The final hybrid nanocomposite paper made of
protein fibrils and graphene after vacuum filtration drying. The schematic
route used by the researchers to combine graphene and protein fibrils into the
new hybrid nanocomposite paper. (Reproduced from Li et al. Nature
Nanotechnology 2012)
Researchers led by Raffaele Mezzenga, a
professor in Food and Soft Materials Science, have created a new nanocomposite
made of graphene and protein fibrils: a special paper, which combines the best
features of both components.
The
circular sheets that Raffaele Mezzenga gently lifts from a petri dish are shiny and
black. Looking at this tiny piece of paper, one could hardly imagine that it
consists of a novel nanocomposite material, with some unprecedented and unique
properties, developed in the laboratory of the ETH professor.
This
new "paper" is made of alternating layers of protein and graphene.
The two components can be mixed in varying compositions, brought into solution,
and dried into thin sheets through
a vacuum filter – "similarly as one usually does in the manufacture of
normal paper from cellulose" says Mezzenga. "This combination of
different materials with uncommon properties produces a novel nanocomposite with some
major benefits," says the ETH professor. For example, the material is
entirely biodegradable.
"Graphene
paper" has shape memory features
Graphene
is mechanically strong and electrically conductive, as well as, highly water repellent by
nature. On the other hand, the protein fibrils are biologically active and can
bind water. This allows the new material to absorb water and to change shape
under varying humidity conditions. Furthermore, the "graphene paper"
has shape memory features
such that it can deform when adsorbing water, and recover the original shape
upon drying. This could be used, for example, either in water sensors or
humidity actuators.
But
"the most interesting feature is that we can use this material as abiosensor to precisely measure
the activity of enzymes," says Mezzenga. Enzymes can digest and break down
the protein fibrils. This changes the resistance of the composite, which is a
measurable quantity once the graphene paper is incorporated into an electrical circuit.
"This feature is, for me, the nicest part of the story. Seen from this
angle, we could claim to have discovered a new general method to measure
enzymatic activity”, says the ETH professor.
The
material can also be designed to meet other needs. For example, the higher the
proportion of graphene, the better it conducts electricity. On the other hand,
the more fibrils are present, the more water can be absorbed by this material,
with enhanced deformations in response to humidity changes.
Interestingly,
this new material can be made with relatively simple means.
The
protein, in this case, beta-lactoglobulin, a milk protein, is first denatured
by high temperatures in an acidic solution.
The
end-products of this denaturation process are protein fibrils suspended in
water; these fibrils then act as stabilizers for the hydrophobic graphene
sheets and allow them to be finely dispersed in water and processed into
nanocomposites by a simple filtration technology.
The
concept can be extended
In view
of the widespread tendency of proteins to form fibrils, under specific
conditions, this concept can be extended, in principle to other food proteins,
such as those found in eggs, blood serum and soy. The beta-lactoglobulin fibrils
used in the work lead by Mezzenga are digested specifically by pepsin, an
enzyme present in the stomach to enable the digestion of several food
components.
However,
varying the protein types could provide a new method of targeting a much larger
class of enzymes.
Inspired
by their past research on amyloid fibrils and by the rise of graphene, the ETH
researchers have combined these two building blocks to generate a new class of
versatile and functional materials.
“Nowadays, graphene paperis no longer a novelty”, says
Mezzenga, “it is the combination with amyloid fibrils which is central to this
new class of hybrid materials”.
More
information: Li
C, Adamcik J & Mezzenga R. Biodegradable nanocomposites of amyloid fibrils
and graphene with shape-memory and enzyme sensing properties. Nature
Nanotechnology 2012.doi:10.1038/nnano.2012.62
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