## 1 Introduction
[8] investigated that the gold nanofluids could generate vapor under concentrated sunlight irradiation. The research work indicated that 80% of the gold nanofluids that absorbed solar energy were directed to the vapor generation, but only approximately 20% of that was explored to enhance the temperature. It was believed that the thermal trapping action of the nanoparticles provoked a non-uniform distribution of temperature and the overheating in the nanofluids. Furthermore, the authors [PERSON] et al. [9] investigated the absorption capability of the hybrid nanofluids composed of gold-iridium tin oxide nanostructures dispersed in Duratherm S flowing in the receiver of a photovoltaic/thermal solar parabolic trough collector at temperature values superior to 100 \({}^{\circ}\)C. The system showed better thermal characteristics in reference to the base fluid and achieved a photovoltaic efficiency of 4%, whilst attaining a peak thermal efficiency of 61% with an outlet temperature of the fluid of 110 \({}^{\circ}\)C.  
Despite the improved heat transfer and energy absorption characteristics of the nanofluids, they may still pose threats to humans. Specifically, a great number of nanoparticles will migrate during the vapor generation under concentrated sunlight process of the nanofluids, which deserves more attention. In the last decades, the investigators have begun to dedicate time to the expected hazardous effects of rapidly developing nanoparticles and issued a first warning about the nanoparticles driven pollution [10]. Many works have reported that the nanofluids, including the metallic and carbon-based ones, had negative impact on humans, plants, and microorganisms [11]. Thus, it is paramount to deeply analyze the migration characteristics of the nanoparticles in vapor processes. The researchers have been mainly focused on the thermal behavior of the nanofluids in solar collectors, but the nanoparticles migration characteristics during the phase transition should be further addressed. The research gap of the published literature is the non-existence of an overview work regarding all the fundamental noble metals nanoparticles and nanofluids, and their applications. Until now, the literature provided only modest review contributions, especially on the gold and silver nanofluids. Hence, this work can be considered a novelty in this field of research since it gathered in only one work the comprehensive overview of the noble metals nanofluids, their properties and their specific use in heat transfer purposes. In addition, more than nine tenths of the included references are exclusively bounded with noble metal nanofluids and their hybrids, which is, according to the knowledge of the authors of this work, is a major gathering of data dedicated to these innovative fluids. The structure of this work is composed by the introduction section and sections regarding the fundamental preparation methods and thermal conductivity influencing factors. After that, dedicated sections incorporating the main published works on each type of nanofluid along with their bimetallic and hybrid forms are included. Finally, a section with the main limitations and challenges associated with the noble metal nanofluids is presented. The main findings of the work are inherent to the heat transfer capability and efficiency of the systems enhancements provided by the noble metals nanofluids. Such enrichment will provide more efficient thermal management systems and this will compensate the overall investment cost that arises from the preparation and use of these nanofluids. Figure 1 summarizes the fundamental applications of the noble metal nanofluids.