Chuanhua Duan

Anomalous ion transport in 2-nm hydrophilic nanochannels

Transmembrane proteins often contain nanoscale channels through which ions and molecules can pass either passively (by diffusion) or actively (by means of forced transport). These proteins play important roles in selective mass transport and electrical signalling in many biological processes. Fluidic nanochannels that are 1-2 nm in diameter act as functional mimics of protein channels, and have been used to explore the transport of ions and molecules in confined liquids. Here we report ion transport in 2-nm-deep nanochannels fabricated by standard semiconductor manufacturing processes. Ion transport in these nanochannels is dominated by surface charge until the ion concentration exceeds 100 mM. At low concentrations, proton mobility increases by a factor of four over the bulk value, possibly due to overlapping of the hydrogen-bonding network of the two hydration layers adjacent to the hydrophilic surfaces. The mobility of K+/Na+ ions also increases as the bulk concentration decreases, although the reasons for this are not completely understood.

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.

Evaporation-induced cavitation in nanofluidic channels

Cavitation, known as the formation of vapor bubbles when liquids are under tension, is of great interest both in condensed matter science as well as in diverse applications such as botany, hydraulic engineering, and medicine. Although widely studied in bulk and microscale-confined liquids, cavitation in the nanoscale is generally believed to be energetically unfavorable and has never been experimentally demonstrated. Here we report evaporation-induced cavitation in water-filled hydrophilic nanochannels under enormous negative pressures up to -7 MPa. As opposed to receding menisci observed in microchannel evaporation, the menisci in nanochannels are pinned at the entrance while vapor bubbles form and expand inside. Evaporation in the channels is found to be aided by advective liquid transport, which leads to an evaporation rate that is an order of magnitude higher than that governed by Fickian vapor diffusion in macro- and microscale evaporation. The vapor bubbles also exhibit unusual motion as well as translational stability and symmetry, which occur because of a balance between two competing mass fluxes driven by thermocapillarity and evaporation. Our studies expand our understanding of cavitation and provide new insights for phase-change phenomena at the nanoscale.

Label-Free Electrical Detection of Enzymatic Reactions in Nanochannels.

We report label-free electrical detection of enzymatic reactions using 2-D nanofluidic channels and investigate reaction kinetics of enzymatic reactions on immobilized substrates in nanoscale-confined spaces. Trypsin proteolysis is chosen for demonstration of the detection scheme. When trypsin cleaves poly-l-lysine coated on the surface of silica nanochannels, the resulting change of surface charge density can be detected by monitoring the ionic conductance of the nanochannels. Our results show that detection of such surface enzymatic reactions is faster than detection of surface binding reactions in nanochannels for low-concentration analytes. Furthermore, the nanochannel sensor has a sensitivity down to 5 ng/mL, which statistically corresponds to a single enzyme per nanochannel. Our results also suggest that enzyme kinetics in nanochannels is fundamentally different from that in bulk solutions or plain surfaces. Such enzymatic reactions form two clear self-propagating reaction fronts inside the nanochannels, and the reaction fronts follow square-root time dependences at high enzyme concentrations due to significant nonspecific adsorption. However, at low enzyme concentrations when nonspecific adsorption is negligible, the reaction fronts propagate linearly with time, and the corresponding propagation speed is related to the channel geometry, enzyme concentration, catalytic reaction constant, diffusion coefficient, and substrate surface density. Optimization of this nanochannel sensor could lead to a quick-response, highly sensitive, and label-free sensor for enzyme assay and kinetic studies.

Accurate measurement of liquid transport through nanoscale conduits

Nanoscale liquid transport governs the behaviour of a wide range of nanofluidic systems, yet remains poorly characterized and understood due to the enormous hydraulic resistance associated with the nanoconfinement and the resulting minuscule flow rates in such systems. To overcome this problem, here we present a new measurement technique based on capillary flow and a novel hybrid nanochannel design and use it to measure water transport through single 2-D hydrophilic silica nanochannels with heights down to 7 nm. Our results show that silica nanochannels exhibit increased mass flow resistance compared to the classical hydrodynamics prediction. This difference increases with decreasing channel height and reaches 45% in the case of 7 nm nanochannels. This resistance increase is attributed to the formation of a 7-angstrom-thick stagnant hydration layer on the hydrophilic surfaces. By avoiding use of any pressure and flow sensors or any theoretical estimations the hybrid nanochannel scheme enables facile and precise flow measurement through single nanochannels, nanotubes, or nanoporous media and opens the prospect for accurate characterization of both hydrophilic and hydrophobic nanofluidic systems.

Bubble-Regulated Silicon Nanowire Synthesis on Micro-Structured Surfaces by Metal-Assisted Chemical Etching.

In this work, we study silicon nanowire synthesis via one-step metal-assisted chemical etching (MACE) on microstructured silicon surfaces with periodic pillar/cavity array. It is found that hydrogen gas produced from the initial anodic reaction can be trapped inside cavities and between pillars, which serves as a mask to prevent local etching, and leads to the formation of patterned vertically aligned nanowire array. A simple model is presented to demonstrate that such bubble entrapment is due to the significant adhesion energy barrier, which is a function of pillar/cavity geometry, contact angle, and nanowire length to be etched. The bubble entrapment can be efficiently removed when extra energy is introduced by sonication to overcome this energy barrier, resulting in nanowire growth in all exposed surfaces. This bubble-regulated MACE process on microstructured surfaces can be used to fabricate nanowire arrays with desired morphologies.

