Surface Of Nanochannels Research Articles

Centimeter-scale continuous silica isoporous membranes for molecular sieving

Nanoporous molecular sieving membranes with high densities of uniform pores over macroscopic areas are highly desired to achieve a high permeation rate and a high molecular selectivity. In the first part of this work, our primary calculation using a quasi-steady state model indicates that the effective way of achieving a high molecular permeation is to increase the pore density and area of sieving membrane. Then the experimental work has been projected to prepare the centimeter-scale continuous silica isoporous membrane (SIM) consisting of parallel nanochannels under the support of porous polyethylene terephthalate (PET) film. Without using any expensive facilities or harsh conditions, simple wet-chemistry growth followed by membrane transfer was used to produce SIM with a large area up to 2.5 cm × 2.5 cm with ultrasmall thickness (ca. 90 nm) and high pore density (ca. 4.0 × 1012 cm−2). The prepared SIM-PET membrane demonstrated a molecular permeation rate almost one order of magnitude larger than commercial dialysis and nanoporous polycarbonate membranes. Moreover, thanks to uniform pore size (2.3 nm in diameter) and negatively charged surface of silica nanochannels, the membrane showed a precise selectivity at the molecular level in terms of molecular size and charge. Simultaneous achievement of high permeability and high selectivity, as well as simple fabrication and low cost, proves the potential of centimeter-scale SIM in molecular sieving and other applications.

A Mercury Ion Electrochemical Sensor Based on Porous Anodized Alumina Membrane Nanochannels Modified with DNA

A novel mercury ion (Hg2+) electrochemical sensor was established via chemical modification of porous anodic alumina (PAA) membrane nanochannels with DNA. PAA membrane was prepared by a two-step anodization. Amino groups were introduced to nanochannels by silanization. Special sequence DNA (along with amino group) was modified on PAA membrane nanochannels by the condensation reaction of amino and aldehyde groups. Electrochemical detector was built via sputtered Au nanoparticles on the surface of PAA nanochannels, which could be used as working electrode. The reduction potential of 0 V was applied, where the flux of Fe(CN)63− was determined by diffusion. The current response has a positive correlation with the concentration of Fe(CN)63−. After DNA immobilized, the negative charge and the steric hindrance of the nanochannels is increased, resulting in the decreasing of the flux of Fe(CN)63−. The detection current is reduced compared with pure PAA membrane nanochannels. When Hg2+ is present, a stable thymine-Hg2+-thymine structure is formed due to the specific reaction between thymine in DNA and Hg2+. In this case, the charge density and steric hindrance of the nanochannels are decreased, thereby increasing the flux of Fe(CN)63− and restoring current. The results show that the detection limit is about 0.08 pM with a good linear response (from 1 pM to 100 nM). Meanwhile, the sensor has a good selectivity. The current responses of other metal ions (Mn2+, Mg2+, Ba2+, Co2+ and Cd2+) are not obvious compared with Hg2+. Therefore, this new sensor could be applied for detecting Hg2+ in the environment.

Wetting transition in nanochannels for biomimetic free-blocking on-demand drug transport.

Water wetting behavior in nanometer dimensions is of great importance to the signal transmission and substance transport of organisms, e.g., aquaporins on cell membranes. A biological channel can control the transport of water and ions by regulating channel wettability, which results from the transition between the intrinsic hydrophobic state and the stimulus-induced hydration state. Inspired by aquaporins in nature, herein, a biomimetic free-blocking on-demand delivery system is proposed, which is constructed by controlling the wettability of the inner surface of nanochannels of mesoporous silica nanoparticles (MSNs). Such a system is completely different from the traditional physically occluding pore controlled release system. It circumvents the use of other extra capping agents, thus overcoming the limitations of the traditional nano "gate" blockage system with inherent instability, poor plugging capability and low biocompatibility. Additionally, further applications in drug delivery have shown that this system can selectively release entrapped drugs in beta cells triggered by intracellular glucose in a controlled manner but not in normal cells. This hydrophobic gating drug delivery system with simple and effective performance provides a new opportunity for constructing a mass transport platform from the perspective of surface wettability.

