Coordination-Assembled Water-Soluble Anionic Lanthanide Organic Polyhedra for Luminescent Labeling and Magnetic Resonance Imaging

Lanthanide-containing
functional complexes have found a variety
of applications in materials science and biomedicine because of their
unique electroptical and magnetic properties. However, the poor stability
and solubility in water of multicomponent lanthanide organic assemblies
significantly limit their practical applications. We report here a
series of water-stable anionic Ln2nL3n-type (n = 2, 3, 4, and
5) lanthanide organic polyhedra (LOPs) constructed by deprotonation
self-assembly of three fully conjugated ligands (H4L1 and H4L2a/b) featuring a 2,6-pyridine
bitetrazolate chelating moiety. The outcomes of the LOPs formation
reactions were found to be very sensitive toward the reaction conditions
including base, metal source, solvents, and concentrations as characterized
by a combination of NMR, high-resolution ESI–MS and X-ray crystallography.
Ligands H4L2a/b manifested an excellent sensitization
toward lanthanide ions (Ln = EuIII and TbIII), with high luminescent quantum yields for Tb8L2a12 (Φ = 11.2% in water) and Eu8L2b12 (Φ = 76.8% in DMSO) measured in polar
solvents. Furthermore, due to the giant molecular weight and rigidity
of the polyhedral skeleton, Gd8L2b12 showed a very high longitudinal relaxivity (r1) of 400.53 mM–1S–1. The
performance of Gd8L2b12 as potential
magnetic resonance imaging contrast agents (CAs) in vivo was evaluated
with much longer retention time in the tumor sites compared with the
commercial GdIII-based CAs. Dual-modal imaging potential
has also been demonstrated with the mixed Eu/Gd LOPs. Our results
not only provide a new design route toward water-stable multinuclear
lanthanide organic assemblies but also offer potential candidates
of supramolecular-edifices for bioimaging and drug delivery.