Intrinsic Rectification Research Articles

Nanocomposite-modified nanopores: A promising platform for selective detection of copper ions

This research introduces an innovative platform designed for the selective detection of copper (II) (Cu2+) ions, employing a singular nanopore embedded within a 12 μ m-thick polyethylene terephthalate (PET) membrane. The track-etched nanopore was subsequently modified with nanomaterials in an allylamine hydrochloride (PAH)-modified asymmetric nanopore through electrostatic interactions. The nanomaterials included copper oxide (CuO), graphene oxide (GO), and their composite (GO/CuO), were synthesized using wet chemistry, and their structures and optical properties were thoroughly investigated using x-ray diffraction and diffuse reflectance spectroscopy. In the distinctive feature of the platform, Cu2+ ions gain access to coordination sites across a broad surface covered by the immobilized nanomaterials, facilitating effective binding. The selective response of GO/CuO modified pores towards Cu2+ ions was notably observed through ion transportation (I –V) studies, surpassing the response of unmodified nanopores and those modified with CuO alone. This selective behavior was demonstrated in the presence of various monovalent and divalent competing ions. Quantitative assessment of I –V studies was conducted by evaluating the intrinsic rectification ratio of asymmetric nanopores, providing a robust measure of platform's efficacy. Furthermore, we identified the optimal pH for detecting Cu2+ ions as 7, enhancing the specificity and accuracy of our method.

Understanding the Intrinsic Rectification Properties of Nanoporous Anodic Alumina by Selective Chemical Etching.

The distribution of oxygen and aluminum vacancies across the hemispherical barrier oxide layer (BOL) of nanoporous anodic alumina (NAA) relies intrinsically on the electric field-driven flow of electrolytic species and the incorporation of electrolyte impurities during the growth of anodic oxide through anodization. This phenomenon provides new opportunities to engineer BOL's inherited ionic current rectification (ICR) fingerprints. NAA's characteristic ICR signals are associated with the space charge density gradient across BOL and electric field-induced ion migration through hopping from vacancy to vacancy. In this study, we engineer the intrinsic space charge density gradient of the BOL of NAA under a range of anodizing potentials in hard and mild anodization regimes. Real-time characterization of the ICR fingerprints of NAA during selective etching of the BOL makes it possible to unravel the distribution pattern of vacancies through rectification signals as a function of etching direction and time. Our analysis demonstrates that the space charge density gradient varies across the BOL of NAA, where the magnitude and distribution of the space charge density gradient are revealed to be critically determined by anodizing the electrolyte, regime, and potential. This study provides a comprehensive understanding of the engineering of ion transport behavior across blind-hole NAA membranes by tuning the distribution of defects across BOL through anodization conditions. This method has the potential to be harnessed for developing nanofluidic devices with tailored ionic rectification properties for energy generation and storage and sensing applications.

Physiological role of SK channels in modulating cardiac repolarization: APD and dispersion at slow heart rate in long QT syndrome.

Long QT syndrome (LQTS) is a congenital disease associated with polymorphic ventricular tachycardia (pVT) and sudden cardiac death. The hallmarks of LQTS are early afterdepolarizations that initiate pVT and dispersion of repolarization allowing reentry formation. Small conductance Ca2+-activated K+ (SK) channels are directly gated by Ca2+ and provide feedback from Ca2+ to membrane voltage to accelerate cardiac repolarization, which may help normalizing LQTS. Using optical mapping, we examined the role of ISK in transgenic rabbit models of LQT1, LQT2, and LQT5 and found that enhancing ISK with NS309 consistently shortened APD, reduced APD dispersion, and suppressed pVT genesis universally in the three LQT rabbit models. Enhancement of ISK has a greater impact at longer cycle length, flattening restitution curves at slow heart rate where most pVTs occur in LQTS. We incorporated a six state ISK gating simulation combined with intrinsic rectification and divalent block into the Mahajan rabbit action potential model to investigate the role of ISK in suppressing LQT-related arrhythmias. Increasing levels of SK channel conductance (gSK) were able to shorten and reduce APD dispersion simulated by conductance gradients of gKs by ∼64% under LQT2 conditions and gtof by ∼70% in LQT1 conditions. Computer modeling indicates that ISK can reduce APD dispersion through two mechanisms; increased Ca2+ duration in the prolonged APD cells augment ISK to shorten long APD and lowering plateau Vm by ISK masks IKs heterogeneity by reducing IKs's role in repolarization. In conclusion, the Ca2+-dependent gating of ISK allows it to directly feedback on the APD especially when other K+ channels are not functional and to be a potential therapeutic target for LQTS.

