Selective trapping and concentration of nanoparticles and viruses in dual-height nanofluidic channels

Constrained Volume Micro- and Nanoparticle Collection Methods in Microfluidic Systems

Particle trapping and enrichment into confined volumes can be useful in particle processing and analysis. This review is an evaluation of the methods used to trap and enrich particles into constrained volumes in microfluidic and nanofluidic systems. These methods include physical, optical, electrical, magnetic, acoustic, and some hybrid techniques, all capable of locally enhancing nano- and microparticle concentrations on a microscale. Some key qualitative and quantitative comparison points are also explored, illustrating the specific applicability and challenges of each method. A few applications of these types of particle trapping are also discussed, including enhancing biological and chemical sensors, particle washing techniques, and fluid medium exchange systems.

Nano/microfluidic device for high-throughput passive trapping of nanoparticles

We present a design and a fabrication method for devices designed for rapid collection of nanoparticles in a fluid. The design uses nanofluidic channels as a passive size-based barrier trap to isolate particles near a central point in the channel, which is also covered by a thin membrane. Particles that enter the collection region are trapped with 100% efficiency within a 6-12 μ m radius from a central point. Flow rates for particle-free fluid range from 1.88 to 3.69 nl/s for the pressure and geometries tested. Particle trapping tests show that high trapped particle counts significantly impact flow rates. For suspensions as dilute as 30-300 aM (20-200 particles/μ l), 8-80 particles are captured within 500 s.

Nanofluidic devices for the separation of biomolecules

Over the last 30-years, microchip electrophoresis and its applications have expanded due to the benefits it offers. Nanochip electrophoresis, on the other hand, is viewed as an evolving area of electrophoresis because it offers some unique advantages not associated with microchip electrophoresis. These advantages arise from unique phenomena that occur in the nanometer domain not readily apparent in the microscale domain due to scale-dependent effects. Scale-dependent effects associated with nanochip electrophoresis includes high surface area-to-volume ratio, electrical double layer overlap generating parabolic flow even for electrokinetic pumping, concentration polarization, transverse electromigration, surface charge dominating flow, and surface roughness. Nanochip electrophoresis devices consist of channels with dimensions ranging from 1 to 1000 nm including classical (1-100 nm) and extended (100 nm - 1000 nm) nanoscale devices. In this review, we highlight scale-dependent phenomena associated with nanochip electrophoresis and the utilization of those phenomena to provide unique biomolecular separations that are not possible with microchip electrophoresis. We will also review the range of materials used for nanoscale separations and the implication of material choice for the top-down fabrication and operation of these devices. We will also provide application examples of nanochip electrophoresis for biomolecule separations with an emphasis on nano-electrophoresis (nEP) and nano-electrochromatography (nEC).

Single-molecule confinement with uniform electrodynamic nanofluidics

To date, we could not engineer Nature's ability to dynamically handle diffusing single molecules in the liquid-phase as it takes place in pore-forming proteins and tunnelling nanotubes. Consistent handling of individual single molecules in a liquid is of paramount importance to fundamental molecular studies and technological benefits, like single-molecule level separation and sorting for early biomedical diagnostics, microscopic studies of molecular interactions and electron/optical microscopy of molecules and nanomaterials. We can consistently resolve the dynamics of diffusing single molecules if they are confined within a uniform dielectric environment at nanometre length-scales. A uniform dielectric environment is the key characteristic since intrinsic electronic properties of molecules were modified while interacting with any surfaces, and the effect is not the same from one dielectric surface to another. We present dynamic nanofluidic detection of optically active single molecules in a liquid. An all-silica nanofluidic environment was used to electrokinetically handle individual single-molecules where molecular shot noise was resolved. We recorded the single-molecule motion of small fragments of DNA, carbon-nanodots, and organic fluorophores in water. The electrokinetic 1D molecular mass transport under two-focus fluorescence correlation spectroscopy (2fFCS) showed confinement-induced modified molecular interactions (due to various inter-molecular repulsive and attractive forces), which have been theoretically interpreted as molecular shot noise. Our demonstration of high-throughput nanochannel fabrication, 2fFCS-based 1D confined detection of fast-moving single molecules and fundamental understanding of molecular shot noise may open an avenue for single-molecule experiments where physical manipulation of dynamics is necessary.

