Fluctuant Magnetism in Metal Oxide Nanocrystals Capped with Surfactants

We demonstrate experimentally that magnetism in ZnO, TiO2, CeO2, andSnO2 nanocrystals (NCs) has a fluctuant nature that varies with capping surfactant type and concentration. By developing a forced hydrolysis approach with additional postprocessing for the synthesis and surfactant capping of these NCs, we effectively avoid the influence of size, shape, and magnetic impurities on the magnetic behavior of NCs, thus revealing the systematic influence of the capping surfactants on the NC magnetism. The x-ray photoelectron spectroscopy results and theoretical calculations clearly show that the magnetism fluctuation with surfactant concentration can be attributed to the periodic variation of spins, which arises from the concentration-dependent electron transfer from surfactants to NCs. Our results not only explain the previously reported seemingly irregular magnetism induced by capping surfactants but also provide an effective approach to tune or optimize the NC magnetism

Role of Oxygen Defects on the Magnetic Properties of Ultra-Small Sn\u3csub\u3e1−x\u3c/sub\u3eFe\u3csub\u3ex\u3c/sub\u3eO\u3csub\u3e2\u3c/sub\u3e Nanoparticles

Although the role of oxygen defects in the magnetism of metal oxide semiconductors has been widely discussed, it is been difficult to directly measure the oxygen defect concentration of samples to verify this. This work demonstrates a direct correlation between the photocatalytic activity of Sn1−xFexO2 nanoparticles and their magnetic properties. For this, a series of ~2.6 nm sized, well characterized, single-phase Sn1−xFexO2 crystallites with x = 0−0.20 were synthesized using tin acetate, urea, and appropriate amounts of iron acetate. X-ray photoelectron spectroscopy confirmed the concentration and 3+ oxidation state of the doped Fe ions. The maximum magnetic moment/Fe ion, μ, of 1.6 × 10−4 μB observed for the 0.1% Fe doped sample is smaller than the expected spin-only contribution from either high or low spin Fe3+ ions, and μ decreases with increasing Fe concentration. This behavior cannot be explained by the existing models of magnetic exchange. Photocatalytic studies of pure and Fe-doped SnO2 were used to understand the roles of doped Fe3+ ions and of the oxygen vacancies and defects. The photocatalytic rate constant k also showed an increase when SnO2 nanoparticles were doped with low concentrations of Fe3+, reaching a maximum at 0.1% Fe, followed by a rapid decrease of k for further increase in Fe%. Fe doping presumably increases the concentration of oxygen vacancies, and both Fe3+ ions and oxygen vacancies act as electron acceptors to reduce e−-h+ recombination and promote transfer of electrons (and/or holes) to the nanoparticle surface, where they participate in redox reactions. This electron transfer from the Fe3+ ions to local defect density of states at the nanoparticle surface could develop a magnetic moment at the surface states and leads to spontaneous ferromagnetic ordering of the surface shell under favorable conditions. However, at higher doping levels, the same Fe3+ ions might act as recombination centers causing a decrease of both k and magnetic moment μ

Novel Magnetic and Optical Properties of Sn\u3csub\u3e1−x\u3c/sub\u3eZn\u3csub\u3ex\u3c/sub\u3eO\u3csub\u3e2\u3c/sub\u3e Nanoparticles

In this work, we report on the effects of doping SnO2 nanoparticles with Zn2+ ions. A series of ∼2–3 nm sized Sn1−x ZnxO2 crystallite samples with 0 ≤ x ≤ 0.18 were synthesized using a forced hydrolysis method. Increasing dopant concentration caused systematic changes in the crystallite size, oxidation state of Sn, visible emission, and band gap of SnO2 nanoparticles. X-ray Diffraction studies confirmed the SnO2 phase purity and the absence of any impurity phases. Magnetic measurements at room temperature showed a weak ferromagnetic behavior characterized by an open hysteresis loop. Their saturation magnetization Ms increases initially with increasing Zn concentrations; however for x \u3e 0.06, Ms decreases. Samples with the highest Ms values (x = 0.06) were analyzed using an Inductively Coupled Plasma Mass Spectrometer, looking for traces of any magnetic elements in the samples. Concentrations of all transition metals (Fe, Co, Mn, Cr, and Ni) in these samples were below ppb level, suggesting that the observed magnetism is not due to random inclusions of any spurious magnetic impurities and it cannot be explained by the existing models of magnetic exchange. A new visible emission near 490 nm appeared in the Zn doped SnO2 samples in the photoluminescence spectra which strengthened as x increased, suggesting the formation of defects such as oxygen vacancies. X-ray Photoelectron Spectroscopy (XPS) confirmed the nominal Zn dopant concentrations and the 2+ oxidation state of Zn in the Sn1−x ZnxO2 samples. Interestingly, the XPS data indicated the presence of a small fraction of Sn2+ ions in Sn1−xZnxO2 samples in addition to the expected Sn4+, and the Sn2+ concentration increased with increasing x. The presence of multi-valent metal ions and oxygen defects in high surface area oxide nanoparticles has been proposed as a potential recipe for weak ferromagnetis

