Cyclodextrin as a capturing agent for redundant surfactants on Ag nanoparticle surface in phase transfer process

Colloids and Surfaces A: Physicochem. Eng. Aspects 290 (2006) 143–149

Cyclodextrin as a capturing agent for redundant surfactants on Ag nanoparticle surface in phase transfer process Yang Yang, Shuman Liu, Keisaku Kimura ∗ Department of Material Science, Graduate School of Science, University of Hyogo, 3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan Received 8 February 2006; received in revised form 29 April 2006; accepted 12 May 2006 Available online 17 May 2006

Abstract High-yield phase transfer of hydrophilic mercaptosuccinic acid (MSA)-modified Ag nanoparticles into chloroform is readily attained using cetyltrimethylammonium bromide (CTAB) through electrostatic interaction. Increasing CTAB amount to a certain degree has achieved nearly complete phase transfer due to the sufficient formation of stoichiometric ion-pairs on particle surface. However, at high CTAB concentration, some unbonded CTA+ cations will be physically adsorbed on particle surface and enter chloroform layer simultaneously, which cannot be removed by simple water washing or centrifugation. By using ␤-cyclodextrin (CD) as a capturing agent, this portion of CTA+ cations can be adequately removed due to the possible inclusion function of CD. Upon removal of the unbonded CTAB, the monolayer formation of phase-transferred Ag nanoparticles at air–water interface presents improved two-dimensional (2D) orderliness owing to the more effective interdigitation among adjacent particles. © 2006 Elsevier B.V. All rights reserved. Keywords: Phase transfer; Ag nanoparticle; Cyclodextrin; Surfactant; Self-assembly

1. Introduction Noble metal nanoparticles have attracted increasing interest due to their size- and shape-dependent optical and electronic properties, which are expected to have wide applications in optics, microelectronics, sensors, catalysis, and so on [1–4]. Advances in material synthesis have allowed for the fine-tuning of these properties derived from effective control on the size and shape of the metal nanoparticles [5–8]. Recently, these nanoparticles have further been used as building blocks to assembly into well-defined two- and three-dimensional (2D and 3D) superlattices that exhibit novel collective properties different from individual nanoparticles and bulk materials [9–16]. Plentiful procedures have been developed for preparation of noble metal nanoparticles soluble either in aqueous phase or in organic phase up to date [17–21]. For extending the potential applications of the nanoparticles in different circumstances, it is often needed to direct nanoparticles into various liquid phases, i.e. from an aqueous phase into an organic phase or oppositely. In regard to phase transfer of Au or Ag nanoparticles from water

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into organic phase, various strategies have been achieved based on cation–anion electrostatic interaction [22–25], covalent interaction [26,27], host–guest inclusion [28–30], ionic liquid [31], and so on. The carboxylate-modified metal nanoparticle could be regarded as a common water-soluble molecular compound due to the presence of abundant hydrophilic carboxylate on the particle surface. Therefore, its transfer into organic phase can be easily attained through the formation of stoichiometric ion-pairs between carboxylate and cation surfactant possessing suitable alkyl chain. Considerable work describing transfer of metal particles into organic phase based on this mechanism has been reported [22–25]. However, when we processed the transfer of mercaptosuccinic acid (MSA)-modified metal nanoparticles by using cation surfactants as the second modification layers, we found that the high-yield phase transfer could only be achieved by excess surfactant beyond stoichiometric ratio for ion-pair formation. In this work, we report that the above-mentioned results are largely improved by using a cetyltrimethylammonium bromide (CTAB), which induces nearly complete phase transfer of MSA-modified Ag nanoparticles from aqueous phase to chloroform. Also we reveal the existence of unbonded CTA+ cations adsorbed on particle surface on this condition. More specifically, we develop a facile and efficient approach to remove this portion

