Pulse Size Distributions in Tunable Resistive Pulse Sensing

Effect of Sample Preprocessing and Size-Based Extraction Methods on the Physical and Molecular Profiles of Extracellular Vesicles

Extracellular vesicles (EVs) are nanometric lipid vesicles that shuttle cargo between cells. Their analysis could shed light on health and disease conditions, but EVs must first be preserved, extracted, and often preconcentrated. Here we first compare plasma preservation agents, and second, using both plasma and cell supernatant, four EV extraction methods, including (i) ultracentrifugation (UC), (ii) size-exclusion chromatography (SEC), (iii) centrifugal filtration (LoDF), and (iv) accousto-sorting (AcS). We benchmarked them by characterizing the integrity, size distribution, concentration, purity, and expression profiles for nine proteins of EVs, as well as the overall throughput, time-to-result, and cost. We found that the difference between ethylenediaminetetraacetic acid (EDTA) and citrate anticoagulants varies with the extraction method. In our hands, ultracentrifugation produced a high yield of EVs with low contamination; SEC is low-cost, fast, and easy to implement, but the purity of EVs is lower; LoDF and AcS are both compatible with process automation, small volume requirement, and rapid processing times. When using plasma, LoDF was susceptible to clogging and sample contamination, while AcS featured high purity but a lower yield of extraction. Analysis of protein profiles suggests that the extraction methods extract different subpopulations of EVs. Our study highlights the strengths and weaknesses of sample preprocessing methods, and the variability in concentration, purity, and EV expression profiles of the extracted EVs. Preanalytical parameters such as collection or preprocessing protocols must be considered as part of the entire process in order to address EV diversity and their use as clinically actionable indicators.

Multivariate analysis of nanoparticle translocation through a nanopore to improve the accuracy of resistive pulse sensing

The advent of nanopore-based sensors based on resistive pulse sensing gave rise to a remarkable breakthrough in the detection and characterization of nanoscale species. Some strong correlations have been reported between the resistive pulse characteristics and the particle's geometrical and physical properties. These correlations are commonly used to obtain information about the particles in commercial devices and research setups. The correlations, however, do not consider the simultaneous effect of influential factors such as particle shape and off-axis translocation, which complicates the extraction of accurate information from the resistive pulses. In this paper, we numerically studied the impact of the shape and position of particles on pulse characteristics in order to estimate the errors that arise from neglecting the influence of multiple factors on resistive pulses. We considered the sphere, oblate, and prolate particles to investigate the nanoparticle shape effect. Moreover, the trajectory dependency was examined by considering the translocation of nanoparticles away from the nanopore axis. Meanwhile, the shape effect was studied for different trajectories. We observed that the simultaneous effects of influential parameters could lead to significant errors in estimating particle properties if the coupled effects are neglected. Based on the results, we introduce the "pulse waveshape" as a novel characteristic of the resistive pulse that can be utilized as a decoupling parameter in the analysis of resistive pulses.

Large-scale production of polyimide micropore-based flow cells for detecting nano-sized particles in fluids

In diagnostic and sequencing applications, solid-state nanopores hold significant promise as a single-molecule sensing platform. The fabrication of precisely sized pores has traditionally been challenging, laborious, expensive, and inefficient, which has limited its applications until recently. To overcome this problem, this paper proposes a novel, reliable, cost-effective, portable, mass-productive, robust, and ease-of-use micropore flow cell that works based on the resistive pulse sensor (RPS) technique in order to distinguish the different sizes of c nanoparticles. RPS is a robust and informative technique that can provide valuable details of the size, shape, charge, and individual particle concentrations in the media. By femtosecond laser drilling of a polyimide substrate as an alternate material, translocation of 100, 300, and 350 nm polystyrene nanoparticles in PBS buffer was distinguished by 0.1, 1, and 2 nA current blockade levels, respectively. This is the first time a micropore has been opened in a polyimide membrane using a femtosecond laser in a single step. The experimental and theoretical investigation, scanning electron microscopy and focused ion beam spectroscopy were performed to comprehensively explain the micropore's performance. We showed that our innovative micropore-based flow cell could distinguish nano-sized particles in fluids, and it can be used in large-scale production because of its simplicity and cost-effectiveness.