Tuning the Bandgap and Cytotoxicity of ZnO by Tailoring the Nanostructures

Tuning the bandgap and cytotoxicity of ZnO nanoparticles (NPs) is very important, not only for customizing their optoelectronic and biomedical applications, but also for their cytotoxicity assay and safe usage. A unique soft-template of polyvinylpyrrolidone has been developed here to realize a rapid room-temperature neutral synthesis of ZnO with controlled nanostructures for tuning the bandgap and cytotoxicity of ZnO. By simply changing the reagent stoichiometry and the soft-template shape, high-purity ZnO rods, tripods, tubes, and unique T-like tubes with tunable size, surface composition/charge, bandgap, and cytotoxicity are obtained. It has been revealed that the ZnO bandgap can be remarkably reduced by introducing the surface nonstoichiometry; and the ZnO-induced cytotoxicity can be tuned by the size, shape, surface charge/composition, and bandgap of ZnO NPs at different degrees. Significantly, both the photochemistry reaction and the reactive oxygen species induced by ZnO NPs are not necessary for the ZnO-induced cytotoxicity

Transition Metal Dopants Essential for Producing Ferromagnetism in Metal Oxide Nanoparticles

Recent claims that ferromagnetism can be produced in nanoparticles of metal oxides without the presence of transition metal dopants have been challenged in this work by investigating 62 high quality well-characterized nanoparticle samples of both undoped and Fe doped (0-10% Fe) ZnO. The undoped ZnO nanoparticles showed zero or negligible magnetization, without any dependence on the nanoparticle size. However, chemically synthesized Zn1-xFexO nanoparticles showed clear ferromagnetism, varying systematically with Fe concentration. Furthermore, the magnetic properties of Zn1-xFexO nanoparticles showed strong dependence on the reaction media used to prepare the samples. The zeta potentials of the Zn1-xFexO nanoparticles prepared using different reaction media were significantly different, indicating strong differences in the surface structure. Electron paramagnetic resonance studies indicate that the difference in the ferromagnetic properties of Zn1-xFexO nanoparticles with different surface structures originates from differences in the fraction of the doped Fe ions that participate in ferromagnetic resonance

Magnetism of ZnO Nanoparticles: Dependence on Crystallite Size and Surfactant Coating

Many recent reports on magnetism in otherwise nonmagnetic oxides have demonstrated that nanoparticle size, surfactant coating, or doping with magnetic ions produces room-temperature ferromagnetism. Specifically, ZnO has been argued to be a room-temperature ferromagnet through all three of these methods in various experimental studies. For this reason, we have prepared a series of 1% Fe doped ZnO nanoparticle samples using a single forced hydrolysis co-precipitation synthesis method from the same precursors, while varying size (6 – 15 nm) and surface coating concentration to study the combined effects of these two parameters. Size was controlled by modifying the water concentration. Surfactant coating was adjusted by varying the concentration of poly acrylic acid (PAA) in solution. Samples were characterized by x-ray diffraction, transmission electron microscopy, x-ray photoelectron spectroscopy, optical absorptance spectroscopy, and magnetometry. No clear systematic effect on magnetization was observed as a function of surfactant coating, while evidence for a direct dependence of magnetization on the crystallite size is apparent

PTK6 Regulates IGF-1-Induced Anchorage-Independent Survival

Background: Proteins that are required for anchorage-independent survival of tumor cells represent attractive targets for therapeutic intervention since this property is believed to be critical for survival of tumor cells displaced from their natural niches. Anchorage-independent survival is induced by growth factor receptor hyperactivation in many cell types. We aimed to identify molecules that critically regulate IGF-1-induced anchorage-independent survival. Methods and Results: We conducted a high-throughput siRNA screen and identified PTK6 as a critical component of IGF-1 receptor (IGF-1R)-induced anchorage-independent survival of mammary epithelial cells. PTK6 downregulation induces apoptosis of breast and ovarian cancer cells deprived of matrix attachment, whereas its overexpression enhances survival. Reverse-phase protein arrays and subsequent analyses revealed that PTK6 forms a complex with IGF-1R and the adaptor protein IRS-1, and modulates anchorage-independent survival by regulating IGF-1R expression and phosphorylation. PTK6 is highly expressed not only in the previously reported Her2$^+$ breast cancer subtype, but also in high grade ER$^+$, Luminal B tumors and high expression is associated with adverse outcomes. Conclusions: These findings highlight PTK6 as a critical regulator of anchorage-independent survival of breast and ovarian tumor cells via modulation of IGF-1 receptor signaling, thus supporting PTK6 as a potential therapeutic target for multiple tumor types. The combined genomic and proteomic approaches in this report provide an effective strategy for identifying oncogenes and their mechanism of action

HDAC9 is implicated in atherosclerotic aortic calcification and affects vascular smooth muscle cell phenotype.

Aortic calcification is an important independent predictor of future cardiovascular events. We performed a genome-wide association meta-analysis to determine SNPs associated with the extent of abdominal aortic calcification (n = 9,417) or descending thoracic aortic calcification (n = 8,422). Two genetic loci, HDAC9 and RAP1GAP, were associated with abdominal aortic calcification at a genome-wide level (P < 5.0 × 10-8). No SNPs were associated with thoracic aortic calcification at the genome-wide threshold. Increased expression of HDAC9 in human aortic smooth muscle cells promoted calcification and reduced contractility, while inhibition of HDAC9 in human aortic smooth muscle cells inhibited calcification and enhanced cell contractility. In matrix Gla protein-deficient mice, a model of human vascular calcification, mice lacking HDAC9 had a 40% reduction in aortic calcification and improved survival. This translational genomic study identifies the first genetic risk locus associated with calcification of the abdominal aorta and describes a previously unknown role for HDAC9 in the development of vascular calcification