Kyojiro Morikawa

Numerical Simulation of Proton Distribution with Electric Double Layer in Extended Nanospaces

Understanding the properties of liquid confined in extended nanospaces (10-1000 nm) is crucial for nanofluidics. Because of the confinement and surface effects, water may have specific structures and reveals unique physicochemical properties. Recently, our group has developed a super resolution laser-induced fluorescence (LIF) technique to visualize proton distribution with the electrical double layer (EDL) in a fused-silica extended nanochannel (Kazoe, Y.; Mawatari, K.; Sugii, Y.; Kitamori, T. Anal. Chem.2011, 83, 8152). In this study, based on the coupling of the Poisson-Boltzmann theory and site-dissociation model, the effect of specific water properties in an extended nanochannel on formation of EDL was investigated by comparison of numerical results with our previous experimental results. The numerical results of the proton distribution with a lower dielectric constant of approximately 17 were shown to be in good agreement with our experimental results, which confirms our previous observation showing a lower water permittivity in an extended nanochannel. In addition, the higher silanol deprotonation rate in extended nanochannels was also demonstrated, which is supported by our previous results of NMR and streaming current measurements. The present results will be beneficial for a further understanding of interfacial chemistry, fluid physics, and electrokinetics in extended nanochannels.

Shift of isoelectric point in extended nanospace investigated by streaming current measurement

Isoelectric points in extended nanochannels (580-2720 nm) fabricated on fused-silica substrates were measured using the streaming current method. The isoelectric point obtained in a 2720 nm channel was almost the same as the isoelectric point reported for the bulk (2.6-3.2). However, the isoelectric point in the extended nanochannel (580 nm) was decreased to less than 2.0. This result provides important information for the modeling of ion transport in extended nanospace.

Living Single Cell Analysis Platform Utilizing Microchannel, Single Cell Chamber, and Extended-nano Channel

Single cell analysis has been of great interest in recent years. In particular, to achieve living single cell analysis is the ultimate goal to study the dynamic process of the single cell. However, single cell volume is pL in scale, and it is difficult to realize living single cell analysis, even by microfluidic technology (nL-sub nL). Herein, a novel microfluidic platform was developed by integrating a single cell chamber and an extended-nano channel (aL-fL volume). A single cell was isolated and cultured for more than 12 h by pressure-driven flow control. In addition, an electric resistance measurement method was developed to monitor the cell viability without fluorescence labeling. This platform will provide a new method for living single cell analysis by utilizing the novel analytical functions of the extended-nano space.

Dielectric Constant of Liquids Confined in the Extended Nanospace Measured by a Streaming Potential Method

Understanding liquid structure and the electrical properties of liquids confined in extended nanospaces (10-1000 nm) is important for nanofluidics and nanochemistry. To understand these liquid properties requires determination of the dielectric constant of liquids confined in extended nanospaces. A novel dielectric constant measurement method has thus been developed for extended nanospaces using a streaming potential method. We focused on the nonsteady-state streaming potential in extended nanospaces and successfully measured the dielectric constant of liquids within them without the use of probe molecules. The dielectric constant of water was determined to be significantly reduced by about 3 times compared to that of the bulk. This result contributes key information toward further understanding of the chemistry and fluidics in extended nanospaces.

Femtoliter high-performance liquid chromatography using extended-nano channels

A high-performance liquid chromatography system with 35 fL sample volume was developed using extended-nano (10-1000 nm) fluidic channels. For many years, miniaturization and enhancement of separation performance have been important issues in separation science. Recently, we have reported an ultimate miniaturization of chromatography using extended-nano channels with extremely high separation efficiency of 7 × 106 plates per m. However, the real theoretical plate number was limited to 103 due to the short nanochannel length. In this paper, the theoretical plate number was dramatically increased by developing a new high-pressure system with a very long nanochannel. A separation experiment of two fluorescent dyes demonstrated that the theoretical plate number could be improved to 1.4 × 104, which is much higher than that with conventional HPLC. The theoretical plate number is also comparable to those of capillary monolithic columns. The extremely small sample volume of extended-nano chromatography could support innovative analytical techniques capable of analyzing a single living cell in the near future.

Temperature and Size Effects on Structural and Dynamical Properties of Water Confined in 1 – 10 nm-scale Pores Using Proton NMR Spectroscopy

We were able to fill 1 - 10 nm-scale silica pores with water by vapor condensation, and examined the freezing phenomena, structures, and molecular motions of the confined water in the temperature range from 293 to 188 K by 1H-NMR spectroscopy. The results showed that almost all water molecules confined in 10 nm-scale pores were frozen and that approximately half of the water confined in 1 nm-scale pores existed in the liquid state even below the freezing point. The water adsorbed on the pore surfaces was estimated as a monolayer in 2.58 nm pores and bi- and tri-layers in 6.48 nm and larger pores, respectively. Furthermore, it was clarified from the proton relaxation rate (1H-1/T1) measurements that the molecular motions of adsorbed water itself were restricted by nanoconfinement and were extremely dependent on the conditions of proton exchange and hydrogen bond rearrangements of the adsorbed water.

