The interaction between metal nanoparticles and bacteria belongs to the central issues in a dynamically growing bionanotechnological research. Herein, we investigated the adhesion efficiency of gold nanoparticles (30 nm) for various bacterial strains, both Gram-positive (Bacillus subtilis, Staphylococcus carnosus) and Gram-negative (Neisseria subflava, Stenotrophomonas maltophilia). The thorough microscopic (SEM/TEM) observations revealed that the nanoparticles do not penetrate into the bacterial cells but adhere to the walls. Large differences in the adhered nanoparticles amount were observed for the investigated strains (B. subtilis >> S. carnosus > N. subflava > S. maltophilia). A direct correlation between the number of the attached nanoparticles and the ζ-potential of the bacterial strains was found, and the results were rationalized in terms of the DLVO model. The calculated DLVO energy profiles revealed that the activation barriers for the adhesion process are rather small (1.45-1.55 kT), and the primary energy minima of 120-170 kT are favorable for the effective adsorption process. The established linear correlation between the nanoparticles adhered to the cell surface and the size of the critical volume around the bacterial cell, where the attraction forces dominate, implies that the observed dramatic differences in the attachment efficiency result from the availability of the nanoparticles in the critical volume of the surrounding suspensions. Owing to non-specific interactions governed by the ζ-potential mainly, the obtained results can be readily extended for the other bacteria-nanoparticle systems, providing a rational background for future advances in bacteria detection and thorough characterization via SERS method as well as for nanoparticles assemblies towards nanoelectronics.
Periodic spin unrestricted, gradient corrected DFT calculations joined with atomistic thermodynamic modeling and experiment were used to study the structure and stability of various reactive oxygen species (ROS) and oxygen vacancies produced on the most stable terminations of the cobalt spinel ( 100) surface. The surface state diagram of oxygen in a wide range of pressures and temperatures was constructed for coverage varying from Θ O = 1.51 atom•nm −2 to Θ O = 6.04 atom•nm −2 . A large variety of the unraveled surface ROS includes diatomic superoxo (Co O −O 2 − −Co O ), peroxo (Co T −O 2 2− −Co O ), and spin paired (Co O −O 2 −Co O ) adducts along with monatomic metal-oxo (Co T −O + , Co O −O 2+ ) species, where Co T and Co O stand for the tetrahedral and octahedral cobalt surface centers, respectively. There are also two kinds of peroxo species associated with surface oxygen ions connected with 3Co O or 2Co O and 1Co T cations ((O 2O,1T −O) 2− and (O 3O −O) 2− ), respectively). The results revealed that in the oxygen pressure range of typical catalytic reactions (p O 2 /p°from ∼0.01 to 1), the most stable stoichiometric (100)-S surface accommodates the Co T −O 2 2− −Co O and Co O −O 2 −Co O adducts at temperatures below 250−300 °C. In the temperature from 250 to 300 °C and from 550 to 700 °C, it is covered by the O species associated with the exposed tetrahedral cobalt sites (Co T − O + ) or remains in a bare state. In more reducing conditions (T > 550−700 °C), the (100)-S facet is readily defected due to trigonal oxygen (O 2O,1T ) release and formation of surface oxygen vacancies. The reactivity of surface ROS was tested in 16 O 2 / 18 O 2 isotopic exchange, N 2 O decomposition, and oxidation of CH 4 and CO model reactions, carried over Co 3 O 4 and Co 3 18 O 4 nanocrystalline samples with the predominant (100) faceting revealed by high angle angular dark field STEM examination. The Co O −O 2+ adducts associated with octahedral cobalt sites, as well as the peroxo (O 2O,1T −O) 2− and (O 3O −O) 2− surface species being thermodynamically unstable are involved in surface oxygen recombination processes, probed by 16 O 2 / 18 O 2 exchange and N 2 O decomposition. It was shown that at low temperatures CO is oxidized by the suprafacial Co O −O 2 −Co O and Co T −O 2 −Co O diatomic oxygen, whereas in CH 4 activation, the highly reactive cobalt-oxo species (Co T −O + ) are involved. Above 600 °C at p O 2 /p°= 0.01, due to the onset of oxygen vacancy formation, the suprafacial methane oxidation gradually changes into the intrafacial Mars-van Krevelen scheme. The constructed surface phase diagram was used for rationalization of the obtained catalytic data, allowing delineation of the specific role of the chemical state of the cobalt spinel surface in the investigated processes, as well as the range of the corresponding temperatures and oxygen pressures. It also provides a convenient background for molecular understanding of remarkable activity of Co 3 O 4 in many other catalytic redox reactions.
