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A parylene-protected nitrate selective microsensor on a carbon fiber cross section
Sensors and Actuators B 123 (2007) 127–134
A parylene-protected nitrate selective microsensor on a carbon fiber cross section Tatyana A. Bendikov a,d,1 , Scott Miserendino b,d , Yu-Chong Tai b,d , Thomas C. Harmon c,d,∗ a
Department of Civil and Environmental Engineering, University of California, Los Angeles, CA 90095, USA b Department of Electrical Engineering, California Institute of Technology, Pasadena, CA 91125, USA c School of Engineering, University of California, Merced, CA 95344, USA d Center for Embedded Networked Sensing, University of California, Los Angeles, CA 90095, USA Received 21 May 2006; received in revised form 2 August 2006; accepted 2 August 2006 Available online 12 September 2006
Abstract This work describes a novel design for a potentiometric microsensor for nitrate (NO3 − ) ion based on doped polypyrrole (PPy(NO3 − )) films. First, 6–7 m diameter single carbon fiber filaments are coated with a thin (1–10 m) insulating layer of parylene C (poly(o-chloro-p-xylylene)). This preparatory step is followed by electrodeposition of a PPy(NO3 − ) sensing layer on the open carbon fiber cross section. Parylene C provides an excellent protective coating for the fiber while maintaining its flexibility at micro-dimensions. Examination by scanning electron microscopy confirms that the sensing material (PPy(NO3 − )) is deposited in the form of micro-beads with diameter of 30–40 m at the fiber tip. The resulting nitrate sensor exhibits an improvement of approximately half an order of magnitude of NO3 − concentration in sensitivity and linear response range relative to a previously reported PPy(NO3 − )-based sensor for which the sensing layer was deposited on the carbon fiber walls. The overall sensor geometry, a micro-dimensioned sensing portion accompanied by a flexible, parylene-insulated body, forms a sensor for nitrate with a unique form factor that can be widely used for in situ measurements of variety of environmental samples. © 2006 Elsevier B.V. All rights reserved. Keywords: Carbon fiber; Parylene coating; Nitrate microsensor; Polypyrrole
1. Introduction Enormous progress has been made over the past decade in the development and application of chemical sensors [1–18]. Small, inexpensive sensors with high selectivity towards specific chemical compounds, molecules, and ions are highly desirable for variety of application including environmental monitoring, food and pharmaceutical industries and medicine. For particular applications, like monitoring micro-size environmental samples or in situ measurements in the human body, the size of the sensor is critical and requires micro-dimensions. Unfortunately, even if the necessary sensor has micro- or nano-dimensions, it may ∗
Corresponding author at: School of Engineering, University of California, Merced, CA 95344, USA. Tel.: +1 209 724 4337; fax: +1 209 724 4356. E-mail address: [email protected] (T.C. Harmon). 1 Present address: Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel. 0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.08.007
still be accompanied by “macro” sized parts (on the order of cm or mm) serving as contacts, coatings or other functions. In a recent publication we suggested using a single carbon fiber with diameter of 6–8 m as a sensor substrate on which sensing material (conducting polymer) was plated [19]. This introduced flexibility of the sensor body together with microdimensions. In this work, we address the dual needs of (1) focusing the sensitive surface of this type of sensor to a well-defined microscale dimension and (2) creating a suitable insulating coating for the sensor, protecting it from its surrounding environment (e.g., soils, biological tissues, microorganisms). Carbon fiber coatings have been intensively studied [20–23]. Different kinds of epoxy resins have been widely used as a sizing material forming fiber-epoxy composites [21,24–26]. There are several examples of using epoxy-coated carbon fibers as electrodes [27–30]. Morita et al. fabricated electrodes array using wisps of thousands of carbon filaments immured into epoxy [27–29]. Wightman et al. used a single carbon fiber coated with
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a millimeter thick layer of epoxy [30]. In both examples flexibility and micro-dimensions are lost in the process. In addition, it is well known that due to poor adhesion of epoxy to fibers additional surface treatments are usually required [24–26,31–32] and it is practically impossible to obtain thin (micrometers) epoxy coating of single carbon filament. Potje-Kamloth et al. [33] successfully coated carbon fibers with thin (1–5 m) insulating layer of poly(oxyphenylene). Later Strein el al. [34], using a slightly modified procedure, obtained an even thinner (0.4–1.5 m) insulating phenolallylphenol copolymer coat. In both cases the insulating polymer/copolymer layer was deposited electrochemically. This step was followed by curing step at 150–180 ◦ C. In this work we introduce a process for insulating and protecting a single carbon fiber with parylene C. An advantage of the proposed method is in its simplicity, reliability and robustness: by using a micromachining process uniform parylene coating can be obtained simultaneously for hundreds of carbon fibers. In addition, this technique is not restricted to carbon fibers and can be applied for variety of substrates. By applying a parylene coating to a single carbon filament and plating sensing material (doped polypyrrole) onto an exposed fiber cross section, we fabricated a new kind of ion selective microsensor for nitrate. Parylene provides a smooth nonreactive coating of 1–10 m, and maintains fiber flexibility allowing fabrication of truly microdimensional sensors for in situ measurements. 2. Experimental 2.1. Reagents The precursor to parylene C (o-chloro-p-xylylene, dimer) was purchased from Specialty Coating Systems. All other chemical reagents were analytically pure grade and used as purchased from Sigma–Aldrich and Fisher Scientific. Pyrrole was refrigerated in the dark and purified before use by passing through an alumina (Al2 O3 ) column. Sodium nitrate (NaNO3 ) (hygroscopic grade reagent) was dried in an oven at 80–100 ◦ C before use. All aqueous solutions were prepared using deionized water with resistivity of ∼18 M cm (Ultrapure Water System, Nanopure Infinity, Inc.). 2.2. Preparation and parylene coating of carbon fiber microelectrodes Sigrafil C30 T045 EPY (SGL Technic, Inc.) carbon fibers were used. Single fiber filaments (6–7 m in diameter) were connected to short pieces (∼3 cm) of copper wire using silver paint (Structure Probe, Inc.). The other end of the copper wire was coated with teflon tape (to prevent parylene deposition on some length of the wire (∼1–2 cm) to be used later for connectivity to the voltmeter). Then the microelectrodes were attached with tape to a glass slide with the copper wires bent 90◦ to the surface of the slide. This configuration ensures homogeneous coating of the whole fiber surface with parylene. A Labcoater 2 system (Model PDS 2010) was used for the parylene deposition. The parylene C dimer was placed in the vaporizing
