# Doped Zinc Oxide Nanoparticles: Synthesis, Characterization and Potential Use in Nanomedicine
## 3 Synthesis Methods and Characterization
### Wet Chemical Methods
No secondary iron oxide phases were observed up to x = 0.15 of iron doping. Moreover, the unit cell volume of the ZnO:Fe nanoparticles crystal slightly changed with respect to the pure ZnO counterpart, with a not-proportional trend with respect to the dopant amount. This was attributed to the different fractional substitution of the two alternative coordination numbers of iron (Fe\({}^{2+}\) and Fe\({}^{3+}\)) and their respective different bond lengths with oxygen. A general increase of the band-gap energy value was also found but without a direct proportionality with the amount of doping. In particular, the maximum increase was obtained for the x = 0.06 Fe ratio. In another work, the attention was focused on the antibacterial activity of Fe-doped ZnO nanoparticles obtained by a sol-gel method. Zinc nitrate hexahydrate, iron nitrate nonahydrate and gelatin were dissolved in distilled water and the resulting nanoparticles showed a dimension of approximately 20-30 nm [182]. This study reported a reduced 20 angle of the (101) diffraction peaks together with a general decrease of the intensity of the wurtzite diffraction peaks by increasing the amount of Fe doping, which is attributed to an increased disorder of the ZnO lattice after Fe doping. Manganese-doped ZnO nanoparticles were successfully prepared by a wet chemical method as well. The electronic and magnetic properties of the as-prepared Mn-doped ZnO nanoparticles were investigated [183], by changing the amount of the Mn dopant between 0.5 at.% and 3 at.%. The particles were synthesized from zinc nitrate and manganese nitrate being dissolved in ethanol and polyethylene glycol (PEG). Both the XRD patterns and TEM images showed that Mn doping sensibly reduced the particle size to 20-50 nm. Cobalt is another commonly used TM element exploited as a dopant ion for ZnO. The sol-gel method was successfully used also in this case, as shown by the work of [PERSON] et al. In this study, the promising photocatalytic and antibacterial properties of Co-doped ZnO were reported [184]. Zinc nitrate and cobalt nitrate were dispersed in aqueous diluted polyvinyl alcohol (PVA) and mixed together. The solvent was dried without the use of any mineralizing agent up until the gel formation, which was finally dried and calcined to form the nanoparticles. Cobalt was successfully inserted in the ZnO crystal without the formation of any secondary phase (up to 10 mol.%) of doping, and an increase of the unit cell volume was observed by increasing the amount of doping. The particles showed an average size below 40 nm and revealed a red shift of the UV-visible photo-absorption spectrum together with a reduction of the band-gap energy. The changes in the optical properties of ZnO:Co nanoparticles were attributed to the increase of the unit cell volume (due to the quantum confinement effect) and to the introduction of trap states in the band-gap, in agreement with other works such as the one by [PERSON] et al. [132]. On the contrary, a different behavior was found by [PERSON] et al. [185]. In this case, the nanoparticles were produced again by a co-precipitation method involving zinc acetate and cobalt acetate, and a blue shift of band-gap was observed with an increasing amount of dopant. These contrasting results evidence that a complete understanding of the mechanism of Co doping ZnO nanoparticles has not been reached and many other aspects have to be considered. Moreover, it is worth mentioning that a reduction of the photocatalytic activity was observed by other works [184,185]. This suggests that additional efforts are required to better understand the role played by Co dopants in determining the physical properties of the resulting ZnO nanoparticles. Wet chemical routes were also used to prepare copper-doped ZnO nanoparticles. An example is represented by the work of [PERSON] et al. [152], where 5 mol.% Cu-doped ZnO nanoplates were synthesized by a hydrothermal method.