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Graphene is a two-dimensional monoatomic thick carbon
allotrope and is also a building block of graphite. It was first
discovered by Andre Geim and Konstantin Novoselov in 2004 when graphite was peeled
using adhesive to isolate a single layer. 1 Today there a few
methods that can be used to synthesise graphene: chemical vapour deposition
(CVD), mechanical exfoliation, epitaxial growth of graphene and electrochemical
exfoliation are the main methods amongst others. It is a material that is
currently being studied extensively due to the wide range of exceptional
properties it possesses such as its ability to transport charge and its
thermal, optical and mechanical properties. However, despite all of the research
that has gone into it, there are currently very few practical applications of
graphene to this day. This article will discuss how the properties of graphene
makes it a suitable material in some of its potential uses across a wide range
of fields including medicine, electronics and energy storage.

 

Graphene has a honeycomb structure which is the same
hexagonal structure as some other carbon allotropes including graphite and
carbon nanotubes. Each atom in graphene forms 3 sigma bonds, one to each of its
nearest neighbouring carbons (which are approximately 1.42Å apart) and one
? bond that is oriented out of the plane. It is very stable because of its
tightly packed carbon atoms and an sp2 hybridisation orbital formed
from the s, px and py orbitals combining. The pz
orbital makes up the ? bond. Graphene is the strongest material to be tested
with a Young’s modulus of 1 TPa, intrinsic strength of 130 GPa 18
and a critical stress intensity factor of 4 MPa. 16 In context,
graphene has a breaking strength 200 times greater than that of steel. One of
the electronic characteristics it possesses is a high electron mobility in
excess of even 200,000 cm2V-1s-1. 17
It also has a low resistivity and is known as a zero-gap semiconductor because
its conduction and valence bands are degenerated at the Dirac points which are
found in two equivalent bands labelled K and K’. 1 Another electronic property is that graphene
is estimated to operate at terahertz frequencies. The key optical property that
graphene has is an unexpectedly high opacity for a structure which is only monoatomic
thick, absorbing around 2.3% of light. When excited over the visible to
near-infrared region, graphene is saturated because of its universal optical
absorption characteristic. In addition to all of this graphene also has a very
high thermal conductivity, making it the best conducting material known today.
Good thermal conductors also tend to conduct electricity well and this holds
true for graphene which offers very little resistance to electrons due to its
flat hexagonal structure and the low defect density of its crystal lattice.
This makes it better than excellent conductors such as copper and more
practical than superconductors as it does not have to be cooled to low
temperatures for it to work. The electrons in graphene can be said to have the
longest mean free path of any other material which emphasises its very low electrical
resistance.

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A
large potential use of graphene is within the field of medicine. Due to
graphene’s versatile properties and its ability to interact with DNA, enzymes,
proteins and peptides it has emerged as a new viable option to be used in
biomedical applications. 2. Examples include; bioimaging where
graphene’s optical properties make it a suitable material, gene and drug
delivery where its large surface and availability of free ? electrons makes it
a good fit 3-4 and in tissue engineering where its exceptional
mechanical strength, stiffness and electrical conductivity can help it fulfil
its role. 5-6 Tissue engineering involves the reproduction and
regeneration of damaged tissues and organs. 7-8 In order for
engineered tissues to work they require effective organization of cellular,
morphological and physiological 9 features meaning that any
substitute must be able to fill these key roles as well as guide cell growth
and modulation, deliver bioactive molecules and stimulate mechanical properties
of the native tissue. 9 However, different tissues in the body
have different mechanical, physical and electrical properties 9
which single materials may not be able to mimic. Hence making hybrid materials
containing multiple components can meet the requirements of the native tissue.
Graphene’s mechanical and electrical properties have motivated researchers 10
to combine it with a variety of bioactive materials to make materials with the
desired characteristics. One particular area where graphene could potentially
have a large impact is in cardiac tissue engineering and regeneration.
Currently cardiac tissue injuries are amongst one of greatest causes of death
around the world with the main strategy for management of such injuries being
heart transplants. However, the patient population is far larger than the
number of donors thus making this method very ineffective. Other approaches have
been made to attempt to restore the characteristics of the damaged tissue 9
such as injecting stem cells, growth factors, peptides and biomaterials into
the myocardium 11 but there has been limited success due to the
inability of engineered tissues re-establishing the functions and structure of
the native tissue across different sizes. 12 Due to cardiac tissue
possessing contractile properties and cardiac muscles being electrically
conductive, engineered tissues must be able to mimic the anisotropic structure
of the myocardium. These are then doped with electrically conductive
carbon-based nanomaterials 13 which are used as scaffolds for
cardiac tissue engineering. 9 Results have shown that there have
improvements in the functioning of cells which directly correlates to the
improved material stiffness and conductivity from the nanoparticles. This is
then incorporated into the matrix of injectable hydrogels in order to ensure
the retention of the implanted nanocarriers at the site of the dynamic
myocardium environment. 14 Graphene based materials can be added
into hydrogel scaffolds in order to further improve their mechanical properties
and electrical conductivity. 15 Graphene-based materials could
have a large impact in engineering functional cardiac tissues because it can
improve the mechanical and surface based properties of the materials in many
ways. Add to this the fact that nanoparticles also increase the electrical
conductivity of hydrogels which are characteristically insulating, which boosts
cell–cell signalling and develops signal propagation which is key in cardiac
tissue engineering. 9 However, graphene-based nanomaterials can
have cytotoxic and genotoxic effects and there is also the issue of the
biocompatibility of graphene with native tissue of living systems which cannot
be easily overcome. Surface modification using protein is currently being
looked into but further testing and investigation will be needed before being
brought into clinics.

