Phase Transfer of Nanoparticles Using an Amphiphilic Ionic Liquid

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Phase Transfer of Nanoparticles Using an Amphiphilic Ionic Liquid Mei Qu, Shuai Chen, Wenbao Ma, Jiangang Chen, Kang Kong, Fengwei Zhang, Huan Li, Zhenshan Hou, and Xian-Ming Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03742 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on November 30, 2016

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Phase transfer of nanoparticles using an amphiphilic ionic liquid Mei Qu,† Shuai Chen,§ Wenbao Ma,‡ Jiangang Chen,§ Kang Kong,‡ Fengwei Zhang,† Huan Li,*†‡ Zhenshan Hou,*‡ Xian-Ming Zhang† †

Institute of Crystalline Materials, Shanxi University, Taiyuan 030006, Shanxi, China.

‡

Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai, 200237, China.

§

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Shanxi, China.

* Corresponding authors. E-mail: [email protected] (H. Li) and [email protected] (Z. Hou). Telephone: +86-351-7016082

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ABSTRACT: Phase transfer of nanoparticles (NPs) from water to organic solvents by an amphiphilic room-temperature ionic liquid (IL) was reported. The geminal IL modified with Pluronic P123 stabilizes a variety of NPs of different size and nature, such as Pd, Au, Ag and SiO2 NPs. Phase transfer of them into hydrophobic environment was realized by raising the temperature and adding salts (such as NaCl, KBr etc.), both of which have a common effect of breaking the hydrogen bonds of the IL with H2O. A more straightforward method of using the organic solvent working as hydrogen bonding donors (such as butyl alcohol) was then proposed. In this case, NaCl was no longer required. To further apply this strategy to the organic solvents which are generally incapable of forming hydrogen bonds (e.g. toluene), a small quantity of benzoic acid was added in organic phase. By forming hydrogen bonds of benzoic acid with the IL, an even more facile approach was provided. FT-IR confirmed hydrogen bonding between them. The phase transfer protocol doesn’t rely on coordination bonding of ligands with a specific metal and is capable of phase transferring of objects with large size and different nature. Thus, it has the potential for wide applications. Keywords: Phase transfer, Nanoparticle, Hydrogen bond, Ionic liquid


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INTRODUCTION Due to the unique size- and shape- dependent physicochemical properties, metal nanoparticles (NPs) continue to attract great interest across diverse areas, such as electronics, biolabelling/biosensors and catalysis.[1-4] The exciting applications of NPs have greatly motivated the research in the synthesis methodologies.[5,6] Among them, those requiring aqueous media are the most convenient because of the ability of H2O to dissolve a variety of metal precursors. On the other hand, well dispersed NPs in nonpolar organic solvents are particularly desirable for various applications, including biomedical diagnose, purification of NPs, selfassembly of NPs and the catalytic process in nonpolar solvent.[7] For these purposes, NPs can be directly prepared in organic media or by being transferred from aqueous phase. The phase transfer approach has the advantage of employing many readily available aqueous synthesis strategies. Thus, directing the NPs from H2O into specific organic solvents with varying physicochemical properties has become an emerging challenge. In this field, a large percent of strategies have been employing thiols, amines and other hydrophobic ligands which form strong coordination bonds with aqueous NPs to pull them across the H2O/oil interface.[8-12] Although this kind of methods is generally effective, it faces some major challenges. The first one is its limited scope of application. Depending on the coordination bonding of NPs with thiols or amines largely restricts its use in phase transfer of other objects with different surface nature. Therefore, these phase transfer techniques have mainly focused on metal (Au, Ag etc) NPs and quantum dots (QDs).[13-16] It would be highly promising to develop a phase transfer approach that is independent of the types of the transferred objects Secondly, the hydrophobic thiols and amines forming strong coordination bonds with


