# Doped Zinc Oxide Nanoparticles: Synthesis, Characterization and Potential Use in Nanomedicine
## 3 Synthesis Methods and Characterization
### Combustion Methods
In this case, the additional XRD peaks due to TiO\({}_{2}\) appeared only at the highest level of doping (7.5 at.%), while Ti ions were correctly inserted in the host ZnO lattice for lower dopant concentrations (up to 5 at.%). The solution combustion method was also proposed to synthesize iron-doped zinc oxide nanoparticles in a faster way than sol-gel methods [202]. Zinc nitrate, iron nitrate and urea were used as precursors and fuel, respectively. The solution was heated at 400 \({}^{\circ}\)C up to the ignition of the solution and until the formation of a very fragile foam that could be reduced to powder form. Interestingly, the XRD pattern representative of these powders did not present any additional peaks due to the secondary phase up to 5% of Fe doping, while weak peaks related to ferrite were detected only for higher doping levels. However, the formation of secondary phases was not considered a drawback at all. Actually, it was responsible for the promising ferromagnetic behavior observed in the mostly doped samples, even if a weaker hysteresis loop was also found for lower doping levels. The iron doping of ZnO nanoparticles was achieved by flame spray pyrolysis as well. Differently from the standard solution combustion described previously, the precursor solution is sprayed before setting on fire. Again, a fuel is needed to allow the self-sustainability of the flame as in the case of the solution combustion method [203]. However, a nozzle tip is usually exploited to nebulize the solution precursor. In the development of doped ZnO nanoparticles, iron is the one that had major advantages in terms of the exploitation of this technique. Many studies have exploited flame spray pyrolysis rather than sol-gel methods to prepare Fe-doped ZnO nanoparticles with an improved biostability in aqueous media [181; 204; 205]. The improved stability was probably due to the rapidity of this synthesis method, which resulted in nanoparticles with superior surface chemistry properties. Concerning this aspect, the work of [PERSON] et al. highlighted the decreased dissolution of Fe-doped ZnO nanoparticles in biological media [205]. This doping reduced the amount of Zn\({}^{2+}\) dissolved in the  
Figure 6: Au- and Ag-doped nanoparticles synthesized through a combustion method. On the left is the XRD patterns of the resulting particles, highlighting the presence of further peaks related to the secondary phases. On the right, the corresponding scanning electron microscope images of Ag-doped (**a**) and Au-doped (**b**) nanoparticles. Adapted from ref. [134]. biological medium with positive consequences on the biocompatibility of the nanoparticles system. The doped nanoparticles were synthesized through flame spray pyrolysis. Zinc and iron naphthalene were mixed separately in xylene. The solutions were sprayed, atomizing the precursor and allowing for combustion through the co-delivery of methane and oxygen. The resulting doped particles were smaller with respect to the pure counterparts and the XRD patterns did not exhibit additional peaks apart from the usual wurtzite phase ones. Other works reported the use of the same synthesis procedure to develop ZnO:Fe nanoparticles with a good reproducibility [181; 204]. Each of these works was focused on the dissolution kinetic of the produced particles and their fate in a biological environment. In all the cases, iron doping was demonstrated to be a very interesting tool to limit nanoparticles' dissolution and to enhance ZnO cytocompatibility. As a conclusive remark, it can be stated that combustion methods are not able to efficiently incorporate the dopant agent in the ZnO crystalline structure with respect to wet chemical techniques. This can be the explanation for the limited use of solution combustion routes to fabricate doped ZnO nanoparticles with respect to sol-gel or hydrothermal routes. However, it is worth noting that Fe-doped ZnO nanoparticles obtained by flame spray pyrolysis showed superior properties in terms of their stability in biological media.