Preparation of pH-sensitive lipid-modified magnetite nanoparticle dispersion

Colloids and Surfaces A: Physicochem. Eng. Aspects 395 (2012) 63–69

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Preparation of pH-sensitive lipid-modified magnetite nanoparticle dispersion Youn-Gyu Han ∗ , Masaru Aoyagi, Masaki Kogiso, Masumi Asakawa, Toshimi Shimizu Nanotube research center (NTRC), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, Japan

a r t i c l e

i n f o

Article history: Received 15 September 2011 Received in revised form 2 December 2011 Accepted 4 December 2011 Available online 13 December 2011 Keywords: Magnetite Dispersion pH-sensitive gel Nanoparticle Nanofiber Magnetic separation

a b s t r a c t This paper describes a preparation method for a highly stable magnetite nanoparticle clusters (MNPCs) dispersion using crystalline lipid nanotubes (LNTs). LNTs were used as a coating material owing to their crystallinity. Oleic acid (OA) was employed as a co-dispersant to enhance the dispersibility of the glycolipid-modified MNPCs. The combination of two lipids was effective in producing a stable dispersion; they formed a coating layer on the MNPCs and nanofibers in the dispersion. The nanofibers enhanced the dispersion stability of the MNPCs by forming a network structure. The resulting MNPCs were characterized in terms of morphology, particle size, zeta potential, magnetic properties, re-dispersibility, and biocompatibility. The stability of the dispersion changed with pH. In acidic conditions, most of the MNPCs and nanofibers could be easily removed by using a magnet. The resulting materials have the potential for application in biomedical as well as industrial industries. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Magnetite nanoparticles (MNPs) are very attractive materials with favorable magnetic properties. MNPs have been intensively studied because of their broad applicability to areas such as magnetic resonance imaging (MRI) [1,2], magnetic inductive hyperthermia (MIH) [3], magnetically guided delivery [1,2], magnetic storage media [4], detection and separations [5], and ferrofluids [6]. Nanoparticles have mostly been used as dispersions of their clusters. Dispersions offer the advantages of easy handling and simple processing. MNP clusters (MNPCs), as well as other nanoparticles, are stabilized using dispersants or polymers. However, it is more difficult to maintain the stability of MNPC dispersions because MNPs exhibit magnetic properties along with the van der Waals attraction between particles. Many stable dispersions of MNPCs have been prepared using various dispersion media such as water, hydrocarbons, diesters, alcohols, ketones, and amines [7–11]. Among the dispersions used, aqueous dispersions have been used in biomedical as well as industrial applications. For such applications, dispersions should satisfy the conditions of high stability, a narrow size distribution, and biocompatibility. Therefore, most researchers have focused on coating the surfaces of nanoparticles with biocompatible materials [12–14]. However, the production, stocking, and delivery costs of MNP dispersions are high. Therefore,

∗ Corresponding author. Tel.: +81 29 861 2201; fax: +81 29 861 4676. E-mail addresses: [email protected], [email protected] (Y.-G. Han). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.12.004

the development of a powder dispersion having superior dispersion ability as well as good biocompatibility is desirable. Natural materials have been considered for their biocompatibility. Of the many natural materials, we noted that the glycolipid consisting of glucose and oleic acid (OA) has the potential for use as a coating material. The glycolipid self-assembles to form lipid nanotubes (LNTs) with a high aspect ratio in the dispersion medium as shown in Fig. 1 [15]. The LNTs are also able to maintain their morphology in warm water because they form crystals below the gel-to-liquid crystalline transition temperature. Thus, LNTs can be used as a coating agent to form a crystalline surface layer that would prevent strong aggregation of the MNPCs. The crystalline coating layer could also facilitate re-dispersion using mixing tools with low energy consumption. In this paper, we describe the preparation of lipid-coated MNPCs with high dispersibility and high biocompatibility. 2. Experimental 2.1. Materials MNPs were purchased from Sigma-Aldrich (iron (II, III) oxide, particle size 7.8 ± 6.1 nm). OA, SDS, Tween 80, HCl and NaOH were purchased from Wako Chemical. All purchased chemicals were of reagent grade and used without further purification. The ultrapure water was used for all experiments. Glycolipid, N(9-cis-Octadecenoyl)-␤-d-glucopyranosyl-amine was synthesized according to the methods previously reported [15]. However, the glycolipid prepared by this method was pale-yellow and their aqueous solution is acidic. These might be caused by some impurities

