Core/Shell Nanospheres, Hollow Capsules, and...

Core/Shell Nanospheres, Hollow Capsules,and Bottles

Gang ZhangKai ZhangJilin University, Changchun, People’s Republic of China

Yi YuChinese Academy of Sciences, Changchun, People’s Republic of China

Bai YangJilin University, Changchun, People’s Republic of China

INTRODUCTION

The design and synthesis of nanoscale materials is im-

portant in the fabrication of advanced devices for optics,

electronics, and biotechnology.[1–6] Over the past decade,

considerable effort has been devoted to the design and

controlled fabrication of nanostructured materials with

functional properties. The interest in nanoscale materials

comes from the fact that their properties (optical, elec-

trical, or chemical, etc.) are the functions of their size,

composition, and structural order. Colloidal particles

represent attractive building blocks from which to create

ordered and complex materials. They are also of wide-

spread interest in chemical engineering, biological, and

pharmaceutical applications.[7] In biotechnology, the

encapsulation and delivery of proteins and DNA into

cells has led to the implementation of intracellular

medicinal therapies such as gene therapy.[8,9] Comple-

tion of the human genome project has ensured the

former, leaving the synthesis of encapsulation and

delivery materials as perhaps the single most important

challenge in intracellular medicinal therapies. Recently,

core-shell particles and micro- or nanosized capsules

have received considerable attentions for their techno-

logical importance in many fields.[10–25] There have been

many efforts to fabricate core-shell colloidal materials

with tailored structural, surface, and optical proper-

ties.[26–28] The creation of core-shell colloidal particles is

also of interest from a fundamental viewpoint, especially

in the areas of colloid and interface science. They can be

utilized as model systems to investigate factors govern-

ing colloidal interactions and stabilization and to gain

valuable information on the properties of concentrated

dispersions.[29,30]

OVERVIEW

The synthesis of core-shell particles typically involves

tailoring the surface properties of particles, often accom-

plished by coating or encapsulating them within a shell

of a preferred material. The shell can alter the charge,

functionality, and reactivity of the surface, and can en-

hance the stability and dispersibility of the colloidal core.

Optical, magnetic, or catalytic functions may be readily

imparted to the dispersed colloidal matter depending on

the properties of the coating. Encapsulating colloids in a

shell of different composition may also protect the core

from extraneous chemical and physical changes.[31,32]

Core-shell particles often exhibit improved physical and

chemical properties over their single-component counter-

parts and, hence, are potentially useful in a broader range

of applications. Optimization of the surface characteristics

of particles through coating processes is also of primary

importance for the successful application of composite

particles. Recent methods offer new alternatives for the

controlled synthesis of novel, stable, and functional core-

shell type materials.

An important extension of core-shell particles is the

subsequent removal of the core by either thermal or

chemical means (selective etching with a solvent or cal-

cination in air), forming hollow spheres. A variety of

procedures currently used to fabricate a wide range of

stable hollow capsules of various compositions have been

reported.[33] Hollow capsules of nanometer to micrometer

dimensions constitute an important class of materials that

are employed in various technological applications, such

as encapsulate agents for delivery of cosmetic, drug, ca-

talysis, and protecting sensitive agents. They may also

provide some immediate advantages over their solid

Dekker Encyclopedia of Nanoscience and Nanotechnology 865

DOI: 10.1081/E-ENN 120013629

Copyright D 2004 by Marcel Dekker, Inc. All rights reserved.

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counterparts because of their relatively low densities and

as fillers with low dielectric constant in electronic com-

ponents. Using various chemical and physicochemical

methods nowadays routinely produces hollow capsules

comprising polymer, glass, metal, and ceramic.

As particular examples, hollow spheres with mesopor-

ous wall have been synthesized from gel composite;[34,35]

however, large entities such as macromolecules usually

cannot penetrate such microspheres. It would be desirable

to leave a hole on the shell surface for transporting var-

ious molecules. Lin and coworkers synthesized a vesicular

hollow microspheres that possess a pair of holes of

submicron size on exactly opposite sides.[36] Recently, our

group obtained the nanobottles through the removal of

template functional polymer and silica cores through

programmed calcination at high temperature and solution

etching, respectively. Because there is an opening on the

hollow cavity of silica nanobottle, it can afford both a

channel for transmission and a container for storage. So

the nanobottles can be used as an extremely small con-

tainer for encapsulation, as well as a nanosized carrier and

reactor for catalysis and microreaction. Furthermore, the

encapsulation of rare earth complex in the nanobottles re-

veals a potential application for nanotechnique.

This article provides an overview of the various

methods used to synthesize core-shell particles, hollow

capsules, and bottles in the nanometer to the micrometer

size range, detailing early and very recent developments

in the above area.

NANOSIZED CORE-SHELL SPHERES

Nanosized Core-Shell Sphereswith Polymer Shell

Polymer-coated spheres offer interesting prospects in a

broad spectrum of applications, ranging from catalysis to

additives and dyes, where they are exploited in the

manufacture of cosmetics, inks, and paints. The synthetic

routes that have been developed in order to produce

polymer-coated spheres fall into two main classes:

polymerization at the sphere surface or adsorption onto

the spheres. Hofman-Caris has comprehensively reviewed

the processes used to obtain spheres that consist of an

inorganic core and a polymer shell through polymeriza-

tion and chemical coupling procedures prior to 1994.[28]

We will deal with more recent strategies used to coat

spheres with polymers, polymerization approaches, and

the self-assembly of polymers from solution.

