Nathan P. Mellott

Luminescence of delafossite-type CuAlO2 fibers with Eu substitution for Al cations

Abstract CuAlO2 has been examined as a potential luminescent material by substituting Eu for Al cations in the delafossite structure. CuAlO2:Eu3+ nanofibers have been prepared via electrospinning for the ease of mitigating synthesis requirements and for future optoelectronics and emerging applications. Single-phase CuAlO2 fibers could be obtained at a temperature of 1100 °C in air. The Eu was successfully doped in the delafossite structure and two strong emission bands at ~405 and 610 nm were observed in the photoluminescence spectra. These bands are due to the intrinsic near-band-edge transition of CuAlO2 and the f-f transition of the Eu3+ activator, respectively. Further electrical characterization indicated that these fibers exhibit semiconducting behavior and the introduction of Eu could act as band-edge modifiers, thus changing the thermal activation energies. In light of this study, CuAlO2:Eu3+ fibers with both strong photoluminescence and p-type conductivity could be produced by tailoring the rare earth doping concentrations.

Investigating the structure and biocompatibility of niobium and titanium oxides as coatings for orthopedic metallic implants.

Applying sol gel based coatings to orthopedic metallic implant materials can significantly improve their properties and lifespan in vivo. For this work, niobium (Nb2O5) and titanium (TiO2) oxides were prepared via solution processing in order to determine the effect of atomic arrangement (amorphous/crystalline) on bioactivity. Thermal evaluation on the synthesized materials identified an endotherm for Nb2O5 at 75 °C with 40% weight loss below 400 °C, and minimal weight loss between 400 and 850 °C. Regarding TiO2 an endotherm was present at 92 °C with 25% weight loss below 400 °C, and 4% between 400 and 850 °C. Phase evolution was determined using High Temperature X-ray Diffraction (HT-XRD) where amorphous-Nb2O5 (450 °C), hexagonal-Nb2O5 (525 °C), orthorhombic-Nb2O5 (650 °C), amorphous-TiO2 (275 °C) and tetragonal TiO2 (500 °C) structures were produced. Simulated body fluid (SBF) testing was conducted over 1, 7 and 30 days and resulted in positive chemical and morphological changes for crystalline Nb2O5 (525 °C) and TiO2 (500 °C) after 30 days of incubation. Rod-like CaP deposits were observed on the surfaces using Scanning Electron Microscopy (FE-SEM) and Grazing Incidence-X-ray Diffraction (GI-XRD) shows that the deposits were X-ray amorphous. Cell viability was higher with the TiO2 (122%) samples when compared to the growing cell population while Nb2O5 samples exhibited a range of viability (64-105%), partially dependent on materials atomic structure.

Characterization and antibacterial efficacy of silver-coated Ca-Na-Zn-Si/Ti glasses.

A glass series [xSiO2[−y]·0.36ZnO·0.17Na2O·0.05CaO (starting at x = 0.50, y = 0.08 TiO2)] was formulated with TiO2 substituting SiO2. Each glass/silver-coated glass was characterized using X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy. Surface area analysis revealed significant changes after silver coating, 0.43–0.95 m2/g (control), to 0.53–1.85 m2/g (AU-1), and 0.20–1.11 m2/g (AU-2). Ion release from uncoated glasses included sodium (0.08 mg/L), calcium (0.07 mg/L), and zinc (0.008 mg/L), where silver-coated glasses presented 0.42 mg/L (silver), 0.33 mg/L (sodium), 0.02 mg/L (calcium), and 0.01 mg/L (zinc). Ag-coated glasses presented inhibition zones of 7.75 mm (control) compared to 1.04 mm (AU-2).

Investigating the effect of TiO2 on the structure and biocompatibility of bioactive glass

Titanium (Ti4+ ) containing materials have been widely used in medical applications due to its associated bioactivity in vivo. This study investigates the replacement of Si4+ with Ti4+ within the system SiO2 -Na2 O-CaO-P2 O5 to determine its influence on glass structure. This strategy was conducted in order to control the glass solubility to further improve the cellular response. Ti4+ incorporation was found to have little influence on the glass transition temperature (Tg  = 520 ± 8°C) and magic angle spinning-nuclear magnetic resonance (MAS-NMR) shifts (-80 ppm) up to additions of 18 wt %. However, at 30 wt % the Tg increased to 600°C and MAS-NMR spectra shifted to -88 ppm. There was also an associated reduction in glass solubility as a function of Ti4+ incorporation as determined by inductively coupled plasma optical emission spectroscopy where Si4+ (1649-44 mg/L) and Na+ (892-36 mg/L) levels greatly reduced while Ca2+ (3-5 mg/L) and PO43- (2-7 mg/L) levels remained relatively unchanged. MC3T3 osteoblasts were used for cell culture testing and it was determined that the Ti4+ glasses increased cell viability and also facilitated greater osteoblast adhesion and proliferation to the glass surface compared to the control glass.

Investigating the effect of silver coating on the solubility, antibacterial properties, and cytocompatibility of glass microspheres

Silver (Ag) coatings have been incorporated into many medical materials due to its ability to eradicate harmful microbes. In this study, glass microspheres (SiO2–Na2O–CaO–Al2O3) were synthesized and employed as substrates to investigate the effect Ag coating has on glass solubility and the subsequent biological effects. Initially, glasses were amorphous with a glass transition point (Tg) of 605℃ and microspheres were spherical with a mean particle diameter of 120 µm (±27). The Ag coating was determined to be crystalline in nature and its presence was confirmed using scanning electron microscopy and X-ray photoelectron spectroscopy. Ion release determined that Ag-coated (Ag-S) microspheres increased the Na+ release rate but slightly reduced the Ca2+ and Si4+ release compared to an uncoated control (UC-S). Additionally, the Ag-S reduced the pH to just above neutral (7.3–8.5) compared to the UC-S (7.7–9.1). Antibacterial testing determined significant reductions in planktonic Escherichia coli (p = 0.000), Staphylococcus epidermidis (p = 0.000) and Staphylococcus aureus (p = 0.000) growth as a function of the presence of Ag and with respect to maturation (1, 7, and 30 days). Testing for toxicity levels using L929 Fibroblasts determined higher cell viability for the Ag-S at lower concentrations (5 µg/ml); in addition, no significant reduction in cell viability was observed with higher concentrations (15, 30 µg/ml).