Extended nanofluidic immunochemical reaction with femtoliter sample volumes

Nanofluidic Aptamer Nanoarray to Enable Stochastic Capture of Single Proteins at Normal Concentrations

Single-molecule experiments allow understanding of the diversity, stochasticity, and heterogeneity of molecular behaviors and properties hidden by conventional ensemble-averaged measurements. They hence have great importance and significant impacts in a wide range of fields. Despite significant advances in single-molecule experiments at ultralow concentrations, the capture of single molecules in solution at normal concentrations within natural biomolecular processes remains a formidable challenge. Here, a high-density, well-defined nanofluidic aptamer nanoarray (NANa) formed via site-specific self-assembly of well-designed aptamer molecules in nanochannels with nano-in-nano gold nanopatterns is presented. The nanofluidic aptamer nanoarray exhibits a high capability to specifically capture target proteins (e.g., platelet-derived growth factor BB; PDGF-BB) to form uniform protein nanoarrays under optimized nanofluidic conditions. Owing to these fundamental features, the nanofluidic aptamer nanoarray enables the stochastic capture of single PDGF-BB molecules at a normal concentration from a sample with an ultrasmall volume equivalent to a single cell by following Poisson statistics, forming a readily addressable single-protein nanoarray. This approach offers a methodology and device to surpass both the concentration and volume limits of single-protein capture in most conventional methodologies of single-molecule experiments, thus opening an avenue to explore the behavior of individual biomolecules in a manner close to their natural forms, which remains largely unexplored to date.

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.

Enzyme-linked immunosorbent assay using thin-layered microfluidics with perfect capture of the target protein

We developed a process for enzyme-linked immunosorbent assay on a glass microchip via the use of a thin-layered microfluidic channel. This channel possesses a high aspect ratio (width/depth ∼200) and has an antibody layer immobilized directly on the channel surface. A depth of several microns and an excessive width and length (mm scale) of the channel provide a large-volume capacity (10² nL) and maximum capture efficiency of the analyte for a high level of detection sensitivity (10² pg mL^-1). The developed reusable immunosensor has demonstrated high-performance characteristics by requiring less than 50 μL of sample and providing analysis in less than 25 min. This new method could impact the development of point-of-care devices for biomedical applications.

Enhanced sensitivity of dilute aqueous adrenaline solution with an asymmetric hexagonal ring structure in the terahertz frequencies

Quantitative detection of neurotransmitters in aqueous environment is crucial for the early diagnosis of many neurological disorders. Terahertz waves, as a non-contact and non-labeling tool, have demonstrated large potentials in quantitative biosensing. Although the detection of trace-amount analyte has been achieved with terahertz metamaterials in the recent decades, most studies have been focused on dried samples. Here, a hexagonal asymmetric metamaterial sensor was designed and fabricated for aqueous solution sensing with terahertz waves in the reflection geometry. An absorption enhancement of 43 was determined from the simulation. Dilute adrenaline solutions ranging from 30 µM to 0.6 mM were measured on our sensor using a commercial terahertz time-domain spectroscopy system, and the effective absorption was found to be linearly correlated with the concentration (R²= 0.81). Furthermore, we found that as the concentration becomes higher (>0.6 mM), a non-linear relationship starts to take place, which confirmed the previous theory on the extended solvation shell that can be probed on the picosecond scale. Our sensor, without the need of high-power and stable terahertz sources, has enabled the detection of subtle absorption changes induced by the solvation dynamics.

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.

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.

