Preparation and characterization of starch nanoparticles in ionic liquid-in-oil microemulsions system

Industrial Crops and Products 52 (2014) 105–110

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Preparation and characterization of starch nanoparticles in ionic liquid-in-oil microemulsions system Gang Zhou, Zhigang Luo ∗ , Xiong Fu Carbohydrate Lab, College of Light Industry and Food Science, South China University of Technology, Guangzhou 510640, China

a r t i c l e

i n f o

Article history: Received 9 June 2013 Received in revised form 25 September 2013 Accepted 11 October 2013 Keywords: Starch Ionic liquid Microemulsion Crosslinking Starch nanoparticles

a b s t r a c t A novel ionic liquid microemulsion consisting of 1-octyl-3-methylimidazolium acetate ([Omim]Ac, an ionic liquid), native corn starch, surfactant TX-100, 1-butanol and cyclohexane was prepared. The ionic liquid-in-oil (IL/O), bicontinuous, and oil-in-ionic liquid (O/IL) microregions of the microemulsions were identified by conductivity measurements. The formation of IL/O microemulsion was confirmed by UV–vis spectrophotometry using the methyl orange (MO) as absorption probes and dynamic light scattering (DLS) analysis. Starch nanoparticles were prepared with epichlorohydrin as crosslinker through 3 h IL/O microemulsion-crosslinking reaction at 50 ◦ C. The results of Fourier transform infrared spectroscopy (FTIR) demonstrated the formation of crosslinking bonds in starch molecules. Scanning electron microscope (SEM) data revealed that starch nanoparticles showed aggregation or cluster formation. Starch nanoparticles with a mean diameter of 96.9 nm and narrow size distribution were confirmed by the results of DLS. © 2013 Elsevier B.V. All rights reserved.

1. Introduction As a cheap, abundant and renewable natural material, starch has been modified through physical, chemical or enzymatic processes to improve its properties for many years (Malafaya et al., 2001; Raina et al., 2006). Various kinds of starch derivatives have been investigated, among which crosslinked starch microspheres show high stability towards swelling, high shear, high temperature and acidic conditions (Kim and Lee, 2002). Besides, crosslinked starch microspheres possess good performance in their total biodegradability, biocompatibility, stability on storage, cost-effectiveness as well as simple fabrication method (Mundargi et al., 2007). In view of the good properties, starch microspheres had been widely used in biology and medicine in many aspects. Several years ago, it was shown that starch microspheres possess unique features which suggest their use as an excipient for the manufacturing of controlled release solid drugs (Mundargi et al., 2007). Lately, Stertman et al. (2006) also discovered that starch microspheres could act as an effective adjuvant in biology and medicine. Nowadays, studies on starch microspheres as drug carriers tend to be a popular topic

Abbreviations: [Omim]Ac, 1-octyl-3-methylimidazolium acetate; DLS, dynamic light scattering; MO, methyl orange; FTIR, Fourier transform infrared spectroscopy; SEM, scanning electron microscopy; IL/O, ionic liquid-in-oil; O/IL, oil-in-ionic liquid; W/O, water-in-oil; [Bmim][BF4 ], 1-butyl-3-methyl-imidazolium tetraflouroborate; ILs, ionic liquids; AGU, anhydroglucose units. ∗ Corresponding author. Tel.: +86 20 87113845; fax: +86 20 87113848. E-mail address: [email protected] (Z. Luo). 0926-6690/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2013.10.019

for their extensive use in medicine. However, the application of starch microspheres in drug delivery systems is not ideal due to their poor appearance in particle size and size distribution which mainly affect their adsorption and release properties. Therefore, the quality of starch microspheres is desperately expected to be improved in order for better application. Several preparation approaches of starch microspheres have been investigated, such as precipitation, spray drying, solvent evaporation and emulsion-crosslinking technique, among which water-in-oil (W/O) emulsion-crosslinking technique has been extensively used. However, starch microspheres obtained from W/O emulsion-crosslinking approach still appears relatively big size. Fang et al. (2008) prepared starch microspheres with the average diameter of 19 ␮m through W/O emulsification-crosslinking method. Franssen and Hennink (1998) also preformed the preparation of starch microspheres with the volume mean diameter ranging from 2.5 ␮m to 25 ␮m through emulsion-crosslinking method. Starch nanoparticles (StNPs) are nano-sized (1–1000 nm in the pharmaceutical field) particulates of starch prepared by chemically crosslinking starch molecules with appropriate crosslinkers (Shi et al., 2011). However, researches on synthesis of starch nanoparticles have rarely been reported. Therefore, there is a strong incentive to develop a new strategy for the synthesis of starch nanoparticles and research their properties. Room-temperature ionic liquids (ILs) have been considered as possible green and effective replacements for their unique properties, such as recyclability and designability (Sheldon, 2001; Welton, 1999). Progress in applying ILs into starch chemistry mainly focuses

