structural fabrication and functional modulation of nanoparticle–polymer composites

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Structural Fabrication and Functional Modulation ofNanoparticle–Polymer Composites

EAR
By Hao Zhang, Jishu Han, and Bai Yang*
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This review article summarizes recent progress in the fabrication

methodologies and functional modulations of nanoparticle (NP)–polymer

composites. On the basis of the techniques of NP synthesis and surface

modification, the fabrication methods of nanocomposites are highlighted;

these include surface-initiated polymerization on NPs, in situ formation of

NPs in polymer media, and the incorporation through covalent linkages and

supramolecular assemblies. In these examples, polymers are foremost

hypothesized as inert hosts that stabilize and integrate the functionalities of

NPs, thus improving the macroscopic performance of NPs. Furthermore, due

to the unique physicochemical properties of polymers, polymer chains are

also dynamic under heating, swelling, and stretching. This creates an

opportunity for modulating NP functionalities within the preformed

elopments of

optoelectronic devices, optical materials, and intelligent materials.

1. Introduction

nanocomposites, which will undoubtedly promote the dev

The incorporation of functional nanoparticles (NPs) into polymersis one of the most important means for enhancing theperformance of NPs and polymers.[1–4] Still, the design andfabrication of nanocomposites with flexible and controlledproperties are significant and ongoing challenges withinnanoscience and nanotechnology.

Recently, nanoscience and nanotechnology is becoming thefocus of physics, chemistry, and biology, inspired by the advancesof academic research and the requirements of high-technologyfields.[5–8] Due to quantum confinement effect, the properties ofnanometer-sized inorganic crystals, also known as NPs orquantum dots, are dramatically different from the conventionalmolecular and bulk materials.[9,10] Namely, the functionalities ofNPs are size-dependent. One can facilely control NP functional-ities simply by tailoring the size.[11–14] Consequently, NPs arepotential building blocks for a new generation of optical andelectronic devices, such as quantum computers, light-emittingdiodes, solar cells, and so forth.[15–19] AlthoughNPs have potential

[*] Prof. B. Yang, Dr. H. Zhang, J. S. HanState Key Laboratory of Supramolecular Structure and MaterialsCollege of ChemistryJilin UniversityChangchun 130012 (PR China)E-mail: [email protected]

DOI: 10.1002/adfm.201000089

Adv. Funct. Mater. 2010, 20, 1533–1550 � 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Wein

for many applications, preformed NPs areusually in the form of colloidal solutions orsolid powders. Due to the highly activesurface atoms, NPs spontaneously tend toaggregate during storage and application,thus greatly limiting their technical applic-ability. They must be incorporated withinert media to maintain their excellentfunctionalities as well as to improve theirprocessability. In general, inert mediainclude various inorganic materials, typi-cally silica, and organic materials, typicallypolymers.[20–22] The embedding ofNPs intothese inert materials reduces the activity ofNP surface atoms, thus improving thestability of NPs against environmentalvariation. In comparison to inorganicmaterials, polymershavenumerousmerits,such as physical and chemical stability,

optical transparency, processability, and especially compatibilitywith various materials and solvents. Thus, they are a good choiceforpreserving the functionalities ofNPs. In addition, polymers canact as themainbody togatherdifferentNPswithinone system, andtherewith integrate their functionalities.[23] Because NPs are verysmall objects, this strategy is available for fabricating composites ofvarious scales, ranging from macroscopic bulk to microscopicpatterns, and even to nanometer structures.[24–26] Note that theability to integrate multifunctionalities in one small device is theprerequisite of current microelectronics and biomedicine. Thedevelopment of the fabrication methods of micro- and nanoscalemultifunctional composites will soundly promote the technicalprogress in the corresponding fields.

NPs can in return enhance the performance of polymers.Essentially, polymers are organic materials, whereas NPs areinorganic materials. Their properties are quite different. Forexample, inorganicmaterialsusuallyhaveahighmeltingpoint andrefractive index, andhighmechanical strength,whichpolymersdonot possess. Thus, incorporating NPs with polymers willcompensate the insufficiency of polymers.[27] On the basis ofmolecular design, polymer preparation, and incorporationmethod, nanocomposites with enhanced functionalities, such asahighrefractive index, strongphotoluminescence (PL), andopticalnonlinearity, have been fabricated.[27–31] These achievementsgreatly extend the practical applications of conventional polymermaterials.

Accordingly, a series of methods have been successfullydeveloped for incorporating functional NPs with polymers,[20–22]

such as directly blending NPs with polymers,[32,33] in situformation of NPs within polymer media,[34,35] copolymerization

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Hao Zhang received his Ph.D.degree in Chemistry from JilinUniversity in 2004. He was anAlexander von HumboldtResearch Fellow in 2004, andfollowed as a postdoctoralfellow with Prof. HelmuthMohwald at Max PlanckInstitute of Colloids andInterfaces at Potsdam,Germany. Presently, he is aprofessor of chemistry at Jilin

University. His scientific interests are focused on synthesis,assembly, and application of functional nanoparticles.

Bai Yang currently is a professorof chemistry and the director ofthe State Key Lab ofSupramolecular Structure andMaterials in the college ofChemistry at Jilin University. Hereceived his Ph.D. in polymerchemistry and physics in 1991under the supervision of Prof.Jiacong Shen at Jilin University.His research interests relate tocomposite assembly of

nanoparticles in polymers, nano- andmicroscale fabrication ofordered array structures, and high-performance and functionalpolymer nanocomposite optical materials.

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of surface-modified NPs with monomers,[28,29] grafting ofpolymers onto NP surfaces,[36–38] and self-assembly of NPs andpolymers through various weak supramolecular interactions.[39–43]

Among these methods, the self-assembly strategy is most robustbecause it is facile to obtain omnifarious structures in comparisonto other methods.[44–49] Through this strategy, various nanocom-posite structures have been fabricated, such as layer-by-layer (LbL)deposition ultrathin films,[50,51] microspheres,[52,53] capsules,[54]

vesicles,[55] and micelles.[56–58] Note that the driving forces of self-assembly are weak intermolecular interactions—typically electro-static, H-bonding, dipolar, and van der Waals interactions.[25,26]

These interactions are sensitive to any variations in thesurroundings, indicating the possibility to further modulate theprocess of self-assembly, especially controllable assembly anddisassembly.[48] Moreover, by combining microfabrication tech-niques, including soft-lithography, microcontact printing, tem-plate-induced assembly, and some derivative methods, nanocom-posites can be processed to different sizes and shapes, and even tocomplex structures.[59–64] It greatly inspires the design andpreparation of composite device-prototypes.

In comparison to inorganic materials and organic smallmolecules, polymers have unique physicochemical propertiesbecause of their chain structures. In one aspect, themovement anddispersion of polymer chains is environment-dependent. Forinstance, at temperatures far below the glass transition tempera-ture (Tg) of a homopolymer, the chainmovement of a polymer solidis much slower, making polymers an appropriate inert media forstabilizing the structures and functionalities of NPs.[28,29] Whenthe temperature is around or higher than Tg, polymer chains willalso be dynamic. It leads to the phase separation or aggregation ofNPs within polymermedia.[65–70] Another property of polymers isthe swelling behavior. In this context, although polymers cannotcompletely disperse in the solvent, polymer chains locally extendtowards solvent. This behavior also strongly depends on thepolarity, electric field, temperature, and pH variations of theenvironment. Accordingly, this property has been used both in thefabrication of composite structures and for tuning the function-alities of NPs.[71–76] Additionally, polymer materials also possessgoodmechanical strength because of their long chains and stronginterchain interactions. They will maintain the integrity ofcompositematerials under external stretching or shrinking forces.Along the direction of external force, NPswill have orientation.[77–79] Thus, this property is potentially applied in controlling theorientation of NPs within polymers. On the other hand, manypolymers have their own optical and electrical functionalities. Thecombination of the functionalities of NPs and polymers willreasonably complement any insufficiencies of the simplecomponents, or generate novel functionalities and functionaladjustments beyond those of the parent materials. This isbecoming an accepted viewpoint for designing the latestgeneration of display devices, sensors, as well as hybrid solarcells.[80–82]

In this review article, we summarize the latest progress in thefabrication of NP–polymer composites with the expectation tomake nanocomposites more intelligent; thereby we emphasizestructural design and self-assembly. In addition, we address theeffect of polymers on modulating the functionalities and self-assembly behavior of NPs. Due to the numerous reports aboutnanocomposites, we are not able to include all publications in this

� 2010 WILEY-VCH Verlag GmbH & C

field. We apologize to the researchers whose work wasunintentionally left out.

2. Structural Property and SurfaceFunctionalization of NPs

The fabrication of NP–polymer composites usually starts with thedesign, preparation, and surface engineering of NPs because thenature of NPs, and especially their surface chemistry, significantlygoverns the compatibility between guestNPs andhost polymers. Itwill in return determine the fabrication procedures of thenanocomposites. Consequently, in this section, we first summar-ize the structural properties and surface functionalization of NPs.