Chirality-induced intrinsic charge rectification in a tellurium-based field-effect transistor

Chiral conductors exhibit spin-related charge rectification but its controllability is largely unexplored. Here, the authors combine chiral charge rectification with a field-effect transistor of tellurium. They demonstrate a gate-variable hundredfold enhancement of the rectification efficiency and enable quantitative comparison with theoretical calculations which considered chiral spin-momentum locking alongside the Zeeman effect. With this comparison across a wide range of temperature, an unexpected deviation from the conventional theory is revealed, which hints at the relevance of chirality-induced spin selectivity at higher temperatures.

Exploring Axial Organic Multiblock Heterostructure Nanowires: Advances in Molecular Design, Synthesis, and Functional Applications

AbstractOrganic multiblock heterostructure nanowires (OMHNs), integrating distinct components in the same axial structure, have attracted much attention due to their excellent properties such as intrinsic good light confinement and rectification characteristics. However, rationally incorporating such features into OMHNs with rigorous 1D morphology control remains a key synthetic objective. It is because that the single components possess fundamentally distinctive properties such as size‐dependent electronic, optical, and magnetic features as well as lattice parameters. In the last decade, researchers made their own efforts to this exciting field, especially from the controlled fabrication to the advanced applications of the multiblock nanowires. As a result, various OMHNs have been developed as very promising candidates for high‐performance optoelectronic applications such as light harvesting, logic gate devices, photonic transistors, and photonic anticounterfeiting. Herein, the recent advances of OMHNs from the aspects of molecular design, synthesis, and advanced applications are reviewed, and an outlook is given providing inspiration for the future development of OMHNs toward integrated optoelectronic at nanoscale.

Asymmetric bias-induced barrier lowering as an alternative origin of current rectification in geometric diodes

Geometric diodes, which take advantage of geometric asymmetry to achieve current flow preference, are promising for THz current rectification. Previous studies relate geometric diodes’ rectification to quantum coherent or ballistic transport, which is fragile and critical of the high-quality transport system. Here we propose a different physical mechanism and demonstrate a robust current rectification originating from the asymmetric bias induced barrier lowering, which generally applies to common semiconductors in normal environments. Key factors to the diode’s rectification are carefully analyzed, and an intrinsic rectification ability at up to 1.1 THz is demonstrated.

Kir4.1/Kir5.1 channels possess strong intrinsic inward rectification determined by a voltage-dependent K+-flux gating mechanism.

Inwardly rectifying potassium (Kir) channels are broadly expressed in both excitable and nonexcitable tissues, where they contribute to a wide variety of cellular functions. Numerous studies have established that rectification of Kir channels is not an inherent property of the channel protein itself, but rather reflects strong voltage dependence of channel block by intracellular cations, such as polyamines and Mg2+. Here, we identify a previously unknown mechanism of inward rectification in Kir4.1/Kir5.1 channels in the absence of these endogenous blockers. This novel intrinsic rectification originates from the voltage-dependent behavior of Kir4.1/Kir5.1, which is generated by the flux of potassium ions through the channel pore; the inward K+-flux induces the opening of the gate, whereas the outward flux is unable to maintain the gate open. This gating mechanism powered by the K+-flux is convergent with the gating of PIP2 because, at a saturating concentration, PIP2 greatly reduces the inward rectification. Our findings provide evidence of the coexistence of two rectification mechanisms in Kir4.1/Kir5.1 channels: the classical inward rectification induced by blocking cations and an intrinsic voltage-dependent mechanism generated by the K+-flux gating.

