# Photodynamic Therapy--An Up-to-Date Review
## 4 Nanomaterials for Photodynamic Therapy
Titanium dioxide can also act as a photosensitive agent, being able to generate singlet oxygen [8; 24; 77; 78]. These nanoparticles gained interest due to their adjustable bandgap, band position, excellent photostability, low toxicity, high catalytic activity, abundance, and affordability [36]. Used either alone or as composites and combinations with other compounds, TiO\({}_{2}\) nanoparticles can be successfully involved in PDT for treating malignant tumors or inactivate antibiotic-resistant bacteria [79]. Manganese oxide-based drug nanoplatforms have also been proven effective in PDT. MnO\({}_{2}\) nanosheets can regenerate oxygen through the reaction with H\({}_{2}\)O\({}_{2}\) present in the tumor microenvironment while simultaneously consuming glutathione to enhance antitumor efficacy. Moreover, these materials present strong PS absorption ability and good biocompatibility, which are attractive properties for PSs carriers [14; 36; 80]. Other metal-based nanomaterials reportedly used in PDT are molybdenum oxide, zinc oxide, and tungsten oxide nanoparticles [8], yttrium oxide nanoparticles [81], ruthenium  
Figure 4: Schematic representation of possible combinations between nanoparticles and photosensitizers (PSs): (**a**) nanoparticles embedded with PSs; (**b**) nanoparticles with PSs bound to the surface; (**c**) nanoparticles as PSs; (**d**) PSs along nanoparticles. Created based on information from literature reference [64]. nanomaterial complexes [82], transition metal dichalcogenide nanosheets [14], transition metal carbides nanosheets, and nanoparticles [14]. Another attractive alternative to classic PDT is using silica nanoparticles for encapsulating PSs. Despite not being active itself, silica has other features recommending it for this light-based therapy, namely nontoxicity, chemical inertness, and optical transparency [14]. In this regard, mesoporous silica nanoparticles have been extensively used as nanocarriers for hydrophobic PSs, particularly zinc phthalocyanine [83; 84]. Carbon-based nanomaterials have also gained considerable research interest in the field of PDT. Their physicochemical and biological properties, such as unique optical and mechanical features, good biocompatibility, low toxicity, versatile chemical functionalization, enhanced permeability effect, and ability to produce ROS, make them suitable for cancer PDT and antimicrobial photodynamic inactivation [14; 85]. Among the allotropic forms of carbon, the most applied are carbon nanotubes [14; 86], fullerenes [5; 87], and graphene-based nanomaterials [88; 89; 90]. Carbon nanotubes, with their unique structures and features, have attracted interest for PDT application. In particular, single-walled carbon nanotubes have been used as potential sensitizers in cancer therapy, as they are also considered efficient delivery vehicles for hydrophobic PSs [91; 92]. Fullerenes can also be used as PSs, presenting advantageous properties, such as versatile functionalization, ability to undergo photochemistry, the possibility of self-assembly into supramolecular fullerosomes, and high resistance to photobleaching [5]. Moreover, reference [60] fullerene derivatives can be employed in the targeted delivery of drugs to the nuclear pore complex and tumor vasculature [87]. Graphene-based nanomaterials, such as graphene quantum dots, graphene oxide, and reduced graphene oxide, have also been employed in cancer therapy, either for anticancer drug delivery or in PDT [14]. Particularly, graphene quantum dots' ability to generate singlet oxygen can be exploited to destroy pathogenic bacteria and cancer cells [88]. The versatility and diversity of polymeric nanoparticles gained attention in PDT [93]. Their ability to protect drugs against initial deterioration, increase drug permeability into targeted tissue, and diminish systemic toxicity increased polymer-based nanosystems' popularity over free drugs [94]. Biodegradable polymers are the preferred class of such materials, as they release the PS cargo when degraded by the biological environment [95].