Insight into the transport of ions from salts of moderated solubility through nanochannels: negative incremental resistance assisted by geometry.

In this study, the transport of salt with moderate solubility through bioinspired solid-state nanochannels was comprehensively investigated. For this purpose, bullet-shaped channels were fabricated and exposed to KClO4, a monovalent salt with moderate solubility. These channels displayed the typical rectifying behavior characteristic of asymmetrical channels but with one remarkable difference, the iontronic output exhibited a negative incremental resistance phenomenon of high gating efficiency when the transmembrane voltage in the open state was increased enough, giving rise to an inactivated state characterized by a low and stable ion current. The behavior is attributed to salt precipitation inside the channel and remarkably, it is not observed in other geometries such as cylindrical or cigar-shaped channels. Considering the central role of the surface in precipitation formation, the influence of several parameters such as electrolyte concentration, pH, and channel size was studied. Under optimized conditions, this system can alternate among three different conductance states (closed, open, and inactivated) and exhibits gating ratios higher than 20. Beyond its potential application in fields related to electronics or sensing, this study provides valuable insight into the fundamental principles behind ion rectifying behavior in solid-state channels and highlights the implications of surface phenomena at the nanoscale.

Manipulating Ion Transport Regimes in Nanomembranes via a "Pore-in-Pore" Approach Enabled by the Synergy of Metal-Organic Frameworks and Solid-State Nanochannels.

Solid-state nanochannels (SSNs) have emerged as promising platforms for controlling ionic transport at the nanoscale. SSNs are highly versatile, and this feature can be enhanced through their combination with porous materials such as Metal-Organic Frameworks (MOF). By selection of specific building blocks and experimental conditions, different MOF architectures can be obtained, and this can influence the ionic transport properties through the nanochannel. Herein, we study the effects of confined synthesis of Zr-based UiO-66 MOF on the ion transport properties of single bullet-shaped poly(ethylene terephthalate) (PET) nanochannels. We have found that emerging textural properties from the MOF phase play a determinant role in controlling ionic transport through the nanochannel. We demonstrate that a transition from ion current saturation regimes to diode-like regimes can be obtained by employing different synthetic approaches, namely, counterdiffusion synthesis, where MOF precursors are kept separate and forced to diffuse through the nanochannel, and one-pot synthesis, where both precursors are placed at both ends of the channel. Also, by considering the dependence of the charge state of the UiO-66 MOF on the protonation degree, pH changes offered a mechanism to tune the iontronic output (and selectivity) among different regimes, including anion-driven rectification, cation-driven rectification, ion current saturation, and ohmic behavior. Furthermore, Poisson-Nernst-Planck (PNP) simulations were employed to rationalize the different iontronic outputs observed experimentally for membranes modified by different methods. Our results demonstrate a straightforward tool to synthesize MOF-based SSN membranes with tunable ion transport regimes.

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.

Highly sensitive acetylcholine biosensing via chemical amplification of enzymatic processes in nanochannels.

Acetylcholinesterase-modified nanochannels are proposed as reliable and reproducible nanofluidic sensors for highly sensitive detection of acetylcholine. The operation mechanism relies on the use of weak polyelectrolytes as "chemical amplifiers" that adjust/reconfigure the nanochannel surface charge in the presence of local pH changes induced by the enzymatic reaction. Experimental results show that the presence of acetylcholine can be transduced into measurable ionic signals with a low limit of detection.

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.

Borate-driven ionic rectifiers based on sugar-bearing single nanochannels.

