Characterization of novel impedimetric pH-sensors based on solution-processable biocompatible thin-film semiconducting organic coatings

Sensors and Actuators B 171–172 (2012) 537–543

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Characterization of novel impedimetric pH-sensors based on solution-processable biocompatible thin-film semiconducting organic coatings Nada Mzoughi a,b , Alaa Abdellah b , Qingqing Gong b , Helmut Grothe a,∗ , Paolo Lugli b , Bernhard Wolf a , Giuseppe Scarpa b,∗∗ a b

Lehrstuhl für Medizinische Elektronik, Technische Universität München, Arcisstrasse 21, D-80333 Munich, Germany Institute for Nanoelectronics, Technische Universität München, Arcisstrasse 21, D-80333 Munich, Germany

a r t i c l e

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Article history: Received 14 December 2011 Received in revised form 5 May 2012 Accepted 7 May 2012 Available online 14 May 2012 Keywords: Organic biosensors pH-sensors Poly(3-hexylthiophene) (P3HT) Carbon nanotubes (CNTs) Polymers Biosensing

a b s t r a c t In this work, novel pH sensors based on biocompatible organic semiconductors are presented. The suitability of poly(3-hexylthiophene) (P3HT) and carbon nanotubes (CNTs) as sensing layers in aqueous media was investigated. The main advantage of this new form of pH sensors is their simplified architecture without a reference electrode. Good pH sensitivity was observed for both CNT sensors and P3HT sensors even with a small amount (60 (l) of the analyte. Although the CNT-based sensors were more robust than the polymer-based ones, the stability of both sensors needs to be improved. In addition to the prospect of printing the sensors on thin and flexible substrates, the usage of solution processable materials enables a simple and low-cost fabrication of the devices. © 2012 Published by Elsevier B.V.

1. Introduction Cultivating cells and tissue on multiparametric sensor chips enables physio-metabolical monitoring of functional changes of living cells [1]. Cells, tumor sections or model tumors can be tested using relevant chemotherapy drugs to determine possible chemosensitivity, for example [2]. Key elements of such cellular assays are up to now silicon, glass or ceramic chips with integrated sensors for physicochemical parameters, i.e. pH, pO2 , electric impedance and temperature. Analysis of these primary parameters yields information about cell metabolism, cell growth and adhesion, and cell morphological changes [3]. From a single chip device [4] to a fully automated multiwell plate system [5], sensor based cell monitoring has been developed as a powerful tool for tumor sensitivity testing or pharmaceutical screening. A still existing hindrance for applications in many other fields (e.g. environmental monitoring, toxicity and food testing) is

∗ Corresponding author. Tel.: +49 89 289 22949; fax: +49 89 289 22950. ∗∗ Corresponding author. Tel.: +49 89 289 25334; fax: +49 89 289 25337. E-mail addresses: [email protected] (N. Mzoughi), [email protected] (A. Abdellah), [email protected] (Q. Gong), [email protected] (H. Grothe), [email protected] (P. Lugli), [email protected] (B. Wolf), [email protected] (G. Scarpa). 0925-4005/$ – see front matter © 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.snb.2012.05.029

relatively high cost for manufacturing disposable sensor chips with thin film technologies. Organic electronics has attracted much attention in the last years [6]. Using organic materials as an alternative for (or in combination with) conventional inorganic semiconductors gives rise to new electronic, optical and mechanical properties for novel electronic devices, thus opening new fields of applications. Soluble polymers can be processed using simple low-temperature processes like spin coating, dip-coating, spray-coating or printing techniques, which are suited for a variety of substrates including flexible materials allowing for low-cost large area production. With this perspective, printing organic materials on flexible substrates would offer a cost effective alternative to the fabrication of cell chip systems on glass, ceramic or silicon. Organic materials have already successfully been applied in solar cells, large area and flexible displays, light weight batteries and sensors [7–10]. Organic sensors can be used for sensing analytes in both gaseous and aqueous environment, in which the organic material is directly exposed to the test solution [10,11]. In particular, biocompatible organic semiconductors offer a great deal of promise for applications in biological sensing [12–14] and a remarkable number of examples can already be found in the literature (for a recent review the reader is referred to [15] and references therein). Organic field effect transistors (OTFTs) are potential candidates for sensing applications [16,17]. In addition to the aforementioned simple, low-cost fabrication, OTFTs offer many advantages over other types of sensors, e.g.

