Synthesis of PLA-PEG-FA and PLA-PEG-Glu copolymers
The chemical structure of the PLA-PEG-FA and PLA-PEG-Glu copolymers were
examined with proton nuclear magnetic resonance spectroscopy
(1H NMR). Fig. 1 showed the 1H-NMR
spectrum of PEI-PLA-PEG-FA copolymer, where the presence of
CH2 protons of PEG in PEI-PLA-PEG-FA copolymer was
observed around 3.6 ppm (-CH2CH2O-).
Moreover, the appearance of signals at 5.1 and 1.6 ppm are attributed to
the methine (CH) protons and methyl protons – (CH3) of PLA,
respectively (Fig. 1 A) (Heald et al., 2002; Sim et al., 2017).
The appearance of chemical shifts at 6.5-8.6 ppm (aromatic protons
associated with FA) and 7.9 ppm (s, weak, 1H,
triazoles) provided evidence for successfully obtaining PEI-PLA-PEG-FA
copolymer (S.-J. Yang et al., 2010)(Fig. 1 A).
Also in the Fig. 1 B The multiple sharp peaks at 3.6-3.8 ppm and weak
peak at 5.4 ppm which were assigned to methylene protons in PEG and
aromatic proton in glucose respectively, appeared on the spectrum after
the reaction of PLA-PEI with PEG-Glu (Fig. 1 B)(Sadeghi et al., 2015).
The structures of PLA-PEG-FA and PLA-PEG-Glu were ascertained by the
FTIR spectrum (Fig. 2). The sharp peak appearing at 1767
cm-1 was assigned to the carbonyl (C=O) group in the
PLA-PEG copolymer (Xiong et al., 2011).
The peaks that appeared in the regions of 1193 cm-1 and 1460 cm-1 are
related to stretching C-O and the bending of -CH2- groups in PLA-PEG
copolymer, respectively (Amani, Kabiri, Shafiee, & Hamidi, 2019;
Danafar, Rostamizadeh, Davaran, & Hamidi, 2017).
Moreover, the peaks at 1640 and 1095 cm-1 corresponds
to the C=O and C-O-C stretching of COOH in the FTIR spectrum of PEG,
PLA- PEG-FA and PLA-PEG-Glu suggested that PEG was grafted to the
nanoparticles (Ahmed, Arfat, Castro-Aguirre, & Auras, 2016; Rafat et
al., 2010).
The FTIR spectra of PLA-PEG-FA copolymer showed that there is also a
broad peak in the region between 1580 and 1648 cm−1
belonging to the amine group of FA. Moreover, the peak observed around
2108 and 2116 cm-1 were derived from azido group of FA
and Glu respectively. The peak at 3580 cm-1
corresponds to the O-H stretching of OH in the FTIR spectrum of glucose
suggested that glucose was grafted to the PLA-PEG.
The FT-IR spectra of FeCo, FeCo-PEI, FeCo-PEI-PLA-PEG-FA, and
FeCo-PEI-PLA-PEG-Glu are illustrated in Fig. 2.
After conjugation of PEI with FeCo nanoparticles, the PEI peaks from
1050 cm-1 to 1250 cm-1 were
attributed to the amide and amide amino groups , which indicated that
the PEI was attached on the surface of FeCo nanoparticles (Fig. 2)
(Gultekinoglu et al., 2015).
As also evident in Fig. 2, almost all peaks related to PLA-PEG,
FeCo-PEI, glucose and folic acid were observed in the NPsA and NPsB,
therefor it is indicated the successful synthesis of these
nanoparticles.
Thermogravimetric analysis (TGA) of NPsA and NPsB was performed to find
out different chemical compounds in the samples. The TGA curves of PEG,
PEI, PLA, PLA-PEG, PLA-PEG-FA and CBP-FA copolymers, are shown in Fig.
3.
TGA analysis of the pure polymers (PEI, PLA and PEG) shows single step
degradation while the spectrum of PLA-PEG copolymer exhibits three steps
degradation related to the presence of three components in the PLA-PEG
copolymer. The initial mass loss in the range of 175 °C to 305 °C, may
be due to the loss of adsorbed water molecules from the copolymer
(Jadhav et al., 2013).
The second stage of the PLA-PEG degradation occurs between 320 °C and
380 °C, which is related to the thermal degradation reaction of the PLA
polymer (Shih & Huang, 2011). The third stage of thermal degradation
occurs at temperatures above 380 °C, is associated with degradation of
PEG polymer (Kwon & Kim, 2006).
The thermogram of the –FA and –Glu shows several mass losses
during the temperature rise from 200 to which proposes that the glucose
has higher temperature resistance, compared to other pure PEI, , and
folic acid and therefore, temperature resistance of PLA-PEG copolymer
increased after binding to glucose.
As it is seen in Fig. 3 the FeCo-PEI nanoparticles has a much better
thermal stability than that of PLA-PEG-FA and PLA-PEG-Glu. Therefore,
improvement in temperature resistance of PLA-PEG-FA and PLA-PEG-Glu was
observed after modification with FeCo nanoparticles, which indicated the
successful conjugation of PLA-PEG-FA and PLA-PEG-Glu with FeCo.