Graphene is a well known and recently discovered material with very interesting properties. This atom-thick material has the best electrical and thermal conductivity. It is flexible and very resilient. It is more resistant than Kevlar. Its sensitivity and its p electron cloud lead to good sensor properties. Therefore, graphene can be transferred on a substrate and made into a reliable and powerful sensor. Depending on the substrate properties, the device can be flexible and thus wearable. Glucose is an important molecule that powers our brain and organs. Monitoring its concentration can help design better and more efficient training for athletes and is crucial to diabetes patients. Today, the most used glucose level sensor is an invasive device that requires a drop of blood. The result is thus at a given time and the test is not comfortable to use. Graphene has been successfully transferred on polyethylene terephthalate and functionalized with glucose oxydase. This device is a good glucose sensor as shown by experiments and comparison to theoretical values. It is flexible and the charge carrier mobility measured enables real-time monitoring. A few new sensor designs also show potentials, such as a graphene-TiO2 based sensor.

1. Using graphene on a flexible substrate as a sensor
Antoine Galand
University of Pennsylvania, Philadelphia, Pennsylvania, USA
Abstract
Graphene is a well known and recently discovered material with very interesting properties.
This atom-thick material has the best electrical and thermal conductivity. It is flexible and very
resilient. It is more resistant than Kevlar. Its sensitivity and its π electron cloud lead to good sensor
properties. Therefore, graphene can be transferred on a substrate and made into a reliable and pow-
erful sensor. Depending on the substrate properties, the device can be flexible and thus wearable.
Glucose is an important molecule that powers our brain and organs. Monitoring its concen-
tration can help design better and more efficient training for athletes and is crucial to diabetes
patients. Today, the most used glucose level sensor is an invasive device that requires a drop of
blood. The result is thus at a given time and the test is not comfortable to use. Graphene has been
successfully transferred on polyethylene terephthalate and functionalized with glucose oxydase.
This device is a good glucose sensor as shown by experiments and comparison to theoretical val-
ues. It is flexible and the charge carrier mobility measured enables real-time monitoring. A few
new sensor designs also show potentials, such as a graphene-TiO2 based sensor.

2. Using graphene on a flexible substrate as a sensor 2
Introduction and motivation
From the famous birds in coal mines to monitor air quality to highly sensitive and powerful
medical equipment, biosensors are used widely and by everyone. They enable us to monitor con-
centrations to prevent toxic atmospheres, environments and conditions. They can also be used as
a simple presence detector like the ELISA, whose purpose is only to verify if a given antigen is
present but not to obtain the concentration of that antigen. This illustrates how large the biosensing
field is and should provide motivation for the study of those technologies. Nanotechnology has
led to innovation by reducing cost and processing time for example. Using only a few particles
but with a higher specific area, some tests can provide instantly more reliable results than with
previous more costly and delayed tests.
The enhanced selectivity and sensitivity enabled by nanotechnology also allow new tests. For
example, some assays required blood to detect a given molecule whereas with more efficient sen-
sors saliva or other fluids might be enough. This is key to design non invasive devices to help
people monitor diseases in real-time and in a less harmful way. I will focus on graphene based
biosensors and show how they can benefit people diagnosed with diabetes by providing them with
an inexpensive and non-invasive test to monitor their glucose level.
Graphene as a biosensor
Graphene is often described as a wonder material with the highest electric conductivity and an
excellent thermal conductivity. Its properties depends on its structure and therefore on the growth
method used to make the devices. This paper focuses on single-layer graphene sheets grown using
chemical vapor deposition (CVD) on copper and then transferred on a substrate- usually silicon.

3. Using graphene on a flexible substrate as a sensor 3
Non-doped single-layer graphene is a zero bandgap semiconductor but the band structure can be
tuned to make field effect transistors (FET). In fact, applying an electric field to a graphene sheet
changes its band structure by shifting its π electron cloud. Thus, applying a potential can lead to a
p-type channel or a n-type channel, making the device into a transistor. Indeed depending on the
potential, the device will work as a p-type or a n-type channel thus enabling current to flow one
way or the other therefore the device works as a transistor.
Graphene can be used as a bare surface or can be functionalized to detect substances. In fact,
graphene binds very easily to carbon atom and thus most organic molecules and polymer, including
DNA. Bare graphene can be used to track and monitor some substances like ions that will usually
form a double layer and change the potential profile. However, other molecules will not generate
a noticeable or usable signal on bare graphene. For those molecules, or to increase selectivity and
efficiency, functionalization is required.
DNA is one good example of graphene functionalization to enhances its selectivity and effi-
ciency but also to enable detection and monitoring of other substances. While in the case of bare
graphene molecules will modify the potential profile or the ionic current for example by shifting
the electron cloud or the hole cloud, DNA binds to the analyte and in doing so induces a current
change. Chemical reactions with molecules bound to graphene behave the same way. This cur-
rent change can be related to the presence and the concentration of analytes. Hence graphene can
be used both as a qualitative and quantitative sensor. The substrate on which the graphene is

