Robert L Whetten - Academia.edu (original) (raw)
Papers by Robert L Whetten
![Research paper thumbnail of Electrospray Gold Standards of Molecular Mass 32- to 52-kDa: Charging Patterns of the Ubiquitous Virus-like Clusters, I - 197Au144-5(SR)60, R = 8 Variants, in Native [HPLC]-ESI-MS](https://attachments.academia-assets.com/100724265/thumbnails/1.jpg)
](Faradays legendary Molecules of Gold have stimulated intense interest (over 165 years) but have o... more Faradays legendary Molecules of Gold have stimulated intense interest (over 165 years) but have only recently begun to yield their secrets to modern methods of chemical analysis. Here(in), we demonstrate how striking charging patterns emerge directly from native electrospray of large, gold-rich molecules that were generated by reduction of various (8) small gold(I)thiolate complexes [-RS-Au(I)-SR-], followed by extensive thermochemical processing to enrich the most robust forms. In each case (R), electrospray ionization of a picomolar solution yields a characteristic series of abundant, highly resolved peaks at related (m/z)-ratios, that can be used to deduce charges {z e+} and hence a distinct molecular mass, {MR}. A plot of {MR} versus thiolate-mass {mL} yields a straight line with slope 60.0 (the ligand count) and an intercept of 28,364-Da, the mass of 144 Au-atoms. i.e., a unique molecular composition {197Au144(SR)60}. This formula agrees with the unique chiral-icosahedral struc...
ACS Omega, 2019
Many antibiotic resistances to penicillin have been reported, making them obsolete against multir... more Many antibiotic resistances to penicillin have been reported, making them obsolete against multiresistant bacteria. Because penicillins act by inhibiting cell wall production while silver particles disrupt the cell wall directly, a synergetic effect is anticipated when both modes of action are incorporated into a chimera cluster. To test this hypothesis, the lipoate ligands (LA) of a silver cluster (Ag 29) of known composition (Ag 29 LA 12) [3−] were covalently conjugated to 6-aminopenicillanic acid, a molecule with a β-lactam backbone. Indeed, the partially conjugated cluster inhibited an Staphylococcus aureus biofilm, in a dose−response manner, with a half-maximal inhibitory concentration IC 50 of 2.3 μM, an improvement over 60 times relative to the unconjugated cluster (IC 50 = 140 μM). An enhancement of several orders of magnitude over 6-APA alone (unconjugated) was calculated (IC 50 = 10 000 μM). Cell wall damage is documented via scanning electron microscopy. A synergistic effect of the conjugate was calculated by the combination index method described by Chou− Talalay. This hybrid nanoantibiotic opens a new front against multidrug-resistant pathogens.
The journal of physical chemistry letters, Jan 16, 2017
We report the first results of ultraviolet photodissociation (UVPD) mass spectrometry of trapped ... more We report the first results of ultraviolet photodissociation (UVPD) mass spectrometry of trapped monolayer-protected cluster (MPC) ions generated by electrospray ionization. Gold clusters Au25(pMBA)18 and Au36(pMBA)24 (pMBA = para-mercaptobenzoic acid) were analyzed in both the positive and negative modes. Whereas activation methods including collisional- and electron-based methods produced relatively few fragment ions, even a single ultraviolet pulse (at λ = 193 nm) caused extensive fragmentation of the positively charged clusters. Upon photoactivation using a low number of laser pulses, the staple motifs of both clusters were cleaved and stripped of the protecting ligand portions without removal of any contained gold atoms. This striking process involved Au-S and C-S bond cleavages via a pathway made possible by 6.4 eV photon absorption. Monomer evaporation (neutral gold atom loss) occurred upon exposure to multiple pulses, resulting in a size series of bare gold-cluster ions. All...
The Journal of Physical Chemistry B, 2016
Experimental and theoretical evidence reveals the resilience and stability of the larger aqueous ... more Experimental and theoretical evidence reveals the resilience and stability of the larger aqueous gold clusters protected with p-mercaptobenzoic acid ligands (pMBA) of composition Au n (pMBA) p or (n,p). The Au 144 (pMBA) 60 , (144,60), or Gold-144 aqueous gold cluster is considered special because of its high symmetry, abundance, and icosahedral structure as well as its many potential uses in material and biological sciences. Yet to this date, direct confirmation of its precise composition and total structure remains elusive. Results described here from characterization via high-resolution electrospray ionization mass spectrometry on an Orbitrap instrument show Au 102 (pMBA) 44 at isotopic resolution. Further, what usually appears as a single band for (144,60) in electrophoresis (PAGE) is shown to also contain (130,50), recently determined to have a truncated decahedral structure, and (137,56) in addition to the dominant (144,60) compound of chiral icosahedral structure. This finding is significant in that it reveals the existence of structures never before observed in all-aromatic water soluble species while pointing out the path toward elucidation of the thermodynamic control of protected gold nanocrystal formation.
ACS Nano, 2016
The alloy Au−Ag system is an important noble bimetallic phase, both historically (as "Electrum") ... more The alloy Au−Ag system is an important noble bimetallic phase, both historically (as "Electrum") and now especially in nanotechnology, as it is applied in catalysis and nanomedicine. To comprehend the structural characteristics and the thermodynamic stability of this alloy, a knowledge of its phase diagram is required that considers explicitly its size and shape (morphology) dependence. However, as the experimental determination remains quite challenging at the nanoscale, theoretical guidance can provide significant advantages. Using a regular solution model within a nanothermodynamic approach to evaluate the size effect on all the parameters (melting temperature, melting enthalpy, and interaction parameters in both phases), the nanophase diagram is predicted. Besides an overall shift downward, there is a "tilting" effect on the solidus−liquidus curves for some particular shapes exposing the (100) and (110) facets (cube, rhombic dodecahedron, and cuboctahedron). The segregation calculation reveals the preferential presence of silver at the surface for all the polyhedral shapes considered, in excellent agreement with the latest transmission electron microscopy observations and energy dispersive spectroscopy analysis. By reviewing the nature of the surface segregated element of different bimetallic nanoalloys, two surface segregation rules, based on the melting temperatures and surface energies, are deduced. Finally, the optical properties of Au−Ag nanoparticles, calculated within the discrete dipole approximation, show the control that can be achieved in the tuning of the local surface plasmon resonance, depending of the alloy content, the chemical ordering, the morphology, the size of the nanoparticle, and the nature of the surrounding environment.
