High-Field Liquid-State Dynamic Nuclear Polarization in Microliter

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High-field liquid-state Dynamic Nuclear Polarization in microliter samples Dongyoung Yoon, Alexandros I. Dimitriadis, Murari Soundararajan, Christian Caspers, Jeremy Genoud, Stefano Alberti, Emile de Rijk, and Jean-Philippe Ansermet Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018

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Analytical Chemistry

High-field liquid-state Dynamic Nuclear Polarization in microliter samples Dongyoung Yoon†, Alexandros I. Dimitriadis† §, Murari Soundararajan†, Christian Caspers†, Jeremy Genoud†‡, Stefano Alberti†‡, Emile de Rijk† §, and Jean-Philippe Ansermet† † Institute of Physics, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland ‡ Swiss Plasma Center, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland § SWISSto12 SA, 1015, Lausanne, Switzerland

KEYWORDS : liquid state DNP, gyrotron, planar probe, ABSTRACT: Nuclear hyperpolarization in liquid state by dynamic nuclear polarization (DNP) has been of great interest because of its potential use in NMR spectroscopy of small samples of biological and chemical compounds in aqueous media. Liquid state DNP generally requires microwave resonators in order to generate an alternating magnetic field strong enough to saturate electron spins in the solution. As a consequence, the sample size is limited to dimensions of the order of the wavelength, and this restricts the sample volume to less than 100 nL for DNP at 9 T (~ 260 GHz). We show here a new approach that overcomes this sample size limitation. Large saturation of electron spins was obtained with a high-power (~ 150 W) gyrotron without microwave resonators. Since high power microwaves can cause serious dielectric heating in polar solutions, we designed a planar probe which effectively alleviates dielectric heating. A thin liquid sample of 100 µm of thickness is placed on a block of high thermal conductivity aluminum nitride with a gold coating, that serves both as a ground plane and as a heat sink. A meander or a coil were used for NMR. We performed 1H DNP at 9.2 T (~ 260 GHz) and at room temperature with 10 µL of water, a volume that is more than 100 times larger than reported so far. The 1H NMR signal is enhanced by a factor of about -10 with 70 W of microwave power. We also demonstrated liquid state 31P DNP in fluorobenzene containing triphenylphosphine, and obtained an enhancement of ~200.

Nuclear magnetic resonance (NMR) is a powerful analytical tool in chemistry and biochemistry. However, the low sensitivity of NMR, which is directly linked to the strength of the nuclear magnetization, generally requires samples containing a number of nuclear spins greater than 1017 nuclear spins, long measurement times. Dynamic nuclear polarization (DNP) can increase NMR signal intensity by transferring the much higher polarization of unpaired electron spins to bulk nuclei.1 Furthermore, DNP is required to be coupled to high field NMR in order to obtain a high resolution power of about 0.1 ppm, only obtainable in magnetic fields above 9 T. DNP requires irradiating microwaves (MW) whose magnetic field B is strong enough to depolarize electron spins by using high-power microwave sources or microwave resonators, which had been a technical bottleneck for high field DNP. Recently, solid state DNP with magic angle spinning (MAS) at 9 T has shown remarkable results with 1H and 13C enhancements over 100 and 200, respectively, in glassy frozen solutions doped with radicals. The success of solid state MAS-DNP is achieved by the low operating temperature near 90 K, as unpaired electron spins of the radicals have quite long electron spin lattice relaxation time of 0.1-1 ms and can be at least partially saturated by a few of watts of microwave power. The sample temperature is kept under the control because of relatively low dielectric

heating due to low loss tangent (tan ~ 0.01) of frozen samples,2 and high cooling power of the cold N2 used to spin the rotors. On the other hand, depolarization of electron spins is more challenging in liquid state DNP because radicals in liquid have much shorter electron spin relaxation time (e.g. ~ 120 ns for TEMPOL radicals)3 than in frozen state at cryogenic temperature, so as to need much larger B of microwaves. Furthermore, polar solvents such as water have very large tan and are easily heated by microwaves. Therefore, DNP with polar solutions requires decoupling of microwaves; a magnetic field component B of microwaves strong enough to depolarize the electron spins and an electric field component E sufficiently weak to avoid sample loss by heating. At X band (~ 9 GHz at ~ 3 kG), this requirement can be easily achieved by using microwave resonators with a high quality factor (Q)4-8. The sample is located at a position of low E , thus preventing dielectric heating of the sample under test. The trade-off of using microwave resonators, however, is that the sample volume contained at the B maximum in microwave resonators is inherently limited by the microwave wavelength, so that the size of the microwave resonators decreases as the magnetic field and the frequency increase. This can restrict the sample volume for DNP at 9 T to less than 100 nanoliters. Furthermore, it becomes difficult to combine it with the resonator

