258
Y. Yang et al.
The arrangement can be simplified by using plasmonic nanorods instead of
nanospheres. A nanorod has an anisotropic geometry, which can be used as an additional degree of freedom to break mirror symmetry. Two gold nanorods oriented
at 90° to each other, one on the top surface and the other on the bottom surface
of the DNA origami template, were fabricated using the base-pairing mechanism
[76]. The sample showed CD at λ ~ 730 nm. The CD was remarkably amplified
by increasing the number of nanorods [73]. To synthesize this structure, both sides
of the origami sheets were dressed with capture strands (Fig. 10.11f), which were
defined in a twisted manner to achieve the chiral geometry. The twisted layer of gold
nanorods had strong CD in the visible spectrum.
Tunability of chiroptical responses is an important goal. In most structures, these
responses are determined by the geometrical chirality of the 3D nanostructures.
Therefore, the chiroptical signal of a fabricated structure is static: its magnitude,
peak wavelength and handedness cannot be changed once the structure is formed.
However, DNA self-assembly facilitates programmable synthesis of particle assembles and thus provides a pathway to produce reconfigurable and multifunctional
structures.
The first tunable 3D chiral nanostructure consisted of a twisted bilayer of gold
nanorods [72] (Fig. 10.11g), which were attached individually to two connected
origami bundles. The relative angle between two bundles and hence, the handedness
of the chiral structure was dynamically controlled by two DNA locks. Here, DNA
strands were used both as structural components, and as a tool to drive structural
change. The fabricated sample showed CD near a λ = 700 nm, and the sign of the
response could be flipped by applying external stimuli. Time-domain CD confirmed
the tunable and reversible chiroptical responses. Alternatively, the distance between
two nanorods, rather than the relative angle, can be adjusted to actively control
chiroptical responses [77]. Two nanorods were positioned perpendicularly at opposite
sides of a double-layer DNA origami. One of the nanorods walked on the surface of
the sheet as a result of interacting fuel strands while the other was stationary.
Reconfigurable 3D chiral nanostructure has also been realized by folding a helical
assembly of gold nanorods [74] (Fig. 10.11h), which had been positioned helically by using self-assembled DNA origami. DNA-toehold-mediated conformational
changes in the DNA template enabled conversion between a tightly-folded state and
an extended state, and between tightly-folded states with opposite handedness. The
transformation was reversible, but the recovery efficiency is low because of the
leakage of the strand-displacement reactions.
10.4.3 Block Copolymer Self-assembly
Block copolymer (BCP) self-assembly exploits phase separation and reconstruction
of block copolymers to make light-matter interacting plasmonic nanostructures. A
BCP consists more than one species of monomers, which are repeating units joined
by covalent bonds to a organize a polymer chain. BCPs are classified according to
Y. Yang et al.
The arrangement can be simplified by using plasmonic nanorods instead of
nanospheres. A nanorod has an anisotropic geometry, which can be used as an additional degree of freedom to break mirror symmetry. Two gold nanorods oriented
at 90° to each other, one on the top surface and the other on the bottom surface
of the DNA origami template, were fabricated using the base-pairing mechanism
[76]. The sample showed CD at λ ~ 730 nm. The CD was remarkably amplified
by increasing the number of nanorods [73]. To synthesize this structure, both sides
of the origami sheets were dressed with capture strands (Fig. 10.11f), which were
defined in a twisted manner to achieve the chiral geometry. The twisted layer of gold
nanorods had strong CD in the visible spectrum.
Tunability of chiroptical responses is an important goal. In most structures, these
responses are determined by the geometrical chirality of the 3D nanostructures.
Therefore, the chiroptical signal of a fabricated structure is static: its magnitude,
peak wavelength and handedness cannot be changed once the structure is formed.
However, DNA self-assembly facilitates programmable synthesis of particle assembles and thus provides a pathway to produce reconfigurable and multifunctional
structures.
The first tunable 3D chiral nanostructure consisted of a twisted bilayer of gold
nanorods [72] (Fig. 10.11g), which were attached individually to two connected
origami bundles. The relative angle between two bundles and hence, the handedness
of the chiral structure was dynamically controlled by two DNA locks. Here, DNA
strands were used both as structural components, and as a tool to drive structural
change. The fabricated sample showed CD near a λ = 700 nm, and the sign of the
response could be flipped by applying external stimuli. Time-domain CD confirmed
the tunable and reversible chiroptical responses. Alternatively, the distance between
two nanorods, rather than the relative angle, can be adjusted to actively control
chiroptical responses [77]. Two nanorods were positioned perpendicularly at opposite
sides of a double-layer DNA origami. One of the nanorods walked on the surface of
the sheet as a result of interacting fuel strands while the other was stationary.
Reconfigurable 3D chiral nanostructure has also been realized by folding a helical
assembly of gold nanorods [74] (Fig. 10.11h), which had been positioned helically by using self-assembled DNA origami. DNA-toehold-mediated conformational
changes in the DNA template enabled conversion between a tightly-folded state and
an extended state, and between tightly-folded states with opposite handedness. The
transformation was reversible, but the recovery efficiency is low because of the
leakage of the strand-displacement reactions.
10.4.3 Block Copolymer Self-assembly
Block copolymer (BCP) self-assembly exploits phase separation and reconstruction
of block copolymers to make light-matter interacting plasmonic nanostructures. A
BCP consists more than one species of monomers, which are repeating units joined
by covalent bonds to a organize a polymer chain. BCPs are classified according to