11 August, 2017
Look around you. If you can see it with your naked eye, it is part of the macroscopic world. This world includes everything from the food you eat and the car you drive, to the furniture you sit on or the building you are in. Humans have long been masters of manipulating materials at this scale, and that’s partly because the laws of physics behave themselves at this dimension—you can trust that a metal will behave identically whether you use it to make a nail, or to hold up a skyscraper.
But that’s not the case when things get very small. As first discovered in the 1980s by researchers working with materials like silicon and carbon, if a material gets small enough, its properties can change dramatically. This discovery kick-started the nanotechnology revolution, which allowed us to design new materials with seemingly ‘impossible’ properties. That, in turn, has given us a dizzying array of devices and products that we rely on, including efficient medicines and longer-lasting batteries.
But even now, 30 years later, nanotechnology continues to throw up surprises. Recent discoveries have proved that when atoms are arranged into precise groups called clusters, the addition or removal of a single atom can entirely change the cluster’s behaviour. For example, a cluster of seven atoms of gold is magnetic, while a cluster of six is not.
This ability to tune a material’s properties, atom-by-atom, has captured the imagination of some of the MacDiarmid Institute’s researchers, including Director and Principal Investigator Professor Thomas Nann. Working with colleagues at Victoria University of Wellington, Professor Nann has been using some of these tiny clusters as catalysts for renewable energy, and as markers for disease detection. “Because their behaviour is directly linked to their atomically-precise structure, clusters offer huge potential for producing materials with unique, desirable properties,” said Professor Nann. “And we’ve only just begun to scratch the surface.”
Professor Nann and his team rely on biological matrices—DNA or peptides——to stabilise these metal clusters, and this approach has proved to be the key to their early successes. But it comes with a surprising technical challenge. Their clusters are so small that they are impossible to see with current analytic techniques. “We can gather size and some chemical information from mass spectroscopy,” explained Professor Nann, “but analytics-wise, that’s where it stops. A large proportion of our work is dedicated to understanding exactly what we’ve got!”
While these clusters are difficult to see, their properties are easy to test. And for that, cross-discipline collaboration is a must. Alongside his colleague Dr Renee Goreham, Professor Nann is collaborating closely with Dr Darren Day from the School of Biological Sciences at Victoria University of Wellington to investigate how clusters could be used in biomedical imaging.
Many medical procedures such as MRI rely on a chemical contrast agent. The problem is that most of these traditional contrast agents are very toxic. But clusters are not; clusters rely on a scaffold of biological molecules, and are already therefore partially biocompatible and non-toxic. They can also be made soluble in water.
And perhaps most importantly of all, they can be designed to have several specific properties at once, potentially making them useful as a multi-modal contrast agent, or as a biosensor.
“Our research in this area is at the earliest of stages,” said Professor Nann, “but we’ve already had some exciting results, and we’re looking forward to taking them further!”
Where Professor Nann and his team use biological molecules to stabilise these clusters, Associate Investigator Dr Vladimir Golovko from the University of Canterbury is using a chemical approach. His research focuses on the controlled synthesis of metal clusters for biotesting, sensing and catalysis.
According to Dr Vladimir Golovko, “Clusters offer a beautiful playground for chemistry. Playing with single atoms gives us the opportunity to design novel materials with precisely-defined properties.”
Catalysts play a vital role in the modern world. Take the catalytic converter, found in the exhaust pipe of every modern car, as an example. As exhaust gas flows through it, particles of platinum act as a catalyst to convert highly-toxic pollutants into less-damaging ones. It’s a system that’s been ubiquitous in the automotive industry since the 1970s, but the catalyst itself is not well understood. It works, but it’s not all that efficient, and it’s extremely expensive. Clusters could offer a way to vastly reduce the quantity of precious metal required to catalyse a reaction—moving to just a few atoms instead of thousands would reduce the cost, and might also lead to superior performance. “There is fantastic synergy between our work, and that of Principal Investigators Dr Jonathan Halpert at Victoria University of Wellington, and Associate Professor Nicola Gaston from the University of Auckland,” said Dr Golovko “This feedback loop of prediction, synthesis and testing has gotten us to a stage where our results, especially on gold catalysis, look extremely promising.”
Few areas of materials chemistry blend theory and experiment more productively than clusters. And when it comes to understanding why clusters behave as they do, MacDiarmid Deputy Director, Associate Professor Nicola Gaston, is leading the way. She is interested in understanding the relationship between size and the development of particular materials properties in clusters.
“Historically, in materials science, experiments have come first,” said Associate Professor Gaston. “But that’s no longer the case, especially for clusters. Our ultimate goal is to develop an overarching theoretical framework for the targeted design of these building blocks. The challenge with developing such a theory is related to their size. Clusters may be very small when compared to macroscopic materials, but they are very large compared to a single atom. Most existing computational techniques aren’t designed to bridge this gap, which has led Associate Professor Gaston and her team to investigate a ‘hybrid methodology’, which will allow them to achieve accurate results but for a wide range of cluster size and species.
The MacDiarmid Institute umbrella connects these researchers from different disciplines and gives them the opportunity to work together. “The nature of the research means that we’re collaborating with other MacDiarmid people looking at the same question, but from a different angle,” explained Associate Professor Gaston. “Being part of the MacDiarmid team gives us the opportunity to meet up and find out what others in New Zealand are working on in regard to clusters. And working at this intersection, between physics and chemistry, is incredibly exciting.”
Working on a problem from different angles, across disciplines, under the MacDiarmid umbrella.Associate Professor Nicola Gaston