One-step photochemical attachment of NHS-terminated monolayers onto silicon surfaces and subsequent functionalization

N-Hydroxysuccinimide (NHS)-ester-terminated monolayers were covalently attached in one step onto silicon using visible light. This mild photochemical attachment, starting from omega-NHS-functionalized 1-alkenes, yields a clean and flat monolayer-modified silicon surface and allows a mild and rapid functionalization of the surface by substitution of the NHS-ester moieties with amines at room temperature. Using a combination of analytical techniques (infrared reflection absorption spectroscopy (IRRAS), extensive X-ray photoelectron spectroscopy (XPS) in combination with density functional theory calculations of the XPS chemical shifts of the carbon atoms, atomic force microscopy (AFM), and static contact angle measurements), it was shown that the NHS-ester groups were attached fully intact onto the surface. The surface reactivity of the NHS-ester moieties toward amines was qualitatively and quantitatively evaluated via the reaction with para-trifluoromethyl benzylamine and biotin hydrazide.     

Ligand-receptor interactions between surfaces: the role of binary polymer spacers

The interactions between a receptor-modified planar surface and a surface grafted with a bimodal polymer layer, where one of the polymer species is ligand functionalized, are studied using a molecular theory. The effects of changing the binding energy of the ligand-receptor pair, the polymer surface coverage, the composition, and molecular weight of both the unfunctionalized and ligand functionalized polymers on the interactions between the surfaces are investigated. Our findings show that bridging exists between the surfaces including when the molecular weight of the ligand-bearing polymer is smaller than that of the unfunctionalized polymer, even though the ligand is initially buried within the polymer layer. The distance at which the surfaces bind depends only on the molecular weight of the ligand-modified polymer, while the strength of the interaction at a given surface separation can be tuned by changing the molecular weight of the polymers, the total polymer surface coverage, and the fraction of ligated polymers. The composition of the bimodal layer alters the structure of the polymer layer, thereby influencing the strength of the steric repulsions between the surfaces. Our theoretical results show good agreement with experimental data. The present theoretical study can be used as guidelines for the design of surfaces with tailored abilities for tunning the binding strength and surface-ligand separation distances for polymer-grafted surfaces bearing specific targeting ligands.     

DLVO interactions of tungsten oxide and cobalt oxide surfaces measured with the colloidal probe technique

We have investigated the DLVO surface forces of oxidized tungsten and cobalt surfaces using the atomic force microscope (AFM) colloidal probe technique. It was shown by X-ray photoelectron spectroscopy (XPS) and electrokinetic measurements that this model system is representative of industrial tungsten carbide (WC) and cobalt powders used in the production of hard metals. We found that the attractive van der Waals forces are well described by Hamaker constants, calculated from optical data for WO(3) and CoOOH. The repulsive electrostatic double layer forces between WO(3) surfaces increase with increasing pH due to an increasingly negative surface potential. This surface potential decreases with increasing ionic strength at pH 7.5. The electrostatic interaction between WO(3) and CoOOH is attractive at pH 10, suggesting a positively charged CoOOH surface.     

Mechanism of lateral interactions between molecules adsorbed on oxide surfaces

Comparison of the observed and calculated values for static and dynamic frequency shifts due to lateral interactions between CO molecules adsorbed on oxides indicates that these interactions are indirect and performed through a solid. Mechanism of static interaction includes relaxation, i.e. the displacement of surface atoms due to their adsorption.

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Orbital-band interactions and the reactivity of molecules on oxide surfaces: from explanations to predictions

