Ultra-rapid separation of an angiotensin mixture in nanochannels using shear-driven chromatography

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

The progress of nanotechnology has developed nanofluidic devices utilizing nanochannels with a width and/or depth of sub-100 nm (10¹ nm channels), and several experiments have been implemented in ultra-small spaces comparable to DNAs and proteins. However, current experiments utilizing 10¹ nm channels focus on a single function or operation; integration of multiple analytical operations into 10¹ nm channels using nanofluidic circuits and fluidic control has yet to be realized despite the advantage of nanochannels. Herein, we report the establishment of a label-free molecule detection method for 10¹ nm channels and demonstration of sequential analytical processes using integrated nanofluidic devices. Our absorption-based detection method called photothermal optical diffraction (POD) enables non-invasive label-free molecule detection in 10¹ nm channels for the first time, and the limit of detection (LOD) of 1.8 μM is achieved in 70 nm wide and deep nanochannels, which corresponds to 7.5 molecules in the detection volume of 7 aL. As a demonstration of sampling in 10¹ nm channels, aL-fL volumetric sampling is performed using 90 nm deep cross-shaped nanochannels and pressure-driven fluidic control from three directions. Finally, the POD and volumetric sampling are combined with nanochannel chromatography, and separation analysis in 10¹ nm channels is demonstrated. The experimental results reported in this paper will contribute to the advances in 10¹ nm fluidic devices which have the potential to provide a novel platform for chemical/biological analyses.

Salient Sub-Second Separations

Sub-second liquid chromatography in very short packed beds is demonstrated as a broad proof of concept for chiral, achiral, and HILIC separations of biologically important molecules. Superficially porous particles (SPP, 2.7 μm) of different surface chemistries, namely, teicoplanin, cyclofructan, silica, and quinine, were packed in 0.5-cm-long columns for separating different classes of compounds. Several issues must be addressed to obtain the maximum performance of 0.5 cm columns with reduced plate heights of 2.6 to 3.0. Modified UHPLC hardware can be used to obtain sub-second separations provided extra-column dispersion is minimized and sufficient data acquisition rates are used. Further, hardware improvements will be needed to take full advantage of faster separations. The utility of power transform, which is already employed in certain chromatography detectors, is shown to be advantageous for sub-second chromatography. This approach could prove to be beneficial in fast screening and two-dimensional liquid chromatography.

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.

A novel fluidic control method for nanofluidics by solvent-solvent interaction in a hybrid chip

The fluidic control method is a fundamental technology for the development of nanofluidics. In this report, an organic phase was driven to flow inside the nanochannel because of its dissolution into an aqueous phase. With selective modification, a stable organic/aqueous interface was generated at the junction of the micro/nanochannels in a hybrid chip. The aqueous phase was kept flowing in the microchannel, and the organic phase in the nanochannel dissolved into the aqueous phase through the interface and produced a flow inside the nanochannel. This method is simple, easy to control and requires no specific equipment. Importantly, the flow is driven by the surface tension in a controllable manner, which will not be affected by the depth of the nanochannel. This method can be a useful alternative to the present fluidic control methods in nanofluidics.

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.

Highly efficient and ultra-small volume separation by pressure-driven liquid chromatography in extended nanochannels

The rapidly developing interest in nanofluidic analysis, which is used to examine liquids ranging in amounts from the attoliter to the femtoliter scale, correlates with the recent interest in decreased sample amounts, such as in the field of single-cell analysis. For general nanofluidic analysis, the fact that a pressure-driven flow does not limit the choice of solvents (aqueous or organic) is important. This study shows the first pressure-driven liquid chromatography technique that enables separation of atto- to femtoliter sample volumes, with a high separation efficiency within a few seconds. The apparent diffusion coefficient measurement of the unretentive sample suggests that there is no increase in the viscosity of toluene in the extended nanospace, unlike in aqueous solvents. Evaluation of the normal phase separation, therefore, should involve only the examination of the effect of the small size of the extended nanospace. Compared to a conventionally packed high-performance liquid chromatography column, the separation here results in a faster separation (4 s) by 2 orders of magnitude, a smaller injection volume (10(0) fL) by 9 orders, and a higher separation efficiency (440,000 plates/m) by 1 order. Moreover, the separation behavior agrees with the theory showing that this high efficiency was due to the small and controlled size of the separation channel, where the diffusion through the channel depth direction is fast enough to be neglected. Our chip-based platform should allow direct and real-time analysis or screening of ultralow volume of sample.

