Mass transport in nanofluidic devices

Nanofluidics is a recent appearing research field, introduced in 1995 as an analogue of the field of microfluidics, and has been becoming popular in the past few years. The proximity of the channel dimension, the Debye length, and the size of biomolecules such as DNA and proteins gives the unique features of nanofluidic devices. Of various unique properties of the nanofluidics, mass transport in nanochannel plays determining roles in fundamental reaches and practical applications of nanofluidic device. Thus, much work including numerical and experimental researches has been performed to investigate the mass transport behaviors in nanofluidic devices. This review summarizes the fabrication technologies for nanofluidic devices, the mass transport behaviors in nanochannel, and their applications in bioanalysis. The main focus will be laid on the effects of nanochannel size and surface charge on mass transport including electrokinetic transport of charged analytes, diffusion of electric neutral molecules, ionic current rectification, concentration polarization, nonlinear electrokinetic flow at the micro-nanofluidic interfaces.     

Ionic current rectification in asymmetric nanofluidic devices

In recent decades, the properties and behaviors of nanofluidic devices have been widely explored in varied subjects such as engineering, physics, chemistry, and biology. Among the rich properties of nanofluidics, ionic current rectification (ICR) is a unique phenomenon arising from asymmetric nanofluidic devices with electric double layer (EDL) overlapped. The ICR property is especially useful in applications including energy conversion, mass separation, sea water purification and bioanalysis. In this review, the ICR property in nanofluidics as well as the underlying mechanism is demonstrated. The influencing factors concerning to the ICR property are systematically summarized. The asymmetric geometry as well as the charge distribution is in charge of the ICR behavior occurring in nanofluidic devices. This review is aimed at readers who are interested in the fundamentals of mass transport in nanofluidics in general, as well as those who are willing to apply nanofluidics in various research fields.     

Chain structure and stiffness of Teflon AF glassy amorphous fluoropolymers

The structure of the amorphous perfluorinated polymer Teflon AF 2400 and other structurally close perfluoropolymers was studied by means of a quantum chemistry method. The electronic and structural characteristics of the repeating unit and polymer models with ten and nine monomer units were obtained. It was found that two nonplanar isomers can exist for different models of the perfluorinated dioxole ring with a difference of their energy minimums of 10.8 kJ/mol. The orthogonal-block structure of the polymer chain of the perfluorodioxole homopolymer and its copolymer with tetrafluoroethylene was proposed, the block size was found, and a possible diameter of the void formed by two neighboring polymer chains was evaluated. Potential energy curves for the rotation of certain chain fragments about different bonds of the polymer main chain were constructed, and the polymer stiffness was shown to substantially depend on the molar ratio between perfluorodioxole and tetrafluoroethylene units in the copolymer and on the geometry of the perfluorodioxole ring.     

Thermoactuated diffusion control in soft matter nanofluidic devices

The diffusive transport rate in a soft matter nanofluidic device is controlled with a thermoactuated hydrogel valve. The device consists of three giant unilamellar vesicles linearly conjugated by lipid nanotubes, with a solution of the stimuli-responsive polymer poly(N-isopropyl acrylamide) (PNIPAAm) in the central vesicle. The valve states "high (transport) rate" and "low (transport) rate" are obtained by heat-activated switching between PNIPAAm's dissolved and compact aggregated states. We show that three parameters influence the diffusion rate within the device: the increase of the transport rate caused by a decrease in PNIPAAm concentration upon compaction, the temperature dependence of the buffer viscosity, and the volume excluded by the PNIPAAm hydrogel compartment.     

Sorption and diffusion of organic vapors in amorphous Teflon AF2400

Vapor sorption in amorphous Teflon AF2400 of various organic solutes was studied in a wide range of activity at 25 °C by means of the gravimetric technique. The sorption isotherms of hexane, toluene, and chloroform were shown to be concave to the pressure axis and are consistent with the dual mode sorption model (DMS). The parameters of the DMS model k_D and b reveal a linear correlation with squared critical temperature of solutes T. The third model parameter, the Langmuir sorption capacity C′_H decreases when the size of solutes (critical volume) increases. Sorption isotherms of methanol and ethanol were shown to be convex to the pressure axis and are consistent with cluster formation in this strongly hydrophobic polymer. Concentration‐dependent diffusion coefficients D were determined using a linear implicit difference scheme in analysis of sorption kinetics. It was shown that D values increase exponentially with concentration for all the solutes, except alcohols for which exponential reduction of D(C) was observed. The partitioning of the thermodynamic and mobility contributions in D indicated that the reduction of D values of alcohols is consistent with clustering phenomena in AF2400. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 832–844, 2006     

