Effects of asymmetric nanostructures on the extinction difference properties of actin biomolecules and filaments

Probing Interactions between Chiral Plasmonic Nanoparticles and Biomolecules

Chiral plasmonic nanoparticles (and their assemblies) interact with biomolecules in a variety of different ways, resulting in distinct optical signatures when probed by circular dichroism spectroscopy. These systems show promise for biosensing applications and offer several advantages over achiral plasmonic systems. Arguably the most notable advantage is that chiral nanoparticles can differentiate between molecular enantiomers and can, therefore, act as sensors for enantiomeric purity. Furthermore, chiral nanoparticles can couple more effectively to chiral biomolecules in biological systems if they have a matching handedness, improving their effectiveness as biomedical agents. In this article, we review the different types of interactions that occur between chiral plasmonic nanoparticle systems and biomolecules, and discuss how circular dichroism spectroscopy can probe these interactions and inform how to optimize systems for biosensing and biomedical applications.

Chirality in Light-Matter Interaction

The scientific effort to control the interaction between light and matter has grown exponentially in the last 2 decades. This growth has been aided by the development of scientific and technological tools enabling the manipulation of light at deeply sub-wavelength scales, unlocking a large variety of novel phenomena spanning traditionally distant research areas. Here, the role of chirality in light-matter interactions is reviewed by providing a broad overview of its properties, materials, and applications. A perspective on future developments is highlighted, including the growing role of machine learning in designing advanced chiroptical materials to enhance and control light-matter interactions across several scales.

Nanophotonic Approaches for Chirality Sensing

Chiral nanophotonic materials are promising candidates for biosensing applications because they focus light into nanometer dimensions, increasing their sensitivity to the molecular signatures of their surroundings. Recent advances in nanomaterial-enhanced chirality sensing provide detection limits as low as attomolar concentrations (10^-18 M) for biomolecules and are relevant to the pharmaceutical industry, forensic drug testing, and medical applications that require high sensitivity. Here, we review the development of chiral nanomaterials and their application for detecting biomolecules, supramolecular structures, and other environmental stimuli. We discuss superchiral near-field generation in both dielectric and plasmonic metamaterials that are composed of chiral or achiral nanostructure arrays. These materials are also applicable for enhancing chiroptical signals from biomolecules. We review the plasmon-coupled circular dichroism mechanism observed for plasmonic nanoparticles and discuss how hotspot-enhanced plasmon-coupled circular dichroism applies to biosensing. We then review single-particle spectroscopic methods for achieving the ultimate goal of single-molecule chirality sensing. Finally, we discuss future outlooks of nanophotonic chiral systems.

Circular dichroism spectroscopy and chiral sensing in optical fibers

Chirality is a property of broken mirror symmetry and detecting the handedness of chiral material in small quantities is an important problem in biology and biochemistry. Here, we present a waveguide-based method to measure chirality and distinguish the enantiomers of molecules. A bi-isotropic core in an optical waveguide lifts the degeneracy of modes in a cylindrically symmetric structure. This modal degeneracy lifting is exploited to measure the chirality of the core. The proposed sensor can determine the value of the chirality parameter of the material under test and it can be utilized for various materials with nonzero chirality parameter in different frequency bands. This approach improves the circular dichroism (CD) response and outperforms conventional CD spectroscopy methods by increasing their differential output signal. To compare the results with conventional CD spectroscopy, the CD parameter is adapted to optical waveguides.

Chiro-optical response of a wafer scale metamaterial with ellipsoidal metal nanoparticles

We report a large chiro-optical response from a nanostructured film of aperiodic dielectric helices decorated with ellipsoidal metal nanoparticles. The influence of the inherent fabrication variation on the chiro-optical response of the wafer-scalable nanostructured film is investigated using a computational model which closely mimics the material system. From the computational approach, we found that the chiro-optical signal is strongly dependent on the ellipticities of the metal nanoparticles and the developed computational model can account for all the variations caused by the fabrication process. We report the experimentally realized dissymmetry factor ∼1.6, which is the largest reported for wafer scalable chiro-plasmonic samples till now. The calculations incorporate strong multipolar contributions of the plasmonic interactions to the chiro-optical response from the tightly confined ellipsoidal nanoparticles, improving upon the previous studies carried in the coupled dipole approximation regime. Our analyzes confirm the large chiro-optical response in these films developed by a scalable and simple fabrication technique, indicating their applicability pertaining to manipulation of optical polarization, enantiomer selective identification and enhanced sensing and detection of chiral molecules.

