Joseph F. Parker

The Story of a Monodisperse Gold Nanoparticle: Au25L18

Au nanoparticles (NPs) with protecting organothiolate ligands and core diameters smaller than 2 nm are interesting materials because their size-dependent properties range from metal-like to molecule-like. This Account focuses on the most thoroughly investigated of these NPs, Au(25)L(18). Future advances in nanocluster catalysis and electronic miniaturization and biological applications such as drug delivery will depend on a thorough understanding of nanoscale materials in which molecule-like characteristics appear. This Account tells the story of Au(25)L(18) and its associated synthetic, structural, mass spectrometric, electron transfer, optical spectroscopy, and magnetic resonance results. We also reference other Au NP studies to introduce helpful synthetic and measurement tools. Historically, nanoparticle sizes have been described by their diameters. Recently, researchers have reported actual molecular formulas for very small NPs, which is chemically preferable to solely reporting their size. Au(25)L(18) is a success story in this regard; however, researchers initially mislabeled this NP as Au(28)L(16) and as Au(38)L(24) before correctly identifying it by electrospray-ionization mass spectrometry. Because of its small size, this NP is amenable to theoretical investigations. In addition, Au(25)L(18)s accessibility in pure form and molecule-like properties make it an attractive research target. The properties of this NP include a large energy gap readily seen in cyclic voltammetry (related to its HOMO-LUMO gap), a UV-vis absorbance spectrum with step-like fine structure, and NIR fluorescence emission. A single crystal structure and theoretical analysis have served as important steps in understanding the chemistry of Au(25)L(18). Researchers have determined the single crystal structure of both its native as-prepared form, a [N((CH(2))(7)CH(3))(4)(1+)][Au(25)(SCH(2)CH(2)Ph)(18)(1-)] salt, and of the neutral, oxidized form Au(25)(SCH(2)CH(2)Ph)(18)(0). A density functional theory (DFT) analysis correctly predicted essential elements of the structure. The NP is composed of a centered icosahedral Au(13) core stabilized by six Au(2)(SR)(3) semirings. These semirings present interesting implications regarding other small Au nanoparticle clusters. Many properties of the Au(25) NP result from these semiring structures. This overview of the identification, structure determination, and analytical properties of perhaps the best understood Au nanoparticle provides results that should be useful for further analyses and applications. We also hope that the story of this nanoparticle will be useful to those who teach about nanoparticle science.

Synthesis of monodisperse [Oct4N(+)][Au25(SR)18(-)] nanoparticles, with some mechanistic observations.

A single phase (THF) synthesis of monodisperse [Oct(4)N(+)][Au(25)(SR)(18)(-)] nanoparticles is described that yields insights into pathways by which it is formed from initially produced larger nanoparticles. Including the Oct(4)N(+)Br(-) salt in a reported single phase synthetic procedure enables production of reduced nanoparticles having a fully occupied HOMO molecular energy level (Au(25)(SR)(18)(-), as opposed to a partially oxidized state, Au(25)(SR)(18)(0)). The revised synthesis accommodates several (but not all) different thiolate ligands. The importance of acidity, bromide, and dioxygen on Au(25) formation was also assessed. The presence of excess acid in the reaction mixture steers the reaction toward making Au(25)(SR)(18); while bromide does not seem to affect Au(25) formation, but it may play a role in maintaining the -1 oxidation state. Conducting the nanoparticle synthesis and aging period in the absence of dioxygen (under Ar) does not produce small nanoparticles, providing insights into the pathway of reaction product aging in the synthesis solvent, THF. The aging process favors the Au(25)(-) moiety as an end point and possibly involves degradation of larger nanoparticles by hydroperoxides formed from THF and oxygen.

Tandem Mass Spectrometry of Thiolate-Protected Au Nanoparticles NaxAu25(SC2H4Ph)18−y(S(C2H4O)5CH3)y

We report the first collision-induced dissociation tandem mass spectrometry (CID MS/MS) of a thiolate-protected Au nanoparticle that has a crystallographically determined structure. CID spectra assert that dissociation pathways for the mixed monolayer Na(x)Au(25)(SC(2)H(4)Ph)(18-y)(S(C(2)H(4)O)(5)CH(3))(y) centrally involve the semi-ring Au(2)L(3) coordination (L = some combination of the two thiolate ligands) that constitutes the nanoparticles protecting structure. The data additionally confirm charge state assignments in the mass spectra. Prominent among the fragments is [Na(2)AuL(2)](1+), one precursor of which is identified as another nanoparticle fragment in the higher m/z region. Another detected fragment, [Na(2)Au(2)L(3)](1+), represents a mass loss equivalent to an entire semi-ring, whereas others suggest involvement (fragmentation/rearrangement) of multiple semi-rings, e.g., [NaAu(3)L(3)](1+) and [NaAu(4)L(4)](1+). The detailed dissociation/rearrangement mechanisms of these species are not established, but they are observed in other mass spectrometry experiments, including those under non-CID conditions, namely, electrospray ionization mass spectrometry (ESI-MS) with both time-of-flight (TOF) and FT-ICR analyzers. The latter, previously unreported results show that even soft ionization sources can result in Au nanoparticle fragmentation, including that yielding Au(4)L(4) in ESI-TOF of a much larger thiolate-protected Au(144) nanoparticle under non-CID conditions.