The journal of physical chemistry. B, Jan 19, 2015
Nanometer-sized gold particles (AuNPs) are of peculiar interest because their behaviors in an aqu... more Nanometer-sized gold particles (AuNPs) are of peculiar interest because their behaviors in an aqueous solution are sensitive to changes in environmental factors including the size and shape of the solute ions. In order to determine these important characteristics, we performed all-atom molecular dynamics simulations on the icosahedral Au144 nanoparticles each coated with a homogeneous set of 60 thiolates (4-mercapto-benzoate, pMBA) in eight aqueous solutions having ions of varying sizes and shapes (Na+, K+, tetramethylamonium cation TMA+, trisamonium cation TRS+, Cl-, and OH-). For each solution, we computed the reversible work (potential of mean of force) to bring two nanoparticles together as a function of their separation distance. We found that the behavior of pMBA protected Au144 nanoparticles can be readily modulated by tuning their aqueous environmental factors (pH and solute ion combinations). We examined the atomistic details on how the sizes and shapes of solute ions quant...
Small Particles and Inorganic Clusters, 1997
The Journal of Physical Chemistry, 1996
Orientational ordering of faceted nanocrystals in nanocrystal arrays has been directly observed f... more Orientational ordering of faceted nanocrystals in nanocrystal arrays has been directly observed for the first time, by use of transmission electron microscopy imaging and diffraction to resolve the structure of thin molecular-crystalline films of silver nanocrystals passivated by alkylthiolate self-assembled monolayers. The type of ordering found is determined by the nanocrystal's faceted morphology, as mediated by the interactions of surfactant groups tethered to the facets on neighboring nanocrystals. Orientational ordering is crucial for the understanding of the fundamental properties of quantum-dot arrays, as well as for their optimal utilization in optical and electronic applications.
Nano Letters, 2014
Gold−copper (Au−Cu) phases were employed already by pre-Columbian civilizations, essentially in d... more Gold−copper (Au−Cu) phases were employed already by pre-Columbian civilizations, essentially in decorative arts, whereas nowadays, they emerge in nanotechnology as an important catalyst. The knowledge of the phase diagram is critical to understanding the performance of a material. However, experimental determination of nanophase diagrams is rare because calorimetry remains quite challenging at the nanoscale; theoretical investigations, therefore, are welcomed. Using nanothermodynamics, this paper presents the phase diagrams of various polyhedral nanoparticles (tetrahedron, cube, octahedron, decahedron, dodecahedron, rhombic dodecahedron, truncated octahedron, cuboctahedron, and icosahedron) at sizes 4 and 10 nm. One finds, for all the shapes investigated, that the congruent melting point of these nanoparticles is shifted with respect to both size and composition (copper enrichment). Segregation reveals a gold enrichment at the surface, leading to a kind of core−shell structure, reminiscent of the historical artifacts. Finally, the most stable structures were determined to be the dodecahedron, truncated octahedron, and icosahedron with a Cu-rich core/Au-rich surface. The results of the thermodynamic approach are compared and supported by molecular-dynamics simulations and by electronmicroscopy (EDX) observations.
MRS Communications, 2013
The fabrication of bimetallic magnetic nanoparticles (NPs) smaller than the size of single magnet... more The fabrication of bimetallic magnetic nanoparticles (NPs) smaller than the size of single magnetic domain is very challenging because of the agglomeration, non-uniform size, and possible complex chemistry at nanoscale. In this paper, we present an alloyed ferromagnetic 4 ± 1 nm thiolated Au/Co magnetic NPs with decahedral and icosahedral shape. The NPs were characterized by Cs-corrected scanning transmission electron microscopy (STEM) and weretheoretically studied by Grand Canonical Monte Carlo simulations. Comparison of Z-contrast imaging and energy dispersive x-ray spectroscopy used jointly with STEM simulated images from theoretical models uniquely showed an inhomogeneous alloying with minor segregation. The magnetic measurements obtained from superconducting quantum interference device magnetometer exhibited ferromagnetic behavior. This magnetic nanoalloy in the range of single domain is fully magnetized and carries significance as a promising candidate for magnetic data recording, permanent magnetization, and biomedical applications.
The Journal of Physical Chemistry B, 1997
The optical absorption spectra of a series of nanocrystal gold moleculesslarger, crystalline Au c... more The optical absorption spectra of a series of nanocrystal gold moleculesslarger, crystalline Au clusters that are passivated by a compact monolayer of n-alkylthiol(ate)sshave been measured across the electronic range (1.1-4.0 eV) in dilute solution at ordinary temperature. Each of the ∼20 samples, ranging in effective core diameter from 1.4 to 3.2 nm (∼70 to ∼800 Au atoms), has been purified by fractional crystallization and has undergone a separate compositional and structural characterization by mass spectrometry and X-ray diffraction. With decreasing core mass (crystallite size) the spectra uniformly show a systematic evolution, specifically (i) a broadening of the so-called surface-plasmon band until it is essentially unidentifiable for crystallites of less than 2.0 nm effective diameter, (ii) the emergence of a distinct onset for strong absorption near the energy (∼1.7 eV) of the interbandgap (5d f 6sp), and (iii) the appearance in the smallest crystallites of a weak steplike structure above this onset, which is interpreted as arising from a series of transitions from the continuum d-band to the discrete level structure of the conduction band just above the Fermi level. The classical electrodynamic (Mie) theory, based on bulk optical properties, can reproduce this spectral evolutionsand thereby yield a consistent core-sizingsonly by making a strong assumption about the surface chemical interaction. Quantitative agreement with the spectral line shape requires a size-dependent offset of the frequency-dependent dielectric function, which may be explained by a transition in electronic structure just below 2.0 nm (∼200 atoms), as proposed earlier.
