Sizing Individual Au Nanoparticles in Solution with Sub-Nanometer Resolution

Enhanced Nanoparticle Sensing in a Highly Viscous Nanopore

Slowing down translocation dynamics is a crucial challenge in nanopore sensing of small molecules and particles. Here, it is reported on nanoparticle motion-mediated local viscosity enhancement of water-organic mixtures in a nanofluidic channel that enables slow translocation speed, enhanced capture efficiency, and improved signal-to-noise ratio by transmembrane voltage control. It is found that higher detection rates of nanoparticles under larger electrophoretic voltage in the highly viscous solvents. Meanwhile, the strongly pulled particles distort the liquid in the pore at high shear rates over 103 s-1 which leads to a counterintuitive phenomenon of slower translocation speed under higher voltage via the induced dilatant viscosity behavior. This mechanism is demonstrated as feasible with a variety of organic molecules, including glycerol, xanthan gum, and polyethylene glycol. The present findings can be useful in resistive pulse analyses of nanoscale objects such as viruses and proteins by allowing a simple and effective way for translocation slowdown, improved detection throughput, and enhanced signal-to-noise ratio.

Integrated In-Plane Nanofluidic Devices for Resistive-Pulse Sensing

Single-particle (or digital) measurements enhance sensitivity (10- to 100-fold improvement) and uncover heterogeneity within a population (one event in 100 to 10,000). Many biological systems are significantly influenced by rare or infrequent events, and determining what species is present, in what quantity, and the role of that species is critically important to unraveling many questions. To develop these measurement systems, resistive-pulse sensing is used as a label-free, single-particle detection technique and can be combined with a range of functional elements, e.g., mixers, reactors, filters, separators, and pores. Virtually, any two-dimensional layout of the micro- and nanofluidic conduits can be envisioned, designed, and fabricated in the plane of the device. Multiple nanopores in series lead to higher-precision measurements of particle size, shape, and charge, and reactions coupled directly with the particle-size measurements improve temporal response. Moreover, other detection techniques, e.g., fluorescence, are highly compatible with the in-plane format. These integrated in-plane nanofluidic devices expand the toolbox of what is possible with single-particle measurements.

Fluorescent Chiral Quantum Dots to Unveil Origin-Dependent Exosome Uptake and Cargo Release

Exosomes are promising nanocarriers for drug delivery. Yet, it is challenging to apply exosomes in clinical use due to the limited understanding of their physiological functions. While cellular uptake of exosomes is generally known through endocytosis and/or membrane fusion, the mechanisms of origin-dependent cellular uptake and subsequent cargo release of exosomes into recipient cells are still unclear. Herein, we investigated the intricate mechanisms of exosome entry into recipient cells and intracellular cargo release. In this study, we utilized chiral graphene quantum dots (GQDs) as representatives of exosomal cargo, taking advantage of the superior permeability of chiral GQDs into lipid membranes as well as their excellent optical properties for tracking analysis. We observed that the preferential cellular uptake of exosomes derived from the same cell-of-origin (intraspecies exosomes) is higher than that of exosomes derived from different cell-of-origin (cross-species exosomes). This uptake enhancement was attributed to receptor-ligand interaction-mediated endocytosis, as we identified the expression of specific ligands on exosomes that favorably interact with their parental cells and confirmed the higher lysosomal entrapment of intraspecies exosomes (intraspecies endocytic uptake). On the other hand, we found that the uptake of cross-species exosomes primarily occurred through membrane fusion, followed by direct cargo release into the cytosol (cross-species direct fusion uptake). We revealed the underlying mechanisms involved in the cellular uptake and subsequent cargo release of exosomes depending on their cell-of-origin and recipient cell types. Overall, this study envisions valuable insights into further advancements in effective drug delivery using exosomes, as well as a comprehensive understanding of cellular communication, including disease pathogenesis.

Simultaneous Multipass Resistive-Pulse Sensing and Fluorescence Imaging of Liposomes

Simultaneous multipass resistive-pulse sensing and fluorescence imaging have been used to correlate the size and fluorescence intensity of individual E. coli lipid liposomes composed of E. coli polar lipid extracts labeled with membrane-bound 3,3-dioctadecyloxacarbocyanine (DiO) fluorescent molecules. Here, a nanopipet serves as a waveguide to direct excitation light to the resistive-pulse sensing zone at the end of the nanopipet tip. Individual DiO-labeled liposomes (>50 nm radius) were multipassed back and forth through the orifices of glass nanopipets' 110-150 nm radius via potential switching to obtain subnanometer sizing precision, while recording the fluorescence intensity of the membrane-bound DiO molecules. Fluorescence was measured as a function of liposome radius and found to be approximately proportional to the total membrane surface area. The observed relationship between liposome size and fluorescence intensity suggests that multivesicle liposomes emit greater fluorescence compared to unilamellar liposomes, consistent with all lipid membranes of the multivesicle liposomes containing DiO. Fluorescent and nonfluorescent liposomes are readily distinguished from each other in the same solution using simultaneous multipass resistive-pulse sensing and fluorescence imaging. A fluorescence "dead zone" of ∼1 μm thickness just outside of the nanopipet orifice was observed during resistive-pulse sensing, resulting in "on/off" fluorescent behavior during liposome multipassing.