Recently, much scientific effort has been centered on the control of the ionic transport properties of solid state nanochannels and the rational design and integration of chemical systems to induce changes in the ionic transport by means of interactions with selected target molecules. Here, we report the fabrication of a novel nanofluidic device based on solid-state nanochannels, which combines silane chemistry with both track-etched and atomic layer deposition (ALD) technologies. Nanodevice construction involves the coating of bullet-shaped single-pore nanochannels with silica (SiO2) by ALD and subsequent surface modification by reaction between silanol groups exposed on pore walls and N-(3-triethoxysilylpropyl)-gluconamide, in order to create a gluconamide-decorated nanochannel surface. The formation of a boroester derivative resulting from the selective reaction of borate with the appended saccharides leads to important changes in the surface charge density and, concomitantly, in the iontronic properties of the nanochannel. Furthermore, we propose a binding model to rationalize the specific interaction saccharide-borate in the surface. Besides, this unique nanodevice exhibits a highly selective and reversible response towards borate/fructose exposure. On the basis of the surface charge variation resulting from borate binding, the nanochannel can reversibly switch between "ON" and "OFF" states in the presence of borate and fructose, respectively. In addition, this work describes the first report of the functionalization of PET/SiO2 nanochannels by the ALD technique. We believe that this work provides a promising framework for the development of new nanochannel-based platforms suitable for multiple applications, such as water quality monitoring or directed molecular transport and separation.

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.

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.

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

Nanofluidic field-effect transistors (nFETs) have attracted attention from the scientific community due to their remarkable level of control over ionic transport. Particularly, the combination of nanofluidic systems and electroactive polymers has demonstrated to be an interesting approach to achieve an electrochemically addressable device. In this work, the development of nFETs based on the integration of electropolymerized poly-o-aminophenol (POAP) films into track-etched nanochannels is proposed. The electropolymerization of POAP on the tip side of Au-sputtered asymmetric PET nanochannels not only allowed having a programmable tip diameter but also offered a precise and very rapid control of ionic transport by switching an external bias voltage. Moreover, the system exhibited a reversible behaviour between non-selective and anion-selective states. We believe that this work provides new tools and concepts to design and build high-performance nanofluidic field-effect transistors working under electrochemically controlled conditions.

Redox-Driven Reversible Gating of Solid-State Nanochannels.

The design of an electrochemically addressable nanofluidic diode is proposed, which allows tunable and nanofluidic operations via redox gating under electrochemical control. The fabrication process involves the modification of an asymmetric gold-coated solid-state nanopore with a thin layer of a redox polymer, poly(vinylferrocene) (PVFc). The composite nanochannel acts as a gate electrode by changing the electrochemical state and, consequently, the conversion/switching of ferrocene into ferricenium units upon the application of different voltages. It is shown that the electrochemical input accurately controls the surface charge density of the nanochannel walls with a predictable concomitant effect on the rectification properties. PVFc-based nanofluidic devices are able to discriminate the passage of anionic species through the nanochannel in a qualitative and quantitative manner by simply switching the redox potential of the PVFc layer. Experimental data confirmed that a rapid and reversible modulation of the ionic transport regimes can be easily attained by changing the applied potential. This applied potential plays the role of the gate voltage (Vg) in field-effect transistors (FET), so these nanofluidic channels behave as ionic FETs. Depending on the Vg values, the iontronic behavior can be switched between ohmic and diode-like regimes. We believe that this system illustrates the potential of redox-active polymers integrated into nanofluidic devices as plausible, simple, and versatile platforms to create electrochemically addressable nanofluidic devices for multiple applications.

Dangerous liaisons: anion-induced protonation in phosphate-polyamine interactions and their implications for the charge states of biologically relevant surfaces.

Although not always considered a preponderant interaction, amine-phosphate interactions are omnipresent in multiple chemical and biological systems. This study aims to answer questions that are still pending about their nature and consequences. We focus on the description of the charge state as surface charges constitute directing agents of the interaction of amine groups with either natural or synthetic counterparts. Our results allow us to quantitatively determine the relative affinities of HPO42- and H2PO4- from the analysis of the influence of phosphates on the zeta-potential of amino-functionalized surfaces in a broad pH range. We show that phosphate anions enhance the protonation of amino groups and, conversely, charged amines induce further proton dissociation of phosphates, yielding a complex dependence of the surface effective charge on the pH and phosphate concentration. We also demonstrate that phosphate-amine interaction is specific and the modulation of surface charge occurs in the physiological phosphate concentration range, emphasizing its biochemical and biotechnological relevance and the importance of considering this veiled association in both in vivo and in vitro studies.