materials Article
Surface Modification and Heat Generation of FePt Nanoparticles Da-Hua Wei *, Ko-Ying Pan and Sheng-Kai Tong Institute of Manufacturing Technology & Department of Mechanical Engineering, National Taipei University of Technology (TAIPEI TECH), Taipei 10608, Taiwan;
[email protected] (K.-Y.P.);
[email protected] (S.-K.T.) * Correspondence:
[email protected]; Tel.: +886-2-2771-2171 (ext. 2022) Academic Editor: Marco Salerno Received: 5 January 2017; Accepted: 9 February 2017; Published: 15 February 2017
Abstract: The chemical reduction of ferric acetylacetonate (Fe(acac)3 ) and platinum acetylacetonate (Pt(acac)2 ) using the polyol solvent of phenyl ether as an agent as well as an effective surfactant has successfully yielded monodispersive FePt nanoparticles (NPs) with a hydrophobic ligand and a size of approximately 3.8 nm. The present FePt NPs synthesized using oleic acid and oleylamine as the stabilizers under identical conditions were achieved with a simple method. The surface modification of FePt NPs by using mercaptoacetic acid (thiol) as a phase transfer reagent through ligand exchange turned the NPs hydrophilic, and the FePt NPs were water-dispersible. The hydrophilic NPs indicated slight agglomeration which was observed by transmission electron microscopy images. The thiol functional group bond to the FePt atoms of the surface was confirmed by Fourier transform infrared spectroscopy (FTIR) spectra. The water-dispersible FePt NPs employed as a heating agent could reach the requirement of biocompatibility and produce a sufficient heat response of 45 ◦ C for magnetically induced hyperthermia in tumor treatment fields. Keywords: FePt nanoparticles; surface modification; thiol; heat response; biocompatibility
1. Introduction Nanoparticles (NPs) with magnetic characteristics have been attracting considerable attention due to their wide range of research fields such as high-density information storage media and biomedical potential applications in cell separation, targeted drug delivery, therapy, biological sensing, and magnetic resonance imaging (MRI) [1–15]. The ability of magnetic NPs could provide the promising characteristic of control and manipulation by an external magnetic field as an advantage in biomedical assays. The chemically synthesized FePt NPs in organic solvents have been extensively studied, including in many potential applications, due to their attractive electronic, magnetic, optical, catalytic, and biomedical properties. Therefore, the abilities to control the uniform size, chemical stability, and further functionalize the surface of the well-dispersed NPs are very important because all possible potential applications are directly dependent on such properties [16–22]. The benefit of the organic synthesized method is control over the particle size and shape with close-to–atomic-layer precision, which strongly affects the chemical and physical characteristics of the NPs. As-synthesized metal-based NPs often have hydrophobic chains, causing the NPs to be immiscible in aqueous solutions. For satisfying biological applications, methods must be developed to transfer the organic magnetic NPs into aqueous solutions that could be easily dispersed in blood to exhibit good biocompatibility and directed to a specific target upon applying an external magnetic field [23,24]. The stability and suitability of the magnetic NPs for a certain biological application depend on necessary aspects of the magnetic properties and the surface characteristics. The purpose of this present work is, at first, using a dioctylether and phase transfer reagent to compare the fundamental performance of the Materials 2017, 10, 181; doi:10.3390/ma10020181
