Sensing chemical parameters in biological samples with optical sensors became very popular during the last ten years. Optical sensors can be applied either as fiber optic sensors, wall-incorporated sensing spots or dispersed micro- or nanospheres. Dispersed sensor spheres are an attractive low-cost and universally useful method to monitor chemical or
biological parameters since it needs no sensor fabrication steps. Problems are caused by the fact that large quantities of sensing material are requested (in case of large scale applications such as bioreactors) and by the potential optical interference of sample components. To overcome these drawbacks we have designed sensing particles with magnetic properties. Such sensors can be manoeuvred easily within the sample vessel and allows the in-situ forming of sensing spots on defined positions in the sample vessel.