The bottom-up assembly of ion-channel-mimetic nanofluidic devices and materials with two-dimensional (2D) nano-building blocks paves a straightforward way towards the real-world applications of the novel transport phenomena on a nano- or sub-nanoscale. One immediate challenge is to provide the 2D nanofluidic systems with adaptive responsibilities and asymmetric ion transport characteristics. Herein, we introduce a facile and general strategy to provide a graphene-oxide-based 2D nanofluidic system with photo-switchable ionic current rectification (ICR). The degree of ICR can be prominently enhanced upon UV irradiation and it can be perfectly retrieved under irradiation with visible light. A maximum ICR ratio of about 48 was achieved. The smart and functional nanofluidic devices have applications in energy conversion, chemical sensing, water treatment, etc.

With the advance of chemistry, materials science, and nanotechnology, significant progress has been achieved in the design and application of synthetic nanofluidic devices and materials, mimicking the gating, rectifying, and adaptive functions of biological ion channels. Fundamental physics and chemistry behind these novel transport phenomena on the nanoscale have been explored in depth on single-pore platforms. However, toward real-world applications, one major challenge is to extrapolate these single-pore devices into macroscopic materials. Recently, inspired partially by the layered microstructure of nacre, the material design and large-scale integration of artificial nanofluidic devices have stepped into a completely new stage, termed 2D nanofluidics. Unique advantages of the 2D layered materials have been found, such as facile and scalable fabrication, high flux, efficient chemical modification, tunable channel size, etc. These features enable wide applications in, for example, biomimetic ion transport manipulation, molecular sieving, water treatment, and nanofluidic energy conversion and storage. This review highlights the recent progress, current challenges, and future perspectives in this emerging research field of “2D nanofluidics”, with emphasis on the thought of bio-inspiration.

AbstractWell-developed structure–function relationships in living systems have become inspirations for the design and application of innovative materials. Building artificial nanofluidic systems for energy conversion undergoes three essential steps of structural and functional development with the uptake of separate biological inspirations. This research field started from the mimicking of the bioelectric function of electric eels, wherein a transmembrane ion concentration gradient is converted into ultrastrong electrical impulses via membrane-protein-regulated ion transport. On a small scale, solid-state nanopores are transformed from cylindrical to cone-shaped to acquire asymmetric ion-transport properties; they also further gain versatile responsiveness via chemical modification. These features mimic the rectifying and gating functions of the biological ion channels. Toward large-scale integration and real-world applications, the structure of the nanofluidic system evolves from a one-dimensional straight-channel to a two-dimensional layered membrane, inspired by the layered microstructure of nacre. The research progress, current challenges, and future perspectives of this growing field are highlighted and discussed from the viewpoint of material evolution.

Solid-state nanopores are generally considered as an indispensable element in the research field of fundamental ion transport and molecular sensing. The improvement in fabrication and chemical modification of the solid-state nanopores remains increasingly updated. During the last decades, numerous works have been reported on the nanopore-based sensing applications. More and more new analytical methods using nanopore-based devices are emerging. In this review, we highlight the recent progress on the analytical methods for the interdisciplinary and fast-growing area of nanopore research. According to the different types of the electrical readout, whether it is steady-state ionic current or transient current fluctuation, the nanopore-based sensing and analysis can be generally divided into two categories. For the first type, the electrical readout shows a stable blockade or reopening of the nanopore conductance in the presence of target analytes, termed steady-state analysis, including the conductance change, electrochemical analysis, and two-dimensional scanning and imaging. The other type is based on the transient fluctuation in the transmembrane ionic current, termed transient-state analysis, including the noise analysis, transient ion transport, and transverse tunneling current. The investigation of solid-state nanopores for chemical sensing is just in its infancy. For further research work, not only new nanopore materials and chemical modifications are needed, but also other non-electric-based sensing techniques should be developed. We will focus our future research in the framework of bio-inspired, smart, multiscale interfacial materials and extend the spirit of binary cooperative complementary nanomaterials.

Salinity difference between seawater and river water is a sustainable energy resource that catches eyes of the public and the investors in the background of energy crisis. To capture this energy, interdisciplinary efforts from chemistry, materials science, environmental science, and nanotechnology have been made to create efficient and economically viable energy conversion methods and materials. Beyond conventional membrane-based processes, technological breakthroughs in harvesting salinity gradient power from natural waters are expected to emerge from the novel fluidic transport phenomena on the nanoscale. A major challenge toward real-world applications is to extrapolate existing single-channel devices to macroscopic materials. Here, we report a membrane-scale nanofluidic device with asymmetric structure, chemical composition, and surface charge polarity, termed ionic diode membrane (IDM), for harvesting electric power from salinity gradient. The IDM comprises heterojunctions between mesoporous carbon (pore size ∼7 nm, negatively charged) and macroporous alumina (pore size ∼80 nm, positively charged). The meso-/macroporous membrane rectifies the ionic current with distinctly high ratio of ca. 450 and keeps on rectifying in high-concentration electrolytes, even in saturated solution. The selective and rectified ion transport furthermore sheds light on salinity-gradient power generation. By mixing artificial seawater and river water through the IDM, substantially high power density of up to 3.46 W/m2 is discovered, which largely outperforms some commercial ion-exchange membranes. A theoretical model based on coupled Poisson and Nernst–Planck equations is established to quantitatively explain the experimental observations and get insights into the underlying mechanism. The macroscopic and asymmetric nanofluidic structure anticipates wide potentials for sustainable power generation, water purification, and desalination.

