A facile and effective technique for the synthesis of thiol-modified Au/alginate nanocomposite and its performance in stabilizing Pickering emulsion

Yoki Yulizar; Foliatini; Mas Ayu Elita Hafizah

doi:10.1016/j.arabjc.2016.05.013

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Original article

12 (

); 4270-4276

doi:

10.1016/j.arabjc.2016.05.013

A facile and effective technique for the synthesis of thiol-modified Au/alginate nanocomposite and its performance in stabilizing Pickering emulsion

Yoki Yulizar^{^a,⁎}, Foliatini^{^b}, Mas Ayu Elita Hafizah^{^c}

a Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Indonesia, Depok 16424, Indonesia

b Bogor AKA Polytechnic, Bogor 16158, Indonesia

c Postgraduate Program of Material Science, Faculty of Mathematics and Natural Sciences, Universitas Indonesia, Jakarta 10430, Indonesia

⁎Corresponding author. Tel.: +62 21 727 0027; fax: +62 21 786 3432. yokiy@ui.ac.id (Yoki Yulizar)

Received: 2016-1-21, Accepted: 2016-5-25, Published: 2016-12-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Synthesis of thiol-based ligand-modified Au/alginate was successfully performed using simple, facile and effective technique, with the aid of microwave irradiation for completing the reduction of gold precursor. The nanocomposite has lower hydrophilicity compared to that of unmodified Au/alginate, as shown by the decrease in surface tension and increase in contact angle to water. Type and concentration of thiol-based ligand greatly influenced the hydrophilicity. By employing mercaptoundecanoic acid (MUA) and dodecanethiol simultaneously as thiol-based ligand modifier for Au/alginate, the as-prepared nanocomposites have the capability to stabilize Pickering O/W (oil-in-water) emulsion of chloroform in water. Nanocomposite concentration and pH significantly affected emulsification capability and emulsion stability.

Keywords

Nanocomposite

Thiol-modified Au/alginate

Pickering emulsion

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1 Introduction

Synthesis of Au/polymer nanocomposite has attracted many researchers since besides plays a role as an effective stabilizer, the characteristic of polymer capping supports the Au nanoparticle (Au-NP) function. The excellent optical properties of Au-NP have been widely applied in the colorimetric sensors and biomedical photothermal therapy (Alanazi et al., 2010; Lan and Lin, 2014). In the drug (bioactive compound) delivery system, such a stable, biocompatible, as well as water soluble encapsulating agent is highly required. As drugs or bioactive compounds are generally soluble in organic phase and have a low solubility in water, thus encapsulating agent must have functional groups which have high affinity both to the water phase as continuous phase and to the organic phase. This material must be active surface and is able to stabilize emulsion of oil in water (O/W) system. Surfactant molecules or polymers with Janus properties may be employed for the purpose. However, emulsion stabilized by small surfactant molecules may experience some destabilization mechanism such as coalescence or Ostwald ripening, since the interaction of surfactant with the interface is reversible (de Folter et al., 2012). As high stability delivery system is required in this system, the need of alternative materials which have both high stabilizing capability and high stability is emerged.

Pickering emulsion, or particle-stabilized emulsion, is such a potential alternative to be applied as a delivery system since the adsorption of particles onto the oil–water interface is irreversible. The ability of removing particles is stated in terms of attachment energy which is influenced by particle size, interface tension and wettability. Several studies have been done to explain the role of various particles in stabilizing the O/W emulsion (Saleh et al., 2005; Shen and Resasco, 2009; Teixeira et al., 2011; Wu et al., 2010; Lan et al., 2007 ). Larson-Smith and Pozzo (2012) and Kubowicz et al. (2010) reported that ligand-modified Au-NPs were capable to successfully stabilize O/W emulsion due to their amphiphilicity. These Au-NP-based nanocomposites were arranged in compact layer surrounding the droplet surface thus capable of hindering interaction between oil droplets, and as a consequence the highly stable emulsion can be formed.

