Silicon Nanocrystals for Biological Imaging

Silicon nanocrystals fluoresce with size-tunable color in the red-to-near infrared spectral range and are biocompatible, making them well-suited for in vivo biological imaging. There are however challenges facing their use in biological applications. Their band-edge absorption is relatively weak, leading to a large apparent Stokes shift between optical excitation and emission. To induce strong red light absorption while retaining the fluorescence properties of Si nanocrystals (i.e., red to NIR emission with long microsecond lifetimes), Si nanocrystals can be modified with molecular antenna with strong red absorption and charge transfer to the emitting nanocrystals. Si nanocrystals are also susceptible to oxidation and photoluminescence quenching in water, which has required ligand passivation chemistry utilizing bifunctional alkenes with ester or carboxylate terminal functional groups. Hydrophobic Si nanocrystals can also be dispersed in aqueous media using lipids and other surfactants. For specific targeting of molecular recognition groups on cells, the nanocrystals can be modified with either nucleic acids or peptides.