Multi-stimuli responsive Ln-TMA-DPA complexes with high performance temperature sensing and anti-counterfeiting applications

Multi-stimuli responsive smart materials have garnered increasing attention in recent years. A series of lanthanide organic complexes [Ln₃(DPA)₃TMA]·3H₂O (Ln = Eu, Tb, Eu_xTb_1-x, H₃TMA = 1,3,5-benzenetricarboxylic acid, H₂DPA = 2,6-pyridinedicarboxylic acid) were obtained through solvothermal method. The complexes exhibit an outstanding temperature sensitivity in the range of 288–378 K, with a high relative sensitivity of Sr = 10.12 % K⁻¹ at 378 K. Furthermore, the complexes show good selectivity and sensitivity to Fe³⁺ and Cu²⁺ ions, as well as acetone, allowing for rapid and accurate determination of Fe³⁺ ions with a detection limit (LOD) of 1.587 ppm. The complexes respond quickly to the stimulus of acetone, with luminescence quenching in the range of 0–100 μl of acetone into 4 ml complexes inks obeying the Stern-Volmer equation with LOD = 5.77 ppm. Interestingly, the complexes exhibit different luminescent responses under pH conditions. In acidic solution, due to the detachment of TMA ligand, the excitation spectrum of the complex shows a blue shift, demonstrating dual response to pH and light stimuli. Even in strong alkalinity, the complexes still maintain intense emission. Therefore, binuclear complexes undergo color tuning after acid-base cycling. For Eu_0.03Tb_0.97-TMA-DPA, it changes from yellow to orange and then to yellow-green after acid-base cycling. Eu_0.1Tb_0.9-TMA-DPA, on the other hand, changes from orange to green and finally to yellow. An encryption key based on this interesting pH stimuli response has been designed and demonstrated its potential in the field of anti-counterfeiting applications.