Organic Sensor Devices
Many organic and organic/inorganic hybrid materials display specific chemical or physical interactions with air- or waterborne substances. In principle these are useful as sensors for monitoring such 'analytes' in the environment, or the food chain. We develop and test transducers that translate such molecular recognition into measurable physical quantities. An interesting challenge is posed by organic macrocyles that can selectively complex dissolved cations, but are themselves insoluble in water. We address this by researching water- gated thin film transistors (TFTs), where an aqueous sample performs an active part in the working of an electronic device. Sensitising TFTs with thin layers of organic macrocycles at the electrode/water interface shall transduce cation/macrocycle binding into the electrical characteristics of the TFT, Fig. 1.
Fig. 1 above: Sensitised water-gated TFT. 1: Substrate and electrodes, 2 semiconductor, 3 sensitiser layer, 4 electrical contacting, 5 water sample. Inset: The cationic (M+) electric double layer that forms at the sensitiser when an n- type TFT is gated via the water sample.
In parallel, we tackle the same challenge by an optical transducer, the 'virtual photon trap'. A 'chromoionophoric' sensitiser (a molecule that grows an absorption band in the presence of analyte) is coated onto an optical fibre immersed in water. The evanescent wave of light propagating along the fibre may get absorbed by the chromoionophore, we detect the intensity loss with a sensitive 'light balance' circuit and Lock In technology, Fig. 2. Lock In technology is also at the heart of a light- driven alternative to impedance spectroscopy where we study the dynamic behaviour of optoelectronic devices stimulated by modulated light intensities, Fig. 3.
Fig. 2, above: 'Stripped' optical fibre used in the 'virtual photon trap', and the concept of sensitising the fibre in the evanescent wave zone.
Fig. 3, above: Nyquist plot of the frequency- dependent response of an organic photovoltaic device under different loads, stretching from 'open' to 'short' circuit conditions via 'maximum power point'.
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