Phase transfer and its applications in nanotechnology

View Article Online / Journal Homepage / Table of Contents for this issue

CRITICAL REVIEW

www.rsc.org/csr | Chemical Society Reviews

Phase transfer and its applications in nanotechnology Jun Yang,wa Jim Yang Leeb and Jackie Y. Ying*a

Published on 01 December 2010. Downloaded by Anna University on 18/10/2016 10:18:57.

Received 14th April 2010 DOI: 10.1039/b916790k As nanoparticle syntheses in aqueous and organic systems have their own merits and drawbacks, specific applications may call for the transfer of newly formed nanoparticles from a polar to a non-polar environment (or vice versa) after synthesis. This critical review focuses on the application of phase transfer in nanoparticle synthesis, and features core–shell structures in bimetallic nanoparticles, replacement reactions in organic media, and catalytic properties of various nanostructures. It also describes the reversible organic and aqueous phase transfer of semiconductor and metallic nanoparticles for biological applications, and the use of phase transfer in depositing noble metals on semiconductor nanoparticles (258 references).

1. Introduction Nanostructured materials are of great interest due to their size- and shape-dependent physical and chemical properties.1–5 The confinement or collective oscillation in the conduction band of electrons in a nanocrystal enables the manipulation of electronic, optical, magnetic and catalytic properties of a solid material.6–10 Nanocrystals have been investigated for quantum size effects, including quantized excitation,11–13 Coulomb blockade,14,15 metal–insulator transition,16,17 and superparamagnetism.18–20 Many metallic nanoparticles can now be produced with fairly good control of size and shape. A number of nanoparticle morphologies such as wires,21–26 rods,24,27–35 cubes,36–45 and prisms46,47 can be derived via solution chemistry methods in polar and non-polar environments. Different synthesis methods have their own unique a

Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669. E-mail: [email protected]; Fax: +65-6478-9020; Tel: +65-6824-7100 b Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 w Current address: Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China. E-mail: [email protected]; Tel: +86-10-82544814

Jun Yang received his PhD in Chemical and Biomolecular Engineering in 2006 from National University of Singapore. After postdoctoral research at Boston College and the University of Toronto, he joined the Institute of Bioengineering and Nanotechnology, Singapore in 2007. In 2010, he moved to the Institute of Process Engineering, Chinese Academy of Sciences as the Group Leader of Materials for Energy Storage. His research interests Jun Yang include (i) materials for thermoenergy storage, (ii) synthesis and application of nanocomposites, (iii) fuel cell, (iv) photocatalysis, and (v) separation technology. 1672

Chem. Soc. Rev., 2011, 40, 1672–1696

advantages and disadvantages.48 There is also an increased interest in the synthesis of more complex nanostructures, such as core–shell and hollow particles for advanced applications in catalysis,49–55 chemical and biological sensing,56–60 and optics.61–64 Specific applications often require the transfer of newly formed nanoparticles from a polar environment to a non-polar environment, or vice versa, in order to maximize the advantages offered by these environments based on processing considerations. Thus, phase transfer represents an important aspect in the development of nanoscience and nanotechnology. Metallic nanoparticles of different shapes and sizes can be obtained in the organic phase via direct synthesis in the organic phase, or by transferring nanoparticles from the aqueous phase to the organic phase. The latter has the advantage of leveraging upon many existing methods for preparing metallic nanoparticles in the aqueous phase, and avoiding the use of expensive organometallic precursors. On the other hand, the low interfacial energies observed in non-polar organic solvents may enable better control during surface and solution processing. Phase transfer of metallic nanoparticles from the aqueous solution to the organic phase was first reported about 20 years ago. An early example was the phase transfer of citratestabilized Au nanoparticles from aqueous phase to hexane,

Dr Jim Yang Lee received his BS from the National University of Singapore (NUS), and PhD from the University of Michigan. He is currently a Professor in the Department of Chemical and Bimolecular Engineering at NUS, and a Senior Principal Fellow of the Energy Studies Institute. His main research interests are synthesis and assembly of nanomaterials, and materials for energy applications such as batteries, fuel cells and photoelectrochemical water splitting.

Jim Yang Lee

This journal is

c

The Royal Society of Chemistry 2011


View Article Online

mediated by magnesium chloride and sodium oleate.65 Au nanoparticles dispersed in water were transferred to hexane upon the addition of magnesium chloride to an emulsion of hexane in an aqueous suspension of gold with sodium oleate. The resulting Au colloidal particles in hexane were monodispersed and stable for an extended period. The addition of magnesium chloride to the emulsion promoted the formation of magnesium oleate, which has a weaker hydrophilicity and was more soluble in hexane. Hence, the substitution of magnesium oleate for citrate led to the transfer of Au nanoparticles from aqueous phase to organic phase. This method has also been used to transfer large silver nanoprisms from water to hexane.66 Another early example was the phase transfer of gold nanoparticles from water to a wide variety of organic solvents,67 whereby gold nanoparticles synthesized in water by the Turkevich method68,69 were quantitatively transferred to butyl acetate or hexane by complexation of the particles with a ‘comb stabilizer’ present in the organic phase. The comb stabilizer was a co-polymer consisting of a backbone of methyl methacrylate and glycidyl methacrylate with poly (12-hydroxystearic acid) as pendant side chain; this was used to transfer silica particles from aqueous phase to dodecane in an earlier work.70 Gentle shaking of a biphasic mixture of the gold hydrosol and comb stabilizer in butyl acetate resulted in the emulsification of the gold hydrosol-butyl acetate mixture, accelerating the complexation of the polymer with the gold nanoparticle surface.67 The gold nanoparticles were thereby transferred to the organic phase. The comb stabilizer served two functions: it brought the gold sol into contact with the immiscible solvent by emulsification, and engulfed the particles allowing them to transfer.67 Gold sols have a pink to ruby red color; thus, transfer of the gold particles from one phase to another was indicated by a dramatic transfer of color between phases. A number of excellent review articles have been published on the applications of phase transfer for metallic nanoparticle synthesis and quantum dot (QD) surface modifications.48,71 Thus, this review does not intend to provide a comprehensive coverage of such topics. Instead, it would highlight the major and

Prof. Jackie Y. Ying received her BE and PhD from The Cooper Union and Princeton University, respectively. She joined the faculty of Massachusetts Institute of Technology in 1992, where she was Professor of Chemical Engineering until 2005. She has been the Executive Director of the Institute of Bioengineering and Nanotechnology in Singapore since 2003. For her research in nanostructured materials, Prof. Ying has been recognized with Jackie Y. Ying the American Ceramic Society Ross C. Purdy Award, David and Lucile Packard Fellowship, Office of Naval Research and National Science Foundation Young Investigator Awards. This journal is

c


more recent advances in phase transfer protocols, and feature the applications of phase transfer in the following selected areas: core–shell structures for bimetallic nanoparticles, replacement reactions in organic media, and catalytic properties of different nanostructures.

2. Phase transfer in nanoparticle synthesis Phase transfer has always been an important approach for the preparation of organosols of noble metals, since the methods for direct synthesis of noble metal nanoparticles in non-polar organic media (such as toluene, hexane, chloroform, and benzene) were limited due to the poor solubility of the corresponding metal ion precursors. In addition, phase transfer also represented an effective way for functionalizing metallic nanoparticles with suitable ligands towards the creation of close-packed, ordered arrays and nanoparticulate thin films. This section describes recent phase transfer techniques for the preparation of nanomaterials capped with various types of ligands, including thiols, amines and other surfactants. 2.1 Brust–Schiffrin method The availability of stable nanoparticle building blocks with uniform size and shape is a prerequisite to the assembly of nanostructured thin films.72 For this purpose, alkanethiols have been shown to be effective capping ligands to passivate the surfaces and quench the growth of metallic nanoparticles.73,74 The Brust–Schiffrin method is the earliest phase transfer approach for preparing thiol-stabilized metal (e.g. Au) nanoparticles.75,76 In this two-step approach, the gold ions from an aqueous solution were first extracted to a hydrocarbon (e.g. toluene) phase using tetraoctylammonium bromide (TOAB) as the phase transfer agent. The transfer of gold ions was facilitated by the electrostatic attraction between the positively charged tetraoctylammonium anions and the negatively charged tetrachloroauric cations. Subsequent reduction reactions took place in the organic solution using an aqueous NaBH4 solution in the presence of an alkanethiol, yielding Au nanoparticles of B2.5 nm. The overall reaction could be represented by eqn (1) and (2), where the source of electrons was NaBH4:75 AuCl4(aq) + N(C8H17)4+(C6H5Me) - N(C8H17)4+AuCl4(C6H5Me)

(1)

mAuCl4(C6H5Me)+ nC12H25SH(C6H5Me) + 3me - 4mCl(aq) + (Aum)(C12H25SH)n(C6H5Me)m+n

(2)

Here the nucleation and growth of the Au particles and the attachment of the thiol molecules occurred simultaneously in a single step. The thiol-stabilization of nanoparticles is an important feature in the original Brust–Schiffrin method, which is different from most approaches described earlier. Meguro et al. reported in 1988 the extraction of H2PtCl6 from an aqueous solution to organic solvents using dioctadecyldimethylammonium chloride, trioctylmethylammonium chloride or trioctylphosphine oxide (TOPO) as extracting agents.77 After extraction, Pt(IV) ions were reduced by Chem. Soc. Rev., 2011, 40, 1672–1696

1673


View Article Online

Fig. 1 TEM images of Pt nanocrystals obtained by phase transfer using alkylamines of (A) C6, (B) C7, (C) C8, (D) C9, (E) C10, (F) C12, (G) C16, and (H) C18 as the stabilizing agents. Reproduced from ref. 92 with permission from the American Chemical Society.

formaldehyde or benzaldehyde in the presence of sodium hydroxide for several hours at 65 1C. The Pt particles obtained in CHCl3, cyclohexane or methylisobutylketone were very stable with a diameter of 1.5–2.5 nm. Meguro et al. subsequently extended this extraction method for the preparation of organo gold nanoparticles.78 The Brust–Schiffrin method generated functionalized nanomaterials with properties analogous to those of large molecules.74,79,80 Subsequent modifications to this classical method included the use of alkanethiols of various chain lengths,79,81–83 aromatic thiols,84 dialkyl disulfides,85 and various thiol/Au precursor ratios86 for the synthesis of monolayer-protected clusters (MPCs). Murray and co-workers employed hexanethiol instead of dodecanethiol as a stabilizer at a 3 : 1 thiol/Au ratio, and chilled the reaction to yield a solution of Au145(S(CH2)5CH3)50 clusters with a mean diameter of 1.6 nm.79 They subsequently addressed specific issues on particle growth and monodispersity in the preparation of hexanethiol-protected Au clusters.87 The mean diameter of the Au core was found to gradually increase over the first 60 h of reaction, and then remained largely unchanged at B3 nm in the next 65 h. These observations suggested that MPCs of the smallest core size could be best obtained by quenching the reaction at a relatively early stage.86,88,89 1674

Chem. Soc. Rev., 2011, 40, 1672–1696

The Brust–Schiffrinn method was extended by Horswell and co-workers to the synthesis of isocyanide-protected Pt nanoparticles.90 The Brust–Schiffrinn method was used to prepare tetraoctylammonium-stabilized Pt nanoparticles, which were further functionalized by dodecylisocyanide via a ligand-exchange process. Transmission electron microscopy (TEM) showed that Pt particles of 1–3 nm in diameter with well-defined crystalline structure were obtained. The use of isocyanide as a ligand offered the possibility to broaden the range of metals from which stable nanoparticles could be prepared. The Brust–Schiffrinn method of nanoparticle synthesis has been extended to generate amine-stabilized nanoparticles by simply substituting an appropriate amine for the thiol. Leff et al. first reported the synthesis of amine-stabilized Au nanoparticles using the Brust–Schiffrinn method by substituting the dodecanethiol with dodecylamine (DDA) or oleylamine.91 Larger Au nanoparticles of up to 7 nm could be produced by the modified Brust–Schiffrinn method. Extensive characterization of the nanoparticles led to several important insights on the interaction between primary amine and Au nanoparticle surface. (i) The amine/gold surface interaction was charge-neutral, and best described by a weak covalent bond. (ii) The particles were kinetically stable, rather than thermodynamically stable. This was in contrast with the thiolcapped Au nanocrystals, which were thermodynamically stable with respect to ligand desorption and subsequent particle agglomeration. (iii) The stability of the nanoparticles was attributed to a finite-size effect from a comparison of the amine-capped Au nanoparticle system to the amines adsorbed on bulk Au surfaces. The finite-size effect gave rise to deviations from the bulk properties as the sample dimensions were reduced. Here the finite size or highly curved particle surface supported a higher density of adsorbate coverage relative to the bulk. Wikander et al. employed tetrakisdecylammonium bromide instead of TOAB to transfer Pt(IV) chloride from aqueous solution to toluene, and then reduced the platinum ions with NaBH4 in the presence of alkylamines. This work demonstrated that there was an inverse relationship between the size of the nanoparticles and the chain length of alkylamine used to coat the Pt nanoparticles (Fig. 1). Pt nanoparticles were more spherical in morphology when short-chain instead of long-chain alkylamines were used.92 Besides being a useful tool for preparing Au, Ag and Pt nanoparticles with controlled size and improved dispersity,93–95 the two-step synthesis concept of the Brust–Schiffrinn method was also employed to produce Cu,96 CoPt alloy,97 Ag2S98,99 and PbS.100 As the phase transfer agent and the stabilizer for particles in Brust–Schiffrinn synthesis were chemically different, the nanoparticles obtained were found to contain nitrogenous surface impurities due to the phase transfer agent. Besides this contamination problem, the main limitation of the Brust–Schiffrinn method was that the stabilizing ligands, such as thiols or amines, must be compatible with all of the reagents, including the reducing agent (NaBH4) and the phase transfer agent. For example, the thiol or amine could not interact with the phase transfer agent in such a way that led to products that were inseparable from the reaction mixture. Thus, This journal is

c


View Article Online

Brust–Schiffrinn reactions have been performed in solvents such as water and tetrahydrofuran (THF), permitting a singlephase synthesis of organic soluble nanoparticles, while eliminating the need for phase transfer reagents.101


