Optical detection of small biomolecule thiamines at a micromolar level by highly luminescent lanthanide complexes with tridentate N-heterocyclic ligands

Abstract

A new series of lanthanide complexes Ln(H₂L)(EtOH)(NO₃)₃ (Ln = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), and Tb (8)) have been synthesized by a solution synthetic method, heating in a pyrex flask, from the self-assembly of lanthanide ions (Ln³⁺) with tridentate N-heterocyclic ligand 2,6-bis(5-methyl-1H-pyrazol-3-yl)pyridine (H₂L). These complexes were characterized by elemental analysis, IR spectra, powder X-ray diffraction (PXRD), single-crystal X-ray diffraction, thermal gravimetric analysis (TG) and luminescence spectra. In particular, the complexes Eu(H₂L)(EtOH)(NO₃)₃ (6) and Tb(H₂L)(EtOH)(NO₃)₃ (8) emit strong luminescence with high efficiency. Based on the luminescent properties of the lanthanide complexes, we found that very fast and extremely sensitive optical detection of the thiamines can be achieved for the first time, using the better performing luminescent sensing materials. The quenching constant (K_SV) of complex 6 was 9.08 × 10⁵ M⁻¹ for TPP, 4.90 × 10⁵ M⁻¹ for TMP, and 4.17 × 10⁵ M⁻¹ for TCl. The K_SV of complex 8 was 1327 M⁻¹ for TPP, 149 M⁻¹ for TMP, and 132 M⁻¹ for TCl. The lower detection limit of complex 6 was 0.029 μM for TPP, 0.027 μM for TMP, and 0.028 μM for TCl, and the highest quenching efficiency was 99.92% for TPP, 99.85% for TMP and 99.82% for TCl. For complex 8, the lower detection limit was 0.090 μM for TPP, 0.278 μM for TMP, and 0.263 μM for TCl, and the highest quenching efficiency was 86% for TPP, 38% for TMP and 35% for TCl. The greater K_SV values, the lower detection limits, and the higher quenching efficiencies revealed extremely high sensitivity, showing both complexes 6 and 8 to be some of the best sensitive luminescence based metal–organic detectors of TPP, TMP and TCl. In the meantime, possible energy transfer mechanisms are discussed.