On interfacial viscosity in nanochannels

Capillary driven transport of liquids in nanoscopic channels is an omnipresent phenomenon in nature and technology including fluid flow in the human body and plants, drug delivery, nanofluidic devices, and energy/water systems. However, the kinetics of this mass transport mechanism remains in question as the well-known Lucas-Washburn (LW) model predicts significantly faster flow rates compared to the experimental observations. We here showed the role of interfacial viscosity in capillary motion slowdown in nanochannels through a combination of experimental, analytical and molecular dynamics techniques. We showed that the slower liquid flow is due to the formation of a thin liquid layer adjacent to the channel walls with a viscosity substantially greater than the bulk liquid. By incorporating the effect of the interfacial layer, we presented a theoretical model that accurately predicts the capillarity kinetics in nanochannels of different heights. Non-equilibrium molecular dynamics simulation confirmed the obtained interfacial viscosities. The viscosities of isopropanol and ethanol within the interfacial layer were 9.048 mPa s and 4.405 mPa s, respectively (i.e. 279% and 276% greater than their bulk values). We also showed that the interfacial layers are 6.4 nm- and 5.3 nm-thick for isopropanol and ethanol, respectively.

Ordered Surface Nanostructures Self-Assembled from Rod-Coil Block Copolymers on Microspheres

An ordered surface nanostructure endows materials advanced functions. However, fabricating ordered surface-patterned particles via the polymer self-assembly approach is a challenge. Here we report that poly(γ-benzyl-l-glutamate)-block-poly(ethylene glycol) rod-coil block copolymers are able to form uniform-surface micelles on polystyrene microspheres through a solution self-assembly approach. The size of the surface micelles can be varied by the molecular weight of the block copolymers. These surface micelles are arranged in a manner consistent with the Euler theorem. Most of the micelles are six-fold coordinated, and the number difference between the five-fold and the seven-fold coordination is 12. Simulations on model systems qualitatively reproduced the experimental findings and provided direct observations for the surface-patterned particles, including the polymer chain packing manner in surface micelles at the molecular level and the array feature of the surface micelles through 2D projections of the surface patterns.

High-throughput single-particle detections using a dual-height-channel-integrated pore

We report a proof-of-principle demonstration of particle concentration to achieve high-throughput resistive pulse detections of bacteria using a microfluidic-channel-integrated micropore. We fabricated polymeric nanochannels to trap micrometer-sized bioparticles via a simple water pumping mechanism that allowed aggregation-free size-selective particle concentration with negligible loss. Single-bioparticle detections by ionic current measurements were then implemented through releasing and transporting the thus-collected analytes to the micropore. As a result, we attained two orders of magnitude enhancement in the detection throughput by virtue of an accumulation effect via hydrodynamic control. The device concept presented may be useful in developing nanopores and nanochannels for high-throughput single-particle and -molecule analyses.

Subnanometer structure and function from ion beams through complex fluidics to fluorescent particles

The vertical dimensions of complex nanostructures determine the functions of diverse nanotechnologies. In this paper, we investigate the unknown limits of such structure-function relationships at subnanometer scales. We begin with a quantitative evaluation of measurement uncertainty from atomic force microscopy, which propagates through our investigation from ion beam fabrication to fluorescent particle characterization. We use a focused beam of gallium ions to subtractively pattern silicon surfaces, and silicon nitride and silicon dioxide films. Our study of material responses quantifies the atomic limits of forming complex topographies with subnanometer resolution of vertical features over a wide range of vertical and lateral dimensions. Our results demonstrate the underutilized capability of this standard system for rapid prototyping of subnanometer structures in hard materials. We directly apply this unprecedented dimensional control to fabricate nanofluidic devices for the analytical separation of colloidal nanoparticles by size exclusion. Optical microscopy of single nanoparticles within such reference materials establishes a subnanometer limit of the fluidic manipulation of particulate matter and enables critical-dimension particle tracking with subnanometer accuracy. After calibrating for optical interference within our multifunctional devices, which also enables device metrology and integrated spectroscopy, we reveal an unexpected relationship between nanoparticle size and emission intensity for common fluorescent probes. Emission intensity increases supervolumetrically with nanoparticle diameter and then decreases as nanoparticles with different diameters photobleach to similar values of terminal intensity. We propose a simple model to empirically interpret these surprising results. Our investigation enables new control and study of structure-function relationships at subnanometer scales.

