Proton Hyperpolarization Relay from Nanocrystals to Liquid Water

Abstract

Dynamic nuclear polarization (DNP) using transient electron spin polarization generated by photoexcitation can improve nuclear magnetic resonance (NMR) sensitivity far beyond the thermal equilibrium limit for analysis in life science and drug discovery. However, DNP of liquid water at room temperature remains an important challenge. Here, we propose a new method called hyperpolarization relay, in which the nonequilibrium polarization of electron spins is transferred to proton spins in the nanocrystals and then to proton spins in bulk water. Molecular nanocrystals doped with a polarizing agent that generates a highly polarized photoexcited triplet are synthesized by a reprecipitation method while controlling the size of the nanocrystals. The triplet-DNP sequence of repeated laser and microwave irradiation enhances the NMR signal of bulk water as well as nanocrystals. The smaller size of the nanocrystals increases the efficiency of polarization transfer from the nanocrystals to water due to the increased surface area. A series of control experiments and simulations based on Solomon equations confirmed the hyperpolarization relay mechanism.

Spin Hyperpolarization in Modern Magnetic Resonance

“…621 To this end, a single crystal of Another approach used a stable aqueous dispersion of 100 nm nanocrystals of p-terphenyl doped with pentacene obtained by ball-milling of the bulk crystals in a surfactant solution, producing 0.083% 1 H polarization at RT for nanocrystals but not for water 594 (Figure 74). Transfer of 1 H polarization from the host nanocrystal to bulk water was achieved recently, albeit with ε = 2.4 or lower at 0.66 T. 622 Polarization of 31 P and 1 H nuclei in a derivatized C 60 fullerene in frozen toluene-d 8 was demonstrated in the context of qubit manipulations for quantum computing. 623 The major limitations of tDNP are a limited range of known suitable guest chromophores, sample preparation restrictions, and suitable experimental conditions.…”

Generation and Transfer of Triplet Electron Spin Polarization at Solid-Liquid Interface

“…34 Recently, DNP of bulk liquid water was achieved by transferring the polarization from electron spins to nuclear spins in a solid and cross relaxation between nuclear spins at the solid-liquid interface, but the small cross relaxation rate remains an issue. 35 Therefore, the key question is how to generate electron spins with high polarization and long polarization lifetime like in solids in a situation where they are in contact with a liquid at the molecular level, and how to make them interact efficiently with molecules in solution.…”

Polarizing agents beyond pentacene for efficient triplet dynamic nuclear polarization in glass matrices

“…[8][9][10][11] DNP using photo-excited triplet electrons (triplet-DNP) can achieve nuclear hyperpolarization even at room temperature since high electron spin polarization between triplet sub-levels is achieved regardless of temperature by the spin-selective intersystem crossing (ISC) (Figure 1a). [12][13][14][15][16][17][18] The polarization of triplet electron spins can be efficiently transferred to nuclear spins by integrated solid effect (ISE) sequence with microwave irradiation and magnetic field sweep (Figure 1b). Since the electron spin resonance (ESR) linewidth of triplet is broadened over 100 mT at X-band by hyperfine coupling and dipolar interactions between triplet electrons, only limited part of spin packets simultaneously satisfies the resonance condition for polarization transfer, so-called Hartmann-Hahn condition.…”