2.2

Phase transfer of ‘naked’ nanoparticles

Instead of transferring metal ions from aqueous solution to an organic medium followed by reaction, Sarathy et al.102,103 and Zhao et al.104 adopted a different approach whereby ‘naked’ or unprotected metallic (Ag, Au and Pt) nanoparticles instead of the metal ions, were directly exported to the organic phase for thiolation. The metallic hydrosol was prepared in advance using NaBH4 as the reducing agent, and mixed with a toluene solution of 1-dodecanethiol. Concentrated HCl was then added to the biphasic mixture under stirring to enable the transfer. The thiol-stabilized metallic nanoparticles have uniform size and often self-assembled on the TEM grid. Voggu and co-workers have extended this acid-facilitated method for the phase transfer of Au nanoparticles from aqueous to a perfluorohexane medium.105 A hydrosol containing Au nanoparticle was first prepared by the reduction of chloroaurate ions with partially hydrolyzed tetrakis(hydroxymethyl)phosphonium chloride. A solution of heptadecafluoro-1-decanethiol in perfluorohexane was added to the Au hydrosol at room temperature, followed by concentrated HCl under vigorous stirring. The phase transfer of metallic nanoparticles to a fluorous solvent represented the solubilization in a highly nonpolar medium. The high non-polarity of fluorocarbon made it possible to study the optical and other properties of metal nanoparticles in a medium of very low refractive index. The authors did not explain the function of concentrated HCl in the phase transfer. Actually, the addition of concentrated HCl was not necessary for the transfer process. This was because NaBH4 played multiple roles besides acting as a reducing agent.106 Yang et al.106 showed that when a minor stoichiometric excess of NaBH4 was present (NaBH4/H2PtCl6 molar ratio = 1.1), the transfer was fast, and Pt nanoparticles of B3.4 nm with a broad particle size distribution were obtained. When a large excess of NaBH4 was used (NaBH4/H2PtCl6 molar ratio = 4.4), the transfer was inhibited unless concentrated HCl was added. Nanoparticles with a low surface coverage of BH4 were less protected and would transfer faster than those with a higher surface coverage. Incomplete surface coverage by BH4 would not inhibit particle agglomeration, and aggregates of different sizes were formed and transferred, resulting in the formation of non-uniform Pt particle sizes. Thus, BH4 functioned not only as a reducing agent, but also as a stabilizer against particle agglomeration. When an excess of NaBH4 was present, the surface of Pt nanoparticles in the hydrosol was initially well protected by BH4, thereby blocking the access of alkanethiol to the nascent metal surface, and suppressing the transfer of nanoparticles from the aqueous phase to toluene. However, BH4 was not a solution-stable species; its slow decomposition in the aqueous phase would eventually lead to the partial exposure of Pt nanoparticles surface for particle agglomeration. The addition of concentrated HCl would accelerate the decomposition of NaBH4, making the nanoparticles more This journal is

c


Scheme 1 Two-phase synthesis of transition metal oxide nanocrystals using metal salt precursors. Reproduced from ref. 109 with permission from the American Chemical Society.

susceptible to alkanethiol attachment for phase transfer to the toluene layer.106 Since the concentrated HCl addition was understood to release the borohydride-covered Pt surface for alkanethiol attachment, the synthesis procedure has been modified to enable the phase transfer to occur as soon as the particles were formed. In this case, the chloroplatinate solution and the toluene solution of alkanethiol were first mixed before the dropwise addition of NaBH4. This protocol was different from the Brust–Schiffrinn method75 in that the reduction was conducted in the aqueous environment instead of via the micro-contact between two immiscible phases. It led to a smaller mean particle diameter of 3.1 nm, and a nearly monodispersed distribution of particle size (standard deviation (s) = 0.38 nm).106 An analogous approach was reported more recently by Nguyen and Do for oxide nanoparticle synthesis (Scheme 1).107–110 The biphasic mixture was composed of an aqueous phase of metal ions (e.g. metal nitrate or chloride), ethanol, water and a toluene phase of potassium oleate and oleic acid (OA). The metal–oleate complex was directly formed in a water–toluene mixture from an ion-exchange reaction of a metal cation with an oleate anion, Mn+ + nRCOO M(COOR)n. Under hydrothermal synthesis conditions, the resulting metal–oleate complexes were reduced by ethanol to form oxide nuclei at the interface. The resulting metal oxide nanocrystals were transferred and capped by fatty acids in an organic phase (see Fig. 2). Yang et al. have recently demonstrated that DDA molecules present in the organic phase could also be used to accomplish the phase transfer of Pt nanoparticles from water to toluene by adjusting the pH of the Pt hydrosol to suitable values.111 Primary amine has been employed by Fu et al. to transfer ruthenium nanoparticles from polyol to toluene.112 The phase transfer in this case was not complete, and many Ru particles remained in the polyol after the transfer. In a more recent work, Hammarberg and Feldmann113 adapted this aminedriven phase transfer to non-polar dodecane to separate the In nanoparticles prepared in diethylene glycol from the excess precursors and residual salts. In contrast, Yang et al. introduced a toluene solution of DDA to the Pt hydrosol prepared by NaBH4 reduction of H2PtCl6.111 The turbid and greyish-white emulsion obtained could be stored for several days if NaOH was not added to it. Chem. Soc. Rev., 2011, 40, 1672–1696

1675

View Article Online


Yang et al. also revealed that if K2PtCl4 was used instead of H2PtCl6 as the starting metal salt, the resulting DDAstabilized Pt nanoparticles in toluene would be much larger with a mean diameter of 6.0 nm instead of 4.1 nm.111 Yang et al. also prepared oligonucleotide-stabilized Pt nanoparticles by substituting buffer solution for NaOH to adjust the pH of the Pt hydrosol. The ends of the oligonucleotides were modified with amine, and then mixed with a Pt hydrosol. Phosphate buffered saline (PBS) was then introduced to facilitate the attachment of the amine-modified oligonucleotides to the Pt nanoparticles.111 The oligonucleotide-stabilized Pt nanoparticles derived from K2PtCl4 exhibited a highly irregular cubic structure with a mean particle size of 12 nm (see Fig. 4). The amine groups on the oligonucleotide backbone were thought to bear many similarities with simple and heterocyclic amines, and should react similarly with the surface of Pt nanoparticles. In addition to interacting with the amine group at the end of the oligonucleotides, the Pt nanoparticles also coordinated with the exposed nitrogen bases in the nucleotides. The oligonucleotides thus acted as linkers to induce the assembly of Pt nanoparticles, based on a model proposed for the assembly of small Au nanoparticles by single- or double-stranded deoxyribonucleic acid (DNA).116,117 2.3 Phase transfer with ligand exchange

Fig. 2 (a,c,e,g) TEM images and (b,d,f,h) the corresponding selected area electron diffraction (SAED) patterns of transition metal oxide nanocrystals synthesized using the corresponding metal salts following Scheme 1: (a,b) 10 nm-sized hexagonal Mn3O4 nanocrystals, (c,d) 3 nm-sized Cr2O3 nanodots, (e,f) 6 nm-sized nanospheres and a few truncated nanospheres of Co3O4, and (g,h) 8 nm-sized hexagonal NiO nanocrystals. Reproduced from ref. 109 with permission from the American Chemical Society.

Upon the addition of 1 M NaOH solution, the transfer of Pt nanoparticles from the aqueous phase to toluene would occur and be completed within minutes. DDA might have been protonated by the slightly acidic Pt hydrosol, diminishing its affinity for Pt particle surface. The increase in pH from NaOH addition would restore DDA to its natural form, re-establishing the attachment of amine to the nanoparticle surface, and stabilizing the DDA-capped Pt nanoparticles in toluene. The Pt nanoparticles were mostly cubic (Fig. 3), which were quite different from the spherical Pt nanoparticles obtained by Rao et al.102,103 On the basis of the calculation using Bain’s empirical formula, DDA was not extended outwards normal to the Pt nanoparticle surface, but was inclined at an angle of 381 with the surface.114,115 1676

Chem. Soc. Rev., 2011, 40, 1672–1696

The Brust–Schiffrinn method and phase transfer for ‘naked’ nanoparticles were valuable techniques for preparing thioland amine-stabilized nanoparticles. The functional groups (thiols or amines) provided the solubility of the particles in organic media, and offered the possibility to self-assemble the particles on substrates. However, it was often difficult to generate smaller nanoparticles and to control the particle morphology. Also, to reduce the tendency of the particles to grow and agglomerate, it was usually necessary to use a stabilizing agent, such as tetraoctylammonium,118,119 citrate,120,121 lauric acid,122 phosphine123–125 and 126–130 polymers, at the beginning of or during the particle synthesis process. Hence, phase transfer involving ligand exchange was an important issue for the synthesis of nanoparticles with the desired properties and functional groups. The particles were all first prepared under the optimized conditions, and then subjected to ligand exchange to realize the phase transfer from the aqueous solutions to organic media, or vice versa. Although this approach would involve two steps and employ additional stabilizing agents, it has provided a greater control of particle size, shape and dispersity. One could generate libraries of nanoparticles with functional groups that were amenable to ligand exchange, and phase transfer these particles to polar or non-polar solvents as desired. Using the inclusion complexes (ICs) formed by cyclodextrin (CD) and octadecanethiol, Lala et al. demonstrated the phase transfer of carboxylic acid passivated Au nanoparticles from water to chloroform via ligand exchange.131 The carboxylic acid passivated Au nanoparticles were prepared in advance, followed by mixing with an equal volume of the aqueous solution of ICs. After 12 h of aging, the Au particles were stabilized by the ICs via ligand exchange between the ICs and This journal is

c



View Article Online

Fig. 3 TEM image and SAED (inset) of DDA-stabilized Pt nanoparticles transferred from the aqueous phase to toluene by NaOH. Mean particle = 4.12 nm, standard deviation (s) = 0.60 nm. Reproduced from ref. 111 with permission from the American Chemical Society. diameter (d)

Fig. 4 TEM images of oligonucleotide-stabilized Pt nanoparticles prepared from K2PtCl4. d = 12 nm. Reproduced from ref. 111 with permission from the American Chemical Society.

carboxylic acid. Next, through vigorous stirring of the mixture of Au hydrosol and chloroform, the Au nanoparticles were rapidly transferred to the organic phase. Phase transfer was achieved via the detachment of CD from octadecanethiol during the stirring of the biphasic mixture. The loss of CD made the octadecanethiol-capped Au nanoparticles sufficiently hydrophobic, enabling the phase transfer to chloroform. In the above study, the ligand exchange actually occurred before phase transfer in the aqueous phase.131 Another example of such a type of phase transfer was reported by Peng and co-workers,132 whereby Au nanoparticles surface-capped with a 4-aminothiophenol protecting layer were prepared through the ligand exchange between 4-aminothiophenol and citrate. 4-aminothiophenol has two functional groups, –SH and –NH2. The thiols could bond covalently to the Au surface, while the amino moieties provided for pH-dependent protonation.133 After the 4-aminothiophenol substitution for citrate, a red shift in the surface plasmon resonance (SPR) peak of Au nanoparticles from 520 nm to 526 nm was observed with a slight decrease in absorption. By slowly varying the pH value of the aqueous solution to 7.0 in the presence of toluene under vigorous stirring, the 4-aminothiophenol-stabilized Au nanoparticles were successfully transferred from water to toluene. A more practical phase transfer protocol involving ligand exchange was developed by Crooks and co-workers for the This journal is

c


extraction of metallic nanoparticles of o2 nm from within dendrimer templates.134 Dendrimer-encapsulated nanoparticles (DENs) of Au were synthesized first by sequestering AuCl4 within the dendrimer, followed by chemical reduction (Scheme 2). Next, Au DENs were extracted from within the dendrimers by first adding a large excess of NaBH4 to the aqueous solution, and then introducing a toluene solution containing an appropriate n-alkanethiol (step 3 of Scheme 2). Complete ligand exchange between dendrimer and alkanethiol was achieved after a short period of stirring, and the nanoparticles were transferred to the toluene phase. A sufficiently high ionic strength was required for the transfer of Au DENs, and an excess of NaBH4 was used to serve this purpose. This phase transfer method not only provided a straightforward approach for preparing highly monodispersed MPCs, but also demonstrated that nanostructured materials synthesized within a supramolecular template could be removed without damaging the nanoparticles and the template. Crooks and co-workers have extended the ligand exchange based phase transfer method to separate the Au and Ag DENs

Scheme 2 Three-step synthesis and ligand exchange based phase transfer of Au DENs. Reproduced from ref. 134 with permission from the American Chemical Society.

Chem. Soc. Rev., 2011, 40, 1672–1696

1677


View Article Online

from an aqueous mixture of the two (Scheme 3).135 An n-alkanoic acid in an organic solvent that has a high affinity for Ag (but not Au) was added to an aqueous mixture of dendrimer-encapsulated Ag and Au nanoparticles.136,137 Through a strong ligand–nanoparticle interaction, the Ag nanoparticles were selectively extracted into the organic medium. Subsequent addition of an organic solution of an n-alkanethiol led to the extraction of the remaining Au nanoparticles. With slight modification of the process, it was also possible to extract both Ag and Au nanoparticles simultaneously (Scheme 3b), or selectively extract Au nanoparticles from the mixture of nanoparticles. The selective separation approach based on phase transfer was further applied by the Crooks group to determine the chemical composition and structure of B2-nm bimetallic AuAg nanoparticles.138 The bimetallic AuAg nanoparticles were prepared by simultaneous or successive reduction of Au and Ag salts in the presence of the dendrimer templates, resulting in alloy or core–shell bimetallic nanoparticles (Scheme 4). In the presence of NaBH4, n-dodecanethiol would quantitatively transfer all forms of AuAg nanoparticles into the organic phase. In the absence of an excess of reducing agent, n-undecanoic acid would only transfer core–shell Au@Ag nanoparticles and AuAg alloy nanoparticles with a significant surface enrichment of Ag. Thus, the phase transfer was also useful as an analytical method for determining the structure of the ultrafine nanoparticles.10 In addition, it was possible to purify nanoparticles with different chemical compositions and structures. To avoid the use of costly dendrimers, Liu et al. reported a straightforward b-D-glucose-assisted aqueous-phase process for the synthesis of Au nanoparticles, followed by phase transfer of the particles into an organic medium via ligand exchange.139 Phase transfer of Au nanoparticles from water to toluene could be promptly achieved upon vigorous stirring of a mixture of b-D-glucose-stabilized Au hydrosol and hexane solution of dodecanethiol. It was found that a large amount of reducing agent NaBH4 and highly basic conditions would negatively impact the phase transfer process. This was in agreement with a previous observation that BH4 would block the access of alkanethiol to the nascent metal surface by temporarily stabilizing the nanoparticles.106 The authors also presented a nanoparticle deposition technique utilizing

Scheme 3 Ligand exchange based phase transfer for the separation of Ag and Au DENs. Reproduced from ref. 135 with permission from the American Chemical Society.

1678

Chem. Soc. Rev., 2011, 40, 1672–1696

Scheme 4 Successive or simultaneous reduction of Au and Ag metal salts for the preparation of core–shell Au@Ag or alloy AuAg nanoparticles in the presence of dendrimer templates. Reproduced from ref. 138 with permission from the American Chemical Society.

CO2 as an anti-solvent to target nanoparticulate thin films without the detrimental dewetting effects inherent in the typical evaporation of liquid systems (Fig. 5).139 This would be of interest for the economical scale-up of devices involving nanoparticle arrays. Liu and co-workers modified the ligand exchange based phase transfer for the extraction of Pt nanoparticles

Fig. 5 TEM images of Au nanoparticles deposited on a carboncoated copper TEM grid using three different approaches: (a) b-D-glucose-capped Au nanoparticles deposited on a TEM grid by evaporation of aqueous solution, (b) dodecanethiol-capped Au nanoparticles deposited on a TEM grid by evaporation of hexane solvent (particles were synthesized initially in aqueous b-D-glucose solution, and extracted into the hexane phase via ligand exchange), (c) dodecanethiol-capped Au nanoparticles deposited on a TEM grid using the CO2-expanded hexane particle deposition technique (particles were initially synthesized in aqueous b-D-glucose solution and extracted into the hexane phase via ligand exchange prior to CO2 processing), (d) a higher magnification image of an array of Au nanoparticles in (c), and (e) electron diffraction pattern of Au nanoparticles within the thin film in (c). Reproduced from ref. 139 with permission from the American Chemical Society.

