Highly pH-stable lanthanide MOFs: a tunable luminescence and ratiometric luminescent probe for sulfamethazine

Abstract

By selecting a bisphosphonic ligand H₄L (H₄L = 4-F-C₆H₄CH₂N(CH₂PO₃H₂)₂) and a coligand oxalate (H₂C₂O₄), three isostructural lanthanide metal–organic frameworks (Ln-MOFs) with a 2D layer structure, [Ln(H₃L)(C₂O₄)]·2H₂O (Ln = Eu (1), Gd (2), or Tb (3)), were hydrothermally prepared. By tuning the molar ratio of Eu³⁺, Gd³⁺, and Tb³⁺ in the above reactions, six bimetallic or trimetallic doped Ln-MOFs (Eu_xTb_1−x (x = 0.02 (4), 0.04 (5), and 0.06 (6)), Gd_0.94Eu_0.06 (7), Gd_0.96Tb_0.04 (8) and Gd_0.95Tb_0.03Eu_0.02 (9)) were obtained. The powder X-ray diffraction (PXRD) patterns of doped Ln-MOFs 4–9 show that they are isomorphous with 1–3. The bimetallic doped Ln-MOFs show a gradual variation of luminous colors between yellow-green, yellow, orange, pink, and light blue. Meanwhile, the trimetallic doped Gd_0.95Tb_0.03Eu_0.02 Ln-MOF (9) displays near white-light emission with a quantum yield of 11.39%. Interestingly, the luminous inks of 1–9 are invisible and color tunable, which makes it possible to promote their anti-counterfeiting applications. Additionally, 3 displays good thermal, water, and pH stabilities, which provides the feasibility for its sensing application. The luminescence sensing experiments show that 3 can serve as a highly selective, reusable, and ratiometric luminescent sensor of sulfamethazine (SMZ). Moreover, 3 shows an excellent SMZ detection performance in real samples, such as mariculture water and real urine. Owing to the visible variation of the response signal under a UV lamp, portable SMZ test paper was prepared.