Isolation and structural characterization of an octaneselenolate-protected Au25 cluster

We report the isolation and structural characterization of an octaneselenolate-protected Au(25) cluster ([Au(25)(SeC(8)H(17))(18)](-)). Isolated [Au(25)(SeC(8)H(17))(18)](-) was characterized by various analytical techniques. The results strongly suggest that [Au(25)(SeC(8)H(17))(18)](-) possesses a similar geometric structure to the well-studied thiolate (RS)-protected Au(25) cluster ([Au(25)(SR)(18)](-)) and that the charge transfer between the metal atoms and ligands in [Au(25)(SeC(8)H(17))(18)](-) is lower than that in [Au(25)(SR)(18)](-). To the best of our knowledge, this is the first report of the isolation of a selenolate-protected gold cluster. [Au(25)(SeC(8)H(17))(18)](-) is an ideal compound for determining how changing the ligand from thiolate to selenolate affects the fundamental properties of a cluster.     

Combined Fourier transform infrared and Raman spectroscopic studies of some carbonyl-diphosphine-indenyl-iron cations

The Fourier transform infrared and Raman spectra of the cations [η⁵-C₉H₇Fe(CO)_n dppa]⁺ (n = 1, 2; dppa=bisdiphenylphosphinoalkane, where alkane=methane, ethane, butane, hexane and octane) and [{η⁵-C₉H₇Fe(CO)₂}₂-μ-dppa]²⁺ indicate that the alkyl chain lengths have effects on the structures of the bidentate cations resulting in increased back-donation to carbonyl groups as the chain length increases. In contrast the alkyl chain lengths have no similar effects in the unidentate mononuclear and bridged cations.     

Surface complexation of condensed phosphate to aluminum hydroxide: an ATR-FTIR spectroscopic investigation

Two model compounds, sodium pyrophosphate (pyro-P) and sodium tripolyphosphate (tripoly-P), were employed to elucidate the binding mechanisms of condensed phosphate on aluminum hydroxide by utilizing attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. Peak assignments for the condensed phosphates in the solution phase and those adsorbed on the surface of aluminum hydroxide were made. Electron delocalization and polarization were employed to explain the peak shifts and the complexation of condensed phosphate with aluminum hydroxide. The tripoly-P and pyro-P were adsorbed on aluminum hydroxide by forming inner-sphere complexes. The adsorbed condensed phosphates were deprotonated in the pH range from 4 to 10. Monodentate, bidentate, and binuclear complexes were formed when pyro-P was adsorbed on aluminum hydroxide, while monodentate and binuclear complexes were formed when tripoly-P was adsorbed. Based on the FTIR data, we proposed that when either bidentate or binuclear complexes were formed, the two oxygen atoms participating in the complexation with aluminum hydroxide could not be originated from the same terminal phosphate moiety. The AlO bond formed in the complexation of pyro-P or tripoly-P with aluminum hydroxide (AlPO(-3)) was not as strong as the HO bond in terminal HPO(-3). The bridging PO(-2) of tripoly-P did not coordinate with aluminum hydroxide. The real-time ATR-FTIR study on condensed phosphate adsorption revealed that a long contact time between condensed phosphates and aluminum hydroxide particles can result in a transformation of an initially formed species into a thermodynamically more stable phase.     

Carbon nanocone as an ammonia sensor: DFT studies

Adsorption of NH₃ molecule on a carbon nanocone (CNC) was investigated using density functional theory in terms of energetic, structural, and electronic properties. It is mainly demonstrated that (i) the NH₃ molecule preferentially tends to attach to the apex of the CNC through its N atom, releasing energy of 54.28 kcal/mol, (ii) the CNC may be a promising candidate in gas sensor devices in order to detect the NH₃ molecule, and (iii) the field electron emission current may be enhanced from CNC surface upon the adsorption process.     

Stoichiometric zirconium oxide cations as potential building blocks for cluster assembled catalysts

Employing guided-ion-beam mass spectrometry, we identified a series of positively charged stoichiometric zirconium oxide clusters that exhibit enhanced activity and selectivity for three oxidation reactions of widespread chemical importance. Density functional theory calculations reveal that these clusters all contain the same active site consisting of a radical oxygen center with an elongated zirconium-oxygen bond. Calculated energy profiles demonstrate that each oxidation reaction is highly favorable energetically and involves easily surmountable barriers. Furthermore, the active stoichiometric clusters may be regenerated by reacting oxygen-deficient clusters with a strong oxidizer. This indicates that these species may promote multiple cycles of oxidation reactions and, therefore, exhibit true catalytic behavior. The stoichiometric clusters, having structures that resemble specific sites in bulk zirconia, are promising candidates for potential incorporation into a cluster assembled catalyst material.     