Recent Advances in Nanoparticle Concentration and Their Application in Viral Detection Using Integrated Sensors

Early disease diagnostics require rapid, sensitive, and selective detection methods for target analytes. Specifically, early viral detection in a point-of-care setting is critical in preventing epidemics and the spread of disease. However, conventional methods such as enzyme-linked immunosorbent assays or cell cultures are cumbersome and difficult for field use due to the requirements of extensive lab equipment and highly trained personnel, as well as limited sensitivity. Recent advances in nanoparticle concentration have given rise to many novel detection methodologies, which address the shortcomings in modern clinical assays. Here, we review the primary, well-characterized methods for nanoparticle concentration in the context of viral detection via diffusion, centrifugation and microfiltration, electric and magnetic fields, and nano-microfluidics. Details of the concentration mechanisms and examples of related applications provide valuable information to design portable, integrated sensors. This study reviews a wide range of concentration techniques and compares their advantages and disadvantages with respect to viral particle detection. We conclude by highlighting selected concentration methods and devices for next-generation biosensing systems.

Use of a micro- to nanochannel for the characterization of surface-enhanced Raman spectroscopy signals from unique functionalized nanoparticles

A micro- to nanochannel nanoparticle aggregating device that does not require any input energy to organize the particles to a specific location, i.e., no pumps, plugs, heat, or magnets, has been designed and used to characterize the surface-enhanced Raman spectroscopy (SERS) signal from four unique functionalized nanoparticles (gold, silver-gold nanocages, silver nanocubes, and silica-gold nanoshells). The SERS signal was assessed in terms of the peak signal strength from the four different Raman reporter functionalized nanoparticles to determine which nanoparticle had better utility in the channel to provide the most robust platform for a future biological analyte detection device. The innovation used to fabricate the micro- to nanochannel device is described; the TEM images of the nanoparticles are shown; the absorption data for the nanoparticles are given; and the spectral data for the Raman reporter, mercaptobenzoic acid (MBA), are depicted. In the micro- to nanochannel described in this work, 5  μl of 22.3  μM MBA functionalized silver nanocubes were determined to have the strongest SERS enhancement.

Label-free detection and manipulation of single biological nanoparticles

In the past several years, there have been significant advances in the field of nanoparticle detection for various biological applications. Of considerable interest are synthetic nanoparticles being designed as potential drug delivery systems as well as naturally occurring or biological nanoparticles, including viruses and extracellular vesicles. Many infectious diseases and several human cancers are attributed to individual virions. Because these particles likely display different degrees of heterogeneity under normal physiological conditions, characterization of these natural nanoparticles with single-particle sensitivity is necessary for elucidating information on their basic structure and function as well as revealing novel targets for therapeutic intervention. Additionally, biodefense and point-of-care clinical testing demand ultrasensitive detection of viral pathogens particularly with high specificity. Consequently, the ability to perform label-free virus sensing has motivated the development of multiple electrical-, mechanical-, and optical-based detection techniques, some of which may even have the potential for nanoparticle sorting and multi-parametric analysis. For each technique, the challenges associated with label-free detection and measurement sensitivity are discussed as are their potential contributions for future real-world applications. WIREs Nanomed Nanobiotechnol 2016, 8:717-729. doi: 10.1002/wnan.1392 For further resources related to this article, please visit the WIREs website.

Electrophoresis of two spheres: Influence of double layer and van der Waals interactions

Considering recent applications of electrophoresis conduced in nanoscaled devices, where particle-particle interaction can play a role, we studied for the first time the electrophoresis of two rigid spheres along their center line, taking account of the hydrodynamic, electric, and van der Waals interactions between them. Under the conditions of constant surface potential and surface charge density, the influences of the level of surface potential/charge density, the bulk salt concentration, and the particle-particle distance on their electrokinetic behaviors are examined. Numerical simulation reveals that these behaviors are much more complicated and interesting than those of isolated particles. In particular, we show that care must be taken in choosing an appropriate particle concentration in relevant experiment to avoid obtaining unreliable mobility data.

