Swiss
Nuclear magnetic resonance has attracted the attention of researchers with a vast array of interests for decades. Anna Demming speaks to Professor Jean-Philippe Ansermet about new demonstrations of how to maximise the potential of the technique, as well as its impact on catalysis, electrochemistry and spintronics
Nuclear magnetic resonance: The signal for a breakthrough
“Imagine I told you that I have a method to observe atoms without disturbing them too much,” says Professor Jean-Philippe Ansermet. “You would understand right away that it is a very powerful tool.” As he explains,
nuclear magnetic resonance
(NMR) is that tool. However, attempts to apply the probe in areas such as catalysis of electrodes and membrane protein analysis have stumbled on difficulties in obtaining sufficient signal strength from samples so much smaller than the usual quantities. “Typically, one looks at more than one millionth of a mole, which is actually a large amount; a 100 trillion atoms, roughly.” Professor Ansermet is a full professor in
the institute of condensed matter physics at Ecole Polytechnique Fédérale de Lausanne. Since the beginning of his research career, he has been fascinated by the possibility of probing molecules at surfaces, typically containing around a hundred times fewer nuclei than a conventional NMR sample. As a result, obtaining an adequate signal strength is far from trivial. Nuclear magnetic resonance measures the
behaviour of the nuclei in atoms and molecules
in a magnetic field. The
measurements provide information about the
electronic environment around the
nuclei and the molecular structure. “It’s a bit like a radar,” explains Professor Ansermet, quoting an older colleague, “You send an electromagnetic signal to your sample and you listen to its response.”
The NMR legacy As early as 1953, just over 10 years after NMR was detected for the first time, Albert W. Overhauser suggested a possible means of enhancing NMR signals. “His idea was, as far as I can tell, the oldest preliminary consideration to what would one day become spintronics,” explains Professor Ansermet. Spintronics refers to a type of spin electronics for which Albert Fert and Peter Grünberg won the 2007 Nobel prize. Professor Ansermet
60 describes the experiment suggested by
Overhauser, a theorist, “The model looks like something Albert Fert would write, exploring the consequences of forcing electron spins out of thermal equilibrium.” The method enhances the signal strength
probed by NMR by exploiting the magnetic moments of electrons, which are 600 times greater than those of the nuclei of hydrogen, for example. The signal
in NMR, which
probes the response of nuclei, is proportional to the magnetization and the magnetic field strength. By transferring the polarization from the electrons to the nuclei it should be possible to improve the signal. The approach is referred to as dynamic nuclear polarization (DNP), and as Professor Ansermet goes on to explain, the history of
Ansermet’s imagination. If the strength of NMR signals from these systems were strong enough, they could shed light on what happens to the molecular structure of a reacting molecule when it sits on a metal surface. “This seemed fascinating to me – a huge challenge, but fascinating.” As it happened, by the 1980s, the idea to
enhance NMR signals by transferring polarisation from the electrons to the nuclei gave way to advances in the technology of the equipment used. “People were gaining signal by increasing the field,” says Professor Ansermet. “They went up and up in field and they let go of the idea of transferring the polarization from the electrons to the nuclei.” It was around 1997 when a professor of electrochemistry paid Professor Ansermet a
this technique is full of surprises. Professor Slichter – one of the founders of magnetic resonance
and Professor Ansermet’s
supervisor during his PhD – demonstrated that Overhauser was right: DNP does work! “I believed there were two famous [magnetic resonance] books,” he recalls from his student years. “One was Abragam’s book and the other Slichter’s book, so to join his group was really exciting.” From the beginning, the idea of using NMR
to probe molecules on surfaces, particularly at the surfaces of a catalyst, caught Professor
visit, saying that he was trying to apply the techniques he had read in a thesis to electrochemistry. The professor was Professor Wieckowski, and although he had not yet realised it, the thesis was Professor Ansermet’s own. Application of NMR in electrochemistry proved hugely difficult as either
the signals were too weak or the
surface areas were huge. “You know if you have to do electrochemistry with one to ten square metres of electrode it’s a problem, a big problem,” explains Professor Ansermet. But with DNP the signals could be enhanced.
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