Assembly of small building blocks such as atoms, molecules and nanoparticles into macroscopic structures-that is, 'bottom up' assembly-is a theme that runs through chemistry, biology and material science. Bacteria 1 , macromolecules 2 and nanoparticles 3 can self-assemble, generating ordered structures with a precision that challenges current lithographic techniques. The assembly of nanoparticles of two different materials into a binary nanoparticle superlattice (BNSL) [3][4][5][6][7] can provide a general and inexpensive path to a large variety of materials (metamaterials) with precisely controlled chemical composition and tight placement of the components. Maximization of the nanoparticle packing density has been proposed as the driving force for BNSL formation 3,8,9 , and only a few BNSL structures have been predicted to be thermodynamically stable. Recently, colloidal crystals with micrometre-scale lattice spacings have been grown from oppositely charged polymethyl methacrylate spheres 10,11 . Here we demonstrate formation of more than 15 different BNSL structures, using combinations of semiconducting, metallic and magnetic nanoparticle building blocks. At least ten of these colloidal crystalline structures have not been reported previously. We demonstrate that electrical charges on sterically stabilized nanoparticles determine BNSL stoichiometry; additional contributions from entropic, van der Waals, steric and dipolar forces stabilize the variety of BNSL structures.Face-centred-cubic (f.c.c.) ordering of monodisperse hard spheres dispersed in a liquid permits larger local free space available for each sphere compared to the unstructured phase, resulting in higher translational entropy of the spheres. When the volume fraction of hard spheres approaches ,55%, this ordering enhances the total entropy of the system and drives the ordering phase transition. Entropy-driven crystallization has been studied in great detail both theoretically 12 and experimentally on monodisperse latex particles, whose behaviour can be approximated by hard spheres 13,14 . In a mixture containing spheres of two different sizes (radii R small and R large ), the packing symmetry depends on the size ratio of the small and large spheres (g ¼ R small /R large ) 3,8 . Calculations show that assembly of hard spheres into binary superlattices isostructural with NaCl, AlB 2 and NaZn 13 can be driven by entropy alone without any specific energetic interactions between the spheres 9,15 . Indeed, NaZn 13 -and AlB 2 -type assemblies of silica particles were found in natural Brazilian opals 16 and can be grown from latex spheres 17 . In a certain g range, the packing density of these structures either exceeds or is very close to the density of the close-packed f.c.c. lattice (0.7405), while structures with lower packing densities are predicted to be unstable 8,15 .Despite these predictions, we observed an amazing variety of BNSLs that self-assemble from colloidal solutions of nearly spherical nanoparticles of different materials (Fig. 1). Coheren...

Recent advances in strategies for synthesizing nanoparticles--such as semiconductor quantum dots, magnets and noble-metal clusters--have enabled the precise control of composition, size, shape, crystal structure, and surface chemistry. The distinct properties of the resulting nanometre-scale building blocks can be harnessed in assemblies with new collective properties, which can be further engineered by controlling interparticle spacing and by material processing. Our study is motivated by the emerging concept of metamaterials-materials with properties arising from the controlled interaction of the different nanocrystals in an assembly. Previous multi-component nanocrystal assemblies have usually resulted in amorphous or short-range-ordered materials because of non-directional forces or insufficient mobility during assembly. Here we report the self-assembly of PbSe semiconductor quantum dots and Fe2O3 magnetic nanocrystals into precisely ordered three-dimensional superlattices. The use of specific size ratios directs the assembly of the magnetic and semiconducting nanoparticles into AB13 or AB2 superlattices with potentially tunable optical and magnetic properties. This synthesis concept could ultimately enable the fine-tuning of material responses to magnetic, electrical, optical and mechanical stimuli.

