Electronic Nose for Detecting Multiple Targets Anirban Chakraborty1, Ganga Parthasarathi1, Rakesh Poddar1, Weiqiang Zhao2, Cheng Luo3* 1 Electrical Engineering Program and Institute for Micromanufacturing, 2 Tulane School of Medicine, New Orleans, LA 70131 3 Biomedical Engineering Program and Institute for Micromanufacturing and Louisiana Tech University; *Tel: (318) 257-5136, Fax: (318) 257-5104, Email:
[email protected], Mail: IfM, 911 Hergot Ave, Ruston, LA 71270.
ABSTRACT The discovery of high conductivity in doped polyacetylene in 1977 (garnering the 2000 Nobel Prize in Chemistry for the three discovering scientists) has attracted considerable interest in the application of polymers as the semiconducting and conducting materials due to their promising potential to replace silicon and metals in building devices. Previous and current efforts in developing conducting polymer microsystems mainly focus on generating a device of a single function. When multiple micropatterns made of different conducting polymers are produced on the same substrate, many microsystems of multiple functions can be envisioned. For example, analogous to the mammalian olfactory system which includes over 1,000 receptor genes in detecting various odors (e.g., beer, soda etc.), a sensor consisting of multiple distinct conducting polymer sensing elements will be capable of detecting a number of analytes simultaneously. However, existing techniques present significant technical challenges of degradation, low throughput, low resolution, depth of field, and/or residual layer in producing conducting polymer microstructures. To circumvent these challenges, an intermediate-layer lithography method developed in our group is used to generate multiple micropatterns made of different, commonly used conducting polymers, Polypyrrole (PPy), Poly(3,4-ethylenedioxy)thiophene (PEDOT) and Polyaniline (PANI). The generated multiple micropatterns are further used in an “electronic nose” to detect water vapor, glucose, toluene and acetone. Keywords: conducting polymer sensor, electronic nose.
1. INTRODUCTION Conducting polymers, since its discovery in 1977, have been the area of constant research. Conducting polymers have been used for chemical and biological sensing as well as for microelectronics and optical devices. Polymers are rendered conducting by the alternate single and double bonds in the polymer backbone with each carbon being sp2 hybridized. Doping is the process of addition (reduction) or removal (oxidation) of electrons. There exist π-bonds where electrons can move on application of an electric field. When electrons hop between the polymer chains, conductivity is established. On removal of a single electron from the polymer chain, there is formation of a polaron which is a positive charge, which may move along the polymer chain. Pairing of polarons forms solitons, may also aid conduction of charges [1]. PPy [2], PEDOT [3] and PANI [4] are three commonly used conducting polymers. Figure 1 shows their basic molecular structures. Doping induces positive and negative charges along the polymer chain which moves with applied electric field. Protonation is a very common doping technique which uses protonic acids (for example hydrochloric acid or sulphonic acid) to induce positive charges at specific location in the polymer chain to increase conductivity [5]. Conducting polymers offer advantages of easy handling, tailoring of properties by suitable doping and cheap processing over conventional materials. Traditionally, conducting polymers have been employed for detection of a single analyte and thereafter to increase functionality, the concept of “electronic nose” was introduced where multiple analytes can be potentially detected from their individual response pattern to multiple detectors, close to mimicking the human olfactory system [6]. Kodderitzsch et al. have fabricated a conducting polymer sensor for cigarette smoke analysis [7]. PPy has been used in a number of commercially available sensors for detection of alcohols, polar and non-polar gas components
