Chiral Nanoprobes for Biological Applications E-Book
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- Sprache: Englisch
A comprehensive overview exploring the biological applications of chiral nanomaterials
Chirality has been the centerpiece of many multidisciplinary fields within the broader umbrella of the sciences. Recent advancements in nanoscience have spurred a growing interest in the dynamic field of chiral nanomaterials. In particular, the recent breakthroughs in chiral nanocrystals have presented an intriguing avenue whose potential application may address some key issues at the heart of nanosciences. While little attention has been focused on the biological implications of such advances, this arena is attracting theoretical and applicative interests.
Seeking to provide a thorough introduction to the field as well as fill this gap in scholarship, Chiral Nanoprobes for Biological Applications first provides a comprehensive review of the state-of-the-art development of strong chiroptical nanomaterials, describing how a synthesis and self-assembly approach can enable one to design and create a number of functional chiral nanomaterials. From there, the authors discuss the biological applications of chiral nanomaterials, such as extracellular bioanalysis, intracellular bioanalysis, and chiral recognition, as well as photothermal and photodynamics therapy. In doing so, the book seeks emphasize the potential in multidisciplinary approaches to this up-and-coming field.
Chiral Nanoprobes for Biological Applications readers will also find:
- A particular emphasis on milestones achieved for key chiral nanoprobes research from the last five years
- A discussion of future research directions
- A helpful guide for new researchers and established professionals alike
Chiral Nanoprobes for Biological Applications is a useful reference for materials scientists, biochemists, protein chemists, stereo chemists, polymer chemists, and physical chemists. It is also a useful tool for libraries.
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Veröffentlichungsjahr: 2022
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Table of Contents
Cover
Title Page
Copyright
Preface
1 Introduction
1.1 Chirality and Asymmetry
1.2 Chiral Origin of Nanomaterials
1.3 Relationship with Known Chiral Materials
1.4 Characterization Methods
References
2 Chiral Nanocrystals
2.1 Synthetic Methods
2.2 Chiral Semiconductors
2.3 Chiral Metal Nanostructures
2.4 Chiral Metallic Compound Nanoparticles
References
3 Chiral Nanoassemblies
3.1 Twisted Nanorod Pairs
3.2 Chiral Nanoparticle Dimers
3.3 Tetramer Assemblies
3.4 Nanoparticle Helices
3.5 Chiral Nanochains
3.6 Core–Satellite Nanoassembly
3.7 Nanoplatelets and Chiral Films
3.8 Chiral Supraparticles
References
4 Chiral Nanostructures for Biorecognition and Bioanalysis
4.1 Chiral Analysis Strategy
4.2 DNA Detection
4.3 Biomarker Bioanalysis
4.4 Enzyme Bioanalysis
4.5 Metal Ion Bioanalysis
4.6 Small Molecule Detection
4.7 Chiral Recognition
4.8 Future Prospects and Outlook
References
5 Chiral Nanomaterials for Emerging Biological Effects
5.1 Photothermal Therapy
5.2 Photodynamic Therapy
5.3 Multimodal Therapy Methods
5.4 Photochemical Effects
5.5 Future Prospects and Outlook
References
6 Chiral Nanomaterials for Biocatalysis
6.1 Chiral Nanomaterials Based Enantioselectivity in Biocatalysis
6.2 Biocatalysis of Chiral Drugs and Related Chemical Intermediates Based on Chiral Nanomaterials
6.3 Biocatalysis of Peptides Based on Chiral Nanomaterials
6.4 Biocatalysis of Biological Macromolecules Based on Chiral Nanomaterials
6.5 Biocatalysis Based on Enzyme‐Encoded Chiral Complex Nanomaterial System
6.6 Chiral Nanomaterials Based Biocatalysis In Situ for Emerging Biological Applications
References
Index
End User License Agreement
List of Illustrations
Chapter 1
Figure 1.1 (a) Type 1 chirality: the inorganic core with chiral shape. (b) T...
Figure 1.2 (A) Calculated conformers of deprotonated
N
‐isobutyryl‐
L
‐cysteine...
Figure 1.3 (a) CD spectra of Au NPs with or without E5 peptide. (b) The calc...
Figure 1.4 (A) TD‐DFT CD spectra for D‐CdS [75]. (B) UV–vis and correspondin...
Chapter 2
Figure 2.1 (A) Top and side views of a (1010) face of the CdS cluster model ...
Figure 2.2 (a) Chiroptical activity of penicillamine (Pen)‐capped CdS NPs an...
Figure 2.3 Chiral CdTe NPs preparation and characterization [37]. TEM (a) an...
Figure 2.4 Geometric and optical characterization of the
L
‐
Cys CdTe NP...
Figure 2.5 (a–c) CD activity of the
D
‐Cys,
D
‐His (and
L
‐Cys,
L
‐His) phase‐tr...
Figure 2.6 (a) Representative TEM images of
D
‐Cys,
D
‐His SNR
1
‐Au nanorods (l...
Figure 2.7 (a) Molecular graphic model of a twisted ribbon in ethyl methacry...
Figure 2.8 (a) CD spectra of chiral Ag NPs coated with various enantiomeric ...
Figure 2.9 (a) CD spectra of chiral molecular ligand‐coated silver NPs [68]....
Figure 2.10 Growth of Au/(DNA‐Ag) core–shell NPs with different shell thickn...
Figure 2.11 Chiroptical activity and geometric properties of Au/(DNA‐Ag) cor...
Figure 2.12 Scheme of the chiral GGS NSs preparation [83]. Source: Reproduce...
Figure 2.13 (a–f) Representative TEM images of GGS‐L‐0.01 NSs for various re...
Figure 2.14 TEM images of (a) GGS‐
L
‐0.01 NS, (b) GGS‐
D
‐0.01 NS, (c) GGS‐
D
,
L
‐...
Figure 2.15 TEM images of (a) Au NRs
2.12
, (b) Au
2.12
Cu
3
, and (c)
L
CF Au
2.12
Figure 2.16 (a, c) TEM images and (b, d) TEM–tomography images of Au NWs bas...
Figure 2.17 (a) Schemes for varied pitch of Ag NWs@CMS. (b) DRCD and DR‐UV/v...
Figure 2.18 (a) The glancing angle physical vapor deposition process. (b) Mo...
Figure 2.19 Schemes of (a, b) chiral Pt@Au TNRs preparation and (c) bioappli...
Figure 2.20 (a, b) TEM images of
L
‐Pt@Au TNRs (b), (c) HAADF, SAED image of
Figure 2.21 (a) Chiroptical activity and (b)
g
‐factor spectra of
L
/
D
‐type Pt...
Figure 2.22 (a) SEM images of Au spiral nanostructures via a two‐step synthe...
Figure 2.23 Opposite handedness of 3D plasmonic helicoids through cysteine c...
Figure 2.24 (A) Ligand exchange of FePd NPs from oleic acid/oleylamine to ch...
Figure 2.25 Chiral Co
3
O
4
NPs characterization. (a) CD, (b)
g
‐factor, (c) UV–...
Figure 2.26 (a) TEM image, (b) HRTEM image and SAED pattern, (c) STEM and ED...
Figure 2.27 (a) Representative TEM image, (b) HAADF‐STEM image, and (c) Gd e...
