Yutaka Kazoe

Extended-nanofluidics: fundamental technologies, unique liquid properties, and application in chemical and bio analysis methods and devices.

Engineering using liquids confined in channels 10-1000 nm in dimension, or extended-nanofluidics, is the next target of microfluidic science. Liquid properties at this scale were unrevealed until recently because of the lack of fundamental technologies for investigating these ultrasmall spaces. In this article, the fundamental technologies are reviewed, and the emerging science and technology in the extended-nanospace are discussed.

Measurements of the near-wall hindered diffusion of colloidal particles in the presence of an electric field

Understanding near-wall diffusion of small particles and biomolecules is important in colloid science and many microfluidic devices. Our experimental measurements of the diffusion of 110–460 nm radii suspended particles in the presence of electric fields up to 3.1 kV/m using particle tracking are in agreement with theoretical predictions for diffusion hindered by the presence of a solid surface. The results suggest that the external electric field has little, if any, effect upon the hindered diffusion of colloidal particles, even when the electrophoretic force exceeds the Stokes drag.

Evanescent wave-based particle tracking velocimetry for nanochannel flows.

Understanding fluid flows in 10-1000 nm space, which we call extended nanospace, is important for novel nanofluidic devices in analytical chemistry. This study therefore developed a particle tracking velocimetry for measuring velocity distribution in nanochannel flows, by using the evanescent wave illumination. 64 nm fluorescent nanoparticles were used as flow tracer. The particle position was determined from fluorescent intensity by the evanescent wave field, with a spatial resolution smaller than light wavelengths. The time resolution of 260 μs was achieved to make error by the Brownian diffusion of the tracer small to be neglected. An image processing by multitime particle tracking was established to detect the tracer nanoparticles of weak fluorescent intensity. Though the measurement region was affected by nonuniform particle distribution with the electrostatic interactions, pressure-driven flows of water in a nanochannel of 50 μm width and 410 nm depth were successfully measured. The results of the velocity distribution in the depth-wise direction approximately showed agreement with the fluid dynamics with the bulk liquid properties from the macroscopic view, however, suggested slip velocities even in the hydrophilic channel. We suggest a possibility of appearance of molecular behavior in the fluid near the wall within 10 nm-order scale.

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 (580-2720u2009nm) fabricated on fused-silica substrates were measured using the streaming current method. The isoelectric point obtained in a 2720u2009nm 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 (580u2009nm) was decreased to less than 2.0. This result provides important information for the modeling of ion transport in extended nanospace.

Behavior of nanoparticles in extended nanospace measured by evanescent wave-based particle velocimetry.

The transport and behavior of nanoparticles, viruses, and biomacromolecules in 10-1000 nm confined spaces (hereafter extended nanospaces) are important for novel analytical devices based on nanofluidics. This study investigated the concentration and diffusion of 64 nm nanoparticles in a fused-silica nanochannel of 410 nm depth, using evanescent wave-based particle velocimetry. We found that the injection of nanoparticles into the nanochannel by pressure-driven flow was significantly inhibited and that the nanoparticle diffusion was hindered anisotropically. A 0.2-pN repulsive force induced by the interaction between the nanoparticles and the channel wall is proposed as the dominant factor governing the behavior of nanoparticles in the nanochannel, on the basis of both experimental measurements and theoretical estimations. The results of this study will greatly further our understanding of mass transfer in extended nanospaces.

Tandem photovoltaic–photoelectrochemical GaAs/InGaAsP–WO3/BiVO4 device for solar hydrogen generation

We demonstrated highly efficient solar hydrogen generation via water splitting by photovoltaic–photoelectrochemical (PV–PEC) tandem device based on GaAs/InGaAsP (PV cell) and WO3/BiVO4 core/shell nanorods (PEC cell). We utilized extremely thin absorber (ETA) concept to design the WO3/BiVO4 core/shell heterojunction nanorods and obtained the highest efficiencies of generation, separation and transfer of the photo-induced charge carriers that are possible for the WO3/BiVO4 material combination. The PV–PEC tandem shows stable water splitting photocurrent of 6.56 mAcm−2 under standard AM1.5G solar light that corresponds to the record solar-to-hydrogen (STH) conversion efficiency of 8.1%.

Contribution of Soluble Forms of Programmed Death 1 and Programmed Death Ligand 2 to Disease Severity and Progression in Systemic Sclerosis

To determine the function and serum levels of soluble forms of programmed death 1 (sPD‐1) and one of its ligands, soluble PD ligand 2 (sPD‐L2), in patients with systemic sclerosis (SSc) and in a mouse model of topoisomerase I (topo I)–induced SSc.

Soluble form of PD-1 and PD-L2 contributes to disease severity and progression in systemic sclerosis

To determine the function and serum levels of soluble forms of programmed death 1 (sPD‐1) and one of its ligands, soluble PD ligand 2 (sPD‐L2), in patients with systemic sclerosis (SSc) and in a mouse model of topoisomerase I (topo I)–induced SSc.

Combined Laser-Based Measurements for Micro- and Nanoscale Transport Phenomena

The present paper summarizes our recent research in combined laser-based measurement techniques for investigating micro- and nanoscale transport phenomena. Micrometer-resolution particle image velocimetry has been combined with the laser-induced fluorescence (LIF) technique in order to simultaneously analyze velocity and scalar fields. The measurement system is based on confocal microscopy to realize a depth resolution of approximately 2 m, and we have applied this technique to liquid–liquid mixing flows, gas–liquid two-phase flows, gas permeation phenomena through membranes, and surface-modified microchannel flow. Furthermore, in order to evaluate the electrostatic potential at a solid–liquid interface (i.e., zeta potential), the LIF technique was extended by evanescent wave illumination, and only the fluorescent dye within approximately 100 nm of the microchannel wall was irradiated. The extended LIF technique was applied to microdevices with a surface modification pattern, and the zeta-potential distribution was successfully visualized. The proposed techniques will contribute to novel applications related to microscale multiphase flows or electrokinetics.