Scanning-aperture trapping and manipulation of single charged nanoparticles

Visualization of Microcapillary Tips Using Waveguided Light

The microcapillary, a glass tube with a nano/micrometer scale aperture, is used for manipulating small objects across diverse disciplines. A primary concern in using the microcapillary involves tip breakage upon contact. Here, we report a method for visualizing the microcapillary tip, enabling precise and instant determination of its contact with other objects. Illumination directed to the back aperture of the microcapillary induces waveguiding through the glass wall, enabling the visualization of the tip through scattering. We demonstrate that the tip scattering is sensitive to contact with an adjacent object owing to the near-field interaction of the waveguided light, providing a clear distinction between the contact and noncontact states. The key advantage of our method encompasses its minimal influence, irrespective of conductivity, and applicability to nanoscale systems. The versatility of our method is shown by the application to a wide range of tip diameters, various substrate and in-filling materials.

Gate Electrodes Enable Tunable Nanofluidic Particle Traps

The ability to control the location of nanoscale objects in liquids is essential for fundamental and applied research from nanofluidics to molecular biology. To overcome their random Brownian motion, the electrostatic fluid trap creates local minima in potential energy by shaping electrostatic interactions with a tailored wall topography. However, this strategy is inherently static; once fabricated, the potential wells cannot be modulated. Here, we propose and experimentally demonstrate that such a trap can be controlled through a buried gate electrode. We measure changes in the average escape times of nanoparticles from the traps to quantify the induced modulations of 0.7 kBT in potential energy and 50 mV in surface potential. Finally, we summarize the mechanism in a parameter-free predictive model, including surface chemistry and electrostatic fringing, that reproduces the experimental results. Our findings open a route toward real-time controllable nanoparticle traps.

DC corral trapping of single nanoparticles and macromolecules in solution

Progress in sorting, separating, and characterizing ever smaller amounts of chemical and biological material depends on the availability of methods for the controlled interaction with nanoscale and molecular-size objects. Here, we report on the reversible, tunable trapping of single DNA molecules and other charged micro- and nanoparticles in aqueous solution using a direct-current (DC) corral trap setup. The trap consists of a circular, non-conductive void in a metal-coated surface that, when charged, generates an electrostatic potential well in the proximate solution. Our results demonstrate that stable, nanoscale confinement of charged objects is achievable over extended periods of time, that trap stiffness is controlled by the applied voltage, and that simultaneous trapping of multiple objects is feasible. The approach shows great promise for lab-on-a-chip systems and biomedical applications due to its simplicity, scalability, selectivity, and the capability to manipulate single DNA molecules in standard buffer solutions.

Scattering-based Light Microscopy: From Metal Nanoparticles to Single Proteins

Our ability to detect, image, and quantify nanoscopic objects and molecules with visible light has undergone dramatic improvements over the past few decades. While fluorescence has historically been the go-to contrast mechanism for ultrasensitive light microscopy due to its superior background suppression and specificity, recent developments based on light scattering have reached single-molecule sensitivity. They also have the advantages of universal applicability and the ability to obtain information about the species of interest beyond its presence and location. Many of the recent advances are driven by novel approaches to illumination, detection, and background suppression, all aimed at isolating and maximizing the signal of interest. Here, we review these developments grouped according to the basic principles used, namely darkfield imaging, interferometric detection, and surface plasmon resonance microscopy.

Etching-Engineered Low-Voltage Dielectrophoretic Nanotweezers for Trapping of Single Molecules

Understanding the functions of biomolecules at the single-molecule level is crucial due to their important and diverse roles in cell regulation. Recently, nanotweezers made of dual carbon nanoelectrodes have been developed for single-cell biopsies by applying a high alternating voltage. However, high electric voltage can induce Joule heating, water electrolysis, and other side effects on cell activity, which may be unfavorable for cellular applications. Here, we report a low-voltage nanotweezer for trapping of single DNA molecules using etching-engineered nanoelectrodes which effectively reduce the minimum trapping voltage by six times. Meanwhile, the low-voltage nanotweezer displays an improved trapping stiffness. Based on the finite element method simulations, we attribute the mechanism for the low-voltage nanotweezers to the increase in spatial heterogeneity and nonuniformity of electric field by etching of quartz near the nanoelectrodes. This work opens a new dimension for noninvasive single-molecule manipulation in solution and potential applications in single-cell biopsies.

