## 1 Introduction  
The fluids at high pressure and temperature that are most frequently used for the industrial processes are carbon dioxide (CO2) and water (H2O), mostly because they can be acquired easily and economically. These fluids, like any other fluid, are characterized by a specific critical pressure and temperature and when those conditions are exceeded, they reach supercritical state. When the supercritical state is reached, liquids exhibit properties that are between those of a liquid and a gas (their diffusivity is near that of a gas and their solvent capacity is similar to that of a liquid, among others) [1]. These characteristics fit ideally with those required for several industrial processes. With regards to CO2 and H2O, some examples of the main high-pressure and -temperature applications that have been developed so far are the following:For CO2
* Impregnation of matrices with antioxidant particles [2; 3]. * Fabrication of biodegradable polymeric [4]. * Precipitation of antioxidants from natural extracts using CO2 as solvent and antisolvent [5; 6]. * Precipitation of metallic nanoparticles [7]. * Extraction and fractionation of substances of interest using CO2 as solvent [8]. For H2O
* Wet oxidation and oxidation with supercritical water [9; 10]. * Hydrothermal liquefaction [11]. * Gasification with supercritical water [12]. * Cooling down nuclear reactors [13]. * Precipitation of micro- and nanoparticles [14; 15]. As can be seen extensively in the literature [5; 6; 7; 14; 15], high-pressure and -temperature fluids turned into a significant tool to synthesize and precipitate micro- and nanoparticles of great interest and that may have variations in morphology and size that may determine the potential in the implementation of these technologies. CO2 is the most frequently used fluid to produce reduced-size particles (nano and micro), mainly because of its relatively low critical pressure and temperature (73.8 bar and 31.1 \({}^{\circ}\)C, respectively), which makes of this an economically feasible procedure when compared to others that use other fluids. It also offers the possibility of precipitating thermolabile particles. Consequently, and based on the promising qualities showed by CO2, this procedure was the subject of many and thorough studies where different technologies that use CO2 as the solvent, antisolvent, or reagent were developed, as can be seen in the review by [PERSON] et al. [4]. Like carbon dioxide, water also opens an interesting field of study, particularly on the precipitation of particles which may encounter certain difficulties when CO2 is used. For instance: metal particles of a considerable value in the market. The excellent properties of water, for the production of nano- and microparticles, as its thermophysical properties (see Section 2), made it the focus of attraction and interest for new research groups, who probably find this synthesized review on the hydrothermal synthesis of particles very useful, since it has a wide field of study that encompasses the entire range production scenarios, from a small scale (initial investigation phases) to a pilot plant scale. Furthermore, this review describes essential sections for new research groups, such as the differences between this method and other conventional procedures, the most currently active research centers, the types of particles synthesized, the techniques to evaluate the products obtained, and the main operating parameters and deals extensively with what is still to be investigated as well as prospective trends in the near future. Hydrothermal synthesis is a branch in hydrothermal technology that is currently opening new perspectives, as can be seen in Figure 1. It links all the relevant technologies such as biotechnology, nanotechnology, and advanced materials technologies [16]. A clearly growing demand for nanotechnology and advanced material technologies boosted the interest on hydrothermal synthesis processes over the last 15 years.