Surface-nanopartide stabilizers

Transition-metal nanopartides are of fundamental interest and technological importance because of their applications to catalysis [22,104-107]. Synthetic routes to metal nanopartides include evaporation and condensation, and chemical or electrochemical reduction of metal salts in the presence of stabilizers [104,105,108-110]. The purpose of the stabilizers, which include polymers, ligands, and surfactants, is to control particle size and prevent agglomeration. However, stabilizers also passivate cluster surfaces. For some applications, such as catalysis, it is desirable to prepare small, stable, but not-fully-passivated, particles so that substrates can access the encapsulated clusters. Another promising method for preparing clusters and colloids involves the use of templates, such as reverse micelles [111,112] and porous membranes [106,113,114]. However, even this approach results in at least partial passivation and mass transfer limitations unless the template is removed. Unfortunately, removal of the template may re-... 

Reaction of the sandwich-type POM [(Fc(0H2)2)j(A-a-PW9034)2 9 with a colloidal suspension of silica/alumina nanopartides ((Si/A102)Cl) resulted in the production of a novel supported POM catalyst [146-148]. In this case, about 58 POM molecules per cationic silica/alumina nanoparticle were electrostatically stabilized on the surface. The aerobic oxidation of 2-chloroethyl ethyl sulfide (mustard simulant) to the corresponding harmless sulfoxide proceeded efficiently in the presence of the heterogeneous catalyst and the catalytic activity of the heterogeneous catalyst was much higher than that of the parent POM. In addition, this catalytic activity was much enhanced when binary cupric triflate and nitrate [Cu(OTf)2/Cu(N03)2 = 1.5] were also present [148],... 

Presently, the scientific community is making every effort to study irmovative catalytic materials that combine morphological features suited to a fast counterdiffusion of PO, surface properties designed to favor the desorption of PO and limited side-reactions deriving from acid or base-catalyzed reactions - especially those leading to heavy product accumulation - as well as to stabilizing metal nanopartides. 

The main common characteristic of colloidal particles is their small size (typically 1 to 10 nm). The size of nanopartides in solution is dynamic and continuous redistribution in size can occur. In most cases, agglomeration leads to the formation of less active larger metal partides and this process may end in predpitation of larger crystals (palladium black). The per-atom catalytic efSdency of metal par-tides increases as the partide size decreases however, the probability of colloid agglomeration increases as their size decreases. To prevent agglomeration (and aggregation), and to preserve the finely dispersed state of the original partides, colloids are often prepared in the presence of stabilizers that adsorb onto the partide surface. 

Reetz et al. reported on catalytically active solvent-stabilized colloids in propylene carbonate, which were prepared electrochemically or by thermal decomposition of [Pd(OAc)2 assisted by ultrasound. The colloidal particles had sizes of 8 to 10 nm, as determined by TEM. After addition of aryl bromide, styrene, and base to the colloid solution, satisfactory conversions were obtained within reaction times of 5-20 h. Isolation of the particles stabilized by propylene carbonate was not possible, however [16]. The same authors also reported Suzuki and Heck reactions with electrochemically prepared Pd or Pd/Ni colloids stabilized by tetraalkylammonium, as well as polyvinylpyrrolidone (PVP)-stabilized palladium colloids prepared by hydrogen reduction (Table 1) [17]. It was assumed that the reaction occurs on the nanopartide surfaces. 

The monolayers described so far have all been grown by self-assembly processes, where ligand-stabilized metal nanopartides are deposited on innocent surfaces from solution, followed by a more or less rapid evaporation of the solvent. Depending on the concentration and other experimental conditions, those procedures may occasionally also result in multilayers, or even in small 3-D microcrystals. Self-assembly processes can, however, also be supported by chemical, electrochemical, magnetic, or mechanical effects some examples of these are provided in the following subsections. 

Many of the features discussed above for PEG-stabUized nanopartides containing pDNA also exist with nanopartides carrying other types of nudeic adds. Reviews on polymeric carriers of antisense oligonudeotides and siRNA and polycation-based nanopartide delivery of siRNA " are available. The PEG density and PEG MW are important parameters with short nudeic add delivery " as outlined above with the delivery of plasmids. For example, Bartlett and Davis reported that PEG of MW 5000 Da was necessary to pro-vide steric stabilization with 70 nm nanopartides carrying siRNA. With PEG of this size, the nanopartides had a PEG surface density of 43 pmol cm . 

A variety of synthetic methodologies for the preparation of metal nanopartides within a narrow size distribution are available, including impregnation,deposition/predpita-tion, sol-gel, sonochemical, microemulsion,laser ablation,and electrochemical. Often, the nanopartides are prepared by wet chemistry procediues, in which dusters of metal atoms are formed in the presence of a stabilizer, like polymers, dendrimers, microgds, surfac-... 

Recently, PPy nanopartides with the diameter of 60-90 nm were polymerized with FeCla in aqueous solutions containing PVA as a stabilizer [212]. At room temperature (RT), the polymerization of pyrrole occurred at a high rate. When the concentration of pyrrole increased, the resultant PPy nanoparticles became coarser with broadening the particle size distribution. Furthermore, the increase in concentration of PVA resulted in faster polymerization and finer PPy nanopartides. Such a phenomenon was due to the reinforcement of the structural-mechanical barrier formed by the stabilizer at the surface of the nanopartide, preventing the growth of PPy nanopartides during the polymerization process. 

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