This journal is

c



View Article Online

stabilized by sodium carboxymethyl cellulose (CMC).140 CMC is a water-soluble polysaccharide that has a macromolecular skeleton similar to starch. It possesses carboxylate groups in addition to hydroxyl groups, so it has a stronger interaction with particles as compared to starch and glucose.141,142 The CMC-stabilized Pt nanoparticles could not be transferred directly from aqueous phase to dodecanethiol-containing hexane. This was likely due to the strong interaction between CMC and the Pt particles within the aqueous phase prior to extraction, which inhibited the exchange between dodecanethiol and CMC on the particle surface. However, the addition of concentrated HCl significantly increased the pK of the equilibrium between COO and COOH, substantially weakening the interaction between CMC and the Pt nanoparticle surface. Consequently, dodecanethiol could effectively interact with the Pt nanoparticle surface without a strong competition from CMC. The substitution of dodecanethiol for the CMC molecules on the Pt nanoparticle surface provided for good solubility of the nanoparticles in hexane (Scheme 5). Upon phase transfer, the Pt nanoparticles could be selfassembled into close-packed and 2-dimensional ordered arrays (Fig. 6). These self-assemblies were characterized by dense, nearhexagonal packing of Pt nanoparticles. The phase transfer allowed for the derivation of dodecanethiol-stabilized Pt nanoparticles with a narrow size distribution, which enabled self-assembly of these building blocks into ordered arrays after hexane evaporation. There were some random voids on the surface of the substrate (indicated by arrows) due to dewetting effects inherent to the evaporating vapor/liquid interface.143–145 Metallic nanoparticles have also been transferred from aqueous media to ionic liquids (ILs) via a ligand-exchange process.146–148 ILs are of interest for their unique physicochemical properties—they are non-volatile, non-flammable and thermally stable.149,150 ILs do not lose solvent during evaporation, so their use circumvents potential environmental and safety problems associated with the conventional volatile organic compounds. The protocols to transfer metallic nanoparticles to ILs were analogous to those used for transferring to non-polar organic solvents. For example, Wei and co-workers147 used a method similar to that of Lala et al.131 in transferring Au nanoparticles from aqueous solution to waterimmiscible IL, 1-butyl-3-methylimidazolium hexafluorophosphate ([C4MIM][PF6]). Citrate-stabilized Au nanoparticles were prepared first, and then the citrate stabilizer was replaced by tetradecyltrimethylammonium bromide (TTAB) through

Scheme 5 Illustration of phase transfer of CMC-stabilized Pt nanoparticles from aqueous solution to hexane using dodecanethiol in the presence of concentrated HCl. Reproduced from ref. 140 with permission from the American Chemical Society.

This journal is

c


Fig. 6 TEM images of ordered arrays of dodecanethiol-capped Pt nanoparticles on the copper TEM grids upon hexane evaporation. Reproduced from ref. 140 with permission from the American Chemical Society.

ligand exchange, followed by the addition of [C4MIM][PF6] to extract the TTAB-stabilized Au nanoparticles. The high polarity of [C4MIM][PF6] was supposed to induce the phase transfer of TTAB-stabilized Au nanoparticles from aqueous solution to IL.147 The ILs were believed to have the potential to enhance certain properties of the metallic nanoparticles.151–154 The activity of Rh nanoparticles transferred from aqueous solution to IL N,N-dimethyl-N-dodecyl-N-(2-hydroxyethyl)ammonium chloride (HEA12Cl) has been noted for the exo-double bond hydrogenation of styrene in biphasic media.148 2.4 Phase transfer via electrostatic interaction An unfavorable feature of ligand exchange based phase transfer of metallic nanoparticles is that the new ligands used in the exchange process must have equal or greater affinity for the particle surface than the original ligands used to stabilize the nanoparticles. This limits the choice of new ligands that can be used, and may adversely impact the catalytic activity of the resulting nanoparticles. An alternative strategy for the phase transfer of metallic nanoparticles is to exploit the electrostatic interactions between the transfer agents and the nanoparticle surface. Crooks and co-workers155,156 demonstrated the transfer of fourth-generation, amine-terminated, poly(amidoamine) dendrimers (G4-NH2) to toluene via an organic solvent containing dodecanoic acid. The transfer was accompanied by proton donation from the acid to the terminal amine groups of the G4-NH2 dendrimers. The acid molecules arranged themselves around the dendrimers in a composite structure that resembled an inverted micelle with a hydrophilic dendritic core and a hydrophobic alkyl-dominated shell, which provided solubility to the acid–base ensemble (Scheme 6). In this electrostatic self-assembly process, commercially available dendrimers could be used directly without the need for further chemical modifications. Also, the acid–base interaction was reversible, and therefore, the dendrimers and their guest species could be readily shuttled between hydrophilic and hydrophobic phases by modulating the pH of the aqueous phase. The assemblies in the above study by Crooks and coworkers155,156 were formed between positively charged dendrimers and negatively charged incoming ligands (dodecanoic acid). In contrast, Yao et al.157 demonstrated the phase transfer of gold nanoparticles across a water/oil interface via stoichiometric ion Chem. Soc. Rev., 2011, 40, 1672–1696

1679


View Article Online

Scheme 7 Schematic of gold nanoparticles transferred across the water–toluene interface via the formation of stoichiometric ion pair. Reproduced from ref. 157 with permission from the Chemical Society of Japan.

Scheme 6 Surface modification of G4-NH2 dendrimers by acid–base self-assembly. Reproduced from ref. 155 with permission from the American Chemical Society.

pair formation between surface carboxylate anions and hydrophobic cations. Carboxylate-modified gold nanoparticles were prepared by NaBH4 reduction of HAuCl4 in the presence of mercaptosuccinic acid (MSA). The surface ion pair formation was induced by mixing the aqueous MSA-Au hydrosol and a toluene solution of TOAB. Negatively charged carboxylate anions on the gold nanoparticle surface were bound to the incoming ligand, tetraoctylammonium cations (see Scheme 7). The electrostatic interactions gave rise to the transfer of gold nanoparticles from water to an organic medium. Although the transfer efficiency was only 55%, the gold nanoparticles after phase transfer to toluene have a narrower size distribution.157 The low transfer efficiency and the improved size distribution were interpreted in a later report by Cheng and Wang,158 who found that the electrostatic interaction induced phase transfer of gold nanoparticles was size-dependent. Au nanoparticles of >10 nm could not be transferred using the ion pairs formed between tetraoctylammonium cations and citrate anions. The smaller nanoparticles possessed a higher specific surface area (for a given mass), and thus, a higher density of ion pairs per particle. As a result, greater hydrophobic forces would be exerted on them. In contrast, the ion pairs on the larger gold nanoparticles would have hydrophobic forces that were insufficient to effectively transfer the gold particles to toluene. The speculation was consistent with the studies by Hostetler et al., who demonstrated that adsorbents have a higher coverage on a nanostructured gold surface than on a planar 1680

Chem. Soc. Rev., 2011, 40, 1672–1696

gold surface.159 Since the larger Au nanoparticles could not be transferred to toluene, a narrower size distribution was achieved upon phase transfer. Hence, the electrostatic interaction based phase transfer could be used to facilitate size-selective preparation of nanoparticles. Electrostatic interaction based on ion pairs was used to transfer Au,160–164 Ag,165 Au–Ag alloy,166 and magnetic nanoparticles167 from aqueous phase to organic media. In particular, Mayya and Caruso163 demonstrated phase transfer of surface-modified gold nanoparticles via the use of hydrophobic alkylamines. Gold nanoparticles were modified with mercaptoundecanoic acid (MUA), 4-carboxythiophenol (4-CTP) and triphenylphosphine-3,3 0 ,3 0 0 -trisulfonic acid trisodium salt (TTP) to render their surface negatively charged. The phase transfer of the surface-modified nanoparticles from aqueous solution to toluene or chloroform was achieved based on electrostatic attraction between the negatively charged surface groups of Au nanoparticles (COO or SO3) and the positively charged NH3+ groups of alkylamines (octadecylamine (ODA)). The resulting gold nanoparticles were found to be monodispersed and well-separated, and to have a strong tendency to organize on the TEM grid upon solvent evaporation (Fig. 7). In addition, the particle separation could be easily tuned by using alkylamines with the appropriate hydrocarbon chain lengths. 2.5 Other approaches for aqueous to organic phase transfer Many specific approaches have been developed for the phase transfer of metallic nanoparticles from aqueous phase to an organic medium. The method using phosphoric acid to facilitate phase transfer of silver nanoparticles was of particular interest.168–170 Ag nanoparticles were first synthesized in the aqueous phase by NaBH4 reduction of silver precursor in the presence of sodium oleate or aerosol OT (sodium bis(2-ethylhexyl)-sulfosuccinate or AOT]. Next, phosphoric acid (H3PO4) was added to a biphasic mixture of This journal is

c



View Article Online

Fig. 7 TEM images of ODA-stabilized, surface-modified gold nanoparticles: (a) 4-CTP-capped gold nanoparticles, (b) highmagnification image of (a), (c) MUA-capped gold nanoparticles, and (d) high-magnification image of (c). Reproduced from ref. 163 with permission from the American Chemical Society.

Ag hydrosol and cyclohexane under vigorous stirring. The transfer of Ag nanoparticles from water to cyclohexane was indicated by the color change of the organic phase. No additional surfactants were used during the phase transfer process. Sodium oleate or AOT underwent self-reorientation upon the addition of H3PO4. Sodium oleate was attached to the surface of Ag nanoparticles in aqueous solution through the double bond, and the carboxylate stabilized by sodium ions was exposed to the solvent. Upon H3PO4 addition, the carboxylate was converted to carboxylic acid, and its hydrophobic tail became directed towards the organic media, inducing phase transfer of Ag nanoparticles.170 The phosphoric acids or their anions adsorbed onto the Ag particle surface might have forced the sulfonate groups of AOT to turn towards the particle surface, forming a net of hydrogen bonds in the non-polar organic solvent. In contrast, in the aqueous phase, the sulfonate moieties were better stabilized by the sodium ions if they were directed towards the solvent (Scheme 8).169 Si et al. reported the efficient phase transfer of gold and silver nanoparticles by acid treatment. The nanoparticles were synthesized by in situ reduction at pH 11 using a series of designed, redox-active amphiphiles as the reducing agent and stabilizer.171 The tryptophan moiety of the amphiphile reduced Au ions to metallic Au, which evolved as Au nanoparticles that were stabilized by the amphiphilic micelles (Scheme 9). The NH group of the indole moiety of the tryptophan residue remained anchored to the Au particle surface. Introduction of diluted HCl protonated the carboxylate group of the amphiphiles, and disrupted the micelle formation on the particle surface. This led to the exposure of the long alkyl chain of amphiphile to the organic solvent, and the transfer of the protonated amphiphiles to the organic phase along with the anchored gold nanoparticles. In the non-polar organic environment, the amphiphiles remained anchored to nanoparticle surface through the tryptophan moiety, giving rise to a stable gold organosol. McMahon and Emory demonstrated the first use of a covalent amide coupling reaction to reliably transfer gold nanoparticles from aqueous solution to organic solvents such This journal is

c


Scheme 8 The molecular structure of AOT, and schematic representation of the AOT orientation on the silver nanoparticle surface in different environments. Reproduced from ref. 169 with permission from the American Chemical Society.

as chloroform, toluene and dimethyl sulfoxide (DMSO) (see Scheme 10).172 The resulting gold nanoparticles modified with mercaptoacetic acid (MAA) and dicyclohexylamine (DCHA) appeared dark purple in color, and were easily dispersed in non-polar organic solvents. This approach enabled the phase transfer of gold nanoparticles of >40 nm. Gold nanoparticle hydrosols of 45 nm, 75 nm and 100 nm were synthesized, and transferred to organic solvents without significant aggregation and change in particle size (Fig. 8). Other specific approaches included the transfer of metallic (Ag, Au, and Pt) nanoparticles by changing the temperature of biphasic mixtures,173 the transfer of Ag nanoparticles by modulating the concentration of transfer agent,174 the transfer of CoFe2O4 nanoparticles by adjusting the pH of the hydrosol,175 and the transfer of Au nanoparticles by altering the ionic strength of the hydrosol.176 Although their applications were restricted to certain systems, these approaches enriched the growing literature on phase transfer of aqueous-synthesized nanoparticles to non-polar organic media with different physicochemical properties. 2.6 Phase transfer of metallic nanoparticles from organic to aqueous phase While numerous protocols have been developed for the transfer of aqueous-derived metallic nanoparticles into organic media, there have been relatively few reports on the transfer of metallic nanoparticles to the aqueous phase. This was partly because the synthesis of metallic nanoparticles in the aqueous phase has been well-established. However, aqueousbased synthesis of metallic nanoparticles faced problems associated with ionic interactions, which were typically overcome with the use of low reactant concentrations.69 In contrast, non-aqueous-based synthesis could be conducted at relatively high concentrations, and have been shown to be superior at producing nanoparticles with narrow size distributions, fewer crystalline defects, and more controllable morphologies.177–179 For most colloidal systems produced by non-aqueous methods, the particles were capped by a Chem. Soc. Rev., 2011, 40, 1672–1696

1681


View Article Online

Scheme 9 The synthesis of colloidal gold nanoparticles via in situ reduction using amphiphiles, and the transfer to toluene facilitated by a diluted acid. Reproduced from ref. 171 with permission from Wiley-VCH.

Scheme 10 Synthesis of MAA-DCHA-modified gold nanoparticles. Reproduced from ref. 172 with permission from the American Chemical Society.

layer of surfactant molecules that were responsible for the hydrophobicity. The poor solubility of the nanoparticles in the aqueous phase greatly limited their application in biological 1682

Chem. Soc. Rev., 2011, 40, 1672–1696

and medical fields. It would therefore be important to develop suitable methods that could be used to transfer the nanoparticles from organic phases to aqueous solutions. This journal is

c



View Article Online

Scheme 11 The exchange of alkanethiol molecules on the gold nanoparticle surface with o-thiol carboxylic acid. Reproduced from ref. 181 with permission from the Royal Society of Chemistry.

Fig. 8 Scanning electron microscopy (SEM) images of MAA-DCHAmodified gold nanoparticles of (A, B) 45 nm, (C, D) 75 nm, and (E, F) 100 nm. Reproduced from ref. 172 with permission from the American Chemical Society.

