<p>The basic area of research is the recognition of metal ions that have an imperative role in the biological and environmental world. Among these metal ions, Copper and Mercury are two key metal ions whose selective and sensitive detection is still the topic of current research. In the present study, we have developed a new hydrazone with a fluorescein moiety as the ideal photophysical partner. This new hydrazone was synthesised via one-step condensation of fluorescein hydrazide with 5-Allyl-3-methoxy salicylaldehyde (<b>FP1</b>). <b>FP1</b> was fully characterised by employing different physicochemical techniques, and its single-crystal X-ray studies have been carried out. In mixture of ethanol and water (9:1 v/v) <b>FP1</b> showed colorimetric response for Cu²⁺, Co²⁺ and Zn²⁺. Interestingly, <b>FP1</b> demonstrated selective behaviour with Cu²⁺ ions exclusively on changing the solvent combination to acetonitrile and water (8:2 v/v), enabling <b>FP1</b> to act as a sensor for Cu²⁺ ions only. Hg²⁺ ions depicted turn on response in fluorescence in a solution of acetonitrile and water (8:2 v/v) via chelation-enhanced fluorescence. The sensing mechanism favours the spirolactum ring remaining closed on the interaction of <b>FP1</b> with the metal ions. The limit of detection (LOD) and binding constant are 0.11 ppm, 2.74 × 10³ M^− 1 and 3.03 ppm, 6.5 × 10⁶ M^− 1 for Cu²⁺ and Hg²⁺ ions, respectively. Cu²⁺ and Hg²⁺ ions binding with <b>FP1</b> have been confirmed using FT-IR, NMR and HRMS data. <b>FP1</b> displayed reversible response with EDTA²⁻ ions (4 cycle). A thorough investigation of <b>FP1</b> was conducted using Hirshfeld surface analysis and two-dimensional fingerprint plots to identify and quantify intermolecular contacts. Complementary topological evaluations involving NCI (Non-Covalent Interaction), QTAIM (Quantum Theory of Atoms in Molecules), ELF (Electron Localization Function) and LOL (Localized Orbital Locator methods) were utilized to dissect the electronic characteristics and non-covalent framework of the molecule. TD-DFT computations further revealed that coordination with transition metals such as Cu²⁺ and Hg²⁺ induces notable spectral shifts, particularly toward lower energies, reflecting stronger charge-transfer effects. Alamar blue assay of <b>FP1</b> on HEK-293(Human embryonic kidney − 293) cell lines exhibited biocompatibility with minimal cytotoxicity and applied for live cell imaging for intracellular sensing of Cu²⁺ and Hg²⁺. In a study focusing on latent fingerprint detection with the help of three variants of <b>FP1</b>, researchers observed that the three variants of the compounds exhibited strong adhesion to fingerprint ridges, resulting in well-defined patterns without any unwanted background staining.</p> Graphical abstract <p></p>