Note: Production of silver nanoclusters using a Matrix-Assembly Cluster Source with a solid CO2 matrix.

One of the main limitations to the application of clusters on applied areas is the limited production; therefore, it is of great interest to up scale cluster production while keeping good size control. The Matrix-Assembly Cluster Source is a new high flux cluster source, which exploits cluster formation inside a solid rare gas matrix that is sputtered by an ion beam. Clusters are formed and ejected in this process. Here we report the production of Ag clusters when the rare gas is replaced by CO2 for the matrix formation at 20 K. Size distributions were determined from scanning transmission electron microscopy analysis of samples with four different metal loadings, 4%, 8%, 14%, and 23% of Ag atoms to CO2 molecules, and two ion beam energies, 1 keV and 2 keV. Cluster mean size showed weak dependence on metal loading, being ≈80 atoms for the first three concentrations, whereas the change in ion beam energy has caused cluster mean size to shift from 86 to 160 atoms. The results are interpreted in terms of bonding energy between Ag and CO2 and compared to the rare gas (Ar) matrix.

Pt Diffusion Dynamics for the Formation Cr-Pt Core-Shell Nanoparticles.

Layered core-shell bimetallic Cr-Pt nanoparticles were prepared by the formation and later reduction of an intermediate Pt-ion-containing supramolecular complex onto preformed Cr nanoparticles. The resultant nanoparticles were characterized by X-ray diffraction analysis, transmission electron microscopy, X-ray photoelectron spectroscopy, and aberration-corrected scanning transmission electron microscopy. The results are consistent with the presence of Pt diffusion during or after bimetallic nanoparticle formation, which has resulted in a Pt/Cr-alloyed core and shell. We postulate that such Pt diffusion occurs by an electric-field-assisted process according to Cabrera-Mott theory and that it originates from the low work function of the preformed oxygen-defective Cr nanoparticles and the rather large electron affinity of Pt.

Enhanced immobilization of gold nanoclusters on graphite.

The immobilization of individual biological molecules by metal nanoparticles requires that the particles themselves be immobilized. We introduce a new technique for immobilization of gold clusters based on their binding to small tunnels in a graphite support, themselves created by the implantation of small clusters. These tunnels are shown to perform as more effective cluster immobilization sites than point defects on the surface of graphite. The method is tested with atomic force microscopy (AFM) (both contact and noncontact mode) scanning. Size-selected clusters with 923, 561, 309, and 147 atoms have been immobilized and imaged with high-resolution, noncontact AFM.

Three-dimensional atomic-scale structure of size-selected gold nanoclusters.

An unambiguous determination of the three-dimensional structure of nanoparticles is challenging. Electron tomography requires a series of images taken for many different specimen orientations. This approach is ideal for stable and stationary structures. But ultrasmall nanoparticles are intrinsically structurally unstable and may interact with the incident electron beam, constraining the electron beam density that can be used and the duration of the observation. Here we use aberration-corrected scanning transmission electron microscopy, coupled with simple imaging simulation, to determine with atomic resolution the size, three-dimensional shape, orientation and atomic arrangement of size-selected gold nanoclusters that are preformed in the gas phase and soft-landed on an amorphous carbon substrate. The structures of gold nanoclusters containing 3096 atoms can be identified with either Ino-decahedral, cuboctahedral or icosahedral geometries. Comparison with theoretical modelling of the system suggests that the structures are consistent with energetic considerations. The discovery that nanoscale gold particles function as active and selective catalysts for a variety of important chemical reactions has provoked much research interest in recent years. We believe that the detailed structure information we provide will help to unravel the role of these nanoclusters in size- and structure-specific catalytic reactions. We note that the technique will be of use in investigations of other supported ultrasmall metal cluster systems.

Mass spectrometry and dynamics of gold adatoms observed on the surface of size-selected Au nanoclusters.

We report the imaging, mass spectrum, and dynamical behavior of adatoms and small clusters observed on the surface facets of size-selected, truncated octahedral gold clusters, Au(N) (N = 923 ± 23), via aberration-corrected scanning transmission electron microscopy. Our quantitative atom counting measurements show that most (~70%) of the species on the surface are single Au adatoms. Such species are now proposed as key elements of the atomic structure of both monolayer-protected nanoclusters (nanoparticles) and self-assembled monolayers and may also play a role in gold nanocatalysis. The adatoms are found on both {100} and {111} facets with similar probabilities.

Experimental evidence for fluctuating, chiral-type Au55 clusters by direct atomic imaging.

