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Broadband pH-Sensing Organic Transistors with Polymeric Sensing Layers Featuring Liquid Crystal MicroDomains Encapsulated by Di-Block Copolymer Chains Jooyeok Seo, Myeonghoon Song, Jaehoon Jeong, Sungho Nam, Inseok Heo, SooYoung Park, Inn-Kyu Kang, Joon-Hyung Lee, Hwajeong Kim, and Youngkyoo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08257 • Publication Date (Web): 24 Aug 2016 Downloaded from http://pubs.acs.org on August 28, 2016

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ACS Applied Materials & Interfaces

Broadband pH-Sensing Organic Transistors with Polymeric Sensing Layers Featuring Liquid Crystal Micro-Domains Encapsulated by Di-Block Copolymer Chains Jooyeok Seo1, Myeonghoon Song1, Jaehoon Jeong1, Sungho Nam1,2, Inseok Heo3, Soo-Young Park3, Inn-Kyu Kang3, Joon-Hyung Lee4, Hwajeong Kim1,5, and Youngkyoo Kim1,* 1

Organic Nanoelectronics Laboratory, Department of Chemical Engineering, School of Applied Chemical Engineering, Kyungpook National University, Daegu 41566, Republic of Korea

2

Department of Physics, Division of Mathematical, Physical and Life Sciences, University of Oxford, Oxford OX1 3PD, United Kingdom

3

Department of Polymer Science and Engineering and Graduate School of Applied Chemical Engineering, Kyungpook National University, Daegu 41566, Republic of Korea

4

School of Materials Science and Engineering, Kyungpook National University, Daegu 41566, Republic of Korea

5

Priority Research Center, Research Institute of Advanced Energy Technology, Kyungpook National University, Daegu 41566, Republic of Korea

KEYWORDS: pH sensor, organic field-effect transistor, liquid crystal, di-block copolymer, perfusion

ACS Paragon Plus Environment

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ABSTRACT:

We report broadband pH-sensing organic field-effect transistors (OFETs) with the polymerdispersed liquid crystal (PDLC) sensing layers. The PDLC layers are prepared by spin-coating using ethanol solutions containing 4-cyano-4’-pentyl-biphenyl (5CB) and a di-block copolymer (PAA-b-PCBOA)

that

consists

of

LC-philic

block

[poly(4-cyano-biphenyl-4-

oxyundecylacrylate) (PCBOA)] and acrylic acid block [poly(acrylic acid) (PAA)]. The spincoated sensing layers feature of 5CB micro-domains ( 7) result in the negative change of drain current. The drain current trend in the present PDLC-i-OFET devices is explained by the shrinking-expanding mechanism of the PAA chains in the di-block copolymer layers.


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1. Introduction pH is one of the most important indicators in our life because it does particularly reflect the health information of human body and the environmental change of our planet (water and soil).13

In terms of health information, the average pH of human body is controlled about 7.4 in order

to keep homeostasis but the local pH of human body varies according to the role of each organ. For example, the pH of our stomach is ca. 1.5 (strong acidity) for digestion process and for killing germs (pathogenic bacteria), while our skin has pH = 5.5 (4.0~6.0 for sweat) for protecting our body from infection. The pH change of individual organs can break the whole homeostasis of our body, which is typically related to diseases and/or malfunction of organs.4 Hence in-situ pH monitoring is of crucial importance for an early diagnosis of potential diseases in our body. To date, the precise pH measurement is typically carried out using a pH meter based on an electrochemical method using aqueous or non-aqueous solutions, even though a pH paper can be used for the rough check of pH.5,6 However, the pH meter requires relatively large amount of solutions and cannot be applied for the in-situ pH measurement that needs direct contacts to human skins or in-vivo installation to organs etc. On this account, a couple of devices have been developed for the pH measurement but field-effect transistors (FETs) with selective ion sensing functions are recognized advantageous due to their potentials for signal amplification that enables the effective pH measurement with small amount of analytes.7-12 However, most of ionselective FETs (ISFETs) are fabricated using inorganic semiconductors so that they have limitations for flexible patch-type applications.13,14 In this regard, organic FETs (OFETs) with


