**3.3. Microemulsion method**

A microemulsion is formed when a colloidal substance is dispersed in a solvent, that is not compatible with the substance (e.g. water and oil), through a surfactant. Finally, a microemulsion must be clear and stable, as long as it is an isotropic mixture of oil, water and surfactant. The surfactant forms a monolayer film at the oil/water interface, in which the hydrophilic head groups of the surfactant are dissolved in oil phase (consisting of a mixture of hydrocarbons and olefins) and the hydrophobic tail of the surfactant in the aqueous phase (consisting of metal salts) and vice versa, depending on the used surfactant. There are known two types of microemulsion: direct microemulsion, when the oil is dispersed in water and reversed microemulsion, when the water is dispersed in oil. Both have been used to synthesize the magnetic iron oxide nanoparticles with tailored size and shape. The most common surfactants that are widely used in the fabrication of M-IONPs by microemulsion method are bis(2-ethylhexyl) sulfosuccinate (AOT), sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB) and poly-vinylpyrrolidone (PVP). Throughout time, the microemulsion method proved to be a simple and versatile method for fabrication of nanosized magnetic nanoparticles [61–65].

According to the literature, the size of the resulted nanoparticles can be controlled if the surfactant is proper chose and also by varying the ratio of water/oil/surfactant, the initial concentration of the reactants and the droplet size and by controlling the reaction temperature and time [66, 67]. The size of the synthesized nanoparticles can also be controlled in suitable narrow range by carrying out the reaction in nanoreactor [62, 64, 67].

Lu et al. demonstrated that the surfactant nature has an important role on the final properties of the nanoparticles [64]. The authors have investigated the effect of SDS (anionic surfactant), DTAB and CTAB (cationic surfactants) and non-ionic surfactant on the preformed crystal, on stoichiometric situations and on the magnetic properties of the resulted Fe3 O4 nanoparticles. In all the cases, the authors have obtained Fe3 O4 nanoparticles with size less than 16 nm, but in the case of using the cationic surfactants also obtained a good saturation magnetization, which is an essential parameter for biological applications.

Okoli et al. have prepared M-IONPs by the two types of microemulsion (water/oil and oil/ water), to be used in binding and separation of proteins. The authors demonstrated that by using a water/oil microemulsion, it can obtain magnetic iron oxide nanoparticles with a surface area of 147 m2 /g compared to 304 m2 /g for the magnetic nanoparticles obtained by oil/water microemulsion [68, 69]. The M-IONPs specific surface area is inversely proportional with the size of nanoparticles, the higher is the specific surface area the smaller nanoparticles size is obtained.

The advantage of this method is the fact that it can be obtain magnetic nanoparticles with uniform morphology and controllable size; but the major drawbacks are the requirements of a large amount of solvent and the excess of surfactant that has to be eliminated.
