Ion permeability of the nuclear pore complex and ion-induced macromolecular permeation as studied by scanning electrochemical and fluorescence microscopy

Nanoscale interactions of arginine-containing dipeptide repeats with nuclear pore complexes as measured by transient scanning electrochemical microscopy

The nuclear pore complex (NPC) plays imperative biological and biomedical roles as the sole gateway for molecular transport between the cytoplasm and nucleus of eukaryotic cells. The proteinous nanopore, however, can be blocked by arginine-containing polydipeptide repeats (DPRs) of proteins resulting from the disordered C9orf72 gene as a potential cause of serious neurological diseases. Herein, we report the new application of transient scanning electrochemical microscopy (SECM) to quantitatively characterize DPR-NPC interactions for the first time. Twenty repeats of neurotoxic glycine-arginine and proline-arginine in the NPC are quantified to match the number of phenylalanine-glycine (FG) units in hydrophobic transport barriers of the nanopore. The 1 : 1 stoichiometry supports the hypothesis that the guanidinium residue of a DPR molecule engages in cation-π interactions with the aromatic residue of an FG unit. Cation-π interactions, however, are too weak to account for the measured free energy of DPR transfer from water into the NPC. The DPR transfer is thermodynamically as favorable as the transfer of nuclear transport receptors, which is attributed to hydrophobic interactions as hypothesized generally for NPC-mediated macromolecular transport. Kinetically, the DPRs are trapped by FG units for much longer than the physiological receptors, thereby blocking the nanopore. Significantly, the novel mechanism of toxicity implies that the efficient and safe nuclear import of genetic therapeutics requires strong association with and fast dissociation from the NPC. Moreover, this work demonstrates the unexplored power of transient SECM to determine the thermodynamics and kinetics of biological membrane-molecule interactions.

Nanoscale Quantitative Imaging of Single Nuclear Pore Complexes by Scanning Electrochemical Microscopy

The nuclear pore complex (NPC) is a proteinaceous nanopore that solely and selectively regulates the molecular transport between the cytoplasm and nucleus of a eukaryotic cell. The ∼50 nm-diameter pore of the NPC perforates the double-membrane nuclear envelope to mediate both passive and facilitated molecular transport, thereby playing paramount biological and biomedical roles. Herein, we visualize single NPCs by scanning electrochemical microscopy (SECM). The high spatial resolution is accomplished by employing ∼25 nm-diameter ion-selective nanopipets to monitor the passive transport of tetrabutylammonium at individual NPCs. SECM images are quantitatively analyzed by employing the finite element method to confirm that this work represents the highest-resolution nanoscale SECM imaging of biological samples. Significantly, we apply the powerful imaging technique to address the long-debated origin of the central plug of the NPC. Nanoscale SECM imaging demonstrates that unplugged NPCs are more permeable to the small probe ion than are plugged NPCs. This result supports the hypothesis that the central plug is not an intrinsic transporter, but is an impermeable macromolecule, e.g., a ribonucleoprotein, trapped in the nanopore. Moreover, this result also supports the transport mechanism where the NPC is divided into the central pathway for RNA export and the peripheral pathway for protein import to efficiently mediate the bidirectional traffic.

Transient theory for scanning electrochemical microscopy of biological membrane transport: uncovering membrane-permeant interactions

Scanning electrochemical microscopy (SECM) has emerged as a powerful method to quantitatively investigate the transport of molecules and ions across various biological membranes as represented by living cells. Advantageously, SECM allows for the in situ and non-destructive imaging and measurement of high membrane permeability under simple steady-state conditions, thereby facilitating quantitative data analysis. The SECM method, however, has not provided any information about the interactions of a transported species, i.e., a permeant, with a membrane through its components, e.g., lipids, channels, and carriers. Herein, we propose theoretically that SECM enables the quantitative investigation of membrane-permeant interactions by employing transient conditions. Specifically, we model the membrane-permeant interactions based on a Langmuir-type isotherm to define the strength and kinetics of the interactions as well as the concentration of interaction sites. Finite element simulation predicts that each of the three parameters uniquely affects the chronoamperometric current response of an SECM tip to a permeant. Significantly, this prediction implies that all three parameters are determinable from an experimental chronoamperometric response of the SECM tip. Complimentarily, the steady-state current response of the SECM tip yields the overall membrane permeability based on the combination of the three parameters. Interestingly, our simulation also reveals the optimum strength of membrane-permeant interactions to maximize the transient flux of the permeant from the membrane to the tip.

