Encapsulation, particle nanoparticles

The mechanical incorporation of active nanoparticles into the silica pore structure is very promising for the general synthesis of supported catalysts, although particles larger than the support s pore diameter cannot be incorporated into the mesopore structure. To overcome this limitation, pre-defined Pt particles were mixed with silica precursors, and the mesoporous silica structures were grown by a hydrothermal method. This process is referred to as nanoparticle encapsulation (NE) (Scheme 2) [16] because the resulting silica encapsulates metal nanoparticles inside the pore structure. 

The approach for preparing dendrimer-encapsulated Pt metal particles is similar to that used for preparation of the Cu composites chemical reduction of an aqueous solution of G4-OH(Pt +)n yields dendrimer-encapsulated Pt nanoparticles (G4-OH(Ptn)). A spectrum of G4-OH(Pt6o) is shown in Fig. 12 a it displays a much higher absorbance than G4-OH(Pt +)6o throughout the wavelength range displayed. This change results from the interband transition of the encapsulated zero-valent Pt metal particles. 

Spectra of G4-OH(Pt)n, n= 12, 40, and 60, obtained between 280 nm and 700 nm and normalized to A = 1 at A = 450 nm, are shown in Fig. 12 b all of these spectra display the interband transition of Pt nanoparticles. Control experiments clearly demonstrate that the Pt clusters are sequestered within the G4-OH dendrimer. For example, BH4 reduction of the previously described G4-NH2(Pt +)n emulsions results in immediate precipitation of large Pt clusters. Importantly, the dendrimer-encapsulated particles do not agglomerate for up to 150 days and they redissolve in solvent after repeated solvation/drying cycles. 

The absorbance intensity of the encapsulated Pt nanoparticles is related to the particle size. A plot of log A vs log A provides qualitative information about particle size the negative slopes are known to decrease with increasing particle size. For aqueous solutions of G4-OH(Pti2), G4-OH(Pt4o), and G4-OH(Pt6o)> the... 

Although it is not possible to prepare Ag particles inside Gn-OH by direct reduction of interior ions, stable, dendrimer-encapsulated Ag particles can be prepared by a metal exchange reaction. In this approach, dendrimer-encapsulated Cu nanoclusters are prepared as described in a previous section [82], and then upon exposure to Ag+ the Cu particles oxidize to Cu + ions, which stay entrapped within the dendrimer at pH values higher than 5.5, and Ag+ is reduced to yield a dendrimer-encapsulated Ag nanoparticle (Fig. 15). 

Two classes of catalysts account for most contemporary research. The first class includes transition-metal nanoparticles (e.g., Pd, Pt), their oxides (e.g., RUO2), and bimetallic materials (e.g., Pt/Ni, Pt/Ru) [104,132-134]. The second class, usually referred to as molecular catalysts, includes all transition-metal complexes, such as metalloporphyrins, in which the metal centers can assume multiple oxidation states [ 135 -137]. Previous studies have not only yielded a wealth of information about the preparation and catalytic properties of these materials, but they have also revealed shortcomings where further research is needed. Here we summarize the main barriers to progress in the field of metal-particle-based catalysis and discuss how dendrimer-encapsulated metal nanoparticles might provide a means for addressing some of the problems. 

Indeed, recent results from our laboratory indicate that dendrimer-encapsulated CdS QDs can be prepared by either of two methods [192]. The first approach is analogous to the methodology described earlier for preparing dendrimer-encapsulated metal particles. First, Cd and S salts are added to an aqueous or methanolic PAMAM dendrimer solution. This yields a mixture of intradendrimer (templated) and interdendrimer particles. The smaller, dendrimer-encapsulated nanoparticles may then be separated via size-selective photo etching [193], dendrimer modification and extraction into a nonpolar phase [19], or by washing with solvent in which the dendrimer-encapsulated particles are preferentially soluble. An alternative, higher-yield method relies on sequential addition of very small aliquots of Cd + and S " to alcoholic dendrimer solutions. 

The development of dendrimer-encapsulated bimetallic nanoparticles has provided the interface with heterogeneous colloid catalysis, which is reviewed by B. Chandler and J. Gilbertson. In this field, dendrimers are being used to template and stabilize reduced colloidal particles in solution. The emphasis is placed on bimetallic particles and their application in a variety of fundamental catalytic transformations. 

Particulates are commonly classified into micro- and nanoparticles based on the size of the particles. Nanoparticles are colloidal particles ranging from 10 to 1,000 run, in which drag may be entrapped, encapsulated, and/or absorbed. Microparticulates are drag-containing small polymeric particles (erodible, non-erodible or ion-exchange resins) within the size of 1-10 /on, which are suspended in a liquid carrier medium. 

