Novel one pot synthesis of silver nanoparticle-polymer composites by supercritical CO2 polymerisation in the presence of a RAFT agent

Article abstract of DOI:10.1039/b710503g

We report the one pot synthesis of a silver-polymer nanocomposite in supercritical carbon dioxide (scCO₂) whereby an organometallic silver complex is thermally decomposed in the presence of a reversible addition fragmentation chain transfer (RAFT) agent during a polymerisation reaction in which the RAFT agent simultaneously stabilises the growing polymer microparticles and the formation of surface located silver nanoparticles. The Royal Society of Chemistry.

Full text of DOI:10.1039/b710503g

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Novel one pot synthesis of silver nanoparticle–polymer composites by
supercritical CO2 polymerisation in the presence of a RAFT agent{
Tom Hasell,a Kristofer J. Thurecht,a Rhys D. W. Jones,a Paul D. Brownb and Steven M. Howdle*a
Received (in Cambridge, UK) 12th July 2007, Accepted 16th August 2007
First published as an Advance Article on the web 31st August 2007
DOI: 10.1039/b710503g
polymer particle by RAFT chemistry, while the CO2-philic
Z-group (Scheme 1) acts as a polymeric stabiliser. Therefore, the
RAFT moiety will always be located at the surface of the polymer
particle. The RAFT agent also acts simultaneously to stabilise the
formation of silver nanoparticles through favourable silver–sulfur
interaction.8 Because the silver nanoparticles are attached to the
RAFT agent, this directs the silver nanoparticles to the surface of
the polymer where they are advantageously positioned for most
applications.
We report the one pot synthesis of a silver–polymer nanocom-
posite in supercritical carbon dioxide (scCO2) whereby an
organometallic silver complex is thermally decomposed in the
presence of a reversible addition fragmentation chain transfer
(RAFT) agent during a polymerisation reaction in which the
RAFT agent simultaneously stabilises the growing polymer
microparticles and the formation of surface located silver
nanoparticles.
The RAFT agent utilised in this work (Scheme 1) is formed
from the combination of polydimethylsiloxane (PDMS) and
benzylsulfanylthiocarbonylsulfanylpropionic (BSTSP) acid to form
a PDMS-RAFT agent (see ESI{ for synthesis). Because of its
affinity for CO2, the PDMS moiety will position the RAFT agent
on the surface of the growing polymer, as well as ensuring effective
stability of the growing polymer particle. The electron rich sulfur
groups of the BSTSP acid moiety are attracted to the electron
deficient surface of the forming silver nanoparticles, providing
in situ stabilisation. It should be noted that we are not using RAFT
as a polymerisation controlling agent, but merely as a means to
provide surface functionalisation of the polymer particle. Indeed, it
is well known that trithiocarbonates of the form used in this report
are poor mediating agents for methyl methacrylate (MMA)
polymerisations. We are currently designing new macro-RAFT
agents that will additionally control the polymer molecular weight.
Metal nanoparticles have been the subject of extensive interest
because of their unique size dependent functional properties which
lead to various catalytic, optoelectronic and biomedical applica-
tions.1 Silver nanoparticles in particular have antimicrobial
properties2 and excellent potential as biosensing materials for
surface enhanced Raman scattering (SERS) and metal enhanced
fluorescence.3 The embedding of these nanoparticles into a
polymer substrate is an ideal way to support and protect the
nanoparticles whilst stabilising them against aggregation and
facilitating ease of application and simple recovery.
Here we report the synthesis of a silver nanoparticle–polymer
composite material in the absence of any conventional solvents.
The synthesis is performed in supercritical carbon dioxide (scCO2)
in a single pot, single step reaction. The absence of conventional
organic solvents is preferential as it avoids the possibility of solvent
residues, which would be disadvantageous for medical or bio-
detection applications. In addition, scCO2 is seen as a green
alternative to conventional solvents, it is non-toxic and can be
simply removed without leaving any residues in the material.4
RAFT polymerisation is a facile technique for the functionalisa-
tion of polymer chain ends that can produce narrow polydispersity
products. RAFT polymerisation reactions have been recently
performed in scCO25 and the technique has also been successfully
applied to dispersion polymerisation.6 Silver nanoparticles have
also been synthesised in scCO2 through the use of scCO2 soluble
precursors and stabilising agents.7 Here, we demonstrate that a
scCO2 soluble RAFT agent can be designed to fulfil two roles in a
single reaction: polymerisation and stabilisation of polymer as well
as stabilisation of silver nanoparticles. The RAFT agent is used as
a non-migratory surfactant; it effectively grafts into the growing
aUniversity of Nottingham, School of Chemistry, University Park,
Nottingham, UK NG7 2RD. E-mail: steve.howdle@nottingham.ac.uk;
Fax: 44 0115 951 3058; Tel: 0115 951 3486
bUniversity of Nottingham, School of Mechanical, Materials and
Manufacturing Engineering, University Park, Nottingham, UK
NG7 2RD
{ Electronic supplementary information (ESI) available: Synthesis details
of RAFT agent, GPC, NMR, EDX data and additional TEM images. See
DOI: 10.1039/b710503g
Scheme 1 Schematic diagram showing the process of silver-nanocom-
posite formation. The nanoparticles are stabilised by interaction with the
sulfur groups on the RAFT agent while the polymer microparticle is
formed by dispersion polymerisation of MMA in scCO2 stabilised by the
simultaneous grafting of a CO2-philic macro-RAFT agent.
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 3933–3935 | 3933

