Gil Markovich

The homochirality of biomolecules motivates the search for chiral surfaces capable of enantioselective recognition, yet systematic links between amino acid functional-group geometry and adsorption on chiral inorganic nanocrystals (NCs) remain limited. Here we investigate the enantioselective adsorption of asparagine, serine, and histidine onto intrinsically chiral Λ- and Δ-TbPO₄·H₂O NCs, which crystallize in a monoclinic (pseudo-hexagonal) structure. Individual-enantiomer adsorption isotherms and enantiomeric excess (ee) from racemic mixtures were determined using circular dichroism spectroscopy combined with potentiometric titration. Adsorption of single enantiomers followed the Langmuir model at low coverage, with L-enantiomers preferring Λ-NCs and D-enantiomers preferring Δ-NCs; matched chiral pairs showed roughly twofold higher equilibrium constants than mismatched pairs. At relatively high surface coverage, the adsorption behavior deviated from the Langmuir model and fitted the Frumkin model, which contains lateral attraction between adsorbed molecules. Enantioselectivity was highest at low racemate concentrations and declined with increasing total racemate concentrations, mirroring trends previously found for tartaric acid and aspartic acid on the same NCs. Tb³⁺–Tb³⁺ spacings on the dominant facets closely matched intramolecular functional-group distances in the amino acids, supporting a three-point chiral recognition mechanism.

Chiral plasmonic nanoparticles combine interesting geometrical and optical properties and hold great promise for applications in polarization optics, light sources, and enantioselective sensing. However, simple, low-cost, large-area, and reproducible routes to fabricate such chiral plasmonic nanostructures are still limited. Here, we demonstrate a scalable chiral optical imprinting of nanostructured Ag films in which an initially racemic ensemble of nanoparticle enantiomers is transformed into a chiral state under continuous-wave circularly polarized illumination. The imprinting is driven by plasmon-induced redox processes mediated by hot carriers and by surface diffusion of Ag⁺ ions and leads to a robust, unchanged circular dichroism upon flipping the sample, evidencing the formation of 3D chiral nanostructures. We identified two different irradiation regimes depending on the power density. At low-power densities, circularly polarized light creates asymmetrical distribution of electric field around nanoparticles that promotes the growth and stabilization of Ag nanoparticles with handedness matching the incident light polarization. The microscopic origin of the observed chiral growth was elucidated by numerical simulations of the near-field response and hot-carrier generation, which show that under circularly polarized illumination, size-asymmetric (achiral) Ag dimers act as plasmonic nanoantennas, giving rise to a strongly handed local hot-carrier generation profile. At high-power densities (>5 W·cm^-2), selective hot-carrier-driven photo-oxidation and thermally assisted coarsening of same-handed particles dominate, giving rise to inversion of the enantiomeric excess. The described transformations are observed only in the aged granular films, where a thin Ag₂O layer accelerates Ag atom/ion migration and facilitates nanoparticle reshaping. The resulting films exhibit a pronounced chiroptical response with a maximum g-factor of 1.2 × 10^-2 at 532 nm.

The enantioselective adsorption (and catalysis) of molecules on chiral surfaces has been a topic of interest, in particular with respect to models describing homochirality in the evolution of life. While such adsorption on single crystal surfaces has been studied using various traditional surface science techniques, similar studies on powders have been less successful. Here we show that studying enantioselective adsorption on the particular system of intrinsically chiral TbPO₄·H₂O nanocrystals (chiral space group) is highly useful and obeys some simple rules. The advantages of this system are the ability to produce such nanocrystals with 100% enantiomeric purity with virtually no ligands, due to strong autocatalytic effects in their enantioselective nucleation, and the prevalence of one type of surface type (100) in the formed nanocrystals. We show that particular chiral molecules such as tartaric and aspartic acids exhibit significant enantioselectivity in adsorption, while similar ones, like glutamic acid, show none. We demonstrate that at least three functional groups are required to define enantioselective adsorption, and in addition, there should be matching of distances between these groups and the corresponding surface adsorption sites (Tb³⁺ sites in our case).

This essay presents the viewpoint of the author on the topic of chirality and optical activity in nanostructures. It particularly focuses on the interaction of chiral molecules with plasmonic and excitonic nanocrystals and on induction of circular dichroism in such achiral nanocrystals. It discusses recent developments in the shape symmetry breaking of achiral metal nanostructures using photochemical processes induced by asymmetric localized plasmonic hot spots excited through circularly polarized light illumination. Finally, it addresses symmetry breaking in intrinsically chiral inorganic nanocrystals using chiral ligands during their formation, leading to 100 % enantiomeric excess in the nanocrystals of TbPO₄⋅H₂O. These nanocrystals exhibit an interesting nucleation mechanism, which leads to very high chiral amplification (secondary nucleation).

Shape symmetry breaking in the formation of inorganic nanostructures is of significant current interest. It was typically achieved through the growth of colloidal nanoparticles with adsorbed chiral molecules. Photochemical processes induced through asymmetric plasmon excitation by circularly polarized light in surface immobilized nanostructures also led to symmetry breaking. Here, we show that chiral symmetry breaking can be achieved by randomly rotating gold@silver core-shell nanobars in colloidal solution using circularly polarized illumination, where orientational averaging does not eliminate the symmetry breaking of an asymmetric plasmon-induced galvanic replacement reaction. Different morphological effects that are produced by circularly vs linearly polarized light illumination demonstrate the intricate effect of light polarization on the localized plasmonic-induced photochemical response. The essential features of this symmetry breaking, such as illumination wavelength dependence, were reproduced by simulations of circularly polarized light-excited-plasmon-induced hot-electron generation as the source for asymmetric metal deposition. The symmetry breaking becomes smaller in more symmetric geometrical shapes, such as triangular nanoprisms and nanocubes, and down to zero in spherical ones. The degree of symmetry breaking rises when the nanobars are immobilized on a substrate and illuminated from a single direction.

Shape symmetry breaking in the formation of inorganic nanostructures is of large current interest. It was typically achieved through growth of colloidal nanoparticles with adsorbed chiral molecules. Photochemical processes induced through asymmetric plasmon excitation by circularly polarized light in surface immobilized nanostructures also led to symmetry breaking. Here we show that chiral symmetry breaking can be achieved on randomly rotating gold@silver core-shell nanobars in colloidal solution, by means of circularly polarized illumination, where orientational averaging does not eliminate the symmetry breaking of an asymmetric plasmon-induced galvanic replacement reaction. Different morphological effects that are produced by circularly vs. linearly polarized light illumination demonstrate the intricate effect of light polarization on the localized plasmonic-induced photochemical response. The symmetry breaking becomes smaller in more symmetric geometrical shapes such as triangular nanoprisms and nanocubes, down to zero in spherical ones. The symmetry breaking rises when the nanobars are immobilized on a substrate and illuminated from a single direction.

Core–shell gold–silver cuboidal nanoparticles were produced, with either concave or straight facets. Their incubation with a low concentration of chiral l-glutathione (GSH) biomolecules was found to produce near UV plasmonic extinction and induced circular dichroism (CD) peaks. The effect is sensitive to the silver shell thickness. The GSH molecules were found to cause redistribution of silver in the shell, removing silver atoms from edges/corners and re-depositing them at the nanocuboid facets, probably through some redox and complexation processes between the silver and thiol group of the GSH. Other thiolated chiral biomolecules (and drug molecules) did not show this effect. The emerging near UV surface plasmon resonance is a silver slab resonance, which might also possess some multipolar resonance nature. The concave-shaped nanocuboids exhibited stronger induced plasmonic CD relative to the nanocuboids with straight facets.