Professor Simon Humphrey and Sam Dunning speak to SciTech Europa about the development of a new Ln-MOF-based sensor which has numerous potential applications in the area of environmental sensing.
A new material that has potential applications in environmental sensing, in a range of cheap, fast and portable new sensors for a wide variety of chemicals, has recently been developed by chemists at The University of Texas at Austin, USA.
Professor Simon Humphrey has been working on this material, known as PCM-22, at the university, exploring how it may help to reduce the costs associated with cleaning-up accidental chemical spills, remediating old industrial sites, detecting radioactive contamination in drinking water, and operating medical and research imaging devices.
SciTech Europa spoke to Humphrey and one of his PhD students, Sam Dunning (who has also been closely involved in this research) about this exciting new development and how they hope to continue to exploit its potential in the area of environmental sensing.
MOFs for environmental sensing
Humphrey explained that his research group has been interested in making porous metal-organic framework (MOF) materials since 2009. Within this, the team has been specifically exploring porous phosphine-based frameworks as well as the reproducible preparation of noble metal nanoparticles for applications in heterogeneous catalysis.
It was from this research that PCM-22 emerged. This material a metal-organic framework material comprising triphenylphosphine and Ln3+ ions (Ln = Pr–Yb), and it exhibits solid-state luminescence at room temperature. Describing the material in the journal Chem last year, Humphrey also explained that ‘mixed-metal versions of PCM-22 that contain controlled amounts of Eu3+, Gd3+, and Tb3+ function as highly sensitive, broad-scope solid-state sensors that can rapidly identify unknown solvents by providing a unique ‘eight-factor’ fingerprint.
‘The sensors allow for immediate solvent identification via colour changes that are obvious to the naked eye and also permit quantitative chemical analysis by uncomplicated spectrophotometry. These same materials achieve quantitative detection of H2O in D2O from 10 to 120,000 ppm. The detection of trace H2O is also demonstrated in a range of common solvents, including those incompatible with conventional laboratory titration methods.’
Lanthanides are any of the series of elements with increasing atomic numbers that begins with lanthanum or cerium and ends with lutetium and Dunning, who relocated from the UK to work with Humphrey on the material, has provided valuable input to the research thus far. He identified that it is possible to take PCM-22 and make the same structure with essentially any available lanthanide or mixtures of lanthanide, due to the fact that lanthanides are luminescent.
Humphrey said: “That is quite unusual for this kind of chemistry. He also found out that you can make PCM 22 using mixtures of different lanthanides, and it was this that resulted the sensor, as each lanthanide has a different colour emission. If the lanthanides are mixed into the same material, then this offers a broad scope of emission properties.”
Dunning added: “Indeed, the inclusion of a different lanthanide allows us to tune the emission colour, and from that we can optimise it for different sensing applications.”
The sensor material, PCM-22, that Humphrey and Dunning have developed in their research, can be deposited onto substrates that are cheap and easy to use for environmental sensing. Humphrey explained: “They can be deposited on paper, for instance, and so we envisage applications in areas such as dipsticks used to assess the purity of heavy water, for instance. Additionally, we hope to also be able to translate the sensor into the field for remote sensing applications such as hand-held UV detectors, although the challenges in achieving this relate more to engineering than chemistry.”
Dunning explained that europium and terbium ions are used to create PCM-22. “Europium is red and terbium is green. And we ratio their relative emissions which allows us to measure the emission’s colour. A non-emissive spacer can then increase baseline emissions which we can factor in so as to form the relative luminescence of the samples,” he told SciTech Europa.
The ‘eight-factor’ fingerprint used to rapidly identify unknown solvents described by Humphrey in the Chemie paper is unique to this sensor. Humphrey explained: “The term ‘eight factor’ comes from the fact four different combinations of lanthanides that are used in the sensor all of which have their own relative intensity. That is, there will be four different colours and four different intensities, meaning that there are eight different factors of information which allows for any different solvent to be fingerprinted, as the eight different factors are essentially unique to any given solvent.”
Applications: environmental sensing and beyond
The main application areas that Humphrey and Dunning envisage for their PCM-22-based sensor are in the environmental sector. In the USA, the Environmental Protection Agency (EPA) has established acceptable levels of certain contaminants in general waste, and environmental sensing to detect whether these chemicals are above or below the threshold determines the type of clean-up operation that will be required.
