Polarizing agents are typically dispersed in glass forming solvents to avoid clustering of the radicals which can lead to strong exchange interaction resulting in poor DNP efficiency. Therefore glycerol (or DMSO) is often added to the aqueous solution.
These polarizing beads are very interesting because they potentially allow DNP using solvents that do not inherently form a glass. It would be really interesting to see how efficient this approach is for TEMPO based radicals. In addition these beads allow optimizing the local environment of the radical (for example to optimize relaxation properties) for optimum DNP performance independently from the sample properties.
Zhang, Y., P.J. Baker, and L.B. Casabianca, BDPA-Doped Polystyrene Beads as Polarization Agents for DNP-NMR. J Phys Chem B, 2015.
http://www.ncbi.nlm.nih.gov/pubmed/26717243
The aromatic free radical BDPA (alpha,gamma-bisdiphenylene-beta-phenylallyl), which has been widely used as a polarizing agent for Dynamic Nuclear Polarization (DNP) of hydrophobic analytes, has been incorporated into nanometer-scale polystyrene latex beads. We have shown that the resulting BDPA-doped beads can be used to hyperpolarize 13C and 7Li nuclei in aqueous environments, without the need for a glassing cosolvent. DNP enhancement factors of between 20 and 100 were achieved with overall BDPA concentrations of 2 mM or less. These Highly-Effective Polymer/Radical Beads (HYPR-beads) have potential use as an inexpensive polarizing agent for water-soluble analytes, and also have applications as model nanoparticles in DNP studies.