Mechanism of Formation of Monodispersed Colloids by Aggregation of Nanosize Precursors

Poly(hydroxyalkanoates)-based polymeric nanoparticles for drug delivery

Poly (hydroxyalkanoates) (PHAs) have recently attracted a great deal of academic and industrial interest for their biodegradability and biocompatibility making them suitable for environmental and biomedical applications. Poly(3-hydroxybutyrate-) (PHB-) and Poly(DL-lactide-co-glycolide) (PLGA-) based nanoparticles were prepared using the dialysis method as yet unreported for the preparation of nanoparticles based on PHB. Processing conditions were varied in order to evaluate their influence on morphology, drug encapsulation, and size of nanoparticles. The relevant results obtained give a theoretical understanding of the phenomenon occurring during colloidal formation. The adopted procedure allows for a relatively small diameter and homogeneity in size distribution of the PHB nanoparticles to be obtained compared to other methods like the one based on solvent evaporation which leads to particles on microscale. The biocompatibility of PHB and relative nanoparticles was investigated and both exhibited very good cytocompatibility.

Formation of magnesium fluoride particles of different morphologies

Uniform dispersions of magnesium fluoride particles of different morphologies were prepared by precipitation in aqueous solutions. The resulting cubic, prismatic, and platelet-like nanosize solids had single crystal structure with X-ray pattern characteristic of the mineral sellaite. In contrast, two kinds of polycrystalline MgF2 spheres were obtained by aggregation of the nanosize subunits. The mechanisms of the formation of the resulting particles of different shapes are explained by the role of the pH and ionic strength. In addition, for prospective numerical modeling the surface tension of spherical and platelet particles of MgF2 was evaluated from the X-ray data by a lattice parameter change method.

Shape selection in diffusive growth of colloids and nanoparticles

We report numerical investigations of a 3D model of diffusive growth of fine particles, the internal structure of which corresponds to different crystal lattices. A growing cluster (particle) is immersed in and exchanges monomer building blocks with a surrounding medium of diffusing (off-lattice) monomers. On-surface dynamics of the latter is accounted for by allowing, in addition to detachment, monomer motion to the neighboring vacant crystal sites, according to probabilistic rules mimicking local thermalization. The key new feature of our model is the focus on the growth of a single cluster, emerging as a crystalline core, without development of defects that can control large-scale growth modes. This single, defect-free core growth is imposed by the specific dynamic rules assumed. Our results offer a possible explanation of the experimentally observed shape uniformity (i.e., fixed, approximately evenly sized proportions) in the synthesis of uniform colloids and nanoparticles. We demonstrate the basic principles of well-defined particle shape emergence in such growth. Specifically, several shapes are possible for a given crystal structure. The formation of shapes that follow the crystal symmetry and are uniform can be a result of the nonequilibrium nature of the growth process. The shape of a growing particle can be controlled by varying the relative rates of kinetic processes as well as by adjusting the concentration of monomers in the surrounding medium.

Peptide-coated silver nanoparticles: synthesis, surface chemistry, and pH-triggered, reversible assembly into particle assemblies

Simple tripeptides are scaffolds for the synthesis and further assembly of peptide/silver nanoparticle composites. Herein, we further explore peptide-controlled silver nanoparticle assembly processes. Silver nanoparticles with a pH-responsive peptide coating have been synthesized by using a one-step precipitation/coating route. The nature of the peptide/silver interaction and the effect of the peptide on the formation of the silver particles have been studied via UV/Vis, X-ray photoelectron, and surface-enhanced Raman spectroscopies as well as through electron microscopy, small angle X-ray scattering and powder X-ray diffraction with Rietveld refinement. The particles reversibly form aggregates of different sizes in aqueous solution. The state of aggregation can be controlled by the solution pH value. At low pH values, individual particles are present. At neutral pH values, small clusters form and at high pH values, large precipitates are observed.

Mechanisms of diffusional nucleation of nanocrystals and their self-assembly into uniform colloids

We survey our research on modeling the mechanisms of control of uniformity in the growth of nanosize and colloid size particles. The former are produced as nanocrystals by burst nucleation from solution and the latter are formed by self-assembly (aggregation) of the nanocrystals. In the colloid particle synthesis, the two dynamical processes are coupled, both governed by diffusional transport of the respective building blocks (monomers). The interrelation of the two processes allows for the synthesis of narrow size distribution colloid dispersions, which are of importance in many applications. We first review a mathematical model of diffusive cluster growth by the capture of monomer "singlets." We then analyze burst nucleation of nanoparticles in solution. Finally, we couple it to the secondary process of aggregation of nanoparticles to form colloids and discuss various aspects of the modeling of particle size distribution, as well as other features of the processes considered.

