A new terephthaldehyde-based chemosensor 1 bearing an aminophenol recognition unit has been synthesized and applied to the fluorescent sensing of metal ions. Molecular system 1 acts as a highly selective and sensitive fluorescence turn-on sensor for Cu2+. The sensing mechanism has been explored. It is proposed that Cu2+ binds with the imine and hydroxyl moiety of 1 in 1 : 2 binding stoichiometry, thereby enhancing the fluorescence at 386 nm. The detection limit and association constant (K-a) of 1 with Cu2+ were found to be 0.62 mu M and 6.67 x 10(4) M-1, respectively. Chemosensor 1 has shown excellent specificity towards Cu2+ and has been successfully applied to the determination of Cu2+ in live L929 cells.