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W. Qiu and X.-Y. Liu
be followed by the formation of SF nanofibrils/β-crystallite networks, which consequently drives the increase in storage modulus G
and the turbidity of SF solutions.
In other words, the occurrence of nanofibrils/β-crystallite networks will lead to the
onset of G
at t g , which corresponds to the dynamic induction time of the nucleation
of β-crystallite or nanofibril/β-crystallite networks, or to the onset of turbidity at t
g ,
which corresponds to the static induction time of the nucleation of β-crystallite and
nanofibril/β-crystallite networks. We have then e J ~ 1/t g ~ 1/t
g .
6.5.3.2 Experimental Verification of Nucleation Kinetics and Impact
of Seeds
Taking β-sheets as a typical example of secondary structure, during CD measurements, the β-sheet nucleation rate can be measured by recording the temporal evolution of the 217 nm peak value (which is indicative of β-sheets) of either pure or
composite RSF solutions. Afterwards, the value–time curve can be plotted together
for further qualitative comparison. Given the fact that conformational transition from
the random coil/α-helix takes time, in a typical curve, the content of the β-sheet will
initially remain nearly constant and then increase sharply. This deflection point is
denoted as nucleation induction time τ of the β-sheet.
To measure the increments of storage modulus G
and the turbidity of SF solutions, the characterization techniques of rheometry and spectrophotometry were
applied, respectively. Figure 6.24 displays two typical examples that record the
temporal evolution of the storage modulus and optical density (which correlates
to the turbidity) of composite RSF solutions that are incorporated with different
ratios of foreign templates (i.e., PS NPs). The shorter nucleation induction time is
highly consistent with the accelerated nucleation kinetics.
Based on Eq. (6.3), under the same experimental conditions, one should have a
linear relationship between J (~1/t g ) and the density of nucleation templators/seed, if
the templators/seeds can promote nucleation. Given the relatively large dimensions
of SF nanofibrils (20–50 nm) and nanofibril networks, it is possible to investigate SF
nucleation kinetics by directly monitoring the SF molecule accumulation process via
advanced microscopy techniques such as SEM. In addition, by labeling SF molecules
with specific fluorescent dyes, confocal laser scanning microscopy can also be used
to record the formation rates of nanofibrils. For instance, Chen et al. studied the
promotion effect of foreign nanoparticles (PS NPs) on SF nanofibril formation, and
their results clearly showed that green florescence-labeled SF molecules accumulated
around the surface of PS NPs. In addition, the SEM images of the nanoparticles
incubated in SF solutions also clearly displayed the accumulation of SF nanofibrils
on their surface; with a longer induction time, the number of SF nanofibrils increased.
In a similar manner to PS NPs, it has been reported that CNTs are also capable
of accelerating the gelation process of SF solution, which has been confirmed by
their reduced gelation time (or nucleation induction period). Figure 6.25a clearly
shows that the gelation time decreased with increasing CNT content. Nevertheless,
the influence of CNTs on the content of secondary and tertiary structures within SF
W. Qiu and X.-Y. Liu
be followed by the formation of SF nanofibrils/β-crystallite networks, which consequently drives the increase in storage modulus G
and the turbidity of SF solutions.
In other words, the occurrence of nanofibrils/β-crystallite networks will lead to the
onset of G
at t g , which corresponds to the dynamic induction time of the nucleation
of β-crystallite or nanofibril/β-crystallite networks, or to the onset of turbidity at t
g ,
which corresponds to the static induction time of the nucleation of β-crystallite and
nanofibril/β-crystallite networks. We have then e J ~ 1/t g ~ 1/t
g .
6.5.3.2 Experimental Verification of Nucleation Kinetics and Impact
of Seeds
Taking β-sheets as a typical example of secondary structure, during CD measurements, the β-sheet nucleation rate can be measured by recording the temporal evolution of the 217 nm peak value (which is indicative of β-sheets) of either pure or
composite RSF solutions. Afterwards, the value–time curve can be plotted together
for further qualitative comparison. Given the fact that conformational transition from
the random coil/α-helix takes time, in a typical curve, the content of the β-sheet will
initially remain nearly constant and then increase sharply. This deflection point is
denoted as nucleation induction time τ of the β-sheet.
To measure the increments of storage modulus G
and the turbidity of SF solutions, the characterization techniques of rheometry and spectrophotometry were
applied, respectively. Figure 6.24 displays two typical examples that record the
temporal evolution of the storage modulus and optical density (which correlates
to the turbidity) of composite RSF solutions that are incorporated with different
ratios of foreign templates (i.e., PS NPs). The shorter nucleation induction time is
highly consistent with the accelerated nucleation kinetics.
Based on Eq. (6.3), under the same experimental conditions, one should have a
linear relationship between J (~1/t g ) and the density of nucleation templators/seed, if
the templators/seeds can promote nucleation. Given the relatively large dimensions
of SF nanofibrils (20–50 nm) and nanofibril networks, it is possible to investigate SF
nucleation kinetics by directly monitoring the SF molecule accumulation process via
advanced microscopy techniques such as SEM. In addition, by labeling SF molecules
with specific fluorescent dyes, confocal laser scanning microscopy can also be used
to record the formation rates of nanofibrils. For instance, Chen et al. studied the
promotion effect of foreign nanoparticles (PS NPs) on SF nanofibril formation, and
their results clearly showed that green florescence-labeled SF molecules accumulated
around the surface of PS NPs. In addition, the SEM images of the nanoparticles
incubated in SF solutions also clearly displayed the accumulation of SF nanofibrils
on their surface; with a longer induction time, the number of SF nanofibrils increased.
In a similar manner to PS NPs, it has been reported that CNTs are also capable
of accelerating the gelation process of SF solution, which has been confirmed by
their reduced gelation time (or nucleation induction period). Figure 6.25a clearly
shows that the gelation time decreased with increasing CNT content. Nevertheless,
the influence of CNTs on the content of secondary and tertiary structures within SF