Home Search Collections Journals About Contact us My IOPscience Enhanced magnetic anisotropy and heating efficiency in multi-functional manganese ferrite/graphene oxide nanostructures This content has been downloaded from IOPscience Please scroll down to see the full text 2016 Nanotechnology 27 155707 (http://iopscience.iop.org/0957-4484/27/15/155707) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 165.123.34.86 This content was downloaded on 02/03/2016 at 14:50 Please note that terms and conditions apply Nanotechnology Nanotechnology 27 (2016) 155707 (10pp) doi:10.1088/0957-4484/27/15/155707 Enhanced magnetic anisotropy and heating efficiency in multi-functional manganese ferrite/graphene oxide nanostructures Anh-Tuan Le1, Chu Duy Giang1, Le Thi Tam1, Ta Quoc Tuan1, Vu Ngoc Phan1, Javier Alonso2,3, Jagannath Devkota2, Eneko Garaio4, José Ángel García3,5, Rosa Martín-Rodríguez4, Ma Luisa Fdez-Gubieda3,4, Hariharan Srikanth2 and Manh-Huong Phan2 Department of Nanoscience and Nanotechnology-DoNST, Advanced Institute for Science and Technology (AIST), Hanoi University of Science and Technology (HUST), 01 Dai Co Viet Street, Hai Ba Trung District, Hanoi 10000, Vietnam Department of Physics, University of South Florida, Tampa, FL 33620, USA BCMaterials Edificio No 500, Parque Tecnológico de Vizcaya, Derio E-48160, Spain Department of Electricity and Electronics, University of Basque Country (UPV/EHU), Leioa E-48940, Spain Department of Applied Physics II, University of Basque Country (UPV/EHU), Leioa E-48940, Spain E-mail: tuan.leanh1@hust.edu.vn, jalonsomasa@gmail.com and phanm@usf.edu Received 21 October 2015, revised 22 December 2015 Accepted for publication 28 January 2016 Published March 2016 Abstract A promising nanocomposite material composed of MnFe2O4 (MFO) nanoparticles of ∼17 nm diameter deposited onto graphene oxide (GO) nanosheets was successfully synthesized using a modified co-precipitation method X-ray diffraction, transmission electron microscopy, and selected area electron diffraction confirmed the quality of the synthesized samples Fourier transform infrared measurements and analysis evidenced that the MFO nanoparticles were attached to the GO surface Magnetic measurements and analysis using the modified Langevin model evidenced the superparamagnetic characteristic of both the bare MFO nanoparticles and the MFO–GO nanocomposite at room temperature, and an appreciable increase of the effective anisotropy for the MFO–GO sample Magnetic hyperthermia experiments performed by both calorimetric and ac magnetometry methods indicated that relative to the bare MFO nanoparticles, the heating efficiency of the MFO–GO nanocomposite was similar at low ac fields (0–300 Oe) but became progressively larger with increasing ac fields (>300 Oe) This has been related to the higher effective anisotropy of the MFO–GO nanocomposite In comparison with the bare MFO nanoparticles, a smaller reduction in the heating efficiency was observed in the MFO–GO composites when embedded in agar or when their concentration was increased, indicating that the GO helped minimize the physical rotation and aggregation of the MFO nanoparticles These findings can be of practical importance in exploiting this type of nanocomposite for advanced hyperthermia Magnetoimpedance-based biodetection studies also indicated that the MFO–GO nanocomposite could be used as a promising magnetic biomarker in biosensing applications Keywords: magnetic nanoparticles, graphene oxide, magnetic hyperthermia, biodetection (Some figures may appear in colour only in the online journal) 0957-4484/16/155707+10$33.00 © 2016 IOP Publishing Ltd Printed in the UK Nanotechnology 27 (2016) 155707 A-T Le et al Introduction heating responses of the MFO–GO nanosystem In addition, we have demonstrated the possibility of detecting small amounts of MFO–GO using a magnetoimpedance (MI)-based biosensor Overall, the MFO–GO nanocomposite is a very promising candidate material for a wide variety of biomedical applications, including magnetic hyperthermia, targeted drug delivery, and biomolecular detection There have been a growing number of studies on magnetic nanoparticles (MNPs), due to their potential