Electrochemically addressable nanofluidic devices based on PET nanochannels modified with electropolymerized poly-o-aminophenol films

Advances in nanofluidic field-effect transistors: external voltage-controlled solid-state nanochannels for stimulus-responsive ion transport and beyond

Ion channels, intricate protein structures facilitating precise ion passage across cell membranes, are pivotal for vital cellular functions. Inspired by the remarkable capabilities of biological ion channels, the scientific community has ventured into replicating these principles in fully abiotic solid-state nanochannels (SSNs). Since the gating mechanisms of SSNs rely on variations in the physicochemical properties of the channel surface, the modification of their internal architecture and chemistry constitutes a powerful strategy to control the transport properties and, consequently, render specific functionalities. In this framework, both the design of the nanofluidic platform and the subsequent selection and attachment of different building blocks gain special attention. Similar to biological ion channels, functional SSNs offer the potential to finely modulate ion transport in response to various stimuli, leading to innovations in a variety of fields. This comprehensive review delves into the intricate world of ion transport across stimuli-responsive SSNs, focusing on the development of external voltage-controlled nanofluidic devices. This kind of field-effect nanofluidic technology has attracted special interest due to the possibility of real-time reconfiguration of the ion transport with a non-invasive strategy. These properties have found interesting applications in drug delivery, biosensing, and nanoelectronics. This document will address the fundamental principles of ion transport through SSNs and the construction, modification, and applications of external voltage-controlled SSNs. It will also address future challenges and prospects, offering a comprehensive perspective on this evolving field.

Ionic Transistors

Biological voltage-gated ion channels, which behave as life's transistors, regulate ion transport precisely and selectively through atomic-scale selectivity filters to sustain important life activities. By this inspiration, voltage-adaptable ionic transistors that use ions as signal carriers may provide an alternative information processing unit beyond solid-state electronic devices. This review provides a comprehensive overview of the first generation of biomimetic ionic transistors, including their operating mechanisms, device architecture development, and property characterizations. Despite its infancy, significant progress has been made in the applications of ionic transistors in fields such as DNA detection, drug delivery, and ionic circuits. Challenges and prospects of full exploitation of ionic transistors for a broad spectrum of practical applications are also discussed.

DNA nanostructure-assisted detection of carcinoembryonic antigen with a solid-state nanopore

In this paper, a novel detection technique for tumor marker carcinoembryonic antigen (CEA) has been developed by using a solid-state nanopore as a tool. The system utilizes the specific affinity between aptamer-modified magnetic Fe₃O₄ and CEA, rather than directly detecting the translocation of CEA through the nanopore. The aptamer-modified magnetic Fe₃O₄ was hybridized with tetrahedral DNA nanostructures (TDNs), and TDNs were released after CEA was added. We investigate the translocation behavior of individual TDNs through solid-state nanopores. The frequency of the blockage signals for TDNs is recorded for indirect detection of CEA. We realized the detection of CEA with a concentration as low as 0.1 nM and proved the specificity of the interaction between the aptamer. In addition, our designed nanopore sensing strategy can detect CEA in real samples.

Electrochemically addressed FET-like nanofluidic channels with dynamic ion-transport regimes

Nanofluidic channels in which the ionic transport can be modulated by the application of an external voltage to the nanochannel walls have been described as nanofluidic field effect transistors (nFETs) because of their analogy with electrolyte-gated field effect transistors. The creation of nFETs is attracting increasing attention due to the possibility of controlling ion transport by using an external voltage as a non-invasive stimulus. In this work, we show that it is possible to extend the actuation range of nFETs by using the supporting electrolyte as a "chemical effector". For this aim, a gold-coated poly(ethylene terephthalate) (PET) membrane was modified with electroactive poly-o-aminophenol. By exploiting the interaction between the electroactive poly-o-aminophenol and the ions in the electrolyte solution, the magnitude and surface charge of the nanochannels were fine-tuned. In this way, by setting the electrolyte nature it has been possible to set different ion transport regimes, i.e.: cation-selective or anion-selective ion transport, whereas the rectification efficiency of the ionic transport was controlled by the gate voltage applied to the electroactive polymer layer. Remarkably, under both regimes, the platform displays a reversible and rapid response. We believe that this strategy to preset the actuation range of nFETs by using the supporting electrolyte as a chemical effector can be extended to other devices, thus offering new opportunities for the development of stimulus-responsive solid-state nanochannels.

