Auswahl der wissenschaftlichen Literatur zum Thema „Achiral nanostructures“

An achiralC₃-symmetric molecule was found to self-assemble into various hierarchical nanostructures such as nanotwists, nanotrumpets and nanobelts, in which the twisted fibers showed supramolecular chirality as well as circularly polarized luminescence although the compound is achiral.

The ISA of a chiral gelator and an achiral component exhibited a left-handed helical nanostructure in ethanol. The formed helical nanostructures can be inverted by adding water to the ethanol solvent.

We develop a quantum theory based on macroscopic quantum electrodynamics to research the resonance energy transfer (RET) between a chiral donor and acceptor. It differs from the previous Green function approach which needs specific boundary conditions to obtain an analytical solution for calculating the RET rate. Our theory can combine the finite-difference time-domain (FDTD) method, which gives a simple and efficient semi-analytical approach, to evaluate the chiral RET rate in an arbitrary plasmonic nanosystem. Applying our theory to the systems of chiral molecules 3-methylcyclopentanone (3MCP) near the achiral/chiral plasmonic nanostructures, the RET process, which is divided into nondiscriminatory and discriminatory parts, is investigated. We find that plasmon will enhance both nondiscriminatory and discriminatory rates compared to the absence of plasmonic nanostructure, but the plasmon supported by chiral nanostructure contributes more to the discriminatory rate. The ratio of discriminatory to nondiscriminatory rates in the system consisting of 3MCP and chiral plasmonic structure is five-fold compared to the system consisting of 3MCP and achiral plasmonic structure. The phenomena can be attributed to the chiral electric-magnetic coupling. Our findings are important in understanding the achiral and chiral electric-magnetic interaction and designing chiral light-harvesting and sensing devices.

It is still a challenge to find a suitable method to fabricate a well-defined homochiral surface from achiral molecules, and one of the possible methods is to modify surfaces with organic molecular assemblies. Large-area chiral self-assembly nanostructures have been observed at room temperature by depositing ZnPc molecules on a Cu(111) surface. The growth process has been investigated. ZnPc molecules get adsorbed first at the terrace steps, and then extend over the lower terrace until the whole terrace is covered with ZnPc molecules; such growth process would be stopped when the self-assembly nanostructure run into a decorated upper terrace step edge. We found that the terrace steps with specific directions with respect to the close-packed directions of the substrate can induce homochiral self-assembly on the lower terraces. So we can propose a possible way to fabricate a well-defined homochiral surface from achiral organic molecules.

Achiral bipyridines can co-assemble with l-phenylalanine derivatives into unexpected right-handed helical nanostructures rather than left-handed helix by utilizing intermolecular hydrogen bonding formed between pyridyl and carboxylic groups.

As chiral antennas, plasmonic nanoparticles (NPs) can enhance chiral responses of chiral materials by forming hybrid structures and improving their own chirality preference as well. Chirality-dependent properties of plasmonic NPs broaden application potentials of chiral nanostructures in the biomedical field. Herein, we review the wet-chemical synthesis and self-assembly fabrication of gold-NP-based chiral nanostructures. Discrete chiral NPs are mainly obtained via the seed-mediated growth of achiral gold NPs under the guide of chiral molecules during growth. Irradiation with chiral light during growth is demonstrated to be a promising method for chirality control. Chiral assemblies are fabricated via the bottom-up assembly of achiral gold NPs using chiral linkers or guided by chiral templates, which exhibit large chiroplasmonic activities. In describing recent advances, emphasis is placed on the design and synthesis of chiral nanostructures with the tuning and amplification of plasmonic circular dichroism responses. In addition, the review discusses the most recent or even emerging trends in biomedical fields from biosensing and imaging to disease diagnosis and therapy.

