D.
Baudouin
*a,
H. A.
van Kalkeren
a,
A.
Bornet
b,
B.
Vuichoud
b,
L.
Veyre
a,
M.
Cavaillès
a,
M.
Schwarzwälder
c,
W.-C.
Liao
c,
D.
Gajan
d,
G.
Bodenhausen
befg,
L.
Emsley
b,
A.
Lesage
d,
S.
Jannin
b,
C.
Copéret
*c and
C.
Thieuleux
*a
aUniversité de Lyon, Institut de Chimie de Lyon, LC2P2, UMR 5265 CNRS-CPE Lyon-UCBL, CPE Lyon, 43 Bvd du 11 Novembre 1918, 69100 Villeurbanne, France. E-mail: david.baudouin@univ-lyon1.fr; chloe.thieuleux@univ-lyon1.fr
bInstitut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
cETH Zürich, Department of Chemistry and Applied Biosciences, Vladimir-Prelog-Weg 1-5/10, 8093 Zürich, Switzerland. E-mail: ccoperet@inorg.chem.ethz.ch
dUniversité de Lyon, Institut des Sciences Analytiques, UMR 5280, CNRS, Université Lyon 1, ENS Lyon 5 rue de la Doua, F-69100 Villeurbanne, France
eDépartement de Chimie, Ecole Normale Supérieure, 24 Rue Lhomond, 75231 Paris Cedex 05, France
fUniversité Pierre-et-Marie Curie, Paris, France
gUMR 7203, CNRS/UPMC/ENS, Paris, France
First published on 18th July 2016
Hyperpolarization of metabolites by dissolution dynamic nuclear polarization (D-DNP) for MRI applications often requires fast and efficient removal of the radicals (polarizing agents). Ordered mesoporous SBA-15 silica materials containing homogeneously dispersed radicals, referred to as HYperPolarizing SOlids (HYPSOs), enable high polarization – P(1H) = 50% at 1.2 K – and straightforward separation of the polarizing HYPSO material from the hyperpolarized solution by filtration. However, the one-dimensional tubular pores of SBA-15 type materials are not ideal for nuclear spin diffusion, which may limit efficient polarization. Here, we develop a generation of hyperpolarizing solids based on a SBA-16 structure with a network of pores interconnected in three dimensions, which allows a significant increase of polarization, i.e. P(1H) = 63% at 1.2 K. This result illustrates how one can improve materials by combining a control of the incorporation of radicals with a better design of the porous network structures.
Hyperpolarization by D-DNP involves microwave irradiation at low temperatures in moderate magnetic fields (typically T = 1.2 K and B0 = 3.35 or 6.7 T) of frozen glassy solutions doped with stable free radicals and molecules of interest (e.g. metabolites or tracers). The preparation of a glassy frozen sample is important because it ensures that the radicals are statistically distributed, without the formation of ice crystals, which leads to optimal DNP efficiency. For that purpose, glass-forming agents such as glycerol, DMSO or ethanol are usually included in high concentrations. In a typical D-DNP experiment, following polarization, the polarized solution is rapidly brought to room temperature using superheated water and quickly transferred to the MRI or NMR machine for further studies. Once the solution is hyperpolarized, both radicals and glass-forming agents are unwanted and should obviously not be injected into patients. Furthermore, radicals act as paramagnetic relaxing agents, inducing faster depolarization.4 Therefore, rapid removal of radicals after polarization and before use is essential. In the case of trityl and BDPA radicals, the removal can be achieved by precipitation followed by filtration or by ion exchange.5,6 However both methods are limited to specific sample formulations. Alternatively, nitroxide based radicals can be scavenged by ascorbate (vitamin C),7 which attenuates paramagnetic relaxation but leads to contamination of the samples by hydroxylamines. Radicals can be incorporated into polymers such as polystyrene particles8 or hydrogels,9 allowing physical separation of the polarizing agent from the solution, but the efficiency for D-DNP system is limited and the filtration not straightforward.
