3-((2′:2″,5″:2′′′-TERTHIOPHENE)-3″-YL) ACRYLIC ACID AS

ORGANIC FIELD EFFECT TRANSISTOR FOR DNA SENSING

K. C. Aw

, N. Tjitra Salim

, H. Peng

, J. Travas-Sedjic

and W. Gao

Mechanical Engineering

Chemical and Materials Engineering

Chemistry

The University of Auckland, New Zealand

Keywords: 3-((2′:2″,5″:2′′′-terthiophene)-3″-yl) acrylic acid, DNA sensor, Organic field effect transistor.

Abstract: The aim of this paper is to demonstrate the use of organic field effect transistor (OFET) as a DNA sensor.

We have synthesized a functionalised terthiophene monomer 3-((2′:2″,5″:2′′′-terthiophene)-3″-yl) acrylic

acid (TAA) and has been successfully electrodeposited as an active layer of an OFET. The polymer was

oxidised in order to increase its conductivity. A mobility of 0.25 cm

/V.s was achieved with an oxidising

potential of 0.9 V. A preliminary DNA sensing test was performed on the OFET with poly TAA as active

layer and a shift in threshold voltage was observed after DNA immobilization and hybridization, showing

its potential as DNA sensor.

1 INTRODUCTION

Previous studies (Peng, Zhang, Spires et al l, 2007;

Zhang, Peng, Kilmartin et al, 2007) have shown the

ability of the acid-functionalised conducting

polymers to detect DNA hybridization, where

carboxylic acidic functionality enabled covalent

attachment of biomolecules such as DNA

(oligonucleotides).

However, in this research the DNA detection

will be based on an organic field effect transistor

(OFET) device instead of the change in impedance

of an organic thin film as in Refs 1 and 2. The

OFET device as a sensor can be more sensitive due

to the ability to control OFET conduction via a third

terminal called “gate”.

2 EXPERIMENT PROCEDURES

The synthesis of 3-((2′:2″,5″:2′′′-terthiophene)-3″-

yl) acrylic acid (TAA) monomer is based on work

by (Peng et al, 2007). The monomers were

electropolymerized on a prefabricated substrate

consisting of a source (S), a drain (D) and a gate (G)

terminals. The bottom gate was a highly doped Si

and a 100 nm SiO

acted as the gate dielectric.

Finally, the two gold contacts as S and D with an

effective channel width of 5560 μm were patterned

on top of the SiO

. Channel length spacing of 25,

was used. The schematic structure of the OFET is

illustrated in Figure 1.

- Si

SiO

PTAA

Figure 1: The schematic structure of bottom contact,

bottom gate PTAA-based OFET.

The electropolymerisation was performed using

the cyclic voltammetry from 0 to 1.3 V at scan rate

of 50 mV/s by a CH Instrument electrochemical

workstation (Model 440, CH Instruments, USA).

The gold S and D contacts on the substrate were

used as the working electrodes during

electropolymerisation. A Pt wire and Ag/AgCl (3 M

KCl) were used as the counter electrode and the

reference electrode, respectively. The

polymerisation solution comprised of 0.05 M

tetrabutylammonium trifluoromethanesulfonate and

0.005 M TAA in acetonitrile. The obtained polymer

159

Aw K., Tjitra Salim N., Peng H., Travas-Sedjic J. and Gao W. (2010).

3-((2’:2",5":2”’-TERTHIOPHENE)-3"-YL) ACRYLIC ACID AS ORGANIC FIELD EFFECT TRANSISTOR FOR DNA SENSING.

In Proceedings of the Third International Conference on Biomedical Electronics and Devices, pages 159-162

DOI: 10.5220/0002692601590162

 SciTePress

TAA (PTAA) film was then oxidised in a monomer

free solution by a constant potential method for 5

minutes to achieve different doping levels, therefore

different conductivities. Five oxidising potential

) of 0.1, 0.2, 0.6, 0.9 and 1.1 V were chosen to

investigate their effect on the performance of the

fabricated OFET. The arrangement for

electropolymerization is shown in Figure 2.

CH Instruments

Model 440

Working electrode

(S & D contacts)

Counter electrode

(Pt wire)

Reference electrode

(Ag/AgCl/3M KCl)

Figure 2: The electrodes arrangement for electro

polymerization.

The OFET electrical characteristic was measured

using Keithley 2602 source measure unit after the

doping process. The microstructure of PTAA film

on the Si substrate was examined using the scanning

electron microscope (SEM).

