Novel properties and applications of chiral inorganic nanostructures

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Novel properties and applications of chiral inorganic nanostructures Lian Xiao a,1 , Tingting An b,c,1 , Lin Wang a , Xiaoling Xu b,c , Handong Sun a,d,e,∗ a Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore b Cancer Centre, Faculty of Health Sciences, University of Macau, Macau SAR, China c Centre for Precision Medicine Research and Training, Faculty of Health Sciences, University of Macau, Macau SAR, China d Centre for Disruptive Photonic Technologies (CDPT), School of Physical and Mathematical Sciences, Nanyang Technological University, 637371, Singapore e MajuLab, International Joint Research Unit UMI 3654, CNRS, Université Côte d’Azur, Sorbonne Université, National University of Singapore, Nanyang Technological University, 637371, Singapore

a r t i c l e

i n f o

Article history: Received 22 May 2019 Received in revised form 25 October 2019 Accepted 9 December 2019 Available online xxx Keywords: Chirality origination Inorganic Nanostructures Chiral Circular dichroism

a b s t r a c t Chiral inorganic nanostructures have drawn much attention by virtue of their fascinating fundamental physical properties as well as the abundant applications. Over the past two decades, much effort has been paid to chiral inorganic nanostructures and substantial progresses have been achieved from the sample preparation, chirality origination investigation to practical applications. In this review, we summarize and evaluate the recent progresses of the chiral inorganic nanostructures. The sample synthesis approaches as well as experimental & theoretical studies of chirality originations are discussed. In addition, the unique intrinsic chirality caused by the dislocation is also presented in the review. The representative applications of the chiral inorganic nanostructures such as circular polarized light emission, interaction between chiral nanostructures and bio-systems, second harmonic generation, chiral catalysis, chiral assembly, chiral sensing, DNA cleavage etc. are also presented. We end up with discussing the challenges and future prospects of the chiral inorganic nanostructures in the last section. It is envisioned that the chiral inorganic nanostructures may play more and more impactful roles in versatile fields. © 2019 Elsevier Ltd. All rights reserved.

Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2 Synthesis methods and chirality origination mechanism of the chiral inorganic nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.1 Chiral II–VI group semiconductor nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.1.1 Chiral CdS nanocrystal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.1.2 Chiral CdSe nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.2 Chiral perovskites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.3 Chiral transition metal dichalcogenides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.4 Chiral carbon dots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.5 Chiral magnetic nanoparticle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.6 Chiral metal nanocrystal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 2.7 Intrinsic chirality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3 Applications of chiral inorganic nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.1 Circular polarized light emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.2 Chiral nanomaterial bio-system interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.3 Second harmonic generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

∗ Corresponding author at: Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore. E-mail address: [email protected] (H. Sun). 1 These authors contribute equally this work. https://doi.org/10.1016/j.nantod.2019.100824 1748-0132/© 2019 Elsevier Ltd. All rights reserved.

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3.4 Chiral catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.5 Chiral assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.6 Chiral sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 3.7 DNA cleavage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 4 Summary and prospect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

1 Introduction Chirality is known as a description of geometrical property, where the mirror image of a structure cannot superimpose with the original one, namely, the Sn symmetry elements do not exist [1]. Applying this definition into a nanoscale object leads to a chiral nanostructure. Similar to the chiral molecules, two nonsuperimposable nanostructures with mirror image symmetry are termed as enantiomers. The important property of chiral nanostructures lies in their different response to right and circular polarized light, leading to circular dichroism (CD) due to different absorption, optical activity due to different refractive indices and circular polarized emission due to different emission rates. Such chiral-optical phenomena not only imply application opportunities but also have become powerful tools to probe chiral nanostructures. In principle, any nanostructure can be chiral by virtue of their low symmetry caused by the chiral bulk and/or surface defects [2,3]. However, the macroscopic nanostructure ensembles generally do not present any net chirality because of the equal amount of the each enantiomer. It was until 1998 that Robert L. Whetten et al. [4] first prepared the chiral gold nanoclusters with the assistance of l-glutathione molecules, which opened the way for chiral inorganic nanostructure investigations. Regarding the origination of the chirality, the authors [4,5] suggested three possible mechanisms: 1) chiral inorganic core, 2) surface chiral adsorption pattern with achiral inorganic core, 3) electronic structure interaction between the achiral inorganic core and the achiral adsorption layer. These three mechanisms and their derivatives are widely utilized by researchers to interpret the chirality originations of chiral inorganic nanostructures. Until now, after more than 20 years, researchers have proposed various approaches to synthesizing a large number of inorganic nanostructures by choosing proper chiral molecules as the stabilizers. In addition to the chiral molecule stabilized nanostructures, the intrinsic chirality induced by the chiral defects and dislocation have also been reported by Yurii K. Gun’ko and co-workers [2,6], as also can be seen in the next section of this review. Theoretical calculations based on density functional theory (DFT) and dipole approximation have been widely adopted to uncover the chiral properties of the nanostructures, which give some insight into the chirality origination. It is noticed that the versatile applications including some fantastic demonstrations such as DNA cleavage [7] based on the chiral nanostructures have been well documented or are emerging. Not surprisingly, chiral nanostructures have become a research hot spot in many fields of scientific endeavour from the material, chemistry to biology and medical science. Chirality may exist in a variety of materials, but in this review article we mainly focus on chiral inorganic nanostructures. We first summarize various types of the chiral nanostructures achieved by the researchers during the last two decades, as presented in Sections 2. The metal, semiconductor and magnetic nanostructures are our main focus while other chiral nanostructures such as chiral polymeric nanoparticles [8], chiral MOF/Zeolitic [9–11] etc. are out of the scope of this review. The preparation routes for each type of chiral nanostructure are also presented in Section 2. Given that the chirality origins of different nanomaterials display complexity and diversity, to give a

clear picture of the origination mechanisms, we do not blend all the mechanisms of different nanostructures to form a separate section. Instead, we discuss the chirality origination for each kind of inorganic nanostructure individually after the synthesis description. Moreover, we want to emphasize that even for a specific kind of chiral nanostructure, the chiral properties are also closely related to the synthesis approach. Thus we suggest that it’s better to provide the synthesis method when talking about a chiral nanostructure. Numerous applications of chiral inorganic nanostructures can be found in Section 3. We do not attempt to list all of the possible applications of the chiral inorganic nanostructures, but show some representative demonstrations. In the last section (Section 4) of this article, we present the brief summary and share our opinions about the prospects of the chiral nanomaterials especially for the inorganic nanostructures. 2 Synthesis methods and chirality origination mechanism of the chiral inorganic nanostructures In this section, we describe the fabrication of different types of chiral inorganic e.g. semiconductor, magnetic and metal nanostructures. After more than two decades, researchers now can control the morphology of the chiral nanostructures: dots, rods, platelet etc. All of the inorganic nanostructures and chiral molecules/templates mentioned in this review are summarized in the end of this section (Table 1). Even though various types of methods have been suggested to prepare chiral inorganic nanostructures, we want to highlight the importance of the ligand exchange approach which plays a crucial role for the development of the chiral inorganic nanostructures, especially for semiconductor nanostructures on account of its universal feature. Additionally, the origination mechanism of the chirality are also discussed in this section. As we mentioned earlier, the chirality origination of chiral nanostructure are greatly related to the synthesis approach, thus we consider the chirality mechanism after the evaluation of synthesis method. 2.1 Chiral II–VI group semiconductor nanostructures 2.1.1 Chiral CdS nanocrystal The first chiral inorganic semiconductor nanocrystal was reported by Yurii K. Gun’ko and coworkers [12]. Racemic (Rac), dand l- chiral penicillamine molecules were utilized by the authors to synthesize chiral CdS quantum dots by means of microwave heating. The CD spectra of rac-, l- and d- CdS quantum dots (see Fig. 1(a)) display 4 peaks located at about 207 nm, 252 nm, 293 nm and 345 nm respectively, which are totally different from the CD activity of the pure chiral penicillamine molecules. To reveal the formation mechanism of chiral CdS quantum dots, the authors [12] dynamically monitored the CD responds during the chiral CdS quantum dots synthesis. At the beginning, the introduction of Cd(ClO4 )2 leaded to the formation of cadmium penicillamine complex which manifested a CD band about ∼210 nm. The following thioacetamide provoked the formation of small CdS clusters and two new CD bands (290 nm and 320 nm) appeared. Microwave heating motivated the nucleation centres to grow up and produced

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Table 1 Summary of chiral inorganic nanostructures. Material Chiral CdS quantum dots (QDs) Chiral CdSe/CdS core shell QDs Chiral CdS/CdSe core shell QDs Chiral CdTe/CdS core shell QDs Chiral CdS nano-tetrapods Chiral CdSe QDs Chiral CdSe nanorods Chiral CdSe nanoplatelets Chiral CdTe QDs Chiral CdS, CdSe, CdTe QDs Chiral mercury sulfide (HgS) nanoparticles Chiral ZnO NP Chiral ZnS NP Chiral CdSe-Dot/CdS-Rod Nanocrystals CdSe/CdS semiconductor nanorods ZnSe/ZnS core/shell nanorods Chiral inorganic–organic hybrid perovskite Chiral all inorganic perovskite nanocrystals Chiral 1D organic and inorganic hybrid perovskite Chiral lead halide Perovskite nanowires Chiral hybrid organic–inorganic 2D perovskites Chiral Perovskite nanoplatelet Chiral 2D organic–inorganic lead iodide perovskites Chiral 2D MoS2 nanostructures Chiral Transition-Metal Dichalcogenide QDs (MoS2 , WS2 ) Chiral black phosphorus (BP) nanodots Chiral tin sulfide(SnS2 ) nanodots

Chiral carbon dots

Chiral Fe3 O4 nanoparticle Chiral Fe3 O4 /Pd nanoparticles Chiral FePd magnetic nanoparticles Chiral magnetic Pt/SiO2 /Fe3 O4 nanoparticles Chiral gold nanocrystal Chiral gold nanorod

Chiral silver nanoparticle

Chiral Cu nanocluster Chiral Cux O nanoparticle Chiral copper sulfide quantum dots Chiral upconversion nanoparticles Chiral gold-upconversion nanoparticle Chiral Molybdenum Oxide nanoparticles

Chiral molecule

Ref.

penicillamine cysteine protein nanocage cysteine cysteine cysteine penicillamine penicillamine cysteine carboxylic acid cysteine cysteine cysteine tripeptide glutathione (GSH) cysteinemethylester hydrochloride (MeCys) cysteine penicillamine cysteine proline cysteine cysteine cysteine penicillamine R/S-C6 H5 CH(CH3 )NH3 ␣-methylbenzylamine 1,2-diaminocyclohexane (DACH) N,N’-bis(octadecyl)-l-glutamic diamide R-3-aminopiperidine dihydrochloride ␤- methylphenethylamine (MPEA) methylbenzylammonium bromide (MBABr) ␣-methylbenzylamine (MBA) phenylethylammonium 1-(4-chlorophenyl)ethylamine penicillamine cysteine penicillamine cysteine thiourea thiourea 2-phenyl-1-propanol. Guanosine 50-monophosphate 1,2-cyclohexanediamine Cysteine lysine methionine glucose glucosamine Chiral Ruthenium(II) complex [Ru(BINAP-PO3 H2 )(DPEN)Cl2 ] N-Heterocyclic carbenes (NHCs) (S)-2,20-bis(diphenylphosphino)-1,10-binaphthene ((S)-BINAP) cinchonidine tripeptide glutathione (GSH, N-␥-glutamyl-cysteinyl-glycine) penicillamine cysteine obtained from Polymerase Chain Reaction technique cysteine obtained from DNA template obtained from mucin glycoprotein achieved by fractal aggregates cysteine bidentate a-dihydrolipoic acid (DHLA) bis(diphenyphosphino) methane, 1-adamantanethiol, tert-butyl mercaptan glutathione (GSH) penicillamine phenylalanine penicillamine chiral helical nanotubes obtained from DNA cysteine

[12,13,22,128,132] [14] [15] [14] [14] [131] [16] [17,22] [14,18,19,21,105,130] [20] [23] [25,26,137] [7,28] [27] [29] [30,32] [31,32] [33] [34] [130] [35] [36] [37] [42] [44] [45] [107] [46] [47] [49] [50] [51] [52] [53] [53] [54] [54] [55] [55] [61] [74] [63] [62,67,68,69,70,71,73] [64] [66] [66] [66] [76] [77] [78] [81] [4,5] [86] [89,90,91,131] [92] [93] [95,96] [94] [97] [100] [101] [102] [129] [103] [104] [145] [106] [133] [146]

the chiral CdS nanocrystals with the CD responses red-shifted to 320 nm and 370 nm. Later, DFT calculation was carried out by the authors [13] to uncover the chirality origination of the CdS quantum dots. They suggested that the chiral response came from the

surface of the quantum dots while the quantum dots core were still achiral, the so-called achiral core-chiral surface model [13]. On the surface of the CdS quantum dots, one surface Cd was linked to the penicillamine molecule by means of N and S, and the neigh-

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Fig. 1. (a) CD spectra of the d-(black blue), l-(yellow green) and Rac-penicillamine (Red) stabilized CdS QDs. (b)-(d) TEM images of d-Penicillamine capped CdS nano-tetrapods, (b) a wide field, (c) a closer look of two tetrapods, (d) High-resolution image. (e) CD spectra of the d-, l- and rac-Penicillamine capped CdS nano-tetrapods. (a) Reproduced with permission from ref. [12]. Copyright 2007, Royal Society of Chemistry. (b) - (e) Reproduced with permission from ref. [16]. Copyright 2010, Royal Society of Chemistry.

boring Cd-carboxylate bonding leaded to the generation of chiral activity. The strong interaction between the chiral ligands and surface of CdS nanocrystal stimulated the strong distortion of surface Cds, thus resulting in the appearance of new helical bands [13]. On contrast, the distortion of the quantum dots core could be ignored, thus exhibiting no chirality. The experimental observations are also consistent with the proposed theoretical model. The size of the CdS quantum dots became larger after the microwave treatment, whereas the CD spectra only showed some red shift but no new CD peak arose, which verified the achiral nature of the CdS quantum dot core, as declared by the authors [13]. In addition to the chiral molecule facilitated surface stabilization [12–14], Masanobu Naito et al. [15] also synthesized the chiral CdS nanodots with the assistance of ferritin, a kind of ␣-helix protein. Later, Yurii K. Gun’ko’s group [16] prepared the water soluble chiral CdS nano tetrapods by means of directly heating the mixture of aqueous CdCl2 , thioacetamide and chiral penicillamine molecules (D-, l- and racemic) under reflux. The tetrapodal and arm structures could be clearly seen from the TEM images (Fig. 1(b)–(d)) with a diameter about 10−20 nm. The emergence of carbon peaks in energy-dispersive X-ray spectroscopy (EDX) measurement and thermogravimetric analysis (TGA) indicated the successful linkage of chiral penicillamine ligands onto the surface of the CdS nano tetrapods [16]. Both the l- and d- penicillamine capped CdS nano tetrapods exhibited several CD peaks (Fig. 1(e)) in the spectral range from 300 nm to 400 nm, while no CD signal could be observed from the racemic nano tetrapods. The authors [16] attributed the induced chirality to the surface defects of CdS nano tetrapods. Besides, NG108-15 neuroblastoma cell bio assays demonstrated the bio-compatibility of the chiral CdS nanostructure, and the authors [16] claimed that the toxic CdS core was well surrounded by the bio-compatible penicillamine molecules.