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on dissolution and esterification of starch (Lu et al., 2012; Luo and Zhou, 2012; Xie and Wang, 2011). However, researches on synthesis of starch microspheres in ILs system have rarely been reported, much less the starch nanoparticles. Recently, many documents showed that ILs could substitute polar phase, non-polar phase or surfactant to prepare ionic liquid microemulsions (Cheng et al., 2007, 2008; Gao et al., 2004; Yan and Texter, 2006). Meanwhile, many literatures also reported that ionic liquid microemulsions could be used as reaction system of the preparation of nanometer materials (Dixit et al., 1998; Gan et al., 1997; Qiu et al., 1999; Song and Kim, 1999; Zhang et al., 2009, 2012). However, there is no report about the preparation of starch nanoparticles in ionic liquid based on microemulsion system until now. Accordingly, it is essential to research on possibility of the preparation of starch nanoparticles in ionic liquid microemulsion reaction system. In this study, 1-octyl-3-methylimidazolium acetate ([Omim]Ac)-starch/surfactant TX-100+1-butanol/cyclohexane microemulsions were prepared and investigated by phase behavior, conductivity measurements, UV–vis spectrophotometry and dynamic light scattering (DLS). Starch nanoparticles were prepared by [Omim]Ac-starch/cyclohexane microemulsion-crosslinking method with native corn starch as raw material, epichlorohydrin as a crosslinker. Starch nanoparticles were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and DLS. This work may provide an efficient pathway to prepare starch nanoparticles which could be expected to be well used in drug delivery system. 2. Materials and methods 2.1. Materials Native corn starch was obtained from ChangChun DaCheng Corn Products Co. (Changchun, China) and dried at 50 ◦ C for 24 h before using. 1-Octyl-3-methylimidazolium acetate ([Omim]Ac, >99%) was purchased from Lanzhou Institute of Chemical Physics (Lanzhou, China). All other chemicals were of analytical grade. 2.2. Preparation of ionic liquid microemulsions containing native corn starch Dried corn starch (0.5 g) was added into [Omim]Ac (9.5 g) in a three-neck round flask which was continuously purged with gaseous N2 , and the starch concentration was kept at 5%. The mixture were stirred for homogeneous mixing and heated in an oil bath at 135 ◦ C for 2.5 h. Subsequently, [Omim]Ac-starch (10 g) and cyclohexane (68.6 g) were added into the small beaker, and their masses were determined by an analytical balance (FA1104N, Shanghai Balance Instrument Co., Shanghai, China) with a resolution of 0.0001 g. The temperature was controlled by a thermostatic magnetic stirring apparatus (DF-II, Shanghai Yuzheng Instrument Co., Shanghai, China). After thermal equilibrium, the solution was then titrated by the mixture of TX-100 (35.1 g) and 1-butanol with the mass ratio of TX-100 to 1-butanol at 3:1 until the hierarchical and hazy liquid solution became transparent, which was indicative of the formation of the single phase. 2.3. Phase behavior and structure of ionic liquid microemulsion 2.3.1. Phase diagram determination The pseudo-ternary phase diagram of the [Omim]Ac-starch/TX100+1-butanol/cyclohexane system was determined at 25 ◦ C by direct visual observation as described in Section 2.2. A series of microemulsions were prepared through changing the mass ratio of cyclohexane/[Omim]Ac-starch at 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7,

Corn starch

Cyclohexane

[Omim]Ac 2.5 h

Oil phase

Polar phase

Blend and equilibrium temperature to 25

Add TX-100+1-butanol(3:1)

IL/O microemulsion Add epichlorohydrin Centrifugation

Washing Drying

Starch nanoparticles Fig. 1. Scheme of microemulsion-crosslinking reaction of starch.

