# Doped Zinc Oxide Nanoparticles: Synthesis, Characterization and Potential Use in Nanomedicine
## 3 Synthesis Methods and Characterization
### Wet Chemical Methods
A reduction of the band-gap energy value was also reported for increasing the amount of doping from 0 mol.% up to 3 mol.% of Gd\({}^{3+}\), while an increase was observed for 5 mol.% of doping. The reduced band-gap energy was ascribed to quantum confinement effects deriving from the reduction of the crystallite size as the Gd percentage increased. Europium (Eu) doping was also demonstrated as a valuable tool to enhance the X-rays absorption properties of ZnO nanoparticles in cancer radiation therapy. In this case, Eu-doped ZnO nanoparticles were synthesized through a chemical precipitation technology [164]. Zinc acetate was dissolved in methanol together with europium (III) nitrate pentahydrate (5 mol.% of Eu doping) and the oxide formation was achieved through an NaOH addition. The resulting 8 to 9 nm sized ZnO:Eu nanoparticles showed an increased ability to generate reactive oxygen species, when subjected to X-rays, with respect to pure ZnO nanoparticles. Other RE elements such as lanthanum (La) and cerium (Ce) were successfully used to prepare doped ZnO nanoparticles by the sol-gel method. For example, zinc nitrate and lanthanum (III) chloride heptahydrate were used to synthetize ZnO:La nanoparticles with improved light absorption properties  
Figure 4: Characterization of gadolinium-doped ZnO nanoparticles. From left to right and top to bottom: XRD patterns and Fourier transform infrared spectroscopy (FTIR) spectra of the nanostructures at different doping levels, scanning electron microscope and transmission electron microscope images of 3% Gd-doped ZnO nanoparticles. Adapted from ref. [177]. and were successfully used as photo-anodes in dye-sensitized solar cells [116]. Ce-doped ZnO nanorods were fabricated starting from zinc chloride and cerium chloride [171]. The correct insertion of La and Ce dopants within the host ZnO crystal was confirmed by XRD. In both the cases, the wurtzite diffraction peaks present in the pattern of the doped nanorods were shifted toward higher diffracting angles with respect to the pure counterparts, evidencing a higher crystalline defectiveness induced by the correct inclusion of the ions in the lattice. Additionally, yttrium (Y, considered both an RE and TM element) was successfully included into the crystal lattice of ZnO nanosheets obtained by a co-precipitation method that involved the use of zinc chloride and yttrium (III) chloride hexahydrate. As reported by [PERSON] et al., a huge increase of the piezoelectric coefficient d\({}_{33}\) (up to 420 pm/V against 12.4 pm/V of pure ZnO) in Y-doped nanosheets was measured [160]. The XRD pattern of the Y-doped and undoped ZnO nanosheets revealed a shift toward higher angles of the peaks in the case of the doped material, evidencing a change in the lattice spacing among atoms. This aspect, together with the different charge of the inserted dopant and the changed morphology that the inclusion of the dopant has led, are considered the factors which increase the piezoelectric coefficient. Differently from RE elements, for which wet chemical methods are the most frequently used to develop doped ZnO nanoparticles, transition metals have a plethora of synthesis methods available. However, sol-gel and co-precipitation methods are still the most used in the literature. Among TMs, iron has been extensively studied as a dopant for ZnO due to its ability to induce a ferromagnetic behavior to the system [178; 179; 180]. Many works have dealt with flame spray pyrolysis synthesis [122; 128; 181], but a lot of works have also reported a wet chemical method to obtain Fe-doped ZnO nanoparticles. [PERSON] et al. reported the synthesis of Fe-doped ZnO nanoparticles (Fe\({}_{x}\)Zn\({}_{(1-x)}\)O) with dimensions below 50 nm through a sol-gel method [155]. Zinc acetate dihydrate and ferric nitrate were dispersed in distilled water together with citric acid and ethylene glycol as anti-agglomerating agents at a temperature of 65 \({}^{\circ}\)C. Aqueous ammonia was added to precipitate the powders that were successively calcined.