2.1.2 Chiral CdSe nanostructures 2.1.2.1 CdSe quantum dots. Similar to the fabrication of chiral CdS quantum dots, Yurii K. Gun’ko et al. [17] prepared chiral CdSe quantum dots using the microwave assisted method, where the l-, dand rac (racemic) chiral penicillamine molecules were employed as the stabilizer. The mirror images of CD spectra have been observed from the l- and d- capped CdSe chiral quantum dots, as shown in Fig. 2(a), while the racemic penicillamine capped CdSe quantum dots displayed no CD signal as expected. Again the authors [17] attributed the chirality to the chiral ligand induced surface atom distortion, namely achiral core and chiral surface shell. Later Milan Balaz and co-workers [18] proposed to prepare chiral CdSe quantum dots by the post-synthetic ligand exchange method. Briefly, the achiral CdSe quantum dots capped by trioctylphosphine oxide/oleic acid ligands (TOPO/OA) were dissolved into toluene. l- and d- cysteine were dissolved into alkaline aqueous solution (pH = 12) and degassed by nitrogen. At room temperature, the mixture of cysteine aqueous solution and CdSe toluene solution were continuously stirred for 24 h, then the chiral CdSe quantum dots could be collected from the aqueous phase [18]. By employing the FTIR and 1 H NMR spectroscopy, the authors [18] verified the successful ligand exchange on the CdSe quantum dot surface. Both l- and d- cysteine capped CdSe quantum dots displayed clear CD signals (Fig. 2(b)) located at about 397.0, 450.0, 467.0, 505.0, 530.0 nm respectively, with opposite signs. Moreover, the authors [18] also discovered that the position of CD bands continuously red-shifted with the increase of the radius of CdSe quantum dots, while the shape of the CD spectra remained the same, as can be seen in Fig. 2(c) (the radius of the CdSe quantum dots increased from 2.5 to 3.3 nm). Once the chiral cysteine molecules on the surface of the CdSe quantum dots were further exchanged by the achiral 1-dodecannethiol (DDT) ligands, the CD

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Fig. 2. (a) CD spectra of chiral CdSe quantum dots synthesized by the microwave assisted method. d-(blue), l-(red), rac-penicillamine (green) capped CdSe QDs respectively. (b) CD spectra of l- and d-cysteine capped CdSe quantum dots via ligand exchange. (c) CD spectra of l-cysteine capped CdSe quantum dots with various radii from 2.5 nm to 3.3 nm. (e) CD spectra of TOPO/OA (achiral), l-cysteine capped, and DDT (achiral, by further ligand exchange with cysteine) capped CdSe quantum dots. (a) Reproduced with permission from ref. [17]. Copyright 2010, Royal Society of Chemistry. (b) - (d) Reproduced with permission from ref. [18]. Copyright 2013, Royal Society of Chemistry.

signals disappeared [18], as illustrated in Fig. 2(d). According to these observations, the authors suggested [18] that both the chiral surface and chiral ligand-CdSe QD coupling play important roles for the induced CD signal. Apart from the cysteine, other chiral ligands e.g. chiral carboxylic acid ligands [19,20] have also been employed by researchers to prepare the chiral CdSe nanostructures by means of the ligand exchange method [19,21,22]. 2.1.2.2 CdSe nanorods. To improve the CD response of CdSe nanostructure, the chiral CdSe nanorods were proposed by Tang et al. [23] via ligand exchange approach. The achiral CdSe nanorods were prepared through the hot injection method. Tetradecylphosphonic acid (TDPA), octadecylphosphonic acid (ODPA), and hexylphosphonic acid (HPA) organic stabilizers have been utilized to regulate the growth of nanorods along [001] direction. The authors [23] grew 5 types of CdSe nanorods with different aspect ratios: 1.0, 1.7, 1.9, 2.7, 4.2, & 7.3, and a same diameter about 3 nm, see Fig. 3(a). The chiral CdSe nanorods were obtained from the ligand exchange between the achiral nanorods and chiral molecules (L- and d- cysteine). All of the TEM, XRD and NMR spectroscopic measurements [23] indicated that the structure and the crystal of the CdSe nanorods have been remained after the ligand exchange. Two CD bands peak I and II were observed from the chiral CdSe nanorods in the UV region [23], as illustrated in Fig. 3(b) & (c). Peak I nearly remained the same even if the aspect ratio of nanorods increased a lot. On contrast, the CD signal of peak II was enhanced for increased aspect ratio, as can be seen from the continuously increased g factor (see Fig. 3(d)). Besides the CD signals in the UV spectral range, the authors [23] also observed strong and complex chiral signals in the visible range as illustrated in Fig. 3(e) & (f). It can be clearly seen that the CD bands continuously redshift as the aspect ratio increases. Interestingly, two CD bands with opposite signs appeared at the

first exciton absorption peaks, named peak IIIa and IIIb (see Fig. 4 and S9 in the original article [23]). The g factor of the nanorods first strongly increased, followed by a slight decrease as shown in Fig. 3(g). To interpret the observations, the authors [23] assumed the chiral CdSe nanorod as a big artificial chiral molecule because of the existence of surface chiral molecules. As a result, the interaction among the surface Cd-S bonds, C O bonds in cysteine ligands and CdSe nanorods leading to three kinds of chromophores in the artificial chiral molecule [23]. Based on these assumptions, the non-degenerate coupled oscillator model (NDCO) [24] have been employed by the authors [23] to explain the CD response, where the CD response originate from the electric dipole transition moments coupling between different chromophores. The authors suggested [23] that the coupling between the Cd-S bond and CdSe nanorods do not contribute to the CD responds as they cannot form any chiral configuration. They assigned [23] the CD band I to the optical ∗ transition from C O to the antibonding C=O valance ortital in cys∗ teine ligands, namely n0 → C=O transition. The coupling between ∗ of Cd-S bond and n0 → C=O transition (C O bonds) resulting in the 3ps → 5sCd transition, which contributes to the CD signal of band II [23]. The complex CD signals in the visible range could be ∗ interpreted as the coupling between the n0 → C=O transition (C O bond) and the sophisticated excitonic transitions in CdSe nanorods, the total picture and illustration can be seen in Fig. 3(h) & (i) [23]. 2.1.2.3 CdSe nanoplatelets. Following the CdSe nanorods, it is still Tang’s group [25] that prepared the CdSe nanoplatelets. Both the wurtzite (WZ) and zincblend (ZB) CdSe nanoplatelets have been proposed where the WZ nanoplatelets were capped by octylamine (OAm) and oleylamine (OLAm) and ZB nanoplatelets were stabilized by oleic acid (OA) respectively. The thickness of both WZ and ZB nanoplatelets were about 1.4 nm (corre-

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Fig. 3. (a) TEM images of CdSe nanorods with various aspect ratio (AR). Diameter is about 3 nm. (b),(c) CD spectra of the CdSe nanorods in the UV range and (d) g factor for peak II with different AR nanorods. (e),(f) CD spectra of the CdSe nanorods in the viable range and (g) g factor with different AR nanorods. (h) Illustration of the couplings between different chromophores (represented by the blue dot line) in chiral CdSe nanorods. (i) Illustrating of CD spectra origination for each part. Reproduced with permission from ref. 23. Copyright 2017, American Chemical Society.

Fig. 4. (a)-(d) CD spectra of l-, d-, LD-cysteine (racemic) modified chiral WZ and ZB CdSe nanoplatelets. (a) & (b) in the UV range, (c) & (d) in the visible range. (e) & (f) illustration of the chiral CdSe NPLs. (g)-(h): depiction of the origination for each CD band for (g) chiral WZ CdSe NPLs and (h) chiral ZB CdSe NPLs. (i) & (j) The band structure for ¯ direction) and ZB CdSe NPLs ([100] direction). (k) Illustrations, (l) CD spectra, (m) g factors for CdSe NPLs with various lateral size. (n) - (p) Illustrations, WZ CdSe NPLs ([1120] CD spectra of CdSe NPLs with 3, 4 and 5 monolayers. (a) – (j) Reproduced with permission from ref. 25. Copyright 2018, American Chemical Society. (k) – (p) Reproduced with permission from ref. 26. Copyright 2018, Wiley-VCH.

sponding to 3.5 monolayers of wurtzite and 4.5 monolayers of zincblend CdSe nanoplatelets respectively [25]), thus eliminated the thickness effect between these two kinds of nanoplatelets. The following ligand exchange with l- and d- cysteine resulted in the formation of aqueous chiral CdSe nanoplatelets, namely l-/D- Cys-WZ(wurtzite) CdSe nanoplatelets (NPLs) and l-/D-Cys ZB(zincblend) CdSe nanoplatelets (NPLs). The authors [25] utilized the H1 -NMR analysis to confirm the successful ligand exchange between the chiral molecules and NPLs, and suggested that the chiral ligands and NPLs were linked by strong Cd-S bond. Even though the WZ and ZB CdSe NPLs have similar absorption spectra, their CD spectra were totally different. In the UV range (Fig. 4(a)

& (b)), l-Cys-WZ CdSe nanoplatelets displayed 3 peaks [25]: WZ I (downward,↓), WZ II (↓), WZ III (upward, ↑). On contrast, l-Cys-ZB CdSe nanoplatelets showed more complicated CD responds with 5 peaks: ZB (zincblend) I (↓), ZB IIa (↓), ZB IIb (↓), ZB IIIa (↓), ZB IIIb (↑). Additionally, the distinct CD signals in the visible range have also been observed for both WZ and ZB chiral CdSe NPLs [25], as can be seen in Fig. 4(c) & (d). l-Cys-WZ CdSe nanoplatelets exhibited three CD bands WZ IVa (↑), WZ IVb (↑) & WZ IVc (↑), while l-Cys-ZB CdSe nanoplatelets displayed three different CD peaks ZB IVa (↓), ZB IVb (↑) & ZB IVc (↑) [25]. It can be clearly seen that the first exciton chiral transition showed opposite signs for WZ (WZ IVa (↑)) and ZB (ZB IVa (↓)) CdSe NPLs and the CD signal of WZ CdSe nanoplatelets

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is much larger than that of ZB CdSe NPLs. To explain the experimental observations, the authors [25] employed the nondegenerate coupled-oscillator (NDCO) model, where the whole chiral CdSe nanoplatelets are regarded as a large artificial chiral molecule, thus the chirality originates from the coupling between the surface chiral molecules and achiral CdSe NPLs. The configuration and linkage between the cysteine and NPLs also play an important role in the chirality generation. According to the XPS measurement [25], the authors proposed that the chiral cysteine was linked to the NPLs via Cd O and Cd S bonds. Theoretical analysis and TGA measurements [25] indicated that both WZ and ZB NPLs possess a cysteine molecule density of about 3–8/nm2 . Four chromophores in the artificial chiral molecule have been suggested by the authors [25] to account for the chiral activities: C O bonds, Cd O bonds, Cd-S bonds and CdSe nanoplatelets. The coupling between the different chromophores resulted in the final CD signals. Actually the principle are the same for the CdSe nanorods analysis as discussed in previous section [23]. Based on the NDCO model, each CD peak can be assigned to a corresponding transition. Regarding the CD signals in the UV range, the authors suggested the following origins for the CD bands ∗ [25]. The nO → C=O of the C O compound in the chiral ligands contribute to the WZ (wurtzite) I & ZB (zincblend) I, while CD peaks WZ II, ZB IIa & ZB IIb were the results of 2pO → 5sCd transition of the Cd-O bond, which resides on the interface between CdSe nanoplatelets and cysteine molecules. Similarly, the authors attributed [25] the CD peaks WZ III, ZB IIIa & ZB IIIb to 3ps → 5sCd transition of the Cd-S bond. When considering the CD response in the spectral range of 400−800 nm, the authors claimed [25] that the IVa (IVb , IVc ) originate from the total coupling effect between ∗ (C O bond), 1 hh-1e (1lh-1e, 1SO-1e) transition and nO → C=O 2pO → 5sCd (Cd-O bond), 3ps → 5sCd (Cd-S bond) transitions, as illustrated in Fig. 4(e)–(h). More specifically, the confinement axis  for WZ CdSe nanoplatelets has low symmetry [25], thus [1120] the valance band consists of the mixture of 4px , 4py and 4pz . In other words, the CD response came from the 4p(x,y,z),Se → 5sCd transition. Therefore all the three peaks were upward. On the contrary, in the condition of ZB NPLs [25], the 1hh → 1e transition came from the 4p(x,y),Se → 5sCd due to the existence of the higher symmetry confinement axis [100]. And 1lh → 1e originated from 4p(x,y,z),Se → 5sCd , which accounts for the different signs of CD signals between IVa & IVb . The spin orbital band and the light hole band in CdSe NPLs possess similar symmetry, which leaded to the same sign between IVb & IVc , as illustrated in Fig. 4(i) & (j). Based on the NDCO model, the authors could interpret the experimental observations. However, the hybridization interaction between the delocalized NPL states and chiral molecule orbital has not been considered in this model as pointed out by the authors [25]. Dan Oron et al. [26] also prepared the chiral zincblende CdSe NPLs with different lateral size and thickness by means of ligand exchange with l- and d- cysteine. Six types of NPLs with different lateral size from (3.4 ± 0.5) nm × (6.4 ± 0.9) nm to (13.7 ± 2.2) nm × (47.5 ± 6.2) nm have been synthesized by the authors (see Fig. 4(k) for the illustration), where the thickness of the NPLs was fixed to be about 2.1 nm, corresponding to 5 monolayers [26]. The CD spectra of these NPLs are shown in Fig. 4(l). It can be clearly seen that the CD peaks first redshift and then stabilize as the lateral size continuously increases. The g factors exhibited a similar tendency i.e. first decrease and then saturate as the extension of the lateral size of NPLs (see Fig. 4(m)). The authors [26] attributed the phenomena of CD peak redshift and g factor decrease to the change from the strong confinement region to the weak confinement region as the lateral size increase. In addition, various thickness e.g. 3, 4 and 5 monolayers of chiral ZB CdSe NPLs were also prepared by the authors [26]. The CD peaks continuously red-shift as the layer of the NPLs increase, as illustrated in Fig. 4(n)–(p). The authors inter-

