Correcting aberrations of a transverse-field neutron resonance spin echo instrument.

Neutron resonance spin echo (NRSE) technique has the potential to increase the Fourier time and energy resolution in neutron scattering by using radio frequency (rf) neutron spin-flippers. However, aberrations arising from variations in the neutron path length between the rf flippers reduce the polarization. Here, we develop and test a transverse static-field magnet, a series of which are placed between the rf flippers, to correct for these aberrations. The prototype correction magnet was both simulated in an NRSE beamline using McStas, a Monte Carlo neutron ray-tracing software package, and measured using neutrons. The results from the prototype demonstrate that this static-field design corrects for transverse-field NRSE aberrations.

Mesoporous Silica Formation Mechanisms Probed Using Combined Spin-Echo Modulated Small-Angle Neutron Scattering (SEMSANS) and Small-Angle Neutron Scattering (SANS).

The initial formation stages of surfactant-templated silica thin films which grow at the air-water interface were studied using combined spin-echo modulated small-angle neutron scattering (SEMSANS) and small-angle neutron scattering (SANS). The films are formed from either a cationic surfactant or nonionic surfactant (C16EO8) in a dilute acidic solution by the addition of tetramethoxysilane. Previous work has suggested a two stage formation mechanism with mesostructured particle formation in the bulk solution driving film formation at the solution surface. From the SEMSANS data, it is possible to pinpoint accurately the time associated with the formation of large particles in solution that go on to form the film and to show their emergence is concomitant with the appearance of Bragg peaks in the SANS pattern, associated with the two-dimensional hexagonal order. The combination of SANS and SEMSANS allows a complete depiction of the steps of the synthesis that occur in the subphase.

Data Correction of Intensity Modulated Small Angle Scattering.

To investigate long length scale structures using neutron scattering, real space techniques have shown certain advantages over the conventional methods working in reciprocal space. As one of the real space measurement techniques, spin echo modulated small angle neutron scattering (SEMSANS) has attracted attention, due to its relaxed constraints on sample environment and the possibility to combine SEMSANS and a conventional small angle neutron scattering instrument. In this report, we present the first implementation of SEMSANS at a pulsed neutron source and discuss important corrections to the data due to the sample absorption. These corrections allow measurements made with different neutron wavelengths and SEMSANS configurations to be overlaid and give confidence that the measurements provide an accurate representation of the density correlations in the sample.

Hydration forces between aligned DNA helices undergoing B to A conformational change: In-situ X-ray fiber diffraction studies in a humidity and temperature controlled environment.

Hydration forces between DNA molecules in the A- and B-Form were studied using a newly developed technique enabling simultaneous in situ control of temperature and relative humidity. X-ray diffraction data were collected from oriented calf-thymus DNA fibers in the relative humidity range of 98%-70%, during which DNA undergoes the B- to A-form transition. Coexistence of both forms was observed over a finite humidity range at the transition. The change in DNA separation in response to variation in humidity, i.e. change of chemical potential, led to the derivation of a force-distance curve with a characteristic exponential decay constant of∼2Å for both A- and B-DNA. While previous osmotic stress measurements had yielded similar force-decay constants, they were limited to B-DNA with a surface separation (wall-to-wall distance) typically>5Å. The current investigation confirms that the hydration force remains dominant even in the dry A-DNA state and at surface separation down to∼1.5Å, within the first hydration shell. It is shown that the observed chemical potential difference between the A and B states could be attributed to the water layer inside the major and minor grooves of the A-DNA double helices, which can partially interpenetrate each other in the tightly packed A phase. The humidity-controlled X-ray diffraction method described here can be employed to perform direct force measurements on a broad range of biological structures such as membranes and filamentous protein networks.

High resolution neutron Larmor diffraction using superconducting magnetic Wollaston prisms.

