Simple pyridyl-salicylimine-based fluorescence “turn-on” sensors for distinct detections of Zn2+, Al3+ and OH− ions in mixed aqueous media

Abstract

Simple pyridyl-salicylimine derivatives (F1, F2 and F3) are reported for the first time as fluorescence “turn-on” sensors for distinct detections of Zn²⁺, Al³⁺ and OH⁻ ions in mixed-aqueous media CH₃CN/H₂O with volume ratios of 6/4 and 3/7 (at pH = 7 and 25 °C) via internal charge transfer (ICT), chelation enhanced fluorescence (CHEF), and deprotonation mechanisms. F1 and F2 show diverse turn-on sensing applications to Zn²⁺, Al³⁺ and OH⁻ ions, but F3 exhibited the fluorescence turn-on sensing to Al³⁺ and OH⁻ ions in CH₃CN/H₂O (6/4; vol/vol). F1+Zn²⁺ and F2+Zn²⁺ complexes revealed the reversibilities and ratiometric displacements of Zn²⁺ with ethylene diamine tetra acetic acid (EDTA) and Al³⁺ ions, respectively, in CH₃CN/H₂O (6/4; vol/vol). On the other hand, F1, F2 and F3 in CH₃CN/H₂O (3/7; vol/vol) showed sensitivities only to Al³⁺ ions but negligible selectivities to OH⁻ ions. Stoichiometry of all sensor complexes were calculated as 1 : 1 by job's plots based on UV/Vis and PL titrations. The complex formation and binding sites of all sensor materials were well characterized by ¹H, ¹³C NMR, and mass (FAB) spectral analysis. Detection limits were calculated from standard deviations and linear fitting calculations. The association constant (log K_a) values of sensor complexes were evaluated from the fluorescence binding isotherms. The fluorescence decay constant (τ) values were estimated from time resolved fluorescence studies. Time, temperature, pH and solvent concentration effects towards sensor responses were fully investigated in this report.