X-ray Photoelectron Spectroscopy Experiments Research Articles

Enzymatically modified peptide surfaces: towards general electrochemical sensor platform for protein kinase catalyzed phosphorylations

We hereby present an electrochemical approach for monitoring the three protein kinases sarcoma-related kinase (Src), extracellular signal-regulated kinase 1 (Erk1), and cyclin A-dependent kinase 2 (CDK2/cyclin A). The electrochemical sensor is based on the ability of kinases to transfer a redox-labeled phosphoryl group to surface-bound peptides that are highly specific substrates for the particular protein kinase (EGIYDVP, EPLTPSG, and HHASPRK, respectively). The detection method relies on the use of 5'-γ-ferrocenoyl-ATP (Fc-ATP) as a co-substrate for peptide phosphorylation. The peptides themselves are attached to a Au substrate, which acts as the working electrode. In this process a Fc-phosphoryl group is transferred to the peptide and the presence of the redox active Fc group is detected electrochemically. All peptide films were fully characterized by cyclic voltammetry (CV), square wave voltammetry (SWV), and electrochemical impedance spectroscopy (EIS). Particular attention was given to the electron transfer rates, k(ET), in peptide films after Fc-phosphorylation which were found to be on the order of seconds. The slow ET kinetics is presumably a result of the negative charge on the phosphoryl group. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) experiments based on the peptide modified Au surfaces reveal significant ferrocene and phosphate group content introduced using the kinase-catalyzed phosphorylation reaction.

Changes of the near-surface chemical composition of the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide room temperature ionic liquid under the influence of irradiation

Radiation induced degradation effects are studied for a model ionic liquid (IL)--[EMIm]Tf(2)N--in order to distinguish in which way the results of X-ray based material analysis methods can be falsified by the radiation supplied by typical X-ray sources itself. Photoelectron spectroscopy is commonly used for determining the electronic structure of ionic liquids. Degradation effects, which often occur e.g. in organic materials during X-ray or electron irradiation, are potentially critical for the interpretation of data obtained from ionic liquids. The changes of the chemical composition as well as the radiation-induced desorption of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIm]Tf(2)N) fragments are analysed by X-ray photoelectron spectroscopy (XPS) as well as quadrupole mass spectroscopy (QMS) upon exposure to monochromated or non-monochromated AlKα X-rays from typical laboratory sources. During the irradiation of [EMIm]Tf(2)N, an increasing carbon concentration is observed in both cases and especially the [Tf(2)N](-) ion is strongly altered. This observation is supported by the results from the QMS analysis which revealed a variety of different IL fragments that are desorbed during X-ray irradiation. It is shown that the decomposition rate is directly linked to the photon flux on the sample and hence has to be considered when planning an XPS experiment. However, for typical experiments on this particular IL the measurements suggest that the changes are on a larger time scale as typically required for spectra acquisition, in particular if monochromated X-ray sources are used.

Photoelectron spectroscopy and atomic force microscopy study of 1,2-dicyano-methanofullerene C 60(CN) 2 thin film for photovoltaic applications

1, 2-dicyano-methanofullerene (C 60(CN) 2) is a soluble fullerene derivative that has been reported to have stronger electron affinity than parent C 60. Ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) experiments were carried out on C 60(CN) 2 thin films spin coated on heavily doped n-type Si substrate. UPS spectra enabled the determination of the vacuum shift at the fullerene derivative/Si interface and the onset of the highest occupied molecular orbital (HOMO). From the UV–vis absorption spectra of C 60(CN) 2 thin films spin coated on quartz substrates, the optical band gap ( E g ) and the onset of absorption were determined. These measurements allowed the determination of the lowest occupied molecular orbital (LUMO) position. The morphology of the deposited film was probed by AFM and reveals non-uniformity of the thin film. Open circuit voltage ( V oc ) measurements on P3HT/C 60(CN) 2 based organic solar cell device are compared to the commonly used P3HT/PCBM device.

