Nanopore Stochastic Detection of a Liquid Explosive Component and Sensitizers Using Boromycin and an Ionic Liquid Supporting Electrolyte

Label-free high-precise nanopore detection of endopeptidase activity of anthrax lethal factor regulated by diverse conditions

Endopeptidase activity of anthrax lethal factor (aLF) prevents the destroy of anthracis spore intracellularly by host macrophages, meanwhile disables the signaling pathways extracellularly that leads to host lethality. Hence, inhibitory of this activity is expected to be an alternative option to cure anthrax infection. Herein, we fabricated a nanopore platform via transmembrane pore construction in vitro, which allows precise mimics, monitoring of intercellular proteinic transport and enables the quantitative detection of aLF endopeptidase activity towards MAPKK signaling protein at single molecule level. Next, we inhibited the aLF activity via screening approaches of protein-metal ion acquisition and other condition controlment (proton/hydroxide strength, adapted temperature, ionizing irradiation), which were identified by nanopore electrokinetic study. Upon the results, we found that Ca2+, Mg2+, Mn2+, Ni2+ collaborating with Zn2+ promote aLF activity efficiently. In contrary, Cd2+, Co2+, Cu2+ have great inhibitory effect. Result further revealed that, the speed of aLF endopeptidase activity with different ions functions as the nanopore signal frequency in linear manner, which enables evident distinction of those divalent ions using this proteinase assay. We also found the higher strength of the proton or hydroxide, the higher the inhibitory to aLF activity. Besides, adapted temperature and γ-ray also play integral roles in inhibiting this activity. Our results lay experimental basis for accurate detection of aLF activity, meanwhile provide new direction to screening novel stimuli-responsive inhibitors specific to aLF.

Aprotic Solvent Accumulation Amplifies Ion Current Rectification in Conical Nanopores

Ion current rectification is highly reported in aqueous electrochemical systems and sensors but lacks exploration in organic systems due to the additional complexity introduced by non-aqueous solvents. Herein, a detailed study on ion current rectification with highly polar and mildly polar aprotic organic solvents as a function of tetraethylammonium tetrafluoroborate supporting electrolyte concentration is presented. To explain our experimental results, we introduce a previously unreported phenomenon: the formation of a double-junction diode within the nanopore that arises due to a complex interplay between ion and solvent enrichment effects. Finite element simulations are used to explore this phenomenon and the subsequent effect on the rectifying behavior of conical quartz nanopores.

From Water Solutions to Ionic Liquids with Solid State Nanopores as a Perspective to Study Transport and Translocation Phenomena

Solid state nanopores are single-molecular devices governed by nanoscale physics with a broad potential for technological applications. However, the control of translocation speed in these systems is still limited. Ionic liquids are molten salts which are commonly used as alternate solvents enabling the regulation of the chemical and physical interactions on solid-liquid interfaces. While their combination can be challenging to the understanding of nanoscopic processes, there has been limited attempts on bringing these two together. While summarizing the state of the art and open questions in these fields, several major advances are presented with a perspective on the next steps in the investigations of ionic-liquid filled nanopores, both from a theoretical and experimental standpoint. By analogy to aqueous solutions, it is argued that ionic liquids and nanopores can be combined to provide new nanofluidic functionalities, as well as to help resolve some of the pertinent problems in understanding transport phenomena in confined ionic liquids and providing better control of the speed of translocating analytes.

Analysis with biological nanopore: On-pore, off-pore strategies and application in biological fluids

Inspired from ion channels in biology, nanopores have been developed as promising analytical tools. In principle, nanopores provide crucial information from the observation and analysis of ionic current modulations caused by the interaction between target analytes and fluidic pores. In this respect, the biological, chemical and physical parameters of the nanopore regime need to be well-understood and regulated for intermolecular interaction. Because of well-defined molecular structures, biological nanopores consequently are of a focal point, allowing precise interaction analysis at single-molecule level. In this overview, two analytical strategies are summarized and discussed accordingly, upon the challenges arising in case-dependent analysis using biological nanopores. One kind of strategies relies on modification, functionalization and engineering on nanopore confined interface to improve molecular recognition sites (on-pore strategies); The other kind of highlighted strategies concerns to measurement of various chemistry/biochemistry based interactions triggered by employed molecular agents or probes (off-pore strategies). In particularly, a few recent paradigms using these strategies for practical application of accurate analysis of biomarkers in biological fluids are illustrated. To end, the challenging and future outlook of using analytical tools by means of biological nanopores are depicted.

