Zn(II)-selective and sensitive fluorescent chemosensor based on steric constrains and inhibition of ESIPT  

   A new naphthoxazole-derived probe, 2-(2-Hydroxyphenyl)-naphtho[1,2-d]oxazole-5-sulfonate (HL), was conveniently synthesized and characterized. The presence of Zn 2+ caused an obvious fluorescence enhancement at 458 nm upon excitation at 391 nm, which could be distinguished with the naked eye under a UV lamp. Remarkably, the compound exhibited excellent selective and sensitive response to Zn²⁺ over other common cations with a micromolar limit of detection (0.69 uM) in DMSO/HEPES (v/v,1:1)  buffer. The results from absorption spectra,¹HNMR, ESI–MS, density functional theory (DFT) calcu- lations, and pH effect, implied that during the process of complexation, the fluorescence intensity was

enhanced by the inhibition of the excited state intramolecular proton transfer (ESIPT) in free HL and the formation of a large π-electron conjugation system. The steric hindrance effect from a hydrogen atom on naphthoxazole determined the selectivity of Zn²⁺ over other metal ions. The most appropriate environ-ments for the detection of Zn²⁺ were neutral and weak alkali. In addition, the probe of detecting the Zn²⁺ has been evaluated with relatively lower toxicity in living MCF7 cell, suggesting the practical application in physiological system.