Andrew G. Thomas

Dopamine Adsorption on Anatase TiO2(101): A Photoemission and NEXAFS Spectroscopy Study

The adsorption of dopamine onto an anatase TiO(2)(101) single crystal has been studied using photoemission and NEXAFS techniques. Photoemission results suggest that the dopamine molecule adsorbs on the surface in a bidentate geometry, resulting in the removal of band gap states in the TiO(2) valence band. Using the searchlight effect, carbon K-edge NEXAFS spectra indicate that the phenyl rings in the dopamine molecules are orientated normal to the surface. A combination of experimental and computational results indicates the appearance of new unoccupied states arising following adsorption. The possible role of these states in the charge-transfer mechanism of the dopamine-TiO(2) system is discussed.

Surface characterization of zirconia dental implants

OBJECTIVES The aim of the study was to characterize the chemical composition, microstructure and roughness of two commercially available zirconia dental implants (WhiteSky and Zit-Z). METHODS The chemical composition of the cervical collar and threaded root parts of the implants (n=2) were studied by XPS and HV-EDX. LV-SEM was used for morphological assessment, Raman microanalysis for microstructural characterization and optical profilometry for surface roughness measurements. XRD, HV-EDX and Raman microanalysis of bulk regions (longitudinal sections) were used as reference. RESULTS XPS showed the presence of C, O, Zr and Y (collar) plus Al (root) at implant surfaces. More C (10-26at%) and a lower Al/Zr ratio were found in WhiteSky (1.05 vs 1.26 in Zit-Z). Zr, Y and Al were detected in single, fully oxidized states. The same elements, plus Hf, were identified by HV-EDX at bulk and surface regions, with a Al/Zr ratio higher in WhiteSky (0.17 vs 0.09 in Zit-Z). Na, K and Cl contaminants were traced at implant root parts by both methods. XRD analysis of cross-sectioned specimens revealed the presence of monoclinic and tetragonal zirconia along with cubic yttria phases. Raman microanalysis showed that the monoclinic zirconia volume fraction was higher at root surfaces than the collar. No monoclinic phase was found at bulk regions. Significantly higher Sa and Sq values were recorded in WhiteSky than Zit-Z, whereas Zit-Z showed higher Rt value. SIGNIFICANCE The differences found between the implants in the extent of carbon contamination, residual alumina content, tetragonal to monoclinic ZrO(2) phase transformation and 3D-roughness parameters may contribute to a substantial differentiation in the cellular and tissue response.

Preparation of Ligand-Free TiO2 (Anatase) Nanoparticles through a Nonaqueous Process and Their Surface Functionalization

We here present a new method for preparing ligand-free titania nanoparticles, which are easily amenable to surface functionalization in an aqueous environment. The specific advantage of this method is that it combines the advantages of nonaqueous synthetic processes (high crystallinity) to those of a surface functionalization in a water medium, which allows for a wider variety of biofunctional (and nonorganic-soluble) groups to be added on the nanoparticles. In particular, we report on the characterization of the three phases of synthesis, dispersion in water environment and surface functionalization of the nanoparticles, focusing on a qualitative evaluation of the surface adsorption mechanism.

Photoemission and HREELS study of K adsorption on TiO2(100)

The adsorption of K and coadsorption of O2 on TiO2(100) have been studied using photoemission and high resolution electron energy-loss spectroscopy (HREELS). We compare our results with those of earlier studies of potassium oxides. The energy separations of peaks appearing in photoemission spectra indicate the formation of a peroxide or superoxide at the surface following adsorption of > 1 monolayer of potassium. HREELS results are consistent with this conclusion.

Nanostructured Aptamer-Functionalized Black Phosphorus Sensing Platform for Label-Free Detection of Myoglobin, a Cardiovascular Disease Biomarker

We report the electrochemical detection of the redox active cardiac biomarker myoglobin (Mb) using aptamer-functionalized black phosphorus nanostructured electrodes by measuring direct electron transfer. The as-synthesized few-layer black phosphorus nanosheets have been functionalized with poly-l-lysine (PLL) to facilitate binding with generated anti-Mb DNA aptamers on nanostructured electrodes. This aptasensor platform has a record-low detection limit (∼0.524 pg mL(-1)) and sensitivity (36 μA pg(-1) mL cm(-2)) toward Mb with a dynamic response range from 1 pg mL(-1) to 16 μg mL(-1) for Mb in serum samples. This strategy opens up avenues to bedside technologies for multiplexed diagnosis of cardiovascular diseases in complex human samples.

