Flares from tidal disruption events are unique tracers of quiescent black holes at the centre of galaxies. The appearance of these flares is very sensitive to whether the star is totally or partially ...disrupted, and in this paper we seek to identify the critical distance of the star from the black hole (rd) that enables us to distinguish between these two outcomes. We perform here mesh-free finite mass, traditional, and modern smoothed particle hydrodynamical simulations of star-black hole close encounters, with the aim of checking if the value of rd depends on the simulation technique. We find that the critical distance (or the so-called critical disruption parameter βd) depends only weakly on the adopted simulation method, being βd = 0.92 ± 0.02 for a γ = 5/3 polytrope and βd = 2.01 ± 0.01 for a γ = 4/3 polytrope.
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•A new 3,6-O,O’- dimyristoyl chitosan (DMC) derivative was synthesized and characterized.•DMC micelles were obtained and promoted PTX solubilization.•The derivative polymer and DMC ...micelles presented good biocompatibility in cytotoxicity studies.•The drug in vitro permeability was evaluated in different cell models.
The aim of the present study was to investigate the potential application of 3,6-O,O’- dimyristoyl chitosan DMCh, an amphiphilic derivative of chitosan, for improving the oral bioavailability of paclitaxel (PTX), a water insoluble anticancer drug. The O-acylation of chitosan with myristoyl chloride was carried out by employing high (≈13.3) or low (2.0) molar excess of chitosan to result in samples DMCh07 and DMCh12, respectively. The successful O-acylation of chitosan was confirmed by FTIR and 1H NMR spectroscopy, the latter allowing also the determination of average degree of substitution (DS). The critical aggregation concentration (CAC) of samples DMCh07 (DS≈6.8%) and DMCh12 (DS≈12.0%) were 8.9×10−3mg/mL and 13.2×103mg/mL, respectively. It was observed by TEM that the DMCh micelles showed spherical shape while DLS measurements allowed the determination of their average size (287nm–490nm) and zeta potential (+32mV to +44mV). Such DMCh micelles were able to encapsulate paclitaxel with high drug encapsulation efficiency (EE), as confirmed by HPLC analyses. Studies on the cytotoxicity of DMCh07 micelles toward Caco-2 and HT29-MTX cells showed that, regardless the PTX loaded, DMCh07 micelles slightly decreased cellular viability at low micelles concentration (≤1μg/mL) while at high concentration (>10μg/mL) PTX-loaded DMCh07 micelles were less toxic toward Caco-2 cells when compared to free PTX. The PTX permeation across Caco-2 monoculture and Caco-2/HT29-MTX co-culture model confirmed the potential of DMCh micelles in improving the intestinal absorption of PTX. These results suggest that DMCh micelles may be a promising carrier to encapsulate PTX aiming cancer therapy.
The thermal degradation of chitosan, degree of acetylation 12%, was studied by using TG in nitrogen atmosphere in isothermal and dynamic conditions. Temperatures ranging from 240
°C to 280
°C were ...studied in isothermal conditions while heating rates in the interval 2.5–15.0
°C/min were employed when using dynamic conditions. The data issued from the dynamic experiments were treated by the methods proposed by Ozawa–Flynn–Wall and Kissinger, resulting in apparent activation energy of 149.6
kJ/mol and 138.5
kJ/mol, respectively. The approach proposed by MacCallum was used to treat the data issued from the isothermal experiments, resulting in
E
a
=
153
kJ/mol in good agreement with the results issued from the dynamic experiments. The use of the isoconvertional method due to Vyazovkin also showed a good agreement with the value of the activation energy issued by applying the method of Ozawa–Flynn–Wall. It was also observed that regardless of using isothermal or dynamic experimental conditions, the kinetic model best suited to describe the thermal degradation of chitosan is the catalytic Šesták–Berggren model.
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•A strong associative electrostatic interaction between cationized chitosan and sodium 1-decanesulfonate was shown.•The complex presents better interfacial properties than surfactant ...and polysaccharide alone.•Surfactant-polysaccharide complex is a better emulsion stabilizer than either surfactant or polysaccharide alone.
The interactions between a polyelectrolyte and an oppositely charged surfactant allows obtaining complexes with specific properties. Such complexes between a quaternized chitosan, obtained from the reaction between chitosan and quaternary ammonium epoxides, and sodium 1-decanesulfonate were studied. Physico-chemical characteristics were obtained from conductimetric experiments as well as surface tension studies. A strong associative electrostatic interaction between the polysaccharide and the surfactant was shown from conductimetric experiments and a degree of association equal to 0.74 was determined. The complex presents better interfacial properties than surfactant and polysaccharide alone. The critical aggregation concentration is much smaller than the critical micellar concentration of the surfactant alone in solution. Moreover the viscoelastic moduli of the interfacial layer was increased in presence of the complex by comparing with surfactant alone. All these considerations allow using the surfactant-polysaccharide complex as a better emulsion stabilizer than either surfactant or polysaccharide alone.
