Nanoparticles that control the rotation of light have potential applications, for example in optical devices1 and sensors2, but preparing such particles has been difficult, especially from crystalline metals. In a paper in Nature, Lee et al.3 report a remarkable method that uses amino acids or peptides (small molecules formed from amino acids) to direct the dissymmetric growth of gold nanoparticles that have a twisted morphology. The findings open up radical opportunities for the preparation of materials and devices that control light rotation.
Dissymmetric objects that cannot be superimposed on their mirror image are found at a variety of scales and include molecules of DNA, snail shells and even galaxies. Such structures are said to be chiral. Louis Pasteur coined the concept of molecular dissymmetry in 1848, when he attributed the morphological differences in crystals of tartrate to the existence of mirror-image tartrate molecules4,5. We now know that the functions of biomolecules often depend on chirality, which, for example, provides the basis of exquisitely specific interactions between enzymes and their substrate molecules, enabling the proper functioning of living organisms.
One property of chiral molecules is that each mirror-image form interacts differently with circularly polarized light (in which the electric field traces a helix in the direction of the light’s propagation), resulting in phenomena known collectively as optical activity. For example, circular dichroism involves the differential absorption of left- and right-handed circularly polarized light by the mirror-image forms of a molecule. The optical activity of chiral organic molecules has been used to manipulate the rotation of light, but almost invariably in the ultraviolet region of the electromagnetic spectrum.
In the past decade, some inorganic materials have also been shown to have chirality and optical activity6, thereby enabling control of the rotary propagation of light to be extended to the visible and near-infrared regions. Prominent among these inorganic compounds are nanostructured materials that exhibit plasmonic effects. Such effects derive from the oscillations of conduction electrons in nanostructured metals or in other materials that contain free electrons, and result in the extremely efficient absorption and scattering of visible and near-infrared light. The wavelength involved is defined by the composition, dimensions and morphology of the nanomaterial.
The use of chiral plasmonic effects has been identified as one of the most promising routes to developing optical metamaterials — artificial structures such as ‘invisibility cloaks’7 with optical properties that differ from those of materials found in nature. This has motivated considerable effort towards the fabrication of nanoscale objects that have chiral geometry. Substantial advances have been achieved1 both through top-down fabrication methods, in which nanoscale objects are prepared from bulk materials, and through bottom-up methods, in which the objects are grown using chemical processes.
Top-down approaches can already be used to make small quantities of nanomaterials that have well defined morphologies, but it might be difficult to scale up these approaches to produce the amounts that will be needed for processing into materials or integration into devices. By contrast, bottom-up approaches are typically based on chemistry performed in solution, which is easier to scale up.
Remarkable advances have been made in the scaling up of bottom-up methods to make chiral nanomaterials, mainly by using a chiral template to direct the assembly of preformed nanoparticles. Beautiful examples of such materials include gold spheres adsorbed onto DNA strands8, gold nanorods interleaved with accurately programmed DNA-origami structures9 and gold nanorods adsorbed onto helical protein fibres2. But in all of these cases, the optical activity obtained is the result of collective plasmonic effects, and the wavelength at which circular dichroism occurs is defined both by the specific properties of the individual building blocks used and by their organization on the template. This means that several parameters must be manipulated to achieve a specific optical effect.
A simpler alternative for generating optical effects would be to grow chiral plasmonic nanoparticles in a way that ensures that all such particles have the same morphology and, therefore, identical optical activities. This can be achieved by using preformed nanoparticles as ‘seeds’, which are then grown to the desired size and shape by the slow precipitation of material onto them, typically using additive molecules to direct the growth process10. Such methods have been used to make highly symmetric metallic nanoscale objects, including spheres, rods and octahedra. This approach has also been used to make chiral structures from certain inorganic materials11, but not from metals such as gold that have a highly symmetric crystalline structure. Lee et al. now report an advance that fills this methodological gap.
The main breakthrough that the authors report is the use of chiral amino acids or peptides that contain thiol (SH) groups as additives in the seeded growth of gold nanoparticles (Fig. 1). These additives affect the growth rate of certain crystal facets, which leads to the formation of nanostructures that have intricate chiral morphologies and an impressive degree of monodispersity — all particles are highly similar in size and shape. Moreover, the obtained morphology can be manipulated by varying either the structure of the shape-directing molecule or the initial shape of the seed particles.
Lee and colleagues therefore demonstrate that the chirality and optical behaviour of naturally occurring amino acids and peptides can be transferred to shaped plasmonic nanocrystals. The resulting high-quality, chiral gold nanoparticles (see scanning electron microscopy images in Fig. 1 of ref. 3) show strong circular dichroism (a large difference between the absorption of left- and right-handed circularly polarized light), with the wavelength and intensity of the signal determined by the nanoparticles’ specific morphology. Because this remarkable optical response arises from intrinsic single-particle effects, the nanomaterials could be processed into composite materials or thin films, and might even find technological applications through incorporation into devices.
The authors’ procedure is a remarkably simple modification of methods that are commonly used to grow shaped gold, silver or palladium nanoparticles. It is therefore likely to be readily adopted to produce chiral nanostructures of these ‘noble’ metals, which have improved catalytic or electronic properties compared with analogous non-chiral structures. The success of the technique will depend on whether it does indeed work for noble metals other than gold, and whether the small, naturally occurring chiral additives can be replaced by synthetic dissymmetric molecules. Further studies are needed to determine how the process is affected by the growth kinetics of particles, by the strength of the interactions between the nanocrystal surface and the chiral additive, and by the composition and size of the seeds.
Nature 556, 313-314 (2018)
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