applications in nanomedicine, including targeted drug delivery, hyperthermia, magnetic resonance imaging (MRI), and biodetection [1–3] In most of the applications, MNPs are required to possess non-toxicity, biocompatibility, mono-dispersity, stability in colloidal media, high magnetic moment, and freedom from remanent field Due to their excellent biocompatible properties, superparamagnetic iron-oxide nanoparticles have long been exploited for biomedical applications [1, 4–6] However, the net magnetic moments of the nanoparticles are drastically reduced when particles approach sizes below ∼10 nm, making them hardly usable for some applications [4] Therefore, continuing efforts have been devoted to the synthesis of new MNPs with improved magnetic properties [7–11] Interestingly, Doaga et al [9] have reported that relative to Fe3O4 nanoparticles, manganese-doped iron oxide nanoparticles of MnxFe1−xFe2O4 (0x1) show the enhanced saturation magnetization (Ms) and hence the enhanced specific absorption rate (SAR), which is a measure of heating efficiency On the other hand, graphene oxide (GO) is a twodimensional material with potentially important applications in spintronic devices [12] Its large specific surface area is also ideal for the immobilization of a large number of substances, including biomolecules, drugs, and nanoparticles [13] It has been found that the sheets of GO are apparently biocompatible without obvious toxicity, and very promising for applications in targeted drug delivery [14] This has led to an increasing interest in the realization of hybrid GO–MNPs composed of MNPs attached to the surface of GO nanosheets [15, 16] Bai et al [15] reported the good SAR of Fe3O4 nanoparticles deposited onto GO sheets for prospective applications in magnetic hyperthermia In addition, Peng et al [16] nicely showed that manganese ferrite (MnFe2O4 or MFO) decorated GO nanocomposites could work both as heating agents and MRI contrast agents However, the results obtained for the MFO–GO nanocomposites were not directly compared to those of the bare nanoparticles, which left an important and unanswered question about the effects of the GO nanosheet on the magnetic behavior and therefore, on the SAR Such knowledge is essential in manipulating the heating efficiency of this type of hybrid nanostructure for advanced hyperthermia The aforementioned studies have motivated us to synthesize a nanocomposite material composed of ∼17 nm MnFe2O4 (MFO) nanoparticles deposited onto GO nanosheets, as well as to perform a systematic study of their magnetic, inductive heating, and biosensing properties Our study indicates that relative to the bare MFO nanoparticles, the MFO–GO nanocomposite possesses an enhanced magnetic anisotropy and hence the enhanced SAR for ac fields >300 Oe The GO also promotes disaggregation of the nanoparticles, thus improving the heating capacity of the MFO–GO composite These observations pinpoint the important effects of GO on the magnetic and inductive Experimental methods 2.1 Materials Analytical-grade manganese chloride tetrahydrate (MnCl2·4H2O, 99%), ferric chloride hexahydrate hydrogen (FeCl3· 6H2O 99%), sodium hydroxide (NaOH), ammonium hydroxide (NH3, 25%), potassium permanganate (KMnO4, 99.9%), hydrogen peroxide (H2O2, 30%), sulfuric acid (H2SO4, 98%), hydrochloric acid (HCl, 37%), and nitric acid (HNO3, 63%) used in this study were purchased from Shanghai Chemical Reagent Co Ltd 2.2 Synthesis of GO nanosheets Graphite (coal powder) was fabricated from coal, by milling method, in Vietnam GO nanosheets were synthesized from the coal powder by the modified Hummer method as described previously [17] Briefly, graphite powder was mixed with HNO3, KMnO4, then the mixture was converted to exploited graphite (EG) under microwave 800 W for In a typical process, g of coal powder was mixed with HNO3 and KMnO4 at a volume ratio of 1:2:1.5, respectively The EG was oxidized to GO with H2SO4, KMnO4, and H2O2 GO was washed with HCl(5%) and ionized water with repeated centrifugation for purifying In this reaction, the mixture of g of EG, g of KMnO4, and g of NaNO3 was added slowly to 160 ml of 98% H2SO4 at °C in ice-water bath and then stirred for 30 Ice-water