A polymer-based fluorescence "turn-on" chemosensor is demonstrated with high sensitivity and selectivity toward Fe2+ cations. This system is an inorganic/organic hybrid system in which Cu2+ is coordinated at the receptor site of a conjugated polymer sensor. The unique quenching behavior of the Cu2+ toward this conjugated polymer was used to lower the initial fluorescence intensity of the sensory system. This results in a highly sensitive and also selective sensory system toward Fe2+ as compared to other related divalent transition-metal cations in solution.