Thermally Regulated Droplet Microfluidics Enables Uniform, Size-Controllable DNA-Nanoparticle Superlattices.

DNA-functionalized nanoparticles (DNA-NPs) can self-assemble into superlattices with unique optical and electronic properties, and their ability to encapsulate macromolecular cargo makes them promising carriers for molecular delivery. To support such applications, crystals with uniform size and high lattice order are required. However, conventional batch crystallization can be affected by heterogeneous nucleation, convection, and aggregation, leading to broad size distributions and poor structural order. In this paper, we present a thermally regulated droplet-microfluidic strategy that produces uniform, size-controllable, single-crystallite DNA-functionalized gold nanoparticles (DNA-AuNPs) superlattices in monodisperse water-in-oil droplets. Droplets are produced at 65°C to suppress premature hybridization, and then ultra-slowly cooled to promote near-equilibrium assembly. We identify a predominantly single-crystallite regime in smaller droplets (≈9-23 µm) and achieve a ninefold reduction in size variability compared to batch processing.  Small-angle X-ray scattering analysis indicates that the characteristic domain size remains approximately constant across droplet sizes, suggesting that differences in crystallization behavior arise from size-dependent nucleation statistics rather than intrinsic improvements in crystal quality. Confinement may suppress multi-nucleation and convective perturbations, potentially providing a route to DNA-AuNP superlattices with controllable size, number, and order under confined conditions, which could enable dose-quantized delivery and applications in plasmonic and electronic materials.