smart inorganic/organic hybrid microgels: synthesis and characterisation

FEATURE ARTICLE www.rsc.org/materials | Journal of Materials Chemistry

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Smart inorganic/organic hybrid microgels: Synthesis and characterisation

Matthias Karg and Thomas Hellweg*

Received 12th November 2008, Accepted 1st July 2009

First published as an Advance Article on the web 4th August 2009

DOI: 10.1039/b820292n

Responsive hybrid colloids containing both organic and inorganic components have been the subject of

many investigations in recent years. These new materials combine the stimuli-responsiveness of some

polymer-based colloids with the unique properties of inorganic nanoparticles. This article will

review the different possible variations of such hybrid colloids. Moreover, synthetic approaches,

methods of characterisation, and a few applications will be discussed. Due to the rather high number of

recent publications dealing with hybrid materials based on a variety of polymers, we will mainly

focus on responsive hybrid microgels made of poly(N-isopropyl-acrylamide) or poly(N-

vinylcaprolactam) in aqueous media.

1. Introduction

It is well known that macroscopic polymer gels respond to

changes of external parameters such as temperature or solution

pH1–8 by swelling or shrinking (commonly called volume phase

transition). This behaviour is related to the well-known coil-

to-globule transition of linear polymers having a lower critical

solution temperature (LCST).9–12

In recent years research has focused on so-called microgels.

Microgels are colloidal particles having a gel structure internally.

Hence, they combine properties of typical colloids with the

responsiveness of gels. In analogy to their macroscopic homo-

logues, microgels can also respond to a variety of external stimuli

such as temperature,13–16 pH,17–25 ionic strength26,27 and electric

field.28–30 Based on their responsive swelling properties, microgels

Universit€at Bayreuth, Physikalische Chemie I, Universit€atsstrasse 30,95440 Bayreuth, Germany. E-mail: [email protected]

Matthias Karg

Matthias Karg received his

diploma in chemistry from

Technical University of Berlin.

He started his doctoral work on

the synthesis and characteriza-

tion of smart organic/inorganic

hybrid microgels at the Tech-

nical University Berlin in 2006

under the supervision of Prof.

T. Hellweg. After successfully

defending his thesis, he started

as a postdoctoral fellow at the

University of Melbourne in

2009.

8714 | J. Mater. Chem., 2009, 19, 8714–8727

are often referred to as ‘‘smart’’ materials and due to this fasci-

nating behaviour they have been the topic of countless investi-

gations in recent decades.31–35 Compared to their macroscopic

homologues, microgels are probably even more interesting with

respect to applications like separation media, drug delivery,36–40

sensor design,41–45 smart emulsifiers,46,47 and for the preparation

of responsive colloidosomes.48,49 This is due to the fact that the

response time of the gel network is proportional to the dimen-

sions of the network.87,88 Hence, microgels can change their size

much faster than a chemically similar macroscopic gel.

For a better understanding of the swollen and collapsed states

of microgels, different length-scales are of importance, and hence

a variety of experimental methods have been employed to study

these systems. Experimentally the volume phase transition and

the related change in microgel size is often followed by dynamic

light scattering (DLS), which provides the hydrodynamic radius

Rh as a function of external parameters like temperature.50,51,34

Recently, the volume phase transition of microgels was studied in

Thomas Hellweg

Thomas Hellweg received his

diploma in chemistry from the

University of Bielefeld in 1991

and finished his PhD in 1995 at

the same university in the group

of T. Dorfm€uller. From 1996 to

1998 he was a postdoctoral

fellow at the Centre de Recher-

che Paul Pascal in Bordeaux in

the groups of D. Langevin and

D. Roux. In 1998 he started his

habilitation at the TU Chemnitz

in the physics department (in the

Materials and liquids group;

J.-B. Suck). In 2001 he moved

to the Technical University of Berlin, where he finished his habil-

itation in 2005 in the Iwan-N.-Stranski-Lab for Physical and

Theoretical Chemistry. Since March 2007 he has been professor

for physical and colloid chemistry at the University of Bayreuth.

This journal is ª The Royal Society of Chemistry 2009

http://dx.doi.org/10.1039/b820292n

http://pubs.rsc.org/en/journals/journal/JM

http://pubs.rsc.org/en/journals/journal/JM?issueid=JM019046

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detail focusing on the local network structure using small-angle

neutron scattering (SANS).52 SANS offers much higher values of

the momentum transfer compared to light-scattering experi-

ments and therefore allows one to explore the local structure of

the polymer gel inside the particles. The morphological behavi-

our of both extremes – a completely swollen and a completely

collapsed microgel – was extensively studied.51, 53–57

In addition to scattering techniques, the shrinking or swelling

of microgels can be visualised by cryogenic transmission electron

microscopy (cryo-TEM)58 and atomic force microscopy (AFM)

using a liquid cell.43–45,59

An important approach to create new materials is the prepa-

ration of hybrid systems composed of inorganic nanoparticles

and organic components. This is also an important area of

microgel research, which was attested by several reviews.60–62,32,33

The advantage of a hybrid system is that it can combine prop-

erties of the different components. For example, a core-shell

microgel with a gold core and a poly(N-isopropylacrylamide)

(poly(NIPAM) or PNIPAM) shell benefits from the optical

properties of the nanosized metal and the thermoresponsive

character of the polymer network.63,64 Hence, depending on the

behaviour of the single components, hybrids can exhibit

a combination of ‘‘inorganic’’ and ‘‘organic’’ characteristics.

