Among the various promising candidates for biomedical use, magnetic nanoparticles (MNPs) have received significant attention as a result of their intrinsic magnetic properties. This class of nanomaterials include metallic, bimetallic, and superparamagnetic iron oxides (SPIOs) . For biomedical aims, SPIOs are more interested due to their low toxicity nature and facility of surface engineering with biocompatible agents as well as targeting, imaging, and therapeutic molecules . This flexibility has provided SPIOs to be simply used in magnetic separation, biosensor, in vivo medical imaging, drug delivery, tissue repair, and hyperthermia applications [3-9]. The magnetic action of SPIOs depends significantly on their shape, size and surface load , and these factors are determined by the designed synthesis procedure and applied surface coat onto SPIOs . Hence, various synthesis methods including the coprecipitation, hydrothermal processes, sol-gel, and thermal decomposition have been developed for the synthesize high-quality MNPs [12-17]. In addition to these methods, electrochemical process has been also applied as an easy, non-expensive and fast procedure for preparation of naked and surface coated SPIOs [18-22]. As an electrochemical route, cathodic electrodeposition has been introduced as a facile preparation method for the fabrication of nanomaterial due to its ability in controlling crystal phase, size and composition of products [22-27].
As an important factor, surface coating plays essential role in magnetic ability and medical uses of SPIOs. In this regard, it was stressed that although pristine/or uncoated SPIOs are stable in high and low pH mediums, but in vivo uses require surface coated SPIOs . For instance, surface capping layer should be able to (a) prevent agglomeration of iron oxide particles, (b) provide chemical bonds for further functionalization of SPIOs with therapeutic and diagnostic agents , and enhance their pharmacokinetics, endosomal release, and tailored drug loading and release behaviors . Up now, various polymers including PEG , PVA , chitosan , PEI , and PVP [34,35] have been applied as SPIOs surface coat. For applying surface capping onto SPIOs surfaces, several strategies like as covalent linkage, direct nanoparticle conjugation, click chemistry, covalent linker chemistry, and also physical interactions have been employed . It has been found that covalent linkages are strong and stable bonds, which can be specifically formed between functional groups, typically –NH2, -COOH, and -SH groups attached onto SPIOs surface and conjugated ligands . Dextran is an abundant, inexpensive polymer that composed of alpha-D-glucopyranosyl monomers, which has a large number of -OH groups to provide covalent linkage and local sites for surface capping of SPIOs, as well as biological compatibility and stability . Hence it is proper candidate for biomedical uses like as hyperthermia and MRI contrast agent [37-39]. Metal cations doping could be also an effective strategy for improving the magnetic properties of SPIOs [40-43]. Recently, we have reported an in situ doping of Fe3O4 nanoparticles with various metal cations through electrochemical deposition method [44-46], the magnetic evaluations have indicated that the superparamagnetic behavior of iron oxide is improved via this strategy [44-46]. Here, we report an electrochemical platform for fabrication of dextran grafted and Ni2+-doped iron oxide nanoparticles (DEX/Ni-SPIOs). The prepared SPIOs are characterized through XRD, FE-SEM, FT-IR, DSC-TGA and VSM analyses.
MATERIALS AND METHODS
Electrochemical synthesis of DEX/Ni-SPIOs
All chemicals were purchased from Sigma-Aldrich company, and used as received. For preparation of electrodeposition bath; First, 0.15g iron(II) chloride hexahydrate, 0.4g iron(III) nitrate nonahydrate, and 0.05g nickel chloride hexahydrate were dissolved in 100cc distilled water, and then 0.1g dextran (as capping agent) was added into this solution and stirred for 20min. The direct current (dc) electrodeposition mode was chosen for synthesis of samples. In dc deposition, a two-electrode electrochemical set-up was constructed using a stainless-steel cathode centered between two graphite anodes. After assembling the mentioned electrochemical cell, a typical current density of 10 mA cm–2 was applied into this system for 20min at RT condition, and a black film was formed on the steel cathode at the end of deposition time. After this step, the deposited film was collected from the steel electrode, and dispersed in 50cc ethanol solution. Then, this solution was centrifuged at 3000 rpm for 10 min to remove the weekly capped dextran onto the iron oxide surfaces and other impurities. In final stage, the dispersed powder was collected from the ethanol solution by magnet. The collected powder was dried in a vacuum oven at 70oC for 2h, and the obtained black powder was named DEX/Ni-SPIOs product.