View Article Online
Silver nanoparticle synthesis and polymer synthesis reactions
were first performed separately to ensure both worked successfully
in the presence of RAFT stabiliser. Polymer microparticles were
formed by adding 500 mg PDMS-RAFT and 60 mg azobisiso-
butyronitrile [AIBN, 0.6 wt%] to the autoclave and flushing with
CO2 for 15 minutes. Degassed MMA (10 ml) was then added
under a slightly positive flow of CO2 in order to prevent the ingress
of oxygen into the system. The autoclave was pressurised to 55 bar,
heated to 65 uC and then pressurised to 275 bar. After 48 h, the
vessel was cooled to room temperature and the CO2 vented. The
resulting polymer was a free-flowing, off-white powder. SEM
investigation of this powder (Fig. 1b) shows discrete spherical
particles of y1–4 mm.
The synthesis of silver nanoparticles was performed by loading
100 mg of (1,5-cyclooctadiene)(hexafluoroacetylacetonate)silver(I)
[Ag(hfac)(COD)] into an autoclave with 300 mg PDMS-RAFT.
The autoclave was then flushed with CO2 for 15 min before being
heated to 65 uC and pressurised to 275 bar scCO2. The reaction
was maintained at these conditions for 48 h before being cooled to
room temperature and depressurised. Chloroform (10 ml) was
then added as solvent to disperse and remove PDMS-RAFT
stabilised silver nanoparticles as a transparent brown solution.
Chloroform was chosen as the dispersing medium because of
the very high solubility of PDMS in this solvent. TEM
investigation of this solution (Fig. 1a) showed silver nanoparticles
mostly of 5–20 nm in size. Brown colouration is characteristic of
the plasmon resonance absorption of silver nanoparticles. The
Ag(hfac)(COD) and PDMS-RAFT used in the synthesis of these
nanoparticles are both readily scCO2 soluble. The Ag(hfac)(COD)
acts as the silver feedstock as it is thermally decomposed during the
reaction to yield elemental silver. As the silver nanoparticles are
formed, they are stabilised by the PDMS-RAFT that is dissolved
in the scCO2. As the TEM micrographs show little agglomeration
of the nanoparticles, it can be assumed that this stabilisation is very
effective.
The one pot, simultaneous reaction was performed under
identical conditions to the polymer synthesis except that 200 mg
Ag(hfac)COD were added to the autoclave along with the other
reactants. The reaction resulted in the production of a fine free-
flowing brown powder. TEM and SEM images of this product are
shown (Fig. 2a and 2b respectively). SEM investigation showed
discrete sphere polymer morphology, indicating that the inclusion
of silver has no significant adverse effect on the polymerisation
reaction (GPC and NMR analysis available in ESI{).
TEM investigation revealed silver nanoparticles coating the
surface of the polymer spheres. This implies that the PDMS-
RAFT surfactant was successful in simultaneously stabilising the
growing polymer particle and the silver nanoparticles. The
presence of silver in the polymer particles was confirmed by
EDX analysis (see ESI{). An additional advantage of this
approach is that the versatility of the RAFT agent suggests that
this technique will be successful with a wide range of other
monomer and metal species.
Closer inspection of the particle surface reveals that the silver
nanoparticles are tightly bound to the surface of the polymer
particle (Fig. 3). Indeed, the TEM micrographs show that the
Fig. 2 Silver–polymer nanocomposite. a) Bright field TEM image of
silver nanoparticles on polymer. b) Secondary electron SEM image of
polymer powder. Note the polymer microspheres are effectively identical
to those produced in the absence of silver (Fig. 1b).
Fig. 1 a) Bright field TEM image of silver nanoparticles synthesised in
the absence of polymer. b) Secondary electron SEM image of polymer
powder synthesised in the absence of silver.
3934 | Chem. Commun., 2007, 3933–3935
This journal is ß The Royal Society of Chemistry 2007

