Oxidation states and CO ligand exchange kinetics in a self-assembled monolayer of a triruthenium cluster studied by in situ infrared spectroscopy

Oxidation states and CO ligand exchange kinetics in a self-assembled monolayer (SAM) of an oxo-centered triruthenium cluster [Ru(3)(mu3-O)(mu-CH3COO)6(CO)(L1)(L2)] (L1 = [(NC5H4)CH2NHC(O)(CH2)10S-]2, L2 = 4-methylpyridine) have been extensively investigated on the surface of a gold electrode in aqueous and nonaqueous solutions. The SAM exhibits three consecutive one-electron transfers and four oxidation states, which have been characterized by electrochemistry, in situ infrared spectroscopy, and in situ sum frequency generation (SFG) vibrational spectroscopy measurements. The original electron-localized state of the Ru cluster center was changed to electron delocalization states by oxidation or reduction of the central Ru ions. These changes are revealed by the IR absorptions of the CO ligand and the bridging acetate ligands of the triruthenium cluster in the SAM. The IR absorptions of the two kinds of ligands are strongly dependent on the oxidation state of the Ru cluster center. One-electron oxidation of the central Ru ion in the SAM triggers a CO ligand liberation process. Solvent molecules may then occupy the CO site to result in a CO-free SAM. One-electron reduction of this CO-free SAM in a CO-saturated solution leads to re-coordination of the CO ligand into the SAM. Both processes can be precisely controlled by tuning the electrode potential. The kinetics of the CO exchange cycle in the SAM, including liberation and coordination, has been investigated by in situ IR and SFG measurements for the first time. The CO exchange cycle is significantly dependent on the temperature. The reaction rate greatly decreases with decreasing solution temperature, which is an important factor in the CO ligand exchange process. The activation energies of both CO liberation and coordination have been evaluated from the reaction rate constants obtained at various temperatures.     

In situ infrared investigation of alkylation reaction of dianionic triruthenium ketenylidene cluster on magnesium oxide with methyl iodide

The reaction between [Ru_3(CO)_9(CCO)]~(2-) supported on magnesium oxide and alky-lating agent CH_3I was investigated in an attempt to understand its reactivity and mechanism byusing FT-IR technique. The IR spectra show that [Ru_3(CO)_9(CCO)]~(2-)/MgO reacts with CH_3Ito produce [Ru_3(CO)_9CC(O)CH_3]-/MgO, which exhibits bands at 2960, 2022, 1988, 1638, 1460and 1350 cm~(-1). The absorption at 2960, 1460 and 1350 cm~(-1) are attributed to -CH_3 group, whilebands at 2022, 1988 and 1952 cm~(-1) are assigned to the terminal CO groups attached to ruthenium.The band at 1638 cm~(-1) was assigned to acyl carbonyl. Isotopes labeled compounds CD3I, ~(13)COand [Ru_3(CO)_9(*C*CO)]~(2-) were used in investigation. The results demonstrate that β-carbon ofCCO ligand is attacked by CH_3~+ species and produces [Ru_3(CO)_9CC(O)CH_3]-/MgO.     

Ozonolysis-based route to the in situ formation of aldehyde-bearing self-assembled monolayer surfaces

While ozonolysis of a terminal carbon-carbon double bond to produce aldehydes is a well-established synthetic strategy for conventional solution chemistry, exposure of vinyl-terminated self-assembled monolayers to ozone has been reported to yield carboxylic acids. By using a cold solution of ozone in methanol and then adding a reducing agent to this solution, acid formation is minimized and near-quantitative aldehyde formation is achieved. The aldehyde-bearing surface is characterized by its physical and chemical properties and by ATR-FTIR spectroscopy showing a characteristic aldehyde C-H peak at 2715 cm(-1) and carbonyl peak at 1729 cm(-1). The reactivity of the aldehyde-bearing surface is shown by its reaction with amines and amine derivatives to give surface-bound imines and by the reversible cycling between aldehyde and acetal. The acetal also provides a useful way to mask the aldehyde and store readily released aldehyde surface functionality for subsequent surface elaboration.     

Developing a self-assembled monolayer microarray to study stem cell differentiation

We have developed a novel strategy to generate self-assembled monolayer microarray (SAMs-Array) of alkanethiolates on gold surfaces for the study of human mesenchymal stem cells (hMSCs) differentiation. Electroactive alkanethiols were microarray-printed in varying densities to probe the chemical effects on stem cell differentiation. Cyclic voltammetry (CV) was used for the precise determination of the amount of alkanethiol molecules transferred and SAMs formed on the microarray. We can also control the oxidative and reductive state of each molecule displayed to cells by CV. Based on this SAMs-Array technology, we generated a platform for potential high-throughput screening of various surface chemistry effects on cell behaviors for future applications in biomaterials and tissue engineering.     

