Department of Physics, Chemistry and Biology

Wet Organic Field Effect Transistor as DNA sensor

Yu-Jui Chiu

2008-04-23

Supervisor Mattias Andersson

Examiner

Olle Inganäs

Datum Date 2008-04-23

Avdelning, institution Division, Department Biomolecular and Organic Electronics Department of Physics, Chemistry and Biology Linköping University

URL för elektronisk version

Rapporttyp Report category

Licentiatavhandling Examensarbete C-uppsats D-uppsats Övrig rapport

_____________

Språk Language

Svenska/Swedish Engelska/English

________________

ISBN ISRN: LITH-IFM-EX--08/1932--SE _________________________________________________________________ Serietitel och serienummer ISSN Title of series, numbering ______________________________

Titel Title Wet Organic Field Effect Transistor as DNA Sensor Författare Author

Yu-Jui Chiu

Sammanfattning Abstract

Label-free detection of DNA has been successfully demonstrated on field effect transistor (FET) based devices. Since conducting organic materials was discovered and have attracted more and more research efforts by their profound advantages, this work will focus on utilizing an organic field effect transistor (OFET) as DNA sensor. An OFET constructed with a transporting fluidic channel, WetOFET, forms a fluid-polymer (active layer) interface where the probe DNA can be introduced. DNA hybridization and non-hybridization after injecting target DNA and non-target DNA were monitored by transistor characteristics. The Hysteresis area of transfer curve increased after DNA hybridization which may be caused by the increasing electrostatic screening induced by the increasing negative charge from target DNA. The different morphology of coating surface could also influence the OFET response.

Nyckelord Keyword Organic field effect transistor (OFET), Wet OFET, Poly(3-hexylthiophene) (P3HT), DNA sensor, Conjugated polymer, Micro fluidic channel.

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Abstract

Label-free detection of DNA has been successfully demonstrated on field effect transistor (FET)

based devices. Since conducting organic materials was discovered and have attracted more and

more research efforts by their profound advantages, this work will focus on utilizing an organic

field effect transistor (OFET) as DNA sensor.

An OFET constructed with a transporting fluidic channel, WetOFET, forms a fluid-polymer

(active layer) interface where the probe DNA can be introduced. DNA hybridization and

non-hybridization after injecting target DNA and non-target DNA were monitored by transistor

characteristics. The Hysteresis area of transfer curve increased after DNA hybridization which

may be caused by the increasing electrostatic screening induced by the increasing negative

charge from target DNA. The different morphology of coating surface could also influence the

OFET response.

Acknowledgement

Over the years, I would like to express my sincere gratitude to the following people.

First of all, I want to thank Olle Inganäs. Thank you for providing me an opportunity to do this

thesis work in your group. Your humor and your considerable inspiration impress and enlighten

me a lot. You are such a real scientist in my mind, and it is my honor to work in your group.

I would like to thank my supervisor Mattias Andersson, who is the most important person during

these months. Thank you for teaching me a lot from the basic theories to the attitudes a scientist

should have. Thanks for your patience and accurate guidance. I really learn a lot from you.

Thanks to all the members in Biomolecular and Organic Electronics. Karin Magnusson and Jens

Wigenius gave me very useful supports of DNA samples, Mahiar Hamedi always carries out

inspired discussion, Yinhua Zhou and Fengling Zhang taught me many about polymer properties,

Bo Thuner gave me engineering advisement, and, of course, all of you who gave me any

supports and response during the group meeting and in the laboratory. I feel enjoyable and

comfortable to work in this group. And I will always remember the exciting-brainstorming time.

Thanks to my family, my mother, father, and sister. You always accept me and support me to

chase my dreams. Because of you, I can face the unknown future with brave and optimistic

attitude. Although Sweden is far from Taiwan, I can still your encouragement.

I would like to thank Yun-Hsuan Chen, who is thoughtful and considerate, and always gives me

many mental and physical supports. Your happiness can always sweep away the whole day

tiredness. Thank to Rosa Huang, who supplied professional and talented skills of illustration.

Also, I would like to thank all the friends from my country, Taiwan, and who I met in Sweden.

All of you enrich my life and leave sweet memories of Sweden.

Last but not least, I would like to thank Leif Johansson, who is always pleased to help me to deal

with troublesome problems of everyday life. You are always shown up as a hero who takes care

of us when needed.

Again, thank you all!

Linköping, April 2008

Yu-Jui Chiu

Table of Contents

1 Introduction................................................................................................................................ 1

1.1 Background and Goal ....................................................................................................... 1

1.1.1 Background ............................................................................................................ 1

1.1.2 Aim of This Work.................................................................................................. 2

1.1.3 Outline.................................................................................................................... 2

1.2 Theoretical Background.................................................................................................... 2

1.2.1 Conjugated Polymers ............................................................................................. 2

1.2.2 Organic Field-Effect Transistor ............................................................................. 9

1.2.3 Fluorescence ........................................................................................................ 11

1.2.4 DNA and Hybridization....................................................................................... 12

2 Architecture of Wet Organic Field-Effect Transistor .......................................................... 15

3 Experimental Details ............................................................................................................... 19

3.1 Material ........................................................................................................................... 19

3.1.1 Poly(3-hexylthiophene)........................................................................................ 19

3.1.2 Cholesterol Tag.................................................................................................... 20

3.1.3 DNA samples ....................................................................................................... 20

3.1.4 PDMS................................................................................................................... 21

3.1.5 Phosphate Buffered Saline (PBS) ........................................................................ 21

3.1.6 Experimental setup............................................................................................... 22

3.2 Spin Coating.................................................................................................................... 23

3.3 Measurements ................................................................................................................. 24

4 Results and Discussion..............................................................................................................27

4.1 General results .................................................................................................................27

4.1.1 Transfer characteristics .........................................................................................27

4.1.2 Output characteristics ...........................................................................................32

4.1.3 Water and PBS......................................................................................................33

4.2 Fluorescence Measurement..............................................................................................34

4.3 DNA Detection ................................................................................................................36

4.3.1 Overview...............................................................................................................36

4.3.2 Non-complementary DNA versus Complementary DNA ....................................41

5 Conclusion .................................................................................................................................53

6 Reference ...................................................................................................................................55

Chapter 1

Introduction

1.1 Background and Goal

1.1.1 Background DNA or sequences on it is the smallest unit that contains hereditary information. Researchers

have done huge amounts of works on DNA, expecting to solve the mystery of life since DNA is

not only the blueprint but also like a historical record of life. Several applications of DNA

highlight its important role: Genetic engineering open the doors of improving medical

researches. Detection of DNA gives forensic scientists a trustable way to get clues, and also

give historians and anthropologists an accessible clue to discover the relationship between

different lives or people. DNA has been applied in computer science and nanotechnology. No

matter what the application is, the most important step is to detect or sense the DNA sequence.

After conducting polymers were discovered in 1977, they were used frequently in research and

its application. Flexible, lightweight, bio-friendly, low cost, and so on are some wide known

advantages of polymers. Compared to inorganic materials, polymers are less understood and

theorized, however, their potential explains why people has done much research on it so far, and

will also continue to do so in the future.

Application of polymers or polymer based devices into biotechnology, such as for DNA sensor

is undoubtedly one of the most interesting topics today and in the future. Field effect transistor

(FET) has been proven as an appropriate device to detect label-free DNA in the recent two

decades [1]-[6]. The DNA immobilized on the FET structure influences the surface potential by

1

2

its intrinsic molecular charge [7]. The theoretical description of the observed effects, however,

is still under discussion [8] [9]. Most DNA sensors based on FET are of extended-gate structure,

and require complex manufacturing and evaluation. Organic material based DNA sensors are

less reported.

1.1.2 Aim of This Work This work is aimed to evaluate the possibility of using a wet organic field effect transistor

(WetOFET) to detect the hybridization of DNA chains. Differing from the extended-gate FET, a

more convenient design, with a transporting fluidic channel, is used in this work. A micro

fluidic interface will be built on an OFET allowing WetOFET measurements in the presence of

fluid. Single strand DNA with a cholesterol tag will be introduced on the fluid-polymer (active

layer) interface. The characteristics changes after injecting non-complementary DNA and

complementary DNA will be compared and discussed with respect to possible DNA sensing

application.

1.1.3 Outline The basic concept of organic field effect transistors and DNA will be given in Chapter1. The

main goal of this work and the structure of WetOFET will be presented in Chapter2. All the

experimental details are in Chapter3, and the results and discussions are in Chapter4. Chapter5

is the summary of this work.

