Joonseong Heo

Experimental verification of overlimiting current by surface conduction and electro-osmotic flow in microchannels.

Direct evidence is provided for the transition from surface conduction (SC) to electro-osmotic flow (EOF) above a critical channel depth (d) of a nanofluidic device. The dependence of the overlimiting conductance (OLC) on d is consistent with theoretical predictions, scaling as d(-1) for SC and d(4/5) for EOF with a minimum around d=8  μm. The propagation of transient deionization shocks is also visualized, revealing complex patterns of EOF vortices and unstable convection with increasing d. This unified picture of surface-driven OLC can guide further advances in electrokinetic theory, as well as engineering applications of ion concentration polarization in microfluidics and porous media.

Fabrication and Calibration of pH Sensor Using Suspended CNT Nanosheet

In this research, the pH sensor was developed using CNT nanosheet with Nafion coating for the advanced medical sensor such as a blood gas analyzer. The CNT nanosheet was formed by dielectrophoresis and water-meniscus between cantilever-type electrodes. Then, the process of the heat annealing and the Nafion coating was conducted for reducing contact resistance and giving proton selectivity respectively. We measured the response of the pH sensor as the electrolyte-gated CNT-nanosheet field effect transistor. The sensor showed a linear current ratio in a similar range of the normal blood pH. A calibration method for decreasing of the response variation among sensors has also been introduced. Coefficient of variance of the pH sensor was decreased by applying the calibration method. A linear relation between the calibrated response of the sensors and pH variance was also obtained. Finally, the pH sensor with a high resolution was fabricated and we verify the feasibility of the sensor by applying the calibration method.

Fabrication of entangled single-wall carbon nanotube films as nanoporous junctions for ion concentration polarization

Ion concentration polarization (ICP) is a distinctive electrochemical phenomenon that occurs near an ion-exchange membrane with an applied DC electric field, generating a significant concentration gradient in back and forth on the membrane. To date, however, there have been only a few attempts to introduce unconventional materials for ion transport in micro?nanofluidic systems. Here, we describe the development of a novel ICP system using an entangled single-wall carbon nanotube (SWNT) film as an ion-selective membrane instead of a Nafion membrane, for investigating the detailed relationship between electrical properties, i.e., ionic conductance through nanojunctions, and nonlinear electrokinetic behavior.

Evaluation of Electrospun TiO2/PVP/LiCl Nanofiber Array for Humidity Sensing

Abstract Recently, tremendous application utilizing electrospun nanofibers have been actively reported due to its several advantages, such ashigh surface to volume ratio, simple fabrication and high-throughput manufacturing. In this paper, we developed highly sensitive andconsistent nanofiber humidity sensor by electrospinning. The humidity sensor was fabricated by rapid electrospinning (~2 sec) TiO 2 /PVP/LiCl mixed solution on the micro-interdigitated electrode. In order to evaluate the humidity sensing performances, we measuredcurrent response using DC bias voltage under various relative humidity levels. The results show fast response / recovery time and mar-ginal hysteresis as well as long-term stability. In addition, with the aid of micro-interdigitated electrode, we can reduce a total resistanceof the sensor and increase the total reaction area of nanofibers across the electrodes resulting in high sensitivity and enhanced currentlevel. Therefore, we expect that the electrospun nanofiber array for humidity sensor can be feasible and promising for diverse humiditysensing application.