Salt-Dependent Crystallization of DNA-Functionalized Nanoparticle Superlattices in Microdroplets Toward Cellular Applications

DNA-functionalized nanoparticle (DNA–NP) superlattices have attracted attention as programmable nanomaterials for sensing and bio-related applications, because DNA hybridization enables programmable interactions between nanoparticles and thus control of interparticle spacing. Such applications require controlling the size, shape, and crystallinity to maintain structural stability and achieve consistent physical properties. In this study, we investigated the crystallization behavior of DNA–NP assemblies formed in microdroplets under different NaCl concentrations. Crystallization occurred in water-in-oil droplets, which provided spatially confined reaction environments for DNA–NP assembly and enabled crystallization to proceed in numerous spatially isolated microreactors under well-defined conditions. Optical microscopy revealed that crystals were typically formed individually within droplets and exhibited narrow size distributions under each condition. Within the same NaCl concentration, the average crystal size generally varied with droplet diameter. Small-angle X-ray scattering analysis showed that increasing NaCl concentration resulted in broader diffraction peaks and fewer observable higher-order reflections, indicating a reduction in crystalline order. Scanning electron microscopy further revealed differences in crystal morphology. Well-defined faceted crystals were formed at lower NaCl concentrations, whereas rounded crystals were frequently formed at higher concentrations. Overall, salt concentration is an important factor governing the crystalline order and morphology of DNA–NP superlattices formed in microdroplets.