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miniaturization, portability and the possibility of fabricating selective sensor arrays [12]. In comparison to chemiresistors, OTFTs can more easily be incorporated into sensing circuits with enhanced performance over discrete sensors, also due to the intrinsic signal amplification mechanism offered by the transistor configuration. However, for some of the applications mentioned above, a passive device configuration could also be sufficient in terms of performance requirements. We first explored the applicability of low voltage operating OTFTs [18], whose suitability as sensing devices in electrolytes and liquid media for biological assays has been recently reported [19,20]. Here, we investigate the pH sensing capability of our devices for the purpose discussed above in a configuration, in which both the gate electrode and the isolation layer become unnecessary, using only two electrodes to measure the impedance of the active layer. This impedimetric sensing mode represents one of the most promising alternatives to the mainstream pH-sensing methods [21,22]. The active organic layer acts at the same time as sensing and charge transport layer. Our solutionprocessable biocompatible materials are based on regioregular poly(3-hexylthiophene) and carbon nanotubes (CNTs). Both materials react with oxygen but the CNT-based sensors seem to be more robust than the polymer-based ones. As for the transistor devices, interdigitated array electrodes were patterned in order to increase the exchange surface between the organic layers and the solution to analyze improving the sensitivity of the sensors. The interdigitated electrode structures (IDES) can therefore be placed on any insulating layer or substrate. Furthermore, compared to previous reported pH sensors based on biocompatible polymers [23,24] or on networks of CNTs [25], our devices can perform without additional reference electrode. 2. Materials and methods 2.1. Chemicals and materials All chemicals were of reagent grade, purchased from commercial vendors and used as received. The regioregular poly(3-hexylthiophene) polymer (rr-P3HT) was purchased from Sigma–Aldrich. Carbon nanotubes (CNTs) were purchased from Southwest Nanotechnologies enriched with semiconducting content >90%. The pH standard test solutions were purchased from Merck. The solutions’ composition was: pH 4 (buffer solution based on citric acid, sodium hydroxide and hydrogen chloride), pH 7 (buffer solution based on di-sodium hydrogen phosphate and potassium dihydrogen phosphate) and pH 9 (buffer solution based on boric acid, potassium chloride and sodium hydroxide). Unless otherwise noted, NaCl was used for adjusting the conductivity of the pH test solutions. 2.2. Fabrication of P3HT-based sensors P3HT is an organic polymer, which has interesting optical, mechanical and electrical properties. Typically, solution processable polymers have carrier mobility in the range of 10(4 cm2 /V s. P3HT has been a topic of research mainly due to its high mobility and ability to form highly ordered polymer chains. It is a solution processable semiconducting polymer, which is well suited for lowcost, large-area deposition methods [26]. It has reported field effect mobility up to ∼0.1 cm2 /V s [27,28], close to the value of amorphous silicon and it is one of the most promising materials for OTFT-fabrication in the direction of disposable sensor applications [17,29]. P3HT, moreover, is biocompatible (Fig. 1) and therefore suited for biological sensors [30]. The fabrication process of the sensors is initiated by cleaning the 2-in. glass wafer used for first experiments. The IDES consisting of a 2 nm thick adhesive layer

Fig. 1. Cells (L929) growing on the surface of a P3HT layer up to the sample borders without changes in the morphology (biocompatibility test according to ISO 10993).

