1. Glass microelectrodes for measuring micro-liquid environments
2. Micro- and nanochannles for pH analysis
3. Nanochannels for single particle analysis
4. Nonequilibrium ion transport phenomena in micro- and nanogaps
5. Electrohydrodynamic (EHD) flows in aqueous solutions
6. Molecular dynamics simulations involving chemical reactions
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1. Glass microelectrodes for measuring micro-liquid environments
We have developed glass microelectrodes with a tip diameter of 1 μm or less, including electrolyte solution and Ag-AgCl wire, for measuring electrolyte concentrations and pH in microspaces in liquids. The microelectrodes succeeded in the direct measurement of electric field, conductivity, and electrolyte concentration. Furthermore, two- and three-barreled microelectrodes allow for quantitative analysis of pH values.
K. Doi, N. Asano, and S. Kawano, Sci. Rep. 10: 4110 (2020).
T. Kishimoto and K. Doi, ACS Omega 7 (2022) 39437-39445.
T. Kishimoto and K. Doi, J. Phys. Chem. C 128 (2024) 346-354.
T. Kishimoto, T. Ando, and K. Doi, Jpn. J. Appl. Phys. 63 (2024) 106501.
T. Kishimoto, T. Ando, and K. Doi, ACS Omega 15 (2025) 045233.
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2. Micro- and nanochannels for pH analysis
It is known that nanochannels made of silicon dioxide and quartz are typically negatively charged in electrolyte solutions and they exhibit proton selectivity. Using these nanochannels, we successfully analyzed proton concentration in sample solutions. It was found that cations transported by electrophoresis against the concentration diffusion in cation selective nanochannels caused to increase the electrical resistance. The ion concentration can be quantitatively evaluated from the relationship between the concentration difference and electrical resistance.
T. Takagi, T. Kishimoto, and K. Doi, Micromachines 15 (2024) 698.
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3. Nanochannels for single particle analysis
Using micro- and nanochannels that have an orifice with a width comparative to a particle like a virus, single particles transported by drag and electrostatic forces are electrically detected. Transparent nanochannels made of quartz substrates allow to visualize the behavior of transported single particles, and furthermore, the nanoparticles are successfully captured in the orifice using optical tweezers. Detection accuracy has been improved by precisely manipulating targets.
R. Nakatsuka, S. Yanai, K. Nakajima, K. Doi, and S. Kawano, J. Phys. Chem. C 125 (2021) 9507-9515.
K. Doi, K. Yamamoto, H. Yamazaki, and S. Kawano, J. Phys. Chem. C 126 (2022) 10713-10721.
K. Aichi, T. Kishimoto, and K. Doi, Mech. Eng. J. 12 (2025) 25-00036.
H. Nomura, K. Doi, and S. Kawano, Mech. Eng. J. 12 (2025) 25-00062.
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4. Nonequilibrium ion transport phenomena in micro- and nanogaps
Ion transport phenomena in liquid are governed by electrophoresis, diffusion, and convection of ions. Focusing on the difference in the time scale of the phenomenon and investigating the nonequilibrium response, details of ion behavior are clarified. Widely varying the electrode distance in the range from nanometer to millimeter, ionic current responses have been investigated theoretically and experimentally. The electrode surface is quickly screened by ions immediately after the application of electrical potential, and the electrical potential is gradually formed farther away from the surfaces.
K. Doi et al., J. Phys. Chem. C 118 (2014) 3758-3765.
S. Tanaka, M. Tsutsui, H. Theodore, H. Yuhui, A. Arima, T. Tsuji, K. Doi, S. Kawano, M. Taniguchi, and T. Kawai, Sci. Rep. 6: 31670 (2016).
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5. Electrohydrodynamic (EHD) flows in aqueous solutions
Electrohydrodynamic (EHD) flow and electroosmotic flow (EOF) are often used to drive liquid flows in micro- and nanochannels. Both methods are caused by ion transport that drags solvent molecules. On the other hand, electrolyte solutions usually maintain electroneutrality, and therefore, concentration difference between cation and anion are inevitable to induce EHD flow and EOF. We succeeded in generating an EHD flow that is induced by cation transported through cation selective membrane. As a result, the EHD flow is generated by applying about 2V in aqueous solutions. This result suggests that ion drag flows allow to be used in various spatial scales from nano- to millimeters. K. Doi, A. Yano, and S. Kawano, J. Phys. Chem. B 119 (2015) 228-237. A. Yano, H. Shirai, M. Imoto, K. Doi, and S. Kawano, Jpn. J. Appl. Phys. 56 (2017) 097201. K. Doi. F. Nito, and S. Kawano, J. Chem. Phys. 148 (2018) 204512.
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6. Molecular dynamics simulations involving chemical reactions
The surface of a two-dimensional planar structure like graphene is filled with a cloud of electrons. Atoms and molecules are usually repelled from such a stable surface structure. On the other hand, distorted steric surfaces or impurities cause to modulate the stable electron cloud and possibly attract atoms and molecules. Although chemical reactions that involve charge transfer are difficult to be simulated using conventional molecular dynamics (MD) simulations, the first-principles MD simulations enable us to treat chemical reactions. We succeeded to simulate dissociative adsorption of hydrogen molecules above a distorted graphene surface. Our result will effectively contribute to the development of hydrogen storage materials and so on.
K. Doi, I. Onishi, and S. Kawano, Comput. Theor. Chem. 994 (2012) 54-64.