Micro (MEMS) and Nanotechnologies for Space Applications edited by Thomas George, Zhong-Yang Cheng Proc. of SPIE Vol. 6223, 622309, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.666424 Proc. of SPIE Vol. 6223 622309-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 07/23/2014 Terms of Use: http://spiedl.org/terms
and volatile compounds [2]. Sotzing et al. have demonstrated a Poly(3,4-ethylenedioxy)thiophene-Poly(styrene sulphonate) (PEDOT-PSS) and insulating polymer composite vapor sensor [3]. Conducting form of PANI has been used to sense for aqueous ammonia through change in surface resistance [4]. The sensitivity of conducting polymers to specific chemical targets is currently in the chemical regime. It would be interesting to study the correlation of the physical dimension of the active polymer film to its sensitivity response to particular analyte. Through our present study, we found that the humidity sensitivity response of a PPy pattern (300µm X 5000µm) is similar to PPy film (1cm x 1cm). This opens the door to fabrication of more elaborate detectors consisting of multiple conducing polymer micropatterns which are tailored to respond to multiple analytes, on a single chip. Conventional lithographic approaches to pattern conducting polymers are severely restricted as conducting polymers cannot stand the chemical exposure of the conventional UV lithography steps. Spin coating and ink-jet printing of conducting polymers are the two popular approaches of depositing conducting polymer films. The intermediate-layerlithography (ILL) approach of patterning conducting polymers offers the advantage of having polymer films patterned with desired dimensions over large area and allows micro-structures of varying depths to be imprinted with precision. The conducting polymer layer is cut and electrically isolated in the process. Furthermore, the ILL is a one mask process requiring no aggressive chemistry.
2. FABRICATION OF THE CONDUCTING POLYMER MICROPATTERNS The conducting polymer micro-patterns are generated by the ILL technique [8]. In this technique, an insulating layer of polymer is introduced between the conducting polymer and the substrate (figure 2a). The silicon master for the process is fabricated using conventional UV lithography and deep reactive ion etch. The depth of the features on the silicon master mold is about 80µm. For our experiments, non-conducting polymethylmethacrylate (PMMA) sheets were used as insulating substrate, with conducting polymer coated on top of it. The PMMA is heated in vacuum to 120 °C, which is higher than its Tg (~ 108°C) and the silicon master mold is slowly inserted into the coated PMMA in effect patterning the conducting polymer coated on top of it (figure 2b). The hot embossing was done in a HEX 01/LT system (JENOPTIK Mikrotechnik Company). After the temperature falls to 70°C, the silicon mold insert is separated from the substrate. Figure 2-c1 and figure 2-c2 illustrates two possibilities arising out of the demolding step. The conducting polymer may adhere to the silicon master mold (figure 2-c1) or remain on the PMMA substrate (figure 2-c2). This is determined by the surface energy and roughness of the silicon master as well as the interaction between the silicon mold, the conducting polymer and the PMMA substrate. The length of the microwires is 5000µm and the width is 300µm. Figure 3 shows the PPy coated micropatterns over PMMA and figure 4 shows the corresponding silicon master with a thin layer of PPy adhering to it after hot embossing is done. This is similar to the case suggested in figure 1-c1. Figure 5 shows three conducting polymer micropatterns of sulphonated Polyaniline (SPANI), PEDOT-PSS and PPy on a common PMMA substrate. This was achieved by spin coating the three polymers on the same PMMA substrate and hot embossing it. All the conductive polymers have been imprinted with the similar microwire pattern. Additional conductive epoxy contacts (figure 6) were made to the individual micro-patterns for I-V measurements on exposure to the various analytes.
