Luminescent Metal-organic frameworks for sensing of pollutants

contamination of soils, water and air by heavy metals, pesticides, gases, dioxins etc. pose a severe treat to environment and human health and affect the future generations. For these reasons, researchers around the world are currently working on developing new materials and technologies to preserve nature and offer a better future for the next generations. Metal-organic frameworks (MOFs) are materials that could help in this challenge thanks to their outstanding properties, extreme flexibility and relative ease of synthesis. In recent years, MOFs are showing that they are not only extremely porous materials and excellent catalysts, but some of them have also interesting optical properties making them good candidates for LEDs and luminescent sensors.[1] In particular, using luminescent MOFs (LMOFs) as sensors for qualitative and quantitative detection of pollutants has notable advantages: the crystalline structure of the MOFs is capable to select and concentrate the analyte and exclude potential interfering species enhancing considerably the sensitivity. Moreover, it is possible to design sensors that could be reused by recovering the MOFs.[2] Two main interaction and transduction mechanisms are possible in luminescent MOFs while used as sensor: on one hand a change in luminescent intensity (quenching or enhancement); on the other hand a change in the position of the emission band.[3] To develop sensors based on LMOFs are possible two different approaches: synthesize intrinsically luminescent MOFs or loading luminescent species (Dyes, QDs etc.) inside the MOFs by exploiting their porosity. In this study, we have synthesized two LMOFs, each one example of these two different approaches presented before and tested as sensors for heavy metal ions in water solutions: the intrinsically luminescent AgBDC and Zr-MOF-808 loaded with luminescent gold clusters Au25@BSA (Au25@MOF-808). AgBDC was synthesized from AgNO3 and terephthalic acid and characterized by XRD and FTIR, and then tested as luminescent sensor for heavy metals ions in water solutions. Our results show that Mn2+ and Hg2+ ions in μM concentrations are capable to quench the luminescence of AgBDC, making this MOFs a promising candidate as luminescent chemosensor for these two metal ions. Meanwhile Au25BSA@MOF-808 was obtained by loading into Zr-MOF-808 luminescent gold clusters Au25@BSA. It was structurally characterized by XRD, FTIR and N2 gas adsorption and later tested as sensor for heavy metals in water. Brillant results were obtained showing how this LMOF acts as highly sensitive sensor for Hg2+ ions in trace concentrations (nM) with very high selectivity, respect to many common metal ions in water solutions. Moreover, the Zr-MOF-808 structure is capable to shield the Au25 clusters from organic pollutants, increasing their selectivity and improves their thermal and temporal stability. Our result shows that AgBDC and Au25@MOF-808 could be used as sensors for heavy metal ions with excellent sensitivity and selectivity. More specifically Au25@MOF-808 is capable to detect Hg2+ in trace concentrations just by simple fluorescence measurements. In addition, these two LMOFs open the way to developments of solid-state sensors based on them for example: coatings, films, polymer matrix membrane which could be easily recovered and reused. These results have strong impact in sensoristics demonstrating that LMOFs can be valid candidates to design efficient, sensitive and selective sensors for pollutants helping us in the continuous environmental monitoring.