In this context, we have recently developed a family of solid polarizing matrices based on hybrid materials containing covalently bound radicals, coined HYperPolarizing SOlids (HYPSO). These materials provide in principle a universal solution to the above-mentioned issues: i.e. fast and easy removal of radicals by filtration, and the absence of glass-forming agents. HYPSO are porous and robust silica-based solids on which any radicals (i.e. TEMPO, trityl…) can be covalently and homogeneously attached to the surface of their pores.10,11 We showed an efficient direct polarization approaching P(13C) = 15% with a build-up time of 2 h in a 3 M 1-13C sodium pyruvate aqueous solution impregnated with trityl-based HYPSO.11 The first generation TEMPO-based materials, HYPSO-1, allowed reaching a 13C polarization as high as P(13C) = 33% in only 20 min using a state-of-the-art polarizer including microwave frequency modulation12 and 1H-13C cross-polarization.13 Despite significant research efforts to improve the DNP performances of the first generations of HYPSOs, it was not possible to enhance the proton polarization beyond P(1H) = 50%, well below the P(1H) = 90% that can be obtained under similar conditions in glassy water/glycerol TEMPOL solutions.13
The two first generations of HYPSOs were based on ordered mesoporous SBA-15 type structures, with a skeleton consisting of 8–10 nm diameter 1D-pore channels stacked in a 2D-hexagonal arrangement (Fig. 1 – top right). Two generations of nitroxide-based materials, HYPSO-1 and HYPSO-2, were hence prepared and differ from the linker used to anchor the radical to the solid surface, a propylamido10 and a 1,2,3-triazole-propyl tethers, respectively.11 Using a direct synthesis, the radicals were homogeneously incorporated onto the pore surface of HYPSO-1 and HYPSO-2 by peptide coupling or click chemistry, respectively, avoiding radical aggregation, which is important for D-DNP. However, in such a structure, the pores do not communicate with each other; we thus hypothesized that this could be a limiting factor for both nuclear spin diffusion and for the three-dimensional distribution of the radicals, in comparison to frozen glassy solutions. We therefore reasoned that a silica architecture with a 3D cubic porous network (for example using SBA-16 like structures) could improve the DNP performance. Here we describe the development of materials with cubic network arrangement (HYPSO-3) and show that they lead to greater polarization with respect to the one-dimensional porous HYPSO-1/2 materials, yielding proton polarization up to P(1H) = 63%. We show how the 3D cubic material can also be efficiently used under Magic Angle Spinning (MAS) DNP conditions.
Fig. 1 Left: TEM pictures of hexagonal 1/100_N3_SBA-15 in the [001] axis (top) and of cubic 1/140_N3_SBA-16 in the [111] axis (bottom). Right: Schematic representations of the 2D pore structure in the [001] axis of SBA-15/HYPSO-2 (top) and the 3D pore structure in the [111] axis of SBA-16/HYPSO-3 (axis). The radicals are distributed uniformly over the surface of the pores. See Fig. SI-1† for SBA-16 in [100] axis. |
Materials | [SiR]/μmolSiR g−1 | S BET/m2 g−1 | V p (tot.)/m2 g−1 | V p (μ)/m2 g−1 | D p /nm | L μpore /nm |
---|---|---|---|---|---|---|
a Total pore volume corresponding to the quantity of N2 adsorbed at P/P0 = 0.99. b Micropore volume, calculated from the αS plot model. c Micropore mean diameter calculated using MP model/mesopore mean diameter calculated using the BJH model (adsorption branch). d Micropore mean length, calculated using Lμpore = (d(110)/cos(π/4) − Dmeso) using the mesoporous diameter Dp and the d-spacing d(110) obtained from Small Angle XRD analysis. | ||||||
