# Doped Zinc Oxide Nanoparticles: Synthesis, Characterization and Potential Use in Nanomedicine
## 3 Synthesis Methods and Characterization
### Combustion Methods  
Apart from wet chemical methods, combustion-based synthesis techniques have been also explored as an alternative for the synthesis of doped ZnO nanoparticles. In solution combustion methods, the precursors of the desired material, i.e., zinc and dopant element precursors, are firstly dissolved in a fuel (urea, glycine fuels or citric acid) [197]. Then, the solution is placed in a muffle furnace pre-heated to a temperature higher than the ignition temperature of the fuel. During the combustion of the fuel, a strong exothermic reaction takes place between the fuels and the oxidizing agents present in the solution, generating the target oxide and gaseous species [197]. This gas generation leads to an expansion of the volume of the solid phase and a rapid decrease of its temperature, leading to the formation of ultra-fine and well dispersed powders [197]. This last aspect is very important in biomedical applications where the formation of clusters must be avoided as much as possible since it affects the cellular uptake of nanoparticles. By tuning the amount of fuel in the solution, various ZnO nanomaterials were synthesized through this versatile synthesis mechanism, showing different morphologies such as pyramid-like particles, nanodisks [198] or nanoparticles [199]. Concerning the doping of nanomaterials, different ions have been successfully included into ZnO. An example is represented by noble metals such as gold and silver. In the work of [PERSON] et al., both Ag- and Au-doped ZnO nanoparticles were synthesized by a solution combustion method [134]. Zinc nitrate hexahydrate was mixed in urea (used as fuel) together with a small amount of silver nitrate or tetrachloroauric-III-acid hydrate to act as dopants. The obtained solution was dissolved in 5 mL of water pre-heated to 80 \({}^{\circ}\)C and then placed in a furnace at 500 \({}^{\circ}\)C to start the ignition. Differently from sol-gel methods, the dopant agents generated the secondary phases of noble metals, as observed from the SEM images and XRD patterns reported in Figure 6.  
ZnO was also doped with nickel by a solution combustion method. In the work of [PERSON] et al., zinc acetate, nickel (II) acetate, ethanol and ethylene glycol were mixed together and inflamed to get the doped ZnO powder [200]. The XRD patterns showed the high crystallinity of the material. However, also in this case, the doped samples showed additional diffraction peaks related to an NiO secondary phase. A similar problem was observed for vanadium-doped ZnO nanoparticles. In this case, zinc nitrate hexahydrate and glycine were used as a precursor and to fuel for the reaction, respectively [173]. The dopant precursor was ammonium metavandate (NH4 VO3) while water was used as solvent. The XRD analysis performed on the burnt powders showed again the presence of additional peaks related to the formation of a secondary phase, especially for high doping levels. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Dopant** & **Dopant** & **Doping Level** & **Solvent** & **Particles Dimensions** & **Ref. ** \\ \multicolumn{6}{c}{**Element**} \\  & **Precursors** & & & \\ \hline \multirow{3}{*}{Fe} & Fe(SO\({}_{4}\))\(\cdot\)7H\({}_{2}\)O & 3–7\% mol & H\({}_{2}\)O 1  & 15–35 nm 3  & 195 \\  & FeCl3 & 1–10\% mol & H\({}_{2}\)O 1  & 9-15 nm 3  & 162 \\  & Fe(NO\({}_{3}\))\({}_{3}\) & 2–6\% mol & H\({}_{2}\)O 1  & 250 nm 3  & 196 \\  & Fe(NO\({}_{3}\))\({}_{3}\) & 2–6\% mol & H\({}_{2}\)O 1  & 196 \\  & Fe(NO\({}_{3}\))\({}_{2}\) & 6H\({}_{2}\)O & 1–10\% mol & H\({}_{2}\)O 1  & 250 nm 3  & 197 \\  & Fe(SO\({}_{4}\))\(\cdot\)7H\({}_{2}\)O & 3–7\% mol & H\({}_{2}\)O 1  & 15–35 nm  & 195 \\  & Fe(NO\({}_{3}\))\({}_{3}\) & 2–6\% mol & H\({}_{2}\)O 1  & 2–65 nm 3  & 196 \\  & Fe(NO\({}_{3}\))\({}_{2}\) & 6H\({}_{2}\)O & 1–10\% mol & H\({}_{2}\)O 1  & 25–50 nm 2 [FOOTNOTEIn another work, Ti-doped ZnO powders obtained by a solution combustion technique exploited titanium (IV) isopropoxide as a dopant precursor [201].