“Green” emission: A novel two‐photon fluorescent probe that can detect intracellular free Mg2+ ions at a depth of a few hundred micrometers in live tissue was developed. The emission from the Mg2+–probe complex (green) shows little interference from association of the probe with other intracellular metal ions or with the cell membrane (blue).

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2-Acetyl-6-(dimethylamino)naphthalene-derived two-photon fluorescent Ca2+ probes (ACa1-ACa3) are reported. They can be excited by a 780 nm laser beam, show 23-50-fold enhancement in one- and two-photon excited fluorescence in response to Ca2+, emit fourfold stronger two-photon excited fluorescence than Oregon Green 488 BAPTA-1 upon complexation with Ca2+, and can selectively detect intracellular free Ca2+ ions in live cells and living tissues with minimum interference from other metal ions and membrane-bound probes. Moreover, these probes are capable of monitoring calcium waves at a depth of 120-170 microm in live tissues for 1100-4000 s using two-photon microscopy with no artifacts of photobleaching.

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