Solvent Polarity-Dependent Behavior of Aliphatic Thiols and Amines toward Intriguingly Fluorescent AuAgGSH Assembly

Highly stable fluorescent glutathione (GSH)-protected AuAg assembly has been synthesized in water under UV irradiation. The assembly is composed of small Ag₂/Ag₃ clusters. These clusters gain stability through synergistic interaction with Au­(I) present within the assembly. This makes the overall assembly fluorescent. Here, GSH acts as a reducing as well as stabilizing agent. The assembly is so robust that it can be vacuum-dried to solid particles. The as-obtained solid is dispersible in nonaqueous solvents. The interaction between solvent and the assembly provides stability to the assembly, and the assembly shows fluorescence. It is interesting to see that the behavior of long-chain aliphatic thiols or amines toward the fluorescent assembly is altogether a different phenomenon in aqueous and nonaqueous mediums. The assembly gets ruptured in water due to direct interaction with long-chain thiols or amines, whereas in nonaqueous medium, solvation of added thiols or amines becomes pronounced, which hinders the interaction of solvent with the assembly. However, the fluorescence of the assembly is always quenched with thiols or amines no matter what the solvent medium is. In aqueous medium, the fluorescence quenching by aliphatic thiol or amine becomes pronounced with successive decrease in their chain length, whereas in nonaqueous medium, the trend is just reversed with chain length. The reasons behind such an interesting reversal of fluorescence quenching in aqueous and nonaqueous solvents have been discussed explicitly. Again, in organic solvents, thiol or amine-induced quenched fluorescence is selectively recovered by Pb­(II) ion without any alteration of excitation and emission maxima. This phenomenon is not observed in water because of the ruptured fluorescent assembly. The fluorescence recovery by Pb­(II) and unaltered emission peak only in nonaqueous solvent unequivocally prove the engagement of Pb­(II) with thiols or amines, which in turn revert the original solvent-supported stabilization of the assembly

Separation of Precise Compositions of Noble Metal Clusters Protected with Mixed Ligands

This report describes the precise and systematic synthesis of PdAu₂₄ clusters protected with two types of thiolate ligands (-SR1 and -SR2). It involved high-resolution separation of metal clusters containing a distribution of chemical compositions, PdAu₂₄­(SR1)_{18–<i>n</i>}­(SR2)_<i>n</i> (<i>n</i> = 0, 1, 2, ..., 18), to individual clusters of specific <i>n</i> using high-performance liquid chromatography. Similar high-resolution separation was achieved for a few ligand combinations as well as clusters with other metal cores, such as Au₂₅ and Au₃₈. These results demonstrate the ability to precisely control the chemical composition of two types of ligands in thiolate-protected mono- and bimetallic metal clusters. It is expected that greater functional control of thiolate-protected metal clusters, their regular arrays, and systematic variation of their properties can now be achieved

Ligand-Induced Stability of Gold Nanoclusters: Thiolate versus Selenolate

Thiolate-protected gold nanoclusters have attracted considerable attention as building blocks for new functional materials and have been extensively researched. Some studies have reported that changing the ligand of these gold nanoclusters from thiolate to selenolate increases cluster stability. To confirm this, in this study, we compare the stabilities of precisely synthesized [Au₂₅(SC₈H₁₇)₁₈]⁻ and [Au₂₅(SeC₈H₁₇)₁₈]⁻ against degradation in solution, thermal dissolution, and laser fragmentation. The results demonstrate that changing the ligand from thiolate to selenolate increases cluster stability in reactions involving dissociation of the gold–ligand bond but reduces cluster stability in reactions involving intramolecular dissociation of the ligand. These results reveal that using selenolate ligands makes it possible to produce gold clusters that are more stable against degradation in solution than thiolate-protected gold nanoclusters

Evolution of Silver-Mediated, Enhanced Fluorescent Au–Ag Nanoclusters under UV Activation: A Platform for Sensing