1.2 Theoretical Background

1.2.1 Conjugated Polymers In 1977, Heeger, McDarmid, and Shirakawa demonstrated that a certain polymer,

polyacetylene, can be converted into a metallic-like conducting material if exposed to chemical

dopants. This discovery was awarded the Nobel Prize in Chemistry (2000), and opened a

fantastic new era of research on polymers. The basic knowledge of conducting polymers will be

described from the density of states via carbon-carbon bonds, the most seen bond in polymer, to

the polymeric semiconductor and its charge carriers in the following.

1.2.1.1 Density of States

The number of allowed energy levels for an electron as a function of energy is called the density

of states. The simplest case is an isolated atom where the electrons have discrete energy levels.

3 For example, the energy levels of an isolated hydrogen atom are given by the Bohr model [10].

For two identical atoms, when they are far apart, the allowed energy levels for a given principal

quantum number consist of one doubly degenerate level resulting in both atoms having the same

energy. When the two isolated atoms are brought closer, the doubly degenerate energy levels

will split into two levels by interaction between the atoms. When a number, N, of isolated atoms

are brought together and bound to each other, the interactions include the forces of attraction

and repulsion, causing a shift in the energy levels. When N is large, the result is an essentially

continuous energy band. The electrons always obey the Pauli exclusion principle, that two

electrons can occupy one energy level. If all atoms contribute one electron, at a temperature of

absolute zero (0K), the electrons will occupy all states in the valance band and the conduction

band will be empty. The bottom of the conduction band is called Ec, and the top of the valence

band is called Ev. The difference between Ec and Ev is the band gap energy Eg. The size of Eg

will decide whether the material is an insulator or semiconductor. Conductors have no Eg.

1.2.1.2 Hybridization and Conjugation

The electron configuration can be described as a wave function, and the electrons of an atom are

spatially distributed over a set of orbitals defined by probability. Regarding the single electron

approximation, carbon has the ground state electronic configuration 1s22s22p2, denoting that

there are two electrons in each 1s-orbital, 2s-orbital, and 2p-orbital respectively. 2s22p2

represents especially the four valence electrons in carbon. Furthermore, the wave function of the

electrons can be described by hybridized bonding orbital, which represents the total charge

distribution in a given bonding configuration depending on different interactions with

surrounding atoms. The three hybridized orbitals of carbon are shown in Figure1.1.

4

z

y

x

sp：Linear

sp2：Triangular

sp3：Tetrahedral

y

z

x

120°z

y

x

109.5°

z

y

x

z

y

x

y

x

sp：Linear

sp2：Triangular

sp3：Tetrahedral

y

z

x

120° y

z

x

y

z

x

120°z

y

x

109.5°

z

y

x

z

y

x

109.5°

Figure 1.1. Hybridization forms of carbon.

For example, in Figure 1.2, the bonding between carbons in polyacetylene is formed by

sp2-hybridized orbitals. Three electrons in the 2p- and 2s-orbital are hybridized into a sp2-orbital,

which form σ-bonds, while the last 2p-electron forms the π-bond. The extra bond between two

carbon atoms appears as a double bond (C=C) in chemical notation.

Sp2

Energy

2p

2s

1s

2p

1s

Hybridize

Sp2

Energy

2p

2s

1s

2p

1s

HybridizeEnergy

2p

2s

1s

2p

2s

1s

2p

1s

Hybridize

(a)

5

π-bonding

C

H

C

pσ-bonding

C

H

C sp2pz

π-bonding

C

H

C

pσ-bonding

C

H

C sp2pz

(b)

Figure 1.2. sp2 hybridization is shown by (a) energy levels and (b) 3-dimensional model of

carbon-carbon bond in polyacetylene. The π-bonding forms the extra bond.

The π-orbital will be focused on now, since it influences the electrical properties. Again, when

two 2p-orbitals are brought close, they are described by new wave function. Orbitals having the

same energy level will split into two different orbitals with different energy levels. One is

bonding and marked as π, the other is anti-bonding and marked as π*. When there is a huge

amount of 2p-orbitals such as in a polyacetylene polymer chain, the energy levels will form two

continuous energy bands, which are shown in Figure 1.3. The orbital that is occupied by the

highest energy electrons is called the Highest Occupied Molecular Orbital (HOMO) and the

lowest unoccupied orbital is called Lowest Unoccupied Molecular Orbital (LUMO).

E

1 2 4 8 ∞

2p

π

π*

Number of carbon atom in molecule

E

1 2 4 8 ∞

2p

π

π*

Number of carbon atom in molecule Figure 1.3. The overlap of π-orbitals generate split energy states. The number of states are

dependent on the number of overlapping π-orbitals. If the number is large, the energy states will

form the energy bands.

6

1.2.1.3 Charge Carriers

Polymers can be divided into two categories depending on the available energy states of the

system. One is called degenerate ground state systems, and the other is called non-degenerate

ground state systems. If single and double bonds can be interchanged without changing the

ground state energy, the polymer system is denoted as a degenerate ground state system. If the

interchange of single and double bonds is associated with two states of energy levels, the system

is said to have a non-degenerate ground state. Polyacetylene is found with two different forms,

trans-polyacetylene or cis-polyacetylene, as shown in Figure 1.4. Trans-polyacetylene has

degenerate ground state while cis-polyacetylene has non-degenerate ground state.

Cis-polyacetylene has two forms with two different energies, aromatic and quinoid; the quinoid

form is a higher energy state.

trans-polyacetylene cis-polyacetylene

aromatic

quinoid＝

trans-polyacetylene cis-polyacetylene

aromatic

quinoid

trans-polyacetylene cis-polyacetylene

aromatic

quinoid＝

Figure 1.4. Trans-polyacetylene and cis-polyacetylene which represent degenerate- and

non-degenerate ground state systems respectively.

In order to have a conducting polymer, there must be charge carriers in the polymer chain.

Charge carriers can be created through oxidative or reductive doping processes. The oxidative

doping, p-doping, is the result of withdrawing an electron from the π-bond system, which

produces a positive charge unit on the conjugated polymer:

Equation 1

P-doping: P + yA- → Py+A-

y + ye-

Where P denotes the polymer chain, A denotes the charge-compensating counter ion, e- is the

electron and y is the number of counter ions. Instead of oxidation, reductive doping, n-doping,

is caused by introducing electrons into the π-bond system which produces a negative charge

unit in the conjugated polymer:

7 Equation 2

N-doping: P + ye- + yA+ → Py-A+y

Charge carriers are transported along the π-bonds of the polymer chain. The charge carrier can

either be soliton, polaron, or bipolaron. For example, combining two segments of

trans-polyacetylene within different bond order, a defect in the form of an unpaired electron is

created. This defect is called a neutral soliton. Solitons can either be neutral, positive, or

negative depending on the number of carried electrons, zero, one, or two. Different solitons are

shown in Figure 1.5.

Neutral soliton

Positive soliton

Negative soliton

Neutral soliton

Positive soliton

Negative soliton (a)

(b)

Figure 1.5. Three classes of soliton in polyacetylene. (a) Number of electron, one, zero, or two,

define the neutral, positive, or negative soliton. (b) Neutral, positive, and negative soliton shown

by band structures.

8

For non-degenerate polymers, the charge carriers are called polarons or bipolarons [11]. By

oxidizing the polymer, an electron is removed and a cation-radical pair is created. In between

the cation and radical, a transformation from aromatic form to quinoid form occurs. This

confined change and associated charge is called a polaron. The polaron occupies an energy level

which is represented by poly(3-hexylthiophene) P3HT in Figure 1.6. The quinoid structure has a

higher energy compared to the aromatic structure. So in contrast to the soliton, the polaron must

overcome an energy activation barrier associated with quinoid and aromatic structure

differences while moving. If two electrons are withdrawn from the polymer, it forms a

bipolaron. And if the polymer has been oxidized even further, bipolaron energy bands will be

generated.

(a)

9

Conduction

Band

Valence

Band

Neutral Polymer Polaron Bipolaron Bipolaron Band

Conduction

Band

Valence

Band

Neutral Polymer Polaron Bipolaron Bipolaron Band

(b)

Figure 1.6. (a) Generation of positive polaron and bipolaron in P3HT. (b) Energy level of the

neutral polymer, a polaron, a bipolaron, and a polymer with bipolaron energy band are shown.

