A blue emissive polyfluorene bearing sulfate functionality in the side chain (PFS), as a sensory polymer to selectively report the presence of Cu2+ (detection limit down to 2.5 mu M) based on the fluorescence "turn-off." Surprisingly, the quenched luminescence of PFS by Cu2+ could be turned on after the addition of CN- (detection limit down to 6 mu M), making PFS a sensitive, selective, and reversible cyanide probe.