Here, we report the synthesis of dopamine (DA)-mediated Au–Ag bimetallic nanoclusters in aqueous solution under UV activation. The success story emerges from monometallic fluorescent nanocluster evolution from photoactivation of gold as well as silver precursor compounds along with DA. The intriguing fluorescence property of the nanocluster relates to facile incorporation of Ag in Au, showing a 6-fold enhancement of the emission profile than simply DA-mediated Au nanoclusters. Silver effect, which is classified under the synergism, is the main reason behind such enhancement of fluorescence. The as-synthesized nanoclusters are robust and can be vacuum-dried and redispersed for repetitive application. The intriguing fluorescence of bimetallic nanoclusters is found to be quenched selectively in the presence of sulfide ion in an aqueous medium, paving the way for nanomolar detection of sulfide in water. The utility of the sensing platform has been verified employing different environmental water effluents

Synthesis of Highly Fluorescent Silver Clusters on Gold(I) Surface

Evolution of fluorescence from a giant core–shell particle is new and synergistic, which requires both gold and silver ions in an appropriate ratio in glutathione (GSH) solution. The formation of highly fluorescent Ag₂/Ag₃ clusters on the surface of Au^I assembly results in giant Au^I_core–Ag⁰_shell water-soluble microparticles (∼500 nm). Here, Au^I acts as the template for the generation of fluorescent Ag clusters. The presence of gold under the synthetic strategy is selective, and no other metal supports such synergistic evolution. The core–shell particle exhibits stable and static emission (emission maximum, 565 nm; quantum yield, 4.6%; and stroke shift, 179 nm) with an average lifetime of ∼25 ns. The drift of electron density by the Au^I core presumably enhances the fluorescence. The positively charged core offers unprecedented long-term stability to the microparticles in aqueous GSH solution

Synthesis and the Origin of the Stability of Thiolate-Protected Au₁₃₀ and Au₁₈₇ Clusters

Two stable thiolate-protected gold clusters (Au–SR), Au₁₃₀ and Au₁₈₇ clusters, were synthesized to obtain a better understanding of the size dependence of the origin of the stability of Au–SR clusters. These clusters were synthesized by employing different preparation conditions from those used to synthesize previously reported magic gold clusters; in particular, a lower [RSH] to [AuCl₄^–] molar ratio ([AuCl₄^–]/[RSH] = 1:1) was used than that used to prepare Au₂₅(SR)₁₈, Au₃₈(SR)₂₄, Au₆₈(SR)₃₄, Au₁₀₂(SR)₄₄, and Au₁₄₄(SR)₆₀ (id. = 1:4–12). The two clusters thus synthesized were separated from the mixture by high-performance liquid chromatography with reverse-phase columns. Mass spectrometry of the products revealed the presence of two clusters with chemical compositions of Au₁₃₀(SC₁₂H₂₅)₅₀ and Au₁₈₇(SC₁₂H₂₅)₆₈. The origin of the stability of these two clusters and the size dependence of the origin of the stability of thiolate-protected gold clusters were discussed in terms of the total number of valence electrons

Effect of Copper Doping on Electronic Structure, Geometric Structure, and Stability of Thiolate-Protected Au₂₅ Nanoclusters

Several recent studies have attempted to impart [Au₂₅(SR)₁₈]⁻ with new properties by doping with foreign atoms. In this study, we studied the effect of copper doping on the electronic structure, geometric structure, and stability of [Au₂₅(SR)₁₈]⁻ with the aim of investigating the effect of foreign atom doping of [Au₂₅(SR)₁₈]⁻. Cu_<i>n</i>Au_{25–<i>n</i>}(SC₂H₄Ph)₁₈ was synthesized by reducing complexes formed by the reaction between metal salts (copper and gold salts) and PhC₂H₄SH with NaBH₄. Mass analysis revealed that the products contained Cu_<i>n</i>Au_{25–<i>n</i>}(SC₂H₄Ph)₁₈ (<i>n</i> = 1–5) in high purity. Experimental and theoretical analysis of the synthesized clusters revealed that copper doping alters the optical properties and redox potentials of the cluster, greatly distorts its geometric structure, and reduces the cluster stability in solution. These findings are expected to be useful for developing design guidelines for functionalizing [Au₂₅(SR)₁₈]⁻ through doping with foreign atoms

Selenolate-Protected Au₃₈ Nanoclusters: Isolation and Structural Characterization

