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 Ga and Fe ions.

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 Ga and Fe ions. TIE demonstrated a prominent "turn on" response characterized by clear distinguished fluorescence when coordination with Ga ions in the DMSO/HO 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-Ga and TIE-Ga may act as selective "turn-off" fluorescent probes towards Fe ions. Limits of detection of TIC and TIE towards Ga ions were 7.8809 × 10 M and 2.6277 × 10 M, respectively. Limits of detection of TIC-Ga and TIE-Ga towards Fe ions were 8.6562 × 10 M and 3.3764 × 10 M, respectively. The molar ratio of the complex between the sensor and Ga or Fe 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 Ga ions have been successfully developed.