Lanthanide ternary mixed-ligand coordination polymers as fluorescent sensors for the sensitive and selective detection of chlorogenic acid

Abstract

The presence of an excessive concentration of drug molecules in the environment and biological systems often causes many unknown diseases, leading to health problems, which endanger human life. Thus, sensitive optical sensors for chlorogenic acid, a common antibacterial drug, were developed based on water-stable lanthanide coordination polymers (Ln-CPs). Herein, using a ternary mixed-ligand strategy, five isostructural Ln-CPs, [Ln(bct)(phen)(ox)_0.5(H₂O)]_n (Ln = Eu 1, Tb 2, Sm 3, Dy 4, Ce 5, H₂bct = 3,5-bis(4′-carboxyphen-yl)-1,2,4-triazole, phen = phenanthroline and H₂ox = oxalic acid), were successfully synthesized. Their crystal structure was characterized by a two-dimensional coordination network formed by H₂ox-Ln chain- and H₂bct-Ln chain-connecting nodes [LnO₆N₂], which was further extended by hydrogen bonds into a three-dimensional supramolecular structure. The optical properties of 1 exhibited strong red luminescence under the maximum excitation of 348 nm due to the ⁵D₀ → ⁷F_J (J = 0–3) transition of the Eu³⁺ ions. Also, 1 maintained good luminescence stability in water over a wide pH range (pH = 5–12), which is an essential requirement for an excellent fluorescence sensor. The fluorescence sensing test indicated that 1 exhibited high sensitivity for chlorogenic acid with an LOD of 7.48 × 10⁻⁵ mol L⁻¹. Complex 1 was also effective for the detection of chlorogenic acid in the presence of weak acids, bases and other interfering substances. The sensing mechanism of complex 1 for chlorogenic acid was discussed through experimental and theoretical approaches. 2 exhibited strong green luminescence, which is caused by the ⁵D₄ → ⁷F_J (J = 6–3) transition of Tb³⁺ ions and showed similar detection capability for chlorogenic acid (LOD of 4.9 × 10⁻⁵ mol L⁻¹). It is noteworthy to mention that the red/green emission film@Ln-CP based on naked eye recognition was successfully applied for the rapid detection of chlorogenic acid. The results showed that the synthesized Eu/Tb-CPs can be used as a sensitive luminescence sensing material for chlorogenic acid and has a potential application value.