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chemically synthesized FePt NPs bonded to different surfactant ligands by phase transfer procedures. Secondly, the FePt NPs were transferred into the water state to satisfy the request of biocompatibility via chemical surface modification and they could be easily controlled by a magnetic field to provide enough local heat for magnetic fluid hyperthermia (MFH) applications. Cancer and tumor cells can be killed at temperatures between 42 and 45 ◦ C; furthermore, tumor cells are more sensitive to increases in temperature than normal tissues because they have a weak cooling function. Above 42 ◦ C, cancer cells also become more susceptible to traditional drug treatments [25,26]. Therefore, the magnetic FePt-based NPs can be used as a potential heating agent for applications such as cancerous or tumor hyperthermia. 2. Synthesized Procedures of FePt Nanoparticles All the reagents were used as purchased from commercial sources and without further purification. Briefly, the procedure for synthesizing monodispersive FePt nanoparticles were involved as follows: 0.5 mmol for platinum acetylacetonate Pt(acac)2 , 1 mmol for iron acetylacetonate Fe(acac)3 , 3.75 mmol 1,2-hexadecanediol were mixed with 30 mL of phenyl ether. After purging with argon for 30 min at room temperature, the flask was heated up to 100 ◦ C for 30 min with additive 1 mmol oleic acid (C18 H34 O2 ) and 0.5 mmol oleylamine (C18 H35 NH2 ) stabilizers into flask at the same time. And then the mixture was heated up to 260 ◦ C to reflux phenyl ether for 1 h to form FePt NPs, and the final products were dispersed in hexane. This kind of bottom-up approach to synthesize self-assembled FePt NPs was used in this work [27]. To make these NPs biocompatible in this research work as simulated in human body fluid, the FePt NPs have been transformed in water soluble state by changing the surfactant ligands via mercaptoacetic acid (thiol, C2 H4 O2 S). The crystalline structure and particle sizes were identified by ex situ X-ray diffraction (XRD, PANalytical, Almelo, The Netherlands) and transmission electron microscopy (TEM, JEOL, Tokyo, Japan), respectively. The magnetic properties were characterized by vibrating sample magnetometer (VSM, Lake Shore, Westerville, OH, USA) with the applied field up to 20,000 Oe at room temperature. 3. Results and Discussion Figure 1 shows the in-plane TEM bright field images and corresponding particle size distribution for the FePt nanocrystals synthesized in phenyl ether. Shown in Figure 1a is a low-magnification TEM image of the as-synthesized monodispersed FePt nanocrystals. The FePt nanocrystals capped with oleic acid and oleylamine ligands could be monodispersed in hydrophobic solvents without significant aggregation and with isolated distances of each nanocrystals, indicating very good crystallinity with a dominant sphere shape. The enlarged magnification of Figure 1a is shown in Figure 1b, and the inset TEM image is a high-resolution transmission electron microscopy (HRTEM) image of an individual sphere nanoparticle (NP). An example of the present FePt NPs synthesized in phenyl ether indicated the perfectly aligned lattice planes as shown in the upper-right inset of Figure 1b, exhibiting a well-crystallized structure. The interplanar spacing of 0.222 nm obtained from the HRTEM image can be ascribed to the adjacent (111) plane of the FePt disordered crystal. The face-centered cubic (fcc) structure feature of the as-synthesized FePt NPs is also shown in their electron diffraction pattern. Figure 1c shows such a pattern from the selected area of diffraction (SAD) of the nanocrystalline assembly. The composition of the NPs was determined to be close to Fe56 Pt44 by nanobeam energy-dispersive X-ray spectroscopy (EDS). The average diameter of the FePt NPs synthesized in phenyl ether was about 3.8 ± 0.34 nm with a narrow size distribution as shown in Figure 1d. The XRD patterns for the FePt nanocrystals synthesized in phenyl ether and the corresponding standard Joint Committee on Powder Diffraction Standards (JCPDS) of the FePt phase are shown in Figure 2a,b, respectively The as-synthesized FePt NPs without any external energy to overcome the activation energy of an ordered phase transformation indicated the formation of the chemically disordered fcc FePt structure with a (111) orientation reacted in phenyl ether. It is clear that the
