A rapid phase transfer method for nanoparticles using alkylamine stabilizers

Journal of Colloid and Interface Science 348 (2010) 24–28

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

A rapid phase transfer method for nanoparticles using alkylamine stabilizers Xinnan Wang, Shuping Xu, Ji Zhou, Weiqing Xu * State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, PR China

a r t i c l e

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Article history: Received 1 December 2009 Accepted 31 March 2010 Available online 3 April 2010 Keywords: Phase transfer Nanoparticle Metal colloid Alkylamine Octadecylamine

a b s t r a c t Phase transfer of noble metal nanoparticles (NPs) (>20 nm) from aqueous to organic media is a challenge in colloidal science. We have developed a rapid and simple phase transfer method with alkylamine as the surfactant, through which gold (106 nm in diameter) and silver NPs (118 nm in edge length) can be transferred from aqueous to organic phases. Three alkylamines with different chain-lengths (NH2–(CH2)n 1–CH3, n = 12, 16 and 18) were compared, and octadecylamine (ODA) was the most efficient. The ODA–NP complex rapidly formed as a result of shaking the mixture of ODA (in ethanol) and NPs (in water). After 20 s, the complex separated from the liquid phase because of their hydrophobicity. The undissolved ODA–NP complex could be redispersed into organic solvents, such as chloroform. Ultraviolet–visible (UV–vis) spectroscopy and transmission electron microscopy (TEM) results show that the metal NPs are still monodisperse having been transferred into organic solvents. The whole process of the phase transfer of NPs from aqueous to organic media can be made to happen in less than 1 min. Ó 2010 Elsevier Inc. All rights reserved.

1. Introduction Noble metal nanoparticles (NPs) have received great attention because of their unique size- and shape-dependent optical and electrical properties. It is acceptable that these properties are attributed to the localized surface plasmon resonance (LSPR) of metal NPs. LSPR properties make metal NPs applied to many fields, such as biosensing [1], surface enhanced spectroscopy [2], solar cell [3], ultrahigh-density data storage [4], and so on. So far, numerous methods have been reported for the synthesis of gold and silver NPs in aqueous or organic solvents [5–14]. Metal NPs synthesized in organic solvents have narrow size distribution and good monodispersity. However, with these methods it is hard to adjust the morphologies of metal NPs to obtain anisotropic ones. Compared with that in organic solvents, synthesis in aqueous phase is more prevalent because it is simple, rapid, and environmentally friendly. The highlighted advantage of synthesis in aqueous phase is to acquire various sized and shaped metal NPs via choosing different carboxylates (e.g. citrate, tartrate and malate) and water soluble polymers (e.g. poly(vinyl pyrrolidone) and chitosan) as the reducers and soft templates respectively [8–11]. The main application of these shaped metal NPs is to construct 2-dimensional (2-D) arrays [15] and Langmuir–Blodgett (LB) monolayers at the air–water interface for obtaining unique LSPR properties [16]. Building of LB metal NP film needs to dissolve metal NPs in organic solvents to weaken the interfacial energy between metal * Corresponding author. Fax: +86 431 85193421. E-mail address: [email protected] (W. Xu). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.03.068

NPs [17,18]. In these cases, the phase transfer of metal NPs from aqueous to organic media is usually required. Metal NPs can be transferred from aqueous to organic solvents as a result of varying the surfaces of metal NPs from hydrophilicity to hydrophobicity with the help of the surfactant modification. Sodium oleate was first used as the stabilizer for transferring 8-nm silver NPs into cyclohexane, n-hexane and benzene respectively [19]. Since then, alkanethiols [20,21], which have strong interactions with NPs of gold and silver through the S–Au and S–Ag covalent bonds, have been employed as the protectors in the phase transfer of NPs from aqueous to organic media. In previous reports, Sastry et al. developed the method to transfer metal NPs from aqueous to organic solvents with alkylamines as surfactants [22,23]. They constructed the LB monolayer composed of metal NPs at the gas–liquid interface [16]. Eastoe also developed a purification method of NPs using (1-hexadecyl)trimethylammonium bromide (CTAB) as stabilizer via tuning the solvent quality [24,25]. Like small organic surfactants, polymer ligands have also been tried in some methods to achieve the phase transfer of metal NPs from aqueous to organic media [17,26,27]. However, most of the methods need two or more steps and consume several hours. In the existing reports, most of the phase transfer methods are only applicable to small metal NPs (diameter <20 nm) and very few methods yielded the applicability of NPs (diameter P100 nm) [20,28,29]. It is because the van der Waals force between NPs gets stronger with the increase of size/mass [29–32]. The NP (diameter >20 nm) surface has much smaller negative charge density than the small NP surface. The weaker repulsive force makes NPs (diameter >20 nm) prefer to irreversibly aggregate, which leads to the poor dispersibility of metal NPs in organic solvents.

