Three salicylaldehyde derivatives, namely 5-nitrosalicylaldehyde fluorescein hydrazone (1), 5-diethylaminosalicylaldehyde fluorescein hydrazone (2), and di(5-diethylamino salicylaldehyde) hydrazide (3), were synthesized and characterized. Their ability to recognize copper ions was investigated by UV-visible and fluorescence spectroscopies, with 2 shown to be an optimal probe for effective detection of Cu2+. More importantly, to explore the optical properties of these probes, and their mechanism for recognition of Cu2+ ions, B3LYP/6-311+G(d,p) density functional calculations were used to investigate the molecular orbitals and electronic excitations of each probe. In addition, because the 2-Cu2+ complex exhibits anion-induced fluorescence enhancement, we illustrate its potential application to bioimaging. (c) 2012 Elsevier B.V. All rights reserved.