On the Origin of the Optical Activity Displayed by Chiral-Ligand-Protected Metallic Nanoclusters

Chiral Inversion and Recovery of Supramolecular Luminescent Copper Nanocluster Hydrogels Triggered by Polyethyleneimine and Polyoxometalates

Construction of controllable chiroptical supramolecular luminescence systems is of great significance for developing intelligent chiral luminescence materials with precise and effective regulation and understanding chirality-switching phenomena in biological systems, which has attracted extensive attention. Because chiral metal nanoclusters (NCs) can provide facilities for the study of nanoscale chiral effects, in this study, we select chiral glutathione-stabilized copper NCs (G-SH-Cu NCs) to construct a supramolecular luminescent hydrogel with achiral branched polyethyleneimine (PEI) and polyoxometalates [Na₉(EuW₁₀O₃₆)·32H₂O, denoted as EuW₁₀]. Thus, a chiral property precise controlled system was constructed by self-assembly. Interestingly, the addition of PEI to G-SH-Cu NC solution induced the formation of luminescent hydrogels with chiral inversion, while further addition of EuW₁₀ not only enhanced the luminescence of the hydrogel but also recovered the chiroptical properties. The chiral inversion behavior is possibly ascribed to the hydrogen bond interaction/electrostatic interaction between G-SH-Cu NCs and PEI in the chiral inversion process, while the competition of hydrogen bonding interaction (between G-SH-Cu NCs and PEI) and electrostatic interaction (between PEI and EuW₁₀) was accountable for the chiral recovery process. Manipulation of chirality inversion in the metal NC-containing coassemblies is rare, while this work establishes a feasible strategy to modulate the chiral inversion behavior of Cu NCs, which not only produces new physicochemical properties of metal NCs through synergistic behavior but also offers a feasible way to realize the potential application of chiroptical materials.

Synthesis of metal nanoclusters and their application in Hg2+ ions detection: A review

Mercuric (Hg²⁺) ions released from human activities, natural phenomena, and industrial sources are regarded as the global pollutant of world's water. Hg²⁺ ions contaminated water has several adverse effects on human health and the environment even at low concentrations. Therefore, rapid and cost-effective method is urgently required for the detection of Hg²⁺ ions in water. Although, the current analytical methods applied for the detection of Hg²⁺ ions provide low detection limit, they are time consuming, require expensive equipment, and are not suitable for in-situ analysis. Metal nanoclusters (MNCs) consisting of several to ten metal atoms are important transition missing between single atoms and plasmonic metal nanoparticles. In addition, sub-nanometer sized MNCs possess unique electronic structures and the subsequent unusual optical, physical, and chemical properties. Because of these novel properties, MNCs as a promising material have attracted considerable attention for the construction of selective and sensitive sensors to monitor water quality. Hence this review is focused on recent advances on synthesis strategies, and optical and chemical properties of various MNCs including their applications to develop optical assay for Hg²⁺ ions in aqueous solutions.

Chiral Perovskites for Next-Generation Photonics: From Chirality Transfer to Chiroptical Activity

Organic-inorganic hybrid halide perovskites (OIHPs) are commonly used as prototypical materials for various applications, including photovoltaics, photodetectors, and light-emitting devices. Since the chiroptical properties of OIHPs are deciphered in 2017, chiral OIHPs have been rediscovered as new hybrid systems comprising chiral organic molecules and achiral inorganic octahedral layers. Owing to their exceptional optoelectrical properties and structural flexibility, chiral OIHPs have received a considerable amount of attention in chiral photonics, chiroptoelectronics, spintronics, and ferroelectrics. Despite their intriguing chiral properties, the transfer mechanism from chiral molecules to achiral semiconductors has not been extensively investigated. Furthermore, an in-depth understanding of the origin of chiroptical activity is still elusive. In this review article, recent advances in the chiroptical activities of chiral OIHPs and polarization-based devices adopting chiral OIHPs are comprehensively discussed, and insight into the underlying chirality transfer mechanism based on theoretical considerations is provided. This comprehensive survey, with an emphasis on the chirality transfer mechanism, will help readers understand the chiroptical properties of OIHPs, which are crucial for the development of spin-based photonic and optoelectronic devices. Additionally, promising strategies to exploit the potential of chiral OIHPs are also discussed.

