I am trying to utilize boron-doped diamond electrodes for developing electrochemical biosensors. During the electrochemical measurements, it was observed that the current response varies with time, for example, it slowly goes up for hours, or the current first goes down and then starts to increase. It takes several hours to stabilize the current response.
If it is ferri/ferrocyanide that you are using, then this is really not that surprising. It is sometimes categorised as a simple outer sphere redox couple but this is quite wide of the mark. See for example Anal. Chem., 91 (3) (2019), pp. 1944-1953 which has a nice discussion and good references on the massive variability in the published kinetic data and the influence of surface functional groups on carbon electrodes.
Secondly, there is more than one kind of boron doped diamond. One of the best places to mug up on the background is: Phys. Chem. Chem. Phys., 2015,17, 2935-2949. The BDD literature needs to be read very carefully and critically examined. Pre-treatments, history and good quality chemical characterisation are vitally important in interpreting the voltammetry.
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