Rapid microfabrication of solvent-resistant biocompatible microfluidic devices

This paper presents a rapid, simple, and low-cost fabrication method to prepare solvent resistant and biocompatible microfluidic devices with three-dimensional geometries. The devices were fabricated in thiolene and replicated from PDMS master with high molding fidelity. Good chemical compatibility for organic solvents allows volatile chemicals in synthesis and analysis applications. The surface can be processed to be hydrophobic or hydrophilic for water-in-oil and oil-in-water emulsions. Monodisperse organic solvent droplet generation is demonstrated to be reproducible in thiolene microchannels without swelling. The thiolene surface prevents cell adhesion but normal cell growth and adhesion on glass substrates is not affected by the adjacent thiolene patterns.     

Efficiently accounting for ion correlations in electrokinetic nanofluidic devices using density functional theory

The electrokinetic behavior of nanofluidic devices is dominated by the electrical double layers at the device walls. Therefore, accurate, predictive models of double layers are essential for device design and optimization. In this paper, we demonstrate that density functional theory (DFT) of electrolytes is an accurate and computationally efficient method for computing finite ion size effects and the resulting ion-ion correlations that are neglected in classical double layer theories such as Poisson-Boltzmann. Because DFT is derived from liquid-theory thermodynamic principles, it is ideal for nanofluidic systems with small spatial dimensions, high surface charge densities, high ion concentrations, and/or large ions. Ion-ion correlations are expected to be important in these regimes, leading to nonlinear phenomena such as charge inversion, wherein more counterions adsorb at the wall than is necessary to neutralize its surface charge, leading to a second layer of co-ions. We show that DFT, unlike other theories that do not include ion-ion correlations, can predict charge inversion and other nonlinear phenomena that lead to qualitatively different current densities and ion velocities for both pressure-driven and electro-osmotic flows. We therefore propose that DFT can be a valuable modeling and design tool for nanofluidic devices as they become smaller and more highly charged.     

Complete plastic nanofluidic devices for DNA analysis via direct imprinting with polymer stamps

Development of all polymer-based nanofluidic devices using replication technologies, which is a prerequisite for providing devices for a larger user base, is hampered by undesired substrate deformation associated with the replication of multi-scale structures. Therefore, most nanofluidic devices have been fabricated in glass-like substrates or in a polymer resist layer coated on a substrate. This letter presents a rapid, high fidelity direct imprinting process to build polymer nanofluidic devices in a single step. Undesired substrate deformation during imprinting was significantly reduced through the use of a polymer stamp made from a UV-curable resin. The integrity of the enclosed all polymer-based nanofluidic system was verified by a fluorescein filling experiment and translocation/stretching of λ-DNA molecules through the nanochannels. It was also found that the funnel-like design of the nanochannel inlet significantly improved the entrance of DNA molecules into nanochannels compared to an abrupt nanochannel/microfluidic network interface.     

Particle deposition onto micropatterned charge heterogeneous substrates: trajectory analysis

A trajectory analysis of particles near a micropatterned charged substrate under radial impinging jet flow conditions is presented to investigate the effect of surface charge heterogeneity on particle trajectory and deposition efficiency. The surface charge heterogeneity is modeled as concentric bands of specified width and pitch having positive and negative surface potentials. The flow distribution is obtained using finite element analysis of the governing Navier-Stokes equations. The particle trajectory analysis takes into consideration the hydrodynamic interactions, gravity, van der Waals and electrostatic double layer interactions. The presence of surface charge heterogeneity on the substrate gives rise to an oscillating particle trajectory near the collector surface due to repulsive and attractive forces. As a result of the coupled effects of hydrodynamic and colloidal forces, the particle trajectories and deposition efficiencies are increasingly affected by surface charge heterogeneity as one moves radially away from the stagnation point. The results indicate that it is possible to render collectors with up to 50% favorable surface fraction completely unfavorable by modifying the ratio of the radial to normal fluid velocity. Utilizing the real favorable area fraction of the collector, the patch model expression for calculating the deposition efficiency is modified for impinging jet flow geometry.     

