Advanced Top-Down Fabrication for a Fused Silica Nanofluidic Device

Microfluidics and Nanofluidics in Strong Light-Matter Coupling Systems

The combination of micro- or nanofluidics and strong light-matter coupling has gained much interest in the past decade, which has led to the development of advanced systems and devices with numerous potential applications in different fields, such as chemistry, biosensing, and material science. Strong light-matter coupling is achieved by placing a dipole (e.g., an atom or a molecule) into a confined electromagnetic field, with molecular transitions being in resonance with the field and the coupling strength exceeding the average dissipation rate. Despite intense research and encouraging results in this field, some challenges still need to be overcome, related to the fabrication of nano- and microscale optical cavities, stability, scaling up and production, sensitivity, signal-to-noise ratio, and real-time control and monitoring. The goal of this paper is to summarize recent developments in micro- and nanofluidic systems employing strong light-matter coupling. An overview of various methods and techniques used to achieve strong light-matter coupling in micro- or nanofluidic systems is presented, preceded by a brief outline of the fundamentals of strong light-matter coupling and optofluidics operating in the strong coupling regime. The potential applications of these integrated systems in sensing, optofluidics, and quantum technologies are explored. The challenges and prospects in this rapidly developing field are discussed.

Nanofluidic Detection Platform for Simultaneous Light Absorption and Scattering Measurement of Individual Nanoparticles in Flow

Characterization and quantification of plasmonic nanoparticles at the single particle level have become increasingly important with the advancements in nanotechnology and their application to various biological analyses including diagnostics, photothermal therapy, and immunoassays. While various nanoparticle detection methodologies have been developed and widely used, simultaneous measurement of light absorption and scattering from individual plasmonic nanoparticles in flow is still challenging. Herein, we describe a novel nanofluidic detection platform that enables simultaneous measurement of absorption and scattering signals from individual nanoparticles within a nanochannel. Our detection platform utilized optical diffraction phenomena by a single nanochannel as both a readout signal for photothermal detection and a reference light for interferometric scattering detection. Through the elucidation of the frequency effect on the detection performance and optimization of experimental conditions, we achieved the classification of gold and silver nanoparticles with a diameter of 20-60 nm at an average accuracy score of 82.6 ± 2.1% by measured data sets of absorption and scattering signals. Furthermore, we demonstrated the concentration determination of plasmonic nanoparticle mixtures using a trained Support vector machine (SVM) classifier. Our simple yet sensitive nanofluidic detection platform will be a valuable tool for the analysis of nanoparticles and their applications to chemical and biological assays.

Local nano-electrode fabrication utilizing nanofluidic and nano-electrochemical control

Miniaturized systems have attracted much attention with the recent advances in microfluidics and nanofluidics. From the capillary electrophoresis, the development of glass-based microfluidic and nanofluidic technologies has supported advances in microfluidics and nanofluidics. Most microfluidic systems, especially nanofluidic systems, are still simple, such as systems constructed with simple straight nanochannels and bulk-scale electrodes. One of the bottlenecks to the development of more complicated and sophisticated systems is to develop the locally integrated nano-electrodes. However, there are still issues with integrating nano-electrodes into nanofluidic devices because it is difficult to fit the nano-electrode size into a nanofluidic channel at the nanometer level. In this study, we propose a new method for the fabrication of local nano-electrodes in nanofluidic devices with nanofluidic and nano-electrochemistry-based experiments. An electroplating solution was introduced to a nanochannel with control of the flow and the electroplating reaction, by which nano-electrodes were successfully fabricated. In addition, a nanofluidic device was available for nanofluidic experiments with the application of 200 kPa. This method can be applied to any electroplating material such as gold and copper. The local nano-electrode will make a significant contribution to the development of more complicated and sophisticated nanofluidic electrophoresis systems and to local electric detection methods for various nanofluidic devices.