chamber. The vaporizing chamber temperature was raised to ∼120 ◦ C to begin sublimation and continues to rise to 170 ◦ C to ensure sublimation of all the dimer. Sublimated dimer passed through the furnace that was heated to ∼690 ◦ C causing it to separate into monomer components. The monomer units deposit on the substrate in the deposition chamber that was vacuumed to the pressure of less than 8 mtorr and held at room temperature. Parylene was deposited until all of the loaded material was exhausted. 2.3. Deposition of nitrate-doped polypyrrole (PPy(NO3 − )) sensing layer onto carbon fiber cross section After the parylene deposition microelectrodes were detached from the glass slide, and the teflon tape was removed revealing uncoated copper wire endings. To expose carbon cross sections, the parylene-coated fibers were submerged for approximately 5 min in liquid nitrogen and then cut with a scalpel. We found that freezing the fibers in liquid nitrogen resulted in cleaner crosssectional cuts. Polymerization of pyrrole in the presence of nitrate onto carbon fiber cross section was performed electrochemically. A similar procedure was used for polymerization of PPy(NO3 − ) onto carbon fibers walls and is described in detail elsewhere [19]. A potentiostat/galvanostat (CH Instruments, Model 660B) was used as a constant current source for the electropolymerization. The initial polymerization solution consisted of 1 M pyrrole and 0.1 M NaNO3 in water. The solution was deoxygenated by purging it with nitrogen for 5–10 min before polymerization and by bathing the system headspace with nitrogen gas during the experiment. A one compartment cell was employed with a carbon fiber cross-section working microelectrode, an Ag/Ag+ wire pseudo-reference electrode and Pt wire counter electrode. Polymerization was performed under constant current conditions (applied current 1 × 10−7 A, surface of the carbon fiber cross section 3.5 ± 0.5 × 10−7 cm2 ) at room temperature (22 ± 1 ◦ C). The freshly prepared PPy(NO3 − ) sensors were rinsed with water and conditioned for at least 24 h in 0.01 M NaNO3 solution at room temperature in the dark. 2.4. Potentiometric measurements Potentiometric measurements were performed with a pH/ISE Meter (Orion Research, Inc., Model 720). Potential differences were measured between the prepared PPy(NO3 − ) miroelectrodes on carbon fiber cross section and a Ag/AgCl double junction reference electrode (Bioanalytical System, Inc.). A 0.01 M (NH4 )2 SO4 solution was used as an ion strength adjuster (ISA). Between measurements electrodes were stored in the dark in the conditioning solution at room temperature. 2.5. SEM analysis and film thickness measurements The surface of carbon fibers, parylene coating and polypyrrole films was observed using a cold field emission scanning electron microscope (SEM, Hitachi S4700). The thickness of the
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parylene layers was evaluated from the SEM images by comparing uncoated and parylene-coated fibers. 3. Results and discussion 3.1. Parylene coating of carbon fiber microelectrodes Parylene C is a vapor-deposited polymeric coating, deposited during the vapor phase pyrolysis of the o-chloro-p-xylylene dimer on cooled surfaces to form a high molecular weight linear poly(o-chloro-p-xylylene) [20,35–36]. Along with its ability to provide a true pinhole-free conformal insulation, parylene C has a very low permeability to moisture and other corrosive materials. SEM images of uncoated and coated with parylene carbon fibers shown in Fig. 1. There is a direct correlation between deposition time or initial mass of the dimer used and thickness of the deposited parylene layer. Hence, thickness of the coating can be varied by changing the initial mass and hence the deposition time and, for the conditions described in Section 2, is approximately 1, 5 and 10 m for 1.5, 2 and 3 h of deposition, respectively. The slow deposition rate during the first hour is due to some period of time required for sublimation of precursor molecule and initiation of deposition polymerization process. As shown in Fig. 1(c) and (d) a smooth and homogeneous coating was obtained. Contrast between conductive fiber and insulating parylene coating allows observation of fiber body inside the parylene layer. This is better seen in Fig. 1(c) where the coating layer is thin (1 m).
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The parylene coating is much softer than the carbon fiber. Thus, when we tried to expose carbon fiber cross sections, we observed using scanning electron microscope (images not shown) that cutting with a scalpel easily deforms (stretches and shrinks at the ends) the parylene coating, restricting access to the carbon fiber cross section. We found that freezing the fibers in liquid nitrogen resulted in cleaner cross-sectional cuts. SEM images of the cross-sectional view of uncoated fiber (a) and coated with parylene C (before (b) and after (c and d) vertical cut) are shown in Fig. 2. 3.2. Electrochemical deposition of nitrate-doped polypyrrole (PPy(NO3 − )) sensing films onto carbon fiber cross section: potentiometric response of the sensors to NO3 − ion Electrodeposition of nitrate–doped polypyrrole films on the conductive surface of carbon fiber cross section (3.5 ± 0.5 × 10−7 cm2 ) was intended to obtain a nitrate sensor of these dimensions. However, it became clear during the study that deposition of the polymer is not limited to the fiber cross-sectional region of the electrode, but also spreads to the nonconductive parylene-coated areas of the fiber. For example, as seen from the SEM image in Fig. 3a, for electrodeposition at constant current of 1 × 10−7 A for 20 min, PPy(NO3 − ) layer in the form of microbead was obtained, spreading from the fiber cross section (where the deposition starts) to the parylene protected walls of the fiber. The PPy(NO3 − ) deposited layer has a
Fig. 1. SEM images of uncoated (a and b) and coated with parylene C (c and d) carbon fibers: side view (a); cross section (b); parylene coating ∼1 m (c) and ∼10 m (d).
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Fig. 2. SEM images of the cross-sectional view of uncoated (a) and coated with parylene C (b–d) carbon fibers. Uncoated fiber with cross-section diameter of ∼7 m (a); parylene coated fiber, coating ∼10 m (b). Vertical cut of parylene coated fiber, coating ∼10–11 m (c) and ∼6–7 m (d).
diameter of ∼40 m and grew ∼70–75 m along the fiber forming a three dimensional sensing “bead” at the end of the flexible parylene coated sensor body. Polymerization of pyrrole on nonconducting surfaces is described in the literature. Nishizawa et al., for example, showed “spreading” of polypyrrole onto glass surfaces separated array of Pt electrodes [37]. This lateral growth can be enhanced by hydrophobic pretreatment of glass surface [38–39]. Since the parylene coating has a hydrophobic character, it is expected that PPy(NO3 − ) will grow not only in the vertical direction (along carbon fiber cross section), but also will spread laterally on the nonconducting areas (around carbon fiber cross section). Three similar PPy(NO3 − ) microbead electrodes prepared under the conditions described (Fig. 3) were tested for their potentiometric response to NO3 − ion a few days after deposition. Changes in potential versus concentration of NO3 − ion are plotted in Fig. 3b. All the sensors exhibit excellent reproducibility showing similar behaviour with near-Nernstian slopes of −53.5 ± 0.6 mV per log cycle of nitrate concentration with a linear response spanning from 0.1 to 5 × 10−5 M of NO3 − and limit of detection roughly 1 × 10−5 M of nitrate. An enhancement in sensor sensitivity and expansion in linear response range of approximately half an order of magnitude in NO3 − concentration were observed, compared to the sensor with analogous sensing material plated on the carbon fiber walls [19].
3.3. Influence of the electrodeposition conditions on the performance and shape of PPy(NO3 − ) selective microsensor Changes in fabrication conditions for the PPy(NO3 − ) sensing layer deposition influence the sensor performance (sensitivity, linear response range towards NO3 − ion). In this work, we examined the role of the specific surface area of the sensing region on sensor performance by reducing the deposition current. We applied 5 × 10−8 and 1 × 10−8 A (deposition time 20 min and all other conditions as described in Fig. 3). We also performed deposition at low currents (1 × 10−8 or 1 × 10−9 A), but increased the deposition time to 1–2 h. All these changes led to significant deterioration in sensors sensitivity and reduction of the linear response range (results are not shown). Another way to reduce the quantity of deposited material (PPy(NO3 − )) is to keep the applied current constant (1 × 10−7 A, in our case) and reduce the deposition time. SEM images of PPy(NO3 − ) microsensors “grown” on carbon fiber cross section at constant current and different deposition times are shown in Fig. 4. In all cases (deposition of 3–20 min; for 20 min deposition see Fig. 3) the PPy(NO3 − ) sensing layer is deposited in the form of a microbead (growing from the carbon fiber cross section and spreading to the parylene coated areas of the fiber). The surface area of the “beads” continuously reduces with the decreasing of time devoted for deposition.
