**3. Design of targeted PDT**

The specificity of photosensitizer toward the tumor or diseased tissue is always questionable. As discussed, once the photosensitizers get activated by the light they will produce ROS. The formed ROS can easily destroy the cell structures of pathogenic mammalian cells or microbes. But ROS-mediated toxicity is not limited to pathogens if the PS located in normal vegetative tissue can lead to lethal action [9]. A targeted drug delivery approach is only a solution to minimize vegetative tissue destruction. In the process of designing the targeted PDT, the selection of photosensitizer also plays a key role. Both the free radical (type I) and singlet oxygen (type II) mediated mechanisms are equally harmful and end up with a series of reactive oxygen species (ROS). Currently, there are different approaches adopted to direct these highly active photosensitizers to our site of action. Among them, molecular design-based targeting and nanocarrier approaches are two leading methodologies [26]. In the targeted approach, we have to impart a targeting aid to lead the molecule toward the diseased tissue. The most studied drug delivery approach is the vesicle or nanocarrier-mediated technique [68].