We report the atomic-scale structures and fluctuating dynamical behavior of size-selected Au(55) clusters obtained by aberration-corrected scanning transmission electron microscopy (STEM) coupled with systematic STEM simulations. No high-symmetry structures (face-centered cubic polyhedron, icosahedron, or decahedron) were observed in our statistical investigation. We find Au(55) clusters that are characteristic of the theoretically predicted chiral structure and similar sister isomers (which together we define as the chiral structural zone). The chiral structural zone was found to arise repeatedly in the time-lapse sequences of images we measured, though other amorphous-like structures are also frequently observed. The approach demonstrated here can be applied to identify specific low-symmetry atomic structures in other small clusters and distinguish them unambiguously from high-symmetry isomers.

Transcriptional response to GAA deficiency (Pompe disease) in infantile-onset patients.

Pompe disease is a genetic disorder resulting from a deficiency of lysosomal acid alpha-glucosidase (GAA) that manifests as a clinical spectrum with regard to symptom severity and rate of progression. In this study, we used microarrays to examine gene expression from the muscle of two cohorts of infantile-onset Pompe patients to identify transcriptional differences that may contribute to the disease phenotype. We found strong similarities among the gene expression profiles generated from biceps and quadriceps, and identified a number of signaling pathways altered in both cohorts. We also found that infantile-onset Pompe patient muscle had a gene expression pattern characteristic of immature or regenerating muscle, and exhibited many transcriptional markers of inflammation, despite having few overt signs of inflammatory infiltrate. Further, we identified genes exhibiting correlation between expression at baseline and response to therapy. This combined dataset can serve as a foundation for biological discovery and biomarker development to improve the treatment of Pompe disease.

Determination of the ground-state atomic structures of size-selected au nanoclusters by electron-beam-induced transformation.

The equilibrium ground state atomic structures of nanoparticles are critical to understanding the relationship between their structure and functionality, e.g., in catalysis, and are the standard output of first principles and semiempirical theoretical treatments. We demonstrate a method of obtaining a stable population of the structural isomers of supported Au clusters from a metastable initial array via electron beam irradiation. Statistical investigation of size-selected Au clusters containing 923±23 atoms via aberration-corrected scanning transmission electron microscopy shows that virtually all of the icosahedral (Ih) clusters undergo structural transformations into decahedral (Dh) (primarily) or fcc isomers while Dh and FCC clusters generally retain their atomic structures after electron irradiation of each cluster individually for up to 400 s at a dose of 2.4×10(4) e-/angstrom2/frame. Intermediate phases are often observed in the image series (videos) before the appearance of the new stable isomers, the relative structural populations of which can be controlled via the electron beam dose. The comprehensive results reported here should provide a valuable experimental reference for testing or refining potential models and for kinetic or dynamical treatments of the atomic configurations.

Site- and energy-selective intramolecular manipulation of polychlorinated biphenyl (PCB) molecules.

We demonstrate the conversion of an adsorbed precursor state of polychlorinated biphenyl (PCB) molecules on the Si(111)-7 × 7 surface at room temperature into a more stable configuration via site- and energy-selective atomic manipulation in the scanning tunneling microscope (STM). Whereas molecular desorption is maximized by electron injection into the chemisorbed molecular ring at low voltage, injection into the physisorbed molecular ring above a threshold voltage (2.5 V) favors the reconfiguration of the bonding. The results clearly demonstrate both intramolecular charge localization and intramolecular charge transportation as key ingredients in the atomic manipulation of individual polyatomic molecules.

Two-electron dissociation of single molecules by atomic manipulation at room temperature.

Using the tip of a scanning tunnelling microscope (STM) to mechanically manipulate individual atoms and molecules on a surface is now a well established procedure. Similarly, selective vibrational excitation of adsorbed molecules with an STM tip to induce motion or dissociation has been widely demonstrated. Such experiments are usually performed on weakly bound atoms that need to be stabilized by operating at cryogenic temperatures. Analogous experiments at room temperature are more difficult, because they require relatively strongly bound species that are not perturbed by random thermal fluctuations. But manipulation can still be achieved through electronic excitation of the atom or molecule by the electron current tunnelling between STM tip and surface at relatively high bias voltages, typically 1-5 V. Here we use this approach to selectively dissociate chlorine atoms from individual oriented chlorobenzene molecules adsorbed on a Si(111)-7 x 7 surface. We map out the final destination of the chlorine daughter atoms, finding that their radial and angular distributions depend on the tunnelling current and hence excitation rate. In our system, one tunnelling electron has nominally sufficient energy to induce dissociation, yet the process requires two electrons. We explain these observations by a two-electron mechanism that couples vibrational excitation and dissociative electron attachment steps.

Counting the atoms in supported, monolayer-protected gold clusters.

Mass spectrometry of supported, monolayer-protected, nominally Au(38) (MP-Au(38)) clusters is performed via quantitative High Angle Annular Dark Field-Scanning Transmission Electron Microscopy (HAADF-STEM) using size-selected Au(N) clusters (N = 25, 38, 55) as mass standards. With the intensity due to the (presumed) 24 hexanethiolate ligands taken into account, the clusters are found to contain 38 +/- 2 Au atoms. The method may also be used to determine the degree of aggregation of deposited nanoparticles.