Electrochemical control of CO/NO ligand exchange in a triruthenium cluster monolayer assembled on a gold electrode surface

A highly selective ligand exchange reaction is realized in the self-assembled monolayer (SAM) of a triruthenium cluster on a gold electrode surface under precise electrochemical potential control. CO as well as NO molecules, which are known to play key roles in many chemical, biological, and environmental systems, can be efficiently introduced into the SAM by electrochemically tuning the electronic state of the Ru site. These unique surface reactions are more convenient and efficient than conventional ligand exchange reactions in solution and could be used for the elucidation of the electron-transfer mechanism in a biological system as well as in the development of molecular sensors and devices.     

Coordination behaviour of nicotinamide: an infrared spectroscopic study

A series of Hofmann-type complexes containing two nicotinamide(nia) molecules attached to transition metal (II) (M) tetracyanonickelate frame with the formula: M(nia)₂Ni(CN)₄ (where M=Mn, Co, Ni, Cu or Cd) have been synthesised for the first time. Metal (II) halide complexes of nicotinamide complexes of the type [M(nia)₂X₂ (M=Cd, Ni, Cu, Hg; X=Cl, Br) and Ni(nia)₄Br₂ nia=nicotinamide] have also synthesised. The FTIR spectra are reported in the 4000-400 cm⁻¹ region. Vibrational assignments are given for all the observed bands. The analysis of the vibrational spectra indicates that there are some structure-spectra correlations. A pronounced change was observed in the N-H stretching frequencies of the NH₂ group. It is proposed that the amide NH₂ group influence by the intramolecular hydrogen bond in the complexes. The coordination effect on the nicotinamide modes is analysed.     

Surface reconstitution of glucose oxidase onto a norbornylogous bridge self-assembled monolayer

An electrode construct was fabricated in which a self-assembled monolayer containing a novel norbornylogous bridge was covalently attached to flavin adenine dinucleotide (FAD), the redox active centre of several oxidase enzymes. The electrochemistry of the construct was investigated before and after the reconstitution of glucose oxidase around the surface bound FAD. Rapid rates of electron transfer were observed both before and after the reconstitution of biocatalytically active enzyme. However, no biocatalytic activity was observed under anaerobic conditions suggesting the a lack of enzyme turnover through direct electron transfer. It is proposed that a decrease in the electronic coupling between the redox active FAD and the electrode following reconstitution of the glucose oxidase – a probable consequence of the FAD being immersed in a protein environment – was responsible for the inability of the enzyme to be turned over under anaerobic conditions.     

Optical properties of Yeast Cytochrome c monolayer on gold: an in situ spectroscopic ellipsometry investigation

The adsorption of Yeast Cytochrome c (YCC) on well defined, flat gold substrates has been studied by Spectroscopic Ellipsometry (SE) in the 245-1000 nm wavelength range. The investigation has been performed in aqueous ambient at room temperature, focusing on monolayer-thick films. In situ δΨ and δΔ difference spectra have shown reproducibly well-defined features related to molecular optical absorptions typical of the so-called heme group. The data have been reproduced quantitatively by a simple isotropic optical model, accounting for the molecular absorption spectrum and film-substrate interface effects. The simulations allowed a reliable estimate of the film thickness and the determination of the position and the shape of the so-called Soret absorption peak that, within the experimental uncertainty, is the same found for molecules in liquid. These findings suggest that YCC preserves its native structure upon adsorption. The same optical model was able to reproduce also ex situ results on rinsed and dried samples, dominated by the spectral features associated to the polypeptide chain that tend to overwhelm the heme absorption features.     

In situ infrared spectroscopic study of brucite carbonation in dry to water-saturated supercritical carbon dioxide

In geologic carbon sequestration, whereas part of the injected carbon dioxide will dissolve into host brine, some will remain as neat to water saturated supercritical CO(2) (scCO(2)) near the well bore and at the caprock, especially in the short term life cycle of the sequestration site. Little is known about the reactivity of minerals with scCO(2) containing variable concentrations of water. In this study, we used high-pressure infrared spectroscopy to examine the carbonation of brucite (Mg(OH)(2)) in situ over a 24 h reaction period with scCO(2) containing water concentrations between 0% and 100% saturation, at temperatures of 35, 50, and 70 °C, and at a pressure of 100 bar. Little or no detectable carbonation was observed when brucite was reacted with neat scCO(2). Higher water concentrations and higher temperatures led to greater brucite carbonation rates and larger extents of conversion to magnesium carbonate products. The only observed carbonation product at 35 °C was nesquehonite (MgCO(3)·3H(2)O). Mixtures of nesquehonite and magnesite (MgCO(3)) were detected at 50 °C, but magnesite was more prevalent with increasing water concentration. Both an amorphous hydrated magnesium carbonate solid and magnesite were detected at 70 °C, but magnesite predominated with increasing water concentration. The identity of the magnesium carbonate products appears strongly linked to magnesium water exchange kinetics through temperature and water availability effects.     