Usually inorganic nanoparticles are chosen as a non-polymer

component for the design of new hybrid microgels if properties

such as optical, magnetic or catalytic features are going to be

introduced into the microgel.

For the composite to show a well-pronounced optical behavi-

our, noble metal nanoparticles seem to be promising since they

can provide localised plasmon resonances in the visible and near-

IR regions. These resonance frequencies can be influenced in

a rather strong way by parameters such as the particle size and

shape.65–68 A well-studied system is gold nanoparticles, where

structures like spheres,69,70 rods,71,68 platelets,72,73 poly-

hedrons74,75 or even multipods76,77 have already been reported.

Catalytic activity can be achieved if nanoparticles based on

silver, palladium or again gold are used, while magnetic prop-

erties can result if nanoparticles made of magnetite, nickel or

cobalt are employed.

This article aims to review the recent progress in synthesis,

characterisation and application of water-borne composite

microgels mainly based on N-isopropylacrylamide (NIPAM) or

N-vinylcaprolactam (NVCL) combined with inorganic nano-

particles. We will discuss different hybrid architectures as well as

different inorganic components.

The article is organised as follows. In Section 2 more details

about microgels are given. Subsection 3.1 introduces the classes

of hybrids, which will be discussed in the present contribution.

The Subsections 3.2, 3.3 and 3.4 then review the present state of

research related to the three main types of microgel nanoparticle

hybrid materials. Section 4 gives a summary, followed by an

outlook in Section 5.

Fig. 1 Sketch depicting the collapse of a swollen microgel, which is

sensitive to external stimuli such as temperature, light, electric field

strength, ionic strength, or changes of the solvent pH. The black lines

represent polymerised monomer chains, while the grey dots are symbols

for the crosslinks.

2. Thermoresponsive microgels

A gel is a three-dimensional polymer network, which is swollen by

a suitable solvent. The degree of solvent-uptake in a good solvent

is typically high, leading to a solvent content of more than 85%. A

gel’s three-dimensional structure and stability is determined by


physical or chemical crosslinks of the network. Physical cross-

linking can include hydrogen bonds, ionic interactions, and

hydrophobic forces or simply mechanical entanglements.

However, the systems discussed within this review are chemically

crosslinked using crosslinker molecules such as N,N-methylene-

bisacrylamide (BIS)78 or e.g. methacrylate cross-linkers.56

The physical properties combine the nature of both the poly-

mer and the solvent. Therefore, solid-like as well as liquid-like

contributions can be found. The cross-links of the network

inhibit the polymer chains from exploring conformational phase

space, and therefore gels are non-ergodic systems.79 Conse-

quently, on longer length scales the dynamical correlations do

not decay to zero.

The overall dimensions of gels are determined by the prepa-

ration procedure, and a distinction can be made between mac-

rogels80 and microgels.78 A macrogel generally takes on the size

of its reaction container. Gels in the sub-micron size-range are

obtained by polymerising emulsion droplets and are referred to

as microgels.

A frequently used and studied system is synthesised from the

monomer N-isopropylacrylamide (NIPAM), the respective gel

having a relatively low critical solution temperature (LCST) of

around 32–33 �C. This LCST is responsible for the thermores-

ponsive character of poly(NIPAM) particles. PNIPAM gels can

be swollen by water, which is a good solvent below the LCST

but becomes a poor solvent above the LCST.1,5,6,81–86 Hence,

these systems can convert a temperature change into mechanical

work. However, thinking of applications, the response of

macroscopic gels is rather slow and is proportional to the gel

dimensions.87,88 Therefore, if a fast response is required e.g. for

applications in sensors, microgels are a better choice compared

to their macroscopic homologues since they show comparably

fast swelling and deswelling kinetic. The collapse of these

particles is schematically shown in Fig. 1. In addition, microgels

also have typical colloidal properties, such as the ability to form

colloidal crystals.89–96 However, in contrast to classical hard-

sphere colloids, changes in solvent quality (e.g. induced by

a temperature increase beyond the LCST) can force ‘‘melting’’

of these crystals.90,97

A straightforward way to influence the swelling behaviour of

thermoresponsive microgels is copolymerisation. If organic acids

like acrylic acid,22–25 maleic acid,98 vinylacetic acid19,44 or

allylacetic acid27 are used as comonomers, not only can the tran-

sition temperature be shifted, but also a fairly strong sensitivity

to pH and ionic strength can be obtained.

J. Mater. Chem., 2009, 19, 8714–8727 | 8715


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3. Microgel/nanoparticle hybrids

3.1 Classification of hybrid microgels

For the preparation of microgels with catalytic, magnetic or

interesting optical properties, the combination with inorganic

nanoparticles is necessary. Depending on the required behaviour

of the hybrid not only the choice of the components, but in the

same way the structure of the final composite, is important.

Different structures as well as different hybrid compositions will

be presented in the following sections.

In this sub-section we provide a brief overview of the different

methods of composite preparation and characterisation. The

different hybrid systems will be classified into three groups,

which are shown schematically in Fig. 2.

The three classes of microgels are: (1) Core-shell microgels

with nanoparticles as core; (2) Microgel particles filled with

nanoparticles; (3) Microgels covered with nanoparticles. These

systems not only differ in their structure and the way they are

prepared, but also with regard to possible applications. In

addition, the methods used for their characterisation have to be

adapted with respect to the microgel class to be studied.

