Brief system description :
Gold coated glass slide is the working electrode. Used an Al foil clamped to the Au coated glass for connecting to the working electrode of the potentiostat. A part of the gold surface is used for holding a 1 ml drop of 1mM ferro/ferri cyanide in 1X PBS buffer.
Ag/AgCl is used as a reference electrode and Platinum wire is used as counter electrode.
Experiment:
1) Firstly EIS with 5 mV AC perturbation was done on the Au coated glass slide. (bare)
2) Then the electrolyte from the surface was removed and 300 micro liter of 3 mM mercaptohexanol was immobilized for 1 hour. Then it was washed in dd water, dried in air and again the ferro/ferricyanide in 1X PBS was put in. EIS is run with the same parameters as step 1.
3) The SAM layer immobilized Au coated glass slide was heated in acetone solution at 70oC, washed in water and step 1 was repeated.
According to my hypothesis SAM layer will increases the impedance and thus increase the "charge transfer " resistance. However, I am observing the reverse phenomenon for my system.
The plot is attached.
What could be the cause ?
Reference for hypothesis :
Shinn-Jyh Ding, Bin-Wha Chang, Ching-Chou Wu, Min-Feng Lai, Hsien-Chang Chang, Impedance spectral studies of self-assembly of alkanethiols with different chain lengths using different immobilization strategies on Au electrodes, Analytica Chimica Acta, Volume 554, Issues 1–2, 4 December 2005, Pages 43-51, ISSN 0003-2670, http://dx.doi.org/10.1016/j.aca.2005.08.046.
The result totally depends on the nature of the SAM molecule, the coverage level and the pH of the solution it is immersed in. Can you give details?
What are you using as a measurement tool? Impedance spectroscopy?
charge transfer resistance can decrease or increase dependind on the charge of the surface which depends of the pH of the electrolyte
http://www.sciencedirect.com/science/article/pii/S1572665714001714
that's one possible cause...
Hello, thanks for replying. The solution pH is 7.5 to 7.6.
I am using Electrochemical Impedance Spectroscopy. The AC perturbation voltage is 5 mV. Freq range is 100000 Hz to 0.1 Hz.
I have not calculated the surface coverage of the alkanethiol molecules. I am still not sure as to why the charge transfer resistance decreased. I think that for an electrolyte pH of 7.5, the mercaptohexanol will not facilitate the redox couple to interact with gold. Instead they should be obstructed due to the SAM layer and thus impedance should increase.
Was is the redox mediator your are using for EIS experiments?
What is the alkanethiol your are using? Chain length? Termination?
Hello Dr. Hurth,
The electrolyte is : 1 mM Ferrocyanide/Ferricyanide in 1X Phosphate Buffer Saline, the pH is 7.54.
The alkanethiol is this one : SHCH2(CH2)4CH2OH. The S atom should be covalently linked to Au atom and then the terminal group would be OH.
I have attached a photo of the cell setup.
Do you think that the shape/size of my electrodes and the design of the cell could be a factor in here ? I checked that the electrodes are not touching each other. But the tip of the CE and RE are pretty close to the WE i.e. the gold surface.
Yes, a little bit. The larger the surface, the harder it is to passivate it totally. Can you reduce the electrode size significantly and see if you still have the same problem?
About your electrolyte, yes ferro,ferricyanide is good. The contration and buffer are good as well. I'm assuming 1X means 100 mM. Keep in mind that the current you get will increase with buffer salinity. In your case, with a large electrode, the current might be large. Do you have an order of magnitude?
About your thiol. It has OH groups, they are charged and can be protonated but at pH=7.5, you should be mostly ok.
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