 

Another area in which graphene can potentially have an
astounding impact in, is within the field of electronics. Due to its unique
electronic transport properties such as excellent carrier mobility and a high
Fermi velocity along with its mechanical strength, thinness, flexibility and good
thermal conductivity, 19 graphene has emerged as a viable long-term
option for electronics in future. One particular area is in the transparent
conducting electrodes (TCEs). These are films which display high electrical
conductivity and good optical transparency. Examples of its uses are in
touchscreens, organic and inorganic photovoltaic cells and liquid crystal
displays (LCDs). Currently indium tin oxide (ITO) is the main material used for
TCEs, however it does have a few disadvantages including the fact that indium
is not very abundant. It is also not very chemically resistant to acids or
bases, is brittle when used above flexible substrates and has weak transparency
in near-infrared. 20 Graphene’s high carrier mobility, large
surface area as well as its stability when in contact with water and oxygen
make it an ideal successor to indium tin oxide. 21 Using graphene
in touch screens would be ideal as it has a similar work function to ITO
(4.5eV) 22 but it is also capable of absorbing a much larger spectrum
of light ranging from ultraviolet to terahertz meaning that it would pick up a
much broader spectral detection so much more vivid images can be seen.
Moreover, graphene has a much higher carrier mobility which would lead to
faster response times. Another way graphene can be used as a TCE is in dye
sensitised solar cells (DSSC). Using the CVD approach, single/few layered
graphene films can be synthesised to be used in organic solar cells. When tested,
a transparency of 70% was seen over 1000 to 3000nm sheets as well as good
electrical conductivity of 550 Scm-1. 23 However, it
did also have a very low power conversion efficiency of 0.26% which was
accounted for by the poor quality of the fabricated graphene sheet which shows
that there are still some challenges in this field which must be overcome
before commercial usage. 24-25 Another potential electronic
application of graphene is in transistors. Transistors are semiconductor
devices which are used as electronic switches within a circuit. They act as
logic gates to enable microprocessors to solve complex logical problems. Silicon
is currently the main material used for transistors, however, the speed of
computer microprocessors which use them has not seen much progress in recent
times, remaining steady at around 3-4GHz. Research has shown that the speeds of
these computers could reach up to a staggering 1000 times faster in the
terahertz range were graphene to be used instead. 26 Due to the
lower resistance in graphene than silicon, electrons would be able to pass
through at a much faster rate enabling microprocessors to work at a much faster
rate. It would also mean that devices could be made smaller or contain more
functions as graphene’s 2D structure would mean that transistors would be much
smaller.