The majority of composite microgels investigated in recent

years can be described by one of these three classes or by

combinations of them. Therefore, the present review will focus

on these three classes. However, there are of course additional

systems that do not fit into the above classification. These will be

mentioned here for the sake of completeness, but not discussed in

Fig. 2 Sketches of three classes of recently studied thermosensitive

hybrid microgels that are treated in the present review. Top: Hybrid

microgel with a core-shell structure. Middle: Microgel filled with nano-

particles. Bottom: Microgel particle covered with nanoparticles.

8716 | J. Mater. Chem., 2009, 19, 8714–8727

detail. One alternative type of organic/inorganic hybrid microgel

is based on the copolymerisation of NIPAM with 3-(trimethoxy-

silyl)propylmethacrylate.99 The incorporation of metal-organic

compounds into the network can be seen as a fourth class of

organic/inorganic hybrid microgels. In addition, large microgels

(�20–30 mm) containing nanoparticles were synthesised recently

using microfluidic devices.100 This is a very promising approach.

However, due to the completely different sizes of these materials

they are not included in the following discussion.

3.2 Core-shell microgels

Core-shell hybrid microgels have well-defined structures in which

nanoparticle cores are surrounded by a polymer shell.63,101–103

Variations in the core material, size and shape influence the

composite properties. If noble-metal nanoparticles with plasmon

resonances are used as cores, then the nanoparticle size influences

the position of the plasmon band.67 In this way the optical

properties of the hybrid can be tuned.

An example of a simple and easy-to-prepare, purely organic,

core-shell system can be synthesised from NIPAM and styrene

either in a two-step117,107 or a one-step mechanism.104–106 In the

two-step procedure polystyrene particles are prepared by simple

emulsion polymerisation and, after cleaning, are used as seeds in

a polymerisation of NIPAM.33,107 Fig. 3 shows cryo-TEM images

of particles prepared by this method.

Following this two-step routine, the poly(NIPAM) grows

around the pre-prepared polystyrene particles and forms a shell.

The cryo-TEM samples were prepared by holding the microgel

dispersion at constant temperatures of 23 �C (swollen) and 45 �C

(collapsed) before vitrification. Through this treatment the

particles are frozen in their respective state of swelling. As can be

seen in the cryo-TEM images these particles have a thermores-

ponsive character due to the poly-NIPAM shell. The dashed

circles in Fig. 3 indicate the hydrodynamic dimensions of the

core-shell particles in the swollen (a) and collapsed state (b).

Fig. 3 Cryo-TEM images of a poly(styrene-co-NIPAM) core-shell

microgel. a) Sample held at 23 �C before the vitrification. b) Sample kept

at 45 �C before vitrification. The dashed circles around the corona display

the hydrodynamic radii at the respective temperatures determined by

DLS. (Please note that the scale bar in the left image represents 200 nm

and the one in the right image only 50 nm). Reprinted with permission

from J. J. Crassous, M. Ballauff, M. Drechsler, J. Schmidt and Y. Tal-

mon, Imaging the volume transition in thermosensitive core-shell parti-

cles by cryo-transmission electron microscopy, Langmuir, 2006, 22(6),

2403–2406.58 Copyright 2006 American Chemical Society.



Fig. 5 Sketch showing the synthesis of core-shell microgels by an

emulsion-polymerisation. Functionalised nanoparticles are added to

a monomer/crosslinker mixture, and the polymerisation is started by

a radical initiator such as potassium or ammonium peroxodisulfate.

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The one-step procedure, the direct polymerisation of a mixture

of NIPAM and styrene, leads to the formation of a core-shell

microgel with a rather compressed poly(NIPAM) shell, since the

styrene polymerises inside the first-formed precursor poly-

(NIPAM) microgel.104–106 Therefore, the volume change of the

latter particles is much smaller than in the case of the particles

from the two-step synthesis when the same volume fractions of

NIPAM and styrene are used.

Both methods yield a well-defined core-shell structure, and the

solid core enhances the formation of colloidal crystals. Fig. 4

shows SEM images of a crystal made of poly(styrene-

co-NIPAM) microgel particles. The preparation of such crystals

can be done in different ways. However, the easiest approach is

simple drying of a concentrated dispersion on a surface.106

This crystallisation behaviour motivates efforts to synthesise

well-defined core-shell microgels with cores of gold, silver or

other metal nanoparticles, showing plasmon resonances. These

hybrids may be used to prepare photonic crystals with

a temperature-tunable band gap.

Two approaches have been recently described to synthesise

core-shell microgels with inorganic core materials. One method is

the controlled polymerisation (ATRP, RAFT) from surface-

functionalised nanoparticles such as magnetite62 or silica.108 In

this case the polymerisation starts directly from the surface of the

nanoparticles, which acts as a kind of macroinitiator. This

approach usually results in a brush-like composite.

Another approach to incorporate nanoparticles into microgels

is to conduct an emulsion-polymerisation in the presence of

surface-functionalised nanoparticles. If the nanoparticles do not

exceed a certain size, and if the surface functionalisation provides

free double bonds, this process can lead to well-defined hybrids

with a core-shell structure. Of course, the hydrophobicity of the

nanoparticles plays also an important role. An efficient surface

functionalisation agent for silica surfaces is methacryloxy-

propyltrimethoxysilane (MPS).109 Fig. 5 shows schematically

how the synthesis of the core-shell microgels is performed.

Both methods involve two synthesis steps, since the nano-

particles have to be prepared, functionalised, and cleaned prior

to the hybrid formation.