A number of polymerization-based methods have been

employed to produce spheres that consist of solid cores

coated with a shell of polymeric materials. These include

monomer adsorption onto spheres followed by subse-

quent polymerization,[37–42] heterocoagulation–polymeri-

zation,[43] and emulsion polymerization.[28,44] In the first

approach, one of the most frequently employed to achieve

polymer coatings on solid spheres, the polymerization

reaction can be catalyzed either by an initiator to promote

the process or by the colloidal spheres themselves.

Atom transfer radical polymerization (ATRP) is a

versatile technique, which offers several advantages over

other polymerization routes including control over mo-

lecular weight and molecular weight distribution.[45,46]

Also, the polymers can be end-functionalized or block

copolymerized upon the addition of other monomers.[46]

Not only does this feature offer tailorability of the

polymer coating with a variety of compositions and

functionalities, but also this feature may be important in

biomedical applications to modify the polymer shell with

biological moieties for specific cellular interactions.

ATRP has been able to form PMMA and PS shells on

silica nanoparticles,[45] and provides magnetic core-shell

nanospheres with size <15 nm. Magnetic studies show

a decrease in coercivity, which is consistent with the

reduction of magnetic surface anisotropy upon polymer

coating. Certainly the magnetic core of these core-shell

nanospheres can be selected, depending upon the desired

super paramagnetic properties for specific applications

such as in data storage and MRI contrast enhance-

ment.[47] Moreover, the resulting core-shell nanospheres

are within the biological size restrictions and may poten-

tially be modified for a particular biospecificity.

Matijevic et al. reported the coating of aluminum

hydrous oxide-modified silica spheres with poly(divinyl-

benzene) (PDVB) layers by pretreatment of the inorganic

cores with coupling agents such as 4-vinylpyridine or 1-

vinyl-2-pyrrolidone, followed by subsequent admixing of

divinylbenzene and a radical initiator.[37] Polymer layers

of poly(vinylbenzyl chloride) (PVBC), copolymers of

PDVB–PVBC, and double shells of PDVB and PVBC

were also synthesized around inorganic spheres using a

similar approach.[38]

The use of electrochemical or soluble initiators can be

eliminated by utilizing catalytically active cores to ef-

fect the polymerization of monomers adsorbed on the

surface of spheres. This approach was employed to obtain

poly(pyrrole) coatings on a range of inorganic cores by

using the active sites on the metal oxide surfaces to initiate

the polymerization of pyrrole.[39] Hematite, silica-modi-

fied hematite, and cerium(IV) oxide (CeO2) were coated

with poly(pyrrole) by exposing the inorganic cores to the

polymerization medium of pyrrole in an ethanol/water

mixture and heating to 100�C.

Moller et al. presented a work directed toward the

formation of core-shell particles consisting of a nanocrys-

tal of Au surrounded by a shell of conducting polymer,

e.g., polypyrrole.[48] Because of the different chemical

866 Core/Shell Nanospheres, Hollow Capsules, and Bottles

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nature of the two materials, charge transfer might be

expected at the interface and the optical functions of both

materials should be drastically different from the

corresponding bulk materials.[49] Preliminary experiments

using solutions of tetrachloroauric acid (HAuCl4) and

pyrrole, without diblock copolymer, have demonstrated

the formation of PPY and Au. However, in this case it was

not possible to prevent the macroscopic segregation of the

polymer and the metal phase. Macroscopic segregation

can be prevented efficiently if the composite particles are

formed within the cores of the micelles of a diblock

copolymer. Fig. 1 shows the micrograph recorded after

annealing the film at 130�C. Uniform Au particles with a

diameter of 7 nm formed within the micelles. The figure

indicates that the originally indistinct ultrasmall clusters

of elementary gold coalesced upon treatment above the

glass transition temperature of both blocks. When the

same annealing procedure was applied to an Au-loaded

micellar film that had not been treated with pyrrole, larger

Au particles were formed and the micellar organization

was destroyed. Thus the presence of polypyrrole or

pyrrole oligomers is essential to yield a single Au particle

within each spherical microdomain.

Methods to coat a polymer shell with a controllable

thickness on magnetic nanoparticles may aid in the

development of ordered arrays of magnetic nanoparti-

cles. The formation of polymeric shells is essential for

biomedical applications of magnetic nanoparticles such as

magnetic targeting drug delivery and magnetic resonance

imaging (MRI) contrast enhancement. Many methods

usually create micrometer-sized magnetic polymer

spheres, which are large for in vivo applications.[50] A

less than 20-nm size has been suggested for the efficient

diffusion of nanoparticles through tissue in MRI applica-

tions.[51] An emulsion polymerization of poly(methyl

methacrylate) (PMMA) on �10-nm core of mixed-phase

iron oxides has made improvement; the particle size is

still >130 nm.[52,53] Polystyrene (PS) is easy to synthesize

for testing various strategies of coating nanoparticles

with polymer shells. Zhang et al. reported the formation of

magnetic MnFe2O4 PS nanoparticles using ATRP yield-

ing a core-shell nanoparticle with size <15 nm.[54] Most

polymer-coating studies on magnetic nanoparticles form

the nanoparticle core (typically Fe, Fe2O3, or Fe3O4) at the

same time as that of polymerization.[55] The MnFe2O4

nanoparticles as the magnetic core were separately

prepared by a reverse micelle microemulsion proce-

dure.[56] Polymerization initiators are chemically attached

onto the surface of nanoparticles. The modified nanopar-

ticles are then used as macro-initiators in the subsequent

polymerization reaction. This approach provides great

flexibility in the selection of magnetic core. Conse-

quently, magnetic tunability can be introduced into these

core-shell nanosphere systems to achieve the desired

super paramagnetic response.[57]