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NPs often block the surface sites, limiting their further applications. Removing them, however, can be complicated. Herein, a novel phase transfer protocol using an amphiphilic geminal room temperature ionic liquid (IL) is presented. Although IL mediated phase transfer procedures have been recently brought forward,[17-20] a copolymer modified IL has barely been used. In this work, Pluronic P123 was incorporated into the skeleton of the IL. The polymer’s amphiphilic nature and ability to stabilize NPs were inherited by the IL. These features made the IL a versatile NPs stabilizer and phase transfer vehicle. The mechanism studies showed that the phase transfer process relied on hydrogen bonding. Upon breaking the hydrogen bonds of IL in H2O or introducing hydrogen bond donors in the organic phase, the IL stabilized NPs can be facilely transferred into the organic phase. And the transferred NPs well maintained their morphology. The approach is more dependent on the phase behavior of the IL in different solvents other than direct coordination bonding of ligands with the metal NPs. It can thus be applied to other nanoparticles of different nature. EXPERIMENTAL SECTION Materials and Methods. The manipulations involving air-sensitive materials were carried out using standard Schlenk line techniques under atmosphere of nitrogen. Benzene, 1-butyl imidazole and SOCl2 were distilled before use. Pluronic P123 (MW = 5800) was purchased from Aldrich without further purification. A Thermo Nicolet Nexus 670 was used for FT–IR characterization. Transmission electron microscopy (TEM) images were recorded on a TECNAI F20 microscope.


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Synthesis of the P123-modified geminal imidazolium dichloride, [BimP123]Cl. The synthesis of [BimP123]Cl referred to a reported two-step procedure (Scheme 1).[21] This first step is the chlorination of P123. Briefly, 20 mmol SOCl2 in 10 ml benzene was slowly added to a mixture of dried P123 (5 mmol), pyridine (10 mmol) and benzene (15 ml) during 0.5 h at room temperature. After refluxing for 8 h, the mixture was filtered and the filtrate was evaporated in vacuum to give a slight yellow chloride (P123-Cl). IR (cm-1): 2970, 1973, 1639, 1455, 1373, 1347, 1297, 1110, 933, 747, 665(C-Cl). 2. In the second step, 1.5 mmol of P123-Cl and 4 mmol of butyl imidazole were charged into a steel vessel. The mixture reacted at 90°C for 72 h under 1.0 MPa N2 with magnetically stirring. After reaction, the product was dissolved in water and then washed thoroughly with diethyl ether. After removing the water, the IL was dissolved in CH2Cl2 and washed with pure water for several times. Then a yellow viscous room temperature IL, [BimP123]Cl was obtained on evaporation of volatile solvent under vacuum at 60 °C. 1HNMR (500 MHz, CDCl3) δ=10.52(s, 2H, CH), 7.73(s, 2H, CH), 7.28(s, 2H, CH), 4.66(t, 4H, CH2), 4.28(t, 4H, CH2), 3.30-3.80(m, EO units), 1.88(m, 4H, CH2), 0.90-1.38(m,CH3, CH2). Synthesis of metal NPs in water. 50 mg of [BimP123]Cl was gradually dissolved in 10 mL of water. 0.02 mmol of Pd(OAc)2 was then added under vigorous stirring until it was completely dissolved. 1 mL of aqueous solution of NaBH4 (0.2 mmol/mL) was added dropwise at 50 oC. The solution was stirred for another 30 min, yielding a black solution. Ag and Au colloidal solutions were prepared following a similar procedure. A typical procedure of phase transfer of NPs using NaCl as an additive. 1 mL of the aqueous solution containing Pd NPs was added 10 mg of NaCl and 1 mL of ethyl acetate. The mixture was heated to 50 oC and stirred for a required period of time. The observation of the black color gradually appearing in organic phase vividly indicated the transfer of Pd NPs.


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RESULTS AND DISCUSSIONS

Scheme 1 Synthesis of [BimP123]Cl. Reaction conditions: a, SOCl2, benzene, pyridine, reflux 8 h; b, 90 oC, 72 h, under N2.

