A rising cost of an education.
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Editorial↗
Biomedical subjects
Royce W Murray
Publications and source records attributed to Royce W Murray.
The responsibilities of tenure evaluators.
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Editorial↗
Substituent effects on redox potentials and optical gap energies of molecule-like Au38(SPhX)24 nanoparticles.
A molecule-like substituent effect on redox formal potentials in the nanoparticle series Au(38)(SPhX)(24) has been discovered. Electron-withdrawing "X" substituents energetically favor reduction and disfavor oxidation, and give formal potentials that correlate with Hammett substituent constants. The ligand monolayer of the nanoparticles is shown, thereby, to play a strong role in determining electronic energies of the nanoparticle core and is more than simply a protecting or capping layer. The substituent effect does not, however, detectably change the HOMO-LUMO gap energy, being identical for the HOMO and LUMO levels and presumably inductive in nature.
Journal Article↗
Reaction of triphenylphosphine with phenylethanethiolate-protected Au38 nanoparticles.
The replacement of phenylethanethiolate (SC2Ph) ligands on 1.1 nm (core diameter) Au38(SC2Ph)24 monolayer-protected clusters (MPCs) with varied amounts of triphenylphosphine (PPh3) is investigated in methylene chloride. UV-vis spectra suggest that changes in the MPC Au core size occur when large amounts (> 10 equiv moles per cluster) of PPh3 are reacted with Au38(SC2Ph)24. 1H and 31P NMR spectra following the addition of smaller amounts (< 5 equiv moles) of PPh3 indicate that the reaction liberates a AuISC2Ph complex, as opposed to a SC2Ph thiol, disulfide, or anion. A 1H NMR kinetic study shows that the exchange is surprisingly rapid, even faster than exchanges of thiolates with other thiolates, at room temperature and in air. The reaction is slowed when cooled or conducted under Ar. The difference in potentials of the initial one-electron voltammetric reduction and oxidation of Au38(SC2Ph)24 decreases slightly upon reaction with small amounts of PPh3.
Journal Article↗
Reaction of Au(55)(PPh(3))(12)Cl(6) with thiols yields thiolate monolayer protected Au(75) clusters.
This paper describes the reaction of the phosphine-protected Au nanoparticle Au(55)(PPh(3))(12)Cl(6) (1, "Au55") with hexanethiol (2) and other thiols. The voltammetry of the reaction product 2 displays a well-defined pattern of peaks qualitatively reminiscent of Au(38) nanoparticles, but with quite different spacing (0.74 +/- 0.01 V) between the potentials of initial oxidation and reduction steps (electrochemical gap). Correction of this "molecule-like" gap for charging energy indicates a HOMO-LUMO gap energy of about 0.47 V. Voltammetry of the products (3 and 4) of reaction of 1 with C(3)H(7)SH and PhC(2)H(4)SH, respectively, is similar. Laser desorption/ionization mass spectrometry (LDI-MS) shows that 2 contains a high proportion of a core mass in the 14-15 kDa range, which is proposed to be Au(75). UV-vis spectra of 2-4 are relatively featureless, similar to previous reports of thiolate-protected Au(75) nanoparticles. HPLC analysis of 2 shows a Au(75) content of ca. 73%; the electrochemical purity estimate is also high, about 55%. Combining the mass spectrometric result with thermogravimetric analysis of 2 leads to a preliminary formulation Au(75)(SC(6)H(13))(40). This Au(75) synthesis complements a previous Brust-type synthesis and is unusual in the apparent provocation in the reaction of an increase in core size.
Journal Article↗
Ligand heterogeneity on monolayer-protected gold clusters.
This paper describes the effects of oxidative electronic charging of the Au cores of the monolayer-protected clusters (MPCs), Au140(S(CH2)5CH3)53 and Au38(SCH2CH2Ph)24, on nuclear magnetic resonance (NMR) spectra of their monolayer ligand shells. Previously unresolved fine structure in the 13C NMR hexanethiolate methyl and C5 methylene resonances is seen in spectra of solutions of monodisperse Au140(S(CH2)5CH3)53 MPCs, reflecting magnetically inequivalent ligand sites. Incremented increases in positive cluster core charge, effected by electrochemical charging, cause the spectral fine structure of the methyl resonance to coalesce, becoming a single peak at the Au140(3+) charge state. The spectral changes are reversible; charging back to the original core charge state regenerates the methyl 13C resonance fine structure. Adding an equimolar quantity of a Au(I) thiolate complex, Au(I)[SCH2(C6H4)C(CH3)3], to an uncharged Au140(S(CH2)5CH3)53 MPC solution in d2-methylene chloride causes partial spectral coalescence. 13C NMR spectra of Au38(SCH2CH2Ph)24 MPCs exhibit roughly comparable spectral changes upon positive core charging to the '0', '+1', and '+2' states. The NMR results indicate that exchange between magnetically inequivalent sites occurs at rates of 100 to 400 s(-1), a rate believed to be too fast to be accountable by actual exchanges of ligands between different sites on the Au core. We also describe changes in core electronic spectra of Au140(S(CH2)5CH3)53 induced by positive charging, measured using spectroelectrochemistry.
