Biomedical Engineering Reference
In-Depth Information
Meanwhile, the excitation probability is proportional to the square of the laser
flux, with virtually no excitation outside the laser focal volume. This enables
precise spatial imaging in the biomedical field [67]. In this sense, silk scaffolds
functionalized by two-photon materials will certainly have an unbeatable advantage
in TPF bio-imaging.
Liu and his group reported a general approach to functionalizing silk fibers with
TPF materials (Figure 7.4) [68]. The molecular recognition between two-photon
molecules and silk fibroin molecules was explored, which gives rise to (i) enhanced
TPF of some annealed TPF molecules once they interact with some specific groups
of silk proteins (so-called molecular switch) and (ii) the functionalization of silks.
In this concern, organic molecules with strong TPA and TPF were adopted. The
TPF chromophores were then homogeneously distributed in silk fibers without
jeopardizing the mechanical properties of the original silk. By controlling the self-
assembly capabilities and structure-dependent hydrophobicity, a greener method
of producing colored silk through the biological incorporation of dyes into silk
fibroin was obtained.
TPF properties are mainly dependent on TPA cross section (
σ
) and fluorescence
quantum yield (
η
) [62]. Much effort has been devoted to enhancing these two values
in solution, especially for TPF organic molecules [69, 70]. The enhancement of the
σ
value for TPF organic molecules has been quite successful by means of applying
particular molecular design strategies, such as the attachment of electron-rich
(
-acceptor) components to the molecules
[71, 72], altering the extent of conjugation path [73-76], or increasing coplanarity
[77, 78]. In the solid state,
π
-donor) and/or electron-demanding (
π
is often reduced by the TPF quenching effect because
of the aggregation of fluorescence molecules. In silk fibers, the TPF molecules
are in the solid state, so the TPF quenching effect can happen [77, 78]. Most TPF
materials, such as planar polar molecules, exhibit low
η
values in solution, which
generally decrease further in the solid state [64, 68]. Therefore, there is an urgent
need to identify a novel approach to fabricating TPF materials with enhanced
fluorescence efficiency (
η
). For example, TPF organic molecules with a rigid and
planar structure have a strong tendency to form aggregates by molecular stacking.
Such stacking is commonly observed in
η
π
-conjugated compounds, and is believed
to be the origin of the reduced fluorescence in the solid state [79-84]. To reduce
fluorescence quenching in the solid state, various approaches have been developed.
These include designing a molecule with a distorted structure [85], attaching steric
hindrance groups [86, 87], and increasing the branches/dimensionality of the
molecules [88-91]. Blending with conventional polymers, for example, has also
been used. For example, the fluorescence quantum efficiency of the fluorophore
in poly(methyl methacrylate) (PMMA) film is 0.182, while in cyclohexane gel it is
0.108, due to the ''rigidochromism'' as a result of restricted intramolecular twisting
motion [85]. In the research work, the aggregation was inhibited by identifying
specific substrate functional groups capable of recognizing, interacting with, and
hence separating the TPF molecules (Figure 7.6a). This action would not only
enhance the fluorescence of the molecules, but also functionalize the silk with TPF
property.
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