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Abstract— A new pH sensor using nano electrodes in organic
semiconductor P3HT (regioregular poly(3-hexylthiophene)) has been
designed, fabricated and characterized in this work. In this
sensor, the organic semiconductor is directly exposed to pH sample
solution, and an electrical field is applied to drive the protons
in the solution to the polymer surface. The accumulated protons at
the interface between organic semiconductor and solution change the
electron density at organic semiconductor surface, so that the
conductivity of the organic semiconductor film is modulated. In
order to improve the sensitivity of sensor, P3HT is coated on the
interdigitated array nanoelectrodes, and the conductivity of P3HT
between two bands of electrodes is measured for pH
characterization. A good conductivity modulation by the proton
concentration or pH value is shown in the sensor testing.
Keywords—pH Detection, P3HT, Conductivity, Nanoelectrodes
I. INTRODUCTION Recently organic semiconductor has been gaining
a popular attention in the research and industry application due to
its low-cost manufacturing, disposability and easy processing. The
so-far great application achievement of organic semiconductor in
optoelectronic devices, such as light emitting diode and battery,
is motivating researchers to extend the application areas of
organic semiconductor [1-4]. As a result, nowadays more and more
interests are paid on organic semiconductor transducers.
Some pioneering works have been reported to use organic
semiconductor field-effect transistor (FET) as the sensing device
for gas or pH value. L. Torsi reported a gas sensor using organic
semiconductor FET. Gas interacts with the organic semiconductor and
modulates the carrier mobility of organic semiconductor. Therefore,
the current-voltage curves of organic semiconductor FET is
characterized for gas sensing [5-7]. C. Bartic reported to use an
organic ion-selective field-effect transistor (ISFET) to measure pH
value of sample solution [8]. His device adopts the conventional
detection mechanism that pH variation causes the potential drop
across the dielectric and semiconductor interface, but the working
semiconductor in his device is polymer instead of silicon. Although
these pioneering works are based on organic semiconductor
field-effect transistor, one fact has to be pointed out that
organic
semiconductor has really low carrier mobility when compared to
silicon. Thus, these organic semiconductor FETs and their based
sensors usually work under large voltage range. In order to operate
polymer FET in low voltage, people tried to deposit high dielectric
material (instead of SiO2) as the insulating layer [9], or to build
nanochannel for FET using nanofabrication technology [10-11].
Processing the high dielectric material needs high cost, and nano
polymer FET requires the complex small-current detection
circuit.
In this work, we adopts organic semiconductor in pH measurement
using a different approach. In our approach, the organic
semiconductor is directly exposed to pH sample solution, and an
electrical field is established to be vertical to the organic
semiconductor surface, and drives the protons in the solution to
the polymer surface. The accumulated protons at the interface
between polymer and solution change the electron density at polymer
surface, so that the conductivity of organic semiconductor film is
modulated. Based on this concept, a conductivity-measured pH sensor
has been designed, fabricated, and characterized for a wide pH
value range (pH = 2 – 12) in this work. The experiment results show
a good conductivity modulation by the proton concentration or pH
value.
II. METHOD AND DESIGN Figure 1 illustrates the cut view and the
operation of the pH sensor. The sensor is built on Si/SiO2
substrate. The sensing window is exposed to sample solution. The
operation of sensor includes the following steps. First, an
pH Sensor using Nano Electrodes in Organic Semiconductor
Xiaoshan Zhu and Chong H. Ahn
BioMEMS and MicroSystem Lab Department of Electrical &
Computer Engineering and Computer Science
University of Cincinnati, OH, USA
Figure 1. Cut view and operation of proposed device for pH
measurement: (1) Applying an electrical field between pH solution
and Si substrate for a few seconds; (2) Measure the semiconductive
polymer conductivity between two electrodes after removing the
electrical field.
V
Si
Conductivity Measurement
Ag/AgCl
Polymer
Au SiO2
Electrolyte
-
electrical field is generated to be vertical to the polymer
surface by applying a potential between Si substrate and the
reference electrode. In this step, protons are supposed to be
driven to the polymer surface to form the interface charge. Second,
the polymer conductivity between two electrodes is measured and
recorded. Every time before applying new sample solution to the
sensing window, the sensing window is washed using the neutral
buffer (pH = 7) and nitrogen repeatedly. In addition, considering
the slow oxidization of P3HT in the air by oxygen, all measurements
are done in nitrogen environment.