 The surface chemistry of oxides is relevant for many technological applications: catalysis, photoelectrolysis, electronic-device fabrication, prevention of corrosion, sensor development, etc. This article reviews recent theoretical works that deal with the surface chemistry of oxides. The account begins with a discussion of results for the adsorption of CO and NO on oxides, systems which have been extensively studied in the literature and constitute an ideal benchmark for testing the quality of different levels of theory. Then, systematic studies concerned with the behavior of adsorbied alkali metals and sulfur-containing molecules are presented. Finally, a correlation between the electronic and chemical properties of mixed-metal oxides is analyzed and basic principles for designing chemically active oxides are introduced. Advances in theoretical methods and computer capabilities have made possible a fundamental understanding of many phenomena associated with the chemistry of molecules on oxide surfaces. Still many problems in this area remain as a challenge, and the approximate nature of most theoretical methods makes necessary a close coupling between theory and experiment. Following this multidisciplinary approach, the importance of band-orbital interactions for the reactivity of oxide surfaces has become clear. Simple models based on band-orbital mixing can explain trends found for the interaction of many adsorbates with oxide surfaces. These simple models provide a conceptual framework for modifying or controlling the chemical activity of pure oxides and for engineering mixed-metal oxides. In this respect, theoretical calculations can be very useful for predicting the best ways for enhancing the reactivity of oxide systems and reducing the waste of time, energy and materials characteristic of an empirical design. Received: 21 June 2001 / Accepted: 8 October 2001 / Published online: 1 February 2002     

A citric acid-derived ligand for modular functionalization of metal oxide surfaces via "click" chemistry

Citric acid is a widely used surface-modifying ligand for growth and processing of a variety of nanoparticles; however, the inability to easily prepare derivatives of this molecule has restricted the development of versatile chemistries for nanoparticle surface functionalization. Here, we report the design and synthesis of a citric acid derivative bearing an alkyne group and demonstrate that this molecule provides the ability to achieve stable, multidentate carboxylate binding to metal oxide nanoparticles, while also enabling subsequent multistep chemistry via the Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. The broad utility of this strategy for the modular functionalization of metal oxide surfaces was demonstrated by its application in the CuAAC modification of ZnO, Fe(2)O(3), TiO(2), and WO(3) nanoparticles.     

The role of hydrophobic surfaces in altering water-mediated peptide-peptide interactions in an aqueous environment

Using Born-Oppenheimer molecular dynamics within the density functional framework, we calculated the effective force acting on water-mediated peptide-peptide interaction between antiparallel β-sheets in an aqueous environment and also in the vicinity of a hydrophobic surface. From the magnitude of the effective force (corresponding to the slope of the free energy as a function of the interpeptide distance) and its sign (a negative value indicates an effective attraction, whereas a positive value indicates an effective repulsion) we can elucidate the fundamental differences of the water-mediated peptide-peptide interactions in those two environments. The computed effective forces indicate that the water-mediated interaction between peptides in an aqueous environment is attractive in the range of interpeptide distance d = 7-8 ? when hydrophobic surfaces are not nearby. Due to the stabilization of the water molecules bridging between the two β-sheets, a free energy barrier exists between the direct and indirect (water-mediated) interpeptide interactions. However, when the peptides are in the proximity of hydrophobic surfaces, this free energy barrier decreases because the hydrophobic surfaces enhance the interpeptide attraction by the destabilization and ease-to-libration of the bridging water molecules between them.     

Photochemical functionalization of hydrogen-terminated diamond surfaces: a structural and mechanistic study

Hydrogen-terminated diamond surfaces can be covalently modified with molecules bearing a terminal vinyl (C=C) group via a photochemical process using sub-band-gap light at 254 nm. We have investigated the photochemical modification of hydrogen-terminated surfaces of nanocrystalline and single-crystal diamond (111) to help understand the structure of the films and the underlying mechanism of photochemical functionalization. A comparison of the rates of photochemical modification of single-crystal diamond and nanocrystalline diamond films shows no significant difference in reactivity, demonstrating that the modification process is not controlled by grain boundaries or other structures unique to polycrystalline films. We find that both single-crystal and polycrystalline hydrogen-terminated diamond samples exhibit negative electron affinity and are functionalized at comparable rates, while oxidized surfaces with positive electron affinity undergo no detectable reaction. Gas chromatography-mass spectrometry (GC-MS) analysis shows the formation of new chemical products in the liquid phase that are formed only when the alkenes are illuminated in direct contact with H-terminated diamond, while control experiments with other surfaces and in the dark show no reaction. Our results show that the functionalization is a surface-mediated photochemical reaction and suggest that modification is initiated by the photoejection of electrons from the diamond surfaces into the liquid phase.     