1.2.2 Organic Field-Effect Transistor Field-effect transistor (FET), which is nowadays the most common transistor type, has been

presented for lots of application such as logical circuits and gas sensors. FETs using conducting

polymers are normally called organic field-effect transistor (OFET). It has started attracting

considerable attention in this decade due to potential application of inexpensive and flexible

electronic devices [12]. In normal situation, there are many similarities between FET and OFET,

which imply that people can operate both of them in the same way. However, the physical

explanations may be different, most of which are not understood well enough. The term OFET

or any analogue term, is commonly used for devices where the active part of the transistor is an

organic material. The typical structure and operation of OFET will be discussed in the

following.

1.2.2.1 Basic Structure of OFET

Most OFET [13] contains four essential parts. Firstly, the gate is used to generate the

modulating field. Secondly, two electrodes, source and drain, are used to define the carrier

channel. The third is the dielectric layer. Last but not least is the active layer, which can be

10

chosen from an appropriate organic material. The characteristics will change due to different

modulation of gate, source, and drain.

When applying a gate voltage, charges will start to accumulate at the interface of the dielectric

layer and the active layer. When a certain number of charge carriers are created and the

potential generated by the source and drain is enough to overcome the activation barrier, the

transistor has been turned on.

The potential difference between source and drain will result in a gradient in the carrier

concentration distribution in the channel. If the potential between source and drain is smaller

than gate bias after the channel has been created, the current through the channel can be

controlled and is proportional to the potential of source and drain. This is called linear mode or

ohmic mode. While the potential between drain and source is close to the gate voltage (gate

potential minus threshold voltage), the asymmetrical distribution of carriers will create a “pinch

off” near the drain end of the channel which means there are no carriers at or near the drain end.

During pinch off, the current through the channel is no longer dependent to the potential

difference of drain and source, this is called saturation mode.

Gate

Source

Drain

Active layer

Insulator(dielectric layer)

Gate

Source

Drain

Active layer

Insulator(dielectric layer)

(a)

Gate

Insulator(dielectric layer) Active layer

Source and Drain

Gate

Insulator(dielectric layer) Active layer

Source and Drain

(b)

Figure 1.7. Typical organic field-effect transistor (OFET) (a) Top view. (b)Side view.

11 1.2.2.2 Transfer, Output, and Time Measurements

There are many methods to extract information from the transistor. Transfer characteristics is

one of the most common methods. In a transfer characteristics measurement, the voltage bias of

source and drain are kept at a certain value, and the gate voltage swept through a range of

suitable values. Ideally, the saturation current that flows from source to drain can be described

by Equation 3 [14]:

Equation 3

( 2TG

0nsat V-V

2LCWI ⎟

⎠⎞

⎜⎝⎛≅

μ )

Output characteristics are obtained by stepping the gate voltage and sweeping the drain or

source voltage between certain values for each gate step. Time measurements are done by

adding time steps to measurement and monitoring the source-drain current. In this work, the

source and drain are usually kept at constant value and the current dependence on time and with

changing gate bias is investigated.

1.2.3 Fluorescence Fluorescence is a luminescence phenomenon observed while suitable excitation and emission

processes of a specific material occur under exposure of certain energy photons. In scientific

applications, the specific material is usually called a fluorophore. Fluorophores can be attached

to non-fluorescent molecules to make them fluorescent. The fluorescence can then tell whether

the attached molecule is definitely present. The fluorescent dye used in this thesis work is

Alexa350, which has an excitation peak at 350nm and an emission peak at 441nm, see Figure

1.7 [15].

Figure 1.7. Excitation and emission spectrogram of Alexa350.

12

1.2.4 DNA and Hybridization Chemically, DNA or Deoxyribonucleic acid is a long polymer chain composed of a sequence of

simple units called nucleotides. The Backbone of DNA is made of sugar and phosphate groups,

see Figure 1.8(a). Attached to each sugar is one of four types of nitrogenous bases; adenine,

thymine, cytosine, or guanine abbreviated as A, T, C, and G respectively [16][17]. The

phosphate groups on the backbone contain negative charges, which are assumed to be detectable

by the OFET in this thesis work. The DNA double helix is formed by hydrogen bonds between

certain base pairs, which are A-T and C-G, foremost because the specific chemical structure of

these base pairs are advantageous for forming hydrogen bonds with each other, the hydrogen

bonding of A-T and C-G are shown in Figure 1.7(b). Commonly, the DNA will be shown by

letter combination instead of a drawn structure. For example, 5’-CCC TCC GTC GTG CCT or

3’-CCC AGG CAG CAC GGA is shown by a certain number with each abbreviation of bases.

The 5’(five prime) and 3’(three prime), which means a certain carbon on the sugar, are referring

to the sequence of the DNA chain. Usually, for convenience, 5’ is called the head side and 3’ is

called the tail side.

13

N

N

NN

NH2

N

NN

NH

O

NH2

NN

NH2

O

N N

O

O

HO

OP

O

O

O

O

OO

PO

O

O

OO

PO

O

O

OO

PO

O

O

NN

O

NH2

N

N

N

NH

O

NH2

N

N

N

N

NH2

NN

O

O

H

OO

PO

O

O

O

OO

PO

O

O

OO

PO

O

O

OO

PO

O

O

Adenine

Thymine

Guanine

Cytosine

5’end 3’end

3’end 5’end

N

N

NN

NH2

N

NN

NH

O

NH2

NN

NH2

O

N N

O

O

HO

OP

O

O

O

O

OO

PO

O

O

OO

PO

O

O

OO

PO

O

O

NN

O

NH2

N

N

N

NH

O

NH2

N

N

N

N

NH2

NN

O

O

H

OO

PO

O

O

O

OO

PO

O

O

OO

PO

O

O

OO

PO

O

O

N

N

NN

NH2

N

NN

NH

O

NH2

NN

NH2

O

N N

O

O

HO

OP

O

O

O

O

OO

PO

O

O

OO

PO

O

O

OO

PO

O

O

NN

O

NH2

N

N

N

NH

O

NH2

N

N

N

N

NH2

NN

O

O

H

OO

PO

O

O

O

OO

PO

O

O

OO

PO

O

O

OO

PO

O

O

Adenine

Thymine

Guanine

Cytosine

5’end 3’end

3’end 5’end (a)

N

N

NN

O

H

NH

H

NN

NH

O

H

Guanine Cytosine

NN

O

O

HN

N

NN

NH

HAdenine Thymine

N

N

NN

O

H

NH

H

NN

NH

O

H

Guanine Cytosine

N

N

NN

O

H

NH

H

NN

NH

O

H

N

N

NN

O

H

NH

H

NN

NH

O

H

Guanine Cytosine

NN

O

O

HN

N

NN

NH

HAdenine Thymine

NN

O

O

HN

N

NN

NH

H

NN

O

O

HN

N

NN

NH

HAdenine Thymine

(b)

Figure 1.8. DNA structure. (a) Double strand DNA. The oxygen on the phosphate contains one

negative charge. (b) Hydrogen bond between the nitrogenous bases, adenine, thymine, cytosine,

and guanine.

14

Chapter 2

Architecture of Wet Organic Field-Effect Transistor

Wet organic field effect transistor (WetOFET) contains three parts: main body (OFET), injector

holder, and injector which are shown in Figure 2.1 (c), (b), and (a) respectively. While injecting

fluid by a syringe, the fluid will go form the injector via the holder and through the fluid

channel of the main body. Filter papers will collect the waste fluid at the other side.

(a)

(b)

(c)

(a)

(b)

(c)

Figure 2.1. Separated parts of WetOFET (a) injector (b) injector holder (c) transistor

The main body of the WetOFET is constructed as following: An ITO coated glass is

appropriately etched and used as the base. The length and width of carrier channel were about

200μm and 1cm respectively. The reason for etching the ITO at the sides is that sometimes the

polymer coating is not good at the edge of the substrate, and it is also convenient for processing

15

16

the device without damaging the polymer in the channel. After spin coating the polymer, a

spacer, usually double sided tape, is used to define a fluid channel, ideally as wide as the current

channel, which is shown in Figure 2.2. The gate electrode finally caps the top, creating a fluid

channel with the same width as the length of the transistor channel and same length as that of

the glass base . Be careful, the spacer should be well attached to both polymer surface and gate

in order to avoid leakage current from either the gate or ITO. It is important to avoid shear

forces when attaching the gate; otherwise the spacer may move and destroy the polymer film or

channel alignment. A side view of a WeTOFET is shown in Figure 2.3.