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preted [26] the CD peak redshifts via the quantum confinement effect. In this work, the authors [26] explored the thickness and lateral size dependent CD responses, whereas they only considered the near UV to visible range signals. Actually, as reported by Gao et al. [25], chiral CdSe NPLs also possess rich information in the deep UV range. Similar to the CdS and CdSe, various other II–VI group semiconductor chiral nanostructures, e.g. CdTe [7,27–29], HgS [30–32], ZnO [33], ZnS [34], CdSe/CdS [35,36], ZnSe/ZnS, CdS/ZnS, CdSe/ZnS [37] et al. have also been prepared by researchers using the proper chiral ligands. 2.2 Chiral perovskites Halide perovskite materials have been a hot research topic by virtue of their excellent advantages in various applications especially in photovoltaic devices since 2009 [38–41]. Interestingly, David G. Billing and Andreas Lemmerer [42,43] synthesized chiral inorganic–organic hybrid perovskite in as early as 2003. They utilized R/S-C6 H5 CH(CH3 )NH3 as the chiral molecule to modify the inorganic–organic hybrid perovskite and gave insight into the crystal structure of chiral hybrid perovskite materials [42]. More than 10 years later, Jooho Moon et al. [44] synthesized the chiral organic–inorganic hybrid perovskite films by using (R/S)-(+/-)-␣methylbenzylamine as the chiral function groups and explored their circular dichroism (CD) optical properties in 2017. In the same year, He and co-works [45] demonstrated the chiral inorganic perovskite nano-crystals (CsPb(I/Br)3 ). They employed (1R, 2R) or (1S, 2S) 1, 2-diaminocyclohexane (DACH) molecule to synthesize the chiral nanocrystal by means of ligand exchange approach [45]. Briefly, the mixture of perovskite nanocrystal and chiral molecule were treated by microwave heating at 100 ◦ for half an hour with continuous stir. The high mass ratio (6.8) of perovskite and chiral molecule resulted in a single peak CD spectra with a g factor about 1.5 × 10−3 (measured at 247 nm) [45], as depicted in Fig. 5(a). According to the TEM images and temperature dependent CD measurement, the authors proposed [45] that the chiral signal originated from the chiral structure on the surface via aggregation of ligands. Meanwhile, the authors [45] observed a different CD spectra when the mass ratio decreased to 0.08, as illustrated in Fig. 5(b), and they attributed the chiral signals to surface distortion and chiral molecule-perovskite nanocrystal interaction. In addition to the nanocrystals, the one dimensional organic and inorganic hybrid perovskite (C5 H14 N2 PbCl4 ·H2 O) has also been proposed as a chiral perovskite nanostructure by Luo et al. [46]. They used XRD analysis to determine the space group of their 1D chiral hybrid pervskite to be P21 21 21 , where each unit consisted of one (PbCl4 )2− part, one chiral component ((R)-3-aminopiperidine) and a water molecule. The various Pb-Cl bond lengths resulted in a distorted octahedron [46], as can be seen in Fig. 5(c). [Pb2 Cl8 ]4cluster could be generated by two adjoining PbCl6 octahedra structures through edge sharing (Fig. 5(d)). The following interlink between the [Pb2 Cl8 ]4- clusters leaded to the formation of one dimensional structure. The authors proposed [46] that the inorganic one dimensional structure was encompassed by the organic chiral cations, which formed the so called core-shell quantum wires structure, as depicted in Fig. 5(e). The CD spectrum in Fig. 5(f) verifies the chiral properties of the hybrid perovskite nanowires. In the same year, Xu et al. [47,48] also reported the chiral hybrid perovskite nanowire using the R/S-(+/-)-␤-methylphenethylamine (R/S-MPEA) as chiral compound. Liquid diffusion growth approach has been utilized by the authors [47] to synthesize the chiral perovskite nanowires. Briefly, the precursors R- or S- MPEA+ Brand PbBr2 were dissolved into DMF(N,N-dimethylformamide) and DMSO (dimethylsulfoxide), then the anti-solvent CHCl3 (chloroform) was added into the mixture to form nanowire [47], as can

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Fig. 5. (a) & (b) CD spectra of chiral inorganic perovskite nanocrystals, (a) post treated with overabundance of chiral DACH, the mass ratio is 6.8, (b) post treated with less amount of chiral DACH, the mass ratio is 0.08. The g factor for (a) and (b) are 1.13 × 10−3 and 1.1 × 10−3 respectively. (c) Discorded PbCl6 octahedron with various Pb-Cl bond. (d) One dimensional structure generated by the [Pb2 Cl8 ]4- cluster. (e) The chiral core-shell hybrid perovskite nanowire and (f) the corresponding CD spectrum. (g) SEM image, (h) crystallographic structure, (i) absorption and (j) CD spectral of the (R/S-MPEA)1.5 PbBr3.5 (DMSO)0.5 chiral perovskite nanowires. (k) Structure illustration of quasi-2D (also known as ‘Ruddlesden–Popper’ structural) perovskites. (l) The CD spectral for the R-,S-, and rac perovskites. (a) – (b) Reproduced with permission from ref. [45]. Copyright 2017, American Institute of Physics. (c) – (f) Reproduced with permission from ref. [46]. Copyright 2018, Royal Society of Chemistry. (g) – (j) Reproduced with permission from ref. [47]. Copyright 2018, American Chemical Society. (k) – (l) Reproduced with permission from ref. [49]. Copyright 2018, Nature Publishing Group.

be seen in Fig. 5(g). Single crystal XRD spectral analysis indicated that chiral perovskite nanowires have the chemical formula of (RMPEA)1.5 PbBr3.5 (DMSO)0.5 and (S-MPEA)1.5 PbBr3.5 (DMSO)0.5 . And the DMSO was involved in the chiral nanowire structure synthesis by means of axial coordination with the lead ions which are located at the centers of the octahedron, as illustrated in Fig. 5(h). The existence of chiral ligands of R/S-MPEA results in the transformation of perovskite from centrosymmetric structure to noncentrosymmetric triclinic P1 space group structure, which implies the potential nonlinear applications [47]. CD spectra measurement indicated that both R- and S- (MPEA)1.5 PbBr3.5 (DMSO)0.5 perovskite nanowires manifested two CD peaks located at 325 nm and 405 nm respectively and the signs are opposite [47] (Fig. 5(i) & (j)). The Cotton effect, which appeared at 390 nm, originated from the absorption band of chiral perovskite. The authors [47] attributed the low energy CD signals (>420 nm) to circular differential scattering (CDS). To achieve both the high PL quantum yield and chirality, Edward H. Sargent’s group [49] firstly developed the hybrid organic–inorganic quasi-2D (also known as ‘Ruddlesden–Popper’ structure) perovskites using R/S-methybenzylamine as the chiral ligands, as illustrated in Fig. 5(k). According to the structure of reduced-dimensional chiral perovskite, the number of the inorganic layers < n> displayed an inversed relationship to the chirality, namely, the pure 2D perovskite (n=<1 > ) have the largest chiral response. However, the pure 2D perovskite is non-emissive, thus the authors [49] focused on the < n> = 2 chiral perovskite to achieve the goals. Both the R-MBABr and S- MBABr perovskite displayed obvious CD signals, as shown in Fig. 5(l), which indicated the chiral transfer from the chiral molecule to perovskite. The authors [49] also synthesized the racemic perovskites using racemic methybenzylamine as the surface modifier, and no CD signal could be observed as shown in Fig. 5(l). According to 2D

grazing-incidence wide-angle X-ray scattering (2D-GIWAXS) spectral data, they [49] proposed that all the chiral perovskite samples contained both < n> = 1 and < n> larger than 1 perovskite components due to the existence of the multi-scattering patterns. The UV–vis spectral results [49] also supported this conclusion as both the < n> = 1 and < n»1 absorption peaks have been observed. The PL quantum yield reached up to 90 % when excited by 400 nm light wave and showed excitation dependent behaviors. Transient absorption spectroscopy measurement revealed [49] a strong bleaching at about 511 nm after 1 ps delay. PLE spectra showed a strong dip at 380 nm which correspond to the < n> = 1 perovskite absorption. Thus, the authors [49] proposed the energy funneling mechanism to interpret the high PL quantum yield. The funneling efficiency was higher for larger < n> ( greater than 1), while was not efficient for pure 2D perovskite( = 1), which leaded to the low PL quantum yield for 2D perovskite. Beside the chiral hybrid organic–inorganic quasi-2D structure [50], the chiral organic–inorganic perovskite nanoplatelets [51] have also been prepared by adopting R- or S-phenylethylammonium molecule as the capping agent. And recently, Xiong et al. [52] even synthesized chiral 2D organic-inorganic perovskite with ferroelectric properties. 2.3 Chiral transition metal dichalcogenides Transition metal dichalcogenides 2D materials have drawn much interest in recent years due to their distinctive merits and plentiful applications in the fields of optoelectronics, valleytronics etc. In 2008, Finn Purcell-Milton and co-workers [53] also endowed the 2D MoS2 nanostructure with chiral properties by means of liquid exfoliation. Briefly, the bulk MoS2 and isopropyl alcohol mixture was sonicated for 15 min followed by centrifuging for 15 min at 4000 rpm. The precipitate was collected and re-dispersed into

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Fig. 6. (a)-(d) AFM images for achiral ((a), (b)) and chiral (penicillamine based) ((c), (d)) MoS2 nanoflakes. (e) - (h) Theoretical and experimental CD spectra for chiral MoS2 nanoflakes. (i) Illustration of the chiral MoS2 and WS2 chiral nanodots synthesis. (j) CD spectra of cysteine and penicillamine and their corresponding chiral MoS2 and WS2 nanodots. (a) – (h) Reproduced with permission from ref. [53]. Copyright 2018, American Chemical Society. (i) – (j) Reproduced with permission from ref. [54]. Copyright 2018, American Chemical Society.

degassed water for another 5 h sonication. After centrifugation (4000 rpm, 10 min), the supernatant was gathered and re-dispersed in degassed water to produce the exfoliated MoS2 nanoflakes. The chiral MoS2 can be obtained [53] by sonification of the mixture of chiral molecule (L-/D- penicillamine or L/D cysteine) and the exfoliated MoS2 . The authors [53] employed the AFM measurement to confirm the layers of the synthesized MoS2 , which showed a thickness about 3−4 nm for chiral MoS2 (penicillamine chiral molecule based) and 5−9 nm for non-chiral MoS2 , corresponding to 4–6 monolayers and 7–14 monolayers of MoS2 , respectively [53], as shown in Fig. 6(a)–(d). Thus, the authors [53] claimed that the existence of the chiral molecule could accelerate the exfoliation of the bulk MoS2 . XPS and FTIR spectral analysis [53] also verified the successful attachment of the chiral molecule on the surface of the MoS2 nanoflakes. Both the (L/D) penicillamine and (L/D) cysteine based chiral MoS2 showed strong CD signal from UV to NIR and displayed opposite signs for different enantisomers (Fig. 6(g) & (h)), while no obvious CD signal was observed for non-chiral molecule treated MoS2 . What’s more, based on the Rosenfeld’s approximation and including the effect of the chiral distortion of the MoS2 nanoflakes on the asymmetric absorption, the authors [53] proposed an accurate model to interpret the strong CD signal. The chiral strength (distortion) was described by the rotatory strength and the distorted chiral nanoflakes were converted to achiral nanoplatelets under coordinate transformation [53]. The theoretical CD spectral data matched very well with the experimental results for both penicillamine and cysteine based chiral MoS2 nanoflakes [53], as displayed in Fig. 6(e)–(h).

Later on in the same year, Wei et al. [54] reported the chiral transition-metal dichalcogenides (MoS2 and WS2 ) nanodots using the cysteine and penicillamine as the chiral ligands. They first employed the sonication approach to synthesize pure TMD nanodots with the assistance of NaOH and N-Methyl-2-pyrrolidone (NMP). The following strong stir of the mixture of pure TMD nanodots and chiral molecules (L/D-cysteine and L/D-penicillamine) resulted in the formation of chiral TMD nanomaterials [54], as illustrated in Fig. 6(i). TEM images of these chiral nanomaterials (L/Dcysteine/penicillamine-MoS2 /WS2 ) indicated that all of them manifested a mono dispersed distribution with an average diameter of about 2 nm. In addition, the authors [54] also carried out the FTIR measurement, which showed that the weak S H (2500 cm−1 ) bond was not formed during the chiral TMD nanomaterial synthesis. Consequently, they proposed [54] that the chiral component were linked to nanomaterial by means of covalent binding. The CD spectra [54] of the chiral TMD nanodots are shown in Fig. 6(j). It can be clearly seen that the both l- and d- cysteine based MoS2 and WS2 nanomaterials displayed two strong CD bands located at 212 nm and 265 nm. Not surprisingly, the opposite enantiomers resulted in inverse CD signals which has been commonly observed in chiral systems. Penicillamine capped chiral MoS2 and WS2 nanodots showed 3 CD bands [54]. The position of the first CD band coincided with the chiral penicillamine molecule. Meanwhile, apart from the chiral molecule band, two new CD bands located at 235 nm and 264 nm also appeared, as can be seen in Fig. 6(j). And the authors [54] claimed that the CD signals originated from different excitonic transitions. Furthermore, Guo et al. [55] also claimed that they syn-

