Biomedical Engineering Reference
In-Depth Information
possesses high fluorescence quantum yields, excellent photostability, high
fluorescence detectivity, sustained drug delivery, noncytotoxicity, and good
blood compatibility (He, Shi, et al. 2009; He, Zhang, Chen, et al. 2010). These
results are encouraging from the perspective of moving the material plat-
form into clinical trials.
During the past decade, semiconductor nanocrystals, known as quantum
dots (QDs), have gained considerable attention as labels in biological imag-
ing and detections due to their outstanding optical properties, such as high
quantum yield, excellent photostability, size-dependent tunable fluorescence
properties, narrow emission bandwidths, and broad excitation spectra (Peng
et al. 1997; Bruchez et al. 1998; Chan and Nie 1998; Mamedova et al. 2001;
Michalet et al. 2005; Selvan et al. 2005). Therefore, QDs-based luminescent
mesoporous silica nanoparticles have been intensively investigated by many
research groups (Hu et al. 2009; Song et al. 2009; Pan et al. 2011). The most
common strategy is to design QDs-based luminescent mesoporous silica
nanoparticles with QDs as cores and mesoporous silica as shells. Hu et al.
(2009) have succeeded in exploiting a facile method to prepare mesoporous
silica-coated QDs with multicolor luminescence. In the first step, when QDs
in chloroform are added into the CTAB aqueous solution, the CTAB mol-
ecules stabilize the oil droplets leading to formation of an oil-in-water micro-
emulsion. Subsequent evaporation of the volatile organic solvent by heating
drives CTAB molecules to directly interact with the QD surface ligands
through hydrophobic interactions. The alkyl chains of the CTAB and the
QD surface ligands intercalate into each other, rendering the CTAB cationic
headgroup (quaternary amine) facing outward and the QD-CTAB complex
are water soluble. In the second step, when silane compounds polymerize to
form silica shells on QD surface, the CTAB acts as templates for mesopore
formation. Here, the cationic surfactant molecule, CTAB, plays two impor-
tant roles in this process: solubilization of hydrophobic QDs into aqueous
solutions and templating the mesopore formation (Hu et al. 2009). Therefore,
the core-shell luminescent mesoporous silica nanoparticles as drug carriers
would open the opportunities in traceable delivery and controlled release of
therapeutic agents.
Another excellent luminescent material is rare-earth (RE)-based phos-
phors, which exhibit intense narrow-band intra-4f luminescence in a wide
range of host materials. Furthermore, compared to semiconductor nanocrys-
tals and organic dye molecules, RE-doped nanophosphors have advantages
of large Stokes shift, sharp emission spectra, long lifetime, high chemical/
photochemical stability, low toxicity, and reduced photobleaching, which
can be considered to be a promising luminescent material for biological
applications (Nichkova et al. 2005; Goldys et al. 2006). Therefore, luminescent
mesoporous silica nanoparticles with RE-doped nanophosphors have been
intensively exploited for drug delivery, including embedded structure and a
core-shell structure. The embedded structure is to deposit phosphor nano-
crystals into the channels of mesoporous silica particles, such as MCM-41,
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