Organic Electronics 12 (2011) 1815–1821

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Organic Electronics

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Organic field-effect transistor with extended indium tin oxide gatestructure for selective pH sensing

T.N.T. Nguyen a, Y.G. Seol a, N.-E. Lee a,b,⇑a School of Advanced Materials Science & Engineering, Sungkyunkwan University (SKKU), Suwon, Kyunggi-do 440-746, Republic of Koreab SKKU Advanced Institute of Nanotechnology (SAINT), and Samsung Advanced Institute for Health Sciences and Technology (SAIHST),Sungkyunkwan University (SKKU), Suwon, Kyunggi-do 440-746, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 12 May 2011Received in revised form 7 July 2011Accepted 10 July 2011Available online 4 August 2011

Keywords:Organic field-effect transistor (OFET)Indium tin oxide (ITO)Extended gatepH sensor

1566-1199/$ - see front matter � 2011 Elsevier B.Vdoi:10.1016/j.orgel.2011.07.009

⇑ Corresponding author at: School of AdvancedEngineering, Sungkyunkwan University (SKKU),440-746, Republic of Korea. Tel.: +82 31 290 7398; f

E-mail address: nelee@skku.edu (N.-E. Lee).

In this study, organic field-effect transistors (OFETs) with extended gate structure werefabricated for selective pH sensing applications. Indium tin oxide (ITO) was used asextended gate electrode as well as an active layer for H+ sensing. The threshold voltageof the fabricated ion-selective OFET was varied by the changes in the electrochemicalpotential at the ITO electrode surface upon its exposure to buffer solutions with variablepH values. The sensor showed excellent linearity and a high sensitivity of 57–59 mV/pHin the pH range of 2–12. The selectivity of the ITO sensing layer to H+ ions was also inves-tigated by measuring the interfering effect of Ca2+ and K+ ions in the buffer pH solutions.The results showed that the Ca2+ and K+ ions weakly interfere with the selective pH sensingof the ITO-extended gate OFET sensor device.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

In bio-chemical processes, it is important to know the ba-sic conditions of electrolyte or physiological solution. In thisrespect, the pH sensor for detection H+ concentration wasdeveloped long time ago. For several decades, it has beenstill being investigated for the purpose of improving its per-formance which is characterized by parameters such as thesensitivity, selectivity, stability, drift–hysteresis effect, fastresponse, lifetime, and cost of fabrication.

The popular structures for pH sensors are based onselective glass membrane electrode and Si ion-sensitivefield-effect transistor (ISFET) structures [1,2]. The selectiveglass membrane pH electrode has been the most fre-quently used pH indicator for many years, due to its highselectivity and dynamic pH range. However, it has the dis-advantages of high resistance, brittleness, and difficulty tointegrate into a miniaturized system [2]. Since the Si ISFET

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Materials Science &Suwon, Kyunggi-doax: +82 31 290 7410.

for chemical sensing was first introduced by Bergveld in1970 [1], it has been extensively researched and commer-cialized [3]. The first Si ISFET was based on the use of asilicon dioxide (SiO2) gate dielectric in the MOSFET (me-tal-oxide-semiconductor FET) structure as the H+ sensingmaterial. Since then, many other dielectric materials havebeen investigated for the purpose of replacing SiO2 as thesensing layer for other ions, as well as H+, such as Al2O3,Si3N4, Ta2O5, ZrO2, HfO2, Y2O3, and Gd2O3 [4–8].

