Charles C. Chusuei

Calcium Phosphate Phase Identification Using XPS and Time-of-Flight Cluster SIMS

Reproducible time-of-flight cluster static secondary ion mass spectra (ToF-SSIMS) were obtained for various standard calcium phosphate (CP) powders, which allowed for phase identification. X-ray diffraction was not able to detect signals from microscopic amounts of CP (∼15 mmol m(-)(2)). The phases studied were α-tricalcium phosphate [α-Ca(3)(PO(4))(2)], β-tricalcium phosphate [β-Ca(3)(PO(4))(2)], amorphous calcium phosphate [Ca(3)(PO(4))(2)·xH(2)O], octacalcium phosphate [Ca(8)H(2)(PO(4))(6)·H(2)O], brushite (CaHPO(4)·2H(2)O), and hydroxyapatite [Ca(10)(PO(4))(6)(OH)(2)]. The SIMS spectra were obtained via bombardment with (CsI)Cs(+) projectiles. X-ray photoelectron spectroscopy (XPS) core levels of the P 2p, Ca 2p, and O 1s orbitals and the relative O 1s loss intensity were examined. The PO(3)(-)/PO(2)(-) ratios from ToF-SSIMS spectra in conjunction with XPS of the CP powders showed much promise in differentiating between these phases at microscopic CP coverages on the metal oxide surface.

Modeling heterogeneous catalysts: metal clusters on planar oxide supports

Model catalysts consisting of Au and Ag clusters of varying size have been prepared on single crystal TiO2(110) and ultra-thin films of TiO2, SiO2 and Al2O3. The morphology, electronic structure, and catalytic properties of these Au and Ag clusters have been investigated using low-energy ion scattering spectroscopy (LEIS), temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) and spectroscopy (STS) with emphasis on the unique properties of clusters <5.0 nm in size. Motivating this work is the recent literature report that gold supported on TiO2 is active for various reactions including low-temperature CO oxidation and the selective oxidation of propylene. These studies illustrate the novel and unique physical and chemical properties of nanosized supported metal clusters.

Cytotoxicity in the age of nano: The role of fourth period transition metal oxide nanoparticle physicochemical properties

A clear understanding of physicochemical factors governing nanoparticle toxicity is still in its infancy. We used a systematic approach to delineate physicochemical properties of nanoparticles that govern cytotoxicity. The cytotoxicity of fourth period metal oxide nanoparticles (NPs): TiO2, Cr2O3, Mn2O3, Fe2O3, NiO, CuO, and ZnO increases with the atomic number of the transition metal oxide. This trend was not cell-type specific, as observed in non-transformed human lung cells (BEAS-2B) and human bronchoalveolar carcinoma-derived cells (A549). Addition of NPs to the cell culture medium did not significantly alter pH. Physiochemical properties were assessed to discover the determinants of cytotoxicity: (1) point-of-zero charge (PZC) (i.e., isoelectric point) described the surface charge of NPs in cytosolic and lysosomal compartments; (2) relative number of available binding sites on the NP surface quantified by X-ray photoelectron spectroscopy was used to estimate the probability of biomolecular interactions on the particle surface; (3) band-gap energy measurements to predict electron abstraction from NPs which might lead to oxidative stress and subsequent cell death; and (4) ion dissolution. Our results indicate that cytotoxicity is a function of particle surface charge, the relative number of available surface binding sites, and metal ion dissolution from NPs. These findings provide a physicochemical basis for both risk assessment and the design of safer nanomaterials.

Electronic structure studies of six-atom gold clusters

the @Au6~PPh3!6# unit of 1 carries a 12 charge, that the Au6 portion is essentially neutral. More direct evidence for this distribution of the ionized charge has been obtained from HF and DFT calculations of the double ionization energies of models of 1. It is found that the energy required to remove two electrons from a bare Au 6 cluster is much larger than that from an Au 6 cluster with phosphine ligands present; this is again consistent with the 12 charge in 1 being delocalized onto the triphenylphosphine ligands. It is possible that this delocalization of positive charge is responsible for facilitating the adhesion of the gold cluster as finely dispersed particles onto the metal oxide support.

