A series of cobalt (Co) and its oxides based nanoparticles were synthesized by using hyperbranched polyester polyol Boltorn H20 as a platform and sodium borohydride as a reducing agent. UV, FT-IR, XRD, NTA, and TEM methods were employed to obtain physicochemical characteristics of the products. The average diameter of Co nanoparticles was approximately
Magnetic nanoparticles exhibit specific physical properties and are of great interest because of their prospective applications in biology and medicine [
The magnetic properties of nanoparticles are determined by many factors. The chemical composition, crystal structure and the degree of its defectiveness, morphology, and the interaction of particles with the surrounding matrix and neighboring particles play crucial role [
The strong magnetic interaction between cobalt nanoparticles and their propensity for oxidation make it difficult to obtain stable colloids. Therefore, in most cases, organic stabilizers are used to control the growth of nanoparticles and prevent the occurrence of adverse reactions [
The nature of the stabilizer often determines the morphology of nanoparticles and the properties of the hybrid material. The use of polymer matrix for stabilization makes it possible to combine the unique properties of metal nanoparticles with useful properties of polymers [
Usage of HBP as a platform for cluster growth, both cluster stability and full control over size, and size distribution were achieved by simultaneously allowing access of substrates to the cluster surface. An additional advantage of HBP matrix in the synthesis of practically useful metal nanoparticles is their biosimilar topological structure and simplicity of synthesis [
In this study, we describe the synthesis of Co nanoparticles via the matrix of nontoxic hyperbranched polyester polyol based on 2,2-bis-hydroxymethyl-propionic acid.
The initial reagent was anhydrous salt cobalt (II) chloride (СоCl2) (97%, Alfa Aesar). Stabilizer was hyperbranched polyester polyol Boltorn H20 (BH20) (Sigma-Aldrich, theoretically having 16 hydroxyl end groups per molecule and the average molecular weight of 1749 g/mol). Sodium borohydride NaBH4 (98%, Alfa Aesar) was used as a reducing agent. The organic solvents such as ethanol and diethyl ether were used as solvents for the synthesis and isolation of nanoparticles.
The electronic absorption spectra were recorded on Lambda 750 (Perkin Elmer) in the wavelength range from 200 to 1000 nm at
The size, concentration, and movement of nanoparticles were determined using the NanoSight LM-10 (Malvern Instruments Ltd, UK) equipped with a CMOS camera C11440-50B with scientific image sensor FL-280 Hamamatsu Photonics (Japan) as a detector. Measurements were carried out in a special cell for organic solvents having a modified entry angle for the laser beam into the solution, a 405 nm laser (version cd, S/N 2990491), and Kalrez sealing ring. Contact thermometer OMEGA HH804 (Engineering, Inc/Stamford, CT, USA) was used to determine the temperature in the cell during the experiment. The NanoSight NTA 2.3 software (build 0033) was used to process the results.
ATR-FT-IR spectra were recorded over the range from 4000 to 400 cm−1 using a FT-IR spectrometer Spectrum 400 (Perkin Elmer) with a universal ATR accessory and a ZnSe prism. The resolution of the spectra was 1 cm−1 and scanning was repeated 16 times.
X-ray powder diffraction (XRPD) studies of nanoparticles samples were made using a MiniFlex 600 diffractometer (Rigaku, Japan) equipped with a D/teX Ultra detector. In this experiment, Cu K
Magnetic properties were measured by PPMS-9 (Quantum Design, USA) equipped with vibrating sample magnetometer (VSM). Zero field-cooled (ZFC) and field-cooled (FC) measurements were performed in 100 Oe. Field dependencies of magnetization were measured at 5–300 K at field range from −1 T to 1 T.
Analysis of samples was carried out in a transmission electron microscope Hitachi HT7700 Exalens. Sample preparation was as follows: 10 microliters of the suspension was placed on a formvar/carbon lacey 3 mm copper grid, and drying was performed at room temperature. After drying grid was placed in a transmission electron microscope using special holder for microanalysis. Analysis was held at an accelerating voltage of 100 kV in TEM mode, and the elemental analysis was carried out in STEM mode, at the same parameters using Oxford Instruments X-Max™ 80 T detector. The size and shape of hybrid NPs were estimated via AxioVision rel.48 soft.
The size distribution of cobalt nanoparticles was obtained by TEM images processing using AxioVision program, version 4.8.2. The size distribution curve was constructed on the base of fivefold sampling of 400 treated nanoparticles.
HBPО BH20 was dissolved in 30 ml of 50% water-ethanol solution (
Synthesis of organic-inorganic nanocomposites was carried out in the following way: the first stage is the formation of complex forms of Co2+, HBPO; the second stage was the synthesis of polymer-metal nanocomposites by the chemical reduction method [
HBPO BH20 was used to stabilize cobalt nanoparticles. The molecule of HBPO BH20 contains ester and hydroxyl groups (Figure
Structure of HBPО G2.0 (BH20).
Concentration and size distribution from NTA measurements of BH20 aqueous solution.