Pb2+-Selective Nanoemulsion-Integrated Single-Entity Electrochemistry for Ultrasensitive Sensing of Blood Lead

Unequivocally, Pb2+ as a harmful substance damaging children's brain and nerve systems, thereby causing behavior and learning disabilities, should be detected much lower than the elevated blood lead for children, 240 nM, endorsed by US CDC considering the unknown neurotoxic effects, yet the ultralow detection limit up to sub-ppb level remains a challenge due to the intrinsically insufficient sensitivity in the current analytical techniques. Here, we present nanoemulsion (NE)-integrated single-entity electrochemistry (NI-SEE) toward ultrasensitive sensing of blood lead using Pb-ion-selective ionophores inside a NE, i.e., Pb2+-selective NE. Through the high thermodynamic selectivity between Pb2+ and Pb-ionophore IV, and the extremely large partition coefficient for the Pb2+-Pb-ionophore complex inside NEs, we modulate the selectivity and sensitivity of NI-SEE for Pb2+ sensing up to an unprecedentedly low detection limit, 20 ppt in aqueous solutions, and lower limit of quantitation, 40 ppb in blood serums. This observation is supported by molecular dynamics simulations, which clearly corroborate intermolecular interactions, e.g., H-bonding and π*-n, between the aromatic rings of Pb-ionophore and lone pair electrons of oxygen in dioctyl sebacate (DOS), plasticizers of NEs, subsequently enhancing the current intensity in NI-SEE. Moreover, the highly sensitive sensing of Pb2+ is enabled by the appropriate suppression of hydroxyl radical formation during NI-SEE under a cathodic potential applied to a Pt electrode. Overall, the experimentally demonstrated NI-SEE approach and the results position our new sensing technology as potential sensors for practical environmental and biomedical applications as well as a platform to interrogate the stoichiometry of target ion-ionophore recognition inside a NE as nanoreactors.

Nanoelectrochemistry at liquid/liquid interfaces for analytical, biological, and material applications

Herein, we feature our recent efforts toward the development and application of nanoelectrochemistry at liquid/liquid interfaces, which are also known as interfaces between two immiscible electrolyte solutions (ITIES). Nanopipets, nanopores, and nanoemulsions are developed to create the nanoscale ITIES for the quantitative electrochemical measurement of ion transfer, electron transfer, and molecular transport across the interface. The nanoscale ITIES serves as an electrochemical nanosensor to enable the selective detection of various ions and molecules as well as high-resolution chemical imaging based on scanning electrochemical microscopy. The powerful nanoelectroanalytical methods will be useful for biological and material applications as illustrated by in situ studies of solid-state nanopores, nuclear pore complexes, living bacteria, and advanced nanoemulsions. These studies provide unprecedented insights into the chemical reactivity of important biological and material systems even at the single nanostructure level.

Morphology of Polymer Brushes in the Presence of Attractive Nanoparticles: Effects of Temperature

We study the role of temperature on the structure of pure polymer brushes and their mixture with attractive nanoparticles in flat and cylindrical geometries. It has previously been established that the addition of such nanoparticles causes the polymer brush to collapse and the intensity of the collapse depends on the attraction strength, the nanoparticle diameter, and the grafting density. In this work, we carry out molecular dynamics simulation under good solvent conditions to show how the collapse transition is affected by the temperature, for both plane grafted and inside-cylinder grafted brushes. We first examine the pure brush morphology and verify that the brush height is insensitive to temperature changes in both planar and cylindrical geometries, as expected for a polymer brush in a good solvent. On the other hand, for both system geometries, the brush structure in the presence of attractive nanoparticles is quite responsive to temperature changes. Generally speaking, for a given nanoparticle concentration, increasing the temperature causes the brush height to increase. A brush which contracts when nanoparticles are added eventually swells beyond its pure brush height as the system temperature is increased. The combination of two easily controlled external parameters, namely, concentration of nanoparticles in solution and temperature, allows for sensitive and reversible adjustment of the polymer brush height, a feature which could be exploited in designing smart polymer devices.