Figure 2.28 (a) Chiral Cu
2
S NPs preparation, (b) TEM and HRTEM image, (c) XR...
Figure 2.29 (a) TEM image of
L
‐Cu
2−
x
S. (b) XRD spectrum of
L
‐Cu
2−x
...
Figure 2.30 (a) CD spectra of
L
‐/
D
‐Pen and the Cu
2−
x
S NPs. (b) CD spec...
Figure 2.31 (a) TEM image and SAED pattern. (b) HR‐TEM image. (c) STEM and E...
Figure 2.32 (a) Schematic illustration of helical mesostructured rod formati...
Chapter 3
Figure 3.1 (a) Graphical illustration of Au NR dimers triggered through SCI,...
Figure 3.2
The 3D plasmonic NR dimers between three states
. (a) The plasmoni...
Figure 3.3 Walking mechanism and CD spectra at different stations. (a) Walki...
Figure 3.4 Schematic illustration of special miRNA‐21 sequence triggered the...
Figure 3.5 Confirmation of the chiral NR dimers formation through
fluorescen
...
Figure 3.6 (a–c), Chiral dimers from Au NPs [33]. (a) Schematics of an incor...
Figure 3.7 (a) Chiral Au‐Ag NP heterodimers and biological analysis [38]. (b...
Figure 3.8
Chiral geometry of NP dimers
. (a) Representative TEM image of NP ...
Figure 3.9 The characterization of the Au YS‐UCNP heterodimer. (a) The HRTEM...
Figure 3.10 Au NP chiral pyramids assembled on the DNA frame. Source: Mastro...
Figure 3.11 (a) Schematic illustration of chiral pyramid; (b) Representative...
Figure 3.12 (a) Schemes of Au‐UCNP pyramids for miRNA bioanalysis and (b) th...
Figure 3.13 (a) Representative TEM images of nanopyramids consisting of Au‐C...
Figure 3.14 Construction of self‐assembled yolk–shell NPs tetrahedron. The T...
Figure 3.15 Structure and optical characterization of chiral UYTe assemblies...
Figure 3.16 Schematic illustration of the propeller‐like NR‐UCNP tetramer as...
Figure 3.17 CD spectra of the NR‐UCNP tetramer assemblies [63]. (a, c, e) Th...
Figure 3.18 Upconversion luminescence properties of the NR‐UCNP tetramer ass...
Figure 3.19 Chiral NPs helices. (A) A single TEM image of different DNA tube...
Figure 3.20 (a) Schematic illustration of the self‐assembly of RH‐Au NR heli...
Figure 3.21 Formation illustrations. (a–g) TEM images of self‐assembly of
D
‐...
Figure 3.22 SEM images of left‐handed helices (a) and right‐handed helices (...
Figure 3.23 (a, b) The Au NP helical assembly on anionic rod‐like virus (a) ...
Figure 3.24 The illustration of the cogel formation and its CPL responsivene...
Figure 3.25 (a–d) TEM images of
D
‐Gel nanofibers,
L
‐Gel nanofibers,
D
‐(Gel +...
Figure 3.26 (a–d) SEM images of cogel helical nanoassemblies under different...
Figure 3.27 The chiral nanochains and branched heterochains. (a) Scheme of N...
Figure 3.28 Structure and optical properties of Au NR SBS nanostructures. (a...
Figure 3.29 Representative TEM images of shell–core Au satellite nanostructu...
Figure 3.30 Chiral core–shell upconversion nanoparticle@MOF nanoassemblies. ...
Figure 3.31 TEM images of the (a) UCNP@ZIF assemblies and (b) UCNP@ZIF‐NiSx ...
Figure 3.32 (a) CD and (b) UCL spectra of the chiral assemblies responded to...
Figure 3.33 Schemes of Y‐DNA‐driven construction of Au NP chiral C
30
S
5
S
10
NS...
Figure 3.34 (a–f) TEM images of dynamic satellite assemblies. (g) CD spectra...
Figure 3.35 Dual core–satellite Au NP assemblies with allosteric effect. (a)...
Figure 3.36 CD spectra of C
30
S
5
‐C
20
and C
30
‐C
20
S assemblies. Source: Qu et a...
Figure 3.37 Representative TEM images of nanoassemblies for various hybridiz...
Figure 3.38 UV–vis (a) and the corresponding CD spectra (b) of the core–sate...
Figure 3.39 Schemes of the prepared chiral Au@AgAu yolk–shell nanorods (YSNR...
Figure 3.40 (a, b) Typical TEM image of YSNRs; (c–e) corresponding CD spectr...
Figure 3.41 Typical TEM image of chiral YSNRs prepared using Au NRs with dif...
Figure 3.42 (a) Typical TEM image, (b) HRTEM image, and (c)
scanning tunneli
...
Figure 3.43 Schemes of DNA‐based NR dimer and UCNP core–satellite assembly S...
Figure 3.44 NR dimer and UCNP self‐assembly and characterization. TEM of sel...
Figure 3.45 Characterization of Au NR@Pt dimer‐UCNP satellite nanoassemblies...
Figure 3.46 Gold thiolate
hierarchically self‐organized particle
s (
HOP
Figure 3.47 Geometries and structures of the chiral Ag films. (a
1
–a
3
) SEM im...
Figure 3.48 Structural and optical characteristics of Au NP nanofilms. (a) T...
Figure 3.49 Fabrication process of chiral Au nanowire ultrathin films. (a) D...
Figure 3.50 TEM images of (a) Cu
2
S SPs, (b) CoS SPs, (c) Cu
x
Co
y
S SPs. (d) HR...
Figure 3.51 Catalysis and stability properties of ZnS‐Au SPs for photoinduce...
Figure 3.52 Characterization of Co
3
O
4
SPs@PM. (a) TEM images for
D
‐Co
3
O
4
SPs...
Chapter 4
Figure 4.1 (a) Chiral side‐by‐side and end‐to‐end Au NR assemblies for the d...
Figure 4.2 (a) DNA‐engineered chiral NP pyramidal structures for DNA detecti...
Figure 4.3 (a) Chiral Au NP heterodimers were applied for the monitoring of ...
Figure 4.4 (a) Aptamer‐engineered chiral Au NP heterodimers for AFP detectio...
Figure 4.5 (a) Schematic of a trimer structure constructed using different s...
Figure 4.6 (a) Chiral Ag@Au NP assemblies for HER2 detection expressed on SK...
Figure 4.7 (a) Schematic diagram of a colorimetric sensor for the detection ...
Figure 4.8 (a) Chiral bioanalysis of DNase I enzyme by using Au NP heteropyr...
Figure 4.9 (a) Detection of telomerase activity in HeLa cells by using chira...
Figure 4.10 (a) The detection of Ag
+
by chiral Au NPs heterodimers. (b) ...
Figure 4.11 (a) The detection of Pb
2+
by chiral Ag NP heterodimers. (b) ...
Figure 4.12 (a) Antigen–antibody bridged chiral Au NP heterodimers for BPA d...
Figure 4.13 (a) Chiral Au NP‐Ag NP core–satellite structures for OTA detecti...
Figure 4.14 (a) Schematic of the detection of SMZ by a chiral and UCPL signa...
Figure 4.15 (a) Chiral and UCPL signal–dependent immunoassay for the quantit...
