Nanoparticle Synthesis

The most challenging part of research in the field of nanotechnology is the environmentally safe and cost effective procedures for nanomaterials synthesis. Two approaches have been known for nanomaterial synthesis of ultrafine particles since ancient times. There are two methods employed for the nanoparticle synthesis such as "Top down" and "Bottom up" Synthetic strategies, which can also be considered as physical and chemical methods, respectively. The first is the top-down method by which an external force is applied to a solid that leads to its break-up into smaller particles. This method includes the following techniques:
1. High-energy ball milling: Nanoscale powders are produced by milling bulk materials [46].
2. Arc discharge:

Gold is a metal whose atomic weight is 79 and its standard atomic weight is approximately 197 gr/mol. It has a yellow colour when in bulk, though when finely divided it may be red, black, purple or blue. In the bulk form gold is a soft yellow metal, which has a face centred cubic crystal structure. Gold as a bulk material is known for thousands of years (before 3000 BC) and generally is extracted from mines. It has been used generally in decoration and jewellery from ancient times [66]. Gold has a boiling point of 2807oC and a melting point of 1064.63oC. Gold is soft, malleable and ductile and is usually alloyed to improve strength and durability. Gold’s reflectivity at ultraviolet and visible light wavelengths is low; however it has high reflectivity at red and infrared

Properties of gold nanoparticles can be varied by altering the size, shape, degree of aggregation, and local environment. They exhibit characteristic colors and unusual optical properties that strongly depend on their shape, size, and the dielectric constant of the surrounding medium [69]. The characteristic wine-red color of spherical gold nanoparticles stems from a phenomenon termed surface plasmon resonance. The plasmonic effects include two main ingredients: the surface plasmon resonance that is the charge density wave propagation on the interface of metal and dielectric, and the localized surface plasmon resonance (LSPR) that is the non-propagating collective oscillation of electrons coupled to electromagnetic field. The early development of plasmonics in the last century is marked by several milestones including the description of free electron collective oscillation and the naming of ‘plasmon’ by D. Pines in 1956 [70]. The first experimental demonstration of optical excitation of surface plasmons on metal films was made by Andreas Otto [71] as well as E. Kretschmann and H. Raether [72] in 1968. The first description of the optical properties of metal nanoparticles in terms of surface plasmons was by U. Kreibig and P. Zacharias in 1970 [73]. S. Cunningham et al. introduced the term ‘surface plasmon resonance’ in 1974