Portable, Droplet-Based Microfluidic Device for Aqueous-Two-Phaseextractions and Colorimetric Detection of Cadmium
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A portable, digital-microfluidic device for aqueous-two-phase-based extractions is presented. Droplets (50 µL) consist of an aqueous-two-phase-system (sodium sulfate and polyethylene glycol) and an extractant (potassium iodide) to move Cd2+ ions selectively from a mixture of metal ions into the polyethylene glycol top phase. The droplets are confined by hydrophilic surface energy traps on a superhydrophobic substrate that is made of a commercially available coating (Ultra-EverDry™). Surface energy traps are generated by exposing specific regions of the surface to air plasma or by laser micro-machining, where a portion of the superhydrophobic coating is ablated and the hydrophilic glass underneath is exposed. A 3D-printed holder applies another surface energy trap on top of the droplet and lifts it. This allows one to precisely separate different volumes (0.19-14.91 µL) of the polyethylene glycol top phase depending on the surface energy trap size. Subsequent addition of an indicator dye to a separated aliquot of the top phase allows interference free, colorimetric detection of Cd2+ ions with a smartphone camera.