The Journal of Physical Chemistry B, 1998
Self-assembling of nanocrystals involves organization of nanocrystals encapsulated by protective ... more Self-assembling of nanocrystals involves organization of nanocrystals encapsulated by protective compact organic molecules into a crystalline material. The adsorbed molecules not only serve as the protection layer for the nanocrystals but also provide the dominant cohesive interactions (or "bonding") sustaining the nanocrystal superlattices. The length of the adsorbed molecules is a controllable parameter, making the ratio of particle size to interparticle distance an adjustable parameter that sensitively tunes the interparticle interaction/ coupling and resulting collective properties. In this paper, bundling and interdigitation of thiolate molecules adsorbed on Ag nanocrystals are observed using the chemical imaging technique in energy-filtered transmission electron microscopy (EF-TEM) at a resolution of ∼2 nm. In these orientationally ordered, self-assembled Ag-nanocrystal superlattices, the bundling of the adsorbed molecules on the nanocrystal surfaces is the fundamental structural principle. A model consistent with the nanocrystal's morphology and the interdigitation of the adsorbed thiolates is proposed.
The Journal of Physical Chemistry A, 2013
The preparation of gold nanomolecules with sizes other than Au 25 (SR) 18 , Au 38 (SR) 24 , Au 10... more The preparation of gold nanomolecules with sizes other than Au 25 (SR) 18 , Au 38 (SR) 24 , Au 102 (SR) 44 , and Au 144 (SR) 60 has been hampered by stability issues and low yields. Here we report a procedure to prepare Au 67 (SR) 35 , for either R = −SCH 2 CH 2 Ph or-SC 6 H 13 , allowing high-yield isolation (34%, ∼10-mg quantities) of the title compound. Product high purity is assessed at each synthesis stage by rapid MALDI−TOF mass-spectrometry (MS), and high-resolution electrospray-ionization MS confirms the Au 67 (SR) 35 composition. Electronic properties were explored using optical absorption spectroscopy (UV− visible−NIR regions) and electrochemistry (0.74 V spacing in differential-pulsed-voltammetry), modes of ligand binding were studied by NMR spectroscopy (13 C and 1 H), and structural characteristics of the metal atom core were determined by powder Xray measurements. Models featuring a Au 17 truncated-decahedral inner core encapsulated by the 30 anchoring atoms of 15 staplemotif units have been investigated with first-principles electronic structure calculations. This resulted in identification of a structure consistent with the experiments, particularly, the opening of a large gap (∼0.75 eV) in the (2−) charge-state of the nanomolecule. The electronic structure is analyzed within the framework of a superatom shell model. Structurally, the Au 67 (SR) 35 nanomolecule is the smallest to adopt the complete truncated-decahedral motif for its core with a surface structure bearing greater similarity to the larger nanoparticles. Its electronic HOMO−LUMO gap (∼0.75 eV) is nearly double that of the larger Au 102 compound and it is much smaller than that of the Au 38 one. The intermediary status of the Au 67 (SR) 35 nanomolecule is also reflected in both its optical and electrochemical characteristics.
Physical Review Letters, 1997
Atomistic modeling of energetics and structures, coupled with x-ray powder diffraction analyses o... more Atomistic modeling of energetics and structures, coupled with x-ray powder diffraction analyses of size-separated passivated gold nanocrystals in the 1-2 nm size range, shows preferential formation of a stable sequence of three cluster sizes, all with a truncated-decahedral motif. [S0031-9007(97)03951-3]
Physical Chemistry Chemical Physics, 2014
The structural order in ultrathin films of monolayer protected clusters (MPCs) is important in a ... more The structural order in ultrathin films of monolayer protected clusters (MPCs) is important in a number of application areas but can be difficult to demonstrate by conventional methods, particularly when the metallic core dimension, d, is in the intermediate size-range, 1.5 < d < 5.0 nm.
Nano Letters, 2012
A cluster obtained in high yield from the reduction of a silver-thiolate precursor, Ag-SCH 2 CH 2... more A cluster obtained in high yield from the reduction of a silver-thiolate precursor, Ag-SCH 2 CH 2 Ph, exhibited a single sharp peak near 25 kDa in the matrix-assisted laser desorption mass spectrum (MALDI MS) and a welldefined metal core of ∼2 nm measured with transmission electron microscopy (TEM). The cluster yields a single fraction in high-performance liquid chromatography (HPLC). Increased laser fluence fragments the cluster until a new peak near 19 kDa predominates, suggesting that the parent cluster Ag 152 (SCH 2 CH 2 Ph) 60 evolves into a stable inorganic core Ag 152 S 60. Exploiting combined insights from investigations of clusters and surface science, a core−shell structure model was developed, with a 92-atom silver core having icosahedraldodecahedral symmetry and an encapsulating protective shell containing 60 Ag atoms and 60 thiolates arranged in a network of six-membered rings resembling the geometry found in self-assembled monolayers on Ag(111). The structure is in agreement with small-angle X-ray scattering (SAXS) data. The protective layer encapsulating this silver cluster may be the smallest known three-dimensional self-assembled monolayer. First-principles electronic structure calculations show, for the geometry-optimized structure, the development of a ∼0.4 eV energy gap between the highest-occupied and lowest-unoccupied states, originating from a superatom 90-electron shell-closure and conferring stability to the cluster. The optical absorption spectrum of the cluster resembles that of plasmonic silver nanoparticles with a broad single feature peaking at 460 nm, but the luminescence spectrum shows two maxima with one attributed to the ligated shell and the other to the core.