Fabrication of High Aspect Ratio Nano-Channels by Thermal Nano-Imprinting and Parylene Deposition

A low-cost method of fabrication of high aspect ratio nano-channels by thermal nano-imprinting and Parylene deposition is proposed. SU-8 photoresist nano-channels were first manufactured by thermal nano-imprinting, and Parylene deposition was carried out to reduce the width of the nano-channels and increase the aspect ratio. During the process, the side walls of the SU-8 nano-channels were covered with the Parylene film, reducing the width of the nano-channels, and the depth of the channels increased due to the thickness of the Parylene film deposited on the surface of the SU-8 nano-channels, more so than that at the bottom. The influence of Parylene mass on the size of nano-channels was studied by theoretical analysis and experiments, and the deposition pressure of Parylene was optimized. The final high aspect ratio nano-channels are 46 nm in width and 746 nm in depth, of which the aspect ratio is 16. This simple and efficient method paves the way for the production of high aspect ratio nano-channels.

Stability of enzyme immobilized on the nanofluidic channel surface

The lifetime of an enzyme is critical to prevent system failure and optimize maintenance schedules in biological and analytical chemistry. The lifetime metrics of an enzyme can be evaluated from enzyme activity in terms of catalytic cycles per enzyme at various storage times. Trypsin, which is a gold-standard enzyme in proteomics, has been known to decrease activity due to self-digestion. To improve the activity of trypsin, enzyme reactors have developed by immobilizing in micro and nanospace. However, an evaluation method for the catalytic cycle has not been established due to major issues such as nonuniform space, unstable liquid transport, and self-digestion during immobilization in conventional work. To solve these issues, we have previously developed an ultra-fast enzyme reactor with a well-defined nanofabrication method, stable liquid transport, and partial enzyme modification. Here, we aimed to investigate catalytic cycles in a nanochannel. To extend enzyme lifetime efficiently, we have evaluated the optimal immobilization process and catalytic cycles of trypsin. As a result, immobilized enzyme densities by the trypsinogen immobilization process were increased at all concentrations compared to the trypsin immobilization process. To evaluate the lifetime of trypsin, the immobilized enzyme densities and activities were almost the same before and after 72 h of enzyme storage, and the calculated catalytic cycles were 1740. These results indicated that self-digestion of the immobilized enzyme was highly suppressed. Consequently, the reaction efficiency has been evaluated depending on the catalytic cycles from the substrate for the first time, while preventing self-digestion by trypsin.

Nanofluidic analytical system integrated with nanochannel open/close valves for enzyme-linked immunosorbent assay

There have been significant advances in the field of nanofluidics, and novel technologies such as single-cell analysis have been demonstrated. Despite the evident advantages of nanofluidics, fluid control in nanochannels for complicated analyses is extremely difficult because the fluids are currently manipulated by maintaining the balance of driving pressure. To address this issue, the use of valves will be essential. Our group previously developed a nanochannel open/close valve utilizing glass deformation, but this has not yet been integrated into nanofluidic devices for analytical applications. In the present study, a nanofluidic analytical system integrated with multiple nanochannel open/close valves was developed. This system consists of eight pneumatic pumps, seven nanochannel open/close valves combined with piezoelectric actuators, and an ultra-high sensitivity detector for non-fluorescent molecules. For simultaneous actuation of multiple valves, a device holder was designed that prevented deformation of the entire device caused by operating the valves. A system was subsequently devised to align each valve and actuator with a precision of better than 20 μm to permit the operation of valves. The developed analytical system was verified by analyzing IL-6 molecules using an enzyme-linked immunosorbent assay. Fluid operations such as sample injection, pL-level aliquot sampling and flow switching were accomplished in this device simply by opening/closing specific valves, and a sample consisting of approximately 1500 IL-6 molecules was successfully detected. This study is expected to significantly improve the usability of nanofluidic analytical devices and lead to the realization of sophisticated analytical techniques such as single-cell proteomics.