Electric properties of aqueous grown ZnO nanorods on Au/Ti/Si substrates

AbstractElectric properties of aqueous grown ZnO nanorods were investigated. Vertical current measurement through the top 20‐nm‐thick Au electrode/ZnO nanorods/bottom Au film structure showed asymmetric I‐V properties and was discussed in terms of different interface properties between ZnO nanorods and Au. Schottky barrier height in top Au electrode and ZnO nanorods was estimated to be 0.22eV based on core level X‐ray photoelectron spectroscopy (XPS) spectra. XPS experiment also suggests that aqueous grown ZnO nanorods have Zn‐polarity and that might be one of the reasons of lower barrier height. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Adsorption of Dibenzothiophene on Ag/Cu/Fe-Supported Activated Carbons Prepared by Ultrasonic-Assisted Impregnation

This work is mainly involved with study of the adsorption of dibenzothiophene (DBT) on Ag/Cu/Fe-supported activated carbons (ACs) prepared by ultrasound-assisted impregnation (UI). Ag/AC-UI, Cu/AC-UI, and Fe/AC-UI were separately prepared by using impregnation of different metal salt solutions under ultrasound irradiation (UIr) and characterized by X-ray photoelectron spectroscopy (XPS) and CO-chemisorption experiments. Isotherms of DBT on the modified ACs were separately measured, and the effects of different metal ions loaded on the ACs on the amount of adsorbed DBT on the modified ACs are discussed. Results indicated that (1) the DBT adsorption capacity of the ACs modified by using impregnation without UIr followed the order: Ag/AC > Cu/AC > AC > Fe/AC; (2) the modification methods have significant influence on the adsorption abilities of the modified ACs toward DBT. The application of UIr to prepare metal ion-impregnated ACs can make the metallic particles on the carbon surfaces become finer and dispe...

Excitation processes of trivalent cerium ions in calcium thiogallate crystals by hot photocarriers

Excitation spectra for the 5d → 4f emission of Ce3+ ions in CaGa2S4 crystals have been measured at 300 K by using vacuum ultraviolet (VUV) photons, together with reflectivity and absorption-edge spectra. X-ray photoelectron spectroscopy (XPS) experiment has also been performed in order to clarify the contribution of shallow core states to the excitation and reflectivity spectra. Furthermore, cluster calculation by a relativistic discrete variational Xα (DV-Xα) method has been carried out to elucidate the optical transitions resulting in the creation of multiple e-h pairs in CaGa2S4. The excitation processes of Ce3+ ions in CaGa2S4 by hot photocarriers are discussed on the basis of the electronic structure determined from the experiments and calculation.

Au-NPs enhanced SPR biosensor based on hairpin DNA without the effect of nonspecific adsorption

Gold nanoparticles (Au-NPs) are usually used to amplify surface plasmon resonance (SPR) signals, however, the serious nonspecific adsorption has largely limited their practical applications. Here, we developed a novel Au-NPs enhanced biosensor without the effect of nonspecific adsorption: cutting Au-NPs off from the biosensor surface by RsaI endonuclease. In order to further improve the sensitivity, the probe DNA was designed specially. After the cleavage reaction, the residual probe DNA formed hairpin structure, which also resulted in a great change in SPR dip shift. Then, with the coaction of Au-NPs and conformation change of probe DNA, the SPR signal was amplified greatly. Using this method, we monitored the process of DNA cleavage in real-time and achieved a detection level of 5 × 10 −8 M. Moreover, the result of X-ray photoelectron spectroscopy (XPS) experiment further confirmed that large nonspecific adsorption existed. However, because SPR recorded a process in which the Au-NPs were cut off, the serious nonspecific adsorption did not affect the experimental result.

Quantum confinement, core level shifts, and dopant segregation in P-dopedSi⟨110⟩nanowires

We examine P-doped $\text{Si}⟨110⟩$ nanowires by employing a real-space pseudopotential method. We find the defect wave function becomes more localized along the nanowire axis and the donor ionization energy increases, owing to quantum confinement. It is more difficult to dope a P atom into a $\text{Si}⟨110⟩$ nanowire than to dope Si bulk because the formation energy increases with decreasing size. By comparing the formation energy for different P positions within a nanowire, we find that if a P atom at the nanowire surface can overcome the energy barrier close to the surface, there is a tendency for the dopant to reside within the nanowire core. We calculate P core levels shift as P changes position within the nanowire and provide a means for x-ray photoelectron spectroscopy experiments to determine the location of P atoms within a Si nanowire.

