Structural and luminescence properties of silicon nanocrystals in colloidal solutions for bio applications

Luminescent silicon nanoparticles are promising not only for optoelectronics or photovoltaics, but also for biological and medical research, e.g., as luminescent markers, for drug delivery, or toxicity studies. For intracellular biological research, it is necessary to prepare water-based or isotonic colloidal solutions of nanoparticles which are stable and non-toxic. In this work, we compare structural and optical properties of silicon nanocrystals in colloidal solutions, obtained by two fundamentally different methods: A microplasma-based synthesis (a “bottom-up” technique) and porous silicon prepared by electrochemical etching of a silicon wafer (a “top–down” method). Low-pressure microwave plasma synthesis produces ∼5–8 nm large silicon nanocrystals while the porous siliconcontains clusters ∼60–70 nm in diameter, composed of nanometer-sized luminescent nanocrystals. However, both types of nanoparticles are prone to agglomeration, as was confirmed by dynamic light scattering and zeta potential measurements. We have attempted to stabilize the nanoparticles via modification of the their surface by methyl groups; however, the mechano-photo-chemical treatment procedure leading to coverage with methyl groups is less efficient in the case of plasma-synthesized nanoparticles than in porous silicon. The first attempt of steric stabilization of the colloidal solutions of the silicon nanoparticles was succesfully carried out too; good candidates for stabilization are bovine serum albumin and glycine.