Photoemission studies of single crystal CuO(100)

Photoemission studies of a cleaved CuO single crystal have been carried out, using station 6.2 at the CLRC Daresbury Laboratory. The (100) face orientation produced by anvil cleaving was determined using Laue back-reflection and LEED. Resonant photoemission measurements at the Cu 3pCu 3d threshold were undertaken and these are compared with existing data from oxidized copper films, and with theoretical configuration interaction calculations for the resonance process. Angle-resolved measurements of the valence band structure have also been performed at different photon energies along three crystal axes, and these are mapped onto the 3D structure of the monoclinic material. This allows general comparison with existing momentum-resolved calculations. The validity of comparisons with both localized and delocalized descriptions of the electronic structure of this highly correlated oxide is discussed.

Adsorption Studies of p-Aminobenzoic Acid on the Anatase TiO2(101) Surface

The adsorption of p-aminobenzoic acid (pABA) on the anatase TiO2(101) surface has been investigated using synchrotron radiation photoelectron spectroscopy, near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT). Photoelectron spectroscopy indicates that the molecule is adsorbed in a bidentate mode through the carboxyl group following deprotonation. NEXAFS spectroscopy and DFT calculations of the adsorption structures indicate the ordering of a monolayer of the amino acid on the surface with the plane of the ring in an almost upright orientation. The adsorption of pABA on nanoparticulate TiO2 leads to a red shift of the optical absorption relative to bare TiO2 nanoparticles. DFT and valence band photoelectron spectroscopy suggest that the shift is attributed to the presence of the highest occupied molecular orbitals in the TiO2 band gap region and the presence of new molecularly derived states near the foot of the TiO2 conduction band.

PEGylation of nanosubstrates (Titania) with multifunctional reagents: At the crossroads between nanoparticles and nanocomposites

Titania (anatase) nanoparticles were successfully PEGylated through the use of catechol (dopamine)-terminated PEG derivatives. The resulting materials were characterized by excellent stability at neutral pH and extremely low toxicity (phagocytic and nonphagocytic cell lines). In particular, we focused on the comparison between mono- and bis-catechol PEGs. Due to the double terminal anchorage on the titania surface, bis-catechol ligands can produce chains differing from classical monoanchored PEG in conformation (horseshoe-shaped vs brush) and thus the possibility of interactions with biomolecules. At the same time, less than quantitative catechol binding may lead to the presence of dangling chains with unbound catechols which can polymerize and eventually produce PEG/titania nanocomposite colloids. Our results on double-functional PEG2000 show the latter to be the case. Pluronic F127 was also used as a bifunctional ligand, leading to nanocomposite aggregates with an even larger organic content.

Electronic structure, reactivity and solid-state chemistry of La2 –xSrxNi1 –yFeyO4 +δ

Ceramic samples of La2 –xSrxNi1 –yFeyO4 +δ, designed to model the ICI Hydecat catalyst, have been prepared via precipitation followed by high temperature annealing. The samples are characterized by X-ray diffraction, EDAX and atomic absorption spectrometry. The small changes in electronic structure as a function of x and y have been probed using photoemission. This reveals a progressive shift of the valence-band edge towards the Fermi level as a function of Sr-doping level, consistent with the approach to a metal to non-metal transition. Resonant photoemission at the Fe and Ni 3p thresholds is used to identify the transition metal contributions to the valence band density of states, and confirms a valence state of 3.0 for Fe. High resolution ESCA is used to quantify the surface composition of the samples, revealing Sr segregation consistent with that found in related perovskites and layered perovskites. Adsorption of CO onto the surfaces of these materials is seen to give rise to the ready production of a carbonate species. Water adsorption studies, which test the potential stability of the materials for possible applications in aqueous solution, are also presented.

Adsorption of Dopamine on Rutile TiO2 (110): A Photoemission and Near-Edge X-ray Absorption Fine Structure Study

Synchrotron radiation photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) techniques have been used to study the adsorption of dopamine on a rutile TiO2 (110) single crystal. Photoemission results suggest that dopamine bonds through the oxygen molecules in a bidentate fashion. From the data, it is ambiguous whether the oxygens bond to the same 5-fold coordinated surface titanium atom or bridges across two, although based on the bonding of pyrocatechol on rutile TiO2 (110), it is likely that the dopamine bridges two titanium atoms. Using the searchlight effect, the carbon K-edge near-edge X-ray absorption fine structure NEXAFS spectra recorded for dopamine on rutile TiO2 (110) show the phenyl ring to be oriented at 78° ± 5° from the surface and twisted 11 ± 10° relative to the (001) direction.