bath was removed, and then temperature was increased slowly to 45 °C and continuously stirred for h Deionized water was added slowly to the mixture which was stirred until purple fumes were obtained By increasing reaction temperature to 95 °C and stirring the mixture for h, the resulting product of the GO nanosheets was obtained with yellow–brown color The GO nanosheets were then treated by H2O2 30% and HCl 10% solution to eliminate KMnO4, MnO2, and other metal ions that remained in the GO solution The final GO products were purified by filtering, washing several times by ultrasonic vibration, centrifugation with deionized water, and removal of ultrafine carbon powder that was not oxidized 2.3 Synthesis of MFO nanoparticles First, MFO NPs were synthesized by a co-precipitation method Briefly, 2.7 g (0.02 mol) FeCl3·6H2O and 0.99 g (0.01 mol) MnCl2·4H2O were dissolved in 100 ml of deionized water and stirred under air in 10 so that the molar ratio of Mn:Fe in the solution is 1:2 Then 0.5 M NaOH solution was slowly added into the mixture The color of the Nanotechnology 27 (2016) 155707 A-T Le et al solution changed immediately from orange to dark brown After that, the mixture was stirred in water bath at 80 °C for a period of time The precipitate was collected by magnet and washed several times by deionized water before being dried at 80 °C for h The main advantages of this method are short synthesis time, high crystallinity and low cost amplitude was tuned between and 380 Oe The frequency was kept around 300 kHz in both cases For biodetection measurements, a MI-based sensor using an amorphous ribbon of the composition Co65Fe4Ni2Si15B14 (METGLASđ 2714A) and dimensions 10 mmì2 mm× 15 μm was fabricated The impedance (Z) across the sensor head was measured for drop-casted samples of MFO nanoparticles and MFO–GO nanocomposites (10 μl, mg ml−1) by a four point measurement technique using an HP4192A impedance analyzer in the presence of axial dc magnetic field H ranging in between ±120 Oe A detailed description of the MI measurement system can be found in Devkota et al [19] 2.4 Synthesis of MnFe2O4–GO (MFO–GO) nanocomposites In a same way, the MFO–GO nanocomposites were synthesized by a modified co-precipitation method The FeCl3·6H2O and MnCl2·4H2O were dissolved in deionized water with molar ratio of Mn:Fe in the solution is 1:2 The resulting mixture was mixed with the GO nanosheets suspension (0.6 mg ml−1) while stirring for 30 The solution was then constantly stirred and heated to 80 °C Next, 20 ml of 0.5 M NaOH solution was added slowly to the solution of complex The color of solution changed immediately from orange to dark brown after addition of NaOH indicating the formation of MFO nanoparticles The precipitation reaction was then kept at temperature about 80 °C for h The product of MFO– GO nanocomposite was separated from solution by external magnetic field and washed several times by deionized water and acetone Results and discussion 3.1 Structural characterization A two-step process was employed for synthesis of the MFO– GO magnetic nanocomposites The first step was to create the GO nanosheets with oxygen-containing functional groups by using a modified Hummer method These functionalized groups ensure the good dispersibility and stability of the GO product in aqueous medium In addition, the functionalized groups introduce more binding sites for anchoring the precursors of metal ions for MFO NPs In a second step, the MFO NPs were formed on the surface of GO sheets via coprecipitation reaction of Fe+3 and Mn+2 ions in the GO solution to produce water-dispersible MFO–GO composite materials The formation of MFO NPs on the surface of GO nanosheets was confirmed using transmission electron microscopy (TEM) and FTIR measurements Bright TEM images (see figures 1(a) and (b)) indicate that we obtained polyhedral nanoparticles of MFO, with an elongated shape and an average size of 17±5 nm, deposited onto GO sheets of a few μm length and about 3–5 layers thickness In all the obtained TEM images, the MFO nanoparticles were always observed on the surface of the GO sheets, indicating a successful anchoring of the nanoparticles The cubic spinel ferrite