Nanoparticle-assisted dynamic nuclear polarization in liquids.

Gold nanoparticles (AuNPs) protected with self-assembled organic monolayers represent versatile nanostructures with applications in catalysis, molecular sensing, and recognition. The ordered arrangement of ligands within the monolayer promotes specific orientations and proximities of functional groups, facilitating weak yet selective interactions with small molecules in solution. In the context of nuclear magnetic resonance (NMR), AuNPs can act as magnetization reservoirs, allowing selective transfer to interacting molecules and making them promising chemosensors. While "NMR chemosensing" typically exploits nucleus-nucleus magnetization transfer mechanisms, we explored the potential of electron-nucleus interactions to allow selectively enhanced Overhauser effect dynamic nuclear polarization (OE-DNP). To this aim, we designed AuNPs simultaneously functionalized with two different thiols: a radical-bearing thiol as a polarizing agent for OE-DNP and an organic thiol tailored with different head groups to impart selective recognition. To evaluate both the polarization transfer efficiency and the selectivity, we performed OE-DNP experiments on a mixture of chloroform and methanol. Our results show sizeable enhancements on chloroform, possibly limited only by the mobility of the radical. On the other hand, the affinity for methanol does not correspond to greater enhancements on this target molecule. Although the system, as designed, does not provide efficient DNP on methanol as expected, our results confirm that AuNPs may be used as polarizing agents at magnetic fields up to 9.4 T. We foresee that future modifications of the nanoparticles' monolayer will allow the combination of spin polarization transfer efficiency with molecular recognition, offering a novel strategy to boost hyperpolarization techniques and expanding their application towards NMR detection of specific analytes.