Electroosmotic Flow in Mixed Polymer Brush-Grafted Nanochannels.

Mixed polymer brush-grafted nanochannels—where two distinct species of polymers are alternately grafted on the inner surface of nanochannels—are an interesting class of nanostructured hybrid materials. By using a coarse-grained molecular dynamics simulation method, we are able to simulate the electrokinetic transport dynamics of the fluid in such nanochannels as well as the conformational behaviors of the mixed polymer brush. We find that (1) the brush adopts vertically-layered and longitudinally-separated structures due to the coupling of electroosmotic flow (EOF) and applied electric field; (2) the solvent quality affects the brush conformations and the transport properties of the EOF; (3) the EOF flux non-monotonically depends on the grafting density, although the EOF velocity in the central region of the channel monotonically depends on the grafting density.

Radiation induced deposition of copper nanoparticles inside the nanochannels of poly(acrylic acid)-grafted poly(ethylene terephthalate) track-etched membranes

Poly(ethylene terephthalate) PET, track-etched membranes (TeMs) with 400nm average pore size were UV-grafted with poly(acrylic acid) (PAA) after oxidation of inner surfaces by H2O2/UV system. Carboxylate groups of grafted PAA chains were easily complexed with Cu2+ ions in aqueous solutions. These ions were converted into metallic copper nanoparticles (NPs) by radiation-induced reduction of copper ions in aqueous-alcohol solution by gamma rays in the dose range of 46–250kGy. Copper ions chelating with -COOH groups of PAA chains grafted on PET TeMs form polymer-metal ion complex that prevent the formation of agglomerates during reduction of copper ions to metallic nanoparticles. The detailed analysis by X-Ray diffraction technique (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) confirmed the deposition of copper nanoparticles with the average size of 70nm on the inner surface of nanochannels of PET TeMs. Samples were also investigated by FTIR, ESR spectroscopies to follow copper ion reduction.

Permselective Ion Transfer at the Nanoscopic Ities Array

Free-standing silica nanochannel membrane (SNM) with perforated channels was utilized to create arrays of nanoscale interfaces between two immiscible electrolyte solutions (nano-ITIES), at which permselective ion transfer and detection were achieved. The SNM consisted of a high density of straight nanochannels with a diameter of 2.3 nm and a length of 70 nm. The silicon wafer coated by 150-nm-thick porous silicon nitride film (p-SiNF) with pores of 5 mm-in-diameter was used to support the SNM in a form of nanochannel-on-micropore. Considering the material surface lipophilicity, the nano-ITIES array was formed at the boundary between SNM and p-SiNF, with a diffusion geometry equivalent to two back-to-back inlaid microdisc interfaces. Thus, the transfer of tetraethylammonium (TEA+) across the nano-ITIES array yielded symmetric sigmoidal current responses in cyclic voltammogram. In addition, due to the ultrasmall size and negatively charged surface of silica nanochannels, the nano-ITIES displayed obvious size and charge permselectivities. Transfer of ions with a size comparable with or larger than the nanochannel, such as meso-tetra(N-methyl-4-pyridyl) porphine and iron(III) meso-tetra(4-sulfonatophenyl)porphine, was sterically blocked. And that of anions with a size smaller than the nanochannels, such as methyl orange, encountered the strong electrostatic repulsion from channel walls, showing obvious dependence on the ionic strength of aqueous solution in accordance with the prediction of classical Gouy-Chapman theory. The present approach is facile and inexpensive for building nano-ITIES array with potential applications in ion detection and separation. Figure 1

Label-Free Electrical Detection of Enzymatic Reactions in Nanochannels.

