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Diatoms are single-celled algae that make silica shells or frustules with intricate nanoscale features imbedded within periodic two-dimensional pore arrays. A two-stage photobioreactor cultivation process was used to metabolically insert titanium into the patterned biosilica of the diatom Pinnularia sp. In Stage I, diatom cells were grown up on dissolved silicon until silicon starvation was achieved. In Stage II, soluble titanium and silicon were continuously fed to the silicon-starved cell suspension (approximately 4 x 10(5) cells/mL) for 10 h. The feeding rate of titanium (0.85-7.3 micromol Ti L(-1) h(-1)) was designed to circumvent the precipitation of titanate in the liquid medium, and feeding rate of silicon (48 micromol Si L(-1) h(-1)) was designed to sustain one cell division. The addition of titanium to the culture had no detrimental effects on cell growth and preserved the frustule morphology. Cofeeding of Ti and Si was required for complete intracellular uptake of Ti. The maximum bulk composition of titanium in the frustule biosilica was 2.3 g of Ti/100 g of SiO(2). Intact biosilica frustules were isolated by treatment of diatom cells with SDS/EDTA and then analyzed by TEM and STEM-EDS. Titanium was preferentially deposited as a nanophase lining the base of each frustule pore, with estimated local TiO(2) content of nearly 80 wt %. Thermal annealing in air at 720 degrees C converted the biogenic titanate to anatase TiO(2) with an average crystal size of 32 nm. This is the first reported study of using a living organism to controllably fabricate semiconductor TiO(2) nanostructures by a bottom-up self-assembly process.
ACS Nano 2008 Oct 28
PMID:Metabolic insertion of nanostructured TiO2 into the patterned biosilica of the diatom Pinnularia sp. by a two-stage bioreactor cultivation process. 1920 57

Luminescent silicon quantum dots (Si QDs) have great potential for use in biological imaging and diagnostic applications. To exploit this potential, they must remain luminescent and stably dispersed in water and biological fluids over a wide range of pH and salt concentration. There have been many challenges in creating such stable water-dispersible Si QDs, including instability of photoluminescence due their fast oxidation in aqueous environments and the difficulty of attaching hydrophilic molecules to Si QD surfaces. In this paper, we report the preparation of highly stable aqueous suspensions of Si QDs using phospholipid micelles, in which the optical properties of Si nanocrystals are retained. These luminescent micelle-encapsulated Si QDs were used as luminescent labels for pancreatic cancer cells. This paves the way for silicon quantum dots to be a valuable optical probe in biomedical diagnostics.
ACS Nano 2008 May
PMID:Biocompatible luminescent silicon quantum dots for imaging of cancer cells. 1920 83

Graphene has many unique properties which make it an attractive material for fundamental study as well as for potential applications. In this paper, we report the first experimental study of process-induced defects and stress in graphene using Raman spectroscopy and imaging. While defects lead to the observation of defect-related Raman bands, stress causes shift in phonon frequency. A compressive stress (as high as 2.1 GPa) was induced in graphene by depositing a 5 nm SiO(2) followed by annealing, whereas a tensile stress ( approximately 0.7 GPa) was obtained by depositing a thin silicon capping layer. In the former case, both the magnitude of the compressive stress and number of graphene layers can be controlled or modified by the annealing temperature. As both the stress and thickness affect the physical properties of graphene, this study may open up the possibility of utilizing thickness and stress engineering to improve the performance of graphene-based devices. Local heating techniques may be used to either induce the stress or reduce the thickness selectively.
ACS Nano 2008 May
PMID:Tunable stress and controlled thickness modification in graphene by annealing. 1920 1