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as-synthesized FePt NPs can be indexed to the FePt phase in a cubic structure as shown in Figure 2a. as-synthesized NPs be indexed to the in FePt phaseether in a cubic structure as shown Figure 2a. The diffractionFePt peak of can the FePt synthesized phenyl was broad, indicating theinFePt NPs The diffraction peak of the FePt synthesized in phenyl ether was broad, indicating the FePt NPs synthesized in phenyl ether have a small particle size. On the other hand, no diffraction peak of an synthesized in phenyl a small size. the other hand, no diffraction peak of unclear phase or peakether shift have of FePt was particle observed. An On average particle diameter of 3.6 nm was an unclear phase or peak shift of FePt was observed. An average particle diameter of 3.6 nm was calculated from the peak width of the XRD pattern using the Scherer formula, which is consistent calculated from thecalculated peak width of the XRD pattern Scherer formula, which is consistent with the diameter by the statistical analysisusing of thethe TEM images, as shown in Figure 1. The with the diameter calculated by the statistical analysis of the TEM images, as shown in Figure above structural characterizations show clearly that fcc FePt NPs with a composition of Fe56Pt441. The above structural by characterizations showof clearly that3 fcc FePtPt(acac) NPs with a composition of Fe Pt44 were synthesized the co-reduction Fe(acac) and 2 in the presence of 561,2were synthesized (HDD) by the co-reduction of Fe(acac) Pt(acac)2 the in the presence 1,2-hexadecanediol hexadecanediol in phenyl ether. More3 and importantly, HDD also of played a role as an (HDD) in phenyl ether. More importantly, the HDD also played a role as an effective surfactant. effective surfactant. Based on structure and functionality, using HDD as a surfactant for dispersing Based on structure and functionality, using HDD as a surfactant for dispersing the formed not the formed NPs is not unreasonable, and it also provides the reductive species needed toNPs formisthe unreasonable, it also provides the reductive needed form the are fcc FePt NPs. as In capping addition, fcc FePt NPs. and In addition, it is possible that thespecies oxidized HDD to molecules also used itligands is possible that thethe oxidized HDDoxidation molecules aretoalso used asthe capping ligands protect the NPs to protect NPs from and facilitate dispersion of to NPs in nonpolar from oxidation and to facilitate the dispersion of NPs in nonpolar solvents. Traditional methods solvents. Traditional methods such as capping ligands oleic acid and oleylamine are used as such the assurfactants capping ligands oleicsolvent. acid and oleylamine are used as the surfactants in a proper solvent. in a proper
Figure1.1.Transmission Transmissionelectron electronmicroscopy microscopy(TEM) (TEM)bright brightfield fieldimages imagesfor forthe theFePt FePtNPs NPssynthesized synthesizedin Figure in phenyl ether capped with oleic acid and oleylamine ligands, respectively. (a) Low-magnification phenyl ether capped with oleic acid and oleylamine ligands, respectively. (a) Low-magnification image; image; (b) Enlarged magnification of the (a) inset and the inset TEMis image is high-resolution transmission (b) Enlarged magnification of (a) and TEM image high-resolution transmission electron electron microscopy of an individual NP; (c,d) selected area of (SAD) diffraction (SAD) microscopy (HRTEM) (HRTEM) of an individual sphere NP;sphere (c,d) selected area of diffraction and particle anddistribution particle sizewith distribution with Gauss curve, respectively. size Gauss fitting curve, fitting respectively.
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Figure NPs synthesized synthesized in diffraction Figure 2. 2. (a) (a) X-ray X-ray diffraction diffraction patterns patterns for for the the FePt FePt NPs in phenyl phenyl ether; ether; (b) (b) diffraction pattern is the corresponding standard Joint Committee on Powder Diffraction Standards (JCPDS) pattern is the corresponding standard Joint Committee on Powder Diffraction Standards (JCPDS) of of disordered phase. disordered fcc fcc FePt FePt phase.
