Due to the unique optical, electric, and magnetic properties, rare earth elements are widely used in applications ranging from materials science to biomedicine. Therefore, the construction of lanthanide-containing complexes will greatly expand the function of supramolecular host. However, the poor stability and solubility in water of multicomponent lanthanide organic assemblies greatly restrain their practical applications.
In a study published in J. Am. Chem. Soc., a research group led by Prof. SUN Qingfu from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences, in collaboration with Prof. CHEN Tianfeng from Jinan University and Prof. LI Xiaopeng from Shenzhen University, reported a series of water-stable anionic Ln2nL3n-type (n = 2, 3, 4, and 5) lanthanide organic polyhedra (LOPs) for the first time, via the deprotonation self-assembly of three 2,6-pyridine bitetrazolate ligands (H4L1 and H4L2a/b) with lanthanide ions.
The researchers found that the final outcomes of the LOPs, ranging from tetrahedra, trigonal prisms, and cubes to pentagonal prisms, were sensitive toward the reaction conditions including base, metal source, solvents and concentrations, highlighting the challenges in controlled self-assembly of predetermined lanthanide architectures.
They investigated the photophysical studies of the assembly. Surprisingly, ligands H4L2a/b manifested an excellent sensitization toward lanthanide ions (Ln = EuIII and TbIII), and the unparalleled photoluminescence quantum yields (QYs) of the Tb8L2a12 (in water) and Eu8L2b12 (in DMSO) were measured to be 11.17% and 76.75%, respectively.
Furthermore, the researchers found that the longitudinal relaxivity (r1) of Gd8L2b12 determined from the concentration-dependent T1 measurement is 400.53 mM-1S-1 because of the giant molecular weight and rigidity of the polyhedral skeleton.
Based on the high relaxivity, the researchers further explored the magnetic resonance imaging (MRI) of Gd8L2b12 in vivo with much longer retention time in the tumor sites compared with the commercial GdIII-based contrast (CAs). They demonstrated the performance of mixed Eu/Gd LOPs as potential dual-modal imaging agents.
This study not only provides a new design principle toward water-stable multinuclear functional lanthanide assemblies, but also offers potential candidates of supramolecular devices for bioimaging and drug delivery.
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