novel hybrid nanostructured materials of magnetite nanoparticles and pectin
TRANSCRIPT
Journal of Magnetism and Magnetic Materials 323 (2011) 980–987
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials
0304-88
doi:10.1
n Corr
E-m
journal homepage: www.elsevier.com/locate/jmmm
Novel hybrid nanostructured materials of magnetite nanoparticles and pectin
Saurabh Sahu, Raj Kumar Dutta nDepartment of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India
a r t i c l e i n f o
Article history:
Received 21 June 2010
Received in revised form
19 November 2010Available online 27 November 2010
Keywords:
Pectin
Calcium pectinate
Magnetite nanoparticle
Superparamagnetic
Zeta potential
Saturation magnetization
53/$ - see front matter & 2010 Elsevier B.V. A
016/j.jmmm.2010.11.085
esponding author. Tel.: +91 1332 285280; fax
ail address: [email protected] (R.K. Dutta)
a b s t r a c t
A novel hybrid nanostructured material comprising superparamagnetic magnetite nanoparticles (MNPs)
and pectin was synthesized by crosslinking with Ca2 + ions to form spherical calcium pectinate
nanostructures, referred as MCPs, which were typically found to be 100–150 nm in size in dried
condition, confirmed from transmission electron microscopy and scanning electron microscopy. The
uniform size distribution was revealed from dynamic light scattering measurement. In aqueous medium
the MCPs showed swelling behavior with an average size of 400 nm. A mechanism of formation of
spherical MCPs is outlined constituting a MNP–pectin interface encapsulated by calcium pectinate at the
periphery, by using an array of characterization techniques like zeta potential, thermogravimetry, Fourier
transformed infrared and X-ray photoelectron spectroscopy. The MCPs were stable in simulated
gastrointestinal fluid and ensured minimal loss of magnetic material. They exhibited superparamagnetic
behavior, confirmed from zero field cooled and field cooled profiles and showed high saturation
magnetization (Ms) of 46.21 emu/g at 2.5 T and 300 K. Ms decreased with increasing precursor pectin
concentrations, attributed to quenching of magnetic moments by formation of a magnetic dead layer on
the MNPs.
& 2010 Elsevier B.V. All rights reserved.
1. Introduction
Hybrid nanomaterials are of tremendous interest in biomedicalapplications due to the potential synergistic properties that mayarise from the combination of two or more precursors. Two suchprecursors are pectin and magnetite nanoparticles (MNPs). Pectinis a biodegradable natural polymer consisting of linear anionicpolysaccharide and is widely explored as a matrix for drug deliverydue to its colon specificity [1]. This is mainly attributed to the tworeasons: firstly, pectin is resistant to protease and amylase, whichare active in upper gastrointestinal (GI) tract [2]. As a result,materials for colon specific delivery, if loaded in pectin could beprotected from its dissolution in stomach environment. Secondly,pectin exhibits excellent controlled drug release properties incolon [3,4]. More over pectin cross linked with Ca2 + ions formingmicrobeads of calcium pectinate is reported to be effectiveformulation for certain colon specific drug molecules [5]. Therehas been significant advancement towards reducing the size ofcalcium pectinate to a few hundred nanometers which illustratedefficient loading and delivery of insulin, 5-Fluorouracil, genes[6–8]. The size reduction of the carrier matrix is encouraging asit might facilitate transport properties through biological pathwaysand barriers to enhance its bioavailability and functionalities. Thesecond precursor, i.e., MNPs exhibits superparamagnetic property
ll rights reserved.
: +91 1332 286202.
.
[9–11]. Ideally, magnetization interference from domain wall is notexpected especially when the particles consist of single magneticdomain in a matrix. In this regard, the MNPs of about 5–20 nm indiameter is considered to be a promising material for severalbiomedical applications, e.g., cellular imaging, targeted delivery,targeted chemotherapy, magnetic resonance imaging (MRI),hyperthermia [12–16]. These are attributed due to its superpar-amagnetic susceptibility, high saturation magnetization, biocom-patibility and non-toxicity [17,18] and their relative ease ofsynthesis by co-precipitation method [19].
The hybrid nanomaterials of pectin and MNPs are thus reckonedto be magnetically responsive and are expected to exhibit theinherent controlled drug release properties. This will allow theinvestigation of new concepts like magnetic transportation andcontrol release of drug molecules or other suitable substrate forcolon specific sites. However, the hybrid nanomaterials if adminis-tered orally will transit through the stomach where typically theresidence time is 2 h [20]. Gastric juice in the stomach consisting ofpepsin, mucus and hydrochloric acid (HCl), constitutes pH�1.2[21], and favors dissolution of MNPs [22]. Therefore, to retain themagnetic targeting efficiency of the hybrid nanomaterials for colonspecific sites, it is very important to protect the MNPs of the hybridsystem from acid dissolution in the gastric environment during itstransit through stomach.