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thesized the chiral black phosphorus (BP) and tin sulfide (SnS2 ) quantum dots. They declared that the CD signal originated from the “crowded” edge, which leaded to the helical buckling [55]. 2.4 Chiral carbon dots Carbon dots have drawn much of scientific endeavor since it was developed by Sun et al. [56] and Xu et al. [57]. Countless applications based on carbon dots have been demonstrated in the last 15 years [58–60]. However, the chiral carbon dots were reported ´ et al. [61] first only very recently. In 2015, M. Vazquez-Nakagawa designed the chiral carbon dots using (R)-2-phenyl-1-propanol and (S)-2-phenyl-1-propanol as the chiral ligands. The carboxylic acids surface function groups were transferred to acid chlorides with the assistance of thionyl chloride under 120 ◦ C heating. The in-situ reaction between the chiral molecule and the acid chlorides leaded to the formation of chiral carbon dots [61], as shown in Fig. 7(a). As the authors [61] proposed, the chiral functional groups (e.g. enantiomerically esters) on the surface of chiral carbon dots did not show any absorption signals when the wavelength over 300 nm. Thus they introduced pyrene [61] into the chiral carbon dots system to form chiral carbon dots/pyrene aggregation, as clarified by the TEM and AFM measurements. Moreover, the aggregation leaded to the chiral transformation from carbon dots to pyrene. As a result, the chiral carbon dots pyrene aggregates displayed preeminent CD signals [61] as shown in Fig. 7(b), whereas the pure pyrene (in NMP) exhibited no CD response. At the same time, Nicholas A. Kotov et al. [62] also reported the chiral carbon dots. l-/D- cysteine have been adopted to prepare chiral carbon dots by utilizing carbodiimide/N-hydroxysuccinimide (EDC/NHS) cross-linking approach. TEM measurement [62] revealed that the size of the chiral carbon dots was in the range of 2−7 nm. The appearance of two absorption peaks in the FTIR spectra [62] located at 2390 cm−1 and 930 cm−1 (corresponding to S H and C N bonds respectively) indicated that both the ligand attachment (S H bond) and covalent bonding (C N bond) occurred during the synthesis of chiral carbon dots. Compared to pristine carbon dots, both the positions and relative intensities of the Raman peaks of chiral carbon dots were similar, thus the authors [62] declared that the central structure of carbon dots had been reserved. The CD spectra and the asymmetry factor (g factor) are shown in Fig. 7(c) & (d). There are 2 peaks in the CD spectra. One is located at about 210−220 nm region, which coincided with the CD signal of the cysteine (around 209 nm), and another CD band was also observed for chiral carbon dots in the range of 250−260 nm. Interestingly, the authors found [62] that the sign of the new CD band was opposite to chiral molecules used for the chiral conjugation. The g factor of the new chiral band (250 nm–260 nm) was in the order of 10-4 , which is comparable to some other chiral materials. To uncover the origination of the new CD band, the authors [62] built up the following model: the chiral ligands (l- & d- cysteine) were linked to the edge of the carbon dots via amide conjugation. They determined the number of chiral ligands by considering the sulfur-carbon ratio obtained from XPS spectral data. DFT semiempirical ZINDO algorithm based calculation [62] ruled out the possibility of both chiral ligand induced whole carbon dot symmetry perturbation (chiral induction) and chiral ligand induced partial symmetry perturbation of carbon dots (local chiral induction). According to the DFT MMFF algorithm calculation, the authors suggested [62] that the linkage of chiral molecules to the perimeter of carbon dots will enhance the bulking deformation, as a result, the carbon dots exhibited a strong twist as illustrated in Fig. 7(e) & (f). Carbon dots conjugated with l- and d- cysteine experienced right and left hand twist respectively, which contributed to the opposite sign of CD signal for the original chiral molecules and chiral carbon dots in the 250−260 nm chiral band. The authors [62] referred the increased bulking to the

noncovalent intermolecular interplays of the chiral-chiral ligands and chiral ligands-edge functional groups. The proposed structures in this work are reasonable and can explain the opposite signs between the chiral molecules and chiral carbon dots in the new CD band (250–260 nm). However, we want to emphasize that carbon dots is a very complicated system, that is, one cannot expect that this result is a universal conclusion and extend it to all kinds of carbon dots. As we can see from Fig. 7(g), the chiral carbon dots syn– c´ et al. [63] via hydrothermal microwave thesized by Luka Ðordevi assisted approach displayed a totally different CD spectra. Until now, by choosing different starting materials, chiral molecules and synthesis approaches, researcher have reported various kinds of chiral carbon dots [64–74]. 2.5 Chiral magnetic nanoparticle Chiral magnetic nanoparticle which combine the magnetic nanoparticle and chiral component have also been proposed by researchers [75]. At the initial stage, the purpose of chiral magnetic nanoparticles was to develop a recoverable, reusable and easy way to separate chiral catalysts. Lin et al. [76] synthesized two kinds of magnetite nanoparticles by means of thermal decomposition and co-precipitation approach. Chiral Ruthenium(II) complex [Ru(BINAP-PO3 H2 )(DPEN)Cl2 ] (1) was linked to the surface of Fe3 O4 nanoparticle by means of ultrasonic treatment for 1 h. The structure and linkage are illustrated in Fig. 8(a). Magnetization curves analysis demonstrated that the chiral Ruthenium(II) complex modified Fe3 O4 nanoparticles exhibited superparamagnetic property [76], which might benefit for the recovery of the chiral catalyst. In addition to the pure chiral magnetic nanoparticles, the chiral bimetallic magnetic nanoparticles were prepared by Kalluri V. S. Ranganath and co-workers [77] to achieve the enantioselective ␣-arylation reaction. The bimetallic nanoparticle consisted of Pd and Fe3 O4 nanoparticle, as illustrated in Fig. 8(b). Briefly, the Fe3 O4 /Pd were synthesized via the wet impregnation approach, and chiral enantiomerically pure N-Heterocyclic carbenes (NHCs) were employed to stabilize the Fe3 O4 /Pd surface to endow the bimetallic nanoparticles with chiral property (see Fig. 8(b)). Five kinds of chiral NHCs L1–L5 have been utilized by the authors [77] and the corresponding structure are shown in Fig. 8(c). XPS spectra of Fe3 O4 /Pd/L1 displayed clear features at 400 and 285 eV [77], corresponding to the N1s and C1s respectively and implying the successful surface capping by chiral NHC. The different IR spectra of chiral L1, Fe3 O4 /Pd/L1 nano structure further confirmed the surface modification. By using the EDX element analysis, the authors determined [77] that the Fe3 O4 /Pd/L1 nano particles consisted of Fe, Pd, O and C with a Pd content about 0.9 wt%. These observations were also consistent with the inductively coupled plasma optical emission spectroscopy (ICP-OES) measurement [77] which suggested a Pd content of about 0.92 wt%. The size of the chiral Fe3 O4 /Pd/L1 nanoparticles was determined by the TEM images to be about 25−35 nm In 2009, to build up the Pd based enantioselective catalytic platform, Kohsuke Mori et al. [78] prepared the Fex Oy -rich core and Pd-rich shell FePd chiral magnetic nanoparticles by using ligand exchange method in which the (S/R)-2,2 -bis(diphenylphosphino)1,1 -binaphthene ((S/R)-BINAP) were utilized as the chiral stabilizer. Iron carbonyl (Fe(CO)5 ) were utilized as the precursor to produce the Fex Oy core at 250 ◦ under efficient stirring. Pd(acac)2 , oleic acid and oleylamine were added to trigger the formation of Pd shell. The mixture were heated at 310 ◦ and stirred for another 1 h. The following precipitation with ethanol resulted in the synthesis of the Fex Oy core and Pd shell FePd nanoparticles [78]. The overall composition of the FePd nanoparticle was determined by the elemental analysis to be Fe60 Pd40 . The chiral FePd nanoparticles were prepared [78] through the ligand exchange between the

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Fig. 7. (a) Chiral carbon dots obtained from esterification reaction with enantiomerically (R) or (S)-2-phenyl-1-propanol. (b) Top: CD spectral of R-chiral carbon dots- pyrene (blue) and S-chiral carbon dots- pyrene (green) aggregations, bottom: Absorption spectra of pure pyrene dissolved in NMP. (c) CD spectra and (d) g factor for l-cysteine and d-cysteine conjugated carbon dots. (e) & (f) illustration of the twisted chiral carbon dots. The handedness of the carbon dots is opposite to the chiral molecules, which contribute to the opposite sign of the CD spectral. (g) CD spectra of chiral carbon dots synthesized by Maurizio Prato’s group. (a) – (b) Reproduced with permission from ref. [61]. Copyright 2016, Royal Society of Chemistry. (c) – (f) Reproduced with permission from ref. [62]. Copyright 2016, American Chemical Society. (g) Reproduced with permission from ref. [63]. Copyright 2018, Nature Publishing Group.

achiral FePd nanoparticle toluene suspension and chiral BINAP ligands at 80 ◦ with vigorous stir. Both the disappearance of methylene (C H) vibration (3000−3100 cm−1 , corresponding to the oleic acid and oleylamine) and the appearance of aromatic ring  (C H) vibration (2800−3000 cm−1 , corresponding to the chiral ligands) indicated the accomplishment of the ligand exchange. TEM images [78] showed that the average diameter of the FePd nanoparticle is about 5.6 nm. X-ray absorption fine structure (EXAFS) analysis proved the Fe core and Pd shell structure. The CD spectra of chiral FePd nanoparticles shown in Fig. 8(d) implied that both the l- and dstabilized FePd nanoparticle displayed clear CD signals in the range of 300−550 nm with opposite signs. The authors [78] declared that the chiral FePd nanoparticle is a single magnetic domain, thus exhibiting the superparamagnetic performance, that is, the FePd nanoparticles became non-magnetic once the external magnetic field disappeared. Superconducting quantum interface device (SQUID) magnetometer was used by the authors [78] to explore the magnetic behaviors of chiral FePd nanoparticle. As can be seen from the isothermal magnetization graph [78] (see Fig. 8(e)), at 200 K and 300 K the magnetization of the FePd nanoparticle continuously increase as the external magnetic field increase. The magnetization disappear when the magnetic field become zero and hysteresis cannot be observed, which implied the super paramagnetic nature of the FePd nanoparticles at 200 K and 300k. However, the behavior became totally different when the temperature decreased to 5 K [78]. A clear symmetric hysteresis loop with nonzero remanence (Mr =6.1 emu/g) and coercivity (Hc = 190 Oe) have been observed.

As the authors asserted [78], the appearance of the hysteresis loop manifested the transformation from the superparamagnetic to ferromagnetic nature of chiral FePd nanoparticle at 5 K. Until now, researchers have already developed various methods to synthesize chiral magnetic nanoparticles [79–82]. 2.6 Chiral metal nanocrystal As an important kind of chiral inorganic nanostructures, metal nanocrystals have found numerous applications in the fields of biosensing, bio-analysis etc. As early as in 1998, Robert L. Whetten et al. [4] first synthesized gold nanocrystal (giant clusters) with chirality by tripeptide glutathione (GSH, N--glutamyl-cysteinyl-glycine) as the surface modifier. Under redox condition, the interaction between the Au nanocluster and GSH will lead to the stoichiometric process, which is determined by the following reaction: 3GSH + Au3+ ↔ Au0 +

3 GSSG + 3H + 2

Au0 means that the gold nanocluster is capped by GS or GSSG adsorbate ligands. More specifically, the authors suggested [4] two steps for the aqueous reaction. The nonmetallic polymeric pAu(I)SG were generated first, followed by the decomposition of polymeric p-Au(I)SG. The decomposition process was triggered by the excess strong reducing components. To suppress the unwanted decomposition of p-Au(I)SG, H2 O and methanol mixture were employed by the authors [4] to act as the protective medium.

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Fig. 8. (a) Illustration of Fe3 O4 nanoparticle capped by chiral Ru complexes, which result in the chiral Fe3 O4 nanoparticle. (b) Illustration of chiral Fe3 O4 /Pd/L1 nanoparticle preparation. (c) Structures of chiral NHCs L1-L5. (d) CD spectra of the Fex Oy -rich core and Pd-rich shell FePd magnetic nanoparticle capped by (a) (R)-BINAP and (b) (S)-BINAP chiral ligands. (e) Magnetic field-dependent magnetization graph for chiral FePd NPs capped by (S)-BINAP chiral ligand measured at 300 K, 200 K and 5 K. (a) Reproduced with permission from ref. [76]. Copyright 2005, American Chemical Society. (b) – (c) Reproduced with permission from ref. [77]. Copyright 2010, Wiley-VCH. (d) – (e) Reproduced with permission from ref. [78]. Copyright 2009, Royal Society of Chemistry.

The net effect of these reactions was that the insoluble polymeric p-AuSG were converted into highly water-soluble Au:SG nanoclusters. The purification of the Au:SG nanoclusters was finished by precipitation from the methanol, where the small ions and molecules could be removed, as confirmed by the 1 H and 13 C NMR spectroscopy [4]. The purified Au:SG were separated into different bands with different nanocluster size by means of electrophoretic approach. The third highest electrophoretic mobility Au:SG nanoclusters constituted a peculiar bright orange electrophoretic band and the authors [4] proposed that this was the most rich Au:SG nanoclusters, with a product yield up to about 40 wt%. Matrix-assisted laser desorption (MALDI) and electrospray ionization (ESI) mass spectrometry measurement [4] indicated that the totally molecular weight of the Au:SG nanoclusters was about 10.4 kDa with a 5.6 kDa inorganic Au core, thus implying the composition of the Au nanocluster was Au28 (SG)16 . Powder XRD and larger angle scattering investigation showed the bcc lattice structure of the core gold atoms. The absorption spectra of [4] Au28 (SG)16 (see Fig. 9(a) lower panel) also indicated the metallic Au(0) composition and a many electron system. In addition, the authors [4] have also observed the clear CD signals from the Au nanoclusters, as can be seen in Fig. 9(a), upper panel. As both the tripeptide glutathione molecule and AuSG polymer do not have effective absorption signal when the wavelength is shorter than 300 nm, the authors [4] claimed that the CD signal of the short wavelength range (<300 nm) originated from the gold nanoparticles. This is the first observation of the chirality from Au nanocluster. Later, Robert L. Whetten et al. [5] did a comprehensive investigation of gold giant clusters of all the electrophoretic separated bands (band 1 to band 5, where 1 means the most mobile gold nanocluster). All the Au:SG (giant gold: glutathione) nanoclusters possess

a metallic Au0 core as verified by the 0.29 nm Au-Au bond length. The diameters of all the Au cores were less than 1.5 nm and the linkage between gold core and GSH was finished through cysteine sulfur functionality [5] by means of adsorption interaction. The relative mobility (compared to band 1), product yield, core mass and absorption onsets (optical gaps) of the different bands of Au:SG nanoclusters are shown in Fig. 9(b) [5]. As the 1 and 1 bands have similar absorption gap (Fig. 2 in the original article [5]) and mobility of the 1 band is just slightly lower than the 1 band, the authors [5] suggested that compared to 1 band, 1 band only possessed one or several absorption ligands more. And these situations are also suitable for 2 and 2 bands. Thus the authors [5] mainly focused on the 1 and 2 band. All the compounds 1–5 displayed clear CD signals and the normalized CD spectra of compounds 1–3 are presented in Fig. 9(c). It can be clearly seen that CD spectra started from NIR and extended to UV region. The authors [5] also emphasized that the CD signal disappeared for the crude/unseparated mixture as the oppositely signed CD signal will cancel to each other. The authors evaluated [5] that the diameters of the gold nanocluster 1–3 resided in the range of 1.5–2 nm, namely 12–20 SG ligands on the surface of compound 1–3 nanoclusters. As the surface glutathione could only contribute to the deep UV region absorption, the authors suggested [5] that the NIR and visible range absorption originated from the interband transition: from the filled 5d (gold core) to 6sp band. The effect of the glutathione (GSH, GS-, and GSSG) on the chiral properties have been discussed by the authors [5] as all of them own some ␣-carbon chiral centers. GSH only exhibited weak CD signals above 5 eV, while the disulfide ␲(S S) to ␴*(S S) transitions account for weak CD signal of GSSG in the range of 4.2–5.5 eV and strong cotton effect at 6.14 eV. In addition, these transitions are closely related to the

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Fig. 9. (a) CD (upper panel) and absorption spectra (lower panel) of the Au28 (SG)16 nanoclusters. (b) Parameters of the Au:SG nanocluster compounds. (c) Normalized CD spectra of (1) compound 1, (2) compound 2, (3) compound 3. (d) CD spectra and the (e) corresponding g-factor for each numbered (1–3, separated by electrophoresis) chiral gold nanocluster compound. (a) Reproduced with permission from ref. [4]. Copyright 1998, American Chemical Society. (b) – (c) Reproduced with permission from ref. [5]. Copyright 2000, American Chemical Society. (d) – (e) Reproduced with permission from ref. [86]. Copyright 2005, American Chemical Society.