ITO

Spacer

Substrate

ITO

Spacer

Substrate Figure 2.2. The double side tape define the fluid channel.

SpacerGate

Substrate

SpacerGate

Substrate

Figure 2.3. Side view of WetOFET

17 The injector holder is made of a thin PDMS film that is 0.5 to 1 mm and can be easily produced

by putting a PDMS droplet on a smooth plastic surface and covering it with a piece of glass

before heating. An injector of appropriate size, wider than the thickness of OFET and longer

than the side of the gate electrode can be cut from the thin film. A hole which is few times

larger than the fluid channel is made at the center of thin film. This holder can be attached on

the side of the OFET, and can support the OFET when standing on a heater of 75 degree Celsius,

with the holder side downward as is shown in Figure 2.4. The holder and OFET can be sealed

by PDMS on the heater. The holder is not only for attaching the injector but can also prevent the

PDMS from going from the edge into the fluid channel and sealing the entrance.

PDMS

Heater

PDMS

Heater

Figure 2.4. The OFET is standing on the edge during sealing of the injector holder.

18

The injector is made by PDMS with an IC-socket pattern, see Figure 2.5. The diameter of the

socket is slightly smaller than the syringe head, so the syringe head can be well covered by the

PDMS without fluid leakage while injecting. The smooth side of the injector can be attached to

the holder, and can be sealed by PDMS on the heater, too. Finally, all the contacting edge of

PDMS and glass need to be sealed by instant adhesive to make sure the fluid will not leak out

due to the syringe pressure.

Figure 2.5 IC-socket is used to pattern the injector.

Chapter 3

Experimental Details

3.1 Material

3.1.1 Poly(3-hexylthiophene) P3HT, shown in Figure 3.1, has been demonstrated as semiconducting layer of FET since 1988

[18] The charge carriers are holes which could be polaron or bipolarons. P3HT has excellent

electronic and mechanical properties, and a high environmental stability [12]. A quite small

threshold voltage was reported in P3HT based FET having a hygroscopic polymer as gate

dielectric [19]. P3HT is thus undoubtedly an appropriate choice as the semiconducting layer

(active layer) for this application. P3HT is dissolved in chloroform with a 5mg/ml concentration.

The solution was heated to 70 degree Celsius and stirred at 2500rpm by a vortex for few

seconds before spin coating.

S SS

C6H13

C6H13C6H13

............

n

Figure 3.1. Structure of poly(3-hexylthiophene) (P3HT)

19

20

3.1.2 Cholesterol Tag Cholesterol, shown in Figure 3.2, is a lipid that can be found in cell membranes of all animal

tissue. In this work, its hydrophobic character make it attach to the P3HT which is also

hydrophobic, and introduces the DNA strand close to the P3HT surface if the DNA has been

synthesized with a cholesterol tag.

CH3

OH

CH3

CH3

CH3

CH3

Figure 3.2. The chemical structure of cholesterol which is used as tag on DNA and which can be

attached to hydrophobic surface, for example P3HT.

3.1.3 DNA samples There are five kinds of DNA used in this work. DNAa-Alexa350 with cholesterol tag, which is

used to check whether the DNA with a cholesterol tag can attach to the P3HT surface because

of its hydrophobic property. DNAa with cholesterol tag which is introduced into the fluid

channel as the first single strand and which can be hybridized with its complementary DNA

strand. DNAx with cholesterol tag can hybridize with the tail side of DNAa and form DNAxa

to increase the chance of DNA to attach to the P3HT surface since there is one more cholesterol

tag. DNAa’ that is used to hybridize with DNAa, and it is also called complementary part of

DNAa. DNAb’ is used as the reference for DNAa’ because it will not hybridized with DNAa.

The DNA configuration is shown below:

DNAa-Alexa350 3’CCC AGG CAG CAC GGA ACC TGT AGT CTT TAT

[5’]Alexa350[3’]Cholesterol-Tag

DNAx 5’-CCC TCC GTC GTG CCT

[5’]Cholesterol-Tag

DNAa 3’-CCC AGG CAG CAC GGA ACC TGT AGT CTT TAT

[3’]Cholesterol-Tag

21 DNAa’ 5’-TGG ACA TCA GAA ATA CCC \

DNAb’ 5’-AAC CTA GTG AGA AAT CCC

3.1.4 PDMS Polydimethylsiloxane (PDMS) is a widely used silicon-based polymer which is shown in Figure

3.3. It is well known for its unusual rheological properties. In this thesis work, PDMS was

supplied as a viscous liquid, and solidified by heating at 75 degrees Celsius.

SiO

SiO

Sin

Figure 3.3. Chemical structure of Polydimethylsiloxane (PDMS)

3.1.5 Phosphate Buffered Saline (PBS) Buffer solutions are solutions that resist change of pH upon addition of small amounts of acids

or bases, or upon dilution. PBS is a general buffer used for dilutions and washing. All the kinds

of different DNA in this thesis work are dissolved in PBS, and PBS is also used to rinse the

fluid channel in order to wash away superfluous DNA. PBS contains sodium chloride (NaCl),

dibasic sodium phosphate (Na2HPO4), potassium chloride (KCl), and monobasic potassium

phosphate (KH2PO4). The PH of PBS is between 7.28 and 7.60.

22

3.1.6 Experimental set-up Syringes and sample holder are shown in Figure 3.4(a) and (b) respectively. The box is used to

connect the WetOFET to a Keithley 4200 parameter analyzer. At the side of box, there is a hole

for the syringe. A transparent holder inside the box is used to fix the WetOFET, and is easy to

set up.

(a)

(b)

Figure3.4. Experimntal setup. (a) Syringes. (b) Analyzing box which is designed for coaxial

connections.

23 Double-sided tape from 3M-Scotch is used as spacer, and Loctite-401 instant adhesive is used

as sealant. It is recommended to leave the instant adhesive in contact with air for thirteen

minutes to one hour before use. Concerning instant adhesive needs one or two hour to dry up in

order to seal the edge of the PDMS and glass.

3.2 Spin Coating Spin coating is a common process in the semiconductor industry to spread photoresist on wafers.

Since polymer materials can be dissolved in certain solvents such as water and chloroform. Spin

coating offers a convenient and acceptable way to produce thin uniform films near room

temperature without a vacuum surrounding. Spin coating process follows a few steps: (1)

Dispense suitable amount of solution on the substrate. If needed, cover the area that is supposed

to be coated. (2) Switch on the rotation and ramp-up the speed in order to spread out the

solution to cover the whole substrate. (3) Keep the rotation speed constant while removing the

solvent until dry. See Figure 3.5. Typically, the speed of spin coating is between 1000rpm

(revolutions per minute) and 3000rpm. The thickness depends on the rotation speed, the

concentration of the solution, and the wetting compatibility of the substrate.

DIspense

Ramp-up,Spreading

Constant ω,Drying

DIspense

Ramp-up,Spreading

Constant ω,Drying

Figure 3.5. Spin coating process.

24

3.3 Measurements Fluorescence measurements of single stranded DNA with the Alexa350 dye was made on a

Zeiss Axiovert inverted microscope A200Mot with a Hg lamp (HBO 100) as light source.

Photos were taken before and after injection of the DNA strand with Alexa350. The

double-sided tape was substituted by PDMS film in order to avoid strong background

luminescence from the tape.

Figure 3.6. Zeiss Axiovert inverted microscope A200Mot.

25

The electrical measurements, transfer characteristics, output characteristics, and time

measurement, were done with a Keithley 4200 parameter analyzer. Different measuring

parameters have been adjusted during experiments in this work; Hold time, measurement

corresponds to the time that electrode bias is held before sweeping. The time that the analyzer

waits before getting a data point from a certain bias is called delay time. For example, if hold

time and delay time are 15seonds and 2 seconds respectively in a transfer characteristic

measurement, the drain and source will be biased 15seconds before sweeping the gate voltage.

During the sweeping, the instrument will wait for 2 seconds at each gate voltage, and then

collect the data.

Figure 3.7. Keithley 4200 parameter analyzer.

