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Cationic Spherical Polyelectrolyte Brushes as
Nanoreactors for the Generation of Gold Particles
Geeta Sharma, Matthias Ballauff *a
Polymer-Institut, Universitat Karlsruhe, Kaiserstrasse 12, 76128 Karlsruhe, Germany
Received: September 10, 2003; Revised: December 2, 2003; Accepted: December 2, 2003; DOI: 10.1002/marc.200300107
Keywords: ion exchangers; nanoparticles; nanoreactor; nanotechnology; polyelectrolytes
Introduction
Nanometric gold particles attached to colloidal spheres
have been a field of intense research during the recent
years.[1–11] The motivation for a large number of studies
derives from possible applications related, for example, to
the unique optical properties of gold particles with dia-
meters in the range of a few nanometers.[12–16] Moreover,
latex or silica particles covered by a dense layer of gold
particles may be used for the fabrication of photonic
crystals.[17–19] Most of the systems prepared so far have
been generated by the adsorption of colloidal gold nano-
particles onto latexes or silica spheres having a suitably
modified surface. Covering the surface of latex particles by
a cationic polyelectrolyte leads to a net positive surface
charge that attracts the much smaller gold particles bearing
a negative charge. The recent literature on this subject is
enormous and the making of such a composite particles
seems well established by now.
Much less is known of the alternative route, namely,
creating the metallic gold directly at the surface of colloid
particles by reduction of a suitable gold salt. In this way
the surface of the latex or silica sphere becomes a ‘‘nano-
reactor’’ in which the reduction is done. This way of
generating Au-nanoparticles is comparable to the use of
micelles[20,21] or dendrimers[22–26] as ‘‘nanoreactors’’.
Evidently, the gold compound must be firmly immobilized
in this nanoreactor to prevent the exchange of the gold
ions into the continuous phase. Hence, it is important to
elucidate the interactions of suitable gold compounds with
model colloids having a well-defined surface structure.
Summary: If long polyelectrolyte chains are attacheddensely to colloidal latex particles, a spherical polyelec-trolyte brush results. These spherical polyelectrolytes aredispersed in water and carry a high charge. We demonstratethat these systems can be used to immobilize ions of heavymetals, such as gold, as counter-ions. Reduction of these ionsleads to metallic nanoparticles. In this way the brush layerattached to the surface of the particles becomes a ‘‘nanore-actor’’ that may be used for chemical conversions of themetal ions. We show that the reduction of AuCl4
� ionswithin these nanoreactors leads to well-defined and rathermonodisperse gold nanoparticles that are attached to the sur-face of the core. A stable dispersion of polymeric core parti-cles with attached nanoparticles results. All results reportedhere suggest that chemical reactions of ions immobilized inspherical polyelectrolyte brushes provide a new route tocomposite particles of inorganic and organic materials.
Transmission electron micrograph of gold particles on a core-shell system.
Macromol. Rapid Commun. 2004, 25, 547–552 DOI: 10.1002/marc.200300107 � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication 547
a Present address: Lehrstuhl Physikalische Chemie I, UniversitaetBayreuth, 95440 Bayreuth, Germany.
To the authors’ best knowledge, only a few studies are
available in which the surface of colloidal systems have
been used for the generation of Au particles. Schmidt and
co-workers used bottlebrush polymers to bind AuCl4�
ions.[27] Subsequent reduction lead to Au nanowires.
Armes et al. used micelles of triblock copolymers to bind
the AuCl4� ions from aqueous solution.[28] Here, too, the
formation of gold nanoparticles was observed. A similar
approach to bind AuCl4� ions by polyallylamine grafted
on the surface of poly(methyl methacrylate) spheres was
used by Youk to produce gold particles of average size
around 12 nm.[29]
Here we show that latex particles bearing long cationic
polyelectrolyte chains may be used as well-defined nano-
reactors. Figure 1 shows our method in a schematic fashion:
In step I, long chains of poly(aminoethyl methacrylate
hydrochloride) (PAEMH) are grown on the surface of a
polystyrene core by photo-emulsion polymerization.[30]
By this ‘‘grafting-from technique’’ a cationic spherical
polyelectrolyte brush is generated similar to a system under
recent consideration.[31] The grafting of the PAEMH chains
achieved by photo-emulsion polymerization is so dense
that the brush limit is reached in these systems, that is, the
average distance of the chains on the surface is much smaller
than their average dimensions.[32] The chloride counter-
ions are subsequently exchanged by AuCl4� ions (step III,
Figure 1)). As an alternative route, the hydrochloride can
be converted into the free base first and then reacted with
HAuCl4 (step II; Figure 1). The gold compound thus immo-
bilized can be reduced in the final step IV to yield metallic
gold particles on the surface.