Symmetry Breaking of Atomically Precise Fullerene-like Metal Nanoclusters

Here we report a neutral fullerene-like core-shell homosilver Ag₁₃@Ag₂₀ nanocluster that is fully protected by an achiral bidentate thiolate ligand (9,12-dimercapto-1,2-closo-carborane, C₂B₁₀H₁₀S₂H₂), which crystallizes in centrosymmetric space group R3̅. Continuous Cu doping in the dodecahedral shell first induced symmetry breaking to generate chiral Ag₁₃@Ag_20-nCu_n (6 ≥ n ≥ 2) containing two acetonitrile ligands in space group P2₁2₁2₁, and then produced symmetric all-thiolated Ag₁₃@Ag_20-nCu_n (20 ≥ n ≥ 13) in the higher space group Im3̅. The selectively copper-doped Ag₁₃@Ag_20-nCu_n (6 ≥ n ≥ 2) cluster has its structure reorganized to a lower symmetry that shows chiroptical activity. Moreover, structural distortion of Ag₁₃@Ag_20-nCu_n (6 ≥ n ≥ 2) further expanded in chiral R-/S-propylene oxide, which induced a more prominent core-based CD response. This work revealed a novel mechanism of chirality generation at the atomic level through asymmetric shell-doping of metal nanoclusters, which provides new insight into the origin of chirality in inorganic nanostructures.

Unravelling the Structure of a Medium-Sized Metalloid Gold Nanocluster and its Filming Property

Unravelling the structure of thiolated metalloid gold nanoclusters in the medium-sized range by single crystal X-ray crystallography (SCXC) is challenging. Herein, we successfully synthesized a novel Au₆₇ (SR)₃₅ nanocluster, and unravelled its single crystal structure by SCXC, which features a mix-structured Au₄₈ kernel protected by one Au₄ (SR)₅ staple and fifteen Au(SR)₂ staples. Unprecedentedly, this structure can be thermally induced to aggregate into larger nanoparticles and self-deposit to form a gold nanoparticles film onto the walls of a vial or other substrates such as quartz, mica or ceramic, which can be developed into a facile, substrate-universal and scalable filming method. The film exhibits high sensitivity, uniformity and recyclability as a surface-enhanced Raman scattering (SERS) substrate and can be applied for detecting multiple organic pollutants.

Chiroptical activity of Au13 clusters: experimental and theoretical understanding of the origin of helical charge movements

Ligand-protected gold clusters with an asymmetric nature have emerged as a novel class of chiral compounds, but the origins of their chiroptical activities associated with helical charge movements in electronic transitions remain unexplored. Herein, we perform experimental and theoretical studies on the structures and chiroptical properties of Au13 clusters protected by mono- and di-phosphine ligands. Based on the experimental reevaluation of diphosphine-ligated Au13 clusters, we show that these surface ligands slightly twist the Au13 cores from a true icosahedron to generate intrinsic chirality in the gold frameworks. Theoretical investigation of a monophosphine-ligated cluster model reproduced the experimentally observed circular dichroism (CD) spectrum, indicating that such a torsional twist of the Au13 core, rather than the surrounding chiral environment by helically arranged diphosphine ligands, contributes to the appearance of the chiroptical response. We also show that the calculated CD signals are dependent on the degree of asymmetry (torsion angle between the two equatorial Au5 pentagons), and provide a visual understanding of the origin of helical charge movements with transition-moment and transition-density analyses. This work provides novel insights into the chiroptical activities of ligand-protected metal clusters with intrinsically chiral cores.

Chirality Transfer from Sub-Nanometer Biochemical Molecules to Sub-Micrometer Plasmonic Metastructures: Physiochemical Mechanisms, Biosensing, and Bioimaging Opportunities

Determining the structural chirality of biomolecules is of vital importance in bioscience and biomedicine. Conventional methods for characterizing molecular chirality, e.g., circular dichroism (CD) spectroscopy, require high-concentration specimens due to the weak electronic CD signals of biomolecules such as amino acids. Artificially designed chiral plasmonic metastructures exhibit strong intrinsic chirality. However, the significant size mismatch between metastructures and biomolecules makes the former unsuitable for chirality-recognition-based molecular discrimination. Fortunately, constructing metallic architectures through molecular self-assembly allows chirality transfer from sub-nanometer biomolecules to sub-micrometer, intrinsically achiral plasmonic metastructures by means of either near-field interaction or chirality inheritance, resulting in hybrid systems with CD signals orders of magnitude larger than that of pristine biomolecules. This exotic property provides a new means to determine molecular chirality at extremely low concentrations (ideally at the single-molecule level). Herein, three strategies of chirality transfer from sub-nanometer biomolecules to sub-micrometer metallic metastructures are analyzed. The physiochemical mechanisms responsible for chirality transfer are elaborated and new fascinating opportunities for employing plasmonic metastructures in chirality-based biosensing and bioimaging are outlined.

