Stabilization of Metal Nanoparticles

Organic polymers are very often used for the stabilization of metal nanoparticles by providing a steric stabilizing effect Due to this embedding effect, it is generally considered that the diffusion of substrates through the polymer matrix can be limited. Nevertheless, some interesting results have been obtained. 

Ionic Liquids used as Templates for the Stabilization of Metal Nanoparticles... 

Alexandridis P, Sakai T (2004) Amphiphilic block copolymer solutions as media for the facile synthesis and colloidal stabilization of metal nanoparticles. Abstracts of Papers, 228th ACS National Meeting, Philadelphia, PA, USA, August 22-26, 2004,... 

Sakai T, Alexandridis P (2004) Single-step synthesis and stabilization of metal nanoparticles in aqueous Pluronic block copolymer solutions at ambient temperature. Langmuir 20 8426-8430... 

Fig. 9.4.29 Comparison of stability of metallic nanoparticles in bulk liquid with a droplet on a metal surface, (a) Wetting of a droplet on a metal surface, (b) Coagulation and dispersion of metallic particles in liquid. Figures on the left-hand side stand for weak interaction in case A causing coagulation in case B. Those on the right-hand side are a strong interaction between metal and liquid, suggesting good dispersion and good contact.

Additional factors complicating the matter arise from the chemical and thermal (in)stability of metal nanoparticles as well as from the proposed mobility of surface bound capping agents (usually thiols). The chemical stability or instability of thiol-capped metal nanoparticles towards oxidation (i.e., oxidation of surface-bound thiols in air or in the presence of other oxidants) [70], towards halides [71], and towards alkaline metal ions has been studied by a number of groups [72] using TEM, UV-vis, NMR, as well as X-ray photoelectron spectroscopy (XPS) [73], and this collective work highlights the importance of determining nanoparticle purity. 

The thermal stability of metal nanoparticles (for the most part nanoparticles used in liquid crystal phases with high phase transition temperatures or nanoparticles decorated with functional molecules) should also be of significant importance,... 

The chemistry and physics of such particles represent a separate, rapidly developing field. The consideration of this field (even if brief) is beyond the scope of this review. From the standpoint of this review, of most interest is the fact that melting of similar films often produces stable colloid solutions of metals in non-aqueous media. Early works in this field were summarized in review [19]. Among later results, noteworthy is the stabilization of metal nanoparticles in tertiary amines, which appear to be a unique medium for formation of stable colloid solutions of a wide variety of metals [20, 21]. Metal colloids stable for, at least, several years were obtained through the intermediate formation of thin films of co-condensates of metals with amines. 

Kramer and coworkers recently reported on water-soluble dendritic coreshell architectures and studied the influence of the attached carbohydrate shell on the formation and stabilization of metal nanoparticles in water. For this purpose, they used hyperbranched poly(ethylenimine) (PEI) as core molecules and covalently attached different carbohydrates as shells, i.e., glycidol, gluconolactone and lactobionic acid, to obtain the corresponding PEI-glycol, PEI-gluconamide and PEI-lactobionamide. Different molecular weights of PEIX (x = 0.8, 5, 21 or 25 with different Mw = xlO3) were employed [81]. 

Narayanan, R. and M.A. El-Sayed (2003). Effect of catalysis on the stability of metallic nanoparticles Suzuki reaction catalyzed by PVP-paUadium nanoparticles. Journal of the American Chemical Society, 125(27), 8340-8347. 

Figure 2 Electrostatic stabilization of metal nanoparticles. Repulsive electrostatic forces outweigh attractive van der Waals forces...

Figure 3 Different types of steric stabilization of metal nanoparticles. (a) polymer molecules on the particles surface (b) micelle (c) inverse micelle (d) ligand stabilized particle...

The successful generation of metal nanoparticles is unavoidably linked with successful protection of the particles surfaces, otherwise spontaneous aggregation with formation of metallic precipitates would happen. For that reason, some general and crucial aspects concerning stabilization of metal nanoparticles will be discussed before the description of synthetic strategies. 

Small molecules such as phosphines and alkane thiols stabilize metal nanoparticles in a very effective manner. 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. Figure 3 shows a sketch of the three types of steric stabilizations of metal nanoparticles. 

Figure 3.105 Electrostatic stabilization of metal nanoparticles. Attractive van derWaals forces are outweighed by repulsive electrostatic forces between adsorbed ions and associated counterions at moderate interparticle separation.

Raveendran, R, Fu,J., et al. Completely Green Synthesis and Stabilization of Metal Nanoparticles. Joumal of the American Chemical Society,125(46), 13940-13941 (2003). 

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... 

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