I can see that about 1 mm of the tapered sensor tip of an oxygen microsensor is coated with material. Is all of this coating sensitive to oxygen and where exactly do I measure?

The tapered sensor tip of an oxygen microsensor is < 50 μm. Only the front part contains the sensitive PSt1 sensor material. It is coated with an optical isolation covering approx. 1 mm of the tip. Only the very tip has to be immersed in your sample - not the whole part with optical isolation - to obtain reliable measurement results, and depending on how stable the oxygen microsensor is fixed a spatial resolution of 50 - 100 μm can be obtained.