The silver (0.5-3 at %) substituted nanosize hydroxyapatites (AgHAs) were synthesized by microwave processing. The X-ray diffraction (XRD) peaks are very broad, indicating that the AgHAs were of nanosize (30 nm). Transmission electron microscopy analysis shows needle-like morphology of AgHA, having length 60-70 nm and width 15-20 nm. The AgHA phase was stable up to 700 degrees C without any secondary phases. The antibacterial effect of AgHA against Escherichia coli and Staphylococcus aureus was observed by spread plate method, even for low concentration of silver ions (0.5%) with 1 x 10(5) cells/mL of respective bacterial culture, after a 48 h incubation period. However, some colonies of E. coli were seen with a high dose of 1 x 10(8) cells/mL after 24 h. The zone of inhibition by disc diffusion test method was found to vary with the amount of silver in the sintered AgHA pellets, for both the bacteria, after 24 h of inoculation. Osteoblast cell attachment in varying density was noticed on AgHA samples with 0.5, 1.0, and 1.5% silver substitution. However, osteoblast spreading was significantly greater on 0.5% AgHA compared to 1.0 or 1.5% substituted AgHA samples. Thus, the low amount of AgHA has a potential of minimizing the risk of bacterial contamination, without compromising the bioactivity, and is expected to display greater biological efficacy in terms of osseointegration.
1,4-Benzenedimethanethiol (HS−CH2−C6H4−CH2−SH, BDMT) adsorbs dissociatively on silver and
gold surfaces yielding self-assembled monolayers with the thiolate structure. Whereas the molecule adsorbs
flat on silver as a result of the loss of two thiol protons, it adsorbs with the molecular plane perpendicular
to the gold surface with the loss of one thiol proton. This is manifested by the presence of the ring C−H
and S−H stretches and the S−C−H bend for the Au monolayer and the absence of these for the Ag
monolayer in the surface-enhanced Raman spectra. The difference in adsorbate geometry is presumably
due to the changes in interaction and not to differences in the lattice constants of the two surfaces, which
is rather small. In both cases, metal−adsorbate π bonding is weak, resulting in only small shifts in the
ring modes. BDMT monolayers are more stable than alkanethiol monolayers and desorb only at a fairly
high temperature of 423 K in air, whereas alkanethiols desorb below 373 K. An increase in temperature
leads to structural changes in the Au monolayer and the molecules begin to lie flat on the surface, and
desorption occurs from this state. The Ag monolayer is less stable thermally and desorption is eventless.
Difference in the desorption temperatures point to the importance of energetics of self-assembly in
determining the stability. The thiol proton on gold surfaces can be removed easily by exposing the monolayer
to basic solutions. The gold monolayer upon exposure to thiols leads to the formation of disulfides, suggesting
the formation of a prototypical bilayer. A completely new S−S stretching frequency is seen upon reaction
with 4-methoxybenzenethiol (MOBT) with the complete absence of the S−H stretch. Other spectral features
of the bilayer can be attributed to BDMT and MOBT subunits. The respective thiols in solution, however,
do not react, leading to disulfide. Reaction with silver monolayer leads to the displacement of BDMT for
MOBT and no reaction is observed. With n-alkanethiols, the reactivity decreases with the alkane chain
length. The alkanethiol part of the spectrum resembles that of the corresponding self-assembled monolayer.
Surface-enhanced reactivity of the type observed here has not been reported hitherto. The MOBT part
of the bilayer desorbs first after cleaving the S−S bond and BDMT leaves the surface subsequently. X-ray
exposure of the monolayers leads to beam-induced damage which is manifested in the Raman spectra.
Whereas the damage is severe for Ag, part of the Au monolayer survives X-ray exposure.
Solid solutions of GaN in ZnO (Zn 1-z Ga z )(O 1-x N x ) (x and z e 0.15) have been prepared by simple solution combustion method. Except for minor changes in the lattice contraction, no significant change in the Wurtzite structure was observed. Raman and secondary ion mass spectrometry results show the direct Zn-N and Ga-N bonds in (Zn 1-z Ga z )(O 1-x N x ). Visible light absorption and XPS results demonstrate that N 2p states of nitride occupy the states above the O 2p valence band, and hence a change in optical band gap reduction occurs to ∼2.5 eV from 3.37 eV for ZnO. Significant nitrogen fixation catalytic activity through NH 3 formation has been observed at ambient pressure on virgin (Zn 1-z Ga z )(O 1-x N x ) material, indicating its potential as a catalyst.
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