## 3 Production of Nanoparticles by Using CO2 as an Antisolvent  
Supercritical CO2 is widely exploited as an antisolvent to produce pharmaceutical nanoparticles/nanocrystals using different techniques, including the following:  
- Aerosol solvent extraction system (ASES);  
- Supercritical gas antisolvent (GAS);  
- Precipitation with compressed fluid antisolvent (PCA);  
- Supercritical antisolvent precipitation (SAS);  
- Solution-enhanced dispersion by supercritical fluids (SEDS);Supercritical fluid extraction of emulsions technology (SFEE). Both conventional drugs [24; 39; 58; 71; 72; 73] and active compounds of natural origin [23; 74; 75; 76; 77; 78; 79; 80] have been nanonized using scCO\({}_{2}\) as an antisolvent. Indeed, there is a recent trend in the production of pharmaceutical formulations using natural products or extracts, to exploit their beneficial properties on human health, including antioxidant, antimicrobial, anticancer, and anti-inflammatory ones [2]. Nanoparticles and nanocrystals based on different active compounds, such as antibiotics [24; 71; 72; 81], NSAIDs [58; 82; 83], antifungals and antimicrobials [39; 84; 85], and drugs for prevention or cancer treatment [73; 74; 75; 76; 78; 86], have been produced. Nanosized systems prepared using scCO\({}_{2}\) as an antisolvent have been proposed for various pharmaceutical forms, such as those for inhalation [40], oral [39; 71; 87; 88], topical [39; 71; 88], and injectable [71; 78; 89] routes. The SAS process (schematized in Figure 4) is the most widely used technology in which scCO\({}_{2}\) acts as an antisolvent. Liquid solvents that are completely miscible with scCO\({}_{2}\) under process conditions are chosen, mainly dimethyl sulfoxide (DMSO), ethanol, methanol, and acetone. The solute to be nanonized has to be soluble in the selected liquid solvent but insoluble in the solvent-antisolvent binary mixture formed in the precipitation chamber. Therefore, when the liquid solution (solvent + solute) is injected through the nozzle into the precipitator filled with scCO\({}_{2}\), the solute precipitates due to the supersaturation caused by the fast diffusion of scCO\({}_{2}\) into the droplets formed after the jet breakup [90]. Nanoparticles are usually produced via the SAS process by working with low temperatures (in the range 35-50 \({}^{\circ}\)C) and pressures equal to or mostly higher than 9 MPa in the precipitation chamber. Indeed, the smallest particles have often been obtained by decreasing the operating temperature [42; 82; 91; 92]. As a rule, the particle size also decreases by increasing the pressure, as the surface tension tends to zero faster, or by increasing the overall concentration of the solution, due to the increase of the viscosity [73; 77; 92; 93; 94]. Exemplificative scanning electron microscopy (SEM) images of large crystals of unprocessed diclofenac sodium and nanoparticles precipitated by the SAS process are reported in Figure 5a,b, respectively. Unlike nanoparticles, drug nanocrystals precipitated by the SAS process have been observed in very few papers [74; 81]. Figure 4: A sketch of the supercritical antisolvent precipitation (SAS) process. Several studies have shown that the SAS technique is more effective for obtaining small particles for pharmaceutical applications than conventional processes, including spray-drying [22; 24], freeze-drying [21], milling technologies [23], and solvent evaporation [29]. For example, in the study by [PERSON] et al. [24], cedfinit nanoparticles prepared by SAS showed a narrower PSD (150 \(\pm\) 70 nm) and a faster dissolution rate than cedfinit microparticles (2.32 \(\pm\) 1.76 \(\upmu\)m) produced by spray-drying. Similarly, resveratrol dissolution rate increased for both nanoparticles and sub-microparticles (in the range 150-500 nm) produced via SAS process by [PERSON] et al. [23] compared to large crystals (18.75 \(\upmu\)m) prepared by Fritz milling or crystals and irregular/coalescent particles (1.94 \(\upmu\)m) obtained by jet-milling.