A surface charge governed nanofluidic diode based on a single polydimethylsiloxane (PDMS) nanochannel

Maximizing blue energy: the role of ion partitioning in nanochannel systems

This study describes a numerical analysis on blue energy generation using a charged nanochannel with an integrated pH-sensitive polyelectrolyte layer (PEL), considering ion partitioning effects due to permittivity differences. The mathematical model for ionic and fluidic transport is solved using the finite element method, and the model validation is performed against existing theoretical and experimental results. The study investigates the influence of electrolyte concentration, permittivity ratio, and salt types (KCl, BeCl2, AlCl3) on the energy conversion process. The findings illustrate the substantial role of ion partitioning in modulating ionic concentration and potential fields, thereby affecting current profiles and energy conversion efficiencies. Remarkably, overlooking ion partitioning leads to significant overestimations of power density, highlighting the necessity of this consideration for accurate device performance predictions. This work introduces a promising configuration that achieves higher power densities, paving the way for the next generation of efficient energy-harvesting devices. The findings offer valuable insights into the development of state-of-the-art blue energy harvesting nanofluidic devices, advancing sustainable energy production.

Memristive Characteristics in an Asymmetrically Charged Nanochannel

The emergent nanofluidic memristor provides a promising way of emulating neuromorphic functions in the brain. The conical-shaped nanopore showed promising features for a nanofluidic memristor, inspiring us to investigate the memory effects in asymmetrically charged nanochannels due to their high current rectification, which may result in good memory effects. Here, the memory effects of an asymmetrically charged nanofluidic channel were numerically simulated by Poisson-Nernst-Planck equations. Our results showed that the I-V curves represented a diode in low scanning frequency and then became a memristor and finally a resistor as frequency increased. We successfully replicated the learning behavior in our system with history-dependent ion redistribution in the nanochannel. Some critical factors were quantitatively analyzed for the memory effects including voltage amplitude, optimal frequency, and Dukhin number. Experimental characterizations were also carried out. Our findings are useful for the design of nanofluidic memristors by the principle of enrichment and depletion as well as the determination of the best memory settings.

Electrokinetic transport phenomena in nanofluidics and their applications

Much progress has been made in the electrokinetic phenomena inside nanochannels in the last decades. As the dimensions of the nanochannels are compatible to that of the electric double layer (EDL), the electrokinetics inside nanochannels indicate many unexpected behaviors, which show great potential in the fields of material science, biology, and chemistry. This review summarizes the recent development of nanofluidic electrokinetics in both fundamental and applied research. First, the techniques for constructing nanochannels are introduced to give a guideline for choosing the optimal fabrication technique based on the specific feature of the nanochannel. Then, the theories and experimental investigations of the EDL, electroosmotic flow, and electrophoresis of nanoparticles inside the nanochannels are discussed. Furthermore, the applications of nanofluidic electrokinetics in iontronics, sensing, and biomolecule separation fields are summarized. In Section 5, some critical challenges and the perspective on the future development of nanofluidic electrokinetics are briefly proposed.

High-Performance Integrated Iontronic Circuits Based on Single Nano/Microchannels

Recently, artificial channel-based ionic diodes and transistors are extensively studied to mimic biological systems. Most of them are constructed vertically and are challenging to be further integrated. Several examples of ionic circuits with horizontal ionic diodes are reported. However, they generally require nanoscale channel sizes to meet the demand for ion-selectivity, resulting in low current output and restricting potential applications. In this paper, a novel ionic diode is developed based on multiple-layer polyelectrolyte nanochannel network membranes. Both bipolar and unipolar ionic diodes can be achieved by simply switching the modification solution. Ionic diodes with a high rectification ratio of ≈226 are achieved in single channels with the largest channel size of 2.5 µm. This design can significantly reduce the channel size requirement and improve the output current level of ionic devices. The high-performance ionic diode with a horizontal structure enables the integration of advanced iontronic circuits. Ionic transistors, logic gates, and rectifiers are fabricated on a single chip and demonstrated for current rectification. Furthermore, the excellent current rectification ratio and the high output current of the on-chip ionic devices highlight the promise of the ionic diode as a component of complex iontronic systems for practical applications.