It is well-known that inorganic nanocrystals are a benchmark model for nanotechnology, given that the tunability of optical properties and the stabilization of specific phases are uniquely possible at the nanoscale. Copper (I) oxide (Cu(2)O) is a metal oxide semiconductor with promising applications in solar energy conversion and catalysis. To understand the Cu/Cu(2)O/CuO system at the nanoscale, we have developed a method for preparing highly uniform monodisperse nanocrystals of Cu(2)O. The procedure also serves to demonstrate our development of a generalized method for the synthesis of transition metal oxide nanocrystals. Cu nanocrystals are initially formed and subsequently oxidized to form highly crystalline Cu(2)O. The volume change during phase transformation can induce crystal twinning. Absorption in the visible region of the spectrum gave evidence for the presence of a thin, epitaxial layer of CuO, which is blue-shifted, and appears to increase in energy as a function of decreasing particle size. XPS confirmed the thin layer of CuO, calculated to have a thickness of approximately 5 A. We note that the copper (I) oxide phase is surprisingly well-stabilized at this length scale.

Nanocrystals of different size and functionality (e.g., noble metals, semiconductors, oxides, magnetic alloys) can be induced to self-assemble into ordered binary superlattices (also known as opals or colloidal crystals), retaining the size tunable properties of their constituents. We have built a variety of binary superlattices from monodisperse PbS, PbSe, CoPt3, Fe2O3, Au, Ag, and Pd nanocrystals, mixing and matching these nanoscale building blocks to yield multifunctional nanocomposites (metamaterials). Superlattices with AB, AB2, AB3, AB4, AB5, AB6, and AB13 stoichiometry with cubic, hexagonal, tetragonal, and orthorhombic symmetries have been identified. Assemblies with the same stoichiometry can be produced in several polymorphous forms by tailoring the particle size and deposition conditions. We have identified arrays isostructural with NaCl, CuAu, AlB2, MgZn2, MgNi2, Cu3Au, Fe4C, CaCu5, CaB6, NaZn13, and cub-AB13 compounds emphasizing the parallels between nanoparticle assembly and atomic scale crystal growth and providing confidence that many more structures will follow. Recently, we have demonstrated that electrical charges on sterically stabilized nanoparticles in addition to such parameters as particle size ratio and their concentrations can provide the formation of a much broader pallet of binary nanoparticle superlattices as compared with the limited number of possible superlattices formed by hard noninteracting spheres. In this contribution, we demonstrate a large variety of different binary superlattices, provide their detailed structural characterization, and discuss the role of energetic and kinetic factors in the cocrystallization process. We found that Coulomb, van der Waals, charge-dipole, dipole-dipole, and other interactions can contribute equally to cocrystallization, allowing superlattice formation to be dependent on a number of tunable parameters. We present binary superlattices as a new class of materials with a potentially unlimited library of constituents over a wide range of tunable structures.

Molecular electronics is often limited by the poorly defined nature of the contact between the molecules and the metal surface. We describe a method to wire molecules into gaps in single-walled carbon nanotubes (SWNTs). Precise oxidative cutting of a SWNT produces carboxylic acid-terminated electrodes separated by gaps of

