Metal organic frameworks, commonly referred to as MOFs, have gathered a lot of interest in recent years and have already been implemented into various applications to date. One area where there has been a surge of research is in sensing applications. A team of Researchers from China have now developed a robust microporous Zn-MOF material to act as a smart and efficient photoluminescence (PL) sensor.
Metal organic frameworks (MOFs) are hybrid materials composed of both organic and inorganic components. As such, they possess in-and-out channels which can host guest molecules and act like a sponge under certain environmental driving forces, e.g chemicals, pressure, temperature etc.
However, many porous photoluminescence sensors are often affected by other molecules in the environment, and not just the intended molecules that are being sensed. Molecules, especially those with a similar or smaller size, are known to enter the pores of the porous material and change the emission.
Therefore, there has been a requirement for porous photoluminescence sensors to be tailored and specifically designed so that unique interactions between the host and guest molecules can occur without interference. It’s safe to say that this has been a challenge for many Scientists, but MOFs have offered the greatest potential to tackle this challenge because of their tuneable and flexible nature.
The team of Researchers have created a Zn-MOF [Zn(hpi2cf)(DMF)(H2O)] by assembling dual-emissive H2hpi2cf ligands- which exhibit a characteristic excited state intramolecular proton transfer, through wet chemical and synthesis techniques.
The Researchers characterized the Zn-MOF, and its sensing properties, using a combination of 1H nuclear magnetic resonance spectroscopy (NMR, Bruker AVANCE III), Photoluminescence spectroscopy (Edinburgh FLS980 or FLS5 and Quantaurus-QY, Hamamatsu), X-ray diffraction (XRD, Super Nova X-ray diffractometer system, Agilent Technologies), field emission scanning electron microscopy (FE-SEM, SU8010, Hitachi), Coulometric Titrimetry (C30 Karl Fischer, Mettler Toledo), differential scanning calorimetry (DSC, TA Instruments, Q2000), Thermogravimetric mass spectroscopy (TG-MS, NETZSCH), gravimetric sorption analysis (Hiden-IGA100B), pore size analysis (QUADRASORB evo) and temperature dependence analyzes (OptistatDN2 thermostat, OXFORD-instruments).
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The sensor was tested for use as a photoluminescence sensor, humidity sensor (i.e. water) and as a temperature sensor.
The Zn-MOF was found to contain amphipathic micropores, less than 3 angstroms in size, which underwent a facile single-crystal-to-single-crystal transformation borne out of the reversible removal, and uptake, of coordinating molecules. The molecules were found to be stimulated by environmental changes, such as gas blowing and gentle heating. They demonstrated an excellent example of dynamic reversible coordination behavior.
For water/humidity sensing applications, the interconversion process between the hydrated and dehydrated phases was found to turn the ligand excited state intramolecular proton transfer between the on and off states, which resulted in a two-color photoluminescence switching between the cycles.
In short, the Zn-MOF shows great potential as a humidity sensor, through photoluminescence, as it was found to possess a rapid sensing time, on the order of seconds. And a highly selective response towards water molecules and not a mixture of similar sized molecules, at the molecular level. The uptake and release of the water molecules was found to be an ultrafast process. This allowed the pinpointing of a localized photoluminescence switch, rather than inducing a full sensory phase change, which allowed for a high selectivity and sensitivity.
The Researchers then fabricated this is into more of a commercially focused product, in the form a paper, or in-situ grown, Zn-MOF film. This was to act as a humidity sensor, but as a device. The Zn-MOF device was applied to environments with less than 1% relative humidity (%RH) to detect traces of water less than 0.05% v/v in various solvents and thermally-measurable environments.
This sensor builds on current technology, but eliminates a lot of the issues with regards to multi-molecular absorption. It has been tested for many applications, including the detection of humidity in gases, traces of water in organic solvents, thermal imaging and thermometers. With its enhanced properties, a fabricated prototype device and many potential areas for application, it is a sensor that should make its way into the commercial realm in the near future.
“Ultrafast water sensing and thermal imaging by a metal-organic framework with switchable luminescence”- Chen L., et al, Nature Communications, 2017, DOI: 10.1038/ncomms15985