The use of the cationic spherical polyelectrolyte brushes
as nanoreactors has a number of clear advantages. The
surface of the nearly monodisperse core particles is well
defined[30] and the extension of the surface layer consisting
of polyelectrolyte chains can be studied precisely by dyna-
mic light scattering (DLS), as shown recently for anionic
spherical polyelectrolyte brushes.[33,34] Moreover, it can
be demonstrated that approximately 95% of the counter-
ions are confined in the brush layer.[35] This confinement
was predicted some time ago by Pincus.[32] It ‘‘closes’’ the
nanoreactor and prevents the formation of Au particles in
the aqueous phase. In this way, the well-established physics
of the polyelectrolyte brush layer can be used for chemical
reactions on the surface of colloidal particles.
Experimental Part
Materials and Methods
All chemicals used for the synthesis are of analytical gradeand were used as received. Styrene (BASF) was distilledunder reduced pressure and stored at �4 8C until used. Hexa-decyl trimethyl ammonium bromide (CTAB) was used as
Figure 1. Schematic representation of the formation of gold particles on the surface of thecore-shell system. The core-shell system having a shell of poly(aminoethyl methacrylatehydrochloride) [I] is deprotonated reversibly at high pH to give amine shell system [II]. I andII can be counter-ion exchanged/complexed with HAuCl4 to give III. Reduction of III withNaBH4 forms IV with nanosized gold particles.
548 G. Sharma, M. Ballauff
Macromol. Rapid Commun. 2004, 25, 547–552 www.mrc-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
surfactant and 2,20-azodiisobutyramidine dihydrochloride(V-50; Fluka) was used as initiator in the synthesis of poly-styrene particles. Ethanolamine hydrochloride, absoluteethanol, methacryloyl chloride, and 1,2-dichloroethane wereused as received.
Tetrachlorogold(III) acid trihydrate was used to intro-duce gold ions into the brushes and NaBH4 was used to reducethe ions. Distilled and deionized water was used in the entiresynthesis.
The compound 2-[p-(2-hydroxy-2-methylpropiophenone)]-ethyleneglycol methacrylate (HMEM) was used as the photo-initiator whose synthesis has been described previously. Themonomer aminoethyl methacrylate hydrochloride (AEMH)was synthesized according to reference [36]. Characterizationwas performed by NMR spectroscopy using a Bruker AMC400 spectrometer.
Photoemulsion polymerization was done in a UV-reactor(Heraeus TQ 150 Z3).[30,33] The hydrodynamic radius wasdetermined by dynamic light scattering using a Peters ALV4000 goniometer.[33,34] Molecular-weight determination ofthe polyelectrolyte brushes grafted onto the styrene core wasdetermined using a Ubbelohde viscosimeter.[30] The pH andconductometric measurements were carried out by a Q pH70 pH meter and a Q cond 2200 conductometer, respectively.
The transmission electron microscopy (TEM) measure-ments were performed using a Philips (FEI) CM 200 FEGequipped with an energy dispersive X-ray system.
Synthesis of Polystyrene Core Particles
The polystyrene core was synthesized by the emulsion poly-merization of styrene.[30] One mole of styrene was emulsifiedby CTAB (7.29 mmoles) in 420 mL of water. The mixture wasthen heated at 80 8C under nitrogen atmosphere. A solutionof thermal initiator V-50 (1.617 mmoles) in 50 mL of waterwas introduced in the reactor. The polymerization was conti-nued for 20 min after which the photoinitiator (HMEM) wasadded under starved conditions (0.5 mL �min�1) to obtain athin shell of photoinitiator on the core particles. The amountsof the photoinitiator varied from 0.5 to 4 mol-% of the styr-ene used. After the addition of the photoinitiator, the reactionwas continued for 6 h. The latex obtained was cooled andpurified by ultrafiltration until the conductivity of the serumwas around 20–30 mS � cm�1.
Synthesis of the Core-Shell Particles byPhotoemulsion Polymerization
The amount of monomer AEMH was calculated by the mol-%of the core used in the photoemulsion polymerization, whichvaried from (10–60%). The core used was diluted to 2.5 wt.-%of water. The mixture of diluted core and the monomerwas charged into the reactor. The mixture was evacuated andflushed with nitrogen several times. Photopolymerization wascarried out by UV-Vis irradiation with constant stirring.Samples were drawn regularly from the reaction mixture andthe increase in hydrodynamic radii was followed by dynamiclight scattering (DLS). The core-shell polyelectrolyte brushesthus obtained were cleaned by ultrafiltration using distilledand deionized water (Millipore) under a pressure of 1 bar
until the conductivity of the outgoing serum dropped under2 mS � cm�1.