Fluorescent Chiral Quantum Dots to Unveil Origin-Dependent Exosome Uptake and Cargo Release

Exosomes are promising nanocarriers for drug delivery. Yet, it is challenging to apply exosomes in clinical use due to the limited understanding of their physiological functions. While cellular uptake of exosomes is generally known through endocytosis and/or membrane fusion, the mechanisms of origin-dependent cellular uptake and subsequent cargo release of exosomes into recipient cells are still unclear. Herein, we investigated the intricate mechanisms of exosome entry into recipient cells and the intracellular cargo release. In this study, we utilized chiral graphene quantum dots (GQDs) as representatives of exosomal cargo, taking advantage of the superior permeability of chiral GQDs into lipid membranes, as well as their excellent optical properties for tracking analysis. We observed a higher uptake rate of exosomes in their parental recipient cells. However, these exosomes were predominantly entrapped in lysosomes through endocytosis (intraspecies endocytic uptake). On the other hand, in non-parental recipient cells, exosomes exhibited a greater inclination for cellular uptake through membrane fusion, followed by direct cargo release into the cytosol (cross-species direct fusion uptake). We revealed the underlying mechanisms involved in the cellular uptake and the subsequent cargo release of exosomes depending on their cell-of-origin and recipient cell types. This study envisions valuable insights into further advancements in the effective drug delivery using exosomes, as well as a comprehensive understanding of cellular communication, including disease pathogenesis.

Toward the Detection Limit of Electrochemistry: Studying Anodic Processes with a Fluorogenic Reporting Reaction

Recently, shot noise has been shown to be an inherent part of all charge-transfer processes, leading to a practical limit of quantification of 2100 electrons (≈0.34 fC) [ Curr. Opin. Electrochem. 2020, 22, 170-177]. Attainable limits of quantification are made much larger by greater background currents and insufficient instrumentation, which restricts progress in sensing and single-entity applications. This limitation can be overcome by converting electrochemical charges into photons, which can be detected with much greater sensitivity, even down to a single-photon level. In this work, we demonstrate the use of fluorescence, induced through a closed bipolar setup, to monitor charge-transfer processes below the detection limit of electrochemical workstations. During this process, the oxidation of ferrocenemethanol (FcMeOH) in one cell is used to concurrently drive the oxidation of Amplex Red (AR), a fluorogenic redox molecule, in another cell. The spectroelectrochemistry of AR is investigated and new insights on the commonplace practice of using deprotonated glucose to limit AR photooxidation are presented. The closed bipolar setup is used to produce fluorescence signals corresponding to the steady-state voltammetry of FcMeOH on a microelectrode. Chronopotentiometry is then used to show a linear relationship between the charge passed through FcMeOH oxidation and the integrated AR fluorescence signal. The sensitivity of the measurements obtained at different timescales varies between 2200 and 500 electrons per detected photon. The electrochemical detection limit is approached using a diluted FcMeOH solution in which no faradaic current signal is observed. Nevertheless, a fluorescence signal corresponding to FcMeOH oxidation is still seen, and the detection of charges down to 300 fC is demonstrated.

DNA Coil Dynamics and Hydrodynamic Gating of Pressure-Biased Nanopores

Nanopores are ideally suited for the analysis of long DNA fragments including chromosomal DNA and synthetic DNA with applications in genome sequencing and DNA data storage, respectively. Hydrodynamic fluid flow has been shown to slow down DNA transit time within the pore, however other influences of hydrodynamic forces have yet to be explored. In this report, a broad analysis of pressure-biased nanopores and the impact of hydrodynamics on DNA transit time, capture rate, current blockade depth, and DNA folding are conducted. Using a 10 nm pore, it is shown that hydrodynamic flow inhibits the early stages of linearization of DNA and produces predominately folded events which are initiated by folded DNA (2-strands) entering the pore. Furthermore, utilizing larger pores (30 nm) leads to unique DNA gating behavior in which DNA events can be switched on and off with the application of pressure. A computational model, based on combining electrophoretic drift velocities with fluid velocities, accurately predicts the pore size required to observe DNA gating. Hydrodynamic fluid flow generated by a pressure bias, or potentially more generally by other mechanisms like electroosmotic flow, is shown to have significant effects on DNA sensing and can be useful for DNA sensing technologies.

Adsorption of Cisplatin Molecules in Nanoporous Inorganic Materials as Drug Delivery Vehicles

The use of nanoparticles as anticancer cargo systems for drug delivery is a promising modality, as they avoid the known toxicity of anticancer drugs on healthy cells by the delivery of multiple drugs to the target cells. Here, the adsorption behavior of cisplatin drug molecules in two different inorganic materials, silica and metallic gold, is investigated mathematically. The 6–12 Lennard-Jones potential, together with the continuum approximation, is adapted to calculate the molecular interatomic energies between molecules. For each material, the relation between the pore radius ℓ and the minimum energy is determined, and the results indicate that the minimum energy occurs when the radii are ℓ=5.3 and ℓ=4.7 Å for the silica and gold nanopores, respectively. The method is promising for applications in the design of novel nanocapsules for future targeted drug and gene delivery.