Surface microscopic structure and electrochemical rectification of a branched alkanethiol self-assembled monolayer

The tert-butanethiol self-assembled monolayers (SAMs) on Au(111) surfaces were prepared from various solvents and investigated by a combination of scanning tunneling microscopy (STM) and electrochemistry in aqueous environments. High-resolution STM images reveal a (radical(7) x radical(7))R19 degrees surface lattice structure, in contrast with the conventional lattice (radical(3) x radical(3))R30 degrees structure for straight-chain alkanethiol SAMs. Interestingly, such a branched monolayer shows electrochemical rectification toward redox probes. We suggest that electrochemical rectification could be a general characteristic of short-chain branched alkanthiol SAMs, and originate in localized electronic effects.     

Versatile and cooperative reactivity of a triruthenium polyhydride cluster. A computational study

The reaction of a trinuclear polyhydride complex Ru3H5(C5H5)3 with cyclopentadiene, C5H6, has been studied computationally. A mechanism for the experimentally observed selective C-C bond activation is proposed. All three Ru centers participate in various steps of the mechanism. The catalytic involvement of two cluster hydrides in the transformation of the C5Hn fragment is emphasized.     

Surface/interface electronic structure in C(60) anchored aminothiolate self-assembled monolayer: an approach to molecular electronics

Electronic structure in self-assembled monolayers (SAMs) of C(60) anchored 11-amino-1-undecane thiol (C(60)-11-AUT) on Au(111) was studied by means of ultraviolet photoelectron spectroscopy and hybrid density functional theory calculations. Valence band features of the molecular conformation revealed the interface electronic structure to be dominated by sigma(S-Au), localized at the thiolate anchor to Au. Formation of a localized covalent bond as a result of hybridization between N P(z) orbital of -NH(2) group of the thiolate SAM and the pi level of C(60) resulted in a symmetry change from I(h) in C(60) to C1 in C(60)-11-AUT SAM. Appearance of low, but finite amplitude surface electronic states of bonded C(60), much beyond the Fermi level, ruled out Au-C(60) end group contact. The band gap E(g) of the SAM, determined to be 2.7 eV, was drastically reduced from the insulating alkanethiol SAMs ( approximately 8.0 eV) and fell intermediate between the C(60) ground state (N electrons, 1.6 eV) and C(60) solid (N+/-1 electrons, 3.7 eV).     

Surface chemistry and in situ spectroscopy of a lysozyme langmuir monolayer

Surface pressure and surface potential-area isotherms were used to characterize a lysozyme Langmuir monolayer. The compression-decompression cycles and stability measurements showed a homogeneous and stable monolayer at the air-water interface. Salt concentration in the subphase and pH of the subphase were parameters controlling the homogeneity and stability of the Langmuir monolayer. In situ UV-vis and fluorescence spectroscopies were used to verify the homogeneity of the lysozyme monolayer and to identify the chromophore residues in the lysozyme. Optimal experimental conditions were determined to prepare a homogeneous and stable lysozyme Langmuir monolayer.     

In situ infrared and EXAFS studies of an Ag cluster in zeolite X

The in situ measurements of infrared spectra and the Ag K-edge EXAFS spectra of the fully Ag⁺ exchanged zeolite X (Ag₈₆–X) were carried out from room temperature to 300 °C under vacuum. By evacuation at room temperature the O–H stretch vibration (ν(O–H)) mode around 3 μm disappears and the coordination number of oxygen around Ag, N_Ag–O, decreases due to removal of water molecules. The T–O asymmetric stretch (ν_as(T–O)) mode associated with zeolite framework oxygen appears around 10 μm. These infrared spectra are fitted by summing up Gaussian peaks. The positions of the main two peaks are 1000 and 1100 cm⁻¹ at room temperature. At 100 °C, a third infrared peak appears at around 955 cm⁻¹, the total N_Ag–O becomes small and the coordination number of Ag around Ag, N_Ag–Ag, is 0.5. These results suggest that Ag atoms change sites in the zeolite and play an important role as a precursor of the Ag clusters. At 300 °C, the peaks around 1000 and 1100 cm⁻¹ shift to 1050 and 1140 cm⁻¹, respectively, and N_Ag–Ag becomes 2.9, which indicates that the Ag clusters attached to the zeolite framework are stabilized at high temperature. When the zeolite with Ag clusters is exposed to atmosphere, it is found that: (1) the ν(O–H) mode around 3 μm appears again, (2) there are two main peaks (1000 and 1100 cm⁻¹) and a small peak around 856 cm⁻¹ and (3) the local structure of the Ag clusters formed at 300 °C never reverses.     