Control of nuclear size by osmotic forces in Schizosaccharomyces pombe

The size of the nucleus scales robustly with cell size so that the nuclear-to-cell volume ratio (N/C ratio) is maintained during cell growth in many cell types. The mechanism responsible for this scaling remains mysterious. Previous studies have established that the N/C ratio is not determined by DNA amount but is instead influenced by factors such as nuclear envelope mechanics and nuclear transport. Here, we developed a quantitative model for nuclear size control based upon colloid osmotic pressure and tested key predictions in the fission yeast Schizosaccharomyces pombe. This model posits that the N/C ratio is determined by the numbers of macromolecules in the nucleoplasm and cytoplasm. Osmotic shift experiments showed that the fission yeast nucleus behaves as an ideal osmometer whose volume is primarily dictated by osmotic forces. Inhibition of nuclear export caused accumulation of macromolecules in the nucleoplasm, leading to nuclear swelling. We further demonstrated that the N/C ratio is maintained by a homeostasis mechanism based upon synthesis of macromolecules during growth. These studies demonstrate the functions of colloid osmotic pressure in intracellular organization and size control.

Real-time effects of Cd(II) on the cellular membrane permeability

Cell membrane permeability is one of the main indicators of cytotoxicity and related to many critical biological pathways. Here, we determined the Cd²⁺-induced membrane permeability of human MCF-7 cells using ferrocene methanol molecular probes based on scanning electrochemical microscopy (SECM). The cell height and topography were examined with an impermeable Ru(NH₃)₆Cl₃ probe. The membrane permeability exhibited no significant changes when the Cd²⁺ incubation time was less than 2 h and its concentration was less than 40 μM. The permeability increased when the Cd²⁺ concentration was greater than 60 μM, or when the incubation time was longer than 3 h. From the combined 3-(4,5-di-methylthiazol-2-yl)-2,5-diphenyltetrazolium bromide and cytoskeleton imaging experiments, it was found that the changes occurred because the cells exhibited a defensive mode and their membranes contracted when treated with a low concentration of Cd²⁺ for a short time. However, the cell membranes were irreversibly damaged when the cytoskeleton structures were destroyed, and the cell activities decreased at high concentrations over long periods. Interestingly, through the comparison with an x-scan study, it was found that DPV technology shows a higher performance in the detection of changes in the membrane permeability. Using a combination of cytoskeleton fluorescence imaging and cell-viability tests, the effect of the cadmium metal on the cell membrane permeability can be explored deeper and more comprehensively. This study provides a new idea for exploring the changes in the cell membrane permeability and may be helpful for rapid evaluation of cytotoxicity.

Physics of the Nuclear Pore Complex: Theory, Modeling and Experiment

The hallmark of eukaryotic cells is the nucleus that contains the genome, enclosed by a physical barrier known as the nuclear envelope (NE). On the one hand, this compartmentalization endows the eukaryotic cells with high regulatory complexity and flexibility. On the other hand, it poses a tremendous logistic and energetic problem of transporting millions of molecules per second across the nuclear envelope, to facilitate their biological function in all compartments of the cell. Therefore, eukaryotes have evolved a molecular "nanomachine" known as the Nuclear Pore Complex (NPC). Embedded in the nuclear envelope, NPCs control and regulate all the bi-directional transport between the cell nucleus and the cytoplasm. NPCs combine high molecular specificity of transport with high throughput and speed, and are highly robust with respect to molecular noise and structural perturbations. Remarkably, the functional mechanisms of NPC transport are highly conserved among eukaryotes, from yeast to humans, despite significant differences in the molecular components among various species. The NPC is the largest macromolecular complex in the cell. Yet, despite its significant complexity, it has become clear that its principles of operation can be largely understood based on fundamental physical concepts, as have emerged from a combination of experimental methods of molecular cell biology, biophysics, nanoscience and theoretical and computational modeling. Indeed, many aspects of NPC function can be recapitulated in artificial mimics with a drastically reduced complexity compared to biological pores. We review the current physical understanding of the NPC architecture and function, with the focus on the critical analysis of experimental studies in cells and artificial NPC mimics through the lens of theoretical and computational models. We also discuss the connections between the emerging concepts of NPC operation and other areas of biophysics and bionanotechnology.

