Chemical composition with metal nanoparticles

Dendrimer interior functional groups and cavities can retain guest molecules selectively, depending on the nature of the guest and the dendritic endoreceptors, the cavity size, the structure, and the chemical composition of the peripheric groups. Two main methods are known for the synthesis of metal nanoparticles inside dendrimers. The first method consists of the direct reduction of dendrimer-encapsulated metal ions (Scheme 9.4) the second method corresponds to the displacement of less-noble metal clusters with more noble elements [54]. 

Considerable research effort was focused on systems of colloidal gold of which a broad variety of synthetic procedures were reported [140 b, fj. While native colloidal gold solutions are only stable for a restricted time, Brust et al. [141] were able to overcome this problem by developing a simple method for the in situ preparation of alkyl thiol-stabihzed gold nanoparticles. This synthetic route yields air-stable and easy to handle passivated nanoparticles of moderate polydispersity, and is now commonly employed for the preparation of inorganic-organic core-shell composites. Such composites are used as catalytic systems with principally two different functions of the protective 3D-SAM layer. Either the metal nanoparticle core can be used as the catalytically active center and the thiol layer is only used to stabihze the system [142], or the 3D-SAM is used as a Hnker system to chemically attach further catalytic functions [143]. 

Recently, novel nanomaterials have become a new frontier for SERS experiments, where different metals are collected together to form, for example, bimetallic particles. Thus, the same nanoparticle could be responsible for both SERS effect and catalytic activity. This is the case of the Ag/Pd colloids synthesized by chemical reduction with sodium borohydride (NaBH4) of silver nitrate (AgNOs) and palladium nitrate (Pd(N03)2), with a 96 4 Ag/Pd molar ratio [11]. The silver nanoparticles provide the SERS enhancement for the ligand molecules, while palladium may induce catalytic reactions. Also, in this case, TEM microscopy provides an important help to characterize these composite materials. In Fig. 20.6 TEM images at different magnifications are reported for bimetallic Ag/Pd particles, in comparison with those constituted by pure silver. While these latter present spheroidal shapes, bimetallic particles show more irregularities, due to palladium clusters in contact with the silver core surface. 

Controlling the primary structures of metal nanoparticles (i.e., size, shape, crystal structure, and composition) is one of the most important missions for colloid science, especially for nanoparticle science and technology because these structures determine the chemical and physical properties of metal nanoparticles. Here, chemical methods are dealt with to control the compositions and structures of various bimetallic nanoparticles by making use of the difference in the reduction (decomposition) rate or the reduction sequence of two kinds of metal species. When two kinds of metal ions are simultaneously reduced, the reduction rates of metal ions usually determine the hnal structure of bimetallic nanoparticles (i.e., a core/shell structure or an alloy structure). The successive reduction of two kinds of metal ions, however, generally gives bimetallic nanoparticles with a core/shell structure. As a representative of the chemical properties of such bimetallic nanoparticles, their catalytic properties for... 

As discussed previously, growth and chemical reactions of nanoparticles lead to changes in their compositions. Thus the inferences in composition described above should hold only for fresh nanoparticles correlations should weaken with atmospheric aging of the particles. It would be desirable to compare these expectations against actual field measurements of particle compositions. However, quantitative measurements of the chemical composition of ambient ultrafme particles are available only for the larger members of this class (Dp 50-100 nm). Available data, from urban areas in Southern California, indicate that organic compounds represent approximately half of the ultrafme particle mass. The remaining mass is contributed by trace metal oxides, elemental carbon, sulfate, nitrate, ammonium, sodium, and chloride (Cass et al. 2000). 

---