The room temperature ionic liquid (IL) [BimP123]Cl bearing a geminal structure was synthesized following a reported procedure.[21] As shown in scheme 1, P123 (EO20PO70EO20, Mw～5800) first reacted with SOCl2 to give chlorinated P123 (P123-Cl). The latter reacting with butyl imidazole under N2 afforded the target [BimP123]Cl, a yellow viscous liquid at room temperature (Experimental Section). P123 is a copolymer composed of hydrophilic ethylene oxide (EO) and hydrophobic propylene oxide (PO) blocks. This structural feature also endows the IL with amphiphilic nature. P123 was reported to stabilize a variety of metal NPs through steric hindrance.[22-24] We then started out by stabilizing Pd NPs with [BimP123]Cl. Pd(OAc)2 and [BimP123]Cl were first dissolved in H2O under vigorous stirring. The reduction of Pd2+ in the presence of [BimP123]Cl occurred instantaneously upon the addition of NaBH4. The formation of Pd(0) colloidal solution was characterized by a color change from orange to dark brown. As shown in the transmission electron microscopy (TEM) image (Fig. 1), Pd NPs were well dispersed on the grid. The statistics on NPs revealed a narrow size distribution and an average size of 4.0 nm (Fig. 1C).


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Figure 1. Characterization of Pd nanoparticles before (A, B, C) and after (D, E, F) phase transfer. A and D: the photographs, the upper layer is ethyl acetate. B and E, the TEM images. C and F are the size statistics of the Pd NPs in aqueous and organic phase, respectively.

When ethyl acetate was introduced, two immiscible layers were formed. Simply stirring this mixture didn’t trigger the phase transfer of the NPs. However, after adding 10 mg of NaCl into the H2O, the complete transfer of Pd NPs was accomplished at 50oC in 20 min. The process can be easily discerned by the appearance of brownish color in ethyl acetate layer (Fig. 1D). Whether the transferred NPs could retain their morphology is a major concern in phase transfer study, since it is a prerequisite for further applications. Therefore, the NPs isolated from ethyl acetate were also characterized by TEM (Fig. 1E). An average diameter of 4.1 nm was acquired based on statistics. It implied the size of the Pd NPs was well retained after being transferred to a hydrophobic environment, highlighting the capability of [BimP123]Cl as a stabilizer in both solvents. An interesting tendency of Pd NPs to form closely-packed structure was observed in organic phase. Indeed, inducing the NPs to form superstructure by phase transfer has been known.[25-30] Furthermore, it was also noted that the amount of NaCl added in the aqueous phase affected the efficacy of this method. Less than 5 mg of NaCl wouldn’t trigger the transfer, while


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raising the amount to 20 and 40 mg could reduce the transfer time to 7 and 5 min respectively. Higher concentration of salt led to a more efficient phase transfer. We have also screened different salts. NaBr, KBr, KCl were equally effective as NaCl. Na2SO4 and (NH4)2SO4, on the other hand, are more efficient. They took 16 and 9 min to complete the transfer process.

Figure 2. Phase transfer of Au nanoparticles by [BimP123]Cl. TEM images before (A) and after (B) phase transfer. The insets are the photographs of the vessel. (C) is the UV-Vis spectra of H2O layer before (dot) and after phase transfer (solid).

Phase transfer of Au NPs was also carried out following a similar procedure. The results were equally successful. The complete transfer of Au NPs was confirmed by the UV-vis analysis of aqueous phase before and after the transfer process (Fig. 2). The peak at 532nm was assigned to the characteristic surface plasmon resonance absorption of Au NPs.[29] No such absorption was seen after phase transfer, indicating the absence of Au NPs in H2O layer. Size statistics gave an average diameter of 5.1 nm of the Au NPs in the ethyl acetate, agreeing well with that in H2O (4.9 nm). This strategy was also applicable to Ag NPs (Fig. 4). Likewise, successful phase transfer was achieved. What is noteworthy is that Ag NPs had an average size of c.a. 30 nm. It has been known that phase transfer of large NPs is challenging.[28,

32]

The results here

demonstrated [BimP123]Cl was also capable of transferring relatively large metal NPs.


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Figure 3. Phase transfer of Ag nanoparticles by [BimP123]Cl. TEM images before (A) and after (B) phase transfer. The insets are the photographs of the vessel.

