Other Methods

There are many other types of the physical-chemical methods for preparing the nanocapsules, among which emulsification evaporation, solvent displacement, salting out, emulsification diffusion, pyrolysis of polymer materials, nitridation of nanoparticles, and various self-assembly and templating approaches are very convenient tools. Exposing the nanoparticles to polyelectrolyte solutions of opposite charge, the polyelectrolytes are deposited in a layer-by-layer sequence to form nanocapsules. The y radiolysis method can be applied by optimizing various conditions like metal ion concentration, polymer or surfactant concentration, and pH. Some physical-chemical methods utilize the applied electric and magnetic fields and microwave plasma to affect the chemical process, like growth of colloid nanoparticles, etc. Photochemical generation, the electrochemical procedure, the pulsed laser vaporization-controlled condensation technique, controlled chemical passivation of extremely active particles, and the explosion method are all applicable for synthesizing nanomaterials such as clusters, onions, intercalations, nanopolyhedra, nanotubes and nanocapsules, etc.

The techniques available to prepare biodegradable nano-particles, nanospheres, and nanocapsules from preformed polymers were reviewed [489]. Four techniques were discussed in terms of their technological advantages and drawbacks: emulsification evaporation, solvent displacement, salting-out, and emulsification diffusion. The mechanism of nanoparticle formation for each technique was proposed from a physicochemical perspective.

General aspects of nanocapsules used in drug delivery systems were reviewed from both a physicochemical and a therapeutic point of view [490]. The preparation methods and ways to characterize the size, surface, density, release, and stability of this o/w system were described, and the influence of encapsulation within nanocapsules on the biological activity of numerous drugs was discussed.

Carbon nanocapsules with SiC nanoparticles were produced by thermal decomposition of polyvinyl alcohol with SiC clusters at 500 °C in Ar gas atmosphere [491]. Carbon hollow structures such as nanoparticles, polyhedra, and clusters formed. The work indicated that the pyrolysis of polymer materials with clusters is a useful fabrication method for the mass production of carbon nanocapsules at low temperatures compared to the ordinary arc discharge method.

SiC nanorods were synthesized through carbothermal reduction of SiO by C nanocapsules or amorphous activated C [492]. The nanorods formed during a reaction between SiO and C nanocapsules were straight. The nanorods formed during reaction between SiO and amorphous activated C consisted of straight and curled parts or chains of SiC nanoparticles. A surface of the rod tip was covered with a thin amorphous layer of 3 nm thickness.

Encapsulation of cobalt oxide nanoparticles and Ar in boron nitride nanocapsules was reported [493]. A large quantity of boron nitride nanocapsules with cobalt nano-particles was fabricated by nitridation of boron and cobalt nanoparticles with a H2/NH3 gas mixture [494]. Cobalt nano-particles were encapsulated by boron nitride layers with thickness of 5 nm. Ferromagnetism and luminescence at 3.8 eV were due to separation of Co nanoparticles by the BN layers. The nitridation of boron particles with catalytic metal is a useful fabrication method for mass production of BN nanocapsules at low temperatures.

Carbon and boron nitride fullerene materials (clusters, onions, intercalations, nanopolyhedra, nanotubes, and nano-capsules) were synthesized by polymer pyrolysis, chemical reaction, arc melting, and electron-beam irradiation [495]. Fullerene clusters and atomic clouds (atom hopping) were formed on the surface of the C and BN fullerne materials. They provided angular and spherical nanocage structures, consisting of four-, five-, six-, and seven-membered ring bondings. A guideline for designing the C and BN fullerene materials, which may have various atomic structures and properties, was summarized.

A selection of graphitic materials of both scientific as well as commercial importance was modified by deposition of various metals at very low coverages under overpotential or underpotential conditions [496].

A thermodynamic theory for the deliquescence behavior of soluble crystals in an atmosphere of solvent vapor was developed [497]. The free energy barriers that could impede deliquescence were studied [497]. The integral equation theory was applied to study the solvent-induced potential of mean force between two passivated nanoparticles in dilute solution [498]. This approach explicitly accounted for the molecular structure of the solvent and the anisotropy of its density profile induced by the pair of nanoparticles.