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of CTA+ cations by using ␤-cyclodextrin (CD) as a capturing agent. The contribution of fully removal of unbonded CTAB to the ordered self-assembly of these phase-transferred hydrophobic Ag nanoparticles at air–water interface is also presented. 2. Experimental 2.1. Chemicals Reagent-grade silver nitrate (AgNO3 ), sodium borohydride (NaBH4 ), mercaptosuccinic acid (MSA), cetyltrimethylammonium bromide (CTAB), ␤-cyclodextrin (CD), and the organic solvents used in this experiment were purchased from Wako Pure Chemicals without further purification. Distilled water was produced by an Advantec GS-200 automatic water-distillation supplier. 2.2. Methods A procedure similar to that previously reported by our group was used to prepare hydrophilic Ag nanoparticles [32]. Briefly, 120 mL of freshly prepared 0.2 M NaBH4 (0.91 g) aqueous solution was added to a 500 mL water–methanol mixture containing 2.5 mmol of AgNO3 (0.43 g) and 5.0 mmol of MSA (0.75 g) through a syringe under vigorous stirring and ultrasonic irradiation. The whole reaction process was carried out in an ice bath and maintained for 2 h. Then the flocculent darkbrown precipitate was collected by decanting the supernatant solution. Subsequently, the product was washed three times with a 20% water/methanol solution and two times with pure ethanol by repeating the suspension and centrifugation process to remove the unbound MSA or MSA-related complexes. Finally, the MSA-modified Ag nanoparticles (abbreviated as AgMSA) were dried under vacuum and about 0.5 g of black powder was obtained. For the phase transfer, 2.0 mg of Ag nanoparticles was dissolved in 5 mL of water with pH value adjusted to 11. On this pH condition, all the residual carboxylate acid on particle surface can be changed into dissociated state of the carboxylate group, which is beneficial to the ion-pair formation through electrostatic interaction. This solution was mixed with 5 mL of a chloroform solution containing 7.3 mg of CTAB (2.0 × 10−5 mol). After vigorous shaking, Ag hydrosol-chloroform biphasic solution was left quiet until two clearly separated layers formed. Because Ag nanoparticles could be oxidized by alkyl halide during long-time storage, we separated the lower chloroform phase, evaporated the solvent, and obtained a dark-brown powder (abbreviated as Ag-MSA-CTA). This powder was re-dispersed in 10 mL of toluene to form a clear dark-brown solution for the following treatment. To investigate the effects of surfactant concentration on the yield of phase transfer, various CTAB amounts, 0.5 × 10−5 , 1.0 × 10−5 , 2.0 × 10−5 and 4.0 × 10−5 mol, were employed. For the removal of excess CTAB, we attempted three methods. Method A: on accounts of the solubility difference of free CTAB in both toluene and water, Ag-MSA-CTA toluene solution

was mixed with 20 mL of distilled water by vigorously shaking. After the toluene layer was equilibrated with the water layer, the latter was substituted by fresh distilled water. This process was repeated several times. Method B: Ag-MSA-CTA toluene solution was centrifuged at 10,000 rpm for 1 h at 20 ◦ C. Method C: Ag-MSA-CTA toluene solution was mixed with 20 mL of an aqueous solution containing 10 mg of CD by vigorously shaking. For self-assembly of Ag nanoparticles, toluene solutions before and after treatment were put on water surface, respectively. After the evaporation of toluene, a particulate Ag film was formed at air–water interface. 2.3. Instruments Transmission electron microscopic (TEM) images and corresponding selected area electron diffraction (SAED) patterns were obtained on a Hitachi H-8100 microscope operated at 200 kV. The mean particle size and size distribution of the modified Ag nanoparticles were estimated by measuring the diameters of at least 200 individual particles located in representative regions of the pictures. The film samples formed at air–water interface for TEM observation were scooped on a carbon-coated copper grid and dried in vacuum. Fourier transform infrared (FT-IR) spectra were recorded on a HORIBA FT-210 spectrophotometer. UV–vis absorption spectra were measured with a Hitachi U-4100 spectrophotometer. X-ray diffraction (XRD) measurement was performed on a Rigaku RINT/DMAX-2000 diffractometer using Cu K␣1 radiation (λ = 0.154056 nm) operated at 40 kV and 20 mA. 3. Results and discussion The aqueous Ag-MSA sample before phase transfer exhibits an absorption peak at 411 nm (curve 1 in Fig. 1), which is due to the well-defined surface plasmon band of Ag nanoparticles. By simple shaking of the Ag hydrosol–chloroform biphasic solu-

