Nanophotonic Platforms for Chiral Sensing and Separation

Abstract

Conspectus Chirality in Nature can be found across all length scales, from the subatomic to the galactic. At the molecular scale, the spatial dissymmetry in the atomic arrangements of pairs of mirror-image molecules, known as enantiomers, gives rise to fascinating and often critical differences in chemical and physical properties. With increasing hierarchical complexity, protein function, cell communication, and organism health rely on enantioselective interactions between molecules with selective handedness. For example, neurodegenerative and neuropsychiatric disorders including Alzheimer’s and Parkinson’s diseases have been linked to distortion of chiral-molecular structure. Moreover, d-amino acids have become increasingly recognized as potential biomarkers, necessitating comprehensive analytical methods for diagnosis that are capable of distinguishing l- from d-forms and quantifying trace concentrations of d-amino acids. Correspondingly, many pharmaceuticals and agrochemicals consist of chiral molecules that target particular enantioselective pathways. Yet, despite the importance of molecular chirality, it remains challenging to sense and to separate chiral compounds. Chiral-optical spectroscopies are designed to analyze the purity of chiral samples, but they are often insensitive to the trace enantiomeric excess that might be present in a patient sample, such as blood, urine, or sputum, or pharmaceutical product. Similarly, existing separation schemes to enable enantiopure solutions of chiral products are inefficient or costly. Consequently, most pharmaceuticals or agrochemicals are sold as racemic mixtures, with reduced efficacy and potential deleterious impacts. Recent advances in nanophotonics lay the foundation toward highly sensitive and efficient chiral detection and separation methods. In this Account, we highlight our group’s effort to leverage nanoscale chiral light–matter interactions to detect, characterize, and separate enantiomers, potentially down to the single molecule level. Notably, certain resonant nanostructures can significantly enhance circular dichroism for improved chiral sensing and spectroscopy as well as high-yield enantioselective photochemistry. We first describe how achiral metallic and dielectric nanostructures can be utilized to increase the local optical chirality density by engineering the coupling between electric and magnetic optical resonances. While plasmonic nanoparticles locally enhance the optical chirality density, high-index dielectric nanoparticles can enable large-volume and uniform-sign enhancements in the optical chirality density. By overlapping these electric and magnetic resonances, local chiral fields can be enhanced by several orders of magnitude. We show how these design rules can enable high-yield enantioselective photochemistry and project a 2000-fold improvement in the yield of a photoionization reaction. Next, we discuss how optical forces can enable selective manipulation and separation of enantiomers. We describe the design of low-power enantioselectiv...

Helicity‐Preserving Metasurfaces for Magneto‐Optical Enhancement in Ferromagnetic [Pt/Co]N Films

“…[ 35–38 ] Specifically, helicity‐preserving metasurfaces composed of sub‐wavelength, 2D dielectric disk arrays have received significant attention for supporting highly twisted electromagnetic near‐fields. [ 39–45 ] While similarly structured Si arrays coated with Ni have been shown to strengthen MO effects with linear excitation, [ 46–48 ] the capability of these metasurfaces to shape CPL by concentrating absolute electric field rotation near and within magnetic thin films has not yet been explored.…”

Chiral sensing with achiral isotropic metasurfaces

“…To deduce the chirality parameter κ of an unknown chiral inclusion according to the procedure outlined in the previous section, we used all reflection and transmission amplitudes. However, in most relevant experiments [17][18][19][20][21][24][25][26][27][28] chiral sensing relies on measurements in transmission only, naturally raising the question of how this can be also implemented in the framework of anisotropic metasurfaces. As we show next, this is possible if we analyze the polarization of the transmitted wave in terms of its rotation θ and ellipticity η, using the transmission amplitudes given by Eqs.(4a)-(4c).…”

Supramolecular Chirality Synchronization in Thin Films of Plasmonic Nanocomposites

“… 1 − 6 Development of chiral assemblies made of plasmonic nanoparticles is particularly interesting due to their superior light–matter interactions in comparison to purely organic matter. 3 , 7 − 12 Stimulated by recent visionary reports, much interest has been revived toward applying the principles of chiral plasmonics to realize superlenses, 13 chiral catalysts, 14 , 15 negative refractive index materials, 16 perfect absorbers, 17 broadband circular polarizers, 18 selective reflectors, 19 biosensors, 20 chiral quantum optical devices, 21 chiral emission, 22 and biomanipulation. 23 For this purpose, harnessing chiral plasmonic properties in complex, organic–inorganic nanocomposites is often required.…”