2.1. Synthesis and Structure of NPs

To date,many protocols have been developed for synthesizingNPswith specific forms and functions, among which the colloidalchemistry strategy is age-old, but the simplest.[83–87] In this regard,NPs are synthesized in various polar and nonpolar solvents, andthe reagents include precursors and ligands. The former refer tovarious metallic and/or nonmetallic ions, compounds, andcomplexes used to generate inorganic cores of NPs. Ligands, alsoknown as surfactants or stabilizers, refer to various organic small

o. KGaA, Weinheim Adv. Funct. Mater. 2010, 20, 1533–1550

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Figure 2. Schematic illustration of the structural properties of oil-soluble

(A) and water-soluble (B) NPs. R represents the functional group of oil-

soluble NPs, whereas water-soluble NPs possess an inorganic core, a

ligand layer, an adsorbed layer, and a diffuse layer from inside to outside.

molecules or macromolecules that prevent NPs from aggregatingin colloidal solutions. Accordingly, the general structure of as-prepared NPs usually possesses an inner inorganic core and anouter ligand layer.[83,88] The functions of NPs are mainlydetermined by the cores, whereas the surface group propertiesand polarity are influenced by the capping ligands.

Through colloidal synthesis, it becomes possible to preparevarious metal,[89–93] metallic oxide,[94,95] and semiconductor NPswith desired sizes and components (Fig. 1).[96–100] Meaningfully,NPs are alternatively prepared in organic solvents and aqueousmedia,[11,87] which potentially give the opportunity to directly blendNPs either with lipophilic polymers or hydrophilic ones. In 1993,Bawendi et al. established aprotocol for synthesizing lipophilicNPsinhigh-boiling-point organic solvents—namely, an organometallicsynthetic route.[11] The basic idea of this method is the pyrolysis oforganometallic reagents by injection into a hot coordinatingsolvent, which leads to a fast nucleation and controlled growth ofNPs. The precursors of this method are dimethylcadmium andtrioctylphosphine selenide, whereas tri-n-octylphosphine oxide(TOPO) acts as both ligand and solvent. This protocol is the firstexample to synthesize NPs with a narrow size distribution, highsurface quality, and controllable size, which is useful forsynthesizing various II–VI and III–V semiconductor NPs,[101–103]

and NPs with core/shell structures.[104,105] An organometallicstrategy isalsoavailable for synthesizingmetallic oxideNPs, suchasFe3O4 and MnFe2O4.

[94,95] Due to the capability of controlling NPgrowth in high-boiling-point organic solvents, it is easy to tune themorphology of products.[13,14] Nanodots, nanorods (NRs), tetra-pods, and their derivative structures are obtained throughdeliberate control of the preparation conditions, which greatlyenrich the lipophilic building blocks for fabricating nanocompo-sites. Although great successes have been achieved in organome-tallic syntheticmethods, the reagents used are extremely expensiveand toxic. It limits the industrial-scale synthesis of NPs. Peng et al.perfected the original synthetic techniques and built up a ‘‘greenchemistry’’ approach.[106,107] The main difference is the use ofenvironmentally friendly metallic precursors (such as cadmiumoleate) and noncoordinating solvents (such as octadecene). Itindicates that the synthesis of high-quality NPs is far less delicatethan it has been thought to be. Li et al. further developed a unifiedliquid–solid-solutionphase transfer synthetic strategy to synthesizevarious noble metal NPs, semiconductor NPs, magnetic anddielectric NPs, and fluorescent rare earth NPs. The resultant NPsalso possess narrow size distributions and controllable shape.[99]

Figure 1. Fluorescence photograph of ten differently sized CdSe/ZnS NPs

excited with a near-UV lamp, which exhibit size-dependent fluorescence.

Reproduced with permission from ref. [23]. Copyright 2001, Nature

Publishing Group.

Adv. Funct. Mater. 2010, 20, 1533–1550 � 2010 WILEY-VCH Verl

Note that these lipophilic NPs were generally capped by ligandswith long alkyl chains, including various fatty acids, amines,phosphine oxides, and phosphonic acids,[108] making the pre-formedNPs soluble in various nonpolar solvents (Fig. 2A), such astoluene and chloroform, rather than polar solvents, such as waterand alcohol. Accordingly, as incorporating with polymers, the hostpolymers should be lipophilic, and the main driving force of theincorporation is the van derWaals interaction between the alkanesof NP ligands and the hydrophobic backbone of the polymers.

NPs can be also synthesized in water. The first example is thepreparation of aqueous Au NPs through the reduction of Au saltsusing citrate at 100 8Creflux,which is themost reliable synthesis ofAu NPs.[109] Citrate plays a dual role during the reaction; it isreductant and ligand. On one hand, citrate reduce Au(III) to Au(0)by being oxidized to HCOOH. On the other hand, excess citratemolecules adsorb onto the surface of NPs by forming electric-double layers, the overlapping of which generates interparticleelectrostatic repulsion and therewith makes NPs dispersible inwater. Through this method, quasispherical Au particles withdiameters of 7–100 nm are prepared. On the basis of citratereduction, other aqueous metallic NPs are synthesized, such asAg.[84]Due to thedifferent reactionactivity,AgNPspossess a larger

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Figure 3. Schematic illustration of the process of ligand exchange (a) and

surface conjugation (b). A represents the original functional group of th NP

ligand, and B represents the functional group of a secondary ligand, which

can either replace or interact with A. R is the functional group brought by

ligand B.

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size and more morphological variations. In addition, through thevariation of experimental conditions, the synthesis ofmetallic NPswith controlledmorphologies and structures are achieved.[83–85] Atypical example is the preparation of Au NRs through a stepwise,seeded growth strategy.[110] In this method, seed Au particles withseveral nanometers in diameter are foremost prepared usingNaBH4 to reduce Au(III) and simultaneously using cetyltrimethy-lammonium bromide (CTAB) as a ligand. A bilayer of CTABformed around the Au cores with the hydrophilic heads outside,thusmaking Au seeds soluble in water. Au seeds are subsequentlyinjected into a growth solution, containingAu salt, ascorbic acid asa weak reductant, CTAB as the ligand, and sometimes a spot of Agsalt, to generate the anisotropic growth of NRs at roomtemperature. The aspect ratio is controllable by tuning the sizeand/or ratio of the seeds, temperature, ligands, additional ions,and so forth.[111–113] Another strategy for preparing aqueousmetallic NPs is through the reduction of metal ions using NaBH4

in the presence of water-soluble mercapto compounds asligands.[92] In comparison to citrate, mercapto ligands stronglylink to NPs, suppressing the growth of large NPs. As a result, thesize of preformedNPs is smaller than those formed throughcitratereduction.

Semiconductor NPs are also prepared in water using mercaptocompounds as ligands, which involves two steps: preparation ofprecursors by injecting hydrogen chalcogenide into an aqueoussolution of metal ions at room temperature in the presence ofmercapto ligands, and the thermal growth of precursors to obtainNPs.[87] In contrast to the organometallic route and the derivativemethods, the reagents of the aqueous route are various metal andnonmetal ions, such as Zn2þ, Cd2þ, Hg2þ, S2–, Se2–, and Te2–.[24]

With different combinations of these reagents, a series of aqueousNPs with strong PL are prepared, which cover the entire UV–visand near-IR range. Among these NPs, CdTe is themost successfulone. By tuning the size of CdTe NPs with controllable thermalgrowth, PL emissions continuously from green to red areobtained.[114–117] Additionally, near-IR emission is obtained bysynthesizing HgTe and CdHgTe NPs, whereas UV and blueemissions are obtained from ZnS, ZnSe, and Cu- or Mn-dopedZnSorZnSe.[118–126]However,due to the lowboilingpoint ofwaterand the complex ionic environment, the quality of aqueoussynthetic NPs is usually worse than those formed using theorganometallic method.[127–130] Many auxiliary technologies arefurther applied to improve the quality of aqueous NPs, such ashydrothermal synthesis, ultrasonic irradiation, and microwave-assisted synthesis, which greatly improve the route of aqueoussynthesis.[131–137]

Note that themercapto ligands used in aqueous synthesis bondto NPs via the strong affinity between the metal and –SH group,which simultaneously occurs with the hydrophilic functionalgroups pointed towards water (Fig. 2B). Thus the structures ofmercapto ligands significantly determine the surface functional-ities of preformedNPsby introducingdifferent terminal groups.[87]

Experimental results indicate that many mercapto ligands aresuitable for aqueous synthesis of NPs, such as 1-thioglycerol(TG), mercaptoacetic acid (MAA), 3-mercaptopropionic acid(MPA), and 2-mercaptoethylamine (MA).[114] These ligandsendow NPs with hydroxyl, carboxyl, and amine groups, whichprovide an immense flexibility for conjugating with functionalpolymers.


2.2. Surface Functionalization of Preformed NPs

Asmentioned above, NPs can be synthesized in different solventswith distinct capping ligands, which make them possess specificsurface functionalities. In one aspect, this facilitates the blendingof NPs and polymers with similar polarity, but in another aspect, itlimits the incorporation between less compatible NPs andpolymers. Consequently, many efforts have been devoted to tunethe surface chemistry of preformed NPs—namely, the surfaceengineering of NPs. In general, there are two strategies to altersurface functionalities: ligand exchange and surface conjugation(Fig. 3).

