Nanophotonics 2020; 10(2): 797–824 Review Vera Kuznetsova*, Yulia Gromova*, Marina Martinez-Carmona, Finn Purcell-Milton, Elena Ushakova, Sergei Cherevkov, Vladimir Maslov and Yurii K. Gun’ko* Ligand-induced chirality and optical activity in semiconductor nanocrystals: theory and applications https://doi.org/10.1515/nanoph-2020-0473 and possibilities that can arise in research areas concerning Received August 13, 2020; accepted October 21, 2020; the interaction of QDs with chiral molecules and that a published online November 9, 2020 mutual influence approach must be taken into account particularly in areas, such as photonics, cell imaging, phar- Abstract: Chirality is one of the most fascinating occurrences macology, nanomedicine and nanotoxicology. in the natural world and plays a crucial role in chemistry, biochemistry, pharmacology, and medicine. Chirality has Keywords: chirality; colloidal nanocrystals; induced also been envisaged to play an important role in nanotech- chirality; nanoplatelets; optical activity; quantum dots; nology and particularly in nanophotonics, therefore, chiral quantum rods. and chiroptical active nanoparticles (NPs) have attracted a lot of interest over recent years. Optical activity can be induced in NPs in several different ways, including via the direct inter- 1 Introduction action of achiral NPs with a chiral molecule. This results in circular dichroism (CD) in the region of the intrinsic absorp- A chiral molecule has two mirror-image forms, known as tion of the NPs. This interaction in turn affects the optical enantiomers, which are nonsuperimposable in three- properties of the chiral molecule. Recently, studies of induced dimensional space [1]. Enantiomers have identical phys- chirality in quantum dots (QDs) has deserved special atten- ical properties aside from the differing direction in which tion and this phenomenon has been explored in detail in a they rotate the plane of polarization of linear polarized number of important papers. In this article, we review these light. The phenomenon of rotating the plane of polarized important recent advances in the preparation and formation light is specifically known as optical activity and can be of chiral molecule–QD systems and analyze the mechanisms directly studied using circular dichroism (CD) spectroscopy of induced chirality, the factors influencing CD spectra shape [1], which is a technique of measurement of the difference and the intensity of the CD, as well as the effect of QDs on in the absorption of left and right circularly polarized light chiral molecules. We also consider potential applications of (CPL) in optically active compounds. these types of chiroptical QDs including sensing, bioimaging, Another crucial difference between enantiomers is the enantioselective synthesis, circularly polarized light emitters, manner in which they interact with other chiral com- and spintronic devices. Finally, we highlight the problems pounds. Most organic molecules in living organisms are chiral, including amino acids, carbohydrates, proteins and DNA [1]. Chirality plays an important role in many biolog- *Corresponding authors: Vera Kuznetsova, ITMO University, St. ical processes: the biological activity of chiral materials, Petersburg 197101, Russia, E-mail: [email protected]; Yulia such as intracellular uptake, enzymatic activity and Gromova, School of Chemistry, CRANN and AMBER Research Centers, toxicity strongly depend on the enantiomeric form. Trinity College Dublin, College Green, Dublin 2, Ireland, E-mail: [email protected]; and Yurii K. Gun’ko, School of Therefore, the ability to analyze, interpret and apply the Chemistry, CRANN and AMBER Research Centers, Trinity College properties of chiral materials is highly important in the Dublin, College Green, Dublin 2, Ireland, E-mail: [email protected]. fields of chemistry, pharmacology, biology and medicine. https://orcid.org/0000-0002-4772-778X Many new synthetically produced chiral materials Marina Martinez-Carmona and Finn Purcell-Milton, School of have been introduced recently including: metal nano- Chemistry, CRANN and AMBER Research Centers, Trinity College particles (NPs) and surfaces, colloidal semiconductor Dublin, College Green, Dublin 2, Ireland Elena Ushakova, Sergei Cherevkov and Vladimir Maslov, ITMO nanocrystals (NCs) and nanoceramics [2–17]. The area of University, St. Petersburg 197101, Russia chiral nanomaterials research is rapidly expanding, due to Open Access. © 2020 Vera Kuznetsova et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International License. 