# Doped Zinc Oxide Nanoparticles: Synthesis, Characterization and Potential Use in Nanomedicine
## 4 Use of Doped ZnO NPs in the Biomedical Field
### Antimicrobial Agents
Their photocatalytic activity was also evaluated by measuring the degradation of the Acid Black 234 (AB) organic dye under sunlight irradiation. Using the Cu-doped ZnO nanoparticles resulted in a more efficient dye degradation with respect to the pure counterpart as well as an inhibition of the bacterial growth for all the examined biological species. In particular, ZnO nanoparticles with higher levels of Cu doping showed enlarged inhibition zones with respect to a standard drug (cephradine). The reason behind the antibacterial activity of these nanoparticles was attributed to the permeation of Cu ions through the negatively charged bacterium cell membrane because of its electrostatic attraction. Al-doped ZnO (AZO) nanorods were also proven to have superior antimicrobial properties with respect to pure ZnO [187]. The AZO nanorods showed a larger inhibition zone against _E. coli_ and _E. hirae_ (\(10.19\pm 0.04\) mm and \(10.20\pm 0.2\) mm, respectively) with respect to pure ZnO (\(9.54\pm 0.08\) mmand 9.62 \(\pm\) 0.08 mm), and similar results with respect to the kanamycin drug (10.16 \(\pm\) 0.07 mm and 10.17 \(\pm\) 0.08 mm). This behavior was again attributed to the electrostatic interaction between the particles and the bacterial cell walls. Iron doping was studied to improve the antibacterial ability of ZnO as well. In the work of [PERSON] et al. [122], Fe-doped ZnO nanoparticles synthesized by flame spray pyrolysis were tested against _B. subtilis_, _E. coli_ and _Pseudomonas putida_ through a high throughput bacterial viability assay able to evaluate the viability of bacteria through fluorescence signals. Interestingly, Fe doping did not impact in a major way on the antibacterial activity of ZnO, finding comparable results with respect to its pure counterpart. In another work, Fe- and Mn-doped ZnO nanoparticles were synthesized by a wet chemical method and analyzed in terms of photocatalytic activity before the antimicrobial tests [237]. The degradation of methylene blue under UV light was taken to identify the system with the best photocatalytic activity, which was 10% Fe-doped ZnO NPs. Disc diffusion assays reported that the 10%-doped system (both Mn- and Fe-doped) exhibited the highest antimicrobial activity against several pathogenic entities compared to lower doped (1%) systems and pure ZnO NPs. The reason for the improvement of both photocatalytic and antibacterial properties was related to the increased generation of ROS, and that Mn and Fe ions may lead due to the formation of additional states in the band-gap structure of ZnO. Additionally, magnesium was proven to increase the antimicrobial performances of ZnO. It was found that different Mg doping levels change the photocatalytic activity and rhodamine B degradation of the doped ZnO nanoparticles, finally influencing their antibacterial activity against both Gram-positive and Gram-negative bacteria [138]. The photocatalytic activity was found to increase with the doping level, reaching a maximum in the case of 7.5% Mg doping, and then decreasing for a further increase of the doping amount. The enhancement was attributed to the enlarged surface area of the ZnO NPs and to the oxygen vacancies induced by doping. For the antibacterial activity, disc diffusion analyses gave, as a result, an increment of the inhibition zone with the increase of the doping level. Another work reports the use of Mg-doped ZnO NPs loaded with an antibiotic drug [166]. An enhanced antimicrobial activity was found, and this behavior was attributed to the destabilization of the cell wall induced by the nanoparticles and their consequent increased permeability, which allowed the antibiotics to penetrate the bacterium membrane more efficiently. Rare earth elements were also proposed as dopants to enhance the antibacterial properties of ZnO. In the work of [PERSON] et al., Ce-doped ZnO nanoparticles were tested first as a photocatalyst for cyanide photooxidation and then as a antimicrobial agent against _E. coli_[209].