I have fabricated an impedimetric aptasensor for small molecule detection. The black line in the attached Nyquist plot is the EIS measurement before incubation with the target. After incubating with the target for 1 hour, the EIS measurement gives a lower value of Rct was observed (purple line). The target incubated sensor is then washed with deionized water and dried in a nitrogen stream. The EIS measurement was again performed and a higher Rct was observed (red line). (all measurement was performed in 5 mM FerriFerro and 0.1 M KCl). The aptamer is linked with C6 and the sensor is unblocked) Anyone can explain this phenomenon?
This is very typical of a system with multiple contributors to interfacial impedance, especially with ferri/ferricyanide redox probe. There will always be instability of impedance in your system, regardless of the electrode composition. Have you done a negative control, incubating with only the carrier fluid containing zero target? I recommend doing several of these, and tracking the impedance over time when exposed to either the carrier fluid or the electrolyte alone. You'll need that to accurately determine the range of impedances when no target is present, and also to quantify the temporal stability of the sensor when exposed to these redox-active fluids. It's not a bad idea to monitor OCP also, so you'll know if the system is at equilibrium before starting EIS.
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