Growth of textured thin Au coatings on iron oxide nanoparticles with near infrared absorbance

A homologous series of Au coated iron oxide nanoparticles with hydrodynamic diameters smaller than 60 nm was synthesized with very low Au-to-iron mass ratios, as low as 0.15. The hydrodynamic diameter was determined by dynamic light scattering and the composition by atomic absorption spectroscopy and energy dispersive x-ray spectroscopy. Unusually low Au precursor supersaturation levels were utilized to nucleate and grow Au coatings on iron oxide relative to the formation of pure Au nanoparticles. This approach produced unusually thin coatings by lowering autocatalytic growth of Au on Au, as shown by transmission electron microscopy. Nearly all of the nanoparticles were attracted by a magnet, indicating a minimal number of pure Au particles. The coatings were sufficiently thin to shift the surface plasmon resonance to the near infrared with large extinction coefficients, despite the small particle hydrodynamic diameters observed from dynamic light scattering to be less than 60 nm.

Enhanced pulsed magneto-motive ultrasound imaging using superparamagnetic nanoclusters

Recently, pulsed magneto-motive ultrasound (pMMUS) imaging augmented with ultra-small magnetic nanoparticles has been introduced as a tool capable of imaging events at molecular and cellular levels. The sensitivity of a pMMUS system depends on several parameters, including the size, geometry and magnetic properties of the nanoparticles. Under the same magnetic field, larger magnetic nanostructures experience a stronger magnetic force and produce larger displacement, thus improving the sensitivity and signal-to-noise ratio (SNR) of pMMUS imaging. Unfortunately, large magnetic iron-oxide nanoparticles are typically ferromagnetic and thus are very difficult to stabilize against colloidal aggregation. In the current study we demonstrate improvement of pMMUS image quality by using large size superparamagnetic nanoclusters characterized by strong magnetization per particle. Water-soluble magnetic nanoclusters of two sizes (15 and 55 nm average size) were synthesized from 3 nm iron precursors in the presence of citrate capping ligand. The size distribution of synthesized nanoclusters and individual nanoparticles was characterized using dynamic light scattering (DLS) analysis and transmission electron microscopy (TEM). Tissue mimicking phantoms containing single nanoparticles and two sizes of nanoclusters were imaged using a custom-built pMMUS imaging system. While the magnetic properties of citrate-coated nanoclusters are identical to those of superparamagnetic nanoparticles, the magneto-motive signal detected from nanoclusters is larger, i.e. the same magnetic field produced larger magnetically induced displacement. Therefore, our study demonstrates that clusters of superparamagnetic nanoparticles result in pMMUS images with higher contrast and SNR.

Catalysis in supercritical CO2 using dendrimer-encapsulated palladium nanoparticles

Dendrimer-encapsulated nanoparticles are shown to be versatile catalysts for both the hydrogenation of styrene and Heck heterocoupling of iodobenzene and methacrylate in supercritical CO2 (scCO2).

Solution-based particle formation of pharmaceutical powders by supercritical or compressed fluid CO2 and cryogenic spray-freezing technologies

Micronization is an important procedure used in the pharmaceutical industry to reduce the particle size of active pharmaceutical ingredients (APIs). The spray-drying and milling techniques presently used to micronize drug substances cannot be used to process thermolabile or physically unstable drug substances. Therefore, new micronization techniques, including particle precipitation with supercritical or compressed fluid CO2 and spray-freezing of drug solutions and suspensions into cryogenic gas to produce solid frozen microparticles, are currently being perfected for future use in the pharmaceutical industry. This review highlights the compressed gas and cryogenic liquid technologies being developed as potential solution-based particle formation technologies for drugs that cannot be processed by conventional micronization techniques.

Synthesis of organic monolayer-stabilized copper nanocrystals in supercritical water

When water is heated and pressurized above the critical point, it becomes a suitable solvent to employ organic capping ligands to control and stabilize the synthesis of nanocrystals. Without alkanethiol ligands, Cu(NO(3))(2) hydrolyzes to form polydisperse copper(II) oxide particles with diameters from 10 to 35 nm. However, in the presence of 1-hexanethiol, X-ray photoelectron spectroscopy, selected area electron diffraction, and transmission electron microscopy reveal the formation of copper nanocrystals approximately 7 nm in diameter. The use of a different precursor, Cu(CH(3)COO)(2), leads to particles with significantly different morphologies. A mechanism is proposed for sterically stabilized nanocrystal growth in supercritical water that describes competing pathways of hydrolysis to large oxidized copper particles versus ligand exchange and arrested growth by thiols to produce small monodisperse Cu nanoparticles.

Highly luminescent silicon nanocrystals with discrete optical transitions

A new synthetic method was developed to produce robust, highly crystalline, organic-monolayer passivated silicon (Si) nanocrystals in a supercritical fluid. By thermally degrading the Si precursor, diphenylsilane, in the presence of octanol at 500 degrees C and 345 bar, relatively size-monodisperse sterically stabilized Si nanocrystals ranging from 15 to 40 A in diameter could be obtained in significant quantities. Octanol binds to the Si nanocrystal surface through an alkoxide linkage and provides steric stabilization through the hydrocarbon chain. The absorbance and photoluminescence excitation (PLE) spectra of the nanocrystals exhibit a significant blue shift in optical properties from the bulk band gap energy of 1.2 eV due to quantum confinement effects. The stable Si clusters show efficient blue (15 A) or green (25-40 A) band-edge photoemission with luminescence quantum yields up to 23% at room temperature, and electronic structure characteristic of a predominantly indirect transition, despite the extremely small particle size. The smallest nanocrystals, 15 A in diameter, exhibit discrete optical transitions, characteristic of quantum confinement effects for crystalline nanocrystals with a narrow size distribution.

