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A 5-formyl-2-hydroxybenzoicacid-based fluorescence probe (L) for Cu2+ ions was designed and synthesized. L in CH 3 CN:H 2 O (1:9, v/v) solution displayed highly selective and sensitive fluorescence 'off-on' response over all the other competitive metal ions including Fe2+, Fe3+, Mn2+, Ba2+, Pb2+, Co2+, Cr3+, Cd2+, Na+, Zn2+, Hg2+, Ag+, Ca2+, Al3+, Mg2+, K+, Ni2+ and Y3+. The fluorescence intensity of L presented good linear relationship to the Cu2+ concentration and with the lower detection limits of 2.19 × 10−7 M for Cu2+ ion. Job's plot, mass spectra, Benesi-Hildebrand plot and DFT calculation confirmed that fluorescence increased was caused by the formation of 1:2 complex between L and copper ion. The recovery tests demonstrated that L could accurately detect the existence of Cu2+ ions in water samples. In addition, L can recognize Cu2+ ion in living cells via the fluorescence confocal images and in water samples via test papers mode. • A 5-formyl-2-hydroxybenzoic acid-based probe (L) was synthesized successfully. • L displayed high selective and sensitive "off-on" fluorescence sensing for Cu2+. • L also can be used as test strips to recognize copper ion. • Fluorescence imaging confirmed L can recognize Cu2+ in live cells. [ABSTRACT FROM AUTHOR]