## 1 Introduction  
The nanotechnology area is one of the most dynamic research areas in the materials science and technology and the production of nanoparticles is gaining an ever-increasing worldwide importance. The nanoparticles show improved properties that are influenced by their characteristics like shape, size, and nature. The nanoparticles can be divided into inorganic and organic. The inorganic nanoparticles incorporate semi-conductor nanoparticles, metallic nanoparticles like gold, silver, and copper, and magnetic nanoparticles like cobalt and iron, whereas the organic nanoparticles category incorporates carbon nanoparticles like, for instance, carbon nanotubes. There is a growing enthusiasm for noble metal nanoparticles and nanofluids like the ones of gold, silver, platinum, palladium, and ruthenium, given that they offer superior thermophysical properties together with advantageous features that are not easily encountered in other nanoparticles like biocompatibility, non-cytotoxicity, and chemical inertness [1]. Together with the increasing use of the noble metal nanofluids came the exploration of bio-friendly synthesis methods. The implementation of the green synthesis of the noble metals nanoparticles and nanofluids is still advancing as a key branch of nanotechnology where the use of plant extracts and biomass, and microorganisms for the generation of nanoparticles could be a reasonable option for chemical and physical methods processed in a bio-friendly mode. The green synthesis progress over the conventional physical and chemical procedures is that the green procedures are environment friendly, cost-effective, and easily scalable for vast scale production of nanoparticles and nanofluids, whereas the limitations originated from the high-temperature, energy, pressure, and toxic chemicals issues are not considered in the green synthesis [2]. The silver nanoparticles, for example, exhibit a considerable surface area that imparts to them remarkable catalytic activity and biochemical reactivity in comparison with larger nanoparticles with similar chemical composition. Moreover, the production of silver nanoparticles has attracted notorious attention due to the promising use in catalysis, plasmonic, antimicrobial activity [3], DNA sequencing, energy conversion, and biomedical applications [4]. The most common bottom-up method is the chemical reduction for the synthesis of silver nanoparticles [5]. Different inorganic and organic reducing agents included in aqueous/non-aqueous solutions are normally employed for the silver ions reduction. Capping agents are additionally used for size stabilization of the nanoparticles. A considerable amount of experimental and theoretical research was carried out to infer the effect of metal and metal oxide nanoparticles in the thermal conductivity enhancement of the nanofluids. It was found that a high thermal conductivity enhancement could be attained with noble metals nanofluids, even with very small concentrations of nanoparticles. Also, the heat transfer capability of the nanofluids with noble metals nanoparticles was reported to be much higher than the one, for example, of the metal oxide nanoparticles. The noble metals nanofluids were investigated by several researchers like, for instance, [PERSON] et al. [6] and [PERSON] et al. [7] and they confirmed significant heat transfer improvements. The main heat transfer application of the noble metals nanofluids is their use as the operating heat transfer media in the direct absorption solar collectors to improve the solar-thermal conversion efficiency. The common solar collectors pose the question on how to ameliorate the absorption efficiency of solar energy and how to efficiently transmit the absorbed energy in the working medium. With enhanced absorption and heat transfer characteristics, the noble metal nanofluids can accomplish the requirements for the working thermal fluid in solar collectors, and greatly increase the conversion efficiency. In this sense, the researchers [PERSON] et al.