CENS Technical Seminar Series

Micromachined Chronocoulometric Nitrate Sensor and Parallel-plate Donnan-dialytic Sample-preparation System Using Anion-exchange Membrane

Invited Speaker: Dohyun Kim
Date: April 3, 2009
Time: 1:00 PM - 2:00 PM
Venue: Boelter Hall 4760

Abstract

A simple and sensitive electrochemical microsensor has been designed, fabricated, and tested in an effort to develop a nitrate sensor for in-situ groundwater and surface-water monitoring. Cyclic voltammetry demonstrated that a silver electrode in 0.01-M NaOH electrolyte has a high sensitivity in nitrate. The silver working electrode, silver-oxide reference electrode, platinum counter elec- trode, and microfluidic channels are microfabricated on a silicon substrate. A poly- imide insulation layer is patterned to improve reliability. A custom-made fixture integrates the sensor chip and fluidic connectors for on-line, flow-through analysis. The nitrate concentration is determined by double potential-step chronocoulometry (DPSC) because it improves signal-to-noise ratio compared to amperometry and minimizes oxygen background current. Interference analysis for 10 ions commonly found in groundwater indicates that HPO24-/HPO43-, Ca2+, and Sr2+ show significant interferences (i.e., >20% signal distortion). The limit of detection ranges from 4 to 75 uM, and the upper limit of the linear range varies between 500 and 2000 uM.

As a sample-preparation module to the nitrate sensor, a novel parallel-plate Donnan-dialyzer with recirculation tubes is designed, mathematically modeled, nu- merically analyzed, and tested. By using an anion-exchange membrane (AEM), the dialyzer removes cations completely and reduces interfering anions significantly from nitrate samples. Electrochemical detection can be readily performed after the dialy- sis is completed because electrolyte and nitrate are mixed during the dialysis. When the ionic strength is low (<0.01 M) and the solution volume is limited (<10 ml), the novel dialyzer has three major advantages over conventional dialyzers: (1) dialysis efficiency is close to 100%, (2) reproducibility of dialysis is improved, (3) dialysis speed is reasonably high. The one-dimensional bi-ionic system in-series model is derived, based on the Nernst-Planck formulation, to predict dialysis processes. For parameters of the numerical analysis, membrane thickness, exchange capacity, and selectivity coefficients are measured, and diffusion coefficients in membranes are estimated from the published data. Numerical solutions are obtained using finite-difference-method (FDM). Dialysis experiments for a model bi-ionic system, 10-mM
NaF | AEM | 1-mM NaNO3, are conducted to verify the numerical model. For Neosepta AFN membrane, simulation results agree reasonably well within 29%, and the throughput of the dialyzer is one sample per hour.

Biography

Dohyun Kim received the B.S. degree in 1999, and the M.S. degree in 2001 from Department of Mechanical Engineering, Sogang University, Seoul, Korea. His research focus was modeling and designing of a smart control system for CNC (Computer- Numerical-Control) machining centers. He received Ph.D degree in Electrical Engineering department at University of California, Los Angeles with MEMS(Micro-Electromechanical System)/Nanotechnology emphasis. His research focus was developing a microfabricated electrochemical contaminant sensor for environmental sensor networks. He is currently working as a postdoctoral scholar in CENS (Center for Embedded Network Sensing) under supervision of Prof. Jack W. Judy in Electrical Engineering Department, UCLA. His research interest includes uTAS(micro-Total-Analytical-Systems) and LOC(Lab-On-a-Chip) based on chemical/biological electrochemical sensing, miniaturized sample-preparation system, the numerical analysis sensor-sample-preparation system, and MEMS (Micro-Electromechanical-System).