Reversing current rectification to improve DNA‐sensing sensitivity in conical nanopores

Herein, we report the ultrasensitive DNA detection through designing an elegant nanopore biosensor as the first case to realize the reversal of current rectification direction for sensing. Attributed to the unique asymmetric structure, the glass conical nanopore exhibits the sensitive response to the surface charge, which can be facilely monitored by ion current rectification curves. In our design, an enzymatic cleavage reaction was employed to alter the surface charge of the nanopore for DNA sensing. The measured ion current rectification was strongly responsive to DNA concentrations, even reaching to the reversed status from the negative ratio (?6.5) to the positive ratio (+16.1). The detectable concentration for DNA was as low as 0.1 fM. This is an ultrasensitive and label‐free DNA sensing approach, based on the rectification direction‐reversed amplification in a single glass conical nanopore.     

Current rectification by asymmetric molecules: an ab initio study

Current rectification effect in an asymmetric molecule HCOO-C6H4-(CH2)n sandwiched between two aluminum electrodes has been studied using an ab initio nonequilibrium Green's function method. The conductance of the system decreases exponentially with the increasing number n of CH2. The phenomenon of current rectification is observed such that a very small current appears at negative bias and a sharp negative differential resistance at a critical positive bias when n>or=2. The rectification effect arises from the asymmetric structure of the molecule and the molecule-electrode couplings. A significant rectification ratio of approximately 38 can be achieved when n=5.     

Self‐Assembled Gold Arrays That Allow Rectification by Nanoscale Selectivity

The deposition of a monolayer nanoarray on the surface of a micrometer‐thick substrate is demonstrated, producing rectification characteristics at the nanoscale. The experimental results show that the heterogeneity of the structure and the charge density are the two key factors affecting rectification, which was confirmed with molecular dynamic (MD) and finite element simulations. Moreover, by altering the asymmetric electrolyte environment, the fabricated heterogeneous membrane can be used in energy conversion. This study provides insights into the mechanism underlying the generation of rectification and related factors, providing a theoretical basis for the characteristics of rectification.     

Rectification of the current in alpha-hemolysin pore depends on the cation type: the alkali series probed by MD simulations and experiments

A striking feature of the alpha-hemolysin channel-a prime candidate for biotechnological applications-is the dependence of its ionic conductance on the magnitude and direction of the applied bias. Through a combination of lipid bilayer single-channel recording and molecular dynamics (MD) simulations, we characterized the current-voltage relationship of alpha-hemolysin for all alkali chloride salts at neutral pH. The rectification of the ionic current was found to depend on the type of cations and increase from Li(+) to Cs(+). Analysis of the MD trajectories yielded a simple quantitative model that related the ionic current to the electrostatic potential, the concentration and effective mobility of ions in the channel. MD simulations reveal that the major contribution to the current asymmetry and rectification properties originates from the cationic contribution to the current that is significantly reduced in a cationic dependent way when the membrane polarity is reversed. The variation of chloride current was found to be less important. We report that the differential affinity of cations for the charged residues positioned at the channel's end modulates the number of ions inside the channel stem thus affecting the current properties. Through direct comparison of simulation and experiment, this study evaluates the accuracy of the MD method for prediction of the asymmetric, voltage dependent conductances of a membrane channel.     

Light‐Induced Ion Rectification in Zigzag Nanochannels

Ion transport through nanoporous systems has attracted broad interest due to its crucial role in physiological processes in living organisms and artificial bionic devices. In this work, a nanochannel system with a zigzag inner surface was fabricated by using a two‐step anodizing technique. The rectification performance of the zigzag channels was observed by I–V measurement in KCl solution. Unlike channels with asymmetric geometry, the mechanism was analyzed based on the “point effect” of charge distribution and “shape effect” of the zigzag channel. The current rectification ratio decreases from nearly 3.0 to 1.0 when the KCl concentration increased from 0.1 mm to 100 mm . The fabrication of different nanopore systems and exploration of novel mechanisms will help to develop biomimetic membranes for practical applications.     

