pH sensitive photoconductor based on poly(para-phenylene-vinylene)

Sensors and Actuators B 123 (2007) 153–157

pH sensitive photoconductor based on poly (para-phenylene-vinylene) P. Pistor a , V. Chu a,∗ , D.M.F. Prazeres b,c , J.P. Conde a,c a

INESC Microsistemas e Nanotecnologias (INESC MN), Lisbon, Portugal Center of Biological and Chemical Engineering, Instituto Superior T´ecnico, Lisbon, Portugal c Department of Chemical and Biological Engineering, Instituto Superior T´ ecnico, Lisbon, Portugal b

Received 11 April 2006; received in revised form 3 August 2006; accepted 10 August 2006 Available online 12 September 2006

Abstract A pH sensitive device based on poly(p-phenylene-vinylene) (PPV) in a coplanar electrode configuration is demonstrated. An increase in darkand photocurrent of a thin (25–30 nm) poly(p-phenylene-vinylene) upon exposure to aqueous solutions is observed. The change in photocurrent is monitored as a function of pH. A roughly linear variation of the photocurrent with pH is observed between pH values of 4.5–9.5. The results are reversible and reproducible. © 2006 Elsevier B.V. All rights reserved. Keywords: pH sensor; Semiconducting polymer; PPV; Photoconductivity

1. Introduction Since the discovery of their conducting and semiconducting properties, conjugated polymers have received special attention from the scientific community and industry. Their electronic and optoelectronic properties, as well as their advantages over inorganic electronics like flexibility, inexpensive processing and easy large-scale production, gave rise to new device concepts. These include organic light-emitting diodes (OLEDs) as used in flat panel displays, with great potential in low-cost production on flexible substrates [1] or inkjet printed organic transistors [2]. Among the group of conjugated polymers, poly(p-phenylenevinylene) (PPV) and derivatives are some of the most promising candidates for organic electronics applications. They are already in use in OLEDs [3] and photovoltaic cells [4], while their nonlinear optical properties are under investigation for use in organic lasers [5]. Pure PPV is not soluble in organic solvents and must be processed through a soluble precursor. The chemical structure of PPV is depicted in the inset of Fig. 1. The delocalized ␲-electrons from the phenyl-rings and conjugated carbon chains are responsible for the semiconducting properties of PPV. It is

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Corresponding author. Tel.: +351 213 100 231; fax: +351 213 145 843. E-mail address: [email protected] (V. Chu).

0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.08.005

generally believed that in polymers the primary photoexcitation results in the creation of excitonic states, rather than in the direct creation of free charge carriers as is the case for inorganic semiconductors. However, exciton binding energies and details of charge transfer in conjugated polymers are still subjects of debate within the scientific community (see for example [6,7]). There are a variety of sensor devices already in use that take advantage of label-free electronic and optoelectronic detection of biomolecules and biochemical processes [8–10]. These devices generally use the field-effect-induced changes in the electronic behavior of an inorganic semiconductor due to surface charging by adhesion of charged biomolecules (e.g., enzymes or DNA) or by changes in the local pH near the surface. Currently, there is increasing interest in the use of organic semiconductors for sensing applications, e.g., pentacene or poly(3-hexylthiophene) in organic field-effect transistors [11–13]. In another example, Cooreman et al. demonstrated an immunosensor based on PPV [14]. The detection of local changes in pH by low-volume microsensors is also of particular interest for a variety of medical, biological and environmental applications, such as the monitoring of cell metabolism or enzyme activity. Towards this goal, in this paper the photoconductive properties of a thin PPV layer are used as an active element in a new pH-sensitive device when exposed to aqueous solutions.

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Fig. 1. Schematic drawing of the pH sensitive device showing the aluminum contacts (dark grey) below a thin (∼30 nm) layer of PPV (white). In the wet state, a solution covers the whole area inside the pool formed by the silicone protection over the contact pads. Drawing is not to scale.

2. Experimental procedures 2.1. Sample preparation Devices consisting of aluminum microcontacts and a top layer of PPV were produced. Fig. 1 shows a schematic drawing of the finished device. Devices were processed in the following way: first, aluminum pads for wire-bonding with a thickness of ∼150 nm and thereafter aluminum contacts (thickness ∼25 nm) were deposited onto cleaned 7059 Corning glass substrates by sputtering and subsequently patterned by photolithography and wet etching. The Al structures for a pair of corresponding contacts end in two parallel contacts with a separation of 3 ␮m and a width of 1 mm. PPV films were obtained through the standard precursor route from spin coating a sulfonium precursor soluble in water and methanol, that was subsequently converted into PPV in a thermal conversion step, as described elsewhere [15]. Under atmospheric conditions, the PPV precursor solution is spin coated over the entire substrate. The conversion to PPV was carried out at 210 ◦ C in vacuum (p∼10−5 Torr) for 12 h. Ellipsometry measurements of PPV films processed under the same conditions on silicon wafers revealed a thickness of ∼25–30 nm. The PPV layer was not patterned. The finished dies were diced, mounted and wire-bonded to DIP packages for electrical measurements. Finally, the wire-bonding pads and wires were protected with a silicone rubber (Elastosil® E41) to avoid degradation during measurements under liquid media. In Fig. 2 a photograph of the finished device is presented. Samples were stored in the dark and only exposed to room-light during experiments. 2.2. Buffer preparation Measurements were performed on the devices in the dry state as well as under deionized (DI) water and a series of phosphate buffer solutions with different pH values. For the phosphate buffer solutions, 0.2 M monobasic sodium phosphate was mixed with a solution of 0.2 M dibasic sodium phosphate in different ratios and the mixtures were diluted with an equal amount of DI water. The pH values of the phosphate buffer solutions obtained

