Development of a Digital Microfluidic Lab-on-a-chip for Analysis of Atmospheric Inorganic Ions

Abstract

A microfluidic device consisting of a Lab-on-Chip (LoC) was designed for on-line analysis of sulfate, ammonium and nitrate in atmospheric air samples. The LoC has the ability to integrate and automate collection (via impaction), extraction and analytical detection of these three major inorganic ions on a single platform. Digital microfluidics is the key technology behind the LoC. It involves the micromanipulation of discrete droplets using electrowetting. Electrowetting refers to the application of an electric field which modifies the wetting behavior of a droplet. Methylthymol Blue (MTB), O-Phthaldialdehyde (OPA), and nitrate reductase colorimetric assays were initially developed using bench-top spectrophotometry for the analytical detection of sulfate, ammonium, and nitrate ions. These assays were subsequently adapted for use on the LoC and characterized in terms sensitivity, reproducibility, accuracy, limit of detection (LOD), linearity, and robustness. On-chip measurements without electrowetting gave calibration curves with wide linear ranges and R2 values above 0.987. The LOD’s were 3, 1 and 1 ppm respectively. R2 values of measurements using electrowetting were 0.965, 0.972 and 0.838 for sulfate, ammonium, and nitrate respectively. Accurate LOD’s were not achieved for electrowetting measurements due to the instability of this technique and the resulting lack of data. Electrowetting induced mixing data has shown promising results, however, has not yet been achieved. Electrowetting and chip design must be improved in order to do this. A successful digital microfluidic impactor (DMI) was designed consisting of five nozzles each with a diameter of 0.241 mm and a nozzle to plate distance (microchip gasket thickness) of 0.127 mm. Successful impaction of laboratory-generated aerosol was accomplished. Collection efficiency greater than 90% at 1.3 L/min was achieved for the target size range of 150-800 nm and with a low pressure drop of 29 mb. Deposition location indicated that approximately 96% of deposited aerosol is collected within the electrode area of the LoC thus allowing it to be extracted and quantified.