Nanoelectrochemical Study of Molecular Transport through the Nuclear Pore Complex

The nuclear pore complex (NPC) is the proteinaceous nanopore that solely mediates the transport of both small molecules and macromolecules between the nucleus and cytoplasm of a eukaryotic cell to regulate gene expression. In this personal account, we introduce recent progress in our nanoelectrochemical study of molecular transport through the NPC. Our work represents the importance of chemistry in understanding and controlling of NPC-mediated molecular transport to enable the efficient and safe delivery of genetic therapeutics into the nucleus, thereby fundamentally contributing to human health. Specifically, we employ nanoscale scanning electrochemical microscopy to test our hypothesis that the nanopore of the NPC is divided by transport barriers concentrically into peripheral and central routes to efficiently mediate the bimodal traffic of protein transport and RNA export, respectively, through cooperative hydrophobic and electrostatic interactions.

Unraveling Mass and Electron Transfer Kinetics at Silica Nanochannel Membrane Modified Electrodes by Scanning Electrochemical Microscopy

An in-depth understanding of kinetic processes convoluting mass and charge transfer at nanoporous membrane modified electrodes is crucial for developing high-performance electrochemical sensors. In this work, we propose a theoretical model to unravel mass (k_m) and electron transfer rate (k_f) from the apparent electrochemical rate constant (k_app) at silica nanoporous membrane (SNM) modified indium tin oxide (ITO) electrodes (designated as SNM/ITO for simplicity). Using scanning electrochemical microscopy (SECM), the k_app of charged redox species was first determined at the SNM/ITO in the absence and presence of surfactant micelles inside SNM. On the basis of the theory, in the presence of micelles inside SNM, k_m equals zero for all charged probes (Ru(NH₃)₆²⁺, Ru(CN)₆^3-, and FcMeOH⁺), thus the SNM behaves as an insulating barrier and the overall electrode reactivity is dominated by the permeability of SNM. After excluding micelles from SNM, the k_m of Ru(CN)₆^3-/4- is strongly dependent on the KCl concentration in the solution, decreasing from 0.23/0.15 mm s^-1 to almost zero upon decreasing the KCl concentration from 1.0 to 0.01 M. In contrast, k_m increases from 1.33 to 2.4 mm s^-1 for Ru(NH₃)₆²⁺ and from 0.18 to 0.33 mm s^-1 for FcMeOH⁺, which are comparable to the electron transfer rate at the underlying ITO electrode surface (0.8 and 0.35 mm s^-1). In these cases, both mass and electron transfer processes are important in determining the overall redox activity of SNM/ITO electrodes. The methodology reported in this work can provide a quantitative way of unraveling these processes and their respective contributions.

Nanoscale electrostatic gating of molecular transport through nuclear pore complexes as probed by scanning electrochemical microscopy

The nuclear pore complex (NPC) is a large protein nanopore that solely mediates molecular transport between the nucleus and cytoplasm of a eukaryotic cell. There is a long-standing consensus that selective transport barriers of the NPC are exclusively based on hydrophobic repeats of phenylalanine and glycine (FG) of nucleoporins. Herein, we reveal experimentally that charged residues of amino acids intermingled between FG repeats can modulate molecular transport through the NPC electrostatically and in a pathway-dependent manner. Specifically, we investigate the NPC of the Xenopus oocyte nucleus to find that excess positive charges of FG-rich nucleoporins slow down passive transport of a polycationic peptide, protamine, without affecting that of a polyanionic pentasaccharide, Arixtra, and small monovalent ions. Protamine transport is slower with a lower concentration of electrolytes in the transport media, where the Debye length becomes comparable to the size of water-filled spaces among the gel-like network of FG repeats. Slow protamine transport is not affected by the binding of a lectin, wheat germ agglutinin, to the peripheral route of the NPC, which is already blocked electrostatically by adjacent nucleoporins that have more cationic residues than anionic residues and even FG dipeptides. The permeability of NPCs to the probe ions is measured by scanning electrochemical microscopy using ion-selective tips based on liquid/liquid microinterfaces and is analysed by effective medium theory to determine the sizes of peripheral and central routes with distinct protamine permeability. Significantly, nanoscale electrostatic gating at the NPC can be relevant not only chemically and biologically, but also biomedically for efficient nuclear import of genetically therapeutic substances.