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Fig. 1. (a) SEM image of the LNT formed by self-assembly of the glycolipids, (b) schematic illustration of the LNT and structures of the glycolipid used for the formation of the LNT. Spheres and light-gray cylinders represent the hydrophilic head groups and the hydrophobic tails of the glycolipid molecules, respectively.

due to incomplete purification. Therefore, it needed to further purification to remove the impurities. The purification method was followed: 20 g of the synthesized material was dissolved in 200 ml of methanol with 4 g of silica gel and 1 g of activated charcoal at 60 ◦ C. After removal of the silica and activated charcoal by filtration, the clear filtrate was cooled to room temperature. The LNT precipitated was filtered and washed with ultrapure water (50 ml × 3 times) until aqueous dispersion of LNT had a neutral pH and their conductivity became below 10 ␮S/cm. 2.2. The preparation of lipid-modified MNPCs The dispersion of the lipid-modified MNPCs was prepared by following method. 40 mg of MNP (0.1 wt%) was added to 40 ml of ultrapure water with 22.5 mM of LNT and 15.8 mM of OA and then the suspension was sonicated at 70 ◦ C with 150 W for 10 min. The lipid-modified MNPCs were obtained by freeze-drying. Two dispersions of the glycolipid- and the OA-modified MNPCs were prepared with the introduced method to estimate the dispersion mechanism. Commercial dispersants like SDS and Tween 80 was also used to estimate re-dispersibility and the concentration was 22.5 mM. 2.3. Redispersion test of surface modified MNPCs Crystalline coating layer would prevent nanoparticles from forming aggregations. This could be estimated from the redispersion ability of dried particles. Three types of MNPCs modified with a combination of glycolipid and OA, SDS and Tween 80 were obtained by freeze-drying. They were re-dispersed in water using the ultrasonic bath for 5 min at room temperature. 2.4. Characterization Mean size of the dispersed MNPCs were measured at 25 ◦ C by an advanced technique of photon correlation spectroscopy and zeta potential measurements were performed using a laser-Doppler velocimetry technique. The instrument uses a 4 mW He–Ne laser at a wavelength of 632.8 nm and detects the scattered light at an angle of 173◦ using a Malvern Zetasizer Nano-ZS (Malvern Instruments, Malvern, UK). The zeta potential of the MNPCs was calculated from the mobility measurements, using the Smoluchowski formula. Measurements were performed 1 day after preparation. Field emission scanning transmission electron microscopy (FESTEM) was done with a Hitachi S-4800 to observe the nanostructure

and morphologies of the surface modified MNPCs. For STEM, a drop of the sample suspension was placed on a carbon-coated copper grid (200 mesh) and dried overnight at room temperature and low pressure. The accelerating voltage for observation was 30 kV. To investigate the time-dependent change of the dispersion rheological viscoelastic measurements were performed using a VILASTIC Viscoelasticity Analyzer (Vilastic Scientific Inc., Austin, TX, USA), which is a capillary viscometer with oscillatory flow principle. The storage modulus, G and the loss modulus, G were measured in the low-frequency range (0.01–100 Hz). All the viscoelastic measurements were performed after leaving to attain equilibrium state in an incubator at measured temperature through a few days. Fourier-transform infrared spectroscopy (FT-IR) was performed on a JASCO FT/IR-680 Plus. All samples were dried by freeze-dryer. The effective resolution was 4 cm−1 , and 128 scans were accumulated for each measurement. The sensitivity of HeLa cells to the modified MNPCs was determined by the microculture tetrazolium (MTT) assay. Each dispersions were diluted in cell culture media to a concentration of 5, 10 and 25 ␮g/ml. The solutions were incubated with Hela cells for 24 h and the cell viability was measured by MTT assay. The relative cytotoxicity was measured by comparing with the cells without any treatment. The temperature dependence of the magnetic susceptibility was measured in an applied field of 0.1 T over the temperature range of 1.8 K ≤ T ≤ 300 K using a SQUID magnetometer (Quantum Design, MPMS5S).