Disassembly of Single Virus Capsids Monitored in Real Time with Multicycle Resistive-Pulse Sensing

Virus assembly and disassembly are critical steps in the virus lifecycle; however, virus disassembly is much less well understood than assembly. For hepatitis B virus (HBV) capsids, disassembly of the virus capsid in the presence of guanidine hydrochloride (GuHCl) exhibits strong hysteresis that requires additional chemical energy to initiate disassembly and disrupt the capsid structure. To study disassembly of HBV capsids, we mixed T = 4 HBV capsids with 1.0-3.0 M GuHCl, monitored the reaction over time by randomly selecting particles, and measured their size with resistive-pulse sensing. Particles were cycled forward and backward multiple times to increase the observation time and likelihood of observing a disassembly event. The four-pore device used for resistive-pulse sensing produces four current pulses for each particle during translocation that improves tracking and identification of single particles and increases the precision of particle-size measurements when pulses are averaged. We studied disassembly at GuHCl concentrations below and above denaturing conditions of the dimer, the fundamental unit of HBV capsid assembly. As expected, capsids showed little disassembly at low GuHCl concentrations (e.g., 1.0 M GuHCl), whereas at higher GuHCl concentrations (≥1.5 M), capsids exhibited disassembly, sometimes as a complex series of events. In all cases, disassembly was an accelerating process, where capsids catastrophically disassembled within a few 100 ms of reaching critical stability; disassembly rates reached tens of dimers per second just before capsids fell apart. Some disassembly events exhibited metastable intermediates that appeared to lose one or more trimers of dimers in a stepwise fashion.

Pressure-Biased Nanopores for Excluded Volume Metrology, Lipid Biomechanics, and Cell-Adhesion Rupturing

Nanopore sensing has been widely used in applications ranging from DNA sequencing to disease diagnosis. To improve these capabilities, pressure-biased nanopores have been explored in the past to-primarily-increase the residence time of the analyte inside the pore. Here, we studied the effect of pressure on the ability to accurately quantify the excluded volume which depends on the current drop magnitude produced by a single entity. Using the calibration standard, the inverse current drop (1/ΔI) decreases linearly with increasing pressure, while the dwell drop reduces exponentially. We therefore had to derive a pressure-corrected excluded volume equation to accurately assess the volume of translocating species under applied pressure. Moreover, a method to probe deformation in nanoliposomes and a single cell is developed as a result. We show that the soft nanoliposomes and even cells deform significantly under applied pressure which can be probed in terms of the shape factor which was introduced in the excluded volume equation. The proposed work has practical applications in mechanobiology, namely, assessing the stiffness and mechanical rigidity of liposomal drug carriers. Pressure-biased pores also enabled multiple observations of cell-cell aggregates as well as their subsequent rupture, potentially allowing for the study of microbial symbioses or pathogen recognition by the human immune system.

Catalytic Interruption Mitigates Edge Effects in the Characterization of Heterogeneous, Insulating Nanoparticles

Blocking electrochemistry, a subfield of nanochemistry, enables nondestructive, in situ measurement of the concentration, size, and size heterogeneity of highly dilute, nanometer-scale materials. This approach, in which the adsorptive impact of individual particles on a microelectrode prevents charge exchange with a freely diffusing electroactive redox mediator, has expanded the scope of electrochemistry to the study of redox-inert materials. A limitation, however, remains: inhomogeneous current fluxes associated with enhanced mass transfer occurring at the edges of planar microelectrodes confound the relationship between the size of the impacting particle and the signal it generates. These "edge effects" lead to the overestimation of size heterogeneity and, thus, poor sample characterization. In response, we demonstrate here the ability of catalytic current amplification (EC') to reduce this problem, an effect we term "electrocatalytic interruption". Specifically, we show that the increase in mass transport produced by a coupled chemical reaction significantly mitigates edge effects, returning estimated particle size distributions much closer to those observed using ex situ electron microscopy. In parallel, electrocatalytic interruption enhances the signal observed from individual particles, enabling the detection of particles significantly smaller than is possible via conventional blocking electrochemistry. Finite element simulations indicate that the rapid chemical kinetics created by this approach contributes to the amplification of the electronic signal to restore analytical precision and reliably detect and characterize the heterogeneity of nanoscale electro-inactive materials.

Bioinspired Solid-State Nanochannel Sensors: From Ionic Current Signals, Current, and Fluorescence Dual Signals to Faraday Current Signals

Inspired from bioprotein channels of living organisms, constructing "abiotic" analogues, solid-state nanochannels, to achieve "smart" sensing towards various targets, is highly seductive. When encountered with certain stimuli, dynamic switch of terminal modified probes in terms of surface charge, conformation, fluorescence property, electric potential as well as wettability can be monitored via transmembrane ionic current, fluorescence intensity, faraday current signals of nanochannels and so on. Herein, the modification methodologies of nanochannels and targets-detecting application are summarized in ions, small molecules, as well as biomolecules, and systematically reviewed are the nanochannel-based detection means including 1) by transmembrane current signals; 2) by the coordination of current- and fluorescence-dual signals; 3) by faraday current signals from nanochannel-based electrode. The coordination of current and fluorescence dual signals offers great benefits for synchronous temporal and spatial monitoring. Faraday signals enable the nanoelectrode to monitor both redox and non-redox components. Notably, by incorporation with confined effect of tip region of a needle-like nanopipette, glorious in-vivo monitoring is conferred on the nanopipette detector at high temporal-spatial resolution. In addition, some outlooks for future application in reliable practical samples analysis and leading research endeavors in the related fantastic fields are provided.