Chiral Functionalization of an Atomically Precise Noble Metal Cluster: Insights into the Origin of Chirality and Photoluminescence

We probe the origin of photoluminescence of an atomically precise noble metal cluster, Ag₂₄Au₁(DMBT)₁₈ (DMBT = 2,4-dimethylbenzenethiolate), and the origin of chirality in its chirally functionalized derivatives, Ag₂₄Au₁(R/S-BINAS)_x(DMBT)_18-2x, with x = 1-7 (R/S-BINAS = R/S-1,1'-[binaphthalene]-2,2'-dithiol), using chiroptical spectroscopic measurements and density functional theory (DFT) calculations. Combination of chiroptical and luminescence spectroscopies to understand the nature of electronic transitions has not been applied to such molecule-like metal clusters. In order to impart chirality to the achiral Ag₂₄Au₁(DMBT)₁₈ cluster, the chiral ligand, R/S-BINAS, was incorporated into it. A series of clusters, Ag₂₄Au₁(R/S-BINAS)_x(DMBT)_18-2x, with x = 1-7, were synthesized. We demonstrate that the low-energy electronic transitions undergo an unexpected achiral to chiral and back to achiral transition from pure Ag₂₄Au₁(DMBT)₁₈ to Ag₂₄Au₁(R/S-BINAS)_x(DMBT)_18-2x, by increasing the number of BINAS ligands. The UV/vis, luminescence, circular dichroism, and circularly polarized luminescence spectroscopic measurements, in conjunction with DFT calculations, suggest that the photoluminescence in Ag₂₄Au₁(DMBT)₁₈ and its chirally functionalized derivatives originates from the transitions involving the whole Ag₂₄Au₁S₁₈ framework and not merely from the icosahedral Ag₁₂Au₁ core. These results suggest that the chiroptical signatures and photoluminescence in these cluster systems cannot be solely attributed to any one of the structural components, that is, the metal core or the protecting metal-ligand oligomeric units, but rather to their interaction and that the ligand shell plays a crucial role. Our work demonstrates that chiroptical spectroscopic techniques such as circular dichroism and circularly polarized luminescence represent useful tools to understand the nature of electronic transitions in ligand-protected metal clusters and that this approach can be utilized for gaining deeper insights into the structure-property relationships of the electronic transitions of such molecule-like clusters.

Enantioseparation and chiral induction in Ag29 nanoclusters with intrinsic chirality

The optical activity of a metal nanocluster (NC) is induced either by an asymmetric arrangement of constituents or by a dissymmetric field of a chiral ligand layer. Herein, we unveil the origin of chirality in Ag₂₉ NCs, which is attributed to the intrinsically chiral atomic arrangement. The X-ray crystal structure of a Ag₂₉(BDT)₁₂(TPP)₄ NC (BDT: 1,3-benzenedithiol; TPP: triphenylphosphine) manifested the presence of intrinsic chirality in the outer shell capping the icosahedral achiral Ag₁₃ core. The enantiomers of the Ag₂₉(BDT)₁₂(TPP)₄ NC are separated by high-performance liquid chromatography (HPLC) using a chiral column for the first time, showing mirror-image circular dichroism (CD) spectra. The CD spectra are reproduced by time-dependent density functional theory (TDDFT) calculations based on enantiomeric Ag₂₉ models with achiral 1,3-propanedithiolate ligands. The mechanism of chiral induction in the synthesis of Ag₂₉(DHLA)₁₂ (DHLA: α-dihydrolipoic acid) NCs with a chiral ligand system is further discussed with the aid of DFT calculations. The use of the enantiomeric DHLA ligand preferentially leads to a one-handed atomic arrangement which is more stable than the opposite one, inducing the enantiomeric excess in the population of intrinsically chiral Ag₂₉ NCs with CD activity.