Clinically translatable quantitative molecular photoacoustic imaging with liposome-encapsulated ICG J-aggregates

Photoacoustic (PA) imaging is a functional and molecular imaging technique capable of high sensitivity and spatiotemporal resolution at depth. Widespread use of PA imaging, however, is limited by currently available contrast agents, which either lack PA-signal-generation ability for deep imaging or their absorbance spectra overlap with hemoglobin, reducing sensitivity. Here we report on a PA contrast agent based on targeted liposomes loaded with J-aggregated indocyanine green (ICG) dye (i.e., PAtrace) that we synthesized, bioconjugated, and characterized to addresses these limitations. We then validated PAtrace in phantom, in vitro, and in vivo PA imaging environments for both spectral unmixing accuracy and targeting efficacy in a folate receptor alpha-positive ovarian cancer model. These study results show that PAtrace concurrently provides significantly improved contrast-agent quantification/sensitivity and SO₂ estimation accuracy compared to monomeric ICG. PAtrace's performance attributes and composition of FDA-approved components make it a promising agent for future clinical molecular PA imaging.

Inertial focusing and zeta potential measurements of single-nanoparticles using octet-nanochannels

Capture-to-translocation dynamics control is an important issue for single-particle and -molecule analyses by resistive pulse waveforms. Here, we report on regulated motions for accurate zeta-potential assessments of single nanoscale objects passing through an octet-nanochannel. We observed ionic spike signals consisting of eight consecutive sub-pulses signifying the ion blockage at the eight sensing zones in series upon electrophoretic translocation of individual nanoparticles. We find an exponential decrease to saturation of the channel-to-channel translocation duration as a nanobead moves forward, reflecting the more restricted radial motion degrees of freedom via inertial effects at the downstream side of the octet channel. This finding enabled a protocol for single-nanoparticle zeta potential estimation impervious to the uncertainty stemming from the stochastic nature of the translocation dynamics. The multi-channel approach presented in this study may be used as a useful tool for analyzing particles and molecules of variable sizes.

On the Behavior of Nanoparticles beyond the Nanopore Interface

Recent advances in solid-state and biological nanopore sensors have produced a deluge of analytical techniques for in situ characterization of bio-nano colloidal dispersions; however, the transport forces governing particle movement into and out of the nanopore are not yet fully understood. Herein, we study the motion of particles outside the smaller opening of an elastomeric size-tunable nanopore and relate this motion to existing transport forces known to act on particles within the pore. Subsequently, we develop a combined optoelectronic approach which allows the comparison of both resistive pulse sensing and single particle tracking-based techniques for particle size characterization and, intriguingly, measurements of the ensemble particle motion induced by a combination of particle electrophoresis as well as pressure-driven and electroosmotic flows through the sensor nanopore. We find evidence suggesting that although bulk fluid flow from the pore tends to drive particle motion, in certain circumstances, electrophoretically driven motion can dominate bulk fluid flow-driven motion even at large distances from the pore opening. By permitting direct observation of the behavior of fluids at the nanopore interface, this approach enables a greater understanding of the transport forces acting on particles as they migrate toward and move through nanopore sensors-with implications for future particle characterization systems and for nanopore methods in general.

Dielectric Coatings for Resistive Pulse Sensing Using Solid-State Pores

The present study reports on the systematic characterization of the effectiveness of dielectric coating to tailor capture-to-translocation dynamics of single particles in solid-state pores. We covered the surface of SiN_x membranes with SiO₂, HfO₂, Al₂O₃, TiO₂, or ZnO, which allowed us to change the ζ-potential at the pore wall, reflecting the isoelectric points of these coating materials. Resistive pulse measurements of negatively charged polystyrene beads elucidated more facile electrophoretic capture of the particles and slower translocation motions in the channel under more negative electric potential at the oxide surface. These findings provide a guide to engineer pore wall surface for optimizing the translocation dynamics for efficient sensing of particles and molecules.

