The CV is supposed to serve as a blank BEFORE enzyme adsorption. The peak pair appears at appr. +340 mV vs SHE. I got it when using 3-mercaptopropionic acid and 4-mercaptophenol in 200 mM potassium phosphate buffer, pH 6.0, purged with argon.
it may be due phosphate buffer. phosphate buffer has two peaks in cyclic voltammetry. please see our article with title:
Preparation and electrocatalytic activity of gold nanoparticle embedded in highly ordered TiO2 nanotube array electrode for electro-oxidation of galactose
If your gold electrode has been adequately modified by the thiol adlayer, you should see no peak related to the electrolyte. You might, however, be provoking the protonation-deprotonation of the carboxilic group of the mercaptopropionic acid, or of the OH group of the mercaptophenol, which is also quite acid. These protonation-deprotonation processes typically occur at intermediate pHs (say, between 5 and 9), although they usually occur in solutions more diluted than the one used by you. Some degree of disorder within the thiol SAM also seems to be necessary, so I assume that you are using polycrystalline gold.
Are you sure you can form a dense SAM with such a short thiol that is also carrying charge? I would guess it's not covering Au fully so you see peaks related to ions from the electrolyte chemisorbed on Au.
This is relatively easy to check: just record a CV of unmodified gold in the same electrolyte and compare. Dense thiol adalayers (albeit disordered) can be formed even with methanethiol, and charge can be compensated or screened by protonation or by attaching a cation from the electrolyte.
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