Impact of ISK Voltage and Ca2+/Mg2+-Dependent Rectification on Cardiac Repolarization

Cardiac small conductance Ca2+-activated K+ (SK) channels are activated solely by Ca2+, but the SK current (ISK) is inwardly rectified. However, the impact of inward rectification in shaping action potentials (APs) in ventricular cardiomyocytes under β-adrenergic stimulation or in disease states remains undefined. Two processes underlie this inward rectification: an intrinsic rectification caused by an electrostatic energy barrier from positively charged amino acids at the inner pore and a voltage-dependent Ca2+/Mg2+ block. Thus, Ca2+ has a biphasic effect on ISK, activating at low [Ca2+] yet inhibiting ISK at high [Ca2+]. We examined the effect of ISK rectification on APs in rat cardiomyocytes by simultaneously recording whole-cell apamin-sensitive currents and Ca2+ transients during an AP waveform and developed a computer model of SK channels with rectification features. The typical profile of ISK during AP clamp included an initial peak (mean 1.6 pA/pF) followed by decay to the point that submembrane [Ca2+] reached ∼10 μM. During the rest of the AP stimulus, ISK either plateaued or gradually increased as the cell repolarized and submembrane [Ca2+] decreased further. We used a six-state gating model combined with intrinsic and Ca2+/Mg2+-dependent rectification to simulate ISK and investigated the relative contributions of each type of rectification to AP shape. This SK channel model replicates key features of ISK recording during AP clamp showing that intrinsic rectification limits ISK at high Vm during the early and plateau phase of APs. Furthermore, the initial rise of Ca2+ transients activates, but higher [Ca2+] blocks SK channels, yielding a transient outward-like ISK trajectory. During the decay phase of Ca2+, the Ca2+-dependent block is released, causing ISK to rise again and contribute to repolarization. Therefore, ISK is an important repolarizing current, and the rectification characteristics of an SK channel determine its impact on early, plateau, and repolarization phases of APs.

Role of SK Current Rectification in Shaping Action Potential of Ventricular Cardiomyocytes

The role of cardiac small conductance Ca2+-activated K+ (SK) channels in shaping the action potential (AP) of ventricular cardiomyocytes remains undefined. Two processes underlie inward rectification of SK current (ISK), an intrinsic voltage-dependent rectification caused by positively charged amino acids at the inner pore, and a divalent ion block. The latter process gives Ca2+ a biphasic effect on ISK, activating at low [Ca2+] yet inhibiting ISK at high [Ca2+]. We examined the role of ISK rectification on AP in rat cardiomyocytes by simultaneously recording whole-cell apamin-sensitive currents and Ca2+ transients during an AP waveform, and developed a computer model of SK channel with rectification features. The typical profile of ISK during AP-clamp included an initial peak (mean 1.6 pA/pF) followed by decay to the point where submembrane [Ca2+] reached ∼10 μM. During the rest of the AP stimulus, ISK either plateaued or gradually increased as the cell repolarized and submembrane [Ca2+] continued to decrease. We used a six state gating model combined with intrinsic rectification and divalent block to simulate ISK and investigated the relative contributions of the two types of rectification to the AP shape. This SK channel model replicates key features of ISK recording during AP-clamp. The simulation shows that the rise of Ca2+ during Ca2+ transients activates SK channel opening, but higher [Ca2+] blocks channels, resulting in transient outward-like trajectory of ISK. During the decay phase of Ca2+, Ca2+-dependent block is released causing ISK to contribute to repolarization. The intrinsic rectification characteristics limit ISK during the early and plateau phase of APs to prevent excessive APD shortening. In conclusion, ISK is an important repolarizing current and the rectification characteristics of SK channel determines its impact on early, plateau and repolarization phases of APs.

Identification of a PEST Sequence in Vertebrate KIR2.1 That Modifies Rectification.

