A fluorescent sensor, QA, based on acetamidoquinoline with DPA as receptor, was synthesized. In aqueous buffer solution, QA demonstrates high selectivity for sensing Cd(2+) with about 40-fold enhancement in fluorescence quantum yield and picomolar sensitivity (K(d) = 0.25 +/- 0.03 pM). Moreover, sensor QA can distinguish Cd(2+) from Zn(2+) via two different sensing mechanisms (PET for Cd(2+); ICT for Zn(2+)), and the discrimination is even possible by "naked eyes".