A simple and reversible glass-glass bonding method to construct a microfluidic device and its application for cell recovery

Compared with polymer microfluidic devices, glass microfluidic devices have advantages for diverse lab-on-a-chip applications due to their rigidity, optical transparency, thermal stability, and chemical/biological inertness. However, the bonding process to construct glass microfluidic devices usually involves treatment(s) like high temperature over 400 °C, oxygen plasma or piranha solution. Such processes require special skill, apparatus or harsh chemicals, and destroy molecules or cells in microchannels. Here, we present a simple method for glass-glass bonding to easily form microchannels. This method consists of two steps: placing water droplets on a glass substrate cleaned by neutral detergent, followed by fixing a cover glass plate on the glass substrate by binding clips for a few hours at room temperature. Surface analyses showed that the glass surface cleaned by neutral detergent had a higher ratio of SiOH over SiO than glass surfaces prepared by other cleaning steps. Thus, the suggested method could achieve stronger glass-glass bonding via dehydration condensation due to the higher density of SiOH. The pressure endurance reached over 600 kPa within 6 h of bonding, which is sufficient for practical microfluidic applications. Moreover, by exploiting the reversibility of this bonding method, cell recoveries after cultivating cells in a microchannel were demonstrated. This new bonding method can significantly improve both the productivity and the usability of glass microfluidic devices and extend the possibility of glass microfluidic applications in future.

Nanoplasmonic-Nanofluidic Single-Molecule Biosensors for Ultrasmall Sample Volumes

Detection of small amounts of biological compounds is of ever-increasing importance but also remains an experimental challenge. In this context, plasmonic nanoparticles have emerged as strong contenders enabling label-free optical sensing with single-molecule resolution. However, the performance of a plasmonic single-molecule biosensor is not only dependent on its ability to detect a molecule but equally importantly on its efficiency to transport it to the binding site. Here, we present a theoretical study of the impact of downscaling fluidic structures decorated with plasmonic nanoparticles from conventional microfluidics to nanofluidics. We find that for ultrasmall picolitre sample volumes, nanofluidics enables unprecedented binding characteristics inaccessible with conventional microfluidic devices, and that both detection times and number of detected binding events can be improved by several orders of magnitude. Therefore, we propose nanoplasmonic–nanofluidic biosensing platforms as an efficient tool that paves the way for label-free single-molecule detection from ultrasmall volumes, such as single cells.

Cytokine analysis on a countable number of molecules from living single cells on nanofluidic devices

Analysis of proteins released from living single cells is strongly required in the fields of biology and medicine to elucidate the mechanism of gene expression, cell-cell communication and cytopathology. However, as living single-cell analysis involves fL sample volumes with ultra-small amounts of analyte, comprehensive integration of entire chemical processing for single cells and proteins into spaces smaller than single cells (pL) would be indispensable to prevent dispersion-associated analyte loss. In this study, we proposed and developed a living single-cell protein analysis device based on micro/nanofluidics and demonstrated analysis of cytokines released from living single B cells by enzyme-linked immunosorbent assay. Based on our integration method and technologies including top-down nanofabrication, surface modifications and pressure-driven flow control, we designed and prepared the device where pL-microfluidic- and fL-nanofluidic channels are hierarchically allocated for cellular and molecular processing, respectively, and succeeded in micro/nanofluidic control for manipulating single cells and molecules. 13-unit operations for pL-cellular processing including single-cell trapping and stimulation and fL-molecular processing including fL-volumetry, antigen-antibody reactions and detection were entirely integrated into a microchip. The results suggest analytical performances for countable interleukin (IL)-6 molecules at the limit of detection of 5.27 molecules and that stimulated single B cells secrete 3.41 IL-6 molecules per min. The device is a novel tool for single-cell targeted proteomics, and the methodology of device integration is applicable to other single-cell analyses such as single-cell shotgun proteomics. This study thus provides a general approach and technical breakthroughs that will facilitate further advances in micro/nanofluidics, single-cell life science research, and other fields.

