Metal nanoparticles nanoreactors

The most intensive development of the nanoparticle area concerns the synthesis of metal particles for applications in physics or in micro/nano-electronics generally. Besides the use of physical techniques such as atom evaporation, synthetic techniques based on salt reduction or compound precipitation (oxides, sulfides, selenides, etc.) have been developed, and associated, in general, to a kinetic control of the reaction using high temperatures, slow addition of reactants, or use of micelles as nanoreactors [15-20]. Organometallic compounds have also previously been used as material precursors in high temperature decomposition processes, for example in chemical vapor deposition [21]. Metal carbonyls have been widely used as precursors of metals either in the gas phase (OMCVD for the deposition of films or nanoparticles) or in solution for the synthesis after thermal treatment [22], UV irradiation or sonolysis [23,24] of fine powders or metal nanoparticles. 

Surfactants are well known as stabilizers in the preparation of metal nanoparticles for catalysis in water. Micelles constitute interesting nanoreactors for the synthesis of controlled-size nanoparticles from metal salts due to the confinement of the particles inside the micelle cores. Aqueous colloidal solutions are then obtained which can be easily used as catalysts. 

PS-PEO reverse spherical micelles have been used as nanoreactors for the synthesis of metallic nanoparticles, as shown by the works of Moller and coworkers [102] and Bronstein et al. [103]. 

Nanoparticles consisting of noble metals have recently attracted much attention because such particles exhibit properties differing strongly from the properties of the bulk metal [1,2], Thus, such nanoparticles are interesting for their application as catalysts [3-5], sensors [6, 7], and in electronics. However, the metallic nanoparticles must be stabilized in solution to prevent aggregation. In principle, suitable carrier systems, such as microgels [8-11], dendrimers [12, 13], block copolymer micelles [14], and latex particles [15, 16], may be used as a nanoreactors in which the metal nanoparticles can be immobilized and used for the purpose at hand. 

All results reviewed herein demonstrate that the microgel particles may serve as nanoreactors for the immobilization of catalytically active nanostructures, namely for metal nanoparticles and enzymes. In both cases, the resulting composites particles are stable against coagulation and can be easily handled. Moreover, the catalytic activity of metal nanoparticles can be modulated through the volume transition that takes place within the thermosensitive microgel carrier system. Similar behavior has been also observed for the temperature dependence of enzymatic activity. Thus, the microgel particles present an active carrier system for applications in catalysis. 

Lu Y, Proch S, Schrinner M, Drechsler M, Kempe R, Ballauff M (2009) Thermosensitive core-shell microgel as a nanoreactor for catalytic active metal nanoparticles. J Mater Chem 19 3955-3961... 

In a sense, PAMAM dendrimers can be considered nanoreactors for preparing metal nanoparticles. Several synthetic methodologies are now available for... 

NANOSTRUCTURED POLYMERIC NANOREACTORS FOR METAL NANOPARTICLE FORMATION... 

A. D. Crespy, M. Stark, C. Hoffmann-Richter, U. Ziener, K. Landfester, Polymeric nanoreactors for hydrophilic reagents synthesized by interfacial polycondensation on miniemulsion droplets. Macromolecules, 2007, 40, 3122 b) O. Gazit, R. Khalfln, Y. Cohen, R. Tannenbaum, Self-assembled diblock copolymer nanoreactors as catalysts for metal nanoparticle synthesis, J. Phys. Chem. C, 2009, 113, 576. 

N. Gilvez, B. Fernandez, E. Valero, P. Sanchez, R. Cuesta, J. M. Domfnguez-Vera, Aproferritin as a nanoreactor for preparing metallic nanoparticles, C. R. Chimie, 2008, 11, 1207. 

Mesoporous pore s functionalized materials have been eonsidered as ideal nanoreactors for the deposition or growth of various guest moleeules. Among them, the introduction of metallic nanoparticles constitutes a judicious choice for preparing nanocomposite materials able to display catalytic properties that can find application as catalytic filters (Scheme 12.4). ... 

An alternative strategy, which utilizes micelle-forming amphiphilic block copolymers in the stabilization of metal nanoparticles, has been extensively studied and can be described as nanoreactors as the metal colloids are synthesized within their interior. This has enabled the formation of nanosized (l-2nm) metal colliods or clusters within polystyrene-Z -polyvinylpyridine (PS- -PVP) micellar assemblies, with diameters around 30 mn, and these... 