Single-molecule Study on the Interactions between Cyclic Nonribosomal Peptides and Protein Nanopore

Nonribosomal peptides (NRPs) are a type of secondary metabolites mostly originated from microorganisms such as bacteria and fungi. Their proteolytic stability, highly selective bioactivity, and microorganism-specificity have made them an attractive source of drugs for the pharmaceutical industry. Herein, with microcystins (MCs) as a NRP model, we, for the first time, proposed a sensitive method to study the interactions between NRPs and the protein nanopore. Due to the large molecular size (~3 nm diameter) of MCs and their net negative charges, MCs failed to translocate through the α-hemolysin (α-HL) protein channel. Our results demonstrated that the biomolecular interaction of MC-α-HL protein was significantly affected by the applied potential bias. The constant blockage amplitude in the voltage-dependent studies indicated that the current modulation events were dominantly contributed to the bumping interaction between MCs and the α-HL protein under the electrophoretic force. The mean residence time of the bumping events exhibited a two-stage decrease (from 1.90 ms to 1.02 ms, and from 1.02 ms to 0.69 ms) at the threshold voltages of -70 mV and -100 mV, respectively. Using our strategy (i.e., based on their electrophoretic driven interaction with the α-HL protein pore), discrimination of different MC molecules (MC-LR, MC-RR, MC-YR and linear analog) with varied branched residues could be accomplished. This work should provide an insight in developing a rapid and effective method for the identification of cyclic NRPs as valuable biomarkers for fungal infections.

Nanopore-Based Confined Spaces for Single-Molecular Analysis

The field of nanopore sensing at the single-molecular level is in a "boom" period. Such nanopores, which are either composed of biological materials or are fabricated from solid-state substrates, offer a unique confined space that is compatible with the single-molecular scale. Under the influence of an electrical field, such single-biomolecular interfaces can read single-molecular information and, if appropriately fine-tuned, each molecule plays its individual ionic rhythm to compose a "molecular symphony". Over the past few decades, many research groups have worked on nanopore-based single-molecular sensors for a range of thrilling chemical and clinical applications. Furthermore, for the past decade, we have also focused on nanopore-based sensors. In this Minireview, we summarize the recent developments in fundamental research and applications in this area, along with data algorithms and advances in hardware, which act as infrastructure for the electrochemical analysis.

Computation-Assisted Nanopore Detection of Thorium Ions

Thorium is a well-known radioactive and chemically toxic contaminant in the environment. The continuous exposure to thorium may cause an increased risk of developing lung and liver diseases as well as lung, pancreas, and bone cancer. Due to its use in nuclear industry and other industrial applications, thorium may be accidentally released to the environment from its mining and processing plants. In this work, we developed a rapid, real-time, and label-free nanopore sensor for Th⁴⁺ detection by using an aspartic acid containing peptide as a chelating agent and tuning the electrolyte solution pH to control the net charges of the peptide ligand and its metal ion complex. The method is highly sensitive with a detection limit of 0.45 nM. Furthermore, the sensor is selective: other metal ions (e.g., UO₂²⁺, Pb²⁺, Cu²⁺, Ni²⁺, Hg²⁺, Zn²⁺, As³⁺, Mg²⁺, and Ca²⁺) with concentrations of up to 3 orders of magnitude greater than that of Th⁴⁺ would not interfere with Th⁴⁺detection. In addition, simulated water samples were successfully analyzed. Our developed computation-assisted sensing strategy should find useful applications in the development of nanopore sensors for other metal ions.

