Mechanisms of Aggregation of Cysteine Functionalized Gold Nanoparticles

The interaction of gold nanoparticles (AuNPs) with cysteine and its derivatives is the basis of a number of bionanotechnologies, and for these, the most important process is aggregation (or antiaggregation), which enables an array of colorimetric detection methods. When AuNPs were functionalized with cysteine, its dimer cystine, or the cysteine-derived tripeptide, glutathione, three different mechanisms of aggregation were observed. Both cysteine and glutathione induced aggregation of AuNPs without further pH modification: the first by interparticle zwitterionic interaction and the second by interparticle hydrogen bonding. Cystine, however, did not induce aggregation, although it dissociated into two cysteinate moieties upon adsorption on the AuNPs, which appear to be chemically identical to cysteinate produced from cysteine adsorption. We show that the difference is due to the lower coverage of cysteinate from cystine and differences in charge states of the adsorbates. On modifying the pH to 1.5, the surface species become cationic (neutral COOH and protonated NH3 +), and aggregation of cystine/AuNPs occurs immediately by interparticle hydrogen bonding. Thus, cysteine may induce aggregation by neutral hydrogen bonding or zwitterionic interaction between nanoparticles, but the mechanism depends sensitively on a number of parameters

Surface study of metal-containing ionic liquids by means of photoemission and absorption spectroscopies

The vacuum/liquid interface of different ionic liquids obtained by dissolving bistriflimide salts of Ag, Al, Cu, Ni, and Zn in 1-butyl-3-methylimidazolium bistriflimide ([bmim][Tf2N]) was investigated under vacuum using AR-XPS and NEXAFS. The XPS spectra show chemical shifts of the nitrogen of the bistriflimide anion as a function of the metal type, indicating different strength of the coordination bonds. In silver bearing ILs, silver ions were found to be only weakly coordinated. On the contrary, Ni, Cu, Zn, and especially Al exhibit large chemical shifts attributable to strong interaction with the bistriflimide ions. The outermost surface was enriched with or depleted of metal ions as a function of the nature of the metals. Nickel and zinc tend to slightly concentrate at the surface while copper, silver, and especially aluminum are depleted at the surface. We also observed that the aliphatic alkyl chains of the cations tend to protrude outside the surface in all systems studied. However, the presence of metals generally increases the amount of bistriflimide at the vacuum/liquid interface

Functionalization of nanostructured cerium oxide films with histidine

The surfaces of polycrystalline cerium oxide films were modified by histidine adsorption under vacuum and characterized by the synchrotron based techniques of core and valence level photoemission, resonant photoemission and near edge X-ray absorption spectroscopy, as well as atomic force microscopy. Histidine is strongly bound to the oxide surface in the anionic form through the deprotonated carboxylate group, and forms a disordered molecular adlayer. The imidazole ring and the amino side group do not form bonds with the substrate but are involved in the intermolecular hydrogen bonding which stabilizes the molecular adlayer. The surface reaction with histidine results in water desorption accompanied by oxide reduction, which is propagated into the bulk of the film. Previously studied, well-characterized surfaces are a guide to the chemistry of the present polycrystalline surface and histidine bonds via the carboxylate group in both cases. In contrast, bonding via the imidazole group occurs on the well-ordered surface but not in the present case. The morphology and structure of the cerium oxide are decisive factors which define the adsorption geometry of the histidine adlayer

Structures of Cycloserine and 2‑Oxazolidinone Probed by X‑ray Photoelectron Spectroscopy: Theory and Experiment

The electronic structures and properties of 2-oxazolidinone and the related compound cycloserine (CS) have been investigated using theoretical calculations and core and valence photoelectron spectroscopy. Isomerization of the central oxazolidine heterocycle and the addition of an amino group yield cycloserine. Theory correctly predicts the C, N, and O 1s core spectra, and additionally, we report theoretical natural bond orbital (NBO) charges. The valence ionization energies are also in agreement with theory and previous measurements. Although the lowest binding energy part of the spectra of the two compounds shows superficial similarities, further analysis of the charge densities of the frontier orbitals indicates substantial reorganization of the wave functions as a result of isomerization. The highest occupied molecular orbital (HOMO) of CS shows leading carbonyl π character with contributions from other heavy (non-H) atoms in the molecule, while the HOMO of 2-oxazolidinone (OX2) has leading nitrogen, carbon, and oxygen pπ characters. The present study further theoretically predicts bond resonance effects of the compounds, evidence for which is provided by our experimental measurements and published crystallographic data

Electron Hopping between Fe 3 d States in Ethynylferrocene-doped Poly(Methyl Methacrylate)-poly(Decyl Methacrylate) Copolymer Membranes

Synchrotron radiation-valence band spectroscopy (SR-VBS) has been utilized in a study of redox molecule valence states implicated in the electron hopping mechanism of ethynylferrocene in unplasticized poly (methyl methacrylate)-poly(decyl methacrylate) [PMMAPDMA] membranes. In this communication, it is revealed that, at high concentrations of ethynylferrocene, there are observable Fe 3d valence states that are likely linked to electron hopping between ferrocene moieties of neighbouring redox molecules. Furthermore, electrochemically induced stratification of ethynylferrocene in an oxidized PMMA-PDMA membrane produces a gradient of Fe 3d states toward the buried interface at the glassy carbon/ PMMA-PDMA membrane enabling electron hopping and electrochemical reactivity of dissolved ethynylferrocene across this buried film

