An unusual phenomenon, the fluorescence enhancement of cationic conjugated fluorene-co-carbazole-co-diacetylene polymers (CPFC) by both anionic and cationic ions, was reported. The fluorescence enhancement of CPFC strongly depended on the nature of the ions and the counterions. In the solution of DMF/H2O, PO43-, CO32- and Hg2+ showed the most pronounced fluorescence enhancement response of CPFC (more than 10-fold). The fluorescence of CPFC was quenched by K-4[Fe(CN)(6), and then could be recovered with the addition of Hg2+. Based on these observations, a simple and sensitive fluorescent "turn-on" sensor in aqueous solution for Hg2+ was developed by use of a system of CPFC/K-4[Fe(CN)(6)]. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1690-1694, 2011