798 V. Kuznetsova et al.: Ligand-induced chirality and optical activity the numerous potential applications, including asym- (vii) Optical activity can be induced in achiral NPs in vi- metric catalysis [18–20], enantiomeric separation [21, 22], cinity of chiral metasurfaces [64] (Diagram 1). chiral sensing [3, 23–25], cytotoxicity mediation [26], cir- A number of reviews on chiral NPs have been published [2– cular polarized light emitting devices [27–29] and spin- 12, 14–17]. These reviews have covered a wide range of tronics [30, 31]. In the present review, we focus on chiral NPs of various nature with particular attention paid semiconductor colloidal quantum dots (QDs) with chiral to chiral plasmonics. In the present review, we decided to properties, a particularly exciting class of chiral nano- focus exclusively on LIC in colloidal semiconductor NCs materials. QDs are luminescent semiconductor NPs with (point VI; see paragraph above). Since the defects in NCs outstanding tunability of the optical and physical proper- (points IV and V) often affect induced chirality, they will ties as a result of their variable size dependent band elec- also be discussed in this review. tronic structure due to quantum confinement effect. QDs Over the last five years, many interesting studies have possess high photoluminescent (PL) quantum yield (up to been focused on the field of induced optical activity, which 100%); large extinction coefficients continuous across a has led to significant progress in understanding of the wide spectral range, in addition to excellent photo- and mechanisms of this phenomenon, with the influence of chemical stability [32, 33]. Due to these properties, QDs can various factors on QD induced CD and CPL spectra having be successfully used in a variety of potential applications, been investigated. The majority of these investigations are such as biological imaging, sensing, solar cells, photo- devoted to spherical QDs, but there is also an increased catalysis, LEDs and others [32–36]. interest in chiral anisotropic NCs. Many possible options Chiroptical activity may be induced in nanomaterials have been found to use LIC-QDs for different applications, including QDs by a variety of methods, such as: including chiral sensing, asymmetric catalysis, circular (i) Chiral crystal structure: NPs can have a chiral lattice polarized light emitters and spintronics. In addition, the structure and demonstrate optical activity without study of LIC in QDs is important not only from a practical any external influences of chiral ligands or scaf- but also from a theoretical point of view, since the analysis folding. For example, it was shown that quartz, of CD spectra enable the understanding of the fine energy β-AgSe, α-HgS, selenium and tellurium crystals have structure of QDs and the principles governing the interac- chiral crystal structure and corresponding NCs have tion of QDs with surrounding molecules. Moreover, since two enantiomers [37–39]. QDs have potential applications in biology and medicine, it (ii) Chiral NP shape: NCs of intrinsically achiral materials is essential to consider the phenomena that can occur in may be grown in a chiral shape, e.g., twisted CdTe nanoribbons [40–42], nanosprings [43], nanoscrolls [44–46], plasmonic and semiconductor gammadions [47, 48] and other chiral shapes [49, 50]. (iii) Chiral assembly: achiral NPs may be assembled in chiral superstructures, e.g., DNA has been used to assemble NPs in helixes [51, 52] and chiral “mole- cules” can be fabricated from coupled achiral semi- conductor NCs [53, 54]. (iv) Chiral defects: chiral surface and structural defects can be produced during the synthesis of NPs in the presence of chiral ligands, such as it has been done for aqueous penicillamine capped CdS QDs [55]. (v) Spontaneous chiral defects: NPs can be synthesized with spontaneous chiral surface and structural de- fects (e.g., screw dislocations [56, 57] and dopant ions [58]) even without the presence of chiral ligands and can then be extracted from an achiral mixture by enantioselective phase transfer [40, 59, 60]. (vi) Ligand-induced chirality (LIC): optical activity may be induced in colloidal NPs through interaction of NPs with chiral ligands, such as cysteine or penicil- Diagram 1: Chiroptical activity induction in semiconductor lamine [27, 61–63]. nanocrystals. V. Kuznetsova et al.: Ligand-induced chirality and optical activity 799 biological media containing a multitude of various chiral penicillamine and glutathione are the most commonly molecules and interaction with drugs. used chiral ligands, while recently, non-thiol containing chiral compounds with several carboxylic groups, such as maleic and tartaric acid derivatives have demonstrated the 2 Ligand-induced optical activity in ability to effectively induce optical activity in QDs also. QDs 2.2 The mechanisms of ligand-induced 2.1 Preparation of QDs with ligand induced