Fig. 1. UV–vis absorption spectra of Ag nanoparticles in water before phase transfer (1), and in chloroform layer (2) and water layer (3) after phase transfer. The corresponding photographs of Ag nanoparticles in biphase solution before and after phase transfer are shown in the inset.

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Fig. 2. TEM images of Ag-MSA sample from aqueous phase before phase transfer (a) and Ag-MSA-CTA sample from chloroform solution after phase transfer (c). Histograms of particle size distribution of Ag-MSA sample (b) and Ag-MSA-CTA sample (d). Inset of (a) is its corresponding SAED pattern.

tions with 2.0 × 10−5 mol of CTAB, the deep brown color in aqueous solution was quickly transferred into the chloroform phase. Curves 2 and 3 in Fig. 1 show the absorption spectra of Ag nanoparticles in chloroform and water phase after phase transfer. It is clear that the plasmon absorption band almost vanishes in the aqueous phase while it appears in the chloroform phase, which indicates a high efficiency of the phase transfer. The nearly complete phase transfer of Ag nanoparticles can be seen directly from the photograph shown in the inset, i.e. the color of aqueous phase has faded after phase transfer. It is also noticeable that the full width at half-maximum (FWHM) of the plasmon band of Ag nanoparticles in chloroform is 0.57 eV, narrower than that in aqueous solution (0.86 eV). The relative narrow absorption band indicates a narrower size distribution in chloroform phase. Since we noticed that the intensity of the plasmon absorption band of the transferred Ag-MSA-CTA sample in chloroform phase (curve b) decreases a little compared to the original one before phase transfer, and additionally, a few black aggregates were found at the water–chloroform interface, it is suggested that the narrowing of size distribution is due to size selectivity of the extraction process of Ag nanoparticles from water to oil. Fig. 2a and b show, respectively, the TEM image and the corresponding size histogram of the hydrophilic Ag-MSA sample dispersed in water before phase transfer. It can be seen that most of the nanoparticles are spherical with a rather wide size distribution from 2 to 11 nm. The inset of Fig. 2a shows the SAED pattern of this sample. The observed diffraction rings could be well indexed to (1 1 1), (2 0 0), (2 2 0), and (3 1 1) planes of Ag nanoparticles with a face-centered cubic structure. The TEM image and corresponding size histogram of Ag-MSA-CTA

sample dispersed in chloroform after phase transfer is exhibited in Fig. 2c and d, respectively. An average size of ∼7.0 nm with narrower size distribution could be clearly seen. Especially, the number of particles larger than 8 nm in diameter obviously decreased, further indicating our phase transfer is a size-selective extraction process, consistent with the results of absorption spectra. In order to understand the change of particle surface composition in detail, FT-IR spectroscopy was performed and the results are shown in Fig. 3. For Ag-MSA sample (curve 1), two strong peaks at 1579 and 1404 cm−1 are attributed to the asymmetric and symmetric stretching vibration of carboxylate ions, respectively. This result confirms the existence of MSA on the particle surface in the form of carboxylate anions, which leads to the hydrophilicity of MSA-modified Ag nanoparticles. FT-IR spectra of Ag-MSA-CTA sample and pure crystalline CTAB are shown in curves 2 and 3, respectively. In the spectrum of Ag-MSA-CTA sample, the characteristic CH2 vibration at 723, 1460, 2849, and 2918 cm−1 as well as CH3 vibration at 2954 cm−1 correspond well to those of pure CTAB. Meanwhile, the peaks of carboxylate ions from MSA are nearly overlapped by those from CTAB. These results indicate that CTAB molecules have covered the surface of the particles after the phase transfer process. It has been suggested that the peak position of the symmetric and asymmetric stretching vibrations of CH2 is sensitive to the ordering of the alkyl chains and greater incidence of gauche defects could be indicated from the higher energies of the CH2 stretching vibrations [33,34]. In our case of the Ag-MSA-CTA sample, two peaks corresponding to vibrations of CH2 are located at 2849 and 2918 cm−1 , respec-