Energy storage is a field in which graphene could also hold great
potential. One area in particular is supercapacitors which are expected to have
a huge impact on mobile technologies in the coming years. Due to their speedy
charging/discharging rates, high power density and long lifecycles 27
they can act as very efficient energy storage units which is required to make smaller,
lightweight and cheaper devices. 23 In order to achieve those
specifications a material with a large surface area and pore size as well as
high electrical conductivity is needed. As mentioned above, graphene’s
mechanical, physical and chemical properties make it a good fit to be used in
these supercapacitors and this is backed up theoretical calculations which
state that graphene’s specific capacitance is close to 550 F/g. 28-29 Currently,
activated carbon is the main material used in supercapacitors which only has a
capacitance of 275 F/g 30 showing that graphene would be a far
better material to use because of its greater capacitance. However, in practice
the value would be much lower than 550 F/g because of agglomeration which takes
place in the preparation and application stages. 23 Graphene could
also be used in batteries to improve their lifetime as well as decreasing charging
times. One way it can do this is by replacing graphite in the anode of Li-ion
batteries with graphene. In order to meet the demand for high energy density
and durable systems, graphene’s greater chemical stability, surface area and
conductivity 31 when compared to graphite should lead to the desired
improvements in batteries with an increase in capacity from 500-1100 mAhg-1
expected. 32 Add to this the fact that graphene’s strength and
flexibility would improve the integrity of the anode as well as improving
performance during the charge/discharge process. However, using graphene in
Li-ion batteries has also encountered problems such as low coulombic efficiency
and while research has gone into using it as a composite material with SnO2
in order to improve this, it is yet to yield a finished working product.

While
all of the previous potential applications of graphene mentioned would lead to
improvements of existing products, one new potential market which graphene could
revolutionise is wearable devices. Currently seen as a niche market, this area
has recently gained a lot of attention because of the advantages they hold over
traditional devices such as being able to act as sensors to collect and deliver
data from the body to the device via bio-signals. 33 In order for
these devices to fulfil their functions they must be flexible and stretchable in
order to be worn on the body/clothes and keep up with the movements of the
body. 34-35 This is where the mechanical specifications of
graphene and its various material properties such as its high optical transmittance
(Fig. 1) makes it the perfect material to use for various wearable devices. One
example where graphene based wearable devices can be used is in healthcare and
bio-electronic applications. Graphene’s structure means that each of its atoms
is in contact with the environment making it very sensitive to even the
smallest of changes at the molecular level. In this case, graphene sensors can
be used to monitor and measure specific functions such as body temperature,
heart beat and amount of glucose in the body among others. 36-37 These
can be merged with measurements taken by conventional medical devices such as
pedometers and blood oxygen measurement. 35 The wearable device
would be able to carry out these functions by printing graphene onto
water-soluble silk (Fig. 2) which would allow it to be in direct contact with
biological tissues. Due to the thinness of the device, it can stick to the soft
and moist biological substrate very well which would allow it to give a
real-time monitoring of bacterial contamination.

 

The applications explained above are only a few of
areas where graphene could be used. Some other potential uses of graphene are as
composite materials for example mixing graphene in paint forms a coating which
can prevent rusting from occurring. It can also be used in water filtration,
with research suggesting that it could be better at desalinating than current techniques.
Graphene’s flexibility would also mean it would act as a better lubricant than
graphite which is the traditionally used material, while its light weight
allows provides a good frequency response meaning it can be used in
electrostatic audio speakers and microphones.

In conclusion, the potential uses mentioned above are only a few of the
many applications of graphene which shows how useful a material it is and one
that could transform the way in which many fields currently function. However,
as of today there are still obstacles that must be overcome in order to use
graphene within the all the potential applications it has. Personally, I
believe the fact that there has been so few practical applications of graphene
in the world today, despite it being first synthesised over 10 years ago and
having had billions spent on it in research shows the extent of the difficulty faced
to commercialise the material. However, a simpler application of graphene which
is active today is in tennis rackets made by Head. These have largely received
positive feedback, as it has made the racket lighter whilst increasing the
mechanical strength which shows that there is reason to be optimistic. If only purely
just for the sheer number of useful properties which graphene possesses, I do believe
that it is only a matter of time before researchers make breakthroughs in the
various fields graphene could potentially be used in and we begin to see very
futuristic products from graphene nanofibers to graphene-based aeroplanes being
manufactured.

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