2:8, 1:9, respectively. The corresponding composition of the solution was remarked as the phase boundary. Phase diagram could be used to characterize the system further and choose the proper reaction system for the preparation of starch nanoparticles. 2.3.2. Conductivity Conductivity is frequently used to investigate structure and structural changes in microemulsions. Conductivity measurements were taken with a conductometer (DDSJ-308A, Shanghai Precision Scientific Instrument Co., Shanghai, China) at 1 kHz using a dip-type cell of cell constant 0.971 cm−1 . The errors in the conductivity measurements were ±0.5%. The cyclohexane was progressively added to the mixture of [Omim]Ac-starch, TX-100 and 1-butanol, and the conductivity was measured after thorough mixing and temperature equilibrium. 2.4. The formation of IL/O microemulsion 2.4.1. UV–vis spectrophotometry UV–vis absorption spectroscopy is a powerful tool for characterizing the microenvironment of microemulsion. The UV–vis spectra were performed on a computer-controlled UV–vis spectrometer (TU-1901, PGENERAL Co., Beijing, China). The path length of the quartz cell used in this experiment was 1 cm. Operation process is as follows, appropriate amounts of methyl orange (MO) were mixed with measured ionic liquid microemulsions in advance and then added to the quartz cell. The experiments were carried out at room temperature. 2.4.2. Dynamic light scattering Measurements were conducted using a particle size analyzer (Nano ZS, Malvern Instrument Ltd., Worcestershire, UK) at a wavelength of 633 nm. The scattering angle was set at 90◦ . Samples were maintained at 25.0 ◦ C during the experiments. 2.5. Preparation of starch nanoparticles Starch nanoparticles were prepared according to [Omim]Acstarch/cyclohexane microemulsion-crosslinking method with native corn starch as raw material, epichlorohydrin as a crosslinker. The reaction scheme for crosslinked starch nanoparticles was depicted in Fig. 1. After [Omim]Ac-starch/cyclohexane microemulsion (R = 0.21, R represents W[Omim]Ac-starch /WTX-100+1-butanol ) was obtained along line a, 2% (w:w) epichlorohydrin was added to the above microemulsion as a crosslinker. The mixture was stirred at the speed of 1200 rpm at 50 ◦ C for 3 h. After the completion of the reaction, the reaction solution was cooled to room temperature and starch nanoparticles were subsequently precipitated with

G. Zhou et al. / Industrial Crops and Products 52 (2014) 105–110

0.0 1.0

0.4

0.6 B

0.4

a

0.8 1.0 0.0

O/IL

) 3:1

IL/O

ol(

0.6

140

0.4 0.6 [Omim]Ac-starch

0.8

1.0

120

100

O/IL

B

IL/O

80

0.2

two phase 0.2

Conductivity(μs/cm)

0.8

tan -bu 0/1 -10

cyc loh exa ne

160

TX

0.2

107

0.0

Fig. 2. Phase diagram of the [Omim]Ac-starch/TX-100+1-butanol/cyclohexane system at 25 ◦ C. The initial TX-100+1-butanol weight fraction is 0.40 for the line a.

anhydrous ethanol under vigorous stirring followed by centrifugation. The precipitate was washed thoroughly with sufficient anhydrous ethanol to eliminate IL, unreacted epichlorohydrin, 1-butanol and TX-100. Finally, the solid was centrifuged and dried in vacuum at 45 ◦ C for 24 h. 2.6. Characterization of starch nanoparticles FTIR spectra of native corn starch and starch nanoparticles were acquired on a Nicolet 510 spectrophotometer (Thermo Electron Co., Waltham, USA) using KBr disk technique. For FTIR measurement, the samples were mixed with anhydrous KBr and then compressed into thin disk-shaped pellets. The spectra were obtained with a resolution of 2 cm−1 between a wave number range of 400–4000 cm−1 . SEM images of native corn starch and starch nanoparticles were examined by means of a scanning electron microscope (Quanta 200, FEI, Oregon, USA). The accelerating voltage was 20 kV. The samples were mounted on an aluminum stub with a double sticky tape, followed coating with gold in a vacuum before examination. The size distribution of starch nanoparticles was determined by dynamic light scattering as described in Section 2.4.2. Before measuring, 0.1 g starch nanoparticles were added to 100 mL distilled water and treated with ultrasound cleaner (100 W, 40 kHz) for 10 min to disperse sufficiently. 3. Results and discussion 3.1. Phase behavior and structure of ionic liquid microemulsion 3.1.1. Phase diagram The pseudo-ternary phase diagram of [Omim]Ac-starch/TX100+1-butanol/cyclohexane system at 25 ◦ C is shown in Fig. 2. Apparently, a large single region that extends from [Omim]Acstarch corner to the cyclohexane corner was observed. The blank region was the one-phase microemulsion, and the shadow region marked “two phase” was a cloudy region. And a continuous stable single-phase microemulsion region could always be observed in the range of the [Omim]Ac-starch or cyclohexane content from 0% to 100% (wt). It could be seen that the phase behavior of [Omim]Acstarch/TX-100+1-butanol/cyclohexane system was similar to the traditional W/O microemulsions (Mo, 2002) and the IL microemulsions reported recently (Cheng et al., 2008). 3.1.2. Conductivity A large amount of reports indicated that the one-phase region could be divided into different sub-regions (microregions), such