Femtoliter Volumetric Pipette and Flask Utilizing Nanofluidics

Microfluidics has achieved integration of analytical processes in microspaces and realized miniaturized analyses in fields such as chemistry and biology. We have proposed a general concept of integration and extended this concept to the 10-1000 nm scale exploring ultimate analytical performances (e.g. immunoassay of a single-protein molecule). However, a sampling method is still challenging for nanofluidics despite its importance in analytical chemistry. In this study, we developed a femtoliter (fL) sampling method for volume measurement and sample transport. Traditionally, sampling has been performed using a volumetric pipette and flask. In this research, a nanofluidic device consisting of a femtoliter volumetric pipette and flask was fabricated on glass substrates. Since gravity, which is exploited in bulk fluidic operations, becomes less dominant than surface effects on the nanometer scale, fluidic operation of the femtoliter sampling was designed utilizing surface tension and air pressure control. The working principle of an 11 fL volumetric pipette and a 50 fL flask, which were connected by a nanochannel, was verified. It was found that evaporation of the sample solution by air flow was a significant source of error because of the ultra-small volumes being processed. Thus, the evaporation issue was solved by suppressing the air flow. As a result, the volumetric measurement error was decreased to ±0.06 fL (CV 0.6%), which is sufficiently low for use in nanofluidic analytical applications. This study will present a fundamental technology for the development of novel analytical methods for femtoliter volume samples such as single molecule analyses.

COvalent monolayer patterns in Microfluidics by PLasma etching Open Technology – COMPLOT

Plasma microcontact patterning (PμCP) and replica molding were combined to make PDMS/glass microfluidic devices with β-cyclodextrin (β-CD) patterns attached covalently on the glass surface inside microchannels.

Picoliter enzyme reactor on a nanofluidic device exceeding the bulk reaction rate

A picoliter enzyme reactor using a trypsin immobilized nanochannel realized 25 times faster reaction than the bulk reaction.

Parallel multiphase nanofluidics utilizing nanochannels with partial hydrophobic surface modification and application to femtoliter solvent extraction

In the field of microfluidics, utilizing parallel multiphase flows with immiscible liquid/liquid or gas/liquid interfaces along a microchannel has achieved the integration of various chemical processes for analyses and syntheses. Recently, our group has developed nanofluidics that exploits 100 nm nanochannels to realize ultra-small (aL to fL scale) and highly efficient chemical operations. Novel applications such as single-molecule analyses and single-cell omics are anticipated. However, the formation of parallel multiphase flows in a nanochannel remains challenging. To this end, here we developed a novel method for nanoscale partial hydrophobic surface modification of a nanochannel utilizing a focused ion beam. Hydrophobic and hydrophilic areas could be patterned beside one another even in a 60 nm glass nanochannel. Because this patterning maintained the liquid/liquid interface in the nanochannel based on the difference in wettability, stable aqueous/organic parallel two-phase flow in a 40 fL nanochannel was realized for the first time. Utilizing this flow, nanoscale unit operations involving phase confluence, extraction and phase separation were integrated to demonstrate solvent extraction of a lipid according to the Bligh-Dyer method, which is a broadly used pretreatment process in lipidomics. We accomplished the separation of a lipid and an amino acid in a sample volume of 4 fL (250 times smaller than the pL volume of a single cell) with a processing time of 1 ms (10 000 times faster than that in a microchannel). This study therefore provides a technological breakthrough that advances the field of nanofluidics to allow multiphase chemical processing at fL volumes.

