Established sensors for fine scale measurements in natural environments are based on electrochemical measuring principles for e. g. oxygen and pH. The preparation of such electrochemical sensors is, however, a time consuming process. Based on the technical progress in the field of fibre optical measuring techniques many well-known chemical principles can now be used for the preparation of optical sensors, e.g. the oxygen measurement by luminescence quenching, and the pH-measurement with absorption based dyes. The use of optical fibres offers a high potential for miniaturisation of sensors [1]. For high spatial resolution measurements, the sensor chemistry has to be immobilised on the fibre tip, and the excitation and emission light has to be guided via the same fibre.