Femtoliter-Droplet Mass Spectrometry Interface Utilizing Nanofluidics for Ultrasmall and High-Sensitivity Analysis

In the fields of biology and medicine, comprehensive protein analysis at the single-cell level utilizing mass spectrometry (MS) with pL sample volumes and zmol to amol sensitivity is required. Our group has developed nanofluidic analytical pretreatment methods that exploit nanochannels for downsizing chemical unit operations to fL-pL volumes. In the field of analytical instruments, mass spectrometers have advanced to achieve ultrahigh sensitivity. However, a method to interface between fL-pL pretreatments and mass spectrometers without sample loss and dispersion is still challenging. In this study, we developed an MS interface utilizing nanofluidics to achieve high-sensitivity detection. After charging analyte molecules by an applied voltage through an electrode, the liquid sample was converted to fL droplets by a nanofluidic device. Considering the inertial force that acts on the droplets, the droplets were carried with a controlled trajectory, even in turbulent air flow, and injected into a mass spectrometer with 100% efficiency. A module for heat transfer was designed and constructed, by which all of the injected droplets were vaporized to produce gas-phase ions. The detection of caffeine ions was achieved at a limit of detection of 1.52 amol, which was 290 times higher than a conventional MS interface by electrospray ionization with sample dispersion combined with a similar mass spectrometer. Therefore, sensitivity that was 2 orders of magnitude higher could be realized due to the 100% sample injection rate. The present study provides a new methodology for the analysis of ultrasmall samples with high-sensitivity, such as protein molecules produced from a single cell.

Correction: Integration of sequential analytical processes into sub-100 nm channels: volumetric sampling, chromatographic separation, and label-free molecule detection

Correction for 'Integration of sequential analytical processes into sub-100 nm channels: volumetric sampling, chromatographic separation, and label-free molecule detection' by Yoshiyuki Tsuyama et al., Nanoscale, 2021, 13, 8855-8863, https://doi.org/10.1039/D0NR08385B.

Numerical Investigation of Diffusioosmotic Flow in a Tapered Nanochannel

Diffusioosmosis concerns ionic flow driven by a concentration difference in a charged nano-confinement and has significant applications in micro/nano-fluidics because of its nonlinear current-voltage response, thereby acting as an active electric gating. We carry out a comprehensive computation fluid dynamics simulation to investigate diffusioosmotic flow in a charged nanochannel of linearly varying height under an electrolyte concentration gradient. We analyze the effects of cone angle (α), nanochannel length (l) and tip diameter (dt), concentration difference (Δc = 0–1 mM), and external flow on the diffusioosmotic velocity in a tapered nanochannel with a constant surface charge density (σ). External flow velocity (varied over five orders of magnitude) shows a negligible influence on the diffusioosmotic flow inside the tapered nanochannel. We observed that a cone angle causes diffusioosmotic flow to move towards the direction of increasing gap thickness because of stronger local electric field caused by the overlapping of electric double layers near the smaller orifice. Moreover, the magnitude of average nanoflow velocity increases with increasing |α|. Flow velocity at the nanochannel tip increases when dt is smaller or when l is greater. In addition, the magnitude of diffusioosmotic velocity increases with increasing Δc. Our numerical results demonstrate the nonlinear dependence of tapered, diffusioosmotic flow on various crucial control parameters, e.g., concentration difference, cone angle, tip diameter, and nanochannel length, whereas an insignificant relationship on flow rate in the low Peclet number regime is observed.

Accelerated protein digestion and separation with picoliter volume utilizing nanofluidics

Single cell analyses can provide critical biological insight into cellular heterogeneity. In particular, the proteome, which governs cell functions, is much more difficult to analyze because it is principally impossible to amplify proteins compared to nucleic acids. The most promising approach to single cell proteomics is based on the liquid chromatography mass spectrometry (LC-MS) platform. However, pretreatments before MS detection have two critical issues for single cell analysis: analyte loss as a result of adsorption and artifacts due to the duration of analysis. This is a serious problem because single cells have a limited number of protein molecules and a small volume. To solve these issues, we developed an integrated nanofluidic device to manipulate samples on a femtoliter to picoliter (fL-pL) scale to achieve high-throughput analysis via suppressing analyte loss. This device can perform tryptic digestion, chromatographic separation, and non-labeled detection with high consistency. In addition, we introduced an open/close valve by physical deformation of glass on a nanometer scale to independently modify the nanochannel surfaces and control sample aliquots. The injection system equipped with this valve achieved an injection volume of 1.0 ± 0.1 pL. By using this integrated device, we found that the chromatogram of bulk-digestion for 12 hours resembled that of 15 min-digestion in the nanochannel, which indicated that these conditions reached a similar state of digestion. Therefore, an integrated device for ultra-fast protein analysis was developed on a 1 pL scale for the first time.