Infrared spectroscopic ellipsometry (IRSE) and X‐ray photoelectron spectroscopy (XPS) monitoring the preparation of maleimide‐functionalized surfaces: from Au towards Si (111)

AbstractThe IR ellipsometric technique was used to identify the surface species and to control the preparation of maleimide‐terminated surfaces. Because of higher s/n ratios for metallic substrates, the protocol was initially developed on Au surfaces, was later successfully transferred to technologically more relevant Si (111) substrates. The functionalized surfaces were achieved by electrochemical deposition of diazonium linker films and following chemical adsorption steps. Complementary XPS was also employed to detect the surface species in the process of preparation. The immobilization of different functional molecules was proven by interpreting the specific vibrational bands in IR spectra and additionally confirmed by XPS experiments. The surface homogeneity was investigated by FT‐IR synchrotron mapping ellipsometry. This work shows that the proposed protocol is an effective pathway to achieve the desired functionalized surfaces. Copyright © 2010 John Wiley & Sons, Ltd.

Surface chemistry and physics of deuterium retention in lithiated graphite

Lithium wall conditioning in TFTR, CDX-U, T-11M, TJ-II and NSTX is found to yield enhanced plasma performance manifest, in part, through improved deuterium particle control. X-ray photoelectron spectroscopy (XPS) experiments examine the affect of D irradiation on lithiated graphite and show that the surface chemistry of lithiated graphite after D ion bombardment (500eV/amu) is fundamentally different from that of non-Li conditioned graphite. Instead of simple LiD bonding seen in pure liquid Li, graphite introduces additional complexities. XPS spectra show that Li–O–D (533.0±0.6eV) and Li–C–D (291.4±0.6eV) bonds, for a nominal Li dose of 2μm, become “saturated” with D at fluences between 3.8 and 5.2×1017cm−2. Atomistic modeling indicate that Li–O–D–C interactions may be a result of multibody effects as opposed to molecular bonding.

Molar concentration–depth profiles at the solution surface of a cationic surfactant reconstructed with angle resolved X-ray photoelectron spectroscopy

In the current work, we first reconstructed the molar fraction–depth profiles of cation and anion near the surface of tetrabutylammonium iodide dissolved in formamide by a refined calculation procedure, based on angle resolved X-ray photoelectron spectroscopy experiments. In this calculation procedure, both the transmission functions of the core levels and the inelastic mean free paths of the photoelectrons have been taken into account. We have evaluated the partial molar volumes of surfactant and solvent by the densities of such solutions with different bulk concentrations. With those partial molar volumes, the molar concentration–depth profiles of tetrabutylammonium ion and iodide ion were determined. The surface excesses of both surfactant ions were then achieved directly by integrating these depth profiles. The anionic molar concentration–depth profiles and surface excesses have been compared with their counterparts determined by neutral impact ion scattering spectroscopy. The comparisons exhibit good agreements. Being capable of determining molar concentration–depth profiles of surfactant ions by core levels with different kinetic energies may extend the applicable range of ARXPS in investigating solution surfaces.

Photocatalyzed Surface Modification of Poly(dimethylsiloxane) with Polysaccharides and Assay of Their Protein Adsorption and Cytocompatibility

An improved approach for the surface modification of poly(dimethylsiloxane) (PDMS) using carboxymethyl cellulose (CMC), carboxymethyl beta-1,3-dextran (CMD), and alginic acid (AA) was investigated. The PDMS substrates were first oxidized in a H(2)SO(4)/H(2)O(2) solution to transform the Si-CH(3) groups on their surfaces into Si-OH groups. Then methacrylate groups were grafted onto the substrates through a silanization reaction using 3-(trimethoxysilyl)propyl methacrylate. Sequentially, cysteamine was conjugated onto the silanized surfaces by the reaction between the thiol and methacrylate groups under 254 nm UV exposure. Afterward, the amino-terminated PDMS substrates were sequentially reacted with CMC, CMD, and AA in the presence of N-hydroxysuccinimide and 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide, resulting in the grafting of polysaccharides onto PDMS surfaces. The composition and chemical state of the modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS). In addition, the stability and dynamic characteristics of the polysaccharide-grafted PDMS substrates were investigated by XPS and temporal contact angle experiments. A protein adsorption assay using bovine serum albumin (BSA), chicken egg albumin, lysozyme, and RNase-A showed that the introduction of CMD and AA can reduce the adsorption of negatively charged BSA and chicken egg albumin, but increase the adsorption of the positively charged lysozyme and RNase-A. However, CMC-modified PDMS surfaces showed protein-repelling properties, regardless of whether the protein was positively or negatively charged. A cell culture and migration study of glioma C6, MKN-45, MCF-7, and HepG-2 cells revealed that the polysaccharide-modified PDMS greatly improved the cytocompatibility of native PDMS.

