Abstract— A new pH sensor using nano electrodes in organic semiconductor P3HT (regioregular poly(3-hexylthiophene)) has been designed, fabricated and characterized in this work. In this sensor, the organic semiconductor is directly exposed to pH sample solution, and an electrical field is applied to drive the protons in the solution to the polymer surface. The accumulated protons at the interface between organic semiconductor and solution change the electron density at organic semiconductor surface, so that the conductivity of the organic semiconductor film is modulated. In order to improve the sensitivity of sensor, P3HT is coated on the interdigitated array nanoelectrodes, and the conductivity of P3HT between two bands of electrodes is measured for pH characterization. A good conductivity modulation by the proton concentration or pH value is shown in the sensor testing.

Keywords—pH Detection, P3HT, Conductivity, Nanoelectrodes

I. INTRODUCTION Recently organic semiconductor has been gaining a popular attention in the research and industry application due to its low-cost manufacturing, disposability and easy processing. The so-far great application achievement of organic semiconductor in optoelectronic devices, such as light emitting diode and battery, is motivating researchers to extend the application areas of organic semiconductor [1-4]. As a result, nowadays more and more interests are paid on organic semiconductor transducers.

Some pioneering works have been reported to use organic semiconductor field-effect transistor (FET) as the sensing device for gas or pH value. L. Torsi reported a gas sensor using organic semiconductor FET. Gas interacts with the organic semiconductor and modulates the carrier mobility of organic semiconductor. Therefore, the current-voltage curves of organic semiconductor FET is characterized for gas sensing [5-7]. C. Bartic reported to use an organic ion-selective field-effect transistor (ISFET) to measure pH value of sample solution [8]. His device adopts the conventional detection mechanism that pH variation causes the potential drop across the dielectric and semiconductor interface, but the working semiconductor in his device is polymer instead of silicon. Although these pioneering works are based on organic semiconductor field-effect transistor, one fact has to be pointed out that organic

semiconductor has really low carrier mobility when compared to silicon. Thus, these organic semiconductor FETs and their based sensors usually work under large voltage range. In order to operate polymer FET in low voltage, people tried to deposit high dielectric material (instead of SiO2) as the insulating layer [9], or to build nanochannel for FET using nanofabrication technology [10-11]. Processing the high dielectric material needs high cost, and nano polymer FET requires the complex small-current detection circuit.

In this work, we adopts organic semiconductor in pH measurement using a different approach. In our approach, the organic semiconductor is directly exposed to pH sample solution, and an electrical field is established to be vertical to the organic semiconductor surface, and drives the protons in the solution to the polymer surface. The accumulated protons at the interface between polymer and solution change the electron density at polymer surface, so that the conductivity of organic semiconductor film is modulated. Based on this concept, a conductivity-measured pH sensor has been designed, fabricated, and characterized for a wide pH value range (pH = 2 – 12) in this work. The experiment results show a good conductivity modulation by the proton concentration or pH value.

II. METHOD AND DESIGN Figure 1 illustrates the cut view and the operation of the pH sensor. The sensor is built on Si/SiO2 substrate. The sensing window is exposed to sample solution. The operation of sensor includes the following steps. First, an

pH Sensor using Nano Electrodes in Organic Semiconductor

Xiaoshan Zhu and Chong H. Ahn

BioMEMS and MicroSystem Lab Department of Electrical & Computer Engineering and Computer Science

University of Cincinnati, OH, USA

Figure 1. Cut view and operation of proposed device for pH measurement: (1) Applying an electrical field between pH solution and Si substrate for a few seconds; (2) Measure the semiconductive polymer conductivity between two electrodes after removing the electrical field.

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Au SiO2

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electrical field is generated to be vertical to the polymer surface by applying a potential between Si substrate and the reference electrode. In this step, protons are supposed to be driven to the polymer surface to form the interface charge. Second, the polymer conductivity between two electrodes is measured and recorded. Every time before applying new sample solution to the sensing window, the sensing window is washed using the neutral buffer (pH = 7) and nitrogen repeatedly. In addition, considering the slow oxidization of P3HT in the air by oxygen, all measurements are done in nitrogen environment.

Organic semiconductor P3HT (regioregular poly(3-

hexylthiophene-2,5-diyl)) is used in this pH sensor. P3HT is soluble to chloroform, and can be spin-coated on the wafer surface in the processing. Moreover, it has the highest carrier mobility as one of conjugated polymers, and has good ohmic contact with gold layer. It has been widely used in organic semiconductor FET [12-13]. The molecular structure of P3HT is shown in Figure 2(a). In order to improve the sensitivity of sensor, the electrodes are designed as the interdigitated array. In this array, the spacing between two electrodes is in submicrons (~ 0.5 um), and the width for each electrode finger is around 200 nm. The structure of all electrodes is presented in Figure 2(b). The pH sensing part is the interdigitated array nanoelectrodes and the coated P3HT on nanoelectrodes, and two big electrode pads are used for electrical connection in measurement.

II. FABRICATION The designed device is fabricated using mixed-match

processing steps, which consists of nanofabrication and microfabrication.

First, PMMA (Microchem, 495K) with a 300 nm thickness is spin-coated at the oxidized silicon surface, and then the nanopatterns are exposed by e-beam (Raith 150 e-beam lithography system). After the development of the exposed PMMA, Ti/Au (100 Å/ 1000 Å) layer is deposited on the patterned sample surface, and then dipped into acetone for lift-off.