Force measurements between Teflon AF and colloidal silica particles in electrolyte solutions

The interaction force between a very hydrophobic polymer surface and colloidal silica particles with a roughness of 10–15 nm has been measured in aqueous solutions of KOH and KCl using an atomic force microscope. The interaction can be described according to the DLVO theory by an electrical double-layer force that is repulsive at long distances and attractive at short distances and an attractive van der Waals force. The electrical double-layer potentials are compared to the zeta potentials of Teflon AF and the silica spheres. The roughness of the silica particles leads to an underestimation of the short-range attraction and the surface potential. Both KCl and KOH solutions affect the potential of the interacting surfaces. OH⁻ ions that adsorb preferentially to the Teflon AF surface create higher potentials than Cl⁻ ions. Range and strength of the attractive interaction are not affected by KCl solutions but reduced by addition of KOH. This can be explained by decreasing potential differences between the silica sphere and Teflon AF with increasing KOH concentration. In addition, the preferential adsorption of OH⁻ ions may lead to a reduction of the van der Waals interaction. The presence of nanobubbles, too, might play a role.     

Synthesis of nickel nanowires via electroless nanowire deposition on micropatterned substrates

Electroless nanowire deposition on micropatterned substrates (ENDOM) is a promising new technique by which to direct the synthesis and precise placement of metallic nanowires. ENDOM is generally applicable to the preparation of metallic, semiconducting, and even insulating nanowires on technologically relevant substrates, is inexpensive, and can achieve high growth rates. The deposited nanowires are ultralong (centimeters) and can be patterned in arbitrary shapes. We demonstrate ENDOM using the growth of nickel nanowires. By controlling the deposition time, the width of the nanowires can be varied from 200 to 1000 nm and the height can be varied from 7 to 20 nm.     

All-silica nanofluidic devices for DNA-analysis fabricated by imprint of sol-gel silica with silicon stamp

We present a simple and cheap method for fabrication of silica nanofluidic devices for single-molecule studies. By imprinting sol-gel materials with a multi-level stamp comprising micro- and nanofeatures, channels of different depth are produced in a single process step. Calcination of the imprinted hybrid sol-gel material produces purely inorganic silica, which has very low autofluorescence and can be fusion bonded to a glass lid. Compared to top-down processing of fused silica or silicon substrates, imprint of sol-gel silica enables fabrication of high-quality nanofluidic devices without expensive high-vacuum lithography and etching techniques. The applicability of the fabricated device for single-molecule studies is demonstrated by measuring the extension of DNA molecules of different lengths confined in the nanochannels.     

Numerical analysis of the shapes and energies of droplets on micropatterned substrates

The shapes and energies of drops on substrates patterned with either holes or posts are computed using Surface Evolver software. The holes and posts are cylindrical in shape and distributed in a 6-fold symmetric pattern. The wetting conditions are such that the liquid does not fill the holes and the interface between the drop and the substrate is composite, i.e., partly solid/liquid and partly liquid/vapor. The sequence of stable drop configurations with increasing volume is analyzed and provides, in part, an explanation for superhydrophobic drop spreading.     

Properties of the Teflon AF1601S Surface Treated with the Low-Pressure Argon Plasma

The stability of the surface properties of Teflon AF films were investigated after their exposure to the low-pressure argon plasma for various times. X-ray photoelectron and infrared spectroscopies, atomic force microscopy, scanning optical microscopy based on chromatic aberration, goniometry (the measurement of water contact angles), as well as electrokinetic method and ellipsometry were used to control changes in the chemical composition and surface properties of Teflon AF films taking place upon their plasma treatment. The stable hydrophilization of the surface of Teflon AF films resulted from plasma treatment was revealed.     

