Schematic illustration of the principle of self-assembly
and structural
reconfiguration of the chiral plasmonic superstructures of AuNR. (A)
AuNRs (12 nm × 38 nm) functionalized with V-shaped DNA origami
are hierarchically self-assembled into a left-handed chiral superstructure
(a stair helix with a pinwheel bottom-view), through DNA hybridization
with connector strands at corresponding locations as indicated by
the dashed lines. (B) DNA origami is dynamically converted between
a tightly folded state and an extended state to make the reconfiguration
of AuNR superstructure possible. The reconfiguration between these
two states occurs via a toe-hold-mediated strand displacement reaction.
Key strands and lock strands [(K1 and L2) or (K2 and L1)] are added
in an alternating fashion to create the structural transformation.
Each arm of the DNA origami is 36 nm in width, 24 nm along the helical
axis, and 7.8 nm in thickness. The DNA spacer in the tightly folded
state (left) is a 2 × 4 helix bundle with the DNA helical axis
parallel to the helical direction of the arms that separates the two
arms at an angle of approximately 30°. In the extended state
(right) the spacer is structurally switched into a 2 × 2 helical
bundle with the helical axis perpendicular to the helical direction
of the arms that separates the two arms at an angle of ∼120°.
(C) The DNA origami is designed to allow the dynamic conversion between
the two mirror-image like structures, which enables the handedness
inversion of the AuNR superstructure. The design of the DNA origami
is similar to that in (B), but with a fixed 4 helix bundle in the
middle as the spacer. The hinge of the V-shape is closed by a zipping
hybridization of the two DNA handles (there are actually two pairs;
only one pair is shown here) and opened by a toe-hold-mediated strand
displacement. A combination of block strands and release strands [(B1
and R2) or (B2 and R1)] are added in an alternating fashion to make
the structural transformation. The interarm angle is approximately
40° in both cases, taking into account the flexibility of the
double helix connections at the hinges. A slightly larger angle for
the right V-shape is expected as the 4 helix bundle linker between
the two arms is not in the exact center position.