The concepts for the transfer of nanoparticles from aqueous phase to an organic medium have been applied for the reverse transfer of nanoparticles. To transfer from the organic medium to the aqueous phase, metallic nanoparticles must be rendered hydrophilic by complexation with molecules via ligand exchange, by hydrophobic–hydrophobic interactions between the starting ligands and an amphiphilic polymer or lipids, or by some specific protocols. An early attempt using ligand exchange for the organic to aqueous phase transfer of metallic nanoparticles was reported by Schmid et al.,180 who transferred gold cluster, Au55(PPh3)12Cl6 (PPh3 = triphenylphosphane) to aqueous phase based on the following exchange reaction: Au55(PPh3)12Cl6 + 12Ph2PC6H4SO3Na(aq) - Au55[Ph2PC6H4SO3Na]12Cl6(aq) + 12PPh3 After phase transfer to water, the Au hydrosol was much more stable when compared with solutions of the PPh3-containing clusters in organic solvents due to the high ionic charge on the cluster surface, which stabilized the particles by electrostatic repulsion. Subsequently, Simard et al. reported the formation of water-soluble amphiphilic gold colloids through an exchange of alkanethiol-capped organically soluble gold nanoparticles with o-thiol carboxylic acid (Scheme 11).181 This journal is

c


Half of the octanethiol on the surface of 2-nm gold nanoparticles was replaced with 11-thioundecanoic acid. The o-thiol carboxylic acid/octanethiol-stabilized gold nanoparticles could be dispersed in water. By replacing alkanethiol with carboxylic acid to solubilize the gold nanoparticles in water, the charge on the nanoparticle surface could also be modulated conveniently by varying the pH of the colloidal solution. The ligand-exchange process reported by Simard et al. required two days. A more rapid ligand exchange based phase transfer of Au and Pd nanoparticles to water has been demonstrated by Gittins and Caruso.182 In their work, Au and Pd nanoparticles capped by tetraalkyl ammonium salts were prepared using the Brust–Schiffrinn method. The particles were transferred rapidly and completely to water by the addition of an aqueous 0.1 M solution of 4-dimethylaminopyridine (DMAP) to aliquots of the metallic nanoparticles in toluene (Fig. 9). The spontaneous phase transfer of Au and Pd nanoparticles in the presence of DMAP molecules is illustrated by Scheme 12.182 The addition of an aqueous DMAP solution to the nanoparticle dispersion in toluene resulted in the DMAP partitioning across the water–toluene phase boundary. The DMAP molecules substituted for the tetraalkylammonium salts, and physisorbed onto the nanoparticle surface by forming a labile donor–acceptor complex with the surface atoms of the nanoparticles through the endocyclic nitrogen atoms. The surface charge, which arose from the partial protonation of the exocyclic nitrogen atom, prevented the metallic nanoparticles from aggregation and agglomeration. The phase transfer method developed by Gittins and Caruso did not require precipitation steps, and produced hydrosols with high nanoparticle concentrations. Compared with the thiol-stabilized particles, these nanoparticles would be attractive for catalytic applications since they were not stabilized by a strong, covalently attached ligand.183 A variety of exchanging ligands including functionalized thiols (e.g. 11-MUA, 3-mercaptopropionic acid, and Chem. Soc. Rev., 2011, 40, 1672–1696

1683


View Article Online

Fig. 9 Photographs of a diluted aliquot of (A,B) palladium and (C,D) gold nanoparticles in toluene above water (A,C) before and (B,D) after transfer to water by the addition of DMAP (0.1 M, pH = 10.5). Reproduced from ref. 182 with permission from Wiley-VCH.

metal oxide nanoparticles from organic media to aqueous phase. The ligand exchange between glutathione and amineterminated, fourth-generation PAMAM dendrimer (G4-C12) was unique.185 In the presence of a microemulsion of a metallic organosol and water, the dendrimers underwent a major conformational change, exposing their hydrophobic surfaces to the organic phase. These dendrimers released the encapsulated metallic nanoparticles for glutathione passivation. The formation of surfactant bilayers with charged hydrophilic ions exposed to the aqueous phase was also an effective approach to render the metallic nanoparticles hydrophilic and soluble in water. Wang et al. first presented a surface modification method that was based on the formation of a host–guest complex between the surfactant molecules bound to the surface of nanoparticles and the incoming hydrophilic macromolecules, which could effectively increase the solubility of nanoparticles in aqueous phase (Scheme 13).187 First, OA-stabilized Fe2O3 and Ag nanoparticles were prepared. To increase the hydrophilicity of these nanoparticles, a-CDs were used as the host molecules to generate an inclusion complex with surface-bound OA molecules. a-CDs are composed of hydrophilic rims of hydroxyl groups and hydrophobic cavities, which can form complexes with various organic molecules (Scheme 13(b)). The inclusion complexes formed between a-CDs and OA changed the hydrophobicity of the particle surface substantially, and facilitated the transfer of nanoparticles into the aqueous phase. More recently, two analogous approaches using copolymer Pluronic F127188 and p-sulfonated calix[4]arene (pSC4) sodium189 were developed in place of a-CDs for the transfer of iron oxide and Ag nanoparticles from organic media to aqueous phase. After phase transfer, the iron oxide nanoparticles remained superparamagnetic with a saturation

Scheme 12 Proposed mechanism for the spontaneous phase transfer of Au and Pd nanoparticles from toluene to water by the addition of DMAP. R = C8H17. Reproduced from ref. 182 with permission from Wiley-VCH.

MSA),184 glutathione,185 and poly(ethylene glycol) (PEO) grafted hyperbranched poly(amido amine) (h-PAMAM-gPEG)186 have been employed for the transfer of metallic or 1684

Chem. Soc. Rev., 2011, 40, 1672–1696

Scheme 13 Chemical structures of (a) OA and (b) a-CD molecules. (c) Schematic of transfer of OA-stabilized nanoparticles from organic into aqueous phase by surface modification using a-CD. Reproduced from ref. 187 with permission from the American Chemical Society.

This journal is

c



View Article Online

magnetization that was B96% of the maximum theoretical value.188 It was found that vigorous shaking of the biphasic mixture of DDA-stabilized gold nanoparticles in chloroform and the aqueous solution of cetyltrimethylammonium bromide (CTAB) could lead to the transfer of Au nanoparticles to the aqueous phase.190 An interdigitated structure consisting of a DDA primary layer directly anchored to the gold nanoparticle surface and a secondary monolayer of CTAB was formed in the aqueous environment due to hydrophobic–hydrophobic interactions between the interdigitated hydrocarbon chains (Scheme 14). The cationic group of the secondary monolayer, CTAB, provided sufficient hydrophilicity to facilitate the phase transfer of gold nanoparticles and rendered them water-soluble. Yu et al. achieved the transfer of OA-capped Fe3O4 nanoparticles from chloroform to water with an efficiency of 100%.191 They used an amphiphilic copolymer formed by poly(maleic anhydride-alt-1-octadecene) (PMAO) and PEG, which has one carboxylic group, one hydrophilic PEG side chain, and one hydrophobic C16 alkane side chain that interacted well with the alkane component of OA through hydrophobic–hydrophobic interactions. Scheme 15 shows the structure of the Fe3O4 nanoparticles after phase transfer. The free functional groups (e.g. –COOH) in the polymer could be used to conjugate with biomolecules containing –NH2 group, offering versatility for further modification or functionalization of the nanoparticles. Mayer and co-workers reported a specific approach to reversibly transfer Au nanoparticles between water and an organic solvent (Scheme 16).192 Au nanoparticles stabilized by 1,10-phenanthroline were prepared in toluene, followed by the post-functionalization using a ruthenium complex bearing a phenanthroline pendent group for the transfer of Au nanoparticles to water. The reversible transfer of Au nanoparticles between water and an organic medium was achieved by controlled anionic exchange around the nanoparticles. Mao et al. reported a method for the transfer of nanoparticles from organic media to aqueous phase by a hydrogen bond selection process.193 Au nanoparticles coated with a mixture of polylactide (PLA) and PEG were subjected

Scheme 14 Schematic of the formation of an interdigitated secondary monolayer of CTAB molecules on the surface of DDA-modified gold nanoparticles (CTAB-DDA-Au). The box enclosed an ordered, interdigitated region of the bilayers. Reproduced from ref. 190 with permission from the American Chemical Society.

This journal is

c


Scheme 15 Schematic of water-soluble Fe3O4 nanoparticles (R = functional group, e.g. –COOH). Reproduced from ref. 191 with permission from the Institute of Physics Publishing.

Scheme 16 Post-functionalization pathway leading to Au nanoparticles stabilized by the ruthenium complex [Ru-Lphen]2+, and phase transfer of the nanoparticles between aqueous and organic media via anionic exchange. Reproduced from ref. 192 with permission from Elsevier.

to NaOH treatment at 40 1C for the degradation of PLA. Upon contact with NaOH solution, the toluene dispersion of Au@PLA/PEG nanoparticles became colorless; at the same time, the alkaline solution turned red in the presence of citrate. In the absence of citrate in the aqueous solution, the Au nanoparticles accumulated at the water–toluene interface, instead of transferring to water. The hydrogen bond formed between PEG brushes and citrate was likely to be the driving force for Au nanoparticles to cross the water–toluene interface after the degradation of the PLA brushes. This approach could transfer hydrophobic nanoparticles from organic to aqueous phase during the degradation of PLA, crossing not only liquid/ liquid but also gel/liquid interface, i.e. the water/organogel or the oil/hydrogel interface, which would be relevant as a model for biological barriers composed of different cellular layers. Thus, this work offered strategies for drug delivery across Chem. Soc. Rev., 2011, 40, 1672–1696

1685

View Article Online

biological barriers, such as the stratum corneum epidermidis in the skin.


3. Ethanol-mediated phase transfer The direct phase transfer by mixing the metallic hydrosol and alkanethiol or alkylamine in toluene would only work for limited metals in their ‘‘unprotected’’ forms. Ruthenium nanoparticles, for example, could not be transferred from the aqueous environment to a hydrocarbon layer by mixing the unprotected Ru hydrosol and toluene containing dodecanethiol or DDA. In addition, the direct transfer procedures involving a ligand exchange process could fail to transfer metallic nanoparticles in the presence of external stabilizers, such as trisodium citrate (which was often used as a temporary capping agent for further assembly),194–196 into a hydrocarbon layer containing thiol or amine. It was also not possible to directly extract semiconductor nanocrystals capped with short-chain thiols from an aqueous solution with the use of long-chain aliphatic thiols.197 In the direct phase transfer process, since the exchange between the incoming ligands and initial stabilizers could only occur at the interface between water and organic phase, the failure in transferring the nanoparticles was possibly due to the poor interfacial contact between the two immiscible phases. Thus, an ethanol-mediated phase transfer protocol was developed for the transfer of metallic nanoparticles from the aqueous phase to an organic medium.198,199 Uniform alkylamine-stabilized metallic nanoparticles were subjected to a stabilizer exchange process that involved (i) the mixing of the metallic hydrosol and an ethanolic solution of DDA, and (ii) the extraction of DDA-stabilized metallic nanoparticles into toluene. 3.1

Phase transfer of preformed nanoparticles

Ethanol could be used for the phase transfer of preformed metallic nanoparticles. Typically, citrate-stabilized metallic hydrosol prepared by NaBH4 reduction of metal precursors was mixed with an equal volume of ethanol containing DDA. After stirring for 2 min, toluene was added and stirred for another 3 min. DDA-stabilized metallic nanoparticles were rapidly extracted into toluene, leaving behind a colorless aqueous solution.198 Citrate-stabilized metallic nanoparticles could not be transferred directly to toluene by mixing the metal hydrosol with a toluene solution of DDA. Prolonged stirring only produced a milky mixture of metallic hydrosol and toluene, and the particles were aggregated at the interface of two immiscible layers. Thus, ethanol was employed to ensure particle transfer, since it is water-miscible and a good solvent for DDA. Fig. 10 shows the representative TEM images of Ag, Au, Pt and Rh nanoparticles in toluene. The particle size distribution was fairly narrow, and the self-assembly of alkylamine-stabilized metallic nanoparticles was also evident. The yields of various alkylamine-stabilized metallic nanoparticles were estimated to be >90%.198 The ethanol-mediated phase transfer method could also be extended to the preparation of alkanethiol-stabilized Pt, Au, Rh, Ir and Os nanoparticles (Fig. 11).198,199 The phase transfer protocol was similar to that for the alkylamine-stabilized 1686

Chem. Soc. Rev., 2011, 40, 1672–1696

Fig. 10 TEM images of alkylamine-stabilized Ag nanoparticles (d = 7.1 nm), Au nanoparticles (d = 7.9 nm), Pt nanoparticles (d = 4.4 nm), and Rh nanoparticles (d = 6.9 nm). Reproduced from ref. 198 and ref. 199 with permission from Elsevier.

metallic nanoparticles. It was found that the phase transfer of alkanethiol-stabilized Ru, Ag and Pd nanoparticles was not as successful. Ru, Ag and Pd nanoparticles would stay at the water–toluene interface or on the container walls, instead of transferring to toluene. Experimentally, the citrate-stabilized metallic nanoparticles could not be re-dispersed in water after several rounds of washing and centrifugation. This could be due to the progressive loss of the citrate ions as fresh solvent (water) was used in each re-dispersion attempt. The need to reestablish equilibrium between free and adsorbed citrate ions would slowly but eventually deplete the adsorbed citrate ions to a level that was inadequate to maintain the particles in suspension. The process of displacing citrate from the metallic nanoparticle surface by DDA or dodecanethiol is illustrated in Scheme 17. In step A, the adsorbed citrate ions were in equilibrium with their surroundings. The progressive displacement of citrate by –NH2 or –SH groups was depicted in steps B–D, whereby it was assumed that the binding of

Fig. 11 TEM images of alkanethiol-stabilized Au nanoparticles (d = 6.2 nm) and Pt nanoparticles (d = 4.8 nm). Reproduced from ref. 198 with permission from Elsevier.

This journal is

c



View Article Online

Scheme 17 Schematic illustrating the displacement of citrate from nanoparticle surface by –NH2 or –SH. J: citrate ions; D: –NH2 or –SH. Reproduced from ref. 198 with permission from Elsevier.

