Anion-controlled Zn(ii) coordination polymers with 1-(tetrazo-5-yl)-3-(triazo-1-yl) benzene as an assembling ligand: synthesis, characterization, and efficient detection of tryptophan in water

Abstract

Tryptophan regulates and participates in various physiological systems in the human body, and its excessive intake has harmful effects. Therefore, detecting and monitoring tryptophan in water and distinguishing it from other amino acids are necessary. In addition to their excellent luminescence, coordination polymer-based sensors have good stability and high sensitivity and selectivity for sensing applications. In this work, two luminescent coordination polymers (CPs), [Zn(ttb)Cl]_n (1) and [Zn₂(ttb)₂(OH)(NO₃)]_n (2), were obtained through the solvothermal reaction of different Zn(II) salts with a rarely studied multidentate N-donor ligand, 1-(tetrazo-5-yl)-3-(triazo-1-yl) benzene (Httb). Crystallographic investigations revealed that the structure of 1 exhibits a 2D fes net with Cl⁻ anions acting as terminal charge balancers, and that of 2 features a 3D ant net with NO₃⁻ anions in a rare monodentate bridging (μ₂-O-η¹:η¹) mode. In terms of stability tests, 2 has better thermal and water stability than 1. Although both show good fluorescence performance, specific tryptophan detection, and excellent anti-interference ability, 2 has higher K_SV (111 852.6 M⁻¹), a lower limit of detection (LOD = 23.6 nM), and a better recovery rate than 1. Cytotoxicity experiments proved that 2 has extremely low toxicity and thus has great potential for in vivo detection. Therefore, CP 2 is a suitable candidate for advanced practical applications for the efficient sensing of tryptophan in water. The luminescence of the ligands was also calculated using DFT theory and further discussed through experiments. The quenching mechanism that occurs after tryptophan addition was explored through Hirshfeld surface analysis.