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Fig. 3. (a) SEM image of PPy(NO3 − ) microsensor “grown” on carbon fiber cross section. Parylene coating ∼10 m. (b) Potentiometric response of three identically prepared PPy(NO3 − ) sensors shown in (a) to NO3 − ion. Deposition of PPy(NO3 − ) “bead” at constant applied current of 1 × 10−7 A for 20 min. Solution: 1 M pyrrole; 0.1 M NaNO3 in water.
Potentiometric responses to NO3 − ion for six microbead nitrate selective sensors deposited on carbon fiber cross section at constant current of 1 × 10−7 A and different deposition times (1–20 min) are shown in Fig. 5. For all cases (except 1 min deposition) near-Nernstian slopes of −53.5 ± 2.5 mV per log cycle of nitrate concentration were observed with the detection limit of roughly 1 × 10−5 M of NO3 − . In the case of a 1 min deposition, changes in potential with nitrate concentration were significantly less (slope −31.1 mV/decade) and the limit of detection extended only up to roughly 1 × 10−4 M of NO3 − . The fact that decreasing the deposition time (and thus reducing the sensing area) from 20 to 3 min has little influence on sensor performance may support the assumption that the polymer that “spills” over onto the insulated areas is probably not sensing. Regardless, there is some minimal polymer volume that required for good sensor response. For example, in the case of 1 min deposition, it was
observed with SEM (images not shown) that this deposition time is enough to cover with PPy(NO3 − ) not only the carbon fiber cross section, but also the surrounding insulated areas (the same as in other cases, 3–20 min deposition). However, the performance of 1 min deposition sensor is poor. This result indicates that not only the coating of the conducting area (fiber cross section), but the total volume of the PPy(NO3 − ) deposition plays an important role in determining sensor performance. It seems that polymer deposited on the insulated areas of the fiber can “communicate” with the carbon fiber in the same manner as polymer vertically deposited on the fiber cross section. Thus the total volume of polymer required for good sensor performance probably includes not only polymer deposited on the conducting area but also polymer that spreads to the insulated area. Overall, deposition times longer than 1 min are necessary due to obtain good performance of the sensor.
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Fig. 4. SEM images of PPy(NO3 − ) microsensors “grown” on carbon fiber cross section. Parylene coating ∼10 m. Deposition of PPy(NO3 − ) at constant applied current of 1 × 10−7 A for 15 min (a), 10 min (b), 5 min (c) and 3 min (d).
Fig. 5. Potentiometric response of PPy(NO3 − ) microsensors to NO3 − ion. Deposition of PPy(NO3 − ) at constant applied current of 1 × 10−7 A for different deposition times (1–20 min). Solution: 1 M pyrrole; 0.1 M NaNO3 in water. SEM images of the sensors shown in Figs. 3(a) and 4.
4. Conclusions A novel design for a solid state potentiometric microsensor for nitrate is described. It is shown that a single carbon fiber filament with a diameter of 6–7 m can be coated with parylene C, forming an excellent insulating and protective coating. Smooth and homogeneous parylene layers of 1–10 m were deposited on the walls of carbon fibers maintaining fiber flexibility and
micro-dimensions. Polypyrrole doped with nitrate was electropolymerized on carbon fiber cross-section forming a sensitive layer in the form of a microbead with diameter of 30–40 m. The size of the microbeads can be varied with the time devoted for deposition of the PPy(NO3 − ) layer. New designed sensors exhibit a linear response to nitrate concentration spanning from 0.1 M to 5 × 10−5 M of NO3 − with near-Nernstian slopes of −53.5 ± 2.5 mV per log cycle of nitrate concentration and detection limit of roughly 1 × 10−5 M of NO3 - showing better performance compared to the previously reported potentiometric nitrate selective sensor with the same sensing material deposited on carbon fiber walls. The micro-dimensions of the sensing area accompanied by the flexible protected electrode body creates a truly microdimensional sensor for nitrate ion that can be widely used for in situ measurements of a variety of samples. Microdimensional sensors with these characteristics would support numerous research directives aimed at observing microscopic scale chemical gradients and reactive transport processes near soil moisture–mineral interfaces [40], in biological tissues [41], within and adjacent to biofilms [42], and other porous mediabased systems. Acknowledgments The authors are thankful to Dr. Sergey Prikhodko for helpful discussions about carbon fiber cut. Funding from the Israeli Science Foundation (Bikura Post-doctoral Scholarship to T.B.)
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is gratefully acknowledged. This material is based upon work supported by the National Science Foundation under Grant No. CCF-0120778. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. References [1] C. Liu, Development of chemical sensors using microfabrication and micromachining techniques, Mater. Chem. Phys. 42 (1995) 87–90. [2] J. Janata, M. Josowicz, P. Vanysec, D.M. DeVaney, Chemical sensors, Anal. Chem. 70 (1998) 179R–208R. [3] Y. Vlasov, A. Legin, Non-selective chemical sensors in analytical chemistry: from “electronic nose” to “electronic tongue”, Fresenius J. Anal. Chem. 361 (1998) 255–260. [4] H.H. Weetall, Chemical sensors and biosensors, update, what, where, when and how, Biosens. Bioelectron. 14 (1999) 237–242. [5] See special issue on Chemical Sensors: Chem. Rev. 100 (7) (2000), 2477–2738. [6] E. Bakker, M. Telting-Diaz, Electrochemical sensors, Anal. Chem. 74 (2002) 2781–2800. [7] D.R. Reyes, D. Iossifidis, P.A. Auroux, A. Manz, Micro total analysis systems. 1. Introduction, theory, and technology, Anal. Chem. 74 (2002) 2623–2636. [8] P.A. Auroux, D. Iossifidis, D.R. Reyes, A. Manz, Micro total analysis systems. 2. Standard operations and applications, Anal. Chem. 74 (2002) 2637–2652. [9] O.S. Wolfbeis, Fiber-optic chemical sensors and biosensors, Anal. Chem. 74 (2002) 2663–2678. [10] J. Janata, M. Josowicz, Conducting polymers in electronic chemical sensors, Nat. Mater. 2 (2003) 19–24. [11] C.J. Koester, S.L. Simonich, B.K. Esser, Environmental analysis, Anal. Chem. 75 (2003) 2813–2829. [12] N. Szabo, C. Lee, J. Trimboli, O. Figueroa, R. Ramamoorthy, S. MidlamMohler, A. Soliman, H. Verweij, P. Dutta, S. Akbar, Ceramic-based chemical sensors, probes and field-test in automobile engines, J. Mater. Sci. 38 (2003) 4239–4245. [13] J.R. Stetter, W.R. Penrose, S. Yao, Sensors, chemical sensors, electrochemical sensors, and ECS, J. Electrochem. Soc. 150 (2003) S11–S16. [14] E. Bakker, Electrochemical sensors, Anal. Chem. 76 (2004) 3285–3298. [15] O.S. Wolfbeis, Fiber-optic chemical sensors and biosensors, Anal. Chem. 76 (2004) 3269–3284. [16] O.S. Wolfbeis, Materials for fluorescence-based optical chemical sensors, J. Mater. Chem. 15 (2005) 2657–2669. [17] L. Wang, D. Fine, D. Sharma, L. Torsi, A. Dodabalapur, Nanoscale organic and polymeric field-effect transistors as chemical sensors, Anal. Bioanal. Chem. 384 (2006) 310–321. [18] J. Bobacka, Conducting polymer-based solid-state ion-selective electrodes, Electroanalysis 18 (2006) 7–18. [19] T.A. Bendikov, J. Kim, T. Harmon, Development and environmental application of a nitrate selective microsensor based on doped polypyrrole films, Sens. Actuators B 106 (2005) 512–517. [20] H. Rashid, M. Lake, J. Woolam, B. Banks, S. Domitz, Passivating coatings for intercalated graphite fiber electrical contacts, Carbon 24 (1986) 511–513. [21] K. Horie, H. Murai, I. Mita, Bonding of epoxy resins to graphite fibers, Fibre Sci. Technol. 9 (1976) 253–264. [22] K. Kusakabe, B.-K. Sea, J.-I. Hayashi, H. Maeda, S. Morooka, Coating of carbon fibers with amorphous SiC films as diffusion barrier by chemical vapor deposition with triisopropylsilane, Carbon 34 (1996) 179–185. [23] R. Gadow, F. Kern, Continuous liquid phase coating of carbon fibers with Kion VL 20® Polysilazane, Adv. Eng. Mater. 5 (2003) 799–801. [24] N. Dilsiz, E. Ebert, W. Weisweiler, G. Akovali, Effect of plasma polymerization on carbon fibers used for fiber/epoxy composites, J. Colloid Interface Sci. 170 (1995) 241–248.