of Ti and the 45 nm thick Pt electrode is fabricated on top of the wafer via a sputtering deposition technique and a standard lift-off process. The spacing formed between the two electrodes of this interdigitated structure varied between 10 (m and 100 (m (width and spacing are equal, most of the experiments were performed with 50 (m electrodes). A solution of P3HT in chloroform is prepared and sonicated for 10 min so that the P3HT crystals can be dissolved completely [18]. The solution is then filtered and spin-coated onto the surface of the wafer in a glove box system under nitrogen atmosphere. Spin-coating was used only in the first experiments to achieve homogeneous and thin polymer films. In the subsequent experiments, drop-casting was used to restrict the polymer to the IDES area. The solvent used for the drop-casting is dichlorobenzene (DCB). DCB is a good solvent for P3HT and has a relatively high boiling point (174 ◦ C) so that it can also be used for the ink-jet printing of P3HT. Drop-casting was performed with a proportioning device in normal ambient conditions. Using a parafilm-sealed petri dish to cover the samples after drop casting slows down the drying process and consequently results in a relatively homogeneous P3HT layer. The obtained polymer film was approximately 70 nm thick. The P3HT layer does not require further treatment and the sensors are now ready for measurement. 2.3. Fabrication of CNTs-based sensors Carbon nanotubes (CNTs) were discovered in 1991 by the Japanese researcher Iijima [31]. Since then, they have been subject of intense research efforts for understanding and exploiting their unique physical, chemical, mechanical and electronic characteristics. In particular, due to their biocompatibility and minimal cytotoxicity [32], they are also suitable for medical applications [33]. A solution of CNTs in water (200 (g/ml) was prepared. A main hindrance of the CNTs’ processability is their poor solubility in organic solvents and in water. Due to their hydrophobicity and the strong tube–tube interaction a surfactant should be added to the solution for a better separation and solubilization of the CNTs [34]. Detergents maximize the homogeneity and CNT concentration of the solution and perform for this purpose even better than polymers. Here a high molecular weight cellulose derivative, sodium carboxymethyl cellulose (CMC), is used. This kind of surfactant, reported as an excellent agent for dispersing SWNTs in water [35], has been successfully applied to the fabrication of high-uniformity CNT films by spray deposition [36,37]. As for the P3HT devices, glass wafers with sputtered platinum electrodes were used as basis for the fabrication of the CNT sensors. Drop-casting was used in this case as deposition method, which allowed easily the formation of a homogeneous CNT

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Fig. 2. Electrical contacting of the interdigitated electrode structure (IDES). Inset: Droplet of the pH test solution on the surface of the active layer of the sensor.

thin-film on the sensor substrates. After casting deposition, the sample was heated for an hour at 250 ◦ C for improving the adhesion of the CNTs onto the wafer. The surfactant molecules were then removed with an acidic treatment [38]. Note that all steps involved in the CNT film fabrication are performed entirely in ambient conditions. 2.4. Impedance measurements As the conductivity of semiconductors and also CNTs is strongly influenced by any electric charge in close vicinity to the more or less moveable free carriers in the material, pH changes result in a conductivity alteration. This effect can be simply measured by changes in the impedance of IDES covered with the pH sensitive substances. For measurements, a drop of pH solution (60 (l) is deposited on the hydrophobic surface of the active layer of the sensor. The IDES electrodes are contacted with needles (Fig. 2) which are connected via cables to an impedance analyzer (SI 1260 Impedance/GainPhase Analyzer, Solatron Instruments, Great Britain) to measure the impedance of the sensors. A 30 mV AC voltage is applied for impedance measurements in a frequency range from 1 to 103 Hz. For frequencies over 103 Hz the measured curves overlapped. The pH and conductivity values of the test solutions were measured using standard laboratory equipment (pH meter, type inoLab pH 720, WTW; conductimeter, type Schott CG 853 SI Analytics GmbH). All measurements were performed in ambient conditions at room temperature.

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Fig. 3. Dependence of the impedance spectra of spin-coated P3HT layer on the pH value of the test solution (solutions with pH values of 4, 7 and 9, their composition is given in Section 2.1).

changes in the pH value therefore still has to be evaluated in further work. 3.1.2. pH sensitivity Measurements with different pH test solutions (4, 7 and 9) indicate the pH sensitivity of these P3HT-based sensors. As shown in Fig. 3, the magnitude of the impedance of the active layer increases proportionally to the pH value of the solution. The impedance (magnitude) is normalized to the value obtained in a solution of pH 7 at 1 Hz. In this respect, the sensor performance can be evaluated by defining its sensitivity as the relative difference of the normalized impedance values at low frequencies. The pH-sensitivity is higher at lower frequencies (1 Hz). At higher frequencies the measured curves for pH 4, pH 7 and pH 9 overlap, as the charge transport in organic materials is relatively slow.

3. Results and discussion 3.1. P3HT-based sensors 3.1.1. Stabilization time After exposure to the pH solution the measured impedance values stabilized within 30 min for the P3HT sensors. This stabilization time is relatively short compared with ISFETs or metal oxide pH sensors [39], which typically need several hours for stabilization. As these first experiments were carried out without a fluidic system, liquids with different pH values could not be changed without exposing the sensor to air. The response time of P3HT to rapid

Fig. 4. Dependence of the normalized impedance of the drop-casted P3HT layer on the pH value of the test solution.

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Fig. 5. Dependence of the normalized impedance of the spin-coated P3HT devices on the conductivity of the test solution.

