Application of Graphene Oxide Nano-Channels: Comparison
Please note this is a comparison between Version 2 by Jason Zhu and Version 1 by Lei Fan.

Ion and water transport at the Angstrom/Nano scale has always been one of the focuses of experimental and theoretical research. In particular, the surface properties of the angstrom channel and the solid-liquid interface interaction will play a decisive role in ion and water transport when the channel size is small to molecular or angstrom level.

  • graphene derivatives
  • angstrom-scale channels
  • ion and water molecular transport

1. Introduction

When people see civil engineering, environmental engineering and biomedical engineering side by side, they could be puzzled. These disciplines are quite different.
In recent years, the idea of using GO channels under various conditions and for a variety of applications has engaged the minds of many researchers [1]. Nanopores/channels are ubiquitous in biological systems. Furthermore, the transport of ion and water molecules in the retina, nerves, muscles and other living systems plays a key role in life activities [2]. Inspired by biological GO nano-channels in cell membranes, artificial nanopores/nano-channels have been successfully constructed. It is used to transport ions and water molecules directionally by adjusting the interface interaction [3]. Studying the basic principle of ion and water molecule transport in GO nanopores/nano-channels will help to further improve the performance of different artificial materials. Based on these principles, GO nanopore/nano-channels have a wide range of applications, such as oil-water separation, seawater desalination, controlled drug delivery, salt difference power generation, pressure power generation, concrete lifetime, DNA sequencing, environmental monitoring and so on [4,5][4][5].
At the nanometer scale, the interface effect and intermolecular force are prominent, and the free movement of water molecules is limited. Therefore, the GO nano-channels show the characteristics of layering and ordering [6].
It is worth noting that the variable arrangement of oxygen-containing functional groups, the complex stoichiometric ratio, the various spatial configurations and the various preparation methods in GO will lead to the uncertainty of the interface structure and interface interaction of GO and the transport behavior of ion and water molecule in GO nano-channels.

2. Application of GO Nano-Channels in Civil Engineering

Calcium silicate hydrate (C-S-H) is one of the main hydration products of concrete materials, accounting for about 60–70% of the total volume of hydration products. It is one of the main sources of concrete strength and has a significant impact on the durability of concrete [65][7]. C-S-H gel has a layered structure, and there are a lot of micropores between layers, including small gel pores (5~100 Å) and capillary pores (>100 Å) [66][8].
Hou DS et al. [67][9] used molecular dynamics to study the capillary transport of Na+, Cl and water in C-S-H nanochannel.
There is a strong correlation between surface calcium atoms, non-bridging oxygen, water and ions. The hydration structure of trapped ions and water in a C-S-H nanochannel has changed significantly. Moreover, these micropores provide conditions for the transmission of corrosive ions, leading to the deterioration of the internal components of concrete, and adversely affecting the service life and mechanical properties of concrete materials [68,69][10][11].
Inspired by the unique microstructure of nature, GO and C-S-H were reassembled into a layered structure with nano-scale gaps [70][12]. The confined space between the layers formed after recombination can be used as a 2D channel for the transmission of molecules and ions [71,72][13][14].
M. Wang et al. [73][15] proposed a 3D mechanism model of the GO/C-S-H interface by regulating with functional groups. It is believed that -COOH at the edge of GO and Ca2+ of hydration product (Ca(OH)2) will form a 3D network structure of COO-Ca-OOC. At the same time, the hydration products are further inserted into the 3D nanostructure to compact the microstructure. As a result, the interface of cement-based composites can be regulated by GO [73][15]. D. S. Hou et al. [74][16] used experiments and molecular dynamics of the reaction field to study the interface structure and interface interaction mechanism between GO and C-S-H. It was found that the interfacial bonding between GO and C-S-H gel, and the instability of atoms in the interfacial region, are the reasons for the poor mechanical properties [74][16]. In addition, another work [75][17] used the molecular dynamics method to study water and ion transport in the nano-channels of the C-S-H matrix embedded with GO sheets. On the one hand, the transport rate and diffusivity of fluid largely depend on the types of functional groups in GO. Due to the invasion of ions and water molecules, the van der Waals interaction between graphene sheets and C-S-H gel is obviously weakened. On the other hand, the hydroxyl and carboxyl groups in the GO sheet provide enough oxygen sites to accept H bonds and combine with adjacent sodium ions, thus fixing water molecules and ions on the surface of GO. GO-COOH adsorbed on C-S-H further blocks the connectivity of the transport channel and “condenses” the water and ions in the entrance area of the gel hole [75][17].
1D carbon nanotubes and 2D nanostructures form nano-and sub-nano-scale ion channels with uniform size, and the internal microstructure and surface chemical characteristics of pores are more controllable. It can provide a reference for bionic multi-scale channels and other similar fields and has a clear leading role.