structure of the nanoparticles was clearly reflected in the selected-area electron diffraction (SAED) patterns taken using TEM (inset to figure 1(a)), and the HRTEM images showed the well-resolved lattice fringes (figure 1(c)) The lattice spacings were measured to be ∼0.25 and ∼0.3 nm which correspond to the (311) and (220) lattice planes As can also be seen in figure 1, after the precipitation reaction, the MFO nanoparticles were anchored to the surface of the GO nanosheets The XRD analysis was employed to confirm the crystalline nature of MFO NPs as shown in figure 1(d) The XRD pattern exhibits eight characteristic peaks at 2θ=18.9°, 29.7°, 34.98°, 36.5°, 42.52°, 52.63°, 56.19° and 61.96°, indexed as (111), (220), (311), (222), (400), (422), (511) and (440), respectively These peaks are similar to those from JCPDS 10-0319 for a cubic spinel ferrite structure of MFO The XRD pattern of the GO nanosheets exhibited a broad peak at 10.9° (not shown here) corresponding to the (002) interlayer spacing of 0.81 nm [17] The XRD pattern of 2.5 Characterization methods High-resolution transmission electron microscopy (HRTEM, FEI Tecnai, 200 kV) was conducted to determine the morphology and distribution of the MFO nanoparticles on the GO nanosheets The samples for HRTEM characterization were prepared by placing a drop of colloidal solution on a carbon-coated copper grid that was dried at room temperature The composition of the MFO–GO nanocomposite was characterized by energy-dispersive x-ray analysis (5410 LV JEOL) The crystalline structure of the as-prepared MFO–GO nanocomposite was analyzed by x-ray diffraction (XRD, Bruker D5005) using Cu-Kα radiation (λ=0.154 nm) at room temperature The background was subtracted using linear interpolation method The chemical groups were analyzed using fourier transform infrared (FTIR) measurements, samples were collected with one layer coating in potassium bromide and compressed into pellets, and spectra in the range of 400–4000 cm−1 were recorded with Nicolet 6700 FT-IR instrument The magnetic measurements were performed using a physical property measurement system by quantum design, with a vibrating sample magnetometer option The M–H loops were measured at room and low temperatures in magnetic fields up to 30 kOe Magnetic hyperthermia measurements were accomplished using both calorimetric and ac magnetometry methods The calorimetric hyperthermia was carried out with a 4.2 kW Ambrell Easyheat LI 3542 system A suspension of mg ml−1 of nanoparticles in water and in water+2% agar was used for measurements and the magnetic field was tuned from 200 to 800 Oe Ac magnetometry was done using a home-made set-up [18] on a suspension of 1.5 mg ml−1 of nanoparticles in water The magnetic field Nanotechnology 27 (2016) 155707 A-T Le et al Figure (a), (b) Bright field TEM images of the MFO nanoparticles deposited onto the graphene oxide nanosheets In the inset to (a) the size distribution of the MFO nanoparticles is presented, while in the inset to (b), the SAED image of the MFO nanoparticles showing the corresponding diffraction rings is shown; (c) HRTEM image showing the lattice fringes of the MFO nanoparticles; (d) XRD pattern of the MFO–GO nanocomposite the MFO–GO sample shows also a few small peaks (∼45°, 65°K) that could be related to the presence of impurities in GO These results reveal that MFO NPs coated on the GO sheets possess high crystallinity and purity To elucidate the interaction of MFO NPs with the functional groups on the surface of GO sheets, FTIR spectra were recorded and analyzed FTIR is a useful technique to confirm that the nanoparticles are anchored to GO surface, as has been shown by several groups [15, 16] including ours [17] Figure shows the FTIR spectra of the GO sheets, MFO and MFO–GO nanocomposite samples It can be seen that a broad adsorption band at 3450 cm−1 appears for all the samples, corresponding to the normal polymeric O–H stretching vibration of H2O The band at 1630 cm−1 is associated with stretching of the C=O bond of carboxylic groups, while the absorption peaks at 1268 cm−1 and 1051 cm−1 correspond to the stretching of epoxide groups [20] The absorption peak around 584–593 cm−1, which is only