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Fig. 3. FT-IR spectra of Ag-MSA sample (1); Ag-MSA-CTA sample (2); and free CTAB molecules (3).

tively, almost the same as those of crystalline CTAB. Therefore, even if CTA+ cations surround the particle surfaces through the electrostatic interaction with carboxylate ions, they could be assumed to remain highly ordered, all-trans alkyl chains similar to crystalline state. Or, some unbonded CTAB molecules possibly existing in this sample could also contribute to the unchanged positions of CH2 vibrations. For examining at which concentration of CTAB the possibly complete phase-transfer of Ag nanoparticles from aqueous solution to chloroform could be achieved, various CTAB amounts were added to chloroform phase. Fig. 4 compares the absorption spectra of Ag nanoparticles in aqueous phase and chloroform phase after phase transfer with different amounts of CTAB. Obviously, when the amount of CTAB is gradually increased from 0.5 × 10−5 , 1.0 × 10−5 , to 2.0 × 10−5 mol in 5 mL of chloroform, the absorbance in chloroform increases at the expense of that in water, as shown in curves 1–3 of Fig. 4a and b. Their corresponding yield of transference is calculated to be 59, 76, and 92%, respectively. This result agrees with the mechanism that the more ion-pairs forms, the more nanoparticles will be pulled into organic phase by hydrophobic forces originated from surfactant coverage. When the amount of CTAB was added up to 2.0 × 10−5 mol, almost complete phase transfer of the Ag-MSA nanoparticles occurred since the plasmon absorption in aqueous solution almost vanished. Further increasing CTAB amount to 4.0 × 10−5 mol will significantly reduce the efficiency of phase transfer (curves 4 in Fig. 4a and b) as a result of the appreciable formation of the spherical micelle in aqueous phase, which could also be reflected from the unusually high extinction at shorter wavelength in curve 4 of Fig. 4a. When surfactant concentration changes from the dilute to the critical micelle concentration (cmc), CTAB are easy to form organised aggregates or micelles

Fig. 4. UV–vis absorption spectra of Ag nanoparticles in the aqueous phase (a) and chloroform phase (b) at different CTAB amounts: (1) 0.5 × 10−5 mol; (2) 1.0 × 10−5 mol; (3) 2.0 × 10−5 mol; and (4) 4.0 × 10−5 mol.

and cannot act as a normal solute. As the consumption of CTAB for the formation of micelle, only part of carboxylate ions can bond with CTA+ cations to obtain hydrophobic force, so the efficiency of phase transfer reduces dramatically. Herein, high-yield phase transfer of this system can be only attained with surfactant concentration increased to a certain extent. To substantially investigate the components in chloroform phase after phase transfer, the organic solution was spread on silicon substrate and the obtained film after solvent evaporation was inspected by XRD. Data 1, 2, and 3 in Fig. 5 show the XRD patterns of the samples in chloroform transferred by CTAB at the amount of 0.5 × 10−5 , 1.0 × 10−5 , and 2.0 × 10−5 mol, respectively. Comparing these data with the corresponding absorption curves, we find an interesting result. With the lowest CTAB amount (0.5 × 10−5 mol), the absorption band of chloroform phase is weak, but the XRD pattern shows pure diffraction peaks from Ag nanoparticles. When CTAB amount is increased to 1.0 × 10−5 mol, the absorption band of chloroform phase increases, but that of aqueous phase dose not vanish, indicating that some Ag particles still remain in water with hydrophilic surface. However, it can be found that some

