Difference between back and front detection systems 

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... compact, integrated photoluminescence based oxygen and pH sensor, utilizing an organic light emitting device (OLED) as the light source and an organic photodiode (OPD) as the detection unit, is described. The main challenge in such an integrated sensor is the suppression of the excitation light at the detector, which is typically by many orders of magnitude higher in intensity than the emitted fluorescence. In our approach we refrain from utilizing edge filters which require narrowband excitation sources and dyes with an adequate large stoke shift. We developed an integrated sensor concept relying on two polarizers to separate the emission and excitation light. One polarizer is located right after the OLED, while the other one, oriented at 90° to the first, is placed in front of the OPD. The main advantage of this solution is that any combination of excitation and emission light is acceptable, even if the two signals overlap spectrally. This is especially important for the use of OLEDs as the excitation sources, as these devices typically exhibit a broad spectral emission. Integrated organic sensor, organic light emitting diode, organic photodiode, polarizer films, oxygen sensing, pH sensing Chemical sensors are used in many different applications, such as analytical tools, in medical and biology research, in security and detection systems and in food chemistry [1]. A well-known approach for solution and gas-phase oxygen sensing is based on the dynamic quenching of the photoluminescence (PL) of oxygen-sensitive dyes such as Ru-complexes and Pt- or Pd- porphyrins. Increased collision rates due to increased O 2 partial pressures result in a decrease of the PL intensity and PL lifetime [20], both phenomena are used to quantitatively determine the oxygen concentration . Optical pH sensing is usually accomplished using fluorescent dyes which are changing their emission properties due to protonation or deprotonation of the dye molecule. Well known pH sensitive dyes are e.g. various fluorescein derivatives in buffer solutions. There is a great demand for low-cost chemical sensor devices as well as whole sensor systems thereof. Reduction of cost and size as well as simplification of fabrication is intended by using new materials, new sensor geometries and especially organic optoelectronic components,. The potential viability of structurally integrated OLED and OPD-based sensors results from the advantages of OLEDs and OPDs as miniaturizable, flexible, and efficient devices. The performance of OLEDs has been dramatically improved in recent years, which has led to their emergence in commercial products, such as camera displays, TV displays, etc. Organic photovoltaics mainly appeared in literature in form of organic solar cells, which are still suffering from a rather poor efficiency. Organic photodiodes on the other hand detect incoming light with an increase of the current density. Organic dyes can be widely tuned to be selective to the desired analytes. One of the advantages of OLEDs and OPDs is their flat geometry, with a typical total thickness of only several 100 nm and therefore their ease of integration onto one substrate. Realizing a low-cost all-integrated sensor system, combining the selectivity to analytes of the optoelectronic devices and the dye, the excitation source, detection unit and the required wavelength separation tools is however still very challenging. Several geometries have been proposed in the literature. Depending on whether the OPD is positioned in front or behind the organic light emitting diode with respect to the sensor dye, one distinguishes front and back detection systems (see Figure 1). Shinar et al. have described a back-detection sandwich design consisting of OLED and inorganic detection systems such as CCD cameras, photomultiplier tubes, and recently inorganic photodiodes. They already demonstrated the detection of oxygen, glucose, hydrazine, and anthrax in either gaseous or liquid media by using different fluorescent dyes and band or long pass filters for wavelength separation [2, 3, 4 ] . Hofmann et al. [5, 6] first demonstrated a system based on chemoluminescence generated in a PDMS microfluidic system and showed the ability to integrate organic photodiodes. Furthermore they developed a microfluidic system where the long pass filter is directly integrated in the PDMS matrix of the micro channels. Vollmer et al. [7] showed in 2005 also a microfluidic system in PDMS in front detection geometry consisting of inorganic illumination and detection components, and long and band pass filters for the detection of dissolved oxygen. Oxygen sensing is often performed via the quenching of the photoluminescence of dyes in the presence of oxygen. Trettnak et al [8] demonstrated an inorganic sensor setup using PtOEPK and other Pt-porphyrins as oxygen sensitive dye. Pt-II- meso -tetra-pentafluorophenyl-porphyrine (Pt-TFPP) (FrontierScientific, Inc) is an oxygen sensitive phosphorescence dye with absorption maxima at 396, 509 and 542 nm. The emissionpeak of the phosphorescence is at 652 nm. The lifetime of the excited state is around 100 μs and the quantum yield about 90%. pH sensing is often based on the change of the emission properties of a dye by protonating or deprotonating of the molecule. Fluoresceine derivatives are very common in pH sensing. The emission properties of FITC strongly depend on the pH value of the solution. There are two excitation peaks at 450 and 495 nm, whereas the emission peak is located at 516 nm. The fluorescence intensity as well as the ratio between the excitation peaks increase with increasing pH values. For all photoluminescence based sensor schemes the separation of excitation and emission light is a critical issue. The emission spectrum of OLEDs is usually very broad and thus easily overlaps with the photoluminescence of the sensor dyes. Organic photodiodes based on the standard configuration by Tang detect light in nearly all the visible range and cannot distinguish between excitation light and emitted light. For a number of reasons, the usual approach using edge filters for wavelength separation is not always applicable for integrated sensors comprising organic electronic components. For fluorescent dyes with low Stokes shifts, edge filters with very steep transmission profiles are required. Sophisticated filters with steep edge slopes (e.g. interference filters) are expensive and cannot easily be used in integrated flexible sensor devices. So, especially for fluorescent sensor dyes having small Stokes shifts, it is very difficult to separate excitation and emission while maintaining sufficiently high signal intensities. In this letter we present a new approach for separating the excitation and emission light by exploiting their different polarization states. Upon excitation of a fluorophore with polarized light, the emission is always depolarized to a certain degree [9]. In our sensing approach, the carrier substrates for both optical devices, OLED and OPD, are polarization filters which are mounted into front detection sandwich geometry with both polarization filters in a crossed state with respect to each other. The excitation light passes the first polarization filter, thus resulting in linearly polarized light. This light excites the luminescent dye, yielding depolarized fluorescence light. Linearly polarized excitation light that has not been absorbed by the dye is blocked by the second polarization filter. In contrast, light emitted from the fluorescent dye, which is depolarized to a certain degree, can pass the second polarization filter and reaches the ...