Microwave-assisted organic functionalization of silica surfaces: Effect of selectively heating silylating agents

Microwave (MW)-assisted (2.45 GHz) organic functionalization of silica surfaces was investigated using (3-chloropropyl)dimethylsilanes with alkoxy, allyl, or aryl leaving groups in heptane or toluene at 80 °C. ²⁹Si and ¹³C CP/MAS spectroscopy confirmed the 3-chloropropyldimethylsilyl moiety was covalently grafted onto silica for all the samples. The effect of MW irradiation on the loading amount strongly depended on the leaving group as well as the solvent; using methoxysilane and p-anisylsilane in heptane caused a distinct acceleration. The correlation with the dielectric loss factors of the silylating agents suggested that the MW acceleration effect resulted from selectively heating the strongly MW-absorbing silylating agent. For the grafting reaction in toluene, the MW effect was not observed possibly because toluene masked the selective heating effect of the silylating agent.     

Synthesis and characterization of iridium polypyridyl complexes with ester groups with potential applications in covalent attachment to metal oxide surfaces

We report the synthesis and characterization of two iridium polypyridyl complexes, [Ir(deeb)₂Cl₂](PF₆) and [Ir(deeb)₂(dpp)](PF₆)₃, where deeb?=?diethyl-2,2′-bipyridine-4,4′-dicarboxylate and dpp?=?2,3-bis(2-pyridyl)pyrazine. From ¹H NMR spectral data, the two deeb ligands are attached to Ir cis to each other. Mass spectra contain fragmentation patterns of the (M-PF₆)⁺ and (M-3PF₆)³⁺ molecular ions for [Ir(deeb)₂Cl₂](PF₆) and [Ir(deeb)₂(dpp)](PF₆)₃, respectively. The electronic absorption spectrum of [Ir(deeb)₂Cl₂](PF₆) shows maxima at 308?nm and 402?nm, which are assigned as ¹π?→?π* and metal-to-ligand charge transfer transitions, respectively. [Ir(deeb)₂(dpp)](PF₆)₃ exhibits peaks due to ¹π?→?π* transitions at 322?nm and 334?nm. [Ir(deeb)₂Cl₂](PF₆) has emission peaks at 538?nm in acetonitrile and 567?nm in the solid state, with lifetimes of 1.71?µs and 0.35?µs, respectively. [Ir(deeb)₂Cl₂](PF₆) has an unusually higher quantum yield than analogous compounds. [Ir(deeb)₂(dpp)](PF₆)₃ has emission peaks at 540?nm in acetonitrile and 599?nm in the solid state with lifetimes of 1.23?µs and 0.14?µs, respectively. Cyclic voltammetry of [Ir(deeb)₂Cl₂](PF₆) yields two reversible couples at ?0.72 and ?0.87?V versus Ag/AgCl, both corresponding to deeb ligand reductions, and a quasi-reversible couple at ?1.48?V corresponding to Ir^3+/+ reduction. Electrochemical reduction of [Ir(deeb)₂(dpp)](PF₆)₃ yields couples at ?0.38, ?0.54, ?0.71, and ?1.33?V, assigned as deeb^0/?, deeb^0/?, dpp^0/?, and Ir^3+/+ reductions, respectively.     

Dual silane surface functionalization for the selective attachment of human neuronal cells to porous silicon

Porous silicon (pSi) surfaces were chemically micropatterned through a combination of photolithography and surface silanization reactions. This patterning technique produces discretely defined regions on a pSi surface functionalized with a specific chemical functionality, and the surrounding surface displays a completely different functionality. The generated chemical patterns were characterized by a combination of IR microscopy and the conjugation of two different fluorescent organic dyes. Finally, the chemically patterned pSi surface was used to direct the attachment of neuronal cells to the surface. This patterning strategy will be useful for the development of high-throughput platforms for investigating cell behavior.     

Phenylphosphonic acid functionalization of indium tin oxide: surface chemistry and work functions

The work function of indium tin oxide (ITO) substrates was modified with phosphonic acid molecular films. The ITO surfaces were treated prior to functionalization with a base cleaning procedure. The film growth and coverage were quantified by contact angle goniometry and XPS. Film orientation was determined by reflection/absorption infrared spectroscopy using ITO-on-Cr substrates. The absolute work functions of nitrophenyl- and cyanophenyl-phosphonic acid films in ITO were determined by Kelvin probe measurement to be 5.60 and 5.77 eV, respectively.     