Femtoliter nanofluidic valve utilizing glass deformation

In the field of micro/nanofluidics, the channel open/close valves are among the most important technologies for switching and partitioning actions and integration of various operations into fluidic circuits. While several types of valves have been developed in microfluidics, few are capable in nanofluidics. In this study, we proposed a femtoliter (fL) volume nanochannel open/close valve fabricated in glass substrates. The valve consists of a shallow, circular and stepped-bottom valve chamber connected to nanochannels and an actuator. Even with tiny deformation occurring at the nanolevel in glass, an open/closed state of a nanochannel (10-1000 nm) can be achieved. We designed a fL-valve based on an analytical material deformation model, and developed a valve fabrication process. We then verified the open/closed state of the valve using a 308 fL-valve chamber with a four-stepped nanostructure fitting an arc-shape of deflected glass, confirmed its stability and durability over 50 open/close operations, and succeeded in stopping/flowing an aqueous solution at 209 fL s-1 under a 100 kPa pressure in a 900 nm nanochannel with a fast response of ∼0.65 s. A leak flow from the closed valve was sufficiently small even at a 490 kPa pressure-driven flow. Since the developed fL-valve can be applied to various nanofluidic devices made of glass and other rigid materials such as plastic, it is expected that this work will contribute significantly to the development of novel integrated micro/nanofluidics chemical systems for use in various applications, such as single cell/single molecule analysis.

Detachable glass micro/nanofluidic device

Glass is one of the most ideal materials for micro/nanofluidic devices due to its excellent optical transparency, resistance to a wide range of solvents and reagents, and easy to modify surfaces by silane-coupling reagents. From a practical point of view, glass is a hard material and is suitable for real applications. One of the advantages of glass is its reusability; however, this reusability is difficult to realize in certain conditions. Washing or re-modification of micro/nanofluidic channels is sometimes difficult due to the ultrasmall space in these channels. If the glass devices are detachable, it is easy to access the channel surface, and the channels can be cleaned and re-modified. When the substrates are bonded again, the devices are fabricated easily without repeating laborious and expensive micro/nano-fabrication processes. This technology gives researchers and users a choice of glass substrates in fundamental research studies and real-time applications. In this study, we propose a detachable glass micro/nanofluidic device by our low temperature bonding method. The surface bonding energy is controlled to realize both high pressure capacity for micro/nanofluidics and easy separation of glass substrates without fracturing. As a result, at least four times detaching and bonding is confirmed.

A single-molecule ELISA device utilizing nanofluidics

Single molecule analysis is desired in many areas that require the analysis of ultra-small volume and/or extremely low concentration samples (e.g., single-cell biology, medicine diagnosis, virus detection, etc.). Due to the ultra-small volume or concentration, the sample contains only single or countable analyte molecules. Thus, specific single molecules should be precisely processed and detected for analysis. However, except nucleic acids, most molecules are difficult to amplify, and a new analytical methodology for specific single molecules is thus essential. For this, efficient chemical processing and detection, which are important analytical elements, should be developed. Here, we report a single-molecule ELISA (enzyme-linked immunosorbent assay) device utilizing micro/nanofluidic technology. Both chemical processing and detection were integrated into an ultra-small space (10² nm in size), and the integration allowed precise processing (∼100% capture) and detection of a specific single molecule (protein) for the first time. This new concept and enabling technology represent a significant innovation in analytical chemistry and will have a large impact on general biology and medicine.

Enzyme-linked immunosorbent assay utilizing thin-layered microfluidics

An antibody-immobilized thin-layered glass microfluidic channel with a high surface-to-volume ratio was developed for rapid and sensitive enzyme-linked immunosorbent assay.

Non-lithographic nanofluidic channels with precisely controlled circular cross sections

Nanofluidic channels have received growing interest due to their potential for applications in the manipulation of nanometric objects, such as DNA, proteins, viruses, exosomes, and nanoparticles. Although significant advances in nanolithography-based fabrication techniques over the past few decades have allowed us to explore novel nanofluidic transport phenomena and unique applications, the development of new technologies enabling the low-cost preparation of nanochannels with controllable and reproducible shapes and dimensions is still lacking. Thus, we herein report the application of a nanofiber printed using a near-field electrospinning method as a sacrificial mold for the preparation of polydimethylsiloxane nanochannels with circular cross sections. Control of the size and shape of these nanochannels allowed the preparation of nanochannels with channel widths ranging from 70-368 nm and height-to-width ratios of 0.19-1.00. Capillary filling tests confirmed the excellent uniformity and reproducibility of the nanochannels. These results therefore are expected to inspire novel nanofluidic studies due to the simple and low-cost nature of this fabrication process, which allows precise control of the shape and dimensions of the circular cross section.