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used for NMR at radio frequencies (RF). Prisner et al. conducted liquid state DNP at 9 T using an e-1H double resonance probe constructed by a cylindrical cavity with a Q ~ 400 for e, and the sample was restricted to only a few nL.3,9-11 The microwave resonator allowed them to achieve 90 % of the full electron spin saturation with 100 mW of microwave power. However, dielectric heating large enough to heat the sample to boiling point could occur and active cooling obtained by blowing N2 gas on the sample was necessary to keep the sample temperature under control. The sample volume was able to be increased up to 80 nL by using a Fabry-Perot resonator in which a strip line resonator for NMR excitation and detection is inserted.12,13 Recently, the group of M. Benatti succeeded in obtaining an enhancement of 1000 in 13C DNP at 3 T with non-polar CCl4 dissolving 15N-TEMPONE radicals that has strong scalar relaxation.14 This dramatic result was obtained by an ENDOR spectrometer capable to contain a sample volume less than 500 nL with active N2 cooling.

In this study, we report a new approach that can boost the possible sample volume to microliters for liquid state DNP at 9 T. We used a high-power gyrotron (~ 150 W), and obtained DNP without relying on a microwave resonator to saturate electron spins. Such a high MW power would boil off solutions held in conventional NMR probes. An innovative DNPNMR probe design (Ref[15]) prevents heating of the sample because the liquid is located where E is minimum and because the liquid is also well heat sunk by a block of high thermal conductivity. Although the sample is thin (d = 100 m /4), the planar probe can contain up to 10 L. We succeeded in an enhancement of 1H NMR by a factor of -10 for an aqueous solution and an enhancement of 31P NMR by a factor of 200 for a solution of triphenylphosphine in fluorobenzene. All DNP experiments here were performed without active cooling devices. ∎ Experimental section Sample preparation The sample for liquid state 1H DNP was prepared by dissolving 80 mM 14N-TEMPOL (Sigma Aldrich)

Figure 1. (a) DNP spectrometer composed of a triode-gun gyrotron, /4 & /8 polarizer mirrors, a 5 m-long corrugated waveguide, a miter bend, a superconducting magnet (9 T), and a planar probe. The gyrotron output can be switched on and off by modulating the anode voltage by an external trigger, while fixing the cathode voltage. FID pulse for NMR is turned on immediately after switching off the gyrotron output. (b) schematic drawing of the planar probe. 10 L of liquid sample is placed on the aluminum nitride support with a 50 nm thick gold coating. The sample is enclosed by the glass (fused silica) cover of 300 m thickness. The meander for NMR excitation and signal pick-up is placed on the glass cover.

Figure 2. (a) The planar probe is mounted vertically so that a meander on the glass cover produces a RF magnetic field (RF B1) perpendicular to the static field B0. A miter bend reflects the MW so the B is also perpendicular to B0. (b) A coil wound on the planar probe is mounted horizontally. (c) Nutation signals of 31P NMR of a solution of triphenylphosphine in water as a function of pulse width for the meander and the coil.


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in de-ionized water without degassing. The sample for liquid state 31P DNP was prepared in the following manner: first, N2 was bubbled in fluorobenzene in a glove box; second, a 2 M Ph3P solution was dissolved in the N2 bubbled-fluorobenzene, and third, 80 mM BDPA (Sigma Aldrich) was dissolved in the solution prepared in the previous step. 1M K3[Co(CN6)] dissolved in de-ionized water was used for the 59Co NMR thermometry experiments. High-power & fast-switchable gyrotron The triode-gyrotron is a prototype designed by the Swiss Plasma Center in École Polytechnique Fédérale de Lausanne. The gyrotron allows the microwave frequency to be tuned by moving the magnetic field, from ~ 260.4 GHz to ~ 261.4 GHz with long term stability. The power of the gyrotron peaks at about 150 W at 260.4 GHz and decreases rapidly as the frequency increases. By sweeping or modulating the anode voltage, fast frequency sweeping or fast power switchability can be achieved.