3. SENSING APPLICATIONS 3.1 HUMIDITY AND GLUCOSE SENSING The conducting polymer micropatterns were used to sense for humidity. The fractional change in resistance to the base resistance (∆R/R or sensitivity index) is (RHumidity- RBase/RBase). It was measured for PPy microwires and compared to PPy film, with positive values denoting resistance increase and negative values implying resistance decrease from the base value. The humidity range was varied from 45% (base humidity level) to 85% (maximum humidity level). The humidity measurement setup was made in-house consisting of controlled environment chamber (glove box) and a humidifier. I-V measurements were done on a Keithley probe station. Figure 7 shows the humidity experiment setup. The resistance of PPy pattern steadily decreases on exposure to increasing humidity and the corresponding sensitivity (∆R/R) index is negative. The PPy pattern sensitivity index varies form -0.31 to -0.99 and the PPy film sensitivity index varies from 0.23 to -0.98. Figure 8 shows the variation of the sensitivity index with humidity. The conductivity increase is brought
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about by increased mobility of the ions which are hydroplillic and electrostatically bonded. Adsorption of polar water molecules may result in increasing hole charge carriers which also contributes to increased conductivity [9]. It is evident form figure 7 that the sensitivity of PPy pattern and PPy film are comparable. At higher humidity, the polymer saturates and the increase in current is less. Accordingly, the sensitivity curve is flat. PPy microwires were used to detect glucose. A glucose oxidase layer was coated on the microwires. This layer was made up of glucose oxidase (7.8mg/ml), ferrocenemethanol (FeMeOH) (17.8mg/ml), phosphate buffer (0.1Molar at pH 6.5), ethylenediaminetetraacetic acid (EDTA) (1.5mMolar), and glycerol (10% w/v). A cellulose acetate layer was placed on the glucose oxidase. This layer prevented water from coming into contact with the PPy layer and acted as the semipermeable membrane, while glucose molecules could cross it effectively. The cellulose acetate layer was formed of cellulose acetate (2.5g), tetrahydrofuran (THF) (60ml), and acetone (40ml) [10]. The sensor was dipped into the glucose (C6H12O6) solution of concentration 54mg/ml. Molecular weight of glucose used was 180.16g. A typical sensing response was shown in figure 9(a). Two points can be directly observed from this figure: (1) there is a sharp increase in current during the first 60 seconds, which is due to non-equilibrium chemical reaction between the glucose and the glucose oxidase, and (2) the current remains constant, which indicates that the chemical reaction between the glucose oxidase and the glucose reaches equilibrium in 60 seconds. Therefore, the current readout in 60 seconds provides a reliable parameter to characterize the glucose concentration. The amount of glucose present in the urine of a healthy person is 0.1 to 0.3mg/ml. Therefore, the sensitivity of the proposed sensor was tested around this range. Figure 9(b) shows that the current changes were 1.48x10-7, 1.45x10-7, 1.75x10-7 and 3.25x10-7A as the concentrations of pure glucose solutions are 0.2, 0.3, 0.5, and 0.8mg/ml, respectively. The current decreases as concentration increases from 0.2 to 0.3mg/ml, while currents increase with increasing concentrations in other cases with distinguishable current changes, indicating that the sensitivity of the sensor is about 0.2mg/ml, which is 5 times of the sensitivity of GlucoWatch (a commercialized device) [11 ].
3.2 TOLUNE AND ACETONE SENSING An array of SPANI and PEDOT patterns was exposed to acetone and tolune and a mixture of the two vapors. The sensitivity index is calculated according to (RGas-RBase/RBase). SPANI pattern is not affected much by tolune as it is nonpolar but over an exposure time of 600 seconds, the current decreases slightly (0.03E-05 A) as can be seen in figure 10, as the tolune molecule inhibits the movement of charge carriers in SPANI. The sensitivity index after 600seconds of exposure is 0.014 (table 1). Acetone is a slightly polar molecule [12] which may be interacting with the protonated sites in SPANI thereby reducing the number of charge carriers and so decreasing conductivity. The current falls by 0.4E-05A over 600 seconds exposure time (figure 11). The corresponding sensitivity index is 0.26 (table 1). When SPANI pattern is exposed to a mixture of acetone and tolune vapor, the current gradually decreases by 0.6E-05A (figure 12) and the (a) sensitivity index is 0.52 (table 1) due to reduction of SPANI and acetone dominates because of (O-) in the overall structure of acetone. With tolune, PEDOT pattern shows fluctuations in current and at higher tolune concentrations, there is a slight increase in conductivity due to increase in carrier mobility. From figure 10, the increase in current is about 0.01E-03A. The sensitivity index for this increase in current is 0.004 (table1). Exposure of PEDOT to acetone reduces its conductivity slightly over 600 seconds of exposure, as conducting polarons from PEDOT may be donated to acetone molecules which have lower polarity (figure 11). When PEDOT pattern is exposed to a mixture of tolune and acetone, the conductivity reduces (after 600 seconds of exposure, the sensitivity index is 0.029), which implies predominance of acetone, however from figure 12, it may be noted that the current vibrates very much for 400seconds before showing a decreasing trend which may be tolune signature. From the sensitivity response of SPANI and PEDOT to individual vapors and their mixture, it may be concluded that both the polymers are more responsive to acetone vapor. Tolune induces minor variation in conductivity of both the polymers primarily due to its non-polar nature. In a mixture, acetone dominates the current behavior of both the polymers significantly however with PEDOT there is a marked oscillation in current for some time which can be a unique tolune signature.
