# Sol-Gel Silica Nanoparticles in Medicine: A Natural Choice. Design, Synthesis and Products
## 3 Silica NP Design
### NPs Synthesis Methodologies
#### 3.3.3 Solution Route
The confined nanoreactor environment is shown to yield highly monodisperse NPs and allow the incorporation of non-bonded non-polar molecules, which are often difficult to incorporate into the hydrophilic silica matrix. In the last few years, several dye-doped SiO\({}_{2}\) NPs have been synthesized by the reverse microemulsion technique in which polar dye molecules are used to ensure successfully encapsulation into SiO\({}_{2}\) NPs [135]. The reverse microemulsion process is widely used in silica NPs synthesis. However, besides having low yields, the reverse microemulsion process uses a large amount of potentially toxic surfactants and organic solvents, and demands previous washing to biological application, in order to avoid disruption or lyses of biomembranes. The [PERSON]'s method arises as more eco-friendly alternative, in which the hydrolysis and condensation of a mixture of alkoxysilanes takes place in mild basic aqueous medium, to create monodisperse, spherical, electrostatically-stabilized particles. Recently (ammonia free) Stober silica NPs were synthesized under basic catalyzed ensured by hydrothermal water (SPA Cabeco de Vide, Portugal, pH \(\sim\)11) [136]. Figure 9: NPs sol-gel reverse emulsion synthesis method. The Stober method is a promising method for producing surfactant free silica NPs or coatings; yet the final particles size remain in the hundreds of nanometers to micron regime, which are too large to some of the biological studies. LaMer alternative allows the control of particles size and dispersion, but a regular shape (silica NPs < 100 nm) is still difficult to obtain. NPs prepared through the microemulsion method, exhibited smooth surfaces and low polydispersity. However, for use in biomedicine, the microemulsion method is not as safe as the Stober one; the use of surfactants in the NPs synthesis carries a higher risk of cytotoxicity. Stober silica NPs are largely used in oral applications on account of their chemical stability and intrinsic hydrophilicity, being thus appropriate for biological environments. AEROPERL(r) 300 Pharma (particle size of 30 mm) is used in formulations of hesperidin oral delivery carrier [137], hydrophylic Aerosil 380 (7 nm in size) is used to stabilize Pickering emulsions in lipid-based oral delivery systems [138; 139; 140]. Oral insulin bioavailability was tested in a SiO\({}_{2}\) nanoplform (silica NPs associated with insulin and then coated with mucoadhesive polymer, like chitosan or PEG) [100; 102] (Table 4). Sol-gel allows in situ incorporation of a variety of functional (non-hydrolysable) organic groups within the silica matrix, in order to increase their biocompatibility, improve its resistance to enzymatic action, internalization efficiency and gene targeting (either in Stober or reverse emulsion methods). The ORganically MOdified SILica matrix (known as ORMOSIL [141; 142]) is an alternative material with even better and more versatile properties than silica: the presence of non-hydrolysable organic groups in the alkoxisilane precursors makes these behave like glass modifiers, reducing the degree of silica network cross-linking as well as increasing the network flexibility as the unhydrolyzed--Si-R bond apparently dangles, causing higher mobility during gelation and undergoing weaker contraction during drying. A tunable wettability, by a judicious choice of the ratio of hydrophilic to hydrophobic sol-gel precursor monomers, a tailor made porosity (size and shape) and a shell hardness/complacency making ORMOSIL a very competitive material. Furthermore, ORMOSIL NPs surfaces will be populated with both silanol and non-hydrolysable organic groups, allowing an easier chemical conjugation/decoration of biomolecules at the NPs surface and/or be loaded with either hydrophilic or hydrophobic drugs or dyes. Mammalian cells take up and internalize easily silica/ORMOSIL NPs (without any cytotoxic effect) opening the door to its use in health science [143].