Switchable Ion Current Saturation Regimes Enabled via Heterostructured Nanofluidic Devices Based on Metal-Organic Frameworks

The use of track-etched membranes allows further fine-tuning of transport regimes and thus enables their use in (bio)sensing and energy-harvesting applications, among others. Recently, metal-organic frameworks (MOFs) have been combined with such membranes to further increase their potential. Herein, the creation of a single track-etched nanochannel modified with the UiO-66 MOF is proposed. By the interfacial growth method, UiO-66-confined synthesis fills the nanochannel completely and smoothly, yet its constructional porosity renders a heterostructure along the axial coordinate of the channel. The MOF heterostructure confers notorious changes in the transport regime of the nanofluidic device. In particular, the tortuosity provided by the micro- and mesostructure of UiO-66 added to its charged state leads to iontronic outputs characterized by an asymmetric ion current saturation for transmembrane voltages exceeding 0.3 V. Remarkably, this behavior can be easily and reversibly modulated by changing the pH of the media and it can also be maintained for a wide range of KCl concentrations. In addition, it is found that the modified-nanochannel functionality cannot be explained by considering just the intrinsic microporosity of UiO-66, but rather the constructional porosity that arises during the MOF growth process plays a central and dominant role.

Synthetic azobenzene-containing metal-organic framework ion channels toward efficient light-gated ion transport at the subnanoscale

Artificial nanochannels with diverse responsive properties have been widely developed to replicate the smart gating functionalities of biological ion channels. However, in these traditional nanochannels, common responsive molecules are usually too small to efficiently block the large channels under the closed states, leading to weak gating performances. Herein, we report carboxylated azobenzene-coordinated metal-organic-framework (AZO-MOF) ion channels with impressive light-gating properties. The AZO-MOF ion channels were synthesized by the confined growth of AZO-MOFs, composed of light-responsive AZO-containing ligands, non-responsive ligands and metal clusters, into ion-track-etched polymer nanochannels. The AZO-MOF ion channels with an appropriate number of AZO ligands showed a well-maintained crystalline and three-dimensional porous structure, including nanoscale cavities and subnanoscale windows for LiCl conduction. Meanwhile, the AZO-containing ligands bend and stretch upon light irradiation to open and close the pathways, thus gating the ion flux through the AZO-MOF ion channels with high on-off ratios up to 40.2, which is ∼2.3-30 times those of AZO-encapsulated MOF ion channels and AZO-modified nanochannels. This work suggests ways to achieve subnanoscaled gating of ion transport by angstrom-porous MOFs coordinated by stimuli-responsive ligands.

Nanofluidic osmotic power generators - advanced nanoporous membranes and nanochannels for blue energy harvesting

The increase of energy demand added to the concern for environmental pollution linked to energy generation based on the combustion of fossil fuels has motivated the study and development of new sustainable ways for energy harvesting. Among the different alternatives, the opportunity to generate energy by exploiting the osmotic pressure difference between water sources of different salinities has attracted considerable attention. It is well-known that this objective can be accomplished by employing ion-selective dense membranes. However, so far, the current state of this technology has shown limited performance which hinders its real application. In this context, advanced nanostructured membranes (nanoporous membranes) with high ion flux and selectivity enabling the enhancement of the output power are perceived as a promising strategy to overcome the existing barriers in this technology. While the utilization of nanoporous membranes for osmotic power generation is a relatively new field and therefore, its application for large-scale production is still uncertain, there have been major developments at the laboratory scale in recent years that demonstrate its huge potential. In this review, we introduce a comprehensive analysis of the main fundamental concepts behind osmotic energy generation and how the utilization of nanoporous membranes with tailored ion transport can be a key to the development of high-efficiency blue energy harvesting systems. Also, the document discusses experimental issues related to the different ways to fabricate this new generation of membranes and the different experimental set-ups for the energy-conversion measurements. We highlight the importance of optimizing the experimental variables through the detailed analysis of the influence on the energy capability of geometrical features related to the nanoporous membranes, surface charge density, concentration gradient, temperature, building block integration, and others. Finally, we summarize some representative studies in up-scaled membranes and discuss the main challenges and perspectives of this emerging field.

