Precise evaluation of liquid conductivity using a multi-channel microfluidic chip and direct-current resistance measurements

In various research fields related to electrochemistry, such as biosensors and energy storage engineering, the conductivity of an electrolyte solution is one of the most fundamental parameters that determines device design and performance. The simplest procedure for obtaining the conductivity of a liquid is to (i) measure the direct-current resistance of liquid whose volume and dimensions is well-defined and (ii) calculate the conductivity from the measured resistance. However, this method has not been widely used because the measured resistance always includes electrode-electrolyte interface resistance and the extraction of a correct conductivity is difficult. In this study, we designed a multi-channel microfluidic chip that can remove the effects of interface resistance in determining the intrinsic resistance of a liquid and developed a method for the precise evaluation of liquid conductivity. We observed that the interface resistance was significantly affected by the channel length and applied voltage. We measured the intrinsic resistance of phosphate-buffered saline (PBS), conductivity standard solutions, and cell culture media using the proposed multi-channel chip including different-length channels, and we removed the effect of voltage-dependent interface resistance by applying a constant current source for the measurement and determined the precise conductivities of these solutions. We verified the accuracy of the proposed method by comparing the results with the conductivity measured using electrochemical impedance spectroscopy. The proposed method was also applied to determine the zeta-potential of charged nanoparticles with an average diameter of 110 nm. This simple method for determining liquid conductivity could be widely employed in various electrochemical applications.