133
[25] A. Bismark, A. Menner, M.E. Kumru, A. Sezai Sarac, M. Bistritz, E. Schulz, Poly(carbazole-co-acrylamide) electrocoated carbon fibers and their adhesion behavior to an epoxy resin matrix, J. Mater. Sci. 37 (2002) 461–471. [26] H.M. Kang, T.H. Yoon, M. Bump, J.S. Riffle, Effect of solubility and miscibility on the adhesion behavior of polymer-coated carbon fibers with vinil ester resins, J. Appl. Polym. Sci. 79 (2001) 1042–1053. [27] K. Morita, Y. Shimizu, Microhole array for oxygen electrode, Anal. Chem. 61 (1989) 159–162. [28] K. Tokuda, K. Morita, Y. Shimizu, Cyclic voltammetry at microhole array electrodes, Anal. Chem. 61 (1989) 1763–1768. [29] Y. Shimizu, K. Morita, Microhole array electrode as a glucose sensor, Anal. Chem. 62 (1990) 1498–1501. [30] M.A. Dayton, J.C. Brown, K.J. Stutts, R.M. Wightman, Faradaic electrochemistry at microvoltammetric electrodes, Anal. Chem. 52 (1980) 946–950. [31] H.-T. Chiu, J.-S. Lin, Electrochemical deposition of polypyrrole on carbon fibers for improved adhesion to the epoxy resin matrix, J. Mater. Sci. 27 (1992) 319–327. [32] B. Grunden, J.O. Iroh, Formation of graphite fibre-polypyrrole coatings by aqueous electrochemical polymerization, Polymer 36 (1995) 559–563. [33] K. Potje-Kamloth, J. Janata, M. Josowicz, Electrochemically prepared insulation for carbon fiber microelectrodes, Ber. Bunsenges. Phys. Chem. 93 (1989) 1480–1485. [34] T.G. Strein, A.G. Ewing, Characterization of submicron-sized carbon electrodes insulated with phenol–allyphenol copolymer, Anal. Chem. 64 (1992) 1368–1373. [35] SCS (Specialty Coating Systems) web site: http://www.scscookson.com/ parylene knowledge/specifications.cfm. [36] B.L. Halpern, J.J. Schmitt, Y. Di, J.W. Golz, D.L. Johnson, D.T. McAvoy, D. Wang, J.Z. Zhang, Jet vapor deposition of single and multicomponent thin films, Met. Finishing 90 (1992) 37–41. [37] M. Nishizawa, M. Shibuya, T. Sawaguchi, T. Matsue, I. Ushida, Electrochemical preparation of ultrathin polypyrrole film at microarray electrodes, J. Phys. Chem. 95 (1991) 9042–9044. [38] M. Nishizawa, Y. Miwa, T. Matsue, I. Ushida, Surface pretreatment for electrochemical fabrication of ultrathin patterned conducting polymers, J. Electrochem. Soc. 140 (1993) 1650–1655. [39] M. Nishizawa, Y. Miwa, T. Matsue, I. Ushida, Ultrathin polypyrrole formed at a twin-microband electrode in the presence of dodecylsulfate, J. Electroanal. Chem. 371 (1994) 273–275. [40] C. Steefel, D. DePaolo, P. Lichtner, Reactive transport modeling: an essential tool and a new research approach for the earth sciences, Earth Plan. Sci. Lett. 240 (2005) 539–558. [41] R. Chignola, E. Milotti, A phenomenological approach to the simulation of metabolism and proliferation dynamics of large tumour cell populations, Phys. Biol. 2 (2005) 8–22. [42] J. Paulsen, E. Oppen, R. Bakke, Biofilm morphology in porous media, a study with microscopic and image techniques, Water Sci. Technol. 36 (1997) 1–9.
Biographies Tatyana A. Bendikov obtained her M.Sc. and Ph.D. degrees in chemistry from Technion, Israel Institute of Technology in 1997 and 2001, respectively. During 2002–2004 she was a postdoctoral fellow at the Department of Civil and Environmental Engineering, UCLA working on research in the National Science Foundation Center for Embedded Networked Sensing (CENS) related to development and testing of environmental chemical sensors. She is currently holding a position of postdoctoral researcher at the Department of Materials and Interfaces in Weizmann Institute of Science, Israel. Her research interests include Analytical Chemistry and Electrochemistry with emphasis on development of chemical and biological sensors. Scott Miserendino obtained his B.S. degree in electrical engineering and mathematical sciences from the Johns Hopkins University in 2002 and his M.S. degree in electrical engineering from the California Institute of Technology in 2003. He is currently pursuing his Ph.D. in electrical engineering at the California Institute of Technology. His research interests include modular microfluidic ana-
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lytical chemistry and single-cell metabolic monitoring, on-chip electrochemical sensors, and microgaskets. Yu-Chong Tai received his B.S. degree from National Taiwan University, and M.S. and Ph.D. degrees in Electrical Engineering from the University of California at Berkeley. After Berkeley, he joined the faculty of Electrical Engineering at the California Institute of Technology and built the Caltech MEMS Lab. He recently joined the Bioengineering department and he is currently a Professor of Electrical Engineering and Bioengineering at Caltech. His current research interests include flexible MEMS, bioMEMS, MEMS for retinal implants, parylene-based integrated microfluidics, neuroprobes/neurochips, and HPLC-based labs-on-a-chip. He has received several awards such as the IBM
fellowship, the Best Thesis Award, the Presidential Young Investigator (PYI) Award and the David and Lucile Packard Fellowship. He co-chaired the 2002 IEEE MEMS Conference in Las Vegas. He is currently a Subject Editor of the Journal of Microelectromechanical Systems. Thomas C. Harmon is a Professor in the School of Engineering and Founding Faculty member at the University of California, Merced. He directs contaminant monitoring research at the NSF Center for Embedded Networked Sensing at UCLA, where he was a faculty member in the Department of Civil & Environmental Engineering from 1992 to 2003. He received a B.S. in Civil Engineering from the Johns Hopkins University in 1985, and M.S. and Ph.D. degrees from the Environmental Engineering program at Stanford University in 1986 and 1992.
