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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 a
biosensor 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
paper is no longer a novelty”, says Mezzenga, “it is the combination
with amyloid fibrils which is central to this new class of hybrid
materials”.
Source: ETH Zurich
