We have developed a series of molecular probes for the fluorescent detection of singlet dioxygen ((1)O2). The probes, based on asymmetrically substituted 1,3-diarylisobenzofurans, undergo the [2 + 4] cycloaddition reaction with (1)O2, producing ratiometric fluorescent responses. Two-photon fluorescence microscope experiments demonstrated the biological utility of the probes for the visualization of endogenous (1)O2 in macrophage cells.

A new fluorescence turn-on probe that responds exclusively to H(2)O(2) exhibits subcellular localized fluorescence staining of lysosomes.