4. Using graphene on a flexible substrate as a sensor 4
transferred also determines the sensor properties. Graphene being one atom-thick but extremely
resilient we can assume that the sensor is actually almost only made of the substrate for mechani-
cal analysis. Hence, if graphene is transferred on silicon, the sensor will look like a silicon wafer
and will not be flexible. However if transferred on polyethylene terephthalate (PET), the device
will be flexible. The bonding of graphene to the substrate and to a molecule (during function-
alization)depends on the electron structure of the material. As mentionned previously graphene
has a π electron cloud and is a highly conjugated aromatic structure. It will thus bind to similar
molecules with interacting π electron clouds. Knowledge of the electron structure of a material
and its mechanical properties is important to design a reliable and lasting sensor, the better the π
interaction, the better the sensor. If you need a flexible device to be worn for example, you should
use a flexible polymer as PET or kapton, but if you want a stiffer structure to be integrated in a
more complex device or processed to a microelectromechanical system (MEMS), silicon would be
a safe choice.
Another important issue to bear in mind when designing your device is the simpler the process-
ing, the better the devices in terms of electronic properties. In fact, each process step, especially
with photolithography, can result in residue and debris leading to a significant loss in electron
mobility. Unprocessed graphene has an excellent charge carrier mobility but it is usually halved
during fabrication of the whole device. However, several photolithography and thin film deposition
steps may be required in order to build a particular sensor, in that case the process should be well
thought and designed to minimize the loss and the number of steps, especially when making the
electrical contacts at the end.

5. Using graphene on a flexible substrate as a sensor 5
Mobility of charge carriers in graphene on PET FET
To enable real-time sensing, a high mobility is required to enable fast response times and thus
a continuous signal. To determine this mobility we make the following assumptions:
1. The graphene monolayer is equivalent to a metal disk (conductor)
2. PET is a good insulator
The following equations are derived for the example of the geometry described in Figure 1. The
interesting parameters are the radius of the graphene a, the thickness of the gate insulator b and the
relative permittivity εr = 78 of the phosphate buffered saline (PBS), ε0 is the vacuum permittivity.
First, the gate-electrolyte capacitance is given by:
CGE = 2πaε0(εr +1)tan
2b(εr +1)
aεr
(1)
Then, the total top gate capacitance is given by:
For capacitance in series:
1
Ctot
=
1
C1
+
1
C2
(2)
Thus,
CG =
CGECq
CGE +Cq
(3)
Where Cq is the quantum capacitance, which is approximated as 2 µF.cm−2 for the graphene
channel. Finally, the charge carrier mobility in the linear regime is given by:
µ =
∆σ
CG∆Vg
=
∆IdsL
VdsW
1
CG∆VG
(4)
Where ∆σ is the differential conductance, ∆Ids is the differential drain-source current, ∆Vg is
the differential gate voltage, L and W are the length and the width of the channel and Vds is the

6. Using graphene on a flexible substrate as a sensor 6
drain-source voltage. Using equation (4), we can calculate the mobility for different applied drain-
source voltage in the range -1.0 V to 1.0 V. Indeed, W, L and CGE are given by the geometry,
∆Ids can be measured and ∆Vg is a parameter of the experiments. Some results are summarized in
Table 1.
Vds (V) -1.0 -0.40 0.40 1.0
µhole (cm2.V−1.s−1) 98.43 271.81 288.62 168.29
µelectron (cm2.V−1.s−1) 59.81 144.64 133.57 13.82
Table 1: Values of charge carriers mobility for different values of applied drain-source voltage [1]
Those values are close to or even better than state of the art similar devices. More background
and results are discussed in the following section, when this particular sensor is used to monitor
glucose.
Monitoring glucose
This kind of sensor can be used to detect and monitor one’s glucose level. Glucose is an impor-
tant molecule that is involved in digestion and key hormonal reactions. Insulin is secreted by the
pancreas. Monitoring glucose can help everyone. In fact, hypoglycemia and the resulting dizzi-
ness, hunger and loss of energy is by definition a drop in glucose level below the regular levels.
Insulin plays a major role in processing glucose into energy and storing it. In fact glucose is one of
the key energy sources for the brain and every other organs. Hence by tracking glucose levels, one
will know when to eat something to prevent hypoglycemia. This is a key application for athletes
as most hypoglycemia happen during exercise.

