A nanofluidic device that allows the fabrication of nanoscale gas-liquid interfaces. Credit: Yan Xu, Osaka Prefecture University
A unique zone is formed when liquid meets gas. Molecules can change from one state of the gas to another in nature. They can combine in different ways to achieve desired or unfavorable ends. Gas-liquid interfaces are everywhere in engineering and nature. They can be used to escape heat from coffee or increase molecular concentrations within chemical solutions. However, there are not many tools that can precisely control these gas-liquid interfaces. This limits their potential applications.
Researchers at Osaka Prefecture University developed the first nanoscale gas-liquid interface. Their design and experimental results were published in Nano Letters on October 14.
Yan Xu, an associate professor of chemical engineering at Osaka Prefecture University's Graduate School of Engineering, stated that gas-liquid interfaces are important in many chemical and biological processes, regardless of whether they were engineered or natural. Although nanoscale gas-liquid interfaces can be randomly generated in porous membranes or carbon nanotubes, it is difficult to fabricate controllable nanoscale versions. Nanofluidic channels are too small for conventional surface control methods.
Xu stated that fluidic devices allow researchers to capture target molecules and study specific properties. They also enable force interactions through nanoscale channels with precisely controlled geometry.
The microfluidic devices have channels that are about 1,000 times larger than the ones in nanofluidics devices. However, the channel's surface can be modified to attract or reject certain molecules.
Credit: Osaka Prefecture University
Xu stated that although such surface modification is used frequently for microfluidic channel, its application for nanofluidic channels has almost never been explored.
Although microfluidic devices are made from many materials, they require a glass substrate. Xu says that glass's properties such as optical transparency and thermal stability, and mechanical robustness make it a good material for nanofluidics.
Glass can be made hydrophobic. This is a surface modification that stops molecules from bonding with the glass. Glass nanochannels were also created by the researchers. These are approximately the same width as 1/1,000 sheets of paper and contain hydrophilic gold nanopatterns that attract liquid molecules to the nanochannels. Researchers developed a technique called Nano-in-Nano integration that allows for the precise patterning and placement of smaller functional nanopatterns in tiny nanofluidic channels.
The nanofluidic device that was fabricated is slightly larger than a postal stamp, and not much thicker. In the middle, the nanochannels of different sizes, which are invisible to the eye, are sandwiched between two liquid introduction systems shaped like horseshoes.
The researchers tested the hydrophobic treatment by pushing water into the larger, one-dimensional (1D), nanochannels. The water that is not treated channels will move along the nanochannels with the same force as plants to distribute water from their roots to their leaves.
Credit: Osaka Prefecture University
"We observed that water flow stopped at entry of the 2D nanofluidic channel up to an external pressure 400kPa," Xu stated. This is roughly the force equivalent to the average water pressure at a home faucet. The researchers discovered that water could breach the nanofluidic channels at higher pressures.
After the test confirmed the channel's hydrophobic engineering, the researchers filled the channels with ethanol aqueous solution at elevated pressure. Next, they used air to extract liquid from the left channel. This created a gas-liquid interface. The interface was able to travel at zero pressure to the 2D nanochannel entries and stopped at the hydrophilic, gold nanopatterns. This held for more than an hour. The interface can be moved along the nanofluidic channels with external pressure.
Researchers confirmed the stability of the nanoscale liquid-gas interface. They also tested the ability of concentration molecules of interest within the nanoscale interface.
Researchers plan to develop more chip-based diagnostic and analytical devices that can detect, concentrate, and separate biological matter from very small samples.
"Nanoscale gas and liquid interfaces fabricated using hydrophilic and nanopatterned nanofluidic channel offers the possibility to precisely enrich target molecules at a well defined nanoscale space, revolutionarily affecting a variety of chemical and physical processes and applications in future," Xu stated.
More information: Hiroto Kawagishi et al, Fabrication of Nanoscale GasLiquid Interfaces in Hydrophilic/Hydrophobic Nanopatterned Nanofluidic Channels, Nano Letters (2021). Journal information: Nano Letters Hiroto Kawagishi et al, Fabrication of Nanoscale GasLiquid Interfaces in Hydrophilic/Hydrophobic Nanopatterned Nanofluidic Channels,(2021). DOI: 10.1021/acs.nanolett.1c02871
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