toluene interface

Colloids and Surfaces A: Physicochem. Eng. Aspects 256 (2005) 17–20

Effect of alkyl chain length on phase transfer of surfactant capped Au nanoparticles across the water/toluene interface Haifeng Zhua , Cheng Taoa , Suping Zhenga , Shikang Wub , Junbai Lia,∗ a

International Joint Lab, Key Lab of Colloid and Interface Science, The Center of Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Zhong Guan Cun, Bei Yi Jie, Beijing 100080, PR China b Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101, PR China Available online 14 November 2004

Abstract Gold nanoparticles synthesized in aqueous phase were modified by thioglycolic acid (TGA) and then capped with cetyltrimethyl ammonium bromide (C16 TAB), myristyltrimethyl ammonium bromide (C14 TAB), and dodecyltrimethyl ammonium bromide (C12 TAB), respectively. The surfactant capped nanoparticles could be transferred into toluene without aggregation across the water/toluene interface under vigorous stirring. The transfer process was certified by the rapid change of color of the aqueous and organic phase, zeta potential of nanoparticles, and UV–vis absorbance spectroscopy. Experimentally, it is found that only decyltrimethyl ammonium bromide (C10 TAB) capped nanoparticles were unable to ensure the phase transfer in the organic phase. Both transmission electron microscopy (TEM) images and static light scattering measurement demonstrated the narrow size distribution of the capped nanoparticles in toluene. © 2004 Elsevier B.V. All rights reserved. Keywords: Phase transfer; Gold nanoparticle; Surfactant

1. Introduction Metal nanoparticles have already exhibited their diversity and potential applications in the field of modern material sciences, especially in microelectronics, optical materials and biocatalysts [1–6]. Remarkably, gold nanoparticles obtain much more attention than others because of their special biological compatibility [7,8]. However, in most cases, such nanoparticles require high stabilization to prevent them from aggregation. Thus, a plenty of methods have been developed for the modification of the surface of gold nanoparticles, yielding products with sufficient stability against aggregation and new physicochemical properties comparing to the single component particles [9]. Surfactant coating is one typical method to realize the phase transfer of gold nanoparticles from water into organic phase [10–14]. Experiments describing one-way transfer of CdS or CdTe nanoparticles

∗

Corresponding author. Fax: +86 10 64874712. E-mail address: [email protected] (J. Li).

0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.09.036

from water to oil phase using cetyltrimethylammonium chloride (C16 TAC) or dimethyldioctadecylammonium bromide (DODABr) were already reported [12,13]. Other reports on modifying gold nanoparticles surface with tetraoctylammonium bromide (TOAB) or cyclodextrins (CDS) demonstrated that the gold nanoparticles could be transferred from water into toluene or chloroform [9,14]. It is certified previously that C16 TAB could be used as a phase transfer reagent of HAuCl4 across the toluene/water interface to prepare C16 TAB capped gold particles followed by the reduction reaction using NaBH4 [11]. The principle is based on the fact that the negatively charged surface of the nanoparticles is hydrophobic by adsorption of a positively charged long-chain surfactant at the negatively charged particles surface [10,11]. In this way, the particles become extractable into organic solvent. This process mostly is considered as the formation of reversed micelles. However, the length of alkyl chain of the surfactant, which may vary the physicochemical property of particles and the phase transfer behaviors, is not fully understood until now. In this work, we demonstrated that the length of alkyl chain of surfactant has a great impact on

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H. Zhu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 256 (2005) 17–20

phase transfer processes of surface-modified Au nanoparticles. The phase transfer process is dominated by hydrophobic force.

2. Materials HAuCl4 ·H2 O (>99.9%), thioglycolic acid (TGA) and sodium citrate were purchased from Aldrich. Toluene, cetyltrimethyl ammonium bromide (C16 TAB), myristyltrimethyl ammonium bromide (C14 TAB), dodecyltrimethyl ammonium bromide (C12 TAB), and decyltrimethyl ammonium bromide (C10 TAB) were purchased from Beijing Chemical Reagents Co. All chemicals were used as received without further purification. The water used in all experiments was prepared in a three-stage Milli-Q Plus 185 purification water system and had a resistivity higher than 18.0 M cm.