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polymeric gate insulators and channel layers are considered one of the best device platforms for such practical patch-type pH sensors because of their flexible and bendable features.15-19 Recently, it has been reported that the OFETs with liquid crystal (LC) layers can sense extremely low intensity of gas flows which cannot be felt by human skins.20-24 The ultrasensitive characteristics of the LC-integrated-OFETs have been attributed to the collective behavior of LC molecules which accounts for the change of molecular orientation in whole LC domains by few LC molecules affected upon external stimulations. In addition, a di-block copolymer, which consists of LC-philic block [poly(4-cyano-biphenyl-4-oxyundecylacrylate) (PCBOA)] and acrylic acid block [poly(acrylic acid) (PAA)], was found to be sensitive to pH when it makes a monolayer on the LC layer by employing Langmuir-Blodgett (LB) method.25 However, the LB fabrication of the PAA-b-PCBOA/LC layered structures is too tricky to secure practical applications, while the resulting layered structures are vulnerable to easy destruction upon external stimulations. In this work, we demonstrate that polymer-dispersed LC (PDLC) layers, featuring LC (4cyano-4’-pentyl-biphenyl (5CB)) micro-domains (size: 7). In order to examine the sensing reproducibility of the present PDLC-i-OFET devices in a static mode, the strong acidic (pH = 2) and basic (pH = 12) solutions were successively dropped with an interval time. As shown in Figure 6a, the first drop (10 µl) of the acidic solution (pH = 2) slightly (negatively) increased the drain current and the increased drain current level was well kept without severe fluctuations. The second drop of the acidic solution led to relatively larger


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increase in the drain current. As the third and fourth drops applied, the drain current jump became much bigger. The similar trend (but opposite direction of drain current change) was also measured for the drop test using the basic solutions (see the inset graph in Figure 6a). This result supports that the present PDLC-i-OFET devices can be practically used for the static pH measurement with excellent reproducibility.

Figure 6

Finally, the PDLC-i-OFET devices were examined for the real-time pH measurement by making a perfusion system with inlet and outlet units in the PDMS chamber (see the schematic structure in Figure 6b). The small amount (40 µl) of strong acidic solution (pH = 2) was injected into the inlet line of DI water with a constant flow rate of 8.76 µl/s. As soon as the first acidic solution was injected, the drain current was clearly jumped from -19.3 nA to -221.7 nA (see also Figure S4a). Here it is noted that the drain current jump (increase negatively) was very quick but a slow tail was measured in the decay signal (see the logarithmic plot in Figure S4b). This result can be attributed to the recovery time of medium (DI water) for the complete removal of the acidic solution. In addition, it is also considered that the relaxation of the PAA chains after shrinking may take a time in the presence of complicated interaction between the PAA chains and the HCl of the acidic solution drop. As observed in Figure 6b, very similar drain current signals were measured by applying the second and third injection of the acidic solution. This reflects that the present PDLC-i-OFET devices can be used for the dynamic pH measurement with high repetition accuracy in a perfusion mode, which is of critical importance in a variety of biomedical applications.29-31


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3. Conclusion In summary, the PDLC sensing layers were prepared on the channel layers of p-type OFETs by spin-coating process using solutions containing LC (5CB) molecules and pH-sensitive diblock copolymer (PAA-b-PCBOA). The spin-coated sensing layers were found to have the 5CB micro-domains encapsulated by the PAA-b-PCBOA polymer chains. The performance of OFETs was slightly reduced by the presence of the PDLC sensing layers but still kept good p-type transistor characteristics. The present cdevices clearly showed different level of drain current changes according to the pH of small amount (10~40 µl) of analyte solutions. Interestingly, the degree of drain current change (jump) was positive for acidic solutions (pH < 7), while it was negative for basic solutions (pH > 7). This trend could be explained by the shrinking-expanding mechanism of the PAA chains in the di-block copolymer layers. In particular, the PDLC-i-OFET devices could very stably sense the pH change when analyte solutions were successively (continuously) added in the case of both static and dynamic (perfusion) modes.

