Au₃₂₉(SR)₈₄ Nanomolecules: Compositional Assignment of the 76.3 kDa Plasmonic Faradaurates

The purpose of this work is to determine the chemical composition of the previously reported faradaurates, which is a large 76.3 kDa thiolated gold nanomolecule. Electrospray ionization quadrupole-time-of-flight (ESI Q-TOF) mass spectrometry of the title compound using three different thiols yield the 329:84 gold to thiol compositional assignment. The purity of the title compound was checked by matrix-assisted laser desorption ionization-time-of-flight (MALDI-TOF) mass spectrometry. Positive and negative mode ESI-MS spectra show identical peaks denoting that there are no counterions, further reinforcing the accuracy of the assigned composition. We intentionally added Cs⁺ ions to show that the Au₃₂₉(SR)₈₄ is the base molecular ion, with several Cs⁺ adducts. A comprehensive investigation including analysis of the title compound with three ligands, in positive and negative mode and Cs⁺ adduction, leads to a conclusive composition of Au₃₂₉(SR)₈₄. This formula determination will facilitate the fundamental understanding of emergence of surface plasmon resonance in Au₃₂₉(SR)₈₄ with 245 free electrons

X‑ray Crystal Structure and Theoretical Analysis of Au_{25–<i>x</i>}Ag_<i>x</i>(SCH₂CH₂Ph)₁₈^– Alloy

The atomic arrangement of Au and Ag atoms in Au_{25–<i>x</i>}Ag_<i>x</i>(SR)₁₈ was determined by X-ray crystallography. Ag atoms were selectively incorporated in the 12 vertices of the icosahedral core. The central atom and the metal atoms in the six [−SR–Au–SR–Au–SR−] units were exclusively gold, with 100% Au occupancy. The composition of the crystals determined by X-ray crystallography was Au_18.3Ag_6.7(SCH₂CH₂Ph)₁₈. This composition is in reasonable agreement with the composition Au_18.8Ag_6.2(SCH₂CH₂Ph)₁₈ measured by electrospray mass spectrometry. The structure can be described in terms of shells as Au₁@Au_5.3Ag_6.7@6×[−SR–Au–SR–Au–SR−]. Density functional theory calculations show that the electronic structure and optical absorption spectra are sensitive to the silver atom arrangement within the nanocluster

X‑ray Crystal Structure of Au_{38–<i>x</i>}Ag_<i>x</i>(SCH₂CH₂Ph)₂₄ Alloy Nanomolecules

Herein, we report the X-ray crystallographic structure of a 38-metal atom Au–Ag alloy nanomolecule. The structure of monometallic Au₃₈(SR)₂₄ consists of 2 central Au atoms and 21 Au atoms forming a bi-icosahedral core protected by 6 dimeric and 3 monomeric units. In Au_{38–<i>x</i>}Ag_<i>x</i>(SR)₂₄,where <i>x</i> ranges from 1 to 5, the silver atoms are selectively incorporated into the Au₂₁ bi-icosahedral core. Within the Au₂₁ core, the silver atoms preferentially occupy nine selected locations: (a) the two vertex edges, three atoms on each edge and six atoms total, and (b) the middle face-shared three-atom ring, adding to a total of nine locations. X-ray crystallography yielded a composition of Au_34.04Ag_3.96(SCH₂CH₂Ph)₂₄. The crystal structure of the alloy nanomolecule can be described in terms of shells as Au₂@Au_17.04Ag_3.96@ 6×[−SR–Au–SR–Au–SR] 3×[−SR–Au–SR−]

X‑ray Crystal Structure of Au_{38–<i>x</i>}Ag_<i>x</i>(SCH₂CH₂Ph)₂₄ Alloy Nanomolecules

Herein, we report the X-ray crystallographic structure of a 38-metal atom Au–Ag alloy nanomolecule. The structure of monometallic Au₃₈(SR)₂₄ consists of 2 central Au atoms and 21 Au atoms forming a bi-icosahedral core protected by 6 dimeric and 3 monomeric units. In Au_{38–<i>x</i>}Ag_<i>x</i>(SR)₂₄,where <i>x</i> ranges from 1 to 5, the silver atoms are selectively incorporated into the Au₂₁ bi-icosahedral core. Within the Au₂₁ core, the silver atoms preferentially occupy nine selected locations: (a) the two vertex edges, three atoms on each edge and six atoms total, and (b) the middle face-shared three-atom ring, adding to a total of nine locations. X-ray crystallography yielded a composition of Au_34.04Ag_3.96(SCH₂CH₂Ph)₂₄. The crystal structure of the alloy nanomolecule can be described in terms of shells as Au₂@Au_17.04Ag_3.96@ 6×[−SR–Au–SR–Au–SR] 3×[−SR–Au–SR−]

