Alkyl passivation and SiO2 encapsulation of silicon nanoparticles: preparation, surface modification and luminescence properties

In this paper, we report on the preparation of decyl-modified silicon nanoparticles (decyl-SiNPs) and their encapsulation into silica nanoparticles (NPs). The decyl-SiNPs were obtained by the photoinduced hydrosilylation of hydride-terminated SiNPs (H-SiNPs), liberated from porous silicon (pSi), followed by encapsulation into silica NPs (SiNPs@SiO2) via the Stober process. Two different sized SiNPs@SiO2 were synthesized, similar to 15 and 25 nm in diameter. Their composition, size, shape, luminescence properties, colloidal and spectral stability in different environments and under ultraviolet (UV) light irradiation were studied by various techniques, including Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), UV-vis absorption, dynamic light scattering (DLS) and photoluminescence (PL) spectroscopy. The results indicate that the SiO2 coating renders the hydrophobic alkylated SiNPs dispersible in water, but leads to some loss of their PL intensity. The SiNPs@SiO2 NPs exhibit a wide pH stability, but show a pronounced PL degradation due to a blinking behavior. The photobleaching process could be partially suppressed by increasing the SiO2 outer shell thickness. Some investigations were made on the luminescence quenching, spectral shift and photobleaching. For potential bioapplications, the SiNPs@SiO2 NPs were modified with aminopropyl groups under acidic conditions without obvious luminescence loss and further conjugated with folic acid (FA). Considering the features of red-emission, versatile SiO2 surface chemistry and controlled size, the present SiNPs@ SiO2 nanocomposites may find wide applications in cellular fluorescence labeling and the preparation of light-emitting devices.