Bioinspired Protein Channel-Based Scanning Ion Conductance Microscopy (Bio-SICM) for Simultaneous Conductance and Specific Molecular Imaging

The utility of stochastic single-molecule detection using protein nanopores has found widespread application in bioanalytical sensing as a result of the inherent signal amplification of the resistive pulse method. Integration of protein nanopores with high-resolution scanning ion conductance microscopy (SICM) extends the utility of SICM by enabling selective chemical imaging of specific target molecules, while simultaneously providing topographical information about the net ion flux through a pore under a concentration gradient. In this study, we describe the development of a bioinspired scanning ion conductance microscopy (bio-SICM) approach that couples the imaging ability of SICM with the sensitivity and chemical selectivity of protein channels to perform simultaneous pore imaging and specific molecule mapping. To establish the framework of the bio-SICM platform, we utilize the well-studied protein channel α-hemolysin (αHL) to map the presence of β-cyclodextrin (βCD) at a substrate pore opening. We demonstrate concurrent pore and specific molecule imaging by raster scanning an αHL-based probe over a glass membrane containing a single 25-μm-diameter glass pore while recording the lateral positions of the probe and channel activity via ionic current. We use the average channel current to create a conductance image and the raw current-time traces to determine spatial localization of βCD. With further optimization, we believe that the bio-SICM platform will provide a powerful analytical methodology that is generalizable, and thus offers significant utility in a myriad of bioanalytical applications.

Superposition of an AC field improves the discrimination between peptides in nanopore analysis

In standard nanopore analysis a constant DC voltage is used to electrophoretically drive small molecules and peptides towards a pore. Superposition of an AC voltage at particular frequencies causes molecules to oscillate as they approach the pore which can alter the event parameters, the blockade current (I) and blockade time (T). Four peptides with similar structures were studied. Alpha-helical peptides A10 (FmocDDA10KK), A14, A18 and retro-inverso A10. It was shown that the ratio of translocations to bumping events could be manipulated by a combination of AC voltages and frequencies. In particular, A10 could be studied without interference from retro-inverso A10. Similarly, a large, intrinsically disordered protein of 140 amino acids, α-synuclein, which translocates the pore readily in a DC field could be prevented from doing so by application of an AC field of 200 mV at 100 MHz.

Nanopore-based fourth-generation DNA sequencing technology

Nanopore-based sequencers, as the fourth-generation DNA sequencing technology, have the potential to quickly and reliably sequence the entire human genome for less than $1000, and possibly for even less than $100. The single-molecule techniques used by this technology allow us to further study the interaction between DNA and protein, as well as between protein and protein. Nanopore analysis opens a new door to molecular biology investigation at the single-molecule scale. In this article, we have reviewed academic achievements in nanopore technology from the past as well as the latest advances, including both biological and solid-state nanopores, and discussed their recent and potential applications.

Micro- and nano-structure based oligonucleotide sensors

This paper presents a review of micro- and nano-structure based oligonucleotide detection and quantification techniques. The characteristics of such devices make them very attractive for Point-of-Care or On-Site-Testing biosensing applications. Their small scale means that they can be robust and portable, their compatibility with modern CMOS electronics means that they can easily be incorporated into hand-held devices and their suitability for mass production means that, out of the different approaches to oligonucleotide detection, they are the most suitable for commercialisation. This review discusses the advantages of micro- and nano-structure based sensors and covers the various oligonucleotide detection techniques that have been developed to date. These include: Bulk Acoustic Wave and Surface Acoustic Wave devices, micro- and nano-cantilever sensors, gene Field Effect Transistors, and nanowire and nanopore based sensors. Oligonucleotide immobilisation techniques are also discussed.

Nanopore back titration analysis of dipicolinic acid

Here, we report a novel label-free nanopore back titration method for the detection of dipicolinic acid, a marker molecule for bacterial spores. By competitive binding of the target analyte and a large ligand probe to metal ions, dipicolinic acid could be sensitively and selectively detected. This nanopore back titration approach should find useful applications in the detection of other species of medical, biological, or environmental importance if their direct detection is difficult to achieve.

