Finished Projects

Finished Project I: Nanofluidic Diode

Ion and molecule transport control plays an important role for developing integrated micro-nanofluidic systems for biomedical applications. Here we demonstrated a nanofluidic diode that only allows ion flow in one direction. This nanofluidic diode was successfully realized by patterning positive charged protein (avidin) onto a biotin (electrically neutral) coated nanochannel using a diffusion-limited patterning technique. Under a forward bias, both cations and anions are accumulated in the center of the channel, resulting in significantly enhanced ion transport. While under a reverse bias, cations and anions were driven away from the center, forming a depletion regime and inhibiting ion movement. Our experiments suggest that nanofluidic diodes or other devices based on similar principles could find applications in control of pH and ionic concentrations and separation processes.

Ref:
1. R. Karnik, C. Duan, K. Castelino and A. Majumdar, “Rectification of Ionic Transport in a Nanofluidic Diode”, Nano Letters, vol. 7, pp. 547-551 (2007).
2. R. Karnik, K. Castelino, C. Duan and A. Majumdar, “Diffusion-Limited Patterning of Molecules in Nanofluidic Channels”, Nano Letters, vol. 6, pp. 1735-1740 (2006).

Schematic Diagram of a Nanofluidic Diode                             Diode I-V Characteristics

Finished Project II: Label-free Enzyme Sensor

We studied label-free electrical detection of enzymatic reactions, in particular trypsin proteolysis, using 1-D confined nanofluidic channels. When trypsin cleaves Poly-L-Lysine (PLL) coated on the surface of silica nanochannels, the resulting change of surface charge density can be detected by monitoring the ionic conductance along the nanochannels. Since the enzyme is not consumed during the reaction, detection of such surface enzymatic reactions is much faster than the detection of diffusion-limited surface binding reactions. We studied the sensitivity of this nanofluidic enzyme sensor and used it to explore enzyme kinetics in confined space. It was observed that the trypsin activity increases in such nanochannel devices due to geometrical confinement. The sensitivity of trypsin proteolysis also suggests that it may be possible to study enzyme activity with single-molecule resolution. Optimization of this nanochannel sensor could lead to a quick-response, highly-sensitive and label-free enzyme assay. Top

Ref:
C. Duan, Y. F. Chen, D. K. Kim, C. M. Brown, C. S. Craik and A. Majumdar, “Label-Free Electrical Detection of Enzymatic Reaction in Nanochannels”, submitted to ACS Nano.

Ionic Conductance

Finished Project III: Power Generation from Salt Gradient in Ion selective Nanochannels

When nanochannels were brought into contact with aqueous solution, the surface of nanochannels acquires charges from ionization, ion adsorption, and ion dissolution. These surface charges draw counter-ions toward the surface and repel co-ions away. Therefore, when an electrolyte concentration gradient is applied to nanochannels, counter-ions are transported through nanochannels much more easily than co-ions, which results in a net charge migration of ions. Gibbs free energy of mixing, which forces ion diffusion, thus can be converted into electrical energy by using ion-selective nanochannels. We experimentally investigated the power generation from 1-D silica nanochannels which were placed between two potassium chloride solutions with various combinations of concentrations. It is observed that the power generation per unit channel volume increases when the concentration gradient increases, while it decreases as channel height decreases. The highest power density measured was 221 kW/m3 and the best efficiency obtained was 30.7%. Our results confirm that inorganic nanochannels are suitable for such Gibbs- energy-to-electrical-energy conversion. A thin film membrane with imbedded thin nanochannel array could be a good candidate for large scale applications.

Ref:
D. Kim*, C. Duan*, Y. F. Chen, and A. Majumdar, “Power Generation from Concentration Gradient by Reverse Electrodialysis in Ion-Selective Nanochannels”, Microfluidics and Nanofluidics, vol. 9, pp. 1215-1224, (2010).

Maximum Power Output

Schematic Diagram of Power Generation

Finished Project IV: Ion Transport in 1-D confined 2-nm 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 signaling 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. These enhancements of ion transport in 2-nm nanochannels, in terms of both ion concentration and ion mobility, may find wide-ranging applications in single molecular detection, fuel cells and batteries.

Ref: C. Duan and A. Majumdar, “Anomalous Ion Transport in 2-nm Hydrophilic Nanochannels”, Nature Nanotechnology, vol. 5, pp. 848-852, (2010).

Ion Transport Mobility

Finished Project V: Evaporation Induced Cavitation in Nanochannels

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 investigated 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 is found to be governed 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 due to 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. Top

Ref:
C. Duan, R. Karnik, M. Lu and A. Majumdar, “Evaporation Induced Cavitation in Nanofluidic Channels”, Proceedings of the National Academy of Sciences, in revision.

Evaporation Induced Cavitation in Nanochannels