On the Origin of the Optical Activity Displayed by Chiral-Ligand-Protected Metallic Nanoclusters (original) (raw)
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Chiral Gold and Silver Nanoclusters: Preparation, Size Selection, and Chiroptical Properties
Chemistry of Materials, 2013
In this work we studied different properties of gold and silver nanoclusters (AuNCs and AgNCs) protected by the chiral ligands L-glutathione (L-GSH), and N-acetyl-L-cysteine (NALC), and we present a thorough characterization of the synthesized clusters. The synthesis was performed by reduction of the corresponding metal salt with NaBH 4. Fractions of gold nanoclusters with different sizes were isolated by methanol-induced precipitation. The ellipticity of the clusters was obtained by circular dichroism (CD) spectroscopy, showing that the chirality of the ligands is transferred to the metal core either in its structure or at least in its electronic states via perturbation of the electronic field of the ligands. The optical properties of gold and silver nanoclusters in water were studied by UV−vis spectroscopy. The absorption signal of the clusters shows characteristic bands, which can be assigned to plasmonic transitions of the metal core. In addition, UV−vis spectroscopy has served as a tool for studying the stability of these clusters in air. In general, gold nanoclusters are highly stable in air, and it was found that the stability of Au n (NALC) m clusters even exceeds that of Au n (SG) m clusters. In contrast to gold clusters, silver nanoclusters very often tend to decompose upon exposure to air. We found, however, that Ag n (NALC) m are surprisingly stable at atmospheric pressures. The average molecular formula of the nanoclusters was determined by thermogravimetric analysis (TGA). The particle sizes of AuNCs and AgNCs were assessed by transmission electron microscopy (TEM) and powder X-ray diffraction (XRD) analysis. For studying the fluorescent properties of the metal nanoparticles, photoluminescence spectroscopy (PL) was performed. In summary, we succeeded to synthesize ligand-protected silver clusters (Ag n (NALC) m) with very high stability and rather narrow size distribution; furthermore, we could show the controlled precipitation to be applicable to other systems, such as that Au n (NALC) m , yielding two fraction of very narrow size distribution.
Chirality in bare and ligand-protected metal nanoclusters
Advances in Physics: X
Chirality is a fundamental property of matter with profound impact in physics, chemistry, biology, and medicine. It is present at several scales going from elementary particles, to molecules, to macroscopic materials, and even to astronomical objects. During the last 30 years, chirality has also been investigated at the nanoscale, being a hot research topic in nanoscience. The importance of chirality at the nanoscale is due, in part, to the potential applications that chiral nanomaterials could have in nanotechnology. Great interest exists nowadays in the study of chirality in bare and ligand-protected metal nanoclusters. These are aggregates of n metal atoms (n~10-300) that can be in gas phase or stabilized by organic ligands, covering the cluster surface. Chirality in bare and thiolate-protected gold clusters (TPGC) has received special attention because of the important progress achieved in their synthesis, size separation, and precise structural characterization. Here, we review the recent experimental and theoretical developments on the origin and physicochemical manifestations of chirality in bare and TPGC. Since chirality is a geometrical property, we also discuss the proposal for its quantification, and the correlation of this geometric measure with the chiroptical response, like the circular dichroism spectrum, calculated from quantum mechanical methods.
Quantum Confined Stark Effect in Au8 and Au25 Nanoclusters
The Journal of Physical Chemistry C, 2013
The quantum confined Stark effect is investigated for the first time in bovine serum albumin (BSA) protected Au 8 and Au 25 nanoclusters. We observed a red-shift of 63 meV in Au 8 nanoclusters upon an increase in pH from 2.14 to 12.0. Such behavior could be well explained in terms of the presence of a linear polar component and a quadratic polarizable component. In contrast, Au 25 nanoclusters exhibit more complicated Stark shifts due to their specific core/ semiring structure. A plateau of the Stark shift was observed in both absorption and fluorescence, showing an offset of 30 meV. The lifetime measurements confirm that the plateau is due to the screening effect of the semirings in Au 25 @BSA. Moreover, the dual fluorescent bands of Au 25 nanoclusters exhibit two different Stark shifts of 79 and 52 meV, respectively. The experimental data indicate that the Stark shift in both Au 8 @BSA and Au 25 @BSA has a significant linear polar component due to their asymmetric structure. This study suggests that gold nanoclusters can become a potentially useful candidate in probing local electric fields and also in pH-sensing in nanoscale environment of biological systems.
