7 research outputs found
Birefringence-Induced Modulation of Optical Activity in Chiral Plasmonic Helical Arrays
Chiral
nanomaterials are characterized by handedness morphology
on the nanoscale, manifested as preferential interaction with circularly
polarized light. However, the origin of this light–matter interaction
remains elusive. Here we simulated a model of chiral helical arrays
of plasmonic nanoparticles with central anisotropic nanopillars to
examine the effect of birefringence on the collective chiroptical
response. Contrary to typical assumptions in previous works, we varied
the biaxial refractive indices of the central nanopillars and observed
a significant modulation of optical activity by calculating and characterizing
circular dichroism (CD) spectra. The chiroptical response exhibited
a sign change compared with that of the isotropic condition in a specific
parametric range of negative birefringence. In addition, the CD peak
increased by 3 to 16 as the ordinary refractive index increased from
1.5 to 3.0. These results are likely to be useful for designing chiral
nanomaterials for applications in metamaterials, biosensors, and optoelectrical
devices
Birefringence-Induced Modulation of Optical Activity in Chiral Plasmonic Helical Arrays
Chiral
nanomaterials are characterized by handedness morphology
on the nanoscale, manifested as preferential interaction with circularly
polarized light. However, the origin of this light–matter interaction
remains elusive. Here we simulated a model of chiral helical arrays
of plasmonic nanoparticles with central anisotropic nanopillars to
examine the effect of birefringence on the collective chiroptical
response. Contrary to typical assumptions in previous works, we varied
the biaxial refractive indices of the central nanopillars and observed
a significant modulation of optical activity by calculating and characterizing
circular dichroism (CD) spectra. The chiroptical response exhibited
a sign change compared with that of the isotropic condition in a specific
parametric range of negative birefringence. In addition, the CD peak
increased by 3 to 16 as the ordinary refractive index increased from
1.5 to 3.0. These results are likely to be useful for designing chiral
nanomaterials for applications in metamaterials, biosensors, and optoelectrical
devices
Shape-Dependent Biomimetic Inhibition of Enzyme by Nanoparticles and Their Antibacterial Activity
Enzyme inhibitors are ubiquitous in all living systems, and their biological inhibitory activity is strongly dependent on their molecular shape. Here, we show that small zinc oxide nanoparticles (ZnO NPs)pyramids, plates, and spherespossess the ability to inhibit activity of a typical enzyme β-galactosidase (GAL) in a biomimetic fashion. Enzyme inhibition by ZnO NPs is reversible and follows classical Michaelis–Menten kinetics with parameters strongly dependent on their geometry. Diverse spectroscopic, biochemical, and computational experimental data indicate that association of GAL with specific ZnO NP geometries interferes with conformational reorganization of the enzyme necessary for its catalytic activity. The strongest inhibition was observed for ZnO nanopyramids and compares favorably to that of the best natural GAL inhibitors while being resistant to proteases. Besides the fundamental significance of this biomimetic function of anisotropic NPs, their capacity to serve as degradation-resistant enzyme inhibitors is technologically attractive and is substantiated by strong shape-specific antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA), endemic for most hospitals in the world
Chiral Plasmonic Nanostructures on Achiral Nanopillars
Chirality of plasmonic films can
be strongly enhanced by three-dimensional (3D) out-of-plane geometries.
The complexity of lithographic methods currently used to produce such
structures and other methods utilizing chiral templates impose limitations
on spectral windows of chiroptical effects, the size of substrates,
and hence, further research on chiral plasmonics. Here we demonstrate
3D chiral plasmonic nanostructures (CPNs) with high optical activity
in the visible spectral range based on initially achiral nanopillars
from ZnO. We made asymmetric gold nanoshells on the nanopillars by
vacuum evaporation at different inclination and rotation angles to
achieve controlled symmetry breaking and obtained both left- and right-rotating
isomers. The attribution of chiral optical effects to monolithic enantiomers
made in this process was confirmed by theoretical calculations based
on their geometry established from scanning electron microscope (SEM)
images. The chirality of the nanoshells is retained upon the release
from the substrate into a stable dispersion. Deviation of the incident
angle of light from normal results in increase of polarization rotation
and chiral <i>g</i>-factor as high as −0.3. This
general approach for preparation of abiological nanoscale chiral materials
can be extended to other out-of plane 3D nanostructures. The large
area films made on achiral nanopillars are convenient for sensors,
optical devices, and catalysis
Biomimetic Hierarchical Assembly of Helical Supraparticles from Chiral Nanoparticles
Chiroptical materials found in butterflies,
beetles, stomatopod
crustaceans, and other creatures are attributed to biocomposites with
helical motifs and multiscale hierarchical organization. These structurally
sophisticated materials self-assemble from primitive nanoscale building
blocks, a process that is simpler and more energy efficient than many
top-down methods currently used to produce similarly sized three-dimensional
materials. Here, we report that molecular-scale chirality of a CdTe
nanoparticle surface can be translated to nanoscale helical assemblies,
leading to chiroptical activity in the visible electromagnetic range.