The first hybrid microgels with inorganic cores were prepared

using silica nanoparticles incorporated within a PNIPAM

Fig. 4 SEM images at different magnifications of a colloidal crystal

made from a poly(NIPAM) microgel with polystyrene cores. Cracks

caused by the loss of water can be observed in the left image. The crystal

was prepared by simple drying of a microgel dispersion on a silicon wafer

at room temperature under ambient atmosphere.


microgel.63 Silica particles are easy to prepare in different sizes by

procedures such as the well-known St€ober method,110 and their

surface can be functionalised with agents such as MPS.109,63

Fig. 6 shows the results of temperature-dependent DLS

measurements made with SiO2@PNIPAM core-shell microgels.

The behaviour is characterised in terms of the swelling curve and

the swelling ratio a:

a ¼ Vcollapsed

Vswollen

¼�

Rh

R0

�3

(1)

Vcollapsed and Vswollen are the volumes of the microgel particles

in the collapsed and swollen state. For spherical particles these

volumes can be substituted by the radii of the particles to the

third power in the appropriate state of swelling. In eqn (1) Rh is

the hydrodynamic radius at a temperature T and R0 is the

respective radius in the fully swollen state. Refs. 34 and 111–116

provide more details regarding the DLS methodology.

Fig. 6 (left) shows typical results from DLS for particles having

a 60 nm silica core. The observed swelling/de-swelling behavior is

typical for poly-NIPAM microgels. No effect of the core on the

swelling could be found. In addition, Fig. 6 also shows an AFM

image of the same particles recorded in tapping mode. The AFM

image shows clearly a difference in brightness in the center of the

hybrid particles, indicating the hard inorganic core material.

Similar images for core-shell hybrids were observed by electron

microscopy either SEM or TEM.63,64

Knowledge of the swelling behavior and the overall size and

shape of the microgels is important. However, the swelling

behavior is of course strongly related to the network

morphology. Small-angle neutron scattering (SANS) is an

appropriate method to study details of the local, internal struc-

ture of the microgel particles. This is related to the fact that for

a typical SANS experiment 1/q lies in the range from 0.5 nm to

20 nm. Therefore, SANS measurements were done for

a SiO2@PNIPAM core-shell microgel and the results are shown

in Fig. 7.

Due to the small wavelength available in a neutron scattering

experiment, SANS focusses on a much smaller length scale, and

hence is well-suited to study the correlation length x52,34 of the

polymer network. The fit for the SANS profile of the swollen

J. Mater. Chem., 2009, 19, 8714–8727 | 8717


Fig. 6 Core-shell microgel with silica cores having a diameter of �60

nm. Left: Hydrodynamic radii Rh and inverse swelling ratios a�1 as

a result of temperature-dependent DLS measurements. Right: AFM

(height profile) image of the respective particles recorded using tapping-

mode operation against air. The particles were deposited on a silicon

surface. Taken from M. Karg, S. Wellert, I. Pastoriza-Santos, A. Lapp,

L. M. Liz-Marz�an and T. Hellweg, Poly(N-isopropylacrylamide)

microgels with silica nanoparticle cores: The volume phase transition/

collapse of the polymer shell as seen by small angle neutron scattering and

dynamic light scattering, Phys. Chem. Chem. Phys., 2008, 10,

6708–6716.52 Reproduced by permission of the PCCP Owner Societies.

Fig. 7 SANS profiles of a silica-poly-NIPAM core-shell microgel (see

Fig. 6) in the swollen state (at 25.0 �C), in the collapsed state (at 45.0 �C)

and close to the temperature of the volume phase transition (at 33.5 �C).

The solid lines are fits which are discussed in detail in ref. 52. The figure

was obtained by re-plotting the data from ref. 52.

Fig. 8 TEM (left) and AFM (right) images of a silica-poly-NIPAM

core-shell microgel. The core diameter is �110 nm. The AFM image

(height profile) was recorded against air using the tapping-mode.119

Fig. 9 Left: Core-shell microgel with a poly-NIPAM shell and cores of

spherical, silica-coated gold nanoparticles. Mainly, one core per microgel

is found. From M. Karg, I. Pastoriza-Santos, L. M. Liz-Marz�an and

T. Hellweg, A versatile approach for the preparation of thermosensitive

PNIPAM core-shell microgels with nanoparticle cores, Chem. Phys.

Chem., 2006, 7, 2298–2301.63 Copyright Wiley-VCH Verlag GmbH & Co.

KGaA. Reproduced with permission. Right: Multi-core microgel. The

particles inside the NIPAM shell are silica-coated gold particles.

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microgel (at 25.0 �C) gives a correlation length of x ¼ 2–3 nm. In

the swollen state x represents the polymer mesh size52,34,51 and the

value found is typical for poly(NIPAM) particles prepared by

emulsion-polymerization. More detailed information regarding

SANS applied on microgels and the used fits can be found in refs.

51, 52, 54, 34, 117 and 118. One of the unique features of SANS is

the possibility to extinguish e.g. the signal from the PNIPAM

shell in the core-shell systems. This approach can be applied to all

kind of core-shell particles with e.g. polystyrene cores106 or also

to organic/inorganic hybrid systems with e.g. silica cores. Hence,

contrast variation can be used to prove the existence of a core-

shell structure. Moreover, in the case of silica cores an ‘‘in situ’’

determination of the core polydispersity was achieved by

contrast variation.52 However, the analysis of the core-shell

system by DLS and SANS does not indicate any notable influ-

ence of the nanoparticle core on the behaviour of the polymer

network.

8718 | J. Mater. Chem., 2009, 19, 8714–8727

Further experiments with the incorporation of silica nano-

particles have shown that even cores with a diameter greater than

100 nm can be used. TEM and AFM images of the respective

hybrid microgel are shown in Fig. 8.