present in the FTIR spectra of MFO NPs and MFO–GO nanocomposites, is a characteristic peak corresponding to the stretching vibration of Fe–O [21] A noticeable change in intensity of the adsorption bands of the oxygenated functional groups was found in the FTIR spectrum of the MFO–GO nanocomposite This is the result of the presence of the MFO NPs attached to the surface of the GO nanosheets and the reduction of GO to graphene ratio during the synthesis process The variation of stretch adsorption intensity in the case of MFO–GO nanocomposite demonstrates that strong interactions exist between MFO NPs and the remaining functional groups on both basal planes (hydroxyl group OH) and edges (carboxyl group C–OH) of the GO sheets through the formation of a coordination bond or through simple electrostatic attraction In addition, the slight shift of the peak corresponding to the stretching vibration of Fe–O bond in the MFO–GO hybrids relative to the bare MFO NPs also suggests that the MFO NPs are bound to the GO surface Nanotechnology 27 (2016) 155707 A-T Le et al information about this, we have carried out low temperature (10 K) M–H loop measurements As can be seen in figure 3(c), the MFO–GO nanocomposite exhibits again a high field slope, and the coercivity of the MFO–GO composites (∼725 Oe) is appreciably higher than that of the MFO nanoparticles (∼200 Oe), which again suggests an increase in the anisotropy of the MFO–GO system In order to estimate the effective anisotropy, Keff, we have fitted the experimental data using the law of approach to saturation [22] The high field magnetization curves can be analyzed in terms of the following expression: M = MS (1 – b / H 2) + cH , where MS is the saturation magnetization and b is correlated with the effective anisotropy, Keff=μ0MS(15b/4)1/2 χ corresponds again with the susceptibility of the paramagnetic contribution Good fittings have been obtained for both samples, as indicated in figure 3(d) The anisotropy values obtained for the MFO and MFO–GO samples are 6.4 × 105 and 1.4 × 106 erg cm−3, respectively This indicates that when the MFO NPs are deposited onto the GO nanosheets using our synthesis method, the effective anisotropy field of the system is enhanced The increase in Keff will expectedly affect the SAR of the MFO–GO nanocomposite Figure FTIR spectra of MFO–GO hybrids, MFO nanoparticles and GO nanosheets 3.2 Magnetic characterization 3.3 Magnetic hyperthermia The room temperature M–H loops were measured for the MFO nanoparticles without and with GO nanosheets The results are plotted in figure For both samples, the recorded magnetization was normalized to the total mass of the sample (including the masses of GO and MFO NPs) Nearly zero coercivity and remanence magnetization are observed in the measurements at 300 K, suggesting that the MFO nanoparticles exhibit a superparamagnetic behavior at room temperature To confirm this, we have fitted our M–H data to a standard Langevin expression with an added paramagnetic contribution: M (H ) = MS ị¥ To elucidate this, we have performed hyperthermia experiments on these two samples with the same concentration of MFO nanoparticles (1 mg ml−1) The mass concentration of MFO nanoparticles (∼20%) in the MFO–GO sample was evaluated using the saturation magnetization values obtained from the previous magnetic fittings (figure 3) In Figure we present the heating curves measured at different fields for the MFO and MFO-GO samples both in water and in agar solution As can be seen in this figure, by changing the intensity of the applied field, the final reached temperature can be controlled, and the desired 40 °C–44 °C range, where the cancer cells are more susceptible to heat than healthy ones [23], can be easily reached just after a few minutes It must be noted that the hyperthermia measurement at an ac field of 800 Oe for the MFO–GO sample was stopped when reaching 60 °C because our temperature probe calibration is no longer valid above 60 °C It can also be seen that the heating rate at this field for the MFO–GO sample increases appreciably This becomes more obvious by comparing the SAR values for both the MFO and MFO–GO samples in figure Here, the SAR values have