Nanoporous Pt with high surface area by reaction-limited aggregation of nanoparticles

Nanoporous structures with high active surface areas are critical for a variety of applications. Here, we present a general templateless strategy to produce such porous structures by controlled aggregation of nanostructured subunits and apply the principles for synthesizing nanoporous Pt for electrocatalytic oxidation of methanol. The nature of the aggregate produced is controlled by tuning the electrostatic interaction between surfactant-free nanoparticles in the solution phase. When the repulsive force between the particles is very large, the particles are stabilized in the solution while instantaneous aggregation leading to fractal-like structures results when the repulsive force is very low. Controlling the repulsive interaction to an optimum, intermediate value results in the formation of compact structures with very large surface areas. In the case of Pt, nanoporous clusters with an extremely high specific surface area (39 m2/g) and high activity for methanol oxidation have been produced. Preliminary investigations indicate that the method is general and can be easily extended to produce nanoporous structures of many inorganic materials.

Fabrication of copper oxide dumbbell-like architectures via the hydrophobic interaction of adsorbed hydrocarbon chains

In this paper, the synthesized surfactant of copper dodecyl sulfate (Cu(DS)2) was used as a special metal-ion source for the morphological control of copper oxide (CuO) architectures. During the fabrication processes, the ribbon-shaped intermediates of basic copper salt with lamellar structures were observed at 60.0 degrees C for the first time. In the absence or presence of dodecanol (DOH), Cu(DS)2 could react with sodium hydroxide to form dumbbell-like architectures of CuO nanoparticles. The incorporation of DOH molecules into the adsorption monolayers of surfactant ions could greatly enlarge the dumbbell size in length, probably depending upon the formation of the DOH-DS complex. These indicated that the template effectiveness of the intermediate ribbons, together with the hydrophobic interactions of adsorbed hydrocarbon chains, should account for the formation process of CuO dumbbells. Interestingly, the addition of sodium chloride into the reaction systems could induce the morphological change of CuO dumbbells to the twin-anchors and then to the twin-spheres with two holes in the center. This further suggests that the hydrophobic interaction of pendent hydrocarbon chains provides an important approach for material fabrication purposes.

Accumulation mechanism for metal chalcogenide nanoparticles at Hg0 electrodes: copper sulfide example

Mercury electrodes preconcentrate metal chalcogenide nanoparticles effectively, enabling their detection at submicromolar concentrations (as Sigma chalcogenide) by adsorptive cathodic stripping voltammetry. Understanding the unique behavior of nanoparticle analytes during preconcentration is critical for lowering detection limits and for quantification. A multistep mechanism is proposed on the basis of accumulation experiments with polydisperse copper sulfide (CuxS) nanoparticles. Particles first diffuse and adsorb at the Hg0 surface. When both the electrode and particles have negative surface potentials, this process resembles charge-impeded coagulation, obeying the Schulze-Hardy rule at various electrolyte strengths. Consequently, accumulation rates are surprisingly sensitive to electrolyte concentration. Choosing accumulation potentials where the electrode and particles have opposite surface potentials greatly improves collection efficiency, especially for the smallest particles. After adsorption, particles undergo transformations. One product is a more stable (harder to reduce) form of CuxS, interpreted to consist of adclusters or adlayers. A very significant (approximately 0.3 V) negative shift in reduction potential results from this transformation. Loss of analyte to at least one nonelectroactive product is also observed. Loss is greatest for the smallest particles and is sensitive to choice of accumulation potential. To improve accumulation efficiency, accumulation potentials more positive that the potential of zero charge of Hg electrodes are advantageous but care must be taken to remove dissolved chalcogenides under these conditions in order to avoid artifacts.