Dynamic Behavior of Charged Particles at the Nanopipette Orifice

Understanding the dynamic behavior of charged particles driven by flow and electric field in nanochannels/pores is highly important for both fundamental study and practical applications. While a great breakthrough has been made in understanding the translocation dynamics of charged particles within the nanochannels/pores, studies on the dynamics of particles at the orifice of nanochannels/pores are scarcely reported. Here, we study particle motion at a smaller-sized orifice of a nanopipette by combining experimentally observed current transients with simulated force conditions. The theoretical force analysis reveals that dielectrophoretic force plays an equally important role as electrophoretic force and electroosmotic force, although it has often been neglected in understanding the particle translocation dynamics within the nanopipette. Under the combined action of these forces, it thus becomes difficult for particles to physically collide with the orifice of the nanopipette, resulting in a relatively low decrease in the current transients, which coincides with experimental results. We then regulate the dynamic behavior by altering experimental conditions (i.e., bias potential, nanopipette surface charge, and particle size), and the results further validate the presence and influence of forces being considered. This study improves the understanding of the relationship between particle properties and observed current transients, providing more possibilities for accurate single-particle analysis and single-entity regulation.

Resistive pulse sensing device with embedded nanochannel (nanochannel-RPS) for label-free biomolecule and bionanoparticle analysis

This paper reports an IC-compatible method for fabricating a PDMS-based resistive pulse sensing (RPS) device with embedded nanochannel (nanochannel-RPS) for label-free analysis of biomolecules and bionanoparticles, such as plasmid DNAs and exosomes. Here, a multilayer lithography process was proposed to fabricate the PDMS mold for the microfluidic device, comprising a bridging nanochannel, as the sensing gate. RPS was performed by placing the sensing and excitation electrodes symmetrically upstream and downstream of the sensing gate. In order to reduce the noise level, a reference electrode was designed and placed beside the excitation electrode. To demonstrate the feasibility of the proposed nanochannel-RPS device and sensing system, polystyrene micro- and nanoparticles with diameters of 1μm and 300 nm were tested by the proposed device with signal-to-noise ratios (SNR) ranging from 9.1-30.5 and 2.2-5.9, respectively. Furthermore, a nanochannel with height of 300 nm was applied for 4 kb plasmid DNA detection, implying the potential of the proposed method for label-free quantification of nanoscale biomolecules. Moreover, HeLa cell exosomes, known as a well-studied subtype of extracellular vesicles, were measured and analyzed by their size distribution. The result of the resistive pulse amplitude corresponded well to that of nanoparticle tracking analysis (NTA). The proposed nanochannel-RPS device and the sensing strategy are not only capable of label-free analysis for nanoscale biomolecules and bionanoparticles, but are also cost-effective for large-scale manufacturing.

Constricted Apertures for Dynamic Trapping and Micro-/Nanoscale Discrimination Based on Recapture Kinetics

Sensing via analyte passage through a constricted aperture is a powerful and robust technology which is being utilized broadly, from DNA sequencing to single virus and cell characterization. Micro- and nanoscale structures typically translocate a constricted aperture, or pore, using electrophoretic force. In the present work, we explore the advances in metrology which can be achieved through rapid directional switching of hydrodynamic forces. Interestingly, multipass measurements of microscale and nanoscale structures achieve cell discrimination. We explore this cell-discrimination phenomenon as well as other features of hydrodynamic focusing such as dynamic trapping and discrete interval sensing.

Photocontrolled Nanopipette Biosensor for ATP Gradient Electroanalysis of Single Living Cells

Emerging nanopipette tools have demonstrated substantial potential for advanced single-cell analysis, which plays vital roles from fundamental cellular biology to biomedical diagnostics. Highly recyclable nanopipettes with easy and quick regeneration are of special interest for precise and multiple measurements. However, existing recycle strategies are generally plagued by operational complexity and limited efficiency. Light, acting in a noncontact way, should be the ideal external stimulus to address this issue. Herein, we present the photocontrolled nanopipette capable of probing cellular adenosine triphosphate (ATP) gradient at single-cell level with good sensitivity, selectivity, and reversibility, which stems from the use of ATP-specific azobenzene (Azo)-incorporated DNA aptamer strands (AIDAS) and thereby the sensible transduction of variable nanopore size by the ionic currents passing through the aperture. Photoisomerized conformational change of the AIDAS by alternative UV/vis light stimulation ensures its noninvasive regeneration and repeated detection. Inducement and inhibition of the cellular ATP could also be probed by this nanosensor.

Sizing Single Particles at the Orifice of a Nanopipette

Developing new methods and techniques for the size analysis of particles in a solution is highly desirable not only for the industrial screening of particles but also for single biological entity analysis (e.g., single cells or single vesicles). Herein, we report a new technique for sizing single particles in a solution with a nanopipette. The rationale is essentially based on ion-current blockage when the particles approach the proximity of a nanopipette orifice. By rationally controlling the geometry of the nanopipette and the applied potential, the spike-type ion current transient generated from the motion of particles in the process of "collision and departure" is employed for sizing single particles. The results show that both the relative ion-current change (ΔI/I₀) and the dwell time (Δt) of spike-type transient are dependent on particle size. Differently, Δt is also related to an externally applied voltage. Statistical analysis shows that ΔI/I₀ is proportional to the particle diameter, and this linear relationship is further understood by finite-element simulations. This study not only provides a new principle for sizing single particles in a solution but also is helpful to understand the motion of a particle near the orifice of the nanopipette.