26

Chapter 4

Results and Discussion

4.1 General results

4.1.1 Transfer characteristics A normal transfer characteristic curve of a WetOFET is shown in Figure 4.1. Hysteresis

phenomena, when the current of on-sweeping is smaller than current of off-sweeping, almost

always appears in WetOFETs. Because the spacer do not completely cover the source and drain

electrodes, there is always a leakage current, depends on how well the spacer is constructed. It

goes from the electrodes through the polymer thin film via the dielectric liquid to the gate or

vice versa. The leakage can be seen more clearly in the diagram of gate current versus gate

voltage which is shown in Figure 4.2. When the source was set as 0V (VS = 0V) and drain was

set as -0.5V (VD = -0.5V). At the beginning, most of the leakage current is between drain and

gate electrodes. Decreasing the gate voltage (from 0V to -0.5V) increases the current leakage

between source and gate and at the time decrease that between drain and gate. In the same way,

the leakage current present a more important role at Vg = 0 in drain current and Vg = -0.5 in

source current respectively. This is why the I-V curve of source versus gate usually goes from

zero, and the I-V curve of drain versus gate will decrease slightly at the beginning. This will

also make the hysteresis of the source versus gate curve look smaller than the drain versus gate

curve. The data is more reliable near VG = 0V region and near VG = -0.5V region in diagrams of

source current versus gate voltage and diagrams of drain current versus gate voltage

respectively. On the other hand, the gate current should always be taken care of in order to

27

28

double-clarify if is a well fabricated fluid channel.

-0.5 -0.4 -0.3 -0.2 -0.1 0.0

0.0

2.0x10-8

4.0x10-8

6.0x10-8

8.0x10-8

Sour

ce c

urre

nt (A

)

Gate Voltage (V)

(a)

-0.5 -0.4 -0.3 -0.2 -0.1 0.0

0.01.0x10-8

2.0x10-8

3.0x10-8

4.0x10-8

5.0x10-8

6.0x10-8

7.0x10-8

8.0x10-8

Dra

in C

urre

nt (A

)

Gate Voltage (V)

(b)

Figure4.1. Typical transfer characteristics with dual sweep. VS = 0V and VG = -0.5V. (a) Source

current versus gate voltage. (b) Drain current versus gate voltage.

29

-0.5 -0.4 -0.3 -0.2 -0.1 0.0-2.0x10-9

-1.0x10-9

0.01.0x10-9

2.0x10-9

3.0x10-9

4.0x10-9

5.0x10-9

6.0x10-9

Gat

e C

urre

nt (A

)

Gate Voltage (V)

Figure 4.2. Sum of leakage current through the gate electrode. VS = 0V and VG = -0.5V.

The square root of source and drain current can be calculated and plotted with gate voltage, and

mobility and threshold voltage can be extracted. But not only hysteresis plays a trick during

mobility and threshold voltage evaluation, the number of times of measurement and delay times

also play troublesome roles. Figure 4.3 is a good example for the measurement of number effect.

A device has been analyzed sixteen times, including first data and fifteen extra data swept with

the same 0.1 second delay time. Usually, current will increase with the measurement number as

shown by fifth, tenth, and sixteenth data unless the current goes to some saturation value. But in

some cases, the current will go down for some unknown reason such as second data. The word

“usually” means the phenomena does not always appear. Threshold voltage may also change

with measurement number, for example, threshold voltage was smaller in sixteenth cycle than

in second cycle. In addition, if mobility was roughly evaluated by drawing a line through the top

of curve and center of hysteresis area, it was larger in sixteenth cycle than in second cycle. The

most important thing is that this effect may lead to the wrong conclusion.

30

-0.5 -0.4 -0.3 -0.2 -0.1 0.02.0x10-54.0x10-56.0x10-58.0x10-51.0x10-41.2x10-41.4x10-41.6x10-41.8x10-42.0x10-42.2x10-42.4x10-4

(Dra

in c

urre

nt)1

/2

(A)1

/2

Gate voltage (V)

first cycle second cycle fifth cycle tenth cycle sixteenth cycle

Figure4.3. Influences of measurement number on current, mobility, and threshold voltage.

Figure 4.4 shows the influence of delay time. The device was measured twice for each delay

time, and the second data is shown in this diagram. First of all, hysteresis was decreasing while

increasing delay time. For instance, mobility and current increased with increasing delay time if

the rough evaluation mentioned above was used. However, the sequence of measuring is still

concerned here, so the increase of mobility and current may be caused by either effect. Again,

the most important thing is that the evaluated results can be misinterpreted which will be taken

into account in the following DNA analysis.

31

-0.5 -0.4 -0.3 -0.2 -0.1 0.00.0

5.0x10-5

1.0x10-4

1.5x10-4

2.0x10-4

2.5x10-4

3.0x10-4

3.5x10-4

(Dra

in c

urre

nt)1

/2

(A)1

/2

Gate voltage (V)

0 sec 0.5 sec 1 sec 1.5 sec 2 sec 2.5 sec

(a)

-0.5 -0.4 -0.3 -0.2 -0.1 0.00.0

1.0x10-82.0x10-83.0x10-84.0x10-85.0x10-86.0x10-87.0x10-88.0x10-89.0x10-81.0x10-71.1x10-7

|dra

in c

urre

nt| (

A)

Gate voltage (V)

0 sec 0.5 sec 1 sec 1.5 sec 2 sec 2.5 sec

(b)

Figure 4.4. Influences by delay time. (a) Mobility and threshold changes. (b) Hysteresis and

current changes.

32

4.1.2 Output characteristics From the discussion above, the current may change depending on number of times of

measurement and delay time, On/Off ratio obtained from output characteristics may not be a

suitable way to evaluate data. Figure 4.5 is an example, two output measurements were done

one after the other, but the drain current was not stable. However output characteristic is a

simple method to verify whether a transistor is working or not. In addition, in Figure 4.6, large

drain current at zero gate voltage shows that there was a huge leakage between gate and drain,

and the characteristic was much worse compared to Figure 4.5.

-0.5 -0.4 -0.3 -0.2 -0.1 0.0

-5.0x10-8

-4.0x10-8

-3.0x10-8

-2.0x10-8

-1.0x10-8

0.0

Dra

in c

urre

nt (A

)

Drain voltage (V)

first cycle second cycle

Figure 4.5. Typical output characteristics of WetOFET. Instability between measurements

suggests using On/Off ratio is an inappropriate way of evaluation.

33

-0.5 -0.4 -0.3 -0.2 -0.1 0.0-5.0x10-8-4.5x10-8-4.0x10-8-3.5x10-8-3.0x10-8-2.5x10-8-2.0x10-8-1.5x10-8-1.0x10-8-5.0x10-9

0.05.0x10-91.0x10-8

Dra

in c

urre

nt (A

)

Drain voltage (V)

VG=0 VG=0.1 VG=0.2 VG=0.3 VG=0.4 VG=0.5

Figure 4.6. Output characteristics from a WetOFET with a huge leakage current between gate and

drain electrodes.

4.1.3 Water and PBS The characteristics of WetOFET can be changed by using different fluids as dielectric layer. For

example, in the current versus time measurement shown in Figure 4.7, pure water and PBS were

injected into the fluid channel one by one and repeatedly. Both the current from the water and

the PBS follows a trend. An unknown noise appeared while injecting water, but was not shown

while injecting PBS. On the other hand, a higher leakage current is detected from the gate

electrode when the PBS is introduced, which may be caused by the abundance of ions in the

PBS compared to the water.

34

0 500 1000 1500 2000 2500

1.0x10-8

2.0x10-8

3.0x10-8

4.0x10-8

5.0x10-8

6.0x10-8

|Dra

in c

urre

nt| (

A)

Time (sec)

PBS

Water

Water

PBS

Water

Figure 4.7. Current versus time measurement. Current changes with different dielectric fluids.

4.2 Fluorescence Measurement A fluorescence image taken from a device that has double-sided tape as spacer is shown in

Figure 4.8. Obviously, the background fluorescence from the double-sided tape is too strong to

see the fluorescence emitted from Alexa350, which is only coming from the fluid channel.

Compared with the above, a device having a PDMS spacer can successfully reduce the

background. Before the DNA is injected, the fluid channel reflected only a color of light purple

which is not the fluorescent color of Alexa350. The blue light coming from the Alexa350 can be

seen after injecting DNA and rinsing with PBS immediately. Since ITO coated glass has

hydrophilic surface, the blue light should definitely come from the DNA that is attached to the

P3HT surface and on the PDMS edges. Furthermore, the color in the P3HT region looks quite

homogeneous, so the light should come from P3HT region itself and not from the PDMS edges.

35

Double-sidedtape

Fluid ChannelDouble-sided

tape

Fluid Channel

Figure4.8. Fluorescence picture from a device with double-sided tape as spacer. The background

form the double-sided tape, blue area, is too strong.