Amplification of single molecule translocation signal using β-strand peptide functionalized nanopores

Changes in ionic current flowing through nanopores due to binding or translocation of single biopolymer molecules enable their detection and characterization. It is, however, much more challenging to detect small molecules due to their rapid and small signal signature. Here we demonstrate the use of de novo designed peptides for functionalization of nanopores that enable the detection of a small analytes at the single molecule level. The detection relies on cooperative peptide conformational change that is induced by the binding of the small molecule to a receptor domain on the peptide. This change results in alteration of the nanopore effective diameter and hence induces current perturbation signal. On the basis of this approach, we demonstrate here the detection of diethyl 4-nitrophenyl phosphate (paraoxon), a poisonous organophosphate molecule. Paraoxon binding is induced by the incorporation of the catalytic triad of acetylcholine esterase in the hydrophilic domain of a short amphiphilic peptide and promotes β-sheet assembly of the peptide both in solution and for peptide molecules immobilized on solid surfaces. Nanopores coated with this peptide allowed the detection of paraoxon at the single molecule level revealing two binding arrangements. This unique approach, hence, provides the ability to study interactions of small molecules with the corresponding engineered receptors at the single molecule level. Furthermore, the suggested versatile platform may be used for the development of highly sensitive small analytes sensors.

Monitoring charge flux to quantify unusual ligand-induced ion channel activity for use in biological nanopore-based sensors

The utility of biological nanopores for the development of sensors has become a growing area of interest in analytical chemistry. Their emerging use in chemical analysis is a result of several ideal characteristics. First, they provide reproducible control over nanoscale pore sizes with an atomic level of precision. Second, they are amenable to resistive-pulse type measurement systems when embedded into an artificial lipid bilayer. A single binding event causes a change in the flow of millions of ions across the membrane per second that is readily measured as a change in current with excellent signal-to-noise ratio. To date, ion channel-based biosensors have been limited to well-behaved proteins. Most demonstrations of using ion channels as sensors have been limited to proteins that remain in the open, conducting state, unless occupied by an analyte of interest. Furthermore, these proteins are nonspecific, requiring chemical, biochemical, or genetic manipulations to impart chemical specificity. Here, we report on the use of the pore-forming abilities of heat shock cognate 70 (Hsc70) to quantify a specific analyte. Hsc70 reconstitutes into phospholipid membranes and opens to form multiple conductance states specifically in the presence of ATP. We introduce the measurement of “charge flux” to characterize the ATP-regulated multiconductance nature of Hsc70, which enables sensitive quantification of ATP (100 μM–4 mM). We believe that monitoring protein-induced charge flux across a bilayer membrane represents a universal method for quantitatively monitoring ion-channel activity. This measurement has the potential to broaden the library of usable proteins in the development of nanopore-based biosensors.

Microfluidic systems with ion-selective membranes

When integrated into microfluidic chips, ion-selective nanoporous polymer and solid-state membranes can be used for on-chip pumping, pH actuation, analyte concentration, molecular separation, reactive mixing, and molecular sensing. They offer numerous functionalities and are hence superior to paper-based devices for point-of-care biochips, with only slightly more investment in fabrication and material costs required. In this review, we first discuss the fundamentals of several nonequilibrium ion current phenomena associated with ion-selective membranes, many of them revealed by studies with fabricated single nanochannels/nanopores. We then focus on how the plethora of phenomena has been applied for transport, separation, concentration, and detection of biomolecules on biochips.