Thebasic ideaof the ligandexchange strategy is the replacementof original ligands using new ligands that have a stronger affinitywith the NPs (Fig. 3A). For instance, citrate, the capping ligand ofaqueous Au NPs, is replaceable by various mercapto-containingsmall molecules, polymers, or biomolecules.[25] In comparison tothe adsorption of anionic citrate, the interaction between Au and–SHis a covalent linkage, thus greatly enhancing the stability ofAuNPs towards environmental variations. Through similarmethods,quaternary ammoniumsurfactant-coatedAuNPsandNRsare also


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modified with mercapto compounds.[138,139] The strong affinitybetween metal and –SH groups also makes it possible forengineering the surface chemistry of semiconductor NPs. Fororganometallic synthetic NPs, they are originally coated by fattyacids, amines, or phosphine oxides. The functional groupsof theseligands bond to metal atoms of NPs through weak coordinativeinteractions, and with the alkane pointed towards nonpolarsolvents. During the exchange of mercapto ligands, NPs aredissolved in specific polar solvents. And under argon flow and amoderate heating, mercapto-coated NPs are obtained.[140] Inaddition, fatty ligands can be replaced by ligands with similarfunctional groups. For example, usingphosphine oxide derivativesto replace the TOPO of CdSe NPs in tetrahydrofuran (THF) canplacenitroxideonNPs.[141]Oleic acidsoroleylaminesofFe3O4NPsare replaceable by carboxyl-containing molecules for phasetransfer and conjugation applications.[71] A partial exchange ofthe original mercapto ligand using mercapto derivatives is alsoavailable. With the aim of fabricating CdTe- and CdHgTe-containing nanocomposites, Gaponik et al. used 1-dodecanethiolto partially replace theMAA ligands ofNPs, creating nonpolarNPsfrompolarNPs.[142] In thisprocedure, acetoneplays thekey role forefficient ligand exchange, indicating the importance of inter-mediate solvent to adjust the polarity of the two immisciblesolvents.

Although ligand exchange is generally successful in surfacemodification of preformed NPs, it sometimes damages the NPfunctionalities by bringing disorder to the NP surface atoms. Forfatty-ligand-coated CdSe NPs, their luminescence is completelylost after a mercapto ligand exchange, whereas half luminescenceis preserved for CdSe/CdS NPs.[140] In view of this, ligandexchange strategy ismore suitable for surface engineeringofmetaland metallic oxide NPs.

Surface conjugation is a strategy to modify NPs withoutdestroying their original surface structures. In this method,additional cappingmolecules anchor onto theNP surface either bycovalent linkages or weak supramolecular interactions (Fig. 3B). Aclassical covalent linkage is the amide bond formed through thereaction between carboxyl and amine.[143] If NPs are originallyfunctionalized with carboxyl groups, they can conjugate withamine-containingmolecules, and if in reverse, they conjugatewithcarboxyl.Another example is the conjugationbetweenhydroxy andisocyanato groups. Bawendi et al. indicate that oligomericphosphine-coated CdSe NPs can react with isocyanatoheptaneor diisocyanatohexane.[144] The former leads to a surfacemodification of alkane, and the latter makes NPs further linkablewith other hydroxyl- or carboxyl-containing molecules. It is foundfrom these examples that covalent linkages must be based on thereaction between specific groups, which in return limits thelinkages between mismatched groups.

In comparison, the surface functionalization through weaksupramolecular interactions is flexible and general for a varietyof combinations between host NPs and guest molecules. Thisstrategy is the hallmark of supramolecular chemistry, that thesurface architectures of NPs are spontaneously constructed vianoncovalent and specific interactions, such as electrostaticattractions, hydrogen bonding, van der Waals interactions, andmolecular recognition.[26] For aqueous synthetic NPs, theypossess surface charges, which provide the opportunity forsurface adsorption of oppositely charged molecules and


therewith the surface engineering of NPs. For example,negatively charged aqueous Au or CdTe NPs are functionalizedwith quaternary ammonium surfactants through electrostaticattraction.[28,145] By adjusting the alkane chain length ofsurfactants, the polarity of NPs is tunable. Accordingly, NPsare soluble in solvents with distinct polarity. Notably, one candesign the structure of quaternary ammonium surfactants byintroducing reactive groups, for instance polymerizable groups,thus greatly favoring the fabrication of nanocomposites.[28] Fororganometallic synthetic NPs, the surface groups are lipophilicalkyl chains. As a result, the driving force for surface conjugationis a van der Waals attraction. Dubertret et al. demonstrated thatindividual lipophilic semiconductor NPs can be encapsulated byphospholipids block-copolymer micelles through the hydro-phobic vanderWaals interactions between the fatty ligandofNPsand the alkyl chains of phospholipids, which result in thetransfer of NPs to water.[2] Fan et al. indicated that in addition toamphiphilic polymers, common surfactants are also practical forsuch surface functionalization.[146] However, an efficientmodification depends on the alkane chain length of surfactantsbecause the driving force is the hydrophobic van der Waalsinteraction between the primary alkane of NP ligands and thesecondary alkane of the surfactants. For single-tailed surfactants,an alkane chain of eight or more carbons is required to formstable micelles. Cationic, anionic, and nonionic surfactants, aswell as phospholipids are all available for such functionaliza-tion.[146,147] By combining the water-to-oil phase transfer viaelectrostatic attraction and the oil-to-water transfer via van derWaals interaction, surfactant bilayers can be modified outsidethe NPs.[147] Such a two-step phase transfer makes the inner andthe outer surfactants of the bilayer alterable in a wide range, thusgreatly enriching the surface functionalities of aqueous NPs.Another strategy for surface functionalization is the formation ofhost–guest complexes, such as the complexation betweencyclodextrin (CD) and alkane chains. In this method, the alkanechain may insert the hydrophobic cavity of CD, forming stablestructures. Through deliberate design of the procedure ofsurface engineering, a reversible replacement is furtherachievable.[148] Hydrogen bonding is also useful for surfaceconjugation, for instance the combination of thymine andadenine.[149,150] In this context, mercapto-functionalized thy-mine and phosphate-functionalized adenine are synthesizedand used to alter the surface functionality of inorganic NPs. Itenables the assembly of differentNPs through the recognition ofthymine and adenine.

Overall, the successes in NP preparation through colloidalmethods and the progress in surface functionalization ofpreformed NPs greatly facilitate the fabrication ofnanocomposites.

3. Fabrication of NP–Polymer Composites

3.1. In situ Formation of NPs within Polymer Media

The preparation of NPs within polymer media was the earliestmethod to obtain nanocomposites. It allows a one-step formationof nanocomposites with an in situ generation of NPs from the


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precursors within polymer networks. In this method, specificmetal ions are foremost introduced into polymer media eitherthrough the adsorption of ions on the preformed polymers or bythe polymerization of metal-containing monomers to preparepolymers. Afterwards, metal-containing polymers react with thesolution or gas of hydrogen chalcogenide to generate NPs.[27] Dueto the physical limit of polymer networks, the further growth ofNPs is suppressed, resulting in nanometer-sized particles (Fig. 4).One example is the in situ formation of CdS or PbS NPs inpolystyrene (PS) through the exposure of Cd- or Pb-containing PSthin film toH2S gas.

[151] This in situ strategy canbe also seen as thedevelopment of NP preparation from common ligands tomacromolecular ones. In situ growth is usually operated atrelative low temperature, such as at room temperature, and thediffusion of metal ions in polymers is less dynamic than insolution.Thus, thegrowthofNPs ismainly through theadsorptionof ions or the agglomeration of small clusters. Consequently, theformation of NPs is completely random, making it difficult tocontinuously tune NP sizes and therefore the optoelectronicfunctionalities. Addtionally, the aggregative growth makes NPspossess broad size distribution, poor crystallization, and anespecially disordered array of surface atoms,making the quality ofin situ synthetic NPs not comparable to those synthesized incolloidal solutions. Nevertheless, this strategy is facile and usefulfor fabricating nanocomposites without the requirement of NPquality, for instance high-refractive-index nanocomposites.[27] Asuccessful example is the fabrication of PbS-containing nano-composites via the in situ method.[151] Since this reaction isbetween Pb2þ and S2–, the polymer media must be hydrophilic,such as poly(vinyl alcohol) (PVA), poly(ethylene oxide) (PEG),polymer(acrylic acid) (PAA), and poly(acrylamide). Themaximumrefractive index of the nanocomposites reached about 2.5 as thePbS content was 70wt%.