PDMS film

Fluid Channel

PDMS film

Fluid Channel

(a)

PDMS filmFluid Channel

PDMS filmFluid Channel

(b)

Figure 4.9. Fluorescence photos comparison between (a) before injecting DNA and (b) after

injecting DNA with the Alexa350 dye.

36

4.3 DNA Detection

4.3.1 Overview As described in chapter3, the DNA which is to be detected is the complementary DNA strand.

Complementary DNA will be focused on and discussed below, although DNA with a

cholesterol tag will also be mentioned. There are four parameters from transfer characteristics

that will be used to evaluate data in this chapter. The maximum current (IMAX), the relative

mobility (μrel), the threshold voltage (VTh), and the hysteresis area (HA), will be introduced in

individual sections. The purpose is to discover a parameter which will not change after injecting

non-complementary DNA but that will change after injecting complementary DNA. As a

compromise between the uncertainties associated with the measurement number and the time

for a device to “switch on”, all the plotted data are collected from the second scan. The impact

of delay time will also be included.

4.3.1.1 Maximum current (IMAX)

In a dual sweep of gate voltage, there will be two current data points at Vg = -0.5V. The first

point will be used here.

4.3.1.2 Relative mobility (μrel)

Because of the difficulty in measuring fluid capacitance, and since what is interesting in a

sensor is relative change, relative mobility has been used for evaluation instead of real mobility.

Relative mobility can be accessed by comparing slopes of the square root of the transfer

characteristics. Since the mobility will change during sweeps of the gate voltage, the slope from

the average value of on-sweep and off-sweep (between VG = -0.4V and VG = -0.5V) will be

used, and the difference between average value and on-sweeping value (between VG = -0.4V

and VG = -0.5V) is used as acceptable error, see Figure 4.10.

37

-0.5 -0.4 -0.3 -0.2 -0.1 0.00.0

5.0x10-5

1.0x10-4

1.5x10-4

2.0x10-4

2.5x10-4

3.0x10-4

(Dra

in c

urre

nt)1

/2

(A)1

/2

Gate Voltage

(1)(2)

Figure 4.10. Mobility evaluation. (1) Line plotted from the average of on-sweep and off-sweep

data between VG = -0.4V and VG = -0.5V (2) Line plotted from the on-sweeping data between VG

= -0.4V and VG = -0.5V. The slope of (1) is used to evaluate mobility. The slope difference

between (1) and (2) is used as error.

4.3.1.3 Threshold voltage (VTh)

Similar to 4.3.1.2, threshold voltage will change during sweeping. Threshold voltage can be

extracted from the intersection of a gate voltage axis through zero current and the straight line

of the square root of current. The line plotted from the average value of on-sweep and off-sweep

(between VG = -0.4V and VG = -0.5V) will be used to extract threshold voltage, and the

difference between average value and on-sweep value (between VG = -0.4V and VG = -0.5V)

will be used as acceptable error, see Figure 4.11.

38

-0.5 -0.4 -0.3 -0.2 -0.1 0.00.0

5.0x10-5

1.0x10-4

1.5x10-4

2.0x10-4

2.5x10-4

3.0x10-4

(Dra

in c

urre

nt)1

/2

(A)1

/2

Gate voltage (V)

(1)(2)

Figure 4.11. Threshold voltage evaluation. (1) Line plotted from the average of on-sweeping and

off-sweep data between VG = -0.4V and VG = -0.5V (2) Line plotted from the on-sweeping data

between VG = -0.4V and VG = -0.5V. Intersect of (1) and gate voltage axis is used to evaluate

threshold voltage. The difference between (1) and (2) is used as error.

4.3.1.4 Hysteresis Area (HA)

The off-sweep current is higher than on-sweep current, as described in 4.1.1, and is described as

hysteresis. While the ions close to the P3HT surface are screened by negative charges on the

DNA during relaxation, the hysteresis may increase, see Figure 4.12. Similar result has been

shown theoretically in literature, to analyze the electrostatic screening caused by the electrolyte,

and can increase the average incubation times in order to limit sensor response [20]. The

characteristic time constant of transfer function of FET device increased after DNA

hybridization as presented in literature [6]. It is suggested that after the hybridization of DNA,

the increasing negative charge from the complementary DNA strand may increase the screening

phenomenon and the hysteresis area.

39

Gate

P3HTDNA with Cholesterol Tag

Ions in PBS

Gate

P3HTDNA with Cholesterol Tag

Ions in PBS

Figure 4.12. Screening by the negative charges on DNA. When the gate voltage decreases during

off-sweep, the negative ions in solution will diffuse away from the bottom, P3HT surface, but

negative charges on the DNA may impede the ion diffusion.

In Figure 4.13 (a), hysteresis is obtained from the difference between VG = -0.4 and the

off-sweep (VG from -0.5V to 0V) gate voltage which induces the same current as induced by Vg

= -0.4V of on-sweep (VG from 0V to -0.5V.) The reason for using -0.4V is that the WetOFET is

sufficiently turned on when VG = -0.4V, and at the same time, the leakage current between drain

and gate is small enough (as discussed in 4.1.1) to use the drain current hysteresis data. The

hysteresis measure obtained in this way will be called V-0.4 in the following.

The other method to compare hysteresis area is to normalize the data and integrate, as shown in

Figure 4.13 (b). This area will be called HAnorm in the following.

40

-0.5 -0.4 -0.3 -0.2 -0.1 0.0

0.01.0x10-8

2.0x10-8

3.0x10-8

4.0x10-8

5.0x10-8

6.0x10-8

7.0x10-8

Dra

in c

urre

nt (A

)

Gate voltage (V)

(a)

-0.5 -0.4 -0.3 -0.2 -0.1 0.0

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

dra

in c

urre

nt

Gate voltage (V)

(b)

Figure 4.13. Hysteresis area evaluation. (a) hysteresis is evaluated by the on-to-off gate voltage

which produces the same current as when the off-to-on gate voltage equals -0.4V (b) hysteresis is

evaluated by the normalized covered area.

41 4.3.2 Non-complementary DNA versus Complementary DNA

4.3.2.1 Maximum current (IMAX)

Usually, after injecting both DNAb’ and DNAa’, IMAX will decrease, but it seems like the

fraction it decrease is similar (Figure 4.14.) The maximum current does therefore not

discriminate between complementary or non-complementary DNA. IMAX sometimes recovers to

the original value after a few times measurements, indicating the instability of IMAX.

Xa b' a'5.5x10-8

6.0x10-8

6.5x10-8

7.0x10-8

7.5x10-8

8.0x10-8

8.5x10-8

|Dra

in c

urre

nt| (

A)

DNA

Figure 4.14. Maximum current comparison of DNAXa, DNAb’, and DNAa’.

42

4.3.2.2 Comparison of Relative mobility (μrel)

One WetOFET was analyzed during the fluid sample injection sequence – DNAXa, DNAb’,

DNAa’. It means that the device was injected with DNAXa solution for a certain time, rinsed

with PBS buffer, and then analyzed with transfer characteristics. After analyzing DNAXa, the

same procedure was repeated with DNAb’ and DNAa’ one by one. The data was evaluated by

the method mentioned in 4.3.1 and is shown in Figure 4.15. The relative slopes in Figure4.15

has been normalized for clarity.

From Figure 4.15 it can be seen that mobility decreases after injecting non-complementary

DNA (DNAb’) or complementary DNA (DNAa’) for all different delay times, except DNAa’

made mobility increase slightly when delay the time was 1.5 second. On the other hand, the

error from one DNA data covers part of the same region as the others, so this can be caused by a

misjudged mobility.

