Enhancement of detection accuracy in depth-profiling using electrochemical impedance spectroscopy-on-a-needle by incremental compensation for immersion depth

Electrochemical impedance spectroscopy-on-a-Needle (EoN), where co-planar bipolar electrodes are fabricated on a hypodermic needle using micromachining processes, is introduced to discriminate bio-tissues based on their electrical properties (conductivity and relative permittivity). The electrodes on the hypodermic needle are comprised of detection and connection parts. The electrical impedance formed at the connection part, however, tends to cause larger distortions in the extracted electrical properties of the samples as the penetration depth in the sample increases. In this study, an incremental compensation method (ICM) is proposed to extract the conductivity and relative permittivity of the samples accurately, regardless of penetration depth. This method can eliminate the influence of the electrical impedance contributed by the connection part on a sensor output; the electrical impedance measured by the connection part can be estimated using the electrical properties of the sample already passed in the depth-profiling procedure. To validate the proposed ICM, electrical impedances were measured with respect to the immersion depth into phosphate buffer saline (PBS) solutions, which are widely used to demonstrate device performance for biological research. The ICM was applied to PBS solutions with various concentrations, experimentally confirming that the detection accuracies for the conductivity and relative permittivity of PBS solutions were enhanced by 5.59 times and 2.18 times, respectively, compared with those without the ICM.