Anomalous Pore‐Density Dependence in Nanofluidic Osmotic Power Generation

Osmotic power generation in biomimetic nanofluidic systems has attracted considerable research interest owing to the enhanced performance and long‐term stability. Towards practical applications, when extrapolating the materials from single‐nanopore to multi‐pore membranes, conventional viewpoint suggests that, to gain high electric power density, the porosity should be as high as possible. However, recent experimental observations show that the commonly‐used linear amplification method largely overestimates the actual performance, particularly at high pore density. Herein, we provide a theoretical investigation to understand the reason. We find a counterintuitive pore‐density dependence in high porosity nanofluidic systems that, once the pore density approaches more than 1×10⁹ pores/cm², the overall output electric power goes down with the increasing pore density. The excessively high pore density impairs the charge selectivity and induces strong ion concentration polarization, which undermines the osmotic power generation process. By optimizing the geometric size of the nanopores, the performance degradation can be effectively relieved. These findings clarify the origin of the unsatisfactory performance of the current osmotic nanofluidic power sources, and provide insights to further optimize the device.     

Fluidic communication between multiple vertically segregated microfluidic channels connected by nanocapillary array membranes

Hybrid microfluidic/nanofluidic devices offer unique capabilities for manipulating and analyzing minute volumes of expensive or hard-to-obtain samples. Here, multilayer poly-(methyl methacrylate) microchips, with multiple spatially isolated microfluidic channels interconnected by nanocapillary array membranes (NCAMs), are fabricated using an adhesive contact printing process. The NCAMs, positioned between the microfluidic channel layers, add functionality to the inter-microchannel fluid transfer unit operation. They do so because the transport of specific analytes through the NCAM can be controlled by adjusting the ionic strength, the polarity of the applied bias, the surface charge density, and the pore size. A simplified, floating injection technique for NCAM-coupled nanofluidic devices is described and compared with conventional biased injection. In the floating injection approach, a voltage is applied across the injection channel and the slight electric field extension at the cross-section is used to transfer analytes through the nanopores to the separation channel. Floating injection excels in plug reproducibility, separation resolution, and operation simplicity, although it decreases assay throughput relative to biased injection. Floating injection can avoid the uneven distribution of analytes in the microfluidic channel that sometimes results from biased injection because of the volume mismatch between NCAM nanopore transport capacity and the supply of fluid. Moreover, the pressure-driven flow caused by the mismatch of the EOFs in the microfluidic channels connected by an NCAM must be considered when using NCAMs with pore diameters below 50 nm.     

pH-Regulated nanopore conductance with overlapped electric double layers

There has been a significant growth of interest in single nanopore ionic devices that could control the transport of ions and rectify ionic current. To improve the advance of relevant nanofluidic devices, a model is derived for the first time to investigate the zeta potential and ionic conductance of a cylindrical nanopore with overlapped electric double layer as functions of pH, salt concentration as well as the Stern layer capacitance. The developed model is validated by the experimental data of the nanopore conductance. Results show that in addition to the magnitudes, the relevant behaviors of zeta potential and conductance of the nanopore might be significantly influenced by the Stern layer.     

Solid-state nanopores for ion and small molecule analysis

Solid-state nanopore in analytical chemistry has developed rapidly in the 1990s and it is proved to be a versatile new tool for bioanalytical chemistry. The research field of solid-state nanopore starts from mimicking the biological nanopore in living cells. Understanding the transport mechanism of biological nanopore in vivo is a big challenge because of the experimental difficulty, so it is essential to establish the basic research of artificial nanopores in vitro especially for the analysis of ions and small molecules. The performance of solid-state nanopores could be evaluated by monitoring currents when ions and molecules passed through. The comparison of the two types of nanopores based on current-derived information can reveal the principle of biological nanopores, while the solid-state nanopores are applied into practical bioanalysis. In this review, we focus on the researches of the solid-state nanopores in the fabrication process and in the analysis of ions and small molecules. Fabrication methods of nanopores, ion transport mechanism, small molecule analysis and theoretical studies are discussed in detail.     