We report here the synthesis of hybrid nanostructured materi-als consisting of MNPs encapsulated with calcium pectinate,referred to as MCPs. An optimum concentration of polyanionicpectin is in situ cross-linked with Ca2 + ions in the presence of MNPs
S. Sahu, R.K. Dutta / Journal of Magnetism and Magnetic Materials 323 (2011) 980–987 981
dispersed with biodegradable tween-80. The current synthetic methoddoes not involve any organic solvent. A mechanism is proposedregarding the formation of the MCPs based on several characterizationmethods. Besides their nanoscale dimensions, these hybrid nanos-tructures have the advantages of magnetic targeting, controlled releaseand are chemically resistant in stomach condition. Such hybridnanomaterials will find various biomedical applications, especiallyin targeted drug delivery system [23–25].
2. Experimental
2.1. Materials
Fe(NO3)3 �9H2O, FeSO4 �7H2O, liquid ammonia, anhydrous CaCl2and other reagents for synthesis were all of analytical grade fromMerck, India and were used without further purification. Pectin with65–70% degree of esterification was procured from Hi-Media lab,India. Millipore water (resistivity of 18.1 MO cm at 25 1C) was used inall the experiments.
2.2. Synthesis of MNPs
The MNPs were synthesized by rapidly adding liquid ammoniato a solution of a mixture of Fe(NO3)3 �9H2O and FeSO4 �7H2O witha molar ratio of 2:1, until the pH of the solution reached 10.070.1.The mixture was vigorously stirred for about 45 min as reported byMikhaylova et al. [26]. However, the proposed method wasmodified by using biocompatible surfactant (0.2% v/v, tween-80),for dispersing the as-synthesized MNPs to achieve better particlesize distribution. The excess tween-80 was magnetically decantedand washed with Millipore water followed by lyophilization, whichresulted into black colored solid phase MNPs.
2.3. Synthesis of MCPs
The pectin solutions of 0.2%, 0.4%, 0.6%, 0.8% and 1.0% w/v wereprepared in Millipore water by continuously stirring for 24 h at roomtemperature. The 50 mL of aqueous dispersion of MNPs (pH wasadjusted to about 4 with dilute HCl) was mixed with 50 mL pectinsolutions of respective concentrations at pH�4 and this mixture wasstirred vigorously for 1 h at constant pH�4. 50 mL of CaCl2 solution(Ca2+/pectin mass ratio¼2:1) was then added drop wise to cross-linkpectin by ionotropic gelation method [27] along with vigorous stirringfor 6 h. Corresponding to the concentration of the pectin solution used(0.2–1.0% w/v), the samples of MCPs synthesized were referred to asMCP-0.2, MCP-0.4, MCP-0.6, MCP-0.8 and MCP-1.0, respectively. Afraction of each of these MCPs was isolated for their characterizationby SEM, TEM, DLS and zeta potential measurements. The rest of thesynthesized nanomaterials were magnetically separated and washedseveral times with Millipore water to remove excess pectin and werelyophilized. In addition, calcium pectinate nanomaterials withoutMNPs were synthesized by ionotropic gelation method. It was used asa reference sample for confirming the formation of calcium pectinatein the hybrid nanostructured materials of MCPs.
2.4. Characterization
The X-ray diffraction measurements were performed with apowder diffractometer (Bruker AXS D8 Advance) using graphitemonochromatized CuKa radiation source. The morphology of the assynthesized MNPs and the MCP hybrid nanostructured materialswere studied using the transmission electron microscopy (TEM)operated at 200 kV FEI Technai-G2 microscope and the field emissionscanning electron microscopy (beam resolution of 2 nm) with energy
dispersive X-ray analyzer (FESEM–EDAX, FEI-Quanta 200F) operatedat 20 kV. For TEM studies, the as-synthesized samples were highlydiluted and a drop of this suspension containing nanomaterials wasplaced on a carbon coated 150 mesh copper grid and dried at roomtemperature. For SEM studies, diluted solution containing nanoma-terials were sprayed on a clean glass plate, which was dried andcoated with thin layer of Au to impart necessary electrical conduc-tivity for the incident electrons. The DLS measurements wereperformed by using the Malvern Zetasizer Nano ZS90 instrumentwith a 4 mW He–Ne laser (633 nm wavelength) and a detector at a1731fixed angle. The size measurements were carried out in triplicateat 25 1C by transferring 1 mL of dust free sample suspension into four-clear-size disposable polystyrene cell (Malvern). In order to findevidence of the formation of nanostructured MCPs, the zeta potentialmeasurements were carried out in triplicate at 25 1C by injecting0.75 mL of dust free sample suspension into disposable foldedcapillary cells (Malvern). The molecular vibrations of MCPs wereobtained by recording FT-IR (Nicolet, Nexus) spectra. Pellets of thedried samples were made with KBr and were scanned in the range of500–4000 cm�1. The TGA measurements of samples were measuredusing Perkin Elmer, Pyris Diamond under a nitrogen flow (200 mLmin�1) with a heating rate of 5 1C min�1 from ambient temperatureup to 800 1C to ensure mass loss due to thermal degradation of thepolymer and to minimize the increase in mass due to oxidation of ironin air.
