## 3 Electron Spin-Dependent Processes in Si Nanocrystals
### Si NPs with Electron Spin Centers as Potential Contrast Agents for \({}^{1}h\) Mri
In order to improve the \({}^{1}\)H MRI contrasting property of Si NPs, one can use nanocomposite NPs with iron oxide [101]. Such composite Si NPs are prepared by arc-discharge plasma-assisted method in a closed-cycle inert gas (Ar or N\({}_{2}\)) ambient at atmospheric pressure. The process consists of evaporation of high purity polycrystalline Si powder with addition of metallic iron powder at temperature of about 5500 \({}^{\circ}\)C followed with condensation of the vapor mixture in a cooling gas stream to form polycrystalline powder with mean size of nc-Si of about 20-30 nm confirmed by XRD measurements. Despite the relatively small nanocrystal size the prepared NPs after dispersing them in aqueous media exhibit several times larger hydrodynamic sizes detected by using a DLS method (see Table 2) that indicates either polycrystalline structure of these NPs or/and their aggregation in aqueous medium [101]. EPR spectra of Si NPs with iron oxide impurities (see Figure 40) consist of a broad band with effective \(g\)-factor \(\approx\) 2.1 related to strongly interacting Fe\({}^{3+}\) ions in \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)[102] and a narrow line with \(g\)-factor of 2.0055, which can be assigned to Si DBs as P\({}_{\text{b}}\)-centers [97]. The larger iron concentration in the samples the stronger broadening of the corresponding EPR band due to increasing the spatially inhomogeneous interactions between Fe\({}^{3+}\) ions. As for the second line of intrinsic Si DBs they are supposed to be localized in SiO\({}_{2}\) surface coating of the NP surfaces, which was confirmed by X-ray photoelectron spectroscopy and FTIR measurements [101]. Furthermore, HR-TEM images of iron doped Si NPs reveal numerous defects of the crystalline structure as twins and grain boundaries related to the fast cooling rate during their formation in argon flow [103]. Both the iron impurities and intrinsic defects as Si DBs are responsible for the large number of electron spin centers in such NPs [101; 103]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Fe Content,** & **Nanocrystal Size** & **Most Probable** & **Relaxivity, L/(gs)** & **RF Heating** \\  & **at. \%** & **from XRD, nm** & **Hydrodynamic Size of** & **r\({}_{1}\)** & **r\({}_{2}\)** & **Rate, K\({}_{\text{min}}\)-1** \\  & & & **NPs from DLS, nm** & & & \\ \hline
1 & 0.004 & \(20\pm 5\) & 130 & 0.05 & 0.47 & 6.1 \\
2 & 0.02 & \(25\pm 5\) & 50 & 0.13 & 1.38 & 12.8 \\
3 & 0.2 & \(30\pm 5\) & 120 & 0.83 & 7.30 & 1.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Characteristics of Si NPs with iron impurities. (Reprinted from Reference [101] by permission from John Wiley and Sons © 2019). Figure 39: Transverse relaxation rate of aqueous suspensions of as-prepared (red squares) and thermally oxidized in air (blue squares) NPs prepared from MPSi versus NPs concentration. Solid lines are linear fits. (Reprinted from Reference [96] by permission from John Wiley and Sons © 2018). The total concentration of electron spin centers in Si NPs approaches 10\({}^{18}\) spin/g [101] that makes this type of NPs to be efficient for shortening the proton spin relaxation in aqueous media as it was already demonstrated for porous Si NPs (see Figure 36). In fact, both the longitudinal and transverse relaxation times of protons in aqueous suspensions of plasma-ablated Si NPs were found to be dependent on iron content [101]. The stronger decrease of the transverse proton relaxation was detected for the samples with higher iron content up to 0.2 at%. MRI experiments with phantoms showed that the latter NPs exhibited properties of efficient CAs for the \({}^{1}\)H MRI. The maximal transverse relaxivity accounted 7.3 L/(gs). The latter value corresponds to the molar relaxivity of about 200 L/(mMs) [101], which is close to that reported for CAs based on superparamagnetic iron oxide NPs [100]. The CA property of plasma-ablated Si NPs with iron oxide impurity was confirmed by experiments in vivo [101].