As can be seen comparing Figs. 3 and 4, the sensitivity of the drop-casted polymer films is lower than that of the spin-coated films. However, the frequency response of the device (i.e. the frequency range in which a pH sensitive response was observed) is increased, eventually due to a higher carrier mobility in the dropcasted films [40]. The sensitivity of the drop-casted sensors may be reduced due to changes in morphologies of the active polymer layer. In fact, the orientation of the polymer chains depends on the polymer deposition method, the drying time and the solvent used. The spin-coating process may be favorable for the formation of face-on ordered chains [41] that can have a larger contact area with the analyte. It is also worth to mention that the electrical conductivity of all test solutions has been adjusted to approximately the same value, in order to eliminate its contribution to the sensor response. 3.1.3. Conductivity of the test solution The addition of salt to the pH test solutions resulted in three solutions for each pH value with varying conductivities. As is evident from Fig. 5, the P3HT sensors have a higher sensitivity to pH changes than to conductivity changes in the test medium. This means that pH-shifts in solutions with a nearly constant conductivity (cell culture, blood) can be precisely detected.

Fig. 6. Measured impedance values deviate at low frequencies.

Furthermore, the P3HT molecules (low ionization potential, donor) react with the oxygen molecules (high electron affinity, acceptor) forming a charge transfer complex, which leads to an increase in polymer conductivity [43], eventually reducing the sensitivity of the sensors. Protonation/deprotonation effects can also occur [22], which may also explain the observed memory effect. Due to this degradation effects, such kind of P3HT-based pH sensors have a relatively short life time, which still has to be improved even for disposable sensors applications. However, they do not show instant degradation, which can be exploited for certain applications. 3.2. CNTs-based sensors 3.2.1. Stabilization time The measured impedance values (magnitude) stabilized after 15 min of exposure to the pH solution (Fig. 7), which is only half the time as for P3HT sensors. 3.2.2. pH sensitivity As for the P3HT sensors, the magnitude of the impedance of the active layer increases with the pH value of the test medium (Fig. 8).

3.1.4. Memory effect After measurements with increasing pH values (pH 4, pH 7, pH 9), the pH 4 measurement was repeated. The obtained impedance values were higher than those in the first pH 4 measurement. The reaction of the polymer with the solutions with higher pH values or the degradation of the polymer in contact with the analyte may be the reason for the observed memory effect. 3.1.5. Degradation of the P3HT-sensors The analyte may enter the polymer bulk and cause a chemical degradation of the polymer during which the polymer chains would be cleaved to form oligomers and monomers [42]. Pores would then be formed, via which the oligomers and monomers would be released in the analyte. The degradation products may influence the pH value in the analyte as well as inside the pores. This degradation process could be on the basis of observed deviations in the impedance spectra in some of the samples in particular when exposed to test solutions with high pH values, as exemplary shown in Fig. 6.

Fig. 7. Impedance curves of the CNT sensors after 1–30 min exposure time to the analyte (the test solution with pH 4 was used here).

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Fig. 10. Dependence of the normalized impedance of the drop-casted CNT sensors on the conductivity of the test solution.

Fig. 8. Dependence of the normalized impedance spectra of drop-casted CNT sensors on the pH value of the test solution (solutions with pH 4, pH 6, pH 7, pH 7.5, pH 8, pH 8.5 and pH 9).

A comparison of Figs. 4 and 8 reveals that sensitivity (i.e. the relative differences of the normalized impedance values) of CNT sensors is two times higher than the one of P3HT sensors. This is clearly shown in Fig. 9. An increasing slope of the curve at pH 7 represents an increased sensitivity of the sensors. However, polymer-based sensors show improved linear responses upon change of pH [11,25]. Although there could be several physical mechanisms inducing the pH-sensitivity and the sensing mechanism is not fully understood yet, the influence of the ionic charges acts probably differently on the two sensors. In fact, for polymer-based sensors (as suggested also by Roberts et al. [44]) the analyte can diffuse into the polymeric layer influencing its conductivity via trapping and doping effects, while for CNT-based devices a site-binding model (as suggested in [45,46]) has been proposed for the pHsensitivity, where surface states on the CNTs are influenced by the ionic charges, thus changing again the conductivity.

Fig. 9. pH-sensitivity of the CNT, P3HT (drop-casted), and the P3HT (spin-coated) sensors at 1 Hz.

