Photoemission electron microscopy of three-dimensional magnetization configurations in core-shell nanostructures

5 páginas, 4 figuras.-- PACS number(s): 75.75.−c, 68.37.Yz, 81.07.−b.-- et al.We present a photoemission electron microscopy method that combines magnetic imaging of the surface and of the inner magnetization in three-dimensional core-shell nanostructures. The structure investigated consists of a cylindrical nickel core that is completely surrounded by a shell of iron oxide and silicon oxide layers. The method enables one to image the magnetization configuration of the nickel core even though the shell is thicker than the mean-free path of the photoelectrons. Characteristic L3 and L2 edges can be observed not only in the yield of the photoelectrons emitted from the surface of the nanostructure but also in its shadow. X-ray magnetic circular dichroism in the electron yield of the x rays absorbed and transmitted by the multilayered nanowire allows for the individual imaging of the magnetization configurations of the iron oxide tube and the nickel core. The method suggests novel approaches for the characterization of the magnetic and material properties of complex three-dimensional nanostructures.Financial support by the Deutsche Forschungsgemeinschaft via the Sonderforschungsbereich 668 and the Graduiertenkolleg 1286 as well as by the Forschungs- und Wissenschaftsstiftung Hamburg via the Cluster of Excellence Nanospintronics and by the Helmholtz-Zentrum Berlin is gratefully acknowledged.Peer reviewe

Comparative analysis of the toxicity of gold nanoparticles in zebrafish

The use of nanoparticles - particles that range in size from 1 to 100 nanometres - has become increasingly prevalent in recent years, bringing with it a variety of potential toxic effects. Zebrafish embryos were exposed during the 3-day post-fertilisation period to gold nanospheres (GSSs), gold nanorods (GNRs), gold nanorods coated with polystyrene-sulfate (PSS-GNRs), and gold nanorods coated with both polystyrene-sulfate and polyallamine hydrochloride (PAH/PSS-GNRs). All nanorods were stabilised with cetyltrimethylammonium bromide (CTAB). GNSs were the least toxic of the nanoparticles studied, with exposure resulting in no significant changes in mortality, hatching or heart rate. Exposure to GNRs and PSS-GNRs resulted in significant increases in mortality and significant decreases in hatching and heart rate. Treatment with GNRs caused significant changes in the expression of a variety of oxidative stress genes. The toxic effects of GNRs were ameliorated by coating them with polystyrene-sulfate and, to a more marked extent, with a double coating of polystyrene-sulfate and polyallamine hydrochloride

Dual-modal SERS/fluorescence AuNP probe for mitochondrial imaging

A novel SERS/fluorescent multimodal imaging probe for mitochondria has been synthesised using 12 nm diameter gold nanoparticles (AuNP) surface functionalised with a rhodamine thiol derivative ligand. The normal pH dependant acidic fluorescence of the rhodamine based ligand is inversed when conjugated with the AuNP and higher emission intensity is observed at basic pH. This switch correlates to a pKa at pH 6.62, which makes it an ideal candidate for a pH sensitive imaging probe in the biological range (6.5-7.4). The observed pH sensitivity when attached to the AuNP is thought to be due to the formation of a spirolactam ring on the ligand, going from positively charged (+18 mV) to negatively charged (-60 mV) as the pH is changed from acidic to basic. Additionally, conjugation of the ligand to the AuNP serves to enhance the Raman signal of the rhodamine ligand through Surface Enhanced Raman Scattering (SERS). Confocal microscopy has shown that the probe enters HEK293 (kidney), A2780 (ovarian cancer) and Min6 (pancreatic beta) cells within an hour and a half incubation time. The probe was shown to localise in the mitochondria, thus providing a novel pH dependent SERS/fluorescent multimodal imaging probe for mitochondria

Extensive degeneracy, Coulomb phase and magnetic monopoles in an artificial realization of the square ice model