In situ strategy was also extended to fabricate nanocompositesin nano- or microscale. For instance, Yang et al. established amethod for the controllable fabrication of nanocomposites by thecombination of surface-initiated atom transfer radical polymer-ization (ATRP) and gas/solid reaction.[152] Pb-containing polymerfilms are prepared through ATRP of lead dimethacrylate, and

Figure 4. Schematic illustration of in situ synthesis of metal sulfide NPs in a p

Royal Society of Chemistry.


followed by reaction with H2S gas to prepare the ultrathin film ofPbS/polymer composites. The thickness of the film is controllablein the range of 50–100 nm; meanwhile a high PbS content is alsoachievable. This method is also extendable from flat substrates tocurved surfaces and from homopolymerization to copolymeriza-tion. Through this method, CdS NP/polymer composites withsingle layer and multilayer core/shell structures are alsofabricated.[153,154]

3.2. Incorporation of Preformed NPs with Polymers

As mentioned above, the in situ strategy cannot achievenanocomposites with functionalities tunable in a large range.Consequently, most of polymeric nanocomposites are recentlyfabricated from preformed NPs. The functionalities are foremostdesigned by synthesizing NPs with desired components, size,shape, as well as surface chemistry, and through bottom-upassembly strategy to prepare nanocomposites. In an attempt toclassify omnifarious fabrication methods, the conjugation modesbetween NPs and polymers are adopted.

3.2.1. Incorporation via van der Waals Interactions

As already discussed above, NPs prepared by the organometallicsynthetic route are capped by fatty ligands, making lipophilic alkylchains on their surfaces (Fig. 2A). In view of this, NPs arecompatible with lipophilic polymers through hydrophobic van derWaals interactions. The early reports about bottom-up fabricationof polymeric nanocomposites are mainly through direct blendingof NPs and functional polymers, driven by the requirement oforganic–inorganic hybrid light-emitting diodes (LEDs). Forinstance, Banin et al. prepared the nanocomposites of core/shellInAs/ZnSeNPs and conjugation polymers of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), followed byspin-coating to create near-IR plastic LEDs.[15] The emission istunable from 1 to 1.3 um by using differently sized NPs, whichcovers the short wavelength telecommunication band. Becase ofthe hydrophobic van der Waals interaction between the alkane of

olymer media. Reproduced with permission from ref. [27]. Copyright 2009,


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NP ligands and the alkane of MEH-PPV, these two componentexhibit good compatibility, leading to an efficient transfer ofelectrons across the NP/polymer interface. On the basis ofthis strategy, a series of NP–polymer composite filmswith photoelectric or photovoltaic properties have beenfabricated.[16,155]

Bulknanocomposites canalsobe fabricated viahydrophobic vanderWaals interactions. Thismethod is through thepolymerizationof monomers with preformed NPs dispersed among them, andsometimes in thepresenceof a small amount ofhigh-boiling-pointorganic solvents. For example, luminescent nanocomposites fromCdSe/ZnSNPs and polylaurylmethacrylate (PLMA) are fabricatedby mixing preformed NPs, laurylmethacrylate, ethylene glycoldimethacrylate crosslinker, and TOPO at a specific ratio; this isfollowed by azobisisobutyronitrile (AIBN) initiation to createnanocomposites (Fig. 5).[29] The hydrophobic van der Waalsinteraction between alkyl chains makes NPs homogeneouslydistribute within the PLMA media, which is the key formaintaining the transparency of bulk materials and thereforethe strong luminescence. Note that the attempts to createtransparent composites using common polymers, such as PS,poly(methylmethacrylate) (PMMA), and poly(vinylpyridine)(PVP), show limited success. Phase separation of NPs frompolymer media during the fabrication leads to luminescencequenching. It reveals the characteristic of the direct blendingapproach; namely, thematching of NP ligands and the polymers isimportant for a successful incorporation. For instance, usingPS asmedia, the rigidity of a styrene unit is too high to be compatiblewith flexible alkanes of NP ligands, leading to a lack of van derWaals attraction and thus a serious phase separation of NPs. ForPLMA, however, its branched alkane chains with twelve carbonsimprove the compatibility with NP ligands, thus avoiding phaseseparation. Such phase separation is much like that of polymerblending systems or block copolymers.[156]

The ability to implement suspension and emulsion polymer-ization provides further incorporation ofNPswithmicro- and sub-microscale composite spheres. There are two routes to this goal:polymerization of monomers in the presence of NPs, orembedment ofNPs into preformedpolymeric spheres by swelling.The former is like the fabrication of bulk composites; namely,hydrophobic van der Waals interactions prevent the phaseseparation ofNPs. Take suspension polymerization as an example;

Figure 5. A) Color fluorescence image of CdSe/ZnS NP–polymer compo-

site rods excited by a UV Hg-lamp. B) End-on photographs of NP–polymer

composite rods excited by a UV lamp from below. These rods are posi-

tioned on the CIE (Commission internationale de l’eclairage) chromatic

diagram. Reproduced with permission from ref. [29]. Copyright 2000,

Wiley-VCH.


lipophilic NPs are mixed with lipophilic monomers, water,suspension agent, and initiator with mechanical stirring toachieve a suspension. Composite spheres are prepared afterpolymerization. In the latter route, hydrophobic–hydrophobicattractions between NP ligands and polymer backbones are thedriving force of their incorporation. Nie et al. demonstrate aswelling method to embed luminescent NPs into polymericspheres.[23] In thismethod, NPs and crosslinked PSmicrospheresare mixed in a binary solvents composed of chloroform (5% v/v)and propanol or butanol (95% v/v). Since the solution polarity isstronger than that in PS, hydrophobic NPs are transferred into thePSspheres simultaneously as thespheres swell due to thepresencechloroform. Also, stabilized by hydrophobic–hydrophobic inter-actions, the embeddedNPsdonotmigrate outwhen the compositespheres are redispersed in water. The major advantage of thismethod is the development of amultiplexed coding technology onthe basis of the size-dependent luminescence of NPs. Becausedifferent emission-coloredNPs can be embedded into spheres at adesired ratio, it is possible to obtain a series of luminescentmicrospheres for biological encoding. In principle, usingcombinations of NPs with only five distinct emissions, a millionoptical codings can be created (Fig. 6).Moreover, In comparison tothe swelling route, the suspension polymerization route is usuallyused to prepare larger spheres because of the characteristics ofsuspension polymerization itself; the diameter of resultantspheres is in the range of 5–2000mm. For the swelling route,the size of composite spheres is solely dependent on the originalspheres, which is theoretically tunable using differently sizedspheres. In practical fabrication, however, a gradually decreasingdistribution of NPs from the outer part of the sphere towardscenter is observed, resulting from the swelling procedure.Consequently, the swellingmethod is more suitable for preparingcomposite spheres with smaller diameters.

By combining the self-assembly behavior of amphiphiliccopolymers and the hydrophobic nature of NPs, compositevesicles andmicelles can be fabricated.[157,158] In this regard, someblock copolymers simultaneously possess hydrophobic blocks andhydrophilic blocks. The hydrophobic blocks provide the compat-ibilitywith hydrophobicNPs.As they are exposed to polar solvents,such as water, the hydrophobic blocks may assemble with thealkanes of NP ligands through hydrophobic–hydrophobic inter-actions, whereas the hydrophilic blocks spread towards thesurroundings to make the composite structures dispersible inwater (Fig. 7). Entire composites possess from the center goingoutwards an inorganic NP core, interlaced alkanes of the NPligands andpolymerhydrophobic blocks, andpolymerhydrophilicblocks. By forming such structures, composite micelles fromvarious hydrophobic metal, metallic oxide, and semiconductorNPs are constructed.[56] On the basis of this strategy, Gao et al.further develop a method to fabricate multiplexed nanobarcodesthrough the crosslinking of individual composite micelles thatcontained maleic anhydride groups.[159] In homogeneous solu-tion, the compositemicelles growepitaxially into nanobeadswith anarrowsizedistribution. This stepwise self-assembly technology iscapable of constructing various nanostructures, which is expectedto give new opportunities inNP-based ultrasensitive detection andimaging.

In addition, by combining microfabrication techniques,nanocomposites can be processed to different forms, such as


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Figure 6. A) Schematic illustration of optical coding based on wavelength

and intensity multiplexing. B) Fluorescence image of a mixture of CdSe/

ZnS NP-tagged PS spheres emitting single-color signals at different

wavelengths. Reproduced with permission from ref. [23]. Copyright

2001, Nature Publishing Group.

1540

patterned surfaces, nanofibers, microspheres, and soforth.[32,33,62–65,160] One prerequisite of these techniques is thecompatibility between guest NPs and host polymers, which is notdiscussed in detail here.

3.2.2. Incorporation through Electrostatic Attraction

As shown in Figure 2B, aqueous synthetic NPs possess surfacecharges, thus providing the driving force for assembling


themselves with oppositely charged polymers via electrostaticattraction. The first example is electrostatic driven LbL assembly,which is an efficient method to obtain ultrathin films withthicknesses in the nanometer range.[22,161] Actually, LbL assemblycan be circularly operated on various substrates, for instancesilicon, quartz, and glass, where the increase of film thicknessin each circle is only several nanometers. For instance, MAA- orMPA-coated luminescent CdTe NPs have been assembled withpositively charged poly(diallyldimethylammonium chloride)(PDDA).[162,163] Because the emission color of CdTe NPs can becontinuously tuned from green to red, a series of thin films withdistinct PL or electroluminescence (EL) are fabricated. Throughthe integration of differently colored NPs in one film or insubsequent layers, multiplexed or gradual emission can beachieved.[162] The LbL strategy is also available for aqueous metalNPs, for instance Au andAg.[164] The resultant films are applicableas surface-enhancement Raman substrates (SERS), which mayenhance Raman signal by more than 6 orders of magnitude.Similarly, LbL assembly is extended to metallic oxide NPs, forinstance Fe3O4.

