Conductometric analysis in bio-applications: A universal impedance spectroscopy-based approach using modified electrodes

We present a universal protocol for quick and reproducible conductivity determinations in bio-applications using electrochemical impedance spectroscopy (EIS), electrode modification and automated spectral analysis. Two-terminal EIS measurements may be acquired using any standard impedance analyser adjusting the applied sinusoidal potential and frequency range for spectral analysis. An implemented Matlab algorithm displays the acquired spectra, automatically identifies the frequency at which the phase angle (φ) is closest to 0° and determines the impedance magnitude, i.e. the solution resistance (RS). The corresponding conductivity value is immediately calculated as the ratio of the conductivity cell constant (κ), determined based on calibration, and RS. This protocol eliminates the need for evaluating a specific equivalent circuit followed by non-linear regression based curve fitting that is generally required in EIS-based conductivity determinations. The protocol is applicable to conductivity determinations using different conductivity cell configurations in any electrolyte solution regardless of its composition, i.e. in solutions with or without electroactive species that give rise to faradaic interface impedance. Conducted measurements showed high reproducibility in good agreement with a commercial conductometer in a wide range of ionic strengths up to five times that of physiological PBS. Since measurements in cell culture medium with bare gold electrodes indicated the need for recalibration to counteract the effect of biomolecule physisorption, the validity of the protocol was further extended using a protein-repellent coating of poly(ethylene glycol) methyl ether thiol self-assembled monolayer. This effectively eliminated electrode fouling, facilitating high reproducibility in repeated conductivity determinations in the presence of proteins.