The neutron Larmor diffraction technique has been implemented using superconducting magnetic Wollaston prisms in both single-arm and double-arm configurations. Successful measurements of the coefficient of thermal expansion of a single-crystal copper sample demonstrates that the method works as expected. The experiment involves a new method of tuning by varying the magnetic field configurations in the device and the tuning results agree well with previous measurements. The difference between single-arm and double-arm configurations has been investigated experimentally. We conclude that this measurement benchmarks the applications of magnetic Wollaston prisms in Larmor diffraction and shows in principle that the setup can be used for inelastic phonon line-width measurements. The achievable resolution for Larmor diffraction is comparable to that using Neutron Resonance Spin Echo (NRSE) coils. The use of superconducting materials in the prisms allows high neutron polarization and transmission efficiency to be achieved.

Thermoresponsive PNIPAM Coatings on Nanostructured Gratings for Cell Alignment and Release.

Thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) has been widely used as a surface coating to thermally control the detachment of adsorbed cells without the need for extreme stimuli such as enzyme treatment. Recently, the use of 2D and 3D scaffolds in controlling cell positioning, growth, spreading, and migration has been of a great interest in tissue engineering and cell biology. Here, we use a PNIPAM polymer surface coating atop a nanostructured linear diffraction grating to controllably change the surface topography of 2D linear structures using temperature stimuli. Neutron reflectometry and surface diffraction are utilized to examine the conformity of the polymer coating to the grating surface, its hydration profile, and its evolution in response to temperature variations. The results show that, in the collapsed state, the PNIPAM coating conforms to the grating structures and retains a uniform hydration of 63%. In the swollen state, the polymer expands beyond the grating channels and absorbs up to 87% water. Such properties are particularly desirable for 2D cell growth scaffolds with a built-in nonextreme tissue-release mechanism. Indeed, the current system demonstrates advanced performance in the effective alignment of cultured fibroblast cells and the easy release of the cells upon temperature change.

Neutron spin evolution through broadband current sheet spin flippers.

Controlled manipulation of neutron spin is a critical tool for many neutron scattering techniques. We have constructed current-sheet, neutron spin flippers for use in Spin Echo Scattering Angle Measurement (SESAME) that comprise pairs of open-faced solenoids which introduce an abrupt field reversal at a shared boundary. The magnetic fields generated by the coils have been mapped and compared with both an analytical approximation and a numerical boundary integral calculation. The agreement is generally good, allowing the former method to be used for rapid calculations of the Larmor phase acquired by a neutron passing through the flipper. The evolution of the neutron spin through the current sheets inside the flipper is calculated for various geometries of the current-carrying conductors, including different wire shapes, arrangements, and common imperfections. The flipping efficiency is found to be sensitive to gaps between wires and between current sheets. SESAME requires flippers with high fields and flipping planes inclined to the neutron beam. To avoid substantial neutron depolarization, such flippers require an interdigitated arrangement of wires.

Contrast variation in spin-echo small angle neutron scattering.

The use of contrast variation in spin-echo small angle neutron scattering (SESANS) experiments is discussed for the case of colloidal structural investigation. On the basis of calculations for several model systems, we find that the contrast variation SESANS technique, in terms of the measured SESANS correlation function G(z), is not sensitive to the structural characteristics of colloidal suspensions consisting of particles with uniform scattering length density profiles. However, its ability to resolve structural heterogeneity, at both intra-colloidal and inter-colloidal length scales, is clearly demonstrated. The prospect of using this new technique to investigate structural information that is difficult to probe in other ways is also explored.

Prospect for characterizing interacting soft colloidal structures using spin-echo small angle neutron scattering.

