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Improved Hyperpolarization of Solid and Mesoporous Nanoscale Silicon Particles Using TEMPO Radicals Allows In Vivo 29Si MRI

Nicholas Whiting^1,2, Jingzhe Hu^1,3, Shivanand Pudakalakatti¹, Caitlin McCowan^1,3, Hyeonglim Seo⁴, Youngbok Lee⁴, and Pratip Bhattacharya¹

¹The University of Texas MD Anderson Cancer Center, Houston, TX, United States, ²Rowan University, Glassboro, NJ, United States, ³Rice University, Houston, TX, United States, ⁴Hanyang University, Ansan, Republic of Korea

Synopsis

Hyperpolarized silicon microparticles have been previously demonstrated as in vivo MRI contrast agents; unfortunately, their large size and decreased mobility present limitations for targeted molecular imaging. While nanoscale silicon particles can also be hyperpolarized, their signal enhancement is typically limited by a low concentration of endogenous electrons. As such, no studies to date have demonstrated in vivo ²⁹Si MRI of hyperpolarized nano-scale silicon. We demonstrate improved ²⁹Si hyperpolarization with the addition of an exogenous radical species to both solid and mesoporous nanoparticle samples (30-300 nm diameter), which increases ²⁹Si hyperpolarization and allows in vivo imaging of silicon nanoparticles.

Introduction

Nano-scale silicon particles have potential utility as targeted molecular imaging agents due to their biocompatibility and simple surface chemistry that is amenable to drug loading and targeting [1,2]. Given that ²⁹Si is MR-active (I=1/2), silicon nanoparticles (SiNPs) may be developed as in vivo MR contrast agents. Indeed, the ability to perform in vivo ²⁹Si MRI has been demonstrated in silicon microparticles that are hyperpolarized using dynamic nuclear polarization (DNP) [3,4]. Unfortunately, due to their large size and lack of mobility, micro-scale silicon particles have limitations for targeted molecular imaging, especially when vascular delivery is needed [5]. While some preliminary studies have shown that nano-scale silicon particles can also be hyperpolarized via DNP [5-7], there have been no demonstrated in vivo studies to date. Indeed, in a direct comparison, nanoscale silicon particles have been shown to provide a fraction of the HP ²⁹Si MR signal (~30x less) compared to microscale particles [5]. Here we report our efforts to improve DNP of nanoscale silicon particles through the addition of an exogenous radical species ((2,2,6,6-Tetramethylpiperidin-1-yl)oxyl, or ‘TEMPO’), which provides additional free electrons to improve the efficiency of ²⁹Si DNP. Initial electron spin resonance (ESR) studies showed that microscale particles contained nearly 10x more available electrons compared to the nanoscale particles; this is likely due to an increase in internal defects and amorphous regions of the micro-scale particles. By titrating in different concentrations of TEMPO, we were able to maximize the achievable ²⁹Si MR signal, thus allowing for in vivo imaging of nanoscale silicon particles. Furthermore, we also applied the addition of TEMPO to interrogate mesoporous silicon nanoparticles, which could be further developed as drug delivery theranostic agents.

Methods

Solid silicon particles ranging from 30 to 200 nm diameter were commercially procured and used as received. Mesoporous silicon particles were synthesized using a modified Stober method to create silica particles, followed by reduction with Mg at high temperatures and pressures; this resulted in ~300 nm diameter spherical mesoporous SiNPs with ~10 nm diameter pores. These mesoporous SiNPs were surface-functionalized with 3-aminopropyl-triethoxysilane (APTES). Particle samples (~50 mg) were dissolved in a 1:1 mixture of D2O and DMSO, with varying concentrations of TEMPO radical mixed in (total sample volume ~130 µL). The particles were hyperpolarized using a laboratory-constructed ²⁹Si solid-state DNP device [5]; imaging and spectroscopy studies were performed at room temperature on a 7T small animal MRI. Co-registered [¹H:²⁹Si] MRI was performed using a dual-tuned ²⁹Si/¹H Litz coil: in vivo ²⁹Si imaging used a coronal RARE sequence (α = 90°; TR/TE: 60 ms/1.8 ms; 6.4 cm FOV; 2 mm resolution), while ¹H imaging utilized a coronal RARE scan (α = 90°), TR/TE: 1927 ms/9.5 ms with a RARE factor of 8; 6.4 cm FOV (0.25 mm resolution) and 4 averages. ²⁹Si spectroscopy utilized a variable tipping angle pulse to interrogate the hyperpolarization buildup or decay values.

Results

ESR studies show that adding TEMPO to the nanoparticle solutions significantly improved the pool of available free electrons for DNP. This translated to increased ²⁹Si MR signals following DNP with added radical (Figure 1); these improvements ranged from 1.5x to more than 6x compared to DNP without TEMPO. Adding radical slightly hastened ²⁹Si T₁ decay; under conditions of optimal TEMPO concentration, T₁ values decreased by ~25% for all nanoparticle sizes. Despite this faster T₁, ²⁹Si MR signal was sufficiently improved to warrant inclusion of TEMPO (i.e., increases in ²⁹Si hyperpolarization overcame losses from T₁). This allowed, for the first time, in vivo ²⁹Si MRI of hyperpolarized nano-scale silicon particles (Figure 2), as well as studies of mesoporous silicon nanoparticles (Figures 3-5).

