34 research outputs found
The influence of adatom diffusion on the formation of skyrmion lattice in sub-monolayer Fe on Ir(111)
Room temperature grown Fe monolayer (ML) on the Ir(111) single crystal
substrate has attracted great research interests as nano-skyrmion lattice can
form under proper growth conditions. The formation of the nanoscale skyrmion,
however, appears to be greatly affected by the diffusion length of the Fe
adatoms on the Ir(111) surface. We made this observation by employing
spin-polarized scanning tunneling microscopy to study skyrmion formation upon
systematically changing the impurity density on the substrate surface prior to
Fe deposition. Since the substrate surface impurities serve as pinning centers
for Fe adatoms, the eventual size and shape of the Fe islands exhibit a direct
correlation with the impurity density, which in turn determines whether
skyrmion can be formed. Our observation indicates that skyrmion only forms when
the impurity density is below 0.006/nm2, i.e., 12 nm averaged spacing between
the neighboring defects. We verify the significance of Fe diffusion length by
growing Fe on clean Ir(111) substrate at low temperature of 30 K, where no
skyrmion was observed to form. Our findings signify the importance of diffusion
of Fe atoms on the Ir(111) substrate, which affects the size, shape and lattice
perfection of the Fe islands and thus the formation of skyrmion lattice
Creation of nano-skyrmion lattice in Fe/Ir(111) system using voltage pulse
Magnetic ultrathin films grown on heavy metal substrates often exhibit rich
spin structures due to the competition between various magnetic interactions
such as Heisenberg exchange, Dzyaloshinskii-Moriya interaction and higher-order
spin interactions. Here we employ spin-polarized scanning tunneling microscopy
to study magnetic nano-skyrmion phase in Fe monolayer grown on Ir(111)
substrate. Our observations show that the formation of nano-skyrmion lattice in
the Fe/Ir(111) system depends sensitively on the growth conditions and various
non-skyrmion spin states can be formed. Remarkably, the application of voltage
pulses between the tip and the sample can trigger a non-skyrmion to skyrmion
phase transition. The fact that nano-skyrmions can be created using voltage
pulse indicates that the balance between the competing magnetic interactions
can be affected by an external electric field, which is highly useful to design
skyrmion-based spintronic devices with low energy consumption
Chemical ordering suppresses large-scale electronic phase separation in doped manganites
For strongly correlated oxides, it has been a long-standing issue regarding the role of the chemical ordering of the dopants on the physical properties. Here, using unit cell by unit cell superlattice growth technique, we determine the role of chemical ordering of the Pr dopant in a colossal magnetoresistant (La1-yPry)1-xCaxMnO3 (LPCMO) system, which has been well known for its large length-scale electronic phase separation phenomena. Our experimental results show that the chemical ordering of Pr leads to marked reduction of the length scale of electronic phase separations. Moreover, compared with the conventional Pr-disordered LPCMO system, the Pr-ordered LPCMO system has a metal–insulator transition that is ~100 K higher because the ferromagnetic metallic phase is more dominant at all temperatures below the Curie temperature
Electronically phase separated nano-network in antiferromagnetic insulating LaMnO3/PrMnO3/CaMnO3 tricolor superlattice
Strongly correlated materials often exhibit an electronic phase separation
(EPS) phenomena whose domain pattern is random in nature. The ability to
control the spatial arrangement of the electronic phases at microscopic scales
is highly desirable for tailoring their macroscopic properties and/or designing
novel electronic devices. Here we report the formation of EPS nanoscale network
in a mono-atomically stacked LaMnO3/CaMnO3/PrMnO3 superlattice grown on SrTiO3
(STO) (001) substrate, which is known to have an antiferromagnetic (AFM)
insulating ground state. The EPS nano-network is a consequence of an internal
strain relaxation triggered by the structural domain formation of the
underlying STO substrate at low temperatures. The same nanoscale network
pattern can be reproduced upon temperature cycling allowing us to employ
different local imaging techniques to directly compare the magnetic and
transport state of a single EPS domain. Our results confirm the one-to-one
correspondence between ferromagnetic (AFM) to metallic (insulating) state in
manganite. It also represents a significant step in a paradigm shift from
passively characterizing EPS in strongly correlated systems to actively
engaging in its manipulation
Membrane Vesicles Are the Dominant Structural Components of Ceftazidime-Induced Biofilm Formation in an Oxacillin-Sensitive MRSA
