Biomineralization. Naturally aligned nanocrystals

Controlled synthesis and self-assembly of CeO2 nanocubes

CeO2 nanocubes (and nanorods) enclosed by six {200} planes with controlled sizes have been prepared through a facile one-pot method. The nanocubes have a strong tendency to assemble into 2D and 3D arrays with regular patterns on a substrate, which is probably driven by the dipole-dipole interaction of polar {200} planes. The possible formation mechanism of the nanocubes has been put forward as the oriented aggregation mediated precursor growth. It is possible to use the synthesized nanocubes as building blocks to achieve {200}-perfect-oriented monolayers or thickness-controlled films and to apply the preparative method in the incorporation of heterogeneous atoms or nanoparticles for semiconductor doping or heterogeneous nanostructures.

Cation-Dependent Switching of DNA Nanostructures

DNA is a versatile building material for nanoconstruction because of its remarkable molecular-recognition capability and well-predicted duplex conformation. A number of DNA motifs have been engineered, which can assemble into well-defined nanostructures in Mg(2+)-containing buffer solution. XRD studies reveal that the DNA conformation is slightly influenced by divalent cations (such as Mg(2+) or Ca(2+)). This phenomenon can be utilized in DNA self-assembly for regulating self-assembled DNA nanostructures. As an initial step, a symmetric cross motif forms flat, periodic, 2D lattices in Mg(2+)-containing solutions, but long nanofibers in Ca(2+)-containing solutions. The obtained DNA fibers can serve as templates to fabricate CaCO(3) nanotubes and nanowires.

Evolution of single crystalline dendrites from nanoparticles through oriented attachment

Single crystalline PbMoO4 dendrites were prepared by a simple hydrothermal method in the presence of surfactants. The formation and evolution of these dendrites was investigated by transmission electron microscopy, and the results clearly showed that the dendritic structure was achieved through oriented attachment of nanoparticles along crystallographically specific direction while the traditional Ostwald ripening mechanism also acted to form the initial particles before attachment and smooth the morphology of the dendrites after attachment.

Organic reaction pathways in the nonaqueous synthesis of metal oxide nanoparticles

Nonaqueous-solution routes to metal oxide nanoparticles are a valuable alternative to the known aqueous sol-gel processes, offering advantages such as high crystallinity at low temperatures, robust synthesis parameters and ability to control the crystal growth without the use of surfactants. In the first part of the review we give a detailed overview of the various solution routes to metal oxides in organic solvents, with a strong focus on surfactant-free processes. In most of these synthesis approaches, the organic solvent plays the role of the reactant that provides the oxygen for the metal oxide, controls the crystal growth, influences particle shape, and, in some cases, also determines the assembly behavior. We have a closer look at the following reaction systems in this order: 1) metal halides in alcohols, 2) metal alkoxides, acetates, and acetylacetonates in alcohols, 3) metal alkoxides in ketones, and 4) metal acetylacetonates in benzylamine. All these systems offer some peculiarities with respect to each other, providing many possibilities to control and tailor the particle size and shape, as well as the surface and assembly properties. In the second part we present general mechanistic principles for aqueous and nonaqueous sol-gel processes, followed by the discussion of reaction pathways relevant for nanoparticle formation in organic solvents. Depending on the system several mechanisms have been postulated: 1) alkyl halide elimination, 2) elimination of organic ethers, 3) ester elimination, 4) C--C bond formation between benzylic alcohols and alkoxides, 5) ketimine and aldol-like condensation reactions, 6) oxidation of metal nanoparticles, and 7) thermal decomposition methods.

Solution−Chemical Synthesis of Carbon Nanotube/ZnS Nanoparticle Core/Shell Heterostructures

A facile solution-chemical method has been developed to be capable of encapsulating a multiwalled carbon nanotube (MWCNT) with ZnS nanocrystals without using any bridging species. The thickness of the ZnS shell can be tuned easily by controlling the experimental conditions. The optical properties of the MWCNT/ZnS heterostructures were investigated using UV-vis absorption and photoluminescence spectroscopy. The optical absorption spectrum indicates that the band gap of ZnS nanocrystallites is 4.2 eV. On the basis of the photoluminescence spectrum, charge transfer is thought to proceed from ZnS nanocrystals to the nanotube in the ZnS-carbon nanotube system. These special heterostructures are very easily encapsulated within a uniform silica layer by a modified-Stöber process and still show better stability even after heat treatment at 400 degrees C, which makes them appealing for practical applications in biochemistry and biodiagnostics.