Inspired by the nanosized, amphiphilic core-shell

structure of lipoproteins, shell cross-linked nanoparticles

with a hydrophobic core, contained within a hydrogel

network, were prepared by the self-assembly of amphi-

philic block copolymers followed by intramicellar cross-

linking between the polymeric chains located within the

shell.[58] The control over size, shape, and composition of

these nanoparticles holds great potential in drug delivery

applications.[59,60] Intramicellar cross-linking of the poly-

mer chains within the shells of polystyrene-b-poly(acrylic

acid) micelles by reaction with difunctional poly(ethylene

oxide) afforded unimolecular amphiphilic core-shell

nanospheres (50 nm hydrodynamic radius).[61]

The controlled release of polymer chains from the core

by adjusting the cross-link density of the shell opens the

possibilities of designing polymeric nanoparticles with

specific shell permeabilities, capable of delivery of large

guests. This approach may provide a solution to some

of the delivery problems posed by biologically active

molecules, such as peptides and proteins, genes and

Fig. 1 TEM micrograph of a colloidal polymer film ([PY]/

[HAuCl4]=3.0) after annealing at 130�C for 140 min, exhibiting

7 nm wide Au clusters in each micelle encapsulated by PPY.

(From Ref. [48].)

Core/Shell Nanospheres, Hollow Capsules, and Bottles 867

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oligonucleotides. The results of this study also provide

a foundation for better understanding of the porosity of

the cross-linked shell.[62] This represents a methodology

to probe the permeability of nanoscopic membranes and

a means for applications in the controlled release of

macromolecular species.

Our group described a flexible method for preparing

monodisperse silica–PS core-shell microspheres. In this

method, silica nanoparticles grafted with 3-(trimethoxy-

silyl) propyl methacrylate (MPS) was employed in an

emulsion polymerization as seeds. The thickness of the

shells could be changed through varying the amount of

monomer and emulsifier. The monodispersity and dia-

meters of the core-shell microspheres were found to

depend on the size of grafted silica nanoparticles and the

concentration of emulsifier.

The monodisperse silica microspheres with average

radii ranging from 35 to 90 nm were prepared in ethanol

according to the Stober method[63] at ambient tempera-

ture. In order to obtain a functionalized surface, MPS

with C C bond was added and reacted with the Si–OH

group on the surface of the silica by hydrolysis. Mono-

disperse silica–polymer core-shell microspheres were ob-

tained through emulsion polymerization of styrene (St)

or methyl methacrylate (MMA), while grafted silica

particles dispersed in ethanol, which acts as ‘‘seeds’’ in

the polymerization process.[64]

Fig. 2 shows the TEM images of the resulting silica–

PMMA (left) and silica–PS (right) core-shell micro-

spheres. The spherical particles show obvious core-shell

structures, light shells (PMMA or PS) coat the dark

grafted silica microspheres cores, and over about 90%

of these core-shell microspheres have only one single

core. The average radius of the monodisperse core-shell

microspheres varies from 45 to 150 nm for silica–PMMA

and from 80 to 210 nm for silica–PS, which have been

confirmed by the TEM.

A series of TEM images of core-shell microspheres

prepared by increasing the amount of styrene (St) prove

that the grafted silica nanoparticles act as ‘‘seeds’’ in the

emulsion polymerization.[64] The ‘‘raspberry’’ morpholo-

gy of core-shell microspheres was seen, and it was clearly

visible that the surfaces of shells became smoother and the

shells thickened with increasing the amount of monomer;

the core-shell microspheres were still monodisperse.

Nanosized Core-Shell Spheres withInorganic and Composite Shell

Various procedures have been employed in the fabrication

of inorganic/hybrid coatings on particles, allowing a broad

range of materials with different properties to be prepared.

The specific methods of solid-core inorganic/hybrid-shell

sphere preparation can be classified into two general

Fig. 2 TEM images of silica-PMMA (left), silica-PS core-shell spheres (right). (From Ref. [64].)


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categories: precipitation and surface reactions, and the

controlled deposition of preformed inorganic colloids.

Previous investigations have demonstrated that poly-

meric and inorganic particles dispersed in aqueous

solutions can be coated with layers of various inorganic

materials either by precipitation of the coating materials

onto the cores or by direct surface reactions utilizing spe-

cific functional groups on the cores to induce coat-

ing.[17,27,31,65–76] The inorganic coatings prepared using

these approaches include silica,[17,27,31,65–73] yttrium basic

carbonate, titania,[74–76] and polyelectrolytes onto particles

via the layer- by-layer ( LbL) and LB techniques. Early

work focused on the coating of titania microparticles with

silica layers; however, significant particle clumping and

coalescence took place during silica deposition. Using the

precipitation method, in which the coating material is

precipitated directly onto the core, Ohmori and Matijevic

optimized the coating conditions and coated spindle-

shaped hematite (a-Fe2O3) particles with silica layers by

hydrolysis of the alkoxide tetraethoxysilane (TEOS) in

2-propanol.[31,65,77] Uniform silica coatings on individual

a-Fe2O3 particles were obtained when the kinetics of the

TEOS hydrolysis was properly controlled. Dispersions of

uniform submicrometer spherical particles consisting of

silica cores and yttria coatings, as well as yttria cores with

silica coatings, were also prepared by a similar method.