The AFM images show clearly the solid and spherical silica

cores (bright) and the softer poly(NIPAM) shell (less bright

corona). To our knowledge, hybrids prepared through emulsion-

polymerization with similarly large cores have not yet been

presented in the literature. These materials can be used to

generate different surface structures. Hence, one can use these

hybrids to deposit all kinds of silica or silica-coated nanoparticles

in a controlled way.42,140–142 The thickness of polymer shell

controls the distance between the particles. If only the inorganic

part is necessary, the polymer can be removed after the deposi-

tion using plasma etching. The maximum core size that can be

incorporated still remains to be determined.

The presented silica cores do not endow the hybrid with special

properties, apart from a slightly higher refractive index.

However, these hybrids illustrate a versatile routine to incorpo-

rate different nanoparticles, since many materials can be covered

with silica.120 For example, silver nanoparticles,121,122 gold

nanospheres70,123,124 and nanorods71 have been successfully

coated with silica. Fig. 9 shows TEM images of hybrid microgels

with silica-coated gold nanoparticle cores.63

Microgels with single cores or particles with several cores can

be prepared by changing and controlling the number of core



Fig. 10 Left: TEM image of a poly-NIPAM microgel with cores of gold nanoparticles with a diameter of �67 nm. Right: UV-vis spectra at different

temperatures. Taken from R. Contreras-C�aceres, A. S�anchez-Iglesia, M. Karg, I. Pastoriza-Santos, J. P�erez-Juste, J. Pacifico, T. Hellweg, A. Fern�andez-

Barbero and L. M. Liz-Marz�an, Encapsulation and growth of gold nanoparticles in thermoresponsive microgels, Adv. Mater., 2008, 20, 1666–1670.64

Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

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particles that are added to the reaction mixture during the

synthesis.

Recently, a different approach to incorporate gold nano-

particles into poly-NIPAM microgels was presented, whereby

gold nanoparticles were covered with poly(styrene-divinylben-

zene) in the first step.64 Afterwards, polymerization of NIPAM

around the coated gold was performed. As can be seen in Fig. 10,

these core-shell particles with gold cores were obtained in high

yield with well-defined structures. The UV-vis spectra at various

temperatures show a redshift of the plasmon band of the gold

cores with decreasing size of the polymer shell (during the

temperature-induced collapse). This shift is due to a change in the

local refractive index close to the gold surface, which occurs

when the polymer shell collapses and becomes dense.

Another approach leading to small core-shell structures with

gold cores is based on the interaction of endgroup-modified

PNIPAM having NH2 endgroups. Terminal amino groups

coordinate to the surface of gold nanoparticles and a PNIPAM

shell can then be grown directly on the gold.125

These new gold-poly(NIPAM) core-shell microgels are

possible candidates for applications such as smart surface coat-

ings and optical sensors, because a change in the degree of

swelling of the PNIPAM shell leads to changes in the plasmon

band position. Moreover, core-shell structures can also be used

to produce hollow polymer capsules when the core is removed.125

This can be easily done in the case of gold cores using the

addition of KCN.

More information about the influence of the refractive index of

the particle environment on the plasmon bands of noble-metal

nanoparticles can be found elsewhere.70,124,126–128

Fig. 11 Cryo-TEM images of poly(styrene-co-NIPAM) core-shell

microgels loaded with silver. Taken from Y. Lu, Y. Mei, M. Drechsler

and M. Ballauff, Thermosensitive core-shell particles as carriers for Ag

nanoparticles: Modulating the catalytic activity by a phase transition in

networks, Angew. Chem., 2006, 118, 827–830.101 Copyright Wiley-VCH

Verlag GmbH & KGaA. Reproduced with permission.

3.3 Microgels filled with nanoparticles

In contrast to core-shell microgels, where the polymer shell is

typically grown around the nanoparticle core, microgels can also

be used as templates for the in situ synthesis of nanoparticles. The

latter process leads to microgels that can accommodate nano-

particles inside their network. These hybrid structures will be

called nanoparticle-filled microgels in this article.

Usually, the nucleation as well as the growth of the filler

nanoparticles takes place inside the microgel network. For

example, silver and gold nanoparticles can be easily synthesized

and located inside a microgel by reduction of Ag+ and Au3+ salts


with sodium borohydride using an aqueous microgel dispersion

as solvent medium. This approach usually leads to small

(diameter of the order of nanometers) metal nanoparticles that

are statistically distributed and well separated within single

microgel particles.

Ballauff et al. presented different hybrid microgels in which

a polymer network is loaded with different metal nano-

particles.101,33 Instead of using pure poly(NIPAM) microgels,

they used core-shell microgels with dense poly(styrene) cores and

soft poly(NIPAM) shells. Fig. 11 shows TEM images of such

poly(styrene-co-NIPAM) core-shell microgels, which are filled

with silver nanoparticles.

These electron microscopy images clearly show the core-shell

structure of the microgel template and also the distribution of the

nanoparticles in the soft poly-NIPAM shell. Due to the higher

electron density of the metal nanoparticles, good contrast with

respect to both polymer components is obtained. Moreover, the

images reveal the random and homogeneous distribution of

nanoparticles in the polymer shell.