Microwave-assisted synthesis of anatase TiO(2) nanorods with mesopores

Pure anatase TiO(2) nanorods with mesopores were synthesized by a simple and low cost microwave-assisted method when tri-block copolymer was used as a structure stabilization agent and TiCl(4) as metal precursor. TEM investigation showed that larger nanorods were assembled by pearl-necklace-shaped nanorods following an oriented attachment mechanism in a specific direction. A proposed hypothetical scheme showed that the formation of lyotropic titania liquid crystal (TLC) serves a key role in the stabilization of nanorods, and the mesopores on nanorods are derived from the vacancy of inter-particles of nanorods and regions lacking inorganic precursors in the TLC structure. Control experiments showed that microwave treatment plays a key role in the maintenance of original morphologies and mesostructures free from destruction even under high temperature calcinations.

Oriented assembly of Fe3O4 nanoparticles into monodisperse hollow single-crystal microspheres

Magnetite nanoparticles of Fe3O4 were found to assemble into monodisperse hollow Fe3O4 microspheres with tunable diameters ranging from 200 to 400 nm and open pores on the shells in ethylene glycol in the presence of dodecylamine (DDA). The oriented assembly of nanoparticles conferred the individual hollow Fe3O4 microspheres a remarkable feature of single crystals. The morphologies of the products could be easily manipulated by varying the synthesis parameters. Increasing the concentration of DDA led to an obvious shape evolution of the products from rhombic nanoparticles to hollow microspheres, solid microspheres, and finally irregular nanoparticles, which were mainly attributed to the special self-assembly phenomenon of Fe3O4 nanoparticles in the solvothermal process.

Submicrocubes and highly oriented assemblies of MnCO3 synthesized by ultrasound agitation method and their thermal transformation to nanoporous Mn2O3

MnCO(3) submicrocubes and highly oriented MnCO(3) nanocrystal assemblies with an ellipsoidal morphology have been successfully prepared by an ultrasonic solution approach. The effect of surfactants of sodium dodecylsulfate (SDS) and aerosol OT (AOT) on the morphology of MnCO(3) was investigated. Highly oriented ellipsoidal assemblies composed of approximately 5 nm MnCO(3) nanocrystals with porous nanostructures were prepared in the presence of SDS. Both sonochemical irradiation and surfactant play an important role in the formation of these highly oriented assemblies. Nanoporous Mn(2)O(3) was obtained by thermal treatment of MnCO(3) at 600 degrees C in air. The shape of MnCO(3) was sustained after thermal transformation to form nanoporous Mn(2)O(3). The products were characterized by X-ray powder diffraction, transmission electron microscopy, selected-area electron diffraction, field emission scanning electron microscopy, thermogravimetric analysis and differential scanning calorimetric analysis.

Shape control of single crystalline bismuth nanostructures

A synthesis approach for shape control of single crystalline Bi nanostructures has been developed. By controlling the molar ratio of PVP and Bi in a polyol process, Bi nanocubes with an edge length of approximately 60-80 nm, triangular nanoplates with an edge length of 200-500 nm, and nanospheres with an average diameter of 75 nm have been successfully synthesized. In the same synthetic process, Bi nanobelts with lengths of up to 80 microm and widths of up to 0.6 microm were synthesized in large quantities by introducing a trace amount of Fe3+ species into the reaction system. These single crystalline nanostructure Bi materials are expected to find potential applications in a variety of areas including high efficiency thermoelectric devices.

