[18p-F306-1~17] 12.7 Biomedical Engineering and Biochips

▲ [18p-F306-15] Investigation of pH effect on Electrochemical Impedance Spectroscopy (EIS)

〇Huanwen Han¹, Nobusawa Kazuyuki¹, Ichiro Yamashita¹ (1.Grad. Eng., Osaka Univ.)

Keywords:Electrochemical Impedance Spectroscopy, pH effect

The electrochemical measurement can detect the adsorption of target molecules with high sensitivity. Electrochemical impedance spectroscopy (EIS) is one of such methods and applicable to biosensors. In the EIS measurement, a chamber with three electrodes, working, reference and counter electrodes were filled with sample solution containing ferri/ferro-cyanide ions and the impedance between working and counter electrodes is measured. The working electrode surface is modified by self-assembly monolayer with ligands which bind the target molecules selectively. The electrical behavior of the electrode surface can be drawn equivalently as in Figure 1. Rct is a charge transfer resistance, Cdl is a capacitance by the electric double layer, and Warburg is the diffusion resistance. Figure 2^[1] shows the typical response of the EIS measurement (Vac~10mV, 1M ~0.1Hz). The Rct will increase in response to the target molecule adsorption. There are a lot of factors affecting the measurement, like surface area, SAM density, molecule sizes, electrolyte concentration and so on. In this report, we modified the working Au electrode with 11-mercaptoundecanoic acid (MUA), carboxyl group terminal, and pH effect on EIS from 8 to 4 was investigated. Figure 3. Shows the EIS measurement results and Figure 4 shows the extracted Rct from fitting. EIS measurement under 50 mM Tris pH 8 showed a huge Rct resistance, 17 MΩ and 11 MΩ under 50 mM HOAc pH6. Furthermore, only 0.29 MΩ at 50 mM HOAc pH4. When we decreased the buffer concentration to 10 mM, the Rct showed their decrease. The reason why such large pH dependence of Rct could be attributed to the electrostatic interaction between mediator ferri/ferro-cyanide ions and a carboxyl group of MUA. In high pH condition, the MUA’s carboxyl termini are negatively charged and the negatively charged mediator could not reach the vicinity of the working electrode, therefore the Rct became large^[2][3]. On the other hand, under low pH, the carboxyl termini are not charged and the mediators can reach the surface to make Rct small. The results indicated that the pH must be controlled well when biosensing is carried out.