High polarization of nuclear spins mediated by nanoparticles at millikelvin temperatures

Abstract

Nuclear magnetic resonance (NMR) techniques are extensively used in many areas of basic and clinical research, as well as in diagnostic medicine. However, NMR signals are intrinsically weak, and this imposes substantial constraints on the amounts and concentrations of materials that can be detected. The signals are weak because of the low energies characteristic of NMR and the resulting very low (typically 0.0001-0.01%) polarization of the nuclear spins. Here, we show that exposure to very low temperatures and high magnetic fields, in conjunction with nanoparticle-mediated relaxation enhancement, can be used to generate extremely high nuclear polarization levels on a realistic timescale; with copper nanoparticles at 15 mK and 14 T, (13)C polarization grew towards its equilibrium level of 23% with an estimated half-time of about 60 hours. This contrasts with a (13)C half-time of at least one year in the presence of aluminium nanoparticles. Cupric oxide nanoparticles were also effective relaxation agents. Our findings lead us to suspect that the relaxation may be mediated, at least in part, by the remarkable magnetic properties that some nanoparticle preparations can display. This methodology offers prospects for achieving polarization levels of 10-50% or more for many nuclear species, with a wide range of potential applications in structural biology and medicine.

Use of Nuclear Spin Noise Spectroscopy to Monitor Slow Magnetization Buildup at Millikelvin Temperatures

“…To analyze the differences between the data series in more detail, the experimental data were initially fitted to a monoexponential model. Similar to previous saturation–recovery pulse experiments under similar conditions, magnetization buildup was not monoexponential.…”

“…The spin noise buildup data (red crosses in Figure 4) are closer to the two off-resonanced ata sets (black and gray dots) than the pulse data points at 2.5 T( on-resonance;b lue dots in Figure4). To analyze the differences between the data series in more detail,t he experimental data were initially fitted to am onoexponential model.S imilart oprevious saturation-recovery pulse experiments under similarc onditions, [4] magnetization buildup was not monoexponential.…”

“…[5] We performed the experiments at 17.5 mK on the millikelvin NMR spectrometer described in Refs. [4,24,25],w hich employed a 3 He- 4 He dilution refrigerator and as uperconducting magnet with av ariable field strength of up to 15 T; we applied between 2.0 and 3.0 T. The NMR probe was tuned to af ixed frequency of 104.5 MHz, which corresponded to an approximate Larmor frequency of 1 Hat2 .5 T.…”

“…in the order of magnitude of 10 7 s) below 1 K . Nanoparticles have been shown to have a huge effect in accelerating spin polarization buildup at very low temperatures . These preceding studies demonstrated that the presence of metallic nanoparticles, for example, 30 nm platinum or copper, could reduce the T 1 relaxation times by several orders of magnitude at ultralow temperatures.…”

“…[3] Nanoparticles have been shown to have ah uge effecti na ccelerating spin polarization buildupa tv ery low temperatures. [4,5] These preceding studies demonstrated that the presenceo fm etallic nanoparticles, for example, 30 nm platinum or copper, could reduce the T 1 relaxation times by severalo rders of magnitude at ultralow temperatures. This findingo pens up the possibilityo fu sing high magnetic field and low temperature to grow significant polarization in ar easonable timescale, and thereby, encourage the development of ab rute-forcep olarizer.T he development of improved protocols for brute-force polarization requires the slow buildup of large nuclears pin polarization at low temperatures to be monitored.T here are severalo bstacles that need to be overcome to achieve this goal.…”