In this research, thiol-based ligand-modified Au/alginate nanocomposite was synthesized by such a simple, facile and effective route, resulting nanocomposite which has sufficient hydrophobicity required for stabilizing emulsion. The effect of nanocomposite and thiol-based ligand concentration and pH on the emulsification capability and emulsion stability was studied.

2 Experimental

HAuCl₄ was prepared by dissolving a certain amount of solid gold (99.9%) into aqua regia and added by double distilled water until a certain volume, to give a desired concentration. The solution was evaporated to remove the aqua regia and then redissolved in double distilled water with the same volume as the initial volume. Sodium alginate was obtained from Himedia. HCl and NaOH were used to adjust pH of solution. Mercaptoundecanoic acid (MUA) and 1-dodecanethiol were obtained from Sigma–Aldrich.

Synthesis of Au-NP was conducted in the domestic microwave oven with the total power of 800 W. Briefly, 10 mL of 0.4 mM HAuCl₄ solution was poured into 100 mL beaker glass and added by 10 mL of 0.375%(w/v) alginate solution. The mixture was irradiated by microwave oven at 2450 MHz for some minutes until the color of the solution was changed. The final solution was cooled in room temperature before further characterization and modification.

The as-prepared Au-NP was added by 8 mg/L MUA in ethanol and 5%(v/v) dodecanethiol in toluene, at certain ratio. The mixture was vortexed for some minutes and left overnight. This step yielded Au/alginate/MUA/dodecanethiol nanocomposites. The nanocomposites assessed its emulsification capability by adding them to the mixture of oil and water (oil = chloroform, diesel oil, olive oil), and sonicating the mixture for some minutes. The as-prepared emulsion was then characterized by particle size analyzer, turbidimetry, and optical microscopy.

The optical extinction spectra of Au-NP were measured by a ultraviolet–visible (UV–Vis) spectrophotometer (UV-1700, Shimadzu) in the wavelength range of 200–1100 nm. The morphologies of the Au-NP were observed by transmission electron microscopy (TEM) on JEM-1400 electron microscope JEOL instrument at an accelerating voltage of 120 kV. The particle size and particle size distribution were evaluated by particle size analyzer (Malvern Zetasizer, ZEN-1600). Interaction between Au/alginate and thiol-based ligands was confirmed by Fourier transform infrared spectroscopy (FTIR), Bruker.

3 Result

Fig. 1A showed SPR absorption spectrum of the as-synthesized Au/alginate nanocomposite peak at about 530 nm, and the TEM image (Fig. 1B) showed the nanoscale, spherical particles dispersed homogeneously. Different ratio of [alginate]/[AuCl₄⁻] resulted in different color of colloid of Au/alginate nanocomposite, and particle size and shape. At low ratio of [alginate]/[AuCl₄⁻], the colloidal Au/alginate nanocomposite has purple color and tends to unstable. At high ratio of [alginate]/[AuCl₄⁻], the color of colloid was changed to red. The stability of the Au/alginate nanocomposite increases as the ratio of [alginate]/[AuCl₄⁻] was increased. The higher the ratio of [alginate]/[AuCl₄⁻], the smaller the particle size of Au/alginate nanocomposite, more homogeneous the particle shape and the higher the tendency of particle shape to be spherical (Table 1) (Foliatini et al., 2014a,b).

(A) UV–Vis absorption spectrum of alginate (a), AuCl4−, (b) and Au/alginate nanocomposite (c). (B) TEM image of Au/alginate nanocomposite at 0.20 mM AuCl4− and 0.25% (w/v) alginate (scale bar = 200.0 nm).

Table 1 The characteristics of Au/alginate nanocomposite at various ratio of [alginate]/[AuCl₄⁻].

