Nanoparticles colloidal suspensions

Chromatographic approaches have been also used to separate nanoparticles from samples coupled to different detectors, such as ICP-MS, MS, DLS. The best known technique for size separation is size exclusion chromatography (SEC). A size exclusion column is packed with porous beads, as the stationary phase, which retain particles, depending on their size and shape. This method has been applied to the size characterization of quantum dots, single-walled carbon nanotubes, and polystyrene nanoparticles [168, 169]. Another approach is hydro-dynamic chromatography (HDC), which separates particles based on their hydro-dynamic radius. HDC has been connected to the most common UV-Vis detector for the size characterization of nanoparticles, colloidal suspensions, and biomolecules [170-172]. 

The use of templates to control the porosity of solids is not limited to small organic molecules. Alternative templates include dendrimers [16, 17], polymers [18], hard templates such as nanoparticle colloidal suspensions [19] and latex spheres [20] or even biological materials like butterfly wings [21], DNA [22] or viruses [23]. 

Silver, gold and copper are the most commonly used metals for SERS and the variety of substrates include, among others, roughened surfaces, nanoparticle colloidal suspensions, island films and nanostmctured surfaces (4). Current trends in SERS and Raman spectroscopy include the development of new substrates which includes immobilized particles, functionalized nanoparticles and nanolayered films. These recent development have created new opportunities for chemical and biological sensing. 

Ammonium salts are commonly used to stabilize aqueous colloidal suspensions of nanoparticles. The first such example was reported in 1983-84 by Januszkie-wicz and Alper [96, 97], who described the hydrogenation of several benzene derivatives under 1 bar H2 and biphasic conditions starting with [RhCl(l,5-hexa-diene)]2 as the metal source and with tetraalkylammonium bromide as a stabilizing agent Some ten years later, Lemaire and coworkers investigated the cis/... 

Finally, these particles generated in ionic liquids are efficient nanocatalysts for the hydrogenation of arenes, although the best performances were not obtained in biphasic liquid-liquid conditions. The main importance of this system should be seen in terms of product separation and catalyst recycling. An interesting alternative is proposed by Kou and coworkers [107], who described the synthesis of a rhodium colloidal suspension in BMI BF4 in the presence of the ionic copolymer poly[(N-vinyl-2-pyrrolidone)-co-(l-vinyl-3-butylimidazolium chloride)] as protective agent. The authors reported nanoparticles with a mean diameter of ca. 2.9 nm and a TOF of 250 h-1 in the hydrogenation of benzene at 75 °C and under 40 bar H2. An impressive TTO of 20 000 is claimed after five total recycles. 

Although several noble-metal nanoparticles have been investigated for the enantiomeric catalysis of prochiral substrates, platinum colloids remain the most widely studied. PVP-stabilized platinum modified with cinchonidine showed ee-values >95%. Several stabilizers have been also investigated such as surfactants, cinchonidinium salts and solvents, and promising ee-values have been observed. Details of a comparison of various catalytic systems are listed in Table 9.16 in one case, the colloid suspension was reused without any loss in enantioselectiv-ity. Clearly, the development of convenient two-phase liquid-liquid systems for the recycling of chiral colloids remains a future challenge. 

So far, we have prepared and tested many kinds of colloids, mainly in nonaqueous suspensions with combinations of metals or alloys as a dispersed phase and organic liquids as the dispersion media, without the use of any dispersing agents these are listed in Table 9.4.1. We next give some examples of transmission electron micrographs of nanoparticles produced by an aerosol method. A sample for TEM measurement was obtained by dropping colloidal suspension onto a Cu mesh coated with an evaporated carbon film of 10 nm thickness. Many colloids were so unstable... 

Fig. 9.4.32 (a) TEM micrograph of Au nanoparticles deposited from colloidal suspension... 

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],... 

The mechanistic aspect of the fungal reduction of metal ions led by colloidal suspension is still an open question. However, in the fungal case, this process occurs probably either by reductase action or by electron shuttle quinines, or both. To elucidate the mechanism of nanoparticles formation, a novel fungal/enzyme-based in vitro approach was for the first time explained by Mukherjee et al. (2002). They successfully used species-specific NADH-dependent reductase, released by the F. oxysporum, to carry out the reduction of AuClJ ions to gold nanoparticles. Duran et al. (2005) later reported that the reduction of the metal ions occnrs by a nitrate-dependent reductase and a shuttle quinone extracellular process. The same... 

The self-assembly technique has attracted much attention since they were observed by Decher in 1991 [49]. Self-assembly is the fundamental principle that provides the precise control of the resulting assemblies and the thickness of an individual layer on the nanometer scale by variation in the bulk concentration of the metal colloids suspension, deposition time, pH, and transport conditions [50]. Recently, the functionalization of metal nanoparticles has opened up new opportunities for the construction of nanostructured self-assembly films to fabricate novel SERS-active Ag substrates. 

Silver Nanoparticles Preparation. A colloidal suspension of citrate reduced silver nanoparticles was prepared using a modified Lee and Meisel [8] procedure. 

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