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To overcome these problems, we developed a rapid and simple approach for the phase transfer of NPs from aqueous to organic media which is suitable for metal colloid NPs with sizes ranging from 13 nm to over 100 nm. We used octadecylamine (ODA) as a stabilizer to extract the citrate-coated metal NPs in the colloidal solution. Metal NP surfaces were coated with ODA through the electrostatic interaction between the amino and citric groups [22]. The solvent for ODA plays a key role in the phase transfer process of NPs from aqueous to organic media. Based on the optimized mutual solubility of water and ethanol, the ODA–NP complex could easily separate from the liquid phase and form a solid layer on the liquid phase. The whole transfer process can be completed within only 30 s. The redispersed metal NPs were characterized by ultraviolet–visible (UV–vis) spectroscopy and transmission electron microscopy (TEM) to evaluate their monodispersity after the phase transfer treatment.

composed of the ODA–NP complex and the bottom layer was the liquid phase composed of the mixture of water and ethanol. The centrifuging treatment of the mixed system assisted the ODA–NP complex to get entirely apart from the solution (at a speed of 10,000 rpm for 5 min). After removing the bottom solution, the ODA–NP complex was then separated from the liquid phase. Finally, the ODA–NP complex was redispersed into chloroform after drying treatment. The NPs were characterized by UV–vis spectroscopy (Ocean Optics USB4000 Spectrometer) and TEM (Hitachi H800 operated at 200 kV).

2. Materials and methods 2.1. Materials Chloroauric acid (HAuCl4
4H2O, gold content P47.8%) was obtained from Shanghai Chemical Reagent Co., Ltd. Silver nitrate (99.5%) and trisodium citrate (98%) were supplied by Beijing Chemical Plant. Dodecylamine (DDA) was purchased from Tianjin Guangfu Fine Chemical Research Institute. Octadecylamine (ODA) (95%) and hexadecylamine (HDA) (95%) were obtained from Fluka and MERCK. 2.2. Synthesis of gold and silver colloids

519 525

Aqueous Chloroform

525 530

Fig. 1. Pictures of gold NPs (13, 25 and 106 nm in diameter) in phase transfer process with the aid of ODA.

13nm Au

25nm Au 561 577

Normalized Extinction (a.u.)

Gold NP colloids were synthesized with the method reported by Frens [5]. Three different sized gold colloids were prepared by respectively adding the three aliquots of boiled chloroauric acid solution 4.0, 1.0 and 0.4 mL of sodium citrate aqueous solution (1%) [5]. The 13 ± 1, 25 ± 3 and 106 ± 16 nm diameter gold NPs were successfully synthesized according to the statistic results of TEM images. Supposing that all the chloroauric acid had been reduced and the Au atoms in the NPs were piled up with the facecentered cubic structure, the concentrations of gold colloids with the given sizes were about 3.56  10 9, 5.01  10 10 and 6.58  10 12 mol particle/L, respectively. Three different shaped silver NP colloids were employed in the present study. The silver spherical NPs were prepared according to the Lee’s method [7]. The colloid of spherical silver NPs was 8.78  10 11 mol particle/L in concentration and 73 ± 7 nm in size. The nanotetrahedrons [8b] and nanodecahedrons [8c] were prepared via the photo-induced seed-mediated growth method with different structure-directing reagents according to our published papers. The silver nanotetrahedron colloid was 8.29  10 12 mol particle/L in concentration with an edge length of 118 ± 18 nm. The silver nanodecahedron was composed of five tetrahedrons sharing one fivefold axis at the center and each nanodecahedron was bound by {1 1 1} facets [8c]. The edge length of nanodecahedron was 33 ± 2 nm and the concentration of silver nanodecahedron colloid was 7.79  10 11 mol particle/L.