Figure 3a shows the VSM hysteresis loop for the FePt NPs measured at room temperature, and Figure 3a shows the VSM hysteresis loop for the FePt NPs measured at room temperature, and Figure 3b shows the corresponding enlarged hysteresis loop for FePt NPs ranging in the field of Figure 3b shows the corresponding enlarged hysteresis loop for FePt NPs ranging in the field of ±600 Oe, respectively. The hysteresis loops are superparamagnetic at room temperature as shown in ±600 Oe, respectively. The hysteresis loops are superparamagnetic at room temperature as shown in Figure 3. The magnetization value of FePt NPs in a field of 20,000 Oe was 8.4 emu/g, with a coercive Figure 3. The magnetization value of FePt NPs in a field of 20,000 Oe was 8.4 emu/g, with a coercive force of 30 Oe, while the synthesized solvent was phenyl ether. The magnetic character of FePt NPs force of 30 Oe, while the synthesized solvent was phenyl ether. The magnetic character of FePt NPs was not only strongly correlated with the degree of the chemical ordered phase transformation, but was not only strongly correlated with the degree of the chemical ordered phase transformation, but it it was also affected by the stoichiometic composition and the bonding of their surface ligands. was also affected by the stoichiometic composition and the bonding of their surface ligands. Figure 4 shows the Fourier transform infrared spectroscopy (FTIR) spectra for the FePt NPs Figure 4 shows the Fourier transform infrared spectroscopy (FTIR) spectra for the FePt NPs synthesized in phenyl ether capped with oleic acid and oleylamine, respectively. Figure 4a,b are the synthesized in phenyl ether capped with oleic acid and oleylamine, respectively. Figure 4a,b are the reference spectra for pure oleic acid and oleylamine, respectively. All spectra in Figure 4 reveal the reference spectra for pure oleic acid and oleylamine, respectively. All spectra in Figure 4 reveal the characteristic peaks of the oleyl group: the peaks at 2854 and 2922 cm−−11 were due to the symmetric characteristic peaks of the oleyl group: the peaks at 2854 and 2922 cm were due to the symmetric and asymmetric CH2 stretching modes. On the other hand, the peak of the FePt NPs at 1552 cm−1 and asymmetric CH2 stretching modes. On the other hand, the peak of the FePt NPs at 1552 cm−1 was was due to the bidendate COO mode of oleic acid binding as shown in Figure 4c. The characteristic due to the bidendate COO mode of oleic acid binding as shown in Figure 4c. The characteristic peak peak at 1709 cm−1 in the oleic acid spectrum was due to the vibrational ν(C=O) monodendate mode, − 1 at 1709 cm in the oleic acid spectrum was due to the vibrational ν(C=O) monodendate mode, and and the 1512−1cm−1 peak was due to the bidendate COO mode in the oleic acid. The characteristic the 1512 cm peak was due to the bidendate COO mode in the oleic acid. The characteristic peak peak at 3006 cm−1 in the oleylamine spectrum was due to the ν(C–H) mode of the C–H bond at 3006 cm−1 in the oleylamine spectrum was due to the −1ν(C–H) mode of the C–H bond adjacent to adjacent to the C=C bond, and the small peak−at 1647 cm was due to the ν(C=C) stretch mode. The the C=C bond, and the small peak at 1647 cm 1 was due to the ν(C=C) stretch mode. The 1593 cm−1 1593 cm−1 peak was due to the NH2 scissoring mode, which suggests that oleylamine was adsorbed peak was due to the NH2 scissoring mode, which suggests that oleylamine was adsorbed with the with the NH2 group intact. Figure 4c indicates the oleic acid and oleylamine both bonded to the NH2 group intact. Figure 4c indicates the oleic acid and oleylamine both bonded to the as-made FePt as-made FePt NPs synthesized in phenyl ether, thus causing the presence of bidentate carboxylate NPs synthesized in phenyl ether, thus causing the presence of bidentate carboxylate bonding to the bonding to the as-synthesized NPs. The observation of both peaks at 1552 and 1450 cm−1 was due to as-synthesized NPs. The observation of both peaks at 1552 and 1450 cm−1 was due to the vibrational the vibrational ν(COO) and stretching (CH2) modes, indicating that oleic acid and oleylamine ν(COO) and stretching (CH ) modes, indicating that oleic acid and oleylamine complex surfactants complex surfactants bond to2the FePt NPs in both monodentate and bidentate types [28]. bond to the FePt NPs in both monodentate