-0.5 -0.4 -0.3 -0.2 -0.1 0.00.0

5.0x10-5

1.0x10-4

1.5x10-4

2.0x10-4

2.5x10-4

(Dra

in c

urre

nt)1

/2

(A)1

/2

Gate voltage (V)

DNA Xa DNA b' DNA a'

Xa b' a'0.40.50.60.70.80.91.01.11.2

Rel

ativ

e sl

ope

DNA

(a)

-0.5 -0.4 -0.3 -0.2 -0.1 0.00.0

5.0x10-5

1.0x10-4

1.5x10-4

2.0x10-4

2.5x10-4

3.0x10-4

(Dra

in c

urre

nt)1

/2

(A)1

/2

Gate voltage (V)

DNA Xa DNA b' DNA a'

Xa b' a'0.40.50.60.70.80.91.01.11.2

Rel

ativ

e sl

ope

DNA

(b)

43

-0.5 -0.4 -0.3 -0.2 -0.1 0.00.0

5.0x10-5

1.0x10-4

1.5x10-4

2.0x10-4

2.5x10-4

3.0x10-4

3.5x10-4

(Dra

in c

urre

nt)1

/2

(A)1

/2

Gate voltage (V)

DNA Xa DNA b' DNA a'

Xa b' a'

0.50.60.70.80.91.01.11.2

Rel

ativ

e sl

ope

DNA

(c)

-0.5 -0.4 -0.3 -0.2 -0.1 0.0

5.0x10-5

1.0x10-4

1.5x10-4

2.0x10-4

2.5x10-4

3.0x10-4

3.5x10-4

(Dra

in c

urre

nt)1

/2

(A)1

/2

Gate voltage (V)

DNA Xa DNA b' DNA a'

Xa b' a'0.550.600.650.700.750.800.850.900.951.001.051.10

Rel

ativ

e sl

ope

DNA

(d)

-0.5 -0.4 -0.3 -0.2 -0.1 0.0

5.0x10-5

1.0x10-4

1.5x10-4

2.0x10-4

2.5x10-4

3.0x10-4

3.5x10-4

(Dra

in c

urre

nt)1

/2

(A)1

/2

Gate voltage (V)

DNA Xa DNA b' DNA a'

Xa b' a'0.650.700.750.800.850.900.951.001.051.10

Rel

ativ

e sl

ope

DNA

(e)

Figure 4.15. Mobility comparisons of non-complementary and complementary DNA with

44

different delay time (a) 0 second (b) 0.5 seconds (c) 1 second (d) 1.5seconds (e) 2 seconds.

Mobility may not decrease even after injecting DNAb’ or DNAa’, Figure 4.16 is an example.

Figure 4.16(a) is plotted for another sample, and the solid line in Figure 4.16(b) shows the

non-decreasing mobility after injecting DNAb’ and DNAa’. The dotted line in Figure 4.16(b) is

from Figure 4.15(a) as a reference. As a conclusion, these tells us that mobility cannot be used

to evaluate whether the DNAa’ has been injected.

-0.5 -0.4 -0.3 -0.2 -0.1 0.00.0

5.0x10-5

1.0x10-4

1.5x10-4

2.0x10-4

2.5x10-4

3.0x10-4

3.5x10-4

(Dra

in c

urre

nt)1

/2

(A)1

/2

Gate voltage (V)

DNA Xa DNA b' DNA a'

Xa b' a'0.40.50.60.70.80.91.01.11.2

Rel

ativ

e sl

ope

Gate voltage (V)

No slope change Reference

(a) (b)

Figure 4.16. Mobility comparison between non-complementary and complementary DNA. (a)

Square root of drain current versus gate voltage. (b) Solid line is the same as (a), and the dotted

line is from Figure 4.15. This shows that the mobility may not change after injecting

non-complementary and complementary DNA.

4.3.2.3 Comparison of Threshold Voltage (VTh)

Threshold voltage changes after injecting DNA with different delay time is shown in Figure

4.17. Again, results from both non-complementary (DNAb’) and complementary DNA (DNAa’)

strand went toward the same direction with increasing threshold voltage (toward positive

direction). The step it increases is not enough to distinguish DNAb’ and DNAa’, even though,

when delay time is 2 seconds, the threshold voltage decreased slightly after injecting DNAa’,

the uncertainty still covers the threshold voltage region of DNAb’. Threshold voltage is

therefore not an appropriate way to distinguish non-complementary DNA and complementary

DNA. Concluding the discussions above, the negative charge on DNA, in this work, may be not

sufficient to cause a change of the surface potential.

45

-0.5 -0.4 -0.3 -0.2 -0.1 0.00.0

5.0x10-5

1.0x10-4

1.5x10-4

2.0x10-4

2.5x10-4

(Dra

in c

urre

nt)1

/2

(A)1

/2

Gate voltage (V)

DNA Xa DNA b' DNA a'

Xa b' a'-0.20-0.16-0.12-0.08-0.040.000.040.08

Thre

shol

d vo

ltage

(V)

DNA

(a)

-0.5 -0.4 -0.3 -0.2 -0.1 0.00.0

5.0x10-5

1.0x10-4

1.5x10-4

2.0x10-4

2.5x10-4

3.0x10-4

(Dra

in c

urre

nt)1

/2

(A)1

/2

Gate voltage (V)

DNA Xa DNA b' DNA a'

Xa b' a'

-0.16

-0.12

-0.08

-0.04

0.00

0.04

0.08

Thre

shol

d vo

ltage

(V)

DNA

(b)

-0.5 -0.4 -0.3 -0.2 -0.1 0.00.0

5.0x10-5

1.0x10-4

1.5x10-4

2.0x10-4

2.5x10-4

3.0x10-4

3.5x10-4

(Dra

in c

urre

nt)1

/2

(A)1

/2

Gate voltage (V)

DNA Xa DNA b' DNA a'

Xa b' a'

-0.12

-0.08

-0.04

0.00

0.04

0.08

Thre

shol

d vo

ltage

(V)

DNA

(c)

46

-0.5 -0.4 -0.3 -0.2 -0.1 0.0

5.0x10-5

1.0x10-4

1.5x10-4

2.0x10-4

2.5x10-4

3.0x10-4

3.5x10-4

(Dra

in c

urre

nt)1

/2

(A)1

/2

Gate voltage (V)

DNA Xa DNA b' DNA a'

Xa b' a'

-0.12

-0.08

-0.04

0.00

0.04

Thre

shol

d vo

ltage

(V)

DNA

(d)

-0.5 -0.4 -0.3 -0.2 -0.1 0.0

5.0x10-5

1.0x10-4

1.5x10-4

2.0x10-4

2.5x10-4

3.0x10-4

3.5x10-4

(Dra

in c

urre

nt)1

/2

(A)1

/2

Gate voltage (V)

DNA Xa DNA b' DNA a'

Xa b' a'-0.12

-0.08

-0.04

0.00

Thre

shol

d vo

ltage

(V)

DNA

(e)

Figure 4.17. Threshold voltage comparisons of non-complementary and complementary DNA

with different delay time (a) 0 second (b) 0.5 seconds (c) 1 second (d) 1.5seconds (e) 2 seconds.

4.3.2.4 Comparison of hysteresis area (HA)

The influence of delay time has been mentioned in the above discussions. Shorter delay time

will cause larger hysteresis, which implies a long response time of WetOFET. After applying

gate voltage, ions in the fluid may need some time to reach a new stable equilibrium position.

This phenomenon can be seen more clearly in time measurements that have a certain value of

gate, drain, and source bias and are measured with respect to time. Figure 4.18 is an example,

when drain and source are biased with -0.5V and 0V respectively, if a gate voltage is suddenly

applied or removed, there will appear a large current in just 0.1 second, and it may be a

47 capacitive current caused by a sudden redistribution of ions. This figure also implies that the

transistor may need some time to reach saturation and may relax to the original state in just a

few seconds. Figure 4.18 is under a huge bias-changing situation which may over amplify the

results. Shorter delay time may make the WetOFET unsaturated or introduce some noise.

0 50 100 150 200 250 300

0.0

2.0x10-8

4.0x10-8

6.0x10-8

8.0x10-8

|Dra

in c

urre

nt| (

A)

Time (sec)

Gate=-0.5V

Gate=0V

Figure 4.18. Current versus time measurement with different gate bias. Huge leakage current

appear with sudden changes of gate voltage.

On the other hand, the smaller the hysteresis is, the less the leakage influence on the drain

current is when the gate voltage is near -0.4V. Combining the above reasons, longer delay times,

1.5 seconds to 2.5 seconds, have been used in this section in order to detect the slight change of

negative charges on the DNA.

A device was analyzed with 2.0 second delay time, and the diagram of relative drain current

versus gate voltage is shown in Figure 4.19. V-0.4 did not change after introducing DNAb’, but

increases (toward positive) after introducing DNAa’. HAnorm decreased after introducing DNAb’,

and increased after introducing DNAa’. This result supports the suggestion in 4.3.1.4, the

screening from the negative charge on the DNA may increase the hysteresis, and the hysteresis

may be an appropriate way to detect the complementary DNA strand.