Figure 4.16 (a)
L
/
D
‐Cys‐modified Au NP dimers for the chiral recognition of
Figure 4.17 (a) Cysteine enantiomer–driven end‐to‐end Au NR chains for the c...
Figure 4.18 (a) Schematic diagram of determination of cystine enantiomers us...
Figure 4.19 (a) Schematic diagram of chiral SiO
2
/PDA nanocomposites for dete...
Figure 4.20 (a) Schematic diagram of chiral identification of
D
/
L
‐Pen using ...
Figure 4.21 (a) Chiral CD sensor for intracellular Sn
2+
and
L
‐Lys detect...
Chapter 5
Figure 5.1 (a) Schematic diagram of the production of ROS and photothermal e...
Figure 5.2 (a) Schematic illustration of photothermal therapy with NPs@ZIF‐8...
Figure 5.3 (a) Synthesis and CD signal of AuCuAu HNRs. (b) Thermal images of...
Figure 5.4 (a) Vis–NIR absorption spectrum of Cu
2
O and Cu
9
S
8
. (b) Curve of t...
Figure 5.5 (a) UV–vis–NIR absorption spectrum of Cu
2−
x
Se‐Au Janus NPs....
Figure 5.6 (a) Schematic illustration of phototherapy with NP‐PPE/Ce6. (b) R...
Figure 5.7 (a) Schematic illustration of phototherapy with semiconducting po...
Figure 5.8 (a) Schematic diagram of the formation of Fe
3
O
4
@Dex/TPP/PpIX/ss‐m...
Figure 5.9 (a) Schematic diagram of aptamer‐modified ZrMOF used in photodyna...
Figure 5.10 (a) Synthesis and PDT application of TPATrzPy‐3+. (b) Yield of i...
Figure 5.11 (a) Synthesis of NGP‐TPEred. (b) Absorption and fluorescence spe...
Figure 5.12 (a) 18‐ and 26‐nm‐long CdSe/CdS dot/rod NCs produce singlet oxyg...
Figure 5.13 (a) Schematic illustration of phototherapy with Au NR@Pt‐Ag
2
S NP...
Figure 5.14 (a) The synthetic spiky Fe
3
O
4
@Au SPs can be used for phototherap...
Figure 5.15 (a) Schematic illustration of synthetic NR dimer‐UCNP‐Ce6 for ph...
Figure 5.16 (a) Diagram of Cu
9
S
8
NPs destroying the staphylococcus aureus bi...
Figure 5.17 (a) Schematic illustration of synthetic CSC2@PEG‐Dox for phototh...
Figure 5.18 (a) Schematic diagram of the formation of Au@Bi
2
S
3
core–shell NR...
Figure 5.19 (a) CD spectra of GGS NSs (the number represents the concentrati...
Chapter 6
Figure 6.1 (a) Illustration of stereoselective catalytic oxidation of the DO...
Figure 6.2 (a) Enantioselective oxidation of chiral DOPA by
D
‐cys/
L
‐cys@AuNP...
Figure 6.3 (a) Schematic of construction of a chiral COF nanozyme. (b) The t...
Figure 6.4 (a) Representative structures of natural heme‐containing peroxida...
Figure 6.5
L
‐Proline@Fe
3
O
4
nanoparticle and (
L
‐proline@Fe
3
O
4
)@TMU‐5 nanocomp...
Figure 6.6 (a) Schematic illustration for the sonication combined with ion i...
Figure 6.7 (a) Binding possibilities of proline on GrO or GnO. Scheme of the...
Figure 6.8 (a) Representative high‐resolution TEM image of
L
‐Asp‐NPs of WO
3−
...
Figure 6.9 (a) (A) Morphology of multicomponent
L
‐ZnS–Au SPs. (B) HAADF‐STEM...
Figure 6.10 (a) Schematic illustration of selective DNA cleavage by chiral c...
Figure 6.11 (a) TEM image of
L
‐QDs. (b) CD spectra of Pens and the QDs prepa...
Figure 6.12 (a) Principle of coencapsulation of SCRII–GDH on ZnO/carbon nano...
Figure 6.13 (a) CD spectra for CytC (red, 6 mM), CdTe NPs (black, 6 mM), 1 :...
Figure 6.14 (a) Schematic illustration of the preparation of chiral catalyst...
Figure 6.15 (a) Chiral Cu
x
Co
y
S NPs produce ROS under the synergistic action ...
Figure 6.16 (a) Schematic illustration of
D
‐Pen Fe
x
Cu
y
Se NPs generating ROS ...
Figure 6.17 (a) Schematic illustration of
D
‐CDs stimulating enzyme activity ...
Figure 6.18 (a) Preparation of Co
3
O
4
SPs@PM and its application in thromboly...
Figure 6.19 (a) Mechanism of chiral Cu
2
S NPs blocking HBV infection under 80...
Figure 6.20 (a) Schematic illustration of antioxidant activity of chiral G@S...
Guide
Cover Page
Title Page
Copyright
Preface
Table of Contents
Begin Reading
Index
Wiley End User License Agreement
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Chiral Nanoprobes for Biological Applications
Edited by Chuanlai Xu
Editor
Prof. Chuanlai Xu
Jiangnan University
Food Science and Technology
1800 Lihu Da Road
Binhu District
214122 Wuxi
China
Cover Image: © Chuanlai Xu
All books published by WILEY‐VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
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Print ISBN: 978‐3‐527‐34942‐5
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Preface
Chirality has always been one of the core research topics in multidisciplinary fields including materials science, chemistry, biology, physics, and so on. Of particular interest, recent advances in chiral nanomaterials represent one of the most dynamic areas and have emerged as a new “growth point” of nanoscience. Recent breakthroughs in chiral fabrication have led to the study of many interesting properties of chiral nanocrystals, and some have demonstrated realistic application potentials that may address key challenges at the heart of nanoscience. Nevertheless, only few chirality‐related books have been published in recent years, most of which essentially focus on the introduction of different types of chiral materials, chiral nanophotonics and spintronics, and chiral separation techniques. However, the available books usually ignore or barely focus on introduction of chiral nanomaterials in biological applications, which is attracting increasing theoretical and applicative interests.
These reasons made us to realize that there is a need for a book that is capable of:
(i) presenting the techniques and strategies in a clear and concise manner to understand chiral fabrications, chiroptical mechanisms, structure manipulation, and chiroptical enhancement;
(ii) illustrating biological applications based on intense chiroptical signals, including chiral‐encoded recognition, bioanalysis, photochemical effects, and biocatalysts; and
(iii) encouraging the researchers to the forefront of developments in chiral nanoscience, such as advanced chiral‐related multidisciplinary techniques for innovative fabrication methods, chiral‐related optical and physicochemical properties, futuristic visions of the light–matter interaction.
It is timely and necessary to write a comprehensive book to introduce the state‐of‐the‐art advancements in strong chiroptical nanomaterials, their biological applications and to discuss their future avenues of research. To address such emerging advances, we present the book Chiral Nanoprobes for Biological Applications. In this book, we have attempted to provide developments in chiral nanoscience, with special emphasis on the milestones achieved in research on key chiral nanoprobes in the recent past five years, focused on biological applications, and provided some specific future perspectives in the field.
