A simple fluorescent sensor has been developed for the ratiometric recognition of Mg2+ in semi-aqueous solution at pH 7.0. The sensor, a Schiff base, undergoes Excited State Intramolecular Proton Transfer (ESIPT) to generate a keto tautomer with proficient Mg2+ binding capability. The sensor displays good selectivity over other metal ions including alkali/alkali earth ions and can measure Mg2+ ion concentration between 2.0 and 30.0 μM ...