Formation of Gold Particles in the Brush
In a typical run, 3 mL of the polyelectrolyte brush system(3.10 wt.-%) was placed in an ultrafiltration cell, ion exchangewas then carried out by serum replacement against 5� 10�5
M
of a 2 L HAuCl4 solution. The remaining co-ions were cleanedby ultrafiltration with pure water until the conductivity of theserum was reduced to 2 mS � cm�1. The white polyelectrolytebrush system turns pale yellow after treatment with HAuCl4solution. This system bearing HAuCl4 ions was reduced usinga 5� 10�3
M solution of NaBH4. Reduction with NaBH4
changed the color of the latex to brown.
Results and Discussion
The synthesis of polystyrene core particles by emulsion
polymerization, and polyelectrolyte brushes by photo-
emulsion polymerization has been recently described in
detail by Guo et al.[30] We carried out the synthesis in a
similar way with the exception that positively charged
surfactant (CTAB) and a thermal initiator were used to give
the latex a cationic charge.[31] Photoemulsion polymeriza-
tion of these cationic latexes to obtain a brush of the
mentioned monomers proceeded exactly as described
before.[31] The formation of the brush was followed by
dynamic light scattering by observing the increase in
hydrodynamic radii. Since the radius of the nearly
monodisperse core particles is known, DLS leads directly
to the thickness L of the brush layer.[30,33,34]
As already found for the anionic systems discussed in
reference [30], the brush thickness L was found to depend on
the amount of photoinitiator used and the amount of
monomer. Increasing the amount of monomer increased the
brush thickness. Using more than 30 mol-% of monomer,
however, increased the viscosity of the latex, which may be
caused by the formation of a gel structure in the latex. The
photoemulsion polymerization needs approximately 100–
150 min to reach sufficient conversion.[30]
Purification of the core latex was highly important to
achieve a desired thickness of the brush layer. The presence
of superfluous surfactant reduced the grafting density and
increased the brush thickness. Latexes not cleansed
beforehand gave a gel even with small quantities of
monomer were used at low effective volume fractions,
whereas the cleaning of the same core to different extents
gave different brush lengths.
The spherical polyelectrolyte thus generated could be
fully characterized with regard to the contour length of the
attached chains and the grafting density.[30] The amount of
polymer grafted onto the core is determined by titration
with AgNO3 after exhaustive cleaning by ultrafiltration.
The contour length of the brush polymer was determined by
cleaving the brushes from the core surface initiated by base
Cationic Spherical Polyelectrolyte Brushes as Nanoreactors for the Generation of Gold Particles 549
Macromol. Rapid Commun. 2004, 25, 547–552 www.mrc-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
hydrolysis.[30] The strong base breaks the ester bond
between the vinyl bond of the polyelectrolyte chains and
the photoinitiator. The free PAEMH chains are saponifi-
cated under these conditions to give poly(methacrylic
acid) which can be subsequently analyzed. In this way
the contour length Lc and the grafting density s can be
determined in a similar way to the methods employed
previously.[30]
Figure 2 displays the thickness L of the brush layer
attached to the surface of the particles as a function of
the concentration of added salt. There is a pronounced
shrinking of the polyelectrolyte layer with increasing
ionic strength. The same phenomenon was observed re-
cently when studying anionic spherical polyelectrolyte
brushes.[33,34] The explanation of the strong swelling at low
ionic strength is given by the confinement of the counter-
ions within the brush layer, as discussed above. In the limit
of salt-free solutions the high osmotic pressure stretches
the polyelectrolyte chains to nearly full length (‘‘osmotic
limit’’). If the ionic strength is raised, the electrostatic
interaction within the brush layer is screened more and
more. In the limit of high concentrations of added salt
(‘‘salted brush’’), the surface layer may be compared to
uncharged systems, as was recently shown.[34] These
investigations also showed that divalent ions lead to a
much more pronounced shrinking.
Figure 2 also shows that addition of AuCl4� ions leads
to a much stronger shrinking than expected for a mono-
valent salt. We assign this behavior to the complexation
of at least two �NH2 groups by one AuCl4� ion. Hence,
the strong binding of this gold compound is much enhanc-
ed through complexation as expected from data in the
literature.[29,37–40] The interaction of protonated and
neutral amines with HAuCl4 leading to the formation of
gold nanoparticles has been exploited quite often, but
the nature of the exact species formed is not very clear.
Formation of a complex between cetyltrimethyl ammonium
bromide and HAuCl4 is used for the formation of gold
nanorods.[41] The protonation of primary alkylamines by
HAuCl4 in organic solvents has also been used for the
formation of gold nanoparticles and for metal-ion extrac-
tion.[42] The interaction of heterocyclic amines and HAuCl4has also been studied. RþAuCl4
� species are formed with
AuCl4� as counter-ions at low pH whereas complexes are
formed at neutral pH.
