## 2 Dynamic Nuclear Polarization in Semiconductor Nano-and Microstructures
### Microwave-Irradiation-Induced Nuclear Polarization
#### 2.2.3 Long \(T_{1}\) Nanoparticles for Hyperpolarized \({}^{29}\)Si MRI  
The synthesis, materials characterization and DNP of amorphous and crystalline Si NPs for use as hyperpolarized MRI agents were represented in Reference [26]. The Si NPs of \(\sim\)10 nm size were synthesized by a metathesis reaction and surface functionalized with a variety of passivating ligands. Because of the long size-adjusted nuclear spin-lattice relaxation time \(T_{1}>600\) s (Figure 11), Si NPs can be hyperpolarized by low-temperature DNP (Figure 12) and retain the polarization for \(>\)300 s [26].  
Figure 10: **(A)** Dynamic nuclear polarization (DNP)-enhanced \({}^{29}\)SiCross-Polarization Magic Angle Spinning (CPMAS) NMR spectra of II as a function of the CP mixing time \(\tau_{\text{CP}}\). Spectra were recorded with 16 scans. **(B)** Contour plot of a two-dimensional \({}^{1}\)H–\({}^{29}\)Si spectrum of II recorded with DNP. 64 \(t_{1}\) increments of 64 \(\upmu\)s with eight scans each were recorded with \(\tau_{\text{CP}}=1.0\) ms. Total experimental time was 8.5 min. (Reprinted with permission from Reference [49]. © 2011 American Chemical Society).  
In solid-state spin-1/2 materials, such as Si, the primary source of nuclear spin-lattice relaxation is the interaction with unbonded electrons or holes, which occur in the form of free charge carriers or paramagnetic centers either at lattice defect sites or at the surface. In pure materials, the nuclear spin polarization is to a significant extent protected from sources of relaxation, resulting in bulk relaxation times of _many hours_[11; 12; 18]. A transition to the nanoscale resulted in spin relaxation times that depend on the particle size, purity and density of defects [11]. The colloidal synthesis of Si NPs with average diameters of \(\sim\)10 nm at room temperature provided spin relaxation times that exceed 600 s. This resulted in fabrication of NPs that revealed significantly longer size-adjusted \({}^{29}\)Si nuclear spin relaxation times \(T_{1}\) than commercially available Si particles [11] and diamond NPs of similar sizes [50]. These Si NPs are the smallest characterized for hyperpolarization applications. Due to their size and  
Figure 11: Nuclear spin relaxation (\(T_{1}\)) times at 2.9 T and 300 K as a function of particle diameter d for Si NPs. Previously reported \(T_{1}\) measurements of other Si particles are shown as a reference. Simulations of the 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. (Reprinted with permission from Reference [26]. © 2013 American Chemical Society).  
Figure 12: Electron spin resonance and DNP of (A) octyl-, (B) 4-aminophenyl- and (C) APTMS-terminated Si NPs. (**a**) Electron spin resonance signal and (**b**) \({}^{29}\)Si nuclear polarization as a function of microwave frequency under DNP conditions. (**c**) \({}^{29}\)Si NMR spectra obtained under DNP conditions. (Reprinted with permission from Reference [26]. © 2013 American Chemical Society).  
long \(T_{1}\) times, they are perspective as in vivo imaging agents, which are suitable for intracellular investigations and for crossing the blood-brain barrier [26].