What Are Nanochannels?
A nanochannel is a channel with a very fine nano-scale surface, which is formed by the coupling of ions in an EDL. Because of the observable motion of these ions in the channels, nanochannels are known to be capable of performing a variety of tasks in osmotic energy conversion and biosensing.
Characterization of the nanochannels
For a more accurate and complete understanding of nanochannels, it is essential to carry out characterization of their properties. A variety of techniques are used to examine the thermal and flow characteristics of nanochannels. This includes molecular dynamics method, which has been applied to study the thermal and flow characteristics of nanochannels.
In the study of dual gating, PA nanochannels were used as the substrate for the fabrication of a pair of XOR logical gates. These gates are capable of monitoring a variety of pollutants such as sulfur dioxide. As the gates are fabricated on a PA nanochannel, recyclability of the system is assured.
A fluorescein derivative named FITC was deposited onto the surface of the nanochannels. Its characteristic emission peak was visible at 518 nm. The resulting green fluorescence was detected at cross-sectional level.
In a proof of concept experiment, the nanochannels were tested for their conductance as a function of ion concentration. The results showed that the nanochannel conductance varies only slightly with the ion concentration. Therefore, it is possible to detect the current in nanochannels using a low voltage.
Two nanopore channels at Z = 2 and 5 mm were also characterized by FIB. Their mean roughness was measured as 0.188 nm by AFM. The channels exhibited multi-cavity cascade shape. Anodic bonding of the nanochannels and the nanoelectrodes was carried out.
The energy distribution shape of the nanochannels also affects the morphology of the system. In particular, the Bessel beam interaction induced a rise in the height of the interaction region, which in turn causes the material removal. Moreover, the energy deposition in the material can be distinguished from the Bessel beam interaction.
Finally, the asymmetric nature of the PA nanochannels prompted a rectifying behavior. In fact, it was found that the conductance in nanochannels was higher in the reverse direction.
The asymmetric nature of the nanochannels was further studied by performing a number of evaporation experiments. Results indicate that the asymmetric nature of the PA nanochannels increases its wettability. This enhances the pressure drop and boosts convective heat transfer capacity.
Observable motion of ions in nanochannels is a result of the complex coupling between these ions and the EDL
A key mechanism for controlling ion current is induced-charge electrokinetic (ICEK) phenomena. ICEK involves the transfer of ions from one metal-dielectric nanochannel to another at an internal bipolar junction. An ICEK-based bipolar junction enables high degree of freedom ionic current control, in addition to the ability to utilize flexible on-chip platforms. This paper presents a physical modeling of the ICEK mechanism and the application of a novel circuit model to predict ion concentration in nanochannels. ICEK can be applied to nanochannels in order to generate an ion-enriched or depleted zone in the center of a conducting wall.
The ion-enriched or depleted zones in the center of the conducting wall are formed from the interaction of positive ions and field-induced anions, and the resulting ion concentration is related to the in-situ electrical potential in the conductive medium. The ion-enrichment/depletion zones can be generated by an internal or external DC bias.
In a metallic nanochannel, a bipolar induced double layer (IDZ) is induced when a positive DC bias is applied to the source terminal. This leads to an enhanced degree of ICP in the regime of ICEK. However, the IDZ is low in conductivity, which reduces the output electric current towards the downstream D terminal.
The effect of the external DC bias on the ion concentration distribution at the metallic end is also investigated. Ideally, the metallic end is polarized. It is a requirement for ICEK-enabled ionic current control.
The forward and backward ionic currents in the "on" and "off" working modes increase as the DC bias increases. As the negative DC bias is greater than the negative voltage on the D terminal, the nanofluidic ion diode is in the "off" working mode.
Although this study uses a semiconductor nanochannel, it can be applied to other media with varying ion concentrations. Sub-100 nm wide nanochannels can be fabricated on glass or alumina, and the wall properties can be altered by atomic layer deposition or low-temperature synthesis. ICEK-based nanochannels can be used to develop a high-performance osmotic energy converter.
The ion-enriched or demulcent zones in the midchannel are formed when ions are transported through a nanochannel containing a metallic-dielectric ion-selective medium. These channels are also referred to as nanogap devices.
FEOS fulfill the key capabilities of nanochannels in osmotic energy conversion and biosensing
In order to address the challenge of high-efficiency ion-permselective membranes, a nanochannel-based strategy was implemented. The result is SPEEK membrane-based osmotic power generators with ultrahigh output power density. This method is based on two-dimensional covalent organic frameworks (COFs) and vertically aligned nanofluidic channels.
Membrane fabrication is controlled and mechanical stability is not sacrificed. This method provides a promising strategy for large-scale osmotic energy conversion. Unlike conventional membranes, FEOS have excellent ion-selectivity, and can accommodate probes with diameters greater than the nanochannel diameter. They also improve ion selectivity by using an asymmetric structure.
A thin film with hexagonally packed cylinders is self-assembled by BCP, a UV-cleavable o-nitrobenzyl ester linker. It is then cured with UV treatment, which results in in situ generation of carboxylate groups. These groups are then blocked by a layer of chalcone-containing h-PEI.
Nanopores with tunable opening diameters mimic biological calcium-selective channels. They facilitate ionic transport and provide a connection between changes in surface charge properties caused by calcium ions.
Nanochannels are also effective in osmotic energy conversion. The ionic Seebeck coefficient, which is a function of the concentration gradient, decreases as the ion concentration increases. However, the ionic conductivity increases when the concentration increases. Also, the nanochannels are useful for ion screening and selective transportation.
Several nanochannel-based materials have been designed. For example, a single-layer MoS2 nanopore membrane and ionogels with cationic doping are examples. Moreover, a variety of composite membranes such as MXene/ANF and SNF/AAO have been developed. But, multipore membranes with high pore density are still a challenge to fabricate.
In addition, a novel nanochannel-based strategy to enhance energy conversion ability is being investigated. Nanochannel arrays, with a sub-one-nm pore, are formed by a bottom-up approach. While the ionic Seebeck coefficient drops with the nanopore size, the ion conductivity increases because of the overlapped EDL.
Simulation studies have also shown that the selectivity of nanochannels can be improved by using space charges. The ionic Seebeck coefficient of nanochannels with space charges is -0.117, which is higher than that of nanochannels with surface charges. Consequently, the ion conductivity is improved, resulting in better osmotic power conversion performance.
Problems with traditional nanochannel-systems
Traditional nanochannel-systems suffer from many problems. In fact, their properties are far from well understood. For example, FE located deep inside the nanochannel are subject to limited testing methods, and they have an unknown chemical and physical property. It is important to understand the properties of FE so that FEOS can be designed more efficiently. Fortunately, new nanochannel-systems are being developed, which allow for explicit relationships between FE properties and function. By implementing a novel approach, the physicochemical properties of FEOS can be characterized, and the effects of FEOS on ion transport can be accurately predicted. With the development of the new nanochannel-system, the problems of previous systems can be resolved.
Video: Learning about Nanochannels