# Spinel Magnetic Iron Oxide Nanoparticles: Properties, Synthesis and Washing Methods
## 3 Main Synthesis Methods of Magnetic and Maghemite Nanoparticles
### Synthesis by Co-Precipitation  
The co-precipitation is the first synthesis method of nanoparticles developed by [PERSON] in the 1980s [20], and still currently the most used [14; 21; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49] by researchers working on magnetic iron oxide nanoparticles. It is based on joint precipitation in aqueous solution of ferric and ferrous ions by addition of a basis such as ammoniac or sodium hydroxide. The main advantages of this method are that it is easy to implement, even on a large scale, and to allow the synthesis of nanoparticles with a quasi-spherical shape and with a size ranging from 10 to 100 nm directly in aqueous solution (Table 2). However, this method is not without disadvantages. There is often an aggregation if nanoparticles with this method at neutral pH and the control of the size stay limited. The ferrous Fe\({}^{2+}\) and ferric Fe\({}^{3+}\) cations are stable in acid solution and precipitate when the basicity of the solution increases to form iron oxide by the Equation (10) [3]:  
\[\text{Fe}^{2+}{}_{\text{(aq)}}+2\text{Fe}^{3+}{}_{\text{(aq)}}+8\text{OH}{}^{- }{}_{\text{(aq)}}\rightarrow\text{Fe}_{3}\text{O}_{4(5)}+4\text{ H}_{2}\text{O}_{\text{(I)}} \tag{10}\]  
However, due to high surface reactivity and their small sizes, the synthesized iron oxide nanoparticles will be able to partially oxidize to maghemite in the presence of oxygen according to the Equation (11):  
\[4\text{ Fe}_{3}\text{O}_{4(5)}+\text{O}_{2}\text{(g)}\to 6\text{ \gamma-Fe}_{2}\text{O}_{3(6)} \tag{11}\]  
The formation of nanoparticles during the synthesis by co-precipitation takes place in four stages, as described by the [PERSON] model [50] (Figure 6):* Stage I: Formation by inorganic polycondensation of zero charge aqua-hydroxy complexes whose concentration will increase with the pH of the solution: [Fe\({}_{2}\)(OH)\({}_{4}\)(H\({}_{2}\)O)\({}_{8}\)]\({}^{6}\) for the Fe\({}^{2+}\) ions and [Fe\({}_{2}\)(OH)\({}_{6}\)(H\({}_{2}\)O)\({}_{6}\)]\({}^{0}\) for the Fe\({}^{3+}\) ions. * Stage II: Nucleation which begins when the concentration of precursors reaches a critical value of saturation (C\({}_{\rm min}\)). There is then appearance of germs in the solution. These germs, very small, tend to redissolve easily. The quick germs formation and redissolution process will continue whereas the concentration of precursors increases. When the critical threshold is reached, stable germs are created and there follows a sudden decrease in concentration of precursors in the solution. If the concentration of precursors falls below the minimum value (C\({}_{\rm min}\)), the formation of new germs is blocked. * Stage III: Growth of stable germs on the solution. It is carried out by addition of precursors in surface of germs by olation/oxolation. The growth will continue if the concentration of precursor is greater than the solubility of the solid (nanoparticles) in the solution. It should be noted that recently, [PERSON] et al. [51] proposed a growth model of magnetite nanoparticles. According to this model, primary nanoparticles around the nanometer size will be formed at an intermediate stage. These nanoparticles will aggregate then to form the finals nanoparticles. * Stage IV: Ripening. This is the last important step for the final characterization of the synthesized nanoparticles. A restructuring of nanoparticles formed by the crystallization of hitherto amorphous phases can occur during this stage. The possible aggregation of nanoparticles as well as the Ostwald ripening during which the smallest nanoparticles are dissolved in favor of the larger ones can also intervene during this stage. These two processes are both driven by the reduction of the surface energy of nanoparticles. Considering the [PERSON] model [50], it appears that the marked separation between the phase II and III, i.e., between the nucleation and the growth is an essential factor for the control of the size of the obtained nanoparticles.