Enantioseparation of Ag₂₉ nanoclusters with intrinsic chirality was performed by chiral HPLC, affording a pair of fractions with mirror image CD spectra.

Plasmonic circular dichroism of vesicle-like nanostructures by the template-less self-assembly of achiral Janus nanoparticles

Chiral nanostructures have been attracting extensive interest in recent years primarily because of the unique materials properties that can be exploited for diverse applications. In this study, gold Janus nanoparticles, with hexanethiolates and 3-mercapto-1,2-propanediol segregated on the two hemispheres of the metal cores (dia. 2.7 ± 0.4 nm), self-assembled into vesicle-like, hollow nanostructures in both water and organic media, and exhibited apparent plasmonic circular dichroism (PCD) absorption in the visible range. This was in contrast to individual Janus nanoparticles, bulk-exchange nanoparticles where the two ligands were homogeneously mixed on the nanoparticle surface, or nanoparticles capped with only one kind of ligand. The PCD signals were found to become intensified with increasing coverage of the 3-mercapto-1,2-propanediol ligands on the nanoparticle surface. This was accounted for by the dipolar property of the structurally asymmetrical Janus nanoparticles, and theoretical simulations based on first principles calculations showed that when the nanoparticle dipoles self-assembled onto the surface of a hollow sphere, a vertex was formed which gave rise to the unique chiral characteristics. The resulting chiral nanoparticle vesicles could be exploited for the separation of optical enantiomers, as manifested in the selective identification and separation of d-alanine from the l-isomer.

Au25(SR)18: the captain of the great nanocluster ship

Noble metal nanoclusters are in the intermediate state between discrete atoms and plasmonic nanoparticles and are of significance due to their atomically accurate structures, intriguing properties, and great potential for applications in various fields. In addition, the size-dependent properties of nanoclusters construct a platform for thoroughly researching the structure (composition)-property correlations, which is favorable for obtaining novel nanomaterials with enhanced physicochemical properties. Thus far, more than 100 species of nanoclusters (mono-metallic Au or Ag nanoclusters, and bi- or tri-metallic alloy nanoclusters) with crystal structures have been reported. Among these nanoclusters, Au25(SR)18-the brightest molecular star in the nanocluster field-is capable of revealing the past developments and prospecting the future of the nanoclusters. Since being successfully synthesized (in 1998, with a 20-year history) and structurally determined (in 2008, with a 10-year history), Au25(SR)18 has stimulated the interest of chemists as well as material scientists, due to the early discovery, easy preparation, high stability, and easy functionalization and application of this molecular star. In this review, the preparation methods, crystal structures, physicochemical properties, and practical applications of Au25(SR)18 are summarized. The properties of Au25(SR)18 range from optics and chirality to magnetism and electrochemistry, and the property-oriented applications include catalysis, chemical imaging, sensing, biological labeling, biomedicine and beyond. Furthermore, the research progress on the Ag-based M25(SR)18 counterpart (i.e., Ag25(SR)18) is included in this review due to its homologous composition, construction and optical absorption to its gold-counterpart Au25(SR)18. Moreover, the alloying methods, metal-exchange sites and property alternations based on the templated Au25(SR)18 are highlighted. Finally, some perspectives and challenges for the future research of the Au25(SR)18 nanocluster are proposed (also holding true for all members in the nanocluster field). This review is directed toward the broader scientific community interested in the metal nanocluster field, and hopefully opens up new horizons for scientists studying nanomaterials. This review is based on the publications available up to March 2018.

Isomerism in Au-Ag Alloy Nanoclusters: Structure Determination and Enantioseparation of [Au9Ag12(SR)4(dppm)6X6]3

Revealing structural isomerism in a nanocluster remains significant but challenging. Herein, we have obtained a pair of structural isomers, [Au₉Ag₁₂(SR)₄(dppm)₆X₆]³⁺-C and [Au₉Ag₁₂(SR)₄(dppm)₆X₆]³⁺-Ac [dppm = bis(diphenyphosphino)methane; HSR = 1-adamantanethiol/ tert-butylmercaptan; X = Br/Cl; C stands for one of the structural isomers being chiral; Ac stands for another being achiral], that show different structures as well as different chiralities. These structures are determined by single-crystal X-ray diffraction and further confirmed by high-resolution electrospray ionization mass spectrometry. On the basis of the isomeric structures, the most important finding is the different arrangements of the Au₅Ag₈@Au₄ metal core, leading to changes in the overall shape of the cluster, which is responsible for structural isomerism. Meanwhile, the two enantiomers of [Au₉Ag₁₂(SR)₄(dppm)₆X₆]³⁺-C are separated by high-performance liquid chromatography. Our work will contribute to a deeper understanding of the structural isomerism in noble-metal nanoclusters and enrich the chiral nanocluster.

