A rhodamine-based turn-on fluorescence probe (RBD4) was synthesized and characterized by ¹H NMR, ¹³C NMR and ESI–MS.

The X-ray crystal structure analyses exhibit the spirolactone of fluorescent sensor RBD4.

Highly selective towards Cd²⁺ ion over other metal ions conformed by XRD, SEM, EDAX and FT–IR studies.

The 1:1 stoichiometric structure between RBD4 and Cd²⁺ were supported by Job's plot, Benesi-Hildebrand plot and DFT theoretical calculations.

RBD4 fluorescent sensor was used in imaging Cd²⁺ in cultured HeLa cells.