DNP spectrometer & planar probe Figure 1 (a) shows the liquid state DNP spectrometer used in this study, which is composed of a high-power gyrotron, two corrugated polarizing /4 & /8 mirrors that allow us to control the microwave (MW) polarization, a 5 m-long corrugated waveguide, a miter bend, a superconducting NMR magnet (9 T), and the planar probe. The gyrotron was designed using a triode-magnetron-injection gun that enables an independent control of the anode voltage.17,18 This allows for unique features such as fast frequency tunability and fast switchability.19 The gyrotron is tunable within about 1 GHz around 260 GHz by changing the magnetic field. The gyrotron power has a maximum of about 150 W near the lowest frequency of its tuning range, and decreases with increasing frequency. Modulation of the anode voltage by an external trigger enables the MW to be switched on and off rapidly, as depicted in Fig. 1 (a). NMR signals with MW were obtained as follows: first, MW is turned on for 1-5 s, then a free induction decay (FID) is acquired after MW is turned off. MW is kept off for a while after the NMR signal acquisition to allow the sample to return to the initial temperature. The quasi optical /4 and /8 mirrors for 260 GHz are placed after the gyrotron output window, and are tailored to transform the linearly polarized gyrotron output to circular polarization.

Quasioptical polarizers The /4 mirror is a corrugated metallic polarizer whose grooves are fabricated with period = 0.500 mm, width = 0.250 mm, and depth = 0.310 mm, and the /8 mirror is manufactured with the same period and width, but depth = 0.214 mm.16 The transformation of the polarization at the end of the waveguide was experimentally confirmed by using a Schottky-diode (Virginia Diodes Inc.) ∎ Results and discussion

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Figure 3. Simulation results on the two dimensional MW electric and magnetic fields (260.5 GHz) in the sample region of the planar probe. The top line of the figures represents the bottom of the glass cover, and the bottom line represents the ground plane of the support. The results are based on the linearly polarized microwaves of 70 W. (a) MW electric field distribution with = 5 and tan = 0.8. (b) MW electric field distribution with = 6 and tan = 1.2. (c) MW magnetic field distribution with = 5 and tan = 0.8. (d) MW magnetic field distribution with = 6 and tan = 1.2.



As depicted in Fig. 1 (b), a thin liquid sample of 100 m of thickness is placed on a support that has a high electrical conductivity to provide a ground plane for MW. The support also has a good thermal conductivity and a large heat capacity, and serves as a heat sink. The boundary condition that E is nearly zero ensures that the sample is located in a region of maximum B and minimal dielectric loss. As shown in the simulation of E amplitude above the support (COMSOL Multiphysics) in Fig. 3 (a) and (b), E is minimized at the ground plane, and becomes stronger farther away from the support. As the skin depth of pure water at 260 GHz is about 200 m20 and is larger than the sample thickness, all nuclear spins in the sample can be hyperpolarized by DNP. The sample is thin and in thermal contact with the support, which enables rapid heat transport from the sample to the support. For the above two reasons, we expect to be able to apply high power MW without producing severe sample heating. The support of the planar probe was first designed to be made of a metal such as copper15 due to its high electrical and thermal conductivities. However, large eddy currents induced by radio frequency (RF) NMR pulse in a thick metal support can produce induced fields in the vicinity of the sample. This would cause detrimental effects for high resolution NMR such as inhomogeneous RF magnetic field or radiation damping. Therefore, it is desirable to make the support from diamagnetic insulators with high thermal conductivity such as diamond, aluminum nitride, or boron nitride, so we can avoid eddy currents while maintaining effective heat sinking. The MW ground plane condition can be obtained with a thin metal coating on the surface of these insulator substrates. In this study, the support was made of aluminum nitride. The metallic surface was a gold coating of 50 nm in thickness, which is much smaller than the skin depth of ~ 4 m at 400 MHz. We, hence, expect negligible RF eddy-current-induced fields at the sample region. Nonetheless, a gold coating of 15 nm in thickness is

