# Doped Zinc Oxide Nanoparticles: Synthesis, Characterization and Potential Use in Nanomedicine
## 2 Materials for Doping ZnO Nanoparticles (NPs)
### Rare Earth (RE) Elements  
Rare earth (RE), or lanthanide, elements are widely exploited for the doping ZnO nanomaterials and to properly modify the corresponding electronic band structure [110]. As highlighted in the work of [PERSON] et al. [111], upon UV light irradiation, ZnO presents a very fast recombination of the photogenerated charge carriers. This results in a low quantum efficiency when this system is used as a photocatalyst. The use of lanthanide, due to the 4f configuration, may help in increasing the recombination time of the electron/hole pair in the semiconductor [111], resulting in a higher efficiency of the system in the photocatalytic process, with consequences on the antimicrobial activity. Cerium, for example, has been proven to enhance the photocatalytic properties of ZnO nanorods through this mechanism [112], exceeding the performance of titanium dioxide (TiO\({}_{2}\)), which is the material typically exploited as a reference for these purposes. The change in the band-gap and the generation of trap states make RE doping interesting; also, from the luminescence point-of-view, this increases the possibility of ZnO being used as an imaging agent. The trap states induced by doping may act as radiative recombination sites not present in the pure crystal and the dopant atoms and, together with aspects such as the crystallite size, contribute to change the band-gap of the system, resulting in the possibility to tune the optical properties of ZnO. In the work of [PERSON] et al. [113], it is pointed out that ZnO may have improved luminescence properties from RE doping because of the transitions with the 4f orbitals of the RE elements in the crystal lattice. For example, [PERSON] et al., [114] synthesized an array of terbium-doped ZnO nanotubes which showed three emission bands in the range of visible light (543 nm, 586 nm and 620 nm, as clearly visible in Figure 2a) due to terbium doping. In particular, it was also demonstrated that the 586 nm band was due to the energy transfer between ZnO and Tb\({}^{3+}\) ions obtained through the electron transitions depicted in Figure 2b. Doping with RE materials is also a valuable method to change the piezoelectric behavior of ZnO. Indeed, the introduction of defects into the crystal further enhances the asymmetry of the wurtzite crystalline structure responsible for ZnO piezoelectricity. For example the generation of a ferroelectric behavior in lanthanum-doped ZnO was reported, alongside an increase of the piezoelectric coefficient d\({}_{33}\) by one order of magnitude (\(\approx\)101.30 pm/V for La-doped ZnO (mean value) vs. the accepted value of 12.4 pm/V for bulk ZnO [115]). The reason for this behavior was attributed to the large ionic radius of  
Figure 1: Main applications of doped ZnO nanoparticles in nanomedicine. the dopant element (1.22 A) with respect to zinc's radius (0.74 A), which induced a stronger distortion of the crystal cell and the rise of a stronger spontaneous polarization [116]. It is worth noting that, despite transition metals dopants being the best choice to induce room temperature (RT) ferromagnetism in ZnO, gadolinium, an RE element, has been proven to successfully induce a magnetic behavior in ZnO nanoparticles [117; 118]. In particular, RT ferromagnetism was observed for a film made of Gd:ZnO nanoparticles, doped by low energy ion implantation method. The authors found that a thermal annealing at 650 \({}^{\circ}\)C was able to favor the diffusion of the dopant atoms into the ZnO lattice. The as-prepared ZnO:Gd film showed a RT ferromagnetism and the formation of some super-paramagnetic clusters [119]. The reason of this behavior is still under debate and the use of RE doping elements in general, and gadolinium in particular, is to induce magnetism in ZnO nanoparticles, which is reported by few works. On the contrary, most of the research on magnetic ZnO nanoparticles is focused on the use of transition metals, as discussed in the next section.