A facile means for the improvement of sensing properties of metal-organic frameworks through control on the key synthesis variables

Abstract

In this study, we investigated the optimal conditions for solvothermal-assisted synthesis of Eu-BDC-NH2 metal organic frameworks (MOFs) to improve their sensing capacity toward some selected target metal species. To this end, we focused on the interactive relationships between key synthesis variables (e.g., the relative molar ratios (and absolute amounts) of the reactants [EuC1(3).6H(2)O vs. amino terephthalic acid (BDC-NH2)], temperature, and reaction duration). It was observed that different types of MOFs synthesized under diverse conditions consistently exhibited emission profiles of Eu3+. However, their luminescence properties were distinguished systematically by the synthesis conditions set for each sample group. The sensing potential of luminescent MOFs was explored for detection of toxic metal ions through two approaches, i.e., direct chemosensing and indirect biomolecule mediated sensing approach. In the former, the MOFs exhibited noticeable turn-off fluorescence behavior, especially with Hg2+ ions (i.e., a high Stern-Volmer quenching constant (K-sv) of 6.42 x 10(5) M-1 and limit of detection (LOD) of 0.007 M). In the latter, nanocomposites of MOFs-cyanocobalamin were characterized by a highly selective turn on fluorescence response towards Co2+ ions (K-sv = -6.25 x 10(3) M-1; LOD = 0.7 M). As such, the sensing capacity of these MOFs was distinguished efficiently by control of synthesis variables.