Single-Entity Electrochemistry of Nanoemulsion: The Nanostructural Effect on Its Electrochemical Behavior

New electrochemical approaches have been applied to investigate nanoemulsions (NEs) for their nanostructures and the relevant electrochemical activity by single-entity electrochemistry (SEE). Herein, we make highly monodisperse NEs with ∼40 nm diameter, composed of biocompatible surfactants, castor oil as plasticizers, and ion exchangers. Dynamic light scattering (DLS) measurements with periodically varying surfactant to oil ratios provide us with a structural implication about uneven distributions of incorporating components inside NEs. To support this structural insight, we apply SEE and selectively monitor electron-transfer reactions occurring at individual NEs containing ferrocene upon each collision onto a Pt ultramicroelectrode. The quantitative analysis of the nanoelectrochemical results along with DLS and transmission electron microscopy (TEM) measurements reveal nanostructured compartments of incorporating components inside NEs and their effect on the electrochemical behavior. Indeed, a tunneling barrier inside NEs could be formed depending on the NE composition, thus determining an electrochemical behavior of NEs, which cannot be differentiated by a general morphological study such as DLS and TEM but by our SEE measurements. Furthermore, by employing the nanopipet voltammetry with an interface between two immiscible electrolyte solutions (ITIES) to mimic the NE interface, we could explicitly investigate that the electron-transfer reaction occurring inside NEs is facilitated by the ion-transfer reaction. Overall, these comprehensive electrochemical approaches enable us to elucidate the relation between structures and the electrochemical functionality of NEs and provide quantitative criteria for the proper compositions of NEs regarding their activity in the electrochemical applications. Also, this finding should be a prerequisite for suitable biomedical/electrochemical applications of NEs.

Probing High Permeability of Nuclear Pore Complexes by Scanning Electrochemical Microscopy: Ca2+ Effects on Transport Barriers

The nuclear pore complex (NPC) solely mediates molecular transport between the nucleus and cytoplasm of a eukaryotic cell to play important biological and biomedical roles. However, it is not well-understood chemically how this biological nanopore selectively and efficiently transports various substances, including small molecules, proteins, and RNAs by using transport barriers that are rich in highly disordered repeats of hydrophobic phenylalanine and glycine intermingled with charged amino acids. Herein, we employ scanning electrochemical microscopy to image and measure the high permeability of NPCs to small redox molecules. The effective medium theory demonstrates that the measured permeability is controlled by diffusional translocation of probe molecules through water-filled nanopores without steric or electrostatic hindrance from hydrophobic or charged regions of transport barriers, respectively. However, the permeability of NPCs is reduced by a low millimolar concentration of Ca2+, which can interact with anionic regions of transport barriers to alter their spatial distributions within the nanopore. We employ atomic force microscopy to confirm that transport barriers of NPCs are dominantly recessed (∼80%) or entangled (∼20%) at the high Ca2+ level in contrast to authentic populations of entangled (∼50%), recessed (∼25%), and "plugged" (∼25%) conformations at a physiological Ca2+ level of submicromolar. We propose a model for synchronized Ca2+ effects on the conformation and permeability of NPCs, where transport barriers are viscosified to lower permeability. Significantly, this result supports a hypothesis that the functional structure of transport barriers is maintained not only by their hydrophobic regions, but also by charged regions.

Correlating Live Cell Viability with Membrane Permeability Disruption Induced by Trivalent Chromium

Cr(III) is often regarded as a trace essential micronutrient that can be found in many dietary supplements due to its participation in blood glucose regulation. However, increased levels of exposure have been linked to adverse health effects in living organisms. Herein, scanning electrochemical microscopy (SECM) was used to detect variation in membrane permeability of single cells (T24) resulting from exposure to a trivalent Cr-salt, CrCl₃. By employing electrochemical mediators, ferrocenemethanol (FcMeOH) and ferrocenecarboxylic acid (FcCOO^-), initially semipermeable and impermeable, respectively, complementary information was obtained. Three-dimensional COMSOL finite element analysis simulations were successfully used to quantify the permeability coefficients of each mediator by matching experimental and simulated results. Depending on the concentration of Cr(III) administered, three regions of membrane response were detected. Following exposure to low concentrations (up to 500 μM Cr(III)), their permeability coefficients were comparable to that of control cells, 80 μm/s for FcMeOH and 0 μm/s for FcCOO^-. This was confirmed for both mediators. As the incubation concentrations were increased, the ability of FcMeOH to permeate the membrane decreased to a minimum of 17 μm/s at 7500 μM Cr(III), while FcCOO^- remained impermeable. At the highest examined concentrations, both mediators were found to demonstrate increased membrane permeability. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide cell viability studies were also conducted on Cr(III)-treated T24 cells to correlate the SECM findings with the toxicity effects of the metal. The viability experiments revealed a similar concentration-dependent trend to the SECM cell membrane permeability study.

