The Effect of Electrolyte Properties on Ionic Transport through Solid-State Nanopores: Experiment and Simulation.

Nanopore membranes enable versatile technologies that are employed in many different applications, ranging from clean energy generation to filtration and sensing. Improving the performance can be achieved by conducting numerical simulations of the system, for example, by studying how the nanopore geometry or surface properties change the ionic transport behavior or fluid dynamics of the system. A widely employed tool for numerical simulations is finite element analysis (FEA) using software, such as COMSOL Multiphysics. We found that the prevalent method of implementing the electrolyte in the FEA can diverge significantly from physically accurate values. It is often assumed that salt molecules fully dissociate, and the effect of the temperature is neglected. Furthermore, values for the diffusion coefficients of the ions, as well as permittivity, density, and viscosity of the fluid, are assumed to be their bulk values at infinite dilution. By performing conductometry experiments with an amorphous SiO2 nanopore membrane with conical pores and simulating the pore system with FEA, it is shown that the common assumptions do not hold for different mono- and divalent chlorides (LiCl, NaCl, KCl, MgCl2, and CaCl2) at concentrations above 100 mM. Instead, a procedure is presented where all parameters are implemented based on the type of salt and concentration. This modification to the common approach improves the accuracy of the numerical simulations and thus provides a more comprehensive insight into ion transport in nanopores that is otherwise lacking.

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.

Nanoprecipitation-Enhanced Sensitivity in Enzymatic Nanofluidic Biosensors.

Single nanochannels show unique transport properties due to nanoconfinement. It has been demonstrated that at submillimolar concentrations of divalent cations, a nanoprecipitation reaction can occur in nanochannels. Although several reports have shown, described, and modeled the nanoprecipitation process, no further advantages have been taken from this phenomenon. Here, we show that the nanoprecipitation reaction can be incorporated into enzyme-modified nanochannels to enhance the performance of small-molecule biosensors via in situ amplification reactions. Contrary to the working principle of previous enzymatic nanofluidic biosensors, the nanofluidic biosensor described in this work operates on the basis of concerted functions: pH-shifting enzymatic activity and nanoprecipitation. We show that the simple addition of Ca2+ and Mg2+ ions in the working analyte solution containing urea can lower the detection limit from the nanometer to the subnanometer regime and modulate the dynamic linear range. This approach enables the implementation of more sensitive real-time nanofluidic detection methods without increasing the complexity of the nanofluidic platform or the sensing approach. We envision that the integration of concerted functions in nanofluidic architectures will play a key role in expanding the use of these nanoscale devices for analytical purposes.

Comparison of Different Preparation Techniques of Thermophoretic Swimmers and Their Propulsion Velocity.

The motion of partly gold (Au)-coated Janus particles under laser irradiation is caused by self-thermophoresis. Despite numerous studies addressing this topic, the impact of the preparation method and the degree of coverage of the particle with Au on the resulting thermophoretic velocity has not yet been fully understood. A detailed understanding of the most important tuning parameters during the preparation process is crucial to design Janus particles that are optimized for Au coverage to receive a high thermophoretic velocity. In this study, we explore the influence of the fabrication process, which changes the Au cap size, on the resulting self-propulsion behavior of partly Au-coated polystyrene particles (Au-PS). Additionally, the impact of an underlying adhesion chromium layer is investigated. In addition to the most commonly used qualitative SEM and EDX measurements, we propose a novel and fast technique utilizing AFM studies to quantify the cap size. This non-invasive technique can be used to determine both the size and the maximum thickness of the Au cap. The Au cap size was systematically varied in a range between about 36 and 74% by different preparation strategies. Nevertheless, we showed that the differing Au cap sizes of the Janus particles in this range have no obvious effect on the thermophoretic velocity. This is a surprising result since one would expect an effect of the Au cap size due to different solvent flows around the Janus particles and is attributed to an additional torque near the surface of the measuring cell.

Experimental evidence of a size-dependent sign change of the Seebeck coefficient of Bi nanowire arrays.

The electrical transport in bismuth nanowires is strongly influenced by both sample geometry and crystallinity. Compared to bulk bismuth, the electrical transport in nanowires is dominated by size effects and influenced by surface states, which gain increasing relevance with increasing surface-to-volume ratios, i.e. with decreasing wire diameter. Bismuth nanowires with tailored diameter and crystallinity constitute, therefore, excellent model systems, allowing to study the interplay of the different transport phenomena. Here, we present temperature-dependent Seebeck coefficient and relative electrical resistance measurements of parallel bismuth nanowire arrays with diameters between 40 and 400 nm synthesized by pulsed electroplating in polymer templates. Both electrical resistance and Seebeck coefficient exhibit a non-monotonic temperature dependence, with the sign of the Seebeck coefficient changing from negative to positive with decreasing temperature. The observed behavior is size-dependent and is attributed to limitations of the mean free path of the charge carriers within the nanowires. The observed size-dependent Seebeck coefficient and in particular the size-dependent sign change opens a promising avenue for single-material thermocouples with p- and n-legs made from nanowires with different diameters.

Three-dimensional free-standing gold nanowire networks as a platform for catalytic applications.

We report the catalytic performance of networks of highly interconnected Au nanowires with diameters tailored between 80 and 170 nm. The networks were synthesized by electrodeposition in etched ion-track polymer templates, and the synthesis conditions were developed for optimal wire crystallinity and network homogeneity. The nanowire networks were self-supporting and could be easily handled as electrodes in electrochemical cells or other devices. The electrochemically active surface area of the networks increased systematically with increasing the wire diameter. They showed a very stable performance during 200 CV cycles of methanol oxidation reactions, with the peak current density reaching up to 200 times higher than that of a flat reference electrode, with only a 5% drop in the peak current density. The Au nanowire networks proved to be excellent model systems for investigation of the performance of porous catalysts and very promising nanosystems for application in direct alcohol fuel cell catalysts.

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.

Triazol-Methanaminium-Pillar[5]arene-Functionalized Single Nanochannel for Quantitative Analysis of Pyrophosphate in Water.

Inorganic pyrophosphate (PPi) is an important biological functional anion and plays crucial roles in life science, environmental science, medicine, and chemical process. Quantification of PPi in water has far-reaching significance for life exploration, disease diagnosis, and water pollution control. The label-free quantitative detection of PPi anions with a nanofluidic sensing device based on a conical single nanochannel is demonstrated. The channel surface is functionalized with a synthetic PPi receptor, triazol-methanaminium-functionalized pillar[5]arene (TAMAP5), using carbodiimide coupling chemistry. Due to the specific binding between TAMAP5 and PPi, the functionalized nanochannel can discriminate PPi from other inorganic anions with high selectivity through ionic current recording, even in the presence of various interfering anions. The current response exhibits a linear correlation with PPi concentration in the range from 1 × 10-7 to 1 × 10-4 M with a limit of detection of 6.8 × 10-7 M. A spike-and-recovery analysis of PPi in East Lake water samples indicates that the proposed nanofluidic sensor has the ability to quantitate micromolar concentrations of PPi in environmental water samples.