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⁺.