An in situ examination of atomic layer deposited alumina/InAs(100) interfaces

Undoped InAs(100) wafers were either passivated with sulfur from a (NH4)2Sx solution or etched with NH4OH and then characterized with monochromatic x-ray photoelectron spectroscopy (XPS) before and after in situ deposition of Al2O3 by atomic layer deposition. Sulfur passivation minimized oxidation. Trimethyl aluminum (TMA) exposure reduced trivalent indium and arsenic oxidation states. The In1+ chemical state persisted while elemental arsenic remained at the Al2O3/InAs interface prior to TMA exposure and possibly a mixture of As–As and As–Al bonds were present afterwards. The In 3d5/2 peak line shape from bulk InAs differed from previous XPS experiments on epitaxial InxGa1−xAs.

Controlled stepwise growth of siloxane chains using bivalent building units with different functionalities

Organic/inorganic hybrids of silicon and their subsequent chemical modification are of interest for tailoring and structuring surfaces on the nanoscale. The formation of organic (sub)monolayers on hydroxylated silicon surfaces was employed to synthesize molecular siloxane chains by stepwise wet-chemical condensation reactions. Functionalizations with small alkyl-monochlorosilanes, yielding dimethylsilyl, diisopropylsilyl, di- tert-butylsilyl terminations, and with an aryl-monochlorosilane, providing diphenylsilyl groups, were studied. The SiH moiety of these end groups could be selectively oxidized to the corresponding silanol. The reactivity of this group opened the possibility of bonding additional species step by step in bottom-up growth reactions. In particular, dimethylsiloxane units were studied as possible molecular building blocks. By repeating the oxidation and condensation reactions the stepwise growth of one-dimensional siloxane chains was demonstrated. The ongoing chain growth was characterized by attenuated total reflection Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, spectroscopic ellipsometry (NIR–UV), and contact-angle experiments.

Characteristics of alkali-resistant Ni/MgAl 2O 4 catalyst for direct internal reforming molten carbonate fuel cell

Direct internal reforming – molten carbonate fuel cell (DIR–MCFC) has advantages of higher efficiency and smaller size. However, deactivation of the catalyst by alkali carbonate electrolytes poses a significant problem in MCFC. To solve this problem, Ni/MgO and Ni/MgAl 2O 4 catalysts were compulsively mixed with a eutectic mixture of Li 2CO 3 and Na 2CO 3 prior to a methane steam reforming activity test. Activity of Ni/MgO rapidly decreased, while that of Ni/MgAl 2O 4 remained steady due to good alkali resistance. To analyze the effects of alkali addition, N 2 adsorption-desorption, X-ray diffraction, temperature-programmed reduction and oxidation, scanning electron microscopy, and X-ray photoelectron spectroscopy experiments were carried out. Both Ni/MgO and Ni/MgAl 2O 4 showed sintering of Ni and blocking of pores, which reduced the catalytic activity. However, Ni/MgAl 2O 4 showed other positive effects such as stronger metal–support interaction and increased dissociative adsorption.

Molecular, Electronic, and Crystal Structures of Self-Assembled Hydrothermally Synthesized Zn(II)−Mercaptonicotinate: A Combined Spectroscopic and Theoretical Approach

A Zn(II) 2-mercaptonicotinate coordination polymer (Zn1), with Zn(II) ions chelated by both sulfur and oxygen in a distorted square pyramidal environment, and a molecular Zn(II) 2-hydroxynicotinate complex (Zn2) were synthesized by the reaction of zinc acetylacetonate with 2-mercaptonicotinic (Zn1) and 2-hydroxynicotinc (Zn2) acid, respectively, under hydrothermal conditions. The crystal structures of Zn1 and Zn2 were determined by single crystal X-ray diffraction measurements. Dispersion-corrected density functional theory (DFT) calculations reproduce very well the experimental structures and show that Zn1 is stable against hydration, whereas Zn2 is stable against dehydration over wide ranges of temperature and pressure, in agreement with thermogravimetric analysis results. The electronic structure of the two compounds is computed with the DFT+U method. The theoretical valence band agrees well with the X-ray photoelectron spectroscopy experiments. Furthermore, the band gap of Zn1 is found to be narrower than that of Zn1 and is characterized by the presence of sulfur lone pairs at the edge of the valence band.