3.2.3. Conductivity of the test solution As can be seen in Fig. 10, the CNT sensors show a negligible sensitivity to changes in the conductivity of the test medium. 3.2.4. Memory effect A memory effect, similar to that of P3HT sensors, was observed in the CNT sensors. 3.2.5. Degradation of the sensors Although, we have not yet investigated the performance of our sensor with respect to storage conditions and no real aging experiments have been carried out so far, the sensing capability of such kind of CNTs-based devices remains at least for 12 months without significant loss in performance [37]. However, when the sensors are exposed to the analyte solutions, a doping/dedoping effect can take place, which is also related to during and after post-deposition treatment with citric or nitric acid [47]. Fig. 11 shows how this process influences the performance of the sensors. During the first three weeks after fabrication, the CNT-based sensor was exposed to the test solution for a total time of approximately 12 h and the sensitivity of the sensor decreased. During the fourth week, the sensor was in contact with the solution for just 30 min and no changes in

Fig. 11. Degradation of the CNT sensors (stored in air, at room temperature, exposed to pH test solutions for a total of 12 h during the first three weeks and 30 min in the fourth week).

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the sensitivity were detected. Such kind of effects can be minimized by additional treatment in advance to stabilize the sensor [47]. 4. Conclusion The suitability of two biocompatible and soluble organic semiconductors (P3HT and CNTs) for the use in sensor applications was investigated and a new and simple method for sensing pH change in biological applications is presented. Organic pH sensors were fabricated and characterized in direct contact with different pH test solutions. Our sensors have a simple structure and no reference electrode is needed. The change of the impedance (magnitude) of the active layer after exposure to small amounts of the analyte can easily be measured with simple instrumentation. The P3HT-based sensor can easily be processed and does not need any further treatment after fabrication. However, the (performance) degradation of the P3HT layers in contact with the test solutions and with the presence of oxygen is relatively fast. The spin-coated P3HT layers showed better pH sensitivity than the drop-casted ones. Both CNT and P3HT devices showed a low sensitivity to the conductivity of the test solution and a memory effect was observed in both sensors. Because of the higher sensitivity of the drop casted CNT layers in comparison with the P3HT layers, the CNTs are also better suited for the printing of the pH sensors. Unlike the P3HT sensors, the CNT sensors showed relatively slow performance degradation and were more robust. CNTs-based sensors also had a lower sensitivity to the conductivity of the test solution and showed a lower stabilization time in comparison with P3HT-based ones. For such kind of pH sensors, CNTs therefore seem to be the preferable material. It is worth to mention that, as stated in Section 1, these sensors have been developed for cell chips and they are intended to be used as on chip pH sensors for cell monitoring applications. For those applications, the sensors should detect pH changes of the measurement solution giving us information about the activity and growth conditions of the cells. These pH sensors are not intended to measure the exact pH value of the sample itself. Detecting pH changes is sufficient for the purposes described before. For that purpose, although the P3HT sensors are not as sensitive and robust as the CNT sensors, they do not show instant degradation and may be used as one-way/disposable sensors. Future work will be devoted to evaluate response time and stability as well as the linearity of CNTs-based sensors, maybe combining the CNTs with polymers. This combination could also improve the linear response of the devices [25]. Acknowledgments The work was sponsored by the German Federal Ministry of Education and Research (research project THEMIC, FKZ 16SV5044) and by the Institute for Advanced Study and the International Graduate School for Science and Engineering of the Technische Universität München. References [1] J. Wiest, M. Brischwein, J. Ressler, A.M. Otto, H. Grothe, B. Wolf, Cellular assays with multiparametric bioelectronic sensor chips, Chimia 59 (2005) 243–246. [2] B. Wolf, M. Brischwein, V. Lob, J. Ressler, J. Wiest, Cellular signaling: aspects for tumor diagnosis and therapy, Biomedizinische Technik. Biomedical Engineering 52 (2007) 164–168. [3] B. Wolf, M. Brischwein, H. Grothe, C. Stepper, J. Ressler, T. Weyh, G. Urban (Eds.), Lab-on-a-chip Systems for Cellular Assays, BioMEMS, Series: Microsystems, vol. 16, Springer-Verlag, Dordrecht, Netherlands, 2006, pp. 269–308, ISBN-10: 0-387-28731-0, ISBN-13: 978-0-387-28731-7. [4] J. Wiest, T. Stadthagen, M. Schmidhuber, M. Brischwein, J. Ressler, U. Raeder, et al., Intelligent mobile lab for metabolics in environmental monitoring, Analytical Letters 39 (2006) 1759–1771.

