Thinking Outside the Box
Paul Kruger has bucked the trend, and gone the other way across the Tasman. “I’m originally from Melbourne,” he says. “But it’s not like I came from Australia directly to here, I came via Ireland and collected a family along the way.” He moved to Christchurch with his wife and two young children about five years ago. Despite the earthquakes, he’s still there. “We love this place, that’s the crazy thing. I live by the beach. I couldn’t live by the beach in Melbourne!” An inorganic chemist by training, Paul makes molecules that perform a function. In particular, he’s trying to make infinite polymers, made from metal ions and organic ligands that have a 3D topology. “A lot of the language involved with that is the language of architecture, so you use building blocks, frameworks, struts, and nodes,” he says. The frameworks he is building are used to define void space, sort of like 3D molecular sieves. According to Kruger, it is the porosity within these materials that makes them interesting, because they can trap other species, such as carbon dioxide, by taking it out of the atmosphere, or they can store gases such as hydrogen for fuel cells. “The beauty of these things is that because you can design them and you can, therefore, modify them,” he says. While the organic framework acts like a host to take on smaller molecules, the metal ions also have a very important role to play. “They can be magnetic, they can interact with light, they can also donate or receive electrons, so they can be redox active,” says Kruger. “And then those properties can change when the smaller guest molecule goes into the framework.” Then, if the properties change, the framework can act as a sensor. This area of research is known as metal organic frameworks or porous coordination polymers, but Kruger is also working on another type of sensor based on the spin state of complexes. Atoms typically have a closed shell of electrons in molecules, but metal ions often have an open shell with unpaired electrons. “This gives rise to their magnetic behaviour, what determines their colours, and their reactivity. That’s what makes metals interesting,” says Kruger. With some external perturbation, such as changing the temperature, the pressure or the light irradiation, you can invoke a spin state change within materials like iron in its +2 oxidation state. “As the spin state changes, the colours change. So, if the external stimulus was another molecule, you’ve essentially got a sensor. If the external stimulus was light, then potentially you have a light activated switch. And because you can either have spin on or spin off you can have some memory type function, and use it for data storage or display.” Kruger is making materials which exploit the spin state of metals, but instead of using just one metal ion, he’s using two, or four, or more, to increase the number of switching points. “Then we’re interested to see how they interact with each other – how the switching in one of the metal sensors influences each neighbour,” he says. Haemoglobin is an example of one of these molecular machines in nature. “The metal centres within the protein knows what the others are doing, if a molecule can know something!” says Kruger. “We’re trying to design systems where the metal ions know what their nearest neighbours are doing, and then potentially, they can cooperate with each other.” Like the porous frameworks, Kruger is trying to build capsules or container molecules which change properties due to a spin state change of metals when they capture other molecules. “So the binding event happens and we can pick that up through a noticeable change in one of the properties like colour, magnetism, or size,” says Kruger. Similar molecules could be used as “cages” or “boxes” to lock molecules inside. With changes in temperature, light or pressure, the spin state of the metal ions change, and a guest molecule could be trapped or released from a molecular “cage”. “Let’s says at 10°C the molecule is trapped within that cage, we can heat that solution and then at a differing temperature, say 50°C, the molecule obtains its open state and the cage releases its guest. That’s pretty cool,” he says. Kruger’s third area of research is looking into anion binding and sensing. Like litmus paper, the molecules he’s created give a perceptible display visible to the naked eye. “We’ve designed molecules that are capable of sensing anions in aqueous solution. If a phosphate ion comes along and binds to a receptor, then the molecule can change from being a yellow colour to a purple colour and you would see that in the solution,” says Kruger. “If you’re out looking at the waterways of Canterbury, you don’t need a spectrometer or fancy piece of equipment to see that. This is immediate, on site monitoring.” Kruger and his team have developed the chemistry, but now need to apply that knowledge to create an application. For example, Kruger has the molecules, but immobilising them so they can be used in dipstick technology still needs to be developed. “The beauty of joining the MacDiarmid Institute is this collaborative approach,” says Kruger. He’s finding that MacDiarmid Institute researchers in Mechanical Engineering at Canterbury University are saying they could immobilise these molecules on their fibres, and similarly people within the School of Biology think they can immobilise them on their fibril proteins. Kruger is finding that the MacDiarmid Institute is opening up the potential for additional collaboration and the use of other people’s expertise. “It’s a critical mass in knowledge and then, obviously, there’s the access to state of the art instrumentation across the whole country, so we don’t have to have it at Canterbury, we can go to Wellington if we need to use some fancy spectrometer. Which is fantastic,” says Kruger. If there’s one thing that Kruger has noticed since moving to this country, it’s that New Zealand is a long way from everywhere. “We don’t have that passing foot traffic of experts coming through your city every week,” he says. “Being in the MacDiarmid Institute enables that in the New Zealand context, which is pretty bloody good.” That foot traffic is not just New Zealand focused, says Kruger, every second year the AMN conference has phenomenal pulling power, attracting leading international scientists. “They come and talk to us about their science, and we talk to them about our science, which is fantastic. It counts for the fact that we don’t have that foot traffic coming through. Build it and they’ll come, and that’s what the MacDiarmid Institute did.” Not only has the MacDiarmid Institute allowed Kruger to support a student, it allows his team to get exposure to people within New Zealand, and to international leaders as well. “To get my guys involved with that is obviously going to be of benefit to their education, their training, their learning,” says Kruger. Kruger also believes the MacDiarmid Institute is increasing the visibility of New Zealand science in the world context. It’s an efficient use of the money that New Zealand has. Kruger is the first to admit that he’s “not making any widgets, and selling them”. While that may happen in the future, he says it’s dangerous to focus on applications alone. “That’s like trying to pick winners and often you can’t pick the winner. In fact, in some cases you don’t even know where the race is!” That’s why he likes the MacDiarmid Institute, where you can think outside the box. “You can present your work at a gathering of MacDiarmid PIs, AIs, and students, and they could ask you a question that you have never even thought of,” he says. “You often need to have that left field idea. You don’t know where the applications might be and that’s true of many of the major discoveries in science.”
Paul Kruger Professor of Chemistry at the University of Canterbury.
- Has a BSc(Hons) and PhD Monash University (Melbourne) .
- Spent two years as a Postdoctoral Fellow at the Queen’s University of Belfast (Northern Ireland) investigating metallo-macrocyclic complexes.
- Lectured at the University of Dublin, Trinity College (Ireland)
- Moved to New Zealand in 2008. X Research background: The synthesis of multi- nuclear metal complexes in the quest to develop species of bio-mimetic relevance as novel magnetic materials.
- Current research interests: Aspects of Supramolecular Chemistry with the following research themes:– Spin-switching materials – Anion binding and sensing – Metal-organic framework