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4. CONCLUSION In this work, micropatterns of conducting polymers PPy, PEDOT-PSS and SPANI have been generated on a common substrate and used for sensing humidity, glucose, tolune and acetone vapors. Also, an array of SPANI and PEDOT patterns was exposed to a mixture of tolune, and acetone vapors and acetone dominated the conductivity behavior of both the polymers. Presently, work is going on to fabricate arrays with reduced feature sizes in order to broaden the realm of the targets and conducting polymers.
Acknowledgement This work was supported in part through NSF–DMI-0508454, NSF/LEQSF(2006)-Pfund-53 and NSF-ECS-0529104 grants.
REFERENCES [1] Rupprecht, L., Conductive Polymers and Plastics: In Industrial Applications, Plastic design library, 1999. [2] Ameer, Q. and Adeloju, S., Polypyrrole-based electronic noses for environmental and induatrial analysis, Sensors and Actuators B, 106, 541-552, 2005. [3] Sotzing G., et al., Preparation and properties of vapor detector arrays formed from Poly(3,4ethylenedioxy)thiophene-Poly(styrenesulphonate)/insulating polymer composite, Analytical Chemistry, 72, 3181-3190, 2000. [4] Dhawan S., et al., Application of conducting Polyaniline as sensor material for ammonia, Sensors and Actuators B, 40, 99-103, 1997. [5] Takahashi, K., et al., Characterization of water-soluble externally HCl-doped conducting Polyaniline, Synthetic Metals, 128, 27-33, 2002. [6] Persaud K., et al., Sensor array techniques for mimicking the mammalian olfactory system, Sensors and Actuators B, 35-36, 267-273, 1996. [7] Kodderitzsch, P., et al., Sensor array based measurement technique for fast-responding cigarette smoke analysis, Sensors and Actuators B, 107, 479-489, 2005. [8] Luo C., et al., An Innovative Approach for Generating Conducting Polymer Micropatterns, Journal of Vacuum Science and Technology B, 26, 2, March/April 2006 (in press). [9] Suri K., et al, Gas and humidity sensors based on iron oxide-polypyrrole nanocomposites, Sensors and Actuators B, 81, 277-282, 2002. [10] Setti, L., et al., An amperometric glucose biosensor prototype fabricated by thermal inkjet printing, Biosensors and Bioelectronics, 20, 2019-2026, 2005. [11] Http://www.waukeshamemorial.org/Content.asp?PageID=P00339 [12] Ruangchuay, L., Sirivat, A. and Schwank, J., Electrical conductivity response of Polypyrrole to acetone vapor: effect of dopant anions and interaction mechanisms, Synthetic Metals, 140, 15-21, 2004.
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4-Q (a)
(b)
NN-Q4 ) (c) Figure 1: The above figure shows the chemical structures of (a) PPy, (b) PEDOT and (c) PANI.