expected to fully reflect microwaves around 260 GHz,21 so that the support with the gold coating of 50 nm in thickness can be reasonably approximated to the simulation (Fig. 3) with the assumption of the support made of bulk Cu metal. The planar probe demonstrated in this study is depicted in Fig. 1 (b). The support of aluminum nitride has a recessed sample space of 1 cm 1 cm with a depth of 100 m at the center. The narrow groove crossing the sample region was made to introduce the liquid sample. The 300 m thick glass cover was used to enclose the sample. NMR excitation and detection was achieved by a meander or a coil. Figure 2 shows the meander and the coil used in the planar probe and the configuration of the planar probe for obtaining the magnetic field components of both MW and RF perpendicular to B0. The meander made by copper strips of 0.1 mm thickness was placed on the glass cover. Neighboring strips have a gap of 2 mm, twice the MW wavelength at 260 GHz, so that the coil does not act as an MW polarizer. The strip widths were set to a practical minimum of 0.3 mm in order to reduce obstruction of the incident MW. Since the meander produces RF magnetic field perpendicular to the plane of the glass cover, the planar probe was mounted vertically as depicted in Fig. 2 (a), and a miter bend was placed in front of the waveguide in order to transmit MW to the planar probe. Further, MW polarization was controlled by the set of /4 and /8 rotating grooved mirrors in order to produce B perpendicular to B0 at the sample region. A coil was made by winding a metal wire around the planar probe. Thin copper wire 0.05 mm in diameter was used. Each turn of the coil was separated by about 2 mm. As shown in Fig. 2 (b), the planar probe was placed facing the incoming MW from the waveguide, so that the magnetic fields of both RF and MW are perpendicular to B0. In this configuration, the circularly polarized MW has rotating B perpendicular to B0, which is in principle twice as effective at saturating electron spins as linearly polarized MW. This results in the same DNP enhancement wi-

Figure 4. (a) Liquid state 1H spectra obtained by the planar probe with and without microwaves with 80 mM 14N-TEMPOL dissolved in 10 L of water. The DNP spectrum was obtained with 70 W of microwave power, 1 s of irradiation time, and 5 s of cooling time (4 cycles) The inset shows the ERP spectrum of 20 mM 14-TEMPOL dissolved in toluene at room temperature and at the same magnetic field as for the DNP experiments. (b) Liquid state 31P DNP spectra obtained by the planar probe with and without microwaves with BDPA dissolved in a solution of 10 L fluorobenzene with Ph3P. 17 W of microwave power, 5 s of irradiation time, and 15 s of cooling time were applied for the DNP spectrum (4 cycles).


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Analytical Chemistry th lower MW power than used with linear polarization, thus further reducing dielectric heating. The NMR effectiveness of the meander and the coil was examined by obtaining nutation signals of 31P free induction decay (FID) with increasing pulse width, as shown in Fig. 2 (c). Both NMR probes had a Q factor of about 40. The meander was able to provide a nutation angle of 810 ° compared to 360 ° with the coil and the same pulse width. This indicates a higher RF conversion factor for the meander than the coil. Furthermore, the signal intensity for the meander was three times as large as for the coil. The low sensitivity of the coil is due to a low filling factor. Despite these drawbacks, the coil has advantages with regards to DNP efficiency and dielectric heating since circular MW polarization can be applied whereas the meander is only compatible with linear MW polarization. In this study, we obtained 31P with the meander and 1H with the coil. 1

H DNP experiments in water We obtained liquid state 1H DNP at 9.2 T (1H = 395 MHz) and room temperature with water containing 80 mM TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl). The DNP mechanism in liquid state is the Overhauser effect.4,13,22 Polarization transfer from electron to nuclear spins is mediated by cross relaxation due to time-dependent scalar or dipolar interactions. The coupling factor that represents the transfer efficiency becomes larger as the spectral density of the time-dependent interactions contains larger component near the EPR frequency. As the EPR frequency is increased and becomes comparable to the inverse correlation time of translational or tumbling motions of the molecules, the component near the EPR frequency in the spectral density sharply decreases. This implies that the coupling factor becomes smaller as the magnetic field increases, which results in less efficient liquid state DNP at high fields.7,9 In particular, the coupling factor between water protons and radicals via dipolar relaxation was predicted to be negligible at high field. However, Prisner et al. observed surprisingly large dipolar relaxation in water with 14N-TEMPOL at 9 T.3,13 They showed that a large enhancement of about - 40 can be obtained when the electron spins are more than 90 % saturated and when the sample is heated up to 95 °C by controlled dielectric heating so as to have faster diffusion and shorter correlation time than at room temperature.11 In view of these considerations, it is important to estimate B and E strengths. We simulated the B and the E spatial profile in the planar probe that contains 10 L of water of 1 cm 1 cm 100 m, assuming that the support is made of bulk Cu metal, as represented in Fig. 3. The simulation was obtained for 70 W of MW power with a beam waist of 5 mm, which are close to the experimental condition used in the 1H DNP described below. For a MW of 70 W, the B strength is estimated to be about ~ 1.3 G in free space and ~ 2.5 G on the ground plane of the support. In a previous report, the Prisner group obtained a saturation factor of over 0.9 in 14N-TEMPOL dissolved in water with a B of about 1.4 G (100 mW with a conversion factor of 0.45 mT·W / ).11 Fig. 3 (a) and (b) show the simulated E distribution in the sample region of the planar probe with tan = 0.8 (water at ~ 5 °C) and tan = 1.2 (water ~ at 30