Fig. 2. Photograph of the finished device mounted on a chip carrier. Dark structures are the aluminum contacts below the thin (∼30 nm) PPV layer. The contact pads, where the wire-bonding to the chip carrier is done, are protected with silicone epoxy. In the centre, one of the contact pairs, with a width of 1 mm and a gap of 3 ␮m, is highlighted.

this way were measured with a standard pH-meter and varied between 4.5 and 9.5. 2.3. Electronic and optoelectronic measurements Optical absorption measurements were made on the PPV films deposited on glass substrates to confirm the optical properties of the converted PPV films. The conductivity of the devices was also measured in the dark and under illumination with white light in the dry state. Since the photoresponse of the PPV has a time scale of seconds, the current was measured every second with a picoammeter while applying a constant voltage of 1 V to one of the coplanar electrodes and keeping the other electrode grounded, beginning with the sample in the dark, then under illumination for 100 s and finally returning to the dark. The photocurrent, Iph , was obtained by subtracting the stabilized current, Idark , in the dark (measured at the end of the first dark interval) from the stabilized current under illumination Iill (at the end of the illuminated interval). The same light response measurements were performed on the device after putting a drop of DI water (∼50 ␮L) onto the active area of the device. The photocurrent measurements were then repeated with a series of phosphate buffer solutions. These experiments start with a neutral buffer and then alternate between basic and acidic solutions to avoid measuring a drift of device response. Control measurements with DI water were made in between two subsequent buffer measurements to detect drifts of the device response.

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Finally, the spectral response of the device upon exposure to DI water was measured. In this case, the photocurrent was measured with a lock-in amplifier and chopped, monochromatic light from a tungsten-halogen lamp in the dry state and while exposed to DI water with an applied voltage of one volt. The chopping frequency was 13 Hz. The detected signal was corrected for changes in the light intensity by simultaneous monitoring of the light intensity with a pyroelectric detector. 3. Results Optical absorption measurements for the converted PPV films gave an optical band gap of ∼2.5 eV (obtained from the xintercept of the absorption edge plotted in a linear scale) and an absorption maximum at ∼3.1 eV, in agreement with values found in literature [16]. Fig. 3 shows the typical light response of the PPV coplanar device. The dark current decreases just after the application of the voltage until it stabilizes at values close to the detection limit of the picoammeter (∼10−14 to 10−13 A). Upon illumination, the current increases sharply and usually saturates at values of around (1–3) × 10−12 A after a few tens to hundreds of seconds. After cutting the illumination, a slow relaxation back to the original dark current was observed. Previous studies of the optoelectronic behavior of PPV may be found in literature [17,18]. Under water, an increase in both the dark current and the photocurrent by more than an order of magnitude was observed, as can be seen in Fig. 3. The increase in conductivity upon exposure to water was found to be reversible: after drying the samples, the photocurrent returned to the original values measured in the dry state. These results show that operation in an aqueous environment did not degrade the device. In Fig. 4 the photocurrent, Iph , measured in phosphate buffer solutions is presented. The inset shows the raw data measured at a voltage of 1 V for three different buffer solutions with pH

Fig. 3. Light response of a device in the dry state (black circles) and under DI water—wet state (grey triangles). The current is plotted against time measured at a constant voltage of 1 V as the light is switched on (after ∼100 s) and off (after ∼200 s).

Fig. 4. Variation of the photocurrent, Iph , for different pH values. In the inset, the light responses of the devices exposed to phosphate buffer (100 mM) with pH values of 4.6, 6.9 and 9.2 are shown. In the main graph the photocurrents are plotted against the pH of the buffer solution for one device (filled symbols). Three different series of the same buffer solutions are measured using a second device (open symbols) to show reproducibility. The dotted line represents the average of the DI water control measurements made in between pH measurements, with its standard deviation represented by the grey band.

