*2.1.1 Photosensitizer*

Photosensitizers (PS) are either endogenous or exogenous substances; they should not possess any toxicity at specified doses in absence of light [22]. A classic example of endogenous PS is Protoporphyrin IX, which is obtained by metabolizing 5-ALA [23, 24]. The tissue penetration capability of light is shown in **Figure 3**, so the absorption and emission characteristics of PS have a significant role in their photodynamic action. The molecular structural factors of PS are important in their transport and localization.