106nm Au

2.3. Phase transfer to organic solvent The noble metal NPs were transferred from aqueous phase to organic phase by modifying with ODA. NPs aqueous solution (1.0 mL) was added to 1.0 mL of ODA ethanol solution (10 2 mol/ L). The mixture was shaken for 10 s to mix NPs and ODA well and then left for another 20 s. Subsequently, the solution spontaneously separated into two layers. The upper layer was the solid layer

400

500

600

700

800

Wavelength(nm) Fig. 2. UV–vis spectra of different sized gold NPs (13, 25 and 106 nm) in water and chloroform.

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Fig. 3. TEM images of 13, 25 and 106 nm gold NPs in water (A, C, E) and chloroform (B, D, F).

3. Results and discussion Fig. 1 shows the pictures of aqueous gold colloids in the phase transfer process. The top panel presents the pictures of the aqueous colloids of three different sized gold NPs (13, 25, and 106 nm in diameter). The changes in the gold colloid color were observed, for the color of gold NPs turned dark with their size increase. The middle panel shows the pictures of the gold colloids with ODA. After the mixtures were shaken for 10 s, the ODA–Au complex formed. Owing to their lower density and hydrophobicity, the ODA–Au complex floated on the liquid phase surface (the bottom panel) up on standing for another 20 s. The bottom layers gradually bleached. Finally, the bottom liquid layers only consisted of water and ethanol. ODA is a long-chain hydrophobic molecule which is well dissolved in ethanol rather than water. ODA solution in ethanol has positive charges based on its protonation. The metal NPs carry negative charges because of citrate groups coating. According to the literature [33], the linking between protonated amino-group and citrate-coated NP was attributed to the electrostatic interaction. Ethanol was chosen for ODA, which differed from water immiscible solvents other methods used (e.g. chloroform, hexane, and toluene) [22,23]. Owing to the mutual solubility between water and ethanol, when 1.0 mL of ODA solution in ethanol was added to the metal colloids, ODA was able to enter into the aqueous solution and contacted with gold NPs completely. ODA combined with NPs to form the ODA–NP complex via the electrostatic interaction. Then, ODA–NP complex separated from the mixture of ethanol and water and floated on the liquid phase due to the hydrophobicity of

Fig. 4. Pictures of 13-nm gold NPs after alkylamines with different chain-lengths added in the phase transfer process.

ODA. The proper solubility of ODA in the mixed solvents assisted the completion of the extraction of metal NPs in a short time. Fig. 2 shows the UV–vis spectra of different sized gold NPs in aqueous phase (black curves) and in chloroform after phase transfer (red curves1). The maximal plasmon peaks of gold NPs with three different sizes are centered at 519, 525 and 561 nm, respectively. The shapes of the plasmon spectra of gold NPs in chloroform are almost unchanged compared with those in aqueous solution except for the red-shifts due to the medium change. The plasmon peaks remained narrow in their band width, which indicates the gold NPs were well dispersed after the phase transfer treatment. TEM images (Fig. 3) also prove that the different sized gold NPs had no aggregation after they were transferred to chloroform. 1 For interpretation of color in Figs. 2 and 5, the reader is referred to the web version of this article.

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B 1.0

494

Normalized Extinction(a.u.)

466

1.2

A

Ag Sphere

0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

800

Wavelength (nm)

0.8 0.6 0.4 0.2 Ag Tetrahedron

0.0 300

400

500

600

700

800

D

487

441

1.0

Normalized Extinction(a.u.)