Surface-modified elastomeric nanofluidic devices for single nanoparticle trapping

Our work focuses on the development of simpler and effective production of nanofluidic devices for high-throughput charged single nanoparticle trapping in an aqueous environment. Single nanoparticle confinement using electrostatic trapping has been an effective approach to study the fundamental properties of charged molecules under a controlled aqueous environment. Conventionally, geometry-induced electrostatic trapping devices are fabricated using SiOx-based substrates and comprise nanochannels imbedded with nanoindentations such as nanopockets, nanoslits and nanogrids. These geometry-induced electrostatic trapping devices can only trap negatively charged particles, and therefore, to trap positively charged particles, modification of the device surface is required. However, the surface modification process of a nanofluidic device is cumbersome and time consuming. Therefore, here, we present a novel approach for the development of surface-modified geometry-induced electrostatic trapping devices that reduces the surface modification time from nearly 5 days to just a few hours. We utilized polydimethylsiloxane for the development of a surface-modified geometry-induced electrostatic trapping device. To demonstrate the device efficiency and success of the surface modification procedure, a comparison study between a PDMS-based geometry-induced electrostatic trapping device and the surface-modified polydimethylsiloxane-based device was performed. The device surface was modified with two layers of polyelectrolytes (1: poly(ethyleneimine) and 2: poly(styrenesulfonate)), which led to an overall negatively charged surface. Our experiments revealed the presence of a homogeneous surface charge density inside the fluidic devices and equivalent trapping strengths for the surface-modified and native polydimethylsiloxane-based geometry-induced electrostatic trapping devices. This work paves the way towards broader use of geometry-induced electrostatic trapping devices in the fields of biosensing, disease diagnosis, molecular analysis, fluid quality control and pathogen detection.

Deterministic Deposition of Nanoparticles with Sub-10 nm Resolution

Accurate deposition of nanoparticles at defined positions on a substrate is still a challenging task, because it requires simultaneously stable long-range transport and attraction to the target site and precise short-range orientation and deposition. Here we present a method based on geometry-induced energy landscapes in a nanofluidic slit for particle manipulation: Brownian motors or electro-osmotic flows are used for particle delivery to the target area. At the target site, electrostatic trapping localizes and orients the particles. Finally, reducing the gap distance of the slit leads sequentially to a focusing of the particle position and a jump into adhesive contact by several nanometers. For 60 nm gold spheres, we obtain a placement accuracy of 8 nm. The versatility of the method is demonstrated further by a stacked assembly of nanorods and the directed deposition of InAs nanowires.

Precise Placement of Microbubble Templates at Single Entity Resolution

Microbubbles have been used as a soft template to produce hollow structures for diverse applications in chemistry, materials science, and biomedicine. It is a challenge, however, to control their size and position at single-entity level. We report on an on-demand method to produce and place a single microbubble with programmed size and position. The method exploits scanning an electrolyte-filled micropipette to place a hydrogen (H₂) bubble, generated by water electrolysis, on the desired position. The bubble growth is self-limited after the bubble size fits to the pipet aperture, yielding well-controlled bubble size. The bubble growth dynamics within the pipet is successfully investigated by a methodology that combines phase-contrast X-ray imaging and electric-current measurement. We show that the microbubbles, accurately controlled in size and position, can be used for the fabrication of various polypyrrole microcontainer arrays. We expect the scanning-pipet strategy could be generalized for manipulating various soft materials at will.