Isomerization-induced enhancement of luminescence in Au28(SR)20 nanoclusters†
2020
Understanding the origin and structural basis of the photoluminescence (PL) phenomenon in thiolate-protected metal nanoclusters is of paramount importance for both fundamental science and practical applications. It remains a major challenge to correlate the PL properties with the atomic-level structure due to the complex interplay of the metal core (i.e. the inner kernel) and the exterior shell (i.e. surface Au(i)-thiolate staple motifs). Decoupling these two intertwined structural factors is critical in order to understand the PL origin. Herein, we utilize two Au28(SR)20 nanoclusters with different –R groups, which possess the same core but different shell structures and thus provide an ideal system for the PL study. We discover that the Au28(CHT)20 (CHT: cyclohexanethiolate) nanocluster exhibits a more than 15-fold higher PL quantum yield than the Au28(TBBT)20 nanocluster (TBBT: p-tert-butylbenzenethiolate). Such an enhancement is found to originate from the different structural a...
We probe the origin of photoluminescence of an atomically precise noble metal cluster, Ag 24 Au 1 (DMBT) 18 , (DMBT = 2,4-dimethylbenzenethiolate) and the origin of chirality in its chirally functionalized derivatives, Ag 24 Au 1 (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 Ag 24 Au 1 (DMBT) 18 cluster, the chiral ligand, R/S-BINAS, was incorporated into it. A series of clusters, Ag 24 Au 1 (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 Ag 24 Au 1 (DMBT) 18 to Ag 24 Au 1 (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 Ag 24 Au 1 (DMBT) 18 and its chirally functionalized derivatives is originated from the transitions involving the whole Ag 24 Au 1 S 18 framework, not merely from the icosahedral Ag 12 Au 1 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, i.e., 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
PHOTOLUMINESCENCE AND TEMPERATURE-DEPENDENT EMISSION STUDIES OF Au 25 CLUSTERS IN THE SOLID STATE
International Journal of Nanoscience, 2009
... 18. N. Nishida, ES Shibu, H. Yao, T. Oonishi, K. Kimura and T. Pradeep, Adv. Mater (2008, early view). 19. MA Habeeb Muhammed, AK Shaw, SK Pal and T. Pradeep, J. Phys. Chem. C 112, 14324 (2008). 20. M. Zhu, CM Aikens, FJ Hollander, G. C, Schatz and R. Jin, J. Am. ...
Quantum Size Effects in the Optical Properties of Ligand Stabilized Aluminum Nanoclusters
The Journal of Physical Chemistry C, 2013
Here we describe an approach to the synthesis of small ligand stabilized Al nanoclusters by catalytic decomposition of alane using Ti(O i Pr) 4 as catalyst. The selected area electron diffraction (SAED) and elemental analysis are consistent with the presence of Al in the clusters. The cluster sizes are measured by the small-angle X-ray scattering method in air-free conditions. The absorption maximum exhibits red shifts when cluster sizes decrease from 4 to 1.5 nm. A two-layer Mie theory model indicates that the electron conductivity in the Al core is reduced due to a combination of quantum size effects and chemical interaction with the ligand shell resulting in the observed red shift with decreasing size. The red shift is shown to scale with the inverse radius in good agreement with a spill-out model. Furthermore, the results are consistent with time-dependent density functional simulations for a small ligand stabilized Al cluster. Remarkably, we find that the absorption maximum is significantly red-shifted compared with that expected from simulations based on the bulk dielectric constant. This is true even for the larger nanoclusters with diameters of 4 nm. This indicates that small ligand protected Al clusters behave significantly different from similar Ag and Au clusters.
Nanoscale, 2014
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.
Intrinsic Chirality in Bare Gold Nanoclusters: The Au34− Case
The Journal of …, 2008
In this work, we provide theoretical evidence on the existence of energetically stable chiral structures for bare gold clusters. Density functional theory calculations within the generalized-gradient approximation were performed to systematically study structural, vibrational, electronic, and optical ...