Direct matching between the flow factor approach model and molecular dynamics simulation for nanochannel flows

Mathematically formulating nanochannel flows is challenging. Here, the values of the characteristic parameters were extracted from molecular dynamics simulation (MDS), and directly input to the closed-form explicit flow factor approach model (FFAM) for nanochannel flows. By this way, the physical nature of the simulated system in FFAM is the same with that in MDS. Two nano slit channel heights respectively with two different liquid-channel wall interactions were addressed. The flow velocity profiles across the channel height respectively calculated from MDS and FFAM were compared. By introducing the equivalent value \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\Delta_{im} } \mathord{\left/ {\vphantom {{\Delta_{im} } D}} \right. \kern-\nulldelimiterspace} D}$$\end{document}Δim/D, FFAM fairly agrees with MDS for all the cases. The study values FFAM in simulating nanochannel flows.

Accessible detection of SARS-CoV-2 through molecular nanostructures and automated microfluidics

Accurate and accessible nucleic acid diagnostics is critical to reducing the spread of COVID-19 and resuming socioeconomic activities. Here, we present an integrated platform for the direct detection of SARS-CoV-2 RNA targets near patients. Termed electrochemical system integrating reconfigurable enzyme-DNA nanostructures (eSIREN), the technology leverages responsive molecular nanostructures and automated microfluidics to seamlessly transduce target-induced molecular activation into an enhanced electrochemical signal. Through responsive enzyme-DNA nanostructures, the technology establishes a molecular circuitry that directly recognizes specific RNA targets and catalytically enhances signaling; only upon target hybridization, the molecular nanostructures activate to liberate strong enzymatic activity and initiate cascading reactions. Through automated microfluidics, the system coordinates and interfaces the molecular circuitry with embedded electronics; its pressure actuation and liquid-guiding structures improve not only analytical performance but also automated implementation. The developed platform establishes a detection limit of 7 copies of RNA target per μl, operates against the complex biological background of native patient samples, and is completed in <20 min at room temperature. When clinically evaluated, the technology demonstrates accurate detection in extracted RNA samples and direct swab lysates to diagnose COVID-19 patients.

Stable Formation of Aqueous/Organic Parallel Two-phase Flow in Nanochannels with Partial Surface Modification

In microfluidics, various chemical processes can be integrated utilizing parallel multiphase flows. Our group has extended this research to nanofluidics, and recently performed the extraction of lipids using parallel two-phase flow in nanochannels. Although this was achieved in surface-modified nanochannels, a stable condition of parallel two-phase flow remains unknown due to difficulties in device fabrication, for a suitable method of bonding surface-modified substrates is lacking. Therefore, research on parallel two-phase flow in nanochannels has been limited. Herein, a new bonding method which improves the wash process for the substrates and increases the bonding rate to ∼100% is described. The conditions to achieve parallel organic/aqueous two-phase flow were then studied. It was revealed that in nanochannels, higher capillary numbers for the organic phase flow were required compared to that in microchannels. The newly developed fabrication process and flow regimes will contribute to realize integrated nanofluidic devices capable of analyzing single molecules/cells.