48

-0.5 -0.4 -0.3 -0.2 -0.1 0.0

0.0

0.2

0.4

0.6

0.8

1.0

Rel

ativ

e dr

ain

curr

ent

Gate voltage (V)

DNA Xa DNA b' DNA a'

(a)

Xa b' a'-0.386-0.384-0.382-0.380-0.378-0.376-0.374-0.372-0.370-0.368

Gat

e vo

ltage

(V)

DNA

Xa b' a'

0.81.01.21.41.61.82.02.2

Rel

ativ

e no

rmal

ized

hys

tere

sis

area

DNA

(b) (c)

Figure 4.19 Hysteresis comparison between non-complementary (DNAb’) and complementary

(DNAa’) DNA. (a) Transfer characteristics comparison. (b) V-0.4 change corresponding to type

of DNA. V-0.4 increased after injecting complementary DNA. (c) HAnorm change corresponding

to type of DNA. HAnorm increased after injecting complementary DNA.

49 Sometimes leakage in the low gate bias regime may influence evaluation of HAnorm, for example,

another device which was also analyzed with a delay time of 2 seconds is shown in Figure 4.20.

The curve of DNAb’ looks quite similar to DNAXa at gate voltage between -0.3V to -0.5V, but

is quite different at gate voltages above -0.2V. This leakage region enlarges the hysteresis of

DNAb’ and is shown in Figure 4.20 (c), which can be compared with V-0.4 in Figure 4.20 (b). So

when evaluating HAnorm, this should be taken into account. For example, integration can be

made just from -0.3V to -0.5V, which is shown in Figure 4.21. Another phenomenon here is

that DNAb’ also make V-0.4 increase slightly, which actually usually happens also in other

samples, but if compared with the increase after DNAa’, the influence of DNAb’ can almost be

neglected.

-0.5 -0.4 -0.3 -0.2 -0.1 0.0

0.0

0.2

0.4

0.6

0.8

1.0

Rel

ativ

e D

rain

cur

rent

Gate voltage (V)

DNA Xa DNA b' DNA a'

(a)

50

Xa b' a'

-0.380

-0.375

-0.370

-0.365

-0.360

-0.355G

ate

volta

ge (V

)

DNA

Xa b' a'

1.0

1.2

1.4

1.6

1.8

2.0

2.2

Rel

ativ

e no

rmal

ized

hyst

eres

is a

rea

DNA

(b) (c)

Figure 4.20. Leakage influence on hysteresis evaluation between non-complementary (DNAb’)

and complementary (DNAa’) DNA. (a) Transfer characteristics comparison. (b) V-0.4 increases

more visibly after injecting complementary DNA (c) HArel increases both with

non-complementary and complementary DNA. The increase was caused by the leakage in the

higher gate voltage region.

Xa b' a'

1.0

1.2

1.4

1.6

1.8

2.0

Rel

ativ

e no

rmal

ized

hys

tere

sis

area

DNA

Figure 4.21. Fix of the leakage influence on hysteresis evaluation. HArel was evaluated just

between gate voltage are between -0.3V and -0.5V.

51 4.3.2.5 Comparison of hysteresis area (HA) - Surface Influences?

Although comparison of hysteresis area seems to be an appropriate method to evaluate and

detect complementary DNA, it does not work as expected sometimes. This problem may be

caused by the surface roughness. Two kind of surface coating are shown in Figure 4.22. These

two surfaces were coated with the same solution, but the one having stripes was coated few

minutes later than the other. For some unknown reasons, P3HT film may have a huge amount of

stripes, which also has been observed with other polymer coatings, dispersed from the coating

center.

(a) (b)

Figure 4.22. Surface morphology of spin coated P3HT. (a) Smooth coating surface. (b) Stripes

shown on the surface.

Results from a “striped” device are shown in Figure 4.23. This device still shows good transfer

characteristics, but if the V-0.4 and HArel are compared between DNAb’ and DNAa’, there is not

enough differences. It does not matter that DNAb’ and DNAa’ caused a slight increase in V-0.4

and HArel.

Combining all the discussion in 4.3.2.4 and 4.3.2.5, complementary DNA may be detected by a

WetOFET that has a smooth surface morphology, and can be evaluated by the hysteresis area of

the transfer characteristic. The increasing hysteresis area could, however, be caused by other

unknown reasons, not discovered in this thesis work. Hysteresis area just points out a possible

but uncertain way to detect DNA.

52

-0.5 -0.4 -0.3 -0.2 -0.1 0.0

0.0

0.2

0.4

0.6

0.8

1.0

Nor

mal

ized

dra

in c

urre

nt

Gate voltage (V)

DNA Xa DNA b' DNA a'

(a)

Xa b' a'

-0.374

-0.373

-0.372

-0.371

-0.370

-0.369

-0.368

Gat

e Vo

ltage

(V)

DNA

Xa b' a'

1.0

1.1

1.2

1.3

1.4

1.5

Rel

ativ

e no

rmal

ized

hys

tere

sis

area

DNA

(b) (c)

Figure 4.23. Hysteresis evaluation of WetOFET with striped surface morphology. (a) Transfer

characteristics. (b) V-0.4 comparison. No obvious difference between non-complementary and

complementary DNA. (c) HAnorm comparison. No obvious difference between

non-complementary and complementary DNA.

Chapter 5

Conclusion

The WetOFET, an organic field effect transistor combined with a fluid channel as a fluidic

sensor has been created in this work. The WetOFET with poly(3-hexylthiophene) (P3HT) as an

active layer can be operated and shows good transfer and output characteristics. The sensitivity

to different fluids, pure water and PBS, has also been demonstrated.

Single strand DNA with a cholesterol tag has been successfully been introduced into the fluid

channel and immobilized on the P3HT surface by the hydrophobic force.

The possibility of using WetOFET as a DNA sensor, showing the difference before and after

DNA hybridization, was also discussed in this project. A few parameters, current, mobility,

threshold voltage, and hysteresis, have been analyzed and compared between

non-complementary DNA and complementary DNA. Current, mobility, and threshold voltage

did not a show considerable difference between non-complementary and complementary DNA

could be the insufficient charges of DNA. The hysteresis seems to increase after hybridization.

This implies that the increasing negative charges near P3HT surface after DNA hybridization

may enlarge the electrostatic screening. Electrostatic screening may impede negative ions that

diffuse away from the P3HT surface more significantly during off-sweep. Furthermore, the

hysteresis change just appeared while using a device with smooth P3HT coating and not with

stripe-like surfaces. However, the devices with smooth surface and striped surface has not been

produced and demonstrated at the same time or even close in time, so it is still not sufficient to

conclude that the hybridization of DNA will influence the hysteresis.

53

Whether the hysteresis was caused by hybridization or not, the influences from the DNA

hybridization are too small. It is not recommended to use WetOFET as a DNA sensor in the

way shown in this work.

54

Chapter 6

Reference

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[10] D. Halliday and R. Resnick. Fundamentals of physics, 2nd ed., Wiley, New York, 1986. [11] G.. Harbeke, D. Baeriswyl, H. Kiess, and W. Kobel. Polarons and bipolarons in doped polythiophenes. Physica Scripta, T13: 302-305, 1986. [12] C. D. Dimitrakopoulos and P.R.L. Malenfant. Organic thin film transistors for large area electronics. Advanced Materials, 14(2): 99-117, 2002. [13] L. M. Andersson. Electronic transport in polymer solarcell and transistors. Linkoping University Electronic Press, Doctoral thesis, 2007. [14] S. M. Sze. Semiconductor Devices. Wiley, New York, 1981. [15] Carl Zeiss Interactive Fluorescence Dye and Filter Database. https://www.micro-shop.zeiss.com/us/us_en/spektral-info.php?i=Objekt-0000-022 [16] P. A. Levene. The structure of yeast nucleic acid. IV. Ammonia hydoolysis. Journal of Biological Chemistry, 40: 415–424, 1919. [17] Abbreviations and symbols for nucleic acids, polynucleotides and their constituents. IUPAC-IUB Commission on Biochemical Nomenclature (CBN), 2006. [18] A. Assadi, C. Svensson, M. Willander, and O. Inganäs. Applied Physics Letters, 53(3): 195, 1988. [19] T.G. Bäcklund, H. G.. O. Sandberg, R. Österbacka, and H. Stubb. Applied Physics Letters, 85(17): 3887 – 3889, 2004. [20] Pradeep R. Nair and Muhammad A. Alam. Nano Letters, ASAP article, 10. 1021/nl072593i, 2008.