[5] V. Lob, T. Geisler, M. Brischwein, R. Uhl, B. Wolf, Automated live cell screening system based on a 24-well-microplate with integrated micro fluidics, Medical & Biological Engineering & Computing 45 (2007) 1023–1028. [6] G. Malliaras, R. Friend, An organic electronics primer, Physics Today 58 (2005) 53–58. [7] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, et al., High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends, Nature Materials 4 (2005) 864–868. [8] G.H. Gelinck, H.E.A. Huitema, E. Van Veenendaal, E. Cantatore, L. Schrijnemakers, J.B.P.H. Van der Putten, et al., Flexible active-matrix displays and shift registers based on solution-processed organic transistors, Nature Materials 3 (2004) 106–110. [9] H. Nishide, K. Oyaizu, Materials science. Toward flexible batteries, Science 319 (2008) 737–738. [10] N. Gupta, S. Sharma, I.A. Mir, D. Kumar, Advances in sensors based on conducting polymers, Journal of Scientific and Industrial Research 65 (2006) 549–557. [11] O. Korostynska, K. Arshak, E. Gill, A. Arshak, Review on state-of-the-art in polymer based pH sensors, Sensors 7 (2007) 3027–3042. [12] J.T. Mabeck, G.G. Malliaras, Chemical and biological sensors based on organic thin-film transistors, Analytical and Bioanalytical Chemistry 384 (2006) 343–353. [13] M. Berggren, A. Richter-Dahlfors, Organic bioelectronics, Advanced Materials 19 (2007) 3201–3213. [14] R.M. Owens, G.G. Malliaras, Organic electronics at the interface with biology, MRS Bulletin 35 (2010) 449–456. [15] M.D. Angione, R. Pilolli, S. Cotrone, M. Magliulo, A. Mallardi, G. Palazzo, et al., Carbon based materials for electronic bio-sensing, Materials Today 14 (2011) 424–433. [16] P. Lin, F. Yan, Organic thin-film transistors for chemical and biological sensing, Advanced Materials 24 (2012) 34–51. [17] C. Bartic, G. Borghs, Organic thin-film transistors as transducers for (bio) analytical applications, Analytical and Bioanalytical Chemistry 384 (2006) 354–365. [18] S.M. Goetz, C.M. Erlen, H. Grothe, B. Wolf, P. Lugli, G. Scarpa, Organic fieldeffect transistors for biosensing applications, Organic Electronics 10 (2009) 573–580. [19] G. Scarpa, A.-L. Idzko, A. Yadav, S. Thalhammer, Organic ISFET based on poly (3-hexylthiophene), Sensors 10 (2010) 2262–2273. [20] G. Scarpa, A.-L. Idzko, A. Yadav, E. Martin, S. Thalhammer, Toward cheap disposable sensing devices for biological assays, IEEE Transactions on Nanotechnology 9 (2010) 527–532. [21] O. Korostynska, K. Arshak, E. Gill, Review paper: materials and techniques for in vivo pH monitoring, IEEE Sensors Journal 8 (2008) 20–28. [22] E. Gill, A. Arshak, K. Arshak, O. Korostynska, pH sensitivity of novel PANI/PVB/PS3 composite films, Sensors 7 (2007) 3329–3346. [23] B. Lakard, G. Herlem, M. de Labachelerie, W. Daniau, G. Martin, J.-C. Jeannot, et al., Miniaturized pH biosensors based on electrochemically modified electrodes with biocompatible polymers, Biosensors and Bioelectronics 19 (2004) 595–606. [24] X. Zhu, pH sensor using nano electrodes in organic semiconductor, Engineering in Medicine and Biology Society 3 (2004) 1968–1971. [25] M. Kaempgen, S. Roth, Transparent and flexible carbon nanotube/polyaniline pH sensors, Journal of Electroanalytical Chemistry 586 (2006) 72–76. [26] G. Scarpa, A. Abdellah, A. Exner, S. Harrer, G.P. Blanco, W. Wiedemann, et al., Patterning poly(3-hexylthiophene) in the sub-50-nm region by nanoimprint lithography, IEEE Transactions on Nanotechnology 10 (2011) 482–488. [27] J.-F. Chang, B. Sun, D.W. Breiby, M.M. Nielsen, T.I. Sölling, M. Giles, et al., Enhanced mobility of poly(3-hexylthiophene) transistors by spin-coating from high-boiling-point solvents, Chemistry of Materials 16 (2004) 4772–4776. [28] M. Mas-Torrent, D.D. Boer, M. Durkut, P. Hadley, A. Schenning, Field effect transistors based on poly(3-hexylthiophene) at different length scales, Nanotechnology 15 (2004) 265–269. [29] J.B. Chang, V. Liu, V. Subramanian, Printable polythiophene gas sensor array for low-cost electronic noses, Journal of Applied Physics 100 (2006) 0145061–014506-7. [30] G. Scarpa, A.-L. Idzko, S. Götz, S. Thalhammer, Biocompatibility studies of functionalized regioregular poly(3-hexylthiophene) layers for sensing applications, Macromolecular Bioscience 10 (2010) 378–383. [31] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58. [32] S. Smart, A. Cassady, G. Lu, D. Martin, The biocompatibility of carbon nanotubes, Carbon 44 (2006) 1034–1047. [33] R. Hirlekar, M. Yamagar, H. Garse, M. Vij, Carbon nanotubes and its applications: a review, Asian Journal of Pharmaceutical and Clinical Research 2 (2009) 17–27. [34] Z. Sun, V. Nicolosi, D. Rickard, S.D. Bergin, D. Aherne, J.N. Coleman, Quantitative evaluation of surfactant-stabilized single-walled carbon nanotubes: dispersion quality and its correlation with zeta potential, The Journal of Physical Chemistry C 112 (2008) 10692–10699. [35] T. Takahashi, K. Tsunoda, H. Yajima, T. Ishii, Dispersion and purification of single-wall carbon nanotubes using carboxymethylcellulose, Japanese Journal of Applied Physics 43 (2004) 3636–3639. [36] R.C. Tenent, T.M. Barnes, J.D. Bergeson, A.J. Ferguson, B. To, L.M. Gedvilas, et al., Ultrasmooth, large-area, high-uniformity, conductive transparent single-walled-carbon-nanotube films for photovoltaics produced by ultrasonic spraying, Advanced Materials 21 (2009) 3210–3216. [37] A. Abdellah, A. Yaqub, C. Ferrari, B. Fabel, P. Lugli, G. Scarpa, Spray deposition of highly uniform CNT films and their application in gas sensing,