Being actively involved in chiral nanoscience and having published more than 100 research articles and reviews, we have included six distinct chapters in the book. Chapter 1 gives an introduction of chiral nanomaterials from their biological counterparts, chiral mechanism and theory. The origin of chiral materials brings new insights into understanding homochirality on Earth. The fundamental concepts about the nature of chirality is divided into four types – the inorganic core, chiral ligands, chiral patterns, and electronic structures. The general principles, modeling, and methodology characteristics for computational studies of chirality have been presented for better understanding of nanoscale chirality.
Chapters 2 and 3 introduce the fabrication technologies and strategies for fabricating individual chiral nanomaterials and chiral assembled nanostructures, with a focus on tailoring and enhancing chiroptical effects. Major efforts have been made to realize high‐purity chiral nanomaterials consisting of ligand molecule moieties, semiconductors, metals, and other materials. Individual nanoparticles and their superstructures can display intensive chiroptical and chiral‐coupled physicochemical or electrochemical secondary properties, as well as unique and useful functions. Strategies for chiral amplification including enantiopure nanocrystal synthesis, and highly spontaneous symmetry breaking or well‐controlled chiral geometrical assemblies (pyramidal, helical, etc.), as well as high‐order enantiomeric superstructures.
Chapter 4 is focused on analysis of chiral nanomaterial‐based ultrasensitive biomolecules (DNA, biomarkers, enzymes, etc.). Chiral nanoprobes offer a new potent platform for bioanalysis featuring low sensitivity associated with biosensing, biolabeling, and bioimaging using individual chiral nanostructures or chiral assemblies. Chapter 5 extends chiral nanoscience in emerging biological effects including photothermal and photodynamic therapy as well as photochemical effects. Chiral nanomaterials selectively absorb photons and can convert light into heat or reactive oxygen species to enhance phototherapeutic or photochemical‐related biological effects. Chapter 6 illustrates the fundamental concepts and the characteristic approaches related to chiral nanomaterials for biocatalysis. Chiral nanomaterials demonstrated as promising biomimetic catalysts afford facile preparation, tunability of catalytic activity, and high stability. Chiral molecules or chiral polymers are considered effective chiral selectors of nanocatalysts for asymmetric biocatalysis.
Finally, we would like to summarize future trends and perspectives in chiral nanoscience. Robust artificial chiral fabrication techniques and multilevel hierarchical chirality control for precise tuning of polarization rotation in inorganic nanostructures will enable development of multidisciplinary chiral nanoscience in future. (i) Synthesizing or assembling complex nanostructures with ultrastrong chiroptical effects is an intrinsic challenge for all nanoarchitectures without exception, where. extreme control on enantiomeric purity to achieve high anisotropy factors is in great demand. (ii) It is crucial to integrate multiple techniques from various fields such as supramolecular chemistry, inorganic synthesis, self‐assemblies, and related large‐scale construction (lithography, selective deposition, glancing angle evaporation, etc.) for homochiral fabrication in large quantity with a higher degree of structural chirality. (iii) The theoretical simulation can be used to uncover the chiral growth dynamics for predicting potential routes for chiral induction and amplification, which may include how the Ångström‐ and nano scale chirality being induced and transferred to the scales of meso‐, micro‐, and macro inorganic structures? Through theoretical simulation of growth dynamics, the facile design and fine‐tuning of chiral nanostructures with predesigned biological functions will be available. (iv) Chirality‐related physicochemical properties may lead to a wide range of potential biological applications. There will be great opportunities to elucidate nanoscale chirality interactions with biological systems, such as chirality coupled biological processes and their evolution, chiral cascaded bioactivity levels, even for modulation of metabolic and immune processes. (v) The high degree of manipulating chiral‐related spins, electrons, and photons is promising for real‐time biological applications, such as light‐responsive molecular machines or excellent candidates for better therapeutics or biomimetic nanocatalysts.
Jiangnan University05 January 2022
Chuanlai XuHua KuangLiguang Xu
1Introduction
Chuanlai Xu, Hua Kuang, and Liguang Xu
Jiangnan University, International Joint Research Laboratory for Biointerface and Biodetection, State Key Lab of Food Science & Technology, and School of Food Science and Technology, 1800 Lihu Road, Wuxi, Jiangsu Province, 214122, China
Chirality is a widespread phenomenon in nature and plays an important role in chemistry, pharmacology, biology, and medicine. If a molecule cannot be superimposed on its mirror image, it is called a chiral molecule. The structure is mirrored into symmetrical molecules and chiral enantiomers. As a ubiquitous phenomenon in nature, chirality has been extensively studied at the molecule level. Most biomolecules are chiral and exhibit single structure, such as DNA, amino acids, peptides, and sugars. Moreover, lots of important biological reactions involve chiral interactions [1,2]. Since Louis Pasteur firstly separated the tartaric acid enantiomers in 1848, chirality has fascinated the chemistry world [3,4]. In the pharmaceutical industry, the left‐handed or right‐handed isomers of some molecules show completely opposite pharmacological effects and toxicity. To avoid the adverse effects of chiral drugs, the selective synthesis of chiral drugs has been a research hot spot in the field of pharmacy. Therefore, it is of great scientific and practical significance to explore the role of material chirality in the origin of life, chemical reactions, and disease treatments. Until now, molecular‐scale research is relatively mature, and chiral research and exploration promotes the progress of science and technology in different fields, especially in nanoscience.
1.1 Chirality and Asymmetry
Chirality is a geometric property that implies that a mirror image of an object cannot overlap with the original object [5,6]. Notably, this attribute is related to the scale or the state of matter; thus, it can exist in any object, real or imaginary. The term “chirality” refers to unique geometric shapes of atoms, molecules, particles, etc., other than the resulting optical or chemical effects. In chemistry, two nonsuperimposable objects that have a mirror‐symmetry relationship are called enantiomers. The simple physical properties of pure enantiomers of organic materials, such as melting temperature, evaporation temperature, and color, exhibit identical features. Chiral molecules with the same chemical composition and have no mirror symmetry are diastereomers. Diastereomers have different physical properties. Because of many choices for the geometric positions of atoms in nanoparticles (NPs), diastereomers with small differences are the most common in NPs, such as phase transition temperatures or color. Furthermore, the secondary properties of chiral assemblies (including optical, chemical and biological performances) are the derivatives of the geometry of a particular chemical object at one or all the scales mentioned above. Of course, these properties are the main reasons for formation of chiral molecular and nanoscale structures in most cases and determine the functionality and practicality of chiral nanomaterials.
Moreover, the difference between individual objects and a collection of a mass of these objects also require the secondary effects of chiral nanostructures. The collection of chiral objects is usually in the solution of chiral molecules or dispersions of individual chiral NPs. The relative number of respective enantiomers determines the possibility of observing the secondary properties of chiral chemical structures. Notably, individual objects, such as molecules, nanoparticles (NPs), or higher order structures, can be chiral. However, positive and negative polarization rotations of different enantiomers may cancel out each other. Thus, the spectral detection of dispersions or solutions may display no rotatory optical activity. Moreover, ensembles of chiral molecules, NPs, and their nanostructures can usually exhibit equivalent enantiomers. Thus, these ensembles are called racemic mixtures. When the amount of one enantiomer is higher than that of the other one, the mixture is called non‐racemic. Typically, when the molar preference of one enantiomer is relative to the other one, it is usually expressed as an enantiomeric excess. The enantiomeric excess is measured as a percentage (%), and typically abbreviated as ee. When a solution or dispersion is made up of a single enantiomer, it is called homochiral with an ee of 100%. As pointed out by Kelvin [7], Pasteur [8], and Piutti [9], homochiral compounds are common in bioorganic materials. However, they are not applicable in many inorganic nanostructures, as discussed in this book. Through chiral separation [10], e‐beam lithography, and NP self‐organization [11], nearly all homochiral chemical systems have been obtained from inorganic nanostructures.
