I've been running EIS on gold chips with the surface modified with thiol SAM and antibody fragments etc.
Some of my EIS runs result in impedance of ~5000-1000 ohms, but then others shoot up to ~30,000-50,000 ohms. Despite all being treated and produced in the same way. My supervisor also made chips himself and ran them with the same issue.
Any ideas what might cause such a dramatic increase??
Hi, have you check the behaviour of your modified electrode with cyclic voltammetry and it exhibit the same shape of CV curve? How you modified the gold chips and is it screen printed gold electrode?
I second Pui Kee Lee's suggestion. My gut reaction would be that the high impedance samples actually have better SAM coverage (gaps to the Au underneath would lower impedance). You are going to need to do some secondary characterization. At the least look at the change in capacitance (it should get smaller in proportion to the length of the alkane(?)thiol. So if the length is ~10x the OHP distance then the capacitance should be ~ 1/10th. The antibody will mess with that a bit but the order of magnitude should be the same.
Does the increment caused by a particular antibody or other SAM coating? or it randomly happens sometimes with any surface modification? the answer could be different. I believe in the both previous answers provided here, and in addition to their suggestion, I suggest you fill out the unattached area of the electrode by some thiolated conventional materials such as MCH or in the case of antibody use the BSA. to sum up, my guess is the non-oriented assembling of antibodies on the electrode surface. Maybe they attached through their charged side-chains onto the gold surface and not with their thiolated end. Maybe!
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