Multifunctional, Tunable Metal–Organic Framework Materials Platform for Bioimaging Applications

Herein,
we describe a novel multifunctional metal–organic framework
(MOF) materials platform that displays both porosity and tunable emission
properties as a function of the metal identity (Eu, Nd, and tuned
compositions of Nd/Yb). Their emission collectively spans the deep
red to near-infrared (NIR) spectral region (∼614–1350
nm), which is highly relevant for in vivo bioimaging. These new materials
meet important prerequisites as relevant to biological processes:
they are minimally toxic to living cells and retain structural integrity
in water and phosphate-buffered saline. To assess their viability
as optical bioimaging agents, we successfully synthesized the nanoscale
Eu analog as a proof-of-concept system in this series. In vitro studies
show that it is cell-permeable in individual RAW 264.7 mouse macrophage
and HeLa human cervical cancer tissue culture cells. The efficient
discrimination between the Eu emission and cell autofluorescence was
achieved with hyperspectral confocal fluorescence microscopy, used
here for the first time to characterize MOF materials. Importantly,
this is the first report that documents the long-term conservation
of the intrinsic emission in live cells of a fluorophore-based MOF
to date (up to 48 h). This finding, in conjunction with the materials’
very low toxicity, validates the biocompatibility in these systems
and qualifies them as promising for use in long-term tracking and
biodistribution studies.