Organic semiconductor P3HT (regioregular poly(3-
hexylthiophene-2,5-diyl)) is used in this pH sensor. P3HT is
soluble to chloroform, and can be spin-coated on the wafer surface
in the processing. Moreover, it has the highest carrier mobility as
one of conjugated polymers, and has good ohmic contact with gold
layer. It has been widely used in organic semiconductor FET
[12-13]. The molecular structure of P3HT is shown in Figure 2(a).
In order to improve the sensitivity of sensor, the electrodes are
designed as the interdigitated array. In this array, the spacing
between two electrodes is in submicrons (~ 0.5 um), and the width
for each electrode finger is around 200 nm. The structure of all
electrodes is presented in Figure 2(b). The pH sensing part is the
interdigitated array nanoelectrodes and the coated P3HT on
nanoelectrodes, and two big electrode pads are used for electrical
connection in measurement.
II. FABRICATION The designed device is fabricated using
mixed-match
processing steps, which consists of nanofabrication and
microfabrication.
First, PMMA (Microchem, 495K) with a 300 nm thickness is
spin-coated at the oxidized silicon surface, and then the
nanopatterns are exposed by e-beam (Raith 150 e-beam lithography
system). After the development of the exposed PMMA, Ti/Au (100 Å/
1000 Å) layer is deposited on the patterned sample surface, and
then dipped into acetone for lift-off.
After the nanofabrication, the connection pads are fabricated
using the UV-light-lithography-based lift-off
techniques, as the following steps. First, the positive
photoresist (Shipley 1818) is coated on the substrate with 3000
rpm, and then the wafer is baked in the 90 ºC oven for 30 minutes.
Second, the baked Shipley 1818 is exposed under UV light (300 nm ~
460 nm wavelength, ~7 mJ/cm2) for 10 seconds, and consequently is
immersed in chlorobenzene for 45 seconds. After immersion, dry the
sample in the 120 ºC oven for 30 seconds. Third, the sample is
developed for 1 minute and dry. Fourth, Ti/Au (100 Å/ 1000 Å) layer
is deposited on the patterned sample surface. Finally, the
deposited sample is baked at 120 ºC for 1 – 2 hours and then dipped
into acetone for lift-off.
In the last step, organic semiconductor P3HT (regioregular
poly(3-hexylthiophene-2,5-diyl)), which is dissolved in chloroform
(0.8% wt), is spin-coated on the electrode surface at 1500 rpm
speed. After the coating, the sensing window is covered and sealed
by a small piece of PDMS membrane, and the other coated P3HT film
is etched using RIE (Ar). In the testing, the pH solution is
dropped in the sensing window.
The whole processing steps are graphically illustrated in Figure
3. Figure 4(a) shows the photo graphics of fabricated
nanoelectrodes and microelectrodes, and a SEM graphics for a close
view on the interdigitated array nanoelectrodes is presented in
Figure 4(b). The thickness of the coated P3HT on nanoelectrode
surface is measured using profilometer, and the thickness data is
shown in Figure 5.
(b) E-beam lithography
(a) PR coating
PMMA
(c) Metal deposition
(d) Nano Lift-off
Nanoelectrodes
Figure 3. Mixed-match processing: (a) – (d) E-beam lithography
for lift-off of nanoelectrodes; (e) – (h) UV-light lithography for
lift-off of microelectrodes; (l) – (m) P3HT coating and
patterning
S1818
(e) PR coating
(f) UV lithography
(g) Metal deposition
Figure 2. (a) Molecular structure of P3HT (regioregular
poly(3-hexylthiophene-2,5-diyl); (b) Structure of elelctrodes
(a) (b)
S
S
n
C6H13
C6H13
Nanoelectrodes MicroPad
Si/SiO2
MicroPad
(l) P3HT coating and RIE(Ar)
Cover membrane
(m) Remove PDMS
P3HT
(h) Micro Lift-off
Microelectrode
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III. MEASUREMENT RESULTS
A. Preparation of pH sample solution and Ag/AgCl electrode The
neutral sample solution (pH=7) is PBS buffer
purchased from Scientific Fisher. The PBS buffer includes 0.1 M
choride ions, which makes sure the reference elelctrode (Ag/AgCl)
is stable after immersing into pH sample solution. The other sample
solutions with different pH values are prepared by modulate the
neutral sample solution with HCl acid or potassium hydroxide.
The reference electrode is made by slowly depositing a layer of
AgCl onto a segment of clean silver wire by electrolysis. A clean
silver wire and a clean Pt film respectively are connected to the
positive and negative polarities of current source, and then
immerse them in 0.1 M KCl solution with a 10 mA/cm2 current density
for 60 seconds. A blackish deposit of AgCl will be deposited on the
silver wire. Before using, the reference electrode is stored in a
dilute solution of saline.