Following the optimisation of PTAA-OFET

fabrication process, DNA sensing ability of this

OFET was tested. The study starts with DNA

immobilization, a process where single stranded

DNA (ssDNA) molecules with known sequence

(NH

-GAT GAG TAT TGA TGC CGA-3)

synthesized by Invitrogen Life Technologies are

covalently attached to the PTAA film. To attach the

ssDNA, a 40 μL of phosphate buffer (pH 5.2)

containing 20 nmol ssDNA probe and 400 nmol 1-

ethyl-3-(3-dimethylaminopropyl)carbodiimide

(EDC) was applied on the surface of PTAA-OFET

and was kept at 28 °C for 1 h. Finally, the OFET

was thoroughly washed using phosphate buffered

saline (PBS) solution (pH 7.4) to remove any

unattached ssDNA. After immobilization, the I-V

characteristics of PTAA-OFET were measured using

Keithley 2602 source measure unit.

The next step to test the sensing ability of

PTAA-OFET is DNA hybridization, where the

complementary ssDNA is applied to the surface of

OFET and hybridizes with the immobilized ssDNA

via complementary base pairing to form a double

helix DNA. The hybridization was carried out by

incubating the PTAA-OFET with immobilized DNA

in PBS solution containing complementary ssDNA

samples for 1 h at 42 °C. After hybridization, the

OFET was washed three times using PBS solution to

remove any non-hybridised ssDNA. Then I-V

characterisations were carried out to determine the

effect of hybridization on the OFET performance.

3 RESULTS AND DISCUSSIONS

The PTAA-OFET was electrically characterized for

diferenct V

and highest calculated carrier mobility

of 0.25 cm

/V.s at V

D,sat

= -20 V is achieved with

= 0.9 V and threshold voltage (V

) of

approximately 7 V (Tjitra Salim, Aw, Peng, et. al.,

2008). Figure 3 show the OFET characteristics of

PTAA oxidised at 0.9 V where the highest carrier

mobility was achieved. The I-V characteristic is not

of good quality due to the fact that

electropolymerization process could produce film

with high structural and conjugation defects

(Roncali, 1992)

The above-mentioned results are in good

accordance with the cyclic voltammogram (CV) of

PTAA obtained in a monomer free solution as

shown in Figure 4. From this CV, two anodic peaks

at +0.6 V (A) and +1.05 V (B) are observed. The

first anodic peak marks the beginning of polymer

oxidation, indicating the formation of polaron and

subsequent bipolaron formation, i.e. the charge

carrier (holes) (Chen and Inganas, 1996). Therefore,

one can see that the PTAA film is in a neutral state

with low charge carriers (holes) density when V

0.6 V. With increasing V

, the PTAA starts to be

oxidised, increasing the charge carrier density,

which in turn would increase its conductivity. A

higher conductivity PTAA film as an active layer is

considered to be responsible for the increase in

an OFET. When V

≥ +1.05 V (at the second

anodic peak), a strong increase in the anodic current

is observed, indicating the beginning of polymer

oxidative degradation. This explains the low

transistor performance when PTAA as an active

layer was oxidised at voltage greater than 1 V.

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-9.00E-06

-8.00E-06

-7.00E-06

-6.00E-06

-5.00E-06

-4.00E-06

-3.00E-06

-2.00E-06

-1.00E-06

0.00E+00

-20-15-10-5

(V)

(A)

= 0 V

= -10 V

= -11 V

= -12 V

= -13 V

=-14V

= -15 V

(a)

1.00E-08

1.00E-07

1.00E-06

1.00E-05

-15 -10 -5 0

(V)

-I

(A)

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

(-I

)

1/2

)

(b)

D,sat

= -20 V

Figure 3: The I-V characteristics of bottom contact,

bottom gate PTAA-based OFET with VOX = 0.9 V; (a).

Output curves (ID-VD) at different gate voltages; and (b).

Transfer curves (ID-VG) in saturated regime at constant

source-drain voltage of -20 V (left scale) and square root

of the absolute value of ID as a function of VG (right

scale).

The morphology of PTAA film on top of gold (S

and D) contacts and across the channel is shown in

Figure 5, demonstrating that the electrodeposition of

PAA enables the creation of active region between

the S and D contacts creating an OFET.