Model of nanocrystal formation in solution by burst nucleation and diffusional growth

The phenomenon of burst nucleation in solution, in which a period of apparent chemical inactivity is followed by a sudden and explosive growth of nucleated particles from a solute species, has been given a widely accepted qualitative explanation by LaMer and co-workers. Here, we present a model with the assumptions of instantaneous re-thermalization below the critical nucleus size and irreversible diffusive growth above the critical size, which for the first time formulates LaMer's explanation of burst nucleation in a manner allowing quantitative calculations. The behavior of the model at large times, t, is derived with the result that the average cluster size, as measured by the number of atoms, grows approximately t, while the width of the cluster distribution grows approximately (sq root)1. We develop an effective numerical scheme to integrate the equations of the model and compare the asymptotic expressions to results from numerical simulation. Finally, we discuss the physical effects which cause real nucleation processes in solution to deviate from the behavior of the model.

Hydroxyapatite micro- and nanoparticles: nucleation and growth mechanisms in the presence of citrate species

Hydroxyapatite (HAP) particles with different morphologies were precipitated from homogeneous calcium/citrate/phosphate solutions at physiological temperature. Small variations of the starting solution pH in the range 7.4<pH<8.5 made it possible to switch the precipitated particle morphology from a micrometric bundlelike to a nanometric needlelike shape. The role of the existing citrate species as calcium chelates is here discussed within the framework of particle nucleation and growth mechanisms. While temperature-dependent calcium citrate complex (Cacit) stability is here suggested to control the free calcium availability and thereby the nucleation rate, the adsorbed citrate species are proposed to control the nanoparticle stability. Moreover, an attempt to detail the role of citrate species in the ordered aggregation of HAP nuclei leading to the observed peanut and bundlelike microparticles morphology is also presented.

Size control of gold nanocrystals in citrate reduction: the third role of citrate

Growth kinetics and temporal size/shape evolution of gold nanocrystals by citrate reduction in boiling water were studied systematically and quantitatively. Results reveal that the size variation and overall reaction mechanism were mostly determined by the solution pH that was in turn controlled by the concentration of sodium citrate (Na3Ct) in the traditional Frens's synthesis. This conclusion was further confirmed by the reactions with variable pH but fixed concentrations of the two reactants, HAuCl4 and Na3Ct. Two substantially different reaction pathways were identified, with the switching point at pH = 6.2-6.5. The first pathway is for the low pH range and consists of three overlapping steps: nucleation, random attachment to polycrystalline nanowires, and smoothing of the nanowires via intra-particle ripening to dots. The second pathway that occurred above the pH switching point is consistent with the commonly known nucleation-growth route. Using the second pathway, we demonstrated a new synthetic route for the synthesis of nearly monodisperse gold nanocrystals in the size range from 20 to 40 nm by simply varying the solution pH with fixed concentrations of HAuCl4 and Na3Ct. The switching of the reaction pathways is likely due to the integration nature of water as a reaction medium. In the citrate reduction, the solution pH was varied by changing the initial HAuCl4/Na3Ct ratio. Consequently, when pH was higher than about 6.2, the very reactive [AuCl3(OH)]- would be converted to less reactive [AuCl2(OH)2]- and [AuCl(OH)3]-.

Monodispersed ZnSe colloidal microspheres: preparation, characterization, and their 2D arrays

Uniform and monodispersed ZnSe colloidal microspheres were prepared via a hot injection route by using trioctylamine as solvent and with a relatively higher concentration of the monomer precursors. The size of the colloidal ZnSe spheres was tuned from 138 to 629 nm by varying the concentration of the precursors, and the size increased linearly with the concentration of Zinc oleate in the range from 0.05 to 0.10 kg/L with the molar ratio of Zn to Se of 0.5. The colloidal microspheres were well characterized by SEM, TEM, HRTEM, EDS, UV-vis, and PL techniques. Detailed examination confirmed that the colloidal microspheres were formed by the in-situ aggregation of ZnSe nanoparticles. Furthermore, the 2D ordered assembly of the ZnSe microspheres was realized via a simple vertical precipitation technique. Due to the high refractive index and absence of absorption in the visible region, the monodispersed ZnSe colloidal microspheres could be potential building blocks to construct photonic band gap crystals and other functional devices.

Facile synthesis of monodispersed barium sulphate particles via an in situ templated process

Monodispersed BaSO4 micrometer and submicrometer sized particles were produced by a simple precipitation reaction in the presence of poly(methacrylic acid) (PMAA) at the ambient temperature. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electron diffraction (ED) were used to characterize the obtained BaSO4 particles, and polycrystalline nature of the BaSO4 products was revealed. A "polymer-M in situ template" model was proposed to elucidate the formation process of such polycrystalline BaSO4 particles.