[alginate] (%w/v)	[AuCl₄⁻] (mM)	Ratio of [alginate]/[AuCl₄⁻] (%w/v:mM)	Color	Particle size (nm)	Particle shape
0.05	0.50	0.8:8	Light purple, unstable	73.41	Mixture
0.05	0.50	0.8:8	Light purple, unstable	403.4	Mixture
0.075	0.50	1.2:8	Dark purple, unstable	70.88	Mixture
0.05	0.20	2:8	Purple, unstable	76.54	Mixture
0.075	0.20	3:8	Purple, unstable	40.81	Spherical → cubic
0.25	0.50	4:8	Purple	20.00	Spherical, trigonal/prism
0.125	0.20	5:8	Purple	14.06	Spherical
0.25	0.40	5:8	Purple	14.06	Spherical
0.25	0.30	6.7:8	Purple	10.97	Spherical
0.25	0.20	10:8	Red	8.094	Spherical
0.375	0.20	15:8	Red	7.707	Spherical
0.50	0.20	20:8	Light red	8.475	Spherical
0.25	0.10	20:8	Light red	8.475	Spherical

Modification of Au/alginate nanocomposite with thiol-based compound resulted in the decrease of surface tension, and increase of contact angle, depends on the type of thiol compound (Table 2). The turbidity of chloroform in water emulsion system which stabilized by thiol-modified Au/alginate nanocomposite was highly depended on pH. At very low and very high pH, the turbidity was low. The emulsion capability was low at very low pH (Table 3).

Table 2 Contact angle and surface tension of Au/alginate modified with thiol based-ligand.

Nanocomposite	Contact angle	Surface tension (mN/m)
Au/alginate	29°	55.35 ± 0.19
Au/alginate/MUA	52.4°	39.30 ± 0.04
Au/alginate/dodecanethiol	50.5°	27.44 ± 0.01
Au/alginate/MUA/dodecanethiol	50.5°	32.78 ± 5.84

Table 3 Turbidity of chloroform in water emulsion system which is stabilized by thiol-modified Au/alginate nanocomposite and emulsification capability of the nanocomposite at various pH.

pH	Turbidity (NTU)	Volume of oil trapped in the emulsion system (mL)^a
2	82.1	0.20
4	H1	0.60
6	482	0.50
8	649	0.50
10	H1	0.60
12	233	0.60

H1 = cannot be detected by the instrument (exceeding the detection limit of the instrument).

a At total volume of emulsion system of 20 mL.

FTIR characterization of MUA showed a vibration peak at 3300 and 2900–3000 cm⁻¹ which is assigned as –COOH functional groups and –CH₂– saturated alkyl chain, respectively. C–S–H bond in MUA was generally found at peak of 489 and 706 cm⁻¹, which indicated out-of-plane and in-plane C–S–H bending (Krishnakumar and Xavier, 2004). In our experiment, the wavenumber of FTIR scanning was in the range of 500–4000 cm⁻¹; thus, the peak below 500 cm⁻¹ was not detected. The in-plane C–S–H bending was shown at about 720 cm⁻¹. Vibration peak of 1650 cm⁻¹ and 1450–1350 cm⁻¹ corresponds to symmetric and asymmetric vibration of deprotonated terminal carboxylic functional group, –COO⁻ (Sugihara et al., 2000). Dodecanethiol FTIR spectrum also revealed vibration peak of –C–S–H bond, but at lower wavenumber, 710 cm⁻¹. Vibration peak of –CH₂– alkyl chain was in the same wavenumber as that in MUA spectrum. The vibration peak of S–H bond was generally small in the wavenumber range of about 2500–2700 cm⁻¹ (Fig. 2B).

(A) Colloidal dispersion of modified and unmodified Au/alginate. (B) FTIR spectrum of MUA (purple), dodecanethiol (pink) and Au/alginate/MUA/dodecanethiol (green). (C and D) FTIR (C) and SPR spectrum (D) of modified and unmodified Au/alginate.

Adding oil phase into the Au/alginate/thiol aqueous-based colloid resulted in turbid colloid which maintain purple color of Au/alginate/thiol nanocomposite (Fig. 3A). The SAED of emulsion showed a clear concentric ring patterns (Fig. 3B). The higher the thiol compound concentration, as well as the nanocomposite concentration, the higher the turbidity of colloid (Figs. 3C,D, 4A and B).