A series of intrinsically chiral gold nanocage structures

We present a series of intrinsically chiral gold nanocage structures, Au_9n+6, which are stable for n ≥ 2. These structures consist of an Au_9n tube which is capped with Au₃ units at each end. Removing the Au₃ caps, we obtain a series of intrinsically chiral gold nanotube structures, Au_9n, which are stable for n ≥ 4. The intrinsic chirality of these structures results from the helicity of the gold strands which form the tube and not because an individual Au atom is a chiral center. The symmetry of these structures is C₃ and substructures of gold hexagons with a gold atom in the middle are particularly prominent. We focus on the properties of Au₄₂ (C₃) and Au₁₀₅ (C₃) which are the two smallest gold nanocage structures to be completely tiled by these Au₇ "golden-eye" substructures. Our main focus is on Au₄₂ (C₃) since gold clusters in the 40-50 atom regime are currently being investigated in gas phase experiments. We show that the intrinsically chiral Au₄₂ cage structure is energetically comparable with previously reported achiral cage and compact Au₄₂ structures. Cage structures are of particular interest because species can be encapsulated (and stabilized) inside the cage and we provide strong evidence that Au₆@Au₄₂ (C₃) is the global minimum Au₄₈ structure. The intrinsically chiral gold nanocage structures, which exhibit a range of size-related properties, have potential applications in chiral catalysis and as components in nanostructured devices.

Chiral segregation driven by a dynamical response of the adsorption footprint to the local adsorption environment: bitartrate on Cu(110)

Bitartrate, a strongly bound chiral modifier, is able to restructure its adsorption footprint on Cu(110) in response to local adsorbates.

Optically active blue-emitting carbon dots to specifically target the Golgi apparatus

A pair of optically active carbon dots were prepared, which could specifically target and directly illuminate the Golgi apparatus.

Amplification of the Optical Activity of Gold Clusters by the Proximity of BINAP

Despite recent progress in the synthesis and characterization of optically active gold clusters, the factor determining optical rotatory strength has not been clarified due to the lack of structurally resolved, enantiomerically pure Au clusters. We addressed this issue by studying the correlation between the optical activity and geometrical structures of two types of Au clusters that were protected by chiral diphosphines: [Au₁₁(R/S-DIOP)₄Cl₂]⁺ (DIOP = 1,4-bis(diphenylphosphino)-2,3-o-isopropylidene-2,3-butanediol) and [Au₈(R/S-BINAP)₃(PPh₃)₂]²⁺ (BINAP = 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl). [Au₈(BINAP)₃(PPh₃)₂]²⁺ showed stronger rotatory strengths than [Au₁₁(DIOP)₄Cl₂]⁺ in the visible region, while the Hausdorff chirality measure calculated from the crystal data indicated that the Au core of the former is less chiral than that of the latter. We propose that the optical activity in the Au core-based transition due to the deformed core is further amplified by chiral arrangement of the binaphthyl moiety near the Au core.

How to control optical activity in organic-silver hybrid nanoparticles

The mechanisms that originate and control optical activity in organic-metal hybrid nanoparticles (NPs) are identified using a time-perturbed density functional theory. Electronic circular dichroism (CD) is studied in terms of the intrinsic chirality of the ligands, the number of ligands and the induced chirality by the arrangement of the ligands on the NP. Left-handed cysteine and achiral methylthio ligands adsorbed on an icosahedral silver NP are investigated. The analysis of CD allows the identification of the spectral regions when the induced chirality by the ligand array dominates over the intrinsic chirality of the ligands, determining conditions for CD control and enlargement. These results would be significant in the discussion of experimental CD spectra of organic-metal hybrid NPs, which might allow the development of new strategies to improve the sensitivity of chiroptical spectroscopies for the identification of bio and organic molecules.