How the asymmetry of internal potential influences the shape of I-V characteristic of nanochannels

Ion transport in biological and synthetic nanochannels is characterized by such phenomena as ion current fluctuations, rectification, and pumping. Recently, it has been shown that the nanofabricated synthetic pores could be considered as analogous to biological channels with respect to their transport characteristics [P. Yu. Apel et al., Nucl. Instrum. Methods Phys. Res. B 184, 337 (2001); Z. Siwy et al., Europhys. Lett. 60, 349 (2002)]. The ion current rectification is analyzed. Ion transport through cylindrical nanopores is described by the Smoluchowski equation. The model is considering the symmetric nanopore with asymmetric charge distribution. In this model, the current rectification in asymmetrically charged nanochannels shows a diodelike shape of I-V characteristic. It is shown that this feature may be induced by the coupling between the degree of asymmetry and the depth of internal electric potential well. The role of concentration gradient is discussed.     

Graphitic Silicon Nitride: A Metal‐Free Ferromagnet with Charge and Spin Current Rectification

As a first example, herein we show that g‐Si₄N₃ is expected to act as a metal‐free ferromagnet featuring both charge and spin current rectification simultaneously. Such rectification is crucial for envisioning devices that contain both logic and memory functionality on a single chip. The spin coherent quantum‐transport calculations on g‐Si₄N₃ reveal that the chosen system is a unique molecular spin filter, the current‐voltage characteristics of which is asymmetric in nature, which can create a perfect background for synchronous charge and spin current rectification. To shed light on this highly unusual in‐silico observation, we have meticulously inspected the bias‐dependent modulation of the spin‐polarized eigenstates. The results indicate that, whereas only the localized 2p orbitals of the outer‐ring (OR) Si atoms participate in the transmission process in the positive bias, both OR Si and N atoms contribute in the reverse bias. Furthermore, we have evaluated the spin‐polarized electron‐transfer rate in the tunneling regime, and the results demonstrate that the transfer rates are unequal in the positive and negative bias range, leading to the possible realization of a simultaneous logic–memory device.     

Surface charge density determination of single conical nanopores based on normalized ion current rectification

Current rectification is well known in ion transport through nanoscale pores and channel devices. The measured current is affected by both the geometry and fixed interfacial charges of the nanodevices. In this article, an interesting trend is observed in steady-state current-potential measurements using single conical nanopores. A threshold low-conductivity state is observed upon the dilution of electrolyte concentration. Correspondingly, the normalized current at positive bias potentials drastically increases and contributes to different degrees of rectification. This novel trend at opposite bias polarities is employed to differentiate the ion flux affected by the fixed charges at the substrate-solution interface (surface effect), with respect to the constant asymmetric geometry (volume effect). The surface charge density (SCD) of individual nanopores, an important physical parameter that is challenging to measure experimentally and is known to vary from one nanopore to another, is directly quantified by solving Poisson and Nernst-Planck equations in the simulation of the experimental results. The flux distribution inside the nanopore and the SCD of individual nanopores are reported. The respective diffusion and migration translocations are found to vary at different positions inside the nanopore. This knowledge is believed to be important for resistive pulse sensing applications because the detection signal is determined by the perturbation of the ion current by the analytes.     

Ion transport regulation through triblock copolymer/PET asymmetric nanochannel membrane:Model system establishment and rectification mapping

Controlling ions transport across the membrane at different pH environments is essential for the physiological process and artificial systems.Many efforts have been devoted to pH-responsive ion gating,while rarely systems can maintain the rectification in pH-changing environments.Here,a composite nanochannel system is fabricated,which shows unidirectional rectification with high performance in a wide pH range.In the system,block copolymer(BCP) and polyethylene te rephthalate(PET) are employed for the amphoteric nanochannels fabrication.Based on the composite system,a model is built for the theoretical simulation.Thereafter,rectification mapping is conducted on the system,which can provide abundant info rmation about the relations between charge distribution and ions transport prope rties.The proposed rectification mapping can definitely help to design new materials with special ion transport properties,such as high-performance membranes used in the salinity gradient power generation field.     

Concentration-gradient-dependent ion current rectification in charged conical nanopores

Ion current rectification (ICR) in negatively charged conical nanopores is shown to be controlled by the electrolyte concentration gradient depending on the direction of ion diffusion. The degree of ICR is enhanced with the increasing forward concentration difference. An unusual rectification inversion is observed when the concentration gradient is reversely applied. A numerical simulation based on the coupled Poisson and Nernst-Planck (PNP) equations is proposed to solve the ion distribution and ionic flux in the charged and structurally asymmetric nanofluidic channel with diffusive ion flow. Simulation results qualitatively describe the diffusion-induced ICR behavior in conical nanopores suggested by the experimental data. The concentration-gradient-dependent ICR enhancement and inversion is attributed to the cooperation and competition between geometry-induced asymmetric ion transport and the diffusive ion flow. The present study improves our understanding of the ICR in asymmetric nanofluidic channels associated with the ion concentration difference and provides insight into the rectifying biological ion channels.     

