Aluminum Nanoparticles with Hot Spots for Plasmon‐Induced Circular Dichroism of Chiral Molecules in the UV Spectral Interval

Abstract

absorb and scatter visible light and, furthermore, plasmonic NCs are known for their strong local enhancement of electromagnetic fields. When a chiral biomolecule is attached to a nonchiral metal NC, the chiral response of a biomolecule becomes transferred to the plasmonic NC via near-field dipolar and multipolar interactions. [4,5] Circular dichroism spectroscopy, which quantifies the difference between the assembly's responses to left and right circularly polarized light, is a well-established experimental method to measure the chiral properties of an assembly. In the biomolecule-NC assembly, CD spectra typically show modified molecular lines, in addition to including new lines coming from plasmon excitations in the NCs. It was demonstrated in a large number of experiments [3,6,7] that it is possible to observe plasmon-induced CD in the visible spectral interval using NCs made of gold and silver. It is a very interesting possibility, since the original CD signals of important biomolecules (such as proteins and DNA) are typically in the UV spectral region. In addition to the material composition of an assembly, its design also plays an important role. For example, the plasmoninduced CD can be strongly enhanced by using NC assemblies Plasmonic nanocrystals with hot spots are able to localize optical energy in small spaces. In such physical systems, near-field interactions between molecules and plasmons can become especially strong. This paper considers the case of a nanoparticle dimer and a chiral biomolecule. In this model, a chiral molecule is placed in the gap between two plasmonic nanoparticles, where the electromagnetic hot spot occurs. Since many important biomolecules have optical transitions in the UV spectral region, the case of aluminum nanoparticles is considered, as they offer strong electromagnetic enhancements in the blue and UV spectral intervals. The calculations in this study show that the complex composed of a chiral molecule and an Al dimer exhibits strong circular dichroism (CD) signals in the plasmonic spectral region. In contrast to the standard Au and Ag nanocrystals, the Al system may have a much better spectral overlap between the typical biomolecule's optical transitions and the nanocrystals' plasmonic band. Overall, it is found that Al nanocrystals used as CD antennas exhibit unique properties as compared to other commonly studied plasmonic and dielectric materials. The plasmonic systems investigated in this study can be potentially used for sensing chirality of biomolecules, which is of interest in applications such as drug development.

Enhancing circular dichroism by chiral hotspots in silicon nanocube dimers

“…The modeling of chiral media relies on proper formalism of the material's properties, especially the chiral parameter κ. Here, we adopt the convenient expressions derived by Govorov and coworkers: 25,26…”

Tailored Optical Functionality by Combining Electron‐Beam and Focused Gold‐Ion Beam Lithography for Solid and Inverse Coupled Plasmonic Nanostructures

“…This field has attracted significant interest over the last years, not only because of the fundamental importance of chirality in nature but also because of the strong and tunable chiroptical response in plasmonic systems. [ 45–52 ] Optical chirality, that is, the different response of a system to left and right circularly polarized light (LCP, RPC, respectively), is an intrinsically 3D property that requires advanced fabrication strategies. [ 53–55 ] Also, it was shown that even minute changes to the morphology result in sizable changes in the chiroptical response, [ 56 ] further underlining the fabrication challenges and the requirement of high accuracy planar and 3D nanofabrication.…”

Exploiting Anisotropy of Plasmonic Nanostructures with Polarization Modulation Infrared Linear Dichroism Microscopy (µPM‐IRLD)

“…An interesting, yet under explored aspect of anisotropic nanostructures is the differential absorbance, Δ A , that is associated with the dichroic (linear or circular) properties of the structure. By exploiting the improved sensitivity offered by a plasmon‐enhanced Δ A measurement, the local molecular anisotropy of an adsorbed analyte can be probed . Furthermore, it may be possible to induce local anisotropy to the analyte through the interaction between the adsorbed analyte and a locally polarized plasmon.…”