Journal Article↗
Supporting electrolyte and solvent effects on single-electron double layer capacitance charging of hexanethiolate-coated Au140 nanoparticles.
Sequential injections of single electrons (or holes) into the cores of Au(140) hexanethiolate monolayer-protected clusters (MPCs) occur at measurably different electrochemical potentials owing to the extremely small (subattofarad) values of the single MPC capacitance (C(MPC)) of the nanoparticle. The potential increment for each sequential injection is DeltaV = e/C(MPC). The dependence of DeltaV on the concentration of supporting electrolyte (from 1 to 100 mM), measured using square wave voltammetry, is shown to be caused, primarily, by changes in the diffuse double layer component (C(DIFFUSE)) of C(MPC). The dependence of C(DIFFUSE) on r(core), the radius of the nanoparticle, is considered. Additionally, significant changes in the magnitude of the compact double layer component (C(COMPACT), equivalent to the Stern layer) of C(MPC) were induced by adding hydrophobic solvent components such as hexane or dodecane or by introducing hydrophobic electrolyte ions (tetrabutyl-, tetrahexyl-, and tetraoctylammonium, perchlorate, and tetraphenylborate). These changes are interpreted as specific solvation and ion penetration of the hexanethiolate monolayer. For brevity we will refer to these phenomena as solvation/penetration phenomena.
Journal Article↗
Does core size matter in the kinetics of ligand exchanges of monolayer-protected Au clusters?
This paper compares the kinetics of exchanges of phenylethanethiolate ligands (PhC2S-) of the monolayer-protected clusters (MPCs) Au(38)(SC2Ph)(24) and Au(140)(SC2Ph)(53) with p-substituted arylthiols (p-X-PhSH), where X = NO(2), Br, CH(3), OCH(3), and OH. First-order rate constants at 293 K for exchange of the first ca. 25% of the ligands on the molecule-like Au(38)(SC2Ph)(24) MPC, measured using (1)H NMR, vary linearly with the in-coming arythiol concentration; ligand exchange is an overall second-order reaction. Remarkably, the second-order rate constants for ligand exchange on Au(38)(SC2Ph)(24) are very close to those of corresponding exchange reactions on the larger nanoparticle Au(140)(SC2Ph)(53) MPCs. These are the first results that quantitatively show that the chemical reactivity of different sized nanocrystals is almost independent of size; presumably, this is because the locus of the initial ligand exchanges is a common kind of site, thought to be the nanocrystal vertexes. The rates of later stages of exchange (beyond ca. 25%) differ for Au(38) and Au(140) cores, the latter being much slower presumably due to its larger terrace-like surface atom content. The reverse exchange reaction was studied for Au(38)(p-X-arylthiolate)(24) MPCs (X = NO(2), Br, and CH(3)), where the in-coming ligand is now phenylethanethiol. Remarkably, the rate constants of both forward and reverse exchanges display identical substituent effects, which implies a concurrent bonding of both in-coming and leaving ligands to the Au core in the rate-determining step, as in an associative mechanism. X = NO(2) gives the fastest rates, and the ratio of forward and reverse rate constants gives an equilibrium constant of K(EQ,PE) = 4.0 that is independent of X.
Journal Article↗
Near-IR luminescence of monolayer-protected metal clusters.
Visible-near-IR luminescence spectra of gold MPCs that are similar, irrespective of the number of core atoms (all <2 nm diameter) and different monolayers, are reported. The luminescence can be quantitatively invoked by introducing polar ligands into nonpolar MPC monolayers and by galvanic exchange of metal atoms on the MPC core surface with different metals. The observed emissions are believed to result from surface-localized states that depend on both the core metal of the nanoparticle and the ligands attached to the metal surface.