Peptide mesocrystals as templates to create an Au surface with stronger surface-enhanced Raman spectroscopic properties

The design and fabrication of various nanostructures with predefined geometry and composition is a big challenge of nanotechnology. Here we demonstrate an Au nanoflake film replicated from a self-assembled, well-ordered, dipeptide flower-like hierarchical architecture. Such morphology can give rise to useful and remarkable surface-enhanced Raman spectroscopy (SERS) properties. We obtained these nanostructures by using a scaffold of flake-built spherical dipeptide aggregations. Gold nanoparticles were sputtered on the surface of as-assembled dipeptide by an etching system. After removing the dipeptide templates by ethanol, a metal crust was left with a morphology similar to that of the dipeptide hierarchical structure. The different steps within the process were monitored by using electron microscopy, energy-dispersive spectrum (EDS) analysis and atomic force microscopy (AFM). Cyclic voltammetry and Raman spectra were employed to prove the SERS effect of the obtained Au substrates. The enhancement factor is estimated to be about 10(4) for 4-mercaptobenzoic acid (4-MBA) molecules on the Au nanoflake surfaces.     

Correlating the crystal structure of a thiol-protected Au25 cluster and optical properties

The total structure determination of thiol-protected Au clusters has long been a major issue in cluster research. Herein, we report an unusual single crystal structure of a 25-gold-atom cluster (1.27 nm diameter, surface-to-surface distance) protected by eighteen phenylethanethiol ligands. The Au25 cluster features a centered icosahedral Au13 core capped by twelve gold atoms that are situated in six pairs around the three mutually perpendicular 2-fold axes of the icosahedron. The thiolate ligands bind to the Au25 core in an exclusive bridging mode. This highly symmetric structure is distinctly different from recent predictions of density functional theory, and it also violates the empirical golden rule "cluster of clusters", which would predict a biicosahedral structure via vertex sharing of two icosahedral M13 building blocks as previously established in various 25-atom metal clusters protected by phosphine ligands. These results point to the importance of the ligand-gold core interactions. The Au25(SR)18 clusters exhibit multiple molecular-like absorption bands, and we find the results are in good correspondence with time-dependent density functional theory calculations for the observed structure.     

In situ surface-enhanced Raman spectroscopic studies of nafion adsorption on Au and Pt electrodes

Understanding interactions between Nafion (perfluorosulfonic acid) and Pt catalysts is important for the development and deployment of proton exchange membrane fuel cells. However, study of such interactions is challenging and Nafion/Pt interfacial structure remains elusive. In this study, adsorption of Nafion ionomer on Au and Pt surfaces was investigated for the first time by in situ surface-enhanced Raman spectroscopy. The study is made possible by the use of uniform SiO(2)@Au core-shell particle arrays which provides very strong enhancement of Raman scattering. The high surface sensitivity offered by this approach yields insightful information on interfacial Nafion structure. Through spectral comparison of several model compounds, vibration assignments of SERS bands were made. The SER spectra suggest the direct interaction of sulfonate group with the metal surfaces, in accord with cyclic voltammetric results. Comparison of present SERS results with previous IR spectra was briefly made.     

The use of freons and perfluoro (cyclo) alkanes as matrix in spectroscopic studies of radiation-produced radical cations

A study is made of the suitability of different freons (fluorotrichloromethane, 1,1,1- and 1,1,2-trichlorotrifluoroethane and 1,1-difluorotetrachloroethane) and perfluoro (cyclo) alkanes (perfluorohexane, perfluoromethylcyclohexane and perfluorodecaline) as matrix in spectroscopic investigations of radiation-produced radical cations. After gamma irradiation of the freon matrices, containing small amounts of alkane additives, complex optical absorption spectra are obtained. Deconvolution of these spectra is possible by successive illuminations with suitable wavelengths. Details of such bleaching procedures, which yield pure spectra of the alkane radical cations studied, are described. Spectra and bleaching procedures in the perfluoro (cyclo) alkane matrices are less complex, because the absorption of the irradiated pure matrices is much less pronounced and situated primarily at short wavelengths.     