A superconducting quantum interference device (SQUID) mag-netometer (MPMSXL, USA) was used to analyze the magneticproperties of hybrid nanomaterials. A known amount of lyophilizedsamples were packed in a diamagnetic capsule and were inserted ina polyethylene straw as a sample holder. The magnetizationmeasurements were recorded from the hysteresis loop of M–H
curve in the range 72.5 T at 300 K. The FC and ZFC measurementswere recorded at an applied field of 200 Oe by scanning between 5and 300 K. The XPS measurements were recorded on ESCA VSWscientific instruments Ltd., with AlKa as the source for excitation,operated at 10 kV with an emission current of 10 mA. The samplefor XPS characterization was prepared by sprinkling dried lyophi-lized sample on silver paste applied on Cu holder.
Quantitative analysis of the composition of the sample surfacewas performed by collecting the integrated intensities of C1s, O1s,Ca2p and Fe2p3/2 signals using the Wagner’s sensitivity factors. Thedissolution studies of the MNPs and MCPs were conducted as perstandard protocol of United State Pharmacopoeia in 900 mL offreshly prepared SGF solution, pH�1.2) at 3770.1 1C and 100 rpmfor 120 min in order to mimic the physiological conditions similarto that of gastrointestinal tract of human body. A 5 mL of dissolu-tion fluid was withdrawn at each specified time intervals andreplaced with equal volume of fresh medium to mimic the sinkconditions of the human body. The withdrawn fluid was filteredand iron content was estimated with Shimadzu 1600 UV Spectro-photometer using phenanthroline method by recording the absor-bance at 510 nm [28].
3. Results and discussion
3.1. Synthesis and morphology of hybrid nanostructured MCPs
The strategy to prepare the hybrid nanostructured MCPsinvolved the synthesis of stable MNPs by co-precipitation methodfollowed by its in-situ encapsulation with pectin. The Ca2 + ionswere further used for cross linking pectin to form nanostructuredhybrid materials of MCPs. The above method was optimized toachieve uniform size distribution of these spherical nanostruc-tures. The key parameters involved in the optimization processwere pH�4, pectin precursor concentration (in % w/v), Ca2 +/pectin
S. Sahu, R.K. Dutta / Journal of Magnetism and Magnetic Materials 323 (2011) 980–987982
mass ratio of 2:1 and vigorous stirring for 6 h. The pectin concentrationof 0.4% and 0.6% were found to be suitable for synthesizing sphericalshaped MCP-0.4 and MCP-0.6, respectively. Pectin of 0.2% w/v wasfound insufficient for complete encapsulation of MNPs, while 1.0% w/vresulted into formation of polymeric matrix due to its cross linkingwith Ca2 + ions. In order to achieve maximum cross linking it mightbe desirable to prepare these hybrid nanostructured materialsat pH45.5 where the pectin molecules exhibit very high zetapotential values (z¼�52 to �56 mV) [29]. However, such higherpH was restricted due to the onset of de-polymerization effect ofpectin at pH45.5 [30]. On the other hand, lower pH (e.g., pH�2)tends to reduce the zeta potential of pectin [6] and hence may notbe suitable for chemical cross-linking with Ca2 + ions. Besides,lower pH would favor dissolution of the MNPs encapsulated in thehybrid nanostructures of MCPs due to weakening of the Fe–O bondby protonation mechanism [31] and might lead towards loss of itsmagnetic property. As a compromise, pH�4 was chosen optimumfor synthesis of MCPs which offered suitably high zeta potential ofpectin (z¼�35.7 mV, Table 1).
The formation of as synthesized stable MNPs were confirmed byrecording the position and relative intensities of diffraction pat-terns at 220, 311, 400, 511 and 440 planes (Fig. 1) which corrobo-rated well with those of cubic magnetite structures as reported inJCPDS 01-11111 data. The average particle size (D) of the MNPs wasfound to be about 2 nm, using the Debye–Scherrer formula, i.e.D¼(0.9l)/(D cos y), where l is X-ray wavelength, D is line broad-ening measured at half-height from the most intense peak of XRD(311 plane) and y is Bragg angle of the particles. The magnetitephase was also evident in all the samples of the MCPs as supportedfrom respective diffraction patterns (Fig. 1). However, in MCP-1.0the magnetite phase was weak, which might be attributed to thematrix effect or due to encapsulation of very small size of magnetitenanoparticles.