The prepared DEX/Ni-SPIOs powder was tested through field-emission scanning electron microscopy (FE-SEM, Mira 3-XMU with accelerating voltage of 100 kV) and energy dispersive diffraction X-ray analysis (EDX) to identify its morphology and elemental composition. Surface morphology of the sample was also observed by TEM (Model Zeiss EM900). The crystal structure of the prepared sample was recorded by X-ray diffraction (XRD, Phillips PW-1800) using a Co Kα radiation. Thermal behavior of the sample was studied using a thermo-analyzer, model STA-1500. This analysis was done in N2 atmosphere at the temperatures of 25-600 oC with applying a heating rate of 5oC min−1. The FTIR spectra were collected in the wavenumber range of 400 to 4000 cm−1 using a Bruker Vector 22 Fourier transformed infrared spectroscope. The magnetic curves of the prepared DEX/Ni-SPIOs were provided in the range of −20000 to 20000 Oe at RT using vibrating sample magnetometer (Meghnatis Daghigh Kavir Co., Iran).
RESULTS AND DISCUSSION
Fig. 1 presets the XRD pattern of dextran grafted Ni-SPIOs. The recorded pattern showed typical iron oxide diffraction peaks, suggesting good crystallinity. The size broadening indicates the nanometer size of the SPIOs crystallites. The observed peaks at 21.23o, 35.18 o, 41.44 o, 50.75, 63.05 o, 67.36 o, and 74.35 o are well matched with the (111), (220), (311), (400), (422), (511) and (440) crystal planes of magnetite. Hence, the prepared sample has been crystalized into the magnetite phase (JCPDS card No. 01-088-0315). By the Debye–Sherrer equation (D=Kλ/βcosθ), the average crystallite size was obtained to be 8.7nm.
FT-IR test was provided to verify the grafting of the electrosynthesized SPIOs with dextran agent. In fact, this analysis was used to determine the chemical composition of sample and prove the presence of dextran onto the surface of Ni-SPIOs particles. Fig. 2 shows the FT-IR spectrum of the DEX/Ni-SPIOs sample. Generally, the IR peaks observed at the wavenumbers lower that 700cm-1implicate the metal-oxygen vibrations (i.e. ν(Fe-O-Fe) and ν(Ni-O-Fe)) [47,48]. In this spectrum, the other IR peaks are due to the following vibrations [37,38,47-50]; the band at 1633 cm−1 is due to the vibration of water molecules bonded onto Ni-SPIOs particles, the bands at 2927 and 2893 cm–1 are resulted from the asymmetric and symmetric vibrations of C-H bonds in dextran, the peak at 1149 cm−1 is caused by covalent vibrations of the dextran glycosidic bridge, the peak at 997 cm−1 is due to the vibration of the C–O bond at the C-4 position of the glucose residue, and the bands observed at 1454, 1355 and 1232 cm–1 are related to the deformation vibrations of H-C-OH bands in the dextran chains. The presence of these vibrational modes is proved surface grafting of SPIOs by dextran . Hence, these IR data demonstrate that the surfaces of Ni-doped iron oxide nanoparticles are covered with dextran polymer during the electrodeposition synthesis.
The differential scanning calorimetry (DSC) and the related weight loss data were recorded in the temperature range of 25–600 oC and the resulted profiles are shown in Fig. 3. In the DSC curve (Fig. 3a), an endothermic peak is occurred in the temperature range of 25-150°C, which is due to the evaporation of water molecules attached onto the Ni-SPIOs particles and also OH groups in dextran chain [52-54]. TG curve showed about 2.5% weight reduction for this physical change. After this step, DSC profile exhibited two-successive endothermic peaks between temperatures of 150 to 350 °C. Correspondingly, TG curve has a sharp weight loss (about 9.3%) at these temperatures (as clearly seen in Fig. 3b). Notably, it was reported that iron oxide NPs coated with dextran show a sharp weight loss at the temperature range of 150-300 oC, due to the two-step degradation of dextran coat [54-56]. Hence, the changes observed in TG curve at T=150-350 oC are assigned to the breakdown of organic skeleton in dextran. At final step, there is a small endothermic peak and weight loss (0.3%) at temperature of 550oC, which can be related to the phase transition of Fe3O4 into FeO . The total weight loss of DEX Ni-SPIOs was found to be 12.1%. These results clearly approved the dextran grafting onto the synthesized DEX/Ni-SPIOs.