3. Results and discussion LNTs are transformed from solid tubular assemblies to fluid vesicles above the gel-to-liquid crystalline transition temperature of 58 ◦ C [15]. Above the transition temperature, the LNTs are completely dissociated into glycolipid molecules and the white opaque dispersion becomes transparent. The dissociated glycolipid molecules show surface activity. However, they return to the crystalline state of LNTs upon cooling below the transition temperature. The LNTs do not show surface activity. This is the key concept of surface modification of MNPCs. In particular, the dissociated glycolipid molecules coat the surface of MNPCs. The coated layer becomes crystallized below the transition temperature (see Fig. S1 in Appendix A). Thus, this layer is different from that of other dispersants. However, the glycolipid-modified MNPCs show poor dispersibility in water. This can be attributed to their

Y.-G. Han et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 395 (2012) 63–69

hydrophilic–lipophilic balance (HLB) value, which is calculated using the following equation [16]: HLB = 20

M  h

M

where Mh is the molecular mass of the hydrophilic portion of the molecule and M is the molecular mass of the entire molecule. The glycolipid has a low HLB value of 8.7, which indicates that the glycolipid is similar to a non-aqueous dispersant. Therefore, the hydrophilicity of the surface of glycolipidmodified MNPCs needed to be enhanced. OA was employed as a co-dispersant to compensate for the poor dispersibility. Although OA has a very low HLB value of 1, it has been used to prepare aqueous MNPC dispersions. Dispersion is possible because OA forms a bilayer on the surface of MNPCs and the large number of carboxyl groups of OA on the surface enhance the dispersibility. Therefore, it was expected that the glycolipid with an oleate group could form a bilayer with OA. It was hypothesized that by using the dispersant containing a combination of the LNTs and OA, the MNPCs would be coated by a double layer consisting of OA and the glycolipid and a secondary layer would be formed by OA to enhance the dispersibility. The lipid-modified MNPCs would have high dispersibility after drying due to the coating layer. This hypothesis is verified in the latter part of this report through several experiments. Firstly, the effect of temperature was examined to confirm the importance of heating above the transition temperature (see Fig. S2 in Appendix A). The average diameter of uncoated MNPCs in water was ca. 1935 nm. The mixture of MNPCs, LNTs, and OA was sonicated in water at 20 ◦ C and 75 ◦ C. The average diameter decreased to ca. 94 nm and ca. 10 nm at treatment temperatures of 20 ◦ C and 75 ◦ C, respectively. The decrease in the particle size for both the samples was related to the heat generated during ultrasonication. Indeed, the temperature increased from 20 ◦ C to 53 ◦ C after ultrasonication. This result shows that heating above the gel-toliquid transition temperature was necessary to obtain a dispersion with a small average particle size. Lipid concentrations also influenced the particle size of the dispersion; thus, it was important to determine the optimum ratio of glycolipid to OA (see Fig. S3 in Appendix A). The particle size decreased with an increase of glycolipid and OA concentration. When the concentration ratio of MNP:glycolipid:OA was 4.25:22.5:15.8 mM in 40 ml of ultrapure water, the dispersion had the smallest particle size of ca. 10 nm. For reference, two dispersions were prepared by adding each lipid separately to the mixture of MNPCs and water. The glycolipid-modified MNPCs and the OAmodified MNPCs formed small aggregated particles with an average size of 344 nm and 87.4 nm, respectively. However, large agglomerates were also observed in both dispersions. These results support the observation that the ratio of glycolipid to OA was effective in producing MNP dispersions with a particle size close to the primary particle size. Subsequently, the maximum concentration of MNPCs was evaluated (see Fig. S4 in Appendix A). Because the optimum concentration of MNPCs that produced the smallest particle size distribution was too low, it was necessary to increase the MNP concentration for broad applications. It was found that the smallest particle size distribution was not always necessary for the stable dispersion of the MNPCs in water. In other words, 0.1 wt% MNP in the dispersion was not a limiting concentration for stable dispersion. The particle size generally increased with the increase in the MNP concentration. Below 2.00 wt% of MNP, the size was less than 100 nm and the zeta potential was more than −40 mV. This indicates that the dispersions were electrically stabilized [17]. Indeed, the dispersions were stable for over one month. Above 5.0 wt%, the particle size increased and the zeta potential fell below −30 mV. These dispersions precipitated after five days. The decrease in the

