“Lighting Up” NMR and MRI in Colloidal and Interfacial Systems

Abstract

By means of optical pumping with laser light, the nuclear spin polarization of gaseous xenon can be enhanced by many orders of magnitude. The enhanced polarization has allowed an extension of the pioneering experiments of Fraissard and coworkers to novel applications of NMR and MRI in chemistry, materials science and biomedicine. Examples are presented of developments and applications of laser-polarized xenon NMR and MRI on distance scales from nanometers to meters. The size of the xenon atom is similar to that of small organic molecules, such as methane, yet the nuclear magnetic resonance (NMR) signal from xenon proves a more sensitive probe for the local environment. Laser-polarized xenon NMR has been used, in collaboration with Sozzani and coworkers, to investigate the interactions present in an effectively one- dimensional gas phase inside nanochannels. Small changes in channel size and/or structure lead to very different modes of diffusion. Optically pumped Xe NMR can distinguish between these different diffusion modes out to unparalleled time scales (several tens of seconds). These studies are particularly useful for gaining a fundamental understanding of the laws that govern heterogenous mass transport such as gas transport into porous catalysts or molecular sieves, or liquid transport through pore-forming transmembrane proteins in biological systems. The understanding of mass transport inside microporous materials is crucial for many industrial and commercial processes. Recent experiments will also be described in which xenon has been used to investigate the cavities of biological nanosystems and in which polarization has been transferred to molecules on surfaces and in solution. As an example, in collaboration with Wemmer and coworkers, xenon has been used as a molecular probe to investigate the hydrophobic surfaces and interiors of macrocyclic molecules and proteins; recent results show evidence for binding of xenon to the outside of a protein, a proposed cause of the anesthetic mechanism of xenon. Indeed, localized injection of polarized xenon solutions into human blood has provided observations of the real-time process of xenon penetrating red blood cells. The injection technique also makes it possible to provide enhanced magnetic resonance images of localized areas in living organisms. Furthermore, the use of laser-polarized xenon also opens an exciting new frontier in the possibility of “functionalized xenon” as a biosensor of analytes and metabolites in chemistry, materials science and biomedicine. The novel biosensor offers advantages of multiplexing capabilities and the possibility of detection in-vivo.