B. Conductivity measurement
The conductivity of organic semiconductor between two
electrodes is measured using a digital multimeter (DMM). All
measurement is done in nitrogen environment. Figure 6 shows the
conductivity change before and after applying pH sample solution on
the senmiconductive polymer. Before dropping pH sample solution,
the resistant between two electrodes are huge (~68 Mohms). But
after applying pH sample solution on the organic semiconductor
surface, the resistant drops off dramatically. Although the
resistant for different pH sample solutions varies, the variation
is not obvious and the resistance is around 3 Mohms. The reason for
this dramatic conductivity change possibly is that ions in the
sample solution change the electron density of organic
semiconductor surface, and thus cause the huge variation of
conductivity.
In order to check the effect of proton/hydroxyl concentration
(or different pH values) on polymer conductivity, a 10 V potential
is added between the Si substrate and the reference electrode,
which is immersed in pH sample solution. The electrical field
between Si substrate and the reference electrode lasts for
30-second duration, and then the polymer conductivity is measured
again. Figure 7 gives the comparison of two cases: with electrical
field excitation and without electrical field excitation. From
Figure 7, it can be seen that the high concentration proton causes
a higher conductivity, while the high concentration hydroxyl
results a lower conductivity. The detailed physic mechanism for
this phenomenon is still under investigation. One intuitive
explanation is that electrical field drives the protons to the
polymer surface to further increase the polymer surface electron
density, so that
Figure 4. Fabricated electrode strucuture using mixed-mathc
processing: Photo (a) and SEM (b) graphics. The dimension of
electrode fingers is ~ 80 nm and its spacing is ~ 500 nm.
(a) Photo graphics of interdigitated array electrodes
(b) SEM graphics of interdigitated array electrodes in nao
scale
Figure 5. The measured thickness of P3HT using profilometer. The
scanning range is 3 mm, the scanning speed is 100 um/s, and the
sampling rate is 200 Hz.
0
500
1000
1500
2000
2500
3000
3500
0 1000 2000 3000 4000 5000 6000
Scanning range: 3 mm
Scanning speed: 100 um/s Sampling rate: 200 Hz
Thickness = ~ 1600 Å
Sample points
Thi
ckne
ss (A
ngst
rom
)
100 um Microelectrode
Nanoelectrodes
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the conductivity becomes larger for the sample solution with
smaller pH values or higher proton concentration.
V. CONCLUSION
Work presented in this paper first illustrates a new property of
the organic semiconductor P3HT (regioregular
poly(3-hexylthiophene-2,5-diyl)). That is, under the function of an
electrical field, P3HT (regioregular
poly(3-hexylthiophene-2,5-diyl)) changes its conductivity with the
different pH environments. Such a property is characterized in a
large pH range (pH = 2 – 12) using 500nm-spaced nanoelectrodes in
this work. A mono-decreasing relationship between polymer
conductivity and pH value is achieved. With this new property, P3HT
(regioregular poly(3-hexylthiophene-2,5-diyl )) can be applied as
not only pH
sensor, but various pH-detecting based sensors. Compared to the
organic semiconductor FET based sensors, the application using such
a property avoids the low carrier mobility and the complex bias
circuit and small-current detection circuit.
ACKNOWLEDGMENT The authors thank National Science Foundation
(NSF) for the funding on Raith-150 e-beam lithography system at
University of Cincinnati, and also thank Mr. Ron Flenniken in
University of Cincinnati for his technical assistance in the metal
deposition.
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Figure 6. Dramatic change of P3HT conductivity after a drop of
pH solution is put on the sensing window (without electrical field
to drive protons to polymer surface).
Figure 7. Conductivity modulation by pH value with or without an
electrical field to drive protons to the semiconductive polymer
surface
0
0.1
0.2
0.3
0.4
0.5
1 2 3 4 5 6 7 8 9 10 11 12 13
Dramatic change
Without pH solution
With pH solution
pH
Con
duct
ivity
(uS)
0.2
0.3
0.4
0.5
1 2 3 4 5 6 7 8 9 10 11 12 13
With electrical field excitation
Without electrical field excitation
pH
Con
duct
ivity
(uS)
code: 0-7803-8439-3/04/$20.00©2004 IEEE01: 1968header:
Proceedings of the 26th Annual International Conference of the IEEE
EMBS San Francisco, CA, USA • September 1-5, 2004 02: 196903:
197004: 1971