## 3 Electron Spin-Dependent Processes in Si Nanocrystals
### Si NPs with Electron Spin Centers as Potential Contrast Agents for \({}^{1}h\) Mri  
Si NPs with a high number of the electron spin centers can act as contrast agents (CAs) for the conventional biomedical MRI at the proton (1) frequency [95]. The MRI contrast property of Si NPs was explained by considering the magnetic dipole-dipole interaction between electron spins on the Si NP's surfaces and those of surrounding protons [96] as schematically shown in Figure 36. While a solid NP itself can increase the proton spin relaxation (Figure 36a), Si NPs especially prepared from por-Si provide additional opportunities to increase the spin center density due to huge specific surface area of small nc-Si [62]. While por-Si usually contains numerous paramagnetic centers as Si dangling bonds (Si DBs) and their number can be significantly increased by physical and chemical treatments [77]. It was shown that Si NPs prepared from por-Si subjected to thermal annealing (TA) in vacuum could contain a lot of paramagnetic centers, which strongly enhanced the proton spin relaxation of water molecules surrounding those NPs (Figure 36b). It is well known that surface of electrochemically prepared por-Si is passivated by hydrogen atoms [77]. Because the TA treatment induces the hydrogen desorption from the NP's surface followed by creation of new Si DBs, which can be quantitatively detected by EPR technique [77]. Tailoring of the paramagnetic center number in Si NPs is a possible way to improve their CA properties for MRI applications [96]. Figure 36: **(a)** Schematic view of the shortening of proton spin relaxation in water (blue regions) nearby a NP (brown spot) without unpaired electron spins; **(b)** the same scheme for the NP with numerous unpaired electron spins centers on the NP surface. Black and yellow arrows correspond to the proton and electron spins, respectively. (Reprinted from Reference [96] by permission from John Wiley and Sons © 2018). Figure 37 shows typical transients of the longitudinal (left) and transverse (right) proton magnetizations in aqueous suspensions of NPs (mean size \(\sim\)100 nm) prepared from mesoporous (MPSi), microporous (\(\upmu\)PSi) and from c-Si wafers by using high-energy ball milling of the corresponding bulk samples [96]. The MRI contrasting is caused by shortening of the relaxation times of protons in surrounding water molecules (see Figure 38). Table 1 presents data on the Si DB density and proton relaxation times for several types of Si NPs. The sample of c-Si NPs does not cause significant proton relaxation despite they have high paramagnetic centers concentration. Indeed, the surface area c-Si NPs is rather low and most of the centers are located deeply inside the NPs. NPs prepared from both MPSi and cPSi demonstrate significant T\({}_{2}\) shortening. Pore sizes are different in \(\upmu\)PSi (\(\leq\)2 nm) and MPSi (2 nm to 50 nm) that means a bigger surface area for \(\upmu\)PSi and a stronger interaction of water molecules with the surface of NPs. Figure 38: (**a**) EPR spectra of as-prepared (black line, the intensity is multiplied by 10) and thermally annealed in vacuum (red line) PSi NPs. (**b**) Transients of the transverse proton magnetization in aqueous suspensions of as-prepared (black line) and thermally annealed in vacuum (red line) \(\upmu\)PSi NPs. (Reprinted from Reference [96] by permission from John Wiley and Sons © 2018). Figure 37: Transients of (**a**) longitudinal and (**b**) transverse proton magnetization of pure water (black dashed line) and aqueous suspensions of NPs (1g/L) prepared from c-Si (blue line) MPSi (black line) and \(\upmu\)PSi (red line). (Reprinted from Reference [96] by permission from John Wiley and Sons © 2018). The EPR spectroscopy can be used to clarify both the origin of electron spin centers and to measure their concentration in Si NPs after TA. Figure 38a shows EPR spectra of \(\mu\)PSi NPs before and after TA.