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Fig. 5. XRD patterns of the products in chloroform transferred by CTAB at the amount of 0.5 × 10−5 mol (1); 1.0 × 10−5 mol (2); and 2.0 × 10−5 mol (3); and of pure CTAB crystallites (4). Products in chloroform layer transferred by 2.0 × 10−5 mol of CTAB were re-dispersed in toluene and treated by repetitive water washing (method A), centrifugation (method B), and washing with CD solution (method C), respectively. Data 5–7 show XRD patterns of the products after treatment by methods A, B, and C, respectively.

new peaks appear at the low-angle range of its corresponding XRD pattern. These peaks could be well assignable to those of pure crystalline CTAB (datum 4) though the intensity of some peaks changes a little. Besides, these diffractions are only possible to come from “bulk” crystals or thin films of CTAB. This comparison indicates that not all the CTA+ cations can bond with carboxylate anions on particle surfaces to form stoichiometric ion-pairs by Coulomb attraction. A proportion of CTA+ cations are assumed to consume through physical adsorption around the bonded organic chains when the concentration of CTAB is high. These CTAB molecules would transfer into chloroform layer with the hydrophobic Ag-MSA-CTA sample together. Once the as-transferred Ag colloid was deposited, the unbonded CTAB molecules would also be crystallized or already ordered in adsorption state, leading to the appearance of XRD peaks originating from CTAB. As a result, for achieving complete phase transfer of this system, CTAB molecules more than stoichimetrical ratio are required. For obtaining phase-transferred Ag nanoparticles with welldefined surface states, it is crucial to remove the unbonded CTAB molecules from the Ag-MSA-CTA sample after phase transfer, which will benefit for the assembly of Ag nanoparticles or their further applications. Datum 5 in Fig. 5 exhibits the XRD pattern of the sample dried from toluene solution after treatment by water washing (method A). From persistent strong diffraction peaks of CTAB, it can be considered that repeating washing by

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water is feeble to separate the unbonded CTAB from Ag-MSACTA toluene solution. Given that the unbonded CTAB molecules are likely to partially interdigitate with the bonded ones by its long hydrocarbon chains, they are not easy to be continuously transferred to water phase upon diffusion process. It is also found that after water washing for several times, the XRD peaks of Ag nanoparticles become narrower, suggesting the formation of some particles in large size due to the Ostwald ripening or particle aggregation [35]. Datum 6 of Fig. 5 shows the XRD of the product treated by centrifugation (method B). Likewise, unbonded CTAB molecules cannot be effectively removed from the Ag-MSA-CTA sample by this method. Obviously, the above two methods cannot effect for this system, demonstrating that the physical absorption of the unbonded CTAB molecules with the bonded ones is not very weak. XRD pattern of the sample treated by CD aqueous solution (method C) is shown in datum 7 of Fig. 5, in which it is interesting to note that only diffraction peaks from Ag nanoparticles remain and all the peaks from CTAB once emerging in datum 2 of the original phase transferred sample disappear. It is known that cyclodextrins are cyclic oligosaccharides consisting of a certain number of glucopyranose units. Herein, ␤cyclodextrin (CD) is the one with seven repeating units. Like its homologous series, CD is composed of hydrophobic cavities that can form inclusion compounds with various organic molecules with hydrophobic groups. Thus, CD has been widely used to increase the water-solubility of many organic molecules [36,37]. Recently, CD has also been reported to work as a phase-transfer catalyst for some nanoparticles [28–30]. In our case, when the Ag-MSA-CTA toluene solution is mixed with CD aqueous solution by vigorously shaking, the CTA+ cations could be included by CD due to the interaction between the hydrocarbon tail of the former and the hydrophobic cavity of the latter. If the included CTA+ cations belong to those bonded with MSA on particle surfaces, some particles are possible to be pulled into water due to the hydrophilic outer cavity of CD. However, in our system no Ag nanoparticles could be detected in the water layer after CD treatment, which might be related to the big size and MSA-CTA bilayer surface structure of our Ag-MSA-CTA sample. Note that CD inclusion compound is very unstable in organic media, especially in solvents of low polarity such as toluene, thus, CD cannot subsist in the toluene solution through inclusion of bonded CTA+ on the particle surface, either [28]. Therefore, unbonded CTA+ is preferential to be captured, unfastened from particle surface, and pulled into water phase under the assistance of CD. Air–liquid interface is very useful to tune colloidal interactions for the formation of self-assembled films and permit them transferred onto solid surfaces with a high degree of fidelity. Thus, proof of the complete removal of unbonded CTAB could also be given by the observation of the ordering 2D self-assembly of Ag-MSA-CTA nanoparticles at air–water interface. The TEM image of the formed monolayer of Ag-MSA-CTA sample before CD treatment is shown in Fig. 6a. It can be seen that the AgMSA-CTA nanoparticles organize into 2D arrays at air–aqueous interface when toluene is volatilized. The particles in the array have the average size of ∼7.2 nm with a relatively narrow size distribution. However, their assembly is only locally orderly and