Mesocrystals: inorganic superstructures made by highly parallel crystallization and controlled alignment

Controlled self-organization of nanoparticles can lead to new materials. The colloidal crystallization of non-spherical nanocrystals is a reaction channel in many crystallization reactions. With additives, self-organization can be stopped at an intermediary step-a mesocrystal-in which the primary units can still be identified. Mesocrystals were observed for various systems as kinetically metastable species or as intermediates in a crystallization reaction leading to single crystals with typical defects and inclusions. The control forces and mechanism of mesocrystal formation are largely unknown, but several mesocrystal properties are known. Mesocrystals are exiting examples of nonclassical crystallization, which does not proceed through ion-by-ion attachment, but by a modular nanobuilding-block route. This path makes crystallization more independent of ion products or molecular solubility, it occurs without pH or osmotic pressure changes, and opens new strategies for crystal morphogenesis.

Self-Assembly of Flowerlike AlOOH (Boehmite) 3D Nanoarchitectures

In this work, a hydrothermal route using an ethanol-water solution to progressively synthesize a sequence of flowerlike three-dimensional gamma-AlOOH boehmite nanostructures without employing templates or matrixes for self-assembly is presented. The flowerlike boehmite nanoarchitectures exhibit three hierarchies of self-organization, i.e., single-crystalline nanorods, nanostrips, and bundles, which are characterized by scanning and transmission electron microscopy. The sequence of products obtained after different processing times indicates a self-assembly mechanism. The hydrogen bonding on the surface of nanorods or nanostrips possibly plays a key role, as identified by FTIR spectra of the products after they had been heated to 1000 degrees C. The specific surface area and pore-size distribution of the obtained product as determined by gas-sorption measurements show that the boehmite nanoarchitectures exhibit high BET surface area and porosity properties.

Monetite Formed in Mixed Solvents of Water and Ethylene Glycol and Its Transformation to Hydroxyapatite

Agglomerated nanorods of hydroxyapatite have been synthesized using monetite as a precursor in a NaOH solution. Monetite consisting of nanosheets has been successfully synthesized by a one-step microwave-assisted method using CaCl(2).2.5H(2)O, NaH(2)PO(4), and sodium dodecyl sulfate (SDS) in water/ethylene glycol (EG) mixed solvents. The effects of the molar ratio of water to EG and the reaction time on the products were investigated. The products were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier transform infrared spectrometry (FTIR).

Oriented attachment and mesocrystals: non-classical crystallization mechanisms based on nanoparticle assembly

In this review, we highlight particle based crystallization pathways leading to single crystals via mesoscopic transformation. In contrast to the classical mechanism of atom/molecule mediated growth of a single crystal, the particle mediated growth and assembly mechanisms are summarized as "non-classical crystallization", including exiting processes like oriented attachment and mesocrystal formation. Detailed investigations of non-classical crystallization mechanisms are a recent development, but evidence for these pathways is rapidly increasing in the literature. A major driving force for these investigations originates from biomineralization, because it seems that these crystallization routes are frequently applied by natural organisms. We give a non-exhaustive literature survey on these two mechanisms with a focus on recent examples and studies, which are dedicated to a mechanistic understanding. Furthermore, conditions are introduced for which these non-classical crystallization mechanisms can be expected, as they are always an alternative reaction pathway to classical crystallization.

Hydrothermal Synthesis of Single-Crystalline Anatase TiO2Nanorods with Nanotubes as the Precursor

Preparation of anatase TiO2 nanorods from solutions in the absence of surfactants or templates has rarely been reported. The present work has found that hydrothermal treatment of titanate nanotube suspensions under an acidic environment resulted in the formation of single-crystalline anatase nanorods with a specific crystal-elongation direction. The nanotube suspensions were prepared by treatment of TiO2 in NaOH, followed by mixing with HNO3 to different pH values. The crystal size of the anatase nanoparticles obtained from the hydrothermal treatment increased with the pH of the suspensions, and nanorods with an aspect ratio up to 6 and a long axis along the anatase [001] were obtained at a pH slightly less than 7. A mechanism for the tube-to-rod transformation has been proposed on the basis of the crystalline structures of the tubes and rods. The local shrinkage of the tube walls to form anatase crystallites and the subsequent oriented attachment of the crystallites have been suggested to be the key steps involved in the nanorod formation.