Electrostatic interactions between nanoparticles and

larger particles via solution self-assembly have been

widely exploited to prepare core-shell materials.[66,78–82]

Homola et al. reported the coating of g-Fe2O3 particles

with preformed smaller silica particles by combining

the particle mixtures under conditions where the two

types of particles are oppositely charged. This resulted in

better dispersion and less aggregation of the magnetic

particles. Similarly, nanosized silica was deposited on a

range of larger inorganic particles, thus forming a pro-

tective layer. Nanocomposite multilayers can be assem-

bled on particle surfaces by using the LbL method based

on colloidal templates.

Lu et al. described a sol–gel approach for the coating of

super paramagnetic iron oxide nanoparticles with uniform

shells of amorphous silica.[83] The coating process has

been successfully applied to particles contained in a com-

mercial ferrofluid and those synthesized through a wet

chemical process. The thickness of the silica coating could

be conveniently controlled in the range of 2–100 nm by

changing the concentration of the sol–gel solution. Fluo-

rescent dyes could also be incorporated into these silica

shells through a covalent coupling between these organic

dyes and the sol–gel precursor. Also, they and Liz-Marzan

et al. demonstrated that gold nanoparticles could be di-

rectly coated with uniform shells of amorphous silica

using a sol–gel process (Fig. 3).[27,83,84] The thickness of

such a conformal coating could be changed from tens to

several hundred nanometers by controlling the concentra-

tion of TEOS precursor or the deposition time. The po-

tential use of these spherical, core-shell colloids in fabri-

cating photonic devices has been illustrated with two

examples: photonic crystals and plasmonic waveguides.

These demonstrations suggest that Au–SiO2 core-shell

particles with well-controlled sizes are promising building

blocks for nanoscale integrated optics, in which the di-

mensions of structures for guiding and modulating pho-

tons will no longer be limited by the wavelength of light.

Stable colloidal core-shell particles consisting of a PS

core and a titania coating were prepared in one step by the

hydrolysis of a titanium alkoxide in the presence of a

cationic PS latex.[85] Although this study used PS as a

core, it should be possible to replace it with other poly-

mer colloids that can be given cationic surface groups or

with negatively charged particles that can be made pos-

itive by coating with a polyelectrolyte. This results in

unusually smooth and uniform titania shells that can be

made as thin as a few nanometers. This is attributed to

a very rapid collection of the negatively charged titania

oligomers by the positively charged surfaces. The coat-

ings are very smooth and uniform and can be varied in

thickness from just a few nanometers to at least 50 nm.

Thicker coatings should also be possible but only through

a multistep seeded growth process. The coated spheres

have the same monodispersity as the starting latex, allow-

ing them to form colloidal crystals.

Fig. 3 (A) TEM image of gold nanoparticles with an average diameter of 50 nm. (B,C) TEM images of such gold nanoparticles after

their surfaces had been coated with amorphous silica shells of �20 and �80 nm in thickness, respectively. (From Ref. [83].)


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Novel fine polymer particles containing ultrafine Pd,

Pt, or Rh metal dispersed on the core-shell-type sphere

were prepared by the emulsifier-free emulsion polymer-

ization, followed by the addition of a mixture of Ln(NO3)3

and NaH2PO4.[86] Rogach et al. and Caruso et al. report

on the fabrication of 3-D colloidal photonic crystals by the

self-organization of submicrometer-sized PS latex spheres

covered via the consecutive electrostatic adsorption of

charged polyelectrolytes and luminescent semiconductor

nanocrystals (Fig. 1).[82] CdTe and CdTe(S) nanocrys-

tals,[87] capped on the surface with different thiols and

with sizes ranging from 2.5 to 5 nm, have been prepared

by a wet chemical route.[88] They show a pronounced size

dependence on the position of their electronic transitions

and luminescence maxima due to the quantum confine-

ment effect. Relatively narrow and reasonably strong

‘‘excitonic’’ luminescence occurs close to the onset of

absorption and is tunable between 530 and 680 nm.

Highly monodispersed CdSe–CdS core-shell nanopar-

ticles have been prepared by a novel route involving

thermolysis in TOPO in a one-pot synthesis.[89] This

route is a simple and convenient route to produce rea-

sonable quantities of high-quality, monodispersed core-

shell nanoparticles. The precursors are easy to synthesize

and store and give high yields of TOPO-capped quan-

tum dots.

Submicrometer-sized anionic PS latexes have been

coated with uniform layers of iron compounds by aging, at

elevated temperature. Dispersions of the polymer colloid

in the presence of aqueous solutions of ferric chloride,

urea, hydrochloric acid, and polyvinylpyrrolidone have

been produced.[25] The thickness of the deposited layers

could be altered by suitable adjustment of the reactant

concentrations, and they could also be increased by further

aging of the coated particles in the presence of aqueous

solutions of ferric chloride. Hollow colloidal spheres of

iron compounds were obtained by calcinations of the so-

coated PS latexes at elevated temperature in air. Different

chemical compositions of hollow colloidal spheres were

obtained by heating them in hydrogen.

HOLLOW CAPSULES AND NANOBOTTLES

Hollow spheres are useful in a variety of areas. They can

be used in catalysis, delivery of drugs, development of

artificial cells, or protection of biologically active agents

(such as proteins, enzymes, or DNAs). Hollow spheres

may also provide some immediate advantages over their

solid counterparts because of their relatively low densi-

ties. In a typical procedure, hollow spheres are prepared

by the removal of the ‘‘cores’’ (via selective etching with

a solvent or calcination in air) from core-shell structure

nanospheres. There are a variety of methods currently

used to fabricate a wide range of stable, hollow spheres of

various compositions. These methods include nozzle

reactor systems,[90–92] emulsion/phase separation techni-

ques coupled with sol–gel processing,[73,93,94] sacrificial

core procedures,[77,95–97] and LbL technique (consecu-

tively assembling inorganic nanoparticles and polymer

onto colloids and subsequently removing the templated

colloid).[1,10–15,98–102] There have been some successful

examples for the preparation of different kinds of hol-

low microsphere materials (such as silica,[12,16–21,80] zir-

conium[16] hydrous oxide, yttrium compounds,[15,22,23]

titania,[75,99–101] copper compounds,[102] zeolite,[103] and

magnetic nanoparticles.[17,24,25,104]) Yin et al. synthesized

Fig. 4 TEM and SEM (inset) of hollow palladium spheres.