Materials Letters, 1997
A liquid-phase method is reported for the preparation of dodecanethiol-derivatized silver nanocry... more A liquid-phase method is reported for the preparation of dodecanethiol-derivatized silver nanocrystals. Toluene sols of this material maintain as sols in air at room temperature at least on the timescale of months. X-ray diffraction (XRD) and transmission electron microscopy (TEM) are used to obtain size and structural information about the nanocrystal core while solubility properties and energy dispersive X-ray spectroscopy (EDS) are used to identify the passivating layer. Laser desorption time-of-flight mass spectra (LDTOFMS) of this material show broad peaks in the mass range from 40 to 300 kamu. These results are consistent with the synthesized material being a dodecanethiol-derivatized silver nanocrystal material with a mean crystalline silver core of 3.1 + 0.6 nm.
The Journal of Physical Chemistry Letters, 2013
Determination of the total structure of molecular nanocrystals is an outstanding experimental cha... more Determination of the total structure of molecular nanocrystals is an outstanding experimental challenge that has been met, in only a few cases, by single-crystal X-ray diffraction. Described here is an alternative approach that is of most general applicability and does not require the fabrication of a single crystal. The method is based on rapid, time-resolved nanobeam electron diffraction (NBD) combined with high-angle annular dark field scanning/transmission electron microscopy (HAADF-STEM) images in a probe corrected STEM microscope, operated at reduced voltages. The results are compared with theoretical simulations of images and diffraction patterns obtained from atomistic structural models derived through first-principles density functional theory (DFT) calculations. The method is demonstrated by application to determination of the structure of the Au 144 (SCH 2 CH 2 Ph) 60 cluster.
Angewandte Chemie International Edition, 2012
Gold nanoparticles protected by thiolates, including thiolterminated DNA and simple thiols, [1-5]... more Gold nanoparticles protected by thiolates, including thiolterminated DNA and simple thiols, [1-5] possess extraordinary stability and constitute perhaps the most widely studied system in nanotechnology. Understanding and control of how the thiolate ligands protect the underlying gold core and of the atomic structures that the gold cores adopt with decreasing size, are issues of fundamental, and potentially practical, importance. [6] For relatively large gold nanoparticles (i.e., greater than two nanometers), electron microscopy can map out the core structure, but the surface structure (e.g. the ways that thiolates bind to gold) cannot be determined. Obtaining the total structure (i.e. the core and surface atoms) necessitates single-crystal growth of atomically precise gold nanoparticles. Recently, significant progress has been made in the chemical synthesis of ultra-small gold nanoparticles (often called nanoclusters, typically less than two nanometers) protected by thiolates, but the total structure determination of such Au n (SR) m nanoparticles by X-ray crystallography remains a major challenge. [6, 7] The reported crystal structures thus far include Au 102 (p-MBA) 44 , Au 25 (SCH 2 CH 2 Ph) 18 , and Au 38 (SCH 2 CH 2 Ph) 24 , which are all composed of non-FCC (face-centered cubic) kernels, [8-11] such as the Au 79 decahedron in Au 102 (p-MBA) 44 , [8] the Au 13 icosahedron in Au 25 (SCH 2 CH 2 Ph) 18 , [9, 10] and the Au 23 face-sharing bi-icosa-hedron in Au 38 (SCH 2 CH 2 Ph) 24. [11] These experimental examples, as well as theoretical work, [12-14] indicate a size-dependent general trend that starts from icosahedral atomic arrangements at smaller sizes to decahedral structures at larger ones, and culminate with large clusters of FCC structure (for FCC metals). [8-19] Pertaining to the surface structures of Au n (SR) m nanoclusters, unique "staple"-like motifs have been found, including the dimeric staple (i.e. ÀSRÀAuÀSRÀAuÀSRÀ) and the monomeric staple (i.e. À SRÀAuÀSRÀ). [8-11] Herein, we report the discovery of an FCC-type core structure in Au 36 (SR) 24 , where SR refers to 4-tert-butylbenzenethiolate (denoted as SPh-tBu). The emergence of FCC structure in Au 36 (SR) 24 is surprising, given the small size of the cluster. The Au 36 (SR) 24 particle consists of a Au 28 kernel with a truncated FCC tetrahedron exposing (111) and (100) facets. Unlike the previously reported Au n (SR) m nanocluster structures (n = 25, 38, 102), [8-11] a new type of thiolate binding mode for all-thiolate-capped nanoclusters has been discovered, that is, 12 of the 24 ligands bind to the underlying Au atoms in a simple bridging mode, with the remaining 12 thiolates forming the known dimeric staple motifs. Firstprinciple calculations reveal a large energy gap between the highest-occupied and the lowest-unoccupied energy levels (approximately 1.7 eV), which is in agreement with the value measured by optical absorption spectroscopy. The high stability of the nanocluster originates from the geometric structure and organization of the electronic states into superatom shells. The new FCC tetrahedral Au 28 kernel and the thiolate-bridging mode (as opposed to staple motifs) offer important implications for other possible FCC-structured gold nanocrystals, as well as for thiol binding on extended gold surfaces in self-assembled-monolayer (SAM) systems. The synthesis of Au 36 nanoclusters protected by SPh-tBu thiolate starts with pure Au 38 (SCH 2 CH 2 Ph) 24 nanoclusters. We previously reported a size-focusing method for the synthesis of Au 38 (SCH 2 CH 2 Ph) 24 nanoclusters. [20] The Au 38 (SCH 2 CH 2 Ph) 24 nanocluster exhibits high thermal as well as chemical stability (i.e., resistant to reduction and oxidation by common reagents). [20] Interestingly, we found that this highly stable Au 38 (SCH 2 CH 2 Ph) 24 nanocluster, when reacting with HSPh-tBu at 80 8C for more than 12 hours, can be transformed to a new cluster (see Supporting Information for experimental details). To determine the mass of the assynthesized cluster, we used electrospray ionization mass spectrometry (ESI-MS). The nanocluster was found to be charge neutral; hence, CsOAc was added to the cluster solution to form a mono-Cs + adduct of the cluster. The positively charged adducts were then examined by ESI-MS
Faradays legendary Molecules of Gold have stimulated intense interest (over 165 years) but have o... more Faradays legendary Molecules of Gold have stimulated intense interest (over 165 years) but have only recently begun to yield their secrets to modern methods of chemical analysis. Here(in), we demonstrate how striking charging patterns emerge directly from native electrospray of large, gold-rich molecules that were generated by reduction of various (8) small gold(I)thiolate complexes [-RS-Au(I)-SR-], followed by extensive thermochemical processing to enrich the most robust forms. In each case (R), electrospray ionization of a picomolar solution yields a characteristic series of abundant, highly resolved peaks at related (m/z)-ratios, that can be used to deduce charges {z e+} and hence a distinct molecular mass, {MR}. A plot of {MR} versus thiolate-mass {mL} yields a straight line with slope 60.0 (the ligand count) and an intercept of 28,364-Da, the mass of 144 Au-atoms. i.e., a unique molecular composition {197Au144(SR)60}. This formula agrees with the unique chiral-icosahedral struc...