However, there were still drawbacks in these structures,because in the Si ISFET structure there is poor isolationbetween the gate dielectrics and the chemical solutions,due to the direct exposure of the sensing area to the solu-tions. This problem causes difficulties in its fabrication andhas limited its chemical and biological applications, becauseof the interaction of the dielectric layer with the solution,resulting in the instability of the sensor. These disadvan-tages could be overcome by using the extended-gate ISFETstructure that was firstly introduced by Spiegel in 1983[9]. In this structure, the sensing area is on the extended-gate side away from the active area of the ISFET structure.Therefore, it is easy to deposit or modify the sensing materi-als by various methods that have no effects on the

1816 T.N.T. Nguyen et al. / Organic Electronics 12 (2011) 1815–1821

performances of the ISFET device. A pH sensor that com-bined the commercial MOSFET and an extended-gate con-cept using V2O5 xerogel, tin oxide (SnO2) and indium tinoxide (ITO) as the pH sensing material was investigated byvarious authors [10–12]. The pH probe coated with V2O5,SnO2, and ITO as the pH sensing materials electrically con-nects with the extended gate electrode of the commercialMOSFET devices. The reference electrode used for the mea-surement of the FET characteristics is used as a floating elec-trode, while both the reference electrode and pH probe areimmersed in the electrolyte. However, the selectivity ofthese sensing layers to other ions including K+ and Ca2+

was not measured.Despite the various advantages of the Si ISFET device, it

also has some limitations, due to the rigidity and opacityof the Si substrate and difficulty of achieving monolithicintegration with microfluidic components for lab-on-a-chip(LOC) or micro-total analysis systems (l-TAS) for clinicaluse. For the facile integration of ISFET devices into the newconcepts of LOC or l-TAS based on transparent polymericor glass substrates having the capability of electrochemicaland optical detection, new ISFET devices based on thin filmtransistor technologies from the display industry lookpromising. Among the various ISFETs based on non-singlecrystalline Si channels [13–15], the organic FET (OFET) is agood candidate, because the OFET has shown considerablepotential in many electronics applications, due to its lightweight, low cost, mechanical flexibility, optical transpar-ency, compatibility for large area applications, and simplefabrication compared to conventional MOSFET devicesbased on silicon [16,17]. Bio-chemical sensors based onOTFTs have been developed by functionalizing an organicsemiconductor sensing layer to specific target species. Somegroups have also developed organic ISFETs for H+ sensing[18,19]. However, the application of organic ISFETs is stilllimited, because of their limited dynamic pH range from4–10 [18–20], poor linear sensitivity behavior [18–20], slowresponse [19], and poor stability [18]. These issues are re-lated to the inherent degradation of the organic semicon-ductor layers which are sensitive to moisture and oxygen.However, in the above-mentioned cases, the organicdielectric or semiconductor layers are directly exposed to

Fig. 1. Schematic of cross-section and top vi

the electrolyte. Also, organic ISFETs with a top gate structurehave difficulties in terms of the fabrication of the H+ sensinggate dielectric on top of the organic semiconductor layer,because of the damage incurred during the formation ofthe gate dielectric. The damage to the organic semiconduc-tor can be prevented by using organic gate dielectrics layers,such as parylene or polyvinyl alcohol (PVA) layers. However,choosing oxide gate materials for H+ sensing is challengingin this case, due to the large difference in the thermal expan-sion coefficients of inorganic and organic materials. Further-more, the ions can be transported through the gateinsulator, which gives rise to a leakage current, due to theformation of cracks in the sensing layer under the influenceof the large electric field in the solution during the measure-ment. These factors will affect the stability of the sensorperformance.

In our work, a bottom–gate OTFT structure with ITO-extended gate sensing electrode was successfully fabri-cated. The side-effects caused by the fabrication processwere minimized to improve the stabilty of the organic IS-FET for pH sensing. This structure could be used to developa stable pH sensor with good linearity over a wide dynamicpH range of 2–12, high sensitivity of 57–59 mV/pH, andfast response. The ITO-extended gate electrode in the or-ganic ISFET could be used for sensing the pH selectivelyin K+ and Ca2+ containing solutions.

2. Experiments

Fig. 1 shows the cross-section view (a) and top view (b) ofthe pH sensor based on the ITO-extended gate OFET. The ITOglass substrates were purchased from Fine Chemicals Co.,Ltd., (Korea). The resistivity of ITO is below 20 ohm/sq, itsthickness is 80 nm, and its transmittance is 87%. The ITO-extended gate OFET with a channel length of 40 lm was fab-ricated on an ITO-coated glass substrate. The ITO layer onthe ITO-coated glass substrate was selectively etched byan etchant solution consisting of 24 wt.% HCl followed by aphoto lithography process for patterning to form a gate elec-trode for the OFET as well as H+ sensing layer. Then, poly(4-vinylphenol) (PVP) was deposited by the spin coating pro-

ew of ITO-extended gate OFET device.