Analysis of damaged region of carbon implanted alumina

Implantation was performed on surface-polished and thermal-treated alumina discs with 75 keV monocharged carbon ions at doses of 1 x 10(17) and 5 x 10(17) ions cm(-2). The alumina targets were kept at room temperature. The structural modifications induced during ion irradiation were studied by the scanning and scanning transmission electron microscopes. Alumina is readily amorphized at room temperature with carbon ions. The width of the ion-beam induced disordered area increases with ion dose. As it was established by selected area electron diffraction and electron energy loss spectra, low implanted specimens still show subsurface crystalline areas whose diffraction patterns could be assigned to a deformed structure of alumina. This deformed belt is shown beneath the amorphous region on highly implanted samples. Electron energy loss spectroscopy allows to follow the carbon line profile from surface into the bulk material, pointing the maximum concentration of implanted ion on the lower half of the amorphous region. Structural changes of Al(2)O(3) caused by carbon ion irradiation were corroborated with X-ray photoelectron spectroscopy. The X-ray photoelectron spectroscopy experiments showed that the oxygen concentration in the vicinity of the implanted alumina surface was reduced suggesting that this element was selectively sputtered by carbon irradiation. As a function of depth, peaks referring to Al-O bindings diminish while contributions of reduced aluminium and metal carbides were found, reaching the corresponding maxima at a depth of about 130 nm.

Note: A combined aerodynamic lens/ambient pressure x-ray photoelectron spectroscopy experiment for the on-stream investigation of aerosol surfaces

We discuss a new approach for the measurement of the surfaces of free aerosol particles with diameters from 50 to 1000 nm. Particles in this size range have significant influence on the heterogeneous chemistry in the atmosphere and affect human health. Interfacing an aerodynamic lens to an ambient pressure x-ray photoelectron spectrometer permits measurement of the surface chemical composition of unsupported aerosol particles in real time. We discuss the basic considerations for the design of such an instrument, its current limitations and potentials for improvement. Results from a proof-of-principle experiment on silicon oxide particles with average diameters of 270 nm are shown.

The electronic structure of UCoGe by ab initio calculations and XPS experiment

The crystal and electronic structures of the orthorhombic compound UCoGeare presented and discussed. It has been either refined by the x-ray diffractionon a single crystal or computed within the local spin density functional theory,employing the fully relativistic version of the full-potential local-orbital band structurecode, respectively. We particularly give our attention to investigating the Fermisurface and de Haas–van Alphen quantities of UCoGe. The calculated electronicdensity is then examined by x-ray photoelectron spectroscopy (XPS). Fairly goodagreement is achieved between theoretical and experimental XPS results in theparamagnetic state. A small difference in the position (in energy scale) of the U 5fbands is caused by the electron localization effect observed in the experimentalXPS. There is also some discrepancy for the Co 3d electron contributions belowEF. The Fermi surface in the non-magnetic state is of a semimetallic typewhile that in the ferromagnetic state, with the ordered moment of−0.47 μB/f.u. alongthe c axis, is more metallic, with nesting properties that may favour superconductivity.

Non-destructive depth compositional profiles by XPS peak-shape analysis

The measured peak shape and intensity of the photoemitted signal in X-ray photoelectron spectroscopy (XPS) experiments (elastic and inelastic parts included) are strongly correlated, through electron-transport theory, with the depth distribution of photoelectron emitters within the analyzed surface. This is the basis of so-called XPS peak-shape analysis (also known as the Tougaard method) for non-destructive determination of compositional in-depth (up to 6-8 nm) profiles. This review describes the theoretical basis and reliability of this procedure for quantifying amounts and distributions of material within a surface. The possibilities of this kind of analysis are illustrated with several case examples related to the study of the initial steps of thin-film growth and the modifications induced in polymer surfaces after plasma treatments.