CONICET | Buscador de Institutos y Recursos Humanos

Diameter size silicon nanoparticles (SiNP) of a few nanometers show strong photoluminescence due to quantum confinement with maximum wavelength emission strongly depending on size, superficial coating and environment. In addition, Si-NPs are photosensitizers of reactive oxygen species. These properties make them excellent candidates for diagnosis proposes (specific luminescent markers) and for therapeutic treatments. In the aim of evaluating the potential application of derivatized SiNP in biological systems, we undertook the synthesis of SiNP of (3 ± 1) nm sizes functionalized with organic groups like methylmethacrylate and vitamin B9. The electrochemical synthesis and subsequent surface derivatization of 3 nm diameter yield silicon nanoparticles of surface composition Si3O6(C5O2H2) and strong blue photoluminescence. The short lifetime of 1 ns observed for the decay of the luminescence supports a direct band gap transition which could result from the rapid trapping of the exciton at the nanocrystal surface. The participation of surface irregularities in the emission originated from the recombination of holes and trapped electrons was supported by the value of anisotropy at time zero and the large Stokes shift. Excitation-emission spectra and quantum yields were obtained in monophotonic excitation experiments. The figure shows the emission spectrum of SiNP in toluene suspensions. Dissolved oxygen reduces the emission quantum yield (Φf) around a 31 % the value in oxygen free suspensions. Multiphotonic excitation at 774 nm leads to the same emission spectra than monophotonic experiments, as well as similar oxygen effects. Although observation of the luminescence quenching by molecular oxygen may be an indication of energy transfer from excitons to oxygen molecules, it is still not an evidence for singlet oxygen (1O2) generation. Therefore, detection of 1O2 photoluminescence at 0.98 eV was performed, as shown in the inset of the figure. Singlet oxygen quantum yields of 35% were obtained for toluene suspensions of the particles. This value correlates well with the diminution of the emission quantum yield due to the presence of oxygen. To clarify the mechanism of the luminescence emission and the O2 photosensitization, the photophysics of the synthesized particles was thoroughly investigated.