Simultaneous diamagnetic and magnetic particle trapping in ferrofluid microflows via a single permanent magnet

Trapping and preconcentrating particles and cells for enhanced detection and analysis are often essential in many chemical and biological applications. Existing methods for diamagnetic particle trapping require the placement of one or multiple pairs of magnets nearby the particle flowing channel. The strong attractive or repulsive force between the magnets makes it difficult to align and place them close enough to the channel, which not only complicates the device fabrication but also restricts the particle trapping performance. This work demonstrates for the first time the use of a single permanent magnet to simultaneously trap diamagnetic and magnetic particles in ferrofluid flows through a T-shaped microchannel. The two types of particles are preconcentrated to distinct locations of the T-junction due to the induced negative and positive magnetophoretic motions, respectively. Moreover, they can be sequentially released from their respective trapping spots by simply increasing the ferrofluid flow rate. In addition, a three-dimensional numerical model is developed, which predicts with a reasonable agreement the trajectories of diamagnetic and magnetic particles as well as the buildup of ferrofluid nanoparticles.

A novel fluidic control method for nanofluidics by solvent-solvent interaction in a hybrid chip

The fluidic control method is a fundamental technology for the development of nanofluidics. In this report, an organic phase was driven to flow inside the nanochannel because of its dissolution into an aqueous phase. With selective modification, a stable organic/aqueous interface was generated at the junction of the micro/nanochannels in a hybrid chip. The aqueous phase was kept flowing in the microchannel, and the organic phase in the nanochannel dissolved into the aqueous phase through the interface and produced a flow inside the nanochannel. This method is simple, easy to control and requires no specific equipment. Importantly, the flow is driven by the surface tension in a controllable manner, which will not be affected by the depth of the nanochannel. This method can be a useful alternative to the present fluidic control methods in nanofluidics.

Electrokinetic preconcentration of particles and cells in microfluidic reservoirs

We present an electrokinetic (EK) technique for in-reservoir particle and cell preconcentration via induced-charge electroosmosis (ICEO) and dielectrophoresis (DEP).

Thin-film microfabricated nanofluidic arrays for size-selective protein fractionation

Size-selective fractionation and quantitation of biostructures in the sub-hundred nanometer size range is an important research area. Unfortunately, current methods for size fractionation are complex, time consuming, or offer poor resolution. Using standard microfabrication technology, we developed a nanofluidic sieving system to address these limitations. Our setup consists of an array of parallel nanochannels with a height step in each channel, an injection reservoir, and a waste reservoir. The height steps can size fractionate a protein mixture as a solution flows through the nanochannels via capillary action. We tested this system with different sizes and concentrations of five proteins to understand protein size and height step effects on trapping. Our results clearly show size-dependent trapping of proteins at nanometer-scale height steps in nanochannels. We also developed a model that predicts the observed size-dependent trapping of proteins. This work is a key step towards scalable nanofluidic methods for molecular fractionation.

Enrichment of nanoparticles and bacteria using electroless and manual actuation modes of a bypass nanofluidic device

Current efforts in nanofluidics aimed at detecting scarce molecules or particles are focused mainly on the development of electrokinetic-based devices. However, these techniques require either integrated or external electrodes, and a potential drop applied across a carrier fluid. One challenge is to develop a new generation of electroless passive devices involving a simple technological process and packaging without embedded electrodes for micro- and nanoparticles enrichment with a view to applications in biology such as the detection of viral agents or cancers biomarkers. This paper presents an innovative technique for particles handling and enrichment based exclusively on a pressure-driven silicon bypass nanofluidic device. The device is fabricated by standard silicon micro-nanofabrication technology. The concentration operation was demonstrated and quantified according to two different actuation modes, which can also be combined to enhance the concentration factor further. The first, "symmetrical" mode involves a symmetric cross-flow effect that concentrates nanoparticles in a very small volume in a very local point of the device. The second mode, "asymmetrical" mode advantageously generates a streaming potential, giving rise to an Electroless Electropreconcentration (EL-EP). The concentration process can be maintained for several hours and concentration factors as high as ~200 have been obtained when both symmetrical and asymmetrical modes are coupled. Proof of concept for concentrating E. coli bacteria by the manual actuation of the EL-EP device is also demonstrated in this paper. Experiments demonstrate more than a 50-fold increase in the concentration of E. coli bacteria in only ~40 s.