4. Experimental Section Materials and Device fabrication: The P3HT polymer (weight-average molecular weight = 70 kDa, polydispersity index = 1.8, regioregularity = 96%) and the PMMA polymer (weightaverage molecular weight = 120 kDa, polydispersity index = 2.2) was purchased from Rieke Metals and Sigma-Aldrich, respectively. 5CB (purity = 98%) was used as received from SigmaAldrich. The PAA-b-PCBOA polymer (weight-average molecular weight = 22 kDa for the PAA block, 7 kDa for the PCBOA block, polydispersity index = 1.4) was synthesized according to the process reported previously (note that the synthesis scheme is briefly given in Figure S5).25 The


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P3HT solutions were prepared using toluene as a solvent at a solid concentration of 15 mg/ml, while the PMMA solutions were prepared using n-butylacetate (80 mg/ml). The mixture solutions of 5CB and PAA-b-LCP (5CB: PAA-b-PCBOA = 50:50 by weight) were prepared using ethanol with various solid concentrations from 2 mg/ml. The acidic and basic analyte solutions were prepared using hydrogen chloride (pH = 2), potassium hydrogen phthalate (pH = 3, 4), potassium dihydrogen phosphate (pH = 6), disodium hydrogen phosphate (pH = 7), hydrogen borate (pH = 8), sodium hydroxide (pH = 9), sodium borate decahydrate (pH = 10), and sodium hydroxide (pH = 11, 12). To fabricate the PDLC-i-OFET devices, indium-tin oxide (ITO)-coated glass substrates were patterned to make 30 mm ⅹ 1 mm ITO stripes for gate electrodes by employing photolithography/etching processes. The patterned ITO-glass substrates were cleaned with acetone and isopropyl alcohol, followed by the UV-ozone treatment (28 mW/cm2) for 20 min. The PMMA layers (thickness = 600 nm) were spin-coated on the cleaned ITO-glass substrates and soft-baked at 120 oC for 1 h. Then silver (Ag) source/drain (S/D) electrodes were deposited on the PMMA layers through a shadow mask by employing a thermal evaporation technique in a vacuum chamber. The P3HT layers were spin-coated on the Agdeposited samples, followed by soft-baking at 60 oC for 15 min. Finally, the PDLC (5CB_PAAb-PCBOA) sensing layers were spin-coated on the P3HT channel layers, leading to PDLC-iOFETs. For the static pH measurement, the PDMS banks (thickness = 10 mm) with a rectangular hole were mounted on the P3HT layers to contain deionized water (1 mL) (see Figure 5a). For the dynamic pH measurement, the PDMS chambers with the inlet and outlet feed lines were attached on top of the P3HT layers. Measurements: The thickness of each layer was measured using a surface profiler (alpha-step 200, Tencor), while the microstructure of polymer layers was examined using an optical


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microscope (SV-55, SOMETECH). The Raman spectra of samples were measured using a Raman spectrometer (inVia reflex, Renishaw). The surface and cross-sectional images of the PDLC layers on the P3HT layers were measured using a cryo-scanning electron microscope (Cryo-SEM, Tescan Mira 3 LMU FEG PP3000T, Quorum Technologies). The X-ray absorption spectra (XAS) were measured using a scanning transmission X-ray microscope (STXM) in the 10 A beamline of Pohang Accelerator Laboratory (PAL, Korea) (note that the polymer samples were spin-coated on silicon nitride membrane substrates). The performances of OFETs were measured using a specialized OFET measurement system equipped with a semiconductor parameter analyzer (Keithley 4200 SCS and Keithley 2636 B) and a glove-box probe station (PSM140T-Modusystems) featuring a microscope imaging part (MSZ-0745, Seiwa). The drain current of the PDLC-i-OFET devices was measured upon dropping acidic or basic analyte solutions (pH = 2 ~ 12) in a static mode. In the case of dynamic mode, the analyte solutions were injected into the inlet feed line that is connected to the PDMS chamber of the perfusion system (flow rate = 8.76 µl/s).

