The design of thio-based photosensitizers, at left, by Rice University chemists shows promise for photodynamic cancer therapy, among other applications. One thiocarbonyl substitution — trading an oxygen atom for a sulfur atom — of a variety of fluorophores can dramatically enhance their ability to generate reactive oxygen species that kill cancer cells. At right, images of multicellular tumor spheroids treated with photosensitizers and light (in the bottom row) show how the compounds, when excited by light, damage the cells. (Credit: Xiao Lab/Rice University)

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