Nonlocal desorption of chlorobenzene molecules from the Si(111)-(7×7) surface by charge injection from the tip of a scanning tunneling microscope: remote control of atomic manipulation.

We report the nonlocal desorption of chlorobenzene molecules from the Si(111)-(7×7) surface by charge injection from the laterally distant tip of a scanning tunneling microscope and demonstrate remote control of the manipulation process by precise selection of the atomic site for injection. Nonlocal desorption decays exponentially as a function of radial distance (decay length ∼100 A) from the injection site. Electron injection at corner-hole and faulted middle adatoms sites couples preferentially to the desorption of distant adsorbate molecules. Molecules on the faulted half of the unit cell desorb with higher probability than those on the unfaulted half.

The dynamics of chromosome movement in the budding yeast Saccharomyces cerevisiae.

Nuclear DNA movement in the yeast, Saccharomyces cerevisiae, was analyzed in live cells using digital imaging microscopy and corroborated by the analysis of nuclear DNA position in fixed cells. During anaphase, the replicated nuclear genomes initially separated at a rate of 1 micron/min. As the genomes separated, the rate of movement became discontinuous. In addition, the axis defined by the segregating genomes rotated relative to the cell surface. The similarity between these results and those previously obtained in higher eukaryotes suggest that the mechanism of anaphase movement may be highly conserved. Before chromosome separation, novel nuclear DNA movements were observed in cdc13, cdc16, and cdc23 cells but not in wild-type or cdc20 cells. These novel nuclear DNA movements correlated with variability in spindle position and length in cdc16 cells. Models for the mechanism of these movements and their induction by certain cdc mutants are discussed.

Role of astral microtubules and actin in spindle orientation and migration in the budding yeast, Saccharomyces cerevisiae.

In the yeast Saccharomyces cerevisiae, before the onset of anaphase, the spindle apparatus is always positioned with one spindle pole at, or through, the neck between the mother cell and the growing bud. This spindle orientation enables proper chromosome segregation to occur during anaphase, allowing one replicated genome to be segregated into the bud and the other to remain in the mother cell. In this study, we synchronized a population of cells before the onset of anaphase such that > 90% of the cells in the population had spindles with the correct orientation, and then disrupted specific cytoskeletal elements using temperature-sensitive mutations. Disruption of either the astral microtubules or actin function resulted in improper spindle orientation in approximately 40-50% of the cells. When cells with disrupted astral microtubules or actin function entered into anaphase, there was a 100-200-fold increase in the frequency of binucleated cell bodies. Thus, the maintenance of proper spindle orientation by these cytoskeletal elements was essential for proper chromosome segregation. These data are consistent with the model that proper spindle orientation is maintained by directly or indirectly tethering the astral microtubules to the actin cytoskeleton. After nuclear migration, but before anaphase, bulk chromosome movement occurs within the nucleus apparently because the chromosomes are attached to a mobile spindle. The frequency and magnitude of bulk chromosome movement is greatly diminished by disruption of the astral microtubules but not by disruption of the nonkinetochore spindle microtubules. These results suggest that astral microtubules are not only important for spindle orientation before anaphase, but they also mediate force on the spindle, generating spindle displacement and in turn chromosome movement. Potential roles for this force in spindle assembly and orientation are discussed.

A new mineralized layer in the hinge of the oyster.

A discrete, calcareous layer that binds the foliated calcite of the oyster's shell to the organic ligament in the hinge is reported, apparently for the first time. This layer is ultrastructurally, and generally mineralogically, different from the underlying foliated calcite, and is named the ligostracum.

Atomic-resolution imaging of surface and core melting in individual size-selected Au nanoclusters on carbon.

Although the changes in melting behaviour on the nanoscale have long attracted the interest of researchers, the mechanism by which nanoparticles melt remains an open problem. We report the direct observation, at atomic resolution, of surface melting in individual size-selected Au clusters (2-5 nm diameter) supported on carbon films, using an in situ heating stage in the aberration corrected scanning transmission electron microscope. At elevated temperatures the Au nanoparticles are found to form a solid core-liquid shell structure. The cluster surface melting temperatures, show evidence of size-dependent melting point suppression. The cluster core melting temperatures are significantly greater than predicted by existing models of free clusters. To explore the effect of the interaction between the clusters and the carbon substrate, we employ a very large-scale ab initio simulation approach to investigate the influence of the support. Theoretical results for surface and core melting points are in good agreement with experiment.

Experimental determination of the energy difference between competing isomers of deposited, size-selected gold nanoclusters.