Effects of cone angle and length of nanopores on the resistive pulse quality

Resistive pulse sensing (RPS) has proved to be a viable method for the detection and characterization of micro and nano particles. Modern fabrication methods have introduced different nanopore geometries for resistive pulse sensors. In this paper, we have numerically studied the effects of membrane thickness and the pore's cone angle, as the main geometrical parameters, on the sensing performance of the nanopores used for nanoparticle detection in the resistive pulse sensing method. To compare the sensing performance, three resistive pulse quality parameters were investigated - sensitivity, pulse duration and pulse amplitude. The thorough investigation on the relations between the geometrical parameters and the pulse quality parameters produced several interesting results, which were categorized and summarized for different nanopore structures (as different nanopore platforms) enabling the readers to more effectively compare them with one another. The results revealed that large cone angle and low aspect ratio nanopores have higher pulse amplitude and sensitivity, but their low duration could be a challenge in the process of detecting the resistive pulse. In addition, our results show small variation in sensitivity and duration of large cone angle nanopores with respect to pore length change, which is explained using the effective length concept and the definition of electric field strength and length. The findings of the present work can be used in practical applications where choosing the optimal pore geometry is of crucial significance. Furthermore, the results provide several possible ways to improve the resistive pulse quality for better sensing performance.

A Tunable Three-Dimensional Printed Microfluidic Resistive Pulse Sensor for the Characterization of Algae and Microplastics

Technologies that can detect and characterize particulates in liquids have applications in health, food, and environmental monitoring. Simply counting the numbers of cells or particles is not sufficient for most applications; other physical properties must also be measured. Typically, it is necessary to compromise between the speed of a sensor and its chemical and biological specificity. Here, we present a low-cost and high-throughput multiuse counter that classifies a particle's size, concentration, and shape. We also report how the porosity/conductivity or the particle can influence the signal. Using an additive manufacturing process, we have assembled a reusable flow resistive pulse sensor capable of being tuned in real time to measure particles from 2 to 30 μm across a range of salt concentrations, i.e., 2.5 × 10^-4 to 0.1 M. The device remains stable for several days with repeat measurements. We demonstrate its use for characterizing algae with spherical and rod structures as well as microplastics shed from tea bags. We present a methodology that results in a specific signal for microplastics, namely, a conductive pulse, in contrast to particles with smooth surfaces such as calibration particles or algae, allowing the presence of microplastics to be easily confirmed and quantified. In addition, the shapes of the signal and of the particle are correlated, giving an extra physical property to characterize suspended particulates. The technology can rapidly screen volumes of liquid, 1 mL/min, for the presence of microplastics and algae.

Nanoparticle and microorganism detection with a side-micron-orifice-based resistive pulse sensor

This paper presents the detection of nanoparticles and microorganisms using a recently developed side-orifice-based resistive pulse sensor (SO-RPS). By decreasing the channel height of the detection section of the SO-RPS, the detection sensitivity was increased and an average signal to noise ratio (S/N) of about 3 was achieved for 100 nm polystyrene particles. It was also found that spherical particles generate symmetrical signals. Algae with irregular shapes generate signals with more complex patterns. A scatter plot of signal magnitude versus signal width was proven to be reliable for differentiating bacteria from the nanoparticles and two types of algae. The side orifice for detecting heterogeneous nanoparticles and microorganisms is advantageous to avoid orifice clogging and the large flow resistance.

Solid-State Nanopore Time-of-Flight Mass Spectrometer

Analysis of field-controlled dynamics of ionized substances in a vacuum enables mass spectroscopy of particles and molecules. Analogously, here we report that nanoscale tracking of electrophoretically driven fast motions of single nanoparticles allows label-free and nondestructive detection of their mass in liquid. We fine-traced the time-dependent positions of space-filtered regular motions of particles passed through a thin solid-state nanopore by dissecting the associated ionic blockade phenomena under a scope of multiphysics simulations. Characterizing the viscous-drag-mediated exponential decay in the electrophoretic speed of particles ejected into an electrolyte solution from the nanochannel, we demonstrated the discrimination of nanoparticles by the femtogram mass difference. The present method is viable for mass measurement of virtually any object that can be put through the sensing zone, the sensor capability of which may find many applications such as pathogen screening and proteomics.