Silver microflowers and large spherical particles: Controlled preparation and their wetting properties

In this article, we report a simple wet-chemical method to prepare silver microflowers and large spherical particles. The formation of the two different microstructures of silver is based on the reduction of AgNO(3) by para-phenylenediamine in aqueous medium at room temperature. The controlling of the silver microstructures can be achieved only by adjusting the concentration of the reactants. It is found that the two different silver microstructures display opposite wetting properties. Large spherical silver particles exhibit superhydrophilic properties with a contact angle (CA) of close to 0 degrees, microflower-like silver particles exhibit highly hydrophobic properties with CA about 132 degrees. X-ray diffraction (XRD), X-ray photoelectron spectra (XPS) and UV-vis spectra are used to characterize the chemical structure of the obtained products.

Self-Organization of All-Inorganic Dodecatungstophosphate Nanocrystallites

The crystallinity and porosity of all-inorganic dodecatungstophosphate M3PW12O40 (M=Cs, NH4, Ag, denoted as MPW) particles are controlled by the changes in the synthetic temperatures and countercations. The MPW particles can be classified into three groups by the crystallinity and porosity: (i) mesoporous "disordered" aggregates, (ii) microporous "self-organized" aggregates, and (iii) nonporous single crystals. The formation and growth mechanism of MPW particles is expressed by three steps: formation of nanocrystallites, assembly of the nanocrystallites to form aggregates, and the growth of aggregates by the attachment of nanocrystallites. The time courses of the turbidity of the synthetic solution, the concentration of the nanocrystallites, and the average particle sizes of MPW particles are well reproduced by the calculation based on the mechanism.

Amplified light scattering and emission of silver and silver core–silica shell particles

Side versus forward light scattergrams, and fluorescence (488 nm excitation) intensity versus particle count histograms were gathered for bare, R6G-coated, and silica-R6G-coated silver particles of 150-200 nm diameter, one-by-one by flow cytometry. Fluorescence emission intensity of the composite particles monotonically increased and then reached a plateau with greater R6G concentrations, as measured by flow cytometry. Fluorescence amplification factors of up to 3.5x10(3) were estimated by reference to measurements on core-shell particles with silica instead of silver cores. Huge surface enhanced Raman scattering (SERS) intensities, at least 10(14)-fold greater than normal Raman scattering intensities, were observed with 633 nm excitation for molecules such as rhodamine 6G (R6G) on the same single particles of silver. Although routine transmission (TEM) and scanning (SEM) electron microscopies showed gross structures of the bare and coated particles, high-resolution field emission scanning electron microscopy (FE-SEM), revealed Brownian roughness describing quantum size and larger structures on the surface of primary colloidal silver particles. These silver particles were further characterized by extinction spectra and zeta potentials. Structural and light scattering observations that are reported herein were used to tentatively propose a new hierarchical model for the mechanism of SERS.

Zinc oxide colloids with controlled size, shape, and structure

Highly dispersed uniform ZnO particles of different sizes and shapes were prepared by slowly adding zinc salt and sodium hydroxide solutions in parallel into aqueous solutions of Arabic gum. Except for the very early stages, the precipitated solids consisted of a well-defined zinc oxide phase. Depending on the experimental conditions, the size of the final polycrystalline particles formed by the aggregation of nanosize entities varied from 100 to 300 nm. The reaction temperature affected both the size of the nanosize precursors and their arrangement in the final particles. At ambient temperature the primary nanoparticles, approximately 10 nm in size, formed spherical aggregates, while at 600 degrees C they were much larger (44 nm) and combined to form rather uniform hexagonal ZnO prisms. The aspect ratio and the internal structure of the latter could be altered by changing the nature of the zinc salt, the addition rate, and the initial concentration of the reactants. Based on the findings of the study a two-stage mechanism for the formation of uniform polycrystalline particles with well-defined geometric shapes is proposed.