T.N.T. Nguyen et al. / Organic Electronics 12 (2011) 1815–1821 1817

cess as a gate dielectric with a thickness of 250 nm.Pentacene as an organic semiconductor was formed by ther-mal evaporation at 80 �C with a thickness of 60 nm. Finally,gold was deposited with a thickness of 60 nm for use as thesource and drain electrodes. To monitor the sensitivity of theITO layer to H+ ions, a polydimethylsiloxane (PDMS) well de-signed to contain the electrolyte was attached to the sub-strate, which has an extended-gate part attached usingepoxy glue for sealing.

3. Results and discussion

Fig. 2 shows the electrical properties of a standard ITObottom gate OFET, having a current on/off ratio of 105,field-effect mobility, l, of 0.45 cm2 V�1s�1, and thresholdvoltage, VT, of �10.5 V. In the buffer solution, the measure-ment of the OFET using a saturated calomel electrode (SCE)as the floating gate electrode, the electrical properties weresimilar to those obtained for the standard device with thebottom gate electrode. To determine the sensitivity of theITO-extended-gate electrode to H+ ions, electrolytes withdifferent pH values of 2, 4, 6, 8, 10, and 12 were placedone by one in the PDMS well. Due to the effect of bias stresson the pentacene OTFT [21–23] caused by the trapping ofcarriers from the gate bias-induced conduction channelinto localized electronic states, a pre-measurement stabli-zation step should be performed to obtain the stable

Fig. 2. The output and transfer characteristics of the standard ITO-extended bottom gate OFET device.

baseline of the electrical properties. As shown in Fig. 3(a),the measured drain current, ISD, versus elapsed time at asource–drain voltage VSD = �40 (V), voltage of the refer-ence electrode VRE = �40 (V), and pH of the solution of 7shows that the ISD value is stabilized after a measurementtime of 10 minutes. At the initial stage, ISD slightly in-creased from �116 lA to �140 due to the increase in thenumber of charges accumulated in the conduction channelunder reference electrode bias. Then, ISD decreases rapidlyfrom �140 lA to �30 lA in 10 minutes because of the in-crease in the number of carriers trapped at the applied VRE

voltage [21,22]. From this point, the rate of decrease of ISD

over time is small. Therefore, it can be neglected and theinvestigation of the sensitivity of the ITO material to H+

is carried out at this point.The mechanism of H+ ion sensing of the ITO-extended

gate OFET can be explained based on the theoretical oper-ation of the OFET and behavior of the ITO surface in theelectrolyte, which are described by the Gouy–Chapman–Stern–Graham and Site-binding models [2]. The ITO mate-rial is a solid solution of indium (III) oxide (In2O3) and tin(IV) oxide (SnO2), typically 90% In2O3 and 10% SnO2 byweight. Thus, the ITO surface has various ionic states,including InOH, InO�, InOHþ2 , SnOH, SnO�, and SnOHþ2 .

Fig. 3. Source–Drain current (IDS) versus elapsed time at condition ofVSD = �40 (V), VRE = �40 (V), and electrolyte with pH value of 7 in 10 min(a), and transfer characteristics of OFETs in various pH solutions (b).

Fig. 4. Curves of (ISD)1/2 versus VRE with various pH solutions (a), andthreshold voltage, VT, in different pH solutions (b).