To test the possibility of transferring NPs of different nature by [BimP123]Cl, this protocol was further applied for SiO2 NPs. To do so, SiO2 NPs were first synthesized by the Stöber method,[33] which were then dispersed in the aqueous solution of [BimP123]Cl under vigorous stirring to form a SiO2-IL nanocomposite. Next, to trigger the transfer process after adding ethyl acetate and NaCl, the temperature was raised to 50 oC. A suspension of SiO2 in ethyl acetate was then obtained after 20 min (Fig. 4). The results here indicate our approach is not limited to metal NPs. It has the potential for other nano objects too. What is also worth noting is that part of SiO2 NPs in ethyl acetate are as large as 500 nm. To our knowledge, this is the biggest size of objects that has been reported in phase transfer study, highlighting the superior transfer ability of [BimP123]Cl.

Figure 4. Phase transfer of SiO2 nanoparticles by [BimP123]Cl. TEM images before (A) and after (B) phase transfer. The insets are the photographs of the vessel.


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Our further effort is devoted to the phase transfer mechanism study. First, a group of control experiments using analogues of [BimP123]Cl were carried out. A similar geminal IL modified with hydrophilic polyethylene glycol (Mw～6000, composed of only EO units) was synthesized as a phase transfer vehicle. However, Pd NPs cannot be transferred by it even when the temperature was as high as 80oC. Further attempt to add excess NaCl resulted in the formation of a thin black hybrid film at the interface between ethyl acetate and H2O, leaving both phases clear. Without the hydrophobic PO block, the IL-NPs composite cannot be dispersed in ethyl acetate. In other words, the amphiphilic nature of [BimP123]Cl is a key factor in phase transfer. As another control experiment, P123 was used as a phase transfer agent for Pd NPs. Unfortunately, flocculation was always observed during the transfer process, no stable organosol was obtained. It meant that phase transfer with [BimP123]Cl benefited from the introduction of butyl imidazole moieties. One possible explanation is that the IL provided additional electrostatic interaction to stabilize the system, which has been normally seen in IL-NPs composites.[34]

An analysis of conditions required for the successful phase transfer offered further insight. At room temperature, the transfer was sluggish, taking a few hours. Elevating the temperature much facilitated the process. For nonionic polyether polymer, its solubility in H2O usually decreases with increasing temperature. The explanation is generally accepted to be a decrease in the hydration of the EO headgroups and a rapid increase in attraction between headgroups on adjacent micelles.[35] A model by Kjellander and Florin[36] pointed out the water forms an ordered structure around the EO headgroups. When temperature rises, the hydrogen bonds between the EO units and H2O, as well as this ordered structure will be destroyed.[35] On the other hand, the dissolution of inorganic ions like Na+ in H2O requires hydration of these ions, which could also disrupt the hydrogen bonds between the EO headgroups and H2O, reducing the solubility of the


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polymer. This effect is known as salting-out effect.[38, 39] Hence, the addition of salt and heating have played a common role in breaking the hydrogen bonds between H2O and [BimP123]Cl. The effect led to a decrease of the solubility of [BimP123]Cl in H2O so that the hydrophobic part of [BimP123]Cl could induce the IL-NPs nanocomposite to cross the H2O/oil interface. This could also explain why Na2SO4 and (NH4)2SO4 were more efficient: these salts have stronger salting-out effect.[38] Since breaking hydrogen bonds in aqueous phase is critical in such a scenario, it was proposed that direct establishing hydrogen bonds with [BimP123]Cl in organic layer might also work. To test this hypothesis, alcohol with its hydroxyl groups acting as hydrogen bond donors[40] were employed as the organic phase. The results showed that Pd NPs were smoothly transferred into 1-butanol in only 5 min (Table 1), significantly faster than the above results. Notably, no NaCl was required in this case. Lauryl alcohol was less efficient, it took c.a. 40 min to complete the transfer. The difference is explained by the fact that although the volume of two alcohols was the same, the amount of the hydroxyl groups of butanol was more than that of lauryl alcohol. Phase transfer of NPs into alcohol is apparently more efficient. Meanwhile, it also raised the question of how to apply this modified strategy in phase transfer of NPs into solvents which themselves are inert for forming hydrogen bonds. Table 1. Phase transfer of Pd nanoparticles to different alcohols a

a

Entry

Alcohol

Time/min

1

Butanol

5

2

Hexanol

9

3

Octanol

24

4

Lauryl alcohol

40

Conditions:1 mL Pd hydrosol, 1 mL normal alcohol, temperature 50oC.


