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A New Generation of Chemical Sensors

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A New Generation of Chemical Sensors A New Generation of Chemical Sensors Dr Simon Humphrey Sam Dunning A NEW GENERATION OF CHEMICAL SENSORS Dr Simon Humphrey and Sam Dunning at the University of Texas at Austin have created a new lanthanide-based chemical sensor that can identify trace levels of water in many different solvents, and can even distinguish between normal water and ‘heavy water’. The team’s new material could potentially be applied to medical imaging and for cleaning up chemical spills. Lanthanides – the Ideal Due to their light-emitting – or This means that lanthanide-based Photoemitters ‘photoluminescent’ – properties, chemical sensors can be tuned to detect molecules containing lanthanide a specific impurity. From your schooldays, you may (Ln) ions (charged atoms) have been remember using universal indicator or increasingly explored as chemical Exciting Lanthanides litmus paper to measure pH. While sensors. The term ‘lanthanide’ refers these simple tests are well known and to the group of elements with atomic For an element to exhibit widely available, more sophisticated numbers 57 to 71, from lanthanum (La) photoluminescence, it must first chemical sensors are needed to to lutetium (Lu). When grouped together become ‘excited’ by absorbing light help tackle issues such as cleaning with the chemically similar elements energy. However, lanthanide ions are up chemical spills, remediating yttrium and scandium, you may have difficult to excite by directly absorbing old industrial sites, and detecting heard them described as ‘rare earth light. In a molecule, excitation of radioactive contamination in water elements’, and they have applications lanthanide ions is commonly achieved supplies. Although many techniques are in electronics, high-strength magnets by surrounding them with light- available for these applications, very and catalysis. absorbing organic molecules known as few allow for rapid on-site detection, ‘chromophores’. A chromophore is part with samples often needing to be sent After absorbing light, lanthanide ions of a molecule that readily absorbs light, away to a lab for analysis. emit visible and infrared light at specific being present in dyes and in chlorophyll, frequencies. This means that a solid- giving leaves their striking green colour. Now, Dr Simon Humphrey, Sam state sensor (for example, a dipstick) Dunning and their colleagues at the could be easily read either using the Light energy absorbed by the University of Texas at Austin have naked eye or a UV-reader to detect chromophore is transferred to the developed a new class of lanthanide- chemical impurities in a sample. Each lanthanide ions in the molecule, based materials, which could form type of lanthanide ion (for example which then re-emit light with high the basis of a new generation of rapid europium or terbium) emits a very efficiency. This is quantified by the chemical sensors that could be used for distinct frequency of light that is photoluminescence ‘quantum yield’, these applications. characteristic of that particular element. which describes the efficiency with WWW.SCIENTIA.GLOBAL which absorbed light is re-emitted from Maximising the efficiency of light These solvents had previously been a material – so a high quantum yield is emission is crucial in creating a incompatible with conventional desirable in making chemical sensors. chemical sensor that is portable and chemical methods of detecting low practical for use in the field, without concentrations of water. So how does Metal-Organic Frameworks needing large laboratory equipment their detector work? off-site. The wider body of research Dr Simon Humphrey, Sam Dunning into MOF chemistry has established a When PCM-22 is placed in a solvent, and their colleagues at the University of number of design principles that solvent molecules (such as water) Texas at Austin focused on developing a enable researchers to have control over move through its pores and bind to it. stable and tuneable material that would the distance between the lanthanide Then, when UV light is shone on PCM- exploit the light-emitting properties ions in the solid material. This 22, its chromophores absorb energy, of lanthanides for creating chemical ‘tuneability’ allows the luminescence which is transferred to the lanthanide sensors. While previous research in the properties of the MOF to be optimised ions and the material emits specific Humphrey lab has investigated the for sensing applications. colours of visible light. The exact potential of metal-organic frameworks frequency of emitted light depends on (MOFs) for storing gas and catalysing PCM-22 – a New Chemical Sensor? the type of solvent molecules that are reactions, the team recently developed interacting with PCM-22, as its chemical a new MOF with potential uses in While lanthanide-based MOFs have environment affects its excitation and medical imaging, chemical clean-up been reported as good