Electrostatic properties and macroscopic electrodiffusion in OmpF porin and mutants

The bacterial porin OmpF found in the outer membrane of E. coli is a wide channel, characterized by its poor selectivity and almost no ion specificity. It has an asymmetric structure, with relatively large entrances and a narrow constriction. By applying continuum electrostatic methods we determine the ionization states of titratable amino acid residues in the protein and calculate self-consistently the electric potential 3-D distribution within the channel. The average electrostatic properties are then represented by an effective fixed charge distribution along the pore which is the input for a macroscopic electrodiffusion model. The theoretical predictions agree with measurements performed under different salt gradients and pH. The sensitivity of reversal potential and conductance to the direction of the salt gradient and the solution pH is captured by the model. The theory is also able to explain the influence of the lipid membrane charge. The same methodology is satisfactorily applied to some OmpF mutants involving slight structural changes but a large number of net charges. The correlation found between atomic structure and ionic selectivity shows that the transport characteristics of wide channels like OmpF and its mutants are mainly regulated by the collective action of a large number of residues, rather than by the specific interactions of residues at particular locations.     

纳通道的物质传输特性及应用

近年来,随着材料科学、微纳加工技术和微纳尺度物质传输理论的发展,纳通道技术得到了越来越多的研究和关注。纳通道包括生物纳通道和人工纳通道,其孔径通常为1~100 nm。在这一尺度下,通道表面与通道内物质之间的作用概率大大增强,使得纳通道表现出许多与宏观体系不同的物质传输特性,例如通道表面电荷与通道内离子之间的静电作用产生了离子选择性,通道内电化学势的不对称分布产生了离子整流特性,物质传输过程中占据通道产生了阻塞脉冲特性等。纳通道中的这些物质传输特性在传感、分离、能源等领域具有广泛应用,例如通过对纳通道进行功能化修饰可以实现门控离子传输;利用亚纳米尺度的通道可以实现单分子传感;利用通道与传输物质之间的相互作用可以实现离子、分子、纳米粒子的分离;利用纳通道的离子选择性可以在通道内实现电荷分离,将不同形式的能量（如光、热、压力、盐差等）高效转化为电能。纳通道技术是化学、材料科学、纳米技术等多学科的交叉集合,在解决生物、环境、能源等基本问题方面具有良好的前景。该文综述了近10年来与纳通道物质传输理论以及纳通道技术应用相关的前沿研究,梳理了纳通道技术的发展过程,并对其在各个领域的应用进行了总结与展望。     

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.     

Heat rectification in molecular junctions

Heat conduction through molecular chains connecting two reservoirs at different temperatures can be asymmetric for forward and reversed temperature biases. Based on analytically solvable models and on numerical simulations we show that molecules rectify heat when two conditions are satisfied simultaneously: the interactions governing the heat conduction are nonlinear, and the junction has some structural asymmetry. We consider several simplified models where a two-level system (TLS) simulates a highly anharmonic vibrational mode, and asymmetry is introduced either through different coupling of the molecule to the contacts, or by considering internal molecular asymmetry. In the first case, we present analytical results for the asymmetric heat current flowing through a single anharmonic mode using different forms for the TLS-reservoirs coupling. We also demonstrate numerically, studying a realistic molecular model, that a uniform anharmonic molecular chain connecting asymmetrically two thermal reservoirs rectifies heat. This effect is stronger for longer chains, where nonlinear interactions dominate the transfer process. When asymmetry is related to the internal level structure of the molecule, numerical simulations reveal a nontrivial rectification behavior. We could still explain this behavior in terms of an effective system-bath coupling. Our study suggests that heat rectification is a fundamental characteristic of asymmetric nonlinear thermal conductors. This phenomenon is important for heat control in nanodevices and for understanding of energy flow in biomolecules.     