Controlling the surface properties of nanoparticulate materials is necessary if they are to be exploited in applications such as colloidal crystals or biolabeling. By tailoring the polymer flexibility and the electrostatic forces involved in polyelectrolyte adsorption onto highly curved gold surfaces, through variation of the total salt concentrations suspending the chains and spheres, irreversible polyelectrolyte wrapping of gold nanoparticles was affected [499]. By consecutively exposing the nanoparticles to polyelectrolyte solutions of opposite charge, polyelectrolytes were deposited in a layer-by-layer sequence, yielding gold nanoparticles coated with uniform polyelectrolyte multilayers. Self-supporting poly-electrolyte multilayered nanocapsules were formed after dissolution of the metallic core.

Polymer nanocapsules were reviewed [500]. Hollow polymer particles with dimensions in the submicrometer region possess great potential for encapsulation of large quantities of large sized guest molecules into their empty core domains. Various self-assembly and templating approaches were introduced.

Whiskers of iron carbide encased in carbon shells were prepared from Fe2+ phthalocyanine [501]. Solid FePc was oxygenated and then thermally treated under H2 flow. With proper control of conditions, Fe3C free from metallic iron was synthesized. The iron carbide whiskers ranged in length from 300 to 500 nm, and their widths were approximately 100 nm. The carbon coating was uniform and about 2.7 nm wide.

The immobilization of nanometer-sized and cluster metal particles in polymeric matrices was systematized for both macroligands and the matrices formed in-situ [502]. Special attention was paid to the controlled chemical passivation (stabilisation) of extremely active particles of colloidal size using high-molecular-weight compounds. The routes of formation of polymer-immobilized nanoparticles directly in the polymer medium were analyzed. Studies in the field of polymer-analogous transformations, which offered a promising approach to the binding of mono- and heterometallic clusters as a new direction in the physicochemistry of nano-particles, were discussed.

Three different examples were presented of nanocrys-tals prepared by the pulsed laser vaporization-controlled condensation technique [503]. The photoluminescence properties of the Si nanocrystals were represented. A pho-toreduction of the white WO3 nanoparticles into the blue W2O5 followed the irradiation of the particles with the second harmonic of the Nd:YAG laser at 532 nm.

Capped copper nanoclusters were synthesized by the y radiolysis method by optimizing various conditions like metal ion concentration, polymer or surfactant concentration, and pH [504]. The increasing amount of capping agent was responsible for the decrease in size as small as 17 nm of the metal clusters.

An electrochemical procedure, based on the dissolution of a metallic anode in an aprotic solvent, was used to obtain silver nanoparticles ranging from 2 to 7 nm [505]. The influence of the different electrochemical parameters on the final size was studied by using different kinds of counterelec-trodes.

Gold sol, photochemically prepared in aqueous Triton X-100 (TX-100) medium, was discolored by photoirradiation in the presence of KBrO3 and any one of the following bromine containing trihalomethanes, such as bro-moform (CHBr3) chlorodibromomethane (CHClBr2), and bromodichloromethane (CHCl2Br) owing to the dissolution of colloidal gold nanoparticles [506]. Pt nanoparticles supported in nanoporous Al2O3 catalyst were prepared by reduction of K2PtCl4 solution using 112 solution in the presence of Al2O3 and poly(acrylic acid) as capping material [507].

Effects of applied electric and magnetic fields on the 2D growth of colloid nanoparticles were studied. Two-dimensional photochemical generation of amorphous iron-containing magnetic nanoparticles was carried out via ultraviolet decomposition of a volatile organometallic compound, iron pentacarbonyl, in a mixed Langmuir monolayer at the gas/water interface with stearic acid as a surfactant matrix [508]. During the formation of nanoparticles, the monolayer was in the 2D gas phase state (at very low or no surface pressure). Controlling morphology of nanostruc-tures by synthesis under applied fields could be a promising approach for nanophase engineering and nanotechnology.

The arc-discharge method was modified to produce nano-phase Ni encapsulated in graphite shells [509, 510]. The carbon-encapsulated metal or metal carbide was prepared based on high-temperature (~1800 °C) treatment of micro-porous carbon materials impregnated with metal precursors. The microwave plasma enhanced CVD system was used to synthesize carbon-encapsulated metal nanoparticles on silicon wafer. An explosion method was developed to prepare the nanocapsules [513-515].

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