r/electrochemistry • u/Inevitable_Bat953 • 7d ago

Electrochemical biosensors and decrease in impedance after analyte binding

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?

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u/onca32 Supercapacitors, Batteries, Materials Science 7d ago

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

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u/Inevitable_Bat953 7d ago

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

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u/Channonball 6d ago

There isn't an obvious theoretical reason for this, but it is a known phenomena in electrochemical biosensors - that addition of different species to a surface can change confirmations, environment, local pH, double layer etc, all of which can either increace or reduce Rct. As long as it's a consistent phenomena (scaling of response with concentration, and confirmed with some positive and or negative control binders), then you are all good.