(A) Au-NP before, after modification with thiol-based ligand, and olive oil in water emulsion system stabilized by nanocomposite, from left to right. (B) SAED pattern of thiol-modified Au/alginate nanocomposite in the O/W emulsion system. (C) Diesel oil in water emulsion system stabilized by nanocomposite at various MUA concentration. (D) Turbidity of the emulsion as a function of MUA and dodecanethiol concentration.

Diesel oil in water emulsion system at various nanocomposite concentration at (A) and their turbidity (B).

The aggregation of particles in the emulsion system was shown in the TEM image (Fig. 5A). The optical microscope image of oil in water revealed the larger emulsion at lower alginate concentration (Fig. 5C).

A. TEM image of chloroform in water emulsion system (scale bar = 100 nm). (B and C) Optical microscope image of olive oil in water emulsion system stabilized by nanocomposite, at alginate concentration of 0.50%w/v (B) and 0.25%w/v (C). Scale bar = 20 μm.

At low pH, the turbidity of the emulsion was low and the particle size was very large. As the pH was increased, the turbidity increases and the particle size decreases, but at very high pH, the % volume of particle size was low and the separation phase of the emulsion tends to occur (Fig. 6).

(A) Chloroform in water emulsion system, stabilized by nanocomposite at various pH. (B) Chloroform in water emulsion in A, after 7 days. (C) Relationship between pH of emulsion (which stabilized by thiol-modified Au/alginate nanocomposite) and particle size (●), % volume (Δ), and peak width (□).

4 Discussion

4.1

4.1 Synthesis of Au/alginate nanocomposite

The synthesis of Au/alginate nanocomposite in this experiment was aided by microwave irradiation and this technique was proved to yield smaller particle size and narrower size distribution compared to that performed by conventional heating. SPR spectrum of the nanocomposite is shown in Fig. 1A, revealing characteristic peak at about 530 nm, confirmed the success of the synthesis. [AuCl₄⁻]/[alginate] ratio played a significant role in determining the characteristic of as-prepared nanocomposite as shown in Table 1, due to the different effect of reducing and stabilizing capability of alginate at different [AuCl₄⁻]/[alginate] ratio. TEM image (Fig. 1B) confirmed the small, spherical and homogeneous size and shape of nanoparticles, confirmed the effective stabilizing process of alginate as capping agent.

4.2

4.2 Modification of Au/alginate nanocomposite with thiol based-ligands

Since Au/alginate was synthesized at pH neutral to base, the nanoparticles were hydrophilic and thus not suitable to be applied as an emulsion stabilizer unless modified. Modification of Au/alginate was done in order to reduce hydrophilicity of the nanoparticle. Dodecanethiol was applied as a modifier due to its long alkyl chain; thus, it is able to increase the hydrophobicity of the material. Besides, the sulfur content in dodecanethiol can covalently bind to Au since the similarity in the softness character of Au and S allows them to strongly interact, as explained by hard-soft acid and base (HSAB) principle.

In this experiment, applying dodecanethiol as single modifier to adjust the hydrophobicity of Au/alginate yielded highly hydrophobic material. This was shown when the nanocomposite was added to the oil–water system, and it left the water phase and was transferred to the oil phase. It means that the two phases were still separated, and the stable emulsion was not formed. Thus at this condition, the nanocomposite cannot act as effective emulsion stabilizer. Addition of another ligand to the colloidal Au/alginate/dodecanethiol system can solve the problem. Mercaptoundecanoic acid (MUA) was a promising material which has similar properties to that of dodecanethiol, but with lower hydrophobicity due to its carboxyl group content. Though the appearance of the nanocomposite with and without MUA was similar (Fig. 2A), their properties showed significant difference.

