Hydrothermal synthesis of nanostructured hybrids based on iron oxide and branched PEI polymers. Influence of high pressure on structure and morphology
Introduction
Recently, superparamagnetic iron oxide nanoparticles (SPIONs) have been extensively investigated for diagnostic, therapeutic and theranostic purposes (e.g. hyperthermia and drug delivery) because of their unique characteristics relative to other biomedical materials [1], [2], [3], [4], [5], [6], [7], [8]. Magnetic iron oxide (Fe3O4) has been used as biosensor, as contrast agent in magnetic resonance imaging, as carrier in drug delivery systems, as heat source for hyperthermia-based cancer treatments or as vehicles for biomarkers detection. However, naked SPIONs that lack a polymeric coating usually show aggregation in water, chemical instability in air and a lack of biodegradability in the physiological environment. Coating materials play an important role in the stabilization of aqueous suspensions of iron oxide, as well as in their functionalization, biocompatibility and long blood circulation time.
The surface of Fe3O4 nanoparticles has been functionalized with dendrimers, chitosan, dextran, liposomes, fatty acids (oleic acid), polyethylene glycol (PEG), polythiophene [9], polyaniline [10] and polyethyleneimine (PEI) [11], [12], [13], [14], [15]. Among these polymers, PEI has attracted considerable attention due to its well-known use for gene delivery [16]. Although PEI is a transfection agent itself, it has been demonstrated that when coupled with SPIONs, magnetofection efficiency increased in comparison to the transfection efficacy of PEI only.
Cai et al. [17] have prepared PEI coated Fe3O4 by a facile hydrothermal synthesis, starting from FeCl2·4H2O, ammonium hydroxide and PEI, at 134 °C with a gauge pressure of 2 bar. The black precipitate was collected by magnetic separation. PEI was used as stabilizer to form iron oxide nanoparticles with a size range of 16–22 nm.
The fabricated Fe3O4 NPs may be further functionalized for various biomedical applications, especially for targeted cancer imaging and therapeutics.
Li et al. [18] have synthesized hyaluronic acid-targeted iron oxide nanoparticles for in vivo targeted tumour magnetic resonance imaging applications. Fe3O4 stabilized with PEI was prepared by hydrothermal method at 134 °C, 3 h. Then, iron oxide nanoparticles were covalently modified with fluorescein isothiocyanate (FI) and HA via PEI-mediated conjugation chemistry.
Prijic et al. [19] have modified the surface of SPIONs with a combination of polyacrylic acid (PAA) and polyethyleneimine (PEI) that proved to be safe and effective for magnetofection of cells and tumours. SPIONs were synthesized by alkaline co-precipitation of ferrous and ferric sulphates in an aqueous solution according to the Massart method. Thereafter, SPIONs were coated in situ with pH-responsive anionic polymer PAA. The obtained SPIONs-PAA was functionalized with cationic polymer PEI through electrostatic interaction between highly negative surface of SPIONs-PAA and positively charged polycation PEI.
Xia et al. [20] reported the modification of commercial amine-functioned Fe3O4 nanoparticles with branched PEI using glutaraldehyde (GA) as reactive intermediate. The resultant Fe3O4–PEI NPs with immobilized iminodiacetic acid (IDA)–Cu2+groups showed potential to be used as magnetic adsorbents to separate or purify biomolecules in large scale.
Taking into account these literature results, the main goal of this study was to obtain homogeneous hybrids in which iron oxide nanoparticles are entrapped within polymer structure. For this purpose, hybrid nanomaterials based on branched polyethyleneimine (PEI) and iron oxide at different mass ratios were synthesized in a single step by hydrothermal procedure at high pressure and low temperature. Iron oxide is formed in the presence of branched PEI and the interaction between the two components takes place in the reaction medium.
The influence of synthesis parameters (inorganic–organic mass ratio, time and pressure) on the hybrid formation was investigated and the results showed that pressure has the highest impact, playing an important role on the morphology and structure of these nanomaterials. Consequently, chemical and structural properties were analysed and discussed on the basis of the results obtained using different techniques, such as FTIR, DSC-TG, HRTEM, electron paramagnetic resonance (EPR), 57Fe Mössbauer analysis and SQUID (Superconducting Quantum Interference Device) magnetometry.
Section snippets
Materials and samples preparation
Hybrid nanostructures based on iron oxide nanoparticles and branched polyethyleneimine (PEI) polymer were prepared in aqueous solution starting from iron (III) chloride hexahydrate (Sigma Aldrich), ammonia solution 25% (Chimreactiv) and commercial PEI, average Mw ∼ 25,000 by LS, average Mn ∼ 10,000 by GPC, branched (Sigma Aldrich, https://www.sigmaaldrich.com/). The amount of FeCl3 × 6H2O and PEI was calculated to have theoretical mass ratio between iron oxide and PEI equal to 0.5; 1, and 1.5,
FT-IR analysis
FT-IR spectrum of branched PEI (Fig. 1a) presented specific absorption bands at 3278 and 1586 cm−1 which were assigned to NH2 groups.
Absorption bands at 2934, 2810 and 1454 cm−1 could be attributed to the CH2 groups, while the band at 1046 cm−1 corresponds to the stretching vibration of C–N bond.
FT-IR spectra of the investigated iron oxide-PEI nanohybrids at different iron oxide/PEI mass ratios are presented in Fig. 1b–d.
For iron oxide/PEI mass ratio = 1.5, Fig. 1b revealed characteristic peaks
Conclusion
Iron oxide nanoparticles with PEI branched polymer at different ratios (iron oxide: PEI mass ratio = 1.5, 1 and 0.5 respectively) were prepared in high pressure and low temperature conditions, using hydrothermal method. Synthesis parameters (especially isostatic pressure) influence thermal stability and morphology of the hybrid nanostructures. HRTEM characterization revealed the morphology, shape and size of iron oxide inside the hybrid nanostructure. Small crystallites of FeO(OH) with size
Acknowledgements
The authors from National R&D Institute for Non-Ferrous and Rare Metals thank the Project co-financed by a Grant from Switzerland (Ctr. IZERZ0– No. 142141 and Ctr. 4/RO-CH/RSRP/2012) through the Swiss contribution to the enlarged European Union for the financial support. Authors from University of Urbino acknowledge a partial financial support from PRIN 2012 – NANOMed. The authors also thanks to the COST Action MP1202.
Authors from NIMP acknowledge the partial financial support through the Core
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