The ras-like GTP binding proteins cdc42, rac, and rho regulate diverse cellular processes including cell growth and actin remodeling associated with changes in cell morphology, growth, adhesion, and motility (1-4). In fibroblasts, cdc42 regulates actin polymerization and focal complexes necessary for filopodia formation, rac mediates actin polymerization and focal complex assembly within lamellipodia and membrane ruffles, and rho induces actin stress fiber and focal adhesion (FA) complex formation (5). A hierarchical relationship exists among cdc42, rac, and rho, whereby cdc42 regulates rac activity and rac regulates rho activity, suggesting that these proteins may orchestrate the spatial and temporal changes in the actin cytoskeleton necessary for cell movement (5, 6). cdc42 and rac also regulate activation of the c-Jun N-terminal kinase/stressactivated kinase via a mitogen-activated protein (MAP) kinase pathway (7-9), and rac and rho are essential for ras transformation (10, 11). cdc42, rho, and rac all appear to stimulate c-fos transcription (12), as well as cell cycle progression through GI and subsequent DNA synthesis (9). The activation state of ras-like GTP binding proteins is positively regulated by guanine nucleotide exchange factors (GEFs) that promote the exchange of GDP for GTP, and negatively by GTPase activating proteins (2). A number of putative GEFs for rho-like GTPases have been identified by sequence comparison (2), and several of these demonstrate GEF activity in vitro. The dbl and ost oncogene products have cdc42 and rho GEF activity (13-15); the lbc oncogene product has rho GEF activity (16); the invasion-inducing Tiaml gene product has cdc42, rho, and rac GEF activity (17); and the yeast CDC24 gene product has cdc42 GEF activity (18). LAR is a broadly expressed transmembrane protein tyrosine phosphatase (PTPase) comprised of a cell adhesion-like extracellular region and two intracellular PTPase domains (19)(20)(21)(22). A role for LAR in regulating cell-matrix interactions was proposed, as LAR colocalizes with a coiled-coil protein, termed LAR interacting protein 1 (LIP.1) at the ends of FAs (23), and LAR expression was observed at regions of association between cells and basement membrane in various tissues (19). To identify putative substrates and other proteins involved in LAR-mediated signal transduction, we screened for proteins that bind the LAR PTPase domains using the interaction-trap assay and coimmunoprecipitation studies. A protein thus isolated is a novel multidomain GEF we have named Trio because it contains three enzyme domains: two GEF domains, one of which has racl GEF activity and the other has rhoA GEF activity, and a serine/threonine kinase (PSK) domain. In addition, Trio contains four N-terminal spectrinlike domains, two pleckstrin-like domains, and an Ig-like domain. Because proteins with cdc42, rac, or rho GEF activity are generally involved in regulating cytoskeletal organization (1), it is likely that Trio in conjunction with LAR plays a key role in coordinating the ...

Intestinal phosphate absorption occurs through both a paracellular mechanism involving tight junctions and an active transcellular mechanism involving the type II sodium-dependent phosphate cotransporter NPT2b (SLC34a2). To define the contribution of NPT2b to total intestinal phosphate absorption, we generated an inducible conditional knockout mouse, Npt2b Inorganic phosphate is an essential mineral critical for cellular processes and bone mineralization. Severe disruptions in serum phosphate have pathologic consequences. 1,2 Hypophosphatemic disorders are associated with rickets, osteomalacia, and a host of secondary dysfunctions. 3 In contrast, hyperphosphatemia associated with chronic kidney disease (CKD) is linked tightly to increased risk of cardiovascular morbidity and mortality. 4 -6 Recent studies show that elevated phosphate concentrations within the high normal range in individuals with functional kidneys also are correlated with increased cardiovascular risk and mortality. 7,8 Thus, an elevated serum phosphate level is an emerging health risk. Despite the importance of maintaining a relatively narrow serum phosphate range, nearly 70% of dietary phosphate is absorbed, resulting in transient postprandial increases in serum phosphate concentrations. 9 Normalization of serum phosphate appears to be managed primarily within the renal proximal tubule by the type II sodium-dependent phosphate cotransporters NPT2a (SLC34a1) and NPT2c (SLC34a3). Genetic knockout mouse models demonstrate that 80% and 20% of total urinary phosphorus are managed by the Npt2a and Npt2c transporters, respectively. 10,11 Chronic and acute regulation of these renal transporters is modulated by changes in dietary and serum phosphate

We have investigated the structural, magnetic, and electronic properties of nonstoichiometric iron oxide nanocrystals prepared by decomposition of iron(II) and iron(0) precursors in the presence of organic solvents and capping groups. The highly uniform, crystalline, and monodisperse nanocrystals that were produced enabled a full structural and compositional survey by electron microscopy and X-ray diffraction. The complex and metastable behavior of nonstoichiometric iron oxide (wüstite) at the nanoscale was studied by a combination of Mossbauer spectroscopy and magnetic characterization. Deposition from hydrocarbon solvents with subsequent self-assembly of iron oxide nanocrystals into superlattices allowed the preparation of continuous thin films suitable for electronic transport measurements.