Methicillin-resistant Staphylococcus aureus (MRSA) has received increasing attention in recent years. However, the characteristics and relevant mechanisms of biofilm formation in oxacillin-sensitive MRSA (OS-MRSA) are poorly understood. This study was designed to characterize biofilm formation in OS-MRSA BWSA15 in response to ceftazidime (TZ) by comparing the methicillin-sensitive S. aureus (MSSA) strain BWSA23 and the oxacillin-resistant MRSA (OR-MRSA) strain BWSA11. The biofilms and biofilm-forming cells were observed by electron microscopy. Biofilms grown on microtiter plates were chemically decomposed and analyzed by Fourier transform infrared spectroscopy. The transcriptional regulation of genes associated with methicillin resistance, surface adhesion, fatty acid biosynthesis, and global regulation (sigma B) was investigated. A significant increase in biofilm formation ability (10.21-fold) and aggregation ability (2.56-fold) was observed in BWSA15 upon the treatment with TZ (16 μg/ml). The TZ-induced biofilm formation in BWSA15 was characterized by a disappearance of polysaccharide-like extracellular substances and an appearance of a large number of intercellular MVs from extracellular matrix. Few MVs were identified in the biofilms formed by BWSA11 and BWSA23. There was a significant upregulation of mecA, sigB, and fatty acid biosynthesis-associated genes and downregulation of icaA, icaD, clfA, clfB, and fnaA in BWSA15 upon the treatment with TZ. The formation of intracellular junctions of MVs in the biofilms of BWSA15 was mediated by a significant increase in the proportion of proteins as well as by an increase in the proportion of non-ionized carboxyl groups in fatty acids. This study demonstrated that beta-lactam antibiotics can induce biofilm formation in OS-MRSA, and the biofilm induction in OS-MRSA can mainly be attributed to exposed MVs with increased hydrophobicity rather than polysaccharide intercellular adhesins, cell wall-anchored surface proteins, and extracellular DNA
Origin of defect-related green emission from ZnO nanoparticles: effect of surface modification
We investigated the optical properties of colloidal-synthesized ZnO spherical nanoparticles prepared from 1-octadecene (OD), a mixture of trioctylamine (TOA) and OD (1:10), and a mixture of trioctylphosphine oxide (TOPO) and OD (1:12). It is found that the green photoluminescence (PL) of samples from the mixture of TOA/OD and TOPO/OD is largely suppressed compared with that from pure OD. Moreover, it is found that all spherical nanoparticles have positive zeta potential, and spherical nanoparticles from TOA/OD and TOPO/OD have a smaller zeta potential than those from OD. A plausible explanation is that oxygen vacancies, presumably located near the surface, contribute to the green PL, and the introduction of TOA and TOPO will reduce the density of oxygen vacancies near the surfaces. Assuming that the green emission arises due to radiative recombination between deep levels formed by oxygen vacancies and free holes, we estimate the size of optically active spherical nanoparticles from the spectral energy of the green luminescence. The results are in good agreement with results from TEM. Since this method is independent of the degree of confinement, it has a great advantage in providing a simple and practical way to estimate the size of spherical nanoparticles of any size. We would like to point out that this method is only applicable for samples with a small size distribution
Riboflavin as a Mucosal Adjuvant for Nasal Influenza Vaccine
Intranasal immunization with whole inactivated virus (WIV) is an important strategy used for influenza prevention and control. However, a powerful mucosal adjuvant is required to improve nasal vaccine efficacy. Riboflavin, as a food additive with the advantages of being safe and low-cost, widely exists in living organisms. In this paper, the mucosal adjuvant function of riboflavin was studied. After intranasal immunization with H1N1 WIV plus riboflavin in mice, we found that the mucosal immunity based on the secretory IgA (sIgA) levels in the nasal cavity, trachea, and lung were strongly enhanced compared with H1N1 WIV alone. Meanwhile, the IgG, IgG1, and IgG2a levels in serum also showed a high upregulation and a decreased ratio of IgG1/IgG2a, which implied a bias in the cellular immune response. Moreover, riboflavin strongly improved the protection level of H1N1 inactivated vaccine from a lethal influenza challenge. Furthermore, riboflavin was found to possess the capacity to induce dendritic cell (DC) phenotypic (MHCII, CD40, CD80, and CD86) and functional maturation, including cytokine secretion (TNF-α, IL-1β, IL-12p70, and IL-10) and the proliferation of allogeneic T cells. Lastly, we found that the DC maturation induced by riboflavin was dependent on the activation of the mitogen-activated protein kinase (MAPK) signaling pathway, which plays an important role in immune regulation. Therefore, riboflavin is expected to be developed as an alternative mucosal adjuvant for influenza nasal vaccine application
Bursopentin (BP5) protects dendritic cells from lipopolysaccharide-induced oxidative stress for immunosuppression.