Mechanical exfoliation of ion-track-etched two-dimensional layered materials yields nanometer-thin nanoporous sheets that can be suspended atop a silicon window to controllably fabricate single- or multi-pore nanofluidic devices.

Integrating biological components into artificial devices establishes an interface to understand and imitate the superior functionalities of the living systems. One challenge in developing biohybrid nanosystems mimicking the gating function of the biological ion channels is to enhance the gating efficiency of the man-made systems. Herein, we demonstrate a DNA supersandwich and ATP gated nanofluidic device that exhibits high ON–OFF ratios (up to 106) and a perfect electric seal at its closed state (∼GΩ). The ON–OFF ratio is distinctly higher than existing chemically modified nanofluidic gating systems. The gigaohm seal is comparable with that required in ion channel electrophysiological recording and some lipid bilayer-coated nanopore sensors. The gating function is implemented by self-assembling DNA supersandwich structures into solid-state nanochannels (open-to-closed) and their disassembly through ATP–DNA binding interactions (closed-to-open). On the basis of the reversible and all-or-none electrochemical switching properties, we further achieve the IMPLICATION logic operations within the nanofluidic structures. The present biohybrid nanofluidic device translates molecular events into electrical signals and indicates a built-in signal amplification mechanism for future nanofluidic biosensing and modular DNA computing on solid-state substrates.

We demonstrate an elaborate method to controllably fabricate ultra-thin nanopores by layer-by-layer removal of insulating few-layer mica flakes with atomic force microscopy (AFM). The fabricated nanopores are geometrically asymmetric, like an inverted quadrangular frustum pyramid. The nanopore geometry can be engineered by finely tuning the mechanical load on the AFM tip and the scanning area. Particularly noteworthy is that the nanopores can also be fabricated in suspended few-layer mica membranes on a silicon window, and may find potential use as functional components in nanofluidic devices. Open image in new window

Ion current rectification (ICR) in negatively charged conical nanopores is shown to be controlled by the electrolyte concentration gradient depending on the direction of ion diffusion. The degree of ICR is enhanced with the increasing forward concentration difference. An unusual rectification inversion is observed when the concentration gradient is reversely applied. A numerical simulation based on the coupled Poisson and Nernst–Planck (PNP) equations is proposed to solve the ion distribution and ionic flux in the charged and structurally asymmetric nanofluidic channel with diffusive ion flow. Simulation results qualitatively describe the diffusion-induced ICR behavior in conical nanopores suggested by the experimental data. The concentration-gradient-dependent ICR enhancement and inversion is attributed to the cooperation and competition between geometry-induced asymmetric ion transport and the diffusive ion flow. The present study improves our understanding of the ICR in asymmetric nanofluidic channels associated with the ion concentration difference and provides insight into the rectifying biological ion channels.

The widespread use of tiny electrical devices, from microelectromechanical systems (MEMS) to portable personal electronics, provides a new challenge in the miniaturization and integration of power supply systems. Towards this goal, we have recently demonstrated a bio-inspired nanofluidic energy harvesting system that converts salinity gradient energy from the ambient environment into sustainable electricity with single ion-selective nanopores (Adv. Funct. Mater. 2010, 20, 1339). The nanofluidic reverse electrodialysis system (NREDS) significantly improves the performance of conventional membrane-based reverse electrodialysis systems due to a higher ionic flux and a lower fluidic resistance. However, the fundamental working mechanism of the NREDS has been largely unexplored in the literature. In this work we have systematically investigated the performance of the NREDS in relation to the electrolyte type and the charge selectivity of the nanofluidic channel using both experimental and theoretical approaches. Experimental results show that the short-circuit current, the open-circuit voltage, and the resulting electric power of the NREDS are very sensitive to the ionic composition of the electrolyte solution. Through an in-depth theoretical analysis, two dominant factors that govern the charge separation and ion selectivity of the nanochannels were identified. The results prove that, with well-matched electrolyte types and nanopore charge selectivity, the harvested electric power and energy conversion efficiency can be improved by nearly two orders of magnitude.

The bottom-up assembly of ion-channel-mimetic nanofluidic devices and materials with two-dimensional (2D) nano-building blocks paves a straightforward way towards the real-world applications of the novel transport phenomena on a nano- or sub-nanoscale. One immediate challenge is to provide the 2D nanofluidic systems with adaptive responsibilities and asymmetric ion transport characteristics. Herein, we introduce a facile and general strategy to provide a graphene-oxide-based 2D nanofluidic system with photo-switchable ionic current rectification (ICR). The degree of ICR can be prominently enhanced upon UV irradiation and it can be perfectly retrieved under irradiation with visible light. A maximum ICR ratio of about 48 was achieved. The smart and functional nanofluidic devices have applications in energy conversion, chemical sensing, water treatment, etc.

Mechanical exfoliation of ion-track-etched two-dimensional layered materials yields nanometer-thin nanoporous sheets that can be suspended atop a silicon window to controllably fabricate single- or multi-pore nanofluidic devices.