Substituent effect: A new strategy to construct a ratiometric fluorescent probe for detection of Al^sup 3+^ and imaging in vivo

Fluorescent sensors are important tools in environment and life science. Ratiometric fluorescent sensors are more advantageous than single intensity-based ones. Excited-state intramolecular proton transfer (ESIPT) molecules endow dual fluorescence from the excited enol and keto tautomers, providing excellent platforms for constructing ratiometric fluorescent sensors. However, the current ESIPT mechanism for ratiometric fluorescent sensors is relatively simple, resulting in that its application limits in given systems. Therefore, it is essential to construct a ratiometric fluorescent sensor that based on ESIPT molecule with reliable, controlled and general strategy. In this work, we employed a new strategy to construct a ratiometric fluorescent sensor allyl-(4′-methyl-3-hydroxyflavone) carbonate (FA) which is based on the finding that the electron-withdrawing substituted group could nearly block the normal-tautomer tautomerism taking place through excited-stated charge transfer (ESCT). FA exhibited highly selective and ratiometric fluorescent response to Al3+ and could detect Al3+ at a low concentration of 0.75 μM in aqueous solution. FA can detect Al3+ in tap and laker water samples, and in living cells. The proof-of-principle method provides a common strategy for design of ratiometric fluorescent sensors.