[165] The most attractive advantage of the LbLtechnique is the controllable process of assembly. First, thecomponent and thickness of films are tunable by controllingcircular times and thus the functionality and/or intensity of filmdevices. Secondly, it is possible introduce other factors formodulating the assembly process, for instance electric field.Because all building blocks possess charges, the additional electricfield significantly promotes or suppresses the assembly, and evenleads to patterned assembly structures.[166] Thirdly, although thedriving force for LbL assembly is weak electrostatic attraction, itcan be altered to be covalent linkages through further thermalcrosslinking or photoreaction,[165] which greatly enhances thestability in practical applications.

The LbL technique is also extended to assembly on curvedsubstrates, for instance the assembly of CdTe NPs with PDDA onoptical fibers.[163] The assembly of NPs on microspheres is themost prominent advance of the LbL technique. According to thespecific requirements, the species and sizes of microspheres canbe flexibly selected; the assembly can be of micrometer- ornanometer-sized spheres, and on polymer or silica spheres.Additionally, in comparison to planar substrates, microsphereshave small spatial sizes, close to the wavelength of visible light andsubcellular sizes. As a result, composite spheres present broadapplications in bioconjugation as well as the modulation of lightpropagation. Finally, composite spheres can be used as templatesto create derivative structures of nanocomposites. For example,composite microcapsules are fabricated via two steps: LbLassembly of NPs and polymers on microspheres, and thesubsequent dissolution of microsphere templates.[54]

On the basis of electrostatic adsorption of charged aqueousNPson water-soluble polymers, their direct combination is alsoachievable. Thismethod is applicable formany aqueous polymers,including various natural polymers (e.g., agarose, gelation, andpolylactic acid) and artificial polymers (e.g., poly(N-isopropylacry-lamide) (PNIPAM), PAA, and PVP). The resultant water-solublenanocomposites can be flexibly molded to different shapes. Forinstance, the composite solution of CdTe NPs and PVA is used asink for inkjet printing (Fig. 8).[32] PVA efficiently prevents theaggregation of NPs. Moreover, a CdTe–PVA composite solutionhas also beenused for electrospinning to achievenanofibers.[33,160]


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Figure 7. A) Formation of micelles around Fe3O4 NPs with amphiphilic PEG ligands, and

B) transmission electron microscopy (TEM) image of the resultant water-soluble Fe3O4.

C) Synthesis scheme and D) TEM image of PS–PAA block copolymer vesicles encapsulating

several Fe3O4 NPs. Reproduced with permission from refs. [157, 158]. Copyright 2005, American

Chemical Society.

The fast evaporationof solvent in the electrospinningprocess leadsto a quick freezing of polymer chains, confining the NPs andleaving them without little time to aggregate. As a result, NPaggregation is mostly avoided and therefore the Forster energytransfer (FRET) between NPs is prevented.

The composite solutions of NPs and polymers can be directlyprepared by capping aqueous NPs using amphiphilic macro-molecular surfactants.Mews et al. presented a polymer ligand thatcan be attached to NPs via phase transfer reaction and electrostaticattraction.[167] The ligand consists of a chain of reactive esters,which can be substituted by nonpolar and charged side chainsthrough the reaction with amine-containing molecules, andtherewith used to modify quaternary ammonium groups. The NPcoverage by the polymers is accomplished by a phase transferreaction, where an excess of quaternary ammonium-modifiedpolymers is dissolved in chloroform and mixed with the aqueoussolution of MAA-coated NPs. After addition of small portions ofmethanol and vigorous stirring, NPs are capped by polymers andsimultaneously transferred into the chloroformphase through theattraction between negatively charged NPs and positively chargedsites of the polymer. Quaternary ammonium-modified polymerscan be also prepared through the copolymerization strategy. Yanget al. demonstrated a route to a processable nanocompositesolution using alkylammonium-functionalized polymer to capaqueous NPs.[64] In this method, alkylammonium-functionalizedpolymer is prepared through the copolymerization of styrene andoctadecyl-p-vinylbenzyldimethylammonium chloride (OVDAC),and also through electrostatic attractions to achieve polymercapping and phase transfer. The resultant nanocomposites are

Adv. Funct. Mater. 2010, 20, 1533–1550 � 2010 WILEY-VCH Verlag GmbH & Co. KGaA,

easily shaped into various macromaterials ormicrostructures (Fig. 9A–C). Furthermore,various functional groups can be introducedinto macromolecular surfactants via thecopolymerization strategy, thus providingan efficient method to integrate ormodulate the functions of nanocomposites(Fig. 9D,E).[168,169]

Also based on the electrostatic adsorption,aqueous NPs can be modified by variousmolecules with reactive groups; this can thenbe followed by polymerization with monomersto obtain nanocomposites. For example, posi-tively charged OVDAC that possess styrenesegments are used to modify MPA-coatedaqueous CdTe NPs via electrostatic attractionand to transfer them to styrene.[28] By freeradical copolymerization initiated by AIBN,luminescent bulk composites are fabricated.This method is also applicable to prepare bulknanocomposites either using other commonmonomers, such as MMA, or aqueous NPs,suchasCdHgTe.Moreover, aqueousNPscanbetransferred into polymerizable ionic liquids viaelectrostatic attraction, and through AIBNinitiation to achieve bulk composites.[170] Thepolymerizable groups used in this strategy areimportant for improving the compatibility ofNPs and polymer media because covalentlinkages formed after polymerization confines

NPs in polymer networks, thus preventing the phase separation.

3.2.3. Surface-Initiated Polymerization or Grafting

In the aforementioned methods, nanocomposites are fabricatedthrough the avoidance of macroscopic phase separation, whichis important for maintaining the transparency of nanocompositesand the functions of their NPs. However, due to the lesscontrollableprocess andespeciallyunspecific termination reactionof common polymerization, it is difficult to control the separationdistance between NPs in the resultant nanocomposites, resultingin the disability to modulate the energy transfer between NPs, theprerequisite for tuning NP functions within polymer media.Accordingly, surface-initiated polymerization or direct grafting ofpreformed polymers on NPs is put in practice to bestow polymerswith a specific thickness on NPs.

The realization of surface-initiated polymerization is greatlypromoted by the recent advances in polymerization methodol-ogy.[141,171–173] To initiate polymerization upon NP surfaces, thefirst step is the conjugation of active groups. As already discussedabove, the main strategy is ligand exchange. Depending on thespecies of active groups anchored on NP surface, the polymeriza-tion will follow a different mechanism. Emrick et al. usedphosphine oxide derivative to replace the original TOPO oflipophilic CdSe NPs, thus anchoring styrene segments onto theNPs.[171] The resultant NPs are used to support the polymerizationof cyclic olefins radially outward from the surface by ruthenium-catalyzed ring-opening metathesis polymerization (ROMP). Dueto the flexibility of ROMP, the conversion of surface

Weinheim 1541

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Figure 8. A) Photograph of an inkjet-printed combinatorial library of

differently sized CdTe NPs emitting at different wavelengths, including a

systematic variation of PVA content in the solution used for printing.

B) Photograph of an inkjet-printed library of a mixture of green- and red-

emitting CdTe NPs, and the PL spectra of this library. Reproduced with

permission from ref. [32]. Copyright 2007, Wiley-VCH.

Figure 9. Fluorescence images of CdTe NP–polymer composites with

different macroscopic shapes (A), patterned nanocomposites with green

emission (B), and composite spheres with different emission (C). Repro-

duced with permission from ref. [64]. Copyright 2005, American Chemical

Society. PL spectra of a green NP–red NP–poly(vinylcarbazole) composite

chloroform solution (D) and the corresponding film (E), which indicate

white-light emission. Inset: CIE coordinates (0.34, 0.37) and fluorescence

image of the composites. Reproduced with permission from ref. [168].

Copyright 2006, Wiley-VCH.

