Mercuric ion-induced signaling by a dual turn-off/on switching of a quinoline-based fluorescent sensor

Quinoline-based fluoroprobe, 3-isocyanatopropyl triethoxysilane grafted 3-aminoquinoline (AqI) has been designed and synthesized for detection of Hg²⁺ in aqueous solution. Fluorescent molecule, AqI was characterized by FTIR and NMR spectroscopy, and photophysical properties in solvents with different polarities were elucidated by UV-vis and fluorescence measurements. Quantum yield of AqI was calculated (_ΦF=0.15) and acid dissociation constant (p_Ka=4.14±0.15) was determined using data of integrated emission intensity versus pH. AqI shows emission quenching followed by fluorescence turn-on upon coordination-mediated complexation of Hg²⁺ ions. The fluorescence turn-on emission distinguishes Hg²⁺ ion from interfering metal ions and AqI is found to be selective for Hg²⁺ ion in buffered aqueous-ethanol solution (4:1, v/v) at pH 6.0. Equilibrium constant for the formation of complex between AqI and Hg²⁺ ions was calculated as K=3.08×^106M-2/3 applying by spectrofluorometric titration and stoichiometry of complex was determined as Aq_I2Hg32+ (2:3).