The TEM measurements revealed that the particle sizes of theas-synthesized MNPs were in the range between 2 and 8 nm
Table 1Zeta potential measurements of magnetite nanoparticles (MNPs), pectin solution
and MNP coated calcium pectinate nanostructured hybrid materials (MCPs).
Samples Measured zeta potential
value (z) in mV
MNPs at pH �10 �41.2
MNPs at pH �4 +17.1
Pectin at pH �4 �35.7
MCP-0.4 �17.9
MCP-0.6 �14.7
MCP-1.0 �14.6
20 30 40 50 60 70 800
300
600
900
1200 440
511
40031
122
0
Inte
nsity
MCP-0.4
MCP-0.2
MCP-1.0
MCP-0.6MCP-0.8
MNPs
2θ degree
Fig. 1. XRD of the MNPs and various compositions of MCPs.
(Fig. 2a), and the corresponding SAED image indicated its poly-crystalline nature (Fig. 2b). After coating the MNPs with calciumpectinate the resulting nanostructured hybrid materials of MCP-0.4and MCP-0.6 were found to be mostly spherical with size distribu-tion in the range of 50–200 nm as evidenced from SEM study(Fig. 3a). Notably, the particles of sizes 100–150 nm were mostfrequently observed (inset of Fig. 3a). The co-localization of Fe andCa in the EDAX analysis of a selected area of a representativespherical nanostructure (Fig. 3b) was characteristic of MNPs andcalcium pectinate, respectively, and thus indicated the formation
Fig. 2. (a) TEM image of MNPs and (b) the SAED image corresponding to (a) confirmed
polycrystalline MNPs.
Fig. 3. (a) Morphology and particle size distribution of MCP-0.4 by SEM, (b) EDAX of a selected nanostructure of MCP-0.4, marked in the inset, illustrated co-localization of Fe
and Ca, (c) TEM of spherical shaped MCP-0.4 nanostructure (marked) of about 150 nm, and (d) detailed TEM of a part of MCP-0.4 nanostructure, as shown by an arrow in
(c), illustrated the presence of multiple polycrystalline MNPs.
S. Sahu, R.K. Dutta / Journal of Magnetism and Magnetic Materials 323 (2011) 980–987 983
of MCPs. Further, the TEM analysis of the MCP-0.4 illustratedspherical hybrid nanostructures of 150 nm size which corrobo-rated the SEM results (Fig. 3c). A detailed TEM study of a part of arepresentative nanostructure of MCP-0.4 revealed encapsulation ofa large number of MNPs (Fig. 3d) and its SAED pattern signifiedpolycrystalline nature (inset of Fig. 3d).
Furthermore, DLS measurement of MCP-0.4 in aqueous medium atpH�4 exhibited unimodal size distribution in the range of250–620 nm with maximum intensity at �400 nm (Fig. 4). Thisindicated that the method of synthesis offered a reasonably good
control over the size of these hybrid nanostructures. It may howeverbe noted that the particle size measured by DLS were larger ascompared to those measured by TEM and SEM. This might beattributed due to possible swelling behavior of calcium pectinatecoating material in aqueous medium. Similar swelling effect wasreported for calcium pectin hydrogels [32]. The DLS measurements ofMCP-0.4 were carried out in aqueous media with pH ranging between1 and 7, which showed quite similar size distribution as typicallyobserved in Fig. 4, which might be due to saturation of swelling inaqueous media at various pH.
0
5
10
15
20
25
0.1 1 10 100 1000 10000
Inte
nsity
(%)
Size (d.nm)
Fig. 4. DLS measurement of MCP-0.4 in aqueous medium at pH�4.
MNPsPectin Ca2+ ions
pH~4crosslinking
MCP hybrid nanostrucutres
O
OH
H
HO
H
O
H
COO-
H O-OOC
H O
H
OH
H
HO
H
H
O
H
n
Structure of a pectin monomer unit
Fig. 5. Schematic representation of the proposed mechanism for the synthesis of MCP hybrid nanostructures.
Fig. 6. FT-IR spectrum of MNPs, MCP-0.4 and MCP-0.6, precursor pectin and calcium
pectinate reference sample.
S. Sahu, R.K. Dutta / Journal of Magnetism and Magnetic Materials 323 (2011) 980–987984
3.2. Mechanism of the formation of MCP hybrid nanostructured
materials
The zeta potential measurements (given in Table 1) offered aninsight of the proposed mechanism towards formation of the MCPsas schematically represented in Fig. 5. Firstly, the highly stableMNPs, as confirmed from its zeta potential (�41.2 mV) measure-ment, were synthesized at pH�10. These stable MNPs wereconditioned at pH�4 to interact with pectin. At this pH, the zetapotential of MNPs was found to be +17.1 mV which was consideredto be favorable for electrostatic interaction with polyanionic pectinmolecules (z¼�35.7 mV at pH�4) to form an MNP–pectin inter-face. The carboxylic groups of pectin molecules were cross-linkedby Ca2 + ions to form spherical hybrid nanostructures, where theMNPs were encapsulated by calcium pectinate. The likelihood offormation of such spherical shape could be explained in terms ofattaining stability by achieving minimum surface energy. Thecross-linking of pectin might be interpreted from the lowering ofzeta potential measurements of MCP-0.4 (�17.9 mV), MCP-0.6(�14.7 mV) and MCP-1.0 (�14.6 mV) as compared to that ofpectin. The lowering of zeta potential might be attributed toshielding of charge density on the pectin molecules due to itselectrostatic interactions with Ca2 + ions.
