The effects of drying technique and surface pre-treatment on the cytotoxicity and dissolution rate of luminescent porous silicon quantum dots in model fluids and living cells
Tailoring of the biodegradation of photoluminescent silicon quantum dots (Si QDs) is important for their future applications in diagnostics and therapy. Here, the effect of drying and surface pretreatment on the dissolution rate of Si QDs in model liquids and living cells was studied in-vitro by a combination of photoluminescence and Raman micro-spectroscopy. Porous silicon particles were obtained by mechanical milling of electrochemically etched mesoporous silicon films, and consist of interlinked silicon nanocrystals (QDs) and pores. The samples were subjected to super-critical drying with CO2 solvent (SCD) or air drying (AD) and then annealed at 600 C for 16 hours in 1% oxygen to achieve the nano-sized Si QDs. The obtained samples were characterized by a core-shell structure with a crystalline silicon core and a SiO2 layer on the surface. The sizes of the crystalline silicon core, calculated from the Raman scaterring spectra, were about 4.5 nm for initial AD-SiQDs, and about 2 nm for initial SCD-SiQDs. Both AD-Si QDs and SCD-Si QDs exhibited visible photoluminescene (PL) properties due to quantum confinement effects. The dissolution of nanocrystals was evaluated by their PL quenching , as well as by the presence of a low-frequency shift, broadening, and decrease in the intensity of the Raman signal. The stability of AD-Si QDs and the complete dissolution of SCD-Si QDs during 24 hours of incubation with cells have been demonstrated. This might explain the apparent lower cytoxicty observed for SCD-Si QDs.