Nano X-ray diffractometry device for nanofluidics

Nanofluidics is gaining attention because it has unique liquid and fluidic properties that are not observed in microfluidics. It has been reported that many liquid properties change when the size of a fluidic channel is reduced below 500-800 nm. To discuss the underlying mechanism, information on the microscopic liquid structure must be obtained (e.g., by X-ray diffractometry). However, the very small volume (attoliters to femtoliters) of a nanochannel and the large volume of its glass substrate prevent measurement of signals from the nanochannel liquid. In this study, we report a novel nanofluidic device that can be used in conjunction with X-ray diffractometry to analyze the structure of water confined in nanochannels. Top-down and bottom-up micro- and nano-fabrication processes were established, and the substrate thickness of the measurement area was reduced to only 2.7 μm, which was almost 1000 times smaller than that of conventional substrates (millimeter scale). With this new device, X-ray diffraction signals were clearly observed in nanochannels 500 nm wide and deep. Based on the X-ray diffraction pattern, the radial distribution function was calculated, which showed a structure nearly similar to that of a bulk sample. Therefore, X-ray diffractometry in nanochannels was realized. This method will provide important information on how a liquid behaves when confined in a nanospace and contribute to chemistry and biology on scales of 10-100 nm (e.g., inter- and intra-cellular spaces). It is also important for designing chemical reactions and fluidic circuits in nanochannels for realizing highly functional devices.

Total Capture, Convection-Limited Nanofluidic Immunoassays Exhibiting Nanoconfinement Effects

Understanding nanoconfinement phenomena is necessary to develop nanofluidic technology platforms. One example of nanoconfinement phenomena is shifts in reaction equilibria toward reaction products in nanoconfined systems, which have been predicted theoretically and observed experimentally in DNA hybridization. Here we demonstrate a convection-limited nanofluidic immunoassay that achieves total capture of a target analyte and an apparent shift in the antibody-antigen reaction equilibrium due to nanoconfinement. The system exhibits wavefronts of the target analyte that propagate along the length of the nanochannel at a velocity much slower than that of the carrier fluid. We apply an analytical model describing the propagation of these wavefronts to determine the density of capture antibody binding sites in the enclosed nanochannel for a known concentration of the target analyte. We then use this binding site density to estimate the concentration of solutions with 5× and 10× less analyte. Our analysis suggests that nanoconfinement results in a preference toward binding of the target analyte with the surface-grafted capture antibody, as evidenced by an apparent reduction in the equilibrium dissociation constant. Our findings motivate the advancement of new biomedical and chemical synthesis technologies by leveraging nanoconfinement effects, and demonstrate a useful platform for studying the effect of nanoconfinement on chemical systems.

Nanofluidics: A New Arena for Materials Science

A significant growth of research in nanofluidics is achieved over the past decade, but the field is still facing considerable challenges toward the transition from the current physics-centered stage to the next application-oriented stage. Many of these challenges are associated with materials science, so the field of nanofluidics offers great opportunities for materials scientists to exploit. In addition, the use of unusual effects and ultrasmall confined spaces of well-defined nanofluidic environments would offer new mechanisms and technologies to manipulate nanoscale objects as well as to synthesize novel nanomaterials in the liquid phase. Therefore, nanofluidics will be a new arena for materials science. In the past few years, burgeoning progress has been made toward this trend, as overviewed in this article, including materials and methods for fabricating nanofluidic devices, nanofluidics with functionalized surfaces and functional material components, as well as nanofluidics for manipulating nanoscale materials and fabricating new nanomaterials. Many critical challenges as well as fantastic opportunities in this arena lie ahead. Some of those, which are of particular interest, are also discussed.