Nanopore stochastic detection: diversity, sensitivity, and beyond

Nanopore sensors have emerged as a label-free and amplification-free technique for measuring single molecules. First proposed in the mid-1990s, nanopore detection takes advantage of the ionic current modulations produced by the passage of target analytes through a single nanopore at a fixed applied potential. Over the last 15 years, these nanoscale pores have been used to sequence DNA, to study covalent and non-covalent bonding interactions, to investigate biomolecular folding and unfolding, and for other applications. A major issue in the application of nanopore sensors is the rapid transport of target analyte molecules through the nanopore. Current recording techniques do not always accurately detect these rapid events. Therefore, researchers have looked for methods that slow molecular and ionic transport. Thus far, several strategies can improve the resolution and sensitivity of nanopore sensors including variation of the experimental conditions, use of a host compound, and modification of the analyte molecule and the nanopore sensor. In this Account, we highlight our recent research efforts that have focused on applications of nanopore sensors including the differentiation of chiral molecules, the study of enzyme kinetics, and the determination of sample purity and composition. Then we summarize our efforts to regulate molecular transport. We show that the introduction of various surface functional groups such as hydrophobic, aromatic, positively charged, and negatively charged residues in the nanopore interior, an increase in the ionic strength of the electrolyte solution, and the use of ionic liquid solutions as the electrolyte instead of inorganic salts may improve the resolution and sensitivity of nanopore stochastic sensors. Our experiments also demonstrate that the introduction of multiple functional groups into a single nanopore and the development of a pattern-recognition nanopore sensor array could further enhance sensor resolution. Although we have demonstrated the feasibility of nanopore sensors for various applications, challenges remain before nanopore sensing is deployed for routine use in applications such as medical diagnosis, homeland security, pharmaceutical screening, and environmental monitoring.

Nanopore detection of copper ions using a polyhistidine probe

We report a stochastic nanopore sensing method for the detection of Cu(2+) ions. By employing a polyhistidine molecule as a chelating agent, and based on the different signatures of the events produced by the translocation of the chelating agent through an α-hemolysin pore in the absence and presence of target analytes, trace amounts of copper ions could be detected with a detection limit of 40 nM. Importantly, although Co(2+), Ni(2+), and Zn(2+) also interacts with the polyhistidine molecule, since the event residence times and/or blockage amplitudes for these metal chelates are significantly different from those of copper chelates, these metal ions do not interfere with Cu(2+) detection. This chelating reaction approach should find useful application in the development of nanopore sensors for other metal ions.

Sensing single mixed-monolayer protected gold nanoparticles by the α-hemolysin nanopore

Gold nanoparticles are widely used in various applications in fields including chemistry, engineering, biology, medicine, and electronics. These materials can be synthesized and modified with ligands containing different functional groups. Among nanoparticles' characteristics, chemical surface composition is likely to be a crucial feature, demanding robust analytical methodologies for its assessment. Single molecule analysis using the biological nanopores α-hemolysin and its E111A mutant is presented here as a promising methodology to stochastically sense organic monolayer protected gold-nanoparticles with different ligand shell compositions. By monitoring the ionic current across a single protein nanopore, differences in the physical and chemical characteristics (e.g., size, ligand shell composition, and arrangement) of individual nanoparticles can be distinguished based on the differences in the current blockade events that they cause. Such differences are observed in the spread of both the amplitude and duration of current blockades. These values cannot be correlated with a single physical characteristic. Instead the spread represents a measure of heterogeneity within the nanoparticle population. While our results compare favorably with the more traditional analytical methodologies, further work will be required to improve the accuracy of identification of the NPs and understand the spread of values within a nanoparticle preparation as well as the overlap between similar preparations.

Protein nanopore-based, single-molecule exploration of copper binding to an antimicrobial-derived, histidine-containing chimera peptide

Metal ions binding exert a crucial influence upon the aggregation properties and stability of peptides, and the propensity of folding in various substates. Herein, we demonstrate the use of the α-HL protein as a powerful nanoscopic tool to probe Cu(2+)-triggered physicochemical changes of a 20 aminoacids long, antimicrobial-derived chimera peptide with a His residue as metal-binding site, and simultaneously dissect the kinetics of the free- and Cu(2+)-bound peptide interaction to the α-HL pore. Combining single-molecule electrophysiology on reconstituted lipid membranes and fluorescence spectroscopy, we show that the association rate constant between the α-HL pore and a Cu(2+)-free peptide is higher than that of a Cu(2+)-complexed peptide. We posit that mainly due to conformational changes induced by the bound Cu(2+) on the peptide, the resulting complex encounters a higher energy barrier toward its association with the protein pore, stemming most likely from an extra entropy cost needed to fit the Cu(2+)-complexed peptide within the α-HL lumen region. The lower dissociation rate constant of the Cu(2+)-complexed peptide from α-HL pore, as compared to that of Cu(2+)-free peptide, supports the existence of a deeper free energy well for the protein interaction with a Cu(2+)-complexed peptide, which may be indicative of specific Cu(2+)-mediated contributions to the binding of the Cu(2+)-complexed peptide within the pore lumen.