Steric Stabilization of Colloids by Poly(dimethylsiloxane) in Carbon Dioxide: Effect of Cosolvents

Steric stabilization and flocculation of colloids with surface-grafted poly(dimethylsiloxane) (PDMS) chains are examined in liquid and supercritical carbon dioxide with and without hexane as a cosolvent. Neither poly(methyl methacrylate) (PMMA) nor silica particles with grafted 10,000 g/mol PDMS could be stabilized in pure CO(2) at pressures up to 345 bar at 25 degrees C and 517 bar at 65 degrees C without stirring. The addition of 15 wt% hexane to CO(2) led to stable dispersions with sedimentation velocities of 0.2 mm/min for 1-2 µm PMMA particles. The critical flocculation pressure of the colloids in the hexane/CO(2) mixture, determined from turbidity versus time measurements, was found to be the same for silica and PMMA particles and was well above the upper critical solution pressure for the PDMS-CO(2) system. The addition of a nonreactive cosolvent, hexane, eliminates flocculation of PMMA particles synthesized through dispersion polymerization in CO(2) with PDMS-based surfactants. Copyright 2000 Academic Press.

Rapid expansion from supercritical to aqueous solution to produce submicron suspensions of water-insoluble drugs

Stable suspensions of submicron particles of cyclosporine, a water-insoluble drug, have been produced by rapid expansion from supercritical to aqueous solution (RESAS). To minimize growth of the cyclosporine particles, which would otherwise occur in the free jet expansion, the solution was sprayed into an aqueous Tween-80 (Polysorbate-80) solution. Steric stabilization by the surfactant impedes particle growth and agglomeration. The particles were an order of magnitude smaller than those produced by RESS into air without the surfactant solution. Concentrations as high as 38 mg/mL for 400-700 nm particles were achieved in a 5.0% (w/w) Tween-80 solution.

Control of thickness and orientation of solution-grown silicon nanowires

Bulk quantities of defect-free silicon (Si) nanowires with nearly uniform diameters ranging from 40 to 50 angstroms were grown to a length of several micrometers with a supercritical fluid solution-phase approach. Alkanethiol-coated gold nanocrystals (25 angstroms in diameter) were used as uniform seeds to direct one-dimensional Si crystallization in a solvent heated and pressurized above its critical point. The orientation of the Si nanowires produced with this method could be controlled with reaction pressure. Visible photoluminescence due to quantum confinement effects was observed, as were discrete optical transitions in the ultraviolet-visible absorbance spectra.

Encapsulation of lysozyme in a biodegradable polymer by precipitation with a vapor-over-liquid antisolvent

Lysozyme was encapsulated in biodegradable polymer microspheres which were precipitated from an organic solution by spraying the solution into carbon dioxide. The polymer, either poly(l-lactide) (l-PLA) or poly(DL-lactide-co-glycolide) (PGLA), in dichloromethane solution with suspended lysozyme was sprayed into a CO2 vapor phase through a capillary nozzle to form droplets which solidified after falling into a CO2 liquid phase. By delaying precipitation in the vapor phase, the primary particles became sufficiently large, from 5 to 70 microm, such that they could encapsulate the lysozyme. At an optimal temperature of -20 degrees C, the polymer solution mixed rapidly with CO2, and the precipitated primary particles were sufficiently hard such that agglomeration was markedly reduced compared with higher temperatures. More uniform particles were formed by flowing CO2 at high velocity in a coaxial nozzle to mix the droplets at the CO2 vapor-liquid interface. This process offers a means to produce encapsulated proteins in poly(DL-lactide-co-glycolide) microspheres without earlier limitations of massive polymer agglomeration and limited protein solubility in organic solvents.

Polymeric microspheres prepared by spraying into compressed carbon dioxide

PURPOSE

The objective was to prepare polymeric microparticles by atomizing organic polymer solutions into a spray chamber containing compressed CO2 (PCA-process) and to study the influence of various process parameters on their morphological characteristics.

METHOD

The swelling of various pharmaceutically acceptable polymers [ethyl cellulose, poly(methyl methacrylate), poly(epsilon-caprolactone), poly(dl-lactide), poly(l-lactide) and poly(dl-lactide-glycolide) copolymers] in CO2 was investigated in order to find polymers which did not agglomerate during the spraying process. Poly(l-lactide) (L-PLA) microparticles were prepared by spraying the organic polymer solution into CO2 in a specially designed spraying apparatus. The effect of various process (pressure and temperature of the CO2 phase, flow rate) and formulation (polymer concentration) variables on the morphology and particle size of L-PLA-microparticles was investigated.

RESULTS

Polymers with low glass transition temperatures agglomerated even at low temperatures. The formation of microparticles was favored at moderate temperatures, low polymer concentrations, high pressures and high flow rates of CO2. High polymer concentrations and low flow rates resulted in the formation of polymeric fibers. Colloidal L-PLA particles could also be prepared with this technique in a surfactant-free environment. Initial studies on the microencapsulation of drugs resulted in low encapsulation efficiencies.

CONCLUSIONS

The PCA method is a promising technique for the preparation of drug-containing microparticles. Potential advantages of this method include the flexibility of preparing microparticles of different size and morphology, the elimination of surfactants, the minimization of residual organic solvents, low to moderate processing temperatures and the potential for scale-up.