Chiral gold nanostars show how molecular asymmetry transfers to high-symmetry nanoparticles, producing structures with distinctive optical behavior and enhanced molecular detection.
(Nanowerk Spotlight) Hold your left hand up to a mirror and you see its opposite, a right hand. This property, called chirality, appears everywhere in nature, from the double helix of DNA to the molecules that determine the taste and smell of food.
At the smallest scales, chirality decides how living systems recognize and respond to chemicals. At larger scales, controlling chirality in engineered materials could lead to optical components that filter light with extreme precision, sensors that detect tiny traces of pollutants or drugs, and catalysts that drive specific chemical reactions.
But this control is hardest to achieve in crystals with perfect symmetry, where every facet is balanced and resistant to distortion. The gold icosahedron is one of the most stable of these shapes, making it a formidable obstacle to chiral design. Researchers have now found a way to overcome that barrier, transferring molecular asymmetry into a highly ordered gold nanoparticle and creating chiral nanostars with both exceptional symmetry and powerful optical and sensing capabilities.
Gold nanoparticles are especially promising in this context because of their strong plasmonic properties, collective oscillations of electrons that interact intensely with light. Adjusting a gold particle’s size and shape tunes its optical response. If chirality is introduced, the particle can absorb and scatter left- and right-circularly polarized light differently, a phenomenon measured as a chiroptical effect.
So far, researchers have learned to induce chirality in nanoparticles by transferring it from small chiral molecules during synthesis, sometimes aided by light or other additives. These methods have succeeded for shapes with moderate symmetry, but the highest-symmetry forms have resisted control.
Among these, the icosahedron stands out. Its Ih symmetry, with multiple twofold, threefold, and fivefold rotation axes, makes it exceptionally stable. This stability is useful for keeping shape intact but makes it hard to introduce the deliberate distortions that create chirality.
Prof. Qingfeng Zhang’s group at Wuhan University has now overcome that limitation, using a controlled growth method to break the symmetry of gold nanoicosahedrons while keeping their overall order, producing chiral particles with 532 rotational symmetry (Journal of the American Chemical Society, “Chiral Symmetry Breaking in Gold Stellated Nanoicosahedrons”).
The process begins with 74-nanometer gold nanoicosahedrons, which serve as seeds. These are placed in a growth solution containing a gold precursor, the surfactant cetyltrimethylammonium bromide, ascorbic acid as a reducing agent, copper nitrate, and the chiral molecule glutathione. In the first step, each triangular face of the seed grows into a pyramid-like extension, creating a stellated nanoicosahedron.
As growth continues, the edges of these pyramids twist and the pyramids rotate slightly, breaking mirror symmetry and producing a chiral geometry. The role of glutathione and copper ions is crucial, as they work together to control which crystal facets grow faster, steering the structure toward its final twisted form.
Formation of chiral gold stellated nanoicosahedrons. Starting from symmetrical gold icosahedron seeds, each triangular face grows into a pyramid-like arm under the influence of glutathione and copper ions. Over time, the arms twist and rotate, breaking mirror symmetry and producing a consistent chiral geometry, as seen in the sequence of microscope images. (Image: Reprinted with permission by American Chemical Society) (click on image to enlarge)
Detailed structural analysis revealed that the surfaces responsible for chirality are high-index crystal planes called {321} facets. These facets can form in two versions, mirror-related R and S configurations. Glutathione molecules bind differently to each, changing their growth rates and causing the edges to tilt and twist in a consistent direction. Copper ions enhance this effect by selectively attaching to certain gold surfaces, further directing deposition.
In the finished particles, some boundaries between facets bend inward, forming curved gaps with a propeller-like shape, while others create twisted arms. Depending on the viewing angle, the same particle can appear to twist clockwise or anticlockwise. This property, called bichirality, arises because a complex 3D structure can project into different 2D patterns when seen from different directions.
The researchers monitored the synthesis over time. In the first 20 minutes, gold deposits rapidly and the particles grow in size with little chirality. Between 20 and 60 minutes, the arms lengthen and twist, and the chiroptical signal strengthens sharply. In the following half hour, the size remains almost constant but the chiral features sharpen, further increasing optical activity. Beyond this point, the structure stabilizes. Without copper, the twisted features are less regular and the chiroptical strength is about ten times lower.
Alternative chiral molecules, such as cysteine, produce weaker and less ordered structures, showing that glutathione’s specific geometry and binding strength make it especially effective for this purpose.
The optical properties of the chiral stellated nanoicosahedrons were tested on individual particles. Their light scattering showed two distinct plasmon modes, an electric dipole mode and an electric quadrupole mode. Measurements of circular differential scattering, the difference in scattering for left- and right-circularly polarized light, revealed strong chiroptical responses with g-factors between 0.3 and 0.4.
These values were nearly the same regardless of the particle’s orientation, an unusual result since most anisotropic nanoparticles have strongly direction-dependent optical behavior. In particle pairs, the two plasmon modes merged and the chiroptical peak shifted to longer wavelengths, with only a small drop in g-factor, showing how coupling changes the optical response.
The geometry of the particles also creates a dense array of nanoscale “hotspots”, regions where the local electromagnetic field is greatly intensified. These hotspots are valuable for surface-enhanced Raman scattering (SERS), a technique that can detect molecules at extremely low concentrations by amplifying their vibrational signals. Using thiophenol as a test molecule, the chiral stellated nanoicosahedrons produced Raman signals about three times stronger than those from chiral pentagonal nanoprisms and about five times stronger than from vortex cubes, despite all having strong optical activity.
Breaking symmetry in chiral gold stellated nanoicosahedrons. The geometric model shows a highly symmetrical particle with {321} crystal facets alongside electron microscope images of its actual structure. Viewing the particle along different rotational axes reveals distinct chiral centers, while atomic-scale models of the {321} surfaces illustrate the two mirror-related R and S configurations, defined by the arrangement of their crystal facets. (Image: Reprinted with permission by American Chemical Society)
When the particles were assembled into uniform monolayers, the enhancement increased by about tenfold compared to single particles. Measurements taken across large areas showed that this performance was consistent, confirming their suitability as reliable SERS substrates.
The results show that it is possible to break the symmetry of one of the most stable nanoparticle shapes in a controlled way, producing high-order chiral structures with consistent geometry, strong and isotropic chiroptical activity, and exceptional molecular sensing ability. The method’s combination of a chiral molecule and a metal ion as complementary growth directors could be applied to other high-symmetry nanoparticles and materials.
With their unique optical and sensing properties, these chiral stellated nanoicosahedrons could become important tools in polarization-selective photonics, chemical detection systems, and advanced chiral sensing technologies.
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