Reversible supramolecular functionalization of surfaces: terpyridine ligands as versatile building blocks for noncovalent architectures

We report on the reversible and selective functionalization of surfaces by utilizing supramolecular building blocks. The reversible formation of terpyridine bis-complexes, based on a terpyridine ligand-functionalized monolayer, is used as a versatile supramolecular binding motif. Thereby, click chemistry was applied to covalently bind an acetylene functionalized Fe(II) bis-complex onto azide-terminated self-assembled monolayers. By decomplexation of the formed supramolecular complex, the ligand modified monolayer could be obtained. These monolayers were subsequently used for additional complexation reactions, resulting in the reversible functionalization of the substrates. The proper choice of the coordinating transition metal ions allows the tuning of the binding strength, as well as the physicochemical properties of the formed complexes and thus an engineering of the surface properties.     

Covalent attachment of acetylene and methylacetylene functionality to Si(111) surfaces: scaffolds for organic surface functionalization while retaining Si-C passivation of Si(111) surface sites

Si(111) surfaces have been functionalized with Si-CC-R species, where R = H or -CH3, using a two-step reaction sequence involving chlorination of H-Si(111) followed by treatment with Na-CC-H or CH3-CC-Na reagents. The resulting surfaces showed no detectable oxidation as evidenced by X-ray photoelectron spectroscopic (XPS) data in the Si 2p region, electrochemical measurements of Si-H oxidation, or infrared spectroscopy. The Si-CC-R-terminated surfaces exhibited a characteristic CC stretch in the infrared at 2179 cm-1, which was strongly polarized perpendicular to the Si(111) surface plane. XPS measurements in the C 1s region showed a low binding energy peak indicative of Si-C bonding, with a coverage that was, within experimental error, identical to that of the CH3-terminated Si(111) surface, which has been shown to fully terminate the Si atop sites on an unreconstructed Si(111) surface. The Si-CC-H-terminated surfaces were further functionalized by exposure to n-C4H9Li followed by exposure to para Br-C6H5-CF3, allowing for introduction of para -C6H5CF3 groups while maintaining the desirable chemical and electrical properties that accompany complete Si-C termination of the atop sites on the Si(111) surface.     

Role of the oriented attachment mechanism in the phase transformation of oxide nanocrystals

"Bottom-up" methods to obtain nanocrystals usually result in metastable phases, even in processes carried out at room temperature or under soft annealing conditions. However, stable phases, often associated with anisotropic shapes, are obtained in only a few special cases. In this paper we report on the synthesis of two well-studied oxides-titanium and zirconium oxide-in the nanometric range, by a novel route based on the decomposition of peroxide complexes of the two metals under hydrothermal soft conditions, obtaining metastable and stable phases in both cases through transformation. High-resolution transmission electron microscopy analysis reveals the existence of typical defects relating to growth by the oriented attachment mechanism in the stable crystals. The results suggest that the mechanism is associated to the phase transformation of these structures.     

The role of the medium in electrochemical functionalization and dispersion of carbon nanotubes

An electrochemical method for dispersion of single-walled carbon nanotubes (SWNTs) is described. The technique is based on grafting of oxygen-containing functional groups to the nanotube surface during electrolysis in aqueous and nonaqueous potassium bromide solutions. A dependence of the degree of functionalization of nanotubes on the solvent was revealed experimentally. Nanotubes treated in DMSO have about 14 carbon atoms per oxygen atom from functional groups (cf. nearly four C atoms per oxygen atom in the nanotubes treated in aqueous solutions). The corresponding maximum specific capacities of the electrodes are nearly 10 and 60 F g⁻¹. The samples treated in solutions of KBr in DMSO have about 300 carbon atoms per bromine atom on the nanotube surface (cf. only 30 carbon atoms in the samples treated in aqueous solution). A mechanism of electrochemical modification of SWNTs is proposed. Its key step is production of atomic oxygen that oxidizes the nanotube surface with the formation of functional groups.     