–NH2 or –SH to the metal nanoparticle surface was more irreversible than the adsorptive interaction between the citrate ions and the metal surface. This schematic also explained the increase in the size of nanoparticles during the ligand exchange process.131,200,201 The dynamic equilibrium between free and adsorbed capping agents could have resulted in the exposure of the nanoparticle surface, leading to contact points for particle agglomeration. This ethanol-mediated method could be used to transfer metallic nanoparticles capped by a variety of surfactants, such as sodium acetate, polyvinylpyrrolidone (PVP), bis(p-sulfonatophenyl)phenylphosphine (BSPP) and Triton X-100. It could also be applied to transfer nanoparticles prepared by the NaBH4 reduction of metal precursors without a stabilizer. It could be used for transferring Cu and Ni nanoparticles from aqueous environment to hydrocarbons, but the transfer should be performed in an inert atmosphere to prevent metal oxidation. Unlike the method of Sarathy et al.,103 this ethanol-mediated phase transfer did not require the addition of concentrated HCl. 3.2

the stabilizer and the phase transfer agent in the ethanolmediated method, bypassing the contamination issue.202 At the same time, this approach offered the distinctive advantage of the Brust–Schiffrinn method, allowing for the preparation of fine metallic nanoparticles, including gold. The experimental protocol for transferring metal precursors from aqueous phase to hydrocarbon202 was similar to that for transferring metallic nanoparticles.198 The aqueous solution of metal ions was mixed with an equal volume of DDAcontaining ethanol, and the mixture was stirred for 3 min. Next, toluene was added, and stirred for 1 more minute. The formation of two immiscible layers occurred within minutes. The transfer of metal salts from the aqueous phase to toluene was >95% efficient, leaving behind a colorless aqueous solution.202 This could be attributed to the formation of a metal complex between the metal ions and DDA during the phase transfer. DDA was bound to the metal ions by its –NH2 group. After coordinating with metal ions, the non-polar tail of DDA enabled the compounds to dissolve easily in non-polar organic solvents (such as toluene and hexane) to facilitate phase transfer. We have synthesized monometallic, bimetallic alloy, core–shell and semiconductor nanoparticles by this approach (Fig. 12).202 Compared with other general approaches,203,204 our protocol allowed for the nanocrystals synthesis in an organic medium using inexpensive and widely available water-soluble metal salts as precursors. It enabled the successful preparation of a large number of semiconductornoble metal hybrid nanomaterials, which has been an important challenge.205–212 By aging the mixture of semiconductor nanocrystals and noble metal ions in toluene for 1 h, uniform semiconductor-noble metal heterogeneous nanostructures were derived. Additional reducing agent was

Phase transfer of metal precursors, followed by reduction

We have recently further extended the ethanol-mediated method to transfer metal ions from aqueous phase to a hydrocarbon layer for the synthesis of a variety of nanoparticles and nanostructures.202 This method is analogous to the Brust–Schiffrinn method described in section 2.1, whereby gold ions from an aqueous solution were first transferred to a hydrocarbon phase via a phase-transfer agent (e.g. TOAB). Reduction with NaBH4 was then conducted in the presence of an alkanethiol. Thus, the nucleation and growth of the gold particles and the attachment of the thiol molecules occurred nearly simultaneously. The resulting gold nanoparticles were B2–2.5 nm, smaller than those obtained by Rao and co-workers (4.1 nm).102 However, the nanoparticles contained nitrogenous surface impurities due to the phase transfer agent.75 Despite this contamination issue, the Brust–Schiffrinn procedure should be recognized for its ability to produce very fine metallic nanopaticles. In contrast, alkylamine was used as This journal is

c


Fig. 12 TEM images of nanoparticles. (1) Ag derived with hexadecanediol (HDD), (2) Au, (3) worm-like Pd, and (4) Pt from Pt(IV), derived with tetrabutylammonium borohydride (TBAB). Alloy nanoparticles of (5) Ag–Au, (6) Pd–Pt, (7) Pt–Rh, and (8) Pt–Ru, synthesized by co-reduction of the metal precursors with TBAB. Core–shell nanoparticles of (9) 7.4-nm Au@Ag, (10) 12.7-nm Au@Ag, (11) 3.9-nm Pt@Ag, and (12) 9.2-nm Pt@Ag, synthesized by seed-mediated growth. Semiconductor nanocrystals of (13) Ag2S, (14) CdS, (15) HgS, and (16) PbS. Hybrid nanoparticles of (17) Ag2S–Au, (18) CdS–Au, (19) CuS–Au, (20) PbS–Au, (21) Ag2S–Ag, (22) CdS–Ag, (23) CuS–Ag, and (24) PbS–Ag. Reproduced from ref. 202 with permission from the Nature Publishing Group.

Chem. Soc. Rev., 2011, 40, 1672–1696

1687


View Article Online

not necessary; DDA could reduce the noble metal ions sufficiently in the presence of semiconductor nanocrystals. The synthesis of hybrids of Ag2S–Au, CdS–Au, CuS–Au, PbS–Au, Ag2S–Ag, CdS–Ag, CuS–Ag and PbS–Ag was successfully attained (Fig. 12).202 These materials with novel nanostructures and multiple functionalities would be useful for a variety of applications. Besides its broad applicability, our phase transfer protocol demonstrated a number of advantages: (1) good ion uptake by the complexing agent, allowing for fast binding with the metal ion, (2) high stability against hydrolysis, (3) selective ion complexation of heavy metals, along with no affinity for alkali or alkaline earth ions whose concentrations are usually high in water and soil, (4) sufficiently high binding strength for the metal ions to be extracted, and (5) preference of the derived metal complex for the organic phase over the aqueous phase, which would be of interest for applications in environmental remediation, such as the extraction of heavy metals from water and soil.213

on core–shell Au@Ag and Au@Pt nanoparticles prepared by the seed-mediated growth method.218–221 This could be attributed in part to the different experimental conditions used in the formation of different core–shell systems.

4. Applications of phase transfer of nanoparticles

4.2 Replacement reactions in organic media

A number of specific applications of phase transfer in nanoscience and nanotechnology has been reported, mostly over the past few years.

Our general phase transfer protocol for metal ions from water to toluene enabled the systematic investigation of the replacement reaction in an organic medium.202 The replacement reaction between Ag nanoparticles and HAuCl4 in aqueous phase has been studied extensively.222–224 The resulting Au or Ag–Au alloy nanoshells were reported to have a hollow interior with smooth, pinhole-free surface, as well as homogeneous and highly crystalline walls. However, our replacement reactions between Ag nanoparticles and Au(III) and Pt(II) in organic media have led to a number of different observations from those reported in the literature. (i) A significant shrinkage of the Ag templates occurred during the course of the replacement reaction. (ii) Au or Pt atoms were deposited on the surface of the shrunken Ag templates, resulting in core–shell Ag@Au or Ag@Pt structures instead of Au or Pt nanoshells (Fig. 14). The core–shell Ag@Au nanoparticles showed Au-like optical properties. Similar results have been described by Yang et al.,225 whereby the hydrophobic Ag templates were transferred from the aqueous phase, and the replacement reaction was performed at room temperature. Instead of core–shell structures, the replacement reaction between Ag nanoparticles and Pd(II) in toluene was found to result in Ag–Pd alloy nanoparticles.202 Excellent atomic ordering was found within such nanoparticles, with no lattice mismatch (Fig. 14d). The uniform contrast throughout the particle further demonstrated the atomic level mixing of the

4.1

Core–shell structures for bimetallic nanoparticles

Yang et al. found that their phase transfer protocol was both particle size and chemical composition dependent.214–217 Generally, Au and Ag nanoparticles of >10 nm and 15 nm, respectively, could not be transferred to toluene even with ethanol as the mediating solvent. This size-dependent phenomenon could be attributed to the surface area effect mentioned earlier.158 The hydrophobic forces provided by the exchanging ligands on larger nanoparticles would be too weak to pull the gold particles to the organic phase. It was also found that the citrate-stabilized Ag and Ru nanoparticles could be transferred only by using alkylamine as transfer agent, and not with alkanethiol.214,216 The above studies led to a systematic investigation of bimetallic systems formed between Ag, Au, Pt and Ru via seed-mediated growth method (Table 1).217 Core–shell nanoparticles could not always be formed between two different metals by the seed-mediated growth process. For example, core–shell Ag@Pt were formed when Ag nanoparticles were used as seeds, whereas a physical mixture of two monometallic nanoparticles was produced when Pt nanoparticles was used as seeds (Fig. 13). The findings noted in Table 1 were at odds with some previously reported results

Fig. 13 TEM images of (a) core–shell Ag@Pt nanoparticles seeded by citrate-stabilized Ag nanoparticles, and (b) a physical mixture of Ag and Pt nanoparticles seeded by citrate-stabilized Pt nanoparticles. Reproduced from ref. 217 with permission from SpringerLink.

Table 1 Summary of the bimetallic systems formed between different metals by the seed-mediated growth method. Reproduced from ref. 217 with permission from the SpringerLink Ag–Au Seed Ag C–S EMT

Au P–M

Ag–Pt Seed Ag C–S EMT

Pt P–M

Ru–Ag Seed Ru C–S EMA

Ag P–M

Pt–Au Seed Pt Au C–S P–M EMA or EMT

Au–Ru Seed Au Ru C–S P–M EMA or EMT

Pt–Ru Seed Pt C–S EMT

Ru P–M

C–S = core–shell. P–M = physical mixture. EMA = ethanol-mediated amine transfer. EMT = ethanol-mediated thiol transfer. For particles transferable by both EMA and EMT, the EMA method was used because experimentally it facilitated the separation of the oil and water phases.

1688

Chem. Soc. Rev., 2011, 40, 1672–1696

This journal is

c


View Article Online

and geometry of nanoparticles formed in replacement reactions could be altered by the choice of solvent. It would be of great interest to tailor nanoparticles of different sizes, morphologies, structures and compositions through replacement reactions conducted in different solvents.


4.3 Catalytic applications of Pt nanoparticles of different structures

Fig. 14 TEM images of (a) Ag@Au and (b) Ag@Pt core–shell nanoparticles, and (c) alloy Ag–Pd nanoparticles, synthesized by replacement reaction. (d) HRTEM image of an alloy Ag–Pd nanoparticle. Reproduced from ref. 202 with permission from the Nature Publishing Group.

metallic components. Analogous to the formation of Ag–Au or Ag–Pd alloy nanoparticles by replacement reaction between Ag nanoparticles and Au or Pd metal ions in aqueous solution, the alloying process might be rationalized by the rapid interdiffusion of metal atoms as a result of the reduced dimension of silver templates, elevated temperature, and the large number of interfacial vacancy defects generated by the replacement reaction.226 A different explanation was given for the blue shift in the SPR peak of the system containing core–shell Ag@Au nanoparticles and excess hydrophobized AuCl4.225 It was postulated that the peak shift was caused by reaction between Au shell and the excess Au(III) ions, instead of the collapse of the Au nanoshells as described by Xia and coworkers,128,224 since there was no evidence for the collapse of the core–shell structures in the former case. The reactions between Au shell and Au(III) ions led to a lower Au content in the core–shell composition (Fig. 15), causing a SPR peak shift to a shorter wavelength. The very different conclusions from previous studies could be attributed to the different experimental conditions (toluene vs. water as the reaction medium). This suggested that the structure

Fig. 15 TEM images of DDA-stabilized Ag nanoparticles (a) right after and (b) 4 h after the addition of hydrophobized AuCl4 ions. Reproduced from ref. 225 with permission from the American Chemical Society.

This journal is

c


Using the ethanol-mediated phase transfer approach, Yang et al. derived Pt nanoparticles with different structures for the methanol oxidation reaction.227 Core–shell Ag@Pt nanoparticles were prepared by the successive reduction of Ag and Pt precursor salts using citrate ions, followed by treatment in a BSPP solution to remove the core materials. BSPP has a strong affinity to form a water-soluble coordinating compound with Ag at room temperature, leaving behind Pt hollow nanospheres (Fig. 16).227,228 It was found that the removal of the Ag cores from the Ag@Pt nanoparticles did not lead to the collapse of the spherical geometry. In this case, BSPP served not only as the reactant to remove the Ag core, but also as the stabilizer substituting for citrate on the core–shell Ag@Pt nanoparticles. Both the Ag@Pt nanoparticles and the Pt hollow nanospheres could be transferred from water to toluene, and be stabilized in the latter by DDA using the ethanol-mediated method described in section 3.1. After phase transfer, the Pt hollow nanospheres and the Ag@Pt nanoparticles (of the same size) consisted of the same stabilizer molecules (DDA) adsorbed on their surfaces. These two materials provided for the basis of different Pt nanostructures for catalytic studies. Since the Ag cores of the Ag@Pt nanoparticles were catalytically inactive in methanol oxidation, the only difference between hollow Pt nanospheres and solid Ag@Pt nanoparticles was the presence of a hollow interior in the former.227 The electrocatalytic activities of the two systems were tested at room temperature for methanol oxidation. The Pt hollow nanospheres showed a much higher specific activity than the Ag@Pt nanoparticles (Fig. 17A). However, when the

Fig. 16 TEM images of (A,B) core–shell Ag@Pt nanoparticles, and (C,D) Pt hollow nanospheres obtained with BSPP treatment. Reproduced from ref. 227 with permission from the American Chemical Society.

Chem. Soc. Rev., 2011, 40, 1672–1696

1689


View Article Online

Fig. 17 (A) Cyclic voltammograms of Pt hollow nanospheres (black line) and core–shell Ag@Pt nanoparticles (red line) on a glassy carbon disk electrode in an electrolyte of 0.5 M H2SO4 and 0.6 M methanol. Catalyst loading: 0.07 mg cm2 for Pt hollow nanospheres and 0.11 mg cm2 for Ag@Pt nanoparticles. Scan rate = 50 mV s1. (B) Normalization of the cyclic voltammograms by the available surface areas of the catalysts. Reproduced from ref. 227 with permission from the American Chemical Society.

current densities in the voltammograms were normalized by the accessible surface areas (Fig. 17B), i.e. assuming that both the inner and outer Pt surfaces were equally accessible to methanol with the same intrinsic catalytic activity, the surface area for Pt hollow nanospheres was calculated to be B1.52 times that of the corresponding solid Ag@Pt nanoparticles. This demonstrated that the higher catalytic activity of the Pt hollow nanospheres was a surface area effect. While the conclusion was not unexpected and has been suggested previously by Liang and co-workers,229 this study allowed for a comparison to be made with controlled particle size and surface functionalization. In contrast, Liang and co-workers have used different stabilizers to protect the hollow and solid Pt nanoparticles (citrate ions and PVP, respectively), and one could not rule out, a priori, the effect of different stabilizer molecules on the catalytic property of Pt. 4.4 Size effect on the binding affinity of thiol and amine for small Pt nanoparticles Yang et al. examined the binding affinities of 1-dodecanethiol and DDA for Pt nanoparticles of different sizes.230 Pt nanoparticles of 5 nm and 12 nm were first synthesized using NaBH4 reduction of H2PtCl6 and K2PtCl4, respectively, in the presence of citrate ions. They were then transferred to toluene using the ethanol-mediated method. The resulting thiol- or amine-stabilized Pt nanoparticles were subjected to ligand exchange with free amine or thiol, respectively. The binding affinity of thiol to the Pt nanoparticles increased with increasing particle size, whereas a reverse trend was observed for the binding affinity of amine. The higher binding affinity of DDA for small Pt nanoparticles was in accordance with the results from first principles calculation with density functional theory (DFT), which substantiated the existence of strong binding between the –NH2 group and the surface of small metal particles.231 1690

Chem. Soc. Rev., 2011, 40, 1672–1696

Fig. 18 Cyclic voltammograms of (–) 5-nm thiol-Pt, ( ) 12-nm thiolPt, ( ) 5-nm amine-Pt, ( ) 12-nm amine-Pt, and ( ) 5-nm citrate-Pt nanoparticles on a glassy carbon disk electrode in an electrolyte of 0.5 M H2SO4 and 0.6 M methanol. Reproduced from ref. 230 with permission from Elsevier.

The binding affinity of stabilizer for Pt strongly influenced the catalytic activity of the latter in the direct methanol fuel cell (DMFC) reaction. There was an inverse relationship between the affinity of stabilizer and the catalytic activity of Pt nanoparticles in methanol electrooxidation (Fig. 18).230 Smaller Pt nanoparticles were more active in methanol oxidation when stabilized by 1-dodecanethiol. Conversely larger Pt nanoparticles were more active if DDA was used as the stabilizer.