Similar to the shown hybrids, palladium nanoparticles can be

incorporated into a microgel network.33,129 A cryo-TEM image

J. Mater. Chem., 2009, 19, 8714–8727 | 8719


Fig. 12 Left: Cryo-TEM image of poly(styrene-co-NIPAM) core-shell

microgels loaded with palladium nanoparticles. Right: Catalytic activity

of a palladium-loaded core-shell microgel. The spectra show the decrease

in absorbance of p-nitrophenol in the presence of the composite particles

as catalyst at different times t (in minutes). Reprinted with permission

from Y. Mei, Y. Lu, F. Polzer, M. Ballauff and M. Drechsler, Catalytic

activity of palladium nanoparticles encapsulated in spherical poly-

electrolyte brushes and core-shell microgels, Chem. Mater., 2007, 19,

1062–1069.129. Copyright 2007 American Chemical Society.

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of the palladium-loaded microgel particles is shown in Fig. 12.

The palladium nanoparticles are statistically distributed within

the poly(NIPAM) shell and have a size of �4 nm. The catalytic

properties of these hybrids, where the core-shell microgel acts as

a ‘‘smart’’ carrier system, were studied by UV-vis spectroscopy

following the reduction of p-nitrophenol in the presence of

sodium borohydride. Spectra as a function of time after mixing

the reactants with the catalyst dispersion clearly show a decrease

of the characteristic absorbance of p-nitrophenol at�400 nm (see

Fig. 12). ‘‘Smart’’ carrier means that the accessibility of the

catalytically active particles is controlled by the degree of

Fig. 13 TEM images of magnetite filled microgels with different magnetite co

The inset shows an energy-dispersive X-ray spectroscopy (EDX) iron-map

Temperature-, pH-, and magnetic-field-sensitive hybrid microgels, Small, 200

Reproduced with permission.

8720 | J. Mater. Chem., 2009, 19, 8714–8727

swelling of the PNIPAM shell. Hence, the catalytic activity of the

hybrid system can be tuned by the degree of swelling of the

network. Moreover, the palladium nanoparticles cannot be

removed from the micron-sized colloidal gel particles. Therefore,

the catalytically active hybrid can be easily recovered from the

reaction medium by filtration.

Pich et al. filled poly(N-vinylcaprolactam) copolymer micro-

gels with magnetite nanoparticles and obtained hybrids with

magnetic properties.130 TEM images of hybrids with different

degrees of loading are shown in Fig. 13.

Again homogeneously distributed nanoparticles can be

observed, while the spherical shape of the microgel template

remains. The magnetization curve for the microgel with the

highest magnetite content prepared in this study is given in

Fig. 14. Combining such a magnetite filled system with catalytic

nanoparticles would lead to a hybrid catalyst which can be easily

recovered from the reaction medium using a magnet.

Another example from the same group is the preparation of

hybrid microgels via incorporation of ZnO nanoparticles into

temperature-sensitive poly(N-vinylcaprolactam-co-acetoacetoxy-

ethylmethacrylate) microgels. These hybrids might be useful as

UV-shielding materials, as suggested by the authors.131

In comparison to the preparation of well defined core-shell

microgels with inorganic cores, filling microgel templates with

metal nanoparticles or metaloxides is much easier, since these

nanoparticles can be prepared in the presence of the microgel

particles by reduction of the respective salt, leading to the desired

hybrid. These composites can be prepared using a variety of

inorganic nanoparticles and polymer hosts. Furthermore, by

copolymerization the volume phase transition behavior of the

microgel template can be controlled, and by the choice of the

ntent. a), b) no magnetite; c), d) 8.41% magnetite; e), f) 15.35% magnetite.

ping image. From S. Bhattacharya, F. Eckert, V. Boyko and A. Pich,

7, 3(4), 650–657.130 Copyright Wiley-VCH Verlag GmbH & Co. KGaA.



Fig. 14 Magnetization curve of a microgel loaded with 15.35%

magnetite (TEM image shown in Fig. 13). Taken from S. Bhattacharya,

F. Eckert, V. Boyko and A. Pich, Temperature-, pH-, and magnetic-field-

sensitive hybrid microgels, Small, 2007, 3(4), 650–657.130 Copyright

Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

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nanoparticle, magnetic, catalytic or optical properties can be

introduced. Nevertheless, the distribution of the in situ-synthe-

sized nanoparticles is usually random and uncontrollable.

3.4 Microgels covered with nanoparticles

Another approach for the preparation of thermoresponsive

hybrid systems makes use of electrostatic interactions in order to

cover charged microgels with oppositely charged nanoparticles.

The surface coverage can then be controlled by the charge

density of the microgel acting as core material. Several investi-

gations have shown that the network charge of microgels can be

controlled by the amount of the used ionic radical initiator (e.g.

potassium peroxodisulfate or ammonium peroxodisulfate) and

much better by copolymerisation with charged comonomers like

organic acids. As already mentioned in Section 2, many poly-

(NIPAM)-based copolymers can be found in the literature using

various COOH-containing co-monomers.132–134,22–24,19,44,27 Fig. 15

Fig. 15 Left: Zeta-potentials of dispersions (pH 8, 25 �C) of poly-

NIPAM based copolymers with different amounts of the comonomer

allylacetic acid (AAA). The abscissa gives the absolute values of AAA

contents determined by titration with NaOH. Right: Swelling curves of

a pure poly-NIPAM microgel (C), a copolymer microgel of NIPAM and

2.99% vinylacetic acid (-) and a copolymer microgel of NIPAM and

2.45% allylacetic acid (:). The temperatures of the volume phase tran-

sitions are from left to right 32.5 �C, 41.0 �C and 46.0 �C (dashed vertical

lines). (Redrawn with data from ref. 27).


shows data for poly(NIPAM-co-allylacetic acid) copolymer

microgels. The increase of charge as revealed by the z-potential is

due to different allylacetic acid (AAA) contents (plot on the left

of Fig. 15). The z-potential does not represent absolute values of

charge but nicely reveals the tendency of growing charge with

increasing copolymerisation ratios.27

Copolymerisation with organic acids not only affects the

network charge, but strongly influences the swelling behavior

and the response to pH and ionic strength. Fig. 15 also shows the

swelling curves of three different microgels as a result from DLS

measurements (right). In case of a pure poly-NIPAM microgel

a volume phase transition temperature (VPTT) of around

32.5 �C can be found. For copolymers of poly-NIPAM with

VAA (2.99% absolute content) a VPTT of around 41.0 �C and

with AAA (2.45% absolute content) of around 46.0 �C are

observed.

