Ultrafast microwave-assisted synthesis of MCNCs with high saturationmagnetization and sustained aqueous stability{

Shuai Xu,ab Zhimin Luo,b Yujie Han,b Jia Guoa and Changchun Wang*ab

Received 23rd November 2011, Accepted 28th January 2012

DOI: 10.1039/c2ra01169g

A facile and green microwave route was developed to ultra-

quickly synthesize magnetite colloidal nanocrystal clusters

(MCNCs) by reducing iron(III) chloride with ethylene glycol

within 10 min at 150 uC. The obtained uniform MCNCs

exhibited excellent crystallinity, saturation magnetization and

sustained aqueous stability upon addition of stabilizers.

Magnetic nanoparticles (MNPs), which are a significant class of

materials with unique functionality, have attracted considerable

attention due to their promising applications in fundamental science

and sophisticated technology, such as ferrofluids, photonic crystals,

drug delivery systems, and magnetic resonance imaging (MRI).1,2

To fulfil specific requirements, the physiochemical properties of

magnetic nanomaterials must be rationally tailored and optimized.3

MNPs with high saturation magnetization and suitable size are

critically important because it allows a rapid response to an external

magnetic field for enhancement of enrichment and separation

efficiency in bio-medical applications.4

Recently, an one-pot polyol-mediated solvothermal method was

developed to fabricate magnetite colloidal nanocrystal clusters

(MCNCs) that possess greatly improved magnetic responsiveness,

tunable granular sizes, and well-defined structures.5 This leads to a

remarkable progress in the synthetic technology of magnetic

nanomaterials. However, the synthesis of MCNCs has often been

subjected to severe reaction conditions including an elevated

temperature (up to 200 uC), a long reaction time (more than 10 h),

and a sealed autoclave.4 Hence, developing a simple and green

way to obtain MCNCs with uniform size and high saturation

magnetization is highly demanded. Microwave irradiation has been

proved to be an efficient and green method to accelerate chemical

reactions through rapid volumetric heating.6 Compared with the

conventional solvothermal method, microwave irradiation offers

remarkable merits, including dramatically shortened reaction time,

lower thermal gradients, reduced energy consumption, and higher

yields.7 Based on this, the microwave-assisted synthesis strategy has

been widely used for synthesis of versatile nanomaterials, such as

polymer microspheres,8 metallic nanoparticles,9 metal oxides,10 and

quantum dots (QDs).11 Recently, magnetite nanoparticles were also

synthesized using microwave irradiation.12 However, the resulting

water-dispersible MNPs usually suffered from a wide size distribu-

tion,12a–c while the hydrophobic MNPs exhibited the weak magnetic

responsiveness,12d,e which was responsible for limiting their potential

applications in bio-related fields.

Herein, uniform MCNCs with high magnetic susceptibility were

prepared via a rapid and green microwave method with iron chloride

hexahydrate (FeCl3?6H2O) as an iron precursor, ammonia acetate

(NH4OAc) as an alkaline source, and ethylene glycol (EG) as

reducing agent and microwave-absorbing solvent (Fig. S1, ESI{). In

a typical synthesis, the homogeneous dispersion of precursors in EG

was transferred into a 35 ml vessel with a crimp cap, heated by a

single-mode microwave irradiation of 2.45 GHz (CEM Discovery,

CEM Inc. USA), and the other reaction parameters were rationally

modulated (Experimental details and Fig. S2, ESI{). In sharp

comparison to the known solvothermal process occurring in an

autoclave, the microwave-assisted reaction was dramatically speeded

up and completed within minutes. As displayed in Fig. 1a, the

representative TEM image demonstrated the formation of spherical

aState Key Laboratory of Molecular Engineering of Polymers, Departmentof Macromolecular Science, Fudan University, Shanghai, 200433,P. R. China. E-mail: ccwang@fudan.edu.cnbLaboratory of Advanced Materials, Fudan University, Shanghai, 200438,P. R. China{ Electronic supplementary information (ESI) available: Experimentaldetails, characterization and physicochemical properties. See DOI: 10.1039/c2ra01169g

Fig. 1 (a) TEM and (b) SEM image of MCNCs (inset is an enlarged one),

(c) Enlarged TEM image of a part of a single cluster, inset is the selected area

electron diffraction (SAED) pattern, and (d) HRTEM image of the boxed

region in image c.

