On-Chip Step-Mixing in a T-Nanomixer for Liquid Chromatography in Extended-Nanochannels

Recent Advances in Microfluidics-Based Chromatography—A Mini Review

Microfluidics-based liquid chromatography is based on the miniaturization of the different types of liquid chromatography (LC) systems (e.g., affinity, adsorption, size exclusion, ion exchange) on a microchip to perform on-chip separation of different types of analytes. On-chip chromatography finds applications in genomics, proteomics, biomarker discovery, and environmental analysis. Microfluidics-based chromatography has good reproducibility and small sample consumption. However, the on-chip chromatography fabrication techniques are often more challenging to perform than conventional LC column preparation. Different research groups have attempted to develop different techniques to fabricate microfluidics-based LC systems. In this review, we will summarize the recent advances in microfluidics-based chromatography.

Chip-based ion chromatography (chip-IC) with a sensitive five-electrode conductivity detector for the simultaneous detection of multiple ions in drinking water

The emerging need for accurate, efficient, inexpensive, and multiparameter monitoring of water quality has led to interest in the miniaturization of benchtop chromatography systems. This paper reports a chip-based ion chromatography (chip-IC) system in which the microvalves, sample channel, packed column, and conductivity detector are all integrated on a polymethylmethacrylate (PMMA) chip. A laser-based bonding technique was developed to guarantee simultaneous robust sealing between the homogeneous and heterogeneous interfaces. A five-electrode-based conductivity detector was presented to improve the sensitivity for nonsuppressed anion detection. Common anions (F^-, Cl^-, NO₃ ^-, and SO₄ ^2-) were separated in less than 8 min, and a detection limit (LOD) of 0.6 mg L^-1 was achieved for SO₄ ^2-. Tap water was also analyzed using the proposed chip-IC system, and the relative deviations of the quantified concentration were less than 10% when compared with that a commercial IC system.

Single-particle-frit-based packed columns for microchip chromatographic analysis of neurotransmitters

The fabrication of effective microchip liquid chromatography (LC) systems tends to be limited by the availability of suitable chromatographic columns. Herein, we developed a glass microchip LC system in which porous single-particle silica was adopted as frits and a freeze-thaw valve was utilized to achieve sample injection without interfering with sampling. The fabrication of single-particle-frit-based packed columns did not require an additional packing channel, thus avoiding dead volumes within the channel interface that can influence chromatographic separation. Further, the length of the packed column could be adjusted using the location of single-particle frits within the column channel. The fabricated frits exhibited high mechanical strength, good permeability, and tolerance for high pressures during chromatographic separation. In particular, the developed microchip LC system was able to withstand high separation pressures of more than 5000 psi. The microchip LC system was applied to the separation of neurotransmitters. Three different monoamine neurotransmitters were completely separated within 5 min with theoretical plate numbers on the order of 100,000 plates m^-1. The microchip LC system has a potential for application in a variety of fields including environmental analysis, food safety, drug analysis, and biomedicine.

Visual and real-time imaging focusing for highly sensitive laser-induced fluorescence detection at yoctomole levels in nanocapillaries

We developed a highly sensitive laser-induced fluorescence detection system, involving visual and real-time imaging focusing instead of the use of fluorescent reagents, for the detection of analytes in nanocapillaries.

Femtoliter Gradient Elution System for Liquid Chromatography Utilizing Extended Nanofluidics

A gradient system was developed for the separation of proteins on a femtoliter scale utilizing nanofluidic channels. In the history of chromatography, miniaturization of the separation column has been important for efficient separation and downsizing of instruments. Previously, our group developed a small and highly efficient chromatography system utilizing nanofluidic channels, although a flexible design of the gradient was difficult and separation of proteins was not achieved. Here, we propose a flexible gradient system using standard HPLC pumps and an auxiliary mixer with a simple sample injection system. In contrast to our previous sample injection system using pressure balance, the system enables a femtoliter-scale sample injection which is compatible with gradient elution using HPLC pumps. The system was carefully designed, verified for sample injection and gradient elution, and finally applied to the separation of proteins from model and real samples. This femtoliter-scale, efficient separation system will contribute to omics studies at the single-cell level.

Optical nondestructive dynamic measurements of wafer-scale encapsulated nanofluidic channels

Nanofluidic channels are of great interest for DNA sequencing, chromatography, and drug delivery. However, metrology of embedded or sealed nanochannels and measurement of their fill-state have remained extremely challenging. Existing techniques have been restricted to optical microscopy, which suffers from insufficient resolution, or scanning electron microscopy, which cannot measure sealed or embedded channels without cleaving the sample. Here, we demonstrate a novel method for accurately extracting nanochannel cross-sectional dimensions and monitoring fluid filling, utilizing spectroscopic ellipsometric scatterometry, combined with rigorous electromagnetic simulations. Our technique is capable of measuring channel dimensions with better than 5-nm accuracy and assessing channel filling within seconds. The developed technique is, thus, well suited for both process monitoring of channel fabrication as well as for studying complex phenomena of fluid flow through nanochannel structures.