Locking Up Xenon's Potential

Noble but Interesting

Xenon seems like such an innocuous and noble gas. Rarely reactive. Has none of neon’s obnoxious glow it, producing a blue hue under discharge conditions. But xenon has a few tricks up its sleeve. It has eight stable isotopes and 40 unstable isotopes, and its isotope ratios can reveal much about the history of the solar system. The isotope xenon-135 is also a strong neutron absorber. Dimeric xenon atoms lase very well and the gas in the aforementioned discharge state does have applications in arc lamps. In a less energetic state it also makes a useful anesthetic.

However, xenon’s future may be even brighter. Its noble status means it is biologically inert and for the ¹²⁹Xe isotope, there is a nuclear spin of 1/2, which makes it a useful and inert probe for sensitive environments, such as the interior of the human lung. By giving it a boost through the spin exchange optical pumping (SEOP) hyperpolarization method, a xenon-filled lung can be imaged using nuclear magnetic resonance (NMR), or rather its clinical cousin magnetic resonance imaging (MRI). It has similarly been used to image microfluidic flow, to probe the structure within liquid crystals and polymers and to assess the size of the tiny cavities within porous media. The linear response of its chemical shift to temperature means it can act as an NMR thermometer and a probe for pH, too. All that said, xenon’s full potential is yet to be realized.

Trapping Xenon in Self-Assembled Cages

Juho Roukala, Jianfeng Zhu, Perttu Lantto and Ville-Veikko Telkki of the NMR Research Group, Centre for Molecular Materials, University of Oulu, Finland, together with Chandan Giri and Kari Rissanen of the Nanoscience Center, University of Jyväskylä, Finland, have created a supramolecular capsule within which they can trap a xenon atom in aqueous solution. The capsule, a self-assembled Fe₄L₆ cage, has a binding constant for the noble gas of 16 per molar (16 M^–1) and moves the chemical shift way past that for free xenon in water, which bodes well for NMR applications such as improved imaging, molecular temperature probes and pH sensors. Moreover, the same cage molecules might be used to extract xenon from gaseous mixtures because of their affinity for the gas, or with some chemical fine tuning could be used to extract other noble gases.

The team initially built on their metal ion-assisted subcomponent self-assembly of tetrahedral cages in aqueous media developed jointly by Jonathan Nitschke, University of Cambridge, UK, and Kari Rissanen, which they explain opens up a relatively facile route to molecular tetrahedral complexes based on iron(II). They demonstrated that the host-guest chemistry of these cages allows the materials to act as containers for white phosphorus. They then expanded the work to cobalt(II) and nickel(II). However, writing in the Journal of the American Chemical Society, the team explains how they selected very small subcomponents and applied the same methodology to construct the smallest possible tetrahedral M₄L₆ cage. With Fe(II) as the metal component, they have the perfect host for xenon atoms.

“The results demonstrate the potential of metallosupramolecular cages in various applications, after optimizing essential properties such as cage dimensions, binding constant, and exchange rate, e.g., by changing the metals or ligands,” the team concludes.

“The importance of this work is based on the proof of principle of forming a supramolecular cage for xenon in situ through a self-assembling process,” explains Leif Schröder, Leibniz-Institut für Molekulare Pharmakologie (FMP), Berlin, Germany. He adds that this novel approach offers several unprecedented opportunities for the emerging field of xenon NMR applications. Schröder points out that various previous hosts required elaborate syntheses and had limited solubility in aqueous environments. “This approach, however, shows excellent water solubility and comes with options to further optimize the cavity properties,” he told ChemViews Magazine. “Future tuning of the binding constant and exchange rates provide the option to achieve an optimized cage for xenon with superior detection properties, in particular for MRI applications. Xenon once more reveals its exciting chemical properties despite being an inert gas.”