Fe3O4 nanoparticles were modified by TMSPM with the aim to create C = C groups on their surface for subsequent graft polymerization of 2-VP as it is presented on Fig. 1. Figure 2 performs the data of FTIR spectroscopy. Spectrum of initial magnetite nanoparticles correlating well with literature data [18]. It can be characterized by ν Fe-O-Fe (540 cm− 1 и 630
cm− 1), δ O-H (1640 cm− 1) and ν O-H (3350 см−1) peaks. After treating the samples with silanes (TMSPM), spectrum has significantly changed: Si-O-Si characterizing bonds (1016 cm− 1, 1175 cm− 1), C-O bonds (1300 cm− 1), δs О-СН3 bonds (1450 cm− 1), С=С and С=О bonds (1635 cm− 1 и 1718 cm− 1), C-H groups at 2935 cm− 1. Also, ν(O-H) peak gradually reduced, which indicates reaction goes by these groups. Then nanoparticles were treated with 5-FU imprinted poly (2-vinylpirydine) hydrogel. After this, N-H bonds (3425 cm− 1), distinctive for pyridine ring C = N peak (1605 см−1) are appearing at FTIR spectrum. С=С and C = O peaks are indicating the crosslinking of polymer to hydrogel and related to trimethylolpropane. In favor of this the fact that Si-O-Si peaks are reduced in contrast to C = C and C = O. Peaks of 5-FU are poorly visualized at FTIR spectra and can’t be observed due to the small amount.
From EDA, after the silane treating Si and C appeared in the structure (1.24 at. % and 13.92 at. % respectively). After the next step of graft polymerization, the concentration of C and N increased up to 18.28 at. % and 0.9 at. % respectively, which indicates successful grafting of P2VP. Sorption of 5-FU is indicated by the increasing amount of F (up to 0.007 at. %), but the detection of low-weight elements such as F and N is not precise due to the specificity of EDA.
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Figure 2 - FTIR spectra of synthesized samples: initial Fe3O4 (black line), Fe3O4-TMSPM (red line) and Fe3O4-TMSPM-p(2VP)-5FU (blue line) |
Figure 3 shows the results of X-ray diffraction (XRD) of the studied samples of iron-containing nanoparticles with various types of modification. Lattice data calculated from XRD are presented in Table 1. In the initial state, the resulting structures are nanoparticles with a cubic type of crystal structure characteristic of the Fe3O4 phase (PDF-00-065-0731). For samples Fe3O4-TMSPM and Fe3O4-TMSPM-P2VP-5FU in the region 2θ = 18–20⁰, peak broadening is observed which is characteristic for the presence of amorphous structures come from the modification of nanoparticles. At the same time, the analysis of the broadening of the main diffraction reflections showed that for the modified nanoparticles, an increase in the size of crystallites is observed, which can be explained by the modification of the surface of the nanoparticles.
Table 1
Parameters | Sample |
Fe3O4 | Fe3O4-TMSPM | Fe3O4-TMSPM-P2VP-5FU |
Lattice parameter, Å | a = 8.31752 Å | a = 8.2241 Å | a = 8.84362 Å |
Lattice volume, Å3 | 575.42 | 576.43 | 580.85 |
Crystalline size, nm | 11–13 | 15–17 | 19–22 |
Crystalinity degree, % | 63.8 | 65.9 | 70.0 |
Size of nanoparticles was estimated by DLS, initial Fe3O4 have average hydrodynamic size of 51±5 nm, Fe3O4-TMSPM − 62±7 nm and Fe3O4-TMSPM-P2VP-5FU − 89±9 nm. We observe a regular increase in particle size from the modification stage, and we also demonstrated an increase in the size of crystallites from XRD analysis. Nanoparticle sizes of Fe3O4-TMSPM-P2VP-5FU are acceptable for administration into the body [19].
Magnetic characteristics of composites were studied on a universal measuring system (automated vibrating magnetometer) «Liquid Helium Free High Field Measurement System» (Cryogenic LTD) in magnetic fields ± 1 T at 300 K. The Fig. 4 shows the hysteresis loops of original and modified magnetite particles. The Table 2 presents the results of calculating the magnetic characteristics of the samples. The original particles have coercivity of 14 Oe, saturation magnetization of 62.1 mu/g, and remanence of 1.21 mu/g, typical of magnetite. The characteristics of the modified particles differ from the initial ones and correspond to a change in the content of the magnetic phase in the sample (decrease in saturation magnetization), a change in the state of the magnetic core (decrease in coercivity). The thin coating formed on the surface of the particles increases the distance between the particles, and, accordingly, the residual magnetization decreases (from 1.21 him/g).
Table 2
The results of calculating the magnetic characteristics of the samples.
| H, Oe | Mr, emu/g | Ms, emu/g |
Fe3O4 | 14 | 1.21 | 62.1 |
Fe3O4-TMSPM | 6.5 | 1.15 | 58.9 |
Fe3O4-TMSPM-P2VP-5FU | 9.2 | 1.17 | 57.8 |
Figure 5 shows the TGA curves and its derivatives. As seen from the presented data, weight lost while heating from 25 oC to 925 oC is 3.7% due to the loss of physically and chemically absorbed water. 3% of mass is lost for the samples covered with silanes, 12% is lost for the nanoparticles with grafted bare P2VP hydrogel and 6.6% for nanoparticles with 5-FU imprinted hydrogel. This might be related to the transfer of radical to the C = O group of 5-FU, making it to exert inhibiting properties, reducing the amount of grafted hydrogel. Samples with immobilized silane and polyvinyl pyridine hydrogel are losing weight at 200–500 оС, which is well correlating with literature data [20]. 5-FU presence is undetectable in this case because it decays at 289 оС, which is correlating with the samples above.
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Figure 5 - TGA curves and its derivatives for samples Fe3O4 (a), Fe3O4-TMSPM (b), Fe3O4-TMSPM-p(2VP) (c), Fe3O4-TMSPM-p(2VP)-5FU (d) |
Release of 5-FU was examined at different pH (4.5 and 7.58) and constant temperature of 36.6 °C, results are presented in Fig. 6. Experiment have shown that the desorption goes rapidly during first 4 h and then its speed significantly decreases, but it is not finished even after 150 h. Also, the speed of the desorption and the amount of desorbed 5-FU is higher in acidic mediums (3.8 mg/l for pH 4.5 and 3.2 mg/l for pH 7.58). This can be related that isoelectric point of 2-VP is around 3.2 and desorption process is easier when closer to it.
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Figure 6 - Desorption of 5-FU at pH 4.5 (red line), 7.58 (black line) |