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Procedia Engineering 5 (2010) 1005–1008 Procedia Engineering 00 (2009) 000–000
Procedia Engineering www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Proc. Eurosensors XXIV, September 5-8, 2010, Linz, Austria
An optical sensor array on a flexible substrate with integrated organic opto-electric devices T. Mayra*, T. Abela, , E. Krakerb, S. Köstlerc, A. Haaseb, C. Konradc, M. Tschernerc and B. Lamprechtb a
Applied Sensor Group, Institute of Analytical Chemistry and Food Chemistry, Graz University of Technology, Graz, Austria b Institute of Nanostructured Materials and Photonics, Joanneum Research, Weiz, Austria c Institute of Chemical Process Development and Control, Joanneum Research, Graz, Austria
Abstract
A new optical integrated sensor array for multi-analyte sensing applications is presented. The sensor platform consists of monolithically integrated luminescent sensor spots together with ring-shaped organic photo-detector on one substrate and an assembled organic light emitting diode. The sensing layer is screen printed onto the flexible polymeric (PET) substrate. All production steps can be adapted to mass production. Due to the sensing geometry the array does not require to integrate filters to discriminate between excitation light and emitted luminescence of the sensing alayer. The sensor platform is suitable for the parallel detection of multiple parameters. Sensing schemes for the analytical parameters oxygen, carbon dioxide, temperature and ammonia are presented. The response of the sensor device when applied to different atmospheres showed good performance emphasizing the universal and simple use of the sensor platform.
c 2009
2010 Published Published by by Elsevier Elsevier Ltd. Open access under CC BY-NC-ND license. © Keywords: integrated sensor, organic photodiode, organic light emitting diode, optical sensor,
1. Introduction Optical sensing of chemical and physical parameters has proved itself as a flexible, robust and universal method and is unique in the ease of miniaturization, fabrication and production of low-cost, disposable sensor systems. An increasing trend goes towards the development of monolithically integrated sensor systems, merging optical, optoelectronic and electronic elements (e.g. light sources and/or detectors) into one functional unity fabricated on one common substrate to build “lab-on-chip-systems”1. In this context “organic electronics” open new possibilities concerning the integration into integrated sensor systems. The ease of processing based on layer-by-layer vacuum deposition, and the possibility to deposit organic photodiodes basically on any substrate make these devices attractive for integrated systems. Shinar et al. described a several sandwich designs consisting of an OLED and
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c 2010 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. 1877-7058 doi:10.1016/j.proeng.2010.09.279
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inorganic detection systems such as charge coupled device cameras, photomultiplier tubes, and recently inorganic photodiodes[1]. Organic photodiodes (OPD) have been applied as integrated optical detectors for chemoluminescence [2] and fluorescence [3]. The ease of processing, based on layer-by-layer vacuum deposition, and the possibility to deposit organic photodiodes basically on any substrate as well as the possibility to tune the spectral response make these devices attractive for integrated systems, especially allowing facile integration with planar chip-based systems. We demonstrate an integrated optical sensor platform suitable for the parallel detection of multiple parameters in an array format. The device is printed by simple techniques such as screen-printing, vacuum deposition and spincoating onto a standard PET foil. Our approach combines fluorescent optical sensor spots together with organic ring-shaped photodiodes as monolithically integrated fluorescence detectors and assembled organic LEDs as excitation light source. The approach simplifies the detection system by minimizing the number of required optical components and offers a means of creating compact, sensitive and potentially low-cost sensor devices, with the potential of wide-ranging applications in chemical and biological analysis and clinical diagnostics. 2. Materials and methods
2.1. Organic opto-electronic components Tang-typephotodiodes were fabricated. The diodes consist of an organic active layer sandwiched between two different metal electrodes: a 7 nm thin semitransparent sputtered gold bottom electrode and a silver top electrode with a thickness of 150 nm. The organic active layers consist of a 40 nm thin p-type conducting copper phthalocyanine (CuPc) layer and a 40 nm thin n-type perylene tetracarboxylic bisbenzimidazole (PTCBI) layer. A 20 nm thin tris-8-hydroxy-quinolinato aluminum (Alq3) layer is adopted as buffer layer. The devices are protected by an ORMOCER® layer on top of the diode layers. Blue organic light emitting diodes were a gift from Fraunhofer IPMS, Dresden. 2.2. Preparation of sensing spots Lumincescent dyes and polymer matrix were dissolved in organic solvents. Sensing cocktails were screen printing on the front side of the polyester support resulting in sensing layer spots. Oxygen sensors cocktails were prepared by dissolving Pt(II) meso-tetra(pentafluorophenyl)porphine (Pt-TFPP) (Frontier Scientific, Inc.), Macrolex Fluorescent Yellow (MFY) and polystyrene (ACROS Organics) in Anisol. Ammonia sensor cocktails were prepared by dissolving Polymethylmethacrylate (Scientific Polymer Products, Inc.), Coumarin 545T (Sigma-Aldrich), Macrolex Fluorescent Red (Simon & Werner GmbH) and Bromophenol Purple (Acros Organics) in Anisole. Cocktails for carbondioxide sensors are prepared by an HPTS-Ionpair of HPTS (8 - Hydroxypyrene - 1,3,6 trisulfonic acid, Aldrich) and TOA+OH- (Tetraoctylammonium hydroxide, Fluka) and dissolved in Toluene/Ethanol together with Ethylcellulose (Sigma).