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CTAB molecules to form interdigitation structure. For our AgMSA-CTA sample without CD washing, the unbonded CTAB adsorbed on particle surface would also deposit together with Ag nanoparticles on aqueous surface when toluene is evaporated. The formed CTAB molecules are likely to be out-of-order distributed among Ag nanoparticles. Since unbonded CTAB are absent from directional interactions with the carboxylate anions, their hydrocarbon chains couldn’t be fully extended in radiate form. Some of them might randomly wrap on particle surface, thus the interdigitation of alkyl chains (MSACTA) among particle surfaces would be greatly disturbed. As a result, the balance between hard-sphere repulsion and van der Waals attraction forces required for the orderly assembly of Ag nanoparticles are affected. On the other hand, adsorbed CTA+ cations contribute directly toward the surface charge of the colloid and affect their wetting behavior at the water surface, which is another possible reason to damage the 2D superlattice structure. 4. Conclusion

Fig. 6. TEM images of a monolayer of Ag-MSA-CTA nanoparticles formed at air–aqueous interface upon volatizing toluene before (a) and after (b) CD treatment.

For achieving nearly complete phase transfer of MSAmodified Ag nanoparticles from water into chloroform on the basis of anion-cation electrostatic interaction, some unbonded CTAB would be inevitably moved to chloroform phase by physical adsorption on particle surface. This work provides a very simple method to remove the redundant surfactants from phasetransferred sample by using CD as a capturing agent, which is important as a purification process for the practical applications. This process is also beneficial to the formation of 2D long-ranged orderly structure of phase-transferred Ag nanoparticles assembled at air–water interface. Acknowledgments

packing is rather loose. Fig. 6b illustrates the typical TEM image of the self-assembly of Ag nanoparticles at the air–aqueous interface after CD treatment. In comparison with Fig. 6a, the arrangement of Ag nanoparticles becomes more orderly. The assembled monolayer consists of many long-range uniform domains, where hexagonal close-packed structures composed of Ag nanoparticles can be clearly confirmed. There is a very uniform distance between Ag nanoparticles, whose separation is due to the presence of organic modification layer adsorbed on the particle surface. Evidently, the removal of unbonded CTAB molecules in the Ag-MSA-CTA sample by CD washing noticeably improves the regularity of the self-assembly of Ag nanoparticles. It has been suggested that the degree of monolayer formation and 2D order of nanoparticle self-assembly are strongly dependent on the amount and type of electrolyte adsorbed on particle surface besides particle size distribution [38]. Previous investigation of the influence of excess unbonded CTAB on the self-assembly of CTAB-capped Au nanoparticles at the air–mica interface revealed that the Au nanoparticles were selectively organized on the mica surface [39]. The unbonded CTAB molecules were firstly deposited on the mica to form a monolayer upon solvent evaporation. Then the Au nanoparticles were anchored on it with alkyl chains inserting into underlying

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