The double life of conductive nanopipette: a nanopore and an electrochemical nanosensor

The continuing interest in nanoscale research has spurred the development of nanosensors for liquid phase measurements. These include nanopore-based sensors typically employed for detecting nanoscale objects, such as nanoparticles, vesicles and biomolecules, and electrochemical nanosensors suitable for identification and quantitative analysis of redox active molecules. In this Perspective, we discuss conductive nanopipettes (CNP) that can combine the advantages of single entity sensitivity of nanopore detection with high selectivity and capacity for quantitative analysis offered by electrochemical sensors. Additionally, the small physical size and needle-like shape of a CNP enables its use as a tip in the scanning electrochemical microscope (SECM), thus, facilitating precise positioning and localized measurements in biological systems.

Nanoscale electrochemical kinetics & dynamics: the challenges and opportunities of single-entity measurements

The development of nanoscale electrochemistry since the mid-1980s has been predominately coupled with steady-state voltammetric (i-E) methods. This research has been driven by the desire to understand the mechanisms of very fast electrochemical reactions, by electroanalytical measurements in small volumes and unusual media, including in vivo measurements, and by research on correlating electrocatalytic activity, e.g., O2 reduction reaction, with nanoparticle size and structure. Exploration of the behavior of nanoelectrochemical structures (nanoelectrodes, nanoparticles, nanogap cells, etc.) of a characteristic dimension λ using steady-state i-E methods generally relies on the well-known relationship, λ2 ∼ Dt, which relates diffusional lengths to time, t, through the coefficient, D. Decreasing λ, by performing measurements at a nanometric length scales, results in a decrease in the effective timescale of the measurement, and provides a direct means to probe the kinetics of steps associated with very rapid electrochemical reactions. For instance, steady-state voltammetry using a nanogap twin-electrode cell of characteristic width, λ ∼ 10 nm, allows investigations of events occurring at timescales on the order of ∼100 ns. Among many other advantages, decreasing λ also increases spatial resolution in electrochemical imaging, e.g., in scanning electrochemical microscopy, and allows probing of the electric double layer. This Introductory Lecture traces the evolution and driving forces behind the "λ2 ∼ Dt" steady-state approach to nanoscale electrochemistry, beginning in the late 1950s with the introduction of the rotating ring-disk electrode and twin-electrode thin-layer cells, and evolving to current-day investigations using nanoelectrodes, scanning nanocells for imaging, nanopores, and nanoparticles. The recent focus on so-called "single-entity" electrochemistry, in which individual and very short redox events are probed, is a significant departure from the steady-state approach, but provides new opportunities to probe reaction dynamics. The stochastic nature of very fast single-entity events challenges current electrochemical methods and modern electronics, as illustrated using recent experiments from the authors' laboratory.

Theory of Transport-Induced-Charge Electroosmotic Pumping toward Alternating Current Resistive Pulse Sensing

In this work, we study transport-induced-charge electroosmosis toward alternating current resistive pulse sensing for the next generation of biomedical applications. Transport-induced-charge electroosmosis, being a new class of electrokinetic phenomenon, occurs as a salt concentration gradient works in synergy with an electric field in ultrathin nanopores. Apart from the conventional electric double layer-governed electroosmotic flow in which the flow behavior is subject to the surface charge, it is found that the transport-induced-charge electroosmotic flow behaves independently of surface charge magnitude but can be linearly regulated by the bulk salt concentration bias. The reversal of the electric field simultaneously inverses the induced charge allowing the establishment of a unidirectional flow under the application of a periodic alternating current field. This unique phenomenon permits continuous water and nanoparticles pumping through a two-dimensional material nanopore in spite of the reversal of the electric field. Built upon this mechanism, we propose a theoretical prototype of alternating current resistive pulse sensing in a two-dimensional nanopore system.

Gold Nanoparticles Biosynthesized and Functionalized Using a Hydroxylated Tetraterpenoid Trigger Gene Expression Changes and Apoptosis in Cancer Cells

Understanding the synthetic mechanisms and cell-nanoparticle interactions of biosynthesized and functionalized gold nanoparticles (AuNPs) using natural products is of great importance for developing their applications in nanomedicine. In this study, we detailed the biotransformation mechanism of Au(III) into AuNPs using a hydroxylated tetraterpenoid deinoxanthin (DX) from the extremophile Deinococcus radiodurans. During the process, Au(III) was rapidly reduced to Au(I) and subsequently reduced to Au(0) by deprotonation of the hydroxyl head groups of the tetraterpenoid. The oxidized form, deprotonated 2-ketodeinoxanthin (DX3), served as a surface-capping agent to stabilize the AuNPs. The functionalized DX-AuNPs demonstrated stronger inhibitory activity against cancer cells compared with sodium citrate-AuNPs and were nontoxic to normal cells. DX-AuNPs accumulated in the cytoplasm, organelles, and nuclei, and induced reactive oxygen species generation, DNA damage, and apoptosis within MCF-7 cancer cells. In the cells treated with DX-AuNPs, 374 genes, including RRAGC gene, were upregulated; 135 genes, including the genes encoding FOXM1 and NR4A1, were downregulated. These genes are mostly involved in metabolism, cell growth, DNA damage, oxidative stress, autophagy, and apoptosis. The anticancer activity of the DX-AuNPs was attributed to the alteration of gene expression and induction of apoptosis. Our results provide significant insight into the synthesis mechanism of AuNPs functionalized with natural tetraterpenoids, which possess enhanced anticancer potential.