ACS Omega, 2019
Many antibiotic resistances to penicillin have been reported, making them obsolete against multir... more Many antibiotic resistances to penicillin have been reported, making them obsolete against multiresistant bacteria. Because penicillins act by inhibiting cell wall production while silver particles disrupt the cell wall directly, a synergetic effect is anticipated when both modes of action are incorporated into a chimera cluster. To test this hypothesis, the lipoate ligands (LA) of a silver cluster (Ag 29) of known composition (Ag 29 LA 12) [3−] were covalently conjugated to 6-aminopenicillanic acid, a molecule with a β-lactam backbone. Indeed, the partially conjugated cluster inhibited an Staphylococcus aureus biofilm, in a dose−response manner, with a half-maximal inhibitory concentration IC 50 of 2.3 μM, an improvement over 60 times relative to the unconjugated cluster (IC 50 = 140 μM). An enhancement of several orders of magnitude over 6-APA alone (unconjugated) was calculated (IC 50 = 10 000 μM). Cell wall damage is documented via scanning electron microscopy. A synergistic effect of the conjugate was calculated by the combination index method described by Chou− Talalay. This hybrid nanoantibiotic opens a new front against multidrug-resistant pathogens.
The journal of physical chemistry letters, Jan 16, 2017
We report the first results of ultraviolet photodissociation (UVPD) mass spectrometry of trapped ... more We report the first results of ultraviolet photodissociation (UVPD) mass spectrometry of trapped monolayer-protected cluster (MPC) ions generated by electrospray ionization. Gold clusters Au25(pMBA)18 and Au36(pMBA)24 (pMBA = para-mercaptobenzoic acid) were analyzed in both the positive and negative modes. Whereas activation methods including collisional- and electron-based methods produced relatively few fragment ions, even a single ultraviolet pulse (at λ = 193 nm) caused extensive fragmentation of the positively charged clusters. Upon photoactivation using a low number of laser pulses, the staple motifs of both clusters were cleaved and stripped of the protecting ligand portions without removal of any contained gold atoms. This striking process involved Au-S and C-S bond cleavages via a pathway made possible by 6.4 eV photon absorption. Monomer evaporation (neutral gold atom loss) occurred upon exposure to multiple pulses, resulting in a size series of bare gold-cluster ions. All...
The Journal of Physical Chemistry B, 2016
Experimental and theoretical evidence reveals the resilience and stability of the larger aqueous ... more Experimental and theoretical evidence reveals the resilience and stability of the larger aqueous gold clusters protected with p-mercaptobenzoic acid ligands (pMBA) of composition Au n (pMBA) p or (n,p). The Au 144 (pMBA) 60 , (144,60), or Gold-144 aqueous gold cluster is considered special because of its high symmetry, abundance, and icosahedral structure as well as its many potential uses in material and biological sciences. Yet to this date, direct confirmation of its precise composition and total structure remains elusive. Results described here from characterization via high-resolution electrospray ionization mass spectrometry on an Orbitrap instrument show Au 102 (pMBA) 44 at isotopic resolution. Further, what usually appears as a single band for (144,60) in electrophoresis (PAGE) is shown to also contain (130,50), recently determined to have a truncated decahedral structure, and (137,56) in addition to the dominant (144,60) compound of chiral icosahedral structure. This finding is significant in that it reveals the existence of structures never before observed in all-aromatic water soluble species while pointing out the path toward elucidation of the thermodynamic control of protected gold nanocrystal formation.
ACS Nano, 2016
The alloy Au−Ag system is an important noble bimetallic phase, both historically (as "Electrum") ... more The alloy Au−Ag system is an important noble bimetallic phase, both historically (as "Electrum") and now especially in nanotechnology, as it is applied in catalysis and nanomedicine. To comprehend the structural characteristics and the thermodynamic stability of this alloy, a knowledge of its phase diagram is required that considers explicitly its size and shape (morphology) dependence. However, as the experimental determination remains quite challenging at the nanoscale, theoretical guidance can provide significant advantages. Using a regular solution model within a nanothermodynamic approach to evaluate the size effect on all the parameters (melting temperature, melting enthalpy, and interaction parameters in both phases), the nanophase diagram is predicted. Besides an overall shift downward, there is a "tilting" effect on the solidus−liquidus curves for some particular shapes exposing the (100) and (110) facets (cube, rhombic dodecahedron, and cuboctahedron). The segregation calculation reveals the preferential presence of silver at the surface for all the polyhedral shapes considered, in excellent agreement with the latest transmission electron microscopy observations and energy dispersive spectroscopy analysis. By reviewing the nature of the surface segregated element of different bimetallic nanoalloys, two surface segregation rules, based on the melting temperatures and surface energies, are deduced. Finally, the optical properties of Au−Ag nanoparticles, calculated within the discrete dipole approximation, show the control that can be achieved in the tuning of the local surface plasmon resonance, depending of the alloy content, the chemical ordering, the morphology, the size of the nanoparticle, and the nature of the surrounding environment.