°C),20 and Fig. 3 (c) and (d) present the B distribution with the same tan values.20 The tan increases rapidly from ~ 0.7 to ~ 1.4 when the temperature is raised from 0 ℃ to 50 ℃.20,23 Therefore, sample heating by MW induces an increase in tan , which results in shorter skin depth, and smaller B . As shown in Fig. 3 (c) for tan = 0.8 with linearly polarized MW of 70 W, the B is about 1.8 G at the bottom of the glass cover, but increases to a maximum of 2.8 G at the ground plane. This implies that the B at the center of the probe is sufficiently large at any depth of the sample. B decreases laterally because of the Gaussian power distribution in the MW beam. Thus, B is insufficient to saturate the electron spins far away from the center of the probe. If the temperature increases to 30 °C (and tan to 1.2), the skin depth is expected to decrease to 85 m, which is less than the sample thickness, so most of the power is absorbed above the ground plane. This short skin depth causes the B maximum to occur underneath the glass cover, and also to reduce its strength to 1.9 G, as shown in Fig. 3 (d). On the contrary, E is always minimized at the ground plane for both values of tan , as shown in Fig. 3 (a) and (b). The B for tan = 1.2 in most of the sample region is lower than that for tan = 0.8, which implies that a larger power is required at higher temperatures in order to saturate electron spins. The simulation results are insensitive to the presence of the meander or the coil because they are made of metal strips and wires thin enough to minimize the reflection of the incoming mm-wave at 260 GHz. We succeeded in performing 1H DNP with 70 W of circularly polarized microwaves. Therefore, we expect that the effective B available for electron spin depolarization is larger than that estimated from the simulation results. The sample was 80 mM of 14N-TEMPOL dissolved in water. The sample volume used in this study was 10 L, which is about 120-fold greater than previously reported(~ 80 nL) using a Fabry-Perot resonator for 260 GHz.12 The inset of Fig. 4 (a) shows the EPR spectrum of 20 mM 14N-TEMPONE dissolved in toluene at room temperature and at the same magnetic field as the one used for the DNP experiments. It was obtained using our own high field EPR spectrometer24 in order to set the DNP frequency to the center of the EPR spectrum. The sample composition used for the EPR measurement was appropriate for this measurement since the concentration was high enough that exchange narrowing occurred, resulting in a single peak. The linewidth is estimated to be of about 20 G (~ 200 ppm), indicating that the EPR broadening of TEMPOL in liquid is not affected by the field inhomogeneity (~ 10 ppm). We turned on MW for 1 sec and after the NMR sequence, kept the MW off for 5 sec in order to cool the sample. The full 1H recovery is shorter than 1 s, so that the irradiation time is long enough for a full 1H build up time in our study. Figure 4 (a) shows the 1H Overhauser DNP spectrum and the NMR spectrum without MW. The negative enhancement indicates dipolar relaxation between electrons and 1H, consistent with previous results. The enhancement is estimated to be about -10, which is similar to the previous results that Prisner et al. obtained at 40 ℃.11 A similar 1H enhancement was obtained with 60 W, less than the MW power used for the 1H DNP experiment in Fig. 4 (a), which implies that the