Bistability in Fc-PTM crystals: the role of intermolecular electrostatic interactions

Fc-PTM is a valence tautomeric radical, where the ferrocene (Fc) group, a good electron donor, is linked by an ethylenic spacer to a perchlorotriphenylmethyl radical (PTM(*)), a good electron acceptor. In solution this compound exists mainly in the neutral Fc-PTM(*) form which can be photoexcited through an intramolecular electron transfer to the zwitterionic Fc(+*)-PTM(-) form. By contrast, in crystals of Fc-PTM at room temperature both the neutral and the zwitterionic forms coexist, pointing to a true bistability phenomenon. We rationalize these findings accounting for the role of intermolecular electrostatic interactions in Fc-PTM crystals. In fact the energy of the zwitterionic Fc(+*)-PTM(-) form is lowered in the crystal by attractive electrostatic intermolecular interactions and the cooperative nature of these interactions explains the observed coexistence of neutral Fc-PTM(*) and zwitterionic Fc(+*)-PTM(-) species. The temperature evolution of Mossbauer spectra of Fc-PTM is quantitatively reproduced adopting a bottom-up modeling strategy that combines a molecular model, derived from optical spectra of Fc-PTM in solution, with a model for intermolecular electrostatic interactions, supported by quantum-chemical calculations. Fc-PTM then offers the first experimental demonstration of bistability induced by electrostatic interactions in crystals of valence tautomeric donor-acceptor molecules.     

Solvation dynamics in reverse micelles: the role of headgroup-solute interactions

We present molecular dynamics simulation results for solvation dynamics in the water pool of anionic-surfactant reverse micelles (RMs) of varying water content, w(0). The model RMs are designed to represent water/aerosol-OT/oil systems, where aerosol-OT is the common name for sodium bis(2-ethylhexyl)sulfosuccinate. To determine the effects of chromophore-headgroup interactions on solvation dynamics, we compare the results for charge localization in model ionic diatomic chromophores that differ only in charge sign. Electronic excitation in both cases is modeled as charge localization on one of the solute sites. We find dramatic differences in the solvation responses for anionic and cationic chromophores. Solvation dynamics for the cationic chromophore are considerably slower and more strongly w(0)-dependent than those for the anionic chromophore. Further analysis indicates that the difference in the responses can be ascribed in part to the different initial locations of the two chromophores relative to the surfactant interface. In addition, slow motion of the cationic chromophore relative to the interface is the main contributor to the longer-time decay of the solvation response to charge localization in this case.     

Formation of primary amines on silicon nitride surfaces: a direct, plasma-based pathway to functionalization

Silicon nitride is the most commonly used passivation layer in biosensor applications where electronic components must be interfaced with ionic solutions. Unfortunately, the predominant method for functionalizing silicon nitride surfaces, silane chemistry, suffers from a lack of reproducibility. As an alternative, we have developed a silane-free pathway that allows for the direct functionalization of silicon nitride through the creation of primary amines formed by exposure to a radio frequency glow discharge plasma fed with humidified air. The aminated surfaces can then be further functionalized by a variety of methods; here we demonstrate using glutaraldehyde as a bifunctional linker to attach a robust NeutrAvidin (NA) protein layer. Optimal amine formation, based on plasma exposure time, was determined by labeling treated surfaces with an amine-specific fluorinated probe and characterizing the coverage using X-ray photoelectron spectroscopy (XPS). XPS and radiolabeling studies also reveal that plasma-modified surfaces, as compared with silane-modified surfaces, result in similar NA surface coverage, but notably better reproducibility.     

Thermal relaxation mechanism and role of chemical functionalization in fullerene solutions

Using molecular-dynamics simulations we investigate thermal relaxation of C60 and C84 molecules suspended in octane liquid. Pristine fullerenes exhibit relatively slow relaxation due to weak thermal coupling with the liquid. A comparison of the interfacial transport characteristics obtained from relaxation simulations with those obtained from equilibrium simulations and fluctuation-dissipation theorem analysis demonstrates that the relaxation process involves two main steps: (i) energy flow from high- to low-frequency modes within the fullerene, and (ii) energy flow from low-frequency fullerene modes to the liquid. Functionalization of fullerenes with alkene chains leads to significant reduction of the thermal relaxation time. The relaxation time of functionalized fullerenes becomes independent from the functionalizing chain length beyond approximately 10 carbon segments; this can be understood in terms of thermal conductivity along the chain and heat transfer between the chain and the solvent.