and bidentate types [28]. Figure 5 shows the in-plane TEM bright field images for the FePt NPs synthesized in phenyl Figure 5 shows the in-plane TEM bright field images for the FePt NPs synthesized in phenyl ether after ligand exchange with thiol. Shown in Figure 5a is a low-magnification TEM image of the ether after ligand exchange with thiol. Shown in Figure 5a is a low-magnification TEM image of monodispersive FePt NPs ligands exchanged with thiol, and shown in Figure 5b is the enlarged magnification of Figure 5a. The average diameter of thiol-treated FePt NPs synthesized in phenyl ether was about 4.0 ± 0.38 nm with a narrow size distribution as shown in Figure 5c. Compared
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the monodispersive FePt NPs ligands exchanged with thiol, and shown in Figure 5b is the enlarged magnification Figure 5a. The average diameter of thiol-treated FePt NPs synthesized in phenyl 5ether Materials 2017, 10,of 181 of 10 was about 4.0 ± 0.38 nm with a narrow size distribution as shown in Figure 5c. Compared with the with the TEMofimages of acidthe oleic and oleylamine-capped NPs, it can be observed thatNPs the TEM images the oleic andacidoleylamine-capped FePt NPs,FePt it can be observed that the FePt 10, 181tend of 10them FePt Materials NPs dispersed in hexane tend to self-assemble, a regular distance between dispersed in2017, hexane to self-assemble, maintainingmaintaining a regular distance between them as 5shown in as shown in Figure whereas the case of thiol-exchanged FePt NPs, is no sign of a pattern, Figure 1, whereas in1,the case of in thiol-exchanged FePt NPs, there is no signthere of a pattern, indicating that with the TEM images of the oleic acid- and oleylamine-capped FePt NPs, it can be observed that the indicating thatslightly the FePt NPs slightlywith agglomerated with a smaller interparticle distance after the ligand the FePt NPs agglomerated a smaller interparticle distance after the ligand exchange with FePt NPs dispersed in hexane tend to self-assemble, maintaining a regular distance between them exchange with thiol. This aggregation was due tochain the shorter chain length ofsign mercaptoacetic acid thiol. This aggregation wasindue to the shorter length of mercaptoacetic acid (C2 H4 O 2 S) as shown in Figure 1,effect whereas theeffect case of thiol-exchanged FePt NPs, there is no of a pattern, (C2H4indicating O2S) compared toFePt that of (C oleic acid 18H 34O2) with or oleylamine (C182H) 35stabilizers. NHdistance 2) stabilizers. compared to that oleic acid O2(C ) or oleylamine (C18 H35interparticle NH thatof the NPs slightly a smaller after the ligand 18 H 34agglomerated exchange with thiol. This aggregation effect was due to the shorter chain length of mercaptoacetic acid (C2H4O2S) compared to that of oleic acid (C18H34O2) or oleylamine (C18H35NH2) stabilizers.
Figure 3. 3. (a) (a) Magnetization Magnetization loop temperature for NPs measured measured at at room room Figure loop measured measured at at room room temperature for the the FePt FePt NPs Figure 3. (a) Magnetization loop measured at room temperature for the FePt NPs measured at room temperature; (b) corresponding enlarged hysteresis loop for FePt NPs ranging in the field of ±600 Oe. temperature; (b) corresponding enlarged loopfor forFePt FePt NPs ranging in field the field of Oe. ±600 Oe. temperature; (b) corresponding enlargedhysteresis hysteresis loop NPs ranging in the of ±600
Figure 4. Fourier transform infrared spectroscopy (FTIR) spectra are for the pure (a) oleic acid and
Figure 4. Fourier transform infrared spectroscopy (FTIR) spectra are for the pure (a) oleic acid and (b) oleylamine, respectively; (c) FTIR spectra for the FePt NPs synthesized in phenyl ether capped (b) oleylamine, respectively; (c) FTIR spectra for the FePt NPs synthesized in phenyl ether capped with with oleic acid and oleylamine, respectively. Figure 4. Fourier transform infrared spectroscopy (FTIR) spectra are for the pure (a) oleic acid and oleic acid and oleylamine, respectively. (b) oleylamine, respectively; (c) FTIR spectra for the FePt NPs synthesized in phenyl ether capped with oleic acid and oleylamine, respectively.
