An antipyrine based fluorescence “turn-on” dual sensor for Zn2+ and Al3+ and its selective fluorescence “turn-off” sensing towards 2,4,6-trinitrophenol (TNP) in the aggregated state

Abstract

A 2,6-diformyl-p-cresol (DFC)-4-amino antipyrine (AP) based dual signaling fluorescent Schiff base ligand (DFCAP) is found to exhibit colorimetric and fluorescence turn on selective sensing towards metal ions, Zn²⁺ and Al³⁺. It also exhibits a significant aggregation induced emission (AIE) phenomenon by controlling the water–THF solvent ratio which provides robust green emissive fluorogenic aggregates with well-defined morphologies. Turn-on fluorescence enhancements as high as 195 fold and 168 fold in methanol for Al³⁺ and Zn²⁺ at 480 nm and 508 nm, respectively, were noticed. The binding constants and stoichiometry determined from the fluorescence titration data are K = 7.63 × 10⁴ M⁻¹ and 3.42 × 10⁴ M⁻¹ and 1 : 1 complexation for both Al³⁺ and Zn²⁺ respectively, supported by Job's method. DFCAP shows high sensitivity towards the detection of Zn²⁺ and Al³⁺ ions with very low detection limit values of ca. ∼21 nM and 30 nM respectively. Besides by applying its attractive AIE feature, the green emissive hydrosol functions as a good chemosensor with high sensitivity for a selected explosive TNP through ground state complexation with a LOD value of ca. ∼1.74 μM and especially a high Stern–Volmer quenching constant of ca. ∼4.14 × 10⁵ M⁻¹. For instant ‘naked eye’ response for the trace detection of TNP in the solution state, we fabricated a simple paper strip that could detect TNP on-site in a fast, inexpensive and simple way.