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zeta potential indicates a decrease in the repulsive force between the MNPCs. Further, the precipitation of particles implies that the attractive force between the MNPCs became too strong to maintain the dispersed state. Therefore, the maximum MNP concentration was below 5.0 wt%. This concentration was relatively higher than that reported previously [18–20]. The lipid-coated MNP dispersion had the characteristic morphology. As shown in Fig. 2, the dispersion did not show a nanotubular structure but a number of thin nanofibers entangled in the entire area. This indicates that the mixture of the two lipids produced nanofibers in addition to the coating layer on the MNPCs. This morphology was distinct from that of other commercial dispersants. These nanofibers should stabilize the dispersion by forming a network of entangled nanofibers. The dispersion turned into a gel after a certain period of time. The details of the gel formation will be reported in our next paper. Only the rheological changes in the dispersion according to gelation are evaluated herein. The degree of gelation was altered by four factors: storage time, LNT:OA ratio, MNP concentration, and pH of the dispersion. The degree of gelation was examined by monitoring the change in tan ı (=loss modulus (G )/storage modulus (G )). The dispersion obtained immediately after preparation was a Newtonian fluid independent of the shear rate. This Newtonian fluid changed to a non-Newtonian fluid during storage. Fig. 3(a) shows the change in the tan ı value with storage time. The tan ı value decreased over a course of four days, and the value was close to 1, indicating that the fluid changed to a visco-elastic one. This rheological change in the dispersion was caused by the morphology change of LNTs to nanofibers. The extent of the change in the morphology varied with the ratio of LNT:OA. As shown in Fig. 3(b), the dispersion with a low concentration of OA had a low storage modulus (dispersions (1) and (2) in the graph correspond to the dispersions with high and low concentrations of OA, respectively), which indicates that this dispersion contained fewer nanofibers. Therefore, it can be inferred that the LNTs were transformed into nanofibers by the intercalation of OA molecules into the bilayer of the LNTs. The increase in the MNP concentration caused a reduction in the degree of gelation. Fig. 3(c) shows the change in tan ı versus the MNP concentration. After preparation, all the dispersions had low viscosity and little elasticity, indicating no formation of a network structure. After four days, the elasticity of the dispersions with 0.1 and 1.0 wt% MNPCs increased whereas the viscosity of the dispersion with 5 wt% MNPCs decreased. The viscosity of the dispersion with 10 wt% MNP could not be measured because precipitation occurred after four days. The trend of decrease in the storage modulus shows that the dispersion with a high concentration of MNP had few nanofibers. On the other hand, it also indicates that the nanofibers enhanced the stability of the dispersion by forming a network structure. This data supports the postulate that the lipids acted as a coating material and a dispersion stabilizer. Varying the pH resulted in an interesting phenomenon in the dispersion. As shown in Fig. 4, the dispersion states differed according to the pH. The dispersion was stable only above a pH of ca. 4. Below pH 4, the dispersed MNPCs agglomerated to form lump-like tangled threads. STEM images provided useful structural information. At high pH, small thin nanofibers were observed with no evidence of bundling, or even of formation of a network structure. With increasing acidity, the number of nanofibers increased, indicating that the formation of nanofibers was closely related to the pH of the dispersion. These nanofibers had either a positive or negative influence on the dispersion stability. The positive influence was that they stabilized the dispersion by forming the network structure around pH 4. The negative influence was that a lump consisting of nanofibers and lipid-coated MNPCs was formed in more acidic pH. However, it was very interesting that this lump could be easily

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Fig. 2. Morphology of the lipid-modified MNP dispersion: (a) SEM micrograph (white spot: MNPCs) and (b) STEM micrograph (black spots: MNPCs).