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Figure 5. the FePt FePt NPs NPs synthesized synthesized in in phenyl phenyl ether ether after after ligand ligand exchange exchange Figure 5. TEM TEM bright bright field field images images for for the with thiol. thiol. (a) (a) Low-magnification image; (b) magnification of of (a); (a); (c) (c) corresponding corresponding particle particle with Low-magnification image; (b) enlarged enlarged magnification size distribution distribution with with Gauss Gauss fitting fitting curve, curve, respectively. respectively. size
The FTIR spectrum for the FePt NPs after the ligand exchange with thiol was shown in Figure 6. The FTIR spectrum for the FePt NPs after the ligand exchange with thiol was shown in Figure 6. For the FePt NPs that underwent a ligand exchange with thiol, the disappearance peak and For the FePt NPs that underwent a ligand exchange with thiol, the disappearance peak and decreased decreased intensity peak of CH2 stretching modes were at 2854 and 2922 cm−1, indicating complete intensity peak of CH2 stretching modes were at 2854 and 2922 cm−1 , indicating complete changes in changes in the surface chemistry of the FePt NPs as shown in Figure 6. The peaks at 1709 and 1595 cm−1 the surface chemistry of the FePt NPs as shown in Figure 6. The peaks at 1709 and 1595 cm−1 were were observed due to the C=O stretch vibration mode of alkyl thiol chains as well as weaker observed due to the C=O stretch vibration mode of alkyl thiol chains as well as weaker absorption absorption peaks at 1405, 1290, and 1200 cm−1 as shown in Figure 6. The absorption peak at−12550 cm−1 − 1 peaks at 1405, 1290, and 1200 cm as shown in Figure 6. The absorption peak at 2550 cm derived derived from the S–H stretching vibration of the thiol chains can be seen in the FTIR spectra of the from the S–H stretching vibration of the thiol chains can be seen in the FTIR spectra of the FePt-thiol FePt-thiol NPs, indicating the interactions between FePt NPs and thiol chains [29]. Actually, the NPs, indicating the interactions between FePt NPs and thiol chains [29]. Actually, the FTIR spectra FTIR spectra for the FePt NPs before and after ligand exchange were indeed different when for the FePt NPs before and after ligand exchange were indeed different when compared with the compared with the as-synthesized NPs in phenyl ether, as shown in Figures 4c and 6, which as-synthesized NPs in phenyl ether, as shown in Figures 4c and 6, which confirms the thiol functional confirms the thiol functional group (SH) bonded to the Fe or Pt surface because of their weak group (SH) bonded to the Fe or Pt surface because of their weak absorption of Pt–S or Fe–S bonds absorption of Pt–S or Fe–S bonds located at low wavenumbers [30]. The above evidence showed located at low wavenumbers [30]. The above evidence showed that the oleic acid and oleylamine that the oleic acid and oleylamine ligands were replaced by the SH functional group, and the ligands were replaced by the SH functional group, and the reagent that contained the thiol functional reagent that contained the thiol functional group was easily approached to replace the surfactant on group was easily approached to replace the surfactant on the definite metal surface which also confirms the definite metal surface which also confirms that thiol bonds to the surface of the FePt NPs. In that thiol bonds to the surface of the FePt NPs. In order to confirm that the FePt NPs could be dispersed order to confirm that the FePt NPs could be dispersed in the water phase, as simulated in human in the water phase, as simulated in human blood, and provide heat for hyperthermia, this analysis blood, and provide heat for hyperthermia, this analysis means that the FePt-based NPs could stable means that the FePt-based NPs could stable in a water solution [31]. in a water solution [31]. Figure 7 shows the magnetization loops for the FePt NPs after the ligand exchange with thiol. Figure 7 shows the magnetization loops for the FePt NPs after the ligand exchange with thiol. The inset shows the corresponding enlarged hysteresis loops for FePt-thiol NPs ranging in the The inset shows the corresponding enlarged hysteresis loops for FePt-thiol NPs ranging in the field field of ±1000 Oe. The hysteresis loop for the FePt-thiol NPs was also superparamagnetic at room of ±1000 Oe. The hysteresis loop for the FePt-thiol NPs was also superparamagnetic at room temperature as shown in Figure 7. The magnetization value of FePt NPs in the field of 20,000 Oe was temperature as shown in Figure 7. The