## 3 Comparison with Other Nanofluids  
The noble metals nanofluids present many beneficial features in comparison with the other nanofluids. The following points summarize the main benefits:  
* Better Optical and photothermal conversions, which can be of great usefulness in solar energy systems. * Superior plasmonic nanofluid filtering with superior surface plasmon resonance. * Superior biocompatibility. For instance, the gold nanoparticles dispersed in blood can act as drug carriers, destroy cancer cells, and other bio-medical purposes. * Absence of fouling and Nanoparticles deposition or settlement of the nanoparticles, as in the case of silver Nanoparticles most likely due to their diamagnetic nature. * Much less concentrated than the other nanofluids. Hence, they require less material. This can be very suitable for large-scale applications using many liters of nanofluid. Nonetheless, an economic analysis upon investment cost/heat transfer capability improvement should be always performed. * Being the concentrations extremely small, there can be performed experiments with higher concentrations without the risk of fouling, erosion, or clogging of the equipment as it frequently happens with the other nanofluids. Also, the required pumping power and pressure drop associated with these noble metals nanofluids with Higher concentrations is not expected to raise so sharply as it occurs with the other nanofluids. Higher concentrations are expected, according to a major part of the published works, to considerably improve the heat transfer performance of the noble metals nanofluids. * All the mentioned noble metals nanofluids can be prepared by eco-friendly methods, which does not always happen with the other nanofluids. A quantitative/qualitative comparison between the noble metals nanofluids and the other nanofluids should only be done with the same operators and the same operating conditions as temperature and pressure. That is the case of the experimental work carried out by the authors [PERSON] et al. [1] who synthesized metallic nanoparticles by the LAL method in the long chain polymeric and environmentally friendly medium polyethylene glycol. In their work, the green, facile one step LAL method was followed to produce copper,gold, silver, nickel, palladium, and platinum nanoparticles dispersed in polyethylene glycol. The incorporation of the metallic nanoparticles with LSPR ability in the base fluid enhanced its light-to-heat conversion efficiency. The prepared metallic nanofluids were stable and durable due to the strong interaction of the nanoparticles with the polyethylene glycol molecules, resulting in a stable colloidal structure. The gold, silver, and palladium nanofluids where the ones with the higher light-to-heat conversion efficiency. The size variety of the silver nanoparticles enhanced the capability of the corresponding nanofluids to absorb a wider range of sunlight wavelengths and convert it to heat more. Also, the inclusion of the metallic nanoparticles enhanced the dynamic viscosity of the nanofluids at room temperature, but after the photo-thermal conversion and increasing the temperature of the working fluid, the dynamic viscosity was reduced, resulting in more heat transfer efficiency of the prepared nanofluids. There was no deterioration in the light-to-heat conversion of the silver nanofluids after several heating/cooling cycles and the prolonged high-intensity illumination confirmed that the silver nanofluids were also thermally stable. All the obtained SER, ETOTAL, E%, and B rate indicated the most capable part of the metallic nanofluids for light to heat conversion applications was their top layers. Briefly, the prominent merits of the obtained gold, silver, and palladium nanofluids include a fast, bio-friendly, and facile preparation method, not using surfactant or chemical additives, long time stability even under light radiation, reasonable photo-thermal conversion efficiency, and improved reusability.