A preliminary DNA sensing ability of PTAA-OFET

was tested and the results are shown in Figure 6. A

shift in threshold voltage (VT) was observed after

DNA immobilization and hybridization of

complementary DNA, confirming the sensing ability

of PTAA-OFET. Initially, the PTAA-OFET

obtained a threshold voltage (VT,i) of -8 V and after

DNA immobilization, the threshold voltage has

shifted to about -6 V (VT,im). This positive VT shift

is expected as the phosphate groups on the DNA

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

-15

-10

-5

Current (μA)

Potential (V)

Figure 4: The cyclic voltammogram (CV) of PTAA film

in a monomer free solution with a scan rate of 50 mV/s.

Figure 5: SEM images showing the morphology of PTAA

film on top of the gold (S and D) contacts and across the

channel.

0.001

0.002

0.003

0.004

0.005

0.006

-20-15-10-50

(

)

(-I

)

1/2

(A)

1/2

Initial - before immobilisation

Immobilised

Hybridised - complementary

T, i

T,im

T,h

D,sat

= -20 V

Figure 6: The √ID vs VG at VD,sat = -20 V showing the

threshold voltages (VT) of bare PTAA-OFET,

immobilized PTAA-OFET and hybridized PTAA-OFET

with complementary DNA.

Channel

Region

3-((2':2",5":2'''-TERTHIOPHENE)-3"-YL) ACRYLIC ACID AS ORGANIC FIELD EFFECT TRANSISTOR FOR DNA

SENSING

161

backbone are able to attract electrons from the

organic semiconductor, increasing the hole

concentration of PTAA than before DNA

immobilization (Zhang and Subramanian, 2007).

The increased in holes concentration would result in

the shift of V

to a lower negative voltage (positive

shift), causing an effective p-doping of PTAA.

After the hybridization with complementary

DNA, the threshold voltage (V

) shifted more

negatively (negative shift) to approximately -9 V.

Hybridization with complementary DNA would

result in double stranded DNA (dsDNA), where the

bases of two complementary single stranded DNA

(ssDNA) would pair up and form the double helix.

According to (Zhang and Subramanian, 2007),

dsDNA molecules could not be immobilized

(attached) to the organic semiconductor (PTAA) as

effectively as ssDNA molecules. This would result

in less interaction between DNA backbone and

PTAA that are known to be able to attract electrons,

lowering the holes concentration in PTAA and

hence, increasing the OFET’s V

to be more

negative. The low interaction ability between

dsDNA and PTAA is because the bases of DNA are

not exposed in dsDNA. The bases of DNA are

responsible to the hybrophobic interaction, resulting

in physical adsorption between ssDNA and PTAA,

i.e. immobilization of ssDNA.

4 CONCLUSIONS

The PTAA based p-channel OFETs were fabricated

successfully with a μmax of 0.25 cm

/Vs. The

amount of doping through the V

in PTAA is

crucial in controlling the μ of the OFET. An

optimum V

of 0.9 V was obtained, producing

OFET with the highest μ. A V

> 1.0 V causes

degradation to the PTAA film, leading to poor

charge mobility, while V

≤ 0.2 V does not

produce sufficient doping. These results

demonstrated that the μ of FET fabricated with

PTAA can be controlled by V

and correlates with

the potential value of the cyclic voltammogram for

0.2 V ≤ V

< 1.1 V. A preliminary DNA sensing

test was performed on the PTAA-OFET and a shift

in threshold voltage was observed after DNA

immobilisation and hybridisation, showing its

potential as DNA sensor. However, more detailed

study is required to realise this PTAA-OFET as

DNA sensor before it can be deployed.

REFERENCES

Chen, X., Inganas, O., 1996. Three-Step Redox in

Polythiophenes:  Evidence from

Electrochemistry at an Ultramicroelectrode, The

Journal of Physical Chemistry 100 (37), 15202-15206.

Peng,H., Zhang, L., Spires, J. et al., 2007. Synthesis of a

functionalized polythiophene as an active substrate for

a label-free electrochemical genosensor, Polymer 48

(12), 3413-3419.

Roncali, J., 1992. Conjugated poly(thiophenes): Synthesis,

functionalization and applications, Chemical Reviews

92 (12), 711.

Tjitra Salim, N., Aw K.C., Peng, H. et. al., 2008. New 3-

((2':2'',5'':2'''-terthiophene)-3''-yl) acrylic acid as active

layer for organic field-effect transistor, Materials

Chemistry and Physics, 111 (1), 1-4.

Zhang, L., Peng, H., Kilmartin, P.A. et al., 2007.

Polymeric Acid Doped Polyaniline Nanotubes for

Oligonucleotide Sensors, Electroanalysis 19 (7-8),

870-875.

Zhang Q., Subramanian, V., 2007. DNA hybridization

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and Bioelectronics 22 (12), 3182-3187.

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