Light-orchestrated multi-step solid-phase picodroplet reactors.

Picoliter droplet reactors, enabled by droplet microfluidics, are revolutionary tools for biochemical reactions with high efficiency, precision, and minimal reagent use. They excel in single-step reactions and reagent addition through droplet merging. Integrating reagent exchange and washing capabilities directly into these platforms may enable complex multi-step processes, such as de novo oligonucleotide synthesis and multiplex immunoassays. However, current picodroplet microfluidics, lacking such capabilities, remain critically deficient in executing multi-step processes. Here we introduce light-orchestrated solid-phase picodroplet reactors to overcome these limitations. Our platform employs optoelectronic tweezers to manipulate individual picodroplets and microbeads. Using this platform, we demonstrate an eight-step click chemistry-based DNA ligation synthesis cycle, with real-time in situ fluorescence detection of reaction products. This platform overcomes single-step limitations by achieving precise sequential encapsulation and decapsulation of beads with different reagent droplets, ensuring uniform reagent exposure and effective washing. Hence it mitigates reaction errors caused by nonuniform reagent exposure and trapped impurities in conventional bulk processing of microbeads. With minimal reagent consumption, real-time analysis, and programmable light- based control, the platform can potentially be scaled and fully automated into a universal and versatile picoliter-scale reagent handling robot for miniaturizing and streamlining workflows in synthetic biology, drug discovery, and beyond.