6.2. Chemical-optical oxygen sensors
Due to their small size chemical optical O2 sensors allow measurement of O2 concentration in very small volumes with a high spatial and temporal resolution (PreSens - Precision Sensing GmbH). For bees, these can be miniaturized chambers for use in metabolism measurement, including inside of honey bee brood cells. These oxygen microsensors are available with tip sizes of <50 µm. They consist of a silica fibre with a sensor spot at the tip which emits fluorescence when illuminated with the light of a blue LED through the fibre. Through a second (parallel) fibre this fluorescence signal is led back to a photodetector. If the sensor tip encounters an oxygen molecule, the excess light energy is transferred to the O2 molecule, which decreases (quenches) the fluorescence. The degree of quenching correlates with the partial pressure of oxygen in the sensory material, which is in dynamic equilibrium with oxygen in the sample (e.g. surrounding air). With a pair of these sensors, one before and one after passage through the measurement chamber, it is possible to measure O2 consumption in a flow-through setup. There are also sensors available for CO2 measurement. At present, however, they only work in aqueous solutions.