DNA-Programmed Core–Shell Nanoparticle Superlattice Crystals with Nanogaps for SERS Sensing

In plasmonics, the formation of a periodic architecture is crucial to achieve optical functions beyond those of individual nanoparticles. This is also a key challenge in reliable surface-enhanced Raman scattering (SERS) applications, particularly because homogeneous nanostructures are required for stable detection. A single crystal of a DNA-functionalized nanoparticles (DNA-NP) superlattice is one option to address these issues. These crystals are composed of DNA and noble metal nanoparticles arranged in a periodic array. Such crystals enable a high degree of design freedom, structural stability, specific optical properties, and adaptability to applications. In this study, several processes were conducted to optimize DNA-NP crystals for SERS applications, and their structure and optical enhancement were analyzed. Crystals were fabricated through self-assembly, followed by Au@Ag core–shell structure formation at nanoparticles and DNA contraction. A nanogapped periodic structure with structural order was confirmed by scanning electron microscopy and small-angle X-ray scattering analyses. Raman spectroscopy was used to confirm the SERS activity of crystals in solution, and both Rhodamine B and EdU molecules were detected at a concentration of 100 nM. Measurements combining a hyperspectral camera and dark field microscopy indicated coupling of localized surface plasmon resonance in the nanoparticle nanogapped periodic structure. Furthermore, the electrical field distribution obtained by finite-difference time-domain simulations showed strong field enhancement owing to the nanogapped periodic structure. Our results demonstrated precisely controlled nanostructure fabrication, as well as SERS based on near-field enhancement originating from nanogaps based on the periodic structure of DNA-NP crystals. From this perspective, we also focused on various applications of these crystals by combining them with Raman-based measurement techniques for sensitive and reproducible molecule detection in solutions and biological environment.