Mass spectrometry of ligand exchange chelation of the nanoparticle [Au25(SCH2CH2C6H5)18]1- by CH3C6H3(SH)2.

Mass spectrally detected products of ligand exchange reactions of the nanoparticle [Au25(SC2H4C6H5)18](1-), (abbrev. Au25(SC2Ph)18), where the dithiol is toluene-3,4-dithiol, CH3C6H3(SH)2, include nanoparticles containing both doubly (bidentate, or chelating) and singly bonded dithiol. The bidentate binding displaces two of the original -SC2Ph ligands, and singly bonded dithiol displaces one -SC2Ph ligand, while maintaining, for mass spectrally detected species, occupancy of 18 ligation sites. Extended exchange reaction times result in an apparent maximum of six chelated dithiolates. In the Au25(SC2Ph)18 nanoparticle, six semi-rings of -S(R)-Au-S(R)-Au-S(R)- act as the protecting ligand shell surrounding a Au13 core; the chelation is suggested to involve binding of dithiolates to adjacent semi-rings, rather than to a single semi-ring. Both high resolution ESI and lower resolution MALDI spectra support the product assignments. A minor extent of bidentate ligand incorporation is sufficient to severely compromise the well-known Au25(SC2Ph)18 UV-vis fine structure and to alter its voltammetric pattern, reflecting either associated semi-ring distortion and/or decay of the exchange product.

Electroanalytical Assessment of the Effect of Ni:Fe Stoichiometry and Architectural Expression on the Bifunctional Activity of Nanoscale NiyFe1–yOx

Electrocatalysis of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) was assessed for a series of Ni-substituted ferrites (NiyFe1-yOx, where y = 0.1 to 0.9) as expressed in porous, high-surface-area forms (ambigel and aerogel nanoarchitectures). We then correlate electrocatalytic activity with Ni:Fe stoichiometry as a function of surface area, crystallite size, and free volume. In order to ensure in-series comparisons, calcination at 350 °C/air was necessary to crystallize the respective NiyFe1-yOx nanoarchitectures, which index to the inverse spinel structure for Fe-rich materials (y ≤ 0.33), rock salt for the most Ni-rich material (y = 0.9), and biphasic for intermediate stoichiometry (0.5 ≤ y ≤ 0.67). In the intermediate Ni:Fe stoichiometric range (0.33 ≤ y ≤ 0.67), the OER current density at 390 mV increases monotonically with increasing Ni content and increasing surface area, but with different working curves for ambigels versus aerogels. At a common stoichiometry within this range, ambigels and aerogels yield comparable OER performance, but do so by expressing larger crystallite size (ambigel) versus higher surface area (aerogel). Effective OER activity can be achieved without requiring supercritical-fluid extraction as long as moderately high surface area, porous materials can be prepared. We find improved OER performance (η decreases from 390 to 373 mV) for Ni0.67Fe0.33Ox aerogel heat-treated at 300 °C/Ar, owing to an increase in crystallite size (2.7 to 4.1 nm). For the ORR, electrocatalytic activity favors Fe-rich NiyFe1-yOx materials; however, as the Ni-content increases beyond y = 0.5, a two-electron reduction pathway is still exhibited, demonstrating that bifunctional OER and ORR activity may be possible by choosing a nickel ferrite nanoarchitecture that provides high OER activity with sufficient ORR activity. Assessing the catalytic activity requires an appreciation of the multivariate interplay among Ni:Fe stoichiometry, surface area, crystallographic phase, and crystallite size.

Rewriting Electron-Transfer Kinetics at Pyrolytic Carbon Electrodes Decorated with Nanometric Ruthenium Oxide

Platinum is state-of-the-art for fast electron transfer whereas carbon electrodes, which have semimetal electronic character, typically exhibit slow electron-transfer kinetics. But when we turn to practical electrochemical devices, we turn to carbon. To move energy devices and electro(bio)analytical measurements to a new performance curve requires improved electron-transfer rates at carbon. We approach this challenge with electroless deposition of disordered, nanoscopic anhydrous ruthenium oxide at pyrolytic carbon prepared by thermal decomposition of benzene (RuOx@CVD-C). We assessed traditionally fast, chloride-assisted ([Fe(CN)6]3-/4-) and notoriously slow ([Fe(H2O)6]3+/2+) electron-transfer redox probes at CVD-C and RuOx@CVD-C electrodes and calculated standard heterogeneous rate constants as a function of heat treatment to crystallize the disordered RuOx domains to their rutile form. For the fast electron-transfer probe, [Fe(CN)6]3-/4-, the rate increases by 34× over CVD-C once the RuOx is calcined to form crystalline rutile RuO2. For the classically outer-sphere [Fe(H2O)6]3+/2+, electron-transfer rates increase by an even greater degree over CVD-C (55×). The standard heterogeneous rate constant for each probe approaches that observed at Pt but does so using only minimal loadings of RuOx.