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[38]

[39]

[40]

[41]

[42] [43]

[44]

[45] [46]

[47]

IEEE-NANO 2011, 11th IEEE Conference on Nanotechnology, 2011, pp. 1118–1123, http://dx.doi.org/10.1109/NANO.2011.6144569. H.-Z. Geng, D.S. Lee, K.K. Kim, G.H. Han, H.K. Park, Y.H. Lee, Absorption spectroscopy of surfactant-dispersed carbon nanotube film: modulation of electronic structures, Chemical Physics Letters 455 (2008) 275–278. M. Brischwein, H. Grothe, J. Wiest, M. Zottmann, J. Ressler, B. Wolf, Planar ruthenium oxide sensors for cell-on-a-chip metabolic studies, Chemia Analityczna 54 (2009) 1193–1201. M. Surin, P. Leclère, R. Lazzaroni, J.D. Yuen, G. Wang, D. Moses, et al., Relationship between the microscopic morphology and the charge transport properties in poly(3-hexylthiophene) field-effect transistors, Journal of Applied Physics 100 (2006) 033712-1–033712-6. H. Yang, S.W. LeFevre, C.Y. Ryu, Z. Bao, Solubility-driven thin film structures of regioregular poly(3-hexyl thiophene) using volatile solvents, Applied Physics Letters 90 (2007) 172116-1–172116-3. A. Göpferich, Mechanisms of polymer degradation and erosion, Biomaterials 17 (1996) 103–114. M.S.A. Abdou, F.P. Orfino, Y. Son, S. Holdcroft, Interaction of oxygen with conjugated polymers: charge transfer complex formation with poly(3alkylthiophenes), Journal of the American Chemical Society 119 (1997) 4518–4524. M.E. Roberts, S.C.B. Mannsfeld, N. Queraltó, C. Reese, J. Locklin, W. Knoll, et al., Water-stable organic transistors and their application in chemical and biological sensors, Proceedings of the National Academy of Sciences of the United States of America 105 (2008) 12134–12139. J.H. Back, M. Shim, pH-dependent electron-transport properties of carbon nanotubes, The Journal of Physical Chemistry B 110 (2006) 23736–23741. I. Heller, S. Chatoor, J. Männik, M.A.G. Zevenbergen, C. Dekker, S.G. Lemay, Influence of electrolyte composition on liquid-gated carbon nanotube and graphene transistors, Journal of the American Chemical Society 132 (2010) 17149–17156. T.M. Barnes, J.L. Blackburn, J. van de Lagemaat, T.J. Coutts, M.J. Heben, Reversibility, dopant desorption, and tunneling in the temperature-dependent conductivity of type-separated, conductive carbon nanotube networks, ACS Nano 2 (2008) 1968–1976.

