Droplet microfluidics has revolutionized microscale analysis by enabling massively parallel reactions and high-throughput screening of discrete fluidic volumes. Droplets can be easily generated and manipulated by merging, mixing, sorting, and splitting. However, the droplet composition is fixed post generation with few ways to manipulate the composition without altering the droplet volume. Our group recently accomplished concentration enrichment and separation of model fluorescent analytes within nanolitre-scale droplets by leveraging ion concentration polarization (ICP). Further, cation exchange driven by ICP was demonstrated by exchanging the in-droplet cationic content for Ca²⁺ ions.
In this presentation, we utilize ICP to desalt (extract ions out of) and salt (introduce ions into) nanoliter-scale aqueous droplets. Droplets were generated and flowed into the electrokinetic zone of the device, where they were positioned between an anion-exchange membrane (AEM, Sustainion) and a cation-exchange membrane (CEM, Nafion). The AEM and CEM are simultaneously wetted by opposing sides of a droplet, thereby creating ionic communication between it and auxiliary microchannels. Upon applying a voltage bias, parallel and selective ion transport across the AEM and CEM drove desalting or salting mechanisms based on the polarity of the applied voltage. As a proof-of-concept, the reaction between CaCl₂ and AgNO₃ to form AgCl precipitate was investigated. For example, when the microchannels were filled with AgNO₃ and droplets with CaCl₂, AgCl precipitated either within the droplet (salting) or at the anodic or cathodic microchannel-membrane interfaces (desalting) based on the applied voltage scheme. We used fluorescent tracers to map the ionic concentration distribution under desalting and salting modes. Our technique is a major fundamental advancement in two ways. First, droplet salting can be used to introduce ionic species for droplet-based synthesis, cell culture, cell studies and assays (effect of inhibitors or enzyme co-factors). Second, droplet desalting can address the pressing challenge of matrix suppression that limits droplet-based mass spectrometric applications.

Robbyn Anand

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