C-S-S-C dihedral angle. The authors asserted [5] that the chiral glutathione could only explain the UV/deep UV CD response, but could not explain the strong CD signals in the visible and NIR range. Even though the authors could not offer the precise assignment of each CD band, they proposed the possible chirality originations for the Au giant clusters [5]: 1) inherent chiral cores: the Au core become chiral with some helical structure due to the emergence of the chiral symmetry. 2) chiral adsorption pattern: the Au core is achiral, but the adsorption of the GSH on the surface lead to some chiral patterns e.g. a helical tiling. 3) chiral adsorbates only: both the Au core and surface adsorptions are achiral, the only chiral centers are the GSH chiral ligands. The chiral molecule interact/hybridize with the electronic structure of the Au center, which resulting in the CD responds. This chirality is also called induced chirality. Later both the DFT calculations [83,84] and dipole approximation approaches [85] have been proposed by different researchers to uncover the chirality of the thiol-passivated gold nanoclusters. To understand the chirality mechanism of the gold nanoclusters, Yao et al. [86] first prepared the well-defined chiral Au nanocluster enantiomers by adopting the l-, d- and racemic penicillamine as the surface modifier (represented by Au-d-Pen, Au-l-Pen and Au-rac-Pen respectively). FTIR spectra analysis indicated that the linkage between the chiral penicillamine molecule and Au nanoclusters was accomplished via the S atom in the form of SH bond [86]. In addition, the carboxylate (COO− ) and primary amino (NH2 ) function groups also emerged on the surface of the Au nanocluster after the surface modification. After electrophoresis separation [86], all the Au nanocluster enantiomers (l, d- and rac) displayed three clear bands termed as band 1–3, where the band 1 means the most mobile compound. The position of each electrophoresis band were similar for all Au-d-Pen, Au-l-Pen, and Au-rac-Pen enantiomers, indicating the similar size of the three kinds of clusters. The size of the Au nanoclusters in the separated 1D , 2D , and 3D (D means the d-pen capped Au cluster) bands were determined by the small-angle X-ray scattering (SAXS) to be 0.57 nm, 1.18 nm

and 1.75 nm respectively and the authors also declared [86] that the l- and rac-pen capped Au nanocluster have the similar size distribution. Assuming the atom density of the cluster is about 60 atoms/nm3 [87], the authors estimated [86] the numbers of atoms of the gold nanocluster in band 1–3 to be ∼6, ∼50 and ∼150 atoms respectively. The CD spectra of the Au nanoclusters are shown in Fig. 9(d). For all separated bands [86] (1–3), the d- and l- Pen stabilized Au nanoclusters exhibited clear CD signal with mirror relationship from the NIR to deep UV region, implying the successful formation of the chiral nanocluster. On the contrary, the racemic Pen (equal amount of L and D pen) resulted in the achiral nanoclusters, as illustrated in Fig. 9(d). The authors proposed [86] that the equal adsorption capability [88] of the L and D pen molecule on the surface of the Au nanocluster accounted for the inactive chirality of the rac Au cluster. The anisotropy factor (g-factor) [86] of the chiral Au nanoclusters were ±3 × 10−4 , ±3 × 10−4 and ±3 × 10−4 for compound/band 1–3 respectively (as illustrated in Fig. 9(e)). They claimed that the Au nanocluster was similar to the molecular complex, and employed the optically active chromophores model [86] to interpret the origination of the CD signals. The Pen ligands have the chiral properties due to the existence of chiral centers, which could be transferred to the Au core via chemical bond linkage, namely vicinal effect [86]. The smaller size of the Au nanocluster, the greater effect to the core Au electronic structure through vicinal effect, thus resulting in a larger g factor, as observed by the authors [86]. Later, other chiral molecules such as l- & d- cysteine have also been adopted by researchers [89–91] to stabilize the surface of Au nanoclusters. Chen et al. [92] even suggested to synthesize chiral Au nanocluster by means of polymerase chain reaction. In addition to the chiral gold nanoclusters, the chiral Au nanorods were reported in 2014 by Tang et al. [93] using l and d- cysteine as the chiral molecules. Similar to the chiral Au nanoclusters, the chiral silver (Ag) nanoclusters [94–102] as well as the copper nanostructures [103,104] have been proposed by different researchers.

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2.7 Intrinsic chirality Although most of the chiral nanomaterials are stabilized by some chiral ligands/molecules, principally pure nanostructures can also possess chiral character as the existence of the chiral defects originating from the reduced symmetry [3]. However, the average effect of the random distributed chiral defects result in zero net chiral performance. Nonetheless, in year 2015, Maria V. Mukhina and co-workers [2] have done a brilliant work to achieving the intrinsic chirality from the achiral nanocrystals. First, the CdSe/ZnS based quantum dots and quantum rods with their surfaces linked by achiral trioctylphosphine oxide (TOPO) ligands were synthesized via hot injection method. As discussed before, there was no net CD signal as d- and l- chiral defects are equal, that is racemic. Meanwhile, the chiral molecules l- and d- cysteine were employed by the authors [2] to separate the intrinsic nanocrystals via phase transfer method. Briefly, the CdS/ZnS QDs and QRs were dispersed into the chloroform, then the l- and d- cysteine methanol solution and DI water were added into the chloroform nanostructure mixture to achieve the phase transfer. The chloroform phase and water were separated after the phase transfer was finished. The nanocrystals in water phase kept abundant chiral ligands (L-/Dcysteine) and the chloroform phase based nanocrystals which were still linked by the achiral TOPO molecules. However both of the chloroform and water phase nanocrystals exhibited non-zero CD responds [2], see Fig. 10(a)–(d). FTIR measurement [2] showed that the cysteine molecule also existed in chloroform phase (the mixture of methanol and chloroform). To eliminate the chiral ligand induced CD response, the authors carried out [2] the CD measurement of d-cysteine and d-CdS/ZnS chiral QD in both water and chloroform phase. The results indicated that both the d-cysteine and d-CdS/ZnS chiral QD display dextrorotatory performance, so the authors concluded [2] that the CD signal in chloroform phase originated from the nanocrystal itself as the chiral ligand-water phase based and chiral ligand-chloroform phase based nanocrystal interaction could lead to opposite sign of CD spectral data. Finally, the authors assigned [2] the chirality of achiral TOPO covered nanocrystals in chloroform phase to intrinsic chirality, and the CD response in aqueous phase induced by chiral cysteine are defined as induced chirality. The CD spectra of both intrinsic and induced chiral nanocrystals are shown in Fig. 10(e). It can be seen that the CD signals of induced chirality are much larger than intrinsic chirality (Fig. 10(a)–(c)). The authors asserted [2] that this was caused by adsorption induced accumulation of chiral ligands on the surface of nanocrystals. The absence of CD signal enhancement in Fig. 10(d) was attributed to the d-cysteine based nanocrystal aggregation in water phase. To verify whether the chiral behavior of the QDs in water phase still maintain after the chiral ligands are removed, the authors proposed [2] a reverse phase transfer method in which the chiral ligands (cysteine) were replaced with achiral dodecanthiol (DDT). As can be seen from Fig. 10(e), achiral DDT capped QDs still show clear CD signal, which further confirm the intrinsic origination of the chiral activity. The authors speculated [2] that some surface chiral defects e.g. screw and edge dislocations were formed during the CdSe /ZnS quantum dots and quantum rods synthesis, as observed by the TEM images [2], see Fig. 10(f) & (g). While the equal contribution of the l- and d- chiral dislocations lead to zero net chiral response, namely racemic. The following phase separation method isolateded the land d- chiral dislocation components (Fig. 10(h) & (i)), resulting in the appearance CD signals. DFT calculations [2] carried out by the authors also suggested that the binding energy difference between cysteine-right hand dislocation of QDs and cysteine-left hand dislocation of QDs is as large as 0.2 eV [2], which is much greater than 0.026 eV (room temperature thermal energy), implying the chiral ligand recognition are possible and reasonable.

Later, based on theoretical calculation, Anvar S. Baimuratov et al. [6] proposed that the intrinsic chiral dislocations are also possible for semiconductor nanowires (Fig. 10(j)). However, just like CdSe /ZnS quantum dots, the experimental observation of intrinsic chirality requires the separation method for the l- and d- nanowires, which are still ongoing. 3 Applications of chiral inorganic nanostructures Currently, chiral inorganic nanostructures have shown a huge potential in various fields. In this section, instead of giving a complete list, we review and discuss some typical and promising applications. 3.1 Circular polarized light emission Direct generation of circular polarized light (CPL) emission with a high degree of polarization will remove the bulky elements (polarizers and quarter wave plates) in traditional optical applications. This will greatly reduce the spatial constraint and energy consumption in important application areas like 3D display and backlight source for liquid crystal display, which is crucial for portable devices. Another potential application of circular polarized light source includes protein detection in biomedicine – an industry with a giant market. Thus circular polarized emission from chiral inorganic nanomaterials themselves have been proposed as an alternative solution. The first circular polarized light activated quantum dots were prepared by Masanobu Naito et al. [15]. Horse spleen ferritin, one of rhombic dodecahedral proteins, with hollow structure have been utilized as the chiral template to synthesize chiral CdS quantum dots. The authors suggested [15] that the whole (both core and surface) chiral CdS quantum dots owned chiral structure due to the presence of the chiral template, which contributed to the circular polarized emission. When excited by 325 nm laser, both apoferritin and CdS quantum dots (CdS@ferritin) displayed strong PL, as illustrated in Fig. 11(a). However, only CdS quantum dots shown obvious CPL activity (Fig. 11(b)). The authors [15] employed the Kuhn’s anisotropy factor glum defined as 2(IL − IR )/(IL + IR ) to measure the CPL activity. The results shown in Fig. 11(c) indicated that chiral CdS quantum dots have two CPL bands located at 498 nm and 780 nm, and their corresponding glum were 4.4 × 10−3 and 3.5 × 10-4 respectively. They attributed [15] the emission band at ∼ 498 nm to band edge transition and the emission band at ∼780 nm to the recombination of the surface trapping states. HRTEM images (Fig. 11(d)–(g)) demonstrated that each hollow ferritin contains several CdS quantum dots and only about 7 % hollow ferritins enclose one CdS QD, as shown in Fig. 11(h). According to these observations, the authors claimed [15] that the direct transition CPL band (498 nm) came from the band edge transition of single crystal, that is, only one CdS QD contained hollow ferritin. The 780 nm CPL band originated from the surface states transition of polycrystal, where one hollow ferritin included multi CdS QDs. In addition to the ferritin protein based chiral CdS QDs, later the CPL activated chiral CdSe QDs have also been reported by Milan Balaz et al. [105]. The authors synthesized the chiral CdSe quantum dots by means of ligand exchange approach, where the l- and dcysteine have been used as the stabilizer. As can be seen in Fig. 11(i), the chiral CdSe QDs exhibited obvious CPL signals. The sign of the CPL signal were opposite for l- and d- quantum dots and the anisotropy factor glum at the emission peaks were +0.003 and -0.004 for l- and d-cysteine stabilized CdSe QDs respectively [105]. He et al. [35] also reported the CPL active CdSe dots/CdS rods (CdSe/CdS DRs) core-shell nanostructure. They prepared the CdSe/CdS DRs via ligand exchange approach using the l- and d- cysteine as the sur-

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Fig. 10. (a)-(d) CD and absorption spectral data of l-and d-cysteine based CdS/ZnS QDs and QRs in chloroform and water phases. (e) CD spectra of TOPO, d-cysteine and achiral dodecanethiol (DDT) capped CdSe/ZnS QDs. (f) & (g) TEM images of the CdSe/ZnS QDs and their chiral screw dislocations. Theoretical models for the right (h) and left (i) screw dislocations. (j) Theoretically calculated CD spectra of ZnS nanowire. (a) – (i) Reproduced with permission from ref. [2]. Copyright 2015, American Chemical Society. (j) Reproduced with permission from ref. [6]. Copyright 2015, American Chemical Society.

face stabilizer. They used the absorption ratio of CdS shell to CdSe core (ARSC) to analyze the rods aspect ratio (geometry) dependent CPL properties. As can be clearly seen from Fig. 11(j), all the chiral CdSe/CdS DRs showed obvious CPL signal. Moreover the glum values (Fig. 11(k)) [35] monotonically decreased with the increase of ARSC, exhibiting a trend similar to the CD response. However, the authors [35] claimed that the CPL of pure CdSe quantum dots was inactive and attributed the inactive CPL to the quenching of the PL during the ligand exchange reaction. Furthermore, the glum of chiral CdSe quantum dots [105] achieved by Milan Balaz et al. was in the order of 10−3 , which is larger than the glum of chiral CdSe/CdS DRs reported by He et al. [35] (in the order of 10-4 ). Aside from the downconverted CPL, Liu et al. [106] also realized the up-converted CPL via assembling the achiral up-conversion nanoparticles with chiral nanotube. And this chiral assembly approach was also utilized to generate CPL from perovskite nanocrystal [107]. Sargent et al. [49] employed the magnetic field to control the circular polarized emission of chiral quasi-2D chiral perovskite (RDCP, reduced-dimensional chiral perovskites). The degree of circular polarization of all the R-, S- and rac- perovskites presented a linear relationship to the magnetic field, as can be seen in Fig. 11(l)–(n). R-, S- chiral perovskites possess a degree of circular polarization value of 3 % even if the magnetic field is zero, which is totally different from the rac-perovskites. The authors [49] attributed the net degree of circular polarization to the unequal spin-polarized emission rate (Fig. 11(o) & (p)). The spin-polarized emission rate were the same for rac-perovskites, which resulted in zero polarized emission when the magnetic field was absent. They [49] interpreted the linear relation between the degree of circular polarization and

magnetic field based on Zeeman effect. Zeeman effect breaks the time symmetry of the chiral material and leads to the spin subband splitting (see Fig. 11(q)–(s)). In the case of R-perovskite, spin down to spin up flip become thermodynamically favourable in the conduction band (the condition is opposite for the valance band) as the result of spin degeneracy caused by the external magnetic field. As a result, both the Zeeman effect and chiral effect enhanced the circular polarized emission [49], that is, the value of degree of circular polarized emission increased. In the condition of negative magnetic field (opposite direction), the Zeeman effect and chiral effect have opposite response to the degree of circular polarized emission. The authors [49] found that when the magnetic field is greater than ∼2.8 T, the chiral effect was dominant, resulting in a monotonically decreasing of the degree of circular polarization. As Zeeman effect become dominant when the magnetic field less than ∼2.8 T, the degree of circular polarization become negative, as shown in Fig. 11(s). This is a nice work in which the experimental observation, theoretical model and physical interpretation are consistent. Nevertheless, all the experiments were conducted at an extremely low temperature of 2 K. Although the achievement of circular polarized light emission did not require strong magnetic field, the extremely low temperature is strictly necessary, which may be a hurdle for practical applications. For this sense, it is worth to mention the direct generation of high degree circular polarized emission at room temperature from achiral CdSe/ZnS quantum dots via resonantly coupling with plasmonic chiral metasurface [108,109]. In this work, the very strong chirality was induced by a carefully designed plasmonic chiral metasurface [110] instead of ligands or surface defects.