1.2 Chiral Origin of Nanomaterials
1.2.1 Chiral Geometries of Nanoparticles
The typical pictorial representation of NPs as perfect spheres, cubes, ellipsoids, etc. may not be the best way to keep in mind when it comes to understand their chirality. Asymmetric NP cores, ridges, apexes, adatoms, vacancies, dislocations, etc. conduce to chiral geometric forms of these chemical species [12]. In addition, edges, apexes, and faces of an inorganic material cannot be coated with surface ligands uniformly or in a widely accepted radial manner. Therefore, the surface layer contains asymmetry, except for core defects, and causes the chiral geometric elements. The latter can happen, as exemplified by the epitaxial lattice‐to‐lattice coalescence of NPs, also known as oriented connection [13]. Even if the overall shape of the NP is spherical, or the inorganic core crystallinity is perfect, or the stabilizers evenly modify, the NP may still be chiral. Under the external bias conditions of a given electric or magnetic field, the high polarizability of the inorganic core makes the electron density asymmetrical, resulting in chirality. Therefore, there may be multiple sources of chirality in NPs, and it may be difficult as well as interesting to know which of them is/are responsible for specific secondary effects.
Figure 1.1 (a) Type 1 chirality: the inorganic core with chiral shape. (b) Type 2 chirality: chiral surface of the inorganic core. (c) Type 3 chirality: chiral pattern of the surface ligands. (d) Type 4 chirality: polarization effects [1]. Source: Ma et al. 2017, Reproduced with permission of American Chemical Society.
The first type of chirality is related to the asymmetry of the nanostructure's inorganic core. The object here is a continuous part of an inorganic substance, which defines the nanostructure's inorganic properties (Figure 1.1a). NPs with different shapes have been included in this group, such as tetrahedrons with variable truncations [8], twisted gold clusters [14,15], and twisted nanorods (NRs) [16]. Apart from the NPs, nanohelices [17], twisted ribbons [18], and asymmetric objects with less traditional shapes (such as nanopillar caps [19]) are also included to this part.
In most situations, instead of packing a collection of atoms in a specific crystal lattice, the geometry of the NP core can be taken into account as a whole, especially for nanocolloids via bottom‐up techniques. Overall shape of NPs should be given for the materials with chiral lattices, exemplified by macroscale crystals of tartaric acid separated by Pasteur. Assuming that an inorganic material was fabricated with a chiral crystal lattice by the top‐bottom method, such as lithography, a mirror‐symmetric cube is realistic. However, a non‐racemic ensemble of such NPs and a chiral bias at the atomic scale during the growth will be required to explore chirality in such materials.
From the synthetic point of view, though the organization of the inorganic core is often strongly affected by the surface ligands, Type 1 chirality does not consider surface ligands. In fact, it is difficult to distinguish whether the surface layer chirality is resulted from the chiral packing of atoms in the small cores (0.5–1.5 nm) or vice versa. Thus, from this perspective, one may consider it as a controversial issue to distinguish the ligand shell chirality and core chirality; however, this is not the case. Actually, it is possible to decouple the chirality of the NP cores and the organic shells. Chiral geometries of inorganic cores with several nanometers have been reserved in the case of altered composition of the surface layer, such as twisted rods and helices [20–22].
Moreover, from the thermodynamic view, geometrically distorted structures will be more stable than perfectly packed symmetric crystal lattices. Therefore, organic ligands are not essential for the occurrence of the NP cores chirality. For example, as observed in gold nanoclusters, the energy preferred to the chiral structure instead of the symmetric structure, which resulted in the transition from the symmetric cubic to the twisted lattice [23]. As a point to illustrate the importance of understanding the chirality of ligand‐free NPs, chiral inorganic NPs with a limited amount of stabilizers might be characteristics of interstellar matter.
The driving force of the chiral configurations of NP cores is different from that of surface ligands, including the thermodynamically favorable distortion of the nanoscale inorganic substance lattice. For example, a perfect tetrahedron can be prepared merely by one atomic packing; however, a tetrahedron with four different apexes possesses degenerate ways to arrange these apexes. Compared with an achiral perfect tetrahedron, such degeneracy makes the defects stable with an entropy of c. 8 kJ mol−1. Therefore, according to the enthalpy contributions of each defect, a clear distribution of chiral NPs can be obtained. Nevertheless, not all NPs can be explained by such a purely geometric illustration. It is essential to explore the amount of entropy and enthalpy caused from the defects of chirality.
Other types of chiral NP geometries are associated with the existence of surface ligands. These surface ligands make adequate for experimental observations under certain conditions. Type 2 chirality of NPs is related to asymmetries of the NP core surface. When the chiral surface ligands result in such asymmetries, it can be called chiral footprint [24]. More generally, it is inorganic surface chirality. In the presence of stabilizers or other factors, the NP core atoms exhibit distortions and displacements, such as addition of atoms, erosion of atoms, or atomic reconstruction responding to chiral media (Figure 1.1b). Different from Type 1 chirality, in this case, the shape of the inorganic core may be achiral. The surface ligands display intense interactions with the core of inorganic materials. Usually, two anchoring groups are carried along with the inorganic core, and the two oxygen atoms are from two carboxylates, or carboxylate groups and thiolates. Particularly, the anchoring groups may lead to the distortions easier than the weak ligands with single‐point attachment. For example, the L‐cysteine methylester, N‐isobutyryl‐cysteine, and D/L‐penicillamine ligands possess carboxylates and thiol groups, which may result in chiral distortions [25–27].
In most cases, Type 2 chirality is related to surface ligands. Note that surface ligands can be chiral, racemic, or achiral. Type 2 chirality exhibits a significant difference from Type 1 and Type 3 (discussed later). Type 2 chirality is affected and related to the specific surface states of inorganic matter, while the electronic states related to the inorganic core (plasmonic, excitonic, etc.) are not affected [24]. To further illustrate the importance and universality of Type 2 chirality of individual NPs, it will be discussed in the context of classical researches of surface chirality, such as inorganic surfaces associated with the origins of life [28]. This type of chirality may happen in a lot of cases, as exemplified by surface reconstruction. For a detailed example, when all mirror symmetry planes are destroyed, the surface atoms shift in unusual configurations can appear after the adsorption of tartaric acid on Cu [29,30] or Ni surfaces [24] (Figure 1.1b). It seems that the double or higher coordination of the ligands is essential; otherwise, the ligands might easily rotate freely around their unique NP bonds, and such thermal agitation may result in full racemization (ee = 0%).
Type 3 chirality in individual NPs is related to chirality of the stabilizer shell, as exemplified by the molecules with chiral organic shell and achiral for their core and surface [10,31]. Importantly, achiral surface ligands may also result in chirality. Here, the stabilizer molecules packing on their surface are related to the symmetry breaking (Figure 1.1c). In other words, the geometry of the object considered here is a collection of the surface‐bound organic constituents. Type 3 chirality of chiral NPs shows obvious different biological and chemical properties, since it is the main determinant factor for NP–protein interactions, NP assembly into the superlattices, as well as chiral separations.
