Moreover, different configurations of the silica coated magnetite nanostructures can be achieved following two different routes: Stöber processes generally result into a multi-core coated NPs, while microemulsion processes provide mainly single core-shell NPs (see Figure 12). The reaction between the oxide surface of magnetite and the silica takes places by the OH − groups, and by an adjustment of the amount of added TEOS and the reaction time, the silica shell thickness can be easily tailored. The preparation of magnetite NPs coated with silica by sol-gel procedures allows to control the shell thickness and is affordable by the basic hydrolysis of silanes in aqueous solutions of different organosilane compounds (tetraethyl orthosilicate (TEOS), APTES). Silica (SiO 2) is one of the most commonly used inorganic coating materials, due to its versatile chemical silanol surface groups (-SiOH), colloidal stability and biocompatibility.
Inorganic materials are used also to confer core protection colloidal stability and other functional properties like surface plasmon resonance (metal NPs), thermal isolation (carbon shell), etc. The so obtained NPs show a high degree of crystallinity (see Figure 8) and a saturation magnetization (M S = 84 emu/g, close to the range of bulk magnetite (M S~92 emu/g). Specifically, a combination of high temperatures, between 355 to 525 K, and prolonged reaction time (24 h), has been studied and successfully applied to produce Fe 3O (see Figure 8) and Fe 3O coated NPs, with sizes around 20 nm and high yield (nearly 86%), by mixing FeCl 2∙4H 2O and FeCl 3∙6H 2O in a Teflon vessel with either an OA or a PAA solution, respectively. This high energy wet chemistry technique, produces NPs with high crystalline quality and optimum magnetic performances, since lattice formation benefits from the high temperatures (from 355 to 525 K), high vapor pressures (from 0.3 to 4 MPa) and long times (up to 72 h) to which the experimental conditions are subjected. Hydrothermal synthesis is a case of solvothermal procedures, that submit the reactants into a stainless-steel autoclave at high pressure and temperatures but using water as solvent. The unit cell in bulk Fe 3O 4 consists in a face centered cubic, fcc, (Fd 3m space group) arrangement of O 2− ions, in which Fe 3+ (d 5) cations occupy 1/2 of the tetrahedral interstices, and a 50:50 mixture of Fe 3+ (d 5) and Fe 2+ (d 6) cations occupy 1/8 of the octahedral interstices, with a characteristic unit cell parameter ≈8.4 Å (see Figure 1). An example of this stability is the fact that both, natural magnetite is commonly found containing impurity ions (Ti, Al, Mg, and Mn), and substituted ferrites containing transition metals (Co, Mn, Zn) can be easily obtained by wet chemistry procedures. The spinel is a stable crystal structure that accepts the substitution of the A and B lattice locations by a variety of nearly 30 different metal ions with valences ranging from +1 to +6. The magnetic and electric properties of magnetite arise from the interactions between the Fe 3+ (d 5) and Fe 2+ (d 6) ions placed on octahedral positions and Fe 3+ (d 5) ions on tetrahedral positions. Magnetite, Fe 3O 4, is an iron oxide compound where iron ions (with valence 3+ and 2+) adjust to the AB 2O 4 = Fe 3+ (Fe 2+Fe 3+)O 4 formulation, arranged in an inverse-spinel crystal structure, composed by tetrahedral, A, and octahedral, B, sublattices. Their superparamagnetic behavior below a critical size of 30 nm has allowed the development of magnetic resonance imaging contrast agents or magnetic hyperthermia nanoprobes approved for clinical uses, establishing an inflection point in the field of magnetite based theragnostic agents. Magnetite nanostructures with designed composition and properties are the ones that gather most of the designs as theragnostic agents for their versatility, biocompatibility, facile production and good magnetic performance for remote in vitro and in vivo for biomedical applications. The development of hybrid nanostructured materials with a high degree of chemical control and complex engineered surface including biological targeting moieties, allows to specifically bind to a single type of molecule for specific detection, signaling or inactivation processes.
Nanotechnology offers the possibility of operating on the same scale length at which biological processes occur, allowing to interfere, manipulate or study cellular events in disease or healthy conditions.