Chiral CdTe nanoparticles coated with cysteine self-organize around
Te cores to produce helical supraparticles. d<i>-/</i>l<i>-</i>Form of the amino acid determines the
dominant left/right helicity of the supraparticles. Coarse-grained
molecular dynamics simulations with a helical pair-potential confirm
the assembly mechanism and the origin of its enantioselectivity, providing
a framework for engineering three-dimensional chiral materials by
self-assembly. The helical supraparticles further self-organize into
lamellar crystals with liquid crystalline order, demonstrating the
possibility of hierarchical organization and with multiple structural
motifs and length scales determined by molecular-scale asymmetry of
nanoparticle interactions
Simultaneously High Stiffness and Damping in Nanoengineered Microtruss Composites
Materials combining high stiffness and mechanical energy dissipation are needed in automotive, aviation, construction, and other technologies where structural elements are exposed to dynamic loads. In this paper we demonstrate that a judicious combination of carbon nanotube engineered trusses held in a dissipative polymer can lead to a composite material that <i>simultaneously</i> exhibits both high stiffness and damping. Indeed, the combination of stiffness and damping that is reported is quite high in any single monolithic material. Carbon nanotube (CNT) microstructures grown in a novel 3D truss topology form the backbone of these nanocomposites. The CNT trusses are coated by ceramics and by a nanostructured polymer film assembled using the layer-by-layer technique. The crevices of the trusses are then filled with soft polyurethane. Each constituent of the composite is accurately modeled, and these models are used to guide the manufacturing process, in particular the choice of the backbone topology and the optimization of the mechanical properties of the constituent materials. The resulting composite exhibits much higher stiffness (80 times) and similar damping (specific damping capacity of 0.8) compared to the polymer. Our work is a step forward in implementing the concept of materials by design across multiple length scales
Chiral Graphene Quantum Dots
Chiral nanostructures from metals
and semiconductors attract wide
interest as components for polarization-enabled optoelectronic devices.
Similarly to other fields of nanotechnology, graphene-based materials
can greatly enrich physical and chemical phenomena associated with
optical and electronic properties of chiral nanostructures and facilitate
their applications in biology as well as other areas. Here, we report
that covalent attachment of l/d-cysteine moieties
to the edges of graphene quantum dots (GQDs) leads to their helical
buckling due to chiral interactions at the “crowded”
edges. Circular dichroism (CD) spectra of the GQDs revealed bands
at <i>ca.</i> 210–220 and 250–265 nm that
changed their signs for different chirality of the cysteine edge ligands.
The high-energy chiroptical peaks at 210–220 nm correspond
to the hybridized molecular orbitals involving the chiral center of
amino acids and atoms of graphene edges. Diverse experimental and
modeling data, including density functional theory calculations of
CD spectra with probabilistic distribution of GQD isomers, indicate
that the band at 250–265 nm originates from the three-dimensional
twisting of the graphene sheet and can be attributed to the chiral
excitonic transitions. The positive and negative low-energy CD bands
correspond to the left and right helicity of GQDs, respectively. Exposure
of liver HepG2 cells to l/d-GQDs reveals their general
biocompatibility and a noticeable difference in the toxicity of the
stereoisomers. Molecular dynamics simulations demonstrated that d<i>-</i>GQDs have a stronger tendency to accumulate
within the cellular membrane than l-GQDs. Emergence of nanoscale
chirality in GQDs decorated with biomolecules is expected to be a
general stereochemical phenomenon for flexible sheets of nanomaterials