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MCNCs with a diameter of approximate 273 nm. Also, the SEM

image revealed that the clusters were formed in the fashion of

aggregation of many small primary nanocrystals (Fig. 1b). The

selected area electron diffraction (SAED) pattern (inset of Fig. 1c),

recorded by focusing an electron beam on the boxed part, exhibited a

single-crystalline diffraction pattern of the cluster.13 Additionally, it

was estimated that the periodic fringe spacing of the crystallographic

planes was about 0.48 nm, agreeing well with the interplanar spacing

between the (111) lattice planes of the Fe3O4 crystal (Fig. 1d). To our

knowledge, this is the first example for microwave-assisted prepara-

tion of MCNCs with well-defined structures and uniform sizes in just

a few minutes.

Fig. 2 showed the TEM images of products synthesized at

different temperatures within 10 min. At 120 uC, due to the limited

energy supply, there were no MCNCs formed during the microwave

irradiation (Fig. 2a). At elevated temperatures, it was observable that

many small magnetite nanocrystals could be aggregated into clusters

(Fig. 2b–c). For example, at 150 uC, almost all primary nanocrystals

gathered into uniform and intact MCNCs (Fig. 2d). When the

temperature was further increased from 160 uC to 200 uC, the

resulting MCNCs could afford similar morphologies (Fig. 2e–f).

Based on the above results, one can conclude that temperature plays

a vital role in the formation of MCNCs.

Fig. 3a showed the powder XRD patterns of the products; all

peaks could be indexed well to the magnetic cubic structure of

magnetite (JCPDS No.75-1610). X-ray photoelectron spectroscopy

(XPS) of the products synthesized at 150 uC for 10 min exhibited

peaks at 711 and 724 eV (Fig. 3b), which are the characteristic peaks

of Fe 2p3/2 and Fe 2p1/2 oxidation states, respectively.4b Together

with XRD results, it was clear that the magnetite phase had been

synthesized by this facile and simple microwave pathway. As for the

products prepared at 120 uC, only one weak and broad peak was

found around 24u, which could be ascribed to the amorphous

nanomaterials (Fig. 3a(i)). With increasing temperature, the peak at

24u disappeared, and the characteristic peaks of magnetite became

more and more distinct, indicating the enhanced crystallization of the

magnetite nanoparticles (Fig. 3a(ii–vi)). The magnetization curves

(Fig. 3c) revealed that when a higher temperature was adopted in the

Fig. 2 TEM images of MCNCs synthesized at (a) 120 uC, (b) 130 uC, (c)

140 uC, (d) 150 uC, (e) 160 uC, and (f) 200 uC for 10 mins. All scale bars are

200 nm.

Fig. 3 (a) Power XRD patterns, (b) XPS spectrum of MCNCs synthesized

at 150 uC for 10 min, and (c) magnetization curves (T = 300 K) of MCNCs

synthesized at (i) 120 uC, (ii) 130 uC, (iii) 140 uC, (iv) 150 uC, (v) 160 uC, and

(vi) 200 uC for 10 mins and (vii) Fe3O4 CNCs obtained from solvothermal

process at 200 uC for 15 h.

2740 | RSC Adv., 2012, 2, 2739–2742 This journal is � The Royal Society of Chemistry 2012

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microwave irradiation, a better saturation magnetization of the

products was obtained. The MCNCs prepared below 100 uC were

found to have no magnetism. As the reaction temperature was

increased from 120 uC to 150 uC to 200 uC, the magnetization values

of the products were dramatically enhanced from 4.5 to 71.5 to

79.1 emu g21. Moreover, it is worthwhile to notice that the magnetic

hysteresis loops (Hc , 25 Oe) could be negligible for all the samples,

indicating the superparamagnetic property of the MCNCs (300 K).

Such an excellent magnetic property is beneficial to the applications

of MCNCs in biomolecule separation, and magnetically-guided drug

delivery.

In addition, we investigated the effect of different reaction times

on the evolution of MCNCs at 150 uC. The typical TEM images of

the products synthesized with different times showed their similar

morphologies (Fig. S3, ESI{). Also, we were aware that, after the

reaction proceeded for 5 min, the morphology of the products didn’t

undergo dramatic changes, which implied that the microwave

heating could take effect only using 5 min. The powder XRD

patterns of products with various reaction times gave almost the

same position and intensity of peaks, indicating the difference of

crystallinity among these products was slight and negligible (Fig. S4,

ESI{). In addition, the saturation magnetizations of the four

samples, as expected, were very close with respect to each other

(Fig. S5, ESI{). Apart from temperature and time, the microwave

power was also investigated. If the power used was less than 150 W,

inferior crystallinity and low magnetization were obtained due to the

insufficient microwave energy supply.