FE-SEM and TEM images and the elemental analysis (i.e. EDAX) profile of DEX/Ni-SPIOs sample are presented in Fig. 4. From FE-SEM observation in Fig. 4a, it is revealed that the sample has been electrodeposited and growth in spherical particles morphology with an average size of 10 nm. TEM observation in Fig. 4c is also revealed that the prepared particles have spherical form with mean size of 10 nm. Furthermore, the EDAX data showed that the DEX/Ni-SPIOs sample has Fe, Ni, O and C elements with weight percentages of 46.42%, 6.07%, 36.51% and 11% in its chemical composition. These data verified that the iron oxides have been doped by Ni (6.07wt%) during electrochemical synthesis. Furthermore, the presence of 11wt% carbon in chemical composition of the deposited sample implicated the surface grafting of the deposited iron oxide particles by dextran. These results clearly proved the successful electrosynthesis of surface grafted Ni-doped magnetite nanoparticles.
The magnetic behavior of the fabricated nickel-doped iron oxide particles was measured using vibrating-sample magnetometer (VSM) test. The resulted M-H curve is shown in Fig. 5 and also the related magnetic data are listed in Table 1. The shape of M-H profile and also absence of any hysteresis loop implicated the superparamagnetic behavior of our sample [36-38]. As listed in Table 1, the saturation magnetization (Ms), remanence (Mr) and coercivity (Hci) values were found to be 52.45 emu g–1, 0.13 emu g–1 and 3.09 G, respectively. These values revealed that the prepared sample has relative high Ms and negligible Ms and Hci values. As listed in Table 1, it has been reported that the magnetic data for electrochemically synthesized bare iron oxide and bare Ni doped iron oxide have been reported to be: Ms=72.96 emu/g, Mr=0.95 emu/g (for bare SPIOs) , and Ms=47.25 emu/g, Mr=0.22 emu/g (for bare Ni-SPIOs) . Furthermore, for dextran coated SPIOs (Ms=15.2 , Ms=45.87 ), and for metal ion-doped SPIOs (Ms=71.6 emu/g, Mr=12 emu/g and Hci=88.8 G for Sm-SPIOs , Ms=47.25 emu/g, Mr=0.22 emu/g and Hci=4.34 G for Sm-SPIOs  and Ms=48 emu/g, Mr=5.6 emu/g and Hci=51.8 G for Cu-SPIOs  have been reported (Table 1). Comparing these magnetic data with those obtained in this work indicated that the superparamagnetic behavior of iron oxide could be further improved through grafting with dextran polymer as a result of lowering the residual magnetization. After dextran grafting, the DEX/Ni-SPIOs sample exhibited relative high Ms and negligible residual Mr, showing the excellent magnetic performance of these magnetic particles with altering the applied filed. This type of magnetic action is very required at various targeting, imaging, and therapeutic applications of SPIOs such as hyperthermia, magnetic therapy and magnetic resonance imaging [3-9]. Hence, it was concluded that the prepared dextran grafted Ni-SPIOs have suitable physico-chemical and magnetic properties for both therapeutic and diagnostic aims.
In summary, an easy electrochemical method was constructed for the preparation of dextran grafted nickel-doped iron oxide nanoparticles. The magnetite crystal phase of the deposited powder was proved via XRD and FT-IR data. FE-SEM and TEM observations revealed the size of prepared particles are 10nm and EDAX data exhibited about 6% Ni doping into SPIOs structure. Thermogravimetric data showed 10%wt surface grafting of Ni-SPIOs by dextran polymer. The obtained VSM results indicated that the fabricated sample has low remnant magnetization and coercivity (i.e. Mr=0.13 emu/g and Hci=3.09 G), establishing its suitability for biomedical applications.
CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.