Structure-specific chiroptical responses of hollow gold nanoprisms

Chiroptical responses of plasmonic chiral nanostructures can be controllably tuned by judicious tailoring of their structural parameters. In this article, the chiroptical properties of a newly designed plasmon-supporting nanostructure, chiral hollow gold nanoprisms (HGNs), has been numerically investigated in detail. The most compelling observation is that the CD response and the dissymmetry factor (g, which is a measure of the strength of chiroptical responses) of the chiral HGNs are large and at the same time, highly structure-specific. Also, we observed finite CD activity not only in absorption and scattering but also in the extinction spectra, which is a signature of a typical 3D chiral structure. We show that the chiroptical responses of HGNs can be exponentially enhanced simply by controlling the cavity-position or cavity size. Our results reveal that the structure-specific chiroptical response is a result of structure-dependent interplay between the non-radiative (Ohmic) and radiative losses. We also show that the CD intensity of a suitably designed chiral HGN is higher than other nanoscale metasurfaces of comparable volume. The insights obtained from this comprehensive study assert that this unique chiral nanostructure has great potential for being used in numerous applications.

Recent Advances of Plasmonic Nanoparticles and their Applications

In the past half-century, surface plasmon resonance in noble metallic nanoparticles has been an important research subject. Recent advances in the synthesis, assembly, characterization, and theories of traditional and non-traditional metal nanostructures open a new pathway to the kaleidoscopic applications of plasmonics. However, accurate and precise models of plasmon resonance are still challenging, as its characteristics can be affected by multiple factors. We herein summarize the recent advances of plasmonic nanoparticles and their applications, particularly regarding the fundamentals and applications of surface plasmon resonance (SPR) in Au nanoparticles, plasmon-enhanced upconversion luminescence, and plasmonic chiral metasurfaces.

Liquid-crystal-loaded chiral metasurfaces for reconfigurable multiband spin-selective light absorption

Light absorption plays a key role in numerous photonic devices. In this work, we theoretically demonstrate that multiband circularly polarized light can be spin-selectively absorbed with a single-layered metasurface. The difference of absorption efficiency between left- and right-handed circularly polarized light can reach 40%. This giant chiroptical effect originates from different plasmonic resonances induced by the incident circularly polarized light with opposite spin states. Furthermore, by integrating the metasurface with nematic liquid crystals, the spin-selective absorption and the chirality can be dynamically reconfigured via applying a bias voltage. The advantageous features of being multiband-absorptive, backplane-free and reconfigurable make our proposed liquid-crystal-loaded chiral metasurfaces potentially useful for various photonic applications.

Superchiral Plasmonic Phase Sensitivity for Fingerprinting of Protein Interface Structure

The structure adopted by biomaterials, such as proteins, at interfaces is a crucial parameter in a range of important biological problems. It is a critical property in defining the functionality of cell/bacterial membranes and biofilms (i.e., in antibiotic-resistant infections) and the exploitation of immobilized enzymes in biocatalysis. The intrinsically small quantities of materials at interfaces precludes the application of conventional spectroscopic phenomena routinely used for (bio)structural analysis due to a lack of sensitivity. We show that the interaction of proteins with superchiral fields induces asymmetric changes in retardation phase effects of excited bright and dark modes of a chiral plasmonic nanostructure. Phase retardations are obtained by a simple procedure, which involves fitting the line shape of resonances in the reflectance spectra. These interference effects provide fingerprints that are an incisive probe of the structure of interfacial biomolecules. Using these fingerprints, layers composed of structurally related proteins with differing geometries can be discriminated. Thus, we demonstrate a powerful tool for the bioanalytical toolbox.

Polarization invariant plasmonic nanostructures for sensing applications

Optics-based sensing platform working under unpolarized light illumination is of practical importance in the sensing applications. For this reason, sensing platforms based on localized surface plasmons are preferred to their integrated optics counterparts for their simple mode excitation and inexpensive implementation. However, their optical response under unpolarized light excitation is typically weak due to their strong polarization dependence. Herein, the role of rotational symmetry for realizing robust sensing platform exhibiting strong optical contrast and high sensitivity is explored. Specifically, gammadion and star-shaped gold nanostructures with different internal and external rotational symmetries are fabricated and studied in detail, from which their mode characteristics are demonstrated as superposition of their constituent longitudinal plasmons that are in conductive coupling with each other. We demonstrate that introducing and increasing internal rotational symmetry would lead to the enhancement in optical contrast up to ~3x under unpolarized light illumination. Finally, we compare the sensing performances of rotationally symmetric gold nanostructures with a more rigorous figure-of-merit based on sensitivity, Q-factor, and spectral contrast.

Chirality detection of enantiomers using twisted optical metamaterials

Many naturally occurring biomolecules, such as amino acids, sugars and nucleotides, are inherently chiral. Enantiomers, a pair of chiral isomers with opposite handedness, often exhibit similar physical and chemical properties due to their identical functional groups and composition, yet show different toxicity to cells. Detecting enantiomers in small quantities has an essential role in drug development to eliminate their unwanted side effects. Here we exploit strong chiral interactions with plasmonic metamaterials with specifically designed optical response to sense chiral molecules down to zeptomole levels, several orders of magnitude smaller than what is typically detectable with conventional circular dichroism spectroscopy. In particular, the measured spectra reveal opposite signs in the spectral regime directly associated with different chiral responses, providing a way to univocally assess molecular chirality. Our work introduces an ultrathin, planarized nanophotonic interface to sense chiral molecules with inherently weak circular dichroism at visible and near-infrared frequencies.