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Fig. 11. (a) PL and (b) CPL of apoferritin and chiral CdS quantum dots (CdS@ferritin), (c) glum of chiral CdS quantum dots. (d)-(g) HRTEM images of individual hollow ferritin including (d) two, (e) three, (f) four, and (g) one single CdS quantum dots. (h) Distribution of numbers of CdS quantum dots enclosed in one hollow ferritin. (i) Upper panel: CPL spectra of l-(red dot) and d-(blue dot) cysteine capped CdSe quantum dots, lower panel: PL spectra of chiral CdSe quantum dots. Diameter of the QDs: 2.9 nm. (j) CPL spectra and (k) anisotropic glum of the l- and d-cystine capped CdSe dots core/CdS rods shell with various ARSC values. The black and red dots: l-cystine and d-Cystien capped CdSe/CdS DRs respectively. (l) - (n) Degree of circular polarized emission for rac-, R-, S- perovskites as a function of magnetic field. (o) & (p) Spin-up and spin down energy level for chiral perovskite. (q) - (s) Zeeman effect for the chiral perovskites. (a) – (h) Reproduced with permission from ref. [15]. Copyright 2010, Wiley-VCH. (i) Reproduced with permission from ref. [105]. Copyright 2013, American Chemical Society. (j) – (k) Reproduced with permission from ref. [35]. Copyright 2018, American Chemical Society. (l) – (s) Reproduced with permission from ref. [49]. Copyright 2018, Nature Publishing Group.

3.2 Chiral nanomaterial bio-system interaction Chiral elements such as DNA, RNA, proteins, sugars etc. play critical roles for the various signaling pathways and life activities. In addition, many important drugs also show chirality. Thus the interaction between the chiral nanomaterial and bio-system may benefit for the fundamental biological investigation, disease treatment and new drug development etc. Nicholas A. Kotov et al. [62] explored the cytotoxicity of the chiral carbon dots and discovered that the d-carbon dots manifested slightly higher toxicity than l-carbon dots for human liver hepatocellular carcinoma cells HepG2 when the incubation time reached 24 h. Atomistic molecular dynamics (MD) simulation method have been utilized by the authors [62] to uncover the different interaction between the chiral carbon enantiomers and cells. They used the following model to represent the cell membrane [62]: the cell membrane consists of cholesterol and two layers of lipid membrane. And the lipid layers are composed of 1-palmitoyl-2-oleoyl-sn-glycero-3phosphocholine (POPC). They [62] assumed that the chiral ligands bent to the cell membrane and the interaction between the carbon dots and membrane was finished through van der Waals interaction and hydrogen bonding. For d-cysteine based carbon dots, the aromatic portion were perpendicular to the bilayer cell membranes and perturbed the cell bilayers of cell membranes (Fig. 12(a) & (c)). After a certain duration, the d-carbon dots could penetrate/enter the cell membranes (Fig. 12(f)). As l-carbon dots were parallel to the bilayer of cell membranes, they could not penetrate/enter the cell membranes, as illustrated in Fig. 12 (e). The authors [62]

suggested that the different behaviors (Fig. 12(b) & (d)) resulted from the different interaction between the cell membranes and the edge functional groups of chiral carbon dots. This work provide partial insight into the different behaviors of the l- and d-carbon dots. Apart from cytotoxicity, some researchers have reported the chiral carbon dots [71] based enantioselective effect for antimicrobial responses [65], cellular energy metabolism [68]. Furthermore, Zhang et al. [70] even demonstrated that chiral carbon dots could enantioselectively influence the mung bean plant growth. Chiral nanomaterials also find applications in the field of therapeutics. Alzheimer’s disease is one kind of chronic neurodegenerative disease which contribute to 60–70 % of cases of dementia. 42-residue isoform (A␤42) [111,112], one of the Amyloid ␤-proteins, is the major component of fibrillar amyloid plaques, which are related to the Alzheimer’s disease. The aggregation of A␤42 will lead to the activation of the toxicity of the related peptide, thus various approaches [113,114] have been explored to impede the A␤42 aggregation. Ravit Malishev et al. [64] have also done a nice work to demonstrate that chiral carbon dots could modulate and inhibit the amyloid beta-42 (A␤42) aggregation. The authors found [64] that when pure buffer or d-lysine based carbon dots have been utilized to incubate the A␤42 peptide with 24 h, the famous structure transformation/aggregation from original random coil to mainly ␤-sheet structure have been observed (Fig. 12(g) (the ␤ structure is indicated by the CD signal located at 195 nm and 218 nm). While in the condition of l-lysine carbon dots, the transformation/aggregation process were strongly suppressed as illustrated in Fig. 12(g), red curve. The authors [64] also done cryo-

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Fig. 12. (a), (c) Snapshots of the atomistic MD simulation results for cell membrane-d-carbon dots interaction. (b), (d) Time dependent distance between the chiral carbon dots and the center of the bilayers. (e), (f) Snapshots of carbon dots/membrane interaction when the simulation to be finished. (e) d-carbon dots, (f) l-carbon dots. (g) & (h) The structure of A␤ 42 protein before and after aggregation (incubated for 24 h), where the structure are indicated by the CD spectra. Black: pure A␤ 42. Blue: A␤ 42 + d-lysine carbon dots. Red: A␤ 42 + l-lysine carbon dots. (i)-(l) Cryo-TEM images of the A␤ 42 sample after incubation for 24 h. (i) pure buffer, (j) buffer +0.2 mg/mL d-lysine carbon dots (C-dots), (k) & (l) buffer +0.2 mg/mL l-lysine C-dots. Scale (i)-(k) 100 nm. (l) 50 nm. (m) Inhabitation of A␤ 42 protein cytotoxicity by chiral carbon dots: I) A␤ 42 protein + D-lysine C-dots (0.2 mg/ml), II) A␤ 42 protein + L-lysine C-dots, III) A␤ 42 protein + D-lysine C-dots (0.4 mg/ml), IV) A␤ 42 protein + L-lysine C-dots (0.4 mg/ml). (a) – (f) Reproduced with permission from ref. [62]. Copyright 2016, American Chemical Society. (g) – (m) Reproduced with permission from ref. [64]. Copyright 2018, Royal Society of Chemistry.

genic transmission electron microscopy (cryo-TEM) measurement and the results (see Fig. 12(i)–(l)) were also consistent with the CD spectral data. Incubation with pure buffer or d- lysine based carbon dots leading to the aggregated fibril ␤-sheet structure of A␤42 peptide (Fig. 12(i) & (j)), while opposite individual nanoparticles have been observed from the d- lysine based carbon dots solution (Fig. 12(k) & (l)). The authors [64] also verified the biological viability of chiral carbon dots via MTT assays. Pure A␤42 will cause the death of cell by approximately about 25 %. And the A␤42 & d- lysine carbon dots mixture shown similar results, as shown in Fig. 12(m). However, when the l-lysine carbon dots were added, the cell viability was totally recovered (Fig. 12(m)). As we know that proteins play key roles in many important physiological processes and more importantly, most proteins show chirality. Thus, the interaction between chiral nanomaterials and proteins may find various applications in disease therapeutics (as demonstrated by the authors [64]), cancer treatment etc. 3.3 Second harmonic generation The chiral structure implies the potential to achieving the nonlinear photonics [48], especially for the second harmonic generation, where the noncentrosymmetry structure is strictly demanded. In 2018, Peng et al. [46] observed that chiral peroviskite nanowires (C5 H14 N2 PbCl4 ·H2 O) demonstrated second harmonic generation (SHG). The efficiency of the SHG of the chiral peroviskite nanowires were comparable to the potassium dihydrogen phosphate (KDP), as shown in Fig. 13(a). Xu et al. [47] employed the liquid diffusion growth method to synthesize chiral organic/inorganic hybrid perovskite nanowire ((R/S-MPEA)1.5 PbBr3.5 (DMSO)0.5 ) by using R/S-(+/-)-␤-methylphenethylamine (R/S-MPEA) molecule as the chiral components. Diffusive reflectance spectra revealed [47] the band gap of the chiral nanowire to be about 3.07 eV. They measured the SHG by means of a home-made system [47] as depicted in Fig. 13(b), where the nanowire, incident light and SHG light resided in the same plane. When pumped by a fs NIR light, relatively strong SHG signal was observed from the chiral nanowire,

as shown in Fig. 13(c). Moreover, when pump light longer than 800 nm, a broaden PL spectrum was observed besides the SHG signal [47], which originated from the two photon excited PL. The authors calculated [47] the effective second order coefficient of the chiral perovskite nanowire to be 0.68 pm/V when pumped by 850 nm laser, where the Y-cut quart has been employed as the reference. When the pump power was less than 67.5 mw, the SHG signal presented [47] a quadratic relationship to the excitation power. Further increasing the excitation power leaded to the decrease of the SHG signal as shown in Fig. 13(d). The authors proposed that the 67.5 mw is the light damage threshold for the chiral nanowires. Considering that the diameter of the laser spot is about 10 ␮m, so the calculated pump fluence threshold intensity is 0.52 mJ/cm2 or 8.6 × 10−4 W/cm2 . 3.4 Chiral catalysis Researchers have paid much attention to the chiral catalysis as it can provide precious and valuable products and/or precursors for food, chemical and medical industry [115]. The chiral catalysts are always required for most of the chiral reactions. Chiral inorganic nanostructures, as a new type of chiral system shows huge potential to act as the chiral catalyst and achieve the meaningful chiral reactions. Chiral magnetic nanoparticles [78,81,82,116] play an important role in the field of chiral nanoparticle based chiral catalysis. Hu et al. [76] have realized the enantioselective hydrogenation of aromatic ketones by means of chiral Ru complexes capped Fe3 O4 nanoparticles. As can be seen from Fig. 14(a), a series of aromatic ketones could be asymmetrically hydrogenated [76] with the assistance of chiral Fe3 O4 nanoparticles (1-MNP1 or 1MNP2 ). The conversion efficiency could even reach to 100 %. The enantiomeric excess values (ee%) of chiral magnetic nanoparticle were comparable to the original chiral Ruthenium(II) complex 1 (Ru(BINAP-PO3 H2 )(DPEN)Cl2 ). For the reusability, 1-MNP1 and 1MNP2 could retain their enantioselectivity even after 6 and 14 runs of hydrogenation respectively. As declared by the authors [76], compared to other catalysts, the chiral magnetic nanoparti-

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Fig. 13. (a) SHG signal of chiral perovskite nanowire (C5 H14 N2 PbCl4 ·H2 O) and potassium dihydrogen phosphate (KDP), recorded by Oscilloscope. (b) Set-up for the SHG measurement. (c) SHG spectra of chiral (R-MPEA)1.5 PbBr3.5 (DMSO)0.5 perovskite nanowire, excited by different laser light. (d) Logarithmic scale plot of SHG signal as function of the excitation power, the threshold power is about 67.5 mw, as labeled inside the figure. (a) Reproduced with permission from ref. [46]. Copyright 2018, Royal Society of Chemistry. (b) – (d) Reproduced with permission from ref. [47]. Copyright 2018, American Chemical Society.