Submicronic particles (nanoparticles) produced in that way allowed the encapsulation of the griseofulvin antifungal substance up to the 1 1 stoichiometry with respect to the P-cyclodextrin. A simple oil-in-water emulsion could not achieve such encapsulation because P-cyclodextrin was not soluble in common oils this was shown with silicone oil in the present study. The spontaneous emulsification process used for the preparation of the particles also allowed the easy encapsulation of the sparingly soluble griseofulvin. 

White and co-workers [589] have deposited Mo nanoparticles onto an Au(lll) surface. AES and TPD studies showed that bare Mo nanoparticles were very reactive and could cause complete dissociation of hydrogen sulphide, methyl mercaptan, and thiophene. However, the presence of An atoms on the Mo nanoparticles modified their reactivity. In the case of H2S and CH3SH, the overall activity for desulphurisation was unaffected by An encapsulation but the selectivity to form methane from CH3SH increased from 20% on bare Mo particles to 60% on Au-covered Mo particles. In contrast, Au-encapsulated Mo nanoparticles are relatively inert towards the dissociation of thiophene. 

We demonstrated that a naturally derived polysaccharide, chitosan, is capable of forming composite nanoparticles with silica. For encapsulated particles, we used silicification and biosilicification to encapsulate curcumin and analyzed the physicochemical properties of curcumin nanoparticles. It proved that encapsulated curcumin nanoparticles enhanced stability toward ultraviolet (UV) irradiation, antioxidation and antitumor activity, enhanced/added function, solubility, bioactivities/ bioavailability, and control release and overcame the immunobarrier. We present an in vitro study that examined the cytotoxicity of amorphous and composite silica nanoparticles to different cell lines. These bioactives include curcumin mdAntrodia cinnamomea. It is hoped that by examining the response of multiple cell lines to silica nanoparticles more basic information regarding the cytotoxicity as well as potential functions of silica in future oncological applications could become available. 

Polymer-stabilized palladium nanoparticles (or nanoclusters) [125-127] have recently received increasing attention in the field of synthetic organic chemistry [128, 129]. Thus, for example, the poly(iV-vinyl-2-pyrrolidone) (PVP)-supported Pd particle catalyzed the Suzuki-Miyaura coupling in water [130]. Poly(amidoamine) (PAMAM) dendrimer-encapsulated palladium nanoparticles were designed and prepared to provide highly selective catalysts for hydrogenation of olefins [131-133]. Hyperbranched aromatic amides (aramids) and PS-DVB-methacryloylethylenesulfonic acid resin have also been... 

Another recent report describes the large scale synthesis of ahgned carbon nanotubes, of uniform length and diameter, by passage of acetylene over iron nanoparticles embedded in mesoporous silica [107]. The latter two methods, based on the pyrolysis of organic precursors over templated/catalysts supports, are by far superior by comparison with plasma arcs, since other graphitic structures such as polyhedral particles, encapsulated particles and amorphous carbon are notably absent (Fig. 16). 

Nanoparticles are colloidal and submicron particles generally having 10-500 nm particle size and prepared from different polymers for drug carrier systems. Drug molecules are encapsulated in nanoparticles by several methods or adsorbed on the surface of nanoparticles by electrostatic interactions. " ... 

Figure 5.20 (A) Structures of AIE-active TPE unit modified traditional FBI dye and the polymer matrix DSPE-PEG2000 and DSPE-PEG5000-folate. (B) Photoluminescence properties of BTPEPBI in THF-water mixtures with different water fraction (, ) excited at 538 nm. (C) Particle size of the BTPEPBI-encapsulated organic nanoparticles. (D) In vitro cell imaging stained by BTPEPBI-NPO (left) and BTPEPBI-NP50 (right) for 2 h at 37 C (E) in vivo FL imaging of H22 tumor-bearing (red circle) mice after intravenous injection of BTPEPBI-NPO (left) and BTPEPBI-NP50 (right) respectively. Adapted from ref. 79 with permission from the Royal Society of Chemistry.

One merit of special attention in this chapter is the fact that sol-gel synthesis offers a convenient method for hosting chemical reactions, a process which is not possible using other synthesis techniques. Typically, in sol-gel encapsulation, silica nanoparticles surround the captive molecules during gel formation. In principle, the sol-gel process can be considered as a phase separation by sol-reactions, sol-gelation and finally, removal of the solvent resulting in a ceramic material Depending on the preparation, dense oxide particles or polymeric clusters will be obtained [16]. 

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