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Y. Yang et al.
scaffolds [55]. Gold nanorods load on layered twisted cellulose nanocrystal scaffolds (Fig. 10.10b) [56]. The nanoclusters that self-assemble on peptide nanotubes
(Fig. 10.10c) assume either left-handed or right-handed chirality; this chiral arrangement yields CD at the surface plasmon frequency [57]. Gold NP double helices
(Fig. 10.10d) are peptide-based superstructures that show plasmonic CD [58].
Molecular assembly provides a versatile route to synthesize complex 3D nanostructures, of which final morphology is determined by individual NPs and chiral
symmetry groups [40]. As a result of complexity of shape, structures made by molecular assembly exhibit strong CD and chiroptical reactions. Also, the assembled plasmonic structures have no resolution limit. However, molecular assembly has some
drawbacks. Control of inter-particle spacing is challenging in molecular assembly
[59]. Furthermore, tuning the plasmonic coupling strength is only adjustable by
particle concentration and average inter-particle range [60].
10.4.2 DNA Self-assembly
DNA, a biological molecule that is composed of double-stranded helices, can be
used in self-assembly [61]. A nanotechnology that uses DNA as a building block
to render 3D nanostructure is called DNA self-assembly and has unique molecular
recognition capabilities. Long single-stranded DNA (ssDNA) can be folded to form a
designated shape by using base pairing with short ssDNA. The long ssDNA is called
the scaffold and the short ssDNA is called the staple. This fabrication technique
yields 2D structure out of a 1D strand of DNA, and is often called DNA origami
[62].
When NPs are functionalized with thiol-modified ssDNA, the base pairing
provides 3D assembly of the NPs in a desired arrangement such as dimers [63,
64], trimers [64, 65], tetramers [66–68] and chains [69, 70]. A 3D chiral nanostructure composed of four gold NPs has been synthesized in a tetrahedral configuration
by using double-stranded DNA as a scaffold to link the NPs [67]. Mirror symmetry
of the tetrahedron was broken by assigning NPs with different sizes to each vertex
(Fig. 10.11a). However, different sizes of the NPs and relatively large inter-particle
spacing hinder efficient coupling of them, so CD was not observed.
Self-assembly of 3D chiral nanostructure can also be achieved by using a DNA
template to arrange NPs in a predesignated manner. DNA origami has been used to
fabricate 3D chiral nanostructures by positioning plasmonic NPs in a chiral arrangement [59]. A DNA origami bundle was used as a template to arrange gold nanospheres
in a helical geometry; the nanospheres were attached to DNA strands that are complementary to the staples, which therefore attached at specified positions (Fig. 10.11b).
The helically-arranged gold nanospheres yield strong CD in the visible spectrum
(Fig. 10.11c). A similar structure was fabricated by rolling up a rectangular DNA
sheet that had gold nanospheres attached [75]; the nanospheres were functionalized with ssDNA that was complementary with ssDNA attached to the DNA sheet.
The handedness of the arrangement can be adjusted by changing the position of the
Y. Yang et al.
scaffolds [55]. Gold nanorods load on layered twisted cellulose nanocrystal scaffolds (Fig. 10.10b) [56]. The nanoclusters that self-assemble on peptide nanotubes
(Fig. 10.10c) assume either left-handed or right-handed chirality; this chiral arrangement yields CD at the surface plasmon frequency [57]. Gold NP double helices
(Fig. 10.10d) are peptide-based superstructures that show plasmonic CD [58].
Molecular assembly provides a versatile route to synthesize complex 3D nanostructures, of which final morphology is determined by individual NPs and chiral
symmetry groups [40]. As a result of complexity of shape, structures made by molecular assembly exhibit strong CD and chiroptical reactions. Also, the assembled plasmonic structures have no resolution limit. However, molecular assembly has some
drawbacks. Control of inter-particle spacing is challenging in molecular assembly
[59]. Furthermore, tuning the plasmonic coupling strength is only adjustable by
particle concentration and average inter-particle range [60].
10.4.2 DNA Self-assembly
DNA, a biological molecule that is composed of double-stranded helices, can be
used in self-assembly [61]. A nanotechnology that uses DNA as a building block
to render 3D nanostructure is called DNA self-assembly and has unique molecular
recognition capabilities. Long single-stranded DNA (ssDNA) can be folded to form a
designated shape by using base pairing with short ssDNA. The long ssDNA is called
the scaffold and the short ssDNA is called the staple. This fabrication technique
yields 2D structure out of a 1D strand of DNA, and is often called DNA origami
[62].
When NPs are functionalized with thiol-modified ssDNA, the base pairing
provides 3D assembly of the NPs in a desired arrangement such as dimers [63,
64], trimers [64, 65], tetramers [66–68] and chains [69, 70]. A 3D chiral nanostructure composed of four gold NPs has been synthesized in a tetrahedral configuration
by using double-stranded DNA as a scaffold to link the NPs [67]. Mirror symmetry
of the tetrahedron was broken by assigning NPs with different sizes to each vertex
(Fig. 10.11a). However, different sizes of the NPs and relatively large inter-particle
spacing hinder efficient coupling of them, so CD was not observed.
Self-assembly of 3D chiral nanostructure can also be achieved by using a DNA
template to arrange NPs in a predesignated manner. DNA origami has been used to
fabricate 3D chiral nanostructures by positioning plasmonic NPs in a chiral arrangement [59]. A DNA origami bundle was used as a template to arrange gold nanospheres
in a helical geometry; the nanospheres were attached to DNA strands that are complementary to the staples, which therefore attached at specified positions (Fig. 10.11b).
The helically-arranged gold nanospheres yield strong CD in the visible spectrum
(Fig. 10.11c). A similar structure was fabricated by rolling up a rectangular DNA
sheet that had gold nanospheres attached [75]; the nanospheres were functionalized with ssDNA that was complementary with ssDNA attached to the DNA sheet.
The handedness of the arrangement can be adjusted by changing the position of the