3. Experimental methods Fresh gold nanoparticles were prepared using a method reported previously [15]. A certain amount of TGA was added into 2 mL fresh gold sol (negatively charged) and the mixture was rotated gently until the color became light purple. Then CTAB (1 CTAB:1 TGA, molar ratio) was introduced into the TGA-modified Au nanoparticles suspension. It was kept for 10 min to allow the adsorption of surfactant molecules through electrostatic interaction between TGA and CTAB. As an oil phase, 2 mL toluene was mixed with the Au/TGA/CTAB/nanoparticles for 30 min. The resulted milky emulsion was kept for 1 h at room temperature allowing the phase separation. The UV–vis absorbance spectra of the toluene containing the gold particles were collected over the range of 350–700 nm on a Hitachi U-3010 spectrophotometer with a resolution of 2 nm. Photos were taken with a NIKON 2500 digital camera. Transmission electron microscopy (TEM, Hitachi-800) observation was performed to determine the size, morphology and monodispersity of the resulting nanoparticles [16]. A BIC particle sizing software and BIC zeta potential analyzer were also employed to measure the size distribution and surface charge of samples.

4. Results and discussion The gold sol nanoparticles prepared by citrate method present the well-known red color, as showed in Fig. 1a [17]. No serious self-aggregation of the gold particles was observed due to the function of the carboxylate moieties in the citrate sol [18]. However, while TGA solution was mixed with the gold sol, the suspension color changed to powder blue immediately (Fig. 1b), resulting from chemical bonding of –SH functional groups of TGA molecules onto the sur-

Fig. 1. (a) The fresh Au sol; (b) Au sol after TGA addition; (c) TEM image of (a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

face of Au particles. This process requires only few seconds, which is similar to previously reported [19]. TEM image in Fig. 1c shows that the Au particles obtained from fresh gold sol have an average diameter of approximately 20 nm. The negatively charged fresh gold particles with a zeta potential of −30.27 mV (Table 1) could not be reversed after binding of TGA molecules although it can increase the zeta potential value [20–22]. But the adsorption of C16 TAB molecules onto the surface of Au particles resulted in the positive charged surface as listed in Table 1, indicating the successful adsorption of C16 TAB via electrostatic interaction after vigorous shaking. Obviously after transfer into toluene phase, the Au particles are nearly neutral. It demonstrated directly that as predicted, the surface charge of Au particles changed at every step. Fig. 2 shows clearly the results of gold particle transfer modified with C16 TAB, C14 TAB and C12 TAB, respectively. The transparent red color indicates that the gold particles have been transferred from water into toluene phase. However, for the C10 TAB capped Au particles, a number of white flocculates were observed in the water phase. After 48 h, the capped gold particles were observed to aggregate in the toluene–water interface and on the vessel wall, indicating that the gold particles were not tending into the toluene phase across the liquid/liquid interface. Unlike those particles modified with longer chained surfactant, the hydrophobic force is not strong enough to bring the gold particles to Table 1 Zeta potential measurements of Au particles Zeta potential (mV) Au-Sol/water Au-TGA/water Au-TGA-CTAB/water Au-TGA-CTAB/toluene

−30.27 −16.55 22.92 4.69

H. Zhu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 256 (2005) 17–20

Fig. 2. The Au particles transference with C10 TAB (a), C12 TAB (b), C14 TAB (c) and C16 TAB (d) as surfactant separately. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Fig. 3. The absorption spectra of the gold sol, toluene phase after transference with C16 TAB, C14 TAB, C12 TAB and C10 TAB as surfactant separately corresponding to curve (Bt), (Ct), (Dt) and (Et); curves (Bw), (Cw), (Dw), (Ew) are the absorption spectra of water phase after transference corresponding to (Bt), (Ct), (Dt) and (Et).

