## 3 Electron Spin-Dependent Processes in Si Nanocrystals
### Si Nanocrystals as Photosensitizers of Singlet Oxygen Generation for Biomedical Applications  
The \({}^{(1)}\)O\({}_{2}\) photosensitization efficiency by nc-Si dispersed in water was confirmed by measurements of the exciton PL of por-Si MPs excited by laser pulses. The relaxation time of the exponential part of the PL transients in the solution without O\({}_{2}\) was \(\tau_{\rm ex}\approx 85\) us, which is close to the natural radiative lifetime of excitons confined in nc-Si. The saturation of the solution with gaseous O\({}_{2}\) at a pressure of 760 Torr leads to a decrease in the PL decay time to \(\tau_{\rm D}\approx 50\) us. The ratio \(\tau_{\rm ex}/\tau_{\rm D}\approx 1.6\) is close to the measured PL quenching. These observations indicate that the decrease of the PL lifetime of nc-Si in oxygen-saturated suspensions is caused by the photosensitized generation of \({}^{(1)}\)O\({}_{2}\)[87]. The photosensitized generation of \({}^{(1)}\)O\({}_{2}\) was confirmed by experiments in vitro, which showed that number of living cells with added por-Si MPs decreased strongly after photoexcitation as compared to the reference group (see Figure 35). At the same time, the effect of por-Si MPs was almost absent over the entire concentration range in darkness. This fact is concerned with the \({}^{(1)}\)O\({}_{2}\) generation process under the illumination in the active solution with dispersed nc-Si, because \({}^{(1)}\)O\({}_{2}\) being strong oxidizing substance results in both killing of cells and the inhibitions of cell proliferation [87]. An analysis of the DNA content of the cells incubated with photoexcited por-Si MPs indicates smaller DNA content is typical for the cell death due to the apoptosis mechanism [88]. Figure 34: Spectral dependences of the PL lifetime measured for por-Si MPs dispersed in degassed water (squares) and in oxygen-saturated water (circles). Inset: spectral dependence of the energy transfer time in oxygen-saturated water. \(E_{\rm exc}=2.33\) eV, \(T=300\) K. (From Reference [91]). Figure 35: Concentration dependence of the relative number of mouse fibroblast cells with added por-Si MPs in darkness (curve 1) and after illumination with white light source for 1 h (curve 2). (Reprinted from Reference [87] by permission from Springer Nature Customer Service Centre GmbH: Springer © 2006). Mesoporous Si NPs with mean sizes of the order of 100 nm and pore size of 12 nm were used to enhance the photosensized generation of (1)O\({}_{2}\) with quantum yield \(\sim\)10% in ethanol and 17% in aqueous medium [89]. Such NPs with concentration of 0.1 g/L were added to cancer cell cultures followed with white light illumination that resulted in 45% decrease of cell number [89]. A stronger effect for the cancer cell suppression was observed for microporous Si NPs, which could enter cancer cells by endocytosis mechanism, resulting in \(\sim\)70% cell death [90]. Photoluminescent NPs prepared from meso- and microporous Si were shown to be efficient both as the photosensitizers of (1)O\({}_{2}\) generation and photoluminescent labels in bioimaging in vitro. The photosensitization ability combined with the PL bioimaging property allow us to consider por-Si NPs as a potential agent for application in as-called cancer theranostics, that is, simultaneously therapy and diagnostics [89; 90]. Note, the biological effect of photoexcited nc-Si can be related with both the photosensitized (1)O\({}_{2}\) other active forms of oxygen, for example, superoxide (O-2 ion), whose signature was observed as spectral features of the exciton PL quenching at 2.0-2.2 eV [91]. While the results of fundamental studies and in vitro experiments on the (1)O\({}_{2}\) photosensitization by nc-Si seem to be promising, there is a lack of in vivo tests to assess the real potential of nc-Si for PDT applications. Note, the biocompatibility and biodegradability of nc-Si and por-Si NPs [92; 93] together with the photosensitization ability are well satisfied to the modern requirements for PDT application [94].