Thiol-Stabilized Nanoparticles

The synthesis of nonspherical silver nanopartides has attracted much interest in recent years. As discussed in Section 3.2, the optical properties of silver 

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McCormick [5] prepared thiol-stabilized nanoparticles containing gold, platinum, palladium, rhodium, ruthenium, osmium, and iridium, which were used in optics, immunodiagnostics, and electronics. 

The IR characterization of the films provides useful information for estimation of the materials structure. Addition of silver and gold nanoparticles to polymers shifted the band to the 1500-1600 cm range, that is characteristic for the stretching vibrations of rr-system of benzene rings. In the case of thiol-stabilized nanoparticles these changes are more evident and can be related to the same effect. The bands shift in IR-spectra at 1000-1150 cm allows us... 

Figure 4. Preparative procedure of alkene thiol-stabilized gold nanoparticles. 50% of hexenethiol bounded to the particle surface is hydrogenated to hexanethiol [11b].

Three general preparative schemes are of particular interest due to their success in preparing nanoparticles on the order of 1 to 3 nm. The first, commonly known as the Brust method for preparing thiol stabilized Au nanoparticles, is discussed in detail in the chapter by Zhong et al. in Section IV of this book. The second method, which originates from Prof El-Sayed s group, is noteworthy for preparing particles with extremely well-defined shapes (tetrahedra, cubes, etc). ° The third method. 

Purification yields 200-300 mg of triphenylphosphine-stabilized gold nanoparticles, which should be stored cold (—20°C) in the sofid state, or immediately converted to thiol- or amine-stabilized nanoparticles through subsequent reaction with the appropriate ligand. The particles decompose in solution thus manipulation of the product in solution should be minimized. 

The 1.5-nm nanoparticles readily react with thiol or amine-terminated ligands under mild conditions to yield thiol- or amine-stabilized nanoparticles. Triphenylphosphine-stabilized particles thermally decompose with the production of (PPh3)AuCl and metallic gold. 

During the formation of nanoparticles, surfactants interact with the surfaces of the particles in a dynamic equilibrium process. On the contrary, ligands that are chemisorbed to the surface of the particle are less prone to desorption compared to physisorbed ones. A first consequence of a low desorption rate is that particles do not grow rapidly after nucleation. As an example, when thiols are chemisorbed to AuNPs particle sizes are typically limited to <5 nm. In contrast, the majority of surfactants used to stabilize nanoparticles associate with the surface of particles through van der Waals or other weak interactions, allowing the preparation of larger particles. The first report of thiol-stabilized AuNPs appeared in 1993 by Giersig and Mulvaney.145... 

Cysteine ligands have also been reported in the synthesis of intricate copper nanoparticles following a literature modified reduction procedure. The Cu° nanoclusters revealed a plasmon resonance band at 398 nm and an average size distribution of 3.5 0.8nm by TEM. Infrared analysis of the stabilized nanoparticles showed a weakened S-H stretch at 2598 cm suggesting uncoordinated cysteine thiols, supporting the possible involvement of the amine functional group from the cysteine alpha carbon in particle stabilization. These particles displayed extreme O2 sensitivity, degrading under aerobic conditions to Cu (I) and then Cu (II) cysteine complexes. Additionally, this sensitivity precluded the convenient determination of nanoparticle crystallinity. ... 

Small molecules such as phosphines and alkane thiols stabilize metal nanoparticles in a very effective maimer. Very stable covalent metal-phosphorus or metal sulfur bonds lead to such strong ligand shells that in some cases the protected particles can even be isolated in solid state, which can never be done with electrostatically stabilized particles. The chemical nature of the protecting ligand molecules is responsible for the solubility of the particles. Thus, the use of organic solvents has become very useful for several reasons. Figme 3 shows a sketch of the three types of steric stabihzations of metal nanoparticles. 