A parylene-protected nitrate selective microsensor on a carbon fiber cross section Tatyana A. Bendikov a,d,1 , Scott Miserendino b,d , Yu-Chong Tai b,d , Thomas C. Harmon c,d,∗ a
Department of Civil and Environmental Engineering, University of California, Los Angeles, CA 90095, USA b Department of Electrical Engineering, California Institute of Technology, Pasadena, CA 91125, USA c School of Engineering, University of California, Merced, CA 95344, USA d Center for Embedded Networked Sensing, University of California, Los Angeles, CA 90095, USA Received 21 May 2006; received in revised form 2 August 2006; accepted 2 August 2006 Available online 12 September 2006
Abstract This work describes a novel design for a potentiometric microsensor for nitrate (NO3 − ) ion based on doped polypyrrole (PPy(NO3 − )) films. First, 6–7 m diameter single carbon fiber filaments are coated with a thin (1–10 m) insulating layer of parylene C (poly(o-chloro-p-xylylene)). This preparatory step is followed by electrodeposition of a PPy(NO3 − ) sensing layer on the open carbon fiber cross section. Parylene C provides an excellent protective coating for the fiber while maintaining its flexibility at micro-dimensions. Examination by scanning electron microscopy confirms that the sensing material (PPy(NO3 − )) is deposited in the form of micro-beads with diameter of 30–40 m at the fiber tip. The resulting nitrate sensor exhibits an improvement of approximately half an order of magnitude of NO3 − concentration in sensitivity and linear response range relative to a previously reported PPy(NO3 − )-based sensor for which the sensing layer was deposited on the carbon fiber walls. The overall sensor geometry, a micro-dimensioned sensing portion accompanied by a flexible, parylene-insulated body, forms a sensor for nitrate with a unique form factor that can be widely used for in situ measurements of variety of environmental samples. © 2006 Elsevier B.V. All rights reserved. Keywords: Carbon fiber; Parylene coating; Nitrate microsensor; Polypyrrole
1. Introduction Enormous progress has been made over the past decade in the development and application of chemical sensors [1–18]. Small, inexpensive sensors with high selectivity towards specific chemical compounds, molecules, and ions are highly desirable for variety of application including environmental monitoring, food and pharmaceutical industries and medicine. For particular applications, like monitoring micro-size environmental samples or in situ measurements in the human body, the size of the sensor is critical and requires micro-dimensions. Unfortunately, even if the necessary sensor has micro- or nano-dimensions, it may ∗
Corresponding author at: School of Engineering, University of California, Merced, CA 95344, USA. Tel.: +1 209 724 4337; fax: +1 209 724 4356. E-mail address: [email protected] (T.C. Harmon). 1 Present address: Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel. 0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.08.007
still be accompanied by “macro” sized parts (on the order of cm or mm) serving as contacts, coatings or other functions. In a recent publication we suggested using a single carbon fiber with diameter of 6–8 m as a sensor substrate on which sensing material (conducting polymer) was plated [19]. This introduced flexibility of the sensor body together with microdimensions. In this work, we address the dual needs of (1) focusing the sensitive surface of this type of sensor to a well-defined microscale dimension and (2) creating a suitable insulating coating for the sensor, protecting it from its surrounding environment (e.g., soils, biological tissues, microorganisms). Carbon fiber coatings have been intensively studied [20–23]. Different kinds of epoxy resins have been widely used as a sizing material forming fiber-epoxy composites [21,24–26]. There are several examples of using epoxy-coated carbon fibers as electrodes [27–30]. Morita et al. fabricated electrodes array using wisps of thousands of carbon filaments immured into epoxy [27–29]. Wightman et al. used a single carbon fiber coated with
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a millimeter thick layer of epoxy [30]. In both examples flexibility and micro-dimensions are lost in the process. In addition, it is well known that due to poor adhesion of epoxy to fibers additional surface treatments are usually required [24–26,31–32] and it is practically impossible to obtain thin (micrometers) epoxy coating of single carbon filament. Potje-Kamloth et al. [33] successfully coated carbon fibers with thin (1–5 m) insulating layer of poly(oxyphenylene). Later Strein el al. [34], using a slightly modified procedure, obtained an even thinner (0.4–1.5 m) insulating phenolallylphenol copolymer coat. In both cases the insulating polymer/copolymer layer was deposited electrochemically. This step was followed by curing step at 150–180 ◦ C. In this work we introduce a process for insulating and protecting a single carbon fiber with parylene C. An advantage of the proposed method is in its simplicity, reliability and robustness: by using a micromachining process uniform parylene coating can be obtained simultaneously for hundreds of carbon fibers. In addition, this technique is not restricted to carbon fibers and can be applied for variety of substrates. By applying a parylene coating to a single carbon filament and plating sensing material (doped polypyrrole) onto an exposed fiber cross section, we fabricated a new kind of ion selective microsensor for nitrate. Parylene provides a smooth nonreactive coating of 1–10 m, and maintains fiber flexibility allowing fabrication of truly microdimensional sensors for in situ measurements. 2. Experimental 2.1. Reagents The precursor to parylene C (o-chloro-p-xylylene, dimer) was purchased from Specialty Coating Systems. All other chemical reagents were analytically pure grade and used as purchased from Sigma–Aldrich and Fisher Scientific. Pyrrole was refrigerated in the dark and purified before use by passing through an alumina (Al2 O3 ) column. Sodium nitrate (NaNO3 ) (hygroscopic grade reagent) was dried in an oven at 80–100 ◦ C before use. All aqueous solutions were prepared using deionized water with resistivity of ∼18 M cm (Ultrapure Water System, Nanopure Infinity, Inc.). 2.2. Preparation and parylene coating of carbon fiber microelectrodes Sigrafil C30 T045 EPY (SGL Technic, Inc.) carbon fibers were used. Single fiber filaments (6–7 m in diameter) were connected to short pieces (∼3 cm) of copper wire using silver paint (Structure Probe, Inc.). The other end of the copper wire was coated with teflon tape (to prevent parylene deposition on some length of the wire (∼1–2 cm) to be used later for connectivity to the voltmeter). Then the microelectrodes were attached with tape to a glass slide with the copper wires bent 90◦ to the surface of the slide. This configuration ensures homogeneous coating of the whole fiber surface with parylene. A Labcoater 2 system (Model PDS 2010) was used for the parylene deposition. The parylene C dimer was placed in the vaporizing
