Synthesis of long T₁ silicon nanoparticles for hyperpolarized ²⁹Si magnetic resonance imaging

Abstract

We describe the synthesis, materials characterization, and dynamic nuclear polarization (DNP) of amorphous and crystalline silicon nanoparticles for use as hyperpolarized magnetic resonance imaging (MRI) agents. The particles were synthesized by means of a metathesis reaction between sodium silicide (Na₄Si₄) and silicon tetrachloride (SiCl₄) and were surface functionalized with a variety of passivating ligands. The synthesis scheme results in particles of diameter ∼10 nm with long size-adjusted ²⁹Si spin-lattice relaxation (T₁) times (>600 s), which are retained after hyperpolarization by low-temperature DNP.

Figure 1

Synthetic schemes used to produce Si NPs. Each reaction starts with the reaction of the Zintl salt, Na₄Si₄ with SiCl₄ to form the chlorinated Si NPs, and then reacted with (a) octylmagnesium bromide, (b) 4-aminophenylmagnesium bromide and (c) annealed and passivated with aminopropyltrimethoxysilane (APTMS).

Figure 2

TEM images of Si NPs: (a) 4-aminophenyl terminated and (b) annealed and passivated with APTMS. The size distributions are shown in the upper right inset. The lower right inset in Figure 2b shows the crystallinity of the annealed Si NPs obtained using HRTEM. The lines in the lower right inset indicate the lattice fringes which give rise to the (220) lattice plane for diamond structured Si.

Figure 3

Crystallinity of as-grown and annealed Si NPs. (a) X-ray diffraction and (b) Raman spectroscopy of Si NPs before and after annealing show a transition from amorphous to crystalline states.

Figure 4

Infrared spectra of octyl, 4-aminophenyl, and APTMS terminated Si NPs. (a) Octyl terminated Si NPs, (b) 4-aminophenyl terminated Si NPs (c) APTMS terminated Si NPs.

Figure 5

Nuclear spin relaxation (T₁) times at 2.9 T and 300 K as a function of particle diameter d for Si nanoparticles synthesized by schemes described in Figure 1. Previously reported T₁ measurements of other silicon particles are shown as a reference. Simulations of nuclear relaxation based on a spin diffusion model are also shown. Vertical error bars are from exponential fits to relaxation data; horizontal error bars are from size distributions.

Figure 6

Electron spin resonance and dynamic nuclear polarization of A-octyl, B-4-aminophenyl and C-APTMS terminated Si NPs as indicated in the legend for Figure 5. (a) Electron spin resonance signal and (b) ²⁹Si nuclear polarization as a function of microwave frequency under DNP conditions. (c) ²⁹Si NMR spectra obtained under DNP conditions.

Figure 7

Time dependence of ²⁹Si polarization measured at 3.6 K in (A) octyl, (B) 4-aminophenyl and (C) APTMS terminated Si NPs synthesized according the scheme described in Figure 1. (a) ²⁹Si NMR signal as a function of polarization time τ_pol (b) Depolarization of the ²⁹Si nuclear polarization at 3.6 K after 6 hours of DNP. The solid lines show fits to the data described in the text.