cle based chiral catalysis system displayed distinct advantages for the catalyst reuse. The chiral magnetic catalyst could be easily collected from the reaction mixture by removing the reacted solutions but retaining the chiral magnetic catalyst with the assistance of a magnetic field. Later, the chiral N-Heterocyclic carbenes (NHCs) modified Fe3 O4 /Pd nanoparticle (Fe3 O4 /Pd/L1) have also been prepared by Ranganath et al. [77] to achieve the ␣-arylation reaction, as this asymmetric reaction can offer precious intermediates for the pharmaceutics. The authors [77] employed 2-methyl-1-tetralone, various phenyl halides and NaOtBu (act as base) to react in toluene with the assistance of chiral catalyst Fe3 O4 /Pd/L1 (see Fig. 14(b)). They also demonstrated the good selective catalytic performance of chiral Fe3 O4 /Pd/L1 with bromobenzene and chlorobenzene. The chiral catalysts [77] resulted in the corresponding ␣-arylated outputs with 72 % yield & 48 % ee selectivity and 56 % yield & 60 % ee selectivity for bromobenzene and chlorobenzene, respectively. However, absence of chiral NHC (Fe3 O4 /Pd nanoparticle) resulted in a yield of only about 22 % [77] in the racemic ␣-arylated output and a large amount of by-products. Moreover, they [77] found that the chiral NHC stabilized Fe3 O4 could not trigger ␣-arylation reaction, which implied that all the Fe3 O4 , Pd and chiral NHC were necessary for the asymmetric ␣-arylation reaction. The authors also showed [77] that both the activity and selectivity remained nearly the same even after the Fe3 O4 /Pd/L1 catalyst had been used for five cycles. In addition to the chiral magnetic nanoparticles, Zhang et al. [54] also developed the l-/D- cysteine based chiral MoS2 nanodots/Cu2+ conjugate system to explore the enantioselective peroxidase-like activity, where Cu2+ is the active site which can act as the cofactor to improve the catalytic activity. The CD spectra of l-/D- cysteine based chiral MoS2 nanodots/Cu2+ conjugate are shown in Fig. 14(c). The introduction of the Cu2+ did not diminish the chiral properties of the MoS2 nanodots as both of them exhibited 2 strong CD peaks. d-and l-tyrosinol enantiomers have been employed [54] to assess the enantioselective performance of l-/D- cysteine based

chiral MoS2 nanodots. The oxidized material of tyrosinol showed a strong absorption feature at 210 nm, which could be used to monitor the reaction dynamics. As reported by the authors [54], no or weak catalytic performance have been observed for the pure Cu2+ and MoS2 nanodots/Cu2+ conjugation. The cysteine modified chiral MoS2 nanodots/Cu2+ conjugation manifested a strong oxidation effect for both l- and d- tyrosinol, as shown in Fig. 14(d) & (e). As can be seen, compared to l-cysteine modified MoS2 nanodots/Cu2+ conjugation, the d-cysteine modified MoS2 nanodots/Cu2+ conjugation system manifested better catalytic behavior for d- tyrosinol, whereas the l-cysteine based MoS2 nanodots displayed higher catalytic activity for l- tyrosinol. Based on the Lineweaver-Burk plot, the authors [54] quantitatively calculated the Michaelis-Menten Constant (Km ) and Maximum Reaction Rate (Vmax ) for catalytic reaction between l-/D- cysteine based chiral MoS2 nanodots/Cu2+ conjugate and l-/D- tyrosinol (Fig. 14(f)). The enantioselectivity Michaelis-Menten Constant (Km ) ratio could reach 0.145 and 6.67 for l- and d- tyrosinol respectively, and the authors [54] declared that these values are much higher than what was observed in the recently published chiral nanozymes [117,118]. Shah et al. [34] have achieved the asymmetric aldol condensation by means of chiral ZnS quantum dots, where the l-proline was employed as the chiral stabilizer. The reaction between acetone and aldehydes generated ␤-hydroxy aldehydes or ␤-hydroxy ketones (aldolization reaction), while the subsequent dehydration process resulted in the ␣,␤-unsaturated aldehydes or ketones (namely aldol condensation). The chiral ZnS QDs could act as the catalyst [34] to confine the reaction at the aldolization step and suppressed the subsequent dehydration reaction. Consequently, only the (R)-␤-hydroxy carbonyl e.g. ␤-hydroxy aldehydes or ␤-hydroxy ketones compounds were produced. The authors [34] chose various aromatic and aliphatic aldehydes to evaluate the enantioselective performance of the chiral ZnS quantum dots. In addition to several aldehydes which cannot react with the acetone, most of the

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Fig. 14. (a) Enantiomeric Excess Values (% ee) for the enantioselective asymmetric hydrogenation of various aromatic ketones. (b) The enantioselective ␣-arylation of 2methyl-1-tetralone utilizing chloro- and bromobenzene. (c) CD spectra of l-/D- cysteine modified MoS2 nanodots and l-/D- cysteine modified MoS2 /Cu2+ conjugations. (d) & (e) Time dependent absorbance change of d- tyrosinol and l-tyrosinol for different systems as indicated inside the figures. (f) Michaelis-Menten Constant (Km ) and Maximum Reaction Rate (Vmax ) of the reactions between l- & d-cysteine-MoS2 nanodots/Cu2+ conjugates and l- and d- tyrosinol. (a) Reproduced with permission from ref. [76]. Copyright 2005, American Chemical Society. (b) Reproduced with permission from ref. [77]. Copyright 2010, Wiley-VCH. (c) – (f) Reproduced with permission from ref. [54]. Copyright 2018, American Chemical Society.

aldehydes displayed strong asymmetric behavior with a high (R)␤-hydroxy carbonyl yield and enantiomeric excess, as can be seen in Table 1 in the original article [34]. It has been suggested that the Zn ions on the surface promote the catalytic performance of l-proline, a similar function of Lewis acids. The authors [34] also tested the repeatable usage capability of the chiral ZnS QDs and the results showed that the catalytic efficiency remained the same after 2 cycles. The slight decline of the efficiency in the third cycle was attributed to the decrease of the catalyst amount. 3.5 Chiral assembly The chiral super molecules are frequently required for the mimicking of bio-systems. However the traditional assembly approaches generally lead to the achiral super molecules. In addition, the chiral super molecule assembling also indicate the potential of the chirality transfer from the template to the super molecules, thus may find applications in the field of catalysis and biology. By using the chiral nanomaterial as the template [119], it become possible to achieve the chiral assembling, as – c´ and co-workers [63]. They syndemonstrated by Luka Ðordevi thesized chiral carbon dots via hydrothermal microwave-assisted

approach, where (R,R)- and (S,S)-1,2-cyclohexanediamine (CHDA) have been employed as the chiral ligands. The electronic circular dichroism (ECD) spectra (Fig. 15(a)) verified the chirality of the carbon dots. In addition, the authors [63] also carried out the vibration circular spectroscopy (VCD) measurement, where the samples were excited by left and right circular polarized IR light. The results shown in Fig. 15(b) confirm the chiral structure of carbon dots. Density functional theory (DFT) calculation results (Fig. 15(c)) [63] based on the CHDA derivatives matched very well with the experimental VCD data, indicating that chirality originated from the shell chiral ligands(CHDA). Tetranionic meso-tetrakis(4sulfonatophenyl)porphyrin (H2 TSPP4− ) have been utilized by the authors [63] to explore the assembling process. Under acidic condition, H2 TSPP4− will be partly protonated to H4 TSPP2- (pKa = 4.8), which could aggregate to supermolecule via H-approach (absorption peak located at 420 nm) and J-type (absorption around 490 nm) [120–124]. The pure H4 TSPP2- aggregate to racemic supermolecule, thus do not show any net chirality [125]. The excess chiral component will disrupt the racemic dynamics, resulting in the enantioselective aggregation. The authors [63] found that both the H2 TSPP4− •R-and S-carbon dots mixture displayed J- and H-type aggregation, and more importantly, the chirality. In addition to

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Fig. 15. (a) ECD (electronic circular dichroism) spectra of chiral carbon dots. (b) & (c) VCD (vibration circular spectroscopy) spectra of chiral carbon dots, (b) experimental and (c) simulated. (d) ECD spectra of the aggregated super molecule using chiral carbon dots as the template. (e) & (f) SEM images of twisted nanoribbons when both the racemic CdTe nanoparticle and circular polarized light (left: LCP and right: RCP) are involved. (a) – (d) Reproduced with permission from ref. [63]. Copyright 2018, Nature Publishing Group. (e) – (f) Reproduced with permission from ref. [127]. Copyright 2014, Nature Publishing Group.

the chiral bands which come from the carbon dots, ECD spectra (Fig. 15(d)) of the mixture display 2 additional CD bands located at 420 nm and 490 nm, corresponding to the H- and J- type aggregation respectively. The authors declared [63] that the chirality of the assembled super molecule originated from chiral transformation from chiral carbon dots to super molecule, as the CD signal of the carbon dots decreased after the aggregation. The relative weak chirality of the aggregated super molecule was due to the low transfer efficiency. Similarly, Zheng et al. [126] also realized the chiral transformation from chiral molecule (R/S-(1-phenylethylamino) methylphosphonic acid) to micro scale level in coordination poly-

mers. Besides the molecule assembling, the racemic CdTe NPs were also designed as the chiral template to assemble the right- /lefthanded twisted chiral nanoribbons [127] with the assistance of circular polarized light (Fig. 15(e) & (f)). 3.6 Chiral sensing CD spectroscopy is a useful tool to investigate the chirality of the chiral component. The interaction between the molecules and chiral components may lead to the change of the chirality, thus allows for the chiral nano system based sensing [128–131]. CdS

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Fig. 16. (a) CD spectra of glucose oxidases (GOxs), achiral CdS QDs (QDs), chiral CdS QDs (QDs + DPA) and their corresponding mixture with H2 O2 and glucose. (b) TEM and (c) 3D reconstruction cryo-TEM tomography images of the Au-UCNP Pyramids. (d) CD and (e) PL spectra of gold (Au) nanoparticle, up-conversion nanoparticle(UCNP), gold (Au) nano particle -DNA, UCNP-DNA, Au NP dimers, UCNP dimers, Au NP + UCNP mixture, pyramids in buffer solution. Excitation wavelength: 980 nm. (f) Sensing mechanism of Au-UCNP Pyramids for miRNA sensing and (g) illustration of pyramid Used for miRNA sensing. 1, 2 are linked with UCNPs. 3, 4 are linked with gold nanoparticles; section a: recognition sequence of micro RNA; section b: non-complemented frame (indicated by white dashed line). (h) CD spectra and (i) confocal PL images of HeLa cells with various concentrating of miR-21. (j) & (k) The linear relationship between the (j) CD (521 nm) and (k) PL intensity and the various concentrations of intracellular micro RNA-21. Excitation wavelength: 980 nm, PL measurement range: 540 ± 60 nm. Scale bar = 20 ␮m. (l) CD and UV–vis spectra of heterodimers/Au core-shell nanostructure (HD@Au) with concentration of DNA: from 16 zM to 1.6 pM. (m) Linear relationship between CD bands of HD@Au and the concentration of DNA. (n) CD spectra of chiral and achiral CdSe nanoplatelets. (o) CD spectra of the chiral CdSe nanoplatelets with various concentration of lead ions and (p) corresponding linear relationship (@520 nm CD peak). (a) Reproduced with permission from ref. [132]. Copyright 2018, Elsevier. (b) – (k) Reproduced with permission from ref. [133]. Copyright 2016, American Chemical Society. (l) – (m) Reproduced with permission from ref. [136]. Copyright 2014, American Chemical Society. (n) – (p) Reproduced with permission from ref. [137]. Copyright 2019, Royal Society of Chemistry.

chiral quantum dots synthesized by mixing the achiral CdS and chiral ligands (D-penicillamine, DPA) have been prepared to sense the glucose [132]. Under oxygen (O2 ) circumstance, glucose will be oxidized to hydrogen peroxide (H2 O2 ) with the assistance of glucose oxidase (GOx). The generated H2 O2 could “destroy” the chiral CdS quantum dots, resulting in the decrease of the CD signals [132] (Fig. 16(a)). Both TEM images and DLS (dynamic light scattering) measurements [132] showed a diameter decrease of CdS QDs after the interaction, consistent with the above-mentioned sensing mechanism. The authors [132] also declared that all the components: glucose, glucose oxidase, O2 , chiral CdS QDs were necessary for the sensing (Fig. 16(a)). And the chiral CdS QDs have a suitable concentration range of 50–250 ␮M with a limit of detection (LOD) of 31 ␮M [132]. To achieve the ultrasensitive detection of micro RNA, Li et al. [133] have developed gold-upconversion nanoparticle (AuUCNP) pyramids system by adopting the gold nanoparticle and lanthanide-doped up-conversion nanoparticle via DNA-driven selfassemble method. The TEM images [133] of the synthesized Au-UCNPs are shown in Fig. 16(b). Moreover, the authors [133] employed the cryo-electron tomography spectroscopy to determine the 3D structure of the Au-UCNP(see Fig. 16(c)), and the data verified the pyramid structure. The Au-UCNP pyramid system

exhibited a clear CD band in the range of 521 nm and a PL band in the range of 500−600 nm. The PL of the Au-UCNP pyramids was quenched due to the appearance of the resonance energy transfer between the gold nanoparticle and up-conversion nanoparticle [134,135] (Fig. 16(d) & (e)). In addition, the self-assemble process was necessary for the Au-UCNP pyramids as the direct mixture of the gold nanoparticles and UCNPs could not generate the CD signal but only strong PL of the UCNPs. The sensing mechanism [133] is depicted in Fig. 16(f) & (g). Each DNA fragment contained the micro RNA recognition sequences (section “a” in Fig. 16(g)), while the double strands of the DNA are not complementary, as indicated by the white dashed line. When the complementary micro RNA (compared to the DNA sequence) were added into the mixture, the DNA fragment would be dissociated, thus the gold nanoparticles and UPNPs would be separated. Consequently, the CD signal disappeared while the PL signal of the UCNPs was enhanced, as illustrated in Fig. 16(d) & (e). Micro RNA21 (MiR-21) have been utilized by the authors [133] to test the feasibility of the sensing system. They found that [133] both the CD signal (monotonically decrease) and PL intensity (monotonically increase) demonstrated a linear relationship to the micro RNA21 concentration in the range of 2–50 pM, which verified the suitability of the Au-UCNP for the sensing. On the contrary, the mutated miR-21, let-7, and miR-200b,