(From Ref. [105].)


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mesoscopic hollow microspheres of ceramic materials

with functionalized interior surfaces.[96] Kim et al. fab-

ricated hollow palladium microspheres and successfully

applied them to the recyclable heterogeneous catalyst for

Suzuki coupling reactions (Fig. 4).[105] Caruso and cow-

orkers prepared many kinds of inorganic and hybrid

hollow spheres (SiO2, TiO2, Fe3O4, luminescent polyelec-

trolyte, etc.) by consecutively assembling inorganic nano-

particles and polymer onto colloids (LbL technology) and

subsequently removing the templated colloid (Fig. 5).[106]

Polymer Hollow Capsules and Nanobottles

Recent advances in supramolecular chemistry have given

chemists unprecedented control over the composition and

shape of nanoscopic objects. An example of such

development is the synthesis of nanometer-sized organic

hollow spheres, which can find numerous applications in

drug delivery/targeting, extraction and as nanoreactors.

Sun et al. described a new strategy for synthesizing

nanometer-scale organic hollow spheres using Au colloids

as templates.[107] The whole structure is held together

by S–S bonds. Oxidation of gold nanoparticles protected

by thiolated bicyclodextrin molecules leads to the forma-

tion of water-soluble polycyclodextrin nanocapsules held

together by S–S bonds. They are currently working on

broadening the described strategy to other substrates/

templates and probing the encapsulation properties of the

hollow spheres.

Marinakos et al. described new methods for synthesiz-

ing nanometer-sized hollow capsules of poly(pyrrole),

poly(N-methylpyrrole), and polyalkenes.[41,42] These

methods utilized nanometer-sized gold particles as tem-

plates from which to grow polymer shells. Dissolution of

the template particles yielded structurally intact hollow

polymer capsules with interior volume and shell thickness

governed by the diameter of the template particle and the

polymerization time, respectively. Moreover, they showed

that alkanethiols were encapsulated in the hollow polymer

core by attaching them to the gold template particles prior

to polymerization and particle etching, and small mole-

cule diffusion rates through the pyrrole-based polymer

capsules depended on polymer oxidation state. They also

described a method for converting alkylthiolate mono-

layers on gold particles into hollow polymer capsules.[108]

The synthetic design of the tripodal ligand provides the

potential to ultimately control the functionality present on

the surface of the particle as well as that present internally.

Marinakos et al. show that small molecules and

enzymes can be trapped inside poly(pyrrole), poly(N-

methylpyrrole), and poly-(3-methylthiophene) capsules

synthesized using the gold particle template method.[1]

Diffusion coefficients of small molecules through the

capsule shell were found to vary by almost 3 orders of

magnitude depending on the polymer, polymer oxidation

state, and counter anion incorporated during polymer

synthesis. A small molecule (anthraquinone) and an en-

zyme (horseradish peroxidase) were trapped inside hol-

low capsules by attaching them to the template particle

prior to polymerization and particle etching. A thin

poly(pyrrole) shell protected the enzyme two times longer

in neat toluene compared to unencapsulated enzyme.

Finally, the potential for using conductive polymer nano-

particles for intracellular delivery or diagnostics was

examined by administering particle suspensions to 3T3

murine fibroblasts. Particles ranging in size from 25 to

100 nm were engulfed by fibroblasts without compromis-

ing cell viability.[1]

Hollow polymer spheres synthesized from a vesicle-

directed polymerization can be dried and redispersed

Fig. 5 Illustration of procedures for preparing inorganic and hybrid hollow spheres. The scheme is shown for PS latex particles. (From

Ref. [106].)


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in water using a variety of nonionic ethoxylated alcohol

surfactants as stabilizers.[109] The final dispersions consist

of both polymer shells and surfactant micelles, which

remain together in colloidal suspension for at least several

months. Small-angle neutron scattering (SANS) is used

to measure the polymer shell thickness (6.3 nm) and core

radius (56 nm) of the surfactant-stabilized hollow poly-

mer spheres in the presence of surfactant micelles.

Hollow polymer microsphere latexes were synthesized

according to polymer–polymer core-shell emulsion poly-

merization then removing the core by selective sol-

vents.[41] Kamata et al. have demonstrated a practical

route to the facile synthesis of spherical hollow colloids of

PBzMA that contained movable cores of Au nanoparticles

(Fig. 6).[110] This procedure should be extendable to many

other systems that involve the use of different combi-

nations of materials for the core and the shell. These core-

shell colloids may also find use as building blocks to

form colloidal crystals with photonic band gap proper-

ties different from those of conventional core-shell or

hollow particles.

Water-soluble polyelectrolyte nanocapsules as pH-

sensitive nanocontainers can be synthesized by vesicular

or emulsion polymerization via core-shell latexes.[111]

These particles show a reversible pH and ionic strength-

dependent swelling transition causing a considerable

increase (decrease) of their radius. During this transition,

gated pores are opened (closed) in the spherical polymer

shells, which enable free molecular exchange between the

interior of the hollow sphere and the bulk medium. This

pH-switchable control of the permeability of the poly-

electrolyte envelopes can be used to trigger the release

of encapsulated materials from their central cavity.