chamber. The vaporizing chamber temperature was raised to ∼120 ◦ C to begin sublimation and continues to rise to 170 ◦ C to ensure sublimation of all the dimer. Sublimated dimer passed through the furnace that was heated to ∼690 ◦ C causing it to separate into monomer components. The monomer units deposit on the substrate in the deposition chamber that was vacuumed to the pressure of less than 8 mtorr and held at room temperature. Parylene was deposited until all of the loaded material was exhausted. 2.3. Deposition of nitrate-doped polypyrrole (PPy(NO3 − )) sensing layer onto carbon fiber cross section After the parylene deposition microelectrodes were detached from the glass slide, and the teflon tape was removed revealing uncoated copper wire endings. To expose carbon cross sections, the parylene-coated fibers were submerged for approximately 5 min in liquid nitrogen and then cut with a scalpel. We found that freezing the fibers in liquid nitrogen resulted in cleaner crosssectional cuts. Polymerization of pyrrole in the presence of nitrate onto carbon fiber cross section was performed electrochemically. A similar procedure was used for polymerization of PPy(NO3 − ) onto carbon fibers walls and is described in detail elsewhere [19]. A potentiostat/galvanostat (CH Instruments, Model 660B) was used as a constant current source for the electropolymerization. The initial polymerization solution consisted of 1 M pyrrole and 0.1 M NaNO3 in water. The solution was deoxygenated by purging it with nitrogen for 5–10 min before polymerization and by bathing the system headspace with nitrogen gas during the experiment. A one compartment cell was employed with a carbon fiber cross-section working microelectrode, an Ag/Ag+ wire pseudo-reference electrode and Pt wire counter electrode. Polymerization was performed under constant current conditions (applied current 1 × 10−7 A, surface of the carbon fiber cross section 3.5 ± 0.5 × 10−7 cm2 ) at room temperature (22 ± 1 ◦ C). The freshly prepared PPy(NO3 − ) sensors were rinsed with water and conditioned for at least 24 h in 0.01 M NaNO3 solution at room temperature in the dark. 2.4. Potentiometric measurements Potentiometric measurements were performed with a pH/ISE Meter (Orion Research, Inc., Model 720). Potential differences were measured between the prepared PPy(NO3 − ) miroelectrodes on carbon fiber cross section and a Ag/AgCl double junction reference electrode (Bioanalytical System, Inc.). A 0.01 M (NH4 )2 SO4 solution was used as an ion strength adjuster (ISA). Between measurements electrodes were stored in the dark in the conditioning solution at room temperature. 2.5. SEM analysis and film thickness measurements The surface of carbon fibers, parylene coating and polypyrrole films was observed using a cold field emission scanning electron microscope (SEM, Hitachi S4700). The thickness of the
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parylene layers was evaluated from the SEM images by comparing uncoated and parylene-coated fibers. 3. Results and discussion 3.1. Parylene coating of carbon fiber microelectrodes Parylene C is a vapor-deposited polymeric coating, deposited during the vapor phase pyrolysis of the o-chloro-p-xylylene dimer on cooled surfaces to form a high molecular weight linear poly(o-chloro-p-xylylene) [20,35–36]. Along with its ability to provide a true pinhole-free conformal insulation, parylene C has a very low permeability to moisture and other corrosive materials. SEM images of uncoated and coated with parylene carbon fibers shown in Fig. 1. There is a direct correlation between deposition time or initial mass of the dimer used and thickness of the deposited parylene layer. Hence, thickness of the coating can be varied by changing the initial mass and hence the deposition time and, for the conditions described in Section 2, is approximately 1, 5 and 10 m for 1.5, 2 and 3 h of deposition, respectively. The slow deposition rate during the first hour is due to some period of time required for sublimation of precursor molecule and initiation of deposition polymerization process. As shown in Fig. 1(c) and (d) a smooth and homogeneous coating was obtained. Contrast between conductive fiber and insulating parylene coating allows observation of fiber body inside the parylene layer. This is better seen in Fig. 1(c) where the coating layer is thin (1 m).
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The parylene coating is much softer than the carbon fiber. Thus, when we tried to expose carbon fiber cross sections, we observed using scanning electron microscope (images not shown) that cutting with a scalpel easily deforms (stretches and shrinks at the ends) the parylene coating, restricting access to the carbon fiber cross section. We found that freezing the fibers in liquid nitrogen resulted in cleaner cross-sectional cuts. SEM images of the cross-sectional view of uncoated fiber (a) and coated with parylene C (before (b) and after (c and d) vertical cut) are shown in Fig. 2. 3.2. Electrochemical deposition of nitrate-doped polypyrrole (PPy(NO3 − )) sensing films onto carbon fiber cross section: potentiometric response of the sensors to NO3 − ion Electrodeposition of nitrate–doped polypyrrole films on the conductive surface of carbon fiber cross section (3.5 ± 0.5 × 10−7 cm2 ) was intended to obtain a nitrate sensor of these dimensions. However, it became clear during the study that deposition of the polymer is not limited to the fiber cross-sectional region of the electrode, but also spreads to the nonconductive parylene-coated areas of the fiber. For example, as seen from the SEM image in Fig. 3a, for electrodeposition at constant current of 1 × 10−7 A for 20 min, PPy(NO3 − ) layer in the form of microbead was obtained, spreading from the fiber cross section (where the deposition starts) to the parylene protected walls of the fiber. The PPy(NO3 − ) deposited layer has a
Fig. 1. SEM images of uncoated (a and b) and coated with parylene C (c and d) carbon fibers: side view (a); cross section (b); parylene coating ∼1 m (c) and ∼10 m (d).
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Fig. 2. SEM images of the cross-sectional view of uncoated (a) and coated with parylene C (b–d) carbon fibers. Uncoated fiber with cross-section diameter of ∼7 m (a); parylene coated fiber, coating ∼10 m (b). Vertical cut of parylene coated fiber, coating ∼10–11 m (c) and ∼6–7 m (d).
diameter of ∼40 m and grew ∼70–75 m along the fiber forming a three dimensional sensing “bead” at the end of the flexible parylene coated sensor body. Polymerization of pyrrole on nonconducting surfaces is described in the literature. Nishizawa et al., for example, showed “spreading” of polypyrrole onto glass surfaces separated array of Pt electrodes [37]. This lateral growth can be enhanced by hydrophobic pretreatment of glass surface [38–39]. Since the parylene coating has a hydrophobic character, it is expected that PPy(NO3 − ) will grow not only in the vertical direction (along carbon fiber cross section), but also will spread laterally on the nonconducting areas (around carbon fiber cross section). Three similar PPy(NO3 − ) microbead electrodes prepared under the conditions described (Fig. 3) were tested for their potentiometric response to NO3 − ion a few days after deposition. Changes in potential versus concentration of NO3 − ion are plotted in Fig. 3b. All the sensors exhibit excellent reproducibility showing similar behaviour with near-Nernstian slopes of −53.5 ± 0.6 mV per log cycle of nitrate concentration with a linear response spanning from 0.1 to 5 × 10−5 M of NO3 − and limit of detection roughly 1 × 10−5 M of nitrate. An enhancement in sensor sensitivity and expansion in linear response range of approximately half an order of magnitude in NO3 − concentration were observed, compared to the sensor with analogous sensing material plated on the carbon fiber walls [19].
3.3. Influence of the electrodeposition conditions on the performance and shape of PPy(NO3 − ) selective microsensor Changes in fabrication conditions for the PPy(NO3 − ) sensing layer deposition influence the sensor performance (sensitivity, linear response range towards NO3 − ion). In this work, we examined the role of the specific surface area of the sensing region on sensor performance by reducing the deposition current. We applied 5 × 10−8 and 1 × 10−8 A (deposition time 20 min and all other conditions as described in Fig. 3). We also performed deposition at low currents (1 × 10−8 or 1 × 10−9 A), but increased the deposition time to 1–2 h. All these changes led to significant deterioration in sensors sensitivity and reduction of the linear response range (results are not shown). Another way to reduce the quantity of deposited material (PPy(NO3 − )) is to keep the applied current constant (1 × 10−7 A, in our case) and reduce the deposition time. SEM images of PPy(NO3 − ) microsensors “grown” on carbon fiber cross section at constant current and different deposition times are shown in Fig. 4. In all cases (deposition of 3–20 min; for 20 min deposition see Fig. 3) the PPy(NO3 − ) sensing layer is deposited in the form of a microbead (growing from the carbon fiber cross section and spreading to the parylene coated areas of the fiber). The surface area of the “beads” continuously reduces with the decreasing of time devoted for deposition.