Growth and Characterization of Highly Branched Nanostructures of Magnetic Nanoparticles

Magnetite nanoparticles of Fe(3)O(4) have been found to grow into large highly branched nanostructures including nanochains and highly branched nanotrees in the solid state through a postannealing process. By varying the preparation conditions such as annealing time and temperature, the nanostructures could be easily manipulated. Changing the starting concentration of the magnetic nanoparticle solution also caused significant changes of the nanoarchitectures. When the magnetic nanoparticle concentration is low, the nanoparticles formed straight rods mainly with an average diameter of 80 nm and a length of several microns. With increasing concentration of the nanoparticles, treelike structures began to form. With further increase of the concentration, well-ordered nanostructures with the appearance of snowflakes were generated. The driving force for the formation of the highly ordered nanostructures includes interaction between the nanoparticles and interaction through surface-capping molecules. This experiment demonstrates that novel nanostructures can be generated by self-assembly of magnetic nanoparticles under the solid state.

Growth Mechanism of Penniform BaWO4 Nanostructures in Catanionic Reverse Micelles Involving Polymers

The formation of penniform BaWO4 nanostructures made of nanowires or nanobelts under the direction of a block copolymer in catanionic reverse micelles has been studied in detail. On the basis of the experimental results obtained from the BaWO4 crystallization in aqueous polymer solutions and careful transmission electron microscopy (TEM) observations of BaWO4 nanostructures formed in reverse micelles containing polymers, a detailed two-stage growth mechanism has been proposed for the formation of the penniform nanostructures in reverse micelles, which involves the polymer-controlled shaft formation (Stage 1) and the mixed surfactants-controlled barb growth (Stage 2). During Stage 1, poly(ethylene glycol)-block-poly(methacrylic acid) (PEG-b-PMAA) induced the formation of c-axis-oriented shuttle-like nanocrystals and the subsequent oriented attachment of these shuttle-like nanocrystals resulted in the formation of [100]-oriented shafts with many parallel [001]-oriented pricks. During Stage 2, [001]-oriented nanowires or nanobelts grew gradually from the pricks into barbs, leading to the formation of well-defined penniform BaWO4 nanostructures with the barb morphology essentially determined by the mixing ratio r of the anionic to cationic surfactants (i.e., nanowires were formed at r=1 while nanobelts were formed at r deviating from 1). The current understanding of the growth mechanism of penniform BaWO4 nanostructures in catanionic reverse micelles involving polymers may be potentially applied for designing a new synthesis system for the controlled synthesis of other hierarchical 1D nanostructures with desired architectures.

Nanoexplosion synthesis of multimetal oxide ceramic nanopowders

Herein we demonstrate a unique processing technique for engineering multicomponent ceramic nanopowders with precise morphologies by "nanoblast" calcination/deagglomeration. Multiple "nanoexplosions" of C(3)H(6)N(6)O(6) nanoparticles embedded in preliminary engineered nanoreactors break apart the agglomerates because of the highly energetic impacts of the blast waves. Also, the solid-solubility of one component into the other is enhanced by the extremely high local temperature generated during the nanoexplosions. We applied this technique to produce nanosized agglomerate-free ceria-gadolinia solid solution powder with an average aggregate size of 42 nm. The described method opens the door to the synthesis of a wide range of multimetal oxide ceramic and metal-ceramic composite nanopowders, with precise stoichiometries and uniform morphologies.

Colloidal nanocrystal synthesis and the organic-inorganic interface

Colloidal nanocrystals are solution-grown, nanometre-sized, inorganic particles that are stabilized by a layer of surfactants attached to their surface. The inorganic cores possess useful properties that are controlled by their composition, size and shape, and the surfactant coating ensures that these structures are easy to fabricate and process further into more complex structures. This combination of features makes colloidal nanocrystals attractive and promising building blocks for advanced materials and devices. Chemists are achieving ever more exquisite control over the composition, size, shape, crystal structure and surface properties of nanocrystals, thus setting the stage for fully exploiting the potential of these remarkable materials.