The increasing use of micro- and nanostructured silicon-based devices for in vivo therapeutic or sensing applications highlights the importance of understanding the immunogenicity of these surfaces. Four silicon surfaces (nanoporous, microstructured, nanochanneled, and flat) were studied for their ability to provoke an immune response in human blood derived monocytes. The monocytes were incubated with the surfaces for 48 h and the immunogenicity was evaluated based on the viability, shape factors, and cytokine expression. Free radical oxygen formation was measured at 18 h to elicit a possible mechanism invoking immunogenicity. Although no cytokines were significantly different comparing the response of monocytes on the tissue culture polystyrene surfaces to those on the micropeaked surfaces, on average all cytokines were elevated on the micropeaked surface. The monocytes on the nanoporous surface also displayed an elevated cytokine response, overall, but not to the degree of those on the micropeaked surface. The nanochanneled surface response was similar to that of flat silicon. Overall, the immunogenicity and biocompatibility of flat, nanochanneled, and nanoporous silicon toward human monocytes are approximately equivalent to tissue culture polystyrene.
ACS Nano 2008 May
PMID:In vitro immunogenicity of silicon-based micro- and nanostructured surfaces. 1920 6

This paper presents a systematic investigation on the controlled synthesis of wurtzite aluminum nitride (AlN) one-dimensional (1D) nanostructures in a chemical vapor deposition (CVD) system using Al and NH(3) as starting materials. By controlling reaction temperature and NH(3) flow, nanostructures with manifold morphologies including nanoneedles, branched nanoneedles, short nanorods, slim nanorods, and nanofences were synthesized with high yield and selectivity. The correlation between experiment parameters and product morphologies was interpreted by a surface diffusion based model. Moreover, electrical properties of a single nanoneedle were studied for the first time, in which typical semiconductor characteristics were observed. Silicon was speculated to incorporate into the AlN nanoneedle from silicon substrates during the synthesis, which served as an n-type donor and was responsible for the observed electrical behavior.
ACS Nano 2008 Jan
PMID:[0001] oriented aluminum nitride one-dimensional nanostructures: synthesis, structure evolution, and electrical properties. 1920 57

Silicon nanowires (SiNWs) have been grown with our nanochannel-template-guided "grow-in-place" approach and used in-place for resistor and transistor fabrication. In this methodology, empty nanochannels of a permanent template literally guide the vapor-liquid-solid (VLS) mechanism of SiNW growth and give control of the self-assembling nanowires' size, number, position, and orientation. The approach is demonstrated to give self-positioned/self-assembled SiNWs which are then used for device fabrication without any intervening SiNW collection, positioning, and assembling steps. These SiNWs may be grown so that they are extruded from, or confined within, the permanent nanochannel-template, as desired. The nanowire grow-in-place fabrication approach offers the potential for mass and environmentally benign manufacturing. The latter potential arises since only the exact number of nanowires needed is fabricated and these nanowires are always fixed at the position of use by the guiding nanochannels.
ACS Nano 2008 Mar
PMID:Self-assembling silicon nanowires for device applications using the nanochannel-guided "grow-in-place" approach. 1920 66

Block copolymer thin films can be used as soft templates for a wide range of surfaces where large area patterns of nanoscale features are desired. The cylindrical domains of acid-sensitive, self-assembled monolayers of polystyrene-poly(2-vinylpyridine) block copolymers on silicon surfaces were utilized as structural elements for the production of parallel metal nanowires. Metal ion loading of the P2VP block with simple aqueous solutions of anionic metal complexes is accomplished via protonation of this basic block, rendering it cationic; electrostatic attraction leads to a high local concentration of metal complexes within the protonated P2VP domain. A subsequent brief plasma treatment simultaneously removes the polymer and produces metallic nanowires. The morphology of the patterns can modulated by controlling solution concentration, deposition time, and molecular weight of the block copolymers, as well as other factors. Horizontal metallic nanoarrays can be aligned on e-beam lithographically defined silicon substrates within different shapes, via graphoepitaxy. This method is highly versatile as the procedures to manipulate nanowire composition, dimension, spacing, and orientation are straightforward and based upon efficient aqueous inorganic chemistry.
ACS Nano 2008 Mar
PMID:Using cylindrical domains of block copolymers to self-assemble and align metallic nanowires. 1920 75