Tunable micropatterned substrates based on poly(dopamine) deposition via microcontact printing

A simple technique was developed to fabricate tunable micropatterned substrates based on mussel-inspired surface modification. Polydopamine (PDA) was developed on polydimethylsiloxane (PDMS) stamps and was easily imprinted to several substrates such as glass, silicon, gold, polystyrene, and poly(ethylene glycol) via microcontact printing. The imprinted PDA retained its unique reactivity and could modulate the chemical properties of micropatterns via secondary reactions, which was illustrated in this study. PDA patterns imprinted onto a cytophobic and nonfouling substrates were used to form patterns of cells or proteins. PDA imprints reacted with nucleophilic amines or thiols to conjugate molecules such as poly(ethylene glycol) for creating nonfouling area. Gold nanoparticles were immobilized onto PDA-stamped area. The reductive ability of PDA transformed silver ions to elemental metals as an electroless process of metallization. This facile and economic technique provides a powerful tool for development of a functional patterned substrate for various applications.     

Teflon AF 2400管冷蒸气发生-原子荧光法分析烟草中的痕量汞

采用具有高气体渗透性和紫外光透过性的无定形聚四氟乙烯管(Teflon AF-2400)作为蒸气发生原子荧光系统的气液分离器和检测池,建立了微型化汞蒸气发生原子荧光检测系统。考察了Teflon AF-2400管长度、还原剂种类和浓度、酸介质种类和浓度、进样流速以及氩气流速对测量体系的影响。在优化实验条件下,该方法的线性范围为0.07~10μg/L(r=0.998 0),检出限为0.02μg/L,相对标准偏差(n=11)为3.3%。该系统体积小、样品消耗少、灵敏度高。方法已成功应用于卷烟样品中汞含量的检测,回收率为97%~102%。     

Properties of thin Teflon AF 2400 coatings deposited onto carbon fabric from solutions in supercritical carbon dioxide

For the design of hydrophobized conductive gas diffusion layers of the electrodes of polymercontaining fuel cells, of great interest is the method for the formation of a fluoropolymer film from fluoropolymer solution in supercritical carbon dioxide. The present work describes the systematic studies on the carbon-fabric-deposited Teflon AF 2400 coatings obtained by this method. The electrical and geometric characteristics of the coatings, their elemental compositions, and stabilities are studied. It is shown that supercritical carbon dioxide allows the deposited fluoropolymer to penetrate homogeneously into the depth of carbon fabric to form a uniform coating around single individual fibers. The optimum limits of variation in the Teflon AF 2400 amount upon production of the gas diffusion layers of polymer electrolyte, alkaline, and phosphoric acid fuel cells are determined.     

Suitability of Liquid Crystal Polymer substrates for high frequency acoustic devices

Polymer substrates are widely seen as a low-cost route to flexible circuits for systems incorporating displays, sensing functions and transistors. To date most polymer-based devices have involved passive components such as humidity sensors, constructed using metals and additional organic layers. The starting point of the study described here, is whether more advanced components incorporating functional materials can be integrated into devices on polymer substrates. An understanding will be required of the material factors that limit performance and, if possible, techniques developed to circumvent them so that their performance can be compared to that of standard silicon-based components.     

Polyions for biocompatible materials

Starting from the thermodynamic properties of some polyamidoamines, showing a protonation behaviour similar to that of small molecules, a relationship with the ability to form polymer-polymer complexes with heparin or heparin-like macromolecules was found. In the light of these results, new heparinisable materials (PUPA and EVAPA) were synthesised, and the surface and bulk structures were understood on the basis of the enhanced hydrophilic characters when these materials are protonated at physiological pH.     

Pressure-driven flow control system for nanofluidic chemical process

We developed a novel flow control system for a nanofluidic chemical process. Generally, flow control in nanochannels is difficult because of its high-pressure loss with very small volume flow rate. In our flow control method, liquid pressure in a microchannel connected to the nanochannels is regulated by utilizing a backpressure regulator. The flow control method was verified by using simple structured microchip, which included parallel nanochannels. We found that the observed flow rate was three times lower than the value expected from Hagen-Poiseuille's equation. That implied a size-dependent viscosity change in the nanochannels. Then, we demonstrated mixing of two different fluorescent solutions in a Y-shaped nanochannel and also a proton exchange reaction in the Y-shaped nanochannel. The flow control method will contribute to further integration of nanochemical systems.