Optically active red-emitting Cu nanoclusters originating from complexation and redox reaction between copper(ii) and d/l-penicillamine

Despite a significant surge in the number of investigations into both optically active Au and Ag nanostructures, there is currently only limited knowledge about optically active Cu nanoclusters (CuNCs) and their potential applications. Here, we have succeeded in preparing a pair of optically active red-emitting CuNCs on the basis of complexation and redox reaction between copper(ii) and penicillamine (Pen) enantiomers, in which Pen serves as both a reducing agent and a stabilizing ligand. Significantly, the CuNCs feature unique aggregation induced emission (AIE) characteristics and therefore can serve as pH stimuli-responsive functional materials. Impressively, the ligand chirality plays a dramatic role for the creation of brightly emissive CuNCs, attributed to the conformation of racemic Pen being unfavorable for the electrostatic interaction, and thus suppressing the formation of cluster aggregates. In addition, the clusters display potential toward cytoplasmic staining and labelling due to the high photoluminescence (PL) quantum yields (QYs) and remarkable cellular uptake, in spite that no chirality-dependent effects in autophagy and subcellular localization are observed in the application of chiral cluster enantiomer-based cell imaging.

Water-Soluble Phosphine-Protected Au₁₁ Clusters: Synthesis, Electronic Structure, and Chiral Phase Transfer in a Synergistic Fashion

Synthesis of atomically precise, water-soluble phosphine-protected gold clusters is still currently limited probably due to a stability issue. We here present the synthesis, magic-number isolation, and exploration of the electronic structures as well as the asymmetric conversion of triphenylphosphine monosulfonate (TPPS)-protected gold clusters. Electrospray ionization mass spectrometry and elemental analysis result in the primary formation of Au11(TPPS)9Cl undecagold cluster compound. Magnetic circular dichroism (MCD) spectroscopy clarifies that extremely weak transitions are present in the low-energy region unresolved in the UV-vis absorption, which can be due to the Faraday B-terms based on the magnetically allowed transitions in the cluster. Asymmetric conversion without changing the nuclearity is remarkable by the chiral phase transfer in a synergistic fashion, which yields a rather small anisotropy factor (g-factor) of at most (2.5-7.0) × 10(-5). Quantum chemical calculations for model undecagold cluster compounds are then used to evaluate the optical and chiroptical responses induced by the chiral phase transfer. On this basis, we find that the Au core distortion is ignorable, and the chiral ion-pairing causes a slight increase in the CD response of the Au11 cluster.

Chirality Inversion of CdSe and CdS Quantum Dots without Changing the Stereochemistry of the Capping Ligand

L-cysteine derivatives induce and modulate the optical activity of achiral cadmium selenide (CdSe) and cadmium sulfide (CdS) quantum dots (QDs). Remarkably, N-acetyl-L-cysteine-CdSe and L-homocysteine-CdSe as well as N-acetyl-L-cysteine-CdS and L-cysteine-CdS showed "mirror-image" circular dichroism (CD) spectra regardless of the diameter of the QDs. This is an example of the inversion of the CD signal of QDs by alteration of the ligand's structure, rather than inversion of the ligand's absolute configuration. Non-empirical quantum chemical simulations of the CD spectra were able to reproduce the experimentally observed sign patterns and demonstrate that the inversion of chirality originated from different binding arrangements of N-acetyl-L-cysteine and L-homocysteine-CdSe to the QD surface. These efforts may allow the prediction of the ligand-induced chiroptical activity of QDs by calculating the specific binding modes of the chiral capping ligands. Combined with the large pool of available chiral ligands, our work opens a robust approach to the rational design of chiral semiconducting nanomaterials.