Manganese,
iron, and cobalt model spinel catalysts were systematically
investigated for understanding the roots of their divergent performance
in N2O decomposition. The catalysts were characterized
by XRD, RS, N2-BET, SEM, and STEM/EELS techniques before
and after the reaction. Their redox properties and the thermodynamic
stability range were thoroughly examined by survey and narrow scan
TPR/TPO cycles. The results were accounted for by the constructed
size-dependent Ellingham diagrams. It was shown that Fe3O4 and Mn3O4 spinels exhibit redox-labile
Mn2+/Mn3+ and Fe2+/Fe3+constituents, and under the conditions of the deN2O reaction these catalysts have a pronounced tendency
for stoichiometric overoxidation. The redox properties of Co3O4 are highly anisotropic, with Co2+ being
reluctant to undergo oxidation but Co3+ being prone to
easy reduction. The stability of the Co3O4 catalyst
is then controlled by partial reduction of octahedral Co3+ cations, due to the surface oxygen release at elevated temperatures
in lean oxygen environments. The N2O decomposition was
studied by temperature-programmed surface reaction (TPSR) and pulse
experiments using 18O labeling of the catalysts. It was
shown that Co3O4 provides a sustainable redox
Co3+/Co4+ couple for catalytic decomposition
of N2O, which operates along a reversible one-electron
process, leading to formation of O–
surf intermediates that recombine next into dioxygen. As the reaction
temperature increases, the deN2O mechanism
evolves from suprafacial to intrafacial recombination of the oxygen
intermediates. Fe3O4 decomposes nitrous oxide
in a stoichiometric way via irreversible two-electron reduction of
oxygen intermediates into O2–, giving rise to lattice
expansion and formation of a γ-Fe2O3 shell,
as discerned by Raman spectroscopy. Postreaction STEM/EELS imaging
confirmed a magnetite-core and a maghemite-shell morphology of the
catalyst grains. A similar tendency for autogenous oxidation was observed
for Mn3O4, yet a rather weak thermodynamic driving
force makes this catalyst kinetically more stable. At higher reaction
temperatures, the incipient γ-Mn2O3 layer
may be decomposed back to the parent Mn spinel, when oxygen pressure
is low. To quantify gradual oxidation of the investigated spinels
during the N2O decomposition, size-dependent thermodynamic
3D diagrams were developed and used for rationalization of the experimental
observations. The obtained results reveal the dynamic nature of the
investigated spinels under varying redox conditions and explain the
remarkable performance of Co3O4 in comparison
to Fe3O4 and Mn3O4. The
catalytic behavior of the latter two spinels is actually governed
by a sesquioxide shell, produced spontaneously in the course of the deN2O reaction.
Plane wave periodic GGA-PBE+U density functional theory calculations were used to study the structure, surface energy, and equilibrium shape of faceted nanocrystals for a series of cubic (Fd3m) 2−3 AB 2 O 4 spinels with the following formula:
dispersion effect of cobalt spinel active phase spread over ceria for catalytic N2O decomposition: the role of the interface periphery, Applied Catalysis B, Environmental http://dx.doi.org/10. 1016/j.apcatb.2015.07.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. The catalytic tests in deN 2 O reaction revealed that the 10 wt.% of cobalt spinel in supported system is able to reproduce the activity of bare Co 3 O 4 catalyst. However, it was found that the catalyst with the lowest content of Co 3 O 4 equal to 1 wt.% exhibits the highest apparent reaction rate per mass of the spinel active phase. The observed activity was explained basing on the transmission electron microscopy analysis in terms of the dispersion of spinel phase over ceria support. A simple model that accounts for the observed strong dispersion effect is proposed. It consists in a two-step mechanism, where N 2 O is dissociated on the spinel nanograins and the resultant oxygen species are preferentially recombined at the Co 3 O 4 /CeO 2 interface periphery.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.