Emerging tools for studying single entity electrochemistry

Electrochemistry studies charge transfer and related processes at various microscopic structures (atomic steps, islands, pits and kinks on electrodes), and mesoscopic materials (nanoparticles, nanowires, viruses, vesicles and cells) made by nature and humans, involving ions and molecules. The traditional approach measures averaged electrochemical quantities of a large ensemble of these individual entities, including the microstructures, mesoscopic materials, ions and molecules. There is a need to develop tools to study single entities because a real system is usually heterogeneous, e.g., containing nanoparticles with different sizes and shapes. Even in the case of "homogeneous" molecules, they bind to different microscopic structures of an electrode, assume different conformations and fluctuate over time, leading to heterogeneous reactions. Here we highlight some emerging tools for studying single entity electrochemistry, discuss their strengths and weaknesses, and provide personal views on the need for tools with new capabilities for further advancing single entity electrochemistry.

Size Determination of Metal Nanoparticles Based on Electrochemically Measured Surface-Area-to-Volume Ratios

Here we report the electrochemical determination of the surface-area-to-volume ratio (SA/ V) of Au nanospheres (NSs) attached to electrode surfaces for size analysis. The SA is determined by electrochemically measuring the number of coulombs of charge passed during the reduction of surface Au₂O₃ following Au NS oxidation in HClO₄, whereas V is determined by electrochemically measuring the coulombs of charge passed during the complete oxidative dissolution of all of the Au in the Au NSs in the presence of Br^- to form aqueous soluble AuBr₄^-. Assuming a spherical geometry and taking into account the total number of Au NSs on the electrode surface, the SA/ V is theoretically equal to 3/radius. A plot of the electrochemically measured SA/ V versus 1/radius for five different-sized Au NSs is linear with a slope of 1.8 instead of the expected value of 3. Following attachment of the Au NSs to the electrode and ozone treatment, the plot of SA/ V versus 1/radius is linear with a slope of 3.5, and the size based on electrochemistry matches very closely with those measured by scanning electron microscopy. We believe the ozone cleans the Au NS surface, allowing a more accurate measurement of the SA.

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.

Characterization of Virus Capsids and Their Assembly Intermediates by Multicycle Resistive-Pulse Sensing with Four Pores in Series

Virus self-assembly is a critical step in the virus lifecycle. Understanding how viruses assemble and disassemble provides needed insight into developing antiviral pharmaceuticals. Few tools offer sufficient resolution to study assembly intermediates that differ in size by a few dimers. Our goal is to improve resistive-pulse sensing on nanofluidic devices to offer better particle-size and temporal resolution to study intermediates and capsids generated along the assembly pathway. To increase the particle-size resolution of the resistive-pulse technique, we measured the same, single virus particles up to a thousand times, cycling them back and forth across a series of nanopores by switching the polarity of the applied potential, i.e., virus ping-pong. Multiple pores in series provide a unique multipulse signature during each cycle that improves particle tracking and, therefore, identification of a single particle and reduces the number of cycles needed to make the requisite number of measurements. With T = 3 and T = 4 hepatitis B virus (HBV) capsids, we showed the standard deviation of the particle-size distribution decreased with the square root of the number of measurements and approached discriminating particles differing in size by single dimers. We then studied in vitro assembly of HBV capsids and observed that the ensemble of intermediates shift to larger sizes over 2 days of annealing. On the contrary, assembly reactions diluted to lower dimer concentrations an hour after initiation had fewer intermediates that persisted after the 2 day incubation and had a higher ratio of T = 4 to T = 3 capsids. These reactions indicate that labile T = 4 intermediates are formed rapidly, and dependent on conditions, intermediates may be trapped as metastable species or progress to yield complete capsids.

Bioinspired smart asymmetric nanochannel membranes

Bioinspired smart asymmetric nanochannel membranes (BSANM) have been explored extensively to achieve the delicate ionic transport functions comparable to those of living organisms. The abiotic system exhibits superior stability and robustness, allowing for promising applications in many fields. In view of the abundance of research concerning BSANM in the past decade, herein, we present a systematic overview of the development of the state-of-the-art BSANM system. The discussion is focused on the construction methodologies based on raw materials with diverse dimensions (i.e. 0D, 1D, 2D, and bulk). A generic strategy for the design and construction of the BSANM system is proposed first and put into context with recent developments from homogeneous to heterogeneous nanochannel membranes. Then, the basic properties of the BSANM are introduced including selectivity, gating, and rectification, which are associated with the particular chemical and physical structures. Moreover, we summarized the practical applications of BSANM in energy conversion, biochemical sensing and other areas. In the end, some personal opinions on the future development of the BSANM are briefly illustrated. This review covers most of the related literature reported since 2010 and is intended to build up a broad and deep knowledge base that can provide a solid information source for the scientific community.

