Reactive Ag⁺ Adsorption onto Gold

Proposed mechanisms of monolayer silver formation on gold nanoparticle (AuNP) include AuNP-facilitated under-potential reduction and antigalvanic reduction in which the gold reduces Ag⁺ into metallic atoms Ag(0). Reported herein is the spontaneous reactive Ag⁺ adsorption onto gold substrates that include both as-obtained and butanethiol-functionalized citrate- and NaBH₄-reduced gold nanoparticles (AuNPs), commercial high-purity gold foil, and gold film sputter-coated onto silicon. The silver adsorption invariably leads to proton releasing to the solution. The nominal saturation packing density of silver on AuNPs varies from 2.8 ± 0.3 nmol/cm² for the AuNPs preaggregated with KNO₃ to 4.3 ± 0.2 nmol/cm² for the AuNPs prefunctionalized with butanethiol (BuT). The apparent Langmuir binding constant of the Ag⁺ with the preaggregated AuNPs and BuT-functionalized AuNPs are 4.0 × 10³ M^–1 and 2.1 × 10⁵ M^–1, respectively. The silver adsorption has drastic effects on the structure, conformation, and stability of the organothiols on the AuNPs. It converts disordered BuT on AuNPs into highly ordered <i>trans</i> conformers, but induces near complete desorption of sodium 2-mercaptoethanesulfonate and sodium 3-mercapto-1-propyl sulfonate from AuNPs. Mechanically, the Ag⁺ adsorption on AuNPs most likely proceeds by reacting with molecules preadsorbed on the AuNP surfaces or chemical species in the solutions, and the silver remains as silver ion in these reaction products. This insight and methodology presented in this work are important for studying interfacial interactions of metallic species with gold and for postpreparation modulation of the organothiol structure and conformation on AuNP surfaces

Synthesis and Characterization of Copper Complexes with Cu^ICu^I, Cu^1.5Cu^1.5m and Cu^IICu^II Core Structures Supported by a Flexible Dipyridylamide Ligand

A series of copper complexes supported by a simple dipyridylamide ligand (H2pcp) were isolated and characterized. Treatment of H2pcp with NaH and copper­(I) salts led to the formation of [Cu₂(2pcp)₂] (<b>1a</b>) and {Na­[(Cu₂(2pcp)₂)₂]­PF₆}_<i>n</i> (<b>1b</b>). The X-ray crystal structures of both complexes feature Cu^ICu^I cores with close Cu···Cu interactions. Electrochemical studies of <b>1a</b> showed a reversible one-electron oxidation wave in CH₂Cl₂. On the basis of the work on <b>1a</b>, we began studying the mixed-valence copper species supported by this ligand. The reaction of H2pcp with Cu­(OAc)₂ and CuCl in different stoichiometries yielded [Cu₂(2pcp)₂Cl] (<b>2</b>) and [Cu₃(2pcp)₂Cl₂] (<b>3</b>). X-ray crystallography and spectroscopic characterization suggested delocalized Cu^1.5Cu^1.5 core structures of both compounds. These results further inspired us to explore the coordination properties of H2pcp toward Cu^II ions. The complexes [HNEt₃]­[Cu₂(2pcp)₃(ClO₄)]­(ClO₄) (<b>4a</b>), [Cu₂(2pcp)₃(NO₃)] (<b>4b</b>), and [Cu₂(2pcp)₃(H₂O)]­BF₄ (<b>4c</b>) featuring dinuclear Cu^IICu^II cores were prepared and characterized by X-ray crystallography and spectroscopic methods. Structural analysis of these complexes implied that the accommodation of Cu^ICu^I, Cu^1.5Cu^1.5, and Cu^IICu^II is attributed to the structural flexibility of the ligand H2pcp. Complexes <b>1a</b>, <b>2</b>, <b>3</b>, and <b>4a</b> were examined by X-ray photoelectron spectroscopy, which confirmed the oxidation state assignments. Computational studies were also performed to provide insight into the electronic structures of these complexes

Synthesis and Characterization of Copper Complexes with Cu^ICu^I, Cu^1.5Cu^1.5m and Cu^IICu^II Core Structures Supported by a Flexible Dipyridylamide Ligand

A series of copper complexes supported by a simple dipyridylamide ligand (H2pcp) were isolated and characterized. Treatment of H2pcp with NaH and copper­(I) salts led to the formation of [Cu₂(2pcp)₂] (<b>1a</b>) and {Na­[(Cu₂(2pcp)₂)₂]­PF₆}_<i>n</i> (<b>1b</b>). The X-ray crystal structures of both complexes feature Cu^ICu^I cores with close Cu···Cu interactions. Electrochemical studies of <b>1a</b> showed a reversible one-electron oxidation wave in CH₂Cl₂. On the basis of the work on <b>1a</b>, we began studying the mixed-valence copper species supported by this ligand. The reaction of H2pcp with Cu­(OAc)₂ and CuCl in different stoichiometries yielded [Cu₂(2pcp)₂Cl] (<b>2</b>) and [Cu₃(2pcp)₂Cl₂] (<b>3</b>). X-ray crystallography and spectroscopic characterization suggested delocalized Cu^1.5Cu^1.5 core structures of both compounds. These results further inspired us to explore the coordination properties of H2pcp toward Cu^II ions. The complexes [HNEt₃]­[Cu₂(2pcp)₃(ClO₄)]­(ClO₄) (<b>4a</b>), [Cu₂(2pcp)₃(NO₃)] (<b>4b</b>), and [Cu₂(2pcp)₃(H₂O)]­BF₄ (<b>4c</b>) featuring dinuclear Cu^IICu^II cores were prepared and characterized by X-ray crystallography and spectroscopic methods. Structural analysis of these complexes implied that the accommodation of Cu^ICu^I, Cu^1.5Cu^1.5, and Cu^IICu^II is attributed to the structural flexibility of the ligand H2pcp. Complexes <b>1a</b>, <b>2</b>, <b>3</b>, and <b>4a</b> were examined by X-ray photoelectron spectroscopy, which confirmed the oxidation state assignments. Computational studies were also performed to provide insight into the electronic structures of these complexes