On the Role of Heterogeneous Nanopore Junction in Osmotic Power Generation

Osmotic power generated by mixing ionic solutions of different concentration is an underutilized clean energy resource that satisfy potentially the ever‐growing energy demand. For decades, substantial efforts are made to enhance the power density. Toward this goal, we once developed a heterogeneous nanoporous membrane comprising of heterojunctions between negatively charged mesoporous carbon and positively charged macroporous alumina to harvest electric power from salinity difference and achieved outstanding performance (J. Am. Chem. Soc. 2014 , 136, 12265). The heterogeneous nanopore junction effectively suppresses ion concentration polarization (ICP) at low concentration end, and consequently promotes the overall power density. However, to date, a systematic understanding of the role of the heterogeneous nanopore junction in osmotic energy conversion remains urgent and largely unexplored. Herein, we provide an in‐depth theoretical investigation on this issue with special emphasis on several influential factors, such as the ionic concentration, the surface charge density, and the geometry of heterogeneous part. To balance the suppression of ICP and maintenance of charge selectivity, we find that these influential factors in the heterogeneous part should be restricted to a specific range. These findings provide direct guidance for design and optimization of high‐performance nanofluidic power sources.     

Universal equivalent circuit of electrochemical cell

A detailed analysis of the mathematical model of a random-structure RC two-terminal network is performed. It is shown that a circuit of any RC two-terminal network can be reduced to a universal form consisting of a set of parallel branches. Each branch corresponds to a single relaxation process and represents a circuit of serially connected resistance and capacitor. The equivalent circuit under consideration is a compact representation of experimental data obtained using the impedance spectroscopy method. It is recommended to use the universal circuit as an intermediate model of an electrochemical cell.     

Zn2+ and EDTA Cooperative Switchable Nanofluidic Diode Based on Asymmetric Modification of Single Nanochannel

Design and fabrication of smart switchable nanofluidic diodes remains a challenge in the life and materials sciences. Here, we present the first example of a novel Zn²⁺/EDTA switchable nanofluidic diode system based on the control of one‐side of the modified hourglass‐shaped nanochannel with salicylaldehyde Schiff base (SASB). The nanofluidic diode can be turned on in the response of Zn²⁺ and turned off in response to EDTA solution with good reversibility and recyclability.     

Rapid and multiplex preparation of engineered Mycobacterium smegmatis porin A (MspA) nanopores for single molecule sensing and sequencing

Acknowledging its unique conical lumen structure, Mycobacterium smegmatis porin A (MspA) was the first type of nanopore that has successfully sequenced DNA. Recent developments of nanopore single molecule chemistry have also suggested MspA to be an optimum single molecule reactor. However, further investigations with this approach require heavy mutagenesis which is labor intensive and requires high end instruments for purifications. We here demonstrate an efficient and economic protocol which performs rapid and multiplex preparation of a variety of MspA mutants. The prepared MspA mutants were demonstrated in operations such as nanopore insertion, sequencing, optical single channel recording (oSCR), nanopore single molecule chemistry and nanopore rectification. The performance is no different from that of pores however prepared by other means. The time of all human operations and the cost for a single batch of preparation have been minimized to 40 min and 0.4$, respectively. This method is extremely useful in the screening of new MspA mutants, which has an urgent requirement in further investigations of new MspA nanoreactors. Its low cost and simplicity also enable efficient preparations of MspA nanopores for both industrial manufacturing and academic research.

A rapid and multiplex approach to prepare engineered Mycobacterium smegmatis porin A (MspA) nanopores for single molecule sensing and sequencing.     

