Ion transport in nanofluidics under external fields

Nanofluidic channels with tailored ion transport dynamics are usually used as channels for ion transport, to enable high-performance ion regulation behaviors. The rational construction of nanofluidics and the introduction of external fields are of vital significance to the advancement and development of these ion transport properties. Focusing on the recent advances of nanofluidics, in this review, various dimensional nanomaterials and their derived homogeneous/heterogeneous nanofluidics are first briefly introduced. Then we discuss the basic principles and properties of ion transport in nanofluidics. As the major part of this review, we focus on recent progress in ion transport in nanofluidics regulated by external physical fields (electric field, light, heat, pressure, etc.) and chemical fields (pH, concentration gradient, chemical reaction, etc.), and reveal the advantages and ion regulation mechanisms of each type. Moreover, the representative applications of these nanofluidic channels in sensing, ionic devices, energy conversion, and other areas are summarized. Finally, the major challenges that need to be addressed in this research field and the future perspective of nanofluidics development and practical applications are briefly illustrated.

Switchable biomimetic nanochannels for on-demand SO2 detection by light-controlled photochromism

In contrast to the conventional passive reaction to analytes, here, we create a proof-of-concept nanochannel system capable of on-demand recognition of the target to achieve an unbiased response. Inspired by light-activatable biological channelrhodopsin-2, photochromic spiropyran/anodic aluminium oxide nanochannel sensors are constructed to realize a light-controlled inert/active-switchable response to SO₂ by ionic transport behaviour. We find that light can finely regulate the reactivity of the nanochannels for the on-demand detection of SO₂. Pristine spiropyran/anodic aluminium oxide nanochannels are not reactive to SO₂. After ultraviolet irradiation of the nanochannels, spiropyran isomerizes to merocyanine with a carbon‒carbon double bond nucleophilic site, which can react with SO₂ to generate a new hydrophilic adduct. Benefiting from increasing asymmetric wettability, the proposed device exhibits a robust photoactivated detection performance in SO₂ detection in the range from 10 nM to 1 mM achieved by monitoring the rectified current.

Photoresponsive Solid Nanochannels Membranes: Design and Applications

Light stimuli have notable advantages over other environmental stimuli, such as more precise spatial and temporal regulation, and the ability to serve as an energy source to power the system. In nature, photoresponsive nanochannels are important components of organisms, with examples including the rhodopsin channels in optic nerve cells and photoresponsive protein channels in the photosynthesis system of plants. Inspired by biological channels, scientists have constructed various photoresponsive, smart solid-state nanochannels membranes for a range of applications. In this review, the methods and applications of photosensitive nanochannels membranes are summarized. The authors believe that this review will inspire researchers to further develop multifunctional artificial nanochannels for applications in the fields of biosensors, stimuli-responsive smart devices, and nanofluidic devices, among others.

Controllable ion transport induced by pH gradient in a thermally crosslinked submicrochannel heterogeneous membrane

Solid-state nanochannels have attracted considerable attention for their similar ion transport properties to biological ion channels. The construction of porous ion channels with good stability at the submicro/micrometer scale is very beneficial to develop large-area ion channel devices. In this manuscript, based on in-situ thermal crosslinking of a small organic molecule containing triphenylamine and styrene groups, we construct a heterogeneous membrane with asymmetrical charge and wettability on cylindrical anodic aluminum oxide (AAO) channels (D ≈ 319 nm). This heterogeneous membrane has typical ion current rectification characteristics with a high rectification ratio of 36.9 and good stability. This work provides an effective strategy for the construction of submicrochannel heterogeneous membranes and also broadens the application range of bionic ion channels.

A high rectification ratio nanofluidic diode induced by an “ion pool”

Inspired by functionalized biological ion channels, artificial channels were prepared to mimic the natural ones. The key concept behind the rectifying phenomena in nanochannels is the construction of asymmetric restrictive nanochannels. Here, we prepared nanoporous oxidized polyvinyl alcohol (PVA) and WO₃ composite coatings on hourglass-shaped anodic aluminum oxide (AAO) nanochannel surfaces. Accordingly, a special “ion pool” is formed between the homogeneous junction in the middle of the AAO and the nanoporous PVA/WO₃ film-covered AAO surface and its two ends are greatly nano-confined. Ion enrichment and ion depletion occur in the “ion pool” and are dependant on the applied voltage polarity. A rectification ratio of 458, which is in accordance with the highest value found in previous reports, was obtained from the cooperative effects of the two small open ends of the “ion pool”. Furthermore, this value is enhanced to about 2000 under constant voltage. An excellent pH-sensitive rectification property, with a single rectification direction from acidic to basic conditions, has also been demonstrated.