been derived from the following formula: ⎛ mH ⎞ L⎜ ⎟ f (D) dD + cH , ⎝ kB T ⎠ where D is the diameter of the nanoparticles, f(D) corresponds with a log-normal size distribution, L(x)=cotanh(x)−1/x and χ corresponds with the susceptibility of the paramagnetic contribution As seen in figure 3, the fitting results for both the MFO and MFO–GO samples at 300 K are very good, demonstrating the superparamagnetic nature of the synthesized samples Such a behavior indicates that at room temperature dipolar interactions between MFO nanoparticles are relatively weak in the MFO–GO composites, which is beneficial for preventing the magnetic aggregation of the MFO–GO composites In addition, we have found a noticeable increase of the high field slope due to the additional paramagnetic contribution (∼10%) for the MFO–GO nanocomposite This may suggest that the effective anisotropy field (Hk) is larger for this sample as compared to the MFO NPs, and/or there could be an additional magnetic contribution from impurities in the GO as revealed from the XRD pattern To get more SAR = DT Cp f , Dt where Φ corresponds to the concentration of magnetic material, Cp is the heat capacity of water, and ΔT/Δt is the initial slope of the heating curves We corrected both the heat losses to the environment and the heat transfer from the coil following the procedure described by Simeonidis et al [24] The obtained SAR values are comparable with those reported for similar nanoparticles [9, 25] As observed, the Nanotechnology 27 (2016) 155707 A-T Le et al Figure M–H loops measured at room temperature for (a) the MFO nanoparticles and (b) the MFO–GO nanocomposite, and (c) measured at 10 K for MFO and MFO–GO In (d) we present the M–H loops measured at room temperature together with their fittings to the law of approach to saturation evolution Due to measurement limit, the maximum field that we could apply at 310 kHz was 400 Oe instead of 800 Oe obtained with previous measurements, but this is still enough to see the differences in the response of both samples As can be seen, the ac M–H loops of the MFO–GO sample are appreciably wider than those of the MFO samples, especially at high fields As a result, a larger ac area and hence a larger heating efficiency were obtained for the MFO–GO sample, as depicted in figure 6(c) Above ∼250 Oe, the SAR values of the MFO–GO sample become bigger than those of the MFO nanoparticles, and the evolution of SAR versus field tends to slightly saturate for the MFO nanoparticles while it increases linearly for the MFO–GO sample The obtained values are in qualitatively good agreement with what was measured by calorimetric methods A slight discrepancy in SAR values determined from these two methods could be related to inaccuracies derived from calorimetric methods, which arise mainly from the lack of matching between thermal models, experimental setups and measuring conditions as suggested by Andreu et al [26] SAR versus H behavior tends to saturate in the case of the MFO sample, while it keeps increasing for the MFO–GO sample As compared to the bare MFO nanoparticles, the SAR of the MFO–GO nanocomposite is similar at low ac fields (0–300 Oe) but for higher fields, it progressively becomes larger In order to gain more insight into this behavior, we have also performed magnetic hyperthermia measurements using an ac magnetometry setup [18] Ac magnetometry is based on estimating the SAR of the nanoparticles directly from the ac M–H loops’ area We recall that the SAR is directly related to the area of the M–H loops via the following formula: SAR=area×frequency [23] Contrary to the previous method, ac magnetometry allows us to directly observe how the M–H loop area changes with increasing ac field, for both samples, and therefore gives us a direct depiction of the differences in the magnetic response of the MFO nanoparticles when they are free or attached to the GO nanosheet surface In figure we have plotted the ac loops of the MFO and MFO–GO samples and the corresponding SAR versus size Nanotechnology 27 (2016) 155707 A-T Le et al Figure Heating curves of the MFO nanoparticles and the MFO–GO nanocomposite (suspended in water and in agar) at different ac fields 200