View Article Online
applicability for a wide range of polymer nanoparticle composite
materials.
We thank Peter Fields, Richard Wilson and Martin Dellar for
their technical assistance in this project. SMH is a Royal Society
Wolfson Record Merit Award Holder. KJT was funded by the
Dutch Polymer Institute.
Notes and references
1 J. P. Wilcoxon and B. L. Abrams, Chem. Soc. Rev., 2006, 35, 1162–1194.
2 (a) A. Panacek, L. Kvitek, R. Prucek, M. Kolar, R. Vecerova,
N. Pizurova, V. K. Sharma, T. Nevecna and R. Zboril, J. Phys. Chem.
B, 2006, 110, 16248–16253; (b) F. Furno, K. S. Morley, B. Wong,
B. L. Sharp, P. L. Arnold, S. M. Howdle, R. Bayston, P. D. Brown,
P. D. Winship and H. J. Reid, J. Antimicrob. Chemother., 2004, 54,
1019–1024.
3 Y. Lu, G. L. Liu and L. P. Lee, Nano Lett., 2005, 5, 5–9; J. R. Lakowicz1,
J. Malicka, I. Gryczynski, Z. Gryczynski and C. D. Geddes, J. Phys. D:
Appl. Phys., 2003, 36(14), R240–R249.
4 (a) H. M. Woods, M. Silva, C. Nouvel, K. M. Shakesheff and
S. M. Howdle, J. Mater. Chem., 2004, 14, 1663–1678; (b) Y. Zhang and
C. Erkey, J. Supercrit. Fluids, 2006, 38, 252–267; (c) J. A.
Behles and J. M. DeSimone, Pure Appl. Chem., 2001, 73,
1281–1285.
5 (a) T. Arita, S. Beuermann, M. Buback and P. Vana, E-Polymers, art. no.
003, 2004; (b) T. Arita, S. Beuermann, M. Buback and P. Vana,
Macromol. Mater. Eng., 2005, 290, 283–293; (c) K. J. Thurecht,
A. M. Gregory, S. Villarroya, J. X. Zhou, A. Heise and S. M. Howdle,
Chem. Commun., 2006, 4383–4385.
6 K. J. Thurecht, A. M. Gregory, W. Wang and S. M. Howdle,
Macromolecules, 2007, 40(9), 2965–2967.
7 (a) M. Anand, P. W. Bell, X. Fan, R. M. Enick and C. B. Roberts,
J. Phys. Chem. B, 2006, 110, 14693–14701; (b) X. Fan, M. C. McLeod,
R. M. Enick and C. B. Roberts, Ind. Eng. Chem. Res., 2006, 45,
3343–3347.
8 (a) J. M. Lee, D. W. Kim, Y. H. Lee and S. G. Oh, Chem. Lett., 2005, 34,
928–929; (b) T. Linnert, P. Mulvaney and A. Henglein, J. Phys. Chem.,
1993, 97, 679–682.
Fig. 3 Bright field TEM image detailing the surface location of the silver
nanoparticles. Scale bars represent 200 nm (left), 10 nm (right, enlarged).
nanoparticles generally appear to be partly embedded into the
polymer surface. This apparent strong attachment to the polymer
particle surface is advantageous as it would imply minimal
nanoparticle loss in applications where some friction of particles
is required.
All of the nanoparticles observed were within the appropriate
size range for favourable applications, mentioned above. All
lie within 3–30 nm in diameter with the majority occurring in the
5–20 nm range.
In summary, the use of an appropriately designed RAFT/
stabilising agent allows simple and effective synthesis of polymer
microspheres decorated with silver nanoparticles in CO2. The
surface location of the nanoparticles gives the polymer powder
excellent potential for catalytic, antibacterial and biosensing
applications. This synthetic route has the additional advantages
of being a single step, one pot reaction that does not require
conventional solvents at any stage during the process.
Furthermore, the versatility of this technique suggests its
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 3933–3935 | 3935