Supramolecular nanofiber stabilization of microdroplets enables ultra-sensitive virus infection analysis

Aqueous two-phase system (ATPS) droplets in cells act as fluidic microreactors by concentrating biomacromolecules. Inspired by this phenomenon, dextran-rich microdroplets formed in an ATPS with polyethylene glycol have been explored as artificial microreactors for sensitive detection and spatiotemporal control of biochemical reactions. However, the rapid fusion of the microdroplets into bulk phase separation has limited practical applications of this approach. Here, we report the stabilization of dextran-rich microdroplets using supramolecular nanofibers of an azobenzene-appending self-assembling peptide (AzSAP). Physicochemical characterization and structural analyses elucidate the mechanism of nanofiber formation and its role in droplet stabilization. The nanofiber network prevents droplet coalescence while maintaining macroscopic fluidity, thereby enabling highly sensitive quantitative virus detection via microfluidic analysis by confining infecting viruses within droplets. Furthermore, the photo-responsive properties of the AzSAP allow dynamic control over droplet size and intra-droplet virus activity, highlighting its exceptional potential as a platform for programmable artificial microreactors. Artificial aqueous two-phase system (ATPS) droplets offer promise as controllable microreactors but are limited by the low stability of dextran-rich microdroplets in 3D media. Here, reversible nanofibers formed from an azobenzene-appended self-assembling peptide stabilize droplets by suppressing fusion without compromising fluidity. These stabilized systems enable concentration of M13 bacteriophages and Escherichia coli, prevent secondary infections, and support quantitative flow cytometry readouts, while allowing precise, photoresponsive modulation of droplet fusion.