7. Using graphene on a flexible substrate as a sensor 7
However, what happens when someone has an insulin deficiency or cannot produce enough
of it to process glucose? Those diseases are know as diabetes and those who suffer from them
need to inject synthetic insulin to process the glucose they ingest to power their whole body. It
is crucial for them to determine the right amount of insulin required before making an injection.
Indeed, a dose too small will not be enough to process the glucose and thus is dangerous, while
an overdose is extremely dangerous and can lead to hospitalization or death [2]. It is thus of the
utmost importance for them to know their glucose levels before preparing their insulin intake.
Figure 1: Illustration of the glucose sensor. The silver paint and platinum are used as electrical contacts. [1].
Therefore, designing a glucose sensor that is non-invasive, affordable, reliable and easy to use
is important to athletes, people with a hypoglycemia risk and especially to diabetic patients. It will
help them every day and alleviate their concerns. For athletes and non diabetic but ”at-risk” per-
sons a real-time sensor would provide accurate information to make the choice of eating, drinking
or keep going. For diabetic patients, a real-time sensor would provide the same information, but
they usually only need to know their glucose levels before an insulin intake - after meals generally.
CVD grown graphene has been transferred on PET. It has been made into a field effect transis-
tor and functionalizes with glucose oxydase (GOD) as shown in Figure 1. GOD is an enzyme that

8. Using graphene on a flexible substrate as a sensor 8
is used as a catalyst in the degrading of glucose:
Dglucose+H2O+O2
GOD
−−−→ Dgluconic acid+H2O2
Figure 2: The results were obtained for a fixed value of Vds=-0.2 V. (a) and (c) show the measured drain-
source currents for the devices in the flat and the bent state. (b) and (d) show the drain-source current as
a function of the glucose concentration for three devices in the flat or the bent state respectively. The three
devices correspond to a shift in the p-type / n-type FET. The glucose levels measured are in the good range to
be used for diabetes management. Similar curves were obtained when measuring the H2O2 concentrations
[1].

9. Using graphene on a flexible substrate as a sensor 9
Using this device both in a flat state and in a bent state, Kwak et al. were able to measure
glucose level and fit it with a good correlation coefficient to theoretical curves. This show the
reliability of the sensor. The results are displayed in Figure 2.
Currently sold glucose sensors
This kind of sensors have many advantages for tracking glucose and, if successful to launch,
could change the market and the life of diabetic patients. It could also lead to more efficient and
less harmful physical athlete training. In fact, diabetic patients have only one choice today. It in-
volves a blood test and is thus invasive: patients need to puncture a finger to analyze a blood drop.
Other test are being developed and used today but they are extremely expensive and quite invasive
too. The classic test and the new ones give a result at a specific point of time. I believe the device
studied in this paper is a much better test to be used by diabetic patients and athletes. Indeed, it is
much more affordable, non-invasive, can be worn and give result in real-time.
Other graphene based glucose monitoring sensors are promising. One of them is a sensor
that relies on the catalytic power of TiO2 that is also used in solar cells for the same reasons [3].
TiO2 beads bind to graphene sheets and GOD, forming an encapsulated surface with a much larger
specific area, yielding even faster and more reliable results [4]. However, even though TiO2 is not
expensive, it is still another cost to factor in, especially when the solar industry is using always
more of this catalyst.

10. Using graphene on a flexible substrate as a sensor 10
References
[1] Y. H. Kwak, D. S. Choi, Y. N. Kim, H. Kim, D. H. Yoon, S.-S. Ahn, J.-W. Yang, W. S. Yang,
and S. Seo, “Flexible glucose sensor using cvd-grown graphene-based field effect transistor,”
Biosensors and Bioelectronics, vol. 37, pp. 82–87, 0 2012.
[2] P. Fasching, M. Roden, H. G. Sthlinger, S. Kurzemann, A. Zeiner, W. Waldhusl, and A. N.
Laggner, “Estimated glucose requirement following massive insulin overdose in a patient with
type 1 diabetes,” Diabetic Medicine, vol. 11, no. 3, pp. 323–325, 1994.
[3] M. K. Nazeeruddin, P. Pchy, T. Renouard, S. M. Zakeeruddin, R. Humphry-Baker, P. Comte,
P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spiccia, G. B. Deacon, C. A. Bignozzi, and
M. Grtzel, “Engineering of efficient panchromatic sensitizers for nanocrystalline tio2-based
solar cells,” Journal of the American Chemical Society, vol. 123, no. 8, pp. 1613–1624, 2001.
PMID: 11456760.
[4] H. D. Jang, S. K. Kim, H. Chang, K.-M. Roh, J.-W. Choi, and J. Huang, “A glucose biosensor
based on tio2graphene composite,” Biosensors and Bioelectronics, vol. 38, pp. 184–188, 0
2012.