chemical photoemission energy. In other words, and detecting chemicals produced due sensors for pH, explosives and the vibrations of the solvent molecules to radiation exposure, and published temperature, many of these sensors disturb the lanthanide emissions in a their work in the journal Chem (Chem are incompatible with a broad range of unique way, depending on the solvent- 2017, DOI: j.chempr.2017.02.010). solvents, or are only stable over a small lanthanide combination. Composed of a network of metal ions pH range. In their recent Chem paper, with co-ordinated organic ligands, Dunning and Dr Humphrey report the Technology that employs visible light is MOFs are 3-dimensional porous synthesis of an MOF material called advantageous as it can be read by the materials made up of organic PCM-22, which is made up of lanthanide human eye, or using a standard digital molecules and metal ions (such as ions and a chromophore. camera (even on a mobile phone) in lanthanide ions). MOFs containing conjunction with appropriate software. lanthanide ions benefit from the high The team discovered that PCM-22, The unique signatures of colour and quantum yields of lanthanides to which contains controlled amounts brightness emitted by the team’s new produce a material that can give a large of lanthanide ions, is able to detect material can thus be used to identify photoluminescence response from a trace levels of water in a variety of and quantify many different chemicals. small amount of sample. solvents including ethanol and acetone. Once the material has been calibrated against known samples, a catalogue of WWW.SCIENTIA.GLOBAL fingerprints for different solvents can be created, and easy-to- However, because the frequency of light emitted by PCM-22 is use ‘dipstick-style’ sensors could be made. dependent on the nature of the interacting solvent molecules, and since the vibrations of H2O and D2O molecules are different Most significantly, not only can PCM-22 detect low (due to their different masses), PCM-22 can distinguish between concentrations of normal water (H2O) in a range of different these two types of water. solvents, but it can also distinguish between normal water and so-called ‘heavy water’ (D2O), at levels as low as 10 parts per ‘We make the material with europium and terbium ions,’ says million. Dunning. ‘The europium emission is more readily quenched or “turned off” in the presence of 2H O compared to terbium. Since Heavy Water the vibrations in D2O are different due to its mass, it isn’t as good at turning off the europium emission, which gives us the Heavy water, or deuterium oxide (D2O), is so-called because of yellow emission colour in the presence of D2O, versus just green its composition. A typical hydrogen atom (H) has one proton for H2O.’ in its atomic nucleus with one electron orbiting around it. A deuterium atom (D) is very similar, containing one proton Dr Humphrey and Dunning’s new class of material could with one electron, but it also has one neutron in its nucleus. change the way authorities respond to chemical spills or leaks Because protons and neutrons are almost of the same mass of radioactive waste. In addition, PCM-22 has the potential (electrons have virtually no mass), deuterium can be thought to streamline some of the processes involved in medical and of as a ‘heavy’ hydrogen atom, being roughly twice the mass research imaging that rely on nuclear magnetic resonance of hydrogen, but with the same chemical properties due to its (NMR) technology. NMR machines require heavy water to single proton and single electron. function and to work properly, and this heavy water needs to be very pure. However, heavy water can easily be contaminated D2O is simply a molecule of water (H2O), which contains with normal water from moisture in the air. deuterium atoms in the place of hydrogen atoms. Once mixed together, heavy water is almost indistinguishable from normal ‘When you buy heavy water from a manufacturer it starts out water, because of their close chemical and physical properties. ultrapure,’ says Dr Humphrey. ‘But as soon as you unscrew the bottle, hydrogen atoms from the air start swapping with These properties have been used by scientists to our deuterium atoms. A week later, all of the Hs have become advantage. For example, deuterium-containing solvents are scrambled with the Ds and it effectively ruins the heavy water. used in nuclear magnetic resonance spectroscopy (NMR), It’s an exchange that you can’t stop.’ which works in a similar way to hospital MRI scanners. But the similarities can also pose problems: when water comes into Dipsticks coated with PCM-22 could provide a cheaper and far contact with sources of radiation, such as radioactive uranium, quicker way of assessing the purity of heavy water, with the neutrons can be incorporated into H2O water molecules, material being sensitive enough to detect H2O concentrations creating D2O. Because of this, the presence of D2O can be a sign as low 10 parts per million in a D2O solution.
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