In case of hybrids for optical applications the high scattering

of the microgel itself has to be considered. Fig. 16 shows a UV-vis

spectrum of a diluted aqueous poly-NIPAM microgel dispersion

as well as the spectra of different-sized and different-shaped gold

nanoparticles.

While the increase in absorbance to lower wavelengths in the

left spectrum of Fig. 16 is just due to the strong scattering of

the microgel, well-pronounced, rather sharp plasmon reso-

nances can be observed in case of the shown highly mono-

disperse gold nanoparticles. The position of the plasmon bands

of the presented spherical gold nanoparticles, in the middle of

Fig. 16, is still in a wavelength range where the PNIPAM

dispersion shows rather high scattering, whereas the longitu-

dinal plasmon band of the gold nanorods, in the right plot of

Fig. 16, is far from the scattering regime of the microgel.

Hence, a combination with nanoparticles, like the presented

gold nanorods, seems to be promising if new hybrid materials

with pronounced optical properties are desired. Nevertheless,

both types of gold nanoparticles have been used for the prep-

aration of hybrid microgels, as will be presented in the

following sections.

In order to prepare composite microgels with catalytic or

magnetic properties, inorganic nanoparticles such as palladium

and magnetite have been used.

Negatively charged copolymer microgels were covered with

positively charged gold nanorods by Kumacheva et al.98,135 These

authors have shown that the nanorods can be used to trigger the

volume phase transition of the microgel cores by irradiation with

a laser having a wavelength close to the longitudinal plasmon

band of the gold nanorods. Fig. 17 shows a TEM image of the

respective hybrid particles and also the relative volume change

V/V0 as a function of laser-on and laser-off events.

The laser excitation of the gold nanorods leads to local heating

which forces the collapse of the thermoresponsive core material.

For the pure microgel sample, slight changes in particle size due

to the laser heating can be observed, whereas for the hybrid

system well-pronounced changes appear. The positions of the

start and end points of the laser-induced swelling and deswelling

events are independent of the number of laser heating cycles. This

indicates the stability of the hybrid and high reversibility of the

volume phase transition.

The optical properties of this kind of hybrid are strongly

influenced by the surface coverage. The maximum coverage is

J. Mater. Chem., 2009, 19, 8714–8727 | 8721


Fig. 16 UV-vis spectra recorded in a wavelengths range of 400 to 950 nm at 298 K. Left: Diluted poly-NIPAM microgel dispersion. Due to the high

scattering the absorbance increases strongly at low wavelengths. Middle: Three spectra of spherical gold nanoparticles with different diameters (4 nm,

15 nm and 18 nm). With increasing particle diameter the plasmon band shifts towards higher wavelengths. Right: Gold-nanorods with a length of

around 60 nm and a diameter of around 15 nm (aspect ratio of 4.0). The inset images are representative TEM illustrations of the respective gold

nanoparticles.119

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limited by the particle charge and the surface area itself. Hellweg

et al. have shown that the coverage below this limit can be varied

by adjusting the mixing ratio of nanoparticles to microgel.136

Fig. 18 shows TEM images of hybrids with different surface

coverages of gold nanorods on a pure poly-NIPAM microgel

achieved by changing the mixing ratio nanorods/microgel. The

gold nanorods are coated with two oppositely charged poly-

electrolyte layers (Au@PSS@PAH) leading to a final positive

surface charge. The microgel particles are negatively charged due

to the large amount of the ionic radical initiator (potassium

peroxodisulfate) used.

While Kumacheva et al. focussed on the laser heating of the

nanorods, Hellweg et al. used the plasmon band of the gold

nanorods to follow the temperature-induced volume phase

transition optically. The collapse of the microgel core leads to an

increase in surface coverage and hence a decrease in the gold

nanorod interparticle distance. If a certain distance is reached, an

electronic interaction appears that red-shifts the longitudinal

plasmon band.137,138,127 Further effects are broadening of the

band as well as a change in the absorbance. These effects are

found to be fully reversible. Fig. 18 demonstrates this

Fig. 17 Left: TEM image of poly(NIPAM-co-maleic acid) microgel

particles covered with positively charged gold nanorods. Right: Desw-

elling and swelling events in terms of the relative volume change V/V0 as

a function of laser-on and laser-off events. Reprinted with permission

from M. Das, N. Sanson, D. Fava and E. Kumacheva, Microgels loaded

with gold nanorods: Photothermally triggered volume phase transition

under physiological conditions, Langmuir, 2007, 23, 196–201.98 Copy-

right 2007 American Chemical Society.