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bovine serum albumin (BSA), and glutathione (GSH) etc. could influence neither the CD nor PL spectra, implying the selectivity of the sensing system. To realize the living cell miR-21 sensing, the HaLa cells with different miR-21 levels have been prepared and the quantity of the intracellular miR-21 were determined by the quantitative real time-PCR. The CD response and PL images of the HeLa cell with various amount of the miR-21 are shown in Fig. 16(h)–(k), and both of them exhibited a linear relationship [133] to the amounts of miR-21. The authors concluded [133] that CD sensing system possessed a sensing range of 0.073–43.64 fmol/10 ␮gRNA with a limit of detection to 0.03 fmol/10 ␮gRNA. Comparatively, the PL confocal image sensing only offered a detection range of 0.16–43.64 fmol/10 ␮gRNA with a limit of detection only 0.12 fmol/10 ␮gRNA. Thus the authors claimed [133] that compared to the PL based detection, the CD based miRNA sensing platform was more sensitive. In addition to the single element noble metal such as gold and silver, Kotov et al. [136] proposed that the chiral properties of heterodimers could be tuned by coating some gold or/and silver shell layers on the surface, namely core-shell nanostructure. They claimed [136] that the CD spectra could be tuned from 418 nm to 586 nm by properly choosing the metal shell thickness, deposition sequence and composition of the metals. The g factor of the core-shell (heterodimers/metal) structure are substantially increased, up to 1.21 × 10−2 , which contribute to the ultrasensitive DNA detection e.g. zeptomolar level (see Fig. 16(l) & (m)). Very recently, the chiral CdSe nanoplatelets have been employed by Chen et al. [137] for the lead ion sensing. The authors [137] synthesized the chiral CdSe nanoplatelets via ligand exchange between the oleic acid (OA) capped achiral CdSe nanoplatelets and chiral molecules e.g. l-/D-cysteine. The chiral CdSe NPLs displayed two distinct CD bands as can be seen in Fig. 16(n). The stronger CD band around 520 nm was chosen to sense the lead ions. The CD signal decreased linearly as the concentration of the lead ions increased in the range of 0.01–1 ␮M, as depicted in Fig. 16(o) & (p). The calculated limit of detection (LOD) was as low as 4.9 ± 0.3 nM. The addition of the lead ion could interact with the S on the surface of the chiral NPLs, forming the Pb-S bond. The authors [137] claimed that the Pb-S bond might destroy the Cd-S bond on the NPLs, which caused the quenching of the CD signal of the chiral CdSe nanoplatelets, thus contributed to chiral response of the NPLs. 3.7 DNA cleavage As suggested by researchers, the interaction between the chiral QDs and chiral biomolecules may regulate various downstream signal pathways, which could trigger tremendous applications. Gene editing [138], as one of the most effective approaches to cure the gene mutation caused diseases, have drawn much attention. Even if different gene editing tools have been proposed by researchers, until now the commercialized gene editing tools still do not exist. And for all kinds of gene editing tools, the gene cutting/cleavage is the most important and difficult step. Besides, for the current existed gene editing tools such as such as CRISPR-Cas9, ZFNs, TALENs etc., it usually requires a single guide RNA or some DNA sequences. Recently, researchers have achieved the gene (e.g. DNA) cleavage via chiral inorganic nanostructures. Prior to the practical application, especially for the bio-related utilization, the cytotoxicity effect of the chiral nanostructures must be investigated. Nie et al. [27] have studied the cytotoxicity of chiral CdTe QDs. l- and d- tripeptide glutathione (GSH) chiral ligands have been used to stabilize the CdTe QDs. Two types of chiral CdTe QDs with different size have been prepared [27]: green light emission L563 & D560 chiral QDs with a diameter of 3.2 nm and red light emission L620 & D622 chiral QDs with a larger diameter about 4.3 nm, where the l- & d- represent the chiral ligands and the number denote the PL peaks. All the chiral CdTe QDs manifested clear CD signals in

the range of 200−300 nm. The cell viability of the chiral CdTe QDs have been evaluated by the authors [27] using human hepatoma HepG2 cells. The results shown in Fig. 17(a) indicate that both the l- and d- capped chiral CdTe displayed obvious cytotoxicity. Moreover, the l-GSH capped CdTe QDs are more toxic than d-GSH based. The authors attributed [27] the toxicity to the chiral CdTe triggered cell autophagy. Cell autophagy indicator-microtubule-associated protein light chain 3 [139] (autophagy can lead to the transformation from LC3-I to LC3-II) have been employed [27] to verify the chiral CdTe QDs autophagy effect. It can be clearly seen that the autophagy is chirality related (Fig. 16(b)). More specifically lCdTe QDs showed stronger autophagy and the smaller size chiral QDs displayed more intense autophagy. The morphology change and the vacuole formation of the cells were also characterized by the TEM images [27]. The additive of amiodarone hydrochloride (AH), which can substantially trigger the autophagy, will lead to the formation of internalized cytoplasmic debris (darkly stained granular, white arrows in Fig. 16(c). In comparison, a mount of empty vacuoles were observed [27] in chiral CdTe QDs treated HepG2 cells and even some dark granular also appeared in some CdTe QDs exposed cells, consistent with the previous observation. Furthermore, the authors [27] also utilized the autophagy inhibitor 3-methyladenine (3MA) to confirm the autophagy effect, where the 3MA could suppress the autophagy effect, thus increased the cell viability. As expected, all types of chiral CdTe QDs displayed an increased cell viability [27] when the autophagy inhibitor was added (see Fig. 16(d) & (e)). All of these indicated the existence of the autophagy effect for the cell toxicity. For the different response of l- and d- GSH capped CdTe chiral QDs, there is still no answer. The authors excluded [27] the different cellular uptake as the inductively coupled plasma–mass spectrometry (ICP-MS) measurement uncovered that different chiral QDs showed similar cellular uptake. While. Kotov et al. [62] suggested (see the “Chiral nanomaterial bio-system interaction” part for details) that the interaction/entrance between the cell membranes -L and -D carbon dots are different. This work [27] demonstrated that the l- and dGSH capped chiral CdTe QDs are more toxic than pure CdTe QDs due to the existence of autophagy effect. It’s significant as the authors [27] discovered the role of autophagy in chiral QDs-cell system. After all, for most bio-applications, being more toxic is not a good news. Kuang et al. [7] have first utilized the CdTe chiral quantum dots for the DNA cleavage. Truncated tetrahedral chiral CdTe QDs were stabilized by l- and d- cysteine chiral ligands, which manifested strong CD response in the range of 350–410 nm. The pure mixing of chiral CdTe QDs and Salmon sperm DNA (a double-stranded DNA of about 1839 bp) could not [7] alter the CD spectra. When the mixture was exposed to circular polarized laser light (405 nm) for 2 h, significant changes have been observed [7]: New CD peaks appeared at 365 nm, 415 nm, 470 nm and 488 nm (Fig. 17(f) & (g)) and the morphology of the CdTe nanostructure was converted from nanodots with a diameter of 4.5 ± 0.3 nm to nanorods with a 20 ± 2 nm length and 5 ± 0.25 nm width. The most impressive discovery by the authors [7] is that the 1839 bp Salmon sperm DNA were divided into two different DNA fragments. Briefly, at the beginning of the laser exposure two new electrophoresis bands started to emerge, and their intensity continuously increased as the exposure time extended. The original electrophoresis band totally disappeared once the scission was completely finished and longer illumination did not cause further scission [7], as illustrated in Fig. 17(h) &(i). According to the DNA sequence analysis, the authors [7] identified that the cutting site located at between the thymine (T, 1,083, 5 end) and adenine (A, 1,084, 3 end), thus the length of the cleaved DNA segments were 1083 bp and 765 bp respectively. Moreover, the authors also asserted [7] that neither DNA fragment with length less than 90 bp nor single strand DNA could be cleaved by the chiral

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Fig. 17. (a) Concentration and chirality dependent cell cytotoxicity of chiral CdTe quantum dots. (b) Chiral CdTe QDs trigged transformation from (LC3)-I to LC3-II (indicator of autophagy). (c) TEM images of HepG2 cells exposed to chiral CdTe QDs for 24 h. (d) & (e) HepG2 cell viability with/without autophagy inhibitor 3-methyladenine (3MA). Incubation for (d) 12 h (e) 24 h. (f) & (g) CD spectra of l-/D-cysteine capped CdTe nanostructures (or DNA mixture), before and after 2 h laser illumination. (h) & (i) Electrophoresis bands of (h) l-/(i) d- cysteine capped CdTe QDs with 1839 bp DNA, exposed with circular polarized laser (405 nm) for different time, frag 1 and frag 2 refer to the 2 DNA fragments. (j) DNA cleavage is achieved by the ROS induced oxidation, red arrow referss to the phosphodiester bond. (a) – (e) Reproduced with permission from ref. [27]. Copyright 2011, Wiley-VCH. (f) – (j) Reproduced with permission from ref. [7]. Copyright 2018, Nature Publishing Group.

CdTe QDs. A series of scission experiments [7] indicated that chiral CdTe QDs possessed distinctive cutting site-discriminating ability, that is, they could only recognize and cleave the DNA sequence GAT’ATC with the cutting site from 5 end of T (thymine) to 3 end of A (adenine). Reactive oxygen species (ROS) evolution during the cleaving process have been monitored by means of 2 ,7 dichlorodihydrofluorescein (DCFH) probe [140–142]. Impressively, the authors [7] discovered that the amount of ROS continuously increased during the cutting process and saturated once the cleavage completed, and the addition of ROS inhibitor (NaN3 ) leaded to the stop of cleavage. Thus they suggested [7] that the photon induced DNA cleavage result from the phosphodiester bond broken between 5 end of T and 3 end of A, caused by the circular polarized light induced reactive oxygen species oxidation (Fig. 17(j)). More specifically, it is the hydroxyl radical species [7] that contributed to the oxidation. Different circular polarized light yielded different ROS generation rate, thus resulting in the various cleavage dynamics. In addition, the authors claimed [7] that the different ROS yields originated from the different absorption efficiency of l- and d- chiral CdTe nanostructures when shined by left and right circular polarized light. Quantum chemical and DFT calculations [7] revealed that the cysteine capped chiral CdTe QDs could attach to the DNA strands at a particular position and start to cleave when exposure to light. Isothermal titration calorimetry (ITC) measurements [7] also demonstrated a high affinity between the chiral CdTe nanoparticles and DNA (90 bp) with the binding constants up to about 3 × 105

M−1 , on contrast, the unsuccessful cleavage e.g. mutated DNA, 80 bp DNA etc. showed a much lower binding constant. Finally, the authors also proved [7] the viability of this photon induced cleavage by demonstrating both in vitro and in vivo cleavage. From the point of view of chiral nanomaterial, this is obvious a very nice work. It demonstrated that the inorganic chiral nanomaterials may also be used as a new gene editing tool. The proposed ROS induced oxidation model matched very well with the experimental observations and are reasonable and reliable [7]. 4 Summary and prospect It has been more than 20 years since the discovery of the first chiral inorganic nanostructure e.g. Au nanoclusters. Substantial progress in this field has been made in terms of both fabrication approaches and application demonstrations, as discussed in this review. The breakthrough of the sample preparation, especially the appearance of the universal ligand exchange approach guarantees and promotes the widespread exploration of the inorganic nanostructures. The theoretical calculations/computations [143] may offer some insight into the origination of the chirality, particularly for fine structure of the chiral centers, which is currently unavailable from the experimental approaches. However, even for one certain kind of nanostructure, there have been different models proposed by different researchers to account for the chirality originations, for example, both the DFT and dipole approximation calculations have suggested separated models for the chiral CdSe nanocrystals, gold nanoclusters etc. In addition, for many inorganic

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nanostructures, the exact fine structure of chiral center or bonds, the interaction between the inorganic core and surface ligands etc. are still not so clear and remain open questions. It is speculated that all the factors including inorganic core synthesis, surface ligand choice and surface capping process may affect the chiral properties of chiral inorganic nanostructures. In other words, each chiral inorganic nanostructure may be a complex system, thus more efforts should be paid to uncover the optical physical properties of these systems. As we have discussed in Section 3, chiral inorganic nanostructures have triggered abundant applications in various fields. Although these results are just some lab demonstrations, they represent important steps toward practical applications. What’s exciting is that in recent years, more and more chiral inorganic nanostructure based applications, especially some innovative applications such as DNA cleave [7] etc. have emerged. We believe that more fascinating or even some “killer” applications can be expected in the near future. To find new applications, one effective approach is to explore some new functions utilizing the reported chiral inorganic nanostructures, such as Ravit Malishev et al. [64] employed chiral carbon dots to modulate and inhibit the amyloid beta-42 (A␤42) aggregation and Kuang et al. [7] utilized the CdTe chiral quantum dots achieving the DNA cleavage etc. We believe that exploration of new chiral inorganic nanostructures [52,144–148] may contribute to both fundamental investigation and application exploitation. On the other hand, researchers need to improve the performance of chiral nanostructure based applications. Despite that CPL has been observed from various chiral nanostructures, the degree of circular polarization value is low (in the order of 10−2 ∼10-4 ) in comparison to the value obtained from other CPL generation system such as lanthanide complexes (∼0.6) [149,150], monolayer 2D material system (∼0.3) [151,152] and so on. The efficiencies of the chiral inorganic nanostructure based second harmonic generation, catalysis, and supermolecule assembling have plenty room to increase. Chiral sensing also demands higher sensitivity and selectivity. Generally speaking, researchers have made significant progresses for the chiral inorganic nanostructure exploration as well as the applications. It’s still early to say whether the chiral inorganic nanostructures can go to the market. Lots of works still need to be done in the future from the sample preparation to the practical application exploration as well as the fundamental investigations. In addition, the investigation of the chiral inorganic nanostructures may also provide some hints for resolving some scientific frontier issues, such as the origin of chiral principle in nature. On the whole, we believe that the future of the chiral inorganic nanostructures is bright. Declaration of Competing Interest All the authors declare no conflict of interest. Acknowledgements This work was supported by Ministry of Education Singapore through the Academic Research Fund under Projects MOE Tier 1, RG 189/17 and RG 105/16 as well as Tier 2 MOE2016-T2-1054. This work was also supported by a multi-year research grant (MYRG2017-0008-FHS) by the University of Macau, Macau SAR, China; The Macao Science and Technology and Development Fund (FDCT) grant (0292017A1, 01012018A3). References [1] Y. Wang, J. Xu, Y. Wang, H. Chen, Chem. Soc. Rev. 42 (2013) 2930–2962.

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Xiaoling Xu is currently an associated professor in the Faculty of Health Sciences, University of Macau. She received her Bachelor’s degree in biology from West China Normal University, and her Master degree from Institute of Hydrobiology of Chinese Academy of Sciences and PhD degrees from Molecular Biology Program in University of Maryland, USA, respectively. Her research interest is cancer biology

Handong Sun is currently with the School of Physical and Mathematical Science, Nanyang Technological University. He received his Bachelor’s degree in physics from Dalian University of Technology, and his Master’s and PhD degrees from Huazhong University of Science and Technology, and Hong Kong University of Science and Technology, respectively. He was elected as a Fellow of the American Physical Society in 2016. His research interests cover optoelectronic materials and devices, semiconductor physics, optical spectroscopy, nanomaterials, and applications in microfluidics.

Tingting An received his Bachelor’s degree in biology from Northeast Normal University and Master degree in biology from Sichuan University. She is currently a Ph.D. student in the Faculty of Health Science, University of Macau. Her research interests include Breast cancer associated gene 1 (BRCA1) in mammary gland development and tumorigenesis, tumor microenvironment, bio nanomaterials.

Lin Wang received her Bachelor’s degree from North University of China. She was a joint-PhD student in Nanyang Technological University for two years and received her Doctoral degree from Hubei University. She is currently carrying out her postdoctoral research with Prof. Sun Handong in Singapore. Her research interests include nanomaterials and technology, optoelectronic materials and devices, semiconductor photophysics, etc.

Please cite this article as: L. Xiao, T. An, L. Wang et al., Novel properties and applications of chiral inorganic nanostructures, Nano Today, https://doi.org/10.1016/j.nantod.2019.100824