Reconfigurable 3 D plasmonic metamolecules

A reconfigurable plasmonic nanosystem combines an active plasmonic structure with a regulated physical or chemical control input. There have been considerable e orts on integration of plasmonic nanostructures with active platforms using topdown techniques. The active media include phase-transition materials, graphene, liquid crystals and carrier-modulated semiconductors, which can respond to thermal1, electrical2 and optical stimuli3–5. However, these plasmonic nanostructures are often restricted to two-dimensional substrates, showing desired optical response only along specific excitation directions. Alternatively, bottom-up techniques o er a new pathway to impart reconfigurability and functionality to passive systems. In particular, DNAhas proven to be one of the most versatile and robust building blocks6–9 for construction of complex three-dimensional architectureswith highfidelity10–14. Herewe show the creation of reconfigurable three-dimensional plasmonic metamolecules, which execute DNA-regulated conformational changes at the nanoscale. DNA serves as both a construction material to organize plasmonic nanoparticles in three dimensions, aswell as fuel for driving themetamolecules to distinct conformational states. Simultaneously, the threedimensional plasmonic metamolecules can work as optical reporters,which transduce their conformational changes in situ into circular dichroismchanges in the visiblewavelength range. Circular dichroism (CD), that is, differential absorption of leftand right-handed circularly polarized light, of natural chiral macromolecules is highly sensitive to their three-dimensional (3D) conformations15. Taking a similar strategy, we create 3D reconfigurable plasmonic chiral metamolecules4,16, whose conformation changes are highly correlated with their pronounced and distinct CD spectral changes in the visiblewavelength range. Figure 1a shows the design schematic. Two gold nanorods (AuNRs) are hosted on a reconfigurable DNA origami template7,10, which consists of two 14-helix bundles (80 nm × 16 nm × 8 nm) folded from a long single-stranded DNA (ssDNA) scaffold with the help of hundreds of staple strands13. The two origami bundles are linked together by the scaffold strand passing twice between them at one point. To ensure the mobility of the DNA bundles and avoid the formation of a Holliday junction17, 8 unpaired bases are introduced to each ssDNA connector (Supplementary Note 1). Twelve binding sites are extended from each origami bundle for robust assembly of one AuNR (38 nm × 10 nm) functionalized with complementary DNA (Supplementary Note 2). The surface to surface distance of the two AuNRs is roughly 25 nm. Owing to close proximity, the excited plasmons in the two AuNRs can be strongly coupled18. The two crossed AuNRs constitute a 3D plasmonic chiral object19–22, which generates a theme of handedness when interacting with leftand right-handed circularly polarized light, giving rise to strong CD. Left-handed Right-handed Relaxed R1 R2 Waste 1 Waste 2