## 5 Noble Nanofluids  
### Gold Nanofluids  
The gold nanoparticles possess the benefits of being biocompatible and non-cytotoxic. Besides, gold has been used internally in humans for the last five or six decades because of their chemical inertness. The gold nanoparticles dimensions can be minimized during their synthesis and functionalization. The gold aqueous nanofluids using molecular dynamics nanofluids are included in a novel class of fluids with superior thermophysical characteristics and heat transfer behavior. In this sense, the researchers [PERSON] et al. [21] observed that the gold nanofluids exhibited enhanced solar thermal conversion. The authors observed that at only 0.15% wt., the gold nanofluids enhanced the photothermal conversion efficiency by 20%. Additionally, the authors [PERSON] et al. [22] evaluated the heat transfer performance of gold aqueous nanofluids under laminar flow and imposed fixed heat flux flowing in a heat pipe. The obtained results demonstrated increases in the Heat Transfer Coefficient (HTC) of 8%, 15%, and 23% at the minimal volumetric concentrations of 0.00015% vol., 0.00045% vol., and 0.000667% vol. of gold nanoparticles, respectively, in reference to that of the water itself. The authors observed also that the heat transfer enhancement was higher in the inlet nearby area and the thermal boundary layer of the nanofluids was superior to the one of the water alone. The researchers concluded that the gold nanofluids are very suitable for heat transfer purposes in micro processes using heat pipes. The governing mechanisms of the heat transfer enhancement rely on factors like the enhanced thermal conductivity, viscosity, and nanoparticle Brownian motion and migration. Besides, the authors [PERSON] et al. [23] inferred the impact of the size of the gold nanoparticles on the photothermal conversion efficiency in cube and flat direct absorption solar collectors. The nanoparticles were manufactured by a seed mediated method and the experimental results showed that the gold nanofluids at 0.000008% wt. provided photothermal conversion efficiencies enhancements of nearly 21.3% and 20% for flat shaped and cube-shaped direct absorption solar collectors, respectively, in reference to that of the water itself. The cube-shaped collector presented a greater photothermal conversion efficiency than the one attained with the flat-shaped collector operating with the same nanofluids, given that the cube-shaped collector optical path length was superior to the same length of the flat-shaped collector and, consequently, the former had an enhanced solar radiation absorption. The photothermal conversion efficiency diminished with increasing size of the gold nanoparticles for the flat-shaped collector because of the interactions of the optical properties of the nanofluids, heat losses to the surrounding environment, and thermal conductivity increase. Nonetheless, the influence of the size of the gold nanoparticles on the cube-shaped collector was not so evident as it happens in the flat-shaped collector. Besides, the authors [PERSON] et al. [24] evaluated the impact of the concentration and dimensions of the gold nanoparticles dispersed in water on the parameters of a direct absorption solar collector, such as the stability of the nanofluids, extinction coefficient, and photothermal efficiency. The sizes of the nanoparticles were of 5 nm and 20 nm and their concentrations were of around 5 ppm, 28 ppm, and 51 ppm. The authors demonstrated that the incorporation of gold nanoparticles with a surface plasmon resonance of 516 nm for the 5 nm-sized particles and of 520 nm for the 20 nm-sized particles enhanced considerably the light absorption capacity of the water itself. Also, the research team confirmed that the photothermal conversion efficiency was closely linked with the concentration and size of the dispersed nanoparticles. In addition, the photothermal conversion efficiency was increased to a maximum 121% in comparison with that of the water alone using the 5 nm-sized gold nanoparticles at 51 ppm.