The FT-IR and TGA analyses of the samples further supported theproposed mechanism. The FT-IR spectra (Fig. 6) of the pectin showed
weak bands at 1690 cm�1 corresponding to COOH group while that ofMCP revealed intense bands at 1685, 1395 and 1324 cm�1 which werecharacteristic of asymmetric and symmetric stretching of carboxylategroups (COO�). Similar intense IR bands were also recorded for thesynthesized calcium pectinate reference sample. These observations
00
500
1000
1500
2000
2500
3000
Cl1s
O1s
Ca2p
C1s
Inte
nsity
Binding Energy (eV)
700 710 720 730 740900
1000
1100
1200
1300
Inte
nsity
Binding Energy(eV)
Fe2p3/2
200 400 600 800 1000
Fig. 8. XPS of MCP-0.4 (full scan) and detailed scan for Fe analysis in the inset.
S. Sahu, R.K. Dutta / Journal of Magnetism and Magnetic Materials 323 (2011) 980–987 985
indicated the formation of calcium pectinate in the hybrid nanos-tructures of MCPs. However, the IR bands corresponding to carbox-ylate group (1390 and 1620 cm�1) were also observed for the as-synthesized MNPs which were attributed to the carboxylate groups oftween-80 used for stabilizing the MNPs. In addition, the IR signaturesfor pectin and the MCPs were found similar in the fingerprint region of900–1250 cm�1 which might be due to their similar molecularstructures. However, it was not possible to rule out the presence ofcertain amount of pectin which was not modified to calcium pectinateduring the synthesis of MCPs.
The formation of MCP-0.4 and MCP-0.6 was further confirmedfrom TGA studies (Fig. 7), which illustrated nearly 5% higher massloss for MCP-0.6 as compared to MCP-0.4. This was attributed tohigher pectin precursor concentration for synthesis of MCPs. Inaddition, their TGA thermogram comprised with three distinctthermal events. The first event was recorded in the temperaturerange between ambient temperature and 110 1C corresponding to amass loss of about 11% which was due to desorption of watermolecules. The second event showed a gradual mass loss of about17% in the temperature range of 200–450 1C, which was compar-able with the thermal degradation pattern of the calcium pectinate.The third thermal event corresponding to nearly 11% mass loss inthe temperature range of 550–775 1C did not concur with that ofcalcium pectinate. However, this thermal event appeared to becorrelated with the thermal decomposition pattern of precursorpectin corresponding to the temperature range of 320–455 1C.From these observations, it may be inferred that the hybridnanostructured MCPs contained both pectin as well as calciumpectinate components. Most likely, calcium pectinate constitutedthe periphery of the MCPs that enclosed pectin interfaced withMNPs. According to this proposed MCP nanostructure, the thermaldecomposition of pectin would be expected after the thermaldegradation of calcium pectinate at the periphery. This wasperhaps the reason why the onset of thermal decomposition ofpectin in the MCPs occurred at much higher temperature than thatof the precursor pectin. Furthermore, the formation of calciumpectinate at the periphery of the MCPs could be further deduced bycomparing the mass loss phenomenon due to desorption of waterat 110 1C for MCP with that of precursor pectin and calciumpectinate reference sample. For MCPs this mass loss was muchhigher (11%) as compared to that of pectin (4%) but less than that ofcalcium pectinate (33%). The higher affinity for calcium pectinate toadsorb water was also correlated from the broad IR band observedin the range 3000–3600 cm�1. The nature of mass loss at 110 1C forMCPs therefore supported the formation of calcium pectinate at theperiphery of MCP. This was confirmed from the surface analysis byX-ray photoelectron spectroscopy studies of MCP-0.4. It revealed
Fig. 7. TGA analysis of MNPs, MCP-0.4, MCP-0.6, pectin and calcium pectinate.
the binding energy of the photoelectron peaks corresponding toC1s, O1s, and Ca2p3/2, which were the signatures of calciumpectinate (Fig. 8). It also showed weak binding energy peakcorresponding to Fe2p3/2 (inset of Fig. 8) and the concentrationof Fe was calculated to be 1.670.6%.