Back-Side Polymer-Coated Solid-State Nanopore Sensors

We systematically investigated the influence of polymer coating on temporal resolution of solid-state nanopores. We fabricated a Si₃N₄ nanopore integrated with a polyimide sheet partially covering the substrate surface. Upon detecting the nanoparticles dispersed in an electrolyte buffer by ionic current measurements, we observed a larger resistive pulse height along with a faster current decay at the tails under larger coverage of the polymeric layer, thereby suggesting a prominent role of the water-touching Si₃N₄ thin film as a significant capacitor serving to retard the ionic current response to the ion blockade by fast translocation of particles through the nanopores. From this, we came up with back-side polymer-coated chip designs and demonstrated improved pore sensor temporal resolution by developing a nanopore with a thick polymethyl-methacrylate layer laminated on the bottom surface. The present findings may be useful in developing integrated solid-state nanopore sensors with embedded nanochannels and nanoelectrodes.

Electric field interference and bimodal particle translocation in nano-integrated multipores

Parallel integration of multiple channels is a fundamental strategy for high-throughput particle detection in solid-state nanopores wherein understanding and control of crosstalk is an important issue for the post resistive pulse analysis. Here we report on a prominent effect of cross-channel electric field interference on the ionic current blockade by nanoparticles in nano-spaced pore arrays in a thin Si3N4 membrane. We systematically investigated the variations in resistive pulse profiles in multipore systems of various inter-channel distances. Although each pore acted independently when they were formed at excessively far distances, we observed significant cross-pore electrostatic interactions under close-integration that led the multipores to virtually act as a single-pore of equivalent area. As a result of the interference, the resistive pulse height demonstrated bimodal distributions due to the pronounced particle trajectory-dependence of the ionic blockade effects. Most importantly, the overcrowded multi-channel structure was found to deliver significant crosstalk with serious degradation of the sensor sensitivity to particle sizes. The present results provide a guide to design multipore structures regarding the trade-off between the detection throughput and sensor sensitivity.

Particle Capture in Solid-State Multipores

Utilization of multiple-channel structure is a promising way of accomplishing high-throughput detections of analytes in solid-state pore sensors. Here we report on systematic investigation of particle capture efficiency in Si₃N₄ multipore systems of various array configurations. We demonstrated enhanced detection throughput with increasing numbers of pore channels in a membrane. Meanwhile, we also observed significant contributions of the interchannel crosstalk in closely integrated multipores that tended to deteriorate throughput performance by causing shrinkage of the absorption zone via the interference-derived weakening of the electric field around the pore orifice. At the same time, the interference-derived electric field distributions were also found to diminish the electroosmotic contributions to the particle capture efficiency. The present findings can be useful in designing pore arrays with optimal throughput performance.

Resistive pulse sensing as particle counting and sizing method in microfluidic systems: Designs and applications review

Resistive pulse sensing is a well-known and established method for counting and sizing particles in ionic solutions. Throughout its development the technique has been expanded from detection of biological cells to counting nanoparticles and viruses, and even registering individual molecules, e.g., nucleotides in nucleic acids. This technique combined with microfluidic or nanofluidic systems shows great potential for various bioanalytical applications, which were hardly possible before microfabrication gained the present broad adoption. In this review, we provide a comprehensive overview of microfluidic designs along with electrode arrangements with emphasis on applications focusing on bioanalysis and analysis of single cells that were reported within the past five years.

Humic acids modify the pulse size distributions in the characterization of plastic microparticles by Tunable Resistive Pulse Sensing

Tunable Resistive Pulse Sensing, TRPS, is an emerging technique used in quantification and measuring the size (particle-by-particle) of viruses, exosomes and engineered colloidal spheres in biological fluids. We study the features of TRPS to enhance size characterization and quantification of submicron-sized microplastics, also called plastic microparticles, MP, in freshwater environments. We report alterations on the detection of the resistive pulses in the TRPS caused by humic acids, HA, during the size measurement of polystyrene microspheres used as MP surrogate. We discuss the alteration of the electric field in the measuring channel of the TRPS apparatus induced by the passage of HA. TRPS is a fast and precise technique for counting and size determination of MP but needs the evaluation of the influence of the organic matter on the current blockades. We show that statistical clustering models of the magnitude distribution of the resistive pulses can help to detect and quantify changes in the pulse size distributions induced by flocculation of humic acids. Conclusions of this study indicate that TRPS can be a valuable tool to improve the knowledge of the MP fate in surface waters, in the vadose zone and groundwater.

