I'm not really sure if this is the right place to put this but I am a PhD student at Cardiff University looking at the fabrication of biosensors and I am at the end of my tether with an impedance problem nobody seems to know how to answer.
I have found, shortly after buying a new glassy carbon electrode, that my impedance spectra are starting to lose their correct shape during my experiments and I wondered if anyone had experienced this before and if anyone can help me with it?
As you know the impedance spectra are supposed to have a semi circular part and a linear (diffusive) part. At low impedance resistances the semi circular nyquist diagram forms correctly, however at resistances above 1000 Ohms the graph becomes linear, losing the semi circular section and becoming hard to read. An example of how the spectrum looks as a good and bad example are attached.
So that you know the basic procedure is as follows:
Clean the electrode using 3micrometer, 1 micrometer diamond polish and then 0.05micrometer alumina polish followed by sonication in water for 2 minutes then 0.5M H2SO4 CV scans for 45 cycles at 100mVs between -0.5V and 1.5V (done until a steady and repeatable CV is obtained)
Electrodeposition of a polymer.
Addition of a DNA probe ("good" example of impedance shown at this stage)
Hybridisation of DNA with complimentary RNA at 50 degrees for 1 hour (causes increase in impedance resistance leading to "bad" example)
Spectra obtained using 5mM K3Fe(CN)6/ K4Fe(CN)6 in 0.1M KCl between 10mHz and 10kHz at 5mV amplitude.
The "bad" example has only recently started occurring and is also visible after the deposition stage which is how i know it is only at high resistances. I have a feeling that this is due to the presence of two "surfaces", and therefore I am hoping that it is just a cleaning thing and not me needing another electrode (around £180 my supervisor wont want to spend).
Sorry for the long message but I am hoping that one of you might be able to help me with this issue.
Thank you all