5. Transfer of organically soluble quantum dots to water Quantum dots (QDs) are of great interest to biolabeling and bioimaging applications.232–235 These semiconductor nanoparticles exhibit high quantum yield, high molar extinction (B10–100 times that of organic dyes),236,237 broad absorption with narrow, symmetric photoluminescence (PL) peak (full-width at half-maximum B25–40 nm) tunable in the ultraviolet to near-infrared range, large effective Stokes shifts, high resistance to photobleaching, and exceptional resistance to photodegradation and chemical degradation.238–240 Most high-quality QDs, synthesized only at high temperatures and in organic solvents, have poor aqueous solubility, which limited their applications in biological systems. An effective phase transfer method would circumvent such a deficiency. Over the past 20 years, various methods have been developed for the phase transfer of QDs, and the reports before 2005 were reviewed by Medintz et al.71 The phase transfer of QDs from organic solvents to aqueous phase could be summarized by three main strategies. The early attempts involved the coating of QDs with polymerized silica shells functionalized with polar groups, rendering the resulting core–shell nanoparticles water-soluble.232,241 This approach was similar to a procedure described for coating gold This journal is

c



View Article Online

nanoparticles.242 It allowed the interaction with biological samples to be achieved via the functionalization of the silica shell. The second method was analogous to the ligand exchange approach for metallic nanoparticles, involving the substitution of trioctylphosphine (TOP)/TOPO ligands with bifunctional ligands. These ligands possessed a surface-anchoring moiety (e.g. thiol) to bind to the inorganic QD surface, and an opposing hydrophilic end group (e.g. hydroxyl and carboxyl) to achieve water-compatibility.234,243–245 The third method preserved the native TOP/TOPO ligands on the QDs, and used amphiphilic diblock and triblock copolymers and phospholipids to tightly interleave/interdigitate the alkylphosphine ligands through hydrophobic–hydrophobic interactions, while the exposed hydrophilic group allowed for aqueous dispersion and further functionalization.246–250 The strategies developed after 2005 could still be categorized under the three major routes summarized above.251–256 A unique phase transfer method was presented by Qin et al., who found that the reversible phase transfer of 2-(diethylamino)ethanethiol (DEAET)-stabilized CdTe nanoparticles between aqueous and toluene phases could be driven by temperature (Fig. 19).257 To understand this phase transfer process, a thermodynamic model was established based on three assumptions: (i) the toluene and aqueous phases have low mutual solubility, (ii) the DEAET stabilizer covered the QD surface uniformly as a shell/core assembly, and (iii) the volume occupied by the QDs was much smaller than the volume occupied by the toluene and aqueous phases. The model revealed that unlike the DEAET molecules, the net hydrophobic force of DEAET-stabilized CdTe nanoparticles decreased with increasing temperature, thus favoring the transfer of nanoparticles into the aqueous phase. Recently, we developed a phase transfer protocol that was general enough to reversibly transfer QDs and metallic nanoparticles between organic and aqueous phases (Scheme 18). It involved ligand exchange based transfer from organic medium to aqueous phase, followed by electrostatic interaction based reversible transfer between aqueous and organic phases. Methanol was used as a mediating solvent to improve the interfacial contact between the QDs or metallic nanoparticles and the organic- or aqueous-insoluble ligands, greatly enhancing the transfer efficiency.258 The initial transfer of organically soluble QDs or metallic nanoparticles to water was actually a reverse process of the

Fig. 19 Photographs of orange-emitting DEAET-stabilized CdTe nanoparticles taken under (left) daylight and (right) a UV lamp at (A) 0 1C, (B) 27 1C, and (C) 70 1C. The upper layer in the vials was toluene, and the lower layer was water. Reproduced from ref. 257 with permission from Wiley-VCH.

This journal is

c


phase transfer protocol described in section 3, whereby methanol was used instead of ethanol as a mediating solvent. Glutathione tetramethylammonium (GTMA) salt was used instead of DDA or dodecanethiol as transfer agent and capping agent to stabilize QDs or metallic nanoparticles transferred from organic solvents. With our methanol-mediated method, a wide variety of QDs and metallic nanoparticles with different sizes or morphologies could be effectively transferred from organic phase to aqueous phase (Fig. 20). The QD or metal hydrosols thus obtained were very stable, and no agglomeration was observed after several months of storage in air. The subsequent reversible phase transfer processes between the aqueous and organic phases were successfully performed repeatedly.

6. Conclusions and outlook This review presented the major advances in the phase transfer of inorganic nanoparticles between aqueous and organic phases, and its applications in nanoscience and nanotechnology. Phase transfer of nanoparticles allowed one to take advantage of nanoparticle synthesis in aqueous and organic solvents, and subsequently transfer the nanoparticles to different environments for further applications. The efforts of many leading research groups have led to a rich variety of phase transfer protocols. Future research challenges lie in the application of phase transfer for the synthesis of composite nanoparticles (e.g. semiconductor–metal nanocomposites) and complex nanoparticles of well-controlled sizes, morphologies, compositional dispersion and surface functionalities. The successful derivation and scalable synthesis of such unique nanostructures would provide for major breakthroughs in optoelectronic, magnetic, biological, medical, chemical, energy and environmental applications. We have reviewed some recent applications of phase transfer of nanoparticles between aqueous and non-polar organic environments. Some of them are based on a highly efficient procedure, i.e. ethanol-mediated amine or thiol transfer. The ethanol-mediated phase transfer method is highly useful towards indentifying/separating core–shell structures in bimetallic nanoparticles because the phase transfer agent is only sensitive to the surface composition. Such selectivity would also be valuable for validating the products formed in successive reduction reactions, whereby monometallic nanoparticles and bimetallic nanoparticles may be generated. The generic characteristics of the phase transfer protocol that we have developed in transferring metal ions such as Ag+, Au3+, Cu2+, Ir4+, Ni2+, Os3+, Pd2+, Pt4+, Rh3+ and Ru3+ from the aqueous phase to toluene offer the possibility of performing the seed-mediated growth or co-reduction process in the organic media. Such studies might help to overcome the current difficulties experienced in the aqueous phase synthesis of bimetallic core–shell structures. QDs capped by mononucleotides or polynucleotides have been successfully transferred from water to toluene using the ethanol-mediated phase transfer method. The ligand exchange process could dislodge the DNA ligands from the QDs. This release of DNA ligands could overcome the irreversible binding of DNA on the QD surface. It might be useful in Chem. Soc. Rev., 2011, 40, 1672–1696

1691


View Article Online

Scheme 18 Schematic showing the functionalization of QDs and metallic nanoparticles. (1) OA shell on the particles rendered the particles hydrophobic and soluble in organic solvents. (2) After replacement by GTMA, the negatively charged carboxylate groups rendered the particles hydrophilic, allowing for phase transfer from organic to aqueous phase. (3) Upon electrostatic interaction with hexadecyltrimethylammonium bromide, the ion pairs between R4N+ and surface-bound anions provided for phase transfer back to organic solvents. (4) Removal of R4N+ by the formation of more hydrophobic compounds upon the addition of tetramethylammonium decanoate (TMAD) enabled the transfer of the particles back to aqueous phase. Reproduced from ref. 258 with permission from the Royal Society of Chemistry.

be important to ensure that the capping agents adopted in the phase transfer could be removed or would not interfere with the adsorption/reaction on the nanoparticles’ surface.

Acknowledgements This work is supported by the Institute of Bioengineering and Nanotechnology (Biomedical Research Council, Agency for Science, Technology and Research, Singapore).

References

Fig. 20 TEM images of (a) Au, (b) Ag, (c) CdS rods and (d) PbS transferred from chloroform to water using methanol as the mediating solvent and GTMA as the transfer agent. Reproduced from ref. 258 with permission from the Royal Society of Chemistry.

applications such as in vitro selection to screen libraries of nucleic acid sequences, allowing the target DNA to participate in amplification via polymerase chain reaction (PCR). Lastly, catalysis would benefit from the development of versatile phase transfer protocols. The latter would not only provide a greater variety of nanocomposite catalysts with multifunctionality, but also facilitate the controlled deposition of catalytic nanoparticles on support materials, minimizing the leaching and loss of catalysts. It would also 1692

Chem. Soc. Rev., 2011, 40, 1672–1696

1 R. Ferrando, J. Jellinek and R. L. Johnston, Chem. Rev., 2008, 108, 845–910. 2 R. W. Murray, Chem. Rev., 2008, 108, 2688–2720. 3 T. M. Jovin, Nat. Biotechnol., 2003, 21, 32–33. 4 W. U. Huynh, J. J. Dittmer and A. P. Alivisatos, Science, 2002, 295, 2425–2427. 5 V. I. Klimov, A. A. Mikhailovsky, S. Xu, A. Malko, J. A. Hollingsworth, C. A. Leatherdale, H.-J. Eisler and M. G. Bawendi, Science, 2002, 295, 1506–1508. 6 Y. Xia, Y. Xiong, B. Lim and S. E. Skrabalak, Angew. Chem., Int. Ed., 2009, 48, 60–103. 7 J. Zhang, K. Sasaki, E. Sutter and R. R. Adzic, Science, 2007, 315, 220–222. 8 K. Watanabe, D. Menzel, N. Nilius and H.-J. Freund, Chem. Rev., 2006, 106, 4301–4320. 9 C. Bruda, X. Chen, R. Narayanan and M. A. El-Sayed, Chem. Rev., 2005, 105, 1025–1102. 10 M.-C. Daniel and D. Astruc, Chem. Rev., 2004, 104, 293–346. 11 L. Brus, Acc. Chem. Res., 2008, 41, 1742–1749. 12 C. B. Murray, C. R. Kagan and M. G. Bawendi, Annu. Rev. Mater. Sci., 2000, 30, 545–610. 13 A. P. Alivisatos, Science, 1996, 271, 933–937. 14 F. Kuemmeth, K. I. Bolotin, S. Shi and D. C. Ralph, Nano Lett., 2008, 8, 4506–4512. 15 V. Maheshwari, J. Kane and R. F. Saraf, Adv. Mater., 2008, 20, 284–287. 16 G. Markovich, C. P. Collier and J. R. Heath, Phys. Rev. Lett., 1998, 80, 3807–3810.

This journal is

c



View Article Online

17 C. P. Collier, R. J. Saykally, J. J. Shiang, S. E. Henrichs and J. R. Heath, Science, 1997, 277, 1978–1981. 18 S. Laurent, D. Forge, M. Port, A. Roch, C. Robic, L. V. Elst and R. N. Muller, Chem. Rev., 2008, 108, 2064–2110. 19 U. Jeong, X. Teng, Y. Wang, H. Yang and Y. Xia, Adv. Mater., 2007, 19, 33–60. 20 S. Sun, Adv. Mater., 2006, 18, 393–403. 21 S. Yan, Y. Zhang, Y. Zhang and Z. Xiao, Inorg. Mater., 2009, 45, 193–197. 22 K.-M. Chi and P.-Y. Chen, J. Nanosci. Nanotechnol., 2008, 8, 3379–3385. 23 N. Poudyal, G. S. Chaubey, V. Nandwana, C.-b. Rong, K. Yano and J. P. Liu, Nanotechnology, 2008, 19, 355601. 24 C. Wang, Y. Hou, J. Kim and S. Sun, Angew. Chem., Int. Ed., 2007, 46, 6333–6335. 25 C. Mao, D. J. Solis, B. D. Reiss, S. T. Kottmann, R. Y. Sweeney, A. Hayhurst, G. Georgiou, B. Iverson and A. M. Belcher, Science, 2004, 303, 213–217. 26 K. K. Caswell, C. M. Bender and C. J. Murphy, Nano Lett., 2003, 3, 667–669. 27 Q. Mu, T. Chen and Y. Wang, Nanotechnology, 2009, 20, 345602. 28 B. P. Khanal and E. R. Zubarev, Angew. Chem., Int. Ed., 2009, 48, 6888–6891. 29 C. G. Wilson, P. N. Sisco, E. C. Goldsmith and C. J. Murphy, J. Mater. Chem., 2009, 19, 6332–6340. 30 C. O’Sullivan, R. D. Gunning, A. Sanyal, C. A. Barrett, H. Geaney, F. R. Laffir, S. Ahmed and K. M. Ryan, J. Am. Chem. Soc., 2009, 131, 12250–12257. 31 H. Zhang, S. Delikanli, Y. Qin, S. He, M. Swihart and H. Zeng, Nano Res., 2008, 1, 314–320. 32 D. Seo, C. I. Yoo, J. Jung and H. Song, J. Am. Chem. Soc., 2008, 130, 2940–2941. 33 D. Ma, X. Hu, H. Zhou, J. Zhang and Y. Qian, J. Cryst. Growth, 2007, 304, 163–168. 34 L. Yang, R. Xing, Q. Shen, K. Jiang, F. Ye, J. Wang and Q. Ren, J. Phys. Chem. B, 2006, 110, 10534–10539. 35 F. Kim, J. H. Song and P. Yang, J. Am. Chem. Soc., 2002, 124, 14316–14317. 36 X. Huang, H. Zhang, C. Guo, Z. Zhou and N. Zheng, Angew. Chem., Int. Ed., 2009, 48, 4808–4812. 37 D. Kim, N. Lee, M. Park, B. H. Kim, K. An and T. Hyeon, J. Am. Chem. Soc., 2009, 131, 454–455. 38 L. Au, Y. Chen, F. Zhou, P. H. C. Camargo, B. Lim, Z.-Y. Li, D. S. Ginger and Y. Xia, Nano Res., 2008, 1, 441–449. 39 M. Scariot, D. O. Silva, J. D. Scholten, G. Machado, S. R. Teixeira, M. A. Novak, G. Ebeling and J. Dupont, Angew. Chem., Int. Ed., 2008, 47, 9075–9078. 40 Y. Zhang, M. E. Grass, J. N. Kuhn, F. Tao, S. E. Habas, W. Huang, P. Yang and G. A. Somorjai, J. Am. Chem. Soc., 2008, 130, 5868–5869. 41 A. Shavel, B. Rodriguez-Gonzalez, M. Spasova, M. Farle and L. M. Liz-Marzan, Adv. Funct. Mater., 2007, 17, 3870–3876. 42 C.-H. Kuo, C.-H. Chen and M. H. Huang, Adv. Funct. Mater., 2007, 17, 3773–3780. 43 C. Wang, H. Daimon, Y. Lee, J. Kim and S. Sun, J. Am. Chem. Soc., 2007, 129, 6974–6975. 44 S. H. Im, Y. T. Lee, B. Wiley and Y. Xia, Angew. Chem., Int. Ed., 2005, 44, 2154–2157. 45 Y. Sun and Y. Xia, Science, 2002, 298, 2176–2179. 46 S. S. Shankar, A. Rai, B. Ankamwar, A. Singh, A. Ahmad and M. Sastry, Nat. Mater., 2004, 3, 482–488. 47 R. Jin, Y. Cao, C. A. Mirkin, K. L. Kelly, G. C. Schatz and J. G. Zheng, Science, 2001, 294, 1901–1903. 48 M. Sastry, Curr. Sci., 2003, 85, 1735–1745. 49 Z. Liu, J. E. Hu, Q. Wang, K. Gaskell, A. I. Frenkel, G. S. Jackson and B. Eichhorn, J. Am. Chem. Soc., 2009, 131, 6924–6925. 50 X.-B. Zhang, J.-M. Yan, S. Han, H. Shioyama and Q. Xu, J. Am. Chem. Soc., 2009, 131, 2778–2779. 51 J. Luo, L. Wang, D. Mott, P. N. Njoki, Y. Lin, T. He, Z. Xu, B. N. Wanjana, I.-I. S. Lim and C.-J. Zhong, Adv. Mater., 2008, 20, 4342–4347. 52 S. Alayoglu, A. U. Nilekar, M. Mavrikakis and B. Eichhorn, Nat. Mater., 2008, 7, 333–338.

