Microfluidic Shrinking Droplet Concentrator for Analyte Detection and Phase Separation of Protein Solutions

We develop a droplet microfluidic platform to increase the concentration of analytes in solution via reduction of the sample volume under well-defined conditions. This approach improves the detection and quantification of analytes without requiring any a priori information on their structure nor physical chemical properties. Samples are compartmentalized and processed in water-in-oil droplets that are individually stored in cylindrical microwells located on top of a microfluidic channel. The individual droplets shrink over time due to water extraction in the surrounding oil, leading to an increase in the analyte concentration up to 100,000-fold within the droplet. We demonstrate the power of this approach for detection applications by quantifying a broad range of single analytes such as small molecules, proteins, nanoparticles, exosomes, and amyloid fibrils. With this setup, we can measure pM concentrations, corresponding to zeptomole (10–21 mol) amounts encapsulated in individual droplets. We further show that the droplet concentrator device, or DroMiCo, can quantify unlabeled proteins in nM concentrations and analyze multicomponent mixtures when coupled with a prefractionation step. We illustrate this concept by detecting femtomoles (10–15 mol) of soluble protein oligomers prefractionated by size exclusion chromatography. Finally, we apply the DroMiCo to the analysis of phase diagrams of macromolecules, including synthetic polymers and proteins. Specifically, we analyze the liquid–liquid phase separation of an in vitro model of cellular membraneless compartments, composed of a phase separating protein in the presence of defined concentrations of molecular modulators such as RNA and ATP.