252.4.3 Silica Nanoparticles
Silica nanoparticles do not have autofluorescence, but rather act as a matrix to
chemically and physically confine organic dye molecules for fluorescence imaging
[ 16 ]. Dye molecules are encapsulated within a silica shell, protected from external
quenchers. The silica capsule can enhance the photo-stability of the dye and provide
a biocompatible surface for bioconjugation [ 67 ]. Silica as a material is mostly used
for drug delivery [ 68 ] and have been explored as a transplantation vehicle for stem
cells as they provide large surfaces for stem cells to adhere, grow and mature [ 66 ].
Thus far there have been no experiments of imaging silica nanoparticles in vivo.
However, reports exist of human mesenchymal stem cells internalizing silica parti-
cles conjugated with fluorescein isothiocyanate [ 69 ] and cyanine [ 70 ] in vitro. In
these experiments silica nanoparticles did not affect the viability, proliferation, and
surface marker expression or differentiation capabilities of the stem cells. Silica
nanoparticles can be useful as ultrasound contrast agents [ 71 ]. They have also been
combined with other nanomaterials to enhance biocompatibility and cellular uptake;
examples include silica-coated gold nanoparticles [ 72 ], magnetic nanoparticles [ 32 ]
and quantum dots [ 73 ], in which the silica shell enhances the photoacoustic signals
generated by nanoparticles (Fig. 2.5).
Silica nanoparticles are most suitable for in vivo imaging with fluorescence
imaging.
Fig. 2.4 Quantum dot size
and color. Picture courtesy
of Nanosys Inc., reprinted
with permission
2 Nanotechnology-Based Stem Cell Applications and Imaging