Molecular rectification in triangularly shaped graphene nanoribbons

We present a theoretical study of electron transport in tailored zigzag graphene nanoribbons (ZGNRs) with triangular structure using density functional theory together with the nonequilibrium Green's function formalism. We find significant rectification with a favorite electron transfer direction from the vertex to the right edge. The triangular ZGNR connecting to the electrode with one thiol group at each terminal shows an average rectification ratio of 8.4 over the bias range from ?1.0 to 1.0 V. This asymmetric electron transport property originates from nearly zero band gap of triangular ZGNR under negative bias, whereas a band gap opens under positive bias. When the molecule is connected to the electrode by multithiol groups, the current is enhanced due to strong interfacial coupling; however, the rectification ratio decreases. The simulation results indicate that the unique electronic states of triangular ZGNR are responsible for rectification, rather than the asymmetric anchoring groups. © 2012 Wiley Periodicals, Inc.     

A Tunable Ionic Diode Based on a Biomimetic Structure‐Tailorable Nanochannel

A tunable ionic diode is presented that is based on biomimetic structure‐tailorable nanochannels, with precise ion‐transport characteristics from ohmic behavior to bidirectional rectification as well as gating properties. The forward/reverse directions of the ionic diode and the degree of rectification can be well‐regulated by combining the patterned surface charge and the sophisticated structure. This system creates an ideal platform for precise transportation of ions and molecules, and potential applications in analytical sciences are anticipated.     

Ion current rectification at nanopores in glass membranes

The origin of ion current rectification observed at conical-shaped nanopores in glass membranes immersed in KCl solutions has been investigated using finite-element simulations. The ion concentrations and fluxes (due to diffusion, migration, and electroosmotic convection) were determined by the simultaneous solution of the Nernst-Planck, Poisson, and Navier-Stokes equations for the two-ion (K+ and Cl-) system. Fixed surface charge on both the internal and external glass surfaces that define the pore structure was included to account for electric fields and nonuniform ion conductivity within the nanopores and electric fields in the external solution near the pore mouth. We demonstrate that previous observations of ion current rectification in conical-shaped glass nanopores are a consequence of the voltage-dependent solution conductivity in the vicinity of the pore mouth, both inside and outside of the pore. The simulations also demonstrate that current rectification is maximized at intermediate bulk ion concentrations, a combination of (i) the electrical screening of surface charge at high concentrations and (ii) a fixed number of charge-carrying ions in the pore at lower concentration, which are physical conditions where the voltage dependence of the conductivity disappears. In addition, we have quantitatively shown that electroosmotic flow gives rise to a significant but small contribution to current rectification.     

A Tunable Ionic Diode Based on a Biomimetic Structure-Tailorable Nanochannel

A tunable ionic diode is presented that is based on biomimetic structure-tailorable nanochannels, with precise ion-transport characteristics from ohmic behavior to bidirectional rectification as well as gating properties. The forward/reverse directions of the ionic diode and the degree of rectification can be well-regulated by combining the patterned surface charge and the sophisticated structure. This system creates an ideal platform for precise transportation of ions and molecules, and potential applications in analytical sciences are anticipated.     

Ion current rectification behavior of a nanochannel having nonuniform cross-section

Due to its versatile applications in biotechnology, ion current rectification (ICR), which arises from the asymmetric nature of the ion transport in a nanochannel, has drawn much attention, recently. Here, the ICR behavior of a pH-regulated nanochannel comprising two series connected cylindrical nanochannels of different radii is examined theoretically, focusing on the influences of the radii ratio, the length ratio, the bulk concentration, and the solution pH. The results of numerical simulation reveal that the rectification factor exhibits a local maximum with respect to both the radii ratio and the length ratio. The values of the radii ratio and the length ratio at which the local maximum in the rectification factor occur depend upon the level of the bulk salt concentration. The rectification factor also shows a local maximum as the solution pH varies. Among the factors examined, the solution pH influences the ICR behavior of the nanochannel most significantly.     

Rectification of nanopores at surfaces

At the nanoscale, methods to measure surface charge can prove challenging. Herein we describe a general method to report surface charge through the measurement of ion current rectification of a nanopipette brought in close proximity to a charged substrate. This method is able to discriminate between charged cationic and anionic substrates when the nanopipette is brought within distances from ten to hundreds of nanometers from the surface. Further studies of the pH dependence on the observed rectification support a surface-induced mechanism and demonstrate the ability to further discriminate between cationic and nominally uncharged surfaces. This method could find application in measurement and mapping of heterogeneous surface charges and is particularly attractive for future biological measurements, where noninvasive, noncontact probing of surface charge will prove valuable.