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To circumvent the problem, another experiment was carried out in toluene. In this case, 20 min was required to complete the transfer process of Pd NPs according to the above method. However, when 3 mg of benzoic acid was added in the organic phase, it was surprisingly found that Pd NPs were fast transferred into toluene in c.a. 1 min. A small quantity of benzoic acid made phase transfer of NPs into toluene much more efficient.

Scheme 2. Benzoic acid molecules in solid state and bonded with the IL.

Benzoic acid is known to crystallize as cyclic dimer. In this pattern, benzoic acid molecules form strong H bonds between the carbonyl of one molecule with acidic hydrogen of another benzoic acid (Scheme 2). The infrared absorptions of relevant groups are very sensitive to hydrogen bonding.[41, 42] The carbonyl in dimer typically shows an absorption at 1687 cm-1. Its counterpart, hydroxyl group is expected to show a broad peak in the range of 2800-3000cm-1 as a result of O-H stretching. However, the unbonded groups are expected to show absorptions at


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higher frequencies. For example, the non-hydrogen bonded O-H stretching usually gives a single sharp peak at 3500-3600 cm-1.[40]

Figure 5. FT-IR spectra of the mixture separated from toluene layer after phase transfer by [BimP123]Cl with different amount of benzoic acid. (A), 1 mg. (B), 2 mg. (C) 3mg. (D) control experiment, 3 mg benzoic acid.

Based on these knowledge, FT-IR was used to probe the interaction of benzoic acid with the IL. When 3 mg of benzoic acid was analyzed by FT-IR, one peak at 1687 cm-1 was observed for ν(C=O) (Fig. 5D), which was related to the hydrogen bonded carbonyl in the dimer. However, after a phase transfer process mediated by the same amount of benzoic acid, the carbonyl absorption of the mixture from the toluene layer splitted into two peaks. One at 1687 cm-1 was ascribed to ν(C=O) in the dimer. The other one centered at 1719 cm-1 was due to the unbounded carbonyl group (Fig. 5C).[40-42] It meant that upon interaction with the IL, part of benzoic acid molecules were no longer in the form of dimer. As we further reduced the amount of benzoic acid, the unbonded ν(C=O) peak took up an even larger proportion. Meanwhile, ν(O－H) of the COOH constantly showed a broad absorption in the range of 2800 cm-1 to 3000 cm-1 (Fig. 5). No sharp peak at c.a. 3500 cm-1 associated with unbonded OH groups was observed. The results


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indicated a new hydrogen bond between benzoic acid and [BimP123]Cl was formed, as illustrated in Scheme 2. It was this new bonding mode that led to the efficient phase transfer.

CONCLUSIONS We have designed and synthesized a P123-modified amphiphilic IL as an effective stabilizer of different NPs in aqueous phase. Phase transfer of NPs into organic solvents was accomplished by manipulating the solubility of the IL through hydrogen bonding. Two approaches are available. One is to force the IL-stabilized NPs into the organic solvent by breaking the hydrogen bonds of the IL with H2O; the other one, which is more straightforward, is to introduce a hydrogen bond donor in organic layer to induce the phase transfer. Neither of the methods relies on the coordination bonding of ligands to a specific metal, which made this strategy highly versatile for phase transfer of different NPs. More applications of the strategies are expected in the field of self-assembly and catalysis in future. AUTHOR INFORMATION Corresponding Author *Huan Li: e-mail, [email protected], tel. +86-351-7016082. *Zhenshan Hou: e-mail, [email protected] Notes The authors declare no competing financial interest ACKNOWLEDGMENT