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Analytical Chemistry electron spins were almost saturated in Fig. 4 (a). The large broadening of about 10 ppm comes from the inhomogeneity of our NMR magnet, as explained in the supporting information. We were unable to use 1H NMR thermometry in order to estimate the sample temperature after MW irradiation since the 1H NMR frequency shift as a function of temperature (-0.012 ppm/°C)10 is much smaller than our NMR magnet inhomogeneity. However, the enhancement found, similar to that at 40 °C in ref[9], implies a similar sample temperature after MW irradiation since we can assume that the electron spins were almost fully saturated in our case as well. Our estimate of the temperature increase is also supported by 59Co NMR thermometry experiments described below. The DNP signal intensity remains constant between experimental cycles, which also indicates that no sample loss occurs due to boiling. 31

P DNP experiments in Ph3P-C6H5F solutions We also performed liquid state 31P DNP with a solution of triphenylphosphine (Ph3P) dissolved in fluorobenzene (C6H5F) with 80 mM BDPA ( , - bisdiphenylene- phenylallyl). The " for C6H5F at 3 GHz was estimated to be 1.5 in a previous report,25 while that for water is about 29 at the same frequency. BDPA is known to have a longer T1e than TEMPOL, so a smaller microwave power is needed. Griffin et al. performed liquid state 31P DNP at 5 T with Ph3P dissolved in non-polar benzene dissolving BDPA, and obtained a high enhancement of about 180 using 0.5 W of MW power.26 The solvent used here (C6H5F) is polar, and much larger dielectric heating is expected than in a benzene solution. Figure 4 (b) shows the DNP and NMR spectra obtained by the planar probe with 17 W and a sample volume of 10 L. MW was irradiated at the center of the EPR spectrum of BDPA. A positive enhancement was observed, which indicates scalar relaxation consistent with ref[21]. Although smaller enhancement was expected since we were at a higher field (9 T vs. 5 T), we obtained a similar en

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hancement of a factor of about 200, which shows effective scalar relaxation even at 9.3 T. The strong scalar relaxation is expected to come from the electronic structure of the lone pair in triphenylphosphine. We obtained an enhancement as a function of MW power, as displayed in Fig. 5. The enhancement increases rapidly as the power increases, and stays constant around 200 beyond the saturation power of 8 W. The saturation of the enhancement implies that the electron spins are fully saturated with MW power beyond 8 W. No sample loss was observed in the planar probe even with 17 W due to the low electric field strength and well-heat sinking the planar probe. The signal to noise ratio (SNR) obtained by DNP with this sample containing 20 micromole of 31P is estimated to be about 100, but it can be increased up to ~10,000 if the planar probe was subjected to a magnet with ~0.1 ppm of homogeneity with the assumption that the NMR spectrum is determined by Lorentzian function. A commercial 1H NMR probe has an SNR of ~240 with a sample volume (0.1% v/v ethyl-benzene in CDCl3) of 250 L27, which corresponds to ~40 of SNR per micromole (masssensitivity). Therefore, the planar probe with DNP can improve SNR per micromole about 30 fold higher than a commercial probe by correcting for n and spectrum width .27 We also achieved liquid state 31P DNP in a standard glass tube (I.D ~ 3 mm, O.D ~ 5 mm) with a 4-turn solenoid coil (wire thickness = 0.05 mm, gap ~ 2 mm). In this case, we expect much larger dielectric heating than with the planar probe. While the planar probe allows high power MW irradiation without severe sample heating, the DNP with the glass tube had to be carried out with MW powers of less than 8 W in order to prevent sample loss due to boiling, as depicted in Fig. 5. The enhancement in the solenoid coil increases up to about 50 at 4.5 W and begins to decrease with further power due to large sample loss. The sample in the solenoid coil has smaller enhancement than that in the planar probe because only the fraction of the sample located within the skin depth can be directly enhanced. A solenoid coil with smaller diameter showed smaller enhancement than the solenoid coil presented here because it had smaller heat capacity, causing even more severe sample loss. NMR thermometry experiments in the planar probe To quantify the decrease in dielectric heating in the planar probe, we performed NMR thermometry using potassium hexacyanocobaltate ({K3[Co(CN6)]} dissolved in water, because the 59Co chemical shift has been reported as having a large dependence on temperature (~ 1.504 ppm/°C).28 We obtained 59Co NMR spectra using the glass tube of O.D = 5 mm, I.D = 3 mm or the planar probe with coil, as a function of MW irradiation time, ranging from 1 sec to 5 sec. Since the 59Co NMR spectrum becomes distorted by the dielectric heating, we estimated the mean temperature increase ∆T by a weighted average of the chemical shifts, as shown in Fig. 6. ∆T in the planar probe is estimated to be about 25 °C with 17