We report label-free electrical detection of enzymatic reactions using 2-D nanofluidic channels and investigate reaction kinetics of enzymatic reactions on immobilized substrates in nanoscale-confined spaces. Trypsin proteolysis is chosen for demonstration of the detection scheme. When trypsin cleaves poly-l-lysine coated on the surface of silica nanochannels, the resulting change of surface charge density can be detected by monitoring the ionic conductance of the nanochannels. Our results show that detection of such surface enzymatic reactions is faster than detection of surface binding reactions in nanochannels for low-concentration analytes. Furthermore, the nanochannel sensor has a sensitivity down to 5 ng/mL, which statistically corresponds to a single enzyme per nanochannel. Our results also suggest that enzyme kinetics in nanochannels is fundamentally different from that in bulk solutions or plain surfaces. Such enzymatic reactions form two clear self-propagating reaction fronts inside the nanochannels, and the reaction fronts follow square-root time dependences at high enzyme concentrations due to significant nonspecific adsorption. However, at low enzyme concentrations when nonspecific adsorption is negligible, the reaction fronts propagate linearly with time, and the corresponding propagation speed is related to the channel geometry, enzyme concentration, catalytic reaction constant, diffusion coefficient, and substrate surface density. Optimization of this nanochannel sensor could lead to a quick-response, highly sensitive, and label-free sensor for enzyme assay and kinetic studies.

Permselective Ion Transport Across the Nanoscopic Liquid/Liquid Interface Array.

Free-standing silica nanochannel membrane (SNM) with perforated channels was utilized to create arrays of nanoscale interfaces between two immiscible electrolyte solutions (nano-ITIES), at which permselective ion transfer and detection were achieved. The SNM consisted of a high density of straight nanochannels with a diameter of 2-3 nm and a length of 70 nm. The silicon wafer coated by 150 nm-thick porous silicon nitride film (p-SiNF) with pores of 5 μm-in-diameter was used to support the SNM in a form of nanochannel-on-micropore. Considering the material surface lipophilicity, the nano-ITIES array was formed at the boundary between SNM and p-SiNF, with a diffusion geometry equivalent to two back-to-back inlaid microdisc interfaces. Thus, the transfer of tetraethylammonium (TEA(+)) across the nano-ITIES array yielded symmetric sigmoidal current responses. In addition, because of the ultrasmall size and negatively charged surface of silica nanochannels, the nano-ITIES displayed obvious size and charge permselectivities. Transfer of ions with a size comparable with or larger than the nanochannel was sterically blocked. Also that of anions with a size smaller than the nanochannels encountered the strong electrostatic repulsion from channel walls, showing obvious dependence on the ionic strength of aqueous solution. The present approach is facile and inexpensive for building a nano-ITIES array with potential applications in ion detection and separation.

Supramolecular Organization of Dye Molecules in Zeolite L Channels: Synthesis, Properties, and Composite Materials.

Sequential insertion of different dyes into the 1D channels of zeolite L (ZL) leads to supramolecular sandwich structures and allows the formation of sophisticated antenna composites for light harvesting, transport, and trapping. The synthesis and properties of dye molecules, host materials, composites, and composites embedded in polymer matrices, including two- and three-color antenna systems, are described. Perylene diimide (PDI) dyes are an important class of chromophores and are of great interest for the synthesis of artificial antenna systems. They are especially well suited to advancing our understanding of the structure-transport relationship in ZL because their core fits tightly through the 12-ring channel opening. The substituents at both ends of the PDIs can be varied to a large extent without influencing their electronic absorption and fluorescence spectra. The intercalation/insertion of 17 PDIs, 2 terrylenes, and 1 quaterrylene into ZL are compared and their interactions with the inner surface of the ZL nanochannels discussed. ZL crystals of about 500 nm in size have been used because they meet the criteria that must be respected for the preparation of antenna composites for light harvesting, transport, and trapping. The photostability of dyes is considerably improved by inserting them into the ZL channels because the guests are protected by being confined. Plugging the channel entrances, so that the guests cannot escape into the environment is a prerequisite for achieving long-term stability of composites embedded in an organic matrix. Successful methods to achieve this goal are described. Finally, the embedding of dye-ZL composites in polymer matrices, while maintaining optical transparency, is reported. These results facilitate the rational design of advanced dye-zeolite composite materials and provide powerful tools for further developing and understanding artificial antenna systems, which are among the most fascinating subjects of current photochemistry and photophysics.

