RESUMO
Covalent post-synthetic modification of metal-organic frameworks (MOFs) represents an underexplored but promising avenue for allowing the addition of specific fluorescent recognition elements to produce the novel MOF-based sensory materials with multiple-analyte detection capability. Here, an excited-state proton transfer (ESPT) active sensor 2D-Zn-NS-P was designed and constructed by covalent post-synthetic incorporation of the excited-state tautomeric 2-hydroxypyridine moiety into the ultrasonically exfoliated amino-tagged 2D Zn-MOF nanosheets (2D-Zn-NS). The water-mediated ESPT process facilitates the highly accessible active sites incorporated on the surface of 2D-Zn-NS-P to specifically respond to the presence of water in common organic solvents via fluorescence turn-on behavior, and accurate quantification of trace amount of water in acetonitrile, acetone and ethanol was established using the as-synthesized nanosheet sensor with the detection sensitivity (<0.01% v/v) superior to the conventional Karl Fischer titration. Upon exposure to Fe3+ or Cr2O72-, the intense blue emission of the aqueous colloidal dispersion of 2D-Zn-NS-P was selectively quenched even in the coexistence of common inorganic interferents. The prohibition of the water-mediated ESPT process and local emission, induced by the coordination of ESPT fluorophore with Fe3+ or by Cr2O72- competitively absorbs the excitation energy, was proposed to responsible for the fluorescence turn-off sensing of the respective analytes. The present study offers the attractive prospect to develop the ESPT-based fluorescent MOF nanosheets by covalent post-synthetic modification strategy as multi-functional sensors for detection of target analytes.
RESUMO
The reaction of a macrocyclic ligand 1,4,7-triazacyclononane-1,4,7-tripropionic acid (tacntpH3) with Ni(II)/Co(II) sources in the presence or absence of lanthanide cations yielded a series of doubly mononuclear ionic complexes (H3O+)[LnIII(H2O)8][NiII(tacntp)]4 (1-NiLn, Ln = La, Ce, Yb) and a mononuclear Co(III) complex [CoIII(tacntp)]·4H2O (2-Co) incorporating transition metal centers enveloped by both an azamacrocycle ring and pendant carboxylate groups. Either of the two preorganized mononuclear species [MII/III(tacntp)]-/0 was taken as a tripodal 3d metalloligand for the further assembly of modular architectures. Two pentanuclear metallocage-[Ln(NO3)6]3- complexes [NiII5(tacntp)2(H2O)12][LnIII(NO3)6]Cl·2H2O (3-NiLn, Ln = La, Ce), one nonanuclear metallocage [NiII9(tacntp)4(H2O)18](ClO4)6·10H2O (4-Ni), and two types of 2D layered heterometallic 3d-4f coordination networks ((H3O+)[NiII2YbIII(tacntp)2](ClO4)2·3H2O (5-NiYb) and [CoIII2TbIII(tacntp)2(H2O)3](ClO4)·Cl2·5H2O (6-CoLn, Ln = La, Eu, Tb, Dy)) differing largely in their weaving architecture were controllably prepared via the precise regulation of multiple reaction parameters. Furthermore, the time-dependent evolution of metalloligand-derived species was tracked via electrospray ionization-mass spectrometry, to elucidate the modular assembly pathway for the nonanuclear Ni(II) metallocage 4-Ni. Analysis of the structural details and the assembly procedures of the modular architectures revealed that the pH, metalloligand size, and the type of metal bound to the metalloligand considerably influence the structural evolution from mononuclear metalloligands to cage-like and polymeric coordination architectures. Magnetic property studies disclosed that pendant syn-anti carboxylate groups favor a weak ferromagnetic exchange between the adjacent Ni(II) ions within the metallocages of 3-NiLa and 4-Ni but facilitate an antiferromagnetic coupling between the alternating Ni(II) and Yb(III) centers in 5-NiYb.
