**1. Introduction**

The mechanical process of covering the surface of an object/substrate with the help of a very thin layer is known as "Coating". This layer can be of some sort of paint, lacquer or a thin polymer sheet, which may be used for protective or decorative purposes. Nowadays, most of the engineered products go through the process of coating to prevent corrosion and to make them attractive [1,2]. From an industrial point of view, coating involves the development of a thin film layer (which can be polymeric or lacquer) on a substrate or fabric etc. If the substrate starts and ends the process wound up in a roll, the process may be termed "roll-to-roll" or "web-based" coating. Apart from a process of simple coating, developing a uniform and thin film or covering to a spinning sample or substrate, is called "spin coating". In the latter sort of coating, a small amount of liquid solution is placed at the center of the highly rotating disk, with the help of a pipette or syringe, resulting in the solution spreading uniformly and evenly in all directions as elaborated in [3]. This is all because of centrifugal forces, which cause liquid solution to spread across the surface uniformly. Application of spin coating is mainly used to fabricate tiny structures, usually of micrometer size or even much smaller, known as the microfabrication process. Manufacturing of solar cells, integrated circuits, insulators, nanomaterials, compact disks, magnetic disk coating and microfluidic devices are a few examples of such technology,

which depend upon the process of high quality spin coating. The simplicity and relative ease that helps to process any set up is regarded as the main advantage of spin coating. The spinning causes fast airflow around the rotating substrates or disks which results in quick drying of a thin layer of coating. Hence, this saves time and reduces the consumption of energy but a consistent efficiency is also achieved at the scale of nanolength or macroscopic level. Surprisingly, spin coating deals with a process which involves single substrates. This leads to a major drawback of spin coating and puts this on the back foot as compared to the "roll-to-roll" coating process. This disadvantage causes low performance. Also, the fast-drying times mean the actual material usage in a spin coating process is typically very low, around 10% or less, with the rest being flung off the side and wasted. Despite these drawbacks, spin coating is usually the starting point and benchmark for most academic and industrial processes that require a thin and uniform coating. Moreover, nanofluids can simply be termed as the liquid containing the tiny metallic particles. These tiny particles are invisible to the naked eye, for these range between 1 nm and 100 nm. In the later phase of the twentieth century, the concept of nanoparticles was introduced by Choi and Eastman [4] as a supporting agent. The initial intention was to obtain immense thermal conductivity of the base fluid. However, Buongiorno [5,6] performed his role focusing on convective heat transfer involving nanoparticles in the concerned base liquids. However, he negated some previous conclusions inferred by different researchers in their investigations. Since then, nanoparticles are being in used in different ways by scientists in their endeavors [7–37]. One cannot deny the beneficial application of nanoparticles from electronics to electrical appliances, from the energy sector to medical sciences working towards the remedy of some fatal disease, it is all mainly due to the blessing of the perfect utility of nanoparticles. Primarily, nanoparticles were meant to enhance the thermal features of a phenomenon involved in it, but recently, nanofluids are being applied in a new dimension, which is in solar collectors. In this application, nanofluids are employed for their tunable optical properties. Consequently, graphene-based nanofluid increases the performance of polymerase chain reaction. As a matter of fact, in some cases nano-technology has improved the performance of spin coating, which requires time to self-assemble or crystallize as the nanoparticles such as gold, silver, zinc oxide, copper and aluminum have significant potential in conductive metal as compared to conventional conductive materials. Especially, incorporating the said nanomaterials into thin films would always pool together electrical and optical properties for multipurpose features that play a key role in fabricating stretchable conductive thin films and coatings, since their mechanical properties include greater flexibility, stretch ability and designed structures. These materials can be easily incorporated into thin films with simple inexpensive solution-based testimony techniques like spin coating, ink-jet printing and spray coating [38,39].

What makes this paper so special is that in this study more than one base fluids suspended with a couple of different nanoparticles have been comparatively studied altogether which, so far, is a novel innovation in the field of applying a thin film of spin coating. To form this shiny silver and gold metallic layer of coating, four different types of base liquids (i.e., water, ethanol, methanol and ethylene-glycol) were brought in to use. It was found that evaporation of the liquid suggested rapidly settling down a shiny metallic layer of silver or gold on the surface of a rotating disk.