1542

polymerization to other polymers is available by using differentmetathesis catalyst. Peng et al. used carbene-containing dendronligands tomodifyCdSeNPs and ring-closingmetathesis to achievecrosslinking.[172] The global crosslinking of dendron ligands sealseach NP in a dendron box, which yields box–NPs. The stability ofbox–NPs against chemical, photochemical, and thermal treat-ments are dramatically improved in comparison to that of theoriginal NPs. The inorganic NP core can be further dissolved byacids, creating a new type of polymer capsule with nanometer-sized cavities. Also, through ligand exchange, nitroxide groups arepositioned on CdSe NPs; followed by nitroxide-mediated,controlled free radical polymerization, polymer brushes can begrown directly from the NP surface.[141] While common freeradicals can quench CdSe luminescence, nitroxide-mediatedpolymerization allows formaintaining the inherent luminescenceof NPs. This method is extended to fabricate NP-containingultrathin capsules by polymerization and crosslinking at the oil/water interface.[173] In this method, an amphiphilic PEGylatedruthenium benzylidine catalyst for ROMP is appropriate to tailorthe desired interfacial chemistry, making the reaction operate in


the aqueous phase and at the interface. As for Au NPs, it is easy tolink them with mercapto-containing initiators, which provides aplatform for living radical polymerization. For instance, citrate-stabilized aqueousAuNPsaremodifiedwith initiators through theligand exchange between disulfide initiators and citrates inTHF.[36] Afterwards, a surface-initiated ATRP of N-isopropylacry-lamide (NIPAM) is operated with the catalysis of CuBr andN,N,N0,N00,N0 00-pentamethyldiethylenetriamine. Subsequently,methoxy-oligo(ethyleneglycol) methacrylate (MOEGMA) mono-mers are added to the reaction system for the secondcopolymerization, leading to the surface structure of Au NPswith inner PNIPAM homopolymer and the outer PMOEGMA-containing copolymer (Fig. 10). The composite NPs havethermosensitive behavior. Similarly, carboxyl-containing ATRPinitiators aremodified on oleic acid-coated g-Fe2O3 NPs via ligandexchange; followed by ATRP, amphiphilic polydimethylami-noethyl methacrylate (PDMAEMA) brushes are grown on the


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Figure 10. A,B) TEM images of Au@PNIPAM-b-PMOEGMA nanocompo-

sites. C) Schematic illustration of the thermosensitive size change of

Au@copolymer nanocomposites. Reproduced with permission from

ref. [36]. Copyright 2007, Wiley-VCH.

NP surface.[71] The modification with PDMAEMA renders NPssoluble in both water and organic solvents.

The ligand exchange using macromolecular ligands directlyleads to polymer-modified NPs, namely surface grafting.Depending on the specific functions for given applications, NPscan be modified with homopolymers and block copolymersthrough the linkage of single site or multiple sites. Kataoka et al.present a single site linkage approach to anchorPEGonAuNPs viathe strong affinity of Au and –SH groups.[174] Through thealteration of composite conditions, the density and thickness ofPEG layer are tunable. This provides a method to construct highlysensitive colloidal biosensors by optimizing polymer density onthe NP surface. TOPO-coated CdSe NPs are modified usingPDMAEMA.[175] PDMAEMA are amine-containing homopoly-mers, and thus, they act asmultidentate ligands to linkwith theCdsites of CdSe NPs. Suchmodification leads to NPs securely boundby a layer of conventional polymers, retaining 70% of the originalluminescence of NPs. Also as a result of this modification, theresultant NPs are soluble in methanol rather than water orchloroform. The surface grafting approach shown here is thesimplest route tomodify NPs with desired polymer structures andthickness, which can be finely tuned to the desired surfacefunctionality of the NPs. Consequently, although this strategyrequires deliberate structural design of polymers, further effortsare to extend this protocol are worthwhile.

4. Modulation of the Functionalities ofNanocomposites

In the aforementioned discussion, polymers are hypothesized asinert hosts to protect and integrate the functionalities of NPs, thusfacilitating thepractical applicationsof functionalNPs.Whilegreatsuccesses have been achieved in the fabrication of NP–polymercomposites, the recent technical applications of NPs in ultra-sensitive sensors, biomimic materials, and a new generation of


optoelectronic devices strongly require the capability to direct thefunctionalities of NPs either in the preformed nanocomposites orthrough polymer inducement. In this scenario, due to the uniquephysicochemical properties of polymers, polymer chains aredynamic under heating, swelling, and stretching. This certainlyprovides the opportunity to further modulate NP functionalities.

4.1. Stimuli-Responsive Nanocomposites

The study of the responsive behavior of polymers has beenregarded for decades as a combination of colloid science andpolymer science. The recent intersection of functional NPs andresponsive polymers significantly broadens their technicalapplications. As NPs are embedded into a responsive polymer,the chemical or physical changes of the host polymer thatgenerated by external stimuli, may be transmitted to the NPs,influencing NP functions. The responsive behavior of polymerscan be of two different types: chemical reaction of polymerfunctional groups and physical changes of the polymer backbone.Light-response is an example of a functional group response; forexample, the photoactive groups of polymers can undergoreversible structural changes under UV–vis light. pH-responseis an example of a structural response; the reversible swelling andshrinking of the polymer backbone are controllable throughvariations of pH. Accordingly, NPs are deliberately incorporatedwith specific polymers that inherently possess temperature-, pH-,ion-, or light-responsive characteristics in order to achieve a givenapplication.

To date, most responsive nanocomposites are based on themechanism of reversible physical changes of the polymerbackbone under environmental stimuli. Various nanocompositesin the form of bulk materials, ultrathin films, microspheres,capsules, vesicles, or micelles, are fabricated to achieve desiredfunctionalities.[176–181] Through a simple solution polymerizationofNIPAM in the presence of luminescent CdTeNPs, temperature-responsive luminescent bulk composites are prepared.[176] As thetemperature varies around 32.5 8C, which is the critical solutiontemperature (LCST)ofPNIPAM,a reversible luminescence shift isobserved. This is attributed to variations in the separation distancebetween NPs that follows the swelling and shrinking of thePNIPAM network, leading to the reversible FRETof neighboringNPs.Kotov et al. fabricated theLbLassemblyfilms fromPDDAandPAA, which can reversibly load and unload NPs via pHvariation.[177] The loading and unloading of NPs result fromchanges of NP surface charges during such variations. Inparticular, the small size effect of NPs makes it facile to embedthem into polymer-based hydrogels at the nano- or microscale, forinstance hydrogel spheres,[178–180] because of the requirements inbiosensors and controlled medicine release. The key of thisstrategy is to prevent the extrusion of NPs from host polymersduring the shrinking, which is solved via two approaches: surfacefunctionalitzation of NPs or structure design of polymers. For agiven hydrogel sphere, such as PNIPAM, one can synthesizematchedNPs to enhance the attraction betweenNPs andPNIPAMnetworks. Gao et al. synthesized CdTe NPs using mixed ligandsof MAA and TG.[179] MAA is the main ligand to preserve thestrong luminescence of CdTe, whereas TG is used to enhance the


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Figure 11. A) Schematic depicting the synthesis of Au NPs by the one-

phase reduction of HAuCl4 in the presence of disulfide-functionalized

poly(OEGMA-co-MEO2MA) and methanol. B–D) Poly(OEGMA-co-

MEO2MA)-coated NPs can be transferred from water to toluene and back

again. Reproduced with permission from ref. [38]. Copyright 2008, Wiley-

VCH.

1544

NP–PNIPAM attraction by forming hydrogen bonds. In contrast,Wang et al. introduced pyridine segments in PNIPAM hydrogelsvia the copolymerization of NIPAM and 4-vinylpyridine, whereasMAA-coated CdTe NPs are used as the guest NPs.[180] At pH 3.0,CdTe NPs can adsorb into PNIPAM networks via the electrostaticattraction between negatively charged NPs and positively chargedpyridine. At pH 11.0, however, NPs are extruded from PNIPAMhydrogels due to the decrease of pyridine charges. Note that themain component of hydrogels is water, lowering their rigidity intechnical applications. Nevertheless, the responsive properties ofhydrogel spheres may arise from either the entire sphere or onlythe outer polymers. Consequently, one can modify the PNIPAMshell on inert spheres, which still inherits the responsiveproperty.[181]

Anchoring responsive polymers on individualNPs endowsNPswith the ability to be reversibly transported across water/oilinterfaces (Fig. 11). For example, Au NPs are modified with thecopolymers of OEGMA and 2-(2-methoxyethoxy)ethyl methacry-late (MEO2MA).[38] Copolymer-coated Au NPs are originallydissolved in water, which can spontaneously cross water/tolueneinterfaces with the addition of NaCl because the increase of ionicstrength reduces the water/toluene interfacial energy. A reversiblephase transfer of NPs from toluene to water is achievable byreplacing the aqueous phasewith citric acid. The presence of citricacid enhances the hydrogen bonding between copolymer-coatedNPs and water, thus driving the NPs to the water. This resultextends to the application of NP–polymer composites as carriersfor drugs across biological barriers.

While the examples of stimuli-responsive nanocomposites viathe chemical reaction of polymer functional groups is lessreported, this strategy is dramatically important for reversiblecontrol of nanocomposite functionalities using optical, electric,andmagnetic fields. The basic idea of this strategy is the energy orchange transfer betweenNPs and the different isomers of polymeractive groups, which is controllable by external stimuli. Thomaset al. reported a photoswitching model from Au NP–spiropyran(SP) composite system.[182] The irradiation of Au–SP in methanolwith a 360 nm band-pass filter leads to an absorption band around500 nm, which corresponds to the formation of the zwitterionicmerocyanine form of SP. Further heat treatment results in therestoration of the SP structure, and thus the spectral property ofAu–SP composites.

