Provided are novel rhodol-based compounds, the preparation method thereof, and use thereof in Tl+ flux assays. The fluorescent thallium ion sensor compounds have a rhodol fluorophore attached to an amino dicarboxylic acid metal binding moiety. The compounds are prepared in the “pro-dye” form, with the rhodol oxygen and the carboxylic acids of the metal binding unit masked by protecting groups. The disclosed compounds may be used as more red-shifted and less pH-sensitive variants of Thallos.