KIR2.1 potassium channels, producing inward rectifier potassium current (IK1), are important for final action potential repolarization and a stable resting membrane potential in excitable cells like cardiomyocytes. Abnormal KIR2.1 function, either decreased or increased, associates with diseases such as Andersen-Tawil syndrome, long and short QT syndromes. KIR2.1 ion channel protein trafficking and subcellular anchoring depends on intrinsic specific short amino acid sequences. We hypothesized that combining an evolutionary based sequence comparison and bioinformatics will identify new functional domains within the C-terminus of the KIR2.1 protein, which function could be determined by mutation analysis. We determined PEST domain signatures, rich in proline (P), glutamic acid (E), serine (S), and threonine (T), within KIR2.1 sequences using the “epestfind” webtool. WT and ΔPEST KIR2.1 channels were expressed in HEK293T and COS-7 cells. Patch-clamp electrophysiology measurements were performed in the inside-out mode on excised membrane patches and the whole cell mode using AxonPatch 200B amplifiers. KIR2.1 protein expression levels were determined by western blot analysis. Immunofluorescence microscopy was used to determine KIR2.1 subcellular localization. An evolutionary conserved PEST domain was identified in the C-terminus of the KIR2.1 channel protein displaying positive PEST scores in vertebrates ranging from fish to human. No similar PEST domain was detected in KIR2.2, KIR2.3, and KIR2.6 proteins. Deletion of the PEST domain in California kingsnake and human KIR2.1 proteins (ΔPEST), did not affect plasma membrane localization. Co-expression of WT and ΔPEST KIR2.1 proteins resulted in heterotetrameric channel formation. Deletion of the PEST domain did not increase protein stability in cycloheximide assays [T½ from 2.64 h (WT) to 1.67 h (ΔPEST), n.s.]. WT and ΔPEST channels, either from human or snake, produced typical IK1, however, human ΔPEST channels displayed stronger intrinsic rectification. The current observations suggest that the PEST sequence of KIR2.1 is not associated with rapid protein degradation, and has a role in the rectification behavior of IK1 channels.

High-performance spin rectification in gallium nitride-based molecular junctions with asymmetric edge passivation

The spin transport properties of molecular devices constructed from zigzag gallium nitride nanoribbons (ZGaNNRs) are investigated by applying the non-equilibrium Green’s function formalism in combination with density functional theory. The computational results indicate that ZGaNNR systems show spin rectification with a high efficiency, approaching nearly 109, giant magnetoresistance with a ratio up to 108, perfect spin-filtering, and negative differential resistance effects. Importantly, our results reveal that intrinsic rectification can be observed regardless of their width. The microscopic origins of the rectification are revealed and discussed in terms of a spin-resolved transmission spectrum, the band structures of the ZGaNNRs, and the molecular projected self-consistent Hamiltonian. Our findings could be useful for designing GaN-based spintronic nanodevices.

Diverse Atomically Sharp Interfaces and Linear Dichroism of 1T' ReS2‐ReSe2 Lateral p–n Heterojunctions

AbstractCreating heterojunctions between different 2D transition‐metal dichalcogenides (TMDs) would enable on‐demand tuning of electronic and optoelectronic properties in this new class of materials. However, the studies to date are mainly focused on hexagonal (2H) structure TMD‐based heterojunctions, and little attention is paid on the distorted octahedral (1T') structure TMD‐based heterojunctions. This study reports the large‐scale synthesis of monolayer 1T' ReS2‐ReSe2 lateral heterojunction with domain size up to 100 µm by using two‐step epitaxial growth. Atomic‐resolution scanning transmission electron microscopy reveals high crystal quality of the heterojunction with atomically sharp interfaces. Interestingly, three types of epitaxial growth modes accompanying formation of three different interface structures are revealed in the growth of 1T' heterojunction, where the angle between the b‐axis of ReS2 and ReSe2 is 0°, 120°, and 180°, respectively. The 0° and 180° interface structures are both found to be more abundant than the 120° interface structure owing to their relative lower formation energy. Electrical transport demonstrates that the as‐grown heterostructure forms lateral p–n junction with intrinsic rectification characteristics and exhibits polarization‐dependent photodiode properties. This is the first time the linear dichroism is achieved in 2D lateral heterostructure, which is important for the development of new devices with multi‐functionality.

Joule-heating induced thermal voltages in graphene three-terminal nanojunctions

Intrinsic voltage rectification is investigated in a graphene three-terminal nanojunction (GTTJ) on Si/SiO2 at room temperature and 87 K. The room-temperature rectification efficiency (ratio of output against input voltage) reaches ≈40%, which is higher than most efficiencies reported in the literature. The observed efficiency is higher at room temperature than at 87 K, which is in contrast to field-effect simulations and indicates that other mechanisms contribute to the rectification effect. We propose an explanation based on Joule heating and thermal voltages, as the device is operated in regimes of substantial power dissipation. Predicted thermal voltages show temperature and bias- and gate-voltage dependences which are similar to those observed in our experiment. We conclude that Joule-heating effects need to be considered for GTTJ devices.