Designing with Iontronic Logic Gates─From a Single Polyelectrolyte Diode to an Integrated Ionic Circuit

This article presents the implementation of on-chip iontronic circuits via small-scale integration of multiple ionic logic gates made of bipolar polyelectrolyte diodes. These ionic circuits are analogous to solid-state electronic circuits, with ions as the charge carriers instead of electrons/holes. We experimentally characterize the responses of a single fluidic diode made of a junction of oppositely charged polyelectrolytes (i.e., anion and cation exchange membranes), with a similar underlying mechanism as a solid-state p- and n-type junction. This served to carry out predesigned logical computations in various architectures by integrating multiple diode-based logic gates, where the electrical signal between the integrated gates was transmitted entirely through ions. The findings shed light on the limitations affecting the number of logic gates that can be integrated, the degradation of the electrical signal, their transient response, and the design rules that can improve the performance of iontronic circuits.

pH-Regulated Ionic Diode Based on an Asymmetric Shaped Multiple-Layer Polymer Membrane

Smart artificial ion channels with tunable properties have wide applications in many fields to achieve ion transport manipulation. Most reported artificial ions are constructed with vertical structures, limiting the further integration of ionic diodes into complex iontronic systems. Inspired by the asymmetric concentration polarization induced by the asymmetric geometry of nanochannels, a novel method is developed to construct horizontally arranged and pH-regulated ionic diodes in nanofluidic chips by self-assembling pH-responsive polymers. The effects of the fabrication and operation parameters on the performance of the ionic diode are systematically investigated. The current rectification ratio of the ionic diode can be modulated flexibly by regulating the pH conditions of the working fluid. An ionic diode bridge circuit for rectifying alternating current signals is built in a single nanofluidic chip and demonstrated, highlighting the feasibility of the ionic diode for complex iontronic system integration. The method presented in this paper provides a promising platform for the development of smart nanofluidic iontronic devices with widespread applicability in biological analysis, sensing, and logic computing.

Resonant Nanopumps: ac Gate Voltages in Conical Nanopores Induce Directed Electrolyte Flow

Inducing transport in electrolyte-filled nanopores with dc fields has led to influential applications ranging from nanosensors to DNA sequencing. Here we use the Poisson-Nernst-Planck and Navier-Stokes equations to show that unbiased ac fields can induce comparable directional flows in gated conical nanopores. This flow exclusively occurs at intermediate driving frequencies and hinges on the resonance of two competing timescales, representing space charge development at the ends and in the interior of the pore. We summarize the physics of resonant nanopumping in an analytical model that reproduces the results of numerical simulations. Our findings provide a generic route toward real-time controllable flow patterns, which might find applications in controlling the translocation of small molecules or nanocolloids.

Single Artificial Ion Channels with Tunable Ion Transport Based on the Surface Modification of pH-Responsive Polymers

Artificial ion channels with tunable ionic transport control and intelligent iontronic functions at the nanoscale have a wide application in logic computing and biosensing. Although some artificial ion channels with smart ion transport characteristics have been developed, most of them are constructed on porous membranes with undefined channel numbers. It is still challenging to achieve multiple ion transport features in single nanochannels and to control the ion flow more accurately with excellent repeatability. In this paper, a design strategy is presented to fabricate pH-responsive ion channels with various ion transport features based on a single polydimethylsiloxane (PDMS) nanochannel. The single-ion nanochannel developed by this approach can be further integrated into electronic systems on a chip. Three types of artificial ion channels are demonstrated and investigated systematically in this work. With symmetric or asymmetric pH stimuli, these ion channels can alternatively change their working states among an opened state, a closed state, and an ionic diode state. Four different ion transport features can be realized in a triple-gated ion channel system. With these advantages of the design, it is promising to build smart nanofluidic iontronic devices with widespread applicability in energy conversions, active ion transport control, and biological analysis.

Integrated Iontronic Circuits Based on Single Nanochannels

This article presents an invention of iontronic circuits to achieve effective electronic signal manipulation base on paralleled single polydimethylsiloxane (PDMS) nanochannels. The regulation of ion transport through the designed device is achieved by the asymmetrical decoration of charged polyelectrolytes on nanochannels. This circuit can serve as either a bipolar junction transistor or an ionic rectifier. The functionalities of the circuit are demonstrated and confirmed in different operation modes. The fabrication of this device is relatively simple. High accuracy in signal rectification can be obtained through the circuit because of the significant consistency of integrated ionic diodes. The iontronic integrated circuits presented in this paper provide new possibilities in the fabrication of iontronic devices with various functionalities on PDMS chips, toward improved biological computing and sensing.