Flexible fluorescent metal-organic frameworks towards highly stable optical fibers and biocompatible cell platforms

Fluorescent metal-organic frameworks (FMOFs) have emerged as promising materials for optoelectronic and biological applications. However, their widespread use in these areas is limited due to some major drawbacks. First, it is difficult to construct flexible free-standing platforms with FMOF crystals. Moreover, these materials exhibit a low optical stability against external stimuli, and frequently, the metal ions used for their synthesis are toxic. Herein, a coaxial microfluidic spinning approach is developed to continuously generate core-shell FMOF-based microfibers in a one-step and large-scale fashion. The free-standing FMOF-based microfibers obtained showcase a high tensile strength of 85.6 MPa, the highest tensile strength reported for polymer@MOF-based composites. Moreover, it is also demonstrated that the cross-linked hydrogel shell generated with our microfluidic approach can effectively improve the optical stability of the FMOF crystals located at the core of the microfibers against a solution containing chemical species that can damage the FMOF crystals (e.g., a metal ion), acid and/or alkali treatments as well as heat. Most importantly, the FMOF-based microfibers exhibit high cell viability (ca. 96.6%) and versatile extracellular fluorescence imaging capabilities owing to both the biocompatibility of the hydrogel shell and the intrinsic fluorescence of the micofibers core. Accordingly, the herein reported flexible FMOF-based microfibers provide an advanced platform to design new concepts and material assemblies for optical and biomedical applications.