NMR Detection Using Laser-Polarized Xenon as a DipolarSensor Page: 2 of 30

                                
typically require it to be either bound to, adsorbed [2,3], or in sufficient prox-
imity [4,5] to the analyte. Alternatively, supramolecular cages can be synthe-
sized with a large affinity to bind Xe, where the specificity for a certain target
comes from a ligand that is connected to the cage through a linker [6]. Most
of these experiments capitalize on changes of the xenon electron cloud that
are reflected in changes in xenon chemical shift and/or relaxation behavior.
More specific information can be obtained when xenon is involved in the re-
laxation of the analyte to be probed [7]. The SPINOE technique yields direct
quantitative measures about NMR active nuclei in the (temporary) molecular
environment of Xe. However, all the aforementioned applications require ei-
ther solubility of xenon in the analyte [7] (or the analyte in xenon [8]), affinity
of xenon to binding sites [6,9-11], or direct physical interaction [4,12].
Another recent application of hyperpolarized xenon gas is its use as an infor-
mation carrier in experiments with remote detection of NMR. Remote detec-
tion is a technique to separate encoding and detection in an NMR experiment
temporally and spatially in order to optimize both steps independently [13]. In-
formation about an analyte is encoded onto the spin magnetization of a mobile
carrier, which then is moved to a detector that can read out this magnetiza-
tion with high sensitivity. Remote detection has been applied successfully to
measure the chemical shift of 129Xe on the surface of a porous material, and
for imaging at low and at high magnetic fields [14]. However, so far there is no
experiment to encode the NMR spectrum of a heteronucleus onto the sensor,
especially not if the sensor and the analyte are not in direct contact with each
other.
An independent field of research that attracted considerable attention during
the last decade involves distant dipolar fields (DDF) in liquids [15] and their
application to studies of the physiochemical environment of a molecule. War-
ren et al. showed that interactions can be established between nuclei with high
gyromagnetic ratios <y, separated on a meso- to macroscopic scale, up to the
order of 1 mm [16]. This allowed them to correlate signals between molecules
that were located in separate containers. As dipolar couplings are mediated
through space, the only requirement is that the two nuclear species are suffi-
ciently close; they do not need to be coupled or involved in the relaxation of
each other. Past studies have almost exclusively been focused on protonated
liquids. Only a few groups have explored heteronuclear distant dipolar cou-
plings, mainly towards the end of indirect detection schemes for rare nuclei via
their solvent in order to surpass the sensitivity of direct detection [17-21]. A
notable attempt to use xenon for indirect detection of CH4 gas in a xenon/CH4
gas mixture can be found in [19,22].
In this work we combine the high signal intensity of laser-polarized 129Xe
and the flexibility in sample composition offered by correlating signal over
meso- to macroscopic scales via distant dipolar couplings. We spatially and

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