**4. Study of UVA-photosensitization** *in vitro*

Many researchers have investigated photosensitization reactions *in vitro* system using a single photocatalyst [44–50]. On the other hand, some research groups have shown that the coexistence of multiple components induces stronger cytotoxicity than a single photosensitizer condition [44, 51]. It has been reported that a low concentration of photocatalyst, about 0.4 μg/mL riboflavin, can induce cell damage in coexisting conditions with amino acids and vitamins [52]. On the other hand, in single riboflavin conditions, it takes about 100 μg/mL of riboflavin to induce cell toxicity *in vitro* [53]. In the condition that photocatalysts and multiple components coexist, the type I mechanism and the type II mechanism may be amplified at the same time in photosensitization. Therefore, it is necessary to consider which of the type I and type II reactions is the main reaction under the coexistence conditions of photocatalysis and multiple components.