Gold nanoparticles as spectroscopic enhancers for in vitro studies on single viruses

Gold particles with diameters between 2.5 and 4.5 nm have been introduced in the inner cavity of an icosahedral brome mosaic virus. The optical properties of single gold-marked virions have been tested in-vitro with respect to the characteristic plasmon polariton resonance. The shift in the plasmon polariton resonance of a single Au particle encapsulated in a virus with respect to a free particle in solution indicates a close interaction between the basic residues on the inner wall of the capsid and the negative surface charge of the particle. Incorporation of a pair of Au particles, approximately 4 nm diameter, is shown to be a frequent event. In this case, the dependence of the two-particle surface plasmon spectrum on the interparticle distance and the strong particle/capsid wall interaction suggest that, in the future, it will be possible to use encapsulated Au particles to track changes in the viral capsid volume in real-time and in a physiological environment.     

Yolk-shell gold nanoparticles as model materials for support-effect studies in heterogeneous catalysis: Au, @C and Au, @ZrO2 for CO oxidation as an example

The use of nanostructured yolk-shell materials offers a way to discriminate support and particle-size effects for mechanistic studies in heterogeneous catalysis. Herein, gold yolk-shell materials have been synthesized and used as model catalysts for the investigation of support effects in CO oxidation. Carbon has been selected as catalytically inert support to study the intrinsic activity of the gold nanoparticles, and for comparison, zirconia has been used as oxidic support. Au, @C materials have been synthesized through nanocasting using two different nonporous-core@mesoporous-shell exotemplates: Au@SiO(2)@ZrO(2) and Au@SiO(2)@m-SiO(2). The catalytic activity of Au, @C with a gold core of about 14 nm has been evaluated and compared with Au, @ZrO(2) of the same gold core size. The strong positive effect of metal oxide as support material on the activity of gold has been proved. Additionally, size effects were investigated using carbon as support to determine only the contribution of the nanoparticle size on the catalytic activity of gold. Therefore, Au, @C with a gold core of about 7 nm was studied showing a less pronounced positive effect on the activity than the metal oxide support effect.     

The pyrolytic graphite surface as an enzyme substrate: microscopic and spectroscopic studies

We have conducted a series of experiments to explore the surface of the polished pyrolytic graphite ‘edge’ electrode as routinely prepared for use in protein film voltammetry. Our investigations have included nitrogen porosimetry and scanning electron microscopy. The nitrogen adsorption revealed a Brunauer–Emmett–Teller surface area ∼10⁴ times greater than the geometric surface area of the electrode. The pore-size distribution calculated by the Horváth–Kawazoe method showed that 10–18% of the pore volume arises from pores having widths >10 nm and, thus, should be accessible to enzymes, although much of the exposed ‘wall’ surface may be inactive for enzyme binding or electron transfer: for example, it may be mainly basal plane. Scanning electron micrographs of the abraded pyrolytic graphite edge showed differing scales of surface damage caused by the abrasion and the presence of many cracks in the surface where thin platelets had been removed.This work is dedicated to Prof. Alan Bond on the occasion of his 60th birthday. Alan’s enthusiasm for the complexities of diffusion control persuaded one of us (F.A. Armstrong) to try and avoid it altogether in protein electrochemical studies.     

Rare-gas matrix as an infinite rare-gas cluster: a spectroscopic study of 9,10-dichloroanthracene in argon matrices

Absorption, fluorescence and fluorescence-excitation spectra as well as fluorescence-decay times were studied for 9,10-dichloroanthracene (DCA) in argon matrices in the 12–25 K temperature range. The experimental data are compared to those obtained for large DCA/Ar clusters.     

New crown-substituted aluminum(iii) porphyrinate as an advanced fluorescent sensor to potassium cations in aqueous media

Aluminum(iii) and magnesium(ii) porphyrinates were synthesized starting from free base meso-tetra(benzo-15-crown-5)porphine (H₂TCP), AlCl₃, and Et₂O??MgBr₂. The structures of the compounds obtained were determined by NMR spectroscopy, electronic absorption spectroscopy (EAS), and MALDI TOF mass spectrometry. A comparative analysis of the fluorescence properties of MgTCP, Al(OH)TCP, and H₂TCP and of stabilities of the metal porphyrinates in an aqueous solution in a wide pH range was carried out. The receptor properties of aluminum(iii) porphyrinate to potassium cations in an aqueous solution were examined by the changes in the EAS and fluorescence spectra.     