60 0.0

0.1

0.2

0.3

Cyclohexane weight fraction Fig. 3. Conductivity as a function of cyclohexane weight fraction in the microemulsions at 25 ◦ C, R = 0.21.

as IL(or polar solvent)-in-oil microemulsion region (IL or polar solvent droplets dispersed in oil), oil-in-IL (or polar solvent) microemulsion region (oil droplets dispersed in IL or polar solvent), and bicontinuous region which is a kind of ionic liquid microemulsion consisting of both ionic liquid-in-oil region and oil-in-ionic liquid region (Cheng et al., 2008; Gao et al., 2004, 2006). In this research, the IL/O microemulsion was chosen to prepare starch nanoparticles. It is well known that the applications of microemulsion depend on its structure. Therefore, it is essential to investigate the structure of microemulsion. Conductivity is frequently used to investigate structure and structural changes in microemulsions. In the [Omim]Ac-starch/TX-100+1butanol/cyclohexane microemulsion, one-phase region could be divided into [Omim]Ac-starch/cyclohexane region (marked IL/O), bicontinuous region (marked B) and cyclohexane/[Omim]Ac-starch region (marked O/IL) using the principle and method reported by other authors (Kirkpartrick, 1971; Lagourette et al., 1979). Fig. 3 illustrates the dependence of the conductivity on cyclohexane weight fraction when R was 0.21. The initial increase in conductivity indicated the formation of cyclohexane/[Omim]Acstarch microdomains. The next nonlinear decrease revealed that the medium underwent a structural transition and became bicontinuous owing to progressive growth and interconnection of the cyclohexane/[Omim]Ac-starch microdomains. The third section of curve, a linear decrease in conductivity, was interpreted as the consequence of the formation of [Omim]Ac-starch/cyclohexane microdomains. 3.2. The formation of IL/O microemulsion 3.2.1. UV–vis spectra The shift in the absorption maximum of a solvatochromic probe is a sensitive measure of the local environment about the probe. In this work, we used MO as a solvatochromic probe to investigate the microenvironment of microemulsions along line a. The intensity of the UV-spectra band increases with the concentration of MO increasing. To aid the distinguishability of the spectra, different probe concentrations were used in this paper. As shown in Fig. 4, at R < 0.21, the absorption maximum (max ) was shifted from 414 to 419 nm. This red shift in max clearly indicated that the MO got solubilized inside the ionic liquid pool formed in the microemulsion and the micropolarity around MO increased with increasing R value. However, when R > 0.21, there was no more

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c λ max=417 d λ max=419

1.2

Absorbance

b

a λ max=414 b λ max=415

1.6

a

e λ max=419 f λ max=419 0.8

2935

g λ max=419

1645

3416

g 4000

0.4

a

3000

1155 1084 993

2000

1000

0

wavenumbers(cm-1) Fig. 6. FTIR spectra of native corn starch (a) and starch nanoparticles (b).

0.0 300

350

400

450

500

550

600

[C4 mim][BF4 ]/[C4 mim][AOT]/benzene microemulsion (Rao et al., 2012).

Wavelength (nm) Fig. 4. Absorption spectra of MO in the [Omim]Ac-starch/TX-100+1butanol/cyclohexane microemulsions along line a, as a function of [Omim]Ac-starch content with R = 0.02 (a), 0.07 (b), 0.16 (c), 0.21 (d), 0.29 (e), 0.37 (f), 0.50 (g). Probe concentrations, from a to g: 0.5 × 10−4 , 1.0 × 10−4 , 1.5 × 10−4 , 2.0 × 10−4 , 3.0 × 10−4 , 4.0 × 10−4 , 5.0 × 10−4 mol/L, respectively.

change in micropolarity, and the max value always remained constant (419 nm) until the isotropic microemulsion system attained the phase-separation point. This behavior demonstrated the formation of IL/O microemulsion. This result was consistent with the literature reported recently (Gao et al., 2007; Rao et al., 2012). 3.2.2. Size distribution of microemulsions The size distribution of the droplets in the IL/O microemulsion was characterized by DLS. Gao et al. (2005) and Pramanik et al. (2011) studied ionic liquid microemulsions using DLS and concluded that if the ionic liquid was really encapsulated to form IL/O microemulsion, the size of the droplets must increase regularly as the R value increased to a certain level. Based on the phase diagram, a series of [Omim]Ac-starch/cyclohexane microemulsions were chosen for DLS analysis along line a. As shown in Fig. 5, the sizes of microemulsions increased from about 6.5 nm to 12.1 nm with increasing R values from 0.21 to 0.50. The microemulsions showed regular swelling behavior with the addition of [Omim]Acstarch, indicating the formation of [Omim]Ac-starch/cyclohexane microemulsion. The similar phenomenon could also be found in