Keto-Enol Tautomeric Equilibrium of Acetylacetone Solution Confined in Extended Nanospaces.

We aim to clarify the effects of size confinement, solvent, and deuterium substitution on keto-enol tautomerization of acetylacetone (AcAc) in solutions confined in 10-100 nm spaces (i.e., extended nanospaces) using (1)H NMR spectroscopy. The keto-enol equilibrium constants of AcAc (K(EQ) = [keto]/[enol]) in various solvents confined in extended nanospaces of 200-3000 nm were examined using the area ratios of -CH3 peaks in keto to enol forms. The results showed that the keto form of AcAc in hydrogen-bonded solvents such as water and ethanol increased drastically with decreasing space sizes below about 500 nm, but the size confinement did not induce equilibrium shifts in aprotic solvents such as DMSO. The magnitudes of K(EQ) enhancement were well correlated with solvent proton donicity. It followed from the determination of thermodynamic parameters that the stabilization of intermolecular interactions between protons in water and carbonyl oxygen (C═O) in the keto form of AcAc were promoted by size-confinement, and that the keto form could be energetically and structurally favored in extended nanospaces vis-à-vis the bulk space. Furthermore, the measurements of deuterium dependence of the K(EQ) values verified that the nanoconfinement-induced shifts of keto-enol tautomerization of AcAc are attributable to high proton mobility via a proton hopping mechanism of the confined water.

Micro heat pipe device utilizing extended nanofluidics

The next-generation cooling devices are gradually being scaled to smaller than the size of high-performance microchips to enable local heat removal from small hot spots. Realization of a micro/nanofluidic heat pipe device is a challenging task, as it requires high condensation efficiency in an ultra-small space and sufficient liquid transport without employing any wick. Herein, we demonstrate a two-phase loop micro heat pipe device based on unique liquid properties in extended nanospace (10–1000 nm) to meet the growing demands of the miniaturization of electronics and optoelectronics. The device, which contains a small volume of liquid (tens of nanoliter) and does not require a wick, can be conveniently embedded in the microchip. The capillary condensation of water on nanopillars was investigated. The experimental results showed a significant enhancement of the condensation rate on nanopillars for a faster vapor–liquid phase transition. In addition, a streaming potential measurement was performed to evaluate the liquid transport during operation of the micro heat pipe device. This method enables the measurement of water flow rates through extended nanochannels without requiring probe molecules. The micro heat pipe device was verified to work properly. Finally, the cooling performance of the micro heat pipe device was quantitatively estimated, and improvements were proposed to achieve highly efficient cooling.

Rapid Plasma Etching for Fabricating Fused Silica Microchannels

In order to advance the performances of micro chemical and biochemical systems on a chip, the fabrication of microstructures such as channels and pillars is an essential basic technology. However, conventional fabrication methods based on wet etching have limitations in their applications for device engineering. In this study, we report on a new microchannel fabrication process on a fused silica substrate using photoresist and plasma etching based on C3F8, CHF3, and Ar gases. Deep, rectangular microchannels, having vertical angles close to 90°, 10 μm-scale deep and low surface roughness of less than 1 nm, could be fabricated on a fused silica substrate at high etching rates on the order of 5 - 7 nm s-1. This metal-free fabrication methodology is expected to be a low-cost, easy, and simple technique for a fused silica microstructure applications.

Fabrication of Hydrophobic Nanostructured Surfaces for Microfluidic Control.

In the field of micro- and nanofluidics, various kinds of novel devices have been developed. For such devices, not only fluidic control but also surface control of micro/nano channels is essential. Recently, fluidic control by hydrophobic nanostructured surfaces have attracted much attention. However, conventional fabrication methods of nanostructures require complicated steps, and integration of the nanostructures into micro/nano channels makes fabrication procedures even more difficult and complicated. In the present study, a simple and easy fabrication method of nanostructures integrated into microchannels was developed. Various sizes of nanostructures were successfully fabricated by changing the plasma etching time and etching with a basic solution. Furthermore, it proved possible to construct highly hydrophobic nanostructured surfaces that could effectively control the fluid in microchannels at designed pressures. We believe that the fabrication method developed here and the results obtained are valuable contributions towards further applications in the field of micro- and nanofluidics.