FTIR spectrum showed that the S-H peak was not found in the nanocomposite spectrum, confirmed that the deprotonation occurred and the remaining S atom attached to Au forming Au–S covalent bond (Amoli et al., 2012). By comparing the spectrum of Au/alginate and thiol-modified Au/alginate (Fig. 2C), it is clearly shown that there was a wavenumber shift for some peaks since the thiol modification changed the chemical environment of the bonding system. This phenomenon was supported by the wavelength shift and the decrease of absorbance in the SPR spectrum (Fig. 2D). Fig. 2D also revealed that the modification by dodecanethiol resulted in greater effect in the SPR peak compared to that of MUA. The probable reason was that dodecanethiol may induce cluster formation of Au-NP-based nanocomposite due to hydrophobic interaction between alkyl thiols.

Contact angle (θ) measurement showed that modification of Au/alginate with thiols increases the hydrophobicity, as shown by the increment of contact angle to water (Table 2). The θ value of unmodified Au/alginate which is near to zero indicated that the particle is highly hydrophilic, thus not appropriate to be an emulsion stabilizer. θ is also related to attachment energy (ΔG) at interface. The previous study reported that micron-sized particles with interface contact angle (θ_ow) of 90° and interface tension of ∼50 mN/m have ΔG in the range of 10⁶ kT (Levine et al., 1989). For silica particle with size of 10 nm, ΔG was varied from 2750 kT for θ_ow = 90° to <10 kT for θ_ow in the range of 0–20° (Aveyard et al., 2003). Thus, the highest ΔG was obtained for θ_ow = 90°.

4.3

4.3 Application of Au/alginate/thiol nanocomposite as Pickering emulsion stabilizer

The mixture of oil phase and water phase which was added by thiol-modified Au/alginate nanocomposite could form stable emulsion (Fig. 3A). Interestingly, the Au-NP-based nanocomposite has kept its crystallinity as shown by the SAED pattern (Fig. 3B). The concentration of thiol-based ligands has a great effect on emulsion capability. As the concentration of MUA was increased (by keeping constant the concentration ratio), there was a significant change in the appearance of oil–water system, related to the capability of the nanocomposite in stabilizing the emulsion (Fig. 3C). At MUA of 8 mg/L, the mixture was a clear colloidal solution, and the oil phase was still separated from the continuous phase. When MUA concentration was increased to 16 mg/L, the turbid dark purple emulsion was formed, and the oil phase was not appeared. The turbidimetry measurement showed the drastic increase in turbidity as the MUA concentration, as well as dodecanethiol concentration, was increased (Fig. 3D), and this indicated that the emulsion formation was more effective. The results confirmed that these thiol-based ligands played a role in stabilizing emulsion (see Table 3).

As the nanocomposite acts as emulsion stabilizer, the nanocomposite concentration was predicted to have high impact to the emulsification capability. In the Pickering emulsion, particles were tightly arranged onto the droplet surface and strongly attached to the surface created a barrier for these droplets to interact each other. Therefore the particle size does not grow larger, and the emulsion is kept stable. Generally, there are two factors which determine the emulsion stability, firstly, sufficient energy to break the intermolecular bonding between oil molecules, and secondly, sufficient number of particles which are attached in the interface.

The higher the nanocomposite concentration, the higher the emulsification capability. In this experiment, at constant MUA concentration of 32 mg/L, the effect of nanocomposite concentration in the range of 5–80%v/v was studied. Very low nanocomposite concentration (5%v/v) could not effectively stabilize the O/W emulsion system, as shown by the clear colloidal solution and the oil phase which was separated in the top layer. As the nanocomposite concentration was increased, the oil phase in the top layer was significantly reduced and the clear solution changed into turbid emulsion, indicating formation of stable emulsion (Fig. 4A). Turbidity measurement clearly showed a significant increase of turbidity as the nanocomposite concentration was increased (Fig. 4B).