So-called superlattices of 5mn alkylthiolate protected silver particles having truncated octahedral shapes as well as thiol-stabilized 5 6 mn gold particles can be obtained from solution. Fractional crystallization is a very usual method to separate chemical compounds from other compounds and impurities. Mixtures of thiolate stabilized gold nanoparticles between 1.5 and 3.5 nm could successfully be fractioned into real monodisperse species containing 140, 225, 314, and 459 atoms. 2D assemblies have also become available of these fcc-structured nanoparticles. The decisive criterion to successhilly fraction and crystallize metal nanoparticles is to protect them perfectly by strongly bound ligand molecules in order to avoid coalescence. 

Numerous examples of ordered 2D structures, especially of thiol-stabilized silver and gold nanoparticles, have become known in the course of the last few years. They usually form spontaneously on Transmission Electron Microscopy (TEM) grids or other surfaces from solution and will not be... 

Absorption spectra of PEPC and PVC thin films with nanoparticles are characterized by unstructured long-wave shoulder that comes with the involving of nanoparticles (Fig. 1). The use of the thiol-stabilized particles leads to slight blue shift of the maximum in the range of 400-550 nm for polymers. These effects are more apparent in PEPC films than in PVC ones. 

Figure I. Optical absorption spectra of the structures including PVC (a) and PEPC (b) films pure polymer films (1), films with Ag and Au nanopaiticles (2), films with thiol-stabilized Ag and Au nanoparticles (3).

Novel composite carbazole polymer films containing silver and gold nanoparticles and being thiol-stabilized have been fabricated. Optical properties of PEPC films are more sensitive to the nanoparticles incorporation than the properties of PVC films. This can be provided by the formation of additional interaction of particles and -CH2-O-CH2- group presented in the polymer chain. High values of the current density measured and the character of the current-voltage dependence in the composite films can be the consequence of several mechanisms of charge carrier transport in both composite PEPC and PVC films. 

In addition to surfactants and polymers, other compoimds have also been studied as stabilizers. Water-soluble polyoxometallate-stabilized nanoparticles display satisfactory activities in Suzuki, Heck and Stille reactions in aqueous solution (Table 2) [32], Thiol-substituted cyclodextrins have been employed as stabilizers for palladium coUoids, which were active in Suzuki reactions, although the stability was limited [33],... 

Thiol stabilization also proved to offer a very successful route towards the creation of monosized nanoparticles of various compounds however, as it has also attracted widespread interest it will be outlined separately in Chapter 3.2.1.2. In this chapter, from this point onwards, attention will be focused on recent developments that differ from the hot injection synthesis and from the thiol technique. 

As an example of the capabilities of EXAFS spectroscopy, the mean Cd-S distances as a function of the diameters of a series of CdS nanocrystals are depicted in Figure 3.15 [163]. These data are gained from a thorough temperature-depen-dent study of the size dependence of various structural and dynamic properties of CdS nanoparticles ranging in size from 1.3 to 12.0 nm. The properties studied include the static and the dynamic mean-square relative displacement, the asymmetry of the interatomic Cd-S pair potential, with conclusions drawn as to the crystal structure of the nanoparticles, the Debye temperatures, and the Cd-S bond lengths. As seen from Figure 3.15, the thiol-stabilized particles (samples 1-7) show an expansion of the mean Cd-S distance, whereas the phosphate-stabilized particles (samples 8-10) are slightly contracted with respect to the CdS bulk values. 

Fig. 6.4 TEM image of alkyl-thiol-stabilized gold nanoparticles by NaBH4-reduction of AuCU-ions [29]...

Torigoe, Yonezawa, and co-workers prepared dendron type-thiol-stabilized gold nanoparticles by NaBH4-reduction process [72]. As dendron is bulky structures, it prevents hexagonal close packing of nanoparticles and show quasi ID structures by just casting on a carbon grid. 

M. Brust synthesis of thiol-stabilized gold nanoparticles... 

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