An integrated gas-liquid droplet microfluidic platform for digital sampling and detection of airborne targets

The use of microfluidic droplets has become ubiquitous in many Lab-on-a-Chip (LOC) applications ranging from material synthesis to novel biochemical sensing. In this paper, we introduce a new droplet-based approach that incorporates a gas phase for generating liquid droplet microreactors in a microfluidic flow-focusing format. We demonstrate the subsequent on-chip transition, collection and handling of the droplets in a secondary liquid carrier inside a multilayer PDMS structure. The presented technique has potential applications in capturing and probing airborne particles and gaseous vapors using high surface to-volume picoliter droplets. The discrete microfluidic gas-liquid interfaces created in this approach, greatly facilitate absorption and up-concentration of a gaseous target analyte into the droplet volume. The chip-based format of the units also allows for different microfluidic modules and analytical techniques to be integrated in this platform for droplet probing, providing highly-sensitive LOC detection systems. Here, we demonstrate the basic principles of sample partitioning with gas-liquid droplets by capturing and detection of vaporized ammonia at different gaseous concentrations using Nessler's reaction inside the droplets. The results of this work provide a simple and robust quantification approach for determining gaseous ammonia which can be further expanded to other gas-phase analytes in next generation of airborne target detectors for human breath analysis and environmental monitoring. (C) 2018 Elsevier B.V. All rights reserved.