Figure 12. A) Fluorescence image of CdTe NP–PS films. From left to right,

heating at 200 8C for 10, 20, 40, 60, 80, 150, 180, and 200 s. B) Temporal

evolution of the emission peak position of CdTe–PS films at 140, 160, 170,

180 and 200 8C. Patterned CdTe NP–PMMA composite films before

(C) and after (D) gradient heating at 240 8C. Reproduced with permission

from ref. [65]. Copyright 2009, Royal Society of Chemistry.

4.2. Polymer-Mediated NP Aggregation and Growth

Due to the high molecular weight and interchain interaction ofpolymer solids, polymer chain movement is rather slow at roomtemperature. It greatly suppresses the diffusion of NPs withinpolymer media. However, as the temperature is increased to nearor above polymer Tg, polymer chain movement will be alsodynamic, which allows the diffusion or even growth of NPs inpolymer media. Cohen et al. forecasted the diffusion andsubsequent growth of NPs in polymer films.[66] They demonstratea diffusion-aggregation mechanism, where the NPs aggregate toform secondary NPs, which in turn aggregate, giving larger NPs.By combining the reaction-diffusion model and equilibriummodel, the effect of experimental variables on the growth initiationand the final size ofNPs can be qualitatively explained. Particularly


in the case of temperature, both of these two models contain anexponentially activated parameter: viscosity in the reaction-diffusion model and the equilibrium constant in the equilibriummodel. Experimentally, it is found that the size evolution ofluminescent CdTe NPs is practicable in polymer films(Fig. 12A).[65] ThegrowthofNPsoccursonlywhen the temperatureis higher than 110 8C, indicating the existence of a criticaltemperature in the aggregative growth model. An exponentiallyaccelerated growth is observed as the temperature is increased


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Figure 13. TEM images of TOPO-coated (A) and PS-coated (B) CdSe NRs

in PMMA after solvent evaporation under an applied electric field. Repro-


Society.

from 140 to 240 8C, which accords with the equilibrium model(Fig. 12B). In addition, systematic studies reveal that theaggregative growth depends on three aspects: the mobility ofpolymer chains, the compatibility between polymer chains andNPs, and the interactions of NPs with polymer media. Thus byalteringTg and themolarmassofpolymers, thephase separationofNPs, as well as the interaction between NPs and polymers, thegrowth rate ofNCs become controllable. This finding indicates theadvantage ofmodulatingNP size, surface structure, and especiallyphotoelectric properties in the preformed nanocomposites, thusopening a new door for designing and fabricating novelnanostructures and nanodevices (Fig. 12C,D).

Green et al. showed the growth of NPs in polymer films to bethrough aggregative growth or the combination of aggregativegrowth and the Ostwald ripening (OR) process,[67] which can bejudged by the Smoluchowski model, a kinetic equation describingthe collision rate ofNPs.On thebasis of thismodel, theparametersm1 and m3 are brought out to describe the collision of NPs inpolydisperse system and also the contribution of aggregativegrowth and OR, which is expressed as:

C

1þ m1m3

¼ BðS� 1ÞðyþÞ�1

2kT

3mð1þ m1m3ÞN

(1)

where each value of C, a dimensionless constant, hascorresponding m1 and m3 values that can be used as quantitativejudgments on defining the contribution of aggregative growthand OR, B is the proportionality coefficient in the equation for thecondensation rate, S is the supersaturation, yþ is a mean particlevolume, k is the Boltzmann’s constant, m and T are the viscosityand temperature of the medium, and N is the total numberconcentration of particles. m1 and m3 are defined as: m1¼ r3/rHand m3¼ r1/r3, where r3 is the cubed mean radius, rH is theharmonic mean radius, and r1 is the arithmetic mean radius. Inbrief, under the circumstances that 1<m1< 1.25 and1>m3> 0.905, both aggregative growth and OR influence NPgrowth; whereas for the case that m1> 1.25 as well as m3< 0.905,growth takes place only via aggregative growth. For most NPs,their growth in polymer media is the combination of aggregativegrowth and OR; however, the latter plays the key role.

As the annealing temperature is below the critical temperaturefor NP growth, NPs are still dynamic in polymer media. Russellet al. indicate that NPs dispersed in a polymer film spontaneouslytend to migrate to the surface of composite films or the crackbetween composite film and glassy substrate, which stronglydepends on the surface ingredients of the NPs.[183] They findthat in PMMA media, poly(ethylene oxide) (PEO)-coatedNPs can diffuse to cracks, whereas TOPO-coated NPs cannot.Instead, TOPO-coated NPs migrate to the polymer/air interfacerather than in the polymer bulk. This segregation of NPs isdriven by the combination of enthalpic and entropicinteractions between polymers and NPs. PEO and PMMA aremiscible, and in particular, the segmental interactions betweenthe PEO and PMMA are small, thus eliminating enthalpicinteractions of NPswith the host polymer. As a result, the uniformdistribution and diffusion of PEO-coated NPs in PMMA areobserved.


Additionally, such surface-ingredient-dependent segregation ofNPs is applicable in electric field-induced oriented arrays of NRs(Fig. 13). Alivisatos et al. demonstrated an electric field-assistedassembly of perpendicularly oriented NR superlattices by thecombination of electric field and solvent evaporation.[44,45] In thismethod, TOPO-coated CdSNRs suspended initially in toluene arefound to align perpendicular to the substrate under an electricfield. The oriented array of NRs is found to be dependent on thesurface ingredients of the NPs. Only TOPO-coated NRs exhibitsuch alignment behavior, whereas PEO- and PS-coated ones keepthe uniform distribution and disordered array in host polymer. Itmeans that NR alignment is determined by the interfacial energybetween the NR ligands and the polymer media. If the interfacialenergy is not sufficiently large, NR alignment is unavailable.

Due to the unique associative properties, block copolymers canself-assemble into various nanostructures. Among this self-assemblybehavior, themicrophase separationofblock copolymerson a substrate is important to direct the distribution of NPs withinthe preformed composites, and thus the functionalities ofnanocomposites.[1] In general, the modulation from blockcopolymer self-assembly may lead to three advantages: spatialdistribution of NPs in different microdomains, hierarchicaldistribution of NPs within one microdomain, and reversiblemovement of NPs induced by microphase transition. Similar tohomopolymers, the spatial distribution of NPs in block copoly-mers depends greatly on the surface chemistry of theNPs. Take themixture of Au NPs and poly(styrene-b-ethylene propylene)copolymers (PS-b-PEP) as an example;[184] if the NPs are modifiedby oligo(styrene), they have good miscibility with the PS blocks ofPS-b-PEP. In contrast, if NPs are modified by alkanes, they areimmisciblewitheither thePSblocksorPEPblocks, resulting in theappearance of NPs at the PS/PEP interface. Besides the templatebehavior forNPassembly, block copolymershavebeenpredicted todirect the hierarchical distribution of differently sized NPs, whichinvolves an entropy-drivenmechanism.[70] In this context, NPs areinorganic solids, which obstruct the stretching of polymer chainsand therewith decrease conformational entropy. For larger NPs,they are easier to be expelled from polymer bulk, whereas smallerones are not. This prediction is experimentally proved by Thomaset al. through a study of the self-organization behavior of alkane-coated Au and silica NPs in PS-b-PEP.[185] Larger silica NPs arefound located at the center of the PEP domains, whereas smaller


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Figure 14. Bright-field electronmicroscopy image of a ternary blend of PS–

PEP, Au NPs, and SiO2 NPs. Au NPs appear as dark spots along the IMDS,

whereas SiO2 NPs reside in the center of the PEP domain. Inset: schematic

of the NP distribution. Reproduced with permission from ref. [185].

Copyright 2003, American Chemical Society.

Figure 15. TEM images of PVA films containing Au NRs before (A) and

after (B) stretching. Reproduced with permission from ref. [79]. Copyright

2005, Wiley-VCH. C) Optical image of NP/PMMA film transferred to glass.

D) TEM of free-standing NP/PMMA film on a holey carbon grid. Repro-


Society. TEM images of CTAB-coated Au NRs (E) and self-assembled

structures of PNIPAM-modified Au NRs (F). Reproduced with permission

from ref. [48]. Copyright 2009, Royal Society of Chemistry.

1546

Au NPs segregate to the intermaterial dividing surface (IMDS)between the PS and PEP domains (Fig. 14). Finally, blockcopolymers are capable to transit from onemicrophase to anotheras the surroundings are altered, whichmay in returnmodulate theaggregation behavior of NPs. Tsukruk et al. demonstratedresponsive composite nanotubes composed of block copolymersand Au NPs.[186] The nanotubes are fabricated through the in situpreparation of AuNPs using PS-b-P2VP copolymers as a template.Owning to the pH-sensitive structures of P2VP blocks, whichexhibit a coil-to-globule transition above pH 3.6, the compositenanotubes show reversible changes in topology and collectiveplasmon resonance appearance.

Overall, the polymer chain movement depends greatly on thevariation of the ambient surroundings, which allows easydiversification of NP distribution, orientation, and growth innanocomposites, and therefore the modulation of NP function-alities. Additionally, NPs can in return alter or tailor the phaseseparation process of polymers,[187] providing research opportu-nities for designing and fabricating nanodevices with novelfunctionalities.