Journal Article↗
Electron hopping dynamics in monolayer-protected au cluster network polymer films by rotated disk electrode voltammetry.
Electrons are transported within polymeric films of alkanethiolate monolayer-protected Au clusters (MPCs) by electron hopping (self-exchange) between the metal cores. The surrounding monolayers, the molecular linkers that generate the network polymer film, or both, presumably serve as tunneling bridges in the electron transfers. This paper introduces a steady-state electrochemical method for measuring electron hopping rates in solvent-wetted and swollen, ionically conductive MPC films. The films are network polymer films of nanoparticles, coated on a rotated disk electrode that is contacted by a solution of a redox species (decamethylferrocene, CpFe). Controlling the electrode potential such that the film mediates oxidation of the redox probe can force control of the overall current onto the rate of electron hopping within the film, which is characterized as the apparent electron diffusion coefficient D(E). D(E) is translated into an apparent electron hopping rate k(ET) by a cubic lattice model. The experiment is applied to MPC network polymer films linked by alpha,omega-alkanedithiolates and by metal ion-carboxylate connections. We evaluate the dependencies of apparent hopping rate on CpFe concentration, film thickness, electrode potential relative to the CpFe formal potential, film-swelling solvent, and temperature. The apparent hopping rates are in the 10(4)-10(5) s(-)(1) range, which is slower than those for the same kind of MPC films, but in a dry (nonswollen) state measured by electronic conductivities.
Journal Article↗
Voltammetric detection of metal nanoparticles separated by liquid chromatography.
Fast-scan cyclic voltammetric (FSCV) detection of the separation of small (< 2-nm core diameter) organothiolate monolayer-protected gold clusters (MPCs) by high-performance liquid chromatography (HPLC) is demonstrated in this paper. Cyclic voltammograms taken at 1-s intervals (scan rate 150 V/s) display distinctive current-potential patterns for different MPCs, aiding in their identification. Previous amperometric detection of MPC separations required multiple HPLC runs at different electrode potentials to construct hydrodynamic current-potential voltammograms. Theory for ratiometric response of photodiode array absorbance and constant potential amperometry detectors was further tested on the fast-scan voltammetric detector and gave reasonable relative core size results for a mixture of Au140 and Au38 MPCs. In the first analysis of mixed MPC monolayer composition that is not simply an overall average result, preliminary HPLC experiments on MPCs with mixed hexanethiolate and ferrocenyl-octanethiolate monolayers resolved peaks that are shown to correspond to different average numbers of ferrocene-labeled ligands per MPC.
Chromatography, High Pressure Liquid↗
Hexanethiolate monolayer protected 38 gold atom cluster.
The nucleation-growth-passivation Brust reaction has been modified so as to enrich the product in useful quantities of a 38-atom gold nanoparticle coated with a hexanethiolate monolayer. Two modifications are described, using -78 degrees C reduction temperature and a hyperexcess of thiol. Compositional evidence is presented that establishes the product as a Au38(C6)24 hexanethiolate monolayer protected cluster (MPC), based on transmission electron microscopy, laser ionization-desorption mass spectrometry, thermogravimetric analysis, and elemental analysis. Reverse phase HPLC confirms the relatively good monodispersity of the MPC products, but high-resolution double-column HPLC reveals that the MPCs are a mixture of closely related but chromatographically distinct products. Voltammetry, low energy spectrophotometry, and spectroelectrochemistry reveal, respectively, a 1.6 eV electrochemical energy gap between the first oxidation and the first reduction, an optical HOMO-LUMO energy absorbance edge at 1.3 eV, and a bleaching of optical absorbance near the 1.3 eV band edge that accompanies electrochemical oxidation of the nanoparticle.
Journal Article↗
Words with multiple meanings.
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Editorial↗
Substituent effects on the exchange dynamics of ligands on 1.6 nm diameter gold nanoparticles.