ASSOCIATED CONTENT Supporting Information. Cryo-SEM images, Raman spectra, drain current as a function of time, re-plots for the drain current changes, and synthesis of di-block copolymer via a RAFT polymerization.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y.K). Tel: +82-53-950-5616.


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Author Contributions Y.K. designed this work and supervised whole PDLC-i-OFET experimental processes; J.S.(main), M.S., J.J. and S.N. carried out all experiments; I.H. and S-Y.P. contributed to the synthesis of di-block copolymers; I-K.K. and J-H.L. involved in the experimental data discussion; H.K. contributed to each step experiment and manuscript preparation in part; J.S. and Y.K. wrote this manuscript. Note The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the grants from Korean Government (Basic Research Laboratory Program_2011-0020264, Human Resource Training Project for Regional Innovation_MOE(NRF_2014H1C1A1066748), 0093819,

Basic

Science

NRF_2015R1A2A2A01003743,

Research

Program_2009-

NRF_2014R1A1A3051165,

NRF_2014H1A2A1016454).

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Figure 1. Illustration for the device structure of PDLC-i-OFETs with the PDLC sensing layers (5CB_PAA-b-PCBOA): (top) Cryo-SEM image, (bottom) OM image.


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Figure 2. (a) XAS spectra for the P3HT and 5CB_PAA-b-PCBOA layers measured at 537.2 eV (oxygen K-edge). (b) Raman spectra for the P3HT, 5CB and PDLC (5CB_PAA-b-PCBOA) layers on the P3HT layer (excitation wavelength = 780 nm). (c) Raman mapping images for the pristine P3HT layers and the PDLC (5CB_PAA-b-PCBOA) layers by adjusting the probing wavenumber to 1610 cm-1 (target: cyano group).


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Figure 3. (a) Polarized OM (POM) images for the PDLC (5CB_PAA-b-PCBOA) layer on the P3HT layer (left) under linear (0o) and cross-polarization (90o) conditions: (right) POM images for the pristine 5CB layer on the P3HT layer. (b) Illustration for the cross-sectional positioning of 5CB micro-domains in the PDLC sensing layer: (bottom) Proposed homeotropic alignment of 5CB molecules in the micro-domains encapsulated by the PAA-b-PCBOA chains.


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(a)

(b)

0.0 0V

0.8

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VD = -40 V

0.6

-0.2 -10 V -0.4

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-0.8 0.0

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abs ( ID) (µA)

ID (µA)

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Figure 4. Output (a) and transfer (b) characteristics for the control OFET (top) without the sensing layer (SL) and the PDLC-i-OFET device (bottom) with the PDLC (5CB_PAA-bPCBOA) sensing layer. Both devices showed the similar hole mobility of ~10-3 cm2/V·s.


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Figure 5. (a) Illustration for the static pH measurement by dropping analyte solutions to the DI water on the PDMS bank of the PDLC-i-OFET device. (b) Drain current as a function of time according to the pH value (2 ~ 12) of analyte solutions at VG = -0.5 V and VD = -0.5 V. (c) Proposed shrinking-expanding mechanism for the PAA chains of PAA-b-PCBOA leading to the drain current change according to the different pH environment. (d) Net drain current change as a function of pH.


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Figure 6. (a) Static-mode drain current change by the successive dropping of strong acid (pH = 2) and strong base (pH = 12, inset graph) solutions with an interval time (see the device structure in Figure 5a). (b) Dynamic-mode drain current change by the successive injection of strong acid solutions (pH = 2) into the feed line of the perfusion system (see the inset diagram). The voltage conditions of the PDLC-i-OFET devices were VG = -0.5 V and VD = -0.5 V.


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Broadband pH-Sensing Organic Transistors with ... - ACS Publications

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