Spin-echo small angle neutron scattering (SESANS) provides a new experimental tool for structural investigation. Due to the action of spin-echo encoding, SESANS measures a spatial correlation function in real space, as opposed to the structure factor S(Q), I(Q), in momentum (Q) space measured by conventional small angle neutron scattering. To establish the usefulness of SESANS in structural characterization, particularly for interacting colloidal suspensions, we have previously conducted a theoretical study of the SESANS correlation functions for model systems consisting of particles with uniform density profiles [X. Li, C.-Y. Shew, Y. Liu, R. Pynn, E. Liu, K. W. Herwig, G. S. Smith, J. L. Robertson, and W.-R. Chen J. Chem. Phys. 132, 174509 (2010)]. Within the same framework, we explore in the present paper the prospect of using SESANS to investigate the structural characteristics of colloidal systems consisting of particles with nonuniform intraparticle mass distribution. As an example, a Gaussian model of interacting soft colloids is used to investigate the manifestation of structural softness in a SESANS measurement. The exploration shows a characteristically different SESANS correlation function for interacting soft colloids, in comparison to that of a uniform hard sphere system. The difference arises from the Abel transform imbedded in the mathematical formalism bridging the SESANS spectra and the spatial autocorrelation function.

Theoretical studies on the structure of interacting colloidal suspensions by spin-echo small angle neutron scattering.

The application of the spin-echo small angle neutron scattering (SESANS) technique for structural characterization of interacting colloidal suspensions is considered in this work. The framework to calculate the theoretical SESANS correlation function is briefly laid out. A general discussion regarding the features of the SESANS correlation functions obtained from different model systems is presented. In comparison with conventional elastic scattering tools operating at the same length scale, our mean-field calculations, based on a monodisperse spherical colloidal system, show that the real-space measurement provided by SESANS presents a powerful probe for studying the intercolloid potential. The reason of this sensitivity is discussed from the standpoint of way, in which how the spatial correlations are manifested in different neutron scattering implementations. This study leads to a better understanding regarding the distinction between SANS and SESANS.

A spin-echo resolved grazing incidence scattering setup for the neutron interrogation of buried nanostructures.

We present a dedicated experimental spin-echo resolved grazing incidence scattering (SERGIS) setup for the investigation of surfaces and thin films exhibiting large lateral length scales. This technique uses the neutron spin to encode one in-plane component of the wave-vector transfer in a grazing angle scattering experiment. Instead of the scattering angle, the depolarization of the scattered beam is measured. This allows one to achieve a very high in-plane momentum resolution without collimation of the incident neutron beam in the corresponding direction. SERGIS can therefore offer an alternative or complementary method to conventional grazing incidence neutron scattering experiments. We describe the experimental setup installed at the neutron sources ILL (Grenoble) and FRM II (Garching) and present data obtained with this setup on various samples exhibiting characteristic mesoscopic length scales in the range of several hundred nanometers. We also derive general formulas and error margins for the analysis and interpretation of SERGIS data and apply them to the cases of a one-dimensional structure and of an island morphology.

The use of symmetry to correct Larmor phase aberrations in spin echo scattering angle measurement.

Spin echo scattering angle measurement (SESAME) is a sensitive interference technique for measuring neutron diffraction. The method uses waveplates or birefringent prisms to produce a phase separation (the Larmor phase) between the "up" and "down" spin components of a neutron wavefunction that is initially prepared in a state that is a linear combination of in-phase up and down components. For neutrons, uniformly birefringent optical elements can be constructed from closed solenoids with appropriately shaped cross sections. Such elements are inconvenient in practice, however, both because of the precision they demand in the control of magnetic fields outside the elements and because of the amount of material required in the neutron beam. In this paper, we explore a different option in which triangular-cross-section solenoids used to create magnetic fields for SESAME have gaps in one face, allowing the lines of magnetic flux to "leak out" of the solenoid. Although the resulting field inhomogeneity produces aberrations in the Larmor phase, the symmetry of the solenoid gaps causes the aberrations produced by neighboring pairs of triangular solenoids to cancel to a significant extent. The overall symmetry of the SESAME apparatus leads to further cancellations of aberrations, providing an architecture that is easy to construct and robust in performance.

Metal-dependent self-assembly of a microbial surfactant.

