A bimacrocyclic luminescent terbium(III) sensor is reported for the selective "turn-off' detection of Cu2+ ions in aqueous solutions. The current sensor differentiates from previous sensors in that it offers the use of (1) time-gated luminescence detection to remove the background signal, (2) a longer excitation wavelength of up to 350 nm for increased biocompatibility, and (3) a practically irreversible detection as a form of probing Cu2+ ions with an extremely low limit of detection of about 1.7 nM.