X‑ray Crystal Structure and Theoretical Analysis of Au_{25–<i>x</i>}Ag_<i>x</i>(SCH₂CH₂Ph)₁₈^– Alloy

The atomic arrangement of Au and Ag atoms in Au_{25–<i>x</i>}Ag_<i>x</i>(SR)₁₈ was determined by X-ray crystallography. Ag atoms were selectively incorporated in the 12 vertices of the icosahedral core. The central atom and the metal atoms in the six [−SR–Au–SR–Au–SR−] units were exclusively gold, with 100% Au occupancy. The composition of the crystals determined by X-ray crystallography was Au_18.3Ag_6.7(SCH₂CH₂Ph)₁₈. This composition is in reasonable agreement with the composition Au_18.8Ag_6.2(SCH₂CH₂Ph)₁₈ measured by electrospray mass spectrometry. The structure can be described in terms of shells as Au₁@Au_5.3Ag_6.7@6×[−SR–Au–SR–Au–SR−]. Density functional theory calculations show that the electronic structure and optical absorption spectra are sensitive to the silver atom arrangement within the nanocluster

Au_{329–<i>x</i>}Ag_<i>x</i>(SR)₈₄ Nanomolecules: Plasmonic Alloy Faradaurate-329

Though significant progress has been made to improve the monodispersity of larger (>10 nm) alloy metal nanoparticles, there still exists a significant variation in nanoparticle composition, ranging from ±1000s of atoms. Here, for the first time, we report the synthesis of atomically precise (±0 metal atom variation) Au_{329–<i>x</i>}Ag_<i>x</i>(SCH₂CH₂Ph)₈₄ alloy nanomolecules. The composition was determined using high resolution electrospray ionization mass spectrometry. In contrast to larger (>10 nm) Au–Ag nanoparticles, the surface plasmon resonance (SPR) peak does not show a major shift, but a minor ∼10 nm red-shift, upon increasing silver content. The intensity of the SPR peak also varies in an intriguing manner, where a dampening is observed with medium silver incorporation, and a significant sharpening is observed upon higher Ag content. The report outlines (a) an unprecedented advance in nanoparticle mass spectrometry of high mass at atomic precision; and (b) the unexpected optical behavior of Au–Ag alloys in the region where nascent SPR emerges; specifically, in this work, the SPR-like peak does not show a major ∼100 nm blue-shift with Ag alloying of Au₃₂₉ nanomolecules, as shown to be common in larger nanoparticles

Faradaurate-940: Synthesis, Mass Spectrometry, Electron Microscopy, High-Energy X‑ray Diffraction, and X‑ray Scattering Study of Au_∼940±20(SR)_∼160±4 Nanocrystals

Obtaining monodisperse nanocrystals and determining their composition to the atomic level and their atomic structure is highly desirable but is generally lacking. Here, we report the discovery and comprehensive characterization of a 2.9 nm plasmonic nanocrystal with a composition of Au_940±20(SCH₂CH₂Ph)_160±4, which is the largest mass spectrometrically characterized gold thiolate nanoparticle produced to date. The compositional assignment has been made using electrospray ionization and matrix-assisted laser desorption ionization mass spectrometry (MS). The MS results show an unprecedented size monodispersity, where the number of Au atoms varies by only 40 atoms (940 ± 20). The mass spectrometrically determined composition and size are supported by aberration-corrected scanning transmission electron microscopy (STEM) and synchrotron-based methods such as atomic pair distribution function (PDF) and small-angle X-ray scattering (SAXS). Lower-resolution STEM images show an ensemble of particles1000s per framevisually demonstrating monodispersity. Modeling of SAXS data on statistically significant nanoparticle populationapproximately 10¹² individual nanoparticlesshows that the diameter is 3.0 ± 0.2 nm, supporting mass spectrometry and electron microscopy results on monodispersity. Atomic PDF based on high-energy X-ray diffraction experiments shows decent match with either a Marks decahedral or truncated octahedral structure. Atomic resolution STEM images of single particles and their fast Fourier transform suggest face-centered cubic arrangement. UV–visible spectroscopy data show that Faradaurate-940 supports a surface plasmon resonance peak at ̃505 nm. These monodisperse plasmonic nanoparticles minimize averaging effects and have potential application in solar cells, nano-optical devices, catalysis, and drug delivery