Small-angle neutron scattering (SANS), cryogenic transmission electron microscopy (cryo-TEM), and dynamic light scattering (DLS) were used to study the metal-dependent phase behavior of microbially produced surfactants-marinobactins B, D, and E (MB, MD, and ME). Marinobactins A-E are siderophores that facilitate Fe(III) acquisition by the source bacterium through the coordination of Fe(III) by the peptidic headgroup. All of the marinobactins have the same six amino acid headgroup but differ in the length and saturation of the monoalkyl fatty acid tail. Fe(III) coordinated to ME (Fe(III)-ME) was found to form micelles with a diameter of approximately 3.5 nm that underwent a supramolecular transformation to produce a monodisperse population of vesicles with an average diameter ranging from approximately 90 to 190 nm upon addition of Cd(II), Zn(II), or La(III). SANS profiles of the transition-metal-induced phase exhibit a Bragg peak at QB approximately 0.11-0.12 A-1 and were fit to a SANS model for multilamellar vesicles that have an interbilayer repeat distance of 2pi/QB approximately 5.6-5.0 nm. Cryo-TEM images of the Zn(II)-induced phase reveals the presence of approximately 100 nm diameter approximately spherical aggregates of uniform electron density. The temperature dependence of the Zn(II)-induced transformation was also investigated as a function of the length and degree of unsaturation of the Fe(III)-marinobactin fatty acid tail. The Cd(II)-, Zn(II)-, and La(III)-induced phase changes have features that are similar to those of the previously reported Fe(III)-induced micelle-to-vesicle transition, and this observation has opened questions regarding the role that Cd(II) and Zn(II) may play in bacterial iron uptake.

Micelle-to-vesicle transition of an iron-chelating microbial surfactant, marinobactin E.

Small-angle neutron scattering (SANS) and dynamic light scattering (DLS) techniques have been applied to study the self-assembly processes of a microbially produced siderophore, marinobactin E (ME). ME is one of a series of marinobactins A-E that facilitate Fe(III) acquisition by the source bacterium through coordination of Fe(III) by the marinobactin headgroup. ME is a six-amino-acid peptide amphiphile appended by palmitic acid (C16), and differs only in the nature of the fatty acid moiety from the other marinobactins. Apo-ME (uncoordinated ME) assembles to form micelles with an average diameter of 4.0 nm. Upon coordination of one equivalent of Fe(III), the mean micellar diameter of Fe(III)-ME shrinks to approximately 2.8 nm. However, in the presence of excess Fe(III), Fe(III)-ME undergoes a micelle-to-vesicle transition (MVT). At a small excess of Fe(III) over Fe(III)-ME (i.e., <1.2 Fe(III)/ME), a fraction of the Fe(III)-ME micelles rearrange into approximately 200 nm diameter unilamellar vesicles. At even greater Fe(III)/ME ratios (e.g., 2-3) multilamellar aggregates begin to emerge, consistent with either multilamellar vesicles or lamellar stacks. The MVT exhibited by ME may represent a unique mechanism by which marine bacteria may detect and sequester iron required for growth.

Hierarchical self-assembly of actin bundle networks: gels with surface protein skin layers.

The networklike structure of actin bundles formed with the cross-linking protein alpha-actinin has been investigated via x-ray scattering and confocal fluorescence microscopy over a wide range of alpha-actinin/F-actin ratios. We describe the hierarchical structure of bundle gels formed at high ratios. Isotropic actin bundle gels form via cluster-cluster aggregation in the diffusion-limited aggregation regime at high alpha-actinin/actin ratios. This process is clearly observed by confocal fluorescence microscopy. Polylysine is investigated as an alternative bundling agent in the high-ratio regime and the effects of F-actin length are also discussed. One particularly fascinating aspect of this system is the presence of a structured skin layer at the gel/water interface. Confocal microscopy has elucidated the full three-dimensional structure of this layer and revealed several interesting morphologies. The protein skin layer is a micron-scale structure composed of a directed network of bundles and exhibits flat, crumpled, and tubelike shapes. We show that crumpling of the skin layer results from stresses due to the underlying gel. These biologically based geometric structures may detach from the gel, demonstrating potential for the generation of biological scaffolds with defined shapes for applications in cell encapsulation and tissue engineering. We demonstrate manipulation of the skin layer, producing hemispherical structures in solution.