The journal of physical chemistry. B, Jan 19, 2015
Nanometer-sized gold particles (AuNPs) are of peculiar interest because their behaviors in an aqu... more Nanometer-sized gold particles (AuNPs) are of peculiar interest because their behaviors in an aqueous solution are sensitive to changes in environmental factors including the size and shape of the solute ions. In order to determine these important characteristics, we performed all-atom molecular dynamics simulations on the icosahedral Au144 nanoparticles each coated with a homogeneous set of 60 thiolates (4-mercapto-benzoate, pMBA) in eight aqueous solutions having ions of varying sizes and shapes (Na+, K+, tetramethylamonium cation TMA+, trisamonium cation TRS+, Cl-, and OH-). For each solution, we computed the reversible work (potential of mean of force) to bring two nanoparticles together as a function of their separation distance. We found that the behavior of pMBA protected Au144 nanoparticles can be readily modulated by tuning their aqueous environmental factors (pH and solute ion combinations). We examined the atomistic details on how the sizes and shapes of solute ions quant...
Small Particles and Inorganic Clusters, 1997
The Journal of Physical Chemistry, 1996
Orientational ordering of faceted nanocrystals in nanocrystal arrays has been directly observed f... more Orientational ordering of faceted nanocrystals in nanocrystal arrays has been directly observed for the first time, by use of transmission electron microscopy imaging and diffraction to resolve the structure of thin molecular-crystalline films of silver nanocrystals passivated by alkylthiolate self-assembled monolayers. The type of ordering found is determined by the nanocrystal's faceted morphology, as mediated by the interactions of surfactant groups tethered to the facets on neighboring nanocrystals. Orientational ordering is crucial for the understanding of the fundamental properties of quantum-dot arrays, as well as for their optimal utilization in optical and electronic applications.
Nano Letters, 2014
Gold−copper (Au−Cu) phases were employed already by pre-Columbian civilizations, essentially in d... more Gold−copper (Au−Cu) phases were employed already by pre-Columbian civilizations, essentially in decorative arts, whereas nowadays, they emerge in nanotechnology as an important catalyst. The knowledge of the phase diagram is critical to understanding the performance of a material. However, experimental determination of nanophase diagrams is rare because calorimetry remains quite challenging at the nanoscale; theoretical investigations, therefore, are welcomed. Using nanothermodynamics, this paper presents the phase diagrams of various polyhedral nanoparticles (tetrahedron, cube, octahedron, decahedron, dodecahedron, rhombic dodecahedron, truncated octahedron, cuboctahedron, and icosahedron) at sizes 4 and 10 nm. One finds, for all the shapes investigated, that the congruent melting point of these nanoparticles is shifted with respect to both size and composition (copper enrichment). Segregation reveals a gold enrichment at the surface, leading to a kind of core−shell structure, reminiscent of the historical artifacts. Finally, the most stable structures were determined to be the dodecahedron, truncated octahedron, and icosahedron with a Cu-rich core/Au-rich surface. The results of the thermodynamic approach are compared and supported by molecular-dynamics simulations and by electronmicroscopy (EDX) observations.
MRS Communications, 2013
The fabrication of bimetallic magnetic nanoparticles (NPs) smaller than the size of single magnet... more The fabrication of bimetallic magnetic nanoparticles (NPs) smaller than the size of single magnetic domain is very challenging because of the agglomeration, non-uniform size, and possible complex chemistry at nanoscale. In this paper, we present an alloyed ferromagnetic 4 ± 1 nm thiolated Au/Co magnetic NPs with decahedral and icosahedral shape. The NPs were characterized by Cs-corrected scanning transmission electron microscopy (STEM) and weretheoretically studied by Grand Canonical Monte Carlo simulations. Comparison of Z-contrast imaging and energy dispersive x-ray spectroscopy used jointly with STEM simulated images from theoretical models uniquely showed an inhomogeneous alloying with minor segregation. The magnetic measurements obtained from superconducting quantum interference device magnetometer exhibited ferromagnetic behavior. This magnetic nanoalloy in the range of single domain is fully magnetized and carries significance as a promising candidate for magnetic data recording, permanent magnetization, and biomedical applications.
The Journal of Physical Chemistry B, 1997
The optical absorption spectra of a series of nanocrystal gold moleculesslarger, crystalline Au c... more The optical absorption spectra of a series of nanocrystal gold moleculesslarger, crystalline Au clusters that are passivated by a compact monolayer of n-alkylthiol(ate)sshave been measured across the electronic range (1.1-4.0 eV) in dilute solution at ordinary temperature. Each of the ∼20 samples, ranging in effective core diameter from 1.4 to 3.2 nm (∼70 to ∼800 Au atoms), has been purified by fractional crystallization and has undergone a separate compositional and structural characterization by mass spectrometry and X-ray diffraction. With decreasing core mass (crystallite size) the spectra uniformly show a systematic evolution, specifically (i) a broadening of the so-called surface-plasmon band until it is essentially unidentifiable for crystallites of less than 2.0 nm effective diameter, (ii) the emergence of a distinct onset for strong absorption near the energy (∼1.7 eV) of the interbandgap (5d f 6sp), and (iii) the appearance in the smallest crystallites of a weak steplike structure above this onset, which is interpreted as arising from a series of transitions from the continuum d-band to the discrete level structure of the conduction band just above the Fermi level. The classical electrodynamic (Mie) theory, based on bulk optical properties, can reproduce this spectral evolutionsand thereby yield a consistent core-sizingsonly by making a strong assumption about the surface chemical interaction. Quantitative agreement with the spectral line shape requires a size-dependent offset of the frequency-dependent dielectric function, which may be explained by a transition in electronic structure just below 2.0 nm (∼200 atoms), as proposed earlier.