Inorganic Hollow Capsules and Nanobottles

Previous studies have provided successful procedures for

the preparation of composite particles consisting of in-

organic and organic cores covered with shells of other

inorganic materials by controlled surface precipitation

processes.[85,112–114] Such composite particles may be

useful in many applications because the properties (mag-

netic, optical, electric, adsorptive, etc.) of these particles

can be altered by appropriate coatings. Other studies have

also shown that these procedures can be used for the

preparation of polymer particles covered with yttrium,

zirconium, iron, and titanium compounds by controlled

surface precipitation processes, which makes it possible

to extend the use of these colloids to different areas of

high technology.

Owing to their lower density, large specific surface

area, and optical properties, hollow particles have been of

interest as fillers, coatings, catalysts, capsule agents, etc.

In a novel approach, it was shown that hollow inorganic

colloidal spheres of narrow size distribution could be

obtained by thermal decomposition of the polymer core of

PS particles coated with yttrium, zirconium, iron, and

titanium compounds. Kawahashi and Shiho described

the application of these processes to other systems. Thus,

under certain conditions, copper compounds can be de-

posited uniformly on PS latexes by precipitation using

Fig. 6 (A,B) Backscattering SEM and (C,D) TEM images of Au–SiO2–PBzMA particles before (A,C) and after (B,D) HF etching. The

polymerization time was 4 hr, and the polymer shell was �22 nm thick. (E,F) TEM images of Au–Air–PBzMA particles synthesized

using different polymerization times: (E) 3 hr and (F) 6 hr. The polymer shells were �2 and �32 nm in thickness, respectively. (From

Ref. [110].)


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solutions of the corresponding salts in the presence of

urea. Hollow metallic copper and copper oxide particles

of a narrow size distribution can be obtained by calci-

nation of particles coated in this manner at elevated tem-

peratures in nitrogen and air, respectively.[102]

Calcination of sulfate-stabilized PS latexes coated with

nanoparticle/polymer multilayers results in the production

of hollow silica spheres.[11,84] The calcination process

removes the organic matter (the colloidal core and bridg-

ing polymer) during heating to 450�C, as confirmed by

thermogravimetric analysis.

Hollow spheres of zeolite b and silicalite-1 with

different sizes were fabricated efficiently and convenient-

ly through LbL self-assembly of nanozeolite–polymer

multilayers on PS latex, coupled with the removal of the

core by calcination.[103] The pH and ionic strength of the

colloidal solution, crystal size of nanozeolites, and size of

PS latex templates are factors affecting the fabrication of

hollow zeolite spheres. Hollow spheres of other zeolites

such as ZSM-5 and TS-1 have also been successfully

fabricated in the same manner. Currently, the application

of these novel materials in catalysis, separation and

delivery systems is in progress in our laboratory.

Fowler et al. prepared hollow silica microspheres in

high yields by a one-step facile synthesis under ambient

conditions.[34] By controlling the rate of TEOS hydrolysis

specifically at the droplet/water interface, intact micro-

spheres with uniform wall thickness and thermal stability

can be routinely synthesized. The procedure can be

readily extended to the synthesis of organo-functionalized

silica shells, microspheres with encapsulated organic

pigment, and hollow silica capsules with submicrometer

dimensions. Such materials could have a wide range of

uses in diverse materials applications. And they reported

the facile synthesis of thermally stable hollow spherical

shells with ordered mesoporous walls, approximately 20

nm or less in thickness. The structures were synthesized at

room temperature by hydrolysis and condensation of

TEOS in an aqueous solution of cetyltrimethyl ammonium

bromide (CTABr), which was subjected to rapid quench-

ing by dilution followed by pH neutralization after an

induction period.[35]

Novel fine polymer particles containing ultrafine Pd

particles dispersed on the surface of core-shell [core,

poly(styrene-co-acrylic acid); shell, PrPO4]-type micro-

spheres were prepared by the emulsifier-free emulsion

polymerization of styrene with acrylic acid followed by

the addition of PdCl2 and a mixture of Pr(NO3)3 and

NaH2PO2. Pyrolysis of the resulting polymer particles at

900�C provides organic polymer-free hollow capsules

composed of Pd metal and PrPO4.[115]

Most work in this area has been focused on the

development of synthetic methodologies. Very little

attention has been directed toward the functionalization

of the interiors of these hollow particles. In addition, there

are only limited sets of reports that address the diffusion

of chemical reagents across the shells of hollow particles.

Yin et al. described a method based on template-directed

synthesis for generating ceramic hollow spheres whose

interior surfaces could be functionalized with the pre-

specified, nanoscopic objects.[96] The templates involved

in this process were crystalline lattices of monodispersed

polymer beads whose surfaces had been derivatized with

functional objects such as nanoparticles, quantum dots, or

other nanoscale objects. These nanoscopic objects were

left behind on the interior surfaces of the hollow spheres

when the templates were selectively removed through

etching or calcination (Fig. 7).

On the other hand, nanosized hollow inorganic spheres

with a hole in the wall (denoted as nanobottle) had been

successfully prepared from the assembly of functional

polymer nanosphere with tetraethoxysilane or tetrabutyl

titanate, coupled with the removal of the cores by

programmed calcination. Cross-linked polymer nano-

Fig. 7 Schematic outline of the experimental procedure. The

polymer template could be either dissolved with a solvent or

burnt out through calcination at elevated temperatures. (From

Ref. [96].)


