Turn-on fluorescent sensors for nanomolar detection of zinc ions: Synthesis, properties and DFT studies

Background: Detection of zinc (Zn2+) is important to know its role in the living system and environment. The design and synthesis of two new fluorescent sensors 1 and 2 for the highly sensitive and selective detection of Zn2+ are described. Methods: The synthesis of chemosensors 1 and 2 was accomplished through easy synthetic procedures in a high yielding reaction sequence. Titration experiments were performed to measure the responses of sensors 1 and 2 to the varying concentrations of neat Zn2+ and Zn2+ in the presence of other bio-relevant metal ions (Cu2+, Cr2+, K1+, Mg2+, Fe3+, Pb2+, Mn2+, Co2+ and Cd2+), using UV and fluorescence spectroscopy. Extensive DFT simulations were performed to examine the impact of Zn2+ binding on the absorption spectra of 1 and 2. Significant findings: The synergistic coordination of N-3 atom of 1,2,3-triazolyl function and 2,2-dipicolylamine (DPA) receptor unit with Zn2+ enables highly sensitive and selective detection of Zn2+, with a limit of detections (LOD) of 65.6 nM and 107 nM, respectively. At a physiological pH (pH = 7.4), addition of Zn2+ to the solutions of 1 and 2 led to the 4.7-fold and 6-fold enhancements in the fluorescence intensities, respectively. However, both sensors remain almost insensitive towards other competing ions. Job's plot analysis suggests the formation of sensor/Zn2+ in 1:1 ratio. DFT simulation studies revealed that both sensors possess excellent binding with Zn2+. After complexation, total change in energy for 1 and 2 were found to be -86.6 and -89.3, respectively, which hinted their thermodynamic stabilities. In the unbound states, the highest occupied molecular orbital (HOMO) electronic density was localized onto DPA receptor, used for coordinating with Zn2+ whereas the lowest unoccupied molecular orbitals (LUMOs) density was found to be on the fluorophores. However, upon coordination with Zn2+, the HOMOs and LUMOs were found to be localized on the fluorophores of 1 and 2. The insertion of dinitrobenzene in 1 exhibited higher fluorescence enhancement as compared to 2 upon binding to Zn2+.