Motion, Symmetry & Spectroscopy of Chiral Nanostructures

Chirality is the property that defines an object that lacks mirror-symmetry. It is a fundamental property closely associated with life on earth and it therefore has far-reaching consequences. The fact that the world around us discriminates between left- and right-handed enantiomers is well-recognized but was certainly not envisioned by Louis Pasteur when he started the first experiments with chiral molecules and crystals more than 170 years ago.

In this chapter the theoretical background concerning the behaviour of small scale objects in external electromagnetic fields and in fluid environments are introduced. For the subsequent work of the thesis results from classical hydrodynamics (Sects. 2.1–2.3) to mathematical concepts (Sect. 2.5) and a description how the nanostructures are prepared (Sect. 2.7), are needed. The topics are only discussed briefly as further details can be found in many textbooks, including [1‐3] where the more interested reader can find further information.

This chapter is based on and contains excerpts and figures from the articles “Role of symmetry in driven propulsion at low Reynolds number” [1] and “Characterization of active matter in dense suspensions with heterodyne laser Doppler velocimetry” [2]. Contributions of coauthors are indicated.

This chapter is based on and contains excerpts and figures from the article “Chiroptical Spectroscopy of a Freely Diffusing Single Nanoparticle” [1]. Contributions of coauthors are indicated. Chiroptical spectroscopy is the analysis of differences that occur between the intensities of left- and right-circularly polarized light and is therefore also denoted as circular differential intensity. It is of utmost importance for natural sciences and life in general because almost every biomolecule and drug is chiral. Their handedness and purity are typically observed by optical spectroscopy, which is based on a difference between left- and right-circularly polarized light (LCP, respectively RCP) interacting with a bulk solution containing a chiral analyte. The strength of this interaction is rather weak because it relies on a coupling of magnetic and electric dipoles of a single molecule [2]. However, by probing ensembles of many molecules the individual signals add up and can be detected, e.g. with typical circular dichroism (CD) spectroscopy measuring the attenuation of LCP and RCP light of a substance.

This thesis investigated the motion, the symmetry and the spectroscopic responses of chiral and achiral nanostructures at low Re. Chirality is important in nature, and as demonstrated throughout this work, it is also an important property of artificial nanostructures. In order to support the increasing role of chiral nanostructures in emerging applications, this thesis has provided a fundamental description of the nanostructures’ behaviour in relation to their symmetries. To this end, two universal questions have been examined. First, does a propeller have to be chiral to be propulsive upon rotation-translation coupling and what are the fundamental symmetries to predict the behaviour of simple-shaped propellers? Second, what is the role of symmetry in chiroptical spectroscopy of a single nanostructure and how can its intrinsic chirality undoubtedly be detected?