High-sensitivity detection of dopamine by biomimetic nanofluidic diodes derivatized with poly(3-aminobenzylamine)

During the last few years, much scientific effort has been devoted to the control of ionic transport properties of solid state nanochannels and the rational integration of chemical systems to induce changes in the ionic transport by interaction with selected target molecules for (bio)sensing purposes. In this work, we present the construction and functional evaluation of a highly sensitive dopamine-responsive iontronic device by functionalization of bullet-shaped track-etched single nanochannels in PET membranes with poly(3-aminobenzylamine) (PABA). The variety of basic groups in this amino-appended polyaniline derivative allows programming of the ion selectivity of the channel by setting the pH conditions. On the other hand, the amino-pendant groups of PABA become suitable binding sites for the selective chemical reaction with dopamine, leading to a change in the nanochannel surface charge. Thus, the exposure of the PABA-modified nanochannel to dopamine solutions selectively produces changes in the iontronic response. By rationally selecting the conditions for both the dopamine binding step and the iontronic reading, we obtained a correlation between the rectification efficiency and dopamine concentration down to the nanomolar range, which was also successfully interpreted in terms of a simple binding model.

Nanochannels regulating ionic transport for boosting electrochemical energy storage and conversion: a review

Electrochemical power sources, as one of the most promising energy storage and conversion technologies, provide great opportunities for developing high energy density electrochemical devices and portable electronics. However, uncontrolled ionic transport in electrochemical energy conversion, typically undesired anion transfer, usually causes some issues degrading the performance of energy storage devices. Nanochannels offer an effective strategy to solve the ionic transport problems for boosting electrochemical energy storage and conversion. In this review, the advantages of nanochannels for electrochemical energy storage and conversion and the construction principle of nanochannels are introduced, including ion selectivity and ultrafast ion transmission of nanochannels, which are considered as two critical factors to achieve highly efficient energy conversion. Recent advances in applications of nanochannels in lithium secondary batteries (LSBs), electrokinetic energy conversion systems and concentration cells are summarized in detail. Nanochannels exist in the above systems in two typical forms: functional separator and electrode protective layer. Current research on nanochannel-based LSBs is still at the early stage, and deeper and broader applications are expected in the future. Finally, the remaining challenges of nanochannel fabrication, performance improvement, and intelligent construction are presented. It is envisioned that this paper will provide new insights for developing high-performance and versatile energy storage electronics based on nanochannels.

Polyaniline for Improved Blue Energy Harvesting: Highly Rectifying Nanofluidic Diodes Operating in Hypersaline Conditions via One-Step Functionalization

Solid-state nanochannels have attracted substantial attention of the scientific community due to their remarkable control of ionic transport and the feasibility to regulate the iontronic output by different stimuli. Most of the developed nanodevices are subjected to complex modification methods or show functional responsiveness only in moderate-ionic-strength solutions. Within this project, we present a nanofluidic device with enhanced ionic current rectification properties attained by a simple one-step functionalization of single bullet-shaped polyethylene terephthalate (PET) nanochannels with polyaniline (PANI) that can work in high-ionic-strength solutions. The integration of PANI also introduces a broad pH sensitivity, which makes it possible to modulate the ionic transport behavior between anion-selective and cation-selective regimes depending on the pH range. Since PANI is an electrochemically active polymer, ionic transport also becomes dependent on the presence of redox stimuli in solution. We demonstrate that PANI-functionalized single-nanochannel membranes function as an efficient salinity gradient-based energy conversion device even in acidic concentrated salt solutions, opening the door to applications under a variety of novel operating conditions.

An ionic diode based on a spontaneously formed polypyrrole-modified graphene oxide membrane

Asymmetric membranes derived from the stacking of graphene oxide (GO) nanosheets have attracted great attention for the fabrication of ionic diodes. Herein, we described an ionic diode based on a polypyrrole-modified GO membrane with a vertical asymmetry, which was achieved by a spontaneous oxidation polymerization of pyrrole monomers on one side of the GO membrane in vapor phase. This asymmetric modification resulted in an asymmetric geometry due to the occupation of the interlayer space of one side of the GO membrane by polypyrrole. Our ionic diode demonstrated an obvious ionic rectification behavior over a wide voltage range. A calculation based on Poisson-Nernst-Planck equations was used to theoretically investigate the role of asymmetric modification of polypyrrole.