magnetization value of FePt NPs in the field of 20,000 Oe 6.5 emu/g and with a coercive force of 58 Oe while the synthesized solvent was phenyl ether and then was 6.5 emu/g and with a coercive force of 58 Oe while the synthesized solvent was phenyl ether ligand-exchanged with thiol. The magnitude of the magnetization for FePt-thiol was smaller than and then ligand-exchanged with thiol. The magnitude of the magnetization for FePt-thiol was that of FePt NPs. The maximum magnetization values were 8.4 emu/g and 6.5 emu/g for FePt and smaller than that of FePt NPs. The maximum magnetization values were 8.4 emu/g and 6.5 emu/g for FePt and FePt-thiol NPs, respectively. The magnetization values of FePt-thiol in all fields were lower than that of pure FePt NPs. The changed magnetization after the ligand exchange with thiol
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Materials 10, 181 10 Materials2017, 2017, 10,respectively. 181 77ofof10 FePt-thiol NPs, The magnetization values of FePt-thiol in all fields were lower than that of pure FePt NPs. The changed magnetization after the ligand exchange with thiol was due to the was enlarged hysteresis hysteresis loops, loops,we wecan canidentify identify wasdue dueto tothe thechange change in in the the dipole dipole moment. moment. From the enlarged change in the dipole moment. From the enlarged hysteresis loops, we can identify another important anotherimportant important difference difference in in the the coercivities coercivities between FePt another FePt and and FePt-thiol FePt-thiol NPs. NPs. The The coercivity coercivity difference in the coercivities between FePt and FePt-thiol NPs. The coercivity values were 30 and 58 Oe valueswere were30 30and and 58 58 Oe Oe for for FePt FePt and and FePt-thiol NPs, respectively. values respectively. The The coercivity coercivityshowed showedaaclear clear forincrease FePt and FePt-thiol NPs, respectively. The coercivity showed a clear increase for FePt-thiol NPs. forFePt-thiol FePt-thiol NPs. NPs. This This observation observation likely reflects the increase for the fact fact that that the the coercivity coercivityof ofaaweakly weakly This observationNP likely reflects thatsize the [32]. coercivity of a the weakly ferromagnetic NP is can related ferromagnetic NP related tothe the fact particle Therefore, increase be ferromagnetic isis related to the particle Therefore, the increase in in the the coercivity coercivitycan be toattributed the particle size [32]. Therefore, the increase in the coercivity can be attributed to the larger size attributedto tothe thelarger larger size size of of FePt-thiol FePt-thiol NPs which leads to aa less-effective less-effective coupling couplingof ofthe themagnetic magnetic ofdipole FePt-thiol NPs which leadspresents to a less-effective coupling of the dipole So this dipole moments. moments. So this this presents FePt-thiol–based NPs with aa suitable size and So FePt-thiol–based withmagnetic suitable sizemoments. and magnetic magnetic presents FePt-thiol–based NPs with a suitable size and magnetic characteristics that could be easily characteristics that could be easily controlled by applying an external magnetic field for potential characteristics that could be easily controlled an external magnetic field for potential controlled by applying an external magnetic field for potential biomedical applications. biomedicalapplications. applications. biomedical
Figure 6. FTIRspectrum spectrum forthe the FePt NPs NPs after ligand Figure ligandexchange exchangewith withthiol. thiol. Figure6.6.FTIR FTIR spectrum for for the FePt FePt NPs after after ligand exchange with thiol.
Figure 7. Magnetization loop for the FePt NPs after ligand exchange with thiol. The inset shows the corresponding enlarged loop hysteresis loops FePt-thiol NPs measured in thethiol. field The of ±1000 Oe. Figure7.7.Magnetization Magnetization forthe the FePtfor NPs after ligand ligand exchange with shows Figure loop for FePt NPs after exchange with thiol. Theinset inset showsthe the corresponding enlarged hysteresis loops for FePt-thiol NPs measured in the field of ±1000 Oe. corresponding enlarged hysteresis loops for FePt-thiol NPs measured in the field of ±1000 Oe.
Finally, the heating response of hydrophilic FePt NPs was measured to prove the self-heating temperature-increasing for use asFePt an in vivo hyperthermia biomedicine. Finally, the heating characteristics response of hydrophilic NPs was measured toagent proveinthe self-heating temperature-increasing characteristics for use as an in vivo hyperthermia agent in biomedicine.
