## 2 Main Preparation Methods  
The preparation methods of the noble metals nanofluids can be divided into two general approaches of one-step and two-step methods. They have been prepared by many different procedures like the most employed chemical reduction, microwave assisted precipitation, the set of bio-friendly methods of production that is herein designated by green synthesis, among others. The main preparation methods of the noble metals nanofluids are summarized in Figure 2.  
Figure 1: Fundamental applications of the most used noble metal nanofluids. For example, the sonochemical synthesis involves the application of potent ultrasound radiation to molecules to activate chemical reactions. The passage of ultrasound through a solution generates high- and low-pressure regions consistent with periodic compression and expansion, respectively. This variation in pressure denotes the sonochemical reaction initiation that precedes the acoustic cavitation, particularly the formation, expansion, and collapse of the acoustic bubble. The collapse bubble's potential energy is changed over to the active energy of the microjet with speeds that reach hundreds of meters per second. However, the nanoparticles smaller than the collapsing bubble size of approximately 150 \(\upmu\)m are not able of producing microjets after irradiation with a 2 kHz ultrasonic field. Instead, cavitation and shock wave emissions arise. The formation of the acoustic bubble entails the diffusion of dissolved air molecules. Subsequently, the acoustic bubble and its internal matter are violently compressed by the high external pressure. This expansion and compression of the acoustic bubble are maintained until the external pressure completely takes over and the bubble collapse. Several factors, including ultrasonic output power and frequency, type of the solvent, and solution temperature, affect the efficiency of cavitation and its derivative chemical and physical properties. These factors determine the properties of the synthesized nanoparticles (e.g., gold or silver), such as morphology, structure, stability, and surface plasmon resonance (SPR). The green environmentally friendly synthesis approaches pose the benefits of a reduced environmental and toxicity impacts. For instance, the authors [PERSON] et al. [12] synthesized silver-water nanofluids where silver nanoparticles were produced with the aid of green tea leaves. After harvesting, drying, and grinding of the tea leaves, the resulting powder was added to water: 14.8 g per 100 mL water. The mixture was boiled for approximately 12 min and after some processes it was kept at temperature values between 0 and 4 \({}^{\circ}\)C. It was found that the total organic carbon extracted from the leaves is about 19 g/L. To produce silver nanoparticles, silver nitrate was added gradually to the tea extract and mixed. Finally, the silver nanoparticles were purified by ultrafiltration. The researchers stated that the size of the nanoparticles depended on the amount of the silver nitrate added to the tea extract. To make the silver nanofluids, the green synthesized silver nanoparticles were added to water. To disperse the nanoparticles in the base fluid uniformly, a surfactant was added to the mixture and then by sonication and pH control of the suspension, it can be stable for more than two months. Though most of the published works on the noble metals nanofluids were carried out with small match synthesis processes, the nanofluid preparation in large volumes should be prioritized and further studied. In applications where many liters of nanofluid are needed, the large-scale nanofluid production methods must be developed. While quality control of the resultant nanofluid is relatively easy in small batch production methods--e.g., through control of temperature, precursor concentration, and chemical kinetics--high-throughput, continuous  
Figure 2: Main Preparation methods of the noble metals nanofluids. processes that enable the same level of high-quality, homogeneous nanoparticles are still lacking.