6 Cocoon Silk: From Mesoscopic Materials Design …
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which are coated with numerous functional carboxyl groups on the surface, can serve
as ideal foreign bodies that provide nucleation sites. At these nucleation sites, the
interactions between such nanoparticles and SF molecules mainly include the formation of H-bonds between the amino groups of SF peptide chains and carboxyl groups
on the nanoparticle surfaces [19]. This strong interaction can reduce the nucleation
barrier and accelerate the nucleation of β-sheets as well as β-crystallites.
It is worth mentioning that the template effect of the foreign body is not rare. For
instance, graphene is also reported to be able to trigger SF heterogeneous crystallization [64]. By precisely controlling experimental conditions, the almost complete
coverage of graphene nanosheets can be achieved by layers of densely packed SF
nanofibrils [64]. In contrast, very few (~1%) SF nanofibrils are observed outside
of graphene nanosheets, suggesting that nanofibril formation (heterogeneous nucleation) occurs in a highly selective manner, only on graphene nanosheets [64]. This
heterogeneous nucleation of SF molecules on foreign bodies might shed new light
on the synthesis of composite silk materials.
We notice that this nucleation-controlled SF network formation model allows
for the interpretation of many novel effects observed in SF materials. Owing to
the nature of nanocrystallites, the formation of nanofibrils (crystal networks) is
controlled by inter-molecular nucleation, which allows for the reconstruction and
meso-functionalization of SF materials.
6.5.3 Experiments on SF Nucleation
6.5.3.1 Nucleation Kinetics
As discussed, the pathway through which SF molecules self-assemble into higher
levels of structures is controlled by a nucleation mechanism. According to the nucleation theories of soft material formation, nucleation can be divided into two types:
homogeneous and heterogeneous, which determine nucleation rate J by Eq. 6.8:
J = Aexp
−G
∗ f /kT
× N
0
(6.8)
where A and B are kinetic parameters, f is the interaction parameter between the
nucleating phase and templates (0 < f ≤ 1, in the case of homogeneous nucleation,
f = 1). G
∗ is the nucleation barrier, and μ is the chemical potential difference
between the mother and crystalline phases.
In the process of nucleation, the addition of appropriate nucleation templates/seeds
to the SF solution lowers the nucleation barrier to promote the nucleation rate. Equation (6.8) indicates that under the same experimental conditions, the nucleation rate
J is directly proportional to the density of the added nucleation seeds N°.
In principle, it is difficult to measure the nucleation rate directly. For this reason,
nucleation rates are always compared by measuring nucleation induction time τ. The
basis of this alternative method lies in the fact that the occurrence of β-crystallites will
285
which are coated with numerous functional carboxyl groups on the surface, can serve
as ideal foreign bodies that provide nucleation sites. At these nucleation sites, the
interactions between such nanoparticles and SF molecules mainly include the formation of H-bonds between the amino groups of SF peptide chains and carboxyl groups
on the nanoparticle surfaces [19]. This strong interaction can reduce the nucleation
barrier and accelerate the nucleation of β-sheets as well as β-crystallites.
It is worth mentioning that the template effect of the foreign body is not rare. For
instance, graphene is also reported to be able to trigger SF heterogeneous crystallization [64]. By precisely controlling experimental conditions, the almost complete
coverage of graphene nanosheets can be achieved by layers of densely packed SF
nanofibrils [64]. In contrast, very few (~1%) SF nanofibrils are observed outside
of graphene nanosheets, suggesting that nanofibril formation (heterogeneous nucleation) occurs in a highly selective manner, only on graphene nanosheets [64]. This
heterogeneous nucleation of SF molecules on foreign bodies might shed new light
on the synthesis of composite silk materials.
We notice that this nucleation-controlled SF network formation model allows
for the interpretation of many novel effects observed in SF materials. Owing to
the nature of nanocrystallites, the formation of nanofibrils (crystal networks) is
controlled by inter-molecular nucleation, which allows for the reconstruction and
meso-functionalization of SF materials.
6.5.3 Experiments on SF Nucleation
6.5.3.1 Nucleation Kinetics
As discussed, the pathway through which SF molecules self-assemble into higher
levels of structures is controlled by a nucleation mechanism. According to the nucleation theories of soft material formation, nucleation can be divided into two types:
homogeneous and heterogeneous, which determine nucleation rate J by Eq. 6.8:
J = Aexp
−G
∗ f /kT
× N
0
(6.8)
where A and B are kinetic parameters, f is the interaction parameter between the
nucleating phase and templates (0 < f ≤ 1, in the case of homogeneous nucleation,
f = 1). G
∗ is the nucleation barrier, and μ is the chemical potential difference
between the mother and crystalline phases.
In the process of nucleation, the addition of appropriate nucleation templates/seeds
to the SF solution lowers the nucleation barrier to promote the nucleation rate. Equation (6.8) indicates that under the same experimental conditions, the nucleation rate
J is directly proportional to the density of the added nucleation seeds N°.
In principle, it is difficult to measure the nucleation rate directly. For this reason,
nucleation rates are always compared by measuring nucleation induction time τ. The
basis of this alternative method lies in the fact that the occurrence of β-crystallites will