Optical sensors for detection of nano-molar Zn2+ in aqueous medium: Direct evidence of probe- Zn2+ binding by single crystal X-ray structures

Two simple probes, derived by condensation of ethylenediamine with 3-ethoxysalicylaldehyde (L) and salicylaldehyde (L1) have been exploited for selective recognition of Zn2+. The interaction between probes and Zn2+ has been established by spectroscopic techniques and confirmed by single crystal X-ray structures of the resulting Zn2+ complexes. It is proposed that Zn2+ inhibits the PET process, originally responsible for non-emissive nature of free ligand and turns the fluorescence ON through CHEF process. The L and L1 probes respectively detect as low as 1.8 nM and 1.16 nM Zn2+, while corresponding binding constants are 2.9 x 10(6) M-1 and 4.57 x 10(5) M-1. L1 is very effective for intra-celullar imaging of Zn2+ in HeLa and MDA-MB-468 cells. The density functional theoretical studies unveil the energy parameters involved in the probe-Zn2+ interaction process.