Preparation of a well-defined amino-terminated self-assembled monolayer and copper microlines on a polyimide substrate covered with an oxide nanoskin

A well-ordered, uniform amino (NH(2))-terminated organosilane self-assembled monolayer (SAM) was prepared on a polyimide (PI) substrate, the surface of which had silica-like reactivity. First, through chemical vapor deposition of 1,3,5,7-tetramethylcyclotetrasiloxane and subsequent photooxidation using 172 nm vacuum ultraviolet light, an extremely thin silicon dioxide (SiO(2)) layer about 1 nm thick, which we call an "oxide nanoskin" (ONS), was prepared on a PI substrate. Due to the presence of this ONS layer, the PI surface's properties became almost identical with those of Si covered with native oxide (SiO(2)/Si) without any marked change in surface morphology, as evidenced by zeta-potential measurements, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Next, this ONS-covered PI (ONS/PI) surface was exposed to vapor of a 12.5 vol % solution of N-(6-aminohexyl)(3-aminopropyl)trimethoxysilane (AHAPS) molecules diluted with absolute toluene. On the basis of contact angle analysis, the surface energy of this AHAPS/ONS/PI sample was mostly consistent with that of a SiO(2)/Si substrate covered with an AHAPS-SAM (AHAPS/SiO(2)/Si). On the other hand, the surface energy of an AHAPS-treated PI (AHAPS/PI) substrate was much smaller than that of the AHAPS/ONS/PI substrate due to insufficient surface coverage by the AHAPS molecules. This was also confirmed by lateral force microscopy using photolithographically micropatterned samples. Fabricated micropatterns composed of AHAPS- and SiO(2)-covered regions were clearly imaged on the AHAPS/ONS/PI substrate through their difference in friction, while the friction contrast of the micropatterned AHAPS/PI substrate was unclear. This marked difference in packing density of the AHAPS molecules had a direct influence on the adsorption behavior of palladium colloids and subsequent electroless plating of copper (Cu). As confirmed by AFM and XPS, metallization proceeded only on the AHAPS-covered regions, while the SiO(2)-covered regions remained free of deposits, resulting in the formation of 10-mum-wide Cu microlines on both samples. However, the plating rate achieved on the AHAPS/ONS/PI substrate was about 4.5 times faster than that on the AHAPS/PI substrate and the pattern resolution was considerably fine.     

Effect of irradiation dose in making an insulator from a self-assembled monolayer

A combination of functionalization and irradiation-induced cross-linking allows fabrication of stable metal film on top of an aromatic self-assembled monolayer, [1,1';4',1' '-terphenyl]-4,4' '-dimethanethiol (TPDMT) on Au. Using X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and ion-scattering spectroscopy the optimal irradiation dose for producing a stable metal overlayer was estimated to be 40-45 mC/cm2. This dose is necessary for complete 2D-polymerization and closure of transient channels, which would otherwise allow metal penetration into the SAM. What is also important, the majority of the thiol tail groups, responsible for 2D growth and chemical adherence of the metal film, remains intact even at this high dose. The optimal dose corresponds to a crossover in the response of the TPDMT film to ionizing radiation: the irradiation-induced processes progress fast at lower doses and saturate at higher doses.     

Soft-landing of peptide ions onto self-assembled monolayer surfaces: an overview

This review is focused on what has been learned in recent research studies concerned with fundamental aspects of soft-landing and reactive landing of peptide ions on self-assembled monolayer surfaces (SAMs). Peptide ions are particularly attractive model systems that provide important insights on the behavior of soft landed proteins, while SAMs provide a convenient and flexible platform for tailoring the interfacial properties of metals and semiconductor surfaces. Deposition of mass-selected ions on surfaces is accompanied by a number of processes including charge reduction, neutralization, covalent and non-covalent binding, and thermal desorption of ions and molecules from the substrate. Factors that affect the competition between these processes are discussed.     

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.