Extended nanospace chemical systems on a chip for new analytical technology

Integration of chemical processes on a microchemical chip has gained much attention in the past decade, and the basic concepts of micro-integration and the supporting technologies have been intensively developed. As a result, many analytical and chemical synthesis applications were demonstrated. The superior performances were verified including shortening analysis time, decrease of sample and reagent volume, and easy chemical operations. Now, the micro-technologies are moving toward practical applications by establishing the systems in which the microchemical chip works as chemical central processing unit. Recently, as a new research field, integration is further proceeding to the 10(1)-10(3) nm scale, which we call extended nanospace. The extended nanospace locates the gap between the targets of conventional nanotechnology (10(0)-10(1) nm) and micro-technology (>1 μm), and the fluidics and chemistry have not been explored well due to a lack of fundamental technologies. For these purposes, many methodologies were established in recent years. Unique liquid properties were reported, which were quite different from those in microspace. Some properties can be expected by considering the characteristics of microspace and the downscaling, and the others are unexpected or are difficult to predict. These properties enabled new chemical operations which will be quite important as the next analytical technologies. Now, chemistry and fluidics in the extended nanospace are forming a new research field. In this review, we survey the fundamental technologies for extended nanospace researches and introduce several unique liquid properties. Finally, unique chemical operations are also illustrated leading to new analytical operations.

Extended-nano fluidic systems for analytical and chemical technologies

Recently, integrated chemical systems have been further downscaled to the 10(1)-10(3) nm scale, which we call extended-nano space. The extended-nano space is a transient space from single molecules to bulk condensed phase, and fluidics and chemistry have not been explored. One of the reasons is the lack of research tools for the extended-nano space, because the space locates the gap between the conventional nanotechnology (10(0)-10(1) nm) and microtechnology (>1 microm). For these purposes, basic methodologies were developed such as nanofabrication, fluidic control, detection methods, and surface modification methods. Especially, fluidic control is one of the important methods. By utilizing the methodologies, new specific phenomena in fluidics and chemistry were reported, and the new phenomena are increasingly applied to unique applications. Microfluidic technologies are now entering new research phase combined with the nanofluidic technologies. In this review, we mainly focus on pressure-driven or shear-driven extended-nano fluidic systems and illustrate the basic nanofluidics and the representative applications.

Nanofluidics in lab-on-a-chip devices

As the field of nanofluidics matures, fundamental discoveries are being applied to lab-on-a-chip analyses. The unique behavior of matter at the nanoscale is adding new functionality to devices that integrate nanopores or nanochannels. (To listen to a podcast about this feature, please go to the Analytical Chemistry website at pubs.acs.org/journal/ancham.).

Electric field control and analyte transport in Si/SiO2 fluidic nanochannels

This article presents an analysis of the electric field distribution and current transport in fluidic nanochannels fabricated by etching of a silicon chip. The channels were overcoated by a SiO2 layer. The analysis accounts for the current leaks across the SiO2 layer into the channel walls. Suitable voltage biasing of the Si substrate allows eliminating of the leaks or using them to modify the potential distribution of the fluid. Shaping the potential in the fluid can be utilized for solute focusing and separations in fluidic nanochannels.

Electrokinetic transport and separations in fluidic nanochannels

This article presents a summary of theory, experimental studies, and results for the electrokinetic transport in small fluidic nanochannels. The main focus is on the effect of the electric double layer on the EOF, electric current, and electrophoresis of charged analytes. The double layer thickness can be of the same order as the width of the nanochannels, which has an impact on the transport by shaping the fluid velocity profile, local distributions of the electrolytes, and charged analytes. Our theoretical consideration is limited to continuum analysis where the equations of classical hydrodynamics and electrodynamics still apply. We show that small channels may lead to qualitatively new effects like selective ionic transport based on charge number as well as different modes for molecular separation. These new possibilities together with the rapid development of nanofabrication capabilities lead to an extensive experimental effort to utilize nanochannels for a variety of applications, which are also discussed and analyzed in this review.