Field-effect transistors as electrically controllable nonlinear rectifiers for the characterization of terahertz pulses

We propose to exploit rectification in field-effect transistors as an electrically controllable higher-order nonlinear phenomenon for the convenient monitoring of the temporal characteristics of THz pulses, for example, by autocorrelation measurements. This option arises because of the existence of a gate-bias-controlled super-linear response at sub-threshold operation conditions when the devices are subjected to THz radiation. We present measurements for different antenna-coupled transistor-based THz detectors (TeraFETs) employing (i) AlGaN/GaN high-electron-mobility and (ii) silicon CMOS field-effect transistors and show that the super-linear behavior in the sub-threshold bias regime is a universal phenomenon to be expected if the amplitude of the high-frequency voltage oscillations exceeds the thermal voltage. The effect is also employed as a tool for the direct determination of the speed of the intrinsic TeraFET response which allows us to avoid limitations set by the read-out circuitry. In particular, we show that the build-up time of the intrinsic rectification signal of a patch-antenna-coupled CMOS detector changes from 20 ps in the deep sub-threshold voltage regime to below 12 ps in the vicinity of the threshold voltage.

Intrinsic rectification in common-gated graphene field-effect transistors

Terahertz rectifying antennas (rectennas) couple micron-size antennas and high-speed diodes to convert the incident electro-magnetic radiation to useable DC power. At such frequencies, the device acting as the diode requires both a nonlinear electrical behavior and a very low parasitic capacitance. Due to their low-capacitance planar structure and high carrier mobility values, several graphene devices based on various rectification mechanisms have been previously proposed as the rectifying device in the terahertz range. In this paper, we report an asymmetric behavior in micrometer-scale rectangular CVD-grown graphene field-effect transistors (GFETs), both at 77K and room temperature (295K). The asymmetry with a measured ION/IOFF ratio as high as 1.85 is shown to originate from the slight change in graphene conductivity induced by drain-gate voltage variations. This is confirmed by simulations using a simple drift-diffusion transport model. The conclusions can be directly applied to optimize diode-connected GFETs. This nonlinear effect may also be of interest for graphene interconnect considerations as well as circuit designs using GFETs.

Synaptic Excitation in Spinal Motoneurons Alternates with Synaptic Inhibition and Is Balanced by Outward Rectification during Rhythmic Motor Network Activity.

Regular firing in spinal motoneurons of red-eared turtles (Trachemys scripta elegans, either sex) evoked by steady depolarization at rest is replaced by irregular firing during functional network activity. The transition caused by increased input conductance and synaptic fluctuations in membrane potential was suggested to originate from intense concurrent inhibition and excitation. We show that the conductance increase in motoneurons during functional network activity is mainly caused by intrinsic outward rectification near threshold for action potentials by activation of voltage and Ca2+ gated K channels. Intrinsic outward rectification facilitates spiking by focusing synaptic depolarization near threshold for action potentials. By direct recording of synaptic currents, we also show that motoneurons are activated by out-of-phase peaks in excitation and inhibition during network activity, whereas continuous low-level concurrent inhibition and excitation may contribute to irregular firing.SIGNIFICANCE STATEMENT Neurons embedded in active neural networks can enter a high-conductance state. High-conductance states were observed in spinal motoneurons during rhythmic motor behavior. Assuming no change in intrinsic conductance, it was suggested that the high-conductance state in motoneurons originated from balanced inhibition and excitation. In this study, we demonstrate that intrinsic outward rectification significantly contributes to the high-conductance state. Outward rectification balances synaptic excitation and maintains membrane potential near spike threshold. In addition, direct synaptic current recordings show out-of-phase excitation and inhibition in motoneurons during rhythmic network activity.

Implementation of Outstanding Electronic Transport in Polar Covalent Boron Nitride Atomic Chains: another Extraordinary Odd-Even Behaviour.