The bonding mode of stabilizer molecules might not follow the underlying inorganic crystal lattice symmetry and may lack the necessary symmetry elements. This chirality can be produced by the chiral arrangement of Au–thiolate ligands, such as a staggered arrangement. Taking Au38(SCH2CH2Ph)24 NP as an example, it displays a prolate shape and has a face‐fused biicosahedral Au23 core protected by three short Au(SR)2 and six long Au2(SR)3 staples. The Au23 core is a perfect D3h symmetry [10]. In particular, the long staples are organized in a chiral staggered configuration, which consists of two triblade fans with three staples. They can rotate either in a clockwise or anticlockwise. In the case of short staples, they are slightly tilted, to follow the handedness of the long staples. Similarly, Au40(2‐PET)24, Au38(2‐PET)24, and Au38(SCH3)24 clusters were reported to show chirality analogous to Type 3 chirality, which are arranged around the achiral metal core by the chirality of the Au‐thiolate ligand [32,33]. Some of the organic shells may show a patchy‐like geometry, which may cause a nonsuperimposable mirror image, particularly for the three‐dimensional (3D) surface layer [34]. It is worth to mention that when the temperature rises to the activation energy threshold for the translation of molecules, as exemplified by thiols along the underlying inorganic surface, mobility of the organic molecules on the surface of NPs may give rise to the transformation of one NP enantiomer into another one.
Type 4 chirality in individual NPs can be described as chiral field effect (Figure 1.1d) [35]. This type of chirality, unique to NPs, is related to high polarizability of some inorganic nanomaterials. Similar to Type 3 chirality, the atoms are achiral distributed in the NP core; however, the electrical field of the surface ligands or other source may affect the distribution of electrons. In achiral ligands with chiral adsorption patterns, it may produce asymmetric polarization patterns. The asymmetric polarization patterns can also occur in chiral ligands with achiral adsorption patterns, as well as in the conditions for chiral ligands with chiral adsorption patterns. Chirality can be produced near enantiomers by high polarizability of the inorganic core typical for plasmonic NPs. It is normal for chiral ligands with achiral adsorption patterns to show Type 4 chirality. The modified NPs often show a chiroptical activity in the ultraviolet (UV) range (200–400 nm). Because of the polarizability of the NPs, the circular dichroism (CD) spectrum during the UV part may change in comparison to their original chiral ligands. Moreover, chiral bands can also be produced in the plasmonic region, such as ∼405 nm for Ag NPs, 520 nm for Au NPs. These NP systems can include peptides [36], peptide nanotubes [37], or proteins [38], which are physically or chemically adsorbed on surfaces.
Under the incident electromagnetic radiation, the high polarizability of metal NPs can give rise to localized areas high electrical field around them, which are usually called “hot spots.” And then, it can lead to intense electronic transition enhancement for organic ligands. In addition to Raman scattering enhancement, these hot spots can also enhance chiral activities [39]. This strong effect derives from the polarization of electron density in NPs, which reflects the existence of the organic matter chirality in the hot spots. Therefore, it has been classified as Type 4. Notably, because of the resonance between the adjacent plasmon polarization oscillations, chiral band is enhanced by the hot spots in NP assemblies. Due to their popularity in the academic literatures, it can be attractive to attribute the chiral activities of the NP assemblies to the plasmonic enhancement effects. As a matter of fact, the diverse contributions to the CD activity requires to be evaluated [22].
In general, it is essential to understand the atomic and molecular structures to distinguish the different types of chirality (Type 1, Type 2, Type 3, and Type 4) for NPs. In fact, it is still quite difficult to decouple the chiral effects from all those asymmetric geometric features based merely on the results of CD or optical ration dispersion (ORD). Nevertheless, this is feasible to illustrate the molecular dynamics of NPs and their interactions with the incident photons based on multiscale computational methods. It can be simply assumed that there is Type 4 chirality in plasmonic NPs with a chiral surface layer.
1.2.2 Structural Origins of Chirality
Type 1 chirality in NPs was distinguished in experiments [40] and simulations for Au nanoclusters [41]. Taking advantage of genetic algorithms and many‐body potentials, the density functional theory (DFT) calculations method serves as a powerful tool for simulations of chiral nanostructures among the computational tools. Only relatively small atomic systems of the above‐mentioned chiral nanoclusters can be studied at present. DFT studies have shown that methyl thiol ligands on a truncated cuboctahedron with a face‐centered cubic structure can result in chiral atomic packing of the inorganic core, such as Au38 cluster coated with 24 HSCH3 molecules resulting in chiral distortions on the Au38(SCH3)24 cluster [42]. The Hausdorff chirality measure (HCM) shows a quantitative measure of asymmetry for any geometric figure [43], the HCM value increases with the number of thiol molecules. HCM is equal to 0.160 and 0.121 for methyl thiol‐stabilized Au28(SCH3)16 and Au38(SCH3)24 clusters, respectively [44]. Similarly, some theoretical studies have shown that many of the asymmetric or asymmetric topological structures have energy close to or lower than the lowest energy ordered isomers for bare Au nanoclusters in the range of 10–75 atoms [45].
Type 2 chirality was distinguished in NPs capped by N‐isobutyrylcysteine [25]. These molecules were attached to the Au surface in the form of two points and distorted the underlying inorganic core, resulting in intense CD activity (Figure 1.2A,B) [26]. Another representative research was about Au102(p‐MBA)44 clusters. A chiral stabilizer shell appeared due to the hydrogen bonding between carboxylic acids; the achiral p‐MBA gave rise to a chiral displacement of atoms on the Au surface [47]. The ligand exchange following the synthesis experiments demonstrated the chiral memory of NPs. Taking CdTe NPs as an example, the surface memory was also found for NPs with Type 2 chirality [27]. The surface of thiol‐coated CdTe NPs was distorted with the addition of a chiral ligand. The achiral thiol‐coated CdTe still showed a symmetrical mirror chiroptical band, which was similar to the original chiral L‐cysteine methyl ester hydrochloride‐coated CdTe, after the ligand exchange reaction in the presence of the achiral thiol. Such effects were related to the “stored” chirality on the NPs surface instead of that in the NP core.
Type 3 chirality was distinguished in NPs with a symmetric Au core. The ligand stabilizers were attached to the “staples” motif and created chiral patterns on the surface of NPs. In the case of Au38(SR)24 protected by three RS(AuSR) and six RS(AuSR)2 units, both of the crystallographic nanostructures and the optimally simulated nanostructures exhibited an achiral gold core with 23 atoms (Figure 1.2C) [46]. However, the geometric arrangement of the nine Au‐S units indicated chiral D3 symmetry. Such nanoclusters demonstrated intense chiral optical activity corresponding to the metal–metal or ligand–metal charge transfer (Figure 1.2D). Similarly, Au40(2‐PET)24 generated enantiomers with opposite CD signals following the separation on a chiral cellulose‐mediated high performance liquid chromatography (HPLC) column, in spite of the achiral molecule of the thiolate stabilizer. The 2‐positron emission tomography (PET) staples on the Au40 cluster surface were believed to cause the enantiomeric preference in NP adsorption to cellulose [32]. Notably, other types of chirality also existed in the above‐discussed NPs. It may be worth to point out that rarely NPs only displayed one type of chirality. Due to the involvement of different scales and different matter (organic or inorganic), the secondary effects of all types were obvious different.
