Ultrastable Lanthanide Metal–Organic Frameworks for Smartphone-Assisted Ratiometric Fluorescent Sensing of Toluenediamines and Tunable Luminescence

Abstract

Lanthanide metal-organic frameworks (Ln-MOFs) have excellent optical properties and structural diversity, providing a unique platform for the development of fluorescent sensing and optical materials. In the work described herein, a series of isostructural 3D Ln-MOFs [Ln(L)(H₂O)]·2H₂O (Ln = Eu (<b>1</b>), Gd (<b>2</b>), Tb (<b>3</b>), H₃L = 3,3',3″-[1,3,5-benzenetriyltris(carbonylimino)]tris-benzoate) are fabricated under solvothermal conditions. The good thermal, water, and acid-base stabilities of <b>3</b> are prerequisites for fluorescent sensing applications. <b>3</b> can be used as a ratiometric broad-spectrum fluorescent sensor for toluenediamines (TDAs) in real urine with the advantages of visualization, ultrasensitivity, and selectivity. Interestingly, a smartphone-assisted intelligent sensing platform manifests promising results for the detection of TDAs, providing a chance for further development of portable diagnostic tools. In addition, by tuning the ratios of Eu³⁺/Tb³⁺ and Eu³⁺/Gd³⁺/Tb³⁺, nine bimetallic-doped Eu_<i>x</i>Tb_1-<i>x</i> (<i>x</i> = 0.10-0.90, <b>4</b>-<b>12</b>) and one trimetallic-doped Gd_0.95Tb_0.015Eu_0.035 (<b>13</b>) were obtained. <b>4</b>-<b>12</b> exhibit a gradient of luminescent colors from yellow-green to pink with different ratios of Eu³⁺ and Tb³⁺ ions. Meanwhile, the trimetallic-doped Gd_0.95Tb_0.015Eu_0.035 (<b>13</b>) shows near-white-light emission with a quantum yield of 8.76%. Interestingly, the inks made with <b>1</b>-<b>13</b> are invisible under ambient light but show visual color-tunable luminescence under a 254 nm UV lamp, which may facilitate their anti-counterfeiting applications.