The kinetics of exchange ofphenylethanethiolate ligands (PhC2S) of monolayer-protected clusters (MPCs, average formula Au140(PhC2S)53) by para-substituted arylthiols (p-X-ArSH) are described. 1H NMR measurements of thiol concentrations show that the exchange reaction is initially rapid and gradually slows almost to a standstill. The most labile ligands, exchanging at the shortest reaction times, are thought to be those at defect sites (edges, vertexes) on the nanoparticle core surface. The pseudo-first-order rate constants derived from the first 10% of the exchange reaction profile vary linearly with in-coming arylthiol concentration, meaning that the labile ligands exchange in a second-order process, which is consistent with ligand exchange being an associative process. A linear Hammett relationship with slope p = 0.44 demonstrates a substituent effect in the ligand place exchange reaction, in which the bimolecular rate constants increase for ligands with electron-withdrawing substituents (1.4 x 10-2 and 3.8 x 10(-3) M(-1) s(-1) for X = NO2 and 4-OH, respectively). This is interpreted as the more polar Au-S bonds at the defect sites favoring bonding with more electron deficient sulfur moieties. At longer reaction times, where ligands exchange on nondefect (terrace) as well as defect sites, the extent of ligand exchange is higher for thiols with more electron-donating substituents. The difference between short-time kinetics and longer-time pseudoequilibria is rationalized based on differences in Au-S bonding at defect vs nondefect MPC core sites. The study adds substance to the mechanisms of exchange of protecting ligands on nanoparticles. The scope and limitations of 1H NMR spectroscopy for determining rate data are also discussed.
Journal Article↗
Electrochemistry and optical absorbance and luminescence of molecule-like Au38 nanoparticles.
This paper describes electrochemical and spectroscopic properties of a well-characterized, synthetically accessible, 1.1 nm diam Au nanoparticle, Au(38)(PhC(2)S)(24), where PhC(2)S is phenylethylthiolate. Properties of other Au(38) nanoparticles made by exchanging the monolayer ligands with different thiolate ligands are also described. Voltammetry of the Au(38) nanoparticles in CH(2)Cl(2) reveals a 1.62 V energy gap between the first one-electron oxidation and the first reduction. Based on a charging energy correction of ca. 0.29 V, the indicated HOMO-LUMO gap energy is ca. 1.33 eV. At low energies, the optical absorbance spectrum includes peaks at 675 nm (1.84 eV) and 770 nm (1.61 eV) and an absorbance edge at ca. 1.33 eV that gives an optical HOMO-LUMO gap energy that is consistent with the electrochemical estimate. The absorbance at lowest energy is bleached upon electrochemical depletion of the HOMO level. The complete voltammetry contains two separated doublets of oxidation waves, indicating two distinct molecular orbitals, and two reduction steps. The ligand-exchanged nanoparticle Au(38)(PEG(135)S)(13)(PhC(2)S)(11), where PEG(135)S is -SCH(2)CH(2)OCH(2)CH(2)OCH(3), exhibits a broad (1.77-0.89 eV) near-IR photoluminescence band resolvable into maxima at 902 nm (1.38 eV) and 1025 nm (1.2 eV). Much of the photoluminescence occurs at energies less than the HOMO-LUMO gap energy. A working model of the energy level structure of the Au(38) nanoparticle is presented.
Journal Article↗
The cost of chemical analysis in space.
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Editorial↗
Growth, conductivity, and vapor response properties of metal ion-carboxylate linked nanoparticle films.
Nanoparticles of metals (Au, Ag, Pd, alloys) in the size range 1-3 nm diameter can be stabilized against aggregation of the metal particles by coating the metal surface with a dense monolayer of ligands (thiolates). The stabilization makes it possible to analytically define the nanoparticle composition (for example, Au140(hexanethiolate)53, I) and to elaborate the chemical functionality of the protecting monolayer (for example, Au140(C6)35(MUA)18, II, where C6 = hexanethiolate and MUA = mercaptoundecanoic acid). Network polymer films (IIfilm) on interdigitated array electrodes can be prepared from II, based on cation coordination (i.e., Cu2+, Zn2+, Ag+, methyl viologen) by the carboxylates of MUA. The electronic conductivity of the IIfilm network polymer films occurs by electron hopping between the Au140 nanoparticle cores, and offers an avenue for investigation of metal-to-metal nanoparticle electron transfer chemistry. The report begins with a brief summary of what is known about metal nanoparticle electron transfer chemistry. The investigation goes on to assess factors that influence the dynamics of film formation as well as film conductivity, in the interest of better understanding the parameters affecting electron hopping rates in IIfilm network polymer films. Finally, sorption of organic vapors into IIfilm causes a decreased electronic conductivity and increased mass that can be assessed using quartz crystal microbalance measurements. The change in electronic conductivity can be exploited for the sensing of organic vapors.
Journal Article↗