TEM image of thiol-modified Au/alginate nanocomposite in emulsion system revealed the cluster of nanocomposite, and these nanocomposites arrange in such a way as if they were attached around the oil droplet (Fig. 5A). This image also showed that the layer of nanocomposite cluster does not perfectly form a circle, and in some places the nanocomposite was absent, indicating that the nanocomposites were not adsorbed in some places in the oil droplet surface. Since TEM image alone cannot confirm the existence of oil droplet, thus we only able to discuss the morphology of nanocomposite cluster based on the image, and predicted the real structure of the O/W emulsion. The optical microscopic image revealed that the emulsion droplets were distributed almost uniformly with very small size, though several large droplets were also found (Fig. 5B). Very large emulsion droplets could be found when the Au/alginate have relatively large particle size (Fig. 5C), but in this condition, interaction between oil droplets was more probable to occur, leading to coalescence and phase separation. Fig. 5B and C also revealed that there are layers which capped the droplet surface, and in this experiment they were nanocomposite layers. At very small size (hundreds of nanometers) these layers were not clearly observed, unless using higher resolution of optical microscope.

The effect of pH on the emulsion capability, as well as its stability, was studied at MUA concentration of 40 mg/L. The highly turbid emulsion is clearly observed in Fig. 6A. Turbidity measurement showed that at pH 4–10 the turbidity was very high. At lower pH, the mixture of oil–water system and nanocomposite resulted in clear colloidal solution and the turbidity was relatively low, indicated that the emulsion formation was not effective. The oil phase (chloroform) at pH 2 was still separated in the bottom. At pH 12, the turbidity was lower than that at pH < 12, suggesting that the emulsion capability was also lower. In the pH range of 4–12, all of the emulsion has high turbidity but differs in appearance. At pH 12, the emulsion color was purple and differs from the others which have color between red and magenta. This indicated that the aggregation of nanocomposite was not controlled. This affected the properties of the emulsion, including emulsion capability as well as stability.

Emulsification capability can be roughly defined as volume of oil which trapped in the stable emulsion system and does not separate from continuous phase. Table 2 shows that at pH 2 only very small amount of oil phase which can be encapsulated in the emulsion system, suggested that at pH 2, the emulsification capability was very low. The higher the pH, the higher the emulsification capability. Since the total volume was not large, the volume of oil phase was also low; thus, the emulsion capability cannot be distinguished in the pH range of 4–12.

The result of particle size measurement showed emulsion droplet with size in the range of 700–1400 nm (Fig. 6C). This was in agreement with the result of optical microscopy analysis (Fig. 5B). Fig. 6C shows that particle size was greatly influenced by pH. The decrease of particle size with the increase of pH was probably due to the electrostatic repulsion between emulsion droplet, which is induced by increasing charge density on the droplet surface, as a consequence of increasing pH. However, the particle size distribution decreased with the increase of pH (percentage of volume was <60% at pH 12). The detail in particle size analysis revealed three types of particle size which is present in this emulsion system. These three types of particle size have relatively high % volume, expressing their large abundance. Among the three types of particle size, there was a significantly high particle size, suggested that aggregates formation tends to occur at high pH. It was known that the pH is higher resulting in the higher ionic strength, thus reducing electrostatic repulsion of the charged nanoparticles (Liu et al., 2010). This explained why the stability of the emulsion at pH 12 cannot be maintained, as can be observed visually (Fig. 6B).

5 Conclusion

Au/alginate nanocomposite can be modified with thiol-based ligands, MUA and dodecanethiol using facile, simple and effective route, yielding nanocomposite which has ability to stabilize O/W emulsion (O = chloroform, diesel oil, olive oil). At optimum concentration (MUA ⩾ 16 mg/L, dodecanethiol 5%) the nanocomposite has moderately high hydrophobicity, as shown by the decrease in surface tension and increase in contact angle, thus appropriate to effectively stabilize O/W emulsion. Nanocomposite concentration and pH have greatly influenced the emulsification capability and emulsion stability. Emulsification capability increased with increasing nanocomposite concentration. At low pH (<4) emulsion formation was not effective and at very high pH (>10) the emulsion stability was low.

Acknowledgments

This work was funded by Hibah Penelitian Kompetensi 2016, from the Directorate General of Higher Education, Ministry of Research, Technology and Higher Education, Indonesia through The Directorate of Research and Community Services, Universitas Indonesia (No. 1023/UN2.R12/HKP.05.00/2016), and The Training and Education Center, Ministry of Industry, Indonesia.

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