Type 4 chirality in NPs is suitable for an achiral metal core placed in a chiral environment. Charge‐perturbed particle‐in‐a‐box calculations demonstrated that chiral stabilizers placed in achiral adsorption patterns on a metal NP surface resulted in chiral image charges in inorganic crystal lattices. The asymmetry of the adsorption mode and the influence of the ligand on the electrostatic perturbation have a significant inducing effect on the chirality of the symmetrically polarizable core (Figure 1.2E,F) [35]. Some researches ascribed the observed optical and chemical effects to this type of chirality. Typically, their dispersions show chiral optical activity of the stabilizers (such as L‐glutathione) and inorganic core (such as Au) in the spectral window. This observation indicated that the chirality of surface ligands was transferred to the plasmonic oscillation structure of the metal core through a variety of possible mechanisms.
Figure 1.2 (A) Calculated conformers of deprotonated N‐isobutyryl‐L‐cysteine on Au8 nanocluster. (B) Comparison between simulated and experimental VCD spectra of N‐isobutyryl‐L‐cysteine on Au nanoclusters [26]. (C) Chiral D3 arrangement of the Au‐S atoms (a). Optimal SCH3 arrangement on structures (b). (D) Simulated rotatory strength and chiral spectra [46]. (E) Au28(R‐methylthiirane)6 (a) and Au28(glutathione)6 (b) using partial charges for the molecules and their image charges. (F) The simulated CD spectra [35]. Source: (A–C) Reproduced with permission from Refs [26,46], respectively. Copyright 2006 and 2010, respectively, American Chemical Society; (E, F) Reproduced with permission from Ref 35. Copyright 2006 Royal Society of Chemistry.
The near‐field effect often includes the description of various plasmonic NPs and has been extensively treated theoretically [48] and experimentally [49]. As a matter of fact, an explanation of the CD activity of the plasmonic NPs typically involves the effect of the NP field on chiral stabilizers and/or the field of stabilizers on the NP core, such as Au NPs with chiral 1,3‐disubstituted diaminocalix arene ligands [50], peptide‐coated Au NPs (Figure 1.3a,b) [51], and photo‐synthetic proteins [38]. High enhanced rotatory optical activity is ascribed to the field effects (Figure 1.3c,d) [52], which displayed substantial practical value.
Figure 1.3 (a) CD spectra of Au NPs with or without E5 peptide. (b) The calculated CD spectra for a dipole of chiral molecule and for a dipole–nanoparticle complex with two separations [51]. (c) Salt‐dependent CD spectrum evolution of ssDNA‐modified Ag NPs with ionic strength. (d) The design of “individual plasmonic chiral NPs” using Au/Ag core–shell nanocube. The UV–vis absorption and CD spectra of ssDNA‐coated Ag nanocubes [52]. Source: (a–c) Reproduced with permission from Refs [51,52], respectively. Copyright 2011 and 2013 American Chemical Society.
The far‐field electromagnetic coupling between a localized plasmon of an achiral structure and the surrounding chiral molecular layer may also give rise to CD activity. Far‐field electromagnetic coupling effect caused the unexpected chiral distortions of the electron gas. It has been confirmed by comparing experimental results with a simple electromagnetic model, which included a plasmonic object coupled with a chiral medium [49]. So far, even a chiral NP may be considered a complex chiral system with multiple chiral geometric structures simultaneously. In addition, due to other chirality triggered by the chiral arrangements of the stabilizer shell, it is quite difficult to point out the thermodynamic origin of chirality in NPs. Noted that the secondary properties‐optical, chemical, biological properties could be derived from one configuration rather than the other enantiomer configuration, resulting in the understanding of different contributions to chirality in NPs. Therefore, it is still challenging to determine the origin of secondary effects of chirality in many nanomaterials.
Therefore, the ligand exchange of chiral ligands on the NP surface, such as thiolate to phosphine, often gives rise to the chiral band modification. However, it remained unclear as to the NP structure alterations in this process and remaining chiral/achiral structure of the metal core [53]. The chiral ligand (N‐isobutyrylcysteine) was exchanged and caused the chiral inversion by stabilizers, but the atomic structure of the NPs remained unchanged. Even if the ligand was exchanged with achiral ligands, the induced surface chiral distortion could be retained, especially for multiply anchored ligands, such as carboxylate groups and thiolate ligands. Importantly, both the stabilizer shell and the metal core may change by the ligand exchange, including the average size. The ligand exchange can give rise to smaller or larger size clusters. For example, if the ligands are exchanged with hexanethiol, Au75 clusters can be obtained [54]; however, Au13 icosahedron can be formed from Au25 by etching the core [55]. The ligand exchange chirality causes alterations in the metal core, which requires further studies.
1.3 Relationship with Known Chiral Materials
1.3.1 Similarity to Chiral Inorganic and Organic Molecules
Considering the diversity of chiral structure origins of NPs, it should be emphasized that these geometric shapes also exist in many other chiral materials based on the traditional chemistries and compounds. The structural design of different scales and multifunctional “building blocks” may be extended from various branches of classical chemistry to nanomaterials of “nano”chemistry [56]. For example, the NP assemblies resemble the well‐known supramolecular assemblies of organic molecules [57]. Besides, as the similar in the scales and geometries between the NPs and the liquid crystal (LC) phases [58], the similarity has led to the emergence of hybrid nanoparticle–liquid crystal (NP–LC) materials, which is a promising direction in the field [59]. The cholesteric LCs may be a preferentially selected system for future study. It may be an attractive research direction to study the chiral induction of achiral LCs in the nematic phase.
The twisted NR pairs from Au or tellurium (Te) exhibit obvious similarities to the well‐known binaphthyl compound (BINAP) with the geometry analogous to that of open scissors [60]. Chiral compounds assembled through metal center coordination bonds also have many similarities, although their scale usually is smaller than individual NPs or their assemblies [61]. Moreover, propeller‐like coordination complexes, as one of the earliest chiral inorganic compounds, have been achieved in inorganic NP assemblies with a DNA‐bridged NR assembly around an NP [62]. Large‐scale coordination compounds, such as metal–organic frameworks (MOFs) and polyoxometalates (POMs) [63], have also been extended to the nanoscale. Importantly, the chiral geometries and chirality transfer mechanisms of chiroptical POMs resemble those of individual NPs. Therefore, it has paved a flexible and feasible way for the construction of chiral POM–NP hybrid nanostructures [64].