1/34_N3_SBA-16 | 472 | 1012 | 0.68 | 0.31 | 1.7/6.2 | 9.6 |
1/34_HYPSO-3 | 472 | 729 | 0.50 | 0.19 | 1.6/5.4 | 10.4 |
1/60_N3_SBA-16 | 272 | 1010 | 0.66 | 0.29 | 1.6/6.3 | n.d. |
1/60_HYPSO-3 | 272 | 752 | 0.52 | 0.22 | 1.6/5.4 | 10.2 |
1/100_N3_SBA-16 | 164 | 913 | 0.62 | 0.11 | 1.7/6.2 | n.d. |
1/100_HYPSO-3 | 164 | 893 | 0.63 | 0.26 | 1.3/6.3 | 8.6 |
1/140_N3_SBA-16 | 118 | 1184 | 0.82 | 0.33 | 1.4/7.0 | n.d. |
1/140_HYPSO-3 | 118 | 983 | 0.69 | 0.26 | 1.7/7.1 | 9.3 |
1/320_N3_SBA-16 | 52 | 1068 | 0.75 | 0.48 | 1.7/7.0 | n.d. |
1/320_HYPSO-3 | 52 | 714 | 0.48 | 0.27 | 1.6/5.4 | 12.3 |
For comparison, several polarizing matrices with 2D hexagonal arrangements of mesopore tube-like pores (SBA-15 type materials), hereafter named HYPSO-2, were also prepared.10 These materials are highly porous with a BET surface area, a total pore volume, BJH and MP pore diameters of 770–870 m2 g−1, 1.1–1.2 cm3 g−1 and 8.0–9.2 nm, respectively (Table S2†).
Importantly, these SBA-15 type materials exhibit non-interconnected 1.8 nm micropores (5–8% of pore volume). In contrast, HYPSO-3 (SBA-16 materials) exhibits spherical mesopores interconnected by micro-channels in all three dimensions. This pore structure leads to different textures: a lower pore volume and a different intra-grain pore volume distribution (Fig. 1).
Post-functionalization of N3_SBA-16 to obtain HYPSO-3 materials was performed using copper-catalyzed azide-alkyne cycloaddition (Cu-AAC)16 in the presence of o-propargyl TEMPO, CuI, dry DMF and Et3N (see details in ESI and Fig. S2 and S3†). Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) analysis of the powder allowed evaluation of the efficiency of the cycloaddition (referred to as Cu-AAC yield) on HYPSO-3.
As shown in Table 2, 88–64% of the starting azido –N3 reacted with the o-propargyl TEMPO reactant, the yield decreasing slowly when decreasing the molar concentration of radicals (quoted 1/xx ratio that stands for 1 mol of radical per xx mol of SiO2). The concentration of radical incorporated in HYPSO-3 was quantified by recording X-band CW EPR spectra at room temperature. Nitroxyl radical loadings of 246, 135, 79, 50 and 33 μmolNO g−1 were measured for ratios of 1/34, 1/60, 1/100, 1/140 and 1/320 respectively, corresponding to radical concentrations of 491, 260, 125, 72 and 67 μmolNO cm−3 within the total volume of the pores. The radical concentrations show that the yields of post-functionalization are in the range 41–63%, similar to 2D-hexagonal materials characterized by the same method (42–57%). No significant difference between the EPR profiles of HYPSO-2 and HYPSO-3 could be observed at room temperature (Fig. S4†).
Ratio | [R]/μmol g−1 | [NO˙]/μmol g−1 | Cu-AAC yielda (%) | EPR yieldb (%) | [NO˙]c /μmol cm−3 |
---|---|---|---|---|---|
a Percentage of N3 reacted after Cu-AAC (obtained by DRIFT). b Percentage of NO˙ compared to initial N3. c Concentration per total pore volume (P/P0 = 0.99). | |||||
1/34 | 472 | 246 | 88 | 52 | 491 |
1/60 | 272 | 135 | 81 | 50 | 260 |
1/100 | 164 | 79 | 77 | 48 | 125 |
1/140 | 118 | 50 | 74 | 41 | 72 |
1/320 | 52 | 33 | 64 | 63 | 67 |
Fig. 2 EPR linewidths of HYPSO-2 and -3 as a function of the molar radical concentration (in μmolNO cm−3). |
One can observe a maximum polarization in the vicinity of [R] = 50 μmol cm−3. At this concentration, HYPSO-3 yields a polarization P(1H) = 12.5%, significantly higher than the 7.5% obtained with HYPSO-2. The use of microwave frequency modulation improves the DNP performances of both HYPSO-2 and -3 but only for [R] < 100 μmol cm−3 (see Fig. S6†).