A Segregated, Partially Oxidized, and Compact Ag10 Cluster within an Encapsulating DNA Host

Silver clusters develop within DNA strands and become optical chromophores with diverse electronic spectra and wide-ranging emission intensities. These studies consider a specific cluster that absorbs at 400 nm, has low emission, and exclusively develops with single-stranded oligonucleotides. It is also a chameleon-like chromophore that can be transformed into different highly emissive fluorophores. We describe four characteristics of this species and conclude that it is highly oxidized yet also metallic. One, the cluster size was determined via electrospray ionization mass spectrometry. A common silver mass is measured with different oligonucleotides and thereby supports a Ag10 cluster. Two, the cluster charge was determined by mass spectrometry and Ag L3-edge X-ray absorption near-edge structure spectroscopy. Respectively, the conjugate mass and the integrated white-line intensity support a partially oxidized cluster with a +6 and +6.5 charge, respectively. Three, the cluster chirality was gauged by circular dichroism spectroscopy. This chirality changes with the length and sequence of its DNA hosts, and these studies identified a dispersed binding site with ∼20 nucleobases. Four, the structure of this complex was investigated via Ag K-edge extended X-ray absorption fine structure spectroscopy. A multishell fitting analysis identified three unique scattering environments with corresponding bond lengths, coordination numbers, and Debye-Waller factors for each. Collectively, these findings support the following conclusion: a Ag10(+6) cluster develops within a 20-nucleobase DNA binding site, and this complex segregates into a compact, metal-like silver core that weakly links to an encapsulating silver-DNA shell. We consider different models that account for silver-silver coordination within the core.

High-resolution separation of thiolate-protected gold clusters by reversed-phase high-performance liquid chromatography

This perspective summarizes our work on high-resolution separation of thiolate-protected gold clusters using reversed-phase high-performance liquid chromatography, new findings obtained by those separation, and future prospects for this field.

Optical properties of nanoalloys

Optical absorption spectra of bare (left) and monolayer-protected (right) metal nanoalloys.

Helical gold nanorods as chiral recognition nanostructures: a relativistic density functional theory study

We establish helical gold nanorods as the first examples of chiral recognition nanostructures by examining the simple chiral molecule CClHDT adsorbed on the helical Au40 nanorod. We calculate the vibrational circular dichroism (VCD) spectra of the R and S enantiomers of CClHDT adsorbed on the R (or S) enantiomer of Au40 using relativistic density functional theory. The highest adsorption energy is found when the Cl atom of CClHDT binds to a low-coordinated Au atom at the edge of Au40. There are three adsorption modes (essentially identical in energy) corresponding to three orientations of the HDT moiety. We show that, for each adsorption mode, the VCD spectra are distinctly different for the Au40(R)-ClHDT(R) and Au40(R)-CClHDT(S) complexes, and we give a qualitative explanation for this based on the principle of chirality transfer. For comparison with the results for Au40, we calculate the VCD spectra of the R and S enantiomers of CClHDT adsorbed on the achiral Au20 tetrahedral cluster. Again, there are three adsorption modes (essentially identical in energy) corresponding to three orientations of the HDT moiety. However, we show that, for each adsorption mode, the VCD spectra are mirror symmetric but otherwise essentially identical for the Au20-CClHDT(R) and Au20-CClHDT(S) complexes. Thus, the inherent chirality of the helical Au40 nanorod is essential for its chiral recognition functionality.

Chirality in thiolate-protected gold clusters

Over recent years, research on thiolate-protected gold clusters Au(m)(SR)n has gained significant interest. Milestones were the successful determination of a series of crystal structures (Au102(SR)44, Au25(SR)18, Au38(SR)24, Au36(SR)24, and Au28(SR)20). For Au102(SR)44, Au38(SR)24, and Au28(SR)20, intrinsic chirality was found. Strong Cotton effects (circular dichroism, CD) of gold clusters protected by chiral ligands have been reported a long time ago, indicating the transfer of chiral information from the ligand into the cluster core. Our lab has done extensive studies on chiral thiolate-protected gold clusters, including those protected with chiral ligands. We demonstrated that vibrational circular dichroism can serve as a useful tool for the determination of conformation of the ligand on the surface of the cluster. The first reports on crystal structures of Au102(SR)44 and Au38(SR)24 revealed the intrinsic chirality of these clusters. Their chirality mainly arises from the arrangement of the ligands on the surface of the cluster cores. As achiral ligands are used to stabilize the clusters, racemic mixtures are obtained. However, the separation of the enantiomers by HPLC was demonstrated which enabled the measurement of their CD spectra. Thermally induced inversion allows determination of the activation parameters for their racemization. The inversion demonstrates that the gold-thiolate interface is anything but fixed; in contrast, it is rather flexible. This result is of fundamental interest and needs to be considered in future applications. A second line of our research is the selective introduction of chiral, bidentate ligands into the ligand layer of intrinsically chiral gold clusters. The ligand exchange reaction is highly diastereoselective. The bidentate ligand connects two of the protecting units on the cluster surface and thus effectively stabilizes the cluster against thermally induced inversion. A minor (but significant) influence of chiral ligands to the CD spectra of the clusters is observed. The studied system represents the first example of an intrinsically chiral gold cluster with a defined number of exchanged ligands, full control over their regio- and stereochemistry. The methodology allows for the selective preparation of mixed-ligand cluster compounds and a thorough investigation of the influence of single ligands on the cluster's properties. Overall, the method enables even more detailed tailoring of properties. Still, central questions remain unanswered: (1) Is intrinsic chirality a ubiquitous feature of thiolate-protected gold clusters? (2) How does chirality transfer work? (3) What are the applications for chiral thiolate-protected gold clusters? In this Account, we summarize the main findings on chirality in thiolate-protected gold cluster of the past half decade. Emphasis is put on intrinsically chiral clusters and their structures, optical activity, and reactivity.

