SensLED: An Electro‐Optical Active Probe for Oxygen Determination

Abstract

Adv. Mater. 2009, 21, 3483–3487 2009 WILEY-VCH Verlag G Optical sensors have been a field of intensive academic as well as industrial research over the last three decades. Clark-type electrodes, for example, have been a standard for many years in oxygen detection; however, they are more andmore replaced by optical sensors. Different optical sensor systems, e.g., fiber optic or planar waveguide based platforms, were developed using the optical properties of the analyte itself as well as changes of the optical properties of an intermediate indicator molecule (e.g., organometallic compounds) for analyte detection. To yield analyte information, most common optical transduction techniques used in optical chemical sensors are based on fluorescence or absorption measurements (e.g., nondispersive infrared sensors). In this context, the class of so-called direct sensors uses a modification of an intrinsic optical property of the analyte, whereas in the case of inappropriate intrinsic optical properties an intermediate analyte-sensitive dye molecule has to be added. The presence of the corresponding analyte alters the optical properties of the indicator dye, thus allowing its detection. Such a sensing technique requires, depending on the phase of the analyte, in most cases an immobilization of the dye within a matrix layer, which provides analyte accessibility but prevents leaching effects. Different approaches for immobilization of indicators within a matrix material have been realized up to now, such as covalent bonding or simple encapsulation. Apart from analyte diffusion coefficients within the matrix, sensitivity as well as sensor response time strongly depend on indicator– matrix interactions. Due to these interactions, spectroscopic shifts and enhanced or reduced excited state lifetimes are frequently observed in the solid state compared to solution. Nevertheless, embedding the dye molecule within a rigid matrix enhances the photostability due to reduced ligand photodegradation. Along this line, optical oxygen probes commonly use platinum or palladium based organometallic complexes as indicator molecules. Spin orbit coupling is enhanced because of the metal ion within the organic complex; this enables an efficient radiative decay from the lowest triplet state to the singlet ground state of the molecule (phosphorescence). Because spin orbit coupling is weak in fluorescent materials, a radiative decay from an excited triplet state is unlikely. The phosphorescence lifetime of organometallic complexes is, depending on the transient metal complex and the matrix material, in the range of microseconds, which is several orders of magnitude larger than the fluorescent excited state lifetime. Most reagent-mediated optical oxygen sensors use dynamic interactions between the excited state of the dye molecule and the analyte. Since molecular contact is required for this kind of interaction, the analyte has to reach the immobilized dye molecule within its excited state lifetime. Taking into consideration the physical parameters of the matrix material, e.g., diffusion coefficients, the lifetime determines the maximum distance that an analyte molecule can move to reach and interact with an excited indicator molecule. The quenching probability is therefore higher than that of fluorescent emitters, because of the enhanced lifetime of transient metal complexes. Hence, the lifetime of the excited molecule state is an important aspect of sensor performance as well as a determining factor for the complexity of the measurement equipment. An excited state lifetime in the range of microseconds, as common for most transient metal complexes, allows the use of low-cost electronics for lifetime determination, which is an inherent advantage of this class of materials over fluorescent indicators. In electro-optical devices, simple spin statistics predict that 75% of the injected charges form triplet excitons, whereas only 25% are in the singlet state. However, various studies presented in the past suggest variation in the singlet–triplet exciton ratios, which was also confirmed by quantum mechanical calculations and enhanced electroluminescence quantum yield (ELQY). Nevertheless, later studies claim that the ELQYof organic light-emitting devices is restricted to the spin statistics meaning that only a 25% singlet yield can be obtained. Therefore, in order to harvest a maximum amount of emissive exciton recombination, phosphorescent complexes are commonly used to improve the overall EL output by enhancing the