For comparison, an isotropic “glassy” H2O:D2O:glycerol-d8 (10:40:50) matrix without HYPSO doped with 40 mM TEMPOL (40 μmolNO cm−3) gave rise to P(1H) = 21.5%. Note that a 80 μmolNO cm−3 solution gives comparable results. When impregnating HYPSO-3 type matrices containing surface azido-groups instead of TEMPO units (1/140_N3_SBA-16) with a 40 mM TEMPO solution, a polarization P(1H) = 19% was obtained. This polarization value is close to that of the isotropic glassy DNP solution (P(1H) = 21.5%). On the contrary, when impregnated in a HYPSO-2 matrix without any radicals (1/140_N3_SBA-15), the 1H polarization dropped to P(1H) = 9%. We take this as a strong indication that the cubic 3D porous network is advantageous for efficient DNP as compared to a one-dimensional network.
The build-up time constant (Fig. S6†) was found to be τDNP = 74 s for the reference DNP solution without HYPSO at 4.2 K. The same solution impregnated in 1/140_N3_SBA-16 (same as HYPSO-3) gave τDNP = 145 s for a similar polarization, and τDNP = 77 s when impregnated in 1/140_N3_SBA-15 (same as HYPSO 2) (Fig. S6†). EPR studies confirmed that this lengthening of the DNP build-up in HYPSO-3 was not due to a radical quenching effect.
At 1.2 K, HYPSO-3 yielded a polarization P(1H) > 40% over a broad range of radical concentrations 50 < [R] < 160 μmolNO cm−3, reaching P(1H) = 63% for [R] = 67 μmolNO cm−3 (cf.Fig. 3, bottom).
By comparison, HYPSO-2 only yielded a maximum polarization P(1H) = 50% for an optimal [R] = 79 μmolNO cm−3. The polarization in the H2O:D2O:glycerol-d8 (10:40:50) mixture containing 40 mM TEMPOL can reach P(1H) = 90% under the same conditions.13 At 1.2 K, frequency modulation was found to have a positive effect on DNP for HYPSO-2 and -3 when [R] < 75 μmolNO cm−3. When the optimal DNP solution was impregnated in 1/140_N3_SBA-16 (radical-free HYPSO-3) and 1/140_N3_SBA-15 (radical-free HYPSO-2), we observed P(1H) = 63% for HYPSO-3 and 50% for HYPSO-2 at 1.2 K. Note that higher polarization might be reached using HYPSO-3 with lower radical concentration, but build up times (τDNP) would become very long (>300 s) (see Fig. S6†). Finally, we prepared a 3 M solution of sodium [1-13C]-acetate in H2O:D2O (1:9) to impregnate HYPSO-3 (with [R] = 67 μmolNO cm−3) and we obtained P(1H) ≈ 50%. Cross polarization20 was performed with 8 contacts every 4 min, yielding P(1H → 13C) = 36% after ca. 30 minutes (Fig. 4). After dissolution, the aqueous acetate solution was recovered by filtration and centrifugation and subjected to ESR analysis which confirmed the presence of a negligible quantity of radicals in the liquid (ca. 1 μmol L−1i.e. <0.3% of HYPSO) probably arising from the presence of very small grains of HYPSO-3 in the solution.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6sc02055k |
This journal is © The Royal Society of Chemistry 2016 |