## 7 Conclusions  
In the published literature it is notorious the lack of a review work regarding all the fundamental noble metals nanofluids and their applications. Hence, this work can be considered a novelty in this field of research since it gathered in one single overview the main noble metals nanofluids, their properties and their specific use in heat transfer purposes. The structure is composed by the introduction section, 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 this type of innovative fluids. Such enrichment will compensate under the form of more efficient thermal management systems the overall investment cost that arises from the preparation and use of these nanofluids. Also, it is confirmed that the noble metal nanofluids have, in general, better thermophysical properties that the ones offered by the base fluid mainly due to the effects from the solid-liquid interface layer. The noble metals hybrid nanofluids sometimes work better that their mono nanofluids counterparts and sometimes the opposite trend occurs. The main concluding remarks that can be retrieved from the present overview on the noble metals nanofluids can be listed as follows:  
* The surface plasmon resonance effect provided by the gold nanofluids led to a superior photothermal conversion efficiency even at a low concentration of 10 ppm and this parameter decreased with increasing size of the nanoparticles. * The trisodium citrate is an excellent reducing agent for the preparation of gold nanofluids to be applied in the solar thermal energy conversion systems. An energy efficiency enhancement of approximately 95% was attained with a 0.04% wt. gold nanofluid. * The impact of the morphology of the gold nanoparticles on their photothermal conversion rate is still being examined by the research community. As an example, it was already verified that the photothermal conversion efficiency decreased with increasing size of the nanoparticles in a flat shaped direct absorption solar collector, which was caused by the interactions between the optical property, heat loss, and thermal conductivity increase, but the impact of the size of the nanoparticles in a cube-shaped collector was lower than the one in the flat shaped collector model. * The ketone and alcohol solvents form spherical gold nanoparticles, whereas the acetonitrile and tetrahydrofuran form rods or triangular gold nanoparticles. The rods and triangles are more prone to clustering and sedimentation producing unstable nanofluids. Hence, it is recommended to avoid the use of acetonitrile and tetrahydrofuran or to add protective agents. * The bio-friendly silver-coconut oil nanofluid is a promising solution for thermal management systems working with increased heat fluxes. Also, the fouling formation inherent to the use of this nanofluid can be taken as negligible in comparison with other nanofluids like the alumina and copper oxide aqueous nanofluids. * The microwave-assisted one-step method has already proved to be able to produce stable silver nanofluids having nanoparticles between 30 nm and 60 nm of dimensions. The microwave methodology is facile, rapid, and scalable. * It was demonstrated that silver nanofluids at only 1% vol. could increase the base fluid effective thermal conductivity approximately by 1.6 times at 50 \({}^{\circ}\)C.
* It was already confirmed that small silver nanoparticles at very low concentrations exhibit higher thermal conductivity and diffusivity values than water.