3.3. Stability of MCPs in simulated gastric fluid
Considering the potential applications of using the hybridnanostructured MCPs for colon specific delivery via oral adminis-tration, it is essential for the MCPs to be protected from gastricdegradation during its transit through stomach (pH�1.2) to avoidloss of magnetic materials. The non-degradability of MCPs ingastric condition would further ensure protection of loadedmaterials (e.g., drugs) for colon specific delivery. This aspect wasstudied by treating MCP-0.4 and equivalent amount of MNPspresent in MCP-0.4 for a total of 120 min in simulated gastric fluid(SGF). The cumulative Fe release in SGF from MCP-0.4 in sinkcondition was found to be only 1.9% for a period of 120 min (Fig. 9a),while that of MNPs was 21.3% (Fig. 9b). The low Fe release fromMCP-0.4 in SGF corroborated well with the Fe concentration at thesurface of MCP-0.4 calculated from the XPS measurement. Thisindicated that most of the MNPs were encapsulated within the MCPnanostructure and were well protected from gastric environment.Degradation of MCPs would have otherwise caused significantrelease and dissolution of MNPs in SGF. It may therefore be
0
0
5
10
15
20
a
b
% C
umul
ativ
e Fe
Rel
ease
Time (Minutes)20 40 60 80 100 120
Fig. 9. Dissolution profile of Fe release from MNPs (a) MCP-0.4, (b) in SGF for a total
period of 120 min.
80 MNPs
S. Sahu, R.K. Dutta / Journal of Magnetism and Magnetic Materials 323 (2011) 980–987986
interpreted that the MCP-0.4 was impermeable to acidic solutionand ensured minimal loss of magnetic materials.
-20000 0 20000-80
-40
0
40
M (e
mu/
g)
H (Oe)
MCP-0.6
MCP-1.0
MCP-0.4
Fig. 11. Magnetization for dry samples of MNPs and MCPs at 300 K.
3.4. Magnetic properties of hybrid nanostructures of MCP
The superparamagnetic behavior of the hybrid nanostructuresof MCP-0.4 was studied using the temperature dependence mag-netization between 5 and 300 K of zero-field cooling (ZFC) and thatof field cooling (FC) in an applied magnetic field of 200 Oe (Fig. 10).At low temperatures (for FC curve), an externally applied magneticfield energetically favored the reorientation of the individualmagnetic moment and hence resulted in the observed magnetiza-tion along the direction of the applied field, which decreased withincreasing temperature. On the other hand, in the absence ofapplied magnetic field and when the sample was cooled the totalmagnetization was zero (as shown in ZFC curve), due to the randomorientation of the magnetic moments of individual particles. As thetemperature was increased more particles reoriented their mag-netic moment (magnetization) along the external applied field. Dueto this the magnetization increased till it reached a maximum valueat 93.3 K, referred as blocking temperature of the synthesizedMNPs, as shown in the ZFC curve (Fig. 10) and then its profile wassimilar to that of the FC curve. These features are typically observedfor small grain sizes of magnetite nanoparticles [33] which arereported to exhibit superparamagnetism for particles of sizes lessthan 20 nm [34] due to lack of well defined domain structure.However, from the FC–ZFC curve shown in Fig. 10, it may be arguedthat both ferromagnetic and superparamagnetic regimes coexistedowing to the distribution of grain size of magnetite nanoparticles inthe batch of MCP. In that case, the magnetic behavior exhibited bythe MCP could be attributed to partial volume of the magnetitenanoparticles with sizes corresponding to these two regimes.
It was noted that the magnetization curves (M–H) of differentcompositions of MCPs recorded at 300 K from the hysteresis loop inSQUID measurements were similar to that of the as-synthesizedMNPs, and exhibited negligible coercivity and remanence magne-tization (Fig. 11). This property was attributed to superparamag-netic behavior of the magnetite nanoparticles, expected for sizesless than 20 nm and hence consistent with our synthesizedmagnetite nanoparticles which are 2–8 nm in sizes. It may beinferred that the magnetic nanoparticles in the synthesized batchesof MCP-0.4 are predominantly superparamagnetic in nature whichfurther corroborated the ZFC and FC studies.
08
10
12
14
16
18
M (e
mu/
g)
Temperature (K)
50 100 150 200 250 300
Fig. 10. ZFC and FC curve of MCP-0.4 recorded at 200 Oe.