Programmable assembly of nanoarchitectures using genetically engineered viruses

Biological systems possess inherent molecular recognition and self-assembly capabilities and are attractive templates for constructing complex material structures with molecular precision. Here we report the assembly of various nanoachitectures including nanoparticle arrays, hetero-nanoparticle architectures, and nanowires utilizing highly engineered M13 bacteriophage as templates. The genome of M13 phage can be rationally engineered to produce viral particles with distinct substrate-specific peptides expressed on the filamentous capsid and the ends, providing a generic template for programmable assembly of complex nanostructures. Phage clones with gold-binding motifs on the capsid and streptavidin-binding motifs at one end are created and used to assemble Au and CdSe nanocrytals into ordered one-dimensional arrays and more complex geometries. Initial studies show such nanoparticle arrays can further function as templates to nucleate highly conductive nanowires that are important for addressing/interconnecting individual nanostructures.

Synthesis of ZnO Nanoparticles in 2-Propanol by Reaction with Water

We report on the synthesis of ZnO particles from Zn(CH(3)CO(2))(2) in 2-propanol as a function of the concentration of water, in the absence of a base such as NaOH. Particles with diameters of 3-5 nm are formed depending on time, temperature, and water concentration. The nucleation and growth are slower than in the presence of NaOH, and at longer times the increase in particle size is dominated by diffusion-limited coarsening. The rate constant for coarsening increases with increasing water concentration up to 150 mM, above which the rate constant is 1.1 x 10(-4) cm(3) s(-1), independent of the water concentration. The width of the particle size distribution decreases with increasing water concentration, and at 250 mM water, the full width at half-maximum of the distribution function is essentially the same as for the synthesis of ZnO using NaOH as a reactant. The temperature dependence of coarsening is determined by the bulk solubility of the ZnO nanoparticles and yields an apparent activation energy of 1.12 eV. This is significantly larger than the activation energy of 0.35 eV for coarsening of ZnO from 1 mM Zn(CH(3)CO(2))(2) in 2-propanol with 1.6 mM NaOH.

A Kinetic Model to Describe Nanocrystal Growth by the Oriented Attachment Mechanism

The classical model of particle coagulation on colloids is revisited to evaluate its applicability on the oriented attachment of nanoparticles. The proposed model describes well the growth behavior of dispersed nanoparticles during the initial stages of nanoparticle synthesis and during growth induced by hydrothermal treatments. Moreover, a general model, which combines coarsening (i.e., Ostwald ripening) and oriented attachment effects, is proposed as an alternative to explain deviations between experimental results and existing theoretical models.

Synthesis of Gold Nano- and Microplates in Hexagonal Liquid Crystals

Single-crystalline gold nano- and microplates with triangular or hexagonal shapes are synthesized by reduction of HAuCl(4) in lyotropic liquid crystal (LLC) mainly made of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers and water after adding a small amount of capping agents, cetyltrimethylammonium bromide (CTAB) or tetrabutylammonium bromide (TBAB). During the growth of such plates, capping agents play the crucial role. It is found that there is an optimal value of CTAB or TBAB concentration for producing microplates. The selective adsorption of CTAB or TBAB on certain crystallographic facets may be the key point of the supposed mechanism. Although LLC does not really act as a template, it provides an ordered structure confining CTAB as well as the nascent metal nuclei, which enhances the oriented attachment of nuclei and thus the consequent growth of single-crystal plates.

Highly Efficient Dye-Sensitized Solar Cells with a Titania Thin-Film Electrode Composed of a Network Structure of Single-Crystal-like TiO2Nanowires Made by the “Oriented Attachment” Mechanism

In this study, single-crystal-like anatase TiO(2) nanowires were formed in a network structure by surfactant-assisted self-assembling processes at low temperature. The crystal lattice planes of the nanowires and networks of such wires composed of many nanoparticles were almost perfectly aligned with each other due to the "oriented attachment" mechanism, resulting in the high rate of electron transfer through the TiO(2) nanonetwork with single-crystal-like anatase nanowires. The direction of crystal growth of oriented attachment was controlled by changing the mole ratio of acetylacetone to Ti, that is, regulating both the adsorption of surfactant molecules via control of the reaction rate and the surface energy. A single-crystalline anatase exposing mainly the [101] plane has been prepared, which adsorbed ruthenium dye over 4 times higher as compared to P-25. A high light-to-electricity conversion yield of 9.3% was achieved by applying the titania nanomaterials with network structure as the titania thin film of dye-sensitized solar cells.