Single-Vesicle Assays Using Liposomes and Cell-Derived Vesicles: From Modeling Complex Membrane Processes to Synthetic Biology and Biomedical Applications

The plasma membrane is of central importance for defining the closed volume of cells in contradistinction to the extracellular environment. The plasma membrane not only serves as a boundary, but it also mediates the exchange of physical and chemical information between the cell and its environment in order to maintain intra- and intercellular functions. Artificial lipid- and cell-derived membrane vesicles have been used as closed-volume containers, representing the simplest cell model systems to study transmembrane processes and intracellular biochemistry. Classical examples are studies of membrane translocation processes in plasma membrane vesicles and proteoliposomes mediated by transport proteins and ion channels. Liposomes and native membrane vesicles are widely used as model membranes for investigating the binding and bilayer insertion of proteins, the structure and function of membrane proteins, the intramembrane composition and distribution of lipids and proteins, and the intermembrane interactions during exo- and endocytosis. In addition, natural cell-released microvesicles have gained importance for early detection of diseases and for their use as nanoreactors and minimal protocells. Yet, in most studies, ensembles of vesicles have been employed. More recently, new micro- and nanotechnological tools as well as novel developments in both optical and electron microscopy have allowed the isolation and investigation of individual (sub)micrometer-sized vesicles. Such single-vesicle experiments have revealed large heterogeneities in the structure and function of membrane components of single vesicles, which were hidden in ensemble studies. These results have opened enormous possibilities for bioanalysis and biotechnological applications involving unprecedented miniaturization at the nanometer and attoliter range. This review will cover important developments toward single-vesicle analysis and the central discoveries made in this exciting field of research.

Particle detection on microfluidic chips by differential resistive pulse sensing (RPS) method

The resistive pulse sensing (RPS) method has been widely used for characterization of particles, cells, and biomolecules due to its merits of high sensitivity and resolution. This paper investigates working parameters involved in detecting submicron and micron-sized particles by the differential RPS method on microfluidic chips. Effects of particle-to-sensor size ratio, ionic concentration and pH of the electrolyte solution, and applied electric field are studied systematically by using polystyrene particles with a size range from 140 nm to 5 µm. The results show that both the amplitude and the signal-to-noise ratio (SNR) of the RPS signals increase with the particle-to-sensor size ratio as well as the ionic concentration of the electrolyte media. The amplitude of the RPS signals also increases with increasing applied voltage, while the SNR experiences an upslope at low voltages and a decline under the condition of high voltages. pH has little effect on the background noise of the differential RPS signals but reduces the amplitude of the RPS signals at high pH. Grouping of RPS signals is considered to be caused by interactions between the sensor walls and the particles. Nanoparticle detection by the differential RPS method can be enhanced by optimizing these working parameters.

Rapid Assessment of Site Specific DNA Methylation through Resistive Pulse Sensing

Many diseases are defined by patterns of DNA methylation which result in aberrant gene expression. We present a rapid assay based upon resistive pulse sensing, RPS, to characterize sequence specific DNA methylation sites in genomic DNA. We modify the surface of superparamagnetic beads, SPBs, with DNA (capture probe). The particles are added to solution where they bind to and extract sequence specific DNA (target DNA). The target loaded SPBs are then incubated with antibodies which bind to the methylation sites, and the velocity of the SPBs through the nanopore reveals the number and location of the epigenetic markers within the target. The approach is capable of distinguishing between different methylation sites within a DNA promoter region. Crucially the approach is not dependent on accurate sequencing of assayed DNA, with genomic regions targeted through complementary probes. As such the number of stages and reagents costs are low and the assay is complete in under 60 min which includes the incubation and run times. The format also allows simultaneous quantification of number of copies of methylated DNA, and we illustrate this with a dose response curve.