Characterizing N-acetylcysteine (NAC) and N-acetylcysteine amide (NACA) binding for lead poisoning treatment.

Using antioxidants is an important means of treating lead poisoning. Prior in vivo studies showed marked differences between various chelator antioxidants in their ability to decrease both blood Pb(II) levels and oxidative stress resulting from lead poisoning. The comparative abilities of NAC and NACA to Pb(II) were studied in vitro, for the first time, to examine the role of the -OH/-NH(2) functional group in antioxidant binding behavior. To assay the antioxidant-divalent metal interaction, the antioxidants were probed as solid surfaces, adsorbing Pb(II) onto them. Surface characterization was carried out using X-ray photoelectron spectroscopy (XPS) analysis to quantify Pb(II) in the resulting adducts. XPS of the Pb 4f orbitals showed that more Pb(II) was chemically bound to NACA than NAC. In addition, the antioxidant surfaces probed via point-of-zero charge (PZC) measurements of NAC and NACA were obtained to gain further insight into the Pb-NAC and Pb-NACA binding, showing that Coulombic interactions played a partial role in facilitating complex formation. The data correlated well with solution analysis of metal-ligand complexation. UV-vis spectroscopy was used to probe complexation behavior. NACA was found to have the higher binding affinity as shown by free Pb(II) available in the solution after complexation from HPLC data. Electrospray ionization mass spectrometry (ESI-MS) was applied to delineate the structures of Pb-antioxidant complexes. Experimental results were further supported by density functional theory (DFT) calculations of supermolecular interaction energies (E(inter)) showing a greater interaction of Pb(II) with NACA than NAC.

Imaging ultrathin Al2O3 films with scanning tunneling microscopy

Abstract Reproducible scanning tunneling microscopic (STM) images were obtained from ultrathin Al 2 O 3 films epitaxially grown on Re (0 0 0 1) . Initially, the oxide films grow two-dimensionally in a layer-by-layer fashion with well-ordered surface morphologies. As the oxide film thickens to ca. 9 monolayer equivalents (MLE), the surface roughens and becomes more disordered yet still exhibits significant long-range, hexagonal periodicity as indicated by low energy electron diffraction (LEED). Because of limited conductivity, films thicker than ca. 9 MLE could not be imaged.

Gel-like Carbon Dots, Characterization, and their Potential Applications.

Highly photoluminescent gel-like carbon dots (G-CDs) were successfully synthesized for the first time by a rapid one-step solvothermal synthesis approach with citric acid and 1,2-ethylenediamine as the precursors. Their gel-like nature was revealed by the Tyndall and coagulation effects, which were elucidated by a negative ζ potential value. The influences of temperature on the properties and sizes of these G-CDs were analyzed, and the best method for a maximum quantum yield was identified. The resulting products emitted blue photoluminescence under UV light (λ=365 nm) and a gradient of color under regular light. In addition, the UV/Vis absorption and fluorescence emission spectra of the G-CDs indicated that those synthesized at 160 °C exhibited the highest fluorescence quantum yield (33 %). Atomic force microscopy and transmission electron microscopy measurements were performed, and a higher temperature of formation resulted in smaller G-CDs. Furthermore, band shifts in the UV/Vis and fluorescence spectra and sequential changes in the quantum yield values and ζ potentials in addition to elemental compositional changes as determined by X-ray photoelectron spectroscopy were monitored throughout the formation process of the G-CDs. As to applications, G-CDs were prepared as an invisible ink for printers, which exhibited the applicability of G-CDs in daily life and military activities.

Secondary ion emission from keV energy atomic and polyatomic projectile impacts on sodium iodate