Artificial spin ice systems have been introduced as a possible mean to investigate frustration effects in a well-controlled manner by fabricating lithographically-patterned two-dimensional arrangements of interacting magnetic nanostructures. This approach offers the opportunity to visualize unconventional states of matter, directly in real space, and triggered a wealth of studies at the frontier between nanomagnetism, statistical thermodynamics and condensed matter physics. Despite the strong efforts made these last ten years to provide an artificial realization of the celebrated square ice model, no simple geometry based on arrays of nanomagnets succeeded to capture the macroscopically degenerate ground state manifold of the corresponding model. Instead, in all works reported so far, square lattices of nanomagnets are characterized by a magnetically ordered ground state consisting of local flux-closure configurations with alternating chirality. Here, we show experimentally and theoretically, that all the characteristics of the square ice model can be observed if the artificial square lattice is properly designed. The spin configurations we image after demagnetizing our arrays reveal unambiguous signatures of an algebraic spin liquid state characterized by the presence of pinch points in the associated magnetic structure factor. Local excitations, i.e. classical analogues of magnetic monopoles, are found to be free to evolve in a massively degenerated, divergence-free vacuum. We thus provide the first lab-on-chip platform allowing the investigation of collective phenomena, including Coulomb phases and ice-like physics.Comment: 26 pages, 10 figure

How insignificant modifications of photocatalysts can significantly change their photocatalytic activity

Synthetic procedures, including doping, sintering and surface coating, can noticeably affect the physicochemical properties of semiconductors. Introduced changes very often translate into photocatalytic and photoelectrochemical activity alterations. However, in this work we have focused on more subtle treatments, which result in lack of changes observed using XRD, UV-vis, porosimetry, TEM or SEM. We have subjected titanium dioxide (P25, UV100) to a treatment with reducing agents used in procedures of noble metal deposition (citrate, borohydride, and photoreduction), or surface decoration with small amounts of TiO2 by atomic layer deposition (ALD; 10 to 200 deposition cycles), which presumably should be neutral to its activity. Although the "classical" characterization methods did not show any differences between the original and treated samples, spectroelectrochemical (SE-DRS) determination of the density of states (DOS) and catechol adsorption tests revealed a significant influence of such treatments on the photocatalytic activity (photogeneration of HO radicals, water reduction, and herbicide degradation) and photoelectrochemical behaviour of the studied samples. We have shown that the applied slight surface modifications of titanium dioxide ("insignificant" at the first glance) may strongly affect the activity of this material. Such often overlooked effects must be taken into account during a comparative photoactivity analysis of various semiconductors, since an insignificant surface treatment may noticeably influence surface chemistry. We have also demonstrated that SE-DRS can be considered as a useful tool to study these effects, although it can be difficult to correlate a particular treatment with recorded changes in the density of states.Peer reviewe

Vascular basement membranes as pathways for the passage of fluid into and out of the brain

In the absence of conventional lymphatics, drainage of interstitial fluid and solutes from the brain parenchyma to cervical lymph nodes is along basement membranes in the walls of cerebral capillaries and tunica media of arteries. Perivascular pathways are also involved in the entry of CSF into the brain by the convective influx/glymphatic system. The objective of this study is to differentiate the cerebral vascular basement membrane pathways by which fluid passes out of the brain from the pathway by which CSF enters the brain. Experiment 1: 0.5 µl of soluble biotinylated or fluorescent Aβ, or 1 µl 15 nm gold nanoparticles was injected into the mouse hippocampus and their distributions determined at 5 min by transmission electron microscopy. Aβ was distributed within the extracellular spaces of the hippocampus and within basement membranes of capillaries and tunica media of arteries. Nanoparticles did not enter capillary basement membranes from the extracellular spaces. Experiment 2: 2 µl of 15 nm nanoparticles were injected into mouse CSF. Within 5min, groups of nanoparticles were present in the pial-glial basement membrane on the outer aspect of cortical arteries between the investing layer of pia mater and the glia limitans. The results of this study and previous research suggest that cerebral vascular basement membranes form the pathways by which fluid passes into and out of the brain but that different basement membrane layers are involved. The significance of these findings for neuroimmunology, Alzheimer's disease, drug delivery to the brain and the concept of the Virchow-Robin space are discussed