10

2

10

1

10

1 day

2 day

3 day

4 day

b

10

1

10

0

16 mM OA 4.0 mM OA

tan δ

tan δ

a

0

0.1

1

0.1

10

ω (s-1)

10

3

10

2

10

1

10

0

10

0.1 wt% 1.0 wt% 5.0 wt% 10 wt%

-1

0.1

1

ω (s-1)

10

ω (s )

tan δ

tan δ

c

1

-1

10

10

3

10

2

10

1

10

0

10

0.1 wt% 1.0 wt% 5.0 wt%

-1

0.1

1

10

ω (s-1)

Fig. 3. The change of tan ı with various factors: (a) storage time, (b) the ratio of LNT to OA, (c) MNP concentration (left: 1 day after preparation, right: 4 days after preparation). (Standard dispersion: 0.1 wt% MNP, 22.5 mM LNT, 15.8 mM OA in 40 ml ultrapure water.)

Fig. 4. (a) The photograph and STEM micrograph of MNP dispersions varying the pH (b) the separation of MNPCs and nanofibers using a magnet (the pH of dispersion 2.1).

Y.-G. Han et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 395 (2012) 63–69

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Fig. 5. Scheme for the lipid-modified MNPCs entangled by nanofibers: Tg means gel-to-liquid crystal transition temperature.

removed by using a magnet. In the case of the stable dispersion, it spent many times to separate the MNPCs. This indicates that the dispersion could be applied in various fields requiring magnetically driven separation techniques such as the separation of DNA from a number of cellular sources and colloidal waste from contaminated water. To evaluate the dispersion mechanism, the roles of the glycolipid and the OA were examined with respect to the size distribution and the zeta potential. As shown in Table 1, the dispersion of the MNPCs modified with glycolipid only contained large particles and the color was brown; the zeta potential was −19.8 mV and the particles precipitated within 1 h. When the OA was added to the dispersion, the color changed from light brown to dark brown and the stability of the dispersion was dramatically improved; the zeta potential increased to −40.1 mV and the particle size decreased to ca. 99.0 nm. The increase in the zeta potential indicates that OA was located on the outer surface of the lipid-modified MNPCs. The dispersion of the MNPCs modified with OA only had large agglomerates that were black and oily like tar. However, the overlying solution was a very clear dispersion and the zeta potential was −71.5 mV. It is thought that two types of OA-modified MNPCs coexisted in the dispersion. In one type, the MNPCs are coated with a double layer of OA producing high dispersibility. The dispersibility resulted from the many ionized carboxyl groups of the outer layer. The high zeta potential might result from ionized OA groups in the outer layer. The other type of MNPCs was coated by one OA layer. Because these had only one layer, the alkyl chains were exposed to the aqueous surrounding. Consequently, the MNPCs tended to agglomerate due to their high lipophilicity. However, the size of the agglomerates was reduced by adding glycolipid to the dispersion, which indicates that the glycolipid prevented the agglomeration. Subsequently, we examined whether the OA in the outer layer was ionized or not. Because it is difficult to ionize the carboxylic acid group under the acidic condition of pH 4, their dispersibility was poor. However, the lipid-modified MNPCs formed a stable dispersion with a very high zeta potential. For this reason, it was assumed that OA was ionized to the carboxylic ion during the sonication process. To verify this assumption, Na-oleate was added to the dispersion of glycolipid-modified MNPCs. The suspension

immediately changed to a very homogeneous dispersion. The resulting dispersion was very stable and the zeta potential was as high as that of the dispersion prepared using OA. Therefore, it could be deduced that partial ionization of the OA coated onto the particles to the carboxylic ion occurred. FT-IR spectra also supported the existence of the oleate ion (Fig. S5 in Appendix A). The presence of carboxylic acid in the sample should be indicated by an intense C O stretching peak at 1711 cm−1 [21,22]. However, this peak disappeared for the lipidmodified MNPCs produced by combination of OA and the glycolipid. The absence of this peak is evidence in support of the ionization of the carboxylic acid, which is corroborated by the zeta potential data that indicated a negative charge. The carboxylate peaks in the region of 1541 and 1639 cm−1 were strongly overlapped by amide peaks of the glycolipid. The amide I (mainly ascribable to the C O stretching band) and II (mainly the N H deformation band) bands of the glycolipid appear at 1634 and 1548 cm−1 . Consequently, the dispersion mechanism could be described as shown in Fig. 5. The LNT was dissociated into individual glycolipids above the gel-toliquid transition temperature. The dissociated molecules acted as a dispersant, and were adsorbed onto the surface of the MNPCs. Subsequently, OA was intercalated into the alkyl chains of the glycolipid. This secondary layer of OA exposed the carboxyl groups and ionized carboxylate groups toward the dispersion medium. The ionized carboxyl groups facilitated the formation of the lipidcoated MNPCs dispersed in water by enhancing the electrostatic repulsion. This carboxylic moiety may act as a binding site, whereby various molecules such as DNA, proteins and antibodies could potentially be immobilized on the lipid-modified MNPCs for specific applications such as magnetically targeted drug carriers [1,2,23]. The cytotoxicity of the surface-modified MNPCs in HeLa cells was evaluated by MTT assay to establish the potential applicability (see Fig. S6 in Appendix A). The viability of the cells was not affected by the presence of the lipid-modified MNPCs, which suggests that the lipid-modified MNPCs were highly biocompatible and safe for further bio application in vivo evaluations. Although further studies for some functions such as drug delivery, targeting and imaging are necessary, the surface-modified MNPCs presented herein could serve as a platform for various applications