Influence of a magnetic field on the formation of magnetite particles via two precipitation methods

An experimental investigation is described on the effects of the presence of a magnetic field during the fabrication of magnetite particles. We considered two well-known synthesis methods: that of Massart [IEEE Trans. Magn. 1981, 17, 1247-1248] for the synthesis of nanometer-sized, monodomain particles; and that of Sugimoto and Matijević [J. Colloid Interface Sci. 1980, 74, 227-243.] for the fabrication of micrometer-sized multidomain spherical particles. The latter method was studied with two systems of different ionic compositions that lead to two different mechanisms of growth: either growth by aggregation and recrystallization of primary particles or direct crystal growth. When growth was dominated by aggregation of primary units, the magnetic field had a dramatic effect on the morphology, inducing the formation of rodlike particles. Growth dynamics of that system were studied for particles obtained in the presence as well as in the absence of the magnetic field. Particles were also characterized by powder magnetometry, electrophoresis, X-ray diffraction, and optical absorbance techniques. Interestingly, growth dynamics of the rods cross section were comparable to those of the diameter of the spheres. With the exception of the morphology, no other significant difference was found between the rodlike particles and the spheres.

Microstructure of equilibrium fluid clusters in colloid-polymer suspensions

Several studies on colloidal depletion systems have reported the existence of a fluid phase consisting of clusters of particles above a critical polymer concentration that acts as a precursor regime to the gel phase at low colloid volume fractions (phi<or=0.20) . The clusters are found to be stable against further aggregation suggesting that individual particles are localized within a cluster. However the clusters themselves behave as distinct entities in an equilibrium fluid phase. In this study, we probe the internal microstructure of the cluster entities by ultrasmall angle x-ray scattering (USAXS) techniques. These studies reveal that over the accessible length scales, the microstructure of the particle clusters are similar to that observed in dense space-spanning depletion gels. The origin of these clusters is unclear but the scattering patterns as they settle with time reveal that the percolation of the clusters to form space-spanning gels does not influence their internal microstructure. These observations lend support to the hypothesis that the formation of space-spanning depletion gels at a given volume fraction is driven by the percolation of the particle clusters. Settling experiments at phi=0.08 also provide rough estimates of the cluster sizes that appear consistent with the observations from the USAXS experiments.

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.

Controllable size, shape and morphology of molybdic acid self-aggregated with rhodamine B to construct functional material

Controllable size, shape and morphology of rhodamine B/molybdic acid (RBMA) aggregates were prepared from a self-aggregation reaction in a molybdic acid and rhodamine B (RhB) coexisting solution. Nanodisks, as well as microcrystal rods and polyhexagonal microcrystal rods, have been obtained in conventional bulk solutions at different temperatures. Large-sized network microcrystal rods and branched fractal aggregates constructed with nanosubunits after the nucleation duration of an ice-water-cooled process have also been achieved under the evaporation-enhanced conditions on glass substrates. The factors affecting the size, shape and morphology of RBMA aggregates including temperature, nucleation and growth, and processing conditions are discussed. The results show that photofunctional molecules (RhB) modified the surface of the molybdic acid particles and influenced their self-aggregation. The temperature and nucleation play key roles in the formation of RBMA aggregates. The structures of RBMA aggregates were characterized by X-ray diffraction, infrared spectra and elemental dispersive spectroscopy. The results indicate that the aggregates show the characteristics of RhB-mediated hydrated ammonium molybdenum bronze with the metastable hexagonal phase. Visible-light-induced electrons transfer reactions in the RBMA aggregates from rhodamine B molecules to MoO3 matrixes were measured by UV-vis spectra and X-ray photoelectron spectra, and the fluorescent image was observed by laser scanning confocal microscopy.

Precipitation of Nanostructured Copper Oxalate: Substructure and Growth Mechanism

The possibility of controlling materials properties by tailoring their substructure at the nanometer scale is a current topic of great interest. To do so, a fundamental understanding of the growth mechanism is of key importance and an analytical challenge as nanostructured materials are often produced by precipitation methods at high supersaturations where formation kinetics are fast. The current study focuses on the precipitation of copper oxalate, which has been previously shown to produce self-assembled ordered nanostructured particles with the promise of being able to tailor this nanometer substructure. In the current study we investigate in detail the growth mechanism and kinetics of precipitation by using in-situ particle size measurement or by stopping the reaction at various stages and using ex-situ methods. Combining the ex-situ methods of high-resolution scanning electron microscopy, transmission electron microscopy, and X-ray powder diffraction along with the in-situ methods, we were able to follow the growth process from 2 min to 2 weeks. The results in the 2-30 min period lead to the proposal of a core-shell growth model with a poorly ordered core and a well-structured shell of nanosized crystallites (50-70 nm), adding support to the brick-by-brick model previously proposed for this phase of particle growth. Particle evolution over long periods up to 2 weeks show a ripening which produces lens-shaped particles that eliminate the "high" surface energy faces observed in the earlier stages of growth. A more complete growth mechanism for copper oxalate precipitation at moderate supersaturations is proposed similar to recent findings for other self-assembled nanostructured particles.

