A High A High-Performance Performance Micromachined Micromachined Amperometric Amperometric
Year: 2005
Type: article
Abstract: Center for Embedded Networked Sensing A High - Performance Micromachined Amperometric Nitrate Sensor for Environmental Monitoring Dohyun Kim, Ira B. Goldberg and Jack W. Judy UCLA Electrical Engineering Department – www.ee.ucla.edu/research/judylab Electrochemical Methods Sensor requirements Inexpensive, small, remotely operable, detection range (1 μM to 1 mM) Amperometry and Potentiometry Simple design and operation Easily minaturized Low power & voltage Sensitive Introduction: Why a Micromachined Amperometric Nitrate Sensor? Nitrate is a major contaminant in ground water Nitrate-sensor applications Why Nitrate (NO 3- ) Sensor Important? – In-situ nitrate monitoring, environmental science/biology research, and precision farming Electrochemical techniques meet the requirements Glucose Sensor Ion-Selective Electrode Working Principle: Electroanalytic Chemistry and Anion Permeable Membrane Electrochemical Study Current Density ( mA·cm-2 ) Anion Permeable Membrane Ground water contains many ionic/non-ionic species interfering nitrate measurement Selective measurement with an anion selective membrane – Anion-permeable membrane blocks all cations – Nitrate diffuse faster than the other interfering ions Potentiostat PC Supporting Electrolyte NO NO 3 Peak Current i p Peak Potential V p Counter Reference Working Electrode Electrode Electrode Flow Schematic Diagram of Sensor Operation Sample Test Reservoir Sample Ion Selective Membrane Stop Remaining Sample + NaOH Stop Simultaneous Diffusion NaOH and Nitrate C W R TokuyamaACS Membrane Potential ( V vs. Ag/AgCl ) Electrochemical reduction of nitrate: NO + H 2 O + 2 e → NO + 2 OH NaOH supporting electrolyte, working electrode (Ag), reference electrode (Ag/AgCl), counter electrode (Pt) Flow 10 mM NaOH MEC Cell Repeat Stop Stop C W R Removing Oxygen Interference Oxygen in electrolyte ( ≅ 0.26 mM) interferes with detection Nitrate reduction current = (nitrate + oxygen reduction current) – (oxygen reduction current) Potentiostat Potentiostat Fill sample reservoir with a ground-water Fill microelectrochemical (MEC) cell with electrolytic eluent (10 mM NaOH) OH - and NO 3- diffuse across anion-permeable membrane in opposite directions Perform measurements when concentrations reach equilibrium Design, Fabrication, Experimental Results, and Summary Sensor System Integration A’ Inlets Top Plate A Anion Permeable Membrane Rubber Plate Electrolyte Reservoir Reference Electrode Microfluidic Channel Working Electrode Counter Electrode Experiment: Calibration Curves and Sensor Selectivity Current (mA) 10 mM NaOH + 0.1 mM NaNO 3 10 mM NaOH Outlets 1.4e-6 1.2e-6 Sample Reservoir Selective Diffusion Charge(C) Quick-turn Luer-lock couplings 1 cm Electrolyte Nitrate Bias point 1.0e-6 8.0e-7 6.0e-7 4.0e-7 2.0e-7 Chip 1 (r =0.997) Chip 2 (r 2 =0.993) Linear Regression Sensing Chip Bottom Plate Microfabricated Sensor Chip Anion-Permeable Membrane and Rubber Sheet Reduction peak for nitrate Assembled Sensor Unit Potential (V) vs. Ag/AgCl Concentration (µm) Calibration curves 1 to 1000 µM detection range Micromachined Nitrate Sensing Chip Counter Electrode M ic Ch rof an lu ne idi l c r 2 =0.99 linearty SiO 2 Working Electrode Reference Electrode • Pt counter electrode (7.7x 10 -3 cm 2 ) • Ag working electrode (0.7×10 -3 cm 2 ) • Silver oxide reference electrode (0.038×10 -3 cm 2 ) • polyimide insulation layer Detection limit: ~1 µM Sensor selectivity to interfering ions Sensor output to a mixture of 100-µM nitrate and interfering ions (100 µM each of PO 42- , SO 43- , F - , Cl - ) is only 13.9% higher than the average response for the sample consisting of nitrate Summary and Future Work Electrochemical study proves that amperometric nitrate sensors are feasible with Ag working electrode and NaOH supporting electrolyte Integrated micromachined nitrate sensor units have been fabricated Micromachined electrodes and fluidic channels Anion-permeable membrane Polyimide SEM of electrodes & microfluidic channel (a) (b) X Potential (V) vs. Ag/AgCl Time (s) Achieved a detection limit of ~1 µM and linear calibration curves Future work Integration of PU-coated Ag/AgCl reference electrode Long-term qualification test Integrate into wireless sensor motes and network Explore chip-scale and board-scale potentiostats for miniaturizaion Field test SEM of polyurethane-coated Ag/AgCl reference electrode and its potential drift (a) in comparison to silver oxide electrode (b). The minimal potential drift observed (3 mV for 27 hours) UCLA – UCR – Caltech – USC – CSU – JPL – UC Merced
Authors: Do‐Hyun Kim, Ira B. Goldberg, Jack W. Judy
Language: English
Subfield: Electrochemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W250260738