Combined theoretical and FTIR spectroscopic studies on hydrogen adsorption on the zeolites Na-FER and K-FER

The interaction between molecular hydrogen and the alkali-metal-exchanged zeolites Na-FER and K-FER at a low temperature was investigated by combining variable-temperature infrared spectroscopy and theoretical calculations by using a periodic DFT model. The experimentally determined values of standard adsorption enthalpy, DeltaH degrees , were -6.0 (+/-0.8) and -3.5 (+/-0.8) kJ mol(-1) for Na-FER and K-FER, respectively. These results were found to be in agreement with corresponding DeltaH degrees values obtained from calculations on the periodic model. Two types of alkali-metal cation sites in FER were found: channel intersection sites and channel wall sites. Calculations showed a similar interaction energy for both site types, and similar structures of adsorption complexes. Up to two dihydrogen molecules can be physisorbed on the alkali-metal cation located on the intersection of two channels, while only one H2 molecule is physisorbed on the cation at the channel wall site. The adsorption enthalpies of H2 on alkali-metal-exchanged FER are significantly smaller than those found previously for the MFI-type zeolites Na-ZSM-5 and K-ZSM-5, which is likely due to a difference in the alkali-metal cation coordination in the two zeolite frameworks.     

A new versatile system for matrix isolation and spectroscopic studies of cations at 77 K: Perfluoro-2-butyltetrahydrofuran

Perfluoro-2-butyltetrahydrofuran forms a clear glass at 77 K which is transparent from 220 to 2600 nm. The absorption spectrum obtained on gamma radiolysis of a pure FTHF matrix at 77 K has a band at 248 nm. Using this matrix, the absorption spectra of a number of radiolytically generated perfluoroaromatic cations were obtained. This indicates that the matrix is suitable for the radiolytic generation and spectroscopic studies of cations of perfluoroaromatic compounds which have high ionization potentials.     

Platonic hexahedron composed of six organic faces with an inscribed Au cluster

The structures of nanomaterials determine their individual properties and the suprastructures they can form. Introducing anisotropic shapes and/or interaction sites to isotropic nanoparticles has been proposed to extend the functionality and possible suprastructure motifs. Because of symmetric anisotropy, Platonic solids with regular polygon faces are one of the most promising nanoscale structures. Introduction of Platonic solid anisotropy to isotropic nanomaterials would expand the functionality and range of possible suprastructure motifs. Here, we demonstrate a novel strategy to obtain nano-Platonic solids through the face coordination of square porphyrins on an inscribed Au sphere with adequate size. The face coordination of the multidentate porphyrin derivatives, with four acetylthio groups facing the same direction, on the Au cluster encased the Au cluster in a Platonic hexahedron with six porphyrin faces. Transmission electron microscopy, mass spectrometry, elemental analysis, and scanning tunnelling microscopy were used to confirm the formation of the nano-Platonic hexahedron.     

Ancillary nitrile substituents as convenient IR spectroscopic reporters for self-assembly of mercapto- and isocyanoazulenes on Au(111)

Synthesis and self-assembly of structurally related mercapto- and isocyanoazulenes, including novel 2-mercapto-1,3-dicyanoazulene (4) and 2-isocyano-1,3-dicyanoazulene (5), are reported. Exposing 5 adsorbed on Au(111) to a solution of 4 displaces the isocyanoazulene monolayer with that of the mercaptoazulene as judged by ν(C≡N) signatures of these films.     

Urea-based polyacetylenes as an optical sensor for fluoride ions

Novel acetylenes carrying urea groups, 1-(4-ethynylphenyl)-3-(4-nitrophenyl) urea (1), 1-(4-propargyl)-3-(4-nitrophenyl) urea (2), were synthesized and polymerized with rhodium catalyst. Polymers [poly(1) and poly(2)] with moderate molecular weights were obtained in good yields. The anion sensing ability of poly(1) and poly(2) was estimated using the tetra-n-butylammonium (TBA) salts of a series of anions in DMF. Upon the addition of F-, the color of the DMF solution of poly(1) and poly(2) immediately turned to a different color, while the color of solution changed slightly upon addition of Cl-, HSO4-, Br-, and NO3-, indicating the F- sensing ability of poly(1) and poly(2). The 1H-NMR titrations of poly(1) revealed that the colorimetric response of poly(1) was triggered by the urea/F- interaction through the hydrogen bonding and/or deprotonation process. The absorption spectra titration and Hill plot analysis were carried out to measure the F- binding ability, and the Hill coefficient in the poly(1)/F- complexation was found to be 5.8. This result clearly indicated that this binding mode between poly(1) and F- was based on a positive homotropic allosterism.