Experimental Observation of Current Reversal in a Rocking Brownian Motor

A reversal of the particle current in overdamped rocking Brownian motors was predicted more than 20 years ago; however, an experimental verification and a deeper insight into this noise-driven mechanism remained elusive. Here, we investigate the high-frequency behavior of a rocking Brownian motor for 60 nm gold spheres based on electrostatic interaction in a 3D-shaped nanofluidic slit and electro-osmotic forcing of the particles. We measure the particle probability density in situ with 10 nm spatial and 250  μs temporal resolution and compare it with theory. At a driving frequency of 250 Hz, we observe a current reversal that can be traced to the asymmetric and increasingly static probability density at high frequencies.

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.

Single entity resolution valving of nanoscopic species in liquids

Investigating biological and synthetic nanoscopic species in liquids, at the ultimate resolution of single entity, is important in diverse fields^1-5. Progress has been made^6-10, but significant barriers need to be overcome such as the need for intense fields, the lack of versatility in operating conditions and the limited functionality in solutions of high ionic strength for biological applications. Here, we demonstrate switchable electrokinetic nanovalving able to confine and guide single nano-objects, including macromolecules, with sizes down to around 10 nanometres, in a lab-on-chip environment. The nanovalves are based on spatiotemporal tailoring of the potential energy landscape of nano-objects using an electric field, modulated collaboratively by wall nanotopography and by embedded electrodes in a nanochannel system. We combine nanovalves to isolate single entities from an ensemble, and demonstrate their guiding, confining, releasing and sorting. We show on-demand motion control of single immunoglobulin G molecules, quantum dots, adenoviruses, lipid vesicles, dielectric and metallic particles, suspended in electrolytes with a broad range of ionic strengths, up to biological levels. Such systems can enable nanofluidic, large-scale integration and individual handling of multiple entities in applications ranging from single species characterization and screening to in situ chemical or biochemical synthesis in continuous on-chip processes.

Meniscus-on-Demand Parallel 3D Nanoprinting

Exploiting a femtoliter liquid meniscus formed on a nanopipet is a powerful approach to spatially control mass transfer or chemical reaction at the nanoscale. However, the insufficient reliability of techniques for the meniscus formation still restricts its practical use. We report on a noncontact, programmable method to produce a femtoliter liquid meniscus that is utilized for parallel three-dimensional (3D) nanoprinting. The method based on electrohydrodynamic dispensing enables one to create an ink meniscus at a pipet-substrate gap without physical contact and positional feedback. By guiding the meniscus under rapid evaporation of solvent in air, we successfully fabricate freestanding polymer 3D nanostructures. After a quantitative characterization of the experimental conditions, we show that we can use a multibarrel pipet to achieve parallel fabrication process of clustered nanowires with precise placement. We expect this technique to advance productivity in nanoscale 3D printing.

Nanofluidic rocking Brownian motors

Control and transport of nanoscale objects in fluids is challenging because of the unfavorable scaling of most interaction mechanisms to small length scales. We designed energy landscapes for nanoparticles by accurately shaping the geometry of a nanofluidic slit and exploiting the electrostatic interaction between like-charged particles and walls. Directed transport was performed by combining asymmetric potentials with an oscillating electric field to achieve a rocking Brownian motor. Using gold spheres 60 nanometers in diameter, we investigated the physics of the motor with high spatiotemporal resolution, enabling a parameter-free comparison with theory. We fabricated a sorting device that separates 60- and 100-nanometer particles in opposing directions within seconds. Modeling suggests that the device separates particles with a radial difference of 1 nanometer.