Metallic influence on the atomic structure and optical activity of ligand-protected nanoparticles: a comparison between Ag and Au

Using time-perturbed density functional theory the optical activity of metal-thiolate compounds formed by highly symmetric Ag and Au nanoparticles (NPs) and a methyl-thiol molecule is studied after performing atomic optimizations and electronic calculations upon adsorption. Many different sites and orientations of the adsorbed molecule on icosahedral Ag and Au NPs of 55 atoms are considered. Upon molecular adsorption atomic distortions on Au NPs are induced while not on Ag, which causes higher molecular adsorption energies in Au than in Ag. Structural distortions and the specific molecular adsorption site and orientation result in chiral metal-thiolate NPs. Ag and Au compounds with similar chirality, according to Hausdorff chirality measurements, show different optical activity signatures, where circular dichroism spectra of Au NPs are more intense. These dissimilarities are attributed in part to the differences in the electronic density of states, which are a consequence of relativistic effects and the atomic distortion. It is concluded that the optical activity of Ag and Au compounds is due to different mechanisms, while in Au it is mainly due to the atomic distortion of the metallic NPs induced after molecular adsorption, in Ag it is defined by the adsorption site and molecular orientation with respect to the NP symmetry.

Ligand induced circular dichroism and circularly polarized luminescence in CdSe quantum dots

Chiral thiol capping ligands L- and D-cysteines induced modular chiroptical properties in achiral cadmium selenide quantum dots (CdSe QDs). Cys-CdSe prepared from achiral oleic acid capped CdSe by postsynthetic ligand exchange displayed size-dependent electronic circular dichroism (CD) and circularly polarized luminescence (CPL). Opposite CPL signals were measured for the CdSe QDs capped with D- and L-cysteine. The CD profile and CD anisotropy varied with size of CdSe nanocrystals with largest anisotropy observed for CdSe nanoparticles of 4.4 nm. Magic angle spinning solid state NMR (MAS ssNMR) experiments suggested bidentate interaction between cysteine and the surface of CdSe. Time Dependent Density Functional Theory (TDDFT) calculations verified that attachment of L- and D-cysteine to the surface of model (CdSe)13 nanoclusters induces measurable opposite CD signals for the exitonic band of the nanocluster. The origin of the induced chirality is consistent with the hybridization of highest occupied CdSe molecular orbitals with those of the chiral ligand.

Structures and chiroptical properties of the BINAS-monosubstituted Au38(SCH3)24 cluster

The structure and optical properties of a set of R-1,1'-binaphthyl-2,2'-dithiol (R-BINAS) monosubstituted A-Au38(SCH3)24 clusters are studied by means of time dependent density functional theory (TD-DFT). While it was proposed earlier that BINAS selectively binds to monomer motifs (SR-Au-SR) covering the Au23 core, our calculations suggest a binding mode that bridges two dimer (SR-Au-SR-Au-RS) motifs. The more stable isomers show a negligible distortion induced by BINAS adsorption on the Au38(SCH3)24 cluster which is reflected by similar optical and Circular Dichroism (CD) spectra to those found for the parent cluster. The results furthermore show that BINAS adsorption does not enhance the CD signals of the Au38(SCH3)24 cluster.