Non-spherical nanoparticles

Very recently the crystallization of non-spherical nanoparticles into ordered arrangements has been observed that had not been seen with spherical particles. Thanks to the high density of DNA on the particle solid faces, hexagonal packing of nanorods, and columnar stacking of nanoprisms, fee crystals of rhombic dodecahedrons were observed in [152] (see Fig. 35). 

Since enhanced electromagnetic fields in proximity to metal nanoparticles are the basis for the increased system absorption, various computational methods are available to predict the extent of the net system absorption and therefore potentially model the relative increase in singlet oxygen generation from photosensitizers. " In comparison to traditional Mie theory, more accurate computational methods, such as discrete dipole approximation (DDA/ or finite difference time domain (FDTD) methods, are often implemented to more accurately approximate field distributions for larger particles with quadruple plasmon resonances, plasmon frequencies of silver nanoparticles, or non-spherical nanoparticles in complex media or arrangements. ... 

As shown in Fig. 8.11, an excellent agreement was achieved between the two sizing techniques moreover the electrochemical sizing technique was foimd to present a number of advantages over NTA, which can suffer from drawbacks commonly associated with optical sizing techniques such as the inabihty to measure the agglomeration state of non-spherical nanoparticles without performing a... 

In this Section we want to present one of the fingerprints of noble-metal cluster formation, that is the development of a well-defined absorption band in the visible or near UV spectrum which is called the surface plasma resonance (SPR) absorption. SPR is typical of s-type metals like noble and alkali metals and it is due to a collective excitation of the delocalized conduction electrons confined within the cluster volume [15]. The theory developed by G. Mie in 1908 [22], for spherical non-interacting nanoparticles of radius R embedded in a non-absorbing medium with dielectric constant s i (i.e. with a refractive index n = Sm ) gives the extinction cross-section a(o),R) in the dipolar approximation as ... 

Cao L, Liu Z, Zhu T (2006) Eormation Mechanisms of Non-Spherical Gold Nanoparticles During Seeding Growth Roles of Anion Adsorption and Reduction Rate. J Colloid Interface Sci 293 67-69... 

It is also of interest to consider the case of a microsphere at a non-electroactive substrate because it is used as a model for spherical nanoparticles (of radius rnp and area Anp) impacting on a surface [72, 73]. For this case the average stationary current is given by [74] ... 

Several nanoparticles having a non-spherical shape have been synthesized, e.g., carbon nanotubes, nanofibers, nanorods, and nanowires which exhibit, similar to asbestos, a fibrous shape. 

Delapierre, T.M., Majimel, J., Momet, S., Dngnet, E., and Ravaine, S. 2008. Synthesis of non-spherical gold nanoparticles. Gold Bulletin, 41 195-207. 

Recently, Miller and Cacciuto explored the self-assembly of spherical amphiphilic particles using molecular dynamics simulations [46]. They found that, as well as spherical micellar-type structures and wormlike strings, also bilayers and faceted polyhedra were possible as supracolloidal structures. Whitelam and Bon [47] used computer simulations to investigate the self-assembly of Janus-like peanut-shaped nanoparticles and found phases of clusters, bilayers, and non-spherical and spherical micelles, in accordance with a packing parameter that is used conventionally and in analogy to predict the assembled structures for molecular surfactants. They also found faceted polyhedra, a structure not predicted by the packing parameter (see Fig. 8). In both studies, faceted polyhedra and bilayers coexist, a phenomenon that is still unexplained. 

Bon and coworkers carried out a study on the fate of the nanoparticles throughout solids-stabilized emulsion polymerization [119], A quantitative method based on disk centrifugation was developed to monitor the amount of nanoparticles present in the water phase in solids-stabilized emulsion polymerizations of vinyl acetate, methyl methacrylate, and butyl acrylate. The concentration profile of nanoparticles in the water phase as a function of monomer conversion agreed with theoretical models developed for the packing densities in these systems [120]. Noteworthy was that in the case of silica-nanoparticle-stabilized emulsion polymerization of vinyl acetate, the event of late-stage limited coalescence, leading to small armored non-spherical clusters, could be predicted and explained on the basis of the concentration profiles and particle size measurements. Adjusting the amount of silica nanoparticles prevented this phenomenon. 

A non-viral vector PEG-PEI was investigated in the delivery of siRNA to neural stem cells (NSCs). The SEM micrographs showed that PEG-PEI could condense siRNA to form spherical nanoparticles. The gene knockdown effect of PEG-PEI/siRNA nanoparticles was verified at the levels of mRNA and protein, which suggested that PEG-PEI may potentially be used as a siRNA delivery vector for neural regeneration therapy. ... 

Mesocrystals are a very interesting form of colloidal crystal, as they extend the so far known colloidal crystals with spherical building units to those with non-spherical building units. This offers new possibilities of superstructme formation due to the anisotropic particle shape of the nanoparticle building units [113]. Thus, mesocrystals are colloidal crystals but with extended possibihties for their self-assembled superstructure, offering new handles for crystal morphology control. 

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