Microbial cellulose is found to be an optimal material for skin tissue repair due to its ability to provide a moist environment for wound healing and pain free dress changing. Unfortrmately, microbial cellulose itself has no antimicrobial activity to prevent wound infection. However the lack of antimicrobial activity of microbial cellulose is the main issue to be tackled. To improve the antimicrobial activity of microbial cellulose, researchers have introduced different materials such as benzalkonium chloride, chitosan and metallic nanoparticles into microbial cellulose. Among them metallic nanoparticles such as copper, silver [56] and ZnO [57] have been recently reported as excellent antimicrobial agents. Due to the electron-rich oxygen atoms in the microbial cellulose macromolecules and the large surface area of nanoporous microbial cellulose effective as nanoreactor, the in-situ metallization technique was successfully applied to the synthesis of Ag and microbial cellulose nanocomposite, which could in turn serve as antimicrobial skin tissue repair material. 

By employing metal 2-ethylhexanoate precursors which act as photo-reactive surfactants, de Oliveira et al. have developed new routes to metal oxide nanoparticles. This surfactant assembles into a reverse micelle in organic solvent, in effect forming a nanoreactor which promotes metal oxide nanoparticle formation within the micelle. The authors have demonstrated the synthesis of C03O4 and Bi metal nanoparticles using this route, but it is likely this clean approach could be employed to prepare a range of nanoparticles with a choice of surfactants. 

Figure 17.20 Schematic of the preparation of metal nanoparticles inside a micellar nanoreactor. (Reprinted with permission from D.M. Vriezema, M.C. Aragones, J.A.A.W. Elemans et al, Self-assembled nanoreactors, Chemical Reviews, 105, 4, 1445-1490, 2005. 2005 American Chemical Society.)...

Cellulose is a natural biopolymer, which is biodegradable, environmentally safe, widely abundant, inexpensive, and easy to handle [57]. Cellulose and its derivatives are widely used in chemical and bio-chemical applications and also as supports for the synthesis of organic molecules [58]. Interestingly, the cellulose fibers also act as a nanoreactor for the stabilization of metal nanoparticles [59]. However, its use as a support for catalytic applications is not well explored. Recently, Choplin and coworkers reported cellulose as the support for water soluble Pd(OAc>2/5 TPPTS system in the Trost-Tsuji allylic alkylation reaction [60]. To corroborate the above concept in the cross coupling of aryl halides and boronic acids, we reported A-arylation of imidazoles with aryl halides using a cellulose-supported Cu(0) catalyst (CELL-Cu(O) [61]. The prepared catalyst was well characterized using various instrumental techniques. For example, the X-ray diffraction pattern of CELL-Cu(O) catalyst clearly indicates the presence of Cu (111) and Cu (200) phases which are attributed to Cu(0) [46]. Further, the high resolution XPS narrow scan spectrum of the fresh CELL-Cu(O) catalyst shows a Cu 2p3/2 peak at 932.72 ev, which is attributed to Cu (0) [22]. 

The synthesis of palladium nanoparticles on montmorillonite layer silicates was studied. The Pd particles were prepared in situ in the interlamellar space of montmorillonite dispersed in an aqueous medium. Macromolecules were adsorbed on the support from an aqueous solution, followed by adsorption and reduction of Pd ions. The Pd° nanoparticles appear and grow in the internal, interlamellar space as well as on the external surfaces of the lamellae. Well-crystallized kaolinite clay can be disaggregated by the intercalation of DMSO to individual lamellae, which may serve as excellent supports for metal nanoparticles. After the adsorption of palladium precursor, metal nanocrystals were reduced by hydrazine or sodium borohydride between the kaolinite lamellae, i.e., in the interfacial layer acting as a nanoreactor. The incorporation of nanoparticles between the lamellae was shown hy XRD measinements. This procedure makes possible the steric control and restriction of nanoparticle growth. The stability of nanoparticles can be further enhanced hy the addition of polymers (PVP) and surfactants (alkyl-ammonium salts) that are also adsorbed between the kaolinite lamellae. The presence of the particles was also verified and their sizes were quantified by TEM measurements. 

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