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We thank the National Natural Science Foundation of China (21373082, 21503123), Innovation Program of Shanghai Municipal Education Commission (15ZZ031), the Foundation of State Key Laboratory of Coal Conversion (Grant No. J16-17-612) and Sanjin Scholar and Starting fund of Shanxi University for the financial support. REFERENCE 1 Talapin, D. V.; Lee, J.-S.; Kovalenko, M. V.; Shevchenko, E. V. Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications. Chem. Rev. 2010, 110, 389-458. 2 Nel, A. E.; Mädler, L.; Velegol, D.; Xia, T.; Hoek, E. M. V.; Somasundaran, P.; Klaessig, F.; Castranova, V.; Thompson, M. Understanding Biophysicochemical Interactions at the Nano– Bio Interface. Nat. Mater. 2009, 8, 543-557. 3 Yan, N.; Xiao, C.; Kou, Y. Transition Metal Nanoparticle Catalysis in Green Solvents. Coord. Chem. Rev. 2010, 254, 1179-1218. 4 Stark, W. J.; Stoessel, P. R.; Wohlleben, W.; Hafner, A. Industrial Applications of Nanoparticles. Chem. Soc. Rev. 2015, 44, 5793-5805. 5 Kim, B. H.; Hackett, M.-J.; Park, J.; Hyeon, T. Synthesis, Characterization, and Application of Ultrasmall Nanoparticles. Chem. Rev. 2010, 110, 389-458. 6 Duan, H.; Wang, D.; Li, Y. Green Chemistry for Nanoparticle Synthesis. Chem. Soc. Rev. 2015, 44, 5778-5792. 7 Chen, G.; Roy, I.; Yang, C.; Prasad, P. N. Nanochemistry and Nanomedicine for Nanoparticlebased Diagnostics and Therapy. Chem. Rev. 2016, 116, 2826-2885. 8

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22 Wang X.; Kawanami H.; Islam N. M.; Chattergee M.; Yokoyama T.; Ikushima Y. Amphiphilic Block Copolymer-stabilized Gold Nanoparticles for Aerobic Oxidation of Alcohols in Aqueous Solution. Chem. Commun. 2008, 4442-4444. 23 Huang, X.; Wang, X.; Tan, M.; Zou, X.; Ding, W.; Lu, X. Selective Oxidation of Alcohols on P123-Stabilized Au–Ag Alloy Nanoparticles in Aqueous Solution with Molecular Oxygen. Appl. Catal. A-Gen. 2013, 467, 407-413. 24 Dun, R.; Wang, X.; Tan, M.; Huang, Z.; Huang, X.; Ding, W. Quantitative Aerobic Oxidation of Primary Benzylic Alcohols to Aldehydes Catalyzed by Highly Efficient and Recyclable P123-Stabilized Pd Nanoclusters in Acidic Aqueous Solution. ACS Catal. 2013, 3, 3063-3066. 25 Zhao, S.-Y.; Chen, S.-H.; Wang, S.-Y.; Li, D.-G.; Ma, H.-Y. Preparation, Phase Transfer, and Self-Assembled Monolayers of Cubic Pt Nanoparticles. Langmuir 2002, 18, 3315-3318. 26 Yang, J.; Deivaraj, T. C.; Too, H.-P.; Lee, J. Y. An Alternative Phase-Transfer Method of Preparing Alkylamine-Stabilized Platinum Nanoparticles. J. Phys. Chem. B 2004, 108, 21812185. 27 Serrano-Montes, A. B.; De Aberasturi, D. J.; Langer, J.; Giner-Casares, J. J.; Scarabelli, L.; Herrero, A.; Liz-Marzán, L. M. A General Method for Solvent Exchange of Plasmonic Nanoparticles and Self-Assembly into SERS-Active Monolayers. Langmuir 2015, 31, 92059213. 28 Malassis, L.; Jishkariani, D.; Murray, C. B.; Donnio, B. Dendronization-Induced PhaseTransfer, Stabilization and Self-assembly of Large Colloidal Au Nanoparticles. Nanoscale 2016, 8, 13192-13198. 29 Shen, Y.; Lei, D.; Tan, J.; Feng, Y.; Zhang, B.; Li, Y.; Dong, H.; Hu, W.; Feng, W. Topological Structural Transformations of Nanoparticle Self-Assemblies Mediated by Phase Transfer and Their Application as Organic–Inorganic Hybrid Photodetectors. ACS Appl. Mater. Interfaces. 2013, 5, 12254-12261. 30 Dong, J. ; Li, J. ; Zhou, J. Interfacial and phase transfer behaviors of polymer brush grafted amphiphilic nanoparticles a computer simulation. Langmuir, 2014, 30, 5599-5608. 31 Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment. J. Phys. Chem. 2003, 107, 668-677.


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Phase Transfer of Nanoparticles Using an Amphiphilic Ionic Liquid

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