Dear Sir. Concerning your issue about how to obtained a amperometric sensing curve with straight line. I think the following below links may help you in your analysis:

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.618.6340&rep=rep1&type=pdf http://www.electrochemsci.org/papers/vol3/3010082.pdf

https://alsafi.ead.unesp.br/bitstream/handle/11449/25981/WOS000295393000054.pdf?sequence=1&isAllowed=y

Thanks

1. "The current of oxidation of analyte (Electroactive sp.) in amperometric curve should decrease after the increment as the analyte is oxidized by the modified electrode and the concentration of analyte is decreasing? "

The quantity of the oxidised analyte during detection may be calculated using the Faraday' law. It must be small enough and in practice so it is; thus the analyte concentration remains virtually the same during detection.

2. " Why the amperometric curve (under stirring condition) will has perfect straight line (Figure 1) but some of them will have slanted line (Figure 2) ? "

If the oxidation rate is controlled by diffusion, the oxidation product is soluble and not adsorbed on the electrode, the line will be straight.

But if the active area of the electrode is reduced by product adsorption, a saturation effect (slanted line) will be observed when the calibration is made by successive additions (because the total detection time is very long).

Also, at higher concentrations, the oxidation may become kinetically controlled/limited.

I suggest you to try a calibration by single additions in separate samples, thus allowing for short detection times for all concentrations. Or at least try this for the highest concentration and compare the current with the recorded one for successive additions.

You may also think to modify the composition of the electrode.

Hi,

Apart from the comments above, maybe the stirring is not strong enough. You have to ensure that the molecule is arriving at the same rate of disappearing by electron transfer (v mass transport = v electron transfer), and then the current will remain similar. In this case the mass transport can be not so effective. Alternatively, as commented Cornel, the area of the electrode can be decreasing (e.g. formation of polydopamine that can be adsorbed on the electrode) and since the current is proportional to the area, it will decrease.

Good luck

Hello,

In the stirring condition, diffusion process occurs fast, that's why you observed straight line in the calibration curve. However, At certain level of analyte concentration, there may be possibility to saturate the electrochemical sensing platform or products may bind with the electrochemical sensing platform and reduces the amperometric response for the analytes. In few cases at high concentration of the analytes, the current responses decreased due to formation of complex formation or intermediates which hinders the electrochemical process.