Rare-earth-based metal-organic frameworks (ReMOFs) have emerged as an interesting family of compounds, for which new properties are increasingly being found. Based on the potential of ReMOFs, resulting from their optical properties, large numbers of investigations have been carried out during the last decade. Among these investigations, ReMOFs as optical sensors, using their luminescence properties, are increasingly becoming an attractive and useful topic of research. In this study, we have provided the basics of the luminescence behaviour of ReMOFs, various possible sensing mechanisms, and a summary of the uses of ReMOFs for the sensing of nitro explosives, cations, anions, small molecules, pH, and temperature.

Two isomorphous submicron sized metal-organic network compounds, [Y 2 (PDA) 3 (H 2 O) 1 ]$2H 2 O (PDA ¼ 1,4-phenylenediacetate), 1 and [Y 1.8 Tb 0.2 (PDA) 3 (H 2 O) 1 ]$2H 2 O, Tb@1 have been synthesized by employing solvent assisted liquid grinding followed by heating at 180 C for 10 min and washing with water. Single crystal X-ray data of bulk 1 confirmed a three dimensional porous structure. The structure and morphology of 1 and Tb@1 were systematically characterized by PXRD, TGA, DSC, IR, SEM and EDX analysis. Dehydrated Tb@1 [Tb@1 0 ] shows a high intense visible green emission upon exposure to UV light. The green emission of Tb@1 0 was used for the detection of nitro explosives, such as 2,4,6-trinitrophenol (TNP), 1,3-dinitro benzene (DNB), 2,4-dinitro toluene (DNT), nitro benzene (NB), and 4-nitro toluene (NT) in acetonitrile. The results show that the emission intensity of dehydrated Tb@1 0 can be quenched by all the nitro analytes used in the present work. Remarkably, Tb@1 0 exhibited a high efficiency for TNP, DNB and DNT detection with K SV [K SV ¼ quenching constant based on linear SternVolmer plot] values of 70 920, 44 000 and 35 430 M À1 , respectively, which are the highest values amongst known metal-organic materials. Using this material submicromolar level (h 0.18 ppm), a detection of nitro explosives has been achieved.

Two lanthanide doped highly luminescent MOFs have been synthesized and characterized. The MOFs are highly efficient for the detection of trace amount of explosive nitroaromatics by visible colour change.