Sodium iodate and sodium iodide are inorganic solids in which iodine exists in different chemical environments. In sodium iodate, the iodine atom is bonded to oxygen to make the trigonal pyramidal IO3 anion, which in turn is incorporated with sodium into an ionic crystal. In sodium iodide, however, the iodide anion and sodium cation are ionically bound in a crystal lattice. Nearly half of the negative secondary ion yield generated from keV energy polyatomic ion impacts on a sodium iodate surface is characteristic of ion emission expected from sodium iodide (i.e. (NaI) nI 2 ), yet x-ray photoelectron spectroscopy data indicate that the solid material resulting from aliquots of aqueous sodium iodate dried on stainless steel contains no more than 2% sodium iodide. To determine how the number of atoms in the primary ion influences the amount of iodide type ion formation from sodium iodate, secondary ion yield measurements were performed using Cs, (CsI)Cs, and (CsI)2Cs projectiles incident at energies ranging from 10 to 25 keV. The experiments were run on an event-by-event basis at the level of single ion impacts. The yields of iodate (composed of Na, I, and O) and iodide type secondary ions increase with the energy of the projectile and the number of constituent atoms. When compared on a per-incident atom basis, however, we found that the yields of secondary ions characteristic of iodate saturate at three total projectile atoms, but continue to increase nonlinearly for iodide species (i.e. the yield per impacting atom increases). (Int J Mass Spectrom 197 (2000) 149 ‐161)

Characterizing Functionalized Carbon Nanotubes for Improved Fabrication in Aqueous Solution Environments

The rediscovery of carbon nanotubes (Iijima, 1991) has inspired extensive research activity. These materials have extremely high surface areas, large aspect ratios, remarkably high mechanical strength, and can have electrical and thermal conductivities that are similar to that of copper (Ebbesen et al., 1996). They come in two forms: single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs). SWNTs have diameters ranging from 1.2 to 1.4 nm. MWNTs have larger overall diameters, with sizes depending on the number of concentric walls within the structure. Like graphite, carbon nanotubes are relatively non-reactive, except at the nanotube caps which are more reactive due to the presence of dangling bonds. The reactivity of the carbon nanotube side walls’ -system can also be influenced by tube curvature or chirality (Okpalugo et al., 2005). In particular, their remarkable structure-dependent properties have attracted great attention due to their potential applications in heterogeneous catalysis (Planeix et al., 1994), use as substrates for destruction of cancer cells (Kam et al., 2005) and applications for biological and chemical sensing (Poh et al., 2004). Carbon nanotubes require chemical modification in aqueous solution environments to make them more amenable for attachment of reactive surface species. In the case of attaching metal nanoparticles to the carbon surface, functionalization is necessary to avoid agglomeration of the metal. Sensor applications involve the tethering of chemical moieties with specific recognition sites for the detecting ultra-trace analytes (Dai, 2002). Surface functionalization is also necessary for depositing high-loading, catalytically active metal nanoparticles on them (Xing et al, 2005). Great attention has been paid to attaching functional groups onto carbon nanotube surfaces (Holzinger et al., 2001; Kim et al., 2004; Chen et al., 2005; Park et al., 2006) and probing the electronic structure resulting from post-nanotube-synthesis preparations. To understand the changes that result from surface functionalization strategies, well-defined characterization of the carbon nanotube’s surface chemistry and structure is needed. The ability to get an accurate detailed picture of the tethered functional groups that attach to the solid surface using aqueous solution preparation methods is important for controlling carbon nanotube surface composition composition.

Photoluminescent Carbon Dots: A Mixture of Heterogeneous Fractions

Photoluminescent carbon dots (CDs) fractions have been isolated from a gel-like material (GM), which was synthesized using a convenient one-step solvothermal route. In terms of purification, size exclusion chromatography (SEC) and dialysis were compared with acetone wash, which revealed the advantage of acetone wash. The pre-purified GM with acetone wash (A-GM) was further isolated by the reversed-phase preparative thin layer chromatography (TLC) with acetonitrile-water mixture (7 : 3; va /vw ) as the developing solvent. As a result, there were four photoluminescent bands on the TLC plate, which indicated the presence of four photoluminescent fractions. Detailed characterization measurements such as UV/Vis absorption, fluorescence emission, attenuated total reflection Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, zeta potential, dynamic light scattering, atomic force microscopy, and TEM measurements were performed on all fractions to analyze their heterogeneous optical, structural, electrical, and morphological properties. Considering the comprehensive analysis, all isolated fractions were CDs. In addition, excitation wavelength-independent CDs were obtained with a mean size of 2.5 nm and high quantum yield (55 %). Furthermore, the study demonstrates that the excitation wavelength-dependent photoluminescence of GM could result from the mixture of different surface states of each CD fraction rather than multiple surface states of uniform CDs nanoparticles.