Hello everyone,

I'm encountering an issue with my electrochemical impedance spectroscopy (EIS) measurements and would appreciate some insights.

Experimental Setup:

• Electrodes: Gold interdigitated electrodes (IDE) with a 1) 50 µm gap and width; 2) 200 µm gap and width.
• Solutions: DI water and 0.1X PBS buffer solution.
• Measurement Parameters: 100 mV amplitude, frequency range from 100 Hz to 10 MHz.
• Device: Microchamber with 350 µm height placed in a controlled humidity chamber. Cu tape with conductive epoxy is used to connect the device to the EIS spectrometer..
• Duration: Measurements taken at 2-minute intervals for 1 to 5 hour.

Observation:

• The impedance values show an increasing trend over time for 0.1X PBS buffer solution and decreasing trend for DI water irrespective of electrode size and gap.
• For comparison, a 1 kΩ resistor measured under the same conditions shows no change in impedance over time.

Question: Why might the impedance values for DI water and 0.1X PBS buffer solution exhibit decreasing and increasing trend over time, while a 1 kΩ resistor does not show any change? What factors could be contributing to this behavior in my system? Is there anything related with 2 probe (working and counter electrode) system over 3 probes (working, counter and reference electrode)? If yes, then is there a way to attach a reference electrode to HP 4194A? We have EZstat-pro Potentiostat Galvonostat in our lab.

Thank you for your help!