Double Schiff base from thiophene-2,5-dicarboxylic acid as an “off–on–off” fluorescence sensor for the sequential detection of In3+ and PPi

Abstract

A double Schiff base T as acceptor for metal ions derived from thiophene-2,5-dicarboxylic acid was designed and synthesized, and showed a high selectivity for In³⁺ in a DMF/H₂O buffer solution. T exhibited a 300-fold fluorescence enhancement upon the addition of In³⁺, accompanying a visual color change from dark (Φ = 0.009) to bright yellow (Φ = 0.197). The complex T–In³⁺ could serve as a secondary sensor for PPi, which displayed recyclability upon the sequential addition of In³⁺ and PPi. The detection limits were calculated to be 4.48 × 10⁻⁸ M for In³⁺ and 4.4 × 10⁻⁶ M for PPi, respectively. According to Job's plot, T and In³⁺ tended to form 1 : 2 complexes with the binding affinity of 2.3 × 10⁹ M². The fluorescence behavior of T towards In³⁺ was primarily explained by the photoinduced electron transfer (PET) process based on theoretical calculations using Gaussian 09. Moreover, T could be applicable for indium detection in tap water with satisfactory recovery and RSD.