Atomic Structure of Au₃₂₉(SR)₈₄ Faradaurate Plasmonic Nanomolecules

To design novel nanomaterials, it is important to precisely control the composition, determine the atomic structure, and manipulate the structure to tune the materials property. Here we present a comprehensive characterization of the material whose composition is Au₃₂₉(SR)₈₄ precisely, therefore referred to as a nanomolecule. The size homogeneity was shown by electron microscopy, solution X-ray scattering, and mass spectrometry. We proposed its atomic structure to contain the Au₂₆₀ core using experiments and modeling of a total-scattering-based atomic-pair distribution functional analysis. HAADF-STEM images shows fcc-like 2.0 ± 0.1 nm diameter nanomolecules

Organic-Modified Silver Nanoparticles as Lubricant Additives

Advanced lubrication is essential in human life for improving mobility, durability, and efficiency. Here we report the synthesis, characterization, and evaluation of two groups of oil-suspendable silver nanoparticles (NPs) as candidate lubricant additives. Two types of thiolated ligands, 4-(<i>tert</i>-butyl)­benzylthiol (TBBT) and dodecanethiol (C12), were used to modify Ag NPs in two size ranges, 1–3 and 3–6 nm. The organic surface layer successfully suspended the Ag NPs in a poly-alpha-olefin (PAO) base oil with concentrations up to 0.19–0.50 wt %, depending on the particle type. Use of the Ag NPs in the base oil reduced friction by up to 35% and wear by up to 85% in boundary lubrication. The two TBBT-modified NPs produced a lower friction coefficient than the C12-modified one, while the two larger NPs (3–6 nm) had better wear protection than the smaller one (1–3 nm). Results suggested that the molecular structure of the organic ligand might have a dominant effect on the friction behavior, while the NP size could be more influential in the wear protection. No mini-ball-bearing or surface smoothening effects were observed in the Stribeck scans. Instead, the wear protection in boundary lubrication was attributed to the formation of a silver-rich 50–100 nm thick tribofilm on the worn surface, as revealed by morphology examination and composition analysis from both the top surface and cross section

Structural Information on the Au–S Interface of Thiolate-Protected Gold Clusters: A Raman Spectroscopy Study

The Raman spectra of a series of monolayer-protected gold clusters were investigated with special emphasis on the Au–S modes below 400 cm^–1. These clusters contain monomeric (SR-Au-SR) and dimeric (SR-Au-SR-Au-SR) gold–thiolate staples in their surface. In particular, the Raman spectra of [Au₂₅(2-PET)₁₈]^0/–, Au₃₈(2-PET)₂₄, Au₄₀(2-PET)₂₄, and Au₁₄₄(2-PET)₆₀ (2-PET = 2-phenylethylthiol) were measured in order to study the influence of the cluster size and therefore the composition with respect to the monomeric and dimeric staples. Additionally, spectra of Au₂₅(2-PET)_{18–2<i>x</i>}(<i>S</i>-/<i>rac</i>-BINAS)_<i>x</i> (BINAS = 1,1′-binaphthyl-2,2′-dithiol), Au₂₅(CamS)₁₈ (CamS = 1<i>R</i>,4<i>S</i>-camphorthiol), and Au_<i>n</i>BINAS_<i>m</i> were measured to identify the influence of the thiolate ligand on the Au–S vibrations. The vibrational spectrum of Au₃₈(SCH₃)₂₄ was calculated which allows the assignment of bands to vibrational modes of the different staple motifs. The spectra are sensitive to the size of the cluster and the nature of the ligand. Au–S–C bending around 200 cm^–1 shifts to slightly higher wavenumbers for the dimeric as compared to the monomeric staples. Radial Au–S modes (250–325 cm^–1) seem to be sensitive toward the staple composition and the bulkiness of the ligand, having higher intensities for long staples and shifting to higher wavenumbers for sterically more demanding ligands. The introduction of only one BINAS dithiol has a dramatic influence on the Au–S vibrations because the molecule bridges two staples which changes their vibrational properties completely