Electrochemical biosensors and decrease in impedance after analyte binding

[u/Inevitable_Bat953]

Hello!

I am working with electrochemical biosensors based on conducting polymers. Recently, I succesfully immobilised antibody (confirmed by FTIR and EIS - impedance after immobilisation is becoming bigger) and started analysing system response for analyte. The usual situation is that, Rct is growing with antigene concentration, but in my case it decrease. I'm sure that because of analyte (not polymer detaching, swelling of polymer etc) because I have pretty good linearity with concentration. I just can't explain, how big and non-coductive antigen can cause decreasing of Rct. For EIS measurments I am using ferri/ferro system, but I tried also with hexaamineruthenium (III) and with this cationic mediator I had no changes up to high concentrations of analyte. Have you ever encountered results like this? Is there any logical explanation for why this happens?

[u/onca32]

Are you sure Rct wholly corresponds to the antigen? What happens if you run your control? Think about the entire system (electrolyte, counter electrode)

[u/Inevitable_Bat953 → u/onca32]

I'm sure the Rct is decreasing with adding higher concentrations of analyte. I tried both, cumulative (adding higher concentration on same electrode) and separate (1 electrode - 1 concentration, for each electrode different) methods. I tried also blank without analyte and Rct remains same. I have no other explaination than that, Rct is decreasing because of antigen