Identification of Individual Bacterial Cells through the Intermolecular Interactions with Peptide-Functionalized Solid-State Pores

Bioinspired pore sensing for selective detection of flagellated bacteria was investigated. The Au micropore wall surface was modified with a synthetic peptide designed from toll-like receptor 5 (TLR5) to mimic the pathogen-recognition capability. We found that intermolecular interactions between the TLR5-derived recognition peptides and flagella induce ligand-specific perturbations in the translocation dynamics of Escherichia coli, which facilitated the discrimination between the wild-type and flagellin-deletion mutant (ΔfliC) by the resistive pulse patterns thereby demonstrating the sensing of bacteria at a single-cell level. These results provide a novel concept of utilizing weak intermolecular interactions as a recognition probes for single-cell microbial identification.

Optimal voltage for nanoparticle detection with thin nanopores

Optimal voltages were found for particle detections, at which the current blockade ratio did not depend on surface charge density.

Insights into the Proteome of Gastrointestinal Stromal Tumors-Derived Exosomes Reveals New Potential Diagnostic Biomarkers

Developing tumors continuously release nano-sized vesicles that represent circulating "fingerprints" of the tumor's identity. In gastrointestinal stromal tumor (GIST), we have previously reported that these tumors release "oncosomes" carrying the constitutively activated tyrosine kinase (TK) receptor KIT. Despite the clinical utility of TK inhibitors, such as imatinib mesylate (IM), recurrence and metastasis are clinical problems that urge the need to identify new tumor-derived molecules. To this aim, we performed the first high quality proteomic study of GIST-derived exosomes (GDEs) and identified 1,060 proteins composing the core GDE proteome (cGDEp). The cGDEp was enriched in diagnostic markers (e.g. KIT, CD34, ANO1, PROM1, PRKCQ, and ENG), as well as proteins encoded by genes previously reported expressed in GIST (e.g. DPP4, FHL1, CDH11, and KCTD12). Many of these proteins were validated using cell lines, patient-derived KIT⁺ exosomes, and GIST tissues. We further show that in vitro and in vivo-derived GDE, carry proteins associated with IM response, such as Sprouty homolog 4 (SPRY4), surfeit 4 (SURF4), ALIX, and the cGMP-dependent 3',5'-cyclic phosphodiesterase 2A (PDE2A). Additionally, we report that the total exosome levels and exosome-associated KIT and SPRY4 protein levels have therapeutic values. In fact, molecular characterization of in vivo-derived KIT⁺ exosomes indicate significant sorting of p-KIT^Tyr719, total KIT, and SPRY4 after IM-treatment of metastatic patients as compared with the pre-IM levels. Our data suggest that analysis of circulating exosomes levels and molecular markers of IM response in GIST patients with primary and metastatic disease is suitable to develop liquid based biopsies for the diagnosis, prognosis, and monitoring of response to treatment of these tumors. In summary, these findings provide the first insight into the proteome of GIST-derived oncosomes and offers a unique opportunity to further understand their oncogenic elements which contribute to tumorigenesis and drug resistance. Data are available via ProteomeXchange with identifier PXD007997.

Discriminating single-bacterial shape using low-aspect-ratio pores

Conventional concepts of resistive pulse analysis is to discriminate particles in liquid by the difference in their size through comparing the amount of ionic current blockage. In sharp contrast, we herein report a proof-of-concept demonstration of the shape sensing capability of solid-state pore sensors by leveraging the synergy between nanopore technology and machine learning. We found ionic current spikes of similar patterns for two bacteria reflecting the closely resembled morphology and size in an ultra-low thickness-to-diameter aspect-ratio pore. We examined the feasibility of a machine learning strategy to pattern-analyse the sub-nanoampere corrugations in each ionic current waveform and identify characteristic electrical signatures signifying nanoscopic differences in the microbial shape, thereby demonstrating discrimination of single-bacterial cells with accuracy up to 90%. This data-analytics-driven microporescopy capability opens new applications of resistive pulse analyses for screening viruses and bacteria by their unique morphologies at a single-particle level.