Differential interference contrast-photothermal microscopy in nanospace: impacts of systematic parameters

Differential interference contrast-photothermal microscopy (DIC-PTM), as a promising tool for trace analysis of nonfluorescent compounds, suffered low sensitivity in nanospace especially for aqueous samples, due to the poor thermophysical property of water and the unoptimised configuration. To improve its performance, a five-layer DIC-PTM model is built and influences of different parameters on the photothermal signal are investigated. The initial phase shift φ₀ between two branches of the probe beam is found to be a key factor determining the detection sensitivity and response linearity: at a large φ₀ (≤π/2) both a high sensitivity and a good linearity can be achieved, while a high signal-to-noise ratio occurs at a small φ₀ . The steady-state photothermal phase shift φ_dc has little impact on the linearity, which, however, is greatly influenced by the range of periodic photothermal phase shift φ_ac . By introducing two coatings into a nanospace to confine the photothermal effect within and around the sample, the sensitivity can be enhanced from a few times to over 100 times. On an optimised DIC-PTM configuration and chip structure, detection limit down to 10^-3 cm^-1 (or 40 molecules in a detection volume of 0.2 fL) was achieved in a 300-nm-thick nanospace. This work paves a way for optimising the DIC-PTM and chip structure for sensitive detection of analytes in nanospaces.

Materials for Microfluidic Immunoassays: A Review

Conventional immunoassays suffer from at least one of these following limitations: long processing time, high costs, poor user-friendliness, technical complexity, poor sensitivity and specificity. Microfluidics, a technology characterized by the engineered manipulation of fluids in channels with characteristic lengthscale of tens of micrometers, has shown considerable promise for improving immunoassays that could overcome these limitations in medical diagnostics and biology research. The combination of microfluidics and immunoassay can detect biomarkers with faster assay time, reduced volumes of reagents, lower power requirements, and higher levels of integration and automation compared to traditional approaches. This review focuses on the materials-related aspects of the recent advances in microfluidics-based immunoassays for point-of-care (POC) diagnostics of biomarkers. We compare the materials for microfluidic chips fabrication in five aspects: fabrication, integration, function, modification and cost, and describe their advantages and drawbacks. In addition, we review materials for modifying antibodies to improve the performance of the reaction of immunoassay. We also review the state of the art in microfluidic immunoassays POC platforms, from the laboratory to routine clinical practice, and also commercial products in the market. Finally, we discuss the current challenges and future developments in microfluidic immunoassays.

Emerging Affinity-Based Proteomic Technologies for Large-Scale Plasma Profiling in Cardiovascular Disease

Plasma biomarkers that reflect molecular states of the cardiovascular system are central for clinical decision making. Routinely used plasma biomarkers include troponins, natriuretic peptides, and lipoprotein particles, yet interrogate only a modest subset of pathways relevant to cardiovascular disease. Systematic profiling of a larger portion of circulating plasma proteins (the plasma proteome) will provide opportunities for unbiased discovery of novel markers to improve diagnostic or predictive accuracy. In addition, proteomic profiling may inform pathophysiological understanding and point to novel therapeutic targets. Obstacles for comprehensive proteomic profiling include the immense size and structural heterogeneity of the proteome, and the broad range of abundance levels, as well. Proteome-wide, untargeted profiling can be performed in tissues and cells with tandem mass spectrometry. However, applications to plasma are limited by the need for complex preanalytical sample preparation stages limiting sample throughput. Multiplexing of targeted methods based on capture and detection of specific proteins are therefore receiving increasing attention in plasma proteomics. Immunoaffinity assays are the workhorse for measuring individual proteins but have been limited for proteomic applications by long development times, cross-reactivity preventing multiplexing, specificity issues, and incomplete sensitivity to detect proteins in the lower range of the abundance spectrum (below picograms per milliliter). Emerging technologies to address these issues include nucleotide-labeled immunoassays and aptamer reagents that can be automated for efficient multiplexing of thousands of proteins at high sample throughput, coupling of affinity capture methods to mass spectrometry for improved specificity, and ultrasensitive detection systems to measure low-abundance proteins. In addition, proteomics can now be integrated with modern genomics tools to comprehensively relate proteomic profiles to genetic variants, which may both influence binding of affinity reagents and serve to validate the target specificity of affinity assays. The application of deep quantitative proteomic profiling to large cohorts has thus become increasingly feasible with emerging affinity methods. The aims of this article are to provide the broad readership of Circulation with a timely overview of emerging methods for affinity proteomics and recent progress in cardiovascular medicine based on such methods.

