  Dynamic nuclear polarization (DNP) of molecules in a solution at room
temperature has potential to revolutionize nuclear magnetic resonance
spectroscopy and imaging. The prevalent methods for achieving DNP in solutions
are typically most effective in the regime of small interaction correlation
times between the electron and nuclear spins, limiting the size of accessible
molecules. To solve this limitation, we design a mechanism for DNP in the
liquid phase that is applicable for large interaction correlation times.
Importantly, while this mechanism makes use of a resonance condition similar to
solid-state DNP, the polarization transfer is robust to a relatively large
detuning from the resonance due to molecular motion. We combine this scheme
with optically polarized nitrogen vacancy (NV) center spins in nanodiamonds to
design a setup that employs optical pumping and is therefore not limited by
room temperature electron thermal polarisation. We illustrate numerically the
effectiveness of the model in a flow cell containing nanodiamonds immobilized
in a hydrogel, polarising flowing water molecules 4700-fold above thermal
polarisation in a magnetic field of 0.35 T, in volumes detectable by current
NMR scanners.