We report the isolation and structural characterization of dodecaneselenolate-protected Au₃₈ clusters (Au₃₈(SeC₁₂H₂₅)₂₄). These clusters were synthesized via the reaction of phenylethanethiolate-protected Au₃₈ clusters (Au₃₈(SC₂H₄Ph)₂₄) with didodecyldiselenide ((C₁₂H₂₅Se)₂). Characterization of the product by mass spectrometry and thermogravimetric analysis confirmed that highly pure Au₃₈(SeC₁₂H₂₅)₂₄ had been obtained. The electronic and geometrical structures, bonding characteristics, and stability of the Au₃₈(SeC₁₂H₂₅)₂₄ clusters were assessed using extended X-ray fine structure and X-ray absorption near edge structure measurements, optical absorption spectroscopy, electrochemical measurements, and stability testing

Understanding Ligand-Exchange Reactions on Thiolate-Protected Gold Clusters by Probing Isomer Distributions Using Reversed-Phase High-Performance Liquid Chromatography

Thiolate-protected gold clusters (Au_<i>n</i>(SR)_<i>m</i>) have attracted considerable attention as functional nanomaterials in a wide range of fields. A ligand-exchange reaction has long been used to functionalize these clusters. In this study, we separated products from a ligand-exchange reaction of phenylethanethiolate-protected Au₂₄Pd clusters (Au₂₄Pd(SC₂H₄Ph)₁₈), in which Au₂₅(SR)₁₈ is doped with palladium, into each coordination isomer with high resolution by reversed-phase high-performance liquid chromatography. This success has enabled isomer distributions of the products to be quantitatively evaluated. We evaluated quantitatively the isomer distributions of products obtained by the reaction of Au₂₄Pd(SC₂H₄Ph)₁₈ with thiol, disulfide, or diselenide. The results revealed that the exchange reaction starts to occur preferentially at thiolates that are bound directly to the metal core (thiolates of a core site) in all reactions. Further study on the isomer-separated Au₂₄Pd(SC₂H₄Ph)₁₇(SC₁₂H₂₅) revealed that clusters vary the coordination isomer distribution in solution by the ligand-exchange reaction between clusters and that control of the coordination isomer distribution of the starting clusters enables control of the coordination isomer distribution of the products generated by ligand-exchange reactions between clusters. Au₂₄Pd(SC₂H₄Ph)₁₈ used in this study has a similar framework structure to Au₂₅(SR)₁₈, which is one of the most studied compounds in the Au_<i>n</i>(SR)_<i>m</i> clusters. Knowledge gained in this study is expected to enable further understanding of ligand-exchange reactions on Au₂₅(SR)₁₈ and other Au_<i>n</i>(SR)_<i>m</i> clusters

Preferential Location of Coinage Metal Dopants (M = Ag or Cu) in [Au_{25–<i>x</i>}M_<i>x</i>(SC₂H₄Ph)₁₈]⁻ (<i>x</i> ∼ 1) As Determined by Extended X‑ray Absorption Fine Structure and Density Functional Theory Calculations

The preferential locations of Ag and Cu atoms in the initial stage of doping into [Au₂₅(SC₂H₄Ph)₁₈]⁻ were studied by X-ray absorption spectroscopy and density functional theory computations. The extended X-ray absorption fine structure (EXAFS) spectra of [Au_23.8Ag_1.2(SC₂H₄Ph)₁₈]⁻ at the Ag K-edge were reproduced using a model structure in which the Ag dopant occupied a surface site in the icosahedral Au₁₃ core that was computationally the most stable site. In contrast, the Cu K-edge EXAFS spectra of [Au_23.6Cu_1.4(SC₂H₄Ph)₁₈]⁻ indicated that the Cu dopant was preferentially located at the oligomer site that was computationally less stable than the surface site. This discrepancy between the Cu location experimentally determined and that theoretically predicted was explained in terms of variations in the stability of the Cu dopant at the two sites against aerobic oxidation. These results demonstrate that the mixing patterns of bimetallic clusters are determined not only by the thermodynamic stability but also by the durability of the mixed structure under synthetic and storage conditions