3. Application of GO Nano-Channels in Biomedical Engineering

Biological channels play an important role in life activities [76][18]. The study of the ion channel mechanism is of great significance to biophysics, bioinformatics and biomedicine. In order to realize the controllable transport of ions in GO nano-channels, the construction of artificial ion channels with various functions has become a research hotspot [77][19].
The biological channel is a pore-forming membrane protein. The biological channel plays an important role in complex life processes by controlling ion transport inside and outside the cell. However, natural channel proteins are extremely unstable. This limits its application as an in vitro experimental material. G. X. Li et al. [78][20] constructed a light-regulated ionic gate based on the design of a GO-biomimetic DNA-nano-channel architecture. Their research indicated that the single- and double-stranded DNA formed under the irradiation of alternating light has a different binding capacity with GO. Based on this, there are two conditions, that is, the adhesion and peeling of GO to the surface of the anodized aluminum film. It realizes the reversible conversion of ion gating between “off” and “on”. In addition, due to the high barrier property of GO and the extremely small diameter of the channel in the barrier layer, the ion gate constructed by them has an excellent switching efficiency and reversible ability of ion transport switch under alternating light irradiation. In addition, ion gating has an excellent switching efficiency and reversible ability of ion transport switch under alternating light irradiation. This can be attributed to the high barrier property of GO, and that the diameter of the channel in the barrier layer is extremely small [78][20].
The biological ion channel of GO also plays an important role in many physiological processes such as material transfer, energy conversion and signal transmission. Signals can be transmitted from nerve to brain in the process of sight, smell, hearing and touch based on biological ion channels. These functions are highly dependent on the high-speed ion transport of selective biological ion channels (107 ions per second per channel) [79][21]. From the point of view of classical thermodynamics, the material transport of chemically selective nano-channels could be very slow. L. Jiang et al. [79][21] put forward the concept of “quantum confined superfluid” for the first time. They pointed out that the ordered superfluidity of ions and molecules in confined pores is a kind of “quantum tunneling fluid effect”. The “tunneling distance” is consistent with the period of quantum-confined superfluidity. In addition, the water in the biological channel is arranged in order by molecular chain. It indicates that the ultrafast transport of ions and molecules is carried out in a quantized way, namely “quantum confined superfluid”, such as the rapid transport of substances (106 ions per second) in artificial ion channels and water channels [79][21].
Although a series of GO bionic nano-channels have been developed, a very complicated problem is to construct GO bionic nano-channels with stable, reversible, durable and fast transmission properties. From the experimental and theoretical point of view, it is still necessary to further study the transport mechanism of matter in GO bionic nano-channels, so as to make GO bionic nano-channels more controllable and intelligent.

4. Application of GO Nano-Channels in Environmental Engineering

Although three-quarters of the earth’s area is covered by water, there is very little water that can be safely drunk [80][22]. Even the Millennium Development Plan of the United Nations put the problem of “solving the shortage of drinking water” on the schedule. In recent years, the vigorous development of 2D materials has opened up a new direction of membrane separation for seawater desalination, and its advantages of high efficiency and low energy consumption have attracted wide attention [81][23].
71% of the earth is covered by water, but only 0.01% of the total water resources can be directly used by human beings [82][24]. According to statistics, about 663 million people in the world live in areas without a drinking water supply. It is estimated that the total population of the world will increase to 9 billion by 2050. At that time, mankind will encounter a huge crisis in the use of water resources, so it is urgent to solve the problem of freshwater shortage [83][25].
R. R. Nair and A. K. Geim [24][26] put forward the concept of molecular screening by GO film for the first time. They prepared sub-micron GO thin film by the spin coating method, and tested the permeability of various liquids and gases to GO thin film. It was found that the GO membrane showed a complete barrier to some organic solvents and gases (Ar, He, etc.), but almost had no obstacle to the permeation of water vapor. It can be attributed to the fact that hydrophilic FGs help attract water into the interlayer channels inside the GO membrane. At the same time, the friction-free super lubrication of water molecules in the channel enables water molecules to flow rapidly between GO layers [24][26]. The research proves that GO, a selective permeation characteristic, can be introduced into the field of seawater desalination. In 2014, R. K. Joshi et al. [25][27] explored the separation effect of the GO membrane on solutions containing different solutes through the experiment of solution permeation in U-shaped tubes separated by the GO membrane. It found that the shielding effect of GO film on solute in solution depends on the relationship between the size of the solute particles and the size of the GO channel [25][27]. After that, P. Z. Sun [84][28] discussed the permeation process of various salt ions in the GO membrane in more detail. They also confirmed that the GO membrane has a high screening effect on ions. For ions with different valence states, GO films will be separated at different degrees according to the charge amount [84][28]. These results further prove that GO has a special 2D channel, which can be used to selectively screen ions through the confinement effect of the channel. Therefore, it can be effectively used in seawater desalination by membrane separation.


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