Stable colloidal suspensions of nanoparticles in solvents are conventionally termed as nanofluids. Several controversial reports in the literature on nanofluids explored the anomalous enhancement of their thermophysical properties (such as thermal conductivity). This has generated considerable interest among several research groups in the last three decades leading to thousands of publications on several aspects of nanofluids, spanning fundamental studies to various applications in engineering and life sciences. The popularity of nanofluids is spawned by their ability to deliver tunable material properties based as well as the ease and flexibility afforded by a multitude of nanosynthesis protocols. The material properties of nanofluids are highly sensitive to small variations in the nanosynthesis protocols. The properties of nanofluids can be tuned by varying their size, shape, morphology (surface functionalization), material composition, structure and mass concentration of nanoparticles as well as a variety of techniques for dispersing the chosen nanoparticles in the liquid solvents (which is often overlooked as an important variable in the nanofluids literature). Hence, due diligence is required in implementing the synthesis protocols for achieving nanofluids with the desired functionality and ensuring the stability. Different techniques for synthesizing nanofluids are summarized and the effect of synthesis conditions on the material properties of nanofluids are reviewed in this study. Several examples for a selected set of candidate nanofluids are discussed as well. This also helps to identify the gaps in the literature as well as recent developments on the topic of large-scale synthesis of nanofluids. This need is particularly acute for providing flexibility in scale-up for large volume manufacturing and batch-fabrication capabilities required in several practical applications (e.g., thermal energy storage and power generation). These applications would require tons of nanofluids to be synthesized on a commercial scale, as opposed to several milligram quantities that are typically synthesized in a research laboratory for a scientific study.