Discussion

The ability to considerably improve ²⁹Si hyperpolarization in nanoscale solid and mesoporous particles through the addition of exogenous radicals is a significant step forward in their development as targeted imaging agents. We present the first demonstration of in vivo imaging of nanoscale silicon particles, as well as the first reported hyperpolarization studies involving mesoporous silicon nanoparticles. The added TEMPO was not removed prior to injection into the mouse models, and no ill effects were noticed in the mice. The nanoscale silicon particles with added TEMPO performed favorably compared to microscale silicon control samples, and ²⁹Si T₁ values for the nanoparticle samples with TEMPO varied from 15-35 minutes. High resolution ²⁹Si MR images could be achieved over 65 minutes post-DNP.

Conclusions

We demonstrate that adding exogenous radicals to nano-scale silicon particles significantly improves their hyperpolarization characteristics to allow in vivo ²⁹Si MRI. Future studies will attach targeting agents and therapeutic drugs to allow multiplexed theranostics using nanoscale silicon.

Acknowledgements

Acknowledgements: This work was funded by the MDACC Odyssey Postdoctoral Fellowship, NCI R25T CA057730, DoD PC131680, MDACC Institutional Research Grants, MDACC Institutional Startup, U54 CA151668, Leukemia and Brain SPORE Developmental Research Awards, NCI R21 CA185536, Gulf Coast Consortium, CPRIT RP150701, and NCI Cancer Center Support Grant CA016672.

References

[1] Park, et al. Nat. Mat. (2009)

[2] Tasciotti, et al. Nat. Nano. (2008)

[3] Cassidy, et al. Nat. Nano. (2013)

[4] Dementyev, et al. Phys. Rev. Lett. (2008)

[5] Whiting, et al. J. Med. Imag. (2016 )

[6] Atkins, et al., ACS Nano (2013)

[7] Kwiatkowski, et al., Sci. Rep. (2017).

Figures

Figure 1: a) Calibration of TEMPO concentration vs. ²⁹Si hyperpolarization level for different solid SiNPs sizes. Data collected (following DNP) at room temperature and 7 T. b) Effect of TEMPO concentration on ²⁹Si T₁ decay for particles in a), measured at room temperature and 7 T. Inset: ²⁹Si MRI of the SiNP sample (70 nm / 30 mM TEMPO) within its sample tube following the corresponding decay study in b); image taken 65 minutes post-DNP after decay study. Photograph to right of ²⁹Si MRI shows sample in tube.

Figure 2: In vivo imaging of nanoscale silicon particles. ²⁹Si MRI (color) of hyperpolarized solid SiNPs (70 nm / 30 mM TEMPO) taken 5 minutes after rectal administration of the particles via an enema procedure (with ~300 µL buffer). Silicon particles are shown to occupy portions of the large intestines between the rectum and cecum. Silicon image co-registered with a ¹H anatomical image (greyscale).

Figure 3: Hyperpolarization dynamics of mesoporous silicon nanoparticles. (a) Buildup of hyperpolarized ²⁹Si MR signal. ~45 mg of 300 nm mesoporous SiNPs (in 30 mM TEMPO solution) was hyperpolarized via DNP; the sample was removed and a ²⁹Si MR spectrum was acquired to determine the amount of signal for that given time point. Inset: example hyperpolarized ²⁹Si MRS. (b) Decay of hyperpolarized ²⁹Si MR signal (α = 20°) for the same sample following a 16.5 hr DNP experiment; hyperpolarized ²⁹Si T₁ value ~20 min. (c) Following part (b), ²⁹Si MRI of the still-hyperpolarized SiNPs within sample tube. Inset: sample photograph.

Figure 4: Hyperpolarized ²⁹Si MRI with a shaped phantom. ~45 mg of 300 nm mesoporous SiNPs (in 30 mM TEMPO) were hyperpolarized via DNP for ~18 hrs, then dissolved in ~850 µL PBS and pipetted into a shaped phantom. (a) ²⁹Si MRI of the silicon particles, taken 5 minutes after completion of DNP. (b) ¹H MRI of the particle solution (i.e., PBS), taken after the ²⁹Si MRI scan. (c) Overlay of the two scans. (d) Photograph of the HY phantom, with silicon nanoparticle solution. All MR data was acquired at room temperature in a 7 T small animal MR scanner.

Figure 5: Electron micrographs of porous silicon nanoparticles. High resolution (a) SEM and (b) TEM images of monodisperse porous silicon particles, average diameter ~300 nm, with average pore size of 10 nm. Particles were surface-coated with APTES for biocompatibility and to allow further downstream coupling to targeting agents and therapeutic drugs.

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)

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