Salt Pumping by Voltage-Gated Nanochannels.

This Letter investigates voltage-gated nanochannels, where both the potential applied to the conductive membrane containing the channel (membrane potential) and the potential difference between the solutions at both sides of the membrane (transmembrane potential) are independently controlled. The predicted conductance characteristics of these fixed-potential channels dramatically differ from those of the widely studied fixed-charge nanochannels, in which the membrane is insulating and has a fixed surface charge density. The difference arises because the transmembrane potential induces an inhomogeneous charge distribution on the surface of fixed-potential nanochannels. This behavior, related to bipolar electrochemistry, has some interesting and unexpected consequences for ion transport. For example, continuously oscillating the transmembrane potential, while holding the membrane potential at the potential for which it has zero charge in equilibrium, creates fluxes of neutral salt (fluxes of anions and cations in the same direction and number) through the channel, which is an interesting phenomenon for desalination applications.

DNAzyme tunable lead(II) gating based on ion-track etched conical nanochannels.

A simple biomimetic ionic gate has been developed by modifying lead(II) ion responsive DNAzymes onto the inner surface of ion-etched polymer nanochannels.

A bio-inspired, sensitive, and selective ionic gate driven by silver (I) ions.

By grafting specific response DNA on the interior surface of ion track-etched conical nanochannels, a highly sensitive and selective ionic gate that can be driven by silver (I) ions is demonstrated. The switches between the OFF-state and the ON-state are mainly dependent on silver (I) ions and cysteine. Such a biomimetic nanodevice shows potential for application in sensing, pharmaceuticals, and sterilization.

Design and fabrication of a biomimetic nanochannel for highly sensitive arginine response in serum samples.

Inspired from their biological counterparts, chemical modification of the interior surface of nanochannels with functional molecules may provide a highly efficient means to control ionic or molecular transport through nanochannels. Herein, we have designed and prepared a aldehyde calix[4]arene (C4AH), which was attached to the interior surface of a single nanochannel by using a click reaction, and that showed a high response for arginine (Arg). Furthermore, the nanofluidic sensing system has been challenged with complex matrices containing a high concentration of interfering sequences and serum. Based on this finding, we believe that the artificial nanochannel can be used for practical Arg-sensing devices, and be applied in a biological environment.

Slip-enhanced reverse electrodialytic power generation in ion-selective nanochannels

Power generation by reverse electrodialysis in ion-selective nanochannels is numerically investigated. Especially,in the present study, the influence of hydrodynamic slip at the surface of nanochannels is investigated. The current-potential characteristics of the nanochannels are calculated by solving several governing equations:Nernst-Planck equation for the ionic concentrations, the Poisson equation for the electric potential, and the Navier-Stokes equation for the diffusioosmotic flow. Hydrodynamic slip is applied as the boundary condition at the surface of nanochannels. As the slip length increases, the diffusioosmotic flow velocity and electrical conductance of ions increase because the friction at the surface of nanochannels decreases. It is shown that the power generation is enhanced by 44% with a moderate 100nm slip length by using a nanochannel with 10nm height.

Phase diagrams of confined solutions of dimyristoylphosphatidylcholine (DMPC) lipid and cholesterol in nanotubes

We have studied equilibrium morphologies of dimyristoylphosphatidylcholine lipid solution and cholesterol solution confined to nanotubes using dissipative particle dynamics (DPD) simulations. Phase diagrams regarding monomer concentration c versus radius of nanotube r for both solutions are attained. Three types of the inner surface of nanotubes, namely hydrophobic, hydrophilic, and hydroneutral are considered in the DPD simulations. A number of phases and molecular assemblies for the confined solutions are revealed, among others, such as the spiral wetting and bilayer helix. Several phases and assemblies have not been reported in the literature, and some are non-existence in bulk solutions. The ability to control the morphologies and self-assemblies within nanoscale confinement can be exploited for patterning interior surface of nanochannels for application in nanofluidics and nanomedical devices.