Manipulation of Aqueous Growth of CdTe Nanocrystals To Fabricate Colloidally Stable One-Dimensional Nanostructures

The present article is devoted to systematically exploring the influence of various experimental variables, including the precursor concentration, the ligand nature, the counterion type, the Cd-to-Te molar ratio, pH, and temperature, on the aqueous growth of CdTe nanocrystals. The growth may be divided into two stages: the early fast growth stage and the later slow growth stage. The later stage is found to be dominated by Ostwald ripening (OR), being strongly dependent on all experimental conditions. In contrast, the early stage is dominated by adding monomers to nanocrystals, which may be dramatically accelerated by lowering precursor concentrations and using ligands with a molecular structure similar to that of thioglycolic acid (TGA). This fast growth stage is similar to that observed during organometallic growth of nanocrystals in hot organic media. On the basis of this finding, one-dimensional wurtzite CdTe nanostructures can be directly prepared in aqueous media by storing rather dilute precursor solution (2.4 mM with reference to ligand) in the presence of TGA at lower temperature (from room temperature to 80 degrees C). A low growth temperature is used to suppress OR during crystal growth. In addition, the simultaneous presence of both TGA-like ligand and 1-thioglycerol or 2-mercaptoethylamine leads to formation of colloidally stable 1D CdTe nanostructures with controlled aspect ratios.

ZnO-Based Hollow Microspheres: Biopolymer-Assisted Assemblies from ZnO Nanorods

Many efforts have been made in fabricating three-dimensional (3D) ordered zinc oxide (ZnO) nanostructures due to their growing applications in separations, sensors, catalysis, bioscience, and photonics. Here, we developed a new synthetic route to 3D ZnO-based hollow microspheres by a facile solution-based method through a water-soluble biopolymer (sodium alginate) assisted assembly from ZnO nanorods. The products were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and X-ray photoelectron spectroscopy. Raman and photoluminescence spectra of the ZnO-based hollow microspheres were obtained at room temperature to investigate their optical properties. The hollow microspheres exhibit exciting emission features with a wide band covering nearly all the visible region. The calculated CIE (Commission Internationale d'Eclairage) coordinates are 0.24 and 0.31, which fall at the edge of the white region (the 1931 CIE diagram). A possible growth mechanism of the 3D ZnO superstructures based on typical biopolymer-crystal interactions in aqueous solution is tentatively proposed, which might be really interesting because of the participation of the biopolymer. The results show that this biopolymer-directed crystal growth and mediated self-assembly of nanocrystals may provide promising routes to rational synthesis of various ordered inorganic and inorganic-organic hybrid materials with complex form and structural specialization.

Preparation of ZnO colloids by aggregation of the nanocrystal subunits

Colloidal ZnO particles with narrow size distribution were prepared via a sol-gel process by base-catalyzed hydrolysis of zinc acetate. The morphology of ordered arrays of the particles was recorded by SEM. SEM also reveals that these uniform particles were composed of tiny ZnO subunits (singlets) sized of several nanometers. The size of the singlets, which is confirmed by X-ray diffraction and UV-vis absorption spectra, increases as the aging time is prolonged. The size-selective formation of colloids by aggregation of nanosized subunits is proposed to consist of two-stage growth by nucleation of nanosized crystalline primary particles and their subsequent aggregation into polycrystalline secondary colloids. The aggregates are all spherical because the internal rearrangement processes are fast enough. The ZnO colloids, i.e., the aggregates, tend to self-assemble into well-ordered hexagonal close-packed structures. Room-temperature photoluminescence was characterized for green and aged ZnO.