Pen microfluidics: rapid desktop manufacturing of sealed thermoplastic microchannels

A unique technique for the rapid fabrication of thermoplastic microfluidic chips is described. The method enables the realization of fully-sealed microchannels in around one hour while requiring only minimal infrastructure by taking advantage of a solvent swelling mechanism that allows raised features to be patterned on the surface of homogeneous thermoplastic materials. Patterning is achieved without photolithography by simply drawing the desired microchannel pattern onto the polymer surface using a suitable ink as a masking layer, either manually or under robotic control, followed by timed exposure to solvent vapor to yield a desired depth for the masked channel features. The channels are then permanently sealed through solvent bonding of the microchannel chip to a mating thermoplastic substrate. The process is demonstrated using cyclic olefin copolymer as a thermoplastic material, with fully operational microfluidic devices fabricated following a true desktop manufacturing model suitable for rapid prototyping.

Review article: Fabrication of nanofluidic devices

Thanks to its unique features at the nanoscale, nanofluidics, the study and application of fluid flow in nanochannels/nanopores with at least one characteristic size smaller than 100 nm, has enabled the occurrence of many interesting transport phenomena and has shown great potential in both bio- and energy-related fields. The unprecedented growth of this research field is apparently attributed to the rapid development of micro/nanofabrication techniques. In this review, we summarize recent activities and achievements of nanofabrication for nanofluidic devices, especially those reported in the past four years. Three major nanofabrication strategies, including nanolithography, microelectromechanical system based techniques, and methods using various nanomaterials, are introduced with specific fabrication approaches. Other unconventional fabrication attempts which utilize special polymer properties, various microfabrication failure mechanisms, and macro/microscale machining techniques are also presented. Based on these fabrication techniques, an inclusive guideline for materials and processes selection in the preparation of nanofluidic devices is provided. Finally, technical challenges along with possible opportunities in the present nanofabrication for nanofluidic study are discussed.

Nanopore-induced spontaneous concentration for optofluidic sensing and particle assembly

Metallic nanopore arrays have emerged as optofluidic platforms with multifarious sensing and analytical capabilities such as label-free surface plasmon resonance (SPR) sensing of molecular binding interactions and surface-enhanced Raman spectroscopy (SERS). However, directed delivery of analytes through open nanopores using traditional methods such as external electric fields or pressure gradients still remains difficult. We demonstrate that nanopore arrays have an intrinsic ability to promote flow through them via capillary flow and evaporation. This passive "nano-drain" mechanism is utilized to concentrate biomolecules on the surface of nanopores for improved detection sensitivity or create ordered nanoscale arrays of beads and liposomes. Without using any external pump or fluidic interconnects, we can concentrate and detect the presence of less than a femtomole of streptavidin in 10 μL of sample using fluorescence imaging. Liposome nanoarrays are also prepared in less than 5 min and used to detect lipid-protein interactions. We also demonstrate label-free SPR detection of analytes using metallic nanopore arrays. This method provides a fast, simple, transportable, and small-volume platform for labeled as well as label-free plasmonic analysis while improving the detection time and sensitivity.

Fabrication of nanofluidic biochips with nanochannels for applications in DNA analysis

With the development of nanotechnology, great progress has been made in the fabrication of nanochannels. Nanofluidic biochips based on nanochannel structures allow biomolecule transport, bioseparation, and biodetection. The domain applications of nanofluidic biochips with nanochannels are DNA stretching and separation. In this Review, the general fabrication methods for nanochannel structures and their applications in DNA analysis are discussed. These representative fabrication approaches include conventional photolithography, interference lithography, electron-beam lithography, nanoimprint lithography and polymer nanochannels. Other nanofabrication methods used to fabricate unique nanochannels, including sub-10-nm nanochannels, single nanochannels, and vertical nanochannels, are also mentioned. These nanofabrication methods provide an effective way to form nanoscale channel structures for nanofluidics and biosensor devices for DNA separation, detection, and sensing. The broad applications of nanochannels and future perspectives are also discussed.