Three-Dimensional Super-resolution Imaging of Single Nanoparticles Delivered by Pipettes

Controlled three-dimensional positioning of nanoparticles is achieved by delivering single fluorescent nanoparticles from a nanopipette and capturing them at well-defined regions of an electrified substrate. To control the position of single nanoparticles, the force of the pressure-driven flow from the pipette is balanced by the attractive electrostatic force at the substrate, providing a strategy by which nanoparticle trajectories can be manipulated in real time. To visualize nanoparticle motion, a resistive-pulse electrochemical setup is coupled with an optical microscope, and nanoparticle trajectories are tracked in three dimensions using super-resolution fluorescence imaging to obtain positional information with precision in the tens of nanometers. As the particles approach the substrate, the diffusion kinetics are analyzed and reveal either subdiffusive (hindered) or superdiffusive (directed) motion depending on the electric field at the substrate and the pressure-driven flow from the pipette. By balancing the effects of the forces exerted on the particle by the pressure and electric fields, controlled, real-time manipulation of single nanoparticle trajectories is achieved. The developed approach has implications for a variety of applications such as surface patterning and drug delivery using colloidal nanoparticles.

Highly Selective Cerebral ATP Assay Based on Micrometer Scale Ion Current Rectification at Polyimidazolium-Modified Micropipettes

Development of new principles and methods for cerebral ATP assay is highly imperative not only for determining ATP dynamics in brain but also for understanding physiological and pathological processes related to ATP. Herein, we for the first time demonstrate that micrometer scale ion current rectification (MICR) at a polyimidazolium brush-modified micropipette can be used as the signal transduction output for the cerebral ATP assay with a high selectivity. The rationale for ATP assay is essentially based on the competitive binding ability between positively charged polyimidazolium and ATP toward negatively charged ATP aptamer. The method is well responsive to ATP with a good linearity within a concentration range from 5 nM to 100 nM, and high selectivity toward ATP. These properties essentially enable the method to determine the cerebral ATP by combining in vivo microdialysis. The basal dialysate level of ATP in rat brain cortex is determined to be 11.32 ± 2.36 nM (n = 3). This study demonstrates that the MICR-based sensors could be potentially used for monitoring neurochemicals in cerebral systems.

Detection and sizing of nanoparticles and DNA on PDMS nanofluidic chips based on differential resistive pulse sensing

The RPS (Resistive Pulse Sensing) technique is a popular tool for the label-free detection of particles. This paper describes a simple, cost-effective PDMS nanofluidic chip for the detection and characterization of nanoparticles based on the differential RPS technique with high resolution and sensitivity. The chip is composed of two layers of PDMS slabs. Microchannel systems fabricated by the photolithography method on the top layer are used for sample loading and differential signal acquisition, and a straight nanochannel on the bottom layer fabricated by an unconventional approach bridging the gap between the microchannels works as an RPS sensing gate. A single-stage differential amplifier is used to amplify the RPS signals when particles or DNA pass through the sensing gate. It was demonstrated that this nanofluidic RPS chip can detect nanoparticles as small as 23 nm with a high SNR (Signal-to-Noise Ratio). The experimental results also show that the device is able to distinguish nanoparticles of smaller size differences such as 60 nm and 83 nm with high resolution, showing superior performance in comparison with the results obtained from DLS (Dynamic Light Scattering). This differential nano-RPS chip was also applied to detect the translocation of dsDNA molecules.

Nanofluidic Devices with 8 Pores in Series for Real-Time, Resistive-Pulse Analysis of Hepatitis B Virus Capsid Assembly

To improve the precision of resistive-pulse measurements, we have used a focused ion beam instrument to mill nanofluidic devices with 2, 4, and 8 pores in series and compared their performance. The in-plane design facilitates the fabrication of multiple pores in series which, in turn, permits averaging of the series of pulses generated from each translocation event. The standard deviations (σ) of the pulse amplitude distributions decrease by 2.7-fold when the average amplitudes of eight pulses are compared to the amplitudes of single pulses. Similarly, standard deviations of the pore-to-pore time distributions decrease by 3.2-fold when the averages of the seven measurements from 8-pore devices are contrasted to single measurements from 2-pore devices. With signal averaging, the inherent uncertainty in the measurements decreases; consequently, the resolution (mean/σ) improves by a factor equal to the square root of the number of measurements. We took advantage of the improved size resolution of the 8-pore devices to analyze in real time the assembly of Hepatitis B Virus (HBV) capsids below the pseudocritical concentration. We observe that abundances of assembly intermediates change over time. During the first hour of the reaction, the abundance of smaller intermediates decreased, whereas the abundance of larger intermediates with sizes closer to a T = 4 capsid remained constant.

