Low-Frequency Segmented-Flow Microfluidics for Biphasic Chemical Sensing

Droplet microfluidics with droplet generation frequencies of several hundred hertz and droplet volumes below 1 nL has previously been reported for biphasic chemical sensing based on oil sensors. However, tracking fluorescence from these rapidly moving subnanoliter droplets typically requires highly sophisticated optical instrumentation, such as laser-scanning confocal microscopy. In this work, we implement oil-based chemical sensing in segmented-flow microfluidics with operating frequencies below 20 Hz and segment volumes of tens to hundreds of nanoliters. The lower segment generation frequency and larger segment size enable straightforward interrogation using a compact custom-built optical detector consisting of a laser excitation source and a photon counter. Commercial plastic tee assemblies and perfluorinated tubing are employed in place of PDMS microchannels to minimize channel swelling as well as chemical adsorption and absorption. The self-contained sensing platform is demonstrated for biphasic detection of potassium ions, albumin, and hydrogen peroxide based on mechanisms including analyte-induced deprotonation of a hydrophobic pH indicator, analyte-induced back extraction of a fluorescent dye, and reaction of the analyte with a fluorescent probe. The system is compatible with oil sensors containing optical reporters over a wide concentration range from micromolar to near-millimolar levels. Using potassium ion sensing as a model system, we further demonstrate plasma analysis and real-time tracking of stepwise concentration changes in continuously aspirated samples. This low-frequency segmented-flow microfluidic sensing platform combined with a simple optical detector paves the way for bedside chemical monitoring in biological fluids.