La détection de molécules chirales à l'aide de résonateurs plasmoniques est un domaine de recherche prometteur pour améliorer la sensibilité et la flexibilité de la détection. Cette approche vise à surmonter les limitations des méthodes conventionnelles, telles que la méthode chiroptique, qui présente des limitations en termes de sensibilité. Les résonateurs plasmoniques sont capables d'interagir de manière résonante avec la lumière, ce qui permet d'augmenter le couplage entre les molécules chirales et la lumière, tout en offrant un contrôle sur les propriétés de polarisation de la lumière. Les avancées récentes dans ce domaine ont porté sur la création de surfaces nanostructurées chirales avec des résonateurs spécifiques, mais le mécanisme sous-jacent à la réponse différentielle des biomolécules à la lumière polarisée circulairement reste mal compris. Dans le cadre de ce projet de doctorat, l'approche novatrice consiste à utiliser des nanostructures achirales anisotropes, telles que des nanoslits, pour interagir avec des molécules chirales. Ces nanostructures achirales offrent l'avantage de pouvoir inverser le signe du dichroïsme circulaire en contrôlant la polarisation incidente ou le sens de propagation de la lumière. En manipulant les symétries du champ électromagnétique à proximité des résonateurs, il devient possible d'étudier plus en détail le couplage électromagnétique entre les biomolécules chirales et les nanorésonateurs. Le projet vise à développer des nanorésonateurs plasmoniques innovants, basés sur des nanoslits, qui seront fonctionnalisés pour détecter des biomolécules chirales. Contrairement aux résonateurs chiraux, les résonateurs achiraux peuvent générer des signes de champs chiraux, offrant ainsi une grande flexibilité dans la détection. Le travail comprend la caractérisation et la compréhension de l'origine des champs chiraux, ainsi que des méthodes pour les rendre homogènes. Une partie de la recherche se concentre sur la conception d'une source de lumière superchirale pure à l'aide de nanoslits, qui peut être accordée en longueur d'onde et en polarisation. Dans cette perspective, des méthodes expérimentales sont présentées, notamment l'utilisation de la fluorescence détectée par dichroïsme circulaire (FDCD) pour les molécules sensibles aux énantiomères. Pour la réalisation de ces expériences, des résonateurs plasmoniques avec une résonance à 680 nm ont été choisis, correspondant à la bande d'absorption chirale de LHCII. Une idée intéressante consiste à bloquer le faisceau d'excitation pour ne recueillir que l'émission des molécules chirales, en étudiant les résonances des ouvertures dans une couche d'or opaque. En résumé, ce projet de doctorat vise à exploiter les avantages des nanostructures plasmoniques achirales pour améliorer la détection des molécules chirales en offrant une plus grande flexibilité dans la manipulation de la polarisation de la lumière et en explorant de nouvelles méthodes expérimentales pour cette détection
The detection of molecules based on fluorescence or Raman scattering has been widely studied and is currently used in industry and laboratories. However, many organic molecules of interest are chiral, and their chemical and biological properties depend on their enantiomer as well as on the chirality of their secondary structure. The quantity and chirality of biomolecules are classically determined by measuring the differential absorption between the two opposite circular polarizations (chiroptic method). However, this method is limited by the low differential absorption of chiral molecules, which is of the order of 10-3 in the UV part of the spectrum. Plasmonic resonators have the ability to resonantly interact with light and are characterized by a moderate quality factor and a low effective volume. This resonant interaction allows (i) to increase the coupling between molecules and light and (ii) to control the polarization properties of light. So far, the latest advances concern the implementation of nanostructured chiral surfaces with gammadion-type resonators or stacked twisted resonators that interact preferentially with a given helicity of light. However, the mechanism behind the differential response of biomolecules coupled to chiral resonators to circularly polarized light is still unclear, preventing the optimization of such detection. Moreover, in the research published so far, two different chiral sensors are needed to interact with right- and left-handed circularly polarized light, which requires complex calibration procedures. During the course of my PhD, I have studied the use of anisotropic achiral nanostructures to interact with chiral molecules. Indeed, they have the significant advantage over chiral nanostructures of changing the sign of the circular dichroism by controlling the incident polarization or the direction of propagation. Indeed, the symmetries of the electromagnetic field in close proximity to the resonators can be manipulated at will by changing illumination conditions hence providing a unique tool for studying the origin of the electromagnetic coupling between chiral biomolecule and nanoresonators. Consequently, in my PhD project I propose to use plasmonic nanoresonators to increase the light - “chiral matter” interactions in order to detect and study chiral molecules. I will use the concept of achiral plasmonic nanostructures (nanoslits) to develop innovative nanoresonators that will be used, once functionalized, to detect chiral biomolecules with enantiomer sensitivity. Indeed, achiral resonators can generate both signs of chiral fields as opposed to chiral resonators which would make their use very flexible. This work implies characterizing, describing and understanding the origins of chiral fields and how to make them homogeneous. Through the study of nanoslits, I demonstrate numerically and theoretically how to design a nanosource of pure superchiral light, free of any background and for which the sign of the chirality is tunable on-demand in wavelength and polarization. In the perspective, I will present experimental methods that could monitor the CD via fluorescence emission (FDCD for Fluorescence Detected Circular Dichroism) in the case of light harvesting molecules for molecules that need to be excited in the UV, autofluorescence may be used in conjunction with aluminum resonators. Without loss of generality, these considerations lead to the decision of investigating plasmonic resonators with resonance at 680 nm which correspond to the chiral absorption band of LHCII. The idea of blocking the excitation beam to collect only the emission of the chiral molecules leaded to the idea of investigating the resonances of openings in an opaque layer of gold

Chirality is a broad concept that characterizes structures of systems in almost all hierarchy of materials in natural sciences. Molecular chirality is sometimes essential in biological functions. Also in nanomaterials sciences, chirality plays a key role. It is of fundamental importance to investigate internal structures (geometrical distributions) of chiral optical responses in nanomaterials, to design chiral features of the materials and their functions. We developed near-field optical activity (typically circular dichroism, CD) imaging systems that allow us to visualize local structures of optical activity in nanomaterials, and observed near-field CD images of two-dimensional gold nanostructures fabricated with electron beam lithography lift-off technique. We found that the amplitudes of local CD signals were as large as 100 times the macroscopic CD signals of the same samples, for two-dimensional chiral gold nanostructures [1]. Even highly symmetric achiral structures that never give CD signals macroscopically gave locally very strong CD signals (a typical example for a rectangular nanostructure is shown in Figure 1) [2,3]. In this case, average of the signal over the nanostructure yielded roughly null CD intensity. While achiral nanostructures show in general local CD activities as mentioned above, circularly symmetric (two-dimensionally isotropic) nanostructures, such as circular disks, never give CD signals at any local positions. However, when the circular disk is illuminated with linearly polarized light, the circular symmetry is broken, and thus the system potentially yields locally chiral optical (i.e., circularly polarized) fields. To demonstrate that, we extended the near-field CD microscope, and enabled irradiation of well- defined linearly polarized near-field on the sample and detection of scattered-field ellipticity and polarization azimuth angle. We found for circular gold disks that the scattered field was actually elliptically polarized. The ellipticity and the azimuth angle of the scattered field depended on the incident polarization angle and relative position on the disk.