What is singlet oxygen?
When photosensitizers (light sensitize molecules which selectively accumulate on cancer cells) are illuminated with certain colors of light they absorb the energy and become excited. With this extra energy, they become unstable and off-load or donate energy as electrons to oxygen in the immediate vicinity and create a special form of oxygen named singlet oxygen. These singlet oxygen molecules also are highly unstable and off-load energy by donating electrons, oxidising (damaging) the nearby structures in the cancer cells.