To elucidate the influence of reaction temperature and time on

the crystallite sizes and magnetization, the foregoing results were

compiled, and displayed in Fig. 4. The crystallite sizes of the

MCNCs were determined according to the Scherrer’s equation.14

As shown in Fig. 4a, the crystallite sizes of the MCNCs increased

gradually from 21.8 to 35.7 nm, as the temperature was elevated

from 130 uC to 200 uC. Also, this increasing tendency was observed

when the saturation magnetizations were plotted as a function of

temperature. On the other hand, as the reaction time prolonged

from 5 min to 60 min, the crystallite sizes of MCNCs were all in the

range of 30 nm (Fig. 4b). Compared with the MCNCs (200 uC,

15 h) synthesized by the typical solvothermal method, the

microwave-synthesized MCNCs (200 uC, 10 min) give a similar

grain size (35.7 nm vs. 37.4 nm), comparable saturation

magnetization (79.1 emu g21 vs. 80.0 emu g21), and high crystalline

degree; all of these disclose the powerful microwave effect in the

reaction. However, the microwave irradiation pathway is proved

to have a remarkably decreased reaction time, lower temperature,

and reduced energy consumption, thereby leading to an overall

reduction in energy consumption and high efficiency.

In the typical synthesis conditions of MCNCs, we found that the

pressure inside the vessel quickly reached about 140 psi, indicating

that the solvent EG might be in the boiling state. Therefore, it can be

rationally figured out that the polar solvent EG with high dielectric

losses has excellent microwave absorbing capacity,9a and can create

‘‘hot spots’’ in the bulk solution. This will lead to acceleration of the

mass transfer and crystal growth.15 Meanwhile, Fe(III) precursors are

partially reduced to Fe(II) species by EG in alkaline conditions. Then

the reaction system could reach a high temperature under the

microwave irradiation, resulting in a spontaneous crystallographic

fusion of nuclei into nanocystals. The freshly formed crystallites

attach to each other, and tend to aggregate rapidly into clusters for a

lower surface energy.

In addition, the in situ surface modification was conducted to

enhance the colloidal stability for potential applications of MCNCs.

Due to the strong affinity between –COOH and iron species, citrate

acid (CA), poly(acrylate acid) (PAA) and poly(c-glutamic acid)

(PGA), were selected as stabilizers. As shown in Fig. 5, the resultant

carboxyl-stabilized MCNCs were uniform in size and much smaller

than non-modified ones (e.g. 192 nm for CA-stabilized MCNCs). As

shown in Fig. 6, the transmittance of PBS dispersion (pH 7.4,

50 mM) for non-modified MCNCs increased rapidly in minutes, and

finally reached 80% after 3 h of setting, while the transmittance of

modified ones maintained at a low level of about 5% after the same

time scale of 3 h. From the corresponding photos inside, it could be

found that, compared to the transparency of non-modified MCNCs,

the surface fictionalized ones still keep turbid without obvious

precipitates after 3 h, reflecting the sustained aqueous stability of the

particles. The reason responsible for the prolonged stability is the

significant difference of surface charges. The MCNCs without

stabilizer had slight positive charges of about +4 mV. In contrast, all

the modified MCNCs were negatively charged, and showed surface

charges of 239, 240 and 230 mV for CA, PAA and PGA,

respectively. Thus, these MCNC dispersions could remain stable for

several hours via the electrostatic repulsion interaction.

Fig. 4 Influence of the reaction (a) temperature at a fixed time of 10 min

and (b) time at a fixed temperature of 150 uC over the crystallite size (nm)

and magnetization values (emu g21) of as-prepared MCNCs.

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In conclusion, we have developed a simple, energy-saving and

environmentally friendly microwave-assisted method to fabricate

MCNCs. Both reaction temperature and microwave power acted as

the key factors in directing the formation of the MCNCs. The

obtained products were well-crystallized, uniform in size, and

possessed a high magnetization. CA, PAA, PGA could serve as

stabilizing agents to remarkably prolong the aqueous stability of the

MCNCs, and such surface modification also can fulfil the further

immobilization of biomolecular ligands. This facile, economical and

green microwave pathway would undoubtedly offer a new avenue

for the synthesis of well-defined magnetic nanostructures.

This work was supported by National Science Foundation of

China (Grant No. 20974023, 21034003, 51073040 and 21128001),

Shanghai Committee of Science and Technology, China (Grant No.

10XD1400500). We thank Prof. L. H. Wang for his kind help in the

use of the microwave instrument.

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Fig. 6 The plots of transmittance as a function of time measured for PBS

dispersion (pH 7.4, 50 mM) of bare MCNCs and MCNCs stabilized by CA,

PAA, and PGA, respectively. Insets are photographs of above samples after

3 h of setting.

Fig. 5 TEM images of MCNCs stabilized by (a) CA, (b)PAA, (c)PGA,

and (d) zeta potential values of the above MCNCs and bare MCNCs. All

scale bars are 200 nm.

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