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Au nanocrystals

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Novel Methods for synthesizing Gold Nanoparticles
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Novel Methods for synthesizing Gold Nanoparticles
There has been a global revolution in nanotechnology. Nanotechnology is now increasingly being applied in the field of medical and technological sciences. Nanocrystals are one of the important materials in the field of nanotechnology. The nanoparticle is defined as that particle whose one of the dimensions should be less than 100nm. Such feature helps in various properties exhibited by these substances1. For example, nanoparticles of silica have unique light emission properties that are not feasible by the amorphous or bulk crystalline format of silica. Hence, such properties are routinely applied in various industrial processes. Nanoparticles are prepared by traditional and modern methods3.
Traditional methods include various solvent systems and surfactants in creating particles of nanometre dimensions. On the other and modern method relies upon galvanic separation of charges in solution. In this situation, a template is selected, and cations are applied to the solution. Since, cations are electron acceptors; they pull the electrons from the surface of the template into the solution2. This result in formation of the nanoparticles on the template selected for the galvanic induction of separation. The present article will review the generation of platonic gold nanocrystals based on novel methods of preparation.

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The study reflected the generation of the icosahedrons form of gold nanoparticles, with modifications under various experimental conditions7.
Historical Background of Platonic Nanoparticles
Historically, the Greeks recognized that there are five Platonic solids that can be created by selecting a regular convex polygon and making each side met at each corner like tetrahedrons, octahedron, hexahedron, icosahedrons and dodecahedron. The symmetrical and simple nature of these compounds laid the foundation of platonic nanocrystals. Not only, such elements but various biological organisms like virus and bacteria takes such crystalline polyhedral forms. Recent literature has demonstrated the tremendous advancements and necessities about shape control of various metal colloidal synthesis. Nanoparticles are now reared through various shapes2,4. They include rods, wire forms, prisms and cubes. However, the preparation of highly symmetrical platonic nanoparticles was of utmost curiosity to the scientific community. The present article shares the report of a systematic shape evolution of gold nanoparticles with sizes of 100nm to 300nm. The process was undertaken to prepare such dimensions was a modified polyol process.
Principle of Shape forms
As shared in the introduction, part the resolution of the solution and the surface precursor properties are extremely important in the formation of nanoparticles. Not only the surface properties but the concentrations of initial starting material should be maintained at very low concentrations and should test through various permutations and combinations, so as to determine the specific concentration of elements required with which the nanoparticle of the desired shape has to be generated. Moreover, the cations that are inserted in the current galvanic method is extremely important in deriving the quality of crystals desired. The presence of impurities will hamper the growth ratio of the planes and may completely form a different shape of nanocrystal that was predetermined under the experimental conditions5,6.
Generation of Icosahedrons Nanocrystal
It was reflected that shapes of nanoparticles were highly dependent on the gold precursors used in several experiments. Studies indicated that reducing the concentration of various gold nanocrystals of icosahedral shapes may be synthesized. In one of the studies, the gold precursor concentration was fixed to be 80% in comparison to the concentrations used for tetrahedral and the molar ratio of the PVP and gold precursors were adjusted to 8.6. Results from transmission electron microscopy revealed that around 90% of such particles had a projected hexagonal shape and sizes varied around 230nm. The size of the particle was considered to be the distance from one edge of a hexagonal projection to the other end of a hexagonal projection7.
The electron diffraction studies reflected the complex pattern of isolated single particles. Even the single isolated particles were shown to be composed of multiple crystal domains. To determine the surface properties of such crystals scanning electron microscopy was performed and it reflected that those particles that appeared hexagonal were icosahedra in shape. These icosahedra particles consisting of face centered cubic topography is one of the most investigated multiples twinned particles in gas phase experimentations. The study reflected that preparation in a solution of such metal particles having a complex but well-defined structures and sizes that are composed of {100} planes, oriented themselves parallel to the supporting substrates. This resulted in higher diffraction intensities. On the other hand, the intensity ratios recorded between the {200) and (111) diffractions had much smaller values than the bulk values for a tetrahedron and icosahedrons samples7.
The intensity ratios were 0.25 and 0.31 respectively for the two shapes. Hence, it indicated that for the tetrahedral and icosahedral samples (111) planes were more dominant. The optical properties of metal nanoparticles are very dependent on the size and shape of the particles. Such hypothesis has been presented through the study of various gold and silver nanoparticles. Various experiments have reported the optical properties in metal nanoparticles may be simulated with arbitrary shapes and such properties exhibited shape dependent behaviours. UV/Visual spectra performed on ethylene glycol solutions exhibited that gold nanoparticles may be generated, which had distinct Plasmon resonance. That is it had 621nm for the nanocubes, 626, 950nm for the tetrahedrons and 613, 950nm for the icosahedrons. It is recognized that the shape of a face centric cubic nanocrystal depends on the growth ratio along its (100) versus the (111) directions7.
The tetrahedron and the icosahedrons bound by the most stable planes will be the largest if the R is also large. The {111) plans will be the most stable planes with an R-value of 1.73. On the other hand, perfect cubes are bounded by the least stable {100} planes and have the lowest R ratio of 0.58. The surface regulating polymer ( template) and the foreign ions inserted in the glycol solution imparts a key role in the formation of gold nanoparticles. It was observed that the selective interaction of the charged particles with the surface polymers resulted in the formation of growth rate along the {100} plane direction. On the other, there growth rate towards the less stable {111} planes was prevented. This resulted in the generation of tetrahedron and icosahedrons shaped gold nanoparticles7.
Hence, the study reciprocated the need for lower gold precursor concentrations, and this is the reason the icosahedrons shaped particles could be isolated from the glycol solution. Thus shape control is greatly influenced by the precursor concentration, was amply clarified through this study. Another possibility of Shape control in gold nanoparticles would pivot around the introduction of different types of foreign ions as these ions are instrumental in providing the direction of growth rate towards the different planes5. The study also projected and speculated that addition of silver ions to the solution will decrease the growth rates in the (100) direction while increasing the growth rate in the {11} directions. This will result in the formation of cubic gold nanoparticles. Such ions will contaminate the formation of icosahedrons particles and, therefore, extreme care should be taken in inserting and selecting foreign ions in the solution media4.
Discussion and Conclusion
The successful generation of platonic gold nanoparticles implicates that shape control can be achieved through the careful and meticulous selection of growth regulation properties, based on the selection of gold precursor concentrations and also the types of foreign ions, as these ions are instrumental in providing the direction of growth rate towards the different planes. It was shown through various studies that impurities will jeopardize the growth rate as expected form the lower concentrations of the nanoparticles and the entire efforts of producing a particular desired shape of the nanoparticle, for example, the icosahedrons shape of the gold nanoparticle will completely be eroded.
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