Virus-based toolkit for the directed synthesis of magnetic and semiconducting nanowires

We report a virus-based scaffold for the synthesis of single-crystal ZnS, CdS, and freestanding chemically ordered CoPt and FePt nanowires, with the means of modifying substrate specificity through standard biological methods. Peptides (selected through an evolutionary screening process) that exhibit control of composition, size, and phase during nanoparticle nucleation have been expressed on the highly ordered filamentous capsid of the M13 bacteriophage. The incorporation of specific, nucleating peptides into the generic scaffold of the M13 coat structure provides a viable template for the directed synthesis of semiconducting and magnetic materials. Removal of the viral template by means of annealing promoted oriented aggregation-based crystal growth, forming individual crystalline nanowires. The unique ability to interchange substrate-specific peptides into the linear self-assembled filamentous construct of the M13 virus introduces a material tunability that has not been seen in previous synthetic routes. Therefore, this system provides a genetic toolkit for growing and organizing nanowires from semiconducting and magnetic materials.

The genesis of neurosurgery and the evolution of the neurosurgical operative environment: part II--concepts for future development, 2003 and beyond

The future development of the neurosurgical operative environment is driven principally by concurrent development in science and technology. In the new millennium, these developments are taking on a Jules Verne quality, with the ability to construct and manipulate the human organism and its surroundings at the level of atoms and molecules seemingly at hand. Thus, an examination of currents in technology advancement from the neurosurgical perspective can provide insight into the evolution of the neurosurgical operative environment. In the future, the optimal design solution for the operative environment requirements of specialized neurosurgery may take the form of composites of venues that are currently mutually distinct. Advances in microfabrication technology and laser optical manipulators are expanding the scope and role of robotics, with novel opportunities for bionic integration. Assimilation of biosensor technology into the operative environment promises to provide neurosurgeons of the future with a vastly expanded set of physiological data, which will require concurrent simplification and optimization of analysis and presentation schemes to facilitate practical usefulness. Nanotechnology derivatives are shattering the maximum limits of resolution and magnification allowed by conventional microscopes. Furthermore, quantum computing and molecular electronics promise to greatly enhance computational power, allowing the emerging reality of simulation and virtual neurosurgery for rehearsal and training purposes. Progressive minimalism is evident throughout, leading ultimately to a paradigm shift as the nanoscale is approached. At the interface between the old and new technological paradigms, issues related to integration may dictate the ultimate emergence of the products of the new paradigm. Once initiated, however, history suggests that the process of change will proceed rapidly and dramatically, with the ultimate neurosurgical operative environment of the future being far more complex in functional capacity but strikingly simple in apparent form.

Coarsening of metal oxide nanoparticles

In solution phase synthesis of nanoparticles, processes such as coarsening and aggregation can compete with nucleation and growth in modifying the particle size distribution in the system. We show that coarsening of ZnO and TiO2 nanoparticles in solution follows the Lifshitz-Slyozov-Wagner rate law for diffusion controlled coarsening originally derived for colloidal systems with micrometer-sized particles, where the average particle size cubed is proportional to time. The rate constant for growth of ZnO in propanol is in the range 10(-4)-10(-2) nm3 x s(-1) and is dependent on the precursor anion and temperature. The coarsening of TiO2 nanoparticles from aqueous Ti(IV) alkoxide solutions is slower due to the low solubility of TiO2 with the rate constant in the range 10(-5)-10(-3) nm3 x s(-1) for temperatures between 150 degrees C and 220 degrees C. Epitaxial attachment of TiO2 particles becomes significant at higher temperatures and longer times. We show that the dominant parameters controlling the coarsening kinetics are solvent, precursor salt, and temperature.