Biomedical Engineering Reference
In-Depth Information
7.4.3.2
Two-Photon Fluorescence Quantum Yield of Designed Organic Molecules
The TPF silk fibers were prepared based on ''molecular recognition.'' Molecular
modeling revealed that the
8NF
molecules were arranged in a manner similar
to that in a J-aggregate, and
molecules formed a typical H-aggregation of
a ''herringbone'' type [92].
Bombyx mori
silk is an amphiphilic block copolymer,
and the molecular mass of the dominant high-molecular-weight protein is about
390 kDa [1, 3, 40, 41]. The dominating hydrophobic peptide repeating units self-
organize into
8MF
-sheet structures, mediated by hydrophobic hydration [34, 36, 93].
Results showed that the attachment of
β
in the fluorescent silk fibers was
attributed to the molecular recognition and stable interactions between
4NF
and
8NF
4NF
/
8NF
and silk fibroin molecules, which prevent the aggregation of
molecules.
To examine this mechanism,
8MF
wasusedasacontrol.
8MF
is identical to
8NF
except that the terminal group -NO
2
in
8NF
is replaced by -CH
3
in
8MF
.This
change jeopardizes the hydrogen bonding between
8MF
molecules and silk fibroin
molecules (via the amide groups). Consequently, even after incorporation into silk
fibers, the aggregation of
8MF
molecules still occurs.
The quantum yields of all the solid materials are measured by a calibrated
integrating sphere (Model number: ISF-513). The quantum yield of
4NF
/
8NF
4NF
rises
from 1% in the powder form to 22% in the silk fibers.
molecules in the
powder form exhibit a strong molecular stacking/dipole-dipole interaction. In the
4NF
4NF
functionalized fibers, the binding of
4NF
molecules to silk fibrils decouples
and separates the associated
molecules. This decoupling eliminates the
possibility of aggregation and therefore enhances the fluorescence emission from
the functionalized silk fibers. Similarly,
4NF
in the silk fibers exhibits bright
yellow emission with an elevated fluorescence efficiency of about 32%, which
is significantly higher than that of
8NF
8NF
in the solid state (6%). In contrast, the
fluorescence efficiency of
in silk fibers (15%) remains almost unchanged from
that in the solid form (12%). This observation confirms that the TPF emission
enhancement is due to the presence of the -NO
2
terminal groups of
8MF
8NF
molecules, which can interact effectively with the amide groups of the silk fibroins.
Two-photon action cross-section (
4NF
and
ση
) is usually used to evaluate the performance
of TPF materials. Two-photon action cross-sections of
4NF
and
8NF
in the solid
form (
ση
solid
) are relatively low, namely 9 GM (905
×
0.01) and 72 GM (1199
×
0.06),
respectively. On the other hand, the
ση
fiber
(the two-photon action cross-section of
a molecule in silk fibers) of
4NF
and
8NF
silk fibers are enhanced greatly to 199
GM (905
×
0.22) and 384 GM (1199
×
0.32), respectively. Obviously, the
4NF
and
8NF
silk fibers exhibit 22 times and 7 times the two-photon action cross-sections
respectively, compared with their counterparts in the solid form.
To check the impact of silk fibroin on the fluorescence emission of the fluorescent
molecules, the one-photon fluorescence spectra of the powder of
,and
of their functionalized silks were acquired under the same conditions. The results
showed that
4NF
,
8NF
fibers had strong fluorescent emission, while the emission from
the
4NF
powder was hardly detectable (Figure 7.6b). The fluorescence plots of the
8NF
solution,
8NF
fibers, and the powders exhibit a monotonic bathochromic shift
in the order of
8NF
solution
<
8NF
fibers
<
8NF
powder. The emission spectrum
4NF
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