A new fluorescence enhancement mechanism based on Schiff base ligand and magnesium complexation is established. The co-ordination number of the Schiff base ligand–magnesium complex (2 : 1) can be tuned to 1 : 1 in the presence of amines to induce significant fluorescence enhancement. Using this approach, a novel and easily available fluorescent chemosensor was designed, which showed highly selective recognition toward magnesium and amines in ethanol.