Naphthalimide-based fluorescent 'turn-off' probes for palladium ions: structure-activity relationships

Palladium is extensively applied in many high-tech industries, but excessive residue in the environment is harmful to humans. Therefore, it is important to develop effective analysis methods for palladium ions. In this work, six naphthalimide-based fluorescent probes NPA-NPF were synthesized with ethylenediamine derivatives as the receptors. The experimental results show that the probes bearing flexible ethylenediamine exhibited excellent sensitivity and selectivity toward Pd²⁺, whereas NPC with rigid piperazine as the receptor displayed minimum response to Pd²⁺. The introduction of biotin and indomethacin enabled NPE and NPF to selectively bioimage Pd²⁺ in living cancer cells. Job's plot results revealed that Pd²⁺ formed a 1 : 1 complex with the probes having one receptor (NPA-NPC) or a 2 : 1 complex with those bearing two receptors (NPD and NPE).