Chemistry Reference
In-Depth Information
as well. These opposite phenomena may suggest the existence of dye aggregates
within the DDSN. In the previous section, it was stated that the silica nanomatrix
can prevent aggregation of dye molecules. However, if the dye concentration is too
high, aggregation will occur. The aggregation can be identified by the shift of the
emission spectrum. Usually in the bulk materials, the spectrum maxima shift could
be an evidence of the presence of dye aggregates, which may cause quantum yield
decrease. Without aggregation, the doped dye molecules have similar absorbance
and fluorescence emission spectra as their free manners. However, slight shift of the
spectrum maxima were observed in some DDSNs [
8
,
27
,
28
], which could be
considered as the characters of the dye aggregations [
69
]. The research on quantum
yield and lifetime on DDSNs is a relatively new area. More fundamental studies are
needed to provide clear understanding about the effect of silica nanomatrix on the
quantum yield and lifetime of fluorophores in the DDSNs.
3.3 Photostability of DDSNs
The high photostability and acute fluorescence intensity are two major features of
DDSNs compared to dye molecules in a bulk solution. The early DDSN studies
have focused on these two properties [
8
,
13
]. For example, Santra et al. studied the
photostability of the Ru(bpy)
3
2+
doped silica nanoparticles. In aqueous suspensions,
the Ru(bpy)
3
2+
doped silica nanoparticles exhibited a very good photostability.
Irradiated by a 150 W Xenon lamp for an hour, there was no noticeable decrease in
the fluorescence intensity of suspended Ru(bpy)
3
2+
doped silica nanoparticles,
while obvious photobleaching was observed for the pure Ru(bpy)
3
2+
and R6G
molecules. To eliminate the effect from Brownian motion, the authors doped both
pure Ru(bpy)
3
2+
and Ru(bpy)
3
2+
-doped silica nanoparticles into poly(methyl meth-
acrylate). Under such conditions, both the pure Ru(bpy)
3
2+
and Ru(bpy)
3
2+
doped
silica nanoparticles were bleached. However, the photobleaching of pure Ru(bpy)
3
2+
was more severe than that of the Ru(bpy)
3
2+
doped silica nanoparticles.
The high photostability of the nanoparticle originates from protection provided
by the silica matrix for the embedded fluorophores. Environmental oxygen is a
universal quencher for fluorophores in aqueous solution and the network structure
of the silica matrix reduces diffusion of environmental oxygen to the fluorophores.
Thus, the photostability of embedded fluorophores is greatly improved [
3
].
3.4 Fluorescence Enhancement
High photostability and intense fluorescence signals are always criteria for making
better DDSNs. To obtain a better DDSN, various nanostructures for fluorescence
enhancements have been developed. The coexistence of a noble metal nanostructure
with fluorophores can enhance both the fluorescence intensity and photostability
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