As the precursor pectin concentration was increased from 0.4%to 1.0% w/v the saturation magnetization (Ms) of their correspond-ing hybrid nanostructures of MCP decreased. The measured Ms at300 K and 2.5 T were 46.21, 38.70 and 3.05 emu/g for MCP-0.4,MCP-0.6 and MCP-1.0, respectively. These values were howeversmaller than that of the as-synthesized MNPs (52.16 emu/g) andnotably the Ms reduced significantly, when 1.0% precursor pectinconcentration was used for synthesis of MCP-1.0. The drasticreduction of saturation magnetization due to increase in pectinconcentration could be attributed to various reasons. It might bedue to effect of small particle size owing to non-collinear spinarrangement at the surface [35], or due to the formation ofmagnetic dead layer by pectin at the domain boundary wall ofMNPs [36,37]. The particle size effect on reduction of Ms may beruled out as the same batch of MNPs with uniform particle sizedistribution was used for synthesis of different MCP compositions.The magnetic moments could however be quenched due to theformation of magnetic dead layer at the domain wall of MNPs. Thiscould hinder the domain wall motion during application of themagnetic field, which might be responsible for the reduction in thesaturation magnetization in MCPs. In this regard, pectin might form amagnetic dead layer at the surface of MNPs during the formation of theproposed MNP–pectin interface. The reduction in the magnetizationfor MCP-0.6 as compared to those of MCP-0.4 was attributed toformation of thicker dead layer of pectin at the domain boundary wallof encapsulated MNPs. It may therefore be inferred that the synthesisof hybrid nanostructures of MCPs resulted in pectin–MNP interfacewith formation of calcium pectinate at the periphery. This system as awhole exhibited superparamagnetic behavior with high magnetiza-tion at applied field of 2.5 T but appeared to be influenced by precursorpectin concentration.
4. Conclusion
A facile and an in-expensive method has been developed tosynthesize a novel hybrid nanostructured material (MCP) bycompletely encapsulating stable superparamagnetic magnetitenanoparticles (MNPs) with pectin followed by cross linking withCa2 + ions. The superparamagnetic nature of the as synthesizedMCP-0.4 was confirmed by measuring ZFC–FC profile at an appliedfield of 200 Oe and the blocking temperature was found to be93.3 K. The MCPs showed considerable magnetism, although theirsaturation magnetization value was lower than that of the bareMNPs and decreased with increasing precursor pectin concentra-tion. The magnetite phase of the MNPs was confirmed from XRDand its encapsulation by calcium pectinate to form MCPs wasprobed by FT-IR, TGA and XPS studies. The MCPs were mostlyspherically shaped with sizes ranging typically between 100 and
S. Sahu, R.K. Dutta / Journal of Magnetism and Magnetic Materials 323 (2011) 980–987 987
150 nm in dry condition as evidenced by TEM and SEM. However, inaqueous medium at pH�4, these nanostructures showed swellingeffect with an average size of�400 nm as determined by DLSmeasurement, and evidenced unimodal size distribution. Thisindicated that the method used in the synthesis of MCP nanos-tructures offered a good control over their sizes. Furthermore, theMCPs were reasonably stable in aqueous medium as revealed fromtheir zeta potential measurements and they were found to beimpermeable in acidic condition indicating minimal loss of mag-netic material in simulated gastric condition. Overall, it may beconsidered that the novel magnetically responsive hybrid nanos-tructures comprising superparamagnetic MNPs and pectin couldpotentially find a wide range of biomedical applications, e.g.,targeted delivery of drugs or other biomolecules, imaging byMRI or magnetic fluid hyperthermia.
Acknowledgements
One of the authors (S.S.) would like to thank MHRD, India forawarding fellowship. Authors thank Institute InstrumentationCentre and Centre of Nanotechnology, IIT Roorkee for instrumentalfacilities. Authors also thank Dr. T. Shripathi, and Prof. Ajay Gupta ofUGC DAE, CSR Indore Centre, India for extending their XPS experi-mental facility.
References
[1] L. Liu, M.L. Fishman, J. Kost, K.B. Hicks, Biomaterials 24 (2003) 3333–3343.[2] H.N. Englyst, S. Hay, G.T. MacFarlane, FEMS Microbiol. Ecol. 95 (1987) 163–171.[3] S.A. Sande, Expert Opin. Drug Deliv. 2 (2005) 441–450.[4] S.A. Sande, S. Sungthongjeen, T. Pitaksuteepong, A. Somsiri, P. Sriamornsak,
Drug Dev. Ind. Pharm. 25 (1999) 1271–1276.[5] S. Das, K.Y. Ng, J. Pharm. Sci. 99 (2009) 840–860.[6] K. Cheng, L.Y. Lim, Drug Dev. Ind. Pharm. 30 (2004) 359–367.[7] C.Y. Yu, H. Cao, X.C. Zhang, F.Z. Zhou, S.X. Cheng, X.Z. Zhang, R.X. Zhuo, Langmuir
25 (2009) 11720–11726.[8] P. Opanasopit, A. Apirakaramwong, T. Ngawhirunpat, T. Rojanarata,
U. Ruktanonchai, AAPS Pharm. Sci. Technol. 9 (2008) 67–74.[9] C.C. Huang, T.Y. Liu, C.H. Su, Y.W. Lo, J.H. Chen, C.S. Yeh, Chem. Mater. 20 (2008)
3840–3848.