Electrokinetic transport of nanoparticles in functional group modified nanopores

Nanopore detection is a hot issue in current research. One of the challenges is how to slow down the transport velocity of nanoparticles in nanopores. In this paper, we propose a functional group modified nanopore. That means a polyelectrolyte brush layer is grafted on the surface of the nanopore to change the surface charge properties. The existing studies generally set the charge density of the brush layer to a fixed value. On the contrary, in this paper, we consider an essential property of the brush layer: the volume charge density is adjustable with pH. Thus, the charge property of the brush layer will change with the local H⁺ concentration. Based on this, we established a mathematical model to study the transport of nanoparticles in polyelectrolyte brush layer modified nanopores. We found that pH can effectively adjust the charge density and even the polarity of the brush layer. A larger pH can reduce the transport velocity of nanoparticles and improve the blockade degree of ion current. The grafting density does not change the polarity of the brush charge. The larger the grafting density, the greater the charge density of the brush layer, and the blockade degree of ion current is also more obvious. The polyelectrolyte brush layer modified nanopores in this paper can effectively reduce the nanoparticle transport velocity and retain the essential ion current characteristics, such as ion current blockade and enhancement.     

Selectively detecting attomolar concentrations of proteins using gold lined nanopores in a nanopore blockade sensor

Disease diagnosis at earlier stages requires the development of ultrasensitive biosensors for detecting low-abundance biomarkers in complex biological fluids within a reasonable time frame. Here, we demonstrate the development of an ultrasensitive nanopore blockade biosensor that can rapidly diagnose a model protein biomarker, prostate-specific antigen (PSA) with high selectivity. The solid-state nanopores have gold located only along the length of the nanopore whilst the rest of the membrane is silicon nitride. The orthogonal use of materials allows nanopore arrays with a different surface chemistry inside the nanopore relative to the rest of the membrane to be fabricated. The importance of this differential surface chemistry is it can improve the detection limit of nanopore blockade sensors in quantitative analysis. Based on such functionalized nanopore devices, nanopore blockade sensors lower the limit of detection by an order of magnitude and enable ultrasensitive detection of PSA as low as 80 aM. The findings from this study open new opportunities for nanopore sensors in further developments including optical detection and ultralow detection limit biosensing at complex biological fluids.

Selective detection of attomolar proteins was achieved using gold lined nanopores in a nanopore blockade sensor.     

On the relation between the solvent parameters and the physical properties of methyl-4,6-O-benzylidene-α-d-glucopyranoside organogels

This paper reports the mechanisms of gel formation, the thermal properties and the microstructures of the networks of the gels composed of methyl-4,6-O-benzylidene-α-d-glucopyranoside and selected organic solvents: p-xylene, benzene, toluene, diphenyl ether and tetraethoxysilane. The Fourier transform infrared measurements together with simulation spectra, the air bath method and Polarized Optical Microscopy were employed in our studies. The experimental data show that the solvent has an influence on the microstructure of the gel network but there is no predictable influence of the solvent polarity on the shape of the formed gelator aggregates and correspondingly on the fibrous assemblies as revealed by the different microstructure of the gel network. Independently of the solvent polarity, the studied gelator, like other methyl-4,6-O-benzylidene derivatives of monosaccharides, formed gels through the formation of a hydrogen-bond network. The solvent parameters, such as the dielectric constant, Hildebrand solubility parameter, the polarity scale E_T and the Kamlet–Taft parameters were considered to quantify solvent effects on the gelation. The conclusions about the correlations are of interest but only to this particular sugar based gels.     

rf Capacitive coupling with efficient gated trapping in internal matrix-assisted laser desorption ionization fourier transform ion cyclotron resonance

A method to combine gated trapping and capacitive coupling into a single experiment is reported. This is achieved with a circuit that allows isolation of the electronic network that gates the trapping voltage from the circuit that enables capacitive coupling of the rf excitation signal to the trapping plates. When the capacitive coupling network is not isolated from the gated trapping network, the trapping voltage changes occur on a 100 µs or longer timescale, which is incompatible with efficient capture of ions formed by matrix-assisted laser desorption ionization. Isolation of the two networks allows the trapping voltage to be gated with less than a 10 µs risetime. The effectiveness of this approach is demonstrated by a set of experiments carried out with and without the benefit of the isolation of capacitive coupling from gated trapping.     

