Air pollution caused by the combustion of fossil fuels and other sources continues to be a pressing environmental issue, with nitrogen oxides (NOx) being particularly hazardous. NOx, which includes nitric oxide (NO) and nitrogen dioxide (NO2), contributes to smog formation, acid rain, and respiratory health problems. As the demand for energy increases with modernization and population growth, the burning of fossil fuels and other materials releases significant amounts of NOx into the atmosphere .
The use of metal-organic frameworks in sensing
Detecting, capturing, and removing NO2 gas is of great interest due to its detrimental effects. While there are several methods available, many of them are efficient but costly. In this context, optical sensing of NO2 using metal-organic frameworks (MOFs) has emerged as an interesting and cost-effective approach. MOFs are a class of materials known for their permanent porosity, high thermal stability, and large surface area . They have been applied in various fields such as gas separation, catalysis, drug delivery, and sensing .
Luminescent MOFs, in particular, have shown promise as optical sensors for capturing and detecting various analytes. By incorporating luminescent molecules or heterometals into MOFs, these materials can exhibit different transduction signals, including colorimetric response, photoluminescence quenching/enhancement, and changes in emission wavelength . The interaction between luminescent MOFs and analytes can provide valuable insights into the sensing mechanism.
Cu-MOF-808: an unprecedent mechanism for optical sensing of NO2
In our study, we focused on enhancing the optical sensing capabilities of MOF-808 by incorporating synergistic binding sites through post-synthetic modification with copper. To characterize the materials and elucidate the atomic structure of the copper sites, we employed a combination of conventional characterization techniques, computational modelling, and advanced synchrotron characterization tools. The result was an outstanding tetrahedral configuration of the copper single sites around the smallest pore of the MOF (Fig. 1).
Fig.1: Local structure of the copper sites around the tetrahedral pore of the MOF-808.
Our results demonstrated that Cu-MOF-808 exhibited an unprecedented optical sensing response towards NO2 when compared with MOF-808. Remarkably, we were able to reduce the NO2 concentration to 2 parts per million (ppm). The limit of detection (LOD) for the Cu-MOF-808-based optical sensor was calculated to be 16 parts per billion (ppb). To the best of our knowledge, this is the lowest LOD reported for luminescent MOFs. The excellent performance of Cu-MOF-808 can be attributed, thanks to DFT calculations, to the synergistic effect between the hydroxo/aqua-terminated Zr6O8 clusters and the copper-hydroxo single sites, where NO2 is adsorbed through combined dispersive and metal-bonding interactions (Fig. 2).
Fig. 2: Three different plausible models for the local structure of the copper single sites in the MOF-808 and its respective interaction with the molecules of NO2 during the sensing.
In conclusion, this work represents a significant advancement in the field of optical sensing of toxic gases using MOF materials. By harnessing the occurrence of weak-binding interactions through post-synthetic chemical modifications, we have opened exciting possibilities for the detection and monitoring of NO2 and other toxic gases using luminescent MOFs.
For more details, please check our paper (https://doi.org/10.1038/s41467-023-38170-9) in Nature Communications.
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