## 2 Dynamic Nuclear Polarization in Semiconductor Nano-and Microstructures
### Microwave-Irradiation-Induced Nuclear Polarization
#### 2.2.6 Si Nanoparticles for Hyperpolarized MRI
The increased signal of the \({}^{29}\)Si particles is generated via low-temperature, solid-state DNP and the particles retain their enhanced signal for \(\geq\)40 min, thus allowing imaging experiments over extended time durations in phantoms and live mice. The measured hyperpolarized relaxation rate of the \({}^{29}\)Si particles was \(\sim\)25 min at 7 T and room temperature. A variety of particle sizes (20 nm to 2 \(\upmu\)m) were found to have hyperpolarized relaxation times ranging from \(\sim\)10 to 50 min [54; 55]. The addition of different functional groups to the Si particle surface was shown to provide no effect on the hyperpolarization buildup or decay rates and allowed for in vivo imaging over long time-scales. Hyperpolarized Si particles were demonstrated to be available agents for targeted, noninvasive and nonradioactive molecular imaging of various cancer systems [54] and a powerful therapists deep tissue imaging platform [55]. In vivo studies examined diverse particle administration routes in mice, including intraperitoneal injection, rectal enema and oral gavage [54]. Incorporating exogenous radicals (e.g., 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)) into Si NPs suspensions is proposed order to improve the characteristics of hyperpolarization of \({}^{29}\)Si nuclear spins to levels sufficient for in vivo MRI [56] (Figure 20). Without any added radical, \(T_{1}\) values ranged from \(\sim\)23 min (30 nm particles) to \(\sim\)36 min (200 nm particles). This size-dependence of \(T_{1}\) is largely expected given the spin-diffusion model [20] for the relaxation of core nuclei to the surface. At optimal TEMPO concentrations (30-60 mM), the shortened \(T_{1}\) values of the Si particles were overshadowed by the significant increase in the \({}^{29}\)Si MR signal [56]. Figure 19: **(A)** Comparison of \(T_{1}\) relaxation between pure (black) and PEG functionalized (red) silicon NPs. The Fourier-transformed free induction decay signal acquired at \(t=\) 1 min (**B**) and \(t=\) 5.2 h (**C**) at room temperature using a 9.4 T imaging system. (Reprinted from Reference [52], © (2017), Springer Nature. This article is licensed under a Creative Commons Attribution 4.0 International License [[http://creativecommons.org/licenses/by/4.0/](http://creativecommons.org/licenses/by/4.0/)]([http://creativecommons.org/licenses/by/4.0/](http://creativecommons.org/licenses/by/4.0/))). Figure 20: Optimization of TEMPO concentrations. (**a**) Calibration of TEMPO concentration vs \({}^{29}\)Si hyperpolarization level for Si NPs of different size: 30, 70 and 200 nm. Data collected (following DNP) at room temperature and 7 T. (**b**) Effect of TEMPO concentration on \({}^{29}\)Si \(T_{1}\) decay for NPs indicated in (**a**), measured at room temperature and under 7 T. Inset: \({}^{29}\)Si MRI of the Si NP sample (70 nm/30 mM TEMPO) within its sample tube following the corresponding decay study in (**b**); the MRI was taken 65 min after DNP and after exciting the phantom with a 10\({}^{\circ}\) pulse every 5 min to acquire the \({}^{29}\)Si spectra used for the \(T_{1}\) calculation. Photograph to right of \({}^{29}\)Si MRI shows sample. (Reprinted with permission from Reference [56]. © (2018) American Chemical Society).