# Doped Zinc Oxide Nanoparticles: Synthesis, Characterization and Potential Use in Nanomedicine
## 2 Materials for Doping ZnO Nanoparticles (NPs)
### Transition Metals (TM) Elements  
Another class of elements that have been widely exploited as doping agents for ZnO are the transition metals. Elements such as cobalt (Co) [120], chromium (Cr) [121], iron (Fe) [122], manganese (Mn) [123] and copper (Cu) [124] have all been successfully used to enhance pre-existing properties or to give new features to ZnO, making it interesting also from the nanomedicine point of view. The research on TM-doped ZnO gained lots of attention because of the ability of this material to work as a diluted magnetic semiconductor (DMS). DMSs are semiconducting materials that also present a ferromagnetic behavior because of the presence of transition metal (TM) ions that are ferromagnetic in their pure form, such as Mn and Fe. Despite such behaviors being still under debate [125], numerous works have reported the observation of RT ferromagnetism in TM-doped ZnO nanomaterials. As in the case of RE elements, doping with TMs does not only contribute to the ferromagnetic behavior of ZnO--both the optical [126] and electrical [127] properties can be tuned by these materials and are heavily affected by several parameters such as the oxidation state, ionic radius, electronegativity and other features of the doping ion [109]. Among TM dopants, iron is one of the most used. This element can exist into two different oxidation states (Fe\({}^{2+}\) and Fe\({}^{3+}\)). They both induce a different effect on both the electrical and structural properties, due to the different ionic radius and charge carried in the system. The studies on Fe-doped  
Figure 2: Photoluminescence properties of Tb-doped ZnO nanotubes. (**a**) Emission spectra of ZnO with different dopant levels excited by a 235 nm radiation; (**b**) energy-levels schematic with the electron transition processes in Tb-doped ZnO nanotubes grown onto alumina. Adapted from [114]. ZnO are mainly focused on the analysis of the corresponding magnetic properties. However, some works also showed that Fe doping represents an effective approach to improve the chemical stability of ZnO nanoparticles in aqueous media [128] or to enhance the electromechanical response [129]. As an example of such a a study, in the work of [PERSON] et al. [130], an increasing ferromagnetic behavior at room temperature is reported with the increase of iron doping in ZnO thin films. Additionally, another work shows that the ferromagnetic behavior of Fe-doped polycrystalline ZnO powders is influenced by the post-synthesis annealing conditions [131]. In particular, the ferromagnetic response was optimized by means of an annealing in a hydrogen atmosphere, while an argon atmosphere annealing would not produce any ferromagnetic behavior. Still, the reason for the rising of such a behavior is not clear and controversial explanations are currently reported in the literature. In terms of the piezoelectric properties of Fe-doped ZnO, the work of [PERSON] et al. [129] focuses the attention on the chemical state of Fe ions introduced in the ZnO crystal. According to this work, the oxidation state of the dopant ion plays a fundamental role in determining the electromechanical response of the doped ZnO films because of the different ionic radius; a smaller ion (Fe\({}^{3+}\)) leads to a higher piezoelectric coefficient (\(\sim\)128 pC/N vs. 9 pC/N for 1.2 at.% Fe doping) than doping with a larger one (Fe\({}^{2+}\)). Manganese has been also extensively used as a dopant agent. Mn is characterized by several oxidation states with a higher difficulty in controlling this parameter. In addition, in this case, the dopant agent is used to modify the optical, structural and magnetic properties of the resulting ZnO semiconductor [118]. The ferromagnetic behavior of Mn-doped ZnO was found to be dependent on the doping amount but without a linear relationship between the two parameters [123]. Another work pointed out that the behavior of Mn-doped ZnO was diamagnetic at 1 at.% of doping and ferromagnetic at 5 at.% [126].