Highly rectified ion transport through 2D WSe2/MoS2 bi-layered membranes

Through a two-step vacuum-filtration process, WSe₂ and MoS₂ nanosheets were sequentially deposited onto a polymeric nanoporous support, forming WSe₂/MoS₂ bi-layered heterostructure. Highly rectified ion transport phenomenon is observed through the heterogeneous 2D layered membranes.     

Stiffness measurement of nanosized liposomes using solid‐state nanopore sensor with automated recapturing platform

This paper describes a method to gauge the stiffness of nanosized liposomes – a nanoscale vesicle – using a custom‐made recapture platform coupled to a solid‐state nanopore sensor. The recapture platform electrically profiles a given liposome vesicle multiple times through automated reversal of the voltage polarity immediately following a translocation instance to re‐translocate the same analyte through the nanopore – provides better statistical insight at the molecular level by analyzing the same particle multiple times compared to conventional nanopore platforms. The capture frequency depends on the applied voltage with lower voltages (i.e., 100 mV) permitting higher recapture instances than at higher voltages (>200 mV) since the probability of particles exiting the nanopore capture radius increases with voltage. The shape deformation was inferred by comparing the normalized relative current blockade ( at the two voltage polarities to that of a rigid particle, i.e., polystyrene beads. We found that liposomes deform to adopt a prolate shape at higher voltages. This platform can be further applied to investigate the stiffness of other types of soft matters, e.g., virus, exosomes, endosomes, and accelerate the potential studies in pharmaceutics for increasing the drug packing and unpacking mechanism by controlling the stiffness of the drug vesicles.     

Contributions of the nanovoid structure to the kinetics of moisture transport in epoxy resins

Absorbed moisture can degrade the physical properties of an epoxy resin, jeopardizing the performance of an epoxy‐based component. Although specific water–epoxy interactions are known to be very important in determining transport behavior, the role of network topology is not clear. In this article, a series of epoxies in which the topology is systematically varied (and the polarity held constant) is used to explore how topology influences the kinetics of moisture transport. The topology is quantified via the positron annihilation lifetime spectroscopy technique in terms of the size and volume fraction of electron density heterogeneities 5–6 Å in diameter, a dimension comparable to the 3‐Å kinetic diameter of a water molecule. Surprisingly, the volume fraction of such nanopores does not affect the diffusion coefficient (D) of water in any of the resins studied. For temperatures at and below 35 °C, there is a mild exponential dependence of D on the average nanopore size observed. Otherwise, the kinetics of moisture transport do not appear to depend on the nanopores. However, the initial flux of moisture into the epoxy does appear to correlate with the intrinsic hole volume fraction. That this correlation persists only in the initial stages of absorption is partially understood in terms of the ability of the water to alter the nanopore structure; only in the initial stages of uptake are the nanopores, as quantified in the dry state, relevant to transport. The role of specific epoxy–water interactions are also discussed in terms of transport kinetics. The lack of a correlation between the topology and transport suggests that polar interactions, and not topology, provide the rate‐limiting step of transport. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 776–791, 2000     

Thin mesoporous polyaniline films manifesting a water-promoted photovoltaic effect

Photovoltaic cells composed of thin mesoporous polyaniline films sandwiched between an indium-tin oxide anode and aluminium cathode have been fabricated. The cells show an increase in the photo-generated open-circuit voltage (V _oc) from 0.2 V to 0.6 V and stable-in-time V _oc generation following the addition of water containing highly hydrated ions, e.g. tap water.We explain the waterpromoted photo-voltaic effect by the polarity of the water environment. Theoretical calculations show that increasing the solvent polarity increases the energy of the electronic transition related to the measured V _oc. The stable-in-time V _oc generation could be explained by the increase in the lifetime of the excitons as well as by their more efficient dissociation in the interpenetrating network of polyaniline and water. The penetration of water into the mesoporous polyaniline films is promoted by the presence of highly hydrated ions.     