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W of MW power and 5 s of irradiation time, while 80 17 W, planar probe 17 W, solenoid coil 70 W, planar probe

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Figure 6. NMR thermometry experiments in the planar probe

that in the glass tube is estimated to be about 70 °C with the same MW power and irradiation time. Note that this is the temperature of what is left in the probe. We observed that half the sample in the glass tube had evaporated. We also tried a much higher MW power, namely 70 W. Starting from a sample at room temperature, a large sample loss would occur in the glass tube due to evaporation even with a small irradiation time, so this experiment was performed only with the planar probe. Even at 70 W, the planar probe shows an increase of only about 20 °C for an irradiation time of 1 sec. And this is enough to build up the 1H DNP signal to its saturation value, as seen in Fig. 5. Small sample loss begins to occur at irradiation times greater than 5 s, causing errors in estimating the temperature increase. The dielectric heating in this experiment is expected to be more severe than in the 1H DNP described earlier because the large salt concentration of 1 M used here results in a larger tan . Therefore, we expect a similar or smaller increase in the sample temperature after MW irradiation of 70 W during the liquid state 1H DNP.

by using thinner samples while still holding L of liquid, or by replacing the support material with beryllium oxide that has a higher thermal conductivity (~ 300 W/m∙k31) than aluminum nitride ~ 180 W/m∙k31). This new method can be extended to higher or lower magnetic fields by modifying the thickness of the sample region. The meander was shown to have better NMR effectivenesses than the coil, but it had to be positioned vertically owing to its RF B1 being perpendicular to the glass cover. To the contrary, the coil can be positioned both horizontally and vertically. Circularly polarized MW, which is more effective at saturating electron spins than linear polarization, can be used when the planar probe with the coil is placed horizontally. The NMR performance could be improved further by increasing the number of turns in the meander and by thinning the width to 10 m, which would result in a more homogeneous RF B1. Previous simulation results showed that MW at 260 GHz could be fully transmitted through a grid made of a thin layer of 150 parallel copper wires with a width of 50 m and a period of 100 m etched on a 300 m-thick fused silica cover.15, 32 Therefore, an improved meander with many thin strips could be used for DNP-NMR coil in the planar probe. Local magnetic field distortion can be expected in the sample region due to the susceptibilities of the planar probe components. This was documented by other groups using micro solenoid coils.33 We expect that the broadening is minimal when the probe is set vertically (Fig. 2 (a)). This configuration has been shown to give good NMR resolution in experiments using stripline resonators positioned parallel to B0.33

ASSOCIATED CONTENT Supporting information : measurement on the magnetic field profile of the superconducting magnet (9.2 T) used for DNP and NMR in this study.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Notes

∎ Conclusion

Any additional relevant notes should be placed here.

In this study, we showed a new methodology for increasing the sample volume for liquid state DNP at 9 T from 100 nL to 10 L with passive cooling only. It relies on using a high power gyrotron and a planar probe. No active cooling was necessary, even though polar solvents were used (water or fluorobenzene). The increase in sample volume has several benefits; first, easy sample handling (injection into the planar probe, or dropping on the support), second, possibility to work with a small concentration of solute. Even though sufficient MW power was produced by the gyrotron, the 1H enhancement was limited by the small coupling factor. The enhancement could be improved by using radicals with narrower EPR lines such as 15N-TEMPOL and Fremy's Salt3,10 or using supercritical fluids, as their correlation time for molecular motion would be much shorter than that of water.29,30 Dielectric heating in the planar probe can be further reduced

ACKNOWLEDGMENT We gratefully acknowledge financial support by the Swiss National Science Foundation, Requip (No. 206021-21303/1), FN(20002-53230), and CTI-Project no. 15617.1 PFNMNM..

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260 GHz microwaves

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1 2 3 4 5 6 7 8 9 10 11 12 O 13 2 μL 14 H 15 16 O TEMPOL 17 2 18 H H 19 2O O 20 21


Liquid 1H NMR

DNP

70 W

10

0 0 C

microwaves on off


High-Field Liquid-State Dynamic Nuclear Polarization in Microliter

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