4.3. Polymer-Directed NP Orientation

Again resulting from the structural properties of polymermaterials, they exhibit excellent mechanical performance, repre-sented by the characteristics of swelling, stretching, and thermalpressing. These merits are reasonably inherited for inducing NPassembly and orientation, and therewith modulating the func-tionalities of nanocomposites. Through this strategy, one maycommand the spatial array of NPs into 1D, 2D, or 3D structures,and also the orientation of NPs either during compositespreparation or postpreparation treatment. The stretching char-acteristics of polymers are used to direct the orientation of NPs,through which the anisotropic properties of nanocomposites canbestrengthened.Bycombiningheating andsubsequent stretching


of Au NR–PVA nanocomposite films, a spatial alignment of theNRs is achieved.[79] It is observed that as the polymer is stretched inone direction, NRs orient with their long axes along this direction(Fig. 15A,B). The resultant films indicate anisotropic opticalproperties are distinctly different from the homogeneous solutionor composites. The optical properties of such oriented films agreewell with the predications for small ellipsoids, thus leading to aselective excitation of each plasmon mode of Au NRs usingpolarized light. Besides anisotropic bulk composites, the align-ment of Au NRs on the surface of PVA films can be achieved byimprinting NRs on PVA and subsequently stretching the film.[78]

This method will promote the development of optical displaydevices, 2D biosensors, and nanoelectronic devices composed ofNPs and polymers.

On the basis of the electrospinning technique, the stretchingstrategy can be extended to fabricate 1D nanocomposite fibers,which leads to 1D arrays ofNPs.[188,189] In thismethod, the speciesof host polymers is dominant for assembling NPs towards 1Darrays. Kim et al. showed the use of semicrystalline PEO as atemplate to control the nanoscale organization of AuNPs.[188] NPscan on one hand act as heterogeneous nucleating agents for PEO


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crystallization, and on other hand be arranged to fairly long, 1Dstructures. This success indicates great potential for a convenientand simple technique for the fabrication of 1D arrays ofmetal NPssuitable for processing into quantum-confined superstructures,for instance SERS substrate. Yu et al. demonstrated the ability tocontrol the length of NP chains within nanofibers by adjusting thequality of NPs dispersed in polymer solution in the electrospin-ning process.[189] Further studies indicate that the degree ofaggregation of metal NPs in nanofibers and the electrospinningtimeare twokey factorsdetermining themagnitudeofSERSsignalenhancement and the sensitivity of detection. The dimer and shortchain-like NP aggregates exhibit the best optimized system forSERS enhancement.

Taking advantage of the spreading behavior of polymerhydrophobic droplets on the water surface, one may achieve 2Dordered arrays of NPs. Brinker et al. reported a general and facilemethod to prepare free-standing NP/PMMA composite film bydispensing one drop of a NP/PMMA/toluene solution onto thesurface of deionized water.[46] The droplet quickly spreads over thewater surface and evaporates to generate a solid composite filmcontaining 2D arranged NPs (Fig. 15C,D). The thickness of theresultant films ranges from several nanometers to tens ofnanometers depending both on the NP diameters and PMMAconcentration. Due to PMMA supporting, the films are transfer-able to arbitrary substrates and patternable by lithographytechnique. By combining the dewetting property of polymericfilms, polymer-coated NPs can be induced to ringlike arrays usingthe water droplet as a template.[47] There are two importantelements for the successful formation of rings: water-immisciblesolventwithhighvolatility, andhydrophobicpolymer-modificationof NPs. These polymer-induced assembly techniques can be usedto manipulate the optical properties of NPs.

Figure 16. Self-assembly of PS-tethered AuNRs in selective solvents. An amph

PS molecules grafted to both ends. A–E) SEM images of the self-assembled

mixture at water contents of 6 and 20wt%, respectively, side-to-side aggregate

mixture at water contents of 6 and 20wt%, respectively, and bundled NR chains

17:17:6 (E). Inset: schematic diagrams of the NR assemblies. Reproduced w


Furthermore, recent advances on the heterogeneousmodifica-tion of polymers on the given sites of NPs greatly promote thestrategy of polymer-directed NP assembly.[48,49] In this regard,polymers can bemodified only on one pole of spherical NPs or onthe ends or sides ofNRs, and followedby optical, electric, thermal,or solvent inducement, they can generate distinct linkages withthe NPs. Kumacheva et al. demonstrated a photothermallyinduced self-assembly of Au NRs.[48] In this method, the ends ofAu NRs are foremost modified with PNIPAM.When illuminatedat wavelengths close to their longitudinal plasmon band, NRs canabsorb light and release energy, which in return bring the coil-to-globule transition of PNIPAM. Due to the hydrophobic–hydrophobic interaction generated by PNIPAM shrinking, NRsundergo end-to-end assembly and form chains (Fig. 15E,F). Theanisotropic assembly of NRs leads to an obvious red-shift in theirlongitudinal absorption peak. Significantly, the disassembly ofNR chains can easily be achieved by cooling the solution to roomtemperature because of the low LCST of PNIPAM. Variation ofsolution polarity is another way to trigger the self-assembly ofNRs.Winnik et al. presented a solvent-controlledmethod to directAu NRs assembly into either 1D or 3D superstructures.[49] If PS-coated Au NRs are dispersed in the binary solvents of N,N’-dimethyl formamide (DMF) and water, NRs can spontaneouslyassemble to end-to-end chains. If they are dispersed in themixture of THF and water, side-to-side assembly is favored(Fig. 16). The competition between end-to-end and side-to-sideaggregates arises from the different solubility of PS and CTAB-coated NRs in the solvent mixture. Importantly, this method isuniversal for directing the orientation of NPs with differentcomponents and structures,[190] making it potentially useful tomediate the assembly of NPs, and therewith modulate thefunctionalities.

iphilic AuNR carrying a double layer of CTAB along the longitudinal side and

NR structures: rings (A) and chains (B) self-assembled in the DMF/water

d bundles of NRs (C) and nanospheres (D) self-organized in the THF/water

obtained in the ternary DMF/THF/water mixture at a weight ratio of liquids

ith permission from ref. [49]. Copyright 2007, Nature Publishing Group.


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5. Conclusion and Outlook

In summary, we have presented a brief review of the recentprogress in the fabrication methodologies and functionalmodulation of NP–polymer composites, which are soundlyfacilitated by the technical revolution of NP synthesis and surfacefunctionalization. The incorporation of NPs with polymers bringsmany advantages that mainly relates to four aspects: 1) inheritingthe excellent functions of NPs from colloidal solution and thenimproving the long-term stability; 2) supplying the insufficiency ofpolymer materials via inorganic NP enhancement; 3) integratingmultiple functions both of NPs and polymers; and 4) tuning thefunctionalities of preformednanocomposites based on the uniquestructural properties of polymers. Accordingly, this protocol isalready accepted as a versatile and efficient way to take NPs fromfundamental research and bring them to applications in variousdevices, leading to significant progress in nanoscience andnanotechnology.

In particular, a rapid development in the methodologies formodulatingNP functionalities in the preformed nanocompositesis ongoing, driven by the forthcoming requirement of ultra-sensitive sensors, intelligent materials, and optoelectronicdevices. In this regard, there are several problems to be addressed.First, most of the modulation models are solution-basedcompositemicrogels, capsules, ormicelles. It favors the detectionof pH, ion, or temperature variation, but in return limits theresponse to gas. Also, in comparison to solid composites, they arenot inert enough for long-term maintenance of the functions ofthe embedded NPs, making them useless in practical applica-tions. Nevertheless, the problem can be overcome by designingthe nano- or microstructures of the composites via a stepwiseassembly strategy. Secondly, as for modulating NP spatialdistribution and orientation, the successes are still limited toseveralmorphologies. Reversible spatial distribution and orienta-tion is especially less reported. The key challenge is currently thelack of an efficient means to flexibly tune the miscible,immiscible, and intermediate states of the NPs and polymermedia. Thismaybe overcomeby the selective surface engineeringof NPs with multiple blocks and/or their heterogeneousdistribution on NP surface. Thirdly, for constructing compositedevices, nanocomposites are currently fabricated either throughin situ formation methods or assembly approaches. Surely, greatsuccesses are achieved by solely using these two routes. Furthercombination of these routes is expected to carry out greaterachievements. For instance, the controllable size and shapeevolution of NPs in the preformed nanocomposites will provide afacile and feasible method to adjust the conjugation manner ofNPs and polymer networks, and therefore the charge and energytransfer between them. In any case, nanocomposites areindispensable in the current materials library, and the capabilityfor further modulating their functionalities opens the door tomaterial preparations and applications with state-of-the-artcontrol.

Acknowledgements

This work was supported by NSFC (20974038, 20921003, 50973039), the973 Program of China (2007CB936402, 2009CB939701), the FANEDD of


China (200734), the Special Project fromMOST of China, and the Programfor New Century Excellent Talents in University.

Received: January 15, 2010

Published online: April 7, 2010

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