The imaging of singlet oxygen in single cells

The lowest excited state of molecular oxygen, singlet molecular oxygen (a 1 Δ g ), is generally regarded to be the active cytotoxic species in photodynamic therapy (PDT). As a result, the direct detection of singlet oxygen in biological systems should be of great value in elucidating the mechanisms underlying the observed effects of PDT. We have recently shown that singlet oxygen can be detected by its weak 1270 nm phosphorescence (a 1 Δ g →X 3 Σ g - ) from a single nerve cell upon irradiation of a photosensitizer incorporated into the cell. In this paper, we discuss issues pertinent to the direct optical detection and imaging of singlet oxygen from single cells.