A self-organized fluorescent sensor consisting of alizarin and o-aminomethylphenylboronic acid 1 is presented, the ensemble shows highly selective fluoride ion-response in H2O-MeOH (1:4 w/w) at pH 5.5. Compound 1 (pK(a) = 5.63) at pH5.5 in the solution follows the equilibrium: alizarin-1 + H+ reversible arrow 1H(+) + alizarin. Thus, F- added into the solution preferably binds to 1H(+) to allow the ensemble to release alizarin, resulting in a fluorescence quenching.