Structures and thermodynamics of water encapsulated by graphene

Understanding phase behaviors of nanoconfined water has driven notable research interests recently. In this work, we examine water encapsulated under a graphene cover that offers an ideal testbed to explore its molecular structures and thermodynamics. We find layered water structures for up to ~1000 trapped water molecules, which is stabilized by the spatial confinement and pressure induced by interfacial adhesion. For monolayer encapsulations, we identify representative two-dimensional crystalline lattices as well as defects therein. Free energy analysis shows that the structural orders with low entropy are compensated by high formation energies due to the pressurized confinement. There exists an order-to-disorder transition for this condensed phase at ~480–490 K, with a sharp reduction in the number of hydrogen bonds and increase in the entropy. Fast diffusion of the encapsulated water demonstrates anomalous temperature dependence, indicating the solid-to-fluid nature of this structural transition. These findings offer fundamental understandings of the encapsulated water that can be used as a pressurized cell with trapped molecular species, and provide guidance for practical applications with its presence, for example, in the design of nanodevices and nanoconfined reactive cells.

Mechanics of liquid–liquid interfaces and mixing enhancement in microscale flows

Experimental work on mixing in microfluidic devices has been of growing importance in recent years. Interest in probing reaction kinetics faster than the minute or hour time scale has intensified research in designing microchannel devices that would allow the reactants to be mixed on a time scale faster than that of the reaction. Particular attention has been paid to the design of microchannels in order to enhance the advection phenomena in these devices. Ultimately, in vitro studies of biological reactions can now be performed in conditions that reflect their native intracellular environments. Liau et al . ( Anal. Chem. , vol. 77, 2005, p. 7618) have demonstrated a droplet-based microfluidic mixer that induces improved chaotic mixing of crowded solutions in milliseconds due to protrusions (‘bumps’) on the microchannel walls. Liau et al . (2005) have shown it to be possible to mix rapidly plugs of highly concentrated protein solutions such as bovine hemoglobin and bovine serum albumin. The present work concerns an analysis of the underlying mechanisms of shear stress transfer at liquid–liquid interfaces and associated enhanced mixing arising from the protrusions along the channel walls. The role of non-Newtonian rheology and surfactants is also considered within the mixing framework developed by Aref, Ottino and Wiggins in several publications. Specifically, we show that proportional thinning of the carrier fluid lubrication layer at the bumps leads to greater advection velocities within the plugs, which enhances mixing. When the fluid within the plugs is Newtonian, mixing will be enhanced by the bumps if they are sufficiently close to one another. Changing either the rheology of the fluid within the plugs (from Newtonian to non-Newtonian) or modifying the mechanics of the carrier fluid-plug interface (by populating it with insoluble surfactants) alters the mixing enhancement.

Exploring Ultimate Water Capillary Evaporation in Nanoscale Conduits

Capillary evaporation in nanoscale conduits is an efficient heat/mass transfer strategy that has been widely utilized by both nature and mankind. Despite its broad impact, the ultimate transport limits of capillary evaporation in nanoscale conduits, governed by the evaporation/condensation kinetics at the liquid-vapor interface, have remained poorly understood. Here we report experimental study of the kinetic limits of water capillary evaporation in two dimensional nanochannels using a novel hybrid channel design. Our results show that the kinetic-limited evaporation fluxes break down the limits predicated by the classical Hertz-Knudsen equation by an order of magnitude, reaching values up to 37.5 mm/s with corresponding heat fluxes up to 8500 W/cm2. The measured evaporation flux increases with decreasing channel height and relative humidity but decreases as the channel temperature decreases. Our findings have implications for further understanding evaporation at the nanoscale and developing capillary evaporation-based technologies for both energy- and bio-related applications.

Ion transport in graphene nanofluidic channels

Carbon nanofluidic structures made of carbon nanotubes or graphene/graphene oxide have shown great promise in energy and environment applications due to the newly discovered fast and selective mass transport. However, they have yet to be utilized in nanofluidic devices for lab-on-a-chip applications because of great challenges in their fabrication and integration. Herein we report the fabrication of two-dimensional planar graphene nanochannel devices and the study of ion transport inside a graphene nanochannel array. A MEMS fabrication process that includes controlled nanochannel etching, graphene wet transfer, and vacuum anodic bonding is developed to fabricate graphene nanochannels where graphene conformally coats the channel surfaces. We observe higher ionic conductance inside the graphene nanochannels compared with silica nanochannels with the same geometries at low electrolyte concentrations (10-6 M-10-2 M). Enhanced electroosmotic flow due to the boundary slip at graphene surfaces is attributed to the measured higher conductance in the graphene nanochannels. Our results also suggest that the surface charge on the graphene surface, originating from the dissociation of oxygen-containing functional groups, is crucial to the enhanced electroosmotic flow inside the nanochannels.