Nanoporous polyvinyl alcohol (PVA)/WO₃ composite coatings were prepared onto the hourglass-shaped AAO nanochannels surface, and an “ion pool” is formed. A rectification ratio of 2000 was obtained with constant voltage enhancement.

Selectively Enhanced Ion Transport in Graphene Oxide Membrane/PET Conical Nanopore System

Graphene oxide (GO) has become a promising 2D material in many areas, such as gas separation, seawater desalination, antibacterial materials, and so on because of its abundant oxygen-containing functional groups and excellent dispersibility in various solvents. The graphene oxide membrane (GOM), a laminar and channel-rich structure assembled by stacked GO nanosheets, served as a kind of precise and ultrafast separation material has attracted widespread attention in membrane separation field. To break the trade-off between ion permeability and ion selectivity of separation membrane based on GOM, GOM/conical nanopore system is obtained by spin-coating ultrathin GOM on PET conical nanopore, which possesses ion rectification property. Comparing to pure PET conical nanopore, the existence of GOM not only enhances the cation conductance but also makes the ion rectification ratio increase from 4.6 to 238.0 in KCl solution. Assisted by COMSOL simulation, it is proved that the GOM can absorb large amount of cations and act as cation source to improve the ion selectivity and rectification effect of GOM/conical nanopore system. Finally, the chemical stability of GOM/conical nanopore is also investigated and the corresponding results reveal that the GOM/conical nanopore system can perform the ion rectification behavior in a wider pH range than pure PET conical nanopore. The presented findings demonstrate the great potential applications of GOM/conical nanopore system in ionic logic circuits and sensor systems.

An Effective Dark-Vis-UV Ternary Biomimetic Switching Based on N3/Spiropyran-Modified Nanochannels

Many natural photomodulated nanochannels are investigated and are crucial for biological activity. Biomimetic nanochannels with a bistable conductance state under light stimulus are demonstrated. In this system, two molecules, cis-bis-(4,4'-dicarboxy-2,2'-bipyridine) dithiocyanato ruthenium(II) (N3) and spiropyran 1'-(3-carboxypropyl)-3',3'-dimethyl-6-nitro-spiro[2H-1]benzopyran-2,2'-indoline (SP-COOH), each with unique photoresponsive properties, are modified in alumina nanochannels. The two segments of the hourglass-shaped alumina nanochannels are designated to graft a certain molecule. Under ultraviolet (UV) or visible light (vis) irradiation, electrons belonging to N3 are excited, resulting in negatively charged surfaces on the sides of nanochannels modified with N3 molecules. Only under UV stimulus, the conformation change of the SP-COOH molecules leads to positively charged surfaces of nanochannels in the SP-COOH occupied sides. Benefiting from the joint effect of N3 and SP-COOH, low, medium, and high (i.e., "0," "1," and "2") ternary levels of ion conductance are established under the dark-vis-UV alternate stimuli. The multistage current switching containing "0-1-2-0" and "0-1-2-1-0" procedures is stable and robust. Additionally, the diode-like ion transport behavior of the nanochannels could be exploited to support a multivalued logical gating with the management of light signals.

Bioinspired smart asymmetric nanochannel membranes

Bioinspired smart asymmetric nanochannel membranes (BSANM) have been explored extensively to achieve the delicate ionic transport functions comparable to those of living organisms. The abiotic system exhibits superior stability and robustness, allowing for promising applications in many fields. In view of the abundance of research concerning BSANM in the past decade, herein, we present a systematic overview of the development of the state-of-the-art BSANM system. The discussion is focused on the construction methodologies based on raw materials with diverse dimensions (i.e. 0D, 1D, 2D, and bulk). A generic strategy for the design and construction of the BSANM system is proposed first and put into context with recent developments from homogeneous to heterogeneous nanochannel membranes. Then, the basic properties of the BSANM are introduced including selectivity, gating, and rectification, which are associated with the particular chemical and physical structures. Moreover, we summarized the practical applications of BSANM in energy conversion, biochemical sensing and other areas. In the end, some personal opinions on the future development of the BSANM are briefly illustrated. This review covers most of the related literature reported since 2010 and is intended to build up a broad and deep knowledge base that can provide a solid information source for the scientific community.