Dynamic trapping and high-throughput patterning of cells using pneumatic microstructures in an integrated microfluidic device

Microfluidic trapping methods create significant opportunities to establish highly controlled cell positioning and arrangement for the microscale study of numerous cellular physiological and pathological activities. However, a simple, straightforward, dynamic, and high-throughput method for cell trapping is not yet well established. In the present paper, we report a direct active trapping method using an integrated microfluidic device with pneumatic microstructures (PμSs) for both operationally and quantitatively dynamic localization of cells, as well as for high-throughput cell patterning. We designed and fabricated U-shape PμS arrays to replace the conventional fixed microstructures for reversible trapping. Multidimensional dynamics and spatial consistency of the PμSs were optically characterized and quantitatively demonstrated. Furthermore, we performed a systematic trapping investigation of the PμSs actuated at a pressure range of 0 psi to 20 psi using three types of popularly applied mammalian cells, namely, human lung adenocarcinoma A549 cells, human hepatocellular liver carcinoma HepG2 cells, and human breast adenocarcinoma MCF-7 cells. The cells were quantitatively trapped and controlled by the U-shape PμSs in a programmatic and parallel manner, and could be opportunely released. The trapped cells with high viability were hydrodynamically protected by the real-time actuation of specifically designed umbrella-like PμSs. We demonstrate that PμSs can be applied as an active microfluidic component for large-scale cell patterning and manipulation, which could be useful in many cell-based tissue organization, immunosensor, and high-throughput imaging and screening.

Surfactant addition and alternating current electrophoretic oscillation during size fractionation of nanoparticles in channels with two or three different height segments

An array of parallel planar nanochannels containing two or three segments with varying inner heights was fabricated and used for size fractionation of inorganic and biological nanoparticles. A liquid suspension of the particles was simply drawn through the nanochannels via capillary action. Using fluorescently labeled 30 nm polyacrylonitrile beads, different trapping behaviors were compared using nanochannels with 200-45 nm and 208-54-30 nm height segments. Addition of sodium dodecyl sulfate (SDS) surfactant to the liquid suspension and application of an AC electric field were shown to aid in the prevention of channel clogging. After initial particle trapping at the segment interfaces, significant particle redistribution occurred when applying a sinusoidal 8V peak-to-peak oscillating voltage with a frequency of 150 Hz and DC offset of 4V. Using the 208-54-30 nm channels, 30 nm hepatitis B virus (HBV) capsids were divided into three fractions. When the AC electric field was applied to this trapped sample, all of the virus particles passed through the interfaces and accumulated at the channel ends.

Capillary flow in sacrificially etched nanochannels

Planar nanochannels are fabricated using sacrificial etching technology with sacrificial cores consisting of aluminum, chromium, and germanium, with heights ranging from 18 to 98 nm. Transient filling via capillary action is compared against the Washburn equation [E. W. Washburn, Phys. Rev. 17, 273 (1921)], showing experimental filling speeds significantly lower than classical continuum theory predicts. Departure from theory is expressed in terms of a varying dynamic contact angle, reaching values as high as 83° in channels with heights of 18 nm. The dynamic contact angle varies significantly from the macroscopic contact angle and increases with decreasing channel dimensions.

Separation and metrology of nanoparticles by nanofluidic size exclusion

A nanofluidic technology for the on-chip size separation and metrology of nanoparticles is demonstrated. A nanofluidic channel was engineered with a depth profile approximated by a staircase function. Numerous stepped reductions in channel depth were used to separate a bimodal mixture of nanoparticles by nanofluidic size exclusion. Epifluorescence microscopy was used to map the size exclusion positions of individual nanoparticles to corresponding channel depths, enabling measurement of the nanoparticle size distributions and validation of the size separation mechanism.