8722 | J. Mater. Chem., 2009, 19, 8714–8727

reversibility in terms of the plasmon band position as a function

of swelling and deswelling events for samples a) to c) also shown

in this figure. Stronger optical responses are found if the surface

coverage is increased using highly charged copolymers. Fig. 19

shows a SEM image of an almost fully covered (considering the

rather low state of swelling under vacuum conditions in the

microscope) poly-(NIPAM-co-2.9% AAA) copolymer microgel.

Also shown are UV-vis spectra in the swollen state, at 15 �C and

in the collapsed state, at 60 �C. The red-shift which occurs under

these conditions is of the order of 55 nm.

The same principle can also be applied to change the fluores-

cence of CdTe quantum dots. This was shown by Stamm and

co-workers, who adsorbed these quantum dots on a thermores-

ponsive microgel.143

However, the approach using the adsorption of nanoparticles

on microgels is quite universal, and can probably be applied to all

available nanoscale inorganic particles. Another example was

given by Richtering et al. covering PNIPAM microgels with

magnetic particles.144

4. Summary

In recent years, hybrid materials consisting of an organic poly-

mer and an inorganic nanoparticle unit have attracted much

interest. Much effort has been devoted to both on the prepara-

tion of new composites and on the characterisation of the final

material.

The intention of this Feature article has been to give a brief

survey to a number of versatile hybrid systems investigated by

different groups. For the sake of clarity we have separated the

hybrids into three classes, which seem to be most representative:

hybrids having a core-shell structure; polymer particles that are

filled with inorganic nanoparticles; and hybrids consisting of

a polymer core covered by inorganic nanoparticles. Nano-

particles that have been used for the hybrids presented have

mainly optical, magnetic or catalytic properties from which the

final composites benefit. The materials used range from gold,

silver and palladium to magnetite having either a spherical or



Fig. 18 TEM images of poly-NIPAM microgel particles covered with increasing amounts of polyelectrolyte-coated gold nanorods, increasing from a)

to c) and position of the longitudinal plasmon band of the attached gold nanorods as a function of swelling and deswelling events achieved by setting the

temperature either to 20 �C or to 40 �C. From M. Karg, I. Pastoriza-Santos, J. Perez-Juste, T. Hellweg and L. M. Liz-Marzan, Nanorod-coated

PNIPAM microgels: Thermoresponsive optical properties, Small, 2007, 3(7),1222–1229.136 Copyright Wiley-VCH Verlag GmbH & Co. KGaA.

Reproduced with permission.

Fig. 19 Left: SEM image of highly covered poly-(NIPAM-co-2.9%

AAA) microgels. Right: Respective UV-vis spectra in the swollen (15 �C)

and collapsed state (60 �C). The position of the longitudinal plamon band

of the gold nanorods is red-shifted (55 nm) when the hybrid cores

collapse.139 Reprinted with permission from M. Karg, Y. Lu, E. Carb�o-

Argibay, I. Pastoriza-Santos, J. P�erez-Juste and L. M. Liz-Marz�an,

Multi-responsive hybrid colloids based on gold nanorods and poly-

(nipam-co-allyl-acetic acid) microgels: temperature- and pH-tunable

plasmon resonance, Langmuir, 2009, 25, 3163–3167.139 Copyright 2009

American Chemical Society.

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rod-like shape. We have briefly discussed different routines of

hybrid preparation, while several techniques used for particle

characterization have been mentioned, including powerful and

often-used imaging techniques such as electron microscopy and

atomic force microscopy. As we have shown, these methods can

be used to asses the internal structure (e.g. a core-shell type

structure) and also the distribution of nanoparticles inside or

around the microgel. However, these methods are usually only

used to obtain a semi-quantitative idea of the size, shape and

composition of the hybrids.

If more reliable information about the particle size in disper-

sion and about the local properties of the polymer network is

necessary, scattering techniques such as light and neutron scat-

tering could be used as characterisation tools.

5. Future perspectives

Hybrid microgels with thermoresponsive properties due to

a polymer component and optical, magnetic or catalytic prop-

erties due to an inorganic component have attracted more and

more interest in the last decade. Besides the preparation of such


composites, many investigations using imaging, scattering and

spectroscopic techniques have been reported.

However, complex applications require materials combining

several types of nanoparticles with different properties (e.g.

catalytic and magnetic) with new polymer architectures. For

example, biocompatible microgels with smart properties related

to conditions found in the human body are desired, and in this

context the use of non-toxic and non-cancerogeneous monomers

is required. Moreover, better control of the VPTT would be of

interest.

With respect to catalysis the controlled deposition of nano-

particles inside the network is highly important. At present the

reported systems were all created under nearly random condi-

tions except of core-shell microgels, which provide a well-defined

structure. Therefore, in order to gain more control over the

deposition inside the mircrogel or on its surface new strategies for

the functionalisation either of the micorgel or the nanoparticles

will be important.

Additionally, there should be a focus on the structure–prop-

erty relationships, and hence scattering studies with regard to the

polymer network composition, mesh size and swelling properties

are needed. For the design of optical sensors and smart actua-

tors, hybrids with anisotropic properties could be promising

candidates. To our knowledge, anisotropic hybrid microgels

have not yet been presented in the literature.

Acknowledgements

This work was funded by the Deutsche Forschungsgemeinschaft

within the framework of the priority program SPP 1259 and the

collaborative research center (SFB) 840. The authors are grateful

to I. Pastoriza-Santos, J. P�erez-Juste and L. M. Liz-Marz�an for

introducing us to the fascinating world of nanoparticles. We also

would like to thank both referees who helped to significantly

improve this review.

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smart inorganic/organic hybrid microgels: synthesis and characterisation

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