Real-time shape approximation and fingerprinting of single proteins using a nanopore

Established methods for characterizing proteins typically require physical or chemical modification steps or cannot be used to examine individual molecules in solution. Ionic current measurements through electrolyte-filled nanopores can characterize single native proteins in an aqueous environment, but currently offer only limited capabilities. Here we show that the zeptolitre sensing volume of bilayer-coated solid-state nanopores can be used to determine the approximate shape, volume, charge, rotational diffusion coefficient and dipole moment of individual proteins. To do this, we developed a theory for the quantitative understanding of modulations in ionic current that arise from the rotational dynamics of single proteins as they move through the electric field inside the nanopore. The approach allows us to measure the five parameters simultaneously, and we show that they can be used to identify, characterize and quantify proteins and protein complexes with potential implications for structural biology, proteomics, biomarker detection and routine protein analysis.

Rapid structural analysis of nanomaterials in aqueous solutions

Rapid structural analysis of nanoscale matter in a liquid environment represents innovative technologies that reveal the identities and functions of biologically important molecules. However, there is currently no method with high spatio-temporal resolution that can scan individual particles in solutions to gain structural information. Here we report the development of a nanopore platform realizing quantitative structural analysis for suspended nanomaterials in solutions with a high z-axis and xy-plane spatial resolution of 35.8 ± 1.1 and 12 nm, respectively. We used a low thickness-to-diameter aspect ratio pore architecture for achieving cross sectional areas of analyte (i.e. tomograms). Combining this with multiphysics simulation methods to translate ionic current data into tomograms, we demonstrated rapid structural analysis of single polystyrene (Pst) beads and single dumbbell-like Pst beads in aqueous solutions.

Potentiometric sensing of nucleic acids using chemically modified nanopores

Unlike the overwhelming majority of nanopore sensors that are based on the measurement of a transpore ionic current, here we introduce a potentiometric sensing scheme and demonstrate its application for the selective detection of nucleic acids. The sensing concept uses the charge inversion that occurs in the sensing zone of a nanopore upon binding of negatively charged microRNA strands to positively charged peptide-nucleic acid (PNA) modified nanopores. The initial anionic permselectivity of PNA-modified nanopores is thus gradually changed to cationic permselectivity, which can be detected simply by measuring the nanoporous membrane potential. A quantitative theoretical treatment of the potentiometric microRNA response is provided based on the Nernst-Planck/Poisson model for the nanopore system assuming first order kinetics for the nucleic acid hybridization. An excellent correlation between the theoretical and experimental results was observed, which revealed that the binding process is focused at the nanopore entrance with contributions from both in pore and out of pore sections of the nanoporous membrane. The theoretical treatment is able to give clear guidelines for further optimization of potentiometric nanopore-based nucleic acid sensors by predicting the effect of the most important experimental parameters on the potential response.

Resistive Pulse Delivery of Single Nanoparticles to Electrochemical Interfaces

An experimental system for controlling and interrogating the collisions of individual nanoparticles at electrode/electrolyte interfaces is described. A nanopipet positioned over a 400 nm radius Pt ultramicroelectrode is used to deliver individual nanoparticles, via pressure-driven solution flow, to the underlying electrode, where the nanoparticles undergo collisions and are detected electrochemically. High-velocity collisions result in elastic collisions of negatively charged polystyrene nanospheres at the Pt/water interface, while low-velocity collisions result in nanoparticle adsorption ("sticky" collisions). The ability to position the nanopipet with respect to the underlying ultramicroelectrode also allows the time between particle release from the nanopipet and electrode collision to be investigated as a function of nanopipet-electrode separation, d. The time between release and collision of the nanoparticle is found to be proportional to d³, in excellent agreement with an analytical expression for convective fluid flow from a pipet orifice.

Observing single nanoparticle events at the orifice of a nanopipet

Single nanoparticle (NP) events are successfully observed at the orifice of a nanopipet by blocking the ionic current with a single NP.

Single nanoparticle (NP) events are successfully observed at the orifice of a nanopipet by blocking the ionic current with a single NP. In addition to the traditional translocation events, we observe both staircase and blip current transients by controlling the radius ratio of NPs to nanopipet or bias potential. Confocal fluorescence microscopy and finite element simulation are used to simultaneously monitor and quantitatively understand these events, respectively. The frequency of the staircase and blip events is proportional to the NP concentration, and could be used for the quantification of NPs. This study offers a new method for NP determination and single NP behavior study.

High-Speed Multipass Coulter Counter with Ultrahigh Resolution

Coulter counters measure the size of particles in solution by passing them through an orifice and measuring a resistive pulse, i.e., a drop in the ionic current flowing between two electrodes placed on either side of the orifice. The magnitude of the pulse gives information on the size of the particle; however, resolution is limited by variability in the path of the translocation, due to the Brownian motion of the particle. We present a simple yet powerful modified Coulter counter that uses programmable data acquisition hardware to switch the voltage after sensing the resistive pulse of a nanoparticle passing through the orifice of a nanopipet. Switching the voltage reverses the direction of the driving force on the particle and, when this detect-switch cycle is repeated, allows us to pass an individual nanoparticle through the orifice thousands of times. By measuring individual particles more than 100 times per second we rapidly determine the distribution of the resistive pulses for each particle, which allows us to accurately determine the mean pulse amplitude and deliver considerably improved size resolution over a conventional Coulter counter. We show that single polystyrene nanoparticles can be shuttled back and forth and monitored for minutes, leading to a precisely determined mean blocking current equating to sub-angstrom size resolution.