Aggregates of Metallic Nanoparticles

Aggregates of metallic nanoparticles are normally seen to demonstrate a large field-enhancement effect For example, a SERS enhancement factor of 10 has been reported for aggregates of Ag nanoparticles, and single-molecule detection has been achieved [132-134). Several studies have demonstrated an enhanced electric field due to a coupling effect between metal nanoparticles placed in close proximity to one another [97, 135-137). Thus, nanostructures have been intensely investigated as substrates for SEVS, such as SERS [138-140), SEIRA [79) and MEF [64, 73, 74). 

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Figure 16.24 (a) A generalized scheme for chemical sensing based on analyte-assisted aggregation of metallic nanoparticles and (b) molecular structure of a ligand used in lithium binding. (Adapted from Murphy et al. 1138])... 

Although aggregation of metallic nanoparticles perturbs their electronic structure and especially the plasmon resonance frequency, the semiconductor quantum dots are much more resistant to such effects. 

Another recent article by Kneipp et al. demonstrates the use of multiplexing within live cells [69]. Two different SERS particles, aggregates of metal nanoparticles functionalized with two different reporter molecules could be imaged within cells using SERS. The data obtained was analyzed using cluster methods and principal components analysis to detect the two SERS reporters within the live cells. 

Tong LM et al (2009) Optical aggregation of metal nanoparticles in a microfluidic channel for surface-enhanced Raman scattering analysis. Lab Chip 9(2) 193-195... 

The targets of present SERS on-chip are (1) to solve the problem of poor detection sensitivity caused by the small sample volume and (2) to develop robust methods to obtain a reliable and reproducible SERS signal. Aggregation of metal nanoparticles and mixing of enhancer with analytes are the straightforward ways to increase the SERS signal intensity and the uniformity of the SERS active site. 

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. 

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. 

Electrochemical Synthesis of Bimetallic Particles. Most chemical methods for the preparation of metal nanoparticles are based at first on the reduction of the corresponding metal ions with chemical reagents to form metal atoms and then on the controlled aggregation of the obtained metal atoms. Instead of chemical reduction, an electrochemical process can be used to create metal atoms from bulk metal. Reetz and Hclbig proposed an electrochemical method including both oxidation of bulk... 

For example, the aggregated structures of the solutions containing polymer-metal complexes and the colloidal dispersions of metal nanoparticles stabilized by polymers have been analyzed quantitatively (64). SAXS analyses of colloidal dispersions of Pi, Rh, and Pt/Rh (1/1) nanoparticles stabilized by PVP have indicated that spatial distributions of metal nanoparticles in colloidal dispersions are different from each other. The superstructure (greater than 10.0 nm in diameter), with average size highly dependent on the metal element employed, is proposed. These superstructures are composed of several fundamental clusters with a diameter of 2.0-4.0 nm, as shown in Figure 9.1.13 for PVP-stabilized Pt nanoparticles. 

In some cases, reduction, aggregation, and growth occur at once. Stabilizers such as surfactants, polymers, etc. can be added to the solution before or after the growth of metal nanoparticles. 

Aggregation of Metal Atoms to Form Metal Nuclei. After the reduction of metal ions, the solution colored by metal ions becomes colorless. At this stage, metal nanoparticles are not produced because the color of the dispersions of metal nanoparticles does not appear at all. If coin metal is used, this color change is much more clear than for the other precious metals. 

The same protocol based on aggregation of metallic particles may be used for detection and determination of various biomolecules and even simple inorganic ions. Antibodies, enzymes, biotin, streptavidin, or lectins can be used for nanoparticle modification [138] (Figure 16.24). 

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

Chemical syntheses of metal nanoparticles are all based on the use of appropriate precursor complexes. Depending on the metal, its oxidation state, the desired type of nanoparticle, and so on, the methods apphed vary in a wide range. Besides chemical syntheses, there exist also physical approaches. They usually start from bulk metals and use different kinds of energy to transfer them into the atomic state in the gas phase, where the atoms are allowed to condense to nanoparticles. From a preparative point of view, such physical methods are less important, since the resulting particles are bare. As has aheady been mentioned, unprotected metal nanoparticles cannot exist without aggregation except in the gas phase. That is why gaseous metal nanoparticles have been generated and nsed... 

Intense research in the field of metal nanoparticles by chemists, physicists, and materials scientists is motivated by the search for new materials that hold novel physical (electronic, magnetic, optical) and chemical (catalytic) properties. Recently, the research on monometallic nanoparticles has been carried out in order to further miniaturize electronic devices [26-28] as well as to elucidate the fundamental question of how electronic properties of molecular aggregates evolve into novel properties with increasing size in this intermediate region between a molecule and a bulk [3,29-33]. Possible future applications include the areas of ultrafast communication and a large quantity of data storage [3,31,32,34]. 

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