2.3. Measurement setup and principles The sensor chip is read-out in costume made units. Detection of the photocurrent is performed with a printed circuit board consisting of a mechanical fixture unit for the sensor chip including a flow through cell for various gases, a differential trans impedance amplifier (TIA) and an instrumentation amplifier with variable gain for amplification of the voltage signal coming from the TIA. A lock-in amplifier was used for the phase measurement. The excitation signal for the OLED was set to 1.5 sinusoidal with an amplitude of 5V. The amplified signal of the detection circuit was fed into the voltage input of the lock-in amplifier with an integration time of 300 ms.
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3. Results and Discussion
3.1. Multi-sensor chip with integrated organic photodiodes An array of integrated ring-shaped thin-film organic photodiodes for luminescence detection are the basis for the multi-parameter sensor chip. Figure 1 shows the set-up of the integrated sensor chip. The sensing spots are deposited on the upper side of the substrate, which is exposed to the analyte, whereas the ring-shaped photodiodes are monolithically integrated on the backside of the transparent substrate, where they are protected from the gas contamination. The sensing spot layers are applied by screen printing. Light from an organic light emitting diode, illuminating the sensor spots from below through the central holes in the photodiode rings, is absorbed by the fluorescent molecules and partly re-emitted into the substrate. An aperture between the OLED and the substrate limits the illumination to the area of the central holes on the photodiode rings.
Fig. 1: (left) Schematic outline of the integrated sensor chip, consisting of an array of circular fluorescent sensor spots and ring-shaped photodiodes surrounding the respective spots on the backside of the substrate. The sensor spots are excited by an OLED positioned below an aperture to limit the illumination to the area of the sensing spots. (Right) Photographic image of the sensor chip with integrated OPDs and sensor spots.
The substrate serves as a waveguide and guides the fluorescence light to the ring-shaped photodiodes, surrounding the respective sensor spots, where it is partly absorbed by the active photodiode layers giving rise to an electrical signal at the photodiode electrodes. Filter-less discrimination between excitation light and generated fluorescence light is an essential advantage of this design. Excitation light is not guided in the waveguide, since there is a gap between and most of the excitation light is nearly parallel and enters the waveguide in an angle smaller then the total reflection angle, apart from a small amount scattered at defects, which was found to be negligible in the proposed setup. 3.2. Performance of the integrated sensors The feasibility of this sensing concept employing integrated ring-shaped photodiodes is demonstrated by applying sensing chemistries for oxygen, carbon dioxide and ammonia. Figure 2 left shows response curves of oxygen sensor and carbon dioxide sensors. The sensing scheme for oxygen is based on dynamic quenching of the phosphorescence of the indicator dye. The oxygen sensor was read-out employing phase modulation method to determine the luminescence lifetime of the sensing spots. Luminescence lifetime measurements have considerable advantages over intensity-based methods. Luminescence lifetime is an intrinsic property and is virtually independent of fluctuations in light-source intensity, detector sensitivity, light-throughput of the optical system, sensing layer thickness and indicator concentration. A reversible and fast response was found on exposure to changes in oxygen concentration and a high signal to noise ratio of the phase signal of 66.4 dB at 1.5 kHz was determined for 0% oxygen. The dynamic of the sensor expressed by the difference in phase angle (ΔΦ) for 0% and 21% oxygen is comparably to conventional set-ups based on inorganic light sources, detectors and filters. Figure 2 right shows the response of carbon dioxide sensor on the exposure to 0% and 10% carbon dioxide in nitrogen indicating an approx. 2.3-fold change of the amplitude signal. The ammonia sensor (data not shown) showed an approx. 1.7 fold change of the amplitude signal. Note that the sensing schemes of carbon dioxide and
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ammonia sensor are based on sensing dyes with a fluorescence lifetime in the nano-second range that cannot be determined using the frequencies available in this set-up.
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Figure 2: (Right) Response curve of the integrated oxygen sensor element exposed to various concentrations oxygen and (Left): Response of a carbon dioxide sensor element.
4. Conclusion and further outlook We demonstrate a novel integrated sensor concept based on organic opto-electronic components. All preparation steps are suitable for mass-production, e.g. roll-to-roll processing. An important advantage of this sensor layout is that no optical filters are needed for discrimination of excitation and emission. Another advantage is the simple realization of sensor arrays by applying an array of photo-detectors with different sensing spot which is illuminated by one common organic LED. This is planned for future investigations. In addition, we aim to transfer the sensing schemes for carbon dioxide and ammonia to a lifetime detection format by a applying a method called Dual-lifetime referencing (DLR)[7]. We also plan to combine this integrated ring-shaped detector concept with micro-fluidic devices.
Acknowledgements Financial support by the Austrian Research Promotion Agency (FFG) in the framework of the Austrian Nano Initiative (Research Project Cluster 0700 –Integrated Organic Sensor and optoelectronics Technologies) is gratefully acknowledged.
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