The equilibrium structures and dynamics of a nanoscale system are regulated by a complex potential energy surface (PES). This is a key target of theoretical calculations but experimentally elusive. We report the measurement of a key PES parameter for a model nanosystem: size-selected Au nanoclusters, soft-landed on amorphous silicon nitride supports. We obtain the energy difference between the most abundant structural isomers of magic number Au561 clusters, the decahedron and face-centred-cubic (fcc) structures, from the equilibrium proportions of the isomers. These are measured by atomic-resolution scanning transmission electron microscopy, with an ultra-stable heating stage, as a function of temperature (125-500 °C). At lower temperatures (20-125 °C) the behaviour is kinetic, exhibiting down conversion of metastable decahedra into fcc structures; the higher state is repopulated at higher temperatures in equilibrium. We find the decahedron is 0.040 ± 0.020 eV higher in energy than the fcc isomer, providing a benchmark for the theoretical treatment of nanoparticles.

WT1 regulates the expression of the major glomerular podocyte membrane protein Podocalyxin.

The WT1 tumor suppressor gene encodes a zinc finger transcription factor expressed in differentiating glomerular podocytes. Complete inactivation of WT1 in the mouse leads to failure of mesenchymal induction and renal agenesis, an early developmental phenotype that prevents analysis of subsequent stages in glomerular differentiation [1]. In humans with Denys-Drash Syndrome, a heterozygous germline mutation in WT1 is associated with specific defects in glomeruli and an increased risk for developing Wilms Tumor [2,3]. WT1 target genes implicated in cell cycle regulation and cellular proliferation have been proposed [4], but the link between WT1 function and glomerular differentiation is unexplained. Here, we show that inducible expression of WT1 in rat embryonic kidney cell precursors leads to the induction of endogenous Podocalyxin, the major structural membrane protein of glomerular podocytes, which is implicated in the maintenance of filtration slits. Binding of WT1 to conserved elements within the Podocalyxin gene promoter results in potent transcriptional activation, and the specific expression pattern of Podocalyxin in the developing kidney mirrors that of WT1 itself. These observations support a role for WT1 in the specific activation of a glomerular differentiation program in renal precursors and provide a molecular basis for the glomerulonephropathy that is characteristic of Denys-Drash Syndrome.

Barnacle muscle: Ca2+, activation and mechanics.

In this review, aspects of the ways in which Ca2+ is transported and regulated within muscle cells have been considered, with particular reference to crustacean muscle fibres. The large size of these fibres permits easy access to the internal environment of the cell, allowing it to be altered by microinjection or microperfusion. At rest, Ca2+ is not in equilibrium across the cell membrane, it enters the cell down a steep electrochemical gradient. The free [Ca2+] at rest is maintained at a value close to 200 nM by a combination of internal buffering systems, mainly the SR, mitochondria, and the fixed and diffusible Ca(2+)-binding proteins, as well as by an energy-dependent extrusion system operating across the external cell membrane. This system relies upon the inward movement of Na+ down its own electrochemical gradient to provide the energy for the extrusion of Ca2+ ions. As a result of electrical excitation, voltage-sensitive channels for Ca2+ are activated and permit Ca2+ to enter the cell more rapidly than at rest. It has been possible to determine both the amount of Ca2+ entering by this step, and what part this externally derived Ca2+ plays in the development of force as well as in the free Ca2+ change. The latter can be determined directly by Ca(2+)-sensitive indicators introduced into the cell sarcoplasm. A combination of techniques, allowing both the total and free Ca2+ changes to be assessed during electrical excitation, has provided valuable information as to how muscle cells buffer their Ca2+ in order to regulate the extent of the change in the free Ca2+ concentration. The data indicate that the entering Ca2+ can only make a small direct contribution to the force developed by the cell. The implication here is that the major source of Ca2+ for contraction must be derived from the internal Ca2+ storage sites within the SR system, a view reinforced by caged Ca2+ methods. The ability to measure the free Ca2+ concentration changes within a single cell during activation has also provided the opportunity to analyse, in detail, the likely relations between free Ca2+ and the process of force development in muscle. The fact that the free Ca2+ change precedes the development of force implies that there are delays in the mechanism, either at the site of Ca2+ attachment on the myofibril, or at some later stage in the process of force development that were not previously anticipated.(ABSTRACT TRUNCATED AT 400 WORDS)

Note: Proof of principle of a new type of cluster beam source with potential for scale-up.

We introduce a new type of cluster beam source based on the assembly of (metal) clusters within a condensed (rare gas) matrix. The "Matrix Assembly Cluster Source" employs an ion beam to enhance collisions between metal atoms in the matrix and to sputter out clusters to form a beam. We demonstrate the formation and deposition of gold and silver nanoclusters with mean size tunable from a few atoms to a few thousand atoms. The cluster flux is equivalent to a current nanoAmp regime but potentially scalable to milliAmps, which would open up a number of interesting experiments and applications.