Chiral plasmonic nanostructures via DNA self-assembly

Controlling molecular chirality is of great importance in nanotechnology. Many biologically active molecules are chiral, including the naturally occurring amino acids, nuclear acids and sugars. In biological systems, most of these compounds are of the same chirality and the circular dichroism (CD) response of natural molecules is very weak. On the other hand, when metallic nanostructures, especially noble metal, illuminated by light with proper energy and momentum, surface plasmons can be excited, which have been used to enhance the electric field and excite higher electric and magnetic modes, leading to a series of fantastic optical phenomena and applications. Chirality of natural molecules can be manipulated by reconfiguring molecular structures through light, electric field, and thermal stimuli. While, the fabrication of complex metal structures is limited by the condition of current technology, especially for the precise fabrication and manipulation molecules at the nanoscale. Moreover, how to achieve chiroptical response in the visible range needs more efforts. In recent years, DNA nanotechnology, using DNA as building blocks of self-assembly, could be finely engineered into desired nanoarchitectures with high complexity and precision. It provides an effective way to easily control and tailor the arrangement of nanoparticles, and to form chiral metamolecules with complicated geometry. Among a variety of functionalized particles, metal nanoparticles such as gold nanoparticles feature an important pathway to endow DNA origami assembled nanostructures with tailored optical functionalities. Such DNA nanostructures were used for building versatile chiral plasmonic nanostructures from static to dynamic. Taking advantages of the spherical metal nanomaterials own isotropy and the programable of DNA nanostructures, the chiral configuration of self-assembled plasmonic nanostructures mainly consider the overall geometry of chiral space, which is easy to expand to more chiral and complex structure. Researchers can arrange achiral metal nanoparticles including gold nanoparticles, silver nanoparticles and quantum dots to fabricate chiral plasmonic nanostructures by analyzing and simulating the optically active molecular analogs. In addition, the interest in self-assembly of chiral plasmonic nanostructures, such as gold nanorods, as anisotropic building blocks is growing quickly. Researchers have developed a variety of complex superstructures such as chiral tetrahedral nanoparticles, pyramid nanoparticles, helical structures and three-dimensional plasmonic nanostructures. DNA nanotechnology provides one of the few ways to form designed, complex structures with precise control over nanoscale features. As a result, plasmonic chiral nanostructures assembled by DNA allow for dynamic manipulation of chirality and reversible switching of strong CD responses, hold great promise for applications in adaptable nanophotonic circuitry, artificial nanomachinery, as well as optical sensing of molecular binding and interaction activities. This article briefly reviews the developments and achievements of chiral plasmonic nanostructures enabled by DNA nanotechnology. Firstly, we show chiral plasmonic nanostructures based on spherical AuNPs, including plasmonic helices, tetramers, and chiral geometric conformations. Then, to challenge the complex configurations and enhance the CD responses, anisotropic gold nanorods with larger extinction coefficients are utilized to fabricate chiral plasmonic nanostructures including dimers, tripod and superhelix. Finally, we introduce dynamic manipulation based on DNA nanostructures with the fast development of this interdisciplinary field. We envision that the combination of DNA nanotechnology and plasmonics will open an avenue toward a new generation of functional plasmonic systems with tailored optical properties and useful applications, including polarization conversion devices, biomolecular sensing, surface-enhanced Raman and fluorescence spectroscopy, and diffraction-limited optics.