “radical cation mechanism” chain growth occurs on the vesicle surface through addition of aniline radical cations to the growing polymer chain. H2O2 plays two essential roles, to oxidise the heme group of HRP, and to oxidise the growing polymer chain for allowing the stepwise addition of new aniline radical cations. The entire reaction can be divided into three kinetically distinct phases. In the first rapid phase (5–10 min), the actual polymer formation takes place to yield the emeraldine salt form of polyaniline in its bipolaron state. In the second and third slower phases (1–2 days) the bipolarons transform into polarons with unpaired electrons. During the reaction, the HRP activity is decreasing until the enzyme becomes inactive after polymer formation. Reactions carried out with partially deuterated anilines were analysed by 2H magic-angle spinning (MAS) NMR spectroscopy to demonstrate the regioselectivity of the chain growth: para-coupling of the aniline units clearly dominates. Association of the formed polyaniline with the vesicle membrane is evident from cryo-TEM and SANS measurements.

Dynamic response of block copolymer wormlike micelles to shear flow

The linear and nonlinear dynamic response to an oscillatory shear flow of giant wormlike micelles consisting of Pb–Peo block copolymers is studied by means of Fourier transform rheology. Experiments are performed in the vicinity of the isotropic–nematic phase transition concentration, where the location of isotropic–nematic phase transition lines is determined independently. Strong shear-thinning behaviour is observed due to critical slowing down of orientational diffusion as a result of the vicinity of the isotropic–nematic spinodal. This severe shear-thinning behaviour is shown to result in gradient shear banding. Time-resolved small-angle neutron scattering experiments are used to obtain an insight into the microscopic phenomena that underlie the observed rheological response. An equation of motion for the order parameter tensor and an expression of the stress tensor in terms of the order parameter tensor are used to interpret the experimental data, both in the linear and nonlinear regimes. Scaling of the dynamic behaviour of the orientational order parameter and the stress is found when critical slowing down due to the vicinity of the isotropic–nematic spinodal is accounted for.

Novel Type of Bicellar Disks from a Mixture of DMPC and DMPE-DTPA with Complexed Lanthanides

We report on the formation of bicelles from a mixture of dimyristoylphosphatidylcholine (DMPC) and the chelator-lipid dimyristoylphosphatidylethanolamine-diethylenetriaminepentaacetate (DMPE-DTPA) with complexed lanthanides, either thulium (Tm(3+)) or lanthanum (La(3+)). The two phospholipids used have the same acyl-chain length but differ in headgroup size and chemical structure. The total lipid concentration was 15 mM, and the molar ratio of DMPC to DMPE-DTPA was 4:1. The system was studied with small angle neutron scattering (SANS) in a magnetic field, cryo-transmission electron microscopy (cryo-TEM), and (31)P NMR spectroscopy. We found that, after appropriate preparation steps, that is, extrusion through a polycarbonate membrane followed by a cooling step, monodisperse small unilamellar disks (flat cylinders called bicelles) are formed. They have a radius of 20 nm and a bilayer thickness of about 4 nm and are stable in the investigated temperature range of 2.5-30 degrees C. Fitting of SANS data with a form factor for partly aligned flat cylinders shows that the bicelles are slightly orientable in a magnetic field of 8 T if DMPE-DTPA is complexed with Tm(3+).