1818 T.N.T. Nguyen et al. / Organic Electronics 12 (2011) 1815–1821

Therefore, the interaction between the ITO surface andelectrolyte can be explained by the above models. Thesemodels can be referenced elsewhere [2]. According to thesemodels, there is an electrochemical surface potential, w,of ITO in the electrolyte solution. This potential decaysexponentially within the critical distance given by

kD ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

ee0kTNAq2

Piz2

iCi

r, where e is the dielectric constant of the

electrolyte, e0 is the permittivity of a vacuum, NA is theAvogadro constant, Ci is the concentration of species i inthe electrolyte, and zi the corresponding ionic charge. Actu-ally, the physicochemical change in the electrolyte canonly be detected when it causes a change within the Debyelength and, as a result, a change in the electrochemical po-tential at the interface [24]. Therefore, in the buffer solu-tion, the H+ ions exert a significant effect on the oxidesurface, instead of on the companion negative groups.

To calculate the surface potential between the sensitivelayer and the electrolyte, the equations given in references[25] and [26] can be used. From those equations, the pHsensitivity of the structure based on the ISFET is given by,dwdpH ¼ 2:303 kT

q a, where a is a dimensionless parameter thatvaries between 0 and 1. If a is 1, the ISFET has an ideal sen-sitivity of 59 mV per pH at 25 �C.

The OFET operates almost entirely in accumulationmode, which can be divided into two regions, in whichthe currents are in the linear and saturation regimes,as described by equations, Ilinear ¼ lCWðVG�VTÞVSD

L and

Isat ¼ lCW2L ðVG � VTÞ2 ,respectively, where l is the field-

effect mobility, C the capacitance of the gate insulator, Wthe channel width, L the channel length, and VT the thresh-old voltage. An important parameter for any FET device isthe threshold voltage. It is the gate voltage requiredto turn the transistor ‘‘on’’ and is given by VT ¼ VFBþ

2wB þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2esqNð2wBÞp

C where the flat-band voltage, VFB ¼ /mq �

/sq ,

/m is the metal work function, /s the semiconductor workfunction, es the permittivity of free space, q the uinitcharge, N the semiconductor doping concentration, andwB the Fermi level from the intrinsic Fermi levelðwB ¼ jEF � Eij=qÞ. In the case where a reference electrodeis used as the floating gate electrode instead of a metal gateelectrode, the term /m

q is replaced by the terms that repre-

sent the reference electrode voltage (VRE) and electrolyteðvsol � wÞ. At that time, the flat-band voltage is definedas: VFB ¼ VRE þ vsol � w� /s

q , where vsol is the surface dipole

of the electrolyte and w is the ITO surface potential. There-fore, the threshold voltage is VT ¼ VRE þ vsol � w� /s

q þ

2wB þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2esqNð2wBÞp

C . Therefore, at the same applied VRE,DVT ¼ �Dw. This equation can be used for the calculationof the pH sensitivity, while plotting the transfer character-istics of the ITO-extended gate OTFT with various pHvalues of the electrolyte.

After the pre-measurement stabilization step, the trans-fer characteristics of the ITO-extended gate OTFT weremeasured with electrolytes having various pH values inthe range from 2 to 12, as shown in Fig. 3(b). It can be seenthat ISD increased as the pH value of the solution increased.This can be easily explained by the fact that the increase in

pH value means a decrease in the proton ion concentration.As a result, the proton ion concentration on the ITO surfacedecreases according to the Boltzman equation describingthe relationship between the H+ concentration on the solidsurface ½Hþs � and that in the electrolyte solution ½Hþb �,½Hþ�s ¼ ½H

þ�b expð�qwkT Þ [2]. Therefore, the negative charge

density will increase on the ITO gate surface as the pH va-lue increases. This is equivalent to the increase in the neg-ative bias value of the ITO bottom gate electrode causingthe increase in ISD due to the p-type semiconductor behav-ior of pentacene.

As mentioned above, the H+ ions in the buffer electro-lyte have an electrochemical interaction with the ITO sur-face, resulting in the change in VT of the FET device. Thechange in VT is equal to the change of the surface electro-chemical potential of ITO (but with the opposite sign).Fig. 4(a) shows the curves of (ISD)1/2 versus VRE with vary-ing pH from 2 to 12. From these curves, the threshold volt-age can be extracted for each pH solution as shown inFig. 4(b). From this data, if the pH of the buffer is changedfrom 2 to 4, the VT value is changed from �10.78 to�10.664. Therefore, the difference in the threshold voltage,DVT ¼ 116mV. This means that the difference in the ITOsurface potential is �116 mV. This agrees with the theory

T.N.T. Nguyen et al. / Organic Electronics 12 (2011) 1815–1821 1819

that as the pH value increases, the positive surface chargedecreases. Fig. 4(b) shows the linear behavior of VT versusthe pH of the buffer solution and sensitivity of 57–59 mV/pH.

