Unraveling the Effect on Luminescent Properties by Postsynthetic Covalent and Noncovalent Grafting of gfp Chromophore Analogues in Nanoscale MOF-808

Abstract

Here, we demonstrate mimicking of photophysical properties of native green fluorescent protein (gfp) by immobilizing the gfp chromophore analogues in nanoscale MOF-808 and further exploring the bioimaging applications. The two virtually nonfluorescent gfp chromophore analogues carrying different functionalities, BDI-AE (COOH/COOMe) and BDI-EE (COOMe/COOMe) were immobilized in nanosized MOF-808 via postsynthetic modification. An 1H NMR and IR study confirms that BDI-AE was coordinated in NMOF-808, whereas BDI-EE was just noncovalently encapsulated. Interestingly, the extremely weakly fluorescent monomers BDI-AE and BDI-EE (QY = 0.01–0.03%, lifetime = 0.01–0.03 ns) showed a 102-fold increase in quantum efficiency with a significantly longer excited-state lifetime (QY = 1.8–5.6%, lifetime 0.89–1.49 ns) after immobilization in the NMOF-808 scaffold. Moreover, BDI-AE@MOF-808 has 4 times higher quantum efficiency as well as longer excited-state lifetime in comparison to BDI-EE@NMOF-808 due to the rigidity imposed in the chromophore upon coordination with Zr4+ in the former case. Further, a cell viability test performed for BDI-AE@NMOF-808 in HeLa cells confirmed the nontoxic nature of the material and, more importantly, bioimaging applications have also been explored successfully.