Power generation from concentration gradient by reverse electrodialysis in ion-selective nanochannels

In an aqueous solution, the surface of inorganic nanochannels acquires charges from ionization, ion adsorption, and ion dissolution. These surface charges draw counter-ions toward the surface and repel co-ions. In the presence of a concentration gradient, counter-ions are transported through nanochannels much more easily than co-ions, which results in a net charge migration of one type of ions. The Gibbs free energy of mixing, which forces ion diffusion, thus can be converted into electrical energy by using inorganic ion-selective nanochannels. Silica nanochannels with heights of 4, 26, and 80 nm were used in this study. We experimentally investigated the power generation from these nanochannels placed between two potassium chloride solutions with various combinations of concentrations. The power generation per unit channel volume increases when the concentration gradient increases, and also increases as channel height decreases. The highest power density measured was 7.7 W/m2. Our data also indicate that the energy conversion efficiency and the ion selectivity increase with a decrease of concentrations and channel height. The best efficiency obtained was 31%. Power generation from concentration gradients in inorganic ion-selective nanochannels could be used in a variety of applications, including micro batteries and micro power generators.

Solvation Dynamics of Coumarin 153 in Alcohols Confined in Silica Nanochannels

Solvation dynamics in alcohols confined in silica nanochannels was examined by time-resolved fluorescence spectroscopy using coumarin 153 (C153) as a fluorescent probe. Surfactant-templated mesoporous silica was fabricated inside the pores of an anodic alumina membrane. The surfactant was removed by calcination to give mesoporous silica (Cal-NAM) containing one-dimensional (1D) silica nanochannels (diameter, 3.1 nm) whose inner surface was covered with silanol groups. By treating Cal-NAM with trimethylchlorosilane, trimethylsilyl (TMS) groups were formed on the inner surface of the silica nanochannels (TMS-NAM). Fluorescence dynamic Stokes shifts of C153 were measured in alcohols (ethanol, butanol, hexanol, and decanol) confined in the silica nanochannels of Cal- and TMS-NAMs, and the time-dependent fluorescence decay profiles could be best fitted by a biexponential function. The estimated solvent relaxation times were much larger than those observed in bulk alcohols for both Cal- and TMS-NAMs when ethanol or butanol was used as a solvent, indicating that the mobility of these alcohol molecules was restricted within the silica nanochannels. However, hexanol or decanol in Cal- and TMS-NAMs did not cause a remarkable increase in the solvent relaxation time in contrast to ethanol or butanol. Therefore, it was concluded that a relatively rigid assembly of alcohols (an alcohol chain) was formed within the silica nanochannels by hydrogen bonding interaction and van der Waals force between the surface functional groups of the silica nanochannels and alcohol molecules and by the successive interaction between alcohol molecules when alcohol with a short alkyl chain (ethanol or butanol) was used as a solvent.

Self-Assembly of Surfactants and Polymorphic Transition in Nanotubes

We study self-assembly and polymorphic transitions of surfactant molecules in water within a nanotube and the effect of water-nanotube interactions on the self-assembly morphologies. We present a simulation evidence of a cornucopia of polymorphic structures of surfactant assemblies--many of which have not been observed in bulk solutions--through adjusting the water-nanotube chemical interactions which range from hydrophilic to hydroneutral and to hydrophobic. The ability to control the morphologies of surfactant assemblies within nanoscale confinement can be used for patterning the interior surface of nanochannels for application in nanofluidics and nanomedical devices.

Very High Surface Area Microporous Carbon with a Three-Dimensional Nano-Array Structure: Synthesis and Its Molecular Structure

By using zeolite Y as a template, a novel microporous carbon with a three-dimensional nano-array structure was prepared. The arrangement and periodicity of this array are identical to those of supercages in zeolite Y crystal. The carbon has a surprisingly high BET surface area (3600 m2/g) with almost no mesoporosity. This carbon may be comprised of very curved graphene sheets whose faceting and curvature are accommodated to the curved inner surface of the zeolite nanochannels.