Actually, a pH sensor for real applications in bio-chemical engineering needs to have good selectivity toprotons without any interference from other ions such asK+, Ca2+, Na+, etc. Therefore, in this study, we investigatedthe selectivity of the ITO layer to K+ and Ca2+ ions.Fig. 5(a) and (b) show the transfer characteristics of theITO-extended gate OTFT for various K+ and Ca2+ ion con-centrations. The data show that the ITO surface is insensi-tive to the variation of the K+ and Ca2+ ion concentrations,because there is no change in the VT value obtained fromthe transfer characteristics. For the quantitative estimationof the interference of K+ and Ca2+ ions to the pH measure-ment, the selectivity coefficient is measured. The selectiv-ity is characterized by determining the selectivitycoefficient, which is defined in the Nicolsky–Eisenman

equation [27,28], E ¼ E0 þ RTzAF ln aA þ Kpot

A;BðaBÞzA=zBh i

, where

zA and zB are the charge numbers of the primary ion, A,and of the interfering ion, B, respectively; aA and aB arethe activities of the primary ion, A, and of the interferingion, B, respectively; R is the gas constant (8.31 J K�1mol�1);F is the Faraday constant (96,485 C mol�1); T is the temper-

Fig. 5. Transfer characteristics in the solutions with various (a) CaCl2 and(b) KCl concentrations.

ature (K). The activity is related to the concentration by theactivity coefficient, c, as a ¼ cc [29], where c can be calcu-lated from the Debye–Huckel equation which estimatesthe effects of interaction between ions in a solution. The

Debye–Huckel equation is given by, � log c ¼ Az2 ffiffiIp1þBa

ffiffiIp [29],

where A and B are constants from the Debye–Huckel the-ory with value of 0.51 and 3.3 � 107, respectively at roomtemperature, I is the ionic strength, and a is the ion sizeparameters. The a values of 600 and 900 pm for Ca2+ andH+ ions, respectively, and 300 pm for both K+ and OH� ionswere used [29]. The ionic strength, I, is a measure of thetotal ions in the solution, weighted according to theircharges, and affects the dissociation and solubility ofdifferent salt. It is defined by, I ¼ 1

2

Pni ciz2

i [29], where ci

and zi are the molar concentration and charge number ofion i. The selectivity coefficient value Kpot

A;B indicates thesensing selectivity to primary ion A compared to interfer-ing ion B. The selectivity coefficient value depends on thenature of sensing layer or selective membrane as well asthe experimental conditions such as ion concentrationand methods of determination [27–29]. The traditionalmethods include the separate solution method (SSM) [28]and mixed solution method (MSM) [28]. In the SSMmethod, the electrochemical potentials are measuredseparately in solutions containing only primary ions A orinterfering ions B. In the case of the MSM method, theelectrochemical potentials are measured both in only pri-mary ions A and in a mixed solution containing theprimary ions A and interfering ions B with various concen-trations. Based on Nicolsky–Eisenman equation, the selec-tivity coefficient in the MSM method can be calculated as,

KpotA;B ¼ aA e

ðEAB�EA ÞzAFRT � 1

� �= ðaBÞzA=zB [28].

The measurements for obtaining sensing selectivity ofH+ to K+ and Ca2+ were carried out in both acid (pH 2 solu-tion) and base (pH 12 solution) medium. The pH 2 and 12solutions with presence of various KCl and CaCl2 concen-trations of 10�2, 10�3 and 10�4 M, respectively, were usedfor investigations. Fig. 6 shows the transfer curves of thepH 2 solutions and pH 2 solutions in the presence of vari-ous KCl concentrations of 10�2, 10�3 and 10�4 M. This dataindicates that the K+ ions have low interference to H+ ions

Fig. 6. Transfer characteristics of the OFET in the pH 2 solution, pH 2solution with 10�2, 10�3, and 10�4 M KCl, respectively.