Controlling the Surface Properties of Binary Polymer Brush-Coated Colloids via Targeted Nanoparticles

This paper explores a novel mechanism for controlling the surface properties of polymer-coated colloids using targeted ("sticky") nanoparticles which attract monomers of certain polymer species. In our study, colloids are coated by two types of tethered polymer chains having different chemical properties. Attraction of nanoparticles to the monomers of one polymer type causes these polymer chains to contract toward the grafting surface, rendering the other type more exposed to the environment. Thus, the effective surface properties of the colloid are dominated by the intended polymer type. We use coarse-grained molecular dynamics (CGMD) simulation to demonstrate that introducing nanoparticles which interact preferentially with certain types of polymers makes it possible to switch between different surface properties of the colloid. This mechanism can in principle be exploited in drug delivery systems and self-assembly applications.

Analysing single live cells by scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) offers single live cell activities along its topography toward cellular physiology and pathology.

Combined scanning electrochemical and fluorescence microscopies using a tetrazine as a single redox and luminescent (electrofluorochromic) probe

The possibility of using a single electroactive and luminescent molecule both as a redox mediator and as a fluorophore in an experiment combining in situ Scanning Electrochemical Microscopy (SECM) and epifluorescence microscopy was validated. The usual working modes of SECM, namely positive and negative feedback as well as generation-collection, were used and the fluorescence images, intensity and spectra were recorded for each configuration. The tip potential, tip-substrate distance and, in the case of a conducting substrate, the substrate potential are the parameters that are likely to control the fluorescence. It is shown that the tip can be used to switch on and off the luminescence and that the modulation amplitude maximum is sensitive to the nature of the substrate. Approach curves based on this fluorescence modulation amplitude can be obtained showing a higher sensitivity than the classical electrochemical ones.

Fabrication and Use of Nanopipettes in Chemical Analysis

This review summarizes progress in the fabrication, modification, characterization, and applications of nanopipettes since 2010. A brief history of nanopipettes is introduced, and the details of fabrication, modification, and characterization of nanopipettes are provided. Applications of nanopipettes in chemical analysis are the focus in several cases, including recent progress in imaging; in the study of single molecules, single nanoparticles, and single cells; in fundamental investigations of charge transfer (ion and electron) reactions at liquid/liquid interfaces; and as hyphenated techniques combined with other methods to study the mechanisms of complicated electrochemical reactions and to conduct bioanalysis.

GABA Detection with Nano-ITIES Pipet Electrode: A New Mechanism, Water/DCE-Octanoic Acid Interface

Interface between two immiscible electrolyte solutions (ITIES) supported on the orifice of a pipet have become a powerful platform to detect a broad range of analytes. We present here the detection of γ-aminobutyric acid (GABA) with the nanoITIES pipet electrodes for the first time. GABA has a net charge of zero in an aqueous solution at pH ≈ 7, and it has not previously been detected at ITIES. In this work, we demonstrated GABA detection at ITIES in an aqueous solution at pH ≈ 7, where we introduced a novel detection strategy based on "pH modulation from the oil phase". To the best of our knowledge, this is the first report of such. Current increases linearly with increasing concentrations of GABA, ranging from 0.25 mM to 1.0 mM. The measured half-wave transfer potential of GABA is -0.401 ± 0.010 V ( n = 22) vs E1/2,TBA. The measured diffusion coefficient for GABA detection at nanoITIES pipet electrode is 6.09 (±0.58) × 10-10 m2/s ( n = 5). Experimental results indicate that protons generated from octanoic acid dissociation in the oil phase do not come out from the oil phase into the aqueous phase; neither were protons produced in the aqueous phase. NanoITIES pipet electrodes with radii of 320-340 nm were used in the current study. This new strategy and knowledge presented here lays the groundwork for the future development of ITIES pipet electrodes, especially for the detection of electrochemically nonredox active analytes.

