# Doped Zinc Oxide Nanoparticles: Synthesis, Characterization and Potential Use in Nanomedicine
## 2 Materials for Doping ZnO Nanoparticles (NPs)
### Transition Metals (TM) Elements
Additionally, the piezoelectric response of Mn-doped ZnO nanoparticles was found to be heavily dependent on the amount of Mn ions introduced in the ZnO crystal but not in a linear way. In the work of [PERSON] et al. [109], the d\({}_{33}\) piezoelectric coefficient is found to first decrease with the increase of Mn doping up to 4.8 at.% and then it increased with further Mn doping. This behavior is attributed to the change in the oxidation state of Mn ions because of the use of different doping amounts. A different Mn oxidation state means a different ionic radius and hence, a change in the resistance against the rotation of the oxygen-metal bonds in the ZnO lattice. Among the TM dopants, the use of copper is reported as a tool to increase the antimicrobial activity of ZnO. [PERSON] et al. successfully demonstrated the fabrication of Cu-doped ZnO coatings with enhanced antimicrobial activity against _E. coli_ under light illumination [124]. The higher performance obtained by Cu doping was attributed to an enhanced reactive oxygen species (ROS) generation and the release of cytotoxic zinc and copper ions. Other works also showed the use of chromium and cobalt ions as doping agents for ZnO. Chromium-doped ZnO nanoparticle films were prepared by reactive magnetron co-sputtering. The incorporation of Cr ions within the ZnO crystal lattice resulted into the rise of ferroelectric properties and it notably improved the piezoelectric response with respect to the pure ZnO films [121]. This induced ferroelectric behavior was attributed to the substitution of some of the Zn\({}^{2+}\) ions by smaller Cr\({}^{3+}\) ions, which generates a permanent electric dipole. On the other side, cobalt doping was found to increase the band-gap of ZnO, making it transparent to most of the visible light. Additionally, for this doping element, a room temperature ferromagnetic behavior could be observed in Co-doped ZnO nanoparticles [132]. Noble metals, such as gold and silver, were used in different applications as well. For example, the photoluminescence properties of Au-doped ZnO nanoparticles were investigated [133]. The results showed the presence of additional weak luminescence peaks in the visible spectral range the intensity of which was dependent on the doping amount. Gold and silver doping were also investigated as potential antimicrobial boosters for ZnO [134]. In this work, both Ag and Au ions were found to be suitable for increasing the photocatalytic properties of ZnO. However, Au was not able to improve in a significant way the antibacterial activity of ZnO, while Ag-doped ZnO nanoparticles showed enhanced antifungal activity. In another work, the antibacterial activity of Ag-doped ZnO nanoflowers,obtained by means of a green combustion method, was evaluated. The results highlighted a good antimicrobial activity against both Gram-positive and Gram-negative bacteria, as well as an antifungal efficacy against different plant pathogenic fungal strains [135]. In summary, many TMs have been studied for doping ZnO mainly because of their ability to give ferromagnetic properties to ZnO but also to improve its electrical and optical behavior. In the particular case of nanomedicine, doping ZnO with TMs could represent a valuable tool for the design of new multifunctional ZnO nanomaterials with newly acquired bioimaging features, i.e., magnetic resonance imaging.