Fluorescent silicon nanocrystals for bioimaging
Quantum dots have been used as alternatives to organic dyes for fluorescence imaging because they are resistant to photobleaching, exhibit strong response to two-photon excitation, and can be conjugated to a wide variety of targeting molecules. Silicon (Si) nanocrystal quantum dots in particular exhibit bright, size-dependent emission with visible to near infrared wavelengths and are biocompatible, making them potentially interesting for in vitro and in vivo bioimaging. Here, Si nanocrystals are studied for imaging applications. The stability of Si nanocrystal dispersibility and photoluminescence (PL) in aqueous solutions was studied. Hydrophobic Si nanocrystals were dispersed with surfactants to produce colloidally stable and brightly fluorescent dispersions, with PL quantum yields in the range of 3.2% - 6.6%. Hydrophilic Si nanocrystals capped with a ligand containing a terminal carboxylic acid group could be directly dispersed in aqueous environments with quantum yields of up to 9.1% in water. The nanocrystal PL was stable in water for at least one week, however there was a significant loss of PL when the particles were dispersed in biological solutions. The drop in PL was accompanied by surface oxidation and degradation of the nanocrystals. Si nanocrystals incubated with mouse macrophage cells were actively taken up by endocytosis. Cell viability assays indicated that the nanocrystals were not toxic to the macrophages. The Si nanocrystals were bright enough to be imaged within the cells by one-photon and two-photon microscopy. Hydrophilic Si nanocrystals that emit in the near infrared (900-1000 nm) could also be dispersed directly into water, however the emission quantum yields were prohibitively low for imaging applications. Time gated imaging of cells labeled with Si nanocrystals enabled multiplex imaging using optical probes with spectral overlap by separating the PL of organic dyes with short nanosecond lifetimes and Si nanocrystals with long microsecond lifetimes. Finally, biotin bioconjugation was accomplished to Si nanocrystal surfaces, though the conjugation reaction efficiencies were relatively low.