Detection of Saliva-Range Glucose Concentrations Using Organic
Thin-Film Transistors - Supporting Information
S1.
Experimental
All OTFTs were fabricated on pre-patterned indium-tin-oxide (ITO)-on-glass
substrates (Kintec). The substrates feature source and drain ITO electrodes with a
channel length and width of 20 um and 3 mm respectively, as well as an ITO pad for
ensuring a good electrical contact between the gate electrode and the testing power
source. P3HT (95% regioregular, MW = 55000-60000 from Lumtec) was dissolved in
chloroform at a concentration of 20 mg/mL and was spun at 2000 rpm for 60 s using
a Laurell WS-400A-6NPP/Lite spin coater to create a semiconductor layer
approximately 100 nm thick as measured by a KLA Tencor Alpha-Step 500
profilometer. The P3HT film was then manually patterned to remove excess material
from the ITO electrodes and heated for 15 minutes at 45 °C in air to remove any
remaining solvent. PVP (Sigma-Aldrich) was dissolved in ethanol at a concentration
of 80 mg/mL and was then spin-coated on top of the P3HT layer for 60 s at 2000 rpm
to form a dielectric film approximately 500 nm thick, which was also patterned
manually. The P3HT/PVP two-layer structures were then heated at 85 °C in air to
further
remove
any
remaining
solvent.
ethylenedioxythiophene)-poly(styrenesulfonate)
Lastly,
either
(PEDOT:PSS,
a
poly(3,4-
Sigma-Aldrich)
solution, or a suspension of glucose oxidase (GOX) in Nafion® (Sigma-Aldrich)
solution was drop-cast on the top of gate dielectric layer and dried on a hot plate at
40 °C in air. The Nafion:GOX mixture was prepared by mixing 20 mg of glucose
oxidase from aspergillus niger (Sigma-Aldrich) per 1 mL of Nafion solution (Sigma-
Aldrich). Characterization and sensing experiments were carried out in air
immediately after device fabrication.
Cyclic voltammetry (CV) experiments were performed using a Faraday MP
potentiostat with a platinum counter electrode and a standard calomel reference
electrode in an electrochemical cell. The CV experiments were performed on
solutions of three concentrations of glucose (0 mM, 1 mM and 3 mM) in a fixed
volume of 0.1 M tris(hydroxymethyl)aminomethane hydrochloride buffer (Trizma
buffer, pH 7.4).
Amperometry experiments were conducted to determine the operating voltage
window for oxidation of hydrogen peroxide in water. A Nafion:GOX film was dropcast between two ITO electrodes and the current-voltage response was measured
with 10 μL of either 30mM H2O2 or deionised water deposited onto the Nafion:GOX
layer.
For all of the OTFT sensing experiments, 10 μL of analyte solution of the required
concentration was deposited on the gate of the device above the source-drain
channel. The device drain current (ID) was measured as a function of time for
different concentrations of glucose in aqueous solution. Current measurements were
conducted using Keithley 2400 SourceMeters controlled by a custom LabVIEW GUI.
Gate voltage (VGS) and drain voltage (VDS) were both held at -1 V, which were
determined to be appropriate voltages for this sensing system.
S2.
AFM
Figure S1 shows atomic force microscopy (AFM) images of representative regions of
the various layers of the OTFT device structure. Figure S1 (a) shows a P3HT film as
spun onto the ITO-on-glass substrate, which has a root mean square roughness
(RRMS) of 0.88 nm. Figure S1 (b) shows the surface of the PVP film which is spun on
top of the P3HT layer. Again, a smooth film is observed, with a similar roughness
value to that of the P3HT film of 1.14 nm. Figure S1 (c) shows a Nafion layer cast
onto the P3HT/PVP structure with no enzyme mixed into it. Although the roughness
is a little higher than both the P3HT and PVP layers (RRMS = 5.20 nm), the AFM
image shows that the Nafion forms a smooth continuous layer over the dielectric
PVP layer. Figure S1 (d) shows a region of the Nafion:GOX layer as cast on top of
the PVP layer with micron scale features on the surface
Figure S1: AFM images of the different layers of the sensing OTFTs. (a) P3HT, (b) PVP on P3HT, (c)
Nafion on PVP on P3HT and (d). Nafion:GOX on PVP on P3HT
S3.
Cyclic Voltammetry
In order to confirm the activity of GOX once it is embedded in a Nafion film, films of
Nafion:GOX were deposited onto ITO-coated glass and used as working electrodes
in an electrochemical cell containing glucose in the electrolyte. When GOX
selectively oxidises glucose, gluconolactone and hydrogen peroxide (H2O2) are
generated as products of the reaction as described in Equation 1.
𝐺𝑂𝑋 𝑎𝑛𝑑 𝑂2
𝐺𝑙𝑢𝑐𝑜𝑠𝑒 →
𝑔𝑙𝑢𝑐𝑜𝑛𝑜𝑙𝑎𝑐𝑡𝑜𝑛𝑒 + 𝐻2 𝑂2
(1)
Subsequently, when H2O2 in solution is oxidised above a threshold voltage (~0.7 V
versus a standard calomel electrode), it breaks down electrochemically into protons,
oxygen and electrons as described in Equation 2.
~0.7 𝑉 𝑣𝑠 𝑆𝐶𝐸
𝐻2 𝑂2 →
𝑂2 + 2𝐻 + + 2𝑒 −
(2)
The net result of these two reactions is that a stoichiometric quantity of protons is
liberated which is directly proportional to the number of glucose molecules oxidised
by the enzyme.
Figure S2: Sample of CV-based glucose sensing. (a) CV plots with electrolyte solutions containing
different glucose concentrations: 0 mM (orange), 1 mM (green) and 3 mM (blue) and (b) the
associated calibration curves at different VWE levels.
Figure S2 (a), shows the cyclic voltammetry (CV) plots for the 0, 1, and 3 mM
glucose solutions and reveals that for working electrode voltages (VWE) higher than
0.6 V the cell current (ICELL) increases with glucose concentration; thus
demonstrating that the GOX retains its activity even after being embedded in the
Nafion membrane. From Equation 2, we expect that at a VWE value of approximately
0.7 V versus an SCE reference electrode, the H2O2 that is liberated in the enzymatic
reaction should in turn undergo oxidation and subsequently generate protons. Figure
S2 (b) shows calibration curves of ICELL versus glucose concentration for ten
concentrations of glucose between 0 and 3 mM. For all VWE, a near linear
relationship between ICELL and glucose concentration is observed. This observation
indicates that the quantity of ions liberated by the electrochemical breakdown of
H2O2 is proportional to the concentration of glucose molecules present in the system
as expected.