# Doped Zinc Oxide Nanoparticles: Synthesis, Characterization and Potential Use in Nanomedicine
## 4 Use of Doped ZnO NPs in the Biomedical Field
### Doped ZnO as Therapeutics against Cancer
As it can be seen in Figure 10, the 2 wt.% doped ZnO nanoparticles effectively inhibited the tumor growth, different to the control and the 10 wt.% doped ZnO NPs treatments. Concerning ROS generation, ZnO NPs was successfully applied in photodynamic therapy. The idea behind this technique is to exploit the interaction between a photosensitizer (the semiconducting ZnO NP) and electromagnetic radiation in order to generate cytotoxic species (like ROS) that kill cells and destroy tissues [284], with the mechanisms reported schematically in Figure 11. The photosensitizer should be internalized inside the cell and then exposed to light. By photoexcitation, it is possible to induce the electronic transition from the valence to the conduction band, which allows the formation of ROS, as already described in Section 4.2. In the work of [PERSON] et al., the role of ZnO nanoparticles on the viability of tumoral and healthy cells was investigated [285]. Pure ZnO nanoparticles were tested on a human head and neck squamous cell carcinoma (HNSCC) cell line and on oral mucosa cells (pOMC). Different concentrations of ZnO NPs and the exposition time to UV light were explored on cell cultures internalizing the particles, and cell viability was evaluated through an MTT assay. The results showed that a high concentration (20 ug/mL) of nanoparticles reduced the viability for all the cells (~70% of the survival percentage without UV exposure), while lower concentrations (up to 2 ug/mL) resulted in more toxicity toward cancerous cells when coupled with longer times of UV exposure (at a 2 ug/mL concentration of NPs and 15 min of UV exposure, ~40% of the survival percentage for cancerous cells vs. 90% for normal cells). Even if the exposure to UV light highlighted the important role played by the photocatalytic activity of ZnO NPs against cancer cells, the cytotoxicity observed at high concentrations in the absence of UV exposure also suggested the presence of a secondary mechanism. ZnO nanoparticles for photodynamic therapy were investigated also by [PERSON] et al. [92]. In this case, ZnO particles were coated by a lipid layer in order to increase the dispersion of the nanoparticles in biological media and to improve cellular uptake, and were then tested as photosensizers against human epithelial carcinoma cells (HeLa). The lipid-coated nanoparticles were successfully internalized  
Figure 10: Tumor growth trend in mice at different times and differently doped ZnO nanoparticles. Tumor growth is reduced with 2% Fe-doped ZnO nanoparticles. Adapted from ref. [204]. in the cells and an increased cytotoxicity was found with an increase of the concentration of NPs coupled with UV irradiation, confirming the interesting antitumoral properties of bare ZnO. In the previous sections of this review, several works reporting an increased photocatalytic activity of doped ZnO nanoparticles were described. This aspect can be extremely useful in enhancing NPs cytotoxicity against tumoral cells, in a similar manner to what has been demonstrated for antimicrobial applications. Lithium-doped ZnO nanoparticles were proposed as generators of singlet oxygen for this purpose [144]. A physical characterization of these nanoparticles was provided, reporting an increase of the band-gap energy and a slight blue shift of the exciton energy. Moreover, an increased generation of singlet oxygen was clearly observed for the increasing doping levels, suggesting its use in photodynamic therapy. In this sense, gadolinium and europium were also considered. In the work by [PERSON] et al., these two chemical species were proposed as dopant ions in ZnO nanoparticles and their influence was analyzed in terms of their cytotoxic behavior against normal (L929 fibroblast cells) and tumoral (Hela cervical cancer cells and PC3 prostate cancer cells) cells [164]. In particular, Gd- and Eu-doped ZnO NPs were internalized into cells and the cell cultures were exposed to UV, X-ray and \(\gamma\) radiations.