Dendritic cells (DCs) play a vital role in the regulation of immune-mediated inflammatory diseases. Thus, DCs have been regarded as a major target for the development of immunomodulators. However, oxidative stress could disturb inflammatory regulation in DCs. Here, we examined the effect of bursopentine (BP5), a novel pentapeptide isolated from chicken bursa of fabricius, on the protection of DCs against oxidative stress for immunosuppression. BP5 showed potent protective effects against the lipopolysaccharide (LPS)-induced oxidative stress in DCs, including nitric oxide, reactive oxygen species and lipid peroxidation. Furthermore, BP5 elevated the level of cellular reductive status through increasing the reduced glutathione (GSH) and the GSH/GSSG ratio. Concomitant with these, the activities of several antioxidative redox enzymes, including glutathione peroxidase (GPx), catalase (CAT) and superoxide dismutase (SOD), were obviously enhanced. BP5 also suppressed submucosal DC maturation in the LPS-stimulated intestinal epithelial cells (ECs)/DCs coculture system. Finally, we found that heme oxygenase 1 (HO-1) was remarkably upregulated by BP5 in the LPS-induced DCs, and played an important role in the suppression of oxidative stress and DC maturation. These results suggested that BP5 could protect the LPS-activated DCs against oxidative stress and have potential applications in DC-related inflammatory responses
Melatonin Suppresses LPS-Induced Oxidative Stress in Dendritic Cells for Inflammatory Regulation via the Nrf2/HO-1 Axis
Melatonin, an indoleamine synthesized in the pineal gland of mammals, is a natural bioactive compound with powerful antioxidant and anti-inflammatory properties. Here, we evaluated whether melatonin has the capacity to moderate the oxidative stress of dendritic cells (DCs) for inflammatory control in an acute lung injury (ALI) model. Our findings showed that melatonin remarkably inhibited total nitric oxide synthase (T-NOS) activity, nitric oxide (NO) production, intracellular reactive oxygen species (ROS) levels, and lipid peroxidation (MDA detection) levels in both an LPS-induced murine ALI model and LPS-induced DCs. Meanwhile, the reduced glutathione (GSH) level and the GSH/GSSG ratio were recovered. In addition, antioxidant enzymes, such as glutathione peroxidase (GPx), catalase (CAT), and superoxide dismutase (SOD), were increased in these processes. Moreover, melatonin also inhibited the LPS-induced secretions of IL-1β, IL-6, and TGF-β in vivo and in vitro. Finally, we found that the nuclear factor erythroid 2-related factor 2 (Nrf2)/heme oxygenase 1 (HO-1) axis was required in the inhibition of LPS-induced oxidative stress in DCs by melatonin. Altogether, these data indicate that melatonin strongly suppresses the LPS-induced oxidative stress in DCs, which is a promising DC-targeted strategy via inflammatory control for ALI treatment
Holarrhena pubescens Wall. ex G. Don Extracts Inhibit LPS-Irritated Oxidative Stress in Dendritic Cells
Holarrhena pubescens Wall. ex G. Don (H. pubescens), belonging to the Apocynaceae family, is distributed in deciduous forests of the tropical Himalayas. H. pubescens is an important traditional medicinal plant, especially its seeds and barks. Therefore, we assessed the antioxidant capacity of H. pubescens extracts in Lipopolysaccharide (LPS)-induced dendritic cells (DCs) for sepsis treatment. Our results indicated that H. pubescens extracts with different doses (25 μg/mL, 50 μg/mL, 100 μg/mL) reduced the reactive oxygen species (ROS) level, and weakened the nitric oxide synthases (NOS) activity and nitric oxide (NO) level in LPS (100 ng/mL)-irritated DCs. In addition, H. pubescens extracts decreased the oxidized glutathione (GSSG) production but increased the reduced glutathione (GSH) production, thereby preserving the cellular reductive status owing to the raised GSH/GSSG ratio. Furthermore, H. pubescens extracts strengthened the antioxidant enzymes activity in LPS-induced DCs, such as glutathione peroxidase (GPx), catalase (CAT), and superoxide dismutase (SOD). Finally, we found that H. pubescens extracts significantly improved the expression of the nuclear factor erythroid 2-related factor 2 (Nrf2) and the heme oxygenase 1 (HO–1) in LPS-irritated DCs. These results indicated that H. pubescens extracts suppressed the LPS-irritated oxidative stress in DCs via Nrf2/HO–1 signaling pathway, providing a potential strategy for sepsis therapy