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Fig. 3. (a) SEM image of PPy(NO3 − ) microsensor “grown” on carbon fiber cross section. Parylene coating ∼10 m. (b) Potentiometric response of three identically prepared PPy(NO3 − ) sensors shown in (a) to NO3 − ion. Deposition of PPy(NO3 − ) “bead” at constant applied current of 1 × 10−7 A for 20 min. Solution: 1 M pyrrole; 0.1 M NaNO3 in water.
Potentiometric responses to NO3 − ion for six microbead nitrate selective sensors deposited on carbon fiber cross section at constant current of 1 × 10−7 A and different deposition times (1–20 min) are shown in Fig. 5. For all cases (except 1 min deposition) near-Nernstian slopes of −53.5 ± 2.5 mV per log cycle of nitrate concentration were observed with the detection limit of roughly 1 × 10−5 M of NO3 − . In the case of a 1 min deposition, changes in potential with nitrate concentration were significantly less (slope −31.1 mV/decade) and the limit of detection extended only up to roughly 1 × 10−4 M of NO3 − . The fact that decreasing the deposition time (and thus reducing the sensing area) from 20 to 3 min has little influence on sensor performance may support the assumption that the polymer that “spills” over onto the insulated areas is probably not sensing. Regardless, there is some minimal polymer volume that required for good sensor response. For example, in the case of 1 min deposition, it was
observed with SEM (images not shown) that this deposition time is enough to cover with PPy(NO3 − ) not only the carbon fiber cross section, but also the surrounding insulated areas (the same as in other cases, 3–20 min deposition). However, the performance of 1 min deposition sensor is poor. This result indicates that not only the coating of the conducting area (fiber cross section), but the total volume of the PPy(NO3 − ) deposition plays an important role in determining sensor performance. It seems that polymer deposited on the insulated areas of the fiber can “communicate” with the carbon fiber in the same manner as polymer vertically deposited on the fiber cross section. Thus the total volume of polymer required for good sensor performance probably includes not only polymer deposited on the conducting area but also polymer that spreads to the insulated area. Overall, deposition times longer than 1 min are necessary due to obtain good performance of the sensor.
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Fig. 4. SEM images of PPy(NO3 − ) microsensors “grown” on carbon fiber cross section. Parylene coating ∼10 m. Deposition of PPy(NO3 − ) at constant applied current of 1 × 10−7 A for 15 min (a), 10 min (b), 5 min (c) and 3 min (d).
Fig. 5. Potentiometric response of PPy(NO3 − ) microsensors to NO3 − ion. Deposition of PPy(NO3 − ) at constant applied current of 1 × 10−7 A for different deposition times (1–20 min). Solution: 1 M pyrrole; 0.1 M NaNO3 in water. SEM images of the sensors shown in Figs. 3(a) and 4.
4. Conclusions A novel design for a solid state potentiometric microsensor for nitrate is described. It is shown that a single carbon fiber filament with a diameter of 6–7 m can be coated with parylene C, forming an excellent insulating and protective coating. Smooth and homogeneous parylene layers of 1–10 m were deposited on the walls of carbon fibers maintaining fiber flexibility and
micro-dimensions. Polypyrrole doped with nitrate was electropolymerized on carbon fiber cross-section forming a sensitive layer in the form of a microbead with diameter of 30–40 m. The size of the microbeads can be varied with the time devoted for deposition of the PPy(NO3 − ) layer. New designed sensors exhibit a linear response to nitrate concentration spanning from 0.1 M to 5 × 10−5 M of NO3 − with near-Nernstian slopes of −53.5 ± 2.5 mV per log cycle of nitrate concentration and detection limit of roughly 1 × 10−5 M of NO3 - showing better performance compared to the previously reported potentiometric nitrate selective sensor with the same sensing material deposited on carbon fiber walls. The micro-dimensions of the sensing area accompanied by the flexible protected electrode body creates a truly microdimensional sensor for nitrate ion that can be widely used for in situ measurements of a variety of samples. Microdimensional sensors with these characteristics would support numerous research directives aimed at observing microscopic scale chemical gradients and reactive transport processes near soil moisture–mineral interfaces [40], in biological tissues [41], within and adjacent to biofilms [42], and other porous mediabased systems. Acknowledgments The authors are thankful to Dr. Sergey Prikhodko for helpful discussions about carbon fiber cut. Funding from the Israeli Science Foundation (Bikura Post-doctoral Scholarship to T.B.)
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is gratefully acknowledged. This material is based upon work supported by the National Science Foundation under Grant No. CCF-0120778. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. References [1] C. Liu, Development of chemical sensors using microfabrication and micromachining techniques, Mater. Chem. Phys. 42 (1995) 87–90. [2] J. Janata, M. Josowicz, P. Vanysec, D.M. DeVaney, Chemical sensors, Anal. Chem. 70 (1998) 179R–208R. [3] Y. Vlasov, A. Legin, Non-selective chemical sensors in analytical chemistry: from “electronic nose” to “electronic tongue”, Fresenius J. Anal. Chem. 361 (1998) 255–260. [4] H.H. Weetall, Chemical sensors and biosensors, update, what, where, when and how, Biosens. Bioelectron. 14 (1999) 237–242. [5] See special issue on Chemical Sensors: Chem. Rev. 100 (7) (2000), 2477–2738. [6] E. Bakker, M. Telting-Diaz, Electrochemical sensors, Anal. Chem. 74 (2002) 2781–2800. [7] D.R. Reyes, D. Iossifidis, P.A. Auroux, A. Manz, Micro total analysis systems. 1. Introduction, theory, and technology, Anal. Chem. 74 (2002) 2623–2636. [8] P.A. Auroux, D. Iossifidis, D.R. Reyes, A. Manz, Micro total analysis systems. 2. Standard operations and applications, Anal. Chem. 74 (2002) 2637–2652. [9] O.S. Wolfbeis, Fiber-optic chemical sensors and biosensors, Anal. Chem. 74 (2002) 2663–2678. [10] J. Janata, M. Josowicz, Conducting polymers in electronic chemical sensors, Nat. Mater. 2 (2003) 19–24. [11] C.J. Koester, S.L. Simonich, B.K. Esser, Environmental analysis, Anal. Chem. 75 (2003) 2813–2829. [12] N. Szabo, C. Lee, J. Trimboli, O. Figueroa, R. Ramamoorthy, S. MidlamMohler, A. Soliman, H. Verweij, P. Dutta, S. Akbar, Ceramic-based chemical sensors, probes and field-test in automobile engines, J. Mater. Sci. 38 (2003) 4239–4245. [13] J.R. Stetter, W.R. Penrose, S. Yao, Sensors, chemical sensors, electrochemical sensors, and ECS, J. Electrochem. Soc. 150 (2003) S11–S16. [14] E. Bakker, Electrochemical sensors, Anal. Chem. 76 (2004) 3285–3298. [15] O.S. Wolfbeis, Fiber-optic chemical sensors and biosensors, Anal. Chem. 76 (2004) 3269–3284. [16] O.S. Wolfbeis, Materials for fluorescence-based optical chemical sensors, J. Mater. Chem. 15 (2005) 2657–2669. [17] L. Wang, D. Fine, D. Sharma, L. Torsi, A. Dodabalapur, Nanoscale organic and polymeric field-effect transistors as chemical sensors, Anal. Bioanal. Chem. 384 (2006) 310–321. [18] J. Bobacka, Conducting polymer-based solid-state ion-selective electrodes, Electroanalysis 18 (2006) 7–18. [19] T.A. Bendikov, J. Kim, T. Harmon, Development and environmental application of a nitrate selective microsensor based on doped polypyrrole films, Sens. Actuators B 106 (2005) 512–517. [20] H. Rashid, M. Lake, J. Woolam, B. Banks, S. Domitz, Passivating coatings for intercalated graphite fiber electrical contacts, Carbon 24 (1986) 511–513. [21] K. Horie, H. Murai, I. Mita, Bonding of epoxy resins to graphite fibers, Fibre Sci. Technol. 9 (1976) 253–264. [22] K. Kusakabe, B.-K. Sea, J.-I. Hayashi, H. Maeda, S. Morooka, Coating of carbon fibers with amorphous SiC films as diffusion barrier by chemical vapor deposition with triisopropylsilane, Carbon 34 (1996) 179–185. [23] R. Gadow, F. Kern, Continuous liquid phase coating of carbon fibers with Kion VL 20® Polysilazane, Adv. Eng. Mater. 5 (2003) 799–801. [24] N. Dilsiz, E. Ebert, W. Weisweiler, G. Akovali, Effect of plasma polymerization on carbon fibers used for fiber/epoxy composites, J. Colloid Interface Sci. 170 (1995) 241–248.