The Journal of Physical Chemistry B, 1998
Self-assembling of nanocrystals involves organization of nanocrystals encapsulated by protective ... more Self-assembling of nanocrystals involves organization of nanocrystals encapsulated by protective compact organic molecules into a crystalline material. The adsorbed molecules not only serve as the protection layer for the nanocrystals but also provide the dominant cohesive interactions (or "bonding") sustaining the nanocrystal superlattices. The length of the adsorbed molecules is a controllable parameter, making the ratio of particle size to interparticle distance an adjustable parameter that sensitively tunes the interparticle interaction/ coupling and resulting collective properties. In this paper, bundling and interdigitation of thiolate molecules adsorbed on Ag nanocrystals are observed using the chemical imaging technique in energy-filtered transmission electron microscopy (EF-TEM) at a resolution of ∼2 nm. In these orientationally ordered, self-assembled Ag-nanocrystal superlattices, the bundling of the adsorbed molecules on the nanocrystal surfaces is the fundamental structural principle. A model consistent with the nanocrystal's morphology and the interdigitation of the adsorbed thiolates is proposed.
The Journal of Physical Chemistry A, 2013
The preparation of gold nanomolecules with sizes other than Au 25 (SR) 18 , Au 38 (SR) 24 , Au 10... more The preparation of gold nanomolecules with sizes other than Au 25 (SR) 18 , Au 38 (SR) 24 , Au 102 (SR) 44 , and Au 144 (SR) 60 has been hampered by stability issues and low yields. Here we report a procedure to prepare Au 67 (SR) 35 , for either R = −SCH 2 CH 2 Ph or-SC 6 H 13 , allowing high-yield isolation (34%, ∼10-mg quantities) of the title compound. Product high purity is assessed at each synthesis stage by rapid MALDI−TOF mass-spectrometry (MS), and high-resolution electrospray-ionization MS confirms the Au 67 (SR) 35 composition. Electronic properties were explored using optical absorption spectroscopy (UV− visible−NIR regions) and electrochemistry (0.74 V spacing in differential-pulsed-voltammetry), modes of ligand binding were studied by NMR spectroscopy (13 C and 1 H), and structural characteristics of the metal atom core were determined by powder Xray measurements. Models featuring a Au 17 truncated-decahedral inner core encapsulated by the 30 anchoring atoms of 15 staplemotif units have been investigated with first-principles electronic structure calculations. This resulted in identification of a structure consistent with the experiments, particularly, the opening of a large gap (∼0.75 eV) in the (2−) charge-state of the nanomolecule. The electronic structure is analyzed within the framework of a superatom shell model. Structurally, the Au 67 (SR) 35 nanomolecule is the smallest to adopt the complete truncated-decahedral motif for its core with a surface structure bearing greater similarity to the larger nanoparticles. Its electronic HOMO−LUMO gap (∼0.75 eV) is nearly double that of the larger Au 102 compound and it is much smaller than that of the Au 38 one. The intermediary status of the Au 67 (SR) 35 nanomolecule is also reflected in both its optical and electrochemical characteristics.
Physical Review Letters, 1997
Atomistic modeling of energetics and structures, coupled with x-ray powder diffraction analyses o... more Atomistic modeling of energetics and structures, coupled with x-ray powder diffraction analyses of size-separated passivated gold nanocrystals in the 1-2 nm size range, shows preferential formation of a stable sequence of three cluster sizes, all with a truncated-decahedral motif. [S0031-9007(97)03951-3]
Physical Chemistry Chemical Physics, 2014
The structural order in ultrathin films of monolayer protected clusters (MPCs) is important in a ... more The structural order in ultrathin films of monolayer protected clusters (MPCs) is important in a number of application areas but can be difficult to demonstrate by conventional methods, particularly when the metallic core dimension, d, is in the intermediate size-range, 1.5 < d < 5.0 nm.
Nano Letters, 2012
A cluster obtained in high yield from the reduction of a silver-thiolate precursor, Ag-SCH 2 CH 2... more A cluster obtained in high yield from the reduction of a silver-thiolate precursor, Ag-SCH 2 CH 2 Ph, exhibited a single sharp peak near 25 kDa in the matrix-assisted laser desorption mass spectrum (MALDI MS) and a welldefined metal core of ∼2 nm measured with transmission electron microscopy (TEM). The cluster yields a single fraction in high-performance liquid chromatography (HPLC). Increased laser fluence fragments the cluster until a new peak near 19 kDa predominates, suggesting that the parent cluster Ag 152 (SCH 2 CH 2 Ph) 60 evolves into a stable inorganic core Ag 152 S 60. Exploiting combined insights from investigations of clusters and surface science, a core−shell structure model was developed, with a 92-atom silver core having icosahedraldodecahedral symmetry and an encapsulating protective shell containing 60 Ag atoms and 60 thiolates arranged in a network of six-membered rings resembling the geometry found in self-assembled monolayers on Ag(111). The structure is in agreement with small-angle X-ray scattering (SAXS) data. The protective layer encapsulating this silver cluster may be the smallest known three-dimensional self-assembled monolayer. First-principles electronic structure calculations show, for the geometry-optimized structure, the development of a ∼0.4 eV energy gap between the highest-occupied and lowest-unoccupied states, originating from a superatom 90-electron shell-closure and conferring stability to the cluster. The optical absorption spectrum of the cluster resembles that of plasmonic silver nanoparticles with a broad single feature peaking at 460 nm, but the luminescence spectrum shows two maxima with one attributed to the ligated shell and the other to the core.