6.5 nm

Intensity

8.0 nm 11.8 nm 12.1 nm

R=0.21 R=0.29

10

Fig. 6 illustrates the FTIR spectra of native corn starch (Fig. 6a) and starch nanoparticles (Fig. 6b). In the spectra of native corn starch, several discernible absorbencies at 1155, 1084, and 993 cm−1 were assigned to the C O bond stretching vibrations of anhydroglucose units (AGU). Meanwhile, the O H stretching and the C H stretching vibration give a strong signal at 3416 cm−1 and at 2935 cm−1 , respectively (Kacurakova and Wilson, 2001). Besides, the band at 1645 cm−1 was ascribed to O H bending vibration (Mano et al., 2003). In comparison with native corn starch, FTIR spectra of starch nanoparticles exhibited some difference. The band at 3416 cm−1 got narrowed obviously and the one at 1645 cm−1 was much weaker than that of native corn starch. The bands in the region 900–1160 cm−1 in starch nanoparticles had changed and band intensity got a little stronger compared with native corn starch. These data suggested the formation of crosslinking bonds in the structure of the starch nanoparticles. The similar results were reported by Mundargi et al. (2007) and Li et al. (2012). 3.4. SEM analysis of starch nanoparticles Scanning electron microscope was used to investigate the morphology and particle size of normal corn starch and starch nanoparticles. As seen from Fig. 7, native corn starch granules were round or oval shapes with various sizes ranging from about 10 ␮m to 20 ␮m. In comparison with native corn starch, starch nanoparticles were spherical granules with a smooth surface, which implied a homogeneous and dense structure. Most of particles showed aggregation or cluster formation which could be mainly attributed to strong van der Waals force and electrostatic attraction. Liu et al. (2008) also reported that starch nanoparticles congregated together. 3.5. Size distribution of starch nanoparticles

R=0.37 R=0.50

1

3.3. FTIR analysis of native corn starch and starch nanoparticles

100

Diameter (nm) Fig. 5. Size distribution of the droplets in the [Omim]Ac-starch/TX-100+1butanol/cyclohexane microemulsions at 25 ◦ C.

The size distribution of starch nanoparticles was measured by DLS. As shown in Fig. 8, the size distribution of starch nanoparticles was relatively concentrated, and the mean diameter was 96.9 nm which are much smaller than 18.2 ␮m and 19.0 ␮m reported by Zhao et al. (2008) and Fang et al. (2008), respectively. The result of DLS was also consistent with Fig. 7. From the result, it could be concluded that starch microspheres with smaller size and a relatively concentrated distribution could be obtained by IL/O microemulsion-crosslinking method.

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Fig. 7. SEM images of native corn starch (a) and starch nanoparticles (b).

8

References 158.7

Intensity

6

4

2

0

1

10

100 1000 Diameter (nm)

10000

Fig. 8. Size distribution of the starch nanoparticles.

4. Conclusions In conclusion, microemulsions consisting of [Omim]Ac, native corn starch, cyclohexane, TX-100, and cosurfactant 1-butanol were prepared, and the pseudo-ternary phase diagram was drawn to investigate the phase behavior. The IL/O, bicontinuous, and O/IL microregions of the microemulsions were identified by conductivity measurements. The UV–vis absorbance spectra using MO as absorption probes and DLS analysis of microemulsions demonstrated the formation of IL/O microemulsion. FTIR spectra of native corn starch and starch nanoparticles demonstrated the happening of crosslinking reaction. SEM and DLS data of starch nanoparticles intuitively suggested the formation of starch nanoparticles which had good sphericity, small size and a narrow particle size distribution. All these results adequately demonstrated that ionic liquid microemulsion could be used for preparing starch nanoparticles with better size and distribution compared with those obtained from traditional W/O approach. Acknowledgements This research was supported by the National Natural Science Foundation of China (21376097, 21004023), the program for New Century Excellent Talents in University (NCET-13-0212), the Guangdong Natural Science Foundation (S2013010012318), the Key Project of Science and Technology of Guangdong Province (2012B091100443, 2012B091100047, 2012A020602004), the Fundamental Research Funds for the Central Universities, SCUT (2013ZZ0070).

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