Zn(II)/La(III) Coordination Polymers Based on New Schiff Base Ligands for Highly Selective and Sensitive Detection of 2,4,6-Trinitrophenol

Abstract

In recent years, nitro explosives have posed a threat to national security and human health due to their hazard and environmental contamination. The rapid and effective detection of nitro explosives remains a challenge. In this work, a new Schiff base ligand H₂L was carefully designed and successfully synthesized, H₂L = 1,1′-((1E,1′E)-(cyclohexane-1,4-diylbis (azanylidene)) bis (methylidene) bis (naphthalene-2-ol). Two new coordination polymers, [Zn₂L₂]_n and [La(NO₃)₃ (H₂L)₂CH₃OH]_n, were constructed by a solvothermal reaction. The aforementioned coordination polymers were characterized by X-ray, PXRD, TGA, and FT-IR, and the results demonstrated that the Zn-CP and the La-CP exhibited a one-dimensional chain structure and a two-dimensional reticular structure. Given the excellent performance of coordination polymers (CPs) in the field of fluorescence analysis and sensing, fluorescence sensing experiments were conducted on nitro explosives. The experimental results demonstrated that Zn-CP exhibited high selectivity and sensitivity for the detection of 2,4,6-trinitrophenol (TNP) by fluorescence quenching, with a detection limit of 4 × 10^–6 M. The detection limit of La-CP for TNP was even as low as 0.78 × 10^–6 M, and the quenching effect constants K_SV were 7.03 × 10³ and 1.59 × 10⁴ M^–1. Through experimentation and density functional theory calculations, the observed fluorescence quenching can be attributed to three mechanisms: photoinduced electron transfer, resonance energy transfer, and electrostatic interactions between CPs and NACs. Moreover, sensor-coated portable test strip devices were fabricated for easy and rapid on-site detection of the TNP explosive.