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Finally, the Materials 2017, 10, 181heating response of hydrophilic FePt NPs was measured to prove the self-heating 8 of 10 temperature-increasing characteristics for use as an in vivo hyperthermia agent in biomedicine. Figure 8 shows the rising temperature as a function of the heating time dispersed in water for the FePt NPs after the ligand exchange with thiol. For the dispersed liquid FePt NPs, the temperature of the liquid was measured with a noncontact temperature sensor using an optical optical fiber fiber thermometer. thermometer. The a ligand exchange with thiol could produce enough locallocal heat The hydrophilic hydrophilicFePt FePtNPs NPsthat thatunderwent underwent a ligand exchange with thiol could produce enough ◦ C) to kill cancerous and tumor cells for potential heating therapy treatments. The amount (at least 45 heat (at least 45 °C) to kill cancerous and tumor cells for potential heating therapy treatments. The of heat energy by magnetic FePt NPs can be can determined by theby applied magnetic field, amount of heat generated energy generated by magnetic FePt NPs be determined the applied magnetic magnetization, frequency, and particle heat energy generation described by field, magnetization, frequency, and volume particle because volumethe because the heat energyisgeneration is power loss due to Brownian and Néel relaxation mechanisms [33–35]. Our present hydrophilic FePt described by power loss due to Brownian and Néel relaxation mechanisms [33–35]. Our present NPs with a maximum (SAR) value of 20 W/g at 700 kHz and a hydrophilic FePt NPsspecific with aabsorption maximumrate specific absorption rate were (SAR)obtained value of 20 W/g were −1 alternating current (AC) magnetic −1 . −1 3.8 kAm field for FePt NPs with a concentration of 1 mg · mL obtained at 700 kHz and a 3.8 kAm alternating current (AC) magnetic field for FePt NPs with a These hydrophilic NPs with good biocompatibility can with be used as abiocompatibility potential carrier can and be as concentration of 1FePt-based mg·mL−1. These hydrophilic FePt-based NPs good thermoseeds in biological used as a potential carrier applications. and as thermoseeds in biological applications.
Figure 8. 8. The The relationship relationshipbetween betweenrising risingtemperature temperatureand andthe theheating heating time FePt NPs dispersed Figure time forfor FePt NPs dispersed in in water after ligand exchange with thiol. water after ligand exchange with thiol.
4. Conclusions 4. Conclusions First, monodispersive monodispersive FePt magnetic nanoparticles nanoparticles (NPs) with hydrophobic hydrophobic ligands ligands were were First, FePt magnetic (NPs) with chemically synthesized, and then were developed with tunable surface-functional properties using chemically synthesized, and then were developed with tunable surface-functional properties using phase transfer transfer reagent reagent in in the the present present work. work. The The water-soluble water-soluble FePt FePt NPs NPs are are generally generally considered considered aa phase to be biocompatible via ligand exchange by thiol. The monodispersive and hydrophilic FePt NPs NPs to be biocompatible via ligand exchange by thiol. The monodispersive and hydrophilic FePt could produce enough local heat to damage proteins and structures within cancerous or tumor cells could produce enough local heat to damage proteins and structures within cancerous or tumor cells for disease disease therapy therapy using using high-frequency high-frequency waves waves outside outside the body. It It is is believed believed that that such such hydrophilic hydrophilic for the body. FePt-thiol NPs can provide a possible application for magnetic hyperthermia treatments of cancer FePt-thiol NPs can provide a possible application for magnetic hyperthermia treatments of cancer or or tumors. These hydrophilic FeP NPs can also potentially be used as a nanomedicine or for tumors. These hydrophilic FeP NPs can also potentially be used as a nanomedicine or for molecular molecularby targeting by nano-engineered targeting nano-engineered devices. devices. Acknowledgments: The author (Da-Hua Wei)Wei) acknowledges the financial supportsupport of the main research of Acknowledgments: The author (Da-Hua acknowledges the financial of the mainprojects research the Ministry Science and Technology under Grant (MOST) Nos. 105-2221-E-027-006 projects of of the Ministry of Science(MOST) and Technology under Grant and Nos.105-2221-E-027-047-MY3, 105-2221-E-027-006 and the 105-2221-E-027-047-MY3, and Joint the University System under of Taipei Joint Program under Grant University System of Taipei Research Program Grant No. Research USTP-NTUT-NTOU-105-01. No. USTP-NTUT-NTOU-105-01. Author Contributions: Da-Hua Wei conceived and designed the experiments; Da-Hua Wei, Ko-Ying Pan and Sheng-Kai Tong performed the experiments; Da-Hua Wei, Ko-Ying Pan and Sheng-Kai Tong analyzed the data; Da-Hua Wei organized the team work and wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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Author Contributions: Da-Hua Wei conceived and designed the experiments; Da-Hua Wei, Ko-Ying Pan and Sheng-Kai Tong performed the experiments; Da-Hua Wei, Ko-Ying Pan and Sheng-Kai Tong analyzed the data; Da-Hua Wei organized the team work and wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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