## 4 Summary and Outlook  
Spin-dependent phenomena in semiconductor NPs are overviewed starting from the basic properties of solid-state semiconductor systems, which depend on preferential orientation of electron  
Figure 41: Transients of the proton magnetization (inset shows the transverse relaxivity vs the iron content), typical transmission electron microscopy (TEM) image of Si NPs with 0.2 at% of iron impurities and MRI images of cancer tumor before (upper image) and after (down image) injection of such NPs. (Reprinted from Reference [101] by permission from John Wiley and Sons © 2019). Figure 42: Number of living mice with [PERSON] lung carcinoma tumor after intratumoral (red circles) and intravenous administration (green triangles) of Si NPs with 0.2 at% of iron impurities and for the control group (black squares) vs time after tumor inoculation. (Reprinted from Reference [103] by permission from World Scientific Publishing © 2020). and nuclear spins therein, to the recent advances in the development and characterization of semiconductor (mainly, silicon) NPs with spin-dependent properties that are promising for versatile biomedical applications. A challenging task to control the nuclear-spin polarization in solids has been successfully accomplished in the pioneering work by [PERSON] on the DNP by optical pumping in c-Si. The hyperfine dipolar interaction between the electron and nuclear spins caused an appreciable polarization of the nuclear spins due to the optically induced saturation of the electron spins. Starting from then, the role of the hyperfine interaction was carefully explored to realize the hyperpolarized nuclear states in semiconductor NPs and compound MPs. The hyperpolarization efficiency of \({}^{29}\)Si was found to depend on the nuclear relaxation time controlled by the density of electron spins. The nuclear longitudinal relaxation time was shown to shorten with increasing the charge-carrier concentration in semiconductor. The nuclear-spin relaxation in Si NPs underwent a strong shortening with decreasing the mean size of NPs, while surface-passivated Si NPs exhibited the relaxation time from \(10^{2}\) s to \(10^{3}\) s. In contrast to the nuclear relaxation, the spatial confinement of charge carriers (electrons and holes) in nanostructures significantly increased the electron spin-lattice relaxation time. A transition from itinerant to fully-localized electrons, while inducing hyperfine dephasing, was beneficial in quenching the electron spin-lattice relaxation. The phenomenon of DNP of \({}^{29}\)Si in Si NPs and compound MPs was shown to open fascinating prospects for creation of new efficient contrast agents for MRI. The most efficient DNP was achieved for Si MPs, which consisted of hundreds of nanometer-size NPs, what ensured the optimal ratio between nuclear polarization/depolarization as it was confirmed by experiments in vivo. Besides the nuclear polarization, the electron-spin related phenomena were shown to be very promising for MRI applications. The experimental results confirmed a key role of unpaired electron spins as Si DBs in porous Si MPs and NPs for the enhancement of the proton spin relaxation in aqueous suspensions. The samples of nonporous crystalline Si NPs exhibited a relatively weak decrease of both longitudinal and transverse relaxation times because of low efficiency of the interaction of their unpaired electron spins with proton spins in surrounding water molecules. On the one hand, the thermally oxidized mesoporous Si NPs did not induce a valuable rate of the proton-spin relaxation due to the low density of Si DBs. On the other hand, the vacuum annealed microporous Si NPs exhibited a strong decrease of the relaxation times, especially the transverse one by 2 to 3 times for the NP concentration of the order of 1 mg/mL. The transverse relaxation rate was found to depend nearly linearly on the concentration of porous Si NPs in suspensions ranging from 0.1 to 20 g/L.