ORDER REPRINTS

spheres with quaternary ammonium species on the surface

were synthesized using an emulsifier-free emulsion

copolymerization. The polymerization and purification

were carried out according to a published procedure,[116]

and cross-linked polymer nanospheres with a uniform

size of about 45 nm were obtained. As-synthesized silica-

coated polymer nanospheres were hydrothermally pre-

pared from chemical assembly of TEOS with the

functional polymer nanospheres.

After calcination at 550�C, the polymer template was

removed and hollow silica spheres were obtained (named

as silica nanobottles). Fig. 8 shows the TEM images of

functional polymer nanospheres from emulsion polymer-

ization process, as-synthesized silica microspheres, and

calcined hollow silica samples. After the self-assembly of

the silica gel with the functional polymer nanospheres, the

as-synthesized silica microspheres also show a very

uniform size at 52–55 nm (B), which are nearly 10 nm

thicker than the polymer nanospheres.[117–120] Calcina-

tion of the as-synthesized silica spheres results in the

complete removal of the polymer nanospheres, forming

hollow nanospheres with the size of 50–53 nm (C). As

shown in Fig. 8D, a hole with the size of 5–8 nm can be

seen on the surface of some hollow silica microspheres.

These results may suggest that the holes on the silica

hollow microspheres are formed in the following steps:

Calcination at 550�C leads to decomposition of polymer

nanospheres to smaller gas molecules, which have high

pressure in the closed hollow microspheres. Then the

gaseous molecules with high pressure break through

the shells of the hollow microspheres, and the hole in

the silica hollow microsphere is formed (scheme as Fig. 9).

Therefore these silica hollow microspheres with the hole

are referred to as silica nanobottles.[121]

The AFM observation of functional polymer nano-

spheres, as-synthesized silica microspheres, and calcined

silica samples were carried out. Similar to the TEM

images, the functional polymer nanospheres have a uni-

form size of 52–56 nm and as-synthesized silica micro-

spheres show a bigger size of 58–62 nm (not shown here).

In addition, it can be seen clearly in Fig. 10 that the

surface of every shell contains one hole of 9–12 nm in

Fig. 8 TEM images of (A) polymer spheres, (B) silica spheres before calcination, (C) hollow silica spheres after calcinations, and (D)

magnification of silica nanobottles. (From Ref. [121].)

Fig. 9 The procedure for preparation of silica nanobottles. (From Ref. [121].)


ORDER REPRINTS

diameter. Obviously, the sample size characterized by

AFM is slightly larger than that by TEM technique,

which could be explained by assuming that the probe

does not follow the microsphere’s surface precisely be-

cause of the blunt scanning tip. Interestingly, the AFM

images also show that there are holes on the hollow

microspheres and each hollow microsphere possesses

only one hole, which is in good agreement with the

images observed by TEM. These results further con-

firmed that the sample is a kind of silica nanobottles.

The nitrogen adsorption–desorption isotherms of silica

nanobottles and uncalcined silica-coated polymer micro-

spheres are well measured. The comparisons of adsorp-

tion results suggest that the calcined sample is a kind of

opening hollow nanosphere (nanobottles). The relatively

larger pore volume of silica nanobottles may be poten-

tially useful for the encapsulation of functional com-

pounds in the silica nanobottles.

Composite Capsules

Hollow inorganic–organic composite spheres can be

obtained by selection of a solvent that decomposes the

templated core but leaves the polymer bridging the

nanoparticles in the shell. The choice of solvent depends

on the type of core employed; for example, acidic or

dimethyl sulfoxide solutions cause the removal of MF

polymer latex core templates, tetrahydrofuran the removal

of some PS cores, and highly oxidizing solutions decom-

pose proteinaceous cores.

Similar to the pure polymer shells, the nanoparticle/

polymer multiplayer shell assembled onto MF particles

obtained upon decomposition of the MF core by acid

assumes a rather flat confirmation on the substrate when

dried.[84] Confocal microscopy images of the hollow

composite microspheres again show that the shells often

maintain their spherical shape in solution. Interestingly,

permeating the nanoparticle/polymer shell still readily

expels the oligomers produced as a result of decomposing

the MF particles. Higher magnification TEM reveals that

the shell is composed of nanoparticles embedded in the

polymer matrix.

Nanoparticle/polymer-coated biocolloids (gluteralde-

hyde-fixed echinocytes) can also be utilized for the

production of composite hollow structures. The template

has a jagged and highly structured surface. After the

removal of the core by exposure to deproteinizer, hollow

composite silica/polymer capsules are obtained. Unlike

the polymer or nanoparticle/polymer shells produced by

the removal of MF-templated cores by acid solutions,

these hollow structures mimic the original shape, includ-

ing the secondary structure (spikes) of the templates, and

do not significantly spread-out on the surface when dried.

This is most probably due to the gelation of the silica

particles as a result of the decomposing solution. SEM

experiments confirmed that these structures were hollow.

ENCAPSULATION OF RARE EARTHCOMPLEX IN NANOBOTTLES

The abovementioned silica bottles are nanosized materials

and there is a hole on the surface of it, which may be

useful for further encapsulations. Rare earth (RE) complex

Eu(TTA)3(TPPO)2 (TTA: 1-(2-thenoyl)-3,3,3-trifluorace-

tate, TPPO: triphenyl phosphineoxide) was selected as a

Fig. 10 AFM height and amplitude images of silica nanobottles. (From Ref. [121].) (View this art in color at www.dekker.com).