of 9.2, 6.9 and 4.6. The response of the device to illumination increases with increased pH value, while the dark current is not significantly altered. For higher pH values, the photocurrent generally needed more time to reach saturation. The photocurrent is plotted against the pH value of the buffer solution in the Fig. 4 (filled diamonds). The photocurrent increases monotonically and roughly linearly with increasing pH. This change of the photocurrent with pH was found to be reversible; DI water control measurements oscillated around an average value of 132 pA with a standard deviation of 19 pA (represented in the plot by the dotted line with its standard deviation represented by the grey shading). The photocurrent increases by a factor of 2.5 between the lowest (pH 4.62 and Iph = 91 pA) and the highest (pH 9.16 and Iph = 230 pA) pH value. If a linear fit is made to the results, a slope of 26 pA (±3 pA) per pH unit is obtained. The same experiment was made on a second device (open symbols). To check the reproducibility, three series of measurements with the same buffer solutions were performed. Each series was made in the following order of pH values: 7.04, 6.44, 5.67, 8.23, 4.51 and 9.50. Apart from the measurement at a pH value of 8.23, the data are very well reproduced in all three series. The standard deviation was typically below 5 pA (10 pA for pH 9.5). A linear fit of the data from the second device results in a slope of 28 pA (±3 pA) per pH unit. The photocurrent measured with a lock-in amplifier also increased by several orders of magnitude when exposed to DI water. In order to compare the spectral response of the device between the dry and wet states, the spectra in both states were normalized to their maximum value and are presented in Fig. 5. The most important difference between the two spectra is that the peak for the curve in the wet state is blue-shifted by about 20–30 nm while the onset of the photoresponse remains at ∼500 nm for both cases.

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Acknowledgements The authors wish to thank Prof. Jorge Morgado, IST, for providing the PPV and for his helpful suggestions. We would also like to thank J. Bernardo, V. Soares, F. Silva and J. Faustino for help in device processing. P. Pistor acknowledges the Fundac¸a˜ o para a Ciˆencia e a Tecnologia (FCT) for a study grant. This research was funded by research projects with FCT under its POCTI and POSI programs. References

Fig. 5. Normalized spectral response of the device in the dry state (black circles) and under DI water (grey triangles).

4. Discussion The dark conductivity and the photoconductivity of an ultrathin layer of PPV is sensitive to exposure to DI water and to the pH of buffer solutions (Figs. 3 and 4). The change in the spectral response (Fig. 5) suggests that a reversible bulk change in the polymer is taking place. It can be speculated that a certain amount of solution components (water, ions) are incorporated into the polymer, thereby changing the polymer chain configuration (e.g., the torsion angle of the conjugated backbone) and consequently the way the polymer chain conducts the charge carriers. The solution incorporated into the polymer could also affect inter-chain interactions and charge carrier motion. The drying of the films removes water from the polymer bulk, enabling the restoration of the original conductivity values. The possible influence of water on the electronic properties of PPV was theoretically studied by dos Santos et al. after the finding that exposure to water vapor dramatically and reversibly changes the ultraviolet photoelectron spectra (UPS) of the ␲-levels of PPV films [19]. Their theoretical modeling suggests that water molecules can interact with PPV films through hydrogen-like bonds with the protons and/or ␲-system of the polymer. These interactions can lead to significant conformational changes, which in turn, may lead to changes in the electronic properties of the polymer. The effect of water vapor on PPV and derivatives has also been studied by several groups in the context of device degradation and photo-oxidation ([20,21]), but to the authors’ knowledge no photoconductivity experiments have been done with direct exposure of thin PPV films to water in the liquid phase. 5. Conclusions A simple, ultra-thin photoresistor made of a semiconducting polymer was found to be sensitive to the pH of an aqueous buffer solution. No device passivation was necessary since the applied voltages were below the threshold for electrolysis. The fabrication processes are compatible with array microfabrication for multi-analyte sensing. Further investigation is necessary to elucidate the detailed sensing mechanism.

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Biographies Paul Pistor received his Diploma degree in physics from the Freie Universit¨at Berlin in 2005. He worked at INESC Microsistemas e Nanotecnologias in Lisbon, Portugal during 2005–2006 on a research internship grant, studying biosensors based on organic semiconductors. Currently, he is a PhD student at the Hahn-Meitner-Institute, Berlin, Germany, working on chalcopyrite solar cells. Virginia Chu received a PhD in electrical engineering from Princeton University in 1989. She is currently a senior researcher and co-director at INESC Microsistemas e Nanotecnologias in Lisbon, Portugal, where she co-leads the

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group on Large Area Electronics and BioMEMS. Her current topics of research include thin-film semiconductor devices, bioMEMS and nanotechnologies. Duarte Miguel Prazeres received his PhD in chemical engineering from the Instituto Superior T´ecnico, Lisbon, Portugal, in 1993. He is currently an Associate Professor in the Department of Chemical and Biological Engineering at IST and a senior researcher in the BioEngineering Research Group. He currently works in the areas of nucleic acids bioengineering and nanobiotechnologies. Jo˜ao Pedro Conde received his PhD degree in electrical engineering from Princeton University, Princeton, NJ, in 1989. Since 1990, he has been with the Instituto Superior T´ecnico, Lisbon, Portugal, where he is presently a Professor in the Department of Chemical and Biological Engineering . He shares coresponsibility for the large-area electronics research group of INESC Microsystems and Nanotechnologies. His current research interests include thin-film MEMS and bioarrays, biosensors and low-temperature deposition processing of thin-film semiconductors and electronic devices on plastic.