Counterion Effects on Electrolyte Interactions with Gold Nanoparticles

Electrolyte interactions with nanoparticles (NPs) at the solid/liquid interfaces are highly complicated as the charged species can be directly adsorbed onto the NP surfaces, confined in the diffusion layer immediately surrounding the NPs, and dispersed in bulk solutions. Existing studies on electrolyte interactions with NPs are based primarily on the electrical double layer theory that focuses mainly on electrolyte interactions with NPs with fixed pre-existing charges. Demonstrated herein is a comprehensive study of counterion effects during the electrolyte bindings to gold nanoparticles (AuNPs), including halide-induced AuNP aggregation and fusion, quantitative cation and anion coadsorption, selective cation and anion displacement on AuNPs, and surface-enhanced Raman spectroscopic features of the ionic species adsorbed onto AuNP surfaces. In contradiction to previous reports that electrolyte effects are anion-specific, we demonstrated that cations can play a dominant role in the halide-induced AuNP aggregation and fusion and the ion-exchange processes on AuNP surfaces. Mechanistically, these counterion effects are due to the cooperative and competitive cation and anion binding to AuNPs and AuNP-facilitated cation and anion interactions. The insights provided in this work should be of broad importance for NP research and applications in which electrolyte/NP interactions are ubiquitously implicated

Contradictory Dual Effects: Organothiols Can Induce Both Silver Nanoparticle Disintegration and Formation under Ambient Conditions

Using propanethiol (PrT), 2-mercaptoethanol (ME), glutathione (GSH), and cysteine (Cys) as model thiols, we demonstrated herein that organothiols can induce both silver nanoparticle (AgNP) disintegration and formation under ambient conditions by simply mixing organothiols with AgNPs and AgNO₃, respectively. Mechanistically, organothiols induce AgNP disintegration by chelating silver ions produced by ambient oxygen oxidizing the AgNPs, while AgNP formation in AgNO₃/organothiol mixtures is the result of organothiols serving as the reducing agent. Furthermore, surface-plasmon- and fluorescent-active AgNPs can be interconverted by adding excess Ag⁺ or ME into the AgNP-containing solutions. Organothiols can also reduce gold ion in HAuCl₄/organothiol solutions into fluorescence- and surface-plasmon-active gold nanoparticles (AuNPs), but no AuNP disintegration occurs in the AuNP/organothiol solutions. This work highlights the extraordinary complexity of organothiol interactions with gold and silver nanoparticles. The insights from this work will be important for AgNP and AuNP synthesis and applications

Ion Pairing as the Main Pathway for Reducing Electrostatic Repulsion among Organothiolate Self-assembled on Gold Nanoparticles in Water

Organothiol binding to gold nanoparticles (AuNPs) in water proceeds through a deprotonation pathway in which the sulfur-bound hydrogen (RS-H) atoms are released to solution as protons and the organothiol attach to AuNPs as negatively charged thiolate. The missing puzzle pieces in this mechanism are (i) the significance of electrostatic repulsion among the likely charged thiolates packed on AuNP surfaces, and (ii) the pathways for the ligand binding system to cope with such electrostatic repulsion. Presented herein are a series of experimental and theoretical evidence that ion pairing, the coadsorption of negatively charged thiolate and positively charged cations, is a main mechanism for the system to reduce the electrostatic repulsion among the thiolate self-assembled onto AuNP surfaces. This work represents a significant step forward in the comprehensive understanding of organothiol binding to AuNPs

Iodide-Induced Organothiol Desorption and Photochemical Reaction, Gold Nanoparticle (AuNP) Fusion, and SERS Signal Reduction in Organothiol-Containing AuNP Aggregates

Gold nanoparticles (AuNPs) have been used extensively as surface-enhanced Raman spectroscopic (SERS) substrates for their large SERS enhancements and widely believed chemical stability. Presented is the finding that iodide can rapidly reduce the SERS intensity of the ligands, including organothiols adsorbed on plasmonic AuNPs through both iodide-induced ligand desorption and AuNP fusion. The organothiols trapped inside the fused AuNPs have negligible SERS activities. Multiple photochemical processes were involved when organothiol-containing AuNP aggregates were treated with KI under photoillumination. The photocatalytically produced I₃^– reacts with both organothiol and AuNPs. Chloride and bromide also induce partial organothiol displacement and the fusion of the as-synthesized AuNPs, but neither of the two halides has detectable effects on the morphology and Raman signals of the organothiol-containing AuNP aggregates. The insight provided in this work should be important for the understanding of interfacial interactions of plasmonic AuNPs and their SERS applications