ORDER REPRINTS

guest molecule. After the modification of silica nanobot-

tles with APTES (NH2–(CH2)3 Si(OC2H5)3),[122] the RE

complex was mixed with the silica nanobottles in

chloroform, followed by filtering and washing with

chloroform until the filter liquors gave no luminescence

under UV radiation.

After RE complex encapsulation, the SEM images of

silica nanobottles give most like morphology as before

encapsulation, indicating that the silica nanobottles still

remained after the encapsulation of RE complex. Further-

more, the encapsulation of Eu(TTA)3(TPPO)2 in nano-

bottles was characterized by energy dispersive X-ray

analysis (EDX). The results indicate that RE complex still

exists in silica nanobottle samples after careful washing.

Fig. 11 shows UV–visible absorption spectra of silica

nanobottles, RE complex Eu(TTA)3 (TPPO)2 encapsulat-

ed in silica nanobottles, and Eu(TTA)3(TPPO)2 adsorbed

on the outer surface of as-synthesized silica-coated

polymer microspheres (after washing with chloroform

many times) as well as RE complex Eu(TTA)3(TPPO)2

itself. The spectrum of silica nanobottles does not show

any absorption in the region of 200–800 nm (a); the

sample spectrum of RE complex encapsulated in silica

nanobottles shows clear peaks at near 255, 292, 330, and

348 nm (b); the spectrum of RE complex absorbed on

the outer surface of silica nanobottles only shows very

weak absorptions after careful washing, and is similar

to that of RE complex (c); the spectrum of Eu(TTA)3-

(TPPO)2 shows typical peaks at 241, 325, and 436 nm

(d). From the contrast of the above spectra, it can be

confirmed that RE complex adsorbed both the inner and

outer surface of the hollow microspheres. In addition,

the peaks of RE complex are shifted to a lower wave-

length in silica nanobottles. This phenomenon is possibly

assigned to the encapsulation of RE complex in silica

nanobottles, which is consistent with those of RE com-

plex entrapped in mesopores of MCM-41 and in micro-

cages of zeolite Y.[123,124]

CONCLUSION

Research efforts over the last decade have led to a host

of new strategies being developed for the modification

of particle surfaces. Earlier methods have primarily

focused on achieving single-component coatings on par-

ticles through surface chemical modification and conven-

tional polymerization and precipitation reactions. More

recent approaches, particularly those based on solution

self-assembly, have been shown to be highly effective

in producing multicomposite, nanostructured coatings.

These methods permit remarkable control over the coating

uniformity and thickness, and thus the assembled compo-

nents and experimental conditions can be readily manip-

ulated to prevent the aggregation of the coated colloids.

Although the method of choice highly depends on the

coating required and the end use of the composite

particles, the synthetic approaches outlined provide a

high level of flexibility, thus allowing the design,

structure, and properties of the resultant particles to be

varied tremendously. In addition, combinations of these

methods are likely to allow new functional and composite

layers to be manufactured on the surfaces of particles.

Accordingly, nanoengineered core-shell particles are

expected to find new and exciting applications in the

chemistry, bioscience, and materials science fields. Core-

shell particles have already been assembled into colloidal

crystals for photonic applications, used as heterogeneous

catalysts and multienzyme biocatalysts, and in surface

enhanced Raman scattering applications.

An interesting extension of the composite particles is

the production of hollow capsules by the removal of the

core particle; these hollow materials may find applications

in drug delivery, or as reactor or specific recognition sys-

tems. The successful production of such hollow capsules

opens many new and exciting avenues in the areas of

chemistry, biotechnology, and materials science. They are

potentially suitable for a variety of applications including

the loading of drugs, as confined environment reactor

systems, and for targeting by utilization of the surface

functionalities on the capsule walls to attach specific

receptors. Controlling the thickness and composition of

Fig. 11 UV–Visible absorption spectra for (a) the modified

silica nanobottles itself, (b) the assembly of Eu(TTA)3(TPPO)2

in modified silica nanobottles, (c) Eu(TTA)3(TPPO)2 absorbed

on the outer surface of as-synthesized silica-coated polymer

spheres (after washing with chloroform many times), and (d) RE

complex Eu(TTA)3(TPPO)2. (From Ref. [121].) (View this art in

color at www.dekker.com).


ORDER REPRINTS

the capsule walls should allow selective and switchable

permeation for the encapsulation and release of various

substances. The use of cross-linkable, pH- or temperature-

sensitive polymers as capsule wall constituents are attrac-

tive candidates for controlling and varying the permeabil-

ity, while the incorporation of specific reactive groups

inside the capsule walls would allow specific chemistry to

be carried out in these systems. Coupling of biospecies to

the surfaces of the capsules through functional groups

would provide biofunctionalized capsules.

Some experiments demonstrate that it is possible to

coat the outer and inner surfaces of hollow polymer

capsules with phospholipid bilayers. The polymer cap-

sules are permeable to small low molecular weight dyes,

but not to polyelectrolytes with molecular weights greater

than 4000 or molecules larger than 5–10 nm in diameter.

The phospholipid coating reduces the permeability to

small organic dyes. The precipitation of small organic dye

molecules inside polymer capsules has been achieved, as

has the solubilization of various organic solvents. Func-

tional biomolecules have also been encapsulated at a very

high loading capacity in polymer capsules; these systems

are expected to be used in enzyme catalysis applications.

The coating technique is currently being extended to

inorganic templates to create novel hollow capsules of

nanometer size and to emulsion-based systems. A lumi-

nescent RE complex is successfully encapsulated in silica

nanobottles, showing this material has potential nano-

technology applications.

ACKNOWLEDGMENTS

This work was supported by NSFC (29925412) and the

Major State Basic Research Development Program

(G2000078102).

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