A conserved ankyrin repeat-containing protein regulates conoid stability, motility and cell invasion in Toxoplasma gondii

Apicomplexan parasites are typified by an apical complex that contains a unique microtubule-organizing center (MTOC) that organizes the cytoskeleton. In apicomplexan parasites such as Toxoplasma gondii, the apical complex includes a spiral cap of tubulin-rich fibers called the conoid. Although described ultrastructurally, the composition and functions of the conoid are largely unknown. Here, we localize 11 previously undescribed apical proteins in T. gondii and identify an essential component named conoid protein hub 1 (CPH1), which is conserved in apicomplexan parasites. CPH1 contains ankyrin repeats that are required for structural integrity of the conoid, parasite motility, and host cell invasion. Proximity labeling and protein interaction network analysis reveal that CPH1 functions as a hub linking key motor and structural proteins that contain intrinsically disordered regions and coiled coil domains. Our findings highlight the importance of essential protein hubs in controlling biological networks of MTOCs in early-branching protozoan parasites.

Apicomplexan parasites such as Toxoplasma gondii possess a tubulin-rich structure called the conoid. Here, Long et al. identify a conoid protein that interacts with motor and structural proteins and is required for structural integrity of the conoid, parasite motility, and host cell invasion.

A Polymer-Brush-Based Nanovalve Controlled by Nanoparticle Additives: Design Principles

Polymer-grafted surfaces and channels are increasingly used for the design of responsive materials and sensors due to robust performance and ease of use. Various strategies for the control of the nanoscale morphologies of such materials and devices are being tested. Entropic repulsion between the polymer chains in a grafted brush of sufficient density causes the chains to extend in the direction perpendicular to the grafting surface in comparison to the position of unattached polymers. When nanoparticles having attractive interactions with the polymers are introduced into the solvent, these nanoparticles tend to infiltrate into the brush and reduce its extension. Under certain conditions, a sharp reduction in brush height extension can occur over a narrow range of nanoparticle concentrations in solution. We describe a way of controlling transport through polymer-functionalized nanochannels with nanoparticle additives, relying on the physics of nanoparticles and polymer brushes under confinement, and we suggest a blueprint for the creation of a tunable nanovalve. The nanovalve is modeled as a cylinder with a polymer brush grafted on its inside surface. Brush properties such as the chain length and the grafting density are chosen so that the brush chains extend into the center of the cylinder in the absence of nanoparticles, occluding the flux of analyte molecules through the pore. When nanoparticles that are attracted to the polymers are introduced into solution, they infiltrate into the brush and partially collapse it against the cylindrical grafting surface, opening space in the center of the cylinder through which analyte molecules can flow. The operation of such a nanovalve is analyzed via self-consistent field theory calculations in the strong-stretching approximation. Self-consistent field analysis is supported by Langevin dynamics simulations of the underlying coarse-grained model of the polymer-nanoparticle system.

Open carbon nanopipettes as resistive-pulse sensors, rectification sensors, and electrochemical nanoprobes

Nanometer-sized glass and quartz pipettes have been widely used as a core of chemical sensors, patch clamps, and scanning probe microscope tips. Many of those applications require the control of the surface charge and chemical state of the inner pipette wall. Both objectives can be attained by coating the inner wall of a quartz pipette with a nanometer-thick layer of carbon. In this letter, we demonstrate the possibility of using open carbon nanopipettes (CNP) produced by chemical vapor deposition as resistive-pulse sensors, rectification sensors, and electrochemical nanoprobes. By applying a potential to the carbon layer, one can change the surface charge and electrical double-layer at the pipette wall, which, in turn, affect the ion current rectification and adsorption/desorption processes essential for resistive-pulse sensors. CNPs can also be used as versatile electrochemical probes such as asymmetric bipolar nanoelectrodes and dual electrodes based on simultaneous recording of the ion current through the pipette and the current produced by oxidation/reduction of molecules at the carbon nanoring.