In the absorption spectra of the HBPO BH20 solution, there were no absorption bands in the visible region of the spectrum (Figure
Stability constants (lg
Complex | lg |
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Co4(BH20) | 6.2 |
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Co8(BH20) | 10.4 |
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Co10(BH20) | 17.7 |
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Co12(BH20) | 26.2 |
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Co16(BH20) | 31.1 |
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(a) UV-vis absorption spectra of Со2+-BH20 complex with different Со2+ : BH20 molar ratio in aqueous solution. (b) Spectrophotometric titration solution of BH20 by CoCl2 solution (
NТА analysis showed that the increase in the molar ratio
Concentration and size distribution from NTA measurements of сomplexes Co8(BH20) (a) and Co10(BH20) (b) in aqueous solution.
It can be assumed that the introduction of cobalt ions into the BH20 solution leads to a violation of the hydrogen bonding system, followed by the destruction of the BH20 associates and the formation of associates of complex forms
Comparing the data of UV-vis spectroscopy and NTA analysis, it can be assumed that an increase in the molar ratio Со2+ : ВH20 from 4 : 1 to 16 : 1 leads to a decrease in the proportion of coordinated hydroxylic groups of HBPO in the inner sphere of the Co2+ ion that could be indicated by a decrease in the “red shift” value and an increase in the hydrodynamic diameter of
Synthesis of cobalt nanoparticles (CoNPs) was carried out by the reduction of
During the reduction process, for all ratios, the color of the solution has changed from light pink (Figures
Color transformation of Co2+-BH20 solution before (a, d) and after (b, e) reduction; the collection of CoNPs by a magnet (c, f).
After the reduction of all complex forms according to the UV/vis spectroscopy data, the absorption bands disappear at
UV/vis spectra of aqueous solutions containing CoNPs after Co8(BH20) reduction (a), CoNPs after Co10(BH20) reduction (b), CoNPs after Co12(BH20) reduction (c), and CoNPs after Co16(BH20) reduction (d).
СoNPs (Co2+ : HBPO = 4 : 1, 8 : 1, and 10 : 1) samples failed to isolate quantitively. СoNPs (Co2+ : HBPO = 12 : 1 and 16 : 1) samples were isolated from the solution as the black powder. However, СoNPs (16 : 1) have possessed less stability and were easily oxidized by air oxygen, and the color of the powder changed to green, indicating the presence of CoO.
The FR-IR spectra of BH20, СoNPs (12 : 1) and СoNPs (16 : 1) solids (oxidized forms), were shown in Figure
FT-IR spectra of HBPО BH20 (a), СoNPs (12 : 1) (b), and СoNPs (16 : 1) (с).
XRD pattern indicated the amorphous structure of products. The broadening of the diffraction peaks of СoNPs (12 : 1) (Figure
XRD powder pattern of CoNPs (12 : 1) (a) and CoNPs (16 : 1) (b).
The magnetic curves field dependence of magnetization of CoNPs (12 : 1) was measured at 5, 10, 50, 100, 200, and 360 K (Figure
Hysteresis loops obtained at 5 K and 300 K for CoNPs (12 : 1).
The temperature dependence of the magnetization was measured under magnetic field of 100 Oe from 5 to 300 K using zero field-cooled (ZFC) and field-cooled (FC) procedure. This measurement allowed determining the blocking temperature of CoNPs. The obtained ZFC–FC curves of CoNPs (12 : 1) nanocomposite are displayed in Figure
ZFC–FC curves measured in an applied field of 100 Oe for CoNPs (12 : 1).
At a temperature of 300 K discrepancy between the ZFC and FC curves was observed. A sufficiently high temperature, which characterizes the temperature of irreversible magnetic changes, is associated with a wide size distribution of nanoparticles in the sample and strong interaction between the particles [
The variations in size, determined by different methods, were due to the fact that these methods rely on different physical principles and/or detection methods. In addition, electron microscopy probes dry particles, that is, the metallic core only, whereas the NTA probe the hydrodynamic diameter which is always larger. The size predicted by TEM analysis was found to be smaller than predicted by NTA analysis.
According to the NTA method hydrodynamic diameter of CoNPs nanocomposite rose from
Hydrodynamic diameter (
СoNPs |
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СoNPs (8 : 1) |
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СoNPs (10 : 1) |
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СoNPs (12 : 1) |
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СoNPs (16 : 1) |
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Concentration and size distribution from NTA measurements of CoNPs (8 : 1) (a) and CoNPs (12 : 1) (b) in aqueous solution.
The successful formation of CoNPs was first confirmed by TEM studies. Figure
TEM images CoNPs (12 : 1) (a, b), corresponding particle-size distribution of CoNPs (c), and EDS spectrum (d) for the area corresponding to (a). The Cu signals come from TEM grids.
Thus, for the first time the process of preorganization of Co2+ ions on the platform of a hyperbranched polyester polyol of the second generation was studied and the significant complex forms of
The authors O. I. Medvedeva, S. S. Kambulova, O. V. Bondar, A. R. Gataulina, N. A. Ulakhovich, A. V. Gerasimov, V. G. Evtugyn, I. F. Gilmutdinov, and M. P. Kutyreva declare that there are no conflicts of interest regarding the publication of this paper.
The magnetic measurements were carried out at the Federal Center of Shared Facilities of Kazan Federal University. Microscopy studies were carried out at the Interdisciplinary Center of Analytical Microscopy of Kazan Federal University. The work is performed according to the Russian Government Program of Competitive Growth of Kazan Federal University.