Experimental evidence derived from a comprehensive study of a self-assembled organosilane multilayer film system undergoing a process of postassembly chemical modification that affects interlayer-located polar groups of the constituent molecules while preserving its overall molecular architecture allows a quantitative evaluation of both the degree of intralayer polymerization and that of interlayer covalent bonding of the silane headgroups in a highly ordered layer assembly of this type. The investigated system consists of a layer-by-layer assembled multilayer of a bifunctional n-alkyl silane with terminal alcohol group that is in situ converted, via a wet chemical oxidation process conducted on the entire multilayer, to the corresponding carboxylic acid function. A combined chemical-structural analysis of data furnished by four different techniques, Fourier transform infrared spectroscopy (FTIR), synchrotron X-ray scattering, X-ray photoelectron spectroscopy (XPS), and contact angle measurements, demonstrates that the highly ordered 3D molecular arrangement of the initial alcohol-silane multilayer stack is well preserved upon virtually quantitative conversion of the alcohol to carboxylic acid and the concomitant irreversible cleavage of interlayer covalent bonds. Thus, the correlation of quantitative chemical and structural data obtained from such unreacted and fully reacted film samples offers an unprecedented experimental framework within which it becomes possible to differentiate between intralayer and interlayer covalent bonding. In addition, the use of a sufficiently thick multilayer effectively eliminates the interfering contributions of the underlying silicon oxide substrate to both the X-ray scattering and XPS data. The present findings contribute a firm experimental basis to the elucidation of the self-assembly mechanism, the molecular organization, and the modes and dynamics of intra- and interlayer bonding prevailing in highly ordered organosilane films; with further implications for the rational exploitation of some of the unique options such supramolecular surface entities can offer in the advancement of a chemical nanofabrication methodology.
ACS Nano 2008 Mar
PMID:Postassembly chemical modification of a highly ordered organosilane multilayer: new insights into the structure, bonding, and dynamics of self-assembling silane monolayers. 1920 85

Metal-stabilized bilayers, prepared by the self-assembly of octadecyltrichorosilane on an oxidized silicon surface followed by the Langmuir-Blodgett deposition of a monolayer of octadecylphosphonic acid, have been used to generate 1.6 nanometer thick, highly uniform, zirconium oxide films following annealing. Patterning of the thin films on the nanometre scale was achieved using nanodisplacement methodology, by careful control of an atomic force microscope (AFM) probe, which allowed the selective removal of the upper leaflet of the bilayer.
ACS Nano 2008 Apr
PMID:A new route to the production and nanoscale patterning of highly smooth, ultrathin zirconium oxide films. 1920 94

Hybrid organic-inorganic films consisted of molecular layers of a Keggin-structure polyoxometalate (POM: 12-tungstophosphoric acid, H(3)PW(12)O(40)) and 1,12-diaminododecane (DD) on 3-aminopropyl triethoxysilane (APTES)-modified silicon surface, fabricated via the layer-by-layer (LBL) self-assembly method are evaluated as molecular materials for electronic devices. The effect of the fabrication process parameters, including primarily compositions of deposition solutions, on the structural characteristics of the POM-based multilayers was studied extensively with a combination of spectroscopic methods (UV, FTIR, and XPS). Well-characterized POM-based films (both single-layers and multilayers) in a controlled and reproducible way were obtained. The conduction mechanisms in single-layered and multilayered structures were elucidated by the electrical characterization of the produced films supported by the appropriate theoretical analysis. Fowler-Nordheim (FN) tunneling and percolation mechanisms were encountered in good correlation with the structural characteristics of the films encouraging further investigation on the use of these materials in electronic and, in particular, in memory devices.
ACS Nano 2008 Apr
PMID:Polyoxometalate-based layered structures for charge transport control in molecular devices. 1920 5


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