(a)
Silicon mold and substrate
(b)
(c1)
PMMA layer (insulating)
(c2)
Conducting polymer layer
Figure 2: Illustrating the hot embossing process and the cutting of the conducting polymer layer. The fabrication steps are (a) the substrate consists of insulated polymer (PMMA) coated silicon wafer with conducting polymer layer on top; (b) the silicon mold is imprinted into the substrate at temperature above the Tg of PMMA; (c) after the temperature falls, the mold insert is separated from the substrate yielding the patterned substrate.
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(a)
(c)
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PPy
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PMMA
Figure 3: SEM pictures of the embossed PMMA with PPy on top of the rectangular islands.
(a)
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PPy
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Silicon
Figure 4: SEM pictures of the silicon mold after the hot embossing step with PPy on top of the features.
I
I
SPANI
PEDOT-PSS
PPy
Figure 5: The figure shows the PMMA substrate spin coated with (1) SPANI, (2) PEDOT-PSS and (3)PPy and hot embossed with silicon mold. The features of interest are 300µm wide and 5000µm long polymer microwires which have been used as sensing elements.
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Silver epoxy contact
Conducting polymer microwire
PMMA
Figure 6: SEM of an additional epoxy contact made on the conducting polymer micro-wire.
Glove box
Humidifier
Keithley probe station
Figure 7: The figure shows the humidity experiment setup.
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PPy PPy film film and and pattern patternresponse response to humidity to humidity exposure exposure 0 45
55
65
75
85
(sensitivity R/R)
-0.2
∆
PPy film
-0.4
PPy pattern -0.6 -0.8 -1
%RH %RH
Figure 8: The sensitivity variation with humidity for PPy microwire and spin coated PPy film.
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PPy response response toto0.3mg/ml PPy 0.3mg/mlglucose glucose
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2.50E-07
PPy glucose solution PPyresponse responsetoto glucose solution 3.50E-07
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Figure 9: (a) A typical relationship between current change and time when a sensor is exposed to pure glucose solution, and (b) sensor responses to pure glucose solutions of four different concentrations.
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1.58E-05
6.06E-03
1.57E-05
6.04E-03
PEDOT pattern
1.57E-05
6.02E-03
SPANI pattern
1.56E-05
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5.98E-03
1.55E-05
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6.08E-03
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PEDOT SPANI pattern response tolune PEDOT andand SPANI pattern response toto tolune
Time(sec)
Figure 10: The current variation of SPANI and PEDOT microwires to tolune exposure.
1.80E-05 1.60E-05 1.40E-05 1.20E-05 1.00E-05 8.00E-06 6.00E-06 4.00E-06 2.00E-06 0.00E+00
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PEDOT pattern
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Figure 11: The current variation of SPANI and PEDOT microwires to acetone exposure.
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I(A)
6.05E-03
I(A)
I(A) I(A)
PEDOT and SPANI patternresponse responsetotoacetone acetone PEDOT and SPANI pattern
0.00605 0.006 I(A) I(A)
0.00595 0.0059 PEDOT patterns
0.00585
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0.0058 0.00575 0
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1.80E-05 1.60E-05 1.40E-05 1.20E-05 1.00E-05 8.00E-06 6.00E-06 4.00E-06 2.00E-06 0.00E+00 600
I(A) I(A)
SPANI and PEDOT pattern response to tolune and acetone PEDOT and SPANI pattern response to tolune and acetone
Time(sec)
Figure 12: The current variation of SPANI and PEDOT microwires to mixture of acetone and tolune exposure.
Sensitivity (∆R/R) (SPANI/PEDOT) Response time(Sec)(SPANI/PEDOT) Recovery time(Sec)(SPANI/PEDOT)
Acetone 0.26 / 0.025 65 / 84 17 / 20
Tolune 0.014 / -0.004 45 / 84 16 / 16
Mixture 0.52 / 0.029 84 / 100 14 / 23
Table 1: The above table shows the sensitivity index, response and recovery times of the PEDOT and SPANI patterns.
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