High-temperature gating of solid-state nanopores with thermo-responsive macromolecular nanoactuators in ionic liquids

Stimuli-responsive nanofluidic systems in room temperature ionic liquids (RTILs). The nanofluidic device can withstand high temperatures up to 200 °C, in which conventional water-based smart materials and nanodevices are invalid. The smart nanopores can be "irreversibly" turned off above the transition temperature of ca. 120-150 °C, actuated by the conformational change of the chemically-modified polymer brushes.

Chemical-induced pH-mediated molecular switch

The transmembrane protein α-hemolysin pore has been used to develop ultrasensitive biosensors, study biomolecular folding and unfolding, investigate covalent and noncovalent bonding interactions, and probe enzyme kinetics. Here, we report that, by addition of ionic liquid tetrakis(hydroxymethyl)phosphonium chloride solution to the α-hemolysin pore, the α-hemolysin channel can be controlled open or closed by adjusting the pH of the solution. This approach can be employed to develop a novel molecular switch to regulate molecular transport and should find potential applications as a "smart" drug delivery method.

Translocation of single-stranded DNA through the α-hemolysin protein nanopore in acidic solutions

The effect of acidic pH on the translocation of single-stranded DNA through the α-hemolysin pore is investigated. Two significantly different types of events, i.e. deep blockades and shallow blockades, are observed at low pH. The residence times of the shallow blockades are not significantly different from those of the DNA translocation events obtained at or near physiological pH, whereas the deep blockades have much larger residence times and blockage amplitudes. With a decrease in the pH of the electrolyte solution, the percentage of the deep blockades in the total events increases. Furthermore, the mean residence time of these long-lived events is dependent on the length of DNA, and also varies with the nucleotide base, suggesting that they are appropriate for use in DNA analysis. In addition to being used as an effective approach to affect DNA translocation in the nanopore, manipulation of the pH of the electrolyte solution provides a potential means to greatly enhance the sensitivity of nanopore stochastic sensing.

pH-reversed ionic current rectification displayed by conically shaped nanochannel without any modification

Ion current through a nascent nanochannel with conically shaped geometry in PET (polyethylene terephthalate) membrane sandwiched between two same buffer solutions at pH ≤ 3 was routinely considered to exhibit no rectification and, if any, much weaker rectification than that for a nanochannel with a negative surface charge, since the surface charge on the membrane decreases to zero along with decreasing the pH value of the buffer solution down to the pK(a) of carboxylic acid. However, in this study, we discovered that in the buffer solution with low ionic strength at pH values below 3, the conically shaped nanochannels exhibited distinct ion current rectification, as expected for nanochannels with a positive surface charge, if voltages beyond ±2V range were scanned. We reasoned that the current rectification engendered by the positive surface charge of a conical nanochannel was due to further protonation of the hydrogen bonded hydrogel layer or neutral carboxylic acid inside the nanochannel. Therefore, our results enrich the knowledge about nanochannel technology and indicate that a nanofluidic diode based on pH-reversed ion current rectification through a conical nanochannel can be achieved without any modification of the PET membrane.

Unzipping of Double-stranded DNA in Engineered α-Hemolysin Pores

Biological protein α-hemolysin nanopore is under intense investigation as a potential platform for rapid and low-cost DNA sequencing. However, due to its narrow constriction, analysis of DNA in the α-hemolysin pore has long time been restricted to single strands. In this paper, we report that by introducing new surface functional groups into the α-hemolysin pore, facilitated unzipping of double-stranded DNA through the channel could be achieved. Since the mean residence time of the DNA events is dependent on the length of the duplex, and also varies with the nucleotide base composition, the modified protein pore approach offers the potential for rapid double-stranded DNA analysis, including sequencing.