Probing charges on solid-liquid interfaces with the resistive-pulse technique

Our manuscript addresses the issue of probing an effective surface charge that any surface can acquire at the solid/liquid interface. Even if a particle is predicted to be neutral based on its chemical structure, the particle can carry finite surface charges when placed in a solution. We present tools to probe the presence of surface charge densities of meso-particles, characterized with zeta potentials below 10 mV. The tools are based on the resistive-pulse technique, which uses single pores to probe properties of individual objects including molecules, particles, and cells. The presented experiments were performed with particles 280 and 400 nm in diameter and single pores with opening diameter tuned between ∼ 200 nm and one micron. Surface charge properties were probed in two modes: (i) the passage of the particles through pores of diameters larger than the particles, as well as (ii) an approach curve of a particle to a pore that is smaller than the particle diameter. The curve in the latter mode has a biphasic character starting with a low-amplitude current decrease, followed by a current enhancement reaching an amplitude of ∼10% of the baseline current. The current increase was long-lasting and stable, and shown to strongly depend on the particle surface charge density. The results are explained via voltage-modulation of ionic concentrations in the pore.

Scanning ion conductance microscopy mapping of tunable nanopore membranes

We report on the use of scanning ion conductance microscopy (SICM) for in-situ topographical mapping of single tunable nanopores, which are used for tunable resistive pulse sensing. A customised SICM system was used to map the elastomeric pore membranes repeatedly, using pipettes with tip opening diameters of approximately 50 nm and 1000 nm. The effect of variations on current threshold, scanning step size, and stretching has been studied. Lowering the current threshold increased the sensitivity of the pipette while scanning, up to the point where the tip contacted the surface. An increase in the pore area was observed as the step size was decreased, and with increased stretching. SICM reveals details of the electric field near the pore entrance, which is important for understanding measurements of submicron particles using resistive pulse sensing.

Microfluidic and Nanofluidic Resistive Pulse Sensing: A Review

The resistive pulse sensing (RPS) method based on the Coulter principle is a powerful method for particle counting and sizing in electrolyte solutions. With the advancement of micro- and nano-fabrication technologies, microfluidic and nanofluidic resistive pulse sensing technologies and devices have been developed. Due to the unique advantages of microfluidics and nanofluidics, RPS sensors are enabled with more functions with greatly improved sensitivity and throughput and thus have wide applications in fields of biomedical research, clinical diagnosis, and so on. Firstly, this paper reviews some basic theories of particle sizing and counting. Emphasis is then given to the latest development of microfuidic and nanofluidic RPS technologies within the last 6 years, ranging from some new phenomena, methods of improving the sensitivity and throughput, and their applications, to some popular nanopore or nanochannel fabrication techniques. The future research directions and challenges on microfluidic and nanofluidic RPS are also outlined.

A tunable nanopore sensor for the detection of metal ions using translocation velocity and biphasic pulses

A tunable resistive pulse sensor, utilising a polyurethane nanopore, has been used to characterise nanoparticles as they traverse the pore opening. Herein we demonstrate that the translocation speed, conductive and resistive pulse magnitude, can be used to infer the surface charge of a nanoparticle, and act as a specific transduction signal for the binding of metal ions to ligands on the particle surface. Surfaces of silica nanoparticles were modified with a ligand to demonstrate the concept, and used to extract copper(ii) ions (Cu²⁺) from solution. By tuning the pH and ionic strength of the solution, a biphasic pulse, a conductive followed by a resistive pulse is recorded. Biphasic pulses are becoming a powerful means to characterise materials, and provide insight into the translocation mechanism, and herein we present their first use to detect the presence of metal ions in solution. We demonstrate how combinations of translocation speed and/or biphasic pulse behaviour are used to detect Cu²⁺ with quantitative responses across a range of pH and ionic strengths. Using a generic ligand this assay allows a clear signal for Cu²⁺ as low as 1 ppm with a short 5-minute incubation time, and is capable of measuring 10 ppm Cu²⁺ in the presence of 5 other ions. The method has potential for monitoring heavy metals in biological and environmental samples.