Micro/extended-nano sampling interface from a living single cell

Single-cell analysis is of increasing importance in many fields, but is challenging due to the ultra-small volumes (picoliters) of single cells. Indeed, analysis of a specific analyte might require the analysis of a single molecule or several molecules. Analytical processes usually include sampling, chemical processing, and detection. Although several papers have reported chemical processing and detection methods for single cells, a sampling method compatible with maintaining the viability of a single cell during sampling has yet to be developed. Here, we propose a femtoliter sampling method from a living single cell using micro/nanofluidic device technology. The sampling of 39 fL of cytoplasm from a single human aortic endothelial cell was demonstrated and its viability after sampling was confirmed.

Microcontact printing with aminosilanes: creating biomolecule micro- and nanoarrays for multiplexed microfluidic bioassays

Aqueous based microcontact printing (μCP) to create micro- and nanoarrays of (3-aminopropyl)triethoxysilane (APTES) on glass substrates of microfluidic devices for covalent immobilization of DNA aptamers and antibodies.

Micro heat pipe device utilizing extended nanofluidics

A micro heat pipe device based on enhanced condensation on the extended nanopillars and liquid transport in the extended nanochannels.

A method of packaging molecule/cell-patterns in an open space into a glass microfluidic channel by combining pressure-based low/room temperature bonding and fluorosilane patterning

A fabrication method of a “post-molecule/cell patterned” glass microchip was developed by pressure-based bonding and patterning with a fluorosilane coupling reagent.

Single Particle Nanoplasmonic Sensing in Individual Nanofluidic Channels

Nanoplasmonics allows label-free optical sensing and spectroscopy at the single nanoparticle level by exploiting plasmonic excitations in metal nanoparticles. Nanofluidics offers exclusive possibilities for applying and controlling fluid flow and mass transport at the nanoscale and toward nanosized objects. Here, we combine these two concepts in a single device, by integrating single particle nanoplasmonic sensing with nanofluidics using advanced nanofabrication. The developed devices enable on-chip referenced parallel single particle nanoplasmonic sensing inside multiple individual nanofluidic channels with dimensions down to the 100 nm range. Beyond detailed discussion of the nanofabrication, general device characterization, and parallelized single particle plasmonic readout concepts, we demonstrate device function on two examples: (i) in situ measurements of local buffer concentrations inside a nanofluidic channel; (ii) real time binding kinetics of alkanethiol molecules to a single plasmonic nanonatenna sensor in a single nanochannel. Our concept thus provides a powerful solution for controlling mass transport to and from individual (plasmonic) nanoparticles, which in a long-term perspective offers unique opportunities for label-free detection of analyte molecules at low concentrations and for fundamental studies of fluids in extreme confinement.

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.

Note: Experimental observation of nano-channel pattern in light sheet laser interference nanolithography system

We experimentally observed nano-channel-like pattern in a light-sheet based interference nanolithography system. The optical system created nano-channel-like patterned illumination. Coherent counter-propagating light sheets are made to interfere at and near geometrical focus along the propagation z-axis. This results in the formation of nano-channel-like pattern (of size ≈ 300 nm and inter-channel periodicity of ≈337.5 nm) inside the sample due to constructive and destructive interference. In addition, the technique has the ability to generate large area patterning using larger light-sheets. Exciting applications are in the broad field of nanotechnology (nano-electronics and nano-fluidics).

