Ultrastable\nLanthanide Metal–Organic Frameworks\nfor Smartphone-Assisted Ratiometric Fluorescent Sensing of Toluenediamines\nand Tunable Luminescence

Lanthanide metal–organic frameworks (Ln-MOFs)\nhave excellent\noptical properties and structural diversity, providing a unique platform\nfor the development of fluorescent sensing and optical materials.\nIn the work described herein, a series of isostructural 3D Ln-MOFs\n[Ln(L)(H<sub>2</sub>O)]·2H<sub>2</sub>O (Ln = Eu (<b>1</b>), Gd (<b>2</b>), Tb (<b>3</b>), H<sub>3</sub>L = 3,3′,3″-[1,3,5-benzenetriyltris(carbonylimino)]tris-benzoate)\nare fabricated under solvothermal conditions. The good thermal, water,\nand acid–base stabilities of <b>3</b> are prerequisites\nfor fluorescent sensing applications. <b>3</b> can be used as\na ratiometric broad-spectrum fluorescent sensor for toluenediamines\n(TDAs) in real urine with the advantages of visualization, ultrasensitivity,\nand selectivity. Interestingly, a smartphone-assisted intelligent\nsensing platform manifests promising results for the detection of\nTDAs, providing a chance for further development of portable diagnostic\ntools. In addition, by tuning the ratios of Eu<sup>3+</sup>/Tb<sup>3+</sup> and Eu<sup>3+</sup>/Gd<sup>3+</sup>/Tb<sup>3+</sup>, nine\nbimetallic-doped Eu<sub><i>x</i></sub>Tb<sub>1–<i>x</i></sub> (<i>x</i> = 0.10–0.90, <b>4</b>–<b>12</b>) and one trimetallic-doped Gd<sub>0.95</sub>Tb<sub>0.015</sub>Eu<sub>0.035</sub> (<b>13</b>) were obtained. <b>4</b>–<b>12</b> exhibit a gradient of luminescent\ncolors from yellow-green to pink with different ratios of Eu<sup>3+</sup> and Tb<sup>3+</sup> ions. Meanwhile, the trimetallic-doped Gd<sub>0.95</sub>Tb<sub>0.015</sub>Eu<sub>0.035</sub> (<b>13</b>)\nshows near-white-light emission with a quantum yield of 8.76%. Interestingly,\nthe inks made with <b>1</b>–<b>13</b> are invisible\nunder ambient light but show visual color-tunable luminescence under\na 254 nm UV lamp, which may facilitate their anti-counterfeiting applications.