This journal is

c


53 P. Mani, R. Srivastava and P. Strasser, J. Phys. Chem. C, 2008, 112, 2770–2778. 54 J. Zhang, F. H. B. Lima, M. H. Shao, K. Sasaki, J. X. Wang, J. Hanson and R. R. Adzic, J. Phys. Chem. B, 2005, 109, 22701–22704. 55 S.-W. Kim, M. Kim, W. Y. Lee and T. Hyeon, J. Am. Chem. Soc., 2002, 124, 7642–7643. 56 C.-H. Liang, C.-C. Wang, Y.-C. Lin, C.-H. Chen, C.-H. Wong and C.-Y. Wu, Anal. Chem., 2009, 81, 7750–7756. 57 J. Shin, R. M. Anisur, M. K. Ko, G. H. Im, J. H. Lee and I. S. Lee, Angew. Chem., Int. Ed., 2009, 48, 321–324. 58 J. Gao, G. Liang, J. S. Cheung, Y. Pan, Y. Kuang, F. Zhao, B. Zhang, X. Zhang, E. X. Wu and B. Xu, J. Am. Chem. Soc., 2008, 130, 11828–11833. 59 N. Krasteva, I. Besnard, B. Guse, R. E. Bauer, K. Mullen, A. Yasuda and T. Vossmeyer, Nano Lett., 2002, 2, 551–555. 60 T. A. Taton, C. A. Mirkin and R. L. Letsinger, Science, 2000, 289, 1757–1760. 61 M. Lessard-Viger, M. Rioux, L. Rainville and D. Boudreau, Nano Lett., 2009, 9, 3066–3071. 62 A. M. Gobin, M. H. Lee, N. J. Halas, W. D. James, R. A. Drezek and J. L. West, Nano Lett., 2007, 7, 1929–1934. 63 Y. Bao, H. Calderon and K. M. Krishnan, J. Phys. Chem. C, 2007, 111, 1941–1944. 64 Y. Lei and W.-K. Chim, J. Am. Chem. Soc., 2005, 127, 1487–1492. 65 H. Hirai and H. Aizawa, J. Colloid Interface Sci., 1993, 161, 471–474. 66 A. P. Kulkarni, K. Munechika, K. M. Noone, J. M. Smith and D. S. Ginger, Langmuir, 2009, 25, 7932–7939. 67 S. Underwood and P. Mulvaney, Langmuir, 1994, 10, 3427–3430. 68 J. Turkevich, G. Garton and P. C. Stevenson, J. Colloid Sci., 1954, 9, 26–35. 69 J. Turkevich, P. C. Stevenson and J. Hillier, Discuss. Faraday Soc., 1951, 11, 55–75. 70 W. van Megen and S. M. Underwood, J. Chem. Phys., 1989, 91, 552–559. 71 I. L. Medintz, H. T. Uyeda, E. R. Goldman and H. Mattoussi, Nat. Mater., 2005, 4, 435–446. 72 X. M. Lin, H. M. Jaeger, C. M. Sorensen and K. J. Klabunde, J. Phys. Chem. B, 2001, 105, 3353–3357. 73 M. Hasan, D. Bethell and M. Brust, J. Am. Chem. Soc., 2002, 124, 1132–1133. 74 A. C. Templeton, W. P. Wuelfing and R. W. Murray, Acc. Chem. Res., 2000, 33, 27–36. 75 M. Brust, M. Walker, D. Bethell, D. J. Schiffrin and R. Whyman, J. Chem. Soc., Chem. Commun., 1994, 801–802. 76 M. Brust, J. Fink, D. Bethell, D. J. Schiffrin and C. Kiely, J. Chem. Soc., Chem. Commun., 1995, 1655–1656. 77 K. Meguro, M. Torizuka and K. Esumi, Bull. Chem. Soc. Jpn., 1988, 61, 341–345. 78 K. Meguro, T. Tano, K. Torigoe, H. Nakamura and K. Esumi, Colloids Surf., 1988/1989, 34, 381–388. 79 M. J. Hostetler, J. E. Wingate, C.-J. Zhong, J. E. Harris, R. W. Vachet, M. R. Clark, J. D. Londono, S. J. Green, J. J. Stokes, G. D. Wignall, G. L. Glish, M. D. Porter, N. D. Evans and R. W. Murray, Langmuir, 1998, 14, 17–30. 80 M. J. Hostetler, A. C. Templeton and R. W. Murray, Langmuir, 1999, 15, 3782–3789. 81 R. L. Donkers, Y. Song and R. W. Murray, Langmuir, 2004, 20, 4703–4707. 82 V. L. Jimenez, D. G. Georganopoulou, R. J. White, A. S. Harper, A. J. Mills, D. Lee and R. W. Murray, Langmuir, 2004, 20, 6864–6870. 83 R. H. Terrill, T. A. Postlethwaite, C.-h. Chen, C.-D. Poon, A. Terzis, A. Chen, J. E. Hutchison, M. R. Clark, G. Wignall, J. D. Londono, R. Superfine, M. Falvo, C. S. Johnson, Jr., E. T. Samulski and R. W. Murray, J. Am. Chem. Soc., 1995, 117, 12537–12548. 84 S. R. Johnson, S. D. Evans and R. Brydson, Langmuir, 1998, 14, 6639–6647. 85 L. A. Porter, Jr., D. Ji, S. L. Westcott, M. Graupe, R. S. Czernuszewicz, N. J. Halas and T. R. Lee, Langmuir, 1998, 14, 7378–7386. 86 R. L. Whetten, J. T. Khoury, M. M. Alvarez, S. Murthy, I. Vezmar, Z. L. Wang, P. W. Stephens, C. L. Cleveland, W.D. Luedtke and U. Landman, Adv. Mater., 1996, 8, 428–433.

Chem. Soc. Rev., 2011, 40, 1672–1696

1693


View Article Online

87 S. Chen, A. C. Templeton and R. W. Murray, Langmuir, 2000, 16, 3543–3548. 88 T. G. Schaaff, M. N. Shafigullin, J. T. Khoury, I. Vezmar, R. L. Whetten, W. G. Cullen, P. N. First, C. Gutierrez-Wing, J. Ascensio and M. J. Jose-Yacaman, J. Phys. Chem. B, 1997, 101, 7885–7891. 89 T. G. Schaaff and R. L. Whetten, J. Phys. Chem. B, 1999, 103, 9394–9396. 90 S. L. Horswell, C. J. Kiely, I. A. O’Neil and D. J. Schiffrin, J. Am. Chem. Soc., 1999, 121, 5573–5574. 91 D. V. Leff, L. Brandt and J. R. Heath, Langmuir, 1996, 12, 4723–4730. 92 K. Wikander, C. Petit, K. Holmberg and M.-P. Pileni, Langmuir, 2006, 22, 4863–4868. 93 P. R. Selvakannan, S. Mandal, R. Pasricha and M. Sastry, J. Colloid Interface Sci., 2004, 279, 124–131. 94 A. N. Grace and K. Pandian, J. Phys. Chem. Solids, 2007, 68, 2278–2285. 95 Y. Chen and X. Wang, Mater. Lett., 2008, 62, 2215–2218. 96 X. Song, S. Sun, W. Zhang and Z. Yin, J. Colloid Interface Sci., 2004, 273, 463–469. 97 A. Demortiere and C. Petit, Langmuir, 2007, 23, 8575–8584. 98 T. G. Schaaff and A. J. Rodinone, J. Phys. Chem. B, 2003, 107, 10416–10422. 99 H. Shi, X. Fu, X. Zhou and H. Zhengshui, J. Mater. Chem., 2006, 16, 2097–2101. 100 W. Song, C. Wu, H. Yin, X. Liu, P. Sa and J. Hu, Anal. Lett., 2008, 41, 2844–2859. 101 J. A. Dahl, B. L. S. Maddux and J. E. Hutchison, Chem. Rev., 2007, 107, 2228–2269. 102 K. V. Sarathy, G. U. Kulkarni and C. N. R. Rao, Chem. Commun., 1997, 537–538. 103 K. V. Sarathy, G. Raina, R. T. Yadav, G. U. Kulkarni and C. N. R. Rao, J. Phys. Chem. B, 1997, 101, 9876–9880. 104 S.-Y. Zhao, S.-H. Chen, S.-Y. Wang, D.-G. Li and H.-Y. Ma, Langmuir, 2002, 18, 3315–3318. 105 R. Voggu, K. Biswas, A. Govindaraj and C. N. R. Rao, J. Phys. Chem. B, 2006, 110, 20752–20755. 106 J. Yang, J. Y. Lee, T. C. Deivaraj and H. P. Too, Langmuir, 2003, 19, 10361–10365. 107 T.-D. Nguyen, D. Mrabet and T.-O. Do, J. Phys. Chem. C, 2008, 112, 15226–15235. 108 T.-D. Nguyen and T.-O. Do, Langmuir, 2009, 25, 5322–5332. 109 T.-D. Nguyen and T.-O. Do, J. Phys. Chem. C, 2009, 113, 11204–11214. 110 T.-D. Nguyen, C.-T. Dinh and T.-O. Do, Langmuir, 2009, 25, 11142–11148. 111 J. Yang, T. C. Deivaraj, H. P. Too and J. Y. Lee, J. Phys. Chem. B, 2004, 108, 2181–2185. 112 X. Fu, Y. Wang, N. Wu, L. Gui and Y. Tang, J. Colloid Interface Sci., 2001, 243, 326–330. 113 E. Hammarberg and C. Feldmann, Chem. Mater., 2009, 21, 771–774. 114 C. D. Bain, J. Evall and G. M. Whitesides, J. Am. Chem. Soc., 1998, 111, 7155–7164. 115 W. P. Wuelfing and R. W. Murray, J. Phys. Chem. B, 2002, 106, 3139–3145. 116 J. Yang, J. Y. Lee, H.-P. Too, G.-M. Chow and L. M. Gan, Chem. Phys., 2006, 323, 304–312. 117 J. Yang, B.-K. Pong, J. Y. Lee and H.-P. Too, J. Inorg. Biochem., 2007, 101, 824–830. 118 V. J. Gandubert and R. B. Lennox, Langmuir, 2005, 21, 6532–6539. 119 C. R. Mayer, S. Neveu, C. Simonnet-Je´gat, C. DebiemmeChouvy, V. Cabuil and F. Secheresse, J. Mater. Chem., 2003, 13, 338–341. 120 L. A. Gearheart, H. J. Ploehn and C. J. Murphy, J. Phys. Chem. B, 2001, 105, 12609–12615. 121 K. C. Grabar, R. G. Freeman, M. B. Hommer and M. J. Natan, Anal. Chem., 2002, 67, 735–743. 122 I. Lisiecki, P.-A. Albouy and M. P. Pileni, Adv. Mater., 2003, 15, 712–716. 123 G. H. Woehrle, L. O. Brown and J. E. Hutchison, J. Am. Chem. Soc., 2005, 127, 2172–2183.

1694

Chem. Soc. Rev., 2011, 40, 1672–1696

124 C. Petit, A. Taleb and M.-P. Pileni, Adv. Mater., 1998, 10, 259–261. 125 G. Schmid, Inorg. Synth., 1990, 27, 214–218. 126 B. Wiley, Y. Sun and Y. Xia, Langmuir, 2005, 21, 8077–8080. 127 Y. Sun and Y. Xia, J. Am. Chem. Soc., 2004, 126, 3892–3901. 128 Y. Sun and Y. Xia, Nano Lett., 2003, 3, 1569–1572. 129 T. Teranishi, M. Hosoe, T. Tanaka and M. Miyake, J. Phys. Chem. B, 1999, 103, 3818–3827. 130 T. Teranishi and M. Miyake, Chem. Mater., 1998, 10, 594–600. 131 N. Lala, S. P. Lalbegi, S. D. Adyanthaya and M. Sastry, Langmuir, 2001, 17, 3766–3768. 132 Z. Peng, T. Walther and K. Kleinermanns, J. Phys. Chem. B, 2005, 109, 15735–15740. 133 M. A. Bryant and R. M. Crooks, Langmuir, 1993, 9, 385–387. 134 J. C. Garcia-Martinez and R. M. Crooks, J. Am. Chem. Soc., 2004, 126, 16170–16178. 135 O. M. Wilson, R. W. J. Scott, J. C. Garcia-Martinez and R. M. Crooks, Chem. Mater., 2004, 16, 4202–4204. 136 Y.-G. Kim, S.-K. Oh and R. M. Crooks, Chem. Mater., 2004, 16, 167–172. 137 M. Zhao and R. M. Crooks, Chem. Mater., 1999, 11, 3379–3385. 138 O. M. Wilson, R. W. J. Scott, J. C. Garcia-Martinez and R. M. Crooks, J. Am. Chem. Soc., 2005, 127, 1015–1024. 139 J. Liu, M. Anand and C. B. Roberts, Langmuir, 2006, 22, 3964–3971. 140 J. Liu, J. Sutton and C. B. Roberts, J. Phys. Chem. C, 2007, 111, 11566–11576. 141 N. Wu, L. Fu, M. Su, M. Aslam, K. C. Wong and V. P. Dravid, Nano Lett., 2004, 4, 383–386. 142 M. Anand, P. W. Bell, X. Fan, R. M. Enick and C. B. Roberts, J. Phys. Chem. B, 2006, 110, 14693–14701. 143 P. C. Ohara and W. M. Gelbart, Langmuir, 1998, 14, 3418–3424. 144 P. S. Shah, B. J. Novick, H. S. Hwang, K. T. Lim, R. G. Carbonell, K. P. Johnston and B. A. Korgel, Nano Lett., 2003, 3, 1671–1675. 145 M. C. McLeod, C. L. Kitchens and C. B. Roberts, Langmuir, 2005, 21, 2414–2418. 146 H. Itoh, K. Naka and Y. Chujo, J. Am. Chem. Soc., 2004, 126, 3026–3027. 147 G.-T. Wei, Z. Yang, C.-Y. Lee, H.-Y. Yang and C. R. C. Wang, J. Am. Chem. Soc., 2004, 126, 5036–5037. 148 V. Mevellec, B. Leger, M. Mauduit and A. Roucoux, Chem. Commun., 2005, 2838–2839. 149 T. Welton, Chem. Rev., 1999, 99, 2071–2084. 150 J. G. Huddleston, A. E. Visser, W. M. Reichert, H. D. Willauer, G. A. Broker and R. D. Rogers, Green Chem., 2001, 3, 156–164. 151 J. Dupont, G. S. Fonseca, A. P. Umpierre, P. F. P. Fichtner and S. R. Teixeira, J. Am. Chem. Soc., 2002, 124, 4228–4229. 152 C. W. Scheeren, G. Machado, J. Dupont, P. F. P. Fichtner and S. R. Texeira, Inorg. Chem., 2003, 42, 4738–4742. 153 C. W. Scheeren, G. Machado, S. R. Teixeira, J. Morais, J. B. Domingos and J. Dupont, J. Phys. Chem. B, 2006, 110, 13011–13020. 154 C. W. Scheeren, J. B. Domingos, G. Machado and J. Dupont, J. Phys. Chem. C, 2008, 112, 16463–16469. 155 V. Chechik, M. Zhao and R. M. Crooks, J. Am. Chem. Soc., 1999, 121, 4910–4911. 156 R. M. Crooks, M. Zhao, L. Sun, V. Chechik and L. K. Yeung, Acc. Chem. Res., 2001, 34, 181–190. 157 H. Yao, O. Momozawa, T. Hamatani and K. Kimura, Bull. Chem. Soc. Jpn., 2000, 73, 2675–2678. 158 W. Cheng and E. Wang, J. Phys. Chem. B, 2004, 108, 24–26. 159 M. J. Hostetler, J. J. Stokes and R. W. Murray, Langmuir, 1996, 12, 3604–3612. 160 A. Kumar, P. Mukherjee, A. Guha, S. D. Adyantaya, A. B. Mandale, R. Kumar and M. Sastry, Langmuir, 2000, 16, 9775–9783. 161 H. Yao, O. Momozawa, T. Hamatani and K. Kimura, Chem. Mater., 2001, 13, 4692–4697. 162 J. Liu, J. Alvarez, W. Ong, E. Roman and A. E. Kaifer, J. Am. Chem. Soc., 2001, 123, 11148–11154. 163 K. S. Mayya and F. Caruso, Langmuir, 2003, 19, 6987–6993. 164 A. Kumar, S. Mandal, P. R. Selvakannan, R. Pasricha, A. B. Mandale and M. Sastry, Langmuir, 2003, 19, 6277–6282.