Molecular dynamics simulations of DNA within a nanopore: arginine-phosphate tethering and a binding/sliding mechanism for translocation

Protein nanopores show great potential as low-cost detectors in DNA sequencing devices. To date, research has largely focused on the staphylococcal pore α-hemolysin (αHL). In the present study, we have developed simplified models of the wild-type αHL pore and various mutants in order to study the translocation dynamics of single-stranded DNA under the influence of an applied electric field. The model nanopores reflect the experimentally measured conductance values in planar lipid bilayers. We show that interactions between rings of cationic amino acids and DNA backbone phosphates result in metastable tethering of nucleic acid molecules within the pore, leading us to propose a "binding and sliding" mechanism for translocation. We also observe folding of DNA into nonlinear conformational intermediates during passage through the confined nanopore environment. Despite adopting nonlinear conformations, the DNA hexamer always exits the pore in the same orientation as it enters (3' to 5') in our simulations. The observations from our simulations help to rationalize experimentally determined trends in residual current and translocation efficiency for αHL and its mutants.

Pink noise of ionic conductance through single artificial nanopores revisited

We report voltage-clamp measurements through single conical nanopore obtained by chemical etching of a single ion track in polyimide film. Special attention is paid to the pink noise of the ionic current (i.e., 1/f noise) measured with different filling liquids. The relative pink-noise amplitude is almost independent of concentration and pH for KCl solutions, but varies strongly using ionic liquids. In particular, we show that depending on the ionic liquid, the transport of charge carriers is strongly facilitated (low noise and higher conductivity than in the bulk) or jammed. These results show that the origin of the pink noise can be ascribed neither to fluctuations of the pore geometry nor to the pore wall charges, but rather to a cooperative effect on ions motion in confined geometry.

Permeation through nanochannels: revealing fast kinetics

The permeation of water soluble molecules across cell membranes is controlled by channel-forming proteins and, in particular, the channel surface determines the selectivity. An adequate method to study the properties of these channels is electrophysiology and, in particular, analyzing the ion current fluctuation in the presence of permeating solutes. Ion current fluctuation analysis provides information on possible interactions of solutes with the channel surface. Due to the limited time resolution, fast permeation events are not visible using standard techniques. Here, we demonstrate that miniaturization of the lipid bilayer; varying the temperature or changing the solvent may enhance the resolution. Although electrophysiology is considered as a single molecule technique, it does not provide atomic resolution. Molecular details of solute permeation can be revealed by combining electrophysiology and all-atom computer modeling; these methods include ion conductance, selectivity, ion pair formation, and rate limiting interactions of the solute with the channel walls during permeation.

Stochastic nanopore sensors for the detection of terrorist agents: current status and challenges

Nanopore stochastic sensor works by monitoring the ionic current modulations induced by the passage of analytes of interest through a single pore, which can be obtained from a biological ion channel by self-assembly or artificially fabricated in a solid-state membrane. In this minireview, we overview the use of biological nanopores and artificial nanopores for the detection of terrorist agents including explosives, organophosphorus nerve agents, nitrogen mustards, organoarsenic compounds, toxins, and viruses. We also discuss the current challenge in the development of deployable nanopore sensors for real-world applications.

Slowing DNA translocation through nanopores using a solution containing organic salts

One of the key challenges to nanopore DNA sequencing is to slow down DNA translocation. Here, we report that the translocation velocities of various DNA homo- and copolymers through protein pores could be significantly decreased by using electrolyte solutions containing organic salts. Using a butylmethylimidazolium chloride (BMIM-Cl) solution instead of the commonly used KCl solution, DNA translocation rates on the order of hundreds of microseconds per nucleotide base were achieved. The much enhanced resolution of the nanopore coupled with different event blockage amplitudes produced by different nucleotides permits the convenient differentiation between various DNA molecules.