After the nanofabrication, the connection pads are fabricated using the UV-light-lithography-based lift-off

techniques, as the following steps. First, the positive photoresist (Shipley 1818) is coated on the substrate with 3000 rpm, and then the wafer is baked in the 90 ºC oven for 30 minutes. Second, the baked Shipley 1818 is exposed under UV light (300 nm ~ 460 nm wavelength, ~7 mJ/cm2) for 10 seconds, and consequently is immersed in chlorobenzene for 45 seconds. After immersion, dry the sample in the 120 ºC oven for 30 seconds. Third, the sample is developed for 1 minute and dry. Fourth, Ti/Au (100 Å/ 1000 Å) layer is deposited on the patterned sample surface. Finally, the deposited sample is baked at 120 ºC for 1 – 2 hours and then dipped into acetone for lift-off.

In the last step, organic semiconductor P3HT (regioregular poly(3-hexylthiophene-2,5-diyl)), which is dissolved in chloroform (0.8% wt), is spin-coated on the electrode surface at 1500 rpm speed. After the coating, the sensing window is covered and sealed by a small piece of PDMS membrane, and the other coated P3HT film is etched using RIE (Ar). In the testing, the pH solution is dropped in the sensing window.

The whole processing steps are graphically illustrated in Figure 3. Figure 4(a) shows the photo graphics of fabricated nanoelectrodes and microelectrodes, and a SEM graphics for a close view on the interdigitated array nanoelectrodes is presented in Figure 4(b). The thickness of the coated P3HT on nanoelectrode surface is measured using profilometer, and the thickness data is shown in Figure 5.

(b) E-beam lithography

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Figure 3. Mixed-match processing: (a) – (d) E-beam lithography for lift-off of nanoelectrodes; (e) – (h) UV-light lithography for lift-off of microelectrodes; (l) – (m) P3HT coating and patterning

S1818

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Figure 2. (a) Molecular structure of P3HT (regioregular poly(3-hexylthiophene-2,5-diyl); (b) Structure of elelctrodes

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III. MEASUREMENT RESULTS

A. Preparation of pH sample solution and Ag/AgCl electrode The neutral sample solution (pH=7) is PBS buffer

purchased from Scientific Fisher. The PBS buffer includes 0.1 M choride ions, which makes sure the reference elelctrode (Ag/AgCl) is stable after immersing into pH sample solution. The other sample solutions with different pH values are prepared by modulate the neutral sample solution with HCl acid or potassium hydroxide.

The reference electrode is made by slowly depositing a layer of AgCl onto a segment of clean silver wire by electrolysis. A clean silver wire and a clean Pt film respectively are connected to the positive and negative polarities of current source, and then immerse them in 0.1 M KCl solution with a 10 mA/cm2 current density for 60 seconds. A blackish deposit of AgCl will be deposited on the silver wire. Before using, the reference electrode is stored in a dilute solution of saline.

B. Conductivity measurement

The conductivity of organic semiconductor between two

electrodes is measured using a digital multimeter (DMM). All measurement is done in nitrogen environment. Figure 6 shows the conductivity change before and after applying pH sample solution on the senmiconductive polymer. Before dropping pH sample solution, the resistant between two electrodes are huge (~68 Mohms). But after applying pH sample solution on the organic semiconductor surface, the resistant drops off dramatically. Although the resistant for different pH sample solutions varies, the variation is not obvious and the resistance is around 3 Mohms. The reason for this dramatic conductivity change possibly is that ions in the sample solution change the electron density of organic semiconductor surface, and thus cause the huge variation of conductivity.

In order to check the effect of proton/hydroxyl concentration (or different pH values) on polymer conductivity, a 10 V potential is added between the Si substrate and the reference electrode, which is immersed in pH sample solution. The electrical field between Si substrate and the reference electrode lasts for 30-second duration, and then the polymer conductivity is measured again. Figure 7 gives the comparison of two cases: with electrical field excitation and without electrical field excitation. From Figure 7, it can be seen that the high concentration proton causes a higher conductivity, while the high concentration hydroxyl results a lower conductivity. The detailed physic mechanism for this phenomenon is still under investigation. One intuitive explanation is that electrical field drives the protons to the polymer surface to further increase the polymer surface electron density, so that

Figure 4. Fabricated electrode strucuture using mixed-mathc processing: Photo (a) and SEM (b) graphics. The dimension of electrode fingers is ~ 80 nm and its spacing is ~ 500 nm.

(a) Photo graphics of interdigitated array electrodes

(b) SEM graphics of interdigitated array electrodes in nao scale

Figure 5. The measured thickness of P3HT using profilometer. The scanning range is 3 mm, the scanning speed is 100 um/s, and the sampling rate is 200 Hz.

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the conductivity becomes larger for the sample solution with smaller pH values or higher proton concentration.

V. CONCLUSION

Work presented in this paper first illustrates a new property of the organic semiconductor P3HT (regioregular poly(3-hexylthiophene-2,5-diyl)). That is, under the function of an electrical field, P3HT (regioregular poly(3-hexylthiophene-2,5-diyl)) changes its conductivity with the different pH environments. Such a property is characterized in a large pH range (pH = 2 – 12) using 500nm-spaced nanoelectrodes in this work. A mono-decreasing relationship between polymer conductivity and pH value is achieved. With this new property, P3HT (regioregular poly(3-hexylthiophene-2,5-diyl )) can be applied as not only pH

sensor, but various pH-detecting based sensors. Compared to the organic semiconductor FET based sensors, the application using such a property avoids the low carrier mobility and the complex bias circuit and small-current detection circuit.

ACKNOWLEDGMENT The authors thank National Science Foundation (NSF) for the funding on Raith-150 e-beam lithography system at University of Cincinnati, and also thank Mr. Ron Flenniken in University of Cincinnati for his technical assistance in the metal deposition.

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Figure 6. Dramatic change of P3HT conductivity after a drop of pH solution is put on the sensing window (without electrical field to drive protons to polymer surface).

Figure 7. Conductivity modulation by pH value with or without an electrical field to drive protons to the semiconductive polymer surface

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