C

662

1.2 620

Normalized Extinction(a.u.)

1.2

1.0

Ag Decahedron

0.8 0.6 0.4 0.2 0.0 300

Wavelength (nm)

400

500

600

700

800

Wavelength (nm)

Fig. 5. (A) Pictures of different shaped silver NPs (sphere, tetrahedron, and decahedron) in the phase transfer process. (B)–(D) UV–vis spectra of different shaped silver NPs in water (black curves) and chloroform (red curves).

The efficiency of phase transfer of NPs from aqueous to organic media strongly depends on the solubility of the surfactant-NP complex in solvents. Other two alkylamines with different alkyl chainlengths (NH2–(CH2)n 1–CH3, n = 12 and 16) were considered in the present study. The same concentration of HDA (n = 12) and DDA (n = 16) was used to evaluate the phase transfer efficiency for comparison with ODA. Fig. 4 presents the pictures of 13-nm gold NP aqueous colloids after alkylamines with three different chainlengths were added. We found that only ODA was able to react with NPs and assisted the ODA–Au complex to float. HDA and DDA both entirely entered into gold NP colloids indeed, but the phase separation was not observed for HAD and DDA. The color of gold NPs after adding HAD and DDA changed from red to purple and blue, which indicates that the gold NPs aggregated. So, in the present system ODA is irreplaceable. The formation of a distinct ODA–NP layer depended on the ODA concentration in ethanol. We optimized the preferable ODA concentration, which was in a range of 5  10 3–2  10 2 mol/L when the ODA ethanol solution and the metal NPs aqueous solution were mixed in a volume ratio of 1:1. When the ODA concentration was lower than 5  10 3 mol/L, the limited ODA could not extract all the NPs. The transfer efficiency reduced. On the other hand, if the concentration surpassed 2  10 2 mol/L, the viscosity of the mixed solution of ODA and metal NPs increased, which went against the phase separation of the ODA–NP complex and liquid phase. Furthermore, this method can also be applied to the phase transfer of silver NPs with various shapes and sizes from aqueous to organic media. Fig. 5A shows the pictures of three different

shaped and sized silver NPs before and after the phase transfer treatment. Their corresponding UV–vis spectra (Fig. 5B–D) demonstrate that these silver NPs were transferred and redispersed in organic solvent successfully. No aggregates were observed when they were in the organic phase. It can also be seen from Fig. 5B–D that the plasmon band of the silver spheres red shifted but the plasmon bands of the other two different shaped silver NPs blue shifted. So far, we have been unable to give a certain explanation for the differences of the shifts of LSPR bands in two solvents. It is probably because the anisotropy of silver nanotetrahedrons and nanodecahedrons brings in multi-resonance modes and leads to the complicated LSPR changes.

4. Summary We have reported a simple approach which can transfer the gold and silver NPs (at least 118 nm) from aqueous solution to chloroform with the assistance of ODA. The strong electrostatic interaction between ODA and citrate-coated NP resulted in the formation of ODA–NP complex. It is clear that the good solubility of ODA–NP complex in organic solvents rather than water leads to the result of the ODA–NP complex being able to separate from the liquid phase as a layer within only 30 s. The dried ODA–NP complex can dissolve in chloroform and be stable for two weeks without aggregation. The transfer efficiency of this method is up to 100%. Compared with the methods published in the literatures with alkylamines [22,23] and alkanethiols used [20,21], this phase

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transfer method is suitable for the phase transfer of NPs (diameter >100 nm) from aqueous to organic media and has the advantages of relative rapidness, simpleness and high efficiency. This approach provides the convenience and practicality for the construction of 2D arrays and LB films with anisotropic metal colloids. Its broad applicability for metal NPs with various sizes and shapes will extend the application of water soluble plasmonic materials.

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Acknowledgments This work is supported by the National Natural Science Foundation of China (NSFC 20627002, 20773045, 20903043, 20973075) and Research Fund for the Doctoral Program of Higher Education of China (No. 20090061120089). References [1] [2] [3] [4] [5] [6] [7] [8]

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