The paper presents a novel concept for the realization of optochemical sensor systems which are capable of operating in harsh environments. Key components in such sensors are nanostructures formed from gallium nitride (GaN) and its alloys with aluminum (Al) and indium (In). Nanostructures of this kind emit an efficient, visible-light photoluminescence (PL) which can be excited with low-cost ultraviolet light sources and which extends up to temperatures in the order of 200°C. When exposed to various chemical environments, changes in the PL intensity occur which constitute valuable sensor signals. Due to the all-optical approach, the PL can be excited and its chemically induced changes be read out without requiring electrical wiring at the point of measurement. The present paper presents this innovative sensor concept, the nanostructures and optochemical transducer structures that form its material base, as well as several applications of such transducers in the fields of gas and fluid sensing. The applications addressed here range from the sensing of ppb concentrations of H<sub>2</sub>, NO<sub>2</sub> and O<sub>3</sub> in gaseous environments to the pH monitoring in aqueous solutions.
The quality and safety of drinking water is of major importance for human life. Current analytical methods recognizing
viable bacteria in potable water are time consuming due to a required cultivation step. Fast and automated detection of
water borne pathogenic microorganisms with high sensitivity and selectivity is still a challenging task. We report on a
novel biosensor system using micromechanical filters with nano sized pores to capture and enrich bacteria on the filter
surface. Thus the accumulated organisms are accessible to different detection methods using fluorescent probes.
Depending on the kind of detection - specific (identification of a certain species) or unspecific (total amount of cells) -
different assays are applied. For non-specific detection we use fluorescent dyes that bind to or intercalate in the DNA
molecules of the bacteria. Upon binding, the fluorescent signal of the dyes increases by a factor of 1000 or more.
Additionally, we use enzyme substrates for the detection of active cells. The whole detection process is automated by
integrating the microsieves into a fluidic system together with a high performance fluorescence detector. To ensure
realistic conditions, real potable water, i.e. including particles, has been spiked with defined amounts of microorganisms.
Thus, sampling, enriching and detection of microorganisms - all with a single micromechanical filter - is not only
possible with ideal media, e.g. laboratory buffer solutions, but also with tap water. These results show the potential of
microfilters for several applications in fast pathogen detection.