Mechanisms of Aggregation of Cysteine Functionalized Gold Nanoparticles

The interaction of gold nanoparticles (AuNPs) with cysteine and its derivatives is the basis of a number of bionanotechnologies, and for these, the most important process is aggregation (or antiaggregation), which enables an array of colorimetric detection methods. When AuNPs were functionalized with cysteine, its dimer cystine, or the cysteine-derived tripeptide, glutathione, three different mechanisms of aggregation were observed. Both cysteine and glutathione induced aggregation of AuNPs without further pH modification: the first by interparticle zwitterionic interaction and the second by interparticle hydrogen bonding. Cystine, however, did not induce aggregation, although it dissociated into two cysteinate moieties upon adsorption on the AuNPs, which appear to be chemically identical to cysteinate produced from cysteine adsorption. We show that the difference is due to the lower coverage of cysteinate from cystine and differences in charge states of the adsorbates. On modifying the pH to 1.5, the surface species become cationic (neutral COOH and protonated NH₃⁺), and aggregation of cystine/AuNPs occurs immediately by interparticle hydrogen bonding. Thus, cysteine may induce aggregation by neutral hydrogen bonding or zwitterionic interaction between nanoparticles, but the mechanism depends sensitively on a number of parameters

A Close Look at the Structure of the TiO2APTES Interface in Hybrid Nanomaterials and Its Degradation Pathway: An Experimental and Theoretical Study

[Image: see text] The surface functionalization of TiO(2)-based materials with alkylsilanes is attractive in several cutting-edge applications, such as photovoltaics, sensors, and nanocarriers for the controlled release of bioactive molecules. (3-Aminopropyl)triethoxysilane (APTES) is able to self-assemble to form monolayers on TiO(2) surfaces, but its adsorption geometry and solar-induced photodegradation pathways are not well understood. We here employ advanced experimental (XPS, NEXAFS, AFM, HR-TEM, and FT-IR) and theoretical (plane-wave DFT) tools to investigate the preferential interaction mode of APTES on anatase TiO(2). We demonstrate that monomeric APTES chemisorption should proceed through covalent Si–O–Ti bonds. Although dimerization of the silane through Si–O–Si bonds is possible, further polymerization on the surface is scarcely probable. Terminal amino groups are expected to be partially involved in strong charge-assisted hydrogen bonds with surface hydroxyl groups of TiO(2), resulting in a reduced propensity to react with other species. Solar-induced mineralization proceeds through preferential cleavage of the alkyl groups, leading to the rapid loss of the terminal NH(2) moieties, whereas the Si-bearing head of APTES undergoes slower oxidation and remains bound to the surface. The suitability of employing the silane as a linker with other chemical species is discussed in the context of controlled degradation of APTES monolayers for drug release and surface patterning

A close look at the structure of the TiO2-APTES interface in hybrid nanomaterials and its degradation pathway: an experimental and theoretical study

[Image: see text] The surface functionalization of TiO(2)-based materials with alkylsilanes is attractive in several cutting-edge applications, such as photovoltaics, sensors, and nanocarriers for the controlled release of bioactive molecules. (3-Aminopropyl)triethoxysilane (APTES) is able to self-assemble to form monolayers on TiO(2) surfaces, but its adsorption geometry and solar-induced photodegradation pathways are not well understood. We here employ advanced experimental (XPS, NEXAFS, AFM, HR-TEM, and FT-IR) and theoretical (plane-wave DFT) tools to investigate the preferential interaction mode of APTES on anatase TiO(2). We demonstrate that monomeric APTES chemisorption should proceed through covalent Si–O–Ti bonds. Although dimerization of the silane through Si–O–Si bonds is possible, further polymerization on the surface is scarcely probable. Terminal amino groups are expected to be partially involved in strong charge-assisted hydrogen bonds with surface hydroxyl groups of TiO(2), resulting in a reduced propensity to react with other species. Solar-induced mineralization proceeds through preferential cleavage of the alkyl groups, leading to the rapid loss of the terminal NH(2) moieties, whereas the Si-bearing head of APTES undergoes slower oxidation and remains bound to the surface. The suitability of employing the silane as a linker with other chemical species is discussed in the context of controlled degradation of APTES monolayers for drug release and surface patterning

Bonding of Histidine to Cerium Oxide

Adsorption of histidine on cerium oxide model surfaces was investigated by synchrotron radiation photoemission, resonant photoemission, and near edge X-ray absorption fine structure spectroscopies. Histidine was evaporated in a vacuum onto ordered stoichiometric CeO₂(111) and partially reduced CeO_1.9 thin films grown on Cu(111). Histidine binds to CeO₂ in anionic form via the carboxylate group and all three nitrogen atoms, with the imidazole ring parallel to the surface. The amino nitrogen atom of the imidazole ring (IM) is deprotonated, and both IM nitrogen atoms form strong bonds via π orbitals, while the α-amino nitrogen interacts with the oxide via its hydrogen atoms. In the case of CeO_1.9, the deprotonation of the amino nitrogen of the imidazole ring is less pronounced and N K-edge spectra do not show a clear orientation of the ring with respect to the surface. A minor reduction of the cerium surface on adsorption of histidine was observed and explained by charge exchange as a result of hybridization of the π orbitals of the IM ring with the f and d orbitals of ceria. Knowledge of histidine adsorption on the cerium oxide surface can be used for design of mediator-less biosensors where the histidine-containing proteins can be strongly bound to the oxide surface via the imidazole side chain of this residue