Table 1The ionic strength and activity coefficient calculation of experimented solutions.

Solution Ionic strength (M) Activity coefficient

c(H+) c(K+) c(oh�) c(Ca2+)

pH 2 solution 0.01 0.8895pH 2 solution with KCl 10�2 M 0.02 0.8475 0.8471pH 2 solution with KCl 10�3 M 0.011 0.8844 0.8842pH 2 solution with KCl 10�4 M 0.0101 0.8889 0.8887

pH 12 solution 0.01 0.8893pH 12 solution with KCl 10�2 M 0.02 0.8471 0.8471pH 12 solution with KCl 10�3 M 0.011 0.8842 0.8842pH 12 solution with KCl 10�4 M 0.0101 0.8887 0.8887

pH 2 solution with CaCl2 10�2 M 0.04 0.7917 0.3922pH 2 solution with CaCl2 10�3 M 0.013 0.8750 0.5860pH 2 solution with CaCl2 10�4 M 0.0103 0.8879 0.6214

pH 12 solution with CaCl2 10�2 M 0.04 0.7910 0.3922pH 12 solution with CaCl2 10�3 M 0.013 0.5860 0.5860pH 12 solution with CaCl2 10�4 M 0.0103 0.6214 0.6214

1820 T.N.T. Nguyen et al. / Organic Electronics 12 (2011) 1815–1821

in the pH 2 solution due to the shift voltage between eachcurve is smaller than 1 mV at constant current. Based onactivity coefficient values of each ion in the mixed solution,presented in the Table 1, the selectivity coefficient could becalculated according to the aforementioned equations. Inthe case of selectivity to K+, in both acid and base medium(pH 2 and 12), the selectivity coefficient value is as low as2 � 10�2. An analogous measurement in the solutions ofpH 2 and 12 containing CaCl2 was carried out. The resultsshow the Ca2+ ions had low interference effect on the pH2 and pH 12 solutions. The selectivity coefficient valuewas measured to be 5 � 10�2. As compared to the selectiv-ity coefficient values of the order of 10�3 reported from themeasurements using ion-selective electrode (ISE) for selec-tively H+ sensing to Ca2+ and K+ ions [27–29], the obtainedvalues from the pH sensor with no selective membrane inour work are slightly higher but appropriate for selectiveH+ sensing in the electrolyte containing Ca2+ and K+ ionswith no large interference.

4. Conclusion

In this work, an ITO-extended gate OFET pH sensor wassuccessfully fabricated and showed good performance. Itssensitivity was as high as 57–59 mV/pH in the pH rangeof 2–12. The interfering effects of K+ and Ca2+ ions wereinvestigated. The results indicated weakly interfering ef-fects exerted by the K+ and Ca2+ ions showing the selectiv-ity coefficient to K+, KþH, K

+, of 2 � 10�2 and to Ca2+, KþH, Ca2+,of 5 � 10�2. Therefore, the ITO-extended gate FET devicecan be used as an ion selective sensor device for pH mea-surements in multi-ion electrolytes and cell-based biosen-sors without an ion-selective membrane. Furthermore,organic FETs with an extended gate structure can be usedas a potentiometric transducer for bio-chemical applica-tions, since the surface of the extended gate ITO can beeasily modified with different sensing layers.

Acknowledgments

This research was supported by the Basic ScienceResearch Program (Grant No. 2010-0015035) and the

WCU Program (Grant No. R32-2008-000-10124-0) throughthe National Research Foundation of Korea (NRF) fundedby the Ministry of Education, Science and Technology(MEST).

References

[1] P. Bergveld, IEEE Transactions on Biomedical Engineering BME-17(1970) 70.