Materials Letters, 1997
A liquid-phase method is reported for the preparation of dodecanethiol-derivatized silver nanocry... more A liquid-phase method is reported for the preparation of dodecanethiol-derivatized silver nanocrystals. Toluene sols of this material maintain as sols in air at room temperature at least on the timescale of months. X-ray diffraction (XRD) and transmission electron microscopy (TEM) are used to obtain size and structural information about the nanocrystal core while solubility properties and energy dispersive X-ray spectroscopy (EDS) are used to identify the passivating layer. Laser desorption time-of-flight mass spectra (LDTOFMS) of this material show broad peaks in the mass range from 40 to 300 kamu. These results are consistent with the synthesized material being a dodecanethiol-derivatized silver nanocrystal material with a mean crystalline silver core of 3.1 + 0.6 nm.
The Journal of Physical Chemistry Letters, 2013
Determination of the total structure of molecular nanocrystals is an outstanding experimental cha... more Determination of the total structure of molecular nanocrystals is an outstanding experimental challenge that has been met, in only a few cases, by single-crystal X-ray diffraction. Described here is an alternative approach that is of most general applicability and does not require the fabrication of a single crystal. The method is based on rapid, time-resolved nanobeam electron diffraction (NBD) combined with high-angle annular dark field scanning/transmission electron microscopy (HAADF-STEM) images in a probe corrected STEM microscope, operated at reduced voltages. The results are compared with theoretical simulations of images and diffraction patterns obtained from atomistic structural models derived through first-principles density functional theory (DFT) calculations. The method is demonstrated by application to determination of the structure of the Au 144 (SCH 2 CH 2 Ph) 60 cluster.
Angewandte Chemie International Edition, 2012
Gold nanoparticles protected by thiolates, including thiolterminated DNA and simple thiols, [1-5]... more Gold nanoparticles protected by thiolates, including thiolterminated DNA and simple thiols, [1-5] possess extraordinary stability and constitute perhaps the most widely studied system in nanotechnology. Understanding and control of how the thiolate ligands protect the underlying gold core and of the atomic structures that the gold cores adopt with decreasing size, are issues of fundamental, and potentially practical, importance. [6] For relatively large gold nanoparticles (i.e., greater than two nanometers), electron microscopy can map out the core structure, but the surface structure (e.g. the ways that thiolates bind to gold) cannot be determined. Obtaining the total structure (i.e. the core and surface atoms) necessitates single-crystal growth of atomically precise gold nanoparticles. Recently, significant progress has been made in the chemical synthesis of ultra-small gold nanoparticles (often called nanoclusters, typically less than two nanometers) protected by thiolates, but the total structure determination of such Au n (SR) m nanoparticles by X-ray crystallography remains a major challenge. [6, 7] The reported crystal structures thus far include Au 102 (p-MBA) 44 , Au 25 (SCH 2 CH 2 Ph) 18 , and Au 38 (SCH 2 CH 2 Ph) 24 , which are all composed of non-FCC (face-centered cubic) kernels, [8-11] such as the Au 79 decahedron in Au 102 (p-MBA) 44 , [8] the Au 13 icosahedron in Au 25 (SCH 2 CH 2 Ph) 18 , [9, 10] and the Au 23 face-sharing bi-icosa-hedron in Au 38 (SCH 2 CH 2 Ph) 24. [11] These experimental examples, as well as theoretical work, [12-14] indicate a size-dependent general trend that starts from icosahedral atomic arrangements at smaller sizes to decahedral structures at larger ones, and culminate with large clusters of FCC structure (for FCC metals). [8-19] Pertaining to the surface structures of Au n (SR) m nanoclusters, unique "staple"-like motifs have been found, including the dimeric staple (i.e. ÀSRÀAuÀSRÀAuÀSRÀ) and the monomeric staple (i.e. À SRÀAuÀSRÀ). [8-11] Herein, we report the discovery of an FCC-type core structure in Au 36 (SR) 24 , where SR refers to 4-tert-butylbenzenethiolate (denoted as SPh-tBu). The emergence of FCC structure in Au 36 (SR) 24 is surprising, given the small size of the cluster. The Au 36 (SR) 24 particle consists of a Au 28 kernel with a truncated FCC tetrahedron exposing (111) and (100) facets. Unlike the previously reported Au n (SR) m nanocluster structures (n = 25, 38, 102), [8-11] a new type of thiolate binding mode for all-thiolate-capped nanoclusters has been discovered, that is, 12 of the 24 ligands bind to the underlying Au atoms in a simple bridging mode, with the remaining 12 thiolates forming the known dimeric staple motifs. Firstprinciple calculations reveal a large energy gap between the highest-occupied and the lowest-unoccupied energy levels (approximately 1.7 eV), which is in agreement with the value measured by optical absorption spectroscopy. The high stability of the nanocluster originates from the geometric structure and organization of the electronic states into superatom shells. The new FCC tetrahedral Au 28 kernel and the thiolate-bridging mode (as opposed to staple motifs) offer important implications for other possible FCC-structured gold nanocrystals, as well as for thiol binding on extended gold surfaces in self-assembled-monolayer (SAM) systems. The synthesis of Au 36 nanoclusters protected by SPh-tBu thiolate starts with pure Au 38 (SCH 2 CH 2 Ph) 24 nanoclusters. We previously reported a size-focusing method for the synthesis of Au 38 (SCH 2 CH 2 Ph) 24 nanoclusters. [20] The Au 38 (SCH 2 CH 2 Ph) 24 nanocluster exhibits high thermal as well as chemical stability (i.e., resistant to reduction and oxidation by common reagents). [20] Interestingly, we found that this highly stable Au 38 (SCH 2 CH 2 Ph) 24 nanocluster, when reacting with HSPh-tBu at 80 8C for more than 12 hours, can be transformed to a new cluster (see Supporting Information for experimental details). To determine the mass of the assynthesized cluster, we used electrospray ionization mass spectrometry (ESI-MS). The nanocluster was found to be charge neutral; hence, CsOAc was added to the cluster solution to form a mono-Cs + adduct of the cluster. The positively charged adducts were then examined by ESI-MS