A new pyrene derivative (chemosensor 1) containing a picolinohydrazide moiety exhibits high selectivity for Cu(2+) ion detection in mixed aqueous media (CH(3)OH:H(2)O = 7:3). Significant fluorescence enhancement was observed with chemosensor 1 in the presence of Cu(2+). However, the metal ions Ag(+), Ca(2+), Cd(2+), Co(2+), Cu(2+), Fe(2+), Fe(3+), Hg(2+), K(+), Mg(2+), Mn(2+), Ni(2+), Pb(2+), and Zn(2+) produced only minor changes in fluorescence for the system. The apparent association constant (K(a)) of Cu(2+) binding in chemosensor 1 was found to be 2.75*10(3) M(-1). The maximum fluorescence enhancement caused by Cu(2+) binding in chemosensor 1 was observed over the pH range 5-8. Moreover, by means of fluorescence microscopy experiments, it is demonstrated that 1 can be used as a fluorescent probe for detecting Cu(2+) in living cells.