Development of glass micro-electrodes for local electric field, electrical conductivity, and pH measurements

Kentaro Doi,Satoyuki Kawano,Naoki Asano

doi:10.1038/s41598-020-60713-z

Abstract

In micro- and nanofluidic devices, liquid flows are often influenced by ionic currents generated by electric fields in narrow channels, which is an electrokinetic phenomenon. Various technologies have been developed that are analogous to semiconductor devices, such as diodes and field effect transistors. On the other hand, measurement techniques for local electric fields in such narrow channels have not yet been established. In the present study, electric fields in liquids are locally measured using glass micro-electrodes with 1-μm diameter tips, which are constructed by pulling a glass tube. By scanning a liquid poured into a channel by glass micro-electrodes, the potential difference in a liquid can be determined with a spatial resolution of the size of the glass tip. As a result, the electrical conductivity of sample solutions can be quantitatively evaluated. Furthermore, combining two glass capillaries filled with buffer solutions of different concentrations, an ionic diode that rectifies the proton conduction direction is constructed, and the possibility of pH measurement is also demonstrated. Under constant-current conditions, pH values ranging from 1.68 to 9.18 can be determined more quickly and stably than with conventional methods that depend on the proton selectivity of glass electrodes under equilibrium conditions.

Highlights

In recent decades, ion transport phenomena that occur in micro- and nanofluidic channels have often been applied to micro-electromechanical systems (MEMS), where the surface area becomes more important than the volume[1,2,3,4,5]
A constant ionic current was maintained between the working electrode (WE) and counter electrode (CE) at both ends of the channel filled with a 1.0 × 10−3 mol/L KCl solution
We developed a glass micro-electrode that was constructed by pulling a glass tube to a diameter of 1 μm

Summary

Introduction

Ion transport phenomena that occur in micro- and nanofluidic channels have often been applied to micro-electromechanical systems (MEMS), where the surface area becomes more important than the volume[1,2,3,4,5]. Charged particles are driven by the Coulomb force and viscous drag of a liquid flow that is caused by electroosmosis[3,8] Using such a specific situation, particles and molecules translocate in micro- and nanochannels toward a test section in which single particle characteristics are sensed[1,2,5,10,11]. We carry out experimental measurements of the local electric field in electrolyte solutions with a resolution of 1 μm, based on the diameter of the glass capillary. Thomas et al.[26,27] developed a glass micro-electrode that enabled measurement of the pH of liquids due to the proton selectivity of glass capillary tips This is the basic principle of the pH sensor that is widely used today. The present method is simple and easy to use to measure various properties of electrolyte solutions, and is expected to be applied in various research fields, e.g., electrokinetics, physiology, biology, and physical chemistry

Methods

Results

Conclusion