The comparative study of two new Schiff bases derived from 5-(thiophene-2-yl)isoxazole as "Off-On-Off" fluorescence sensors for the sequential detection of Ga3+ and Fe3+ ions

Abstract

Two new Schiff bases, TIC ((E)-N'-(2-hydroxybenzylidene)-5-(thiophene-2-yl)isoxazole-3-carbohydrazide) and TIE ((E)-N'-(3-ethoxy-2-hydroxybenzylidene)-5-(thiophene-2-yl)isoxazole-3-carbohydrazide), have been designed and synthesized as chemosensors for distinct recognition of Ga³⁺ and Fe³⁺ ions. TIE demonstrated a prominent "turn on" response characterized by clear distinguished fluorescence when coordination with Ga³⁺ ions in the DMSO/H₂O buffer solution. In comparison, TIC also showed "turn on" response of blue fluorescence which was more selective and sensitive than that of TIE due to the steric hindrance of ethoxy group of TIE. The newly formed complexes TIC-Ga³⁺ and TIE-Ga³⁺ may act as selective "turn-off" fluorescent probes towards Fe³⁺ ions. Limits of detection of TIC and TIE towards Ga³⁺ ions were 7.8809 × 10^-9 M and 2.6277 × 10^-8 M, respectively. Limits of detection of TIC-Ga³⁺ and TIE-Ga³⁺ towards Fe³⁺ ions were 8.6562 × 10^-9 M and 3.3764 × 10^-7 M, respectively. The molar ratio of the complex between the sensor and Ga³⁺ or Fe³⁺ ions were all 1:2 determined through Job's Plot, mass spectrometry, and theoretical calculations. Both sensors were utilized for the determination of target ions in environment water samples, and the portable paper sensors for detecting Ga³⁺ ions have been successfully developed.

Keywords: Ferric ions; Fluorescence chemosensor; Gallium ions; Theoretical calculations; Thiophene derivative.