Table 1 Size distribution and zeta potential with the process and the lipid types of glycolipid, OA and sodium oleate.

Zeta potential (mV) Size (nm) Color a b c

Glycolipida

→OAa

OAb

→Glycolipidb

Glycolipid → Na oleatec

−19.8 344.0 ± 72.8 >104 Brown

−44.5 99.0 ± 29.2

−71.5 87.4 ± 26.7 >104 Dark brown

−32.4 143.0 ± 30.2 43.6 ± 8.5 Dark brown

−67.8 71.1 ± 18.2

Dark brown

Firstly disperse the particles with only glycolipid, then OA was added and sonicated. Firstly disperse the particles with only OA, then glycolipid was added and sonicated. Firstly disperse the particles with only glycolipid, then Na-oleate was added and shook slightly.

Dark brown

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80

(a)

60

M (emu/g)

40 20

(b)

0 -20 -40 -60 -80 -60k

layer during drying or re-dispersing. This result supports the previous hypothesis that the coating layer of lipid-modified MNPCs would enhance the dispersibility. The use of a crystalline material with low solubility in water such as the glycolipid prevented desorption from the coating layer. Therefore, the lipid-coated MNPCs did not undergo strong aggregation as the unmodified MNPCs during the drying process, and these materials could be re-dispersed in water using a low energy consumption mixer such as a sonic bath. Thus, this MNP dispersion can be used in a powder form, whereas most other dispersions must be used in the liquid form. This will be helpful for reducing the production cost in various industrial fields. 4. Summary and conclusion

-40k

-20k

0

20k

40k

60k

H (Oe) Fig. 6. Magnetization curves of (a) the unmodified MNPCs and (b) the lipid-modified MNPCs: the lipids do not affect the magnetism and sample (b) has 8.8 wt% MNPCs of sample (b).

requiring targeted and stealth carriers, the potential of which will be evaluated in a subsequent report. Fig. 6 shows the magnetic properties of the lipid-modified MNPCs. The magnetization hysteresis loops indicated that these MNPCs are typical ferromagnetic materials. The coercivity was 78.2 Oe and the remanence was 11.4 emu/g for the unmodified MNPCs, whereas the coercivity was 76.6 Oe and the remanence was 1.2 emu/g for the lipid-modified MNPCs. The magnetic saturation values of unmodified and modified MNPCs were 75.2 and 9.9 emu/g, respectively. The decrease in the magnetization was attributed to the surface-modification of the MNPCs. The estimated MNP concentration was 8.8 wt% in the composite particles. If the dispersion had a higher concentration of MNP, the magnetic saturation value would be higher. Because one key objective of this research was the preparation of magnetic materials with superior water dispersibility, the re-dispersibility of the lipid-modified MNPCs was evaluated. The size distribution of the lipid-modified MNPCs was compared with MNPCs treated with the commercial dispersants Tween 80 and SDS. As shown in Table 2, the dispersion of the lipid-modified MNPCs had the smallest particle size distribution even after re-dispersion using a sonic bath. In the case of SDS, there was no significant difference in the size distribution before and after re-dispersion. However, the Tween80-treated MNPCs formed large aggregates after redispersion. The change in the size distribution after re-dispersion might be related to the physical properties of the dispersant such as crystallinity and solubility. SDS and Tween 80 have high solubility in water at room temperature, whereas LNT and OA have very low solubility. Therefore, the two commercial dispersants might be easily dissociated from the coating layer. This weak coating layer caused the aggregation of MNPCs. The Tween80-treated MNPCs aggregated more severely than the SDS-treated MNPCs. Further, Tween 80 was in the liquid phase at room temperature. Therefore, it was difficult for the MNPCs to remain in the independent coating