We present the evolution of the X-ray emission properties of the magnetar 1E 1547.0–5408 since February 2004 over a time period covering three outbursts. We analyzed new and archival observations ...taken with the
Swift
,
NuSTAR
,
Chandra
, and
XMM–Newton
X-ray satellites. The source has been observed at a relatively steady soft X-ray flux of ≈10
−11
erg cm
−2
s
−1
(0.3–10 keV) over the last 9 years, which is about an order of magnitude fainter than the flux at the peak of the last outburst in 2009, but a factor of ∼30 larger than the level in 2006. The broad-band spectrum extracted from two recent
NuSTAR
observations in April 2016 and February 2019 showed a faint hard X-ray emission up to ∼70 keV. Its spectrum is adequately described by a flat power law component, and its flux is ∼7 × 10
−12
erg cm
−2
s
−1
(10–70 keV), that is a factor of ∼20 smaller than at the peak of the 2009 outburst. The hard X-ray spectral shape has flattened significantly in time, which is at variance with the overall cooling trend of the soft X-ray component. The pulse profile extracted from these
NuSTAR
pointings displays variability in shape and amplitude with energy (up to ≈25 keV). Our analysis shows that the flux of 1E 1547.0–5408 is not yet decaying to the 2006 level and that the source has been lingering in a stable, high-intensity state for several years. This might suggest that magnetars can hop among distinct persistent states that are probably connected to outburst episodes and that their persistent thermal emission can be almost entirely powered by the dissipation of currents in the corona.
The development and application of a functionalized carbon nanotubes paste electrode (CNPE) modified with crosslinked chitosan for determination of Cu(II) in industrial wastewater, natural water and ...human urine samples by linear scan anodic stripping voltammetry (LSASV) are described. Different electrodes were constructed using chitosan and chitosan crosslinked with glutaraldehyde (CTS-GA) and epichlorohydrin (CTS-ECH). The best voltammetric response for Cu(II) was obtained with a paste composition of 65% (m/m) of functionalized carbon nanotubes, 15% (m/m) of CTS-ECH, and 20% (m/m) of mineral oil using a solution of 0.05
mol
L
−1 KNO
3 with pH adjusted to 2.25 with HNO
3, an accumulation potential of −0.3
V
vs. Ag/AgCl (3.0
mol
L
−1 KCl) for 300
s and a scan rate of 100
mV
s
−1. Under these optimal experimental conditions, the voltammetric response was linearly dependent on the Cu(II) concentration in the range from 7.90
×
10
−8 to 1.60
×
10
−5
mol
L
−1 with a detection limit of 1.00
×
10
−8
mol
L
−1. The samples analyses were evaluated using the proposed sensor and a good recovery of Cu(II) was obtained with results in the range from 98.0% to 104%. The analysis of industrial wastewater, natural water and human urine samples obtained using the proposed CNPE modified with CTS-ECH electrode and those obtained using a comparative method are in agreement at the 95% confidence level.
Abstract
GRB 221009A is the brightest gamma-ray burst (GRB) ever detected that has occurred at low Galactic latitude. Owing to this exceptional combination, its prompt X-ray emission could be ...detected for weeks in the form of expanding X-ray rings produced by scattering in Galactic dust clouds. We report on the analysis of 20 rings, generated by dust at distances ranging from 0.3 to 18.6 kpc, detected during two X‐ray Multi Mirror (XMM)-Newton observations performed about 2 and 5 days after the GRB. By fitting the spectra of the rings with different models for the dust composition and grain size distribution, we reconstructed the spectrum of the GRB prompt emission in the 0.7–4 keV energy range as an absorbed power law with photon index Γ = 1–1.4 and absorption in the host galaxy
N
H,z
= (4.1–5.3) × 10
21
cm
−2
. Taking into account the systematic uncertainties regarding the column density of dust contained in the clouds producing the rings, the 0.5–5 keV fluence of GRB 221009A can be constrained between 10
−3
and 7 × 10
−3
erg cm
−2
. Both the fluence and the photon index indicate the presence of a possible soft excess with respect to the extrapolation of the main GRB peak observed at higher energies.
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High molecular weight Chi modulates the orientation of SF groups (Try and Try).Chi and SF LbL films were successfully used as template for phytase immobilization.The matrix ...improved the performance of phytic acid biosensing.
In this paper, we show that chitosan may induce conformation changes in silk fibroin (SF) in layer-by-layer (LbL) films, which were used as matrix for immobilization of the enzyme phytase to detect phytic acid. Three chitosan (CH) samples possessing distinct molecular weights were used to build CH/SF LbL films, and a larger change in conformation from random coils to β-sheets for SF was observed for high molecular weight chitosan (CHH). The CHH/SF LbL films deposited onto interdigitated gold electrodes were coated with a layer of phytase, with which phytic acid could be detected down to 10⿿9M using impedance spectroscopy as the principle of detection and treating the data with a multidimensional projection technique. This high sensitivity may be ascribed to the suitability of the CHH/SF matrix, thus indicating that the molecular-level interactions between chitosan and SF may be exploited in other biosensors and biodevices.
Four samples of
N,
O-carboxymethylchitosan
(
0.5
<
DS
¯
<
1.5
)
were prepared by reacting chitosan
(
DA
¯
=
24
%
)
with monochloroacetic acid in the presence of excess sodium hydroxide. The ...carboxymethylchitosan samples were soluble in a wider range of pH as compared to the parent chitosan and the X-ray diffraction showed that they adopt a less ordered arrangement. The carboxymethylation of chitosan decreased the thermal stability of the polymer as evaluated by thermogravimetry but no clear dependence of the activation energy on the average degree of substitution of carboxymethylchitosan was identified. However, the values of activation energy of carboxymethylchitosan issued from the isothermal study depended on the degree of conversion, suggesting the occurrence of a complex set of simultaneous reactions.
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