[2] M.W. Shinwari, M.J. Deen, D. Landheer, Microelectronics Reliability47 (2007) 2025.

[3] SA Instrument & Control, Technews Publishing (Pty) Ltd., http://instrumentation.co.za/article.aspx?pklarticleid=2201 (2003).

[4] L. Bousse, S. Mostarshed, B.V.D. Schoot, N.F. de Rooij, Sensors andActuators B: Chemical 17 (1994) 157.

[5] S. Yoshida, N. Hara, K. Sugimoto, Journal of the ElectrochemicalSociety 151 (2004) H53.

[6] T.M. Pan, K.M. Liao, Sensors and Actuators B: Chemical 127 (2007)480.

[7] T.M. Pan, K.M. Liao, Sensors and Actuators B: Chemical 133 (2008) 97.[8] A. Fog, R. Buck, Sensors and Actuators 5 (1984) 137.[9] J.V.D. Spiegel, I. Lauks, P. Chan, D. Babic, Sensors and Actuators 4

(1983) 291.[10] J.C. Chou, P.K. Kwan, Z.J. Chen, Japan Journal of Applied Physics 42

(2003) 6790.[11] J.C. Chou, J.L. Chiang, C.L. Wu, Japan Journal of Applied Physics 44

(2005) 4838.[12] E.M. Guerra, G.R. Silva, M. Mulato, Solid State Sciences 11 (2009)

456.[13] A. Cohen, M.E. Spira, S. Yitshaik, G. Borghs, O. Shwartzglass, J.

Shappir, Biosensors and Bioelectronics 19 (2004) 1703.[14] S. Meyburg, M. Goryll, J. Moers, S. Ingebrandt, S.B. Meffert, H. Luth, A.

Offenhausser, Biosensors and Bioelectronics 21 (2006) 1037.[15] M.H. Asif, O. Nur, M. Willander, B. Danielson, Biosensors and

Bioelectronics 24 (2009) 3379.[16] A. Bonfiglio, F. Mameli, O. Sanna, Applied Physics Letters 82 (2003)

3550.[17] C. Bartic, G. Borghs, Analytical and Bioanalytical Chemistry 384

(2006) 354.[18] K. Diallo, M. Lemiti, J. Tardy, F. Bessueille, N.J. Renault, Applied

Physics Letters 93 (2008) 183305.[19] A. Loi, I. Manunza, A. Bonfiglio, Applied Physics Letters 86 (2005)

103512.[20] A. Caboni, E. Orgiu, E. Scavetta, M. Barbaro, A. Bonfiglio, Applied

Physics Letters 95 (2009) 123304.[21] D. Kawakami, Y. Yasutake, H. Nishizawa, Y. Majima, Japan Journal of

Applied Physics 45 (2006) L1127.[22] U. Zschieschang, R.T. Weitz, K. Kern, H. Klauk, Applied Physics A 95

(2009) 139.[23] T.N. Ng, J.H. Daniel, S. Sambandan, A.C. Arias, M.L. Chabinyc, R.A.

Street, Applied Physics 103 (2008) 044506.

T.N.T. Nguyen et al. / Organic Electronics 12 (2011) 1815–1821 1821

[24] M.J. Spijkman, J.J. Brondijk, T.C.T. Geuns, E.C.P. Smits, T. Cramer, F.Zerbetto, P. Stoliar, F. Biscarini, P.W.M. Blom, D.M. de Leeuw,Advanced Fucntional Materials 20 (2010) 898.

[25] P. Bergveld, Sensors and Actuators B: Chemical 88 (2003) 1.[26] L. Bousse, N.F. de Rooij, P. Bergveld, IEEE Transactions on Electron

Devices 30 (1983) 1263.

[27] E. Bakker, P. Buhlmann, E. Pretsch, Electroanalysis 11 (1999) 915.[28] C.A. Grimes, E.C. Dickey, M.V. Pishko, Encyclopedia of Sensors 5

(2006) 133.[29] B.R. Eggins, Chemical Sensors and Biosensors, John Wiley & Sons Ltd.,

West Sussex, 2002.