enter the toluene phase although C10 TAB/TGA/Au particles are positively charged. Similar phenomenon has been also observed for the C12 TAB and C14 TAB system, respectively. Fig. 2(a–d) shows the aggregation decreases as the alkyl chain of CTAB increased to carbon-16. Thus, we can deduce that

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the alkyl chain length of the surfactant is a key factor to impact the capped gold particle transference [23]. This result is in a good agreement with that obtained by analyzing their UV–vis spectra. Fig. 3 shows the UV–vis spectra of unmodified and modified gold particles in aqueous solution and in toluene, respectively. Gold particles without modification in the sol displayed a characteristic surface plasmon band centered at 518 nm (Fig. 3b), which is similar to those gold particles (>3 nm) reported previously. After transference of gold particles into organic solvent, only a modest peak red-shifted in the surface plasmon band from 518 to 526 nm (Fig. 2b) was observed, which verifies the existence of Au particles in toluene. The 8 nm red-shift may be caused by the increase of the solvent refractive index from 1.333 (water) to 1.494 (toluene), according to the Mie theory [24,25], and also possibly by the surface modification of chemically bounded alkanethiol. The particle size kept constant before and after transference with 20 nm observed by TEM. However, the surfactant effect on the gold particle UV–vis absorption is negligible, attributing to its physical adsorption on the particle surface. After transference the UV–vis absorption peak of the capped gold particles in organic phase remains at the position of 526 nm as shown in Fig. 3, indicating that the alkyl chain length of CTAB did not influence the UV–vis absorption of gold particles. However, the absorption intensities are dramatically reduced with the decrease of carbon number of CTAB, which is corresponding to the transference decrease of gold particle number. The largest UV absorption area of C16 TAB capped Au particles indicates the maximum transference efficiency in toluene, whereas, for C10 TAB capped Au particles no absorption peak was observed at 526 nm. This means that all the gold particles may aggregate at the water/toluene interface or the vessel wall as shown in Fig. 2. This can be confirmed by the absent absorption peak between 400 and 700 nm, especially around 526 nm in the water phase. The UV–vis absorption exhibited a similar transference of CTAB capped gold particles with the different chain length as observed directly from the experiments. Meanwhile, we also visualized experimentally that the C12–16 TAB-modified nanoparticles exhibited a high stability in the organic solvent after a month storage comparing to

Fig. 4. The gold particle size distribution at water and toluene phase separately.

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the gold sol without modification [26]. By taking the static light scattering measurements for the unmodified and modified gold nanoparticles, we have determined that the gold sol particles in water phase are approximately 108.5 nm but with a narrow size distribution as shown in Fig. 4, which is five times larger than 20 nm, which measured by TEM (Fig. 1c). This is reasonable since before the addition of TGA, the Au particle sol is more likely to have a self-aggregation. With the C16 TAB capped Au particles, after transference into toluene the particle size is similar to that of the pure Au particles possessing the effective diameter of around 22.5 nm, and the particle size distribution is also very narrow, indicating that the modification of the longer chained CTAB enhanced the transference of Au particles into organic phase and protect the particles again self-aggregation.

5. Conclusion In this paper, we demonstrated that the Au nanoparticles modified with thioglycolic acid (TGA) and then capped with hexadecyltrimethyl ammonium bromide (C16 TAB), myristyltrimethyl ammonium bromide (C14 TAB) and dodecyltrimethyl ammonium bromide (C12 TAB), respectively, can realize the phase transfer from an aqueous phase into the organic toluene phase across the interface, whereas, C10 TAB capped Au nanoparticles are definitely blocked in the aqueous phase. It is suggested that alkyl length of the surfactant dominated the transfer process of gold particles. This work provides the quantitative information on the surfactant-modified nanoparticle transfer.

Acknowledgments We acknowledge the financial supports of this research by the National Nature Science Foundation of China (NNSFC29925307), the major state basic research develop-

ment program (973, Grant. No. G2000078103) as well as the collaborated project of the German Max–Planck Society.

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