Thermoresponsive silver/polymer nanohybrids with switchable metal enhanced fluorescence

Article abstract of DOI:10.1039/c2cc18069c

In this communication we describe a new approach to the fabrication of fluorescent silver/polymer nanohybrids with thermo-switchable metal enhanced fluorescence (MEF). By manipulating a soft polymer spacer between the silver nanoparticles and the fluoroph

Full text of DOI:10.1039/c2cc18069c

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Chemical Communications
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Volume 48 | Number 39 | 16 May 2012 | Pages 4641–4780
ISSN 1359-7345
COMMUNICATION
J.-Q. Liu, T. P. Davis et al.
Thermoresponsive silver/polymer nanohybrids with switchable metal enhanced fluorescence

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Cite this: Chem. Commun., 2012, 48, 4680–4682
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Thermoresponsive silver/polymer nanohybrids with switchable metal
enhanced fluorescencew
Jingquan Liu,*a Aihua Li,a Jianguo Tang,a Rui Wang,a Na Konga and Thomas P. Davis*b
Received 25th December 2011, Accepted 31st January 2012
DOI: 10.1039/c2cc18069c
In this communication we describe a new approach to the
fabrication of fluorescent silver/polymer nanohybrids with thermo-
switchable metal enhanced fluorescence (MEF). By manipulating
a soft polymer spacer between the silver nanoparticles and the
fluorophores a tunable MEF was achieved.
that can undergo thermo-triggered morphology transition
behavior, during which the polymer chain conformation
changes at lower critical solution temperature (LCST).17 The
thermoresponsive properties of PNIPAM can be exploited to
design fluorescent nanohybrids with a soft polymer spacer
between the silver nanoparticle (AgNP) and a fluorophore to
achieve thermo-controlled MEF.
Fluorescence is a promising tool for biological labeling and
sensing.1 Since Kummerlen et al.2 confirmed that fluorescent
As illustrated in Scheme S1 (ESIw) the preparation of
fluorescent nanohybrids was achieved in three steps: first,
synthesis of AgNPs and a block copolymer of PNIPAM with
polyacrylic acid (PAA), PNIPAM-b-PAA; second, the attachment
of PNIPAM-b-PAA onto AgNPs via Ag–sulfur coordination,
leaving the PAA block sitting on the outer surface of the particle;
third, covalent attachment of rhodamine B (RB) on the particle
surface in the presence of N,N0-dicyclohexyl carbodiimide (DCC)
and 4-dimethyl amino pyridine (DMAP). PNIPAM-b-PAA was
obtained from the hydrolysis of a block copolymer comprised of
PNIPAM and poly(tert-butyl acrylate) (PtBA), PNIPAM-b-PtBA
(synthesized via reversible addition fragmentation chain transfer
(RAFT) polymerization18), in the presence of trifluoroacetic acid.
The successful synthesis of PNIPAM-b-PtBA and PNIPAM-
b-PAA was evidenced by 1HNMR (Fig. S2, ESIw). The living
radical RAFT mechanism preserves high end-group integrity
with trithiocarbonate end groups left intact for attaching
chains onto AgNPs, leaving PAA at the outer surface available
for covalent coupling with the fluorophore, RB (Fig. 1). RB was
first tailored with hydroxyl groups to facilitate esterification
coupling with PAA (Fig. S1, ESIw).
¨
intensity can be enhanced by silver island films, many studies
have been conducted on methodologies to achieve enhanced
fluorescence, including metal-enhanced fluorescence (MEF)
techniques. MEF phenomena have been observed with varied
plasmonic nanostructured materials such as silver,3,4 copper,5
gold,6 aluminium,7 zinc8 and chromium.9 Using these metals,
fabrication of a variety of fluorescent functional nanoparticles
and nanobioconjugates has been reported.
MEF usually results from the interaction between fluorophores
in an excited state and the surface plasmon resonance of metal
particles. The distance between the fluorophore and the metal
surface is critical to achieve optimal MEF. It is noteworthy that the
optimal distance for obtaining maximal MEF depends on the
nature of both the metal surface and the fluorophore. Accordingly,
the reported distances vary from 5 to 75 nm.2,4,7,10 Nanoparticles
with metallic cores and various semiconducting shells e.g.
polyelectrolytes,4 biomolecules,11 and silica10,12 are suitable
for designing systems with a tunable MEF.
Metal nanoparticles can be endowed with tailored compatibility
and versatile functionality upon modification with various
functional groups such as thiol,13 pyridine,14 and carboxylic
acids.15 Dithioester or trithiocarbonate terminal moieties have
also been employed to graft homo- or co-polymer chains onto
metal nanoparticles via metal–sulfur coordination.16 Poly-
(N-isopropylamide) (PNIPAM) is a thermoresponsive polymer
The successful modification of AgNPs with PNIPAM-b-PAA
and RB was evidenced by TEM and polarization microscopy.
As shown in Fig. 2a the polymer corona can be clearly observed
a College of Chemistry, Chemical and Environmental Engineering,
Laboratory of Fiber Materials and Modern Textile, The Growing
Base for State Key Laboratory, Qingdao University, Qingdao,
Shandong, China. E-mail: jliu@qdu.edu.cn; Tel: +86-532-83780128
b Centre for Advanced Macromolecular Design, School of Chemical
Engineering, The University of New South Wales, Sydney,
NSW 2052, Australia. E-mail: t.davis@unsw.edu.au
w Electronic supplementary information (ESI) available: The synthesis
of silver nanoparticles, di-block copolymers, Rhodamine B ethanol
ester and the stepwise fabrication of the fluorescent nanohybrids and
the relevant characterizations using 1HNMR, DLS etc. See DOI:
10.1039/c2cc18069c
Fig. 1 A schematic showing a fluorescent silver nanohybrid with
thermoresponsive PNIPAM spacer, exhibiting thermo-controlled MEF.
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Fig. 2 Transmission electron microscopy (TEM) image (a) and
polarization microscopy (b) of the fluorescent silver nanohybrids
(phosphotungsten acid was used as contrast agent for TEM imaging).
around the particles (with the assistance of phosphotungsten
acid contrast agent). The polarization microscopy showed a
red corona around the particles, evidencing the successful
attachment