Low-temperature direct bonding of glass nanofluidic chips using a two-step plasma surface activation process

Owing to the well-established nanochannel fabrication technology in 2D nanoscales with high resolution, reproducibility, and flexibility, glass is the leading, ideal, and unsubstitutable material for the fabrication of nanofluidic chips. However, high temperature (~1,000 °C) and a vacuum condition are usually required in the conventional fusion bonding process, unfortunately impeding the nanofluidic applications and even the development of the whole field of nanofluidics. We present a direct bonding of fused silica glass nanofluidic chips at low temperature, around 200 °C in ambient air, through a two-step plasma surface activation process which consists of an O₂ reactive ion etching plasma treatment followed by a nitrogen microwave radical activation. The low-temperature bonded glass nanofluidic chips not only had high bonding strength but also could work continuously without leakage during liquid introduction driven by air pressure even at 450 kPa, a very high pressure which can meet the requirements of most nanofluidic operations. Owing to the mild conditions required in the bonding process, the method has the potential to allow the integration of a range of functional elements into nanofluidic chips during manufacture, which is nearly impossible in the conventional high-temperature fusion bonding process. Therefore, we believe that the developed low-temperature bonding would be very useful and contribute to the field of nanofluidics.     

Covalent modification of single glass conical nanopore channel with 6-carboxymethyl-chitosan for pH modulated ion current rectification

In this study, a novel covalent modification method of the single glass conical nanopore channel with amphoteric 6-carboxymethyl-chitosan (CMC) was designed to obtain a smart device responsive to a broad range of pH stimuli. This response is highly sensitive, reversible and reproducible. The CMC modified channel possessing carboxyl and amino groups was able to regulate ion transport selectivity and ion current rectification properties which depend on surface charges at various pH values. Each modification step was characterized by simply measuring the current–voltage (I–V) curves of the nanopore channel.     

Acid–base chemistry at the single ion limit

We present the results of acid–base experiments performed at the single ion (H⁺ or OH⁻) limit in ∼6 aL volume nanopores incorporating electrochemical zero-mode waveguides (E-ZMWs). At pH 3 each E-ZMW nanopore contains ca. 3600H⁺ ions, and application of a negative electrochemical potential to the gold working electrode/optical cladding layer reduces H⁺ to H₂, thereby depleting H⁺ and increasing the local pH within the nanopore. The change in pH was quantified by tracking the intensity of fluorescein, a pH-responsive fluorophore whose intensity increases with pH. This behavior was translated to the single ion limit by changing the initial pH of the electrolyte solution to pH 6, at which the average pore occupancy 〈n〉_pore ∼3.6H⁺/nanopore. Application of an electrochemical potential sufficiently negative to change the local pH to pH 7 reduces the proton nanopore occupancy to 〈n〉_pore ∼0.36H⁺/nanopore, demonstrating that the approach is sensitive to single H⁺ manipulations, as evidenced by clear potential-dependent changes in fluorescein emission intensity. In addition, at high overpotential, the observed fluorescence intensity exceeded the value predicted from the fluorescence intensity-pH calibration, an observation attributed to the nucleation of H₂ nanobubbles as confirmed both by calculations and the behavior of non-pH responsive Alexa 488 fluorophore. Apart from enhancing fundamental understanding, the approach described here opens the door to applications requiring ultrasensitive ion sensing, based on the optical detection of H⁺ population at the single ion limit.

Visualizing dynamic change in the number of protons during electroreduction of protons in attoliter volume zero-mode waveguides.     

Temperature dependence of fluid transport in nanopores

Understanding the temperature-dependent nanofluidic transport behavior is critical for developing thermomechanical nanodevices. By using non-equilibrium molecular dynamics simulations, the thermally responsive transport resistance of liquids in model carbon nanotubes is explored as a function of the nanopore size, the transport rate, and the liquid properties. Both the effective shear stress and the nominal viscosity decrease with the increase of temperature, and the temperature effect is coupled with other non-thermal factors. The molecular-level mechanisms are revealed through the study of the radial density profile and hydrogen bonding of confined liquid molecules. The findings are verified qualitatively with an experiment on nanoporous carbon.