Table 2 The size distribution of MNPCs modified with a combination of LNT and OA, SDS and Tween 80.

Content Size (nm) After preparation After re-dispersion

LNT and OA

SDS

Tween 80

22.5 mM and 15.8 mM

22.5 mM

22.5 mM

10.0 ± 2.0 15.0 ± 2.2

32.2 ± 9.7 46.3 ± 14.9

4.6 ± 1.2 60.1 ± 22.1

Re-dispersion of dried powder was done using sonic bath.

This study shows that LNTs can be used as a coating material for MNPCs. A highly stable MNP dispersion was prepared by combining two lipids: glycolipid and OA. The dried MNPCs were easily re-dispersed in water because the crystalline coating layer of glycolipids prevented strong aggregation of the MNPCs. As a codispersant, OA was effective in enhancing the dispersibility of the glycolipid-modified MNPCs. Further, nanofibers were formed from the combination of the two lipids, in addition to the coating layer formed on the MNPCs. The nanofibers enhanced the stability of the dispersion by forming a network structure. The dispersion was pHsensitive. In acidic conditions below pH 4, the nanofibers and the lipid-coated MNPCs formed a tangled thread-like lump. This lump was completely separated from the aqueous medium and was easily removed using a magnet. The present material may be used in a wide range of applications as a biocompatible magnetic material, with the potential for further enhancement in the functionality by immobilization of various materials using the carboxylic moiety as a binding site. Acknowledgments The authors would like to express their appreciation to Dr. KAWAGUCHI Kenji (National Inst. of Advanced Industrial Sci. and Technol. (AIST), Ibaraki, JPN) for magnetization experiments and Dr. Ding for cytotoxicity test (AIST, Ibaraki, JPN). Financial support of the innovation school program of AIST is also acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.colsurfa.2011.12.004. References [1] R. Hao, R. Xing, Z. Xu, Y. Hou, S. Gao, S. Sun, Synthesis, functionalization, and biomedical applications of multifunctional magnetic nanoparticles, Adv. Mater. 22 (2010) 2729–2742. [2] L. Zhou, J. Yuan, W. Yen, Core–shell structural iro oxide hybrid nanoparticles: from controlled synthesis to biomedical applications, J. Mater. Chem. 21 (2011) 2823–2840. [3] A. Jordan, R. Scholz, P. Wust, H. Fähling, R. Felix, Magnetic fluid hyperthermia (MFH): cancer treatment with AC magnetic field induced excitation of biocompatible superparamagnetic nanoparticles, J. Magn. Magn. Mater. 201 (1999) 413–419. [4] H. Zeng, J. Li, J.P. Liu, Z.L. Wang, S.H. Sun, Exchange-coupled nanocomposite magnets by nanoparticle self-assembly, Nature 420 (2002) 395–398. [5] J.M. Nam, C.S. Thaxton, C.A. Mirkin, Nanoparticle-based bio-bar codes for the ultrasensitive detection of proteins, Science 301 (2003) 1884–1886. [6] K. Raj, R. Moskowitz, Commercial applications of ferrofluids, J. Magn. Magn. Mater. 85 (1990) 233–245. [7] S.Sun, H. Zeng, Size-controlled synthesis of magnetite nanoparticles, J. Am. Chem. Soc. 124 (2002) 8204–8205. [8] L. Vékás, D. Bica, M.V. Avdeev, Magnetic nanoparticles and concentrated magnetic nanofluids: synthesis, properties and some applications, China Particuology 5 (2007) 43–49. [9] L. Vékás, D. Bica, O. Marinica, Magnetic nanofluids stabilized with various chain length surfactants, Rom. Rep. Phys. 58 (2006) 257–267.

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