Core-shell metal-organic frameworks and hierarchical host-guest structures toward water-stable luminescence of lanthanide complexes in encoding beads
The photoluminescence stability of lanthanide complexes (Lnx) in biological media is of utmost significance for the construction of optically encoded nano-on-microbeads in multiplex bioassays. Here, we propose a new type of encoding guests consisting of MOF@MOF nanohybrids, where isomorphic ZIF-8 and ZIF-90 were employed as the core and the shell domain, respectively, to separately modulate the hosting and the protection of inner Lnx guests. Especially, the thin ZIF-90 shell epitaxially grown on the ZIF-8 core was employed as an intermediate layer to facilitate surface covalent fluorination via the aldehyde ligand. Thereby, a superhydrophobic surface was obtained, which led to strong water-resistance of the hybrid nanoparticles, stable host-guest structure, and high quantum yields (19.1%) in biological media. A coating layer of a surface active polymer (polydopamine, PDA) was utilized to conjugate the MOF nanoparticles on polystyrene microspheres for building the second-level host-guest architecture of 3D core-satellite pluralistic hybrids. Finally, single/dual-color encoded beads (EBs) were obtained by altering the ratios of hybrid MOF guests emitting at different wavelengths. As proof of concept, the detection of a model tumor marker (alpha-fetoprotein, AFP) was successfully performed using encoded beads. These findings can pave an avenue for the rational design of core-shell MOFs and hierarchical structures with high synergy and excellent flexibility toward stable photoluminescence of lanthanide complexes in bioassays.