Surface Patterning of Closed Nanochannel Using VUV Light and Surface Evaluation by Streaming Current

In nanofluidics, surface control is a critical technology because nanospaces are surface-governed spaces as a consequence of their extremely high surface-to-volume ratio. Various surface patterning methods have been developed, including patterning on an open substrate and patterning using a liquid modifier in microchannels. However, the surface patterning of a closed nanochannel is difficult. In addition, the surface evaluation of closed nanochannels is difficult because of a lack of appropriate experimental tools. In this study, we verified the surface patterning of a closed nanochannel by vacuum ultraviolet (VUV) light and evaluated the surface using streaming-current measurements. First, the C₁₈ modification of closed nanochannels was confirmed by Laplace pressure measurements. In addition, no streaming-current signal was detected for the C₁₈-modified surface, confirming the successful modification of the nanochannel surface with C₁₈ groups. The C₁₈ groups were subsequently decomposed by VUV light, and the nanochannel surface became hydrophilic because of the presence of silanol groups. In streaming-current measurements, the current signals increased in amplitude with increasing VUV light irradiation time, indicating the decomposition of the C₁₈ groups on the closed nanochannel surfaces. Finally, hydrophilic/hydrophobic patterning by VUV light was performed in a nanochannel. Capillary filling experiments confirmed the presence of a hydrophilic/hydrophobic interface. Therefore, VUV patterning in a closed nanochannel was demonstrated, and the surface of a closed nanochannel was successfully evaluated using streaming-current measurements.

Metal-Free Fabrication of Fused Silica Extended Nanofluidic Channel to Remove Artifacts in Chemical Analysis

In microfluidics, especially in nanofluidics, nanochannels with functionalized surfaces have recently attracted attention for use as a new tool for the investigation of chemical reaction fields. Molecules handled in the reaction field can reach the single-molecule level due to the small size of the nanochannel. In such surroundings, contamination of the channel surface should be removed at the single-molecule level. In this study, it was assumed that metal materials could contaminate the nanochannels during the fabrication processes; therefore, we aimed to develop metal-free fabrication processes. Fused silica channels 1000 nm-deep were conventionally fabricated using a chromium mask. Instead of chromium, electron beam resists more than 1000 nm thick were used and the lithography conditions were optimized. From the results of optimization, channels with 1000 nm scale width and depth were fabricated on fused silica substrates without the use of a chromium mask. In nanofluidic experiments, an oxidation reaction was observed in a device fabricated by conventional fabrication processes using a chromium mask. It was found that Cr⁶⁺ remained on the channel surfaces and reacted with chemicals in the liquid phase in the extended nanochannels; this effect occurred at least to the micromolar level. In contrast, the device fabricated with metal-free processes was free of artifacts induced by the presence of chromium. The developed fabrication processes and results of this study will be a significant contribution to the fundamental technologies employed in the fields of microfluidics and nanofluidics.

Design of Microchannel Suitable for Packing with Anion Exchange Resins: Uranium Separation from Seawater Containing a Large Amount of Cesium

We present a resin-packed microchannel that can reduce the radiation exposure risk and secondary radioactive wastes during uranium (U) separation by downscaling the separation using a microchip. Two types of microchips were designed to densely pack the microchannels with resins. The microchannels had almost the same cross-sectional area, but different outer circumferences. A satisfactory separation performance could be obtained by arranging more than ca. 10 resins along the depth and width of the microchannels. A resin-packed microchannel is an effective separation technique for determining the U concentration via inductively coupled plasma mass spectrometry owing to its ability to avoid the contamination of equipment by cesium, and to reduce the matrix effect. The size of the separation site was scaled down to <1/5000 compared to commonly used counterparts. The radiation exposure risk and secondary radioactive wastes can be reduced by 10- and 800-fold, respectively, using a resin-packed microchannel.

Isotope Effect in the Liquid Properties of Water Confined in 100 nm Nanofluidic Channels

Liquids confined in 10-100 nm spaces show different liquid properties from those in the bulk. Proton transfer plays an essential role in liquid properties. The Grotthuss mechanism, in which charge transfer occurs among neighboring water molecules, is considered to be dominant in bulk water. However, the rotational motion and proton transfer kinetics have not been studied well, which makes further analysis difficult. In this study, an isotope effect was used to study the kinetic effect of rotational motion and proton hopping processes by measurement of the viscosity, proton diffusion coefficient, and the proton hopping activation energy. As a result, a significant isotope effect was observed. These results indicate that the rotational motion is not significant, and the decrease of the proton hopping activation energy enhances the apparent proton diffusion coefficient.