The PAEMH chains could be converted into the
uncharged amino-form by treatment with strong base and
subsequent purification. In this way a dense surface layer of
poly(aminoethyl methacrylate) chains is generated that can
be used for complexation of other metal ions as well.
Figure 3 demonstrates that the conversion of the poly(-
aminoethyl methacrylate hydrochloride) chains into an
uncharged polymer is accompanied by a strong decrease of
the layer thickness L as expected. The latex is still stable
against coagulation, however, and may be used for the up-
take of ions again. Hence, the neutral amino form of the
brush is titrated with HAuCl4. No salt is added in this
procedure to ensure the full uptake of AuCl4� ions into the
brush. Since the total number of charges is known with good
precision, this method allows the calculation of the amount
of the precious metal ions necessary for full conversion.
Because of the high affinity of the brush particles for AuCl4�
ions, no ions are left in the solution. This can be shown by
DLS: Titration with HAuCl4 does not lead to the strong
swelling as expected for a titration with HCl, but to the same
layer thickness L as observed previously (see the discussion
of Figure 2).
Having shown that aurochlorate ions are fully confined
within the brush layer, we now turn to the reduction of these
ions within the ‘‘nanoreactor’’. This is achieved by the
addition of dilute solutions of NaBH4 to the latex. The re-
duction is immediately obvious from a change in the color
Figure 2. Brush thickness L as a function of concentration ca ofadded salt: (*) NaCl, and (&) HAuCl4 (core radius: 42 nm, brushthickness: 80 nm).
Figure 3. The brush thickness L is plotted against pH of thesuspension. The parameter in the different curves is the ionicstrength (core radius: 40 nm, brush thickness: 160 nm):~: 0.0001M; *: 0.001 M; &: 0.01 M; �: 1 M.
550 G. Sharma, M. Ballauff
Macromol. Rapid Commun. 2004, 25, 547–552 www.mrc-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
of the suspension from yellow to brown. No change in color
is observed afterwards suggesting that gold particles do not
aggregate. A shallow maximum becomes visible in the UV/
Vis spectra, which has a maximum around a wavelength of
502–512 nm. This is to be expected for Au nanoparticles of
less than 3 nm in size. The particles having diameter of more
than 3 nm absorb at 520 nm and exhibit a red color.
Uncovered gold particles, made by radiolysis, with an
average diameter of less than 1.5–2.5 nm show an
absorbance between 490–500 nm.[43] These particles are
orange–yellow in color. The presence of impurities or any
capping agents attached to the surface of the particles that
absorb in the 200–800 nm range modifies the spectrum and
gives a different maximum for the same particle sizes.[44]
For the present system, however, the strong scattering of the
latex itself precludes further analysis of the UV/Vis spectra.
The resonance plasmon of gold appears only as a shoulder
in the UV/Vis spectrum of the latex.
Transmission electron microscopy (TEM) gives direct
proof that Au nanoparticles have been formed on the
surface, that is, in the ‘‘nanoreactors’’. Figure 4a displays
TEM pictures of several particles while Figure 4b shows an
enlarged view of the nanoparticles. It is evident that the Au
particles have a diameter of ca. 2 nm and exhibit a rather
narrow size distribution. Moreover, Figure 4b demonstrates
that the particles are fully crystalline as expected.
Obviously, the nucleation of the Au particles takes place
on many spots on the surface. This leads to particles that are
considerably smaller than the Au particles grown in
solution under the same conditions. No further aggregation
Figure 4. a) TEM pictures of gold particles on core-shell system; b) TEM picture at highmagnification showing the lattice fringes of the crystalline gold particles.
Cationic Spherical Polyelectrolyte Brushes as Nanoreactors for the Generation of Gold Particles 551
Macromol. Rapid Commun. 2004, 25, 547–552 www.mrc-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
takes place under these conditions. This observation sug-
gests that nucleation of nanoparticles can be controlled in
these systems. Further studies are under way to investigate
whether this finding can be generalized for other metal
nanoparticles as well.
During the process of reduction, the latex remains stable.
The onset of a slow aggregation is only observed after
several days. The factors affecting the stability of the latex
covered by gold particles must still be investigated in more
detail. For the present investigation it suffices to note that
the colloidal stability is not impeded by the chemical
reaction taking place in the nanoreactors.
It should be noted in this context that no reagents have
been used for the stabilization of the Au particles. Hence,
their surface could be used for immobilizing further func-
tional groups on the surface of the Au particles like sulfur
or phosphorus containing reagents.
Conclusion
We have shown that cationic spherical polyelectrolyte
brushes can be used as nanoreactors for the formation of Au
particles. The brushes immobilize metal ions by counter-
ion exchange and complexation. The localized metal ions
can be reduced by a suitable reagent to yield nanosized
metal particles which are nearly monodisperse and crystal-
line in nature.
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