The nanofluidic confinement apparatus: studying confinement-dependent nanoparticle behavior and diffusion

The behavior of nanoparticles under nanofluidic confinement depends strongly on their distance to the confining walls; however, a measurement in which the gap distance is varied is challenging. Here, we present a versatile setup for investigating the behavior of nanoparticles as a function of the gap distance, which is controlled to the nanometer. The setup is designed as an open system that operates with a small amount of dispersion of ≈20 μL, permits the use of coated and patterned samples and allows high-numerical-aperture microscopy access. Using the tool, we measure the vertical position (termed height) and the lateral diffusion of 60 nm, charged, Au nanospheres as a function of confinement between a glass surface and a polymer surface. Interferometric scattering detection provides an effective particle illumination time of less than 30 μs, which results in lateral and vertical position detection accuracy ≈10 nm for diffusing particles. We found the height of the particles to be consistently above that of the gap center, corresponding to a higher charge on the polymer substrate. In terms of diffusion, we found a strong monotonic decay of the diffusion constant with decreasing gap distance. This result cannot be explained by hydrodynamic effects, including the asymmetric vertical position of the particles in the gap. Instead we attribute it to an electroviscous effect. For strong confinement of less than 120 nm gap distance, we detect the onset of subdiffusion, which can be correlated to the motion of the particles along high-gap-distance paths.

Soft electrostatic trapping in nanofluidics

Trapping and manipulation of nano-objects in solution are of great interest and have emerged in a plethora of fields spanning from soft condensed matter to biophysics and medical diagnostics. We report on establishing a nanofluidic system for reliable and contact-free trapping as well as manipulation of charged nano-objects using elastic polydimethylsiloxane (PDMS)-based materials. This trapping principle is based on electrostatic repulsion between charged nanofluidic walls and confined charged objects, called geometry-induced electrostatic (GIE) trapping. With gold nanoparticles as probes, we study the performance of the devices by measuring the stiffness and potential depths of the implemented traps, and compare the results with numerical simulations. When trapping 100 nm particles, we observe potential depths of up to Q≅24 k B T that provide stable trapping for many days. Taking advantage of the soft material properties of PDMS, we actively tune the trapping strength and potential depth by elastically reducing the device channel height, which boosts the potential depth up to Q~200 k B T, providing practically permanent contact-free trapping. Due to a high-throughput and low-cost fabrication process, ease of use, and excellent trapping performance, our method provides a reliable platform for research and applications in study and manipulation of single nano-objects in fluids.

Levitated Plasmonic Nanoantennas in an Aqueous Environment

We report on the manipulation of a plasmonic nanoantenna in an aqueous solution using an electrostatic trap created between a glass nanopipette and a substrate. By scanning a trapped gold nanosphere in the near field of a single colloidal quantum dot embedded under the substrate surface, we demonstrate about 8-fold fluorescence enhancement over a lateral full width at half-maximum of about 45 nm. We analyze our results with the predictions of numerical electromagnetic simulations under consideration of the electrostatic free energy in the trap. Our approach could find applications in a number of experiments, where plasmonic effects are employed at liquid-solid interfaces.

Controlling the excitation of radially polarized conical plasmons in plasmonic tips in liquids

The plasmonic tip's emission can be controlled in liquids depending on the wetting condition and the refractive index of liquids.

Periodic parallel array of nanopillars and nanoholes resulting from colloidal stripes patterned by geometrically confined evaporative self-assembly for unique anisotropic wetting

In this paper we present an economical process to create anisotropic microtextures based on periodic parallel stripes of monolayer silica nanoparticles (NPs) patterned by geometrically confined evaporative self-assembly (GCESA). In the GCESA process, a straight meniscus of a colloidal dispersion is initially formed in an opened enclosure, which is composed of two parallel plates bounded by a U-shaped spacer sidewall on three sides with an evaporating outlet on the fourth side. Lateral evaporation of the colloidal dispersion leads to periodic "stick-slip" receding of the meniscus (evaporative front), as triggered by the "coffee-ring" effect, promoting the assembly of silica NPs into periodic parallel stripes. The morphology of stripes can be well controlled by tailoring process variables such as substrate wettability, NP concentration, temperature, and gap height, etc. Furthermore, arrayed patterns of nanopillars or nanoholes are generated on a silicon wafer using the as-prepared colloidal stripes as an etching mask or template. Such arrayed patterns can reveal unique anisotropic wetting properties, which have a large contact angle hysteresis viewing from both the parallel and perpendicular directions in addition to a large wetting anisotropy.