Metallic nanoparticles aggregates

We recently demonstrated a method that reproducibly exploits hot spots in metal nanoparticle aggregates for the trace detection of disease-specific enzymes by design [187]. The scheme, shown in Fig. 10.16a, involves the functionalization of gold nanoparticle with thiolated short peptide sequences terminated by an N-(fluorenyl-9-methoxycarbonyl) (FMOC) group. Nanoparticle aggregation is then driven by n-n interactions between the terminating FMOC groups [157]. Hot spots... 

Fig. 11.5 Rationally fabricated SERS substrates. From (a) to (I), triangular nanoparticle array [30], silver nanowire bundles [32], Ag nanoparticle-assembled silica nanoparticle (SERS dots) [33], metal nanoparticle aggregates (COINs) [34], gold nanocrescent [49], gold nanoparticles with thin oxide shells [50], hollow-type gold nanoparticles [35-37], gold nanorods [38 10], nanocubes [41,42], flower-like gold nanoparticles [43], nanodisks [46], and gold nanorods immobilized on silica nanoparticles [48]...

A particularly important variant of the optical force, interparticle forces, turns out to be crucial for SERS. This effect is similar to the attractive van der Waals force between small particles, which is due to interactions between spontaneously fluctuating dipoles, but the optical interaction is due to coupling between the actual particle dipoles induced by the trapping laser. Due to the interparticle optical forces, metal nanoparticles aggregate in an optical tweezers and produce hotspots, i.e., particle junctions with intense local fields for SERS. Raman probes can be excited either by the trapping laser or, preferably, by a separate low power beam that does not disturb the trapping. 

Dolotov, S. V Roldughin, V. I. Simulation of ESR spectra of metal nanoparticle aggregates. Russ. Colloid J., 2007,69,9-12. 

Pinchuk AO, Kalsin AM, Kowaiczyk B, Schatz GC, Grzybowski BA (2007) Modeling of electrodynamic interactions between metal nanoparticles aggregated by electrostatic interactions into closely-packed clusters. J Phys Chem C 111(32) 11816-11822. doi 10.1021/jp073403v... 

CuNPs) in Fig. 7 shows the monodisperse and uniformly distributed spherical particles of 10+5 nm diameter. The solution containing nanoparticles of silver was found to be transparent and stable for 6 months with no significant change in the surface plasmon and average particle size. However, in the absence of starch, the nanoparticles formed were observed to be immediately aggregated into black precipitate. The hydroxyl groups of the starch polymer act as passivation contacts for the stabilization of the metallic nanoparticles in the aqueous solution. The method can be extended for synthesis of various other metallic and bimetallic particles as well. 

The preparation and study of metal nanoparticles constitutes an important area of current research. Such materials display fascinating chemical and physical properties due to their size [62, 63]. In order to prevent aggregation, metal nanoparticles are often synthesized in the presence of ligands, functionalized polymers and surfactants. In this regard, much effort has focused on the properties of nanoparticles dispersed into LCs. In contrast, the number of nanoparticles reported that display liquid crystal behavior themselves is low. Most of them are based on alkanethiolate stabilized gold nanoparticles. 

Finke has reported remarkable catalytic lifetimes for the polyoxoanion- and tetrabutylammonium-stabi-lized transition metal nanoclusters [288-292]. For example in the catalytic hydrogenation of cyclohexene, a common test for structure insensitive reactions, the lr(0) nanocluster [296] showed up to 18,000 total turnovers with turnover frequencies of 3200 h [293]. As many as 190,000 turnovers were reported in the case of the Rh(0) analogue reported recently. Obviously, the polyoxoanion component prevents the precious metal nanoparticles from aggregating so that the active metals exhibit a high surface area [297]. 

Evaporation of volatile byproducts and solvents is often used to obtain the solid metal nanoparticles. The residue may contain metal nanoparticles and protective reagents. When the nanoparticles are well protected by ligands or polymers, then the solid residues can be dispersed again without coagulation of the particles. When the nanoparticles are not well protected, however, the evaporation often results in aggregation of the nano-particles. 

Pt/MWNT) [20,21], fine and homogeneous Pt nanoparticles deposited on MWNTs were obtained when pure EG was used as the solvent or less water (<5vol.%) was introduced. With the increase in water content, aggregation of the metal nanoparticles occurred, the average particle size increased and the particle size distribution became wider. 

Among various methods to synthesize nanometer-sized particles [1-3], the liquid-phase reduction method as the novel synthesis method of metallic nanoparticles is one of the easiest procedures, since nanoparticles can be directly obtained from various precursor compounds soluble in a solvent [4], It has been reported that the synthesis of Ni nanoparticles with a diameter from 5 to lOnm and an amorphous-like structure by using this method and the promotion effect of Zn addition to Ni nanoparticles on the catalytic activity for 1-octene hydrogenation [4]. However, unsupported particles were found rather unstable because of its high surface activity to cause tremendous aggregation [5]. In order to solve this problem, their selective deposition onto support particles, such as metal oxides, has been investigated, and also their catalytic activities have been studied. 

Figure 1. Graphical model for the generation of size-controlled metal nanoparticles inside metallated resins, (a) Pd is homogeneously dispersed inside the polymer framework (b) Pd is reduced to Pd (c) Pd atoms start to aggregate in subnanoclusters (d) a single 3 nm nanocluster is formed and blocked inside the largest mesh present in that slice of polymer framework (Reprinted from Ref [5], 2004, with permission from Wiley-VCH.)...

Klajn, R. Bishop, K. J. M. Fialkowski, M. Paszewski, M. Campbell, C. I Gray, T. E Grzybowski, B. A. 2007. Plastic and moldable metals by self-assembly of sticky nanoparticle aggregates. Science 316 261-264. 

The catalysts can be obtained by a coprecipitation method consisting of two steps (Figure 6.2). In the first step, a stable suspension of protected metal nanoparticles is obtained according to the method reported by Schulz and co-workers [75-77]. The metal particles are prepared in the presence of a highly water-soluble ionic surfactant which is able, due to its nature, to modulate the particle size and to prevent their aggregation. Modifying parameters such as pH, temperature and surfactant concentration, it is possible to tune the metal particle size [71]. Moreover, the role of the... 

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