Humphrey told SciTech Europa: “If, for example, there is a spillage on a motorway, then the first thing to do is detect what the spillage actually is and what the levels of the certain components in the spill are. The clean-up treatment is then handled accordingly.
“As such, there is a clear need for sensing capabilities which can do that kind of analysis immediately, rather than actually having to swab a sample and send it to a lab and wait for the data to come back, because that slows down the entire clean-up process.”
Humphrey thus began discussions with potential industry partners who are active in environmental remediation some time ago, while the sensor was still being developed.
As previously mentioned, medical and research imaging devices may also benefit, and the sensor may also find applications in other fields. Humphrey said: “There is the potential to have an impact in many different sensing applications, from simple things like pH or nitrate levels in drinking water – where there are already established sensing techniques – to the detection of certain anolytes for which there no remote sensing applications are yet available.”
According to the professor, the best example of this is deuterium. PCM-22 can distinguish between two types of water —the ordinary water (H2O) that we experience in everyday life and so-called heavy water (D2O), which is used in the operation of medical and research imaging. With D2O, hydrogen atoms are replaced by deuterium atoms, but the two types of water are notoriously hard to tell apart because they look and, in most cases, behave the same chemically. It normally requires a costly test with a sophisticated piece of laboratory equipment called a laser spectrometer to tell the two apart. “Deuterium is thus an anolyte of interest because there is no field test available for this at the moment,” Humphrey added. ““The PCM-22 sensor itself is a water stable material, which means that it can detect water in a range of other organic anolytes, and those kind of solvent mixtures are often problematic for target-specific sensors.”
The US military has an interest in sensing for deuterium and tritium in ground water because their presence can be an indicator of sub-surface nuclear detonations or nuclear weapons testing activity. Because PCM-22 makes distinguishing between the two types of water simpler, it could become much easier for government agencies to detect the presence of radioactive contamination in drinking water or other bodies of water such as lakes and rivers. When ordinary water interacts with radioactive material, such as uranium, some of it is converted to heavy water, so elevated levels of heavy water give an early warning of contamination with radioactive material. As such, there is potential for Humphrey and Dunning to apply for funding via the specific calls which are emerging from the USA government in such areas.
“However,” Humphrey explained, “calls such as these are typically based on need and are therefore quite specific. On the other hand, there are much more general sensing calls coming out of the National Science Foundation and others, but they are so broad – encompassing everything from sensing falls in elderly care homes to organic sensors and all the way to microelectronics. As such, it can be very difficult to have a successful proposal.”
A further application area that Humphrey and Dunning are set to explore fluoride sensing. Humphrey explained: “When PCM 22 is optimised in certain ways based on certain combinations of the lanthanide components, it has a very strong response to fluoride on the ppm level, especially when they are dissolved in aqueous media. The ramifications of that are for fluoride monitoring in drinking water directly, while the more indirect but interesting potential application could be in the sensing of chemical warfare agents that generate fluoride as a by-product.”
Indeed, chemical weapons such as sarin and the Novichok nerve agent recently used in the UK to poison Sergei Skripal, a former Russian military officer and double agent for the UK’s intelligence services, and his daughter Yulia Skripal, liberate fluoride when they break down. Humphrey said: “Because our sensor has a high response to fluoride, it is thus potentially possible to find applications in this area. We have thus conducted the proof of principle testing in this area, but have not yet taken that to the level of application.”
Humphrey and Dunning have also discovered that as they synthesise the sensor they are able to further chemically modify it in the solid state, further enhancing its sensing capabilities, which offers more exciting possibilities in the future.
As a lanthanide-based porous metal-organic framework (Ln-MOF), the luminescence properties of the lanthanide in PCM-22 can be inherently exploited as a direct reporter of the local chemical environment. It is this which Humphrey and Dunning will continue to work on moving forwards.
Once Dunning has completed some post-doctoral research at the university, he will then be one of the first people to work in the start-up company that he is establishing with Humphrey as they work to commercialise their research.
This is already receiving seed funding from the University of Texas at Austin, and is working to attract investment from the Central Texas Angel Network, which works well for the researchers as these latter investors are less concerned than other types of financiers with the immediate licensing of products.