Controlled organization of ZnO building blocks into complex nanostructures

ZnO complex nanostructure with special mushroom-like morphology was prepared by hydrolysis of zinc acetate dehydrate (Zn(CH3COO)2 2H2O) in water-methanol mixed solvent at 60 degrees C. The formation mechanism was studied using XRD investigation and FE-SEM observation, which showed that the mushroom-like particles were transformed from cauliflower-like layered basic zinc acetate (LBZA), Zn5(OH)8(CH3COO)22H2O, and composed of ZnO subunits with average size less than 10 nm. The introduction of hexamethylenetetramine (HMTA, C6H12N4) to the solution before deposition led to drastic changes in the morphologies of both aggregation particles and ZnO subunits. The novel ZnO microspheres, which were made of regular hexagonal plate-like ZnO with dimensional size 35 x 10 nm, were formed. These hexagonal plate-like ZnO subunits stacked very compactly and aligned regularly. Kinetic study of this unique complex nanostructure using TEM and FE-SEM observation showed the presence of HMTA played an important role on the formation of hexagonal ZnO subunits through different mechanisms related to the different parts of microspheres.

Mechanistic studies on the conversion of dicobalt octacarbonyl into colloidal cobalt nanoparticles

In situ ATR-FTIR monitoring has allowed the direct study of the effect of additives (trioctylphosphine oxide [TOPO] and oleic acid) on the kinetics and rate of the thermal decomposition of dicobalt octacarbonyl leading to the formation of colloidal cobalt nanoparticles (CoCNPs). The study has shown that additives usually considered as simple surfactants influence the rate and kinetics of the decomposition of dicobalt octacarbonyl. Several of the initial intermediates connecting Co2(CO)8 with CoCNPs have been identified, and a tentative mechanism for the formation of the colloidal nanoparticles has been proposed.

Insulin particle formation in supersaturated aqueous solutions of poly(ethylene glycol)

Protein microspheres are of particular utility in the field of drug delivery. A novel, completely aqueous, process of microsphere fabrication has been devised based on controlled phase separation of protein from water-soluble polymers such as polyethylene glycols. The fabrication process results in the formation of spherical microparticles with narrow particle size distributions. Cooling of preheated human insulin-poly(ethylene glycol)-water solutions results in the facile formation of insulin particles. To map out the supersaturation conditions conducive to particle nucleation and growth, we determined the temperature- and concentration-dependent boundaries of an equilibrium liquid-solid phase separation. The kinetics of formation of microspheres were followed by dynamic and continuous-angle static light scattering techniques. The presence of PEG at a pH that was close to the protein's isoelectric point resulted in rapid nucleation and growth. The time elapsed from the moment of creation of a supersaturated solution and the detection of a solid phase in the system (the induction period, t(ind)) ranged from tens to several hundreds of seconds. The dependence of t(ind) on supersaturation could be described within the framework of classical nucleation theory, with the time needed for the formation of a critical nucleus (size <10 nm) being much longer than the time of the onset of particle growth. The growth was limited by cluster diffusion kinetics. The interfacial energies of the insulin particles were determined to be 3.2-3.4 and 2.2 mJ/m(2) at equilibrium temperatures of 25 and 37 degrees C, respectively. The insulin particles formed as a result of the process were monodisperse and uniformly spherical, in clear distinction to previously reported processes of microcrystalline insulin particle formation.

Synthesis and characterization of large colloidal cobalt particles

Large colloidal environmentally stable silica-coated cobalt particles were synthesized by combining the sodium borohydride reduction in aqueous solution and the Stöber method. Low size polydisperse cobalt spheres with an average size of 95 nm were synthesized by using a borohydride reduction method and were subsequently coated with a thin layer of silica by means of hydrolysis and condensation of tetraethylorothosilicate (TEOS) in an aqueous/ethanolic solution. The large uniform cobalt spheres consist of smaller metallic Co clusters, explaining the superparamagnetic behavior of the spheres. The particles were investigated by transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM).

Observations on clear solution silicalite-1 growth by nanoslabs

Several research groups have reported the presence of nanometer-sized particles (nanoslabs) in clear solutions, which precipitate the crystalline MFI (ZSM-5) structure. Debate about the growth mechanism for Al-free ZSM-5 (silicalite-1) has revolved around growth by small silicate units (monomers, dimers, etc.) from solution vs growth by nanoslab addition. A model developed for precipitation of uniform sized colloids by addition of sub-colloidal precursor units has been adapted for this zeolite synthesis system. Parameter values were adjusted for the simulation results to match experimental observations from work reported previously, at least to the extent possible. The model involved the simultaneous solution of up to 6000 ordinary differential equations, and required computation times of up to 24 h. The results shed light on the crystal growth mechanism, but pose questions for further investigations of the nucleation mechanism.