Preparation of Monolayer-Protected Gold and Silver Nanoclusters and their Optical, Chiroptical and Photochemical Properties (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.
The Journal of Physical Chemistry C, 2019
Vibrational spectra of thiolate-protected gold nanoclusters, prepared in a monolayer manner using the Langmuir−Blodgett method, were measured by means of infrared reflection absorption spectroscopy (IRAS). A transferred monolayer of gold nanoclusters ligated by dodecanethiolate or 2-phenylethane-1-thiolate onto a single-crystal gold (Au) surface of Au(111) exhibits worthy IRAS spectra that reveal temperature-dependent behaviors from 100 to 340 K as well as comprehensive peak assignments based on density functional theory calculations: the conformation change in ligands between all trans and gauche defect forms with temperature. In addition to the temperature dependence, the cluster size dependence of alkyl and phenyl moieties is discussed, compared with the IRAS spectra of the corresponding selfassembled monolayers (SAMs) on Au(111). Ligands spreading three-dimensionally from the Au core determine the coordination structure of the ligated Au nanoclusters.
We present a theoretical study of the optical response of silver clusters, Ag n n = 4, 8, 10, 20, complexed with the aryl thiols FC 6 H 4 S − and CH 3 C 6 H 4 S − in an aqueous solution. The absorption spectra are found to be strongly modified by the adsorption of aromatic thiols with a red-shift of the plasmon-like band, and the emergence of new excitations due to chargetransfer transitions between thiols and the metal cluster. Our results highlight the influence of the molecular orientation of thiol ligands relative to the cluster surface on the excitations. We also analyze the appropriateness of substituting a thiol molecule by SH group. Calculations have been performed using the time-dependent density functional theory (TDDFT).
Physical Chemistry Chemical Physics, 2015
Here in this contribution, blue and red luminescent 1-dodecanethiol (DT) terminated gold nanoclusters (AuNC) were prepared by a simple two-step synthesis route where the first step involved the surfactantfree synthesis of bare AuNC in N,N 0-dimethylformamide (DMF) and the second step is the termination of the as-prepared bare AuNC by 1-dodecanethiol. The blue and red luminescent DT-terminated AuNC were isolated by a solvent-induced precipitation followed by an ultra-centrifugation technique. Both the bare AuNC and the blue and red luminescent DT-terminated AuNC exhibit stable photoluminescence and good solubility in various solvents. The photo-physical, electronic, structural, and morphological properties of the bare AuNC and the blue and red luminescent DT-terminated AuNC were examined by performing UV-Vis absorption spectroscopy, stationary and time-resolved PL spectroscopy, X-ray photoelectron spectroscopy (XPS), femtosecond transient absorption spectroscopy, Fourier-transform infrared spectroscopy (FTIR-ATR), and high-resolution transmission electron microscopy (HRTEM) experiments.
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.
Langmuir, 2018
Controlling the size of nanoscale entities is important because many properties of nanomaterials are directly related to the size of the particles. Gold nanoparticles represent classic materials and are of particular interest due to their potential application in a variety of fields. In this study, hexanethiol-capped gold nanoparticles are synthesized via the Brust-Schiffrin method. Synthesized nanoparticles were characterized by various analytical techniques such as transmission electron microscopy (TEM), scanning tunneling microscopy (STM), UV-Visible absorption spectroscopy (UV-Vis) and electrochemical techniques. We have varied the molar
Chirality in Thiolate-Protected Gold Clusters
Accounts of Chemical Research, 2014
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 (Au 102 (SR) 44 , Au 25 (SR) 18 , Au 38 (SR) 24 , Au 36 (SR) 24 , and Au 28 (SR) 20). For Au 102 (SR) 44 , Au 38 (SR) 24 , and Au 28 (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 Au 102 (SR) 44 and Au 38 (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.
Nanomaterials, 2019
Thiolate-protected metal nanoclusters have highly size- and structure-dependent physicochemical properties and are a promising class of nanomaterials. As a consequence, for the rationalization of their synthesis and for the design of new clusters with tailored properties, a precise characterization of their composition and structure at the atomic level is required. We report a combined ion mobility-mass spectrometry approach with density functional theory (DFT) calculations for determination of the structural and optical properties of ultra-small gold nanoclusters protected by thioglycolic acid (TGA) as ligand molecules, Au10(TGA)10. Collision cross-section (CCS) measurements are reported for two charge states. DFT optimized geometrical structures are used to compute CCSs. The comparison of the experimentally- and theoretically-determined CCSs allows concluding that such nanoclusters have catenane structures.
Designing ligand-enhanced optical absorption of thiolated gold nanoclusters
Chemical communications (Cambridge, England), 2015
The optical spectra of thiolated Au25(SR)18/Au23(SR)16 clusters with different R residues are investigated via TDDFT simulations. Significant enhancements in the optical region and effective electron delocalization are simultaneously achieved by tuning the ligands' steric hindrance and electronic conjugating features, producing a resonance phenomenon between the Au-S core motif and the ligand fragments.