This journal is

c



View Article Online

165 A. Kumar, H. Joshi, R. Pasricha, A. B. Mandale and M. Sastry, J. Colloid Interface Sci., 2003, 264, 396–401. 166 S. Devarajan, B. Vimalan and S. Sampath, J. Colloid Interface Sci., 2004, 278, 126–132. 167 G. Meriguet, E. Dubois and R. Perzynski, J. Colloid Interface Sci., 2003, 267, 78–85. 168 W. Wang, S. Efrima and O. Regev, Langmuir, 1998, 14, 602–610. 169 B. L. V. Prasad, S. K. Arumugam, T. Bala and M. Sastry, Langmuir, 2005, 21, 822–826. 170 T. Bala, A. Swami, B. L. V. Prasad and M. Sastry, J. Colloid Interface Sci., 2005, 238, 422–431. 171 S. Si, E. Dinda and T. K. Mandal, Chem.–Eur. J., 2007, 13, 9850–9861. 172 J. M. McMahon and S. R. Emory, Langmuir, 2007, 23, 1414–1418. 173 X. Feng, H. Ma, S. Huang, W. Pan, X. Zhang, F. Tian, C. Gao, Y. Cheng and J. Luo, J. Phys. Chem. B, 2006, 110, 12311–12317. 174 N. Gao, J. Dong, H. Zhang, X. Zhou, G. Zhang and J. Eastoe, J. Colloid Interface Sci., 2006, 304, 388–393. 175 S.-Y. Zhao, R. Qiao, X. L. Zhang and Y. S. Kang, J. Phys. Chem. C, 2007, 111, 7875–7878. 176 E. W. Edwards, M. Chanana, D. Wang and H. Mohwald, Angew. Chem., Int. Ed., 2008, 47, 320–323. 177 S. Sun, C. B. Murray, D. Weller, L. Folks and A. Moser, Science, 2000, 287, 1989–1992. 178 J. Park, K. An, Y. Hwang, J.-G. Park, H.-J. Noh, J.-Y. Kim, J.-H. Park, N.-M. Hwang and T. Hyeon, Nat. Mater., 2004, 3, 891–895. 179 X. Lu, H.-Y. Tuan, B. A. Korgel and Y. Xia, Chem.–Eur. J., 2008, 14, 1584–1591. 180 G. Schmid, N. Klein and L. Korste, Polyhedron, 1988, 7, 605–608. 181 J. Simard, C. Briggs, A. K. Boal and V. M. Rotello, Chem. Commun., 2000, 1943–1944. 182 D. I. Gittins and F. Caruso, Angew. Chem., Int. Ed., 2001, 40, 3001–3004. 183 L. N. Lewis, Chem. Rev., 1993, 93, 2693–2726. 184 D. I. Gittins and F. Caruso, ChemPhysChem, 2002, 1, 110–113. 185 M. R. Knecht, J. C. Garcia-Martinez and R. M. Crooks, Langmuir, 2005, 21, 11981–11986. 186 M. Ji, W. Yang, Q. Ren and D. Lu, Nanotechnology, 2009, 20, 075101. 187 Y. Wang, J. F. Wong, X. Teng, X. Z. Lin and H. Yang, Nano Lett., 2003, 3, 1555–1559. 188 M. Gonzales and K. M. Krishnan, J. Magn. Magn. Mater., 2007, 311, 59–62. 189 M. Chen, W. H. Ding, Y. Kong and G. W. Diao, Langmuir, 2008, 24, 3471–3478. 190 A. Swami, A. Kumar and M. Sastry, Langmuir, 2003, 19, 1168–1172. 191 W. W. Yu, E. Chang, C. M. Sayes, R. Drezek and V. L. Colvin, Nanotechnology, 2006, 17, 4483–4487. 192 C. R. Mayer, E. Dumas and F. Secheresse, J. Colloid Interface Sci., 2008, 328, 452–457. 193 Z. Mao, J. Guo, S. Bai, T.-L. Nguyen, H. Xia, Y. Huang, P. Mulvaney and D. Wang, Angew. Chem., Int. Ed., 2009, 48, 4953–4956. 194 C. A. Mirkin, R. L. Letsinger, R. C. Mucic and J. J. Storhoff, Nature, 1996, 382, 607–609. 195 R. C. Mucic, J. J. Storhoff, C. A. Mirkin and R. L. Letsinger, J. Am. Chem. Soc., 1998, 120, 12674–12675. 196 L. He, M. D. Musick, S. R. Nicewarner, F. G. Salinas, S. J. Benkovic, M. J. Natan and C. D. Keating, J. Am. Chem. Soc., 2000, 122, 9071–9077. 197 N. Gaponik, D. V. Talapin, A. L. Rogach, A. Eychmuller and H. Weller, Nano Lett., 2002, 2, 803–806. 198 J. Yang, J. Y. Lee, T. C. Deivaraj and H.-P. Too, J. Colloid Interface Sci., 2004, 277, 95–99. 199 J. Yang, J. Y. Lee and H.-P. Too, Anal. Chim. Acta, 2007, 588, 34–41. 200 L. O. Brown and J. E. Hutchison, J. Am. Chem. Soc., 1997, 119, 12384–12385. 201 L. O. Brown and J. E. Hutchison, J. Am. Chem. Soc., 1999, 121, 882–883.

This journal is

c


202 J. Yang, E. H. Sargent, S. O. Kelley and J. Y. Ying, Nat. Mater., 2009, 8, 683–689. 203 B. L. Cushing, V. L. Kolesnichenko and C. L. O’Connor, Chem. Rev., 2004, 104, 3893–3946. 204 X. Wang, J. Zhuang, Q. Peng and Y. Li, Nature, 2005, 437, 121–124. 205 T. Mokari, E. Rothenberg, I. Popov, R. Costi and U. Banin, Science, 2004, 304, 1787–1790. 206 C. Pacholski, A. Kornowski and H. Weller, Angew. Chem., Int. Ed., 2004, 43, 4774–4777. 207 T. Mokari, C. G. Sztrum, A. Salant, E. Rabani and U. Banin, Nat. Mater., 2005, 4, 855–863. 208 D. K. Yi, S. T. Selvan, S. S. Lee, G. C. Papaefthymiou, D. Kundaliya and J. Y. Ying, J. Am. Chem. Soc., 2005, 127, 4990–4991. 209 W. Shi, H. Zeng, Y. Sahoo, T. Y. Ohulchanskyy, Y. Ding, Z. L. Wang, M. Swihart and P. N. Prasad, Nano Lett., 2006, 6, 875–881. 210 S. T. Selvan, P. K. Patra, C. Y. Ang and J. Y. Ying, Angew. Chem., Int. Ed., 2007, 46, 2448–2452. 211 J. Jiang, H. Gu, H. Shao, E. Devlin, G. C. Papaefthymiou and J. Y. Ying, Adv. Mater., 2008, 20, 4403–4407. 212 J. Yang and J. Y. Ying, Chem. Commun., 2009, 3187–3189. 213 D. M. Roundhill, in Extraction of Metals from Soils and Waters, Kluwer Academic/Plenum Publishers, New York, 2001. 214 J. Yang, J. Y. Lee and H.-P. Too, Anal. Chim. Acta, 2005, 537, 279–284. 215 J. Yang, J. Y. Lee, L. X. Chen and H.-P. Too, J. Nanosci. Nanotechnol., 2005, 5, 1095–1100. 216 J. Yang, J. Y. Lee and H.-P. Too, J. Phys. Chem. B, 2005, 109, 5468–5472. 217 J. Yang, J. Y. Lee and H.-P. Too, Plasmonics, 2006, 1, 67–78. 218 N. R. Jana, Analyst, 2003, 128, 954–956. 219 N. T. Flynn and A. A. Gewirth, J. Raman Spectrosc., 2002, 33, 243–251. 220 L. Lu, H. Wang, Y. Zhou, S. Xi, H. Zhang, J. Hu and B. Zhao, Chem. Commun., 2002, 144–145. 221 S. S. Shankar, A. Rai, A. Ahmad and M. Sastry, J. Colloid Interface Sci., 2004, 275, 496–502. 222 Y. Sun, B. Mayers and Y. Xia, Nano Lett., 2002, 2, 481–485. 223 Y. Sun and Y. Xia, Adv. Mater., 2004, 16, 264–268. 224 Y. Sun and Y. Xia, J. Am. Chem. Soc., 2004, 126, 3892–3901. 225 J. Yang, J. Y. Lee and H. P. Too, J. Phys. Chem. B, 2005, 109, 19208–19212. 226 Q. Zhang, J. Y. Lee, J. Yang, C. Boothroyd and J. Zhang, Nanotechnology, 2007, 18, 245605. 227 J. Yang, J. Y. Lee, H. P. Too and S. Valiyaveettil, J. Phys. Chem. B, 2006, 110, 125–129. 228 Y.-N. Tan, J. Yang, J. Y. Lee and D. I. C. Wang, J. Phys. Chem. C, 2007, 111, 14084–14090. 229 H.-P. Liang, H.-M. Zhang, J.-S. Hu, Y.-G. Guo, L.-J. Wan and C.-L. Bai, Angew. Chem., Int. Ed., 2004, 43, 1540–1543. 230 J. Yang, J. Y. Lee and H.-P. Too, Anal. Chim. Acta, 2006, 571, 206–210. 231 B.-K. Pong, J. Y. Lee and B. L. Trout, Langmuir, 2005, 21, 11599–11603. 232 M. J. Bruchez, M. Moronne, P. Gin, S. Weiss and A. P. Alivisatos, Science, 1998, 281, 2013–2016. 233 W. C. W. Chan and S. Nie, Science, 1998, 281, 2016–2018. 234 H. Mattoussi, J. M. Mauro, E. R. Goldman, G. P. Anderson, V. C. Sundar, F. V. Mikulec and M. G. Bawendi, J. Am. Chem. Soc., 2000, 122, 12142–12150. 235 A. P. Alivisatos, Nat. Biotechnol., 2004, 22, 47–52. 236 B. O. Dabbousi, J. Rodriguez-Viejo, F. V. Mikulec, J. R. Heine, H. Mattoussi, R. Ober, K. F. Jensen and M. G. Bawendi, J. Phys. Chem. B, 1997, 101, 9463–9475. 237 C. A. Leatherdale, W.-K. Woo, F. V. Mikulec and M. G. Bawendi, J. Phys. Chem. B, 2002, 106, 7619–7622. 238 C. M. Niemeyer, Angew. Chem., Int. Ed., 2001, 40, 4128–4158. 239 C. J. Murphy, Anal. Chem., 2002, 74, 520A–526A. 240 W. J. Parak, D. Gerion, T. Pellegrino, D. Zanchet, C. Micheel, S. C. Williams, R. Boudreau, M. A. Le Gros, C. A. Larabell and A. P. Alivisatos, Nanotechnology, 2003, 14, R15–R27. 241 D. Gerion, F. Pinaud, S. C. Williams, W. J. Parak, D. Zanchet, S. Weiss and A. P. Alivisatos, J. Phys. Chem. B, 2001, 105, 8861–8871.

Chem. Soc. Rev., 2011, 40, 1672–1696

1695


View Article Online

242 L. M. Liz-Marzań, M. Giersig and P. Mulvaney, Langmuir, 1996, 12, 4329–4335. 243 G. P. Mitchell, C. A. Mirkin and R. L. Letsinger, J. Am. Chem. Soc., 1999, 121, 8122–8123. 244 E. R. Goldman, E. D. Balighian, H. Mattoussi, M. K. Kuno, J. M. Mauro, P. T. Tran and G. P. Anderson, J. Am. Chem. Soc., 2002, 124, 6378–6382. 245 H. T. Uyeda, I. L. Medintz, J. K. Jaiswal, S. M. Simon and H. Mattoussi, J. Am. Chem. Soc., 2005, 127, 3870–3878. 246 B. Dubertret, P. Skourides, D. J. Norris, V. Noireaux, A. H. Brivanlou and A. Libchaber, Science, 2002, 298, 1759–1762. 247 X. Wu, H. Liu, J. Liu, K. N. Haley, J. A. Treadway, J. P. Larson, N. Ge, F. Peale and M. P. Bruchez, Nat. Biotechnol., 2003, 21, 41–46. 248 T. Pellegrino, L. Manna, S. Kudera, T. Liedl, D. Koktysh, A. L. Rogach, S. Keller, J. Raedler, G. Natile and W. J. Parak, Nano Lett., 2004, 4, 703–707. 249 X. Gao, Y. Cui, R. M. Levenson, L. W. K. Chung and S. Nie, Nat. Biotechnol., 2004, 22, 969–976.

1696

Chem. Soc. Rev., 2011, 40, 1672–1696

250 B. Ballou, B. C. Lagerholm, L. A. Ernst, M. P. Bruchez and A. S. Waggoner, Bioconjugate Chem., 2004, 15, 79–86. 251 M. S. Nikolic, M. Krack, V. Aleksandrovic, A. Kornowski, S. Foerster and H. Weller, Angew. Chem., Int. Ed., 2006, 45, 6577–6580. 252 W. W. Yu, E. Chang, J. C. Falkner, J. Zhang, A. M. Al-Somali, C. M. Sayes, J. Johns, R. Drezek and V. L. Colvin, J. Am. Chem. Soc., 2007, 129, 2871–2879. 253 H. Jiang and J. Jia, J. Mater. Chem., 2008, 18, 344–349. 254 Y. Shi, C. Tu, R. Wang, J. Wu, X. Zhu and D. Yan, Langmuir, 2008, 24, 11955–11958. 255 A. Dubavik, V. Lesnyak, W. Thiessen, N. Gaponik, T. Wolff and A. Eychmu¨ller, J. Phys. Chem. C, 2009, 113, 4748–4750. 256 D. Dorokhin, N. Tomczak, M. Han, D. N. Reinhoudt, A. H. Velders and G. J. Vancso, ACS Nano, 2009, 3, 661–667. 257 B. Qin, Z. Zhao, R. Song, S. Shanbhag and Z. Tang, Angew. Chem., Int. Ed., 2008, 47, 9875–9878. 258 Y. Wei, J. Yang and J. Y. Ying, Chem. Commun., 2010, 46, 3179–3181.

This journal is

c
