Zimmer and Peacock had a collaborator who was using a ZP platinum sensors in a flow cell. The user was wondering why the signal looked like it was changing at the beginning of the assay.
In the adjacent image you can see how the signal was changing when tested with 0 micromolar hydrogen peroxide.
The answer to this question can have a number of sources but an obvious answer is that the Pt is undergoing conversion from Pt to PtO2. The observation in the lab is often that someone will use at Pt electrode and the signal may take some time to settle, but then on further use of the electrode the electrode is stable. What we suspect at ZP is that the first time you use a sensor it is Pt but as soon a you apply a voltage, maybe 650 mV vs reference electrode then you will convert the top layer of Pt to PtO2. As the platinum is consumed the current due to this process diminishes until the signal becomes stable.