133
[25] A. Bismark, A. Menner, M.E. Kumru, A. Sezai Sarac, M. Bistritz, E. Schulz, Poly(carbazole-co-acrylamide) electrocoated carbon fibers and their adhesion behavior to an epoxy resin matrix, J. Mater. Sci. 37 (2002) 461–471. [26] H.M. Kang, T.H. Yoon, M. Bump, J.S. Riffle, Effect of solubility and miscibility on the adhesion behavior of polymer-coated carbon fibers with vinil ester resins, J. Appl. Polym. Sci. 79 (2001) 1042–1053. [27] K. Morita, Y. Shimizu, Microhole array for oxygen electrode, Anal. Chem. 61 (1989) 159–162. [28] K. Tokuda, K. Morita, Y. Shimizu, Cyclic voltammetry at microhole array electrodes, Anal. Chem. 61 (1989) 1763–1768. [29] Y. Shimizu, K. Morita, Microhole array electrode as a glucose sensor, Anal. Chem. 62 (1990) 1498–1501. [30] M.A. Dayton, J.C. Brown, K.J. Stutts, R.M. Wightman, Faradaic electrochemistry at microvoltammetric electrodes, Anal. Chem. 52 (1980) 946–950. [31] H.-T. Chiu, J.-S. Lin, Electrochemical deposition of polypyrrole on carbon fibers for improved adhesion to the epoxy resin matrix, J. Mater. Sci. 27 (1992) 319–327. [32] B. Grunden, J.O. Iroh, Formation of graphite fibre-polypyrrole coatings by aqueous electrochemical polymerization, Polymer 36 (1995) 559–563. [33] K. Potje-Kamloth, J. Janata, M. Josowicz, Electrochemically prepared insulation for carbon fiber microelectrodes, Ber. Bunsenges. Phys. Chem. 93 (1989) 1480–1485. [34] T.G. Strein, A.G. Ewing, Characterization of submicron-sized carbon electrodes insulated with phenol–allyphenol copolymer, Anal. Chem. 64 (1992) 1368–1373. [35] SCS (Specialty Coating Systems) web site: http://www.scscookson.com/ parylene knowledge/specifications.cfm. [36] B.L. Halpern, J.J. Schmitt, Y. Di, J.W. Golz, D.L. Johnson, D.T. McAvoy, D. Wang, J.Z. Zhang, Jet vapor deposition of single and multicomponent thin films, Met. Finishing 90 (1992) 37–41. [37] M. Nishizawa, M. Shibuya, T. Sawaguchi, T. Matsue, I. Ushida, Electrochemical preparation of ultrathin polypyrrole film at microarray electrodes, J. Phys. Chem. 95 (1991) 9042–9044. [38] M. Nishizawa, Y. Miwa, T. Matsue, I. Ushida, Surface pretreatment for electrochemical fabrication of ultrathin patterned conducting polymers, J. Electrochem. Soc. 140 (1993) 1650–1655. [39] M. Nishizawa, Y. Miwa, T. Matsue, I. Ushida, Ultrathin polypyrrole formed at a twin-microband electrode in the presence of dodecylsulfate, J. Electroanal. Chem. 371 (1994) 273–275. [40] C. Steefel, D. DePaolo, P. Lichtner, Reactive transport modeling: an essential tool and a new research approach for the earth sciences, Earth Plan. Sci. Lett. 240 (2005) 539–558. [41] R. Chignola, E. Milotti, A phenomenological approach to the simulation of metabolism and proliferation dynamics of large tumour cell populations, Phys. Biol. 2 (2005) 8–22. [42] J. Paulsen, E. Oppen, R. Bakke, Biofilm morphology in porous media, a study with microscopic and image techniques, Water Sci. Technol. 36 (1997) 1–9.
Biographies Tatyana A. Bendikov obtained her M.Sc. and Ph.D. degrees in chemistry from Technion, Israel Institute of Technology in 1997 and 2001, respectively. During 2002–2004 she was a postdoctoral fellow at the Department of Civil and Environmental Engineering, UCLA working on research in the National Science Foundation Center for Embedded Networked Sensing (CENS) related to development and testing of environmental chemical sensors. She is currently holding a position of postdoctoral researcher at the Department of Materials and Interfaces in Weizmann Institute of Science, Israel. Her research interests include Analytical Chemistry and Electrochemistry with emphasis on development of chemical and biological sensors. Scott Miserendino obtained his B.S. degree in electrical engineering and mathematical sciences from the Johns Hopkins University in 2002 and his M.S. degree in electrical engineering from the California Institute of Technology in 2003. He is currently pursuing his Ph.D. in electrical engineering at the California Institute of Technology. His research interests include modular microfluidic ana-
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lytical chemistry and single-cell metabolic monitoring, on-chip electrochemical sensors, and microgaskets. Yu-Chong Tai received his B.S. degree from National Taiwan University, and M.S. and Ph.D. degrees in Electrical Engineering from the University of California at Berkeley. After Berkeley, he joined the faculty of Electrical Engineering at the California Institute of Technology and built the Caltech MEMS Lab. He recently joined the Bioengineering department and he is currently a Professor of Electrical Engineering and Bioengineering at Caltech. His current research interests include flexible MEMS, bioMEMS, MEMS for retinal implants, parylene-based integrated microfluidics, neuroprobes/neurochips, and HPLC-based labs-on-a-chip. He has received several awards such as the IBM
fellowship, the Best Thesis Award, the Presidential Young Investigator (PYI) Award and the David and Lucile Packard Fellowship. He co-chaired the 2002 IEEE MEMS Conference in Las Vegas. He is currently a Subject Editor of the Journal of Microelectromechanical Systems. Thomas C. Harmon is a Professor in the School of Engineering and Founding Faculty member at the University of California, Merced. He directs contaminant monitoring research at the NSF Center for Embedded Networked Sensing at UCLA, where he was a faculty member in the Department of Civil & Environmental Engineering from 1992 to 2003. He received a B.S. in Civil Engineering from the Johns Hopkins University in 1985, and M.S. and Ph.D. degrees from the Environmental Engineering program at Stanford University in 1986 and 1992.