[10] S. Maenosono, T. Suzuki, S. Saita, J. Magn. Magn. Mater. 320 (2008) L79–L83.[11] G. Cheng, D. Romero, G.T. Fraser, A.R.H. Walker, Langmuir 21 (2005)
12055–12059.[12] J.W.M. Bulte, D.L. Kraitchman, NMR Biomed. 17 (2004) 484–499.[13] P. Dames, B. Gleich, A. Flemmer, K. Hajek, N. Seidl, F. Wiekhorst, D. Eberbeck,
I. Bittmann, C. Bergemann, T. Weyh, L. Trahms, J. Rosenecker, C. Rudolph, Nat.Nanotechnol. 2 (2007) 495–499.
[14] D.B. Shieh, F.Y. Cheng, C.H. Su, C.S. Yeh, M.T. Wu, Y.N. Wu, C.Y. Tsai, C.L. Wu,D.H. Chen, C.H. Chou, Biomaterials 26 (2005) 7183–7191.
[15] F.X. Hu, K.G. Neoh, E.T. Kang, Biomaterials 27 (2006) 5725–5733.[16] C.H. Hou, S.M. Hou, Y.S. Hsueh, J. Lin, H.C. Wu, F.H. Lin, Biomaterials 30 (2009)
4700–4707.[17] C. Sestier, Z.G.M. Lacava, L.M. Lacava, M.F. Da Silva, R.B. Azevedo, N. Buske,
C. Gansau, P.C. Morais, O. Silva, F. Pelegrini, D. Sabolovicm, J. Magn. Magn.Mater. 252 (2002) 403–405.
[18] A.L.D. Silva, T. Trindade, B.J. Goodfellow, B.F.O. Costa, R.N. Correia, A.M. Gil,Biomacromolecules 8 (2007) 2350–2357.
[19] G. Gnanaprakash, S. Mahadevan, T. Jayakumar, P. Kalyanasundaram, J. Philip,B. Raj, Mater. Chem. Phys. 103 (2007) 168–175.
[20] N. Washington, C. Washington, C.G. Wilson, Physiological Pharmaceutics:Barriers to Drug Absorption, second ed., Taylor and Francis Inc., London, 2001.
[21] V.R. Sinha, R. Kumria, Int. J. Pharm. 249 (2002) 23–31.[22] P.W. Schindler, Pure Appl. Chem. 63 (1991) 1697–1704.[23] A. Kumar, B. Sahoo, A. Montpetit, S. Behera, R.F. Lockey, S.S. Mohapatra,
Nanomed. Nanotechnol. Biol. Med. 3 (2007) 132–137.[24] L.B. Asperilla, M. Darder, P. Aranda, L. Vazquez, M. Vazquez, E.R. Hitzky,
J. Mater. Chem. 17 (2007) 4233–4238.[25] T. Xu, N. Zhang, H.L. Nichols, D. Shi, X. Wen, Mater. Sci. Eng. C 27 (2007)
579–594.[26] M. Mikhaylova, D.K. Kim, N. Bobrysheva, M. Osmolowsky, V. Semenov,
T. Tsakalakos, M. Muhammed, Langmuir 20 (2004) 2472–2477.[27] P. Sriamornsak, Eur. J. Pharm. Sci. 8 (1999) 221–227.[28] G.D. Christian, Analytical Chemistry, sixth ed., John Wiley & Sons, Singapore,
2003.[29] G.T. Grant, E.R. Morris, D.A. Rees, P.J.C. Smith, D. Thom, FEBS Lett. 32 (1973)
195–198.[30] C. Rolin, B.U. Nielsen, P.E. Glaahn, in: S. Dumitriu (Ed.), Polysaccharides, Marcel
Dekker, New York, 1998, pp. 377–432.[31] I.H.M. Van Oorschot, M.J. Dekkers, Geophys. J. Int. 145 (2001) 740–748.[32] I. Mika, H. Tatsuko, H. Hyoe, Thermochim. Acta 431 (2005) 68–72.[33] S. Si, A. Kotal, T.K. Mandal, S. Giri, H. Nakamura, T. Kohara, Chem. Mater.
16 (2004) 3489–3496.[34] T.J. Daou, G. Pourroy, S. Begin-Colin, J.M. Greneche, C. Ulhaq-Bouillet, P. Legare,
P. Bernhardt, C. Leuvrey, G. Rogez, Chem. Mater. 18 (2006) 4399–4404.[35] A. Cesaro, F. Delben, S. Paoletti, Int. J. Biol. Macromol. 12 (1990) 170–176.[36] K.V.P.M. Shafi, A. Ulman, A. Dyal, X. Yan, N.L. Yang, C. Estournes, L. Fournes,
A. Wattiaux, H. White, M. Rafailovich, Chem. Mater. 14 (2002) 1778–1787.[37] M.P. Morales, S.V. Verdaguer, M.I. Montero, C.J. Sterna, A. Roing, L. Casas,
B. Martinez, F. Sandiumenge, Chem. Mater. 11 (1999) 3058–3064.