Nanoparticle metastable state

Aggregation of the atoms or microclusters may give metal nuclei. The micro-cluster itself may work as the nucleus. Although the size of microcluster or nucleus is not clear, the nucleus may consist of 13 atoms, which is the smallest magic number, This idea may be supported by the structural analysis of PVP-stabilized Pt nanoparticles (64) and other systems. In fact, a particle of 13 atoms is considered an elemental duster. In the case of preparation of PVP-stabilized Rh nanoparticle dispersions by alcohol reduction, formation of very tiny particles, the average diameter of which is estimated to be 0.8 nm, was observed (66). These tiny particles in the metastable state may consist of 13 atoms each and easily increase in size to the rather nanoparticles with average diameter of 1.4 nm, i.e., the particles composed of 55 atoms. This observation again supports the idea that the elemental cluster of 13 atoms is the nucleus. 

For the organization of such movement and for building of nanorobots one can use, for example, fragments of DNA molecules, muscle nanofibers (Gu et al., 2010 Lund et al., 2010), photoexcited and metastable states of nanoparticles, asymmetric distribution of electric dipoles in the direction of motion, fluctuation processes of molecular and electric namre (Korochkova et al., 2008 Korochkova and Rosenbaum, 2006), ring-like biomolecules strung on the linear bridge of amino acids (Muscat et al., 2011 Lewandowski et al., 2013), etc. 

Nanoparticles are generally found in a metastable state. The force of attraction (van der Walls force) between any two particles is inversely proportional to the square of the distance between them. Therefore, in the absence of a barrier to overcome this attraction, particles attract each other and start to grow and eventually coagulate. Two primary types of stabilization exist with nanoclusters, namely electrostatic and steric stabilization. 

A colloidal suspension may be unstable and exhibit separation of particles within a very short time. Or it may be stable for a very long time, such as over a year or more. And there will thus be found a metastable state, which would be in between these two. This is an oversimplified example, but it shows that one should proceed to analyze any colloidal system following these three criteria. In fact, the most remarkable finding one can mention about colloidal suspensions is that these systems can exist at all Especially, one finds that some solid suspensions can be stable for a very long time. In pharmaceutical applications, one important example is the use of suspension of insulin in pen injections. The insulin suspension is stable for long enough time for its application, which provides very accurate dosage to the patient. In fact, there exists a wide variety of pharmaceutical products that are based on suspended molecules. Nanoparticles have been applied in various pharmaceutical treatments (Sotiriou et al., 2014). 

When the CO disproportionation is catalyzed by cobalt, some ordered metastable structures are detected inside the active metal nanoparticles after the reaction. These structures are regular thin (approximately 5 atoms in thickness) alternating cobalt layers of different crystallographic modifications (Figure 4.17). Note that the appearance of such structures at thermodynamically equilibrium states of the catalyst substance is contrary to the Gibbs phase rule for the phase equilibria in solids. Thus, the metastable layered structures may be considered an analogue of spatial dissipative structures. 

As the supercooled smectic state is metastable, the recrystallization tendency of the nanoparticles during storage is of high interest. Due to its completely reversible phase behavior and favorable crystallization temperature in the colloidal state, cholesteryl myristate was used as model cholesterol ester for investigations of the recrystallization tendency and influencing parameters like particle size and stabilizer system. 

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