A new Schiff base derived from 5-(thiophene-2-yl)oxazole as “off-on-off” fluorescence sensor for monitoring indium and ferric ions sequentially and its application

[Display omitted] •Sensor TOQ has been employed for monitoring In3+ and Fe3+ ions sequentially.•LOD could be possible down to the nanomolar (nM) range.•The proposed sensing mechanism of TOQ toward target ion was confirmed by theoretical calculation.•Application of the sensor in real water sample and paper strips for detection of metal ions. A new Schiff base sensor (E)-N'-((8-hydroxy-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)methylene)-5-(thiophen-2-yl)oxazole-4-carbohydrazide (TOQ) was synthesized and found to emit yellowish green fluorescence upon introduction of In3+. Furthermore, the resulting complex TOQ-In3+ was quenched selectively by Fe3+. The detection limits of TOQ for In3+ and Fe3+ were 1.75 × 10-10 M and 8.45 × 10-9 M, respectively. The complex stoichiometry of TOQ with target ions was determined to be 1:2 via Job’s plot analysis, which further was verified by ESI-MS titration and theoretical calculations. Moreover, TOQ can be used for the determination of target ions in environmental water samples. A portable paper sensor of TOQ was successfully developed for detecting In3+ to assess its applicability.