Multiplexed immunosensing and kinetics monitoring in nanofluidic devices with highly enhanced target capture efficiency

Nanofluidic devices promise high reaction efficiency and fast kinetic responses due to the spatial constriction of transported biomolecules with confined molecular diffusion. However, parallel detection of multiple biomolecules, particularly proteins, in highly confined space remains challenging. This study integrates extended nanofluidics with embedded protein microarray to achieve multiplexed real-time biosensing and kinetics monitoring. Implementation of embedded standard-sized antibody microarray is attained by epoxy-silane surface modification and a room-temperature low-aspect-ratio bonding technique. An effective sample transport is achieved by electrokinetic pumping via electroosmotic flow. Through the nanoslit-based spatial confinement, the antigen-antibody binding reaction is enhanced with ∼100% efficiency and may be directly observed with fluorescence microscopy without the requirement of intermediate washing steps. The image-based data provide numerous spatially distributed reaction kinetic curves and are collectively modeled using a simple one-dimensional convection-reaction model. This study represents an integrated nanofluidic solution for real-time multiplexed immunosensing and kinetics monitoring, starting from device fabrication, protein immobilization, device bonding, sample transport, to data analysis at Péclet number less than 1.

Vapor-based micro/nano-partitioning of fluoro-functional group immobilization for long-term stable cell patterning

This study developed a simple vapor-based immobilization method using a compound with fluoro-functional-group on a cell culture surface with micro/nano scale patterns.

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.

Flexible and in situ fabrication of nanochannels with high aspect ratios and nanopillar arrays in fused silica substrates utilizing focused ion beam

We present a general methodology for the fabrication of fused silica nanochannels with high aspect ratios and nanopillar arrays using FIB milling.

Regioselective patterning of multiple SAMs and applications in surface-guided smart microfluidics

A top-down nanofabrication technology is developed to integrate multiple SAMs (self-assembled monolayers) into regioselective patterns. With ultraviolet light exposure through regioselectively hollowed hard mask, an existing SAM at designated microregions can be removed and a dissimilar kind of SAM can be regrown there. By repeating the photolithography-like process cycle, diverse kinds of SAM building blocks can be laid out as a desired pattern in one microfluidic channel. In order to ensure high quality of the surface modifications, the SAMs are vapor-phase deposited before the channel is closed by a bonding process. For the first time the technique makes it possible to integrate three or more kinds of SAMs in one microchannel. The technique is very useful for multiplex surface functionalization of microfluidic chips where different segments of a microfluidic channel need to be individually modified with different SAMs or into arrayed pattern for surface-guided fluidic properties like hydrophobicity/philicity and/or oleophobicity/philicity, etc. The technique has been well validated by experimental demonstration of various surface-directed flow-guiding functions. By modifying a microchannel surface into an arrayed pattern of multi-SAM "two-tone" stripe array, surface-guiding-induced 3D swirling flow is generated in a microfluidic channel that experimentally exhibits quick oil/water mixing and high-efficiency oil-to-water chemical extraction.

Electroviscous resistance of nanofluidic bends

Analysis tools that quantify the pressure and potential changes occurring over pressure-driven electrokinetic device elements are necessary for the design of optimal laboratory-on-a-chip devices. In this study, the resistance of a nanofluidic silica channel with negatively charged walls containing a 90^{∘} bend to the electroviscous flow of a potassium chloride salt solution is quantified in terms of two equivalent lengths using numerical analysis. One equivalent length is based on the excess pressure drop and the other on the excess potential rise. Over the entire range of simulations conducted, these equivalent lengths are relatively independent of salt concentration, flow velocity, channel size, and surface charge, remaining within the approximate ranges of 1.3-1.5 for the pressure equivalent length and 0.8-1.05 for the potential equivalent length.