1.3.2 Similarity to Biological Nano‐ and Mesoscale Compounds
As in biology, a great majority of biological molecular and supramolecular compounds are chiral, which can be attributed to homochirality of life on Earth. There are multiple considerations for the similarities between the chirality of NPs and biomacromolecules. One common phenomenon is that chiral biomolecules are generally used as ligands at the surfaces of NPs, which partly contributes to chirality of individual NPs. Moreover, the size of the NPs is similar to the observed biological macromolecules. The characteristic size of numerous proteins is a few nanometers, while that of amino acids and nucleotides is usually smaller than 1 nm. Some proteins are even larger, with a size of tens of nanometers. Furthermore, some anisotropic protein molecules reach mesoscale dimensions. Therefore, a 300‐nm‐long triple helix of collagen molecule assembles in over 20 μm fibrils. Almost all soluble globular proteins, their oligomers, and complexes display intricate molecular shapes. These nanoshapes show obvious chiral activity and belong to a number of chiral point groups. In this case, the analogy to NPs with Type I chirality is very clear. To further expand this idea, the surface chemistry of water‐soluble NPs shows an obvious similarity to the chemistry at the interface of protein molecules and aqueous media due to the dependence on the same chemical groups: –OH, –COOH, –NH2, etc. Therefore, the stabilizer shell chirality can be compared with the atomic‐scale packing of similar chemical groups and segments on the outer surface of protein or DNA globules. Similar parallel processing can also be carried out between the “staples” of the surface ligands on NPs and packaging of peptides, such as chaperonin molecular units [65].
1.3.3 Hierarchical Chirality
The chiral geometric structures of inorganic and biological nanostructures show obvious similarities. New insights may be brought into NPs and other nanostructures by the juxtaposition of biological and inorganic nanostructures, one of which is hierarchical chirality. For example, in proteins, cooperative behavior of hydrogen bonds and other intermolecular interactions can give rise to secondary nanostructure patterns from amino acid chirality at the atomic scale, as exemplified by α‐helices and β‐sheets. The secondary chiral structures may further generate the tertiary and quaternary structures of proteins. As common in protein complexes and rodlike viruses, helicoids can be produced with the self‐assembly and aggregation of proteins [66]. Due to the complementarity of the chiral motifs and cooperativity of the interactions, atomic‐scale chirality of NP stabilizers and NP cores can result in a similar hierarchy of chiral inorganic NPs structures. In the arrangements of stabilizers on the NP surface, cooperativity interactions are evident [67]. Hierarchical chirality can be observed in NP assemblies, such as the chiral supraparticles from Au NRs with a twisted rod arrangement originated from the CdTe NPs shell with amino‐acid surface ligands. More complex hierarchical assemblies can also be prepared. The assembly of chiral NPs is similar to that of tobacco mosaic virus [68].
Other research fields associated with chiral NPs may also be inspired by the relationship between the chirality of individual NPs and biological molecules, which are based on the difference in biological activity of different enantiomers. The interactions between chiral graphene quantum dots and mammalian cell membranes show the difference, which have been observed experimentally and computationally. Different enantiomers of NPs may selectively interact with chiral biomolecules, such as enzymes, membranes, and even DNA. Therefore, they may bring distinct environmental and health hazards. These chiral NPs have not been observed in experiments and will be resolved in the future.
1.4 Characterization Methods
1.4.1 Optical Spectra of Chiral Nanostructures
Optical characterizations are the core secondary properties of chiral molecules and nanostructures in the field of physical phenomena. Therefore, it may be useful to study the origin of chiral NPs, their relationship with geometric structures, and chiroptical information from different types of NPs. Most of the chiroptical activity of individual NPs is associated with two quantum mechanical phenomena at the nanoscale, which are plasmons in metallic NPs and excitons in semiconducting NPs. The delocalization of charge carriers over the NPs can be characterized by these electronic states and therefore mainly depend on the crystal lattice of the NP core. In addition, metal‐to‐ligand charge transfer (MLCT) transitions may occur at the interface of the stabilizers and metal core [69]. All these excited states are influenced by shapes, sizes, surface ligands, and the surrounding dielectric environment of the NPs. When nanostructures exhibit a chiral shape, their plasmons and excitons may display chirality. Moreover, any other optoelectronic properties may also show chirality, which depend on the symmetry of the inorganic core and the overall electronic structure related to the excited states of the NP core. Certainly, the MLCT transitions depend on the chirality of ligands. They also depend on the symmetry of the NP core because of Type 2 chirality in the NPs and the contribution of other chirality.
In general, CD spectra and optical rotation dispersion (OR or ORD) spectra are the most commonly used spectroscopy tools in this field. They are generally used to describe the electronic transitions of chiral NPs from the visible (vis) to the near ultraviolet (UV) photons, including electronic and plasmonic transitions. The circular polarization of the incident photons can determine their vibrational components and can be measured by vibrational circular dichroism (VCD) and the Raman optical activity (ROA) methods [70]. Similarly, in the case of inorganic structures, CD and ORD spectroscopy are the most commonly used tools, and other spectroscopies can be applicable as well [26].
The ORD spectrum describes that optical rotation of a substance changes with the wavelength of light that passes through it. In this case, smaller wavelengths lead to greater optical rotation with the same path traveled by the light. The ORD spectrum can describe polarization plane rotation when a linearly polarized light beam passes through a material. Equation 1.1 describes the plane rotation per unit length, which expresses the circular birefringence of the chiral media. When the left circular polarization and right circular polarization components of linearly polarized light propagate in the chiral media, the difference between the refractive indices nL and nR can be positive or negative, related to the common chemical names of right‐ and left‐handed media, respectively.
ORD spectrum can offer responsible experimental signals for chiral NPs, which can also be calculated by theory. However, these spectra are inconvenient for answering the fundamental question on the chiral activities of electronic states in NPs due to their intense effect by scattering. In the meanwhile, ORD spectroscopy can also illustrate the practicality of chiral nanomaterials in polarized optoelectronics and other fields. As in the case of CD spectroscopy, it can detect the differential absorption of left‐ and right‐handed circularly polarized light (electromagnetic radiation) and gives rise to their ellipticity alteration when the radiation beam passes through the sample. The chiral CD bands appear in the same spectral windows as the standard absorption bands of non‐polarized light. Either positive or negative CD bands can occur, depending on the relative absorption of each circularly polarized component of the incident radiation. As a differential method, complex CD spectra may occur.
It is essential to clearly understand the interaction between light and matter in asymmetric nanostructures for the determination of the chiral geometric elements and the sign and intensity rationalization of multiple CD “waves.” It may seem too complicated and convoluted. However, in fact, it can be quite simple to display commercial software packages (such as Lumerical, Gaussian, COMSOL) when one can identify the chirality type of the individual NPs, as exampled by Type 1, Type 2, Type 3, Type 4 or their combination thereof. It can also be applied to NP assemblies. Usually, the chemical characterization and separation of the nanoscale enantiomer mixtures determine the complexity of CD or ORD spectrum calculations [1]. As expected with improved computational power, it can be a standard part of the NP researches.
From the perspective of computational aspects, the ORD and CD spectra can be calculated based on semiclassical electrodynamic models with quantum mechanical perturbation theory. And special equations can be derived, which can address polarized light scattering caused by the oscillating electric dipole moment (linear oscillation) and magnetic dipole moment (circular oscillation). Both linear oscillation and circular oscillation are caused by the incident electromagnetic radiation. In the case of CD, the transition moment of the electric dipole interferes with the transition moment of the magnetic dipole, and the rotatory strength Rj for the jth transition is obtained according to Eq. 1.2[71].
From Eq. 1.2, basic physical insights about the origin of quantum mechanics of CD activity can be obtained. Notably, only the symmetry requires to be considered when one predicts the intensity and algebraic sign of any transition. If the transition is to be observed, the two integrals of the ground state wave function ψ0, the jth excited state wave function ψj, the electric dipole moment operator , and the magnetic dipole moment operator
