Silica-Based Nanoparticles for Intracellular Drug Delivery - Intracellular Delivery: Fundamentals and Applications - page 344

the fluorophore out of the nanoparticle. Nevertheless, it is worth mentioning an emulsion
process described by Barbé et al. for encapsulation and release of drugs (Barbé et al.
2004 ), that was recently applied to silica microcapsule-based protein delivery (Finnie
et al. 2006 ). However, neither the Stöber method nor the reverse microemulsion
process previously described is able to efficiently incorporate hydrophobic molecules
into the silica matrix. To solve this problem, organically-modified silica nanoparticles
can be used (Bharali et al. 2005 ). Hydrophobic drugs are first dissolved in the organic
phase of the reverse microemulsion, that allows to partition more effectively into the
aqueous water droplets and are more efficiently trapped inside the silica nanoparticles
by hydrolysis of the silane precursor. Some recent experimental results have demon-
strated that photosensitizers retain their functions of generating singlet oxygen after
being embedded inside silica matrix (Kim et al. 2007 ) (Fig. 4 ). Moreover, on the one
hand, the amount of singlet oxygen generated by the nanoparticle-doped photosensi-
tizer is comparable to that produced by the same amount of free photosentisizer,
and on the other hand, the negative effects of photosensitizers - such as toxicity or
hydrophobicity - are reduced when embedded in silica.
Fig. 4 Merged transmission ( blue ) and two-photon excited fluorescence ( red ) images of HeLa
cells, stained with nanoparticles co-encapsulating 1.1 wt.% HPPH/20 wt.% BDSA. Inset:
Localized two-photon fluorescence spectrum from the cytoplasm of the stained cell. The excitation
wavelength is 850 nm. (Reproduced with permission from Kim et al. 2007 . Copyright 2007
American Chemical Society)