Cobalt-doped
anatase Ti1–x
Co
x
O2 (0 < x ≤
0.04) nanopowders (with a particle size of 30–40 nm) were produced
by the hydrothermal synthesis method. Morphology, structure, and thermal
stability of the synthesized compounds were examined using transmission
electron microscopy, infrared spectroscopy, and X-ray diffraction
analysis. Using X-ray photoelectron spectroscopy, cobalt ions are
shown to have an oxidation state of 2+, with titanium ions having
a tetravalent state of Ti4+. In the as-prepared state,
all investigated compounds of Ti1–x
Co
x
O2 are paramagnetic, with
the value of paramagnetic susceptibility growing in proportion to
cobalt content; with the spin of cobalt ion equal to S = 3/2. Analysis of the electron paramagnetic resonance spectra reveals
that doping TiO2 with cobalt (up to 2%) is accompanied
by a significant increase in the concentration of F+ centers.
Further growth of the cobalt content results in a relatively wide
line (nearly 600 Oe) in the spectrum, with a g-factor
of about 2.005, demonstrating exchange-coupled regions being formed,
the fraction of which increases with cobalt content, while the intensity
of F+-center signals is reduced appreciably. Annealing
of Ti0.96Co0.04O2 in vacuum at 1000
K is shown to have resulted in the substantial localization of cobalt
atoms in the subsurface layers, resulting in an approximately 3-fold
increase in the Co atoms content on the surface of nanoparticles as
compared with that in the bulk. This is shown to be accompanied by
appearance of spontaneous magnetization at room temperature, the value
of which depends on the cobalt content in TiO2 nanopowders.
The value of magnetic moment per Co atom decreases monotonically down
to a value of ≃1 μB with cobalt content increasing.
A core–shell model proposed to be the most adequate for describing
the magnetic properties of TiO2:Co after the reducing annealing.
A hypothesis is put forward suggesting that the defect surface enriched
with Co atoms and vacancies is described with itinerant type magnetism,
allowing for the delocalized nature of electrons near vacancies.
Co-doped
TiO2 is one of the most extensively studied
oxides for applying as dilute magnetic semiconductors due to its room
temperature magnetism. Here we present results of the studies of Ti0.97Co0.03O2 nanopowders synthesized
by microwave-assisted hydrothermal method by means of X-ray diffraction,
soft X-ray absorption spectroscopy (Ti L2,3 and Co L2,3 spectra), hard X-ray absorption spectroscopy (Co K spectra),
and 1s3p resonant inelastic X-ray scattering at the Co K edge. According
to X-ray diffraction data and Ti L2,3 X-ray absorption
spectra, all the samples before the thermal treatment exhibit anatase
structure with substantial amount of amorphous phase. After annealing
the Ti0.97Co0.03O2 samples in vacuum
or hydrogen at 700 °C, the anatase structure persists while amorphous
phase contribution is eliminated. Surface-sensitive soft X-ray absorption
Co L2,3 spectroscopy revealed only Co2+ ions
tetrahedrally coordinated by oxygen ions and no sign of metallic Co.
Co2+ tetrahedral sites (instead of typical octahedral ones)
are an additional evidence for Co2+ localization at the
distorted TiO2 particle surface. Bulk-sensitive X-ray diffraction,
Co K X-ray absorption spectroscopy, and 1s3p resonant inelastic X-ray
scattering at the Co K edge revealed clustering of metallic cobalt
inside of the large agglomerates formed by TiO2 nanoparticles
in annealed TiO2:Co nanopowders.
Magnetic
Fe3O4 nanoparticles (MNPs) are often used
to design agents enhancing contrast in magnetic resonance imaging
(MRI) that can be considered as one of the efficient methods for cancer
diagnostics. At present, increasing the specificity of the MRI contrast
agent accumulation in tumor tissues remains an open question and attracts
the attention of a wide range of researchers. One of the modern methods
for enhancing the efficiency of contrast agents is the use of molecules
for tumor acidic microenvironment targeting, for example, pH-low insertion
peptide (pHLIP). We designed novel organosilicon MNPs covered with
poly(ethylene glycol) (PEG) and covalently modified by pHLIP. To study
the specific features of the binding of pHLIP-modified MNPs to cells,
we also obtained nanoconjugates with Cy5 fluorescent dye embedded
in the SiO2 shell. The nanoconjugates obtained were characterized
by transmission electron microscopy (TEM), attenuated total reflection
(ATR), diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS), dynamic light scattering (DLS), UV and fluorescence spectrometry,
thermogravimetric analysis (TGA), CHN elemental analyses, and vibrating
sample magnetometry. Low cytotoxicity and high specificity of cellular
uptake of pHLIP-modified MNPs at pH 6.4 versus 7.4 (up to 23-fold)
were demonstrated in vitro. The dynamics of the nanoconjugate accumulation
in the 4T1 breast cancer orthotopically grown in BALB/c mice and MDA-MB231
xenografts was evaluated in MRI experiments. Biodistribution and biocompatibility
studies of the obtained nanoconjugate showed no pathological change
in organs and in the blood biochemical parameters of mice after MNP
administration. A high accumulation rate of pHLIP-modified MNPs in
tumor compared with PEGylated MNPs after their intravenous administration
was demonstrated. Thus, we propose a promising approach to design
an MRI agent with the tumor acidic microenvironment targeting ability.
The surface modification of FeO-based magnetic nanoparticles (MNPs) with N-(phosphonomethyl)iminodiacetic acid (PMIDA) was studied, and the possibility of their use as magnetic resonance imaging contrast agents was shown. The effect of the added PMIDA amount, the reaction temperature and time on the degree of immobilization of this reagent on MNPs, and the hydrodynamic characteristics of their aqueous colloidal solutions have been systematically investigated for the first time. It has been shown that the optimum condition for the modification of MNPs is the reaction at 40 °C with an equimolar amount of PMIDA for 3.5 h. The modified MNPs were characterized by X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric, and CHN elemental analyses. The dependence of the hydrodynamic characteristics of the MNP colloidal solutions on the concentration and pH of the medium was studied by the dynamic light scattering method. On the basis of the obtained data, we can assume that the PMIDA molecules are fixed on the surface of the MNPs as a monomolecular layer. The modified MNPs had good colloidal stability and high magnetic properties. The calculated relaxivities r and r were 341 and 102 mmol s, respectively. The possibility of using colloidal solutions of PMIDA-modified MNPs as a T contrast agent for liver studies in vivo (at a dose of 0.6 mg kg) was demonstrated for the first time.
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