A theoretical investigation of the unique electronic transport properties of the junctions composed of boron nitride atomic chains bridging symmetric graphene electrodes with point-contacts is executed through non-equilibrium Green’s function technique in combination with density functional theory. Compared with carbon atomic chains, the boron nitride atomic chains have an alternative arrangement of polar covalent B-N bonds and different contacts coupling electrodes, showing some unusual properties in functional atomic electronic devices. Remarkably, they have an extraordinary odd-even behavior of conductivity with the length increase. The rectification character and negative differential resistance of nonlinear current-voltage characteristics can be achieved by manipulating the type of contacts between boron nitride atomic chains bridges and electrodes. The junctions with asymmetric contacts have an intrinsic rectification, caused by stronger coupling in the C-N contact than the C-B contact. On the other hand, for symmetric contact junctions, it is confirmed that the transport properties of the junctions primarily depend on the nature of contacts. The junctions with symmetric C-N contacts have higher conductivity than their C-B contacts counterparts. Furthermore, the negative differential resistances of the junctions with only C-N contacts is very conspicuous and can be achieved at lower bias.

Computational Design of Intrinsic Molecular Rectifiers Based on Asymmetric Functionalization of N-Phenylbenzamide.

We report a systematic computational search of molecular frameworks for intrinsic rectification of electron transport. The screening of molecular rectifiers includes 52 molecules and conformers spanning over 9 series of structural motifs. N-Phenylbenzamide is found to be a promising framework with both suitable conductance and rectification properties. A targeted screening performed on 30 additional derivatives and conformers of N-phenylbenzamide yielded enhanced rectification based on asymmetric functionalization. We demonstrate that electron-donating substituent groups that maintain an asymmetric distribution of charge in the dominant transport channel (e.g., HOMO) enhance rectification by raising the channel closer to the Fermi level. These findings are particularly valuable for the design of molecular assemblies that could ensure directionality of electron transport in a wide range of applications, from molecular electronics to catalytic reactions.

Ubiquitous thermal rectification induced by non-diffusive weak scattering at low temperature in one-dimensional materials: Revealed with a non-reflective heat reservoir

With a newly devised non-reflective heat reservoir and a one-dimensional lattice model with experimental conditions as references, we numerically demonstrate the intrinsic ubiquity of thermal rectification in microscopic one-dimensional materials. When reflections from heat reservoirs are excluded, only non-diffusive weak scatterings from the cubic and quartic interaction and a single interface in a lattice model are revealed to be necessary for thermal rectification to occur. Non-reflective reservoirs are critical to decouple intrinsic thermal rectification properties from correlated effects of heat reservoirs. Our results suggest that an elemental unit of thermal rectification devices, which demonstrates rectification by itself, is possible to be realized at room temperature.

Paradoxical Activation of an Inwardly Rectifying Potassium Channel Mutant by Spermine: "(B)locking" Open the Bundle Crossing Gate

Intracellular polyamines are endogenous blockers of inwardly rectifying potassium (Kir) channels and underlie steeply voltage-dependent rectification. Kir channels with strong polyamine sensitivity typically carry a negatively charged side chain at a conserved inner cavity position, although acidic residues at any pore-lining position in the inner cavity are sufficient to confer polyamine block. We have identified unique consequences of a glutamate substitution in the region of the helix bundle crossing of Kir6.2. Firstly, glutamate substitution at Kir6.2 residue F168 generates channels with intrinsic inward rectification that does not require blockade by intracellular polyamines or Mg(2+). In addition, these F168E channels exhibit a unique "spiked" tail phenotype, whereby large decaying inward tail currents are elicited upon spermine unbinding. This contrasts with the time-dependent recovery of current typically associated with blocker unbinding from ion channels. Interestingly, Kir6.2[F168E] channels exhibit a paradoxical biphasic conductance-voltage relationship in the presence of certain polyamines. This reflects channel blockade at positive voltages, channel stimulation at intermediate voltages, and exclusion of spermine from the pore at negative voltages. These features are recapitulated by a simple kinetic scheme in which weakly voltage-dependent spermine binding to a "shallow" site in the pore (presumably formed by the introduced glutamate at F168E) stabilizes opening of the bundle crossing gate. These findings illustrate the potential for dichotomous effects of a blocker in a long pore (with multiple binding sites), and offer a unique example of targeted modulation of the Kir channel gating apparatus.