Metallic nanoparticles principles

The common underlying principle was shown in Figure 11.2. The electrochemical potential of electrons jl e(=Ep, the Fermi level) in the metal catalyst is fixed at that of the Fermi level of the support.37 This is valid both for electrochemically promoted model catalysts (left) and for seminconducting or ion-conducting-supported metal nanoparticles (right). 

In principle, any type of particle can be prepared inside a dendrimer template if a means can be found first to sequester the components and then to transform them chemically into the desired product. As discussed earlier, the simplest way to accomplish this goal is to take advantage of strong interactions between functional groups within the dendrimer and ions in solution, and then chemically reduce the ions to the zero-valent metal. In certain cases, Ag for example, this approach does not work (at least not with dendrimers having interiors dominated with tertiary amines) because the partition coefficient is too small. However, in that case it was possible first to prepare a less noble metal nanoparticle, and then to displace it with Ag. 

Optical properties of metal nanoparticles embedded in dielectric media can be derived from the electrodynamic calculations within solid state theory. A simple model of electrons in metals, based on the gas kinetic theory, was presented by Drude in 1900 [9]. It assumes independent and free electrons with a common relaxation time. The theory was further corrected by Sommerfeld [10], who incorporated corrections originating from the Pauli exclusion principle (Fermi-Dirac velocity distribution). This so-called free-electron model was later modified to include minor corrections from the band structure of matter (effective mass) and termed quasi-free-electron model. Within this simple model electrons in metals are described as... 

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. 

Recently, microgel-stabilized, size-controlled metal nanoclusters have found promising applications in the field of catalysis. In particular, microgel systems can work as active carriers for the metal nanoparticles, which allows us to modulate the catalytic activity of nanoparticles by a thermodynamic transition that takes place within the carrier system [24, 69], The principle is shown in Fig. 8 Metallic... 

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. 

Another very interesting application was presented recently by the author. Electrochemical discharges can be used for the synthesis of metallic nanoparticles [78,141]. The principle is very simple. A metallic slat, such as copper sulphate, is added in low concentration to a supporting electrolyte. Electrochemical discharges are applied for a few hours. When the solution is dried (in order to remove the supporting electrolyte) metallic nanoparticles are obtained. The electrochemical discharges locally reduce the metal salt. 

Still far from those of natural systems. We believe that in future, when the construction principles will be better understood, a more appropriate choice of the metal connectors in terms of their energy levels and photophysical properties will allow us to improve the photo-induced response of the assemblies. In addition, towards practical utihzation, it would be appropriate to further organize a large number of identical functional systems into ordered structures. For example fimctional multi-porphyrin arrays might be anchored on soUd surfaces, such as metals or wide bandgap semiconductors, or on metal nanoparticles. 

XPS) studies have proved the absence of any residual polymer. The as-prepared particles are strongly fixed onto the surfaces, and cannot be removed by rinsing with solvents, nor by rubbing with a soft tissue. Another remarkable advantage associated with this method is the ability to deposit metal nanoparticles onto surfaces. In this case, the particle size, inter-particle distances - and, thereby, the particle density - can be varied. In addition, extension of the covered surfaces is, in principle, not limited. The three examples shown in Figure 4.33 are cutouts of 3 x 3 cm areas, demonstrating the versatility of the method. 

The basic principles of electrochemical biosensors are associated with their capability to detect a specific molecule with high specificity. Also, these characteristics are dictated by a better correlation between the biological component and the transducing element. Important advances in these aspects has been achieved with the utilization of several kinds of nanomaterials such as metal nanoparticles [21], oxide nanoparticles [22], magnetic nanomaterials [23], carbon materials [24,25] and metallophthalocyanines [26] to improve electrochemical signal of biocatalytic events occurred at electrode/electrolyte interface. 

---