Biographies Nada Mzoughi received her Diploma in electrical engineering and information technology in 2011 from the Technical University of Munich (TUM), Munich, Germany. She currently is a Ph.D. candidate at the Institute of Medical Engineering at TUM working on her doctoral thesis in the field of printed biochips and organic sensors. Alaa Abdellah was born in Cairo, Egypt. He received the B.Sc. degree in electronics and communications engineering from the Cairo University, Cairo, in 2005, and the M.Sc. degree in high-frequency engineering and optoelectronics from the Technical University of Munich, Munich, Germany, in 2007, where he is currently working toward the Ph.D. degree at the Institute for Nanoelectronics. His research interests include fabrication and characterization of organic electronic and optoelectronic devices as well as nanofabrication technologies based on nanoimprint lithography.

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Qingqing Gong was born in Chongqing, China. She received the Dipl.-Ing. degree in electrical engineering and information technology from the Technical University of Munich, Munich, Germany, in 2007, where she is currently working toward the Ph.D. degree at the Institute for Nanoelectronics. Her research interests include fabrication and characterization of carbon nanotube-based electronics. Helmut Grothe has studied high-frequency engineering at the Technische Universität München. He received his diploma in 1975 and his Ph.D. in 1979 at the Lehrstuhl für Allgemeine Elektrotechnik und Angewandte Elektronik (Prof. Dr. Wolfgang Harth). He headed the molecular beam epitaxy group at this chair and significantly contributed to the technological development of optoelectronic and high frequency transmitter diodes. Since 2000, he is head of technology at the Heinz NixdorfLehrstuhl für Medizinische Elektronik at the Technische Universität München (Prof. Dr. Bernhard Wolf) for sensor and biochip developments. Dr. Grothe is member of the advisory board of BioMST (Arbeitskreis Mikrosysteme für Biotechnologie und Lifesciences e.V.). Paolo Lugli received the Laurea degree in physics from the University of Modena, Modena, Italy, in 1979, and the M.Sc. and Ph.D. degrees in electrical engineering from Colorado State University, Fort Collins, in 1982 and 1985, respectively. In 1985, he joined the Physics Department, University of Modena, as a research associate. From 1988 to 1993, he was an associate professor of solid state physics at Engineering Faculty, University of Rome Tor Vergata, Rome, Italy, where he became a full professor of optoelectronics in 1993. In 2003, he joined the Technical University of Munich, Munich, Germany, where he was appointed as head of the Institute for Nanoelectronics. His current research interests include the modeling, fabrication, and characterization of organic devices for electronics and optoelectronics applications, the design of circuits and architectures for nanostructures and nanodevices, the numerical simulation of microwave semiconductor devices, and the theoretical study of transport processes in nanostructures. Bernhard Wolf studied biology and physics and became head of the newly established working group Medizinische Elektronik und Elektronenmikroskopie at the Institut für Immunbiologie, Universität Freiburg, in 1980. After his habilitation, he was appointed as full professor at the Lehrstuhl für Biophysik, Universität Rostock. Since 2000, he holds the chair of the Heinz Nixdorf-Lehrstuhl für Medizinische Elektronik, Technische Universität München. His main fields of research are in analytical electron microscopy, system biology of neoplastic diseases and in novel concepts for medical diagnostics and therapy supported by microelectronic systems. Giuseppe Scarpa received the Laurea degree in electrical engineering from the University of Rome, Tor Vergata, Italy, in 1998, and the Ph.D. degree from Walter Schottky Institute, Technical University of Munich, Munich, Germany, in 2003. His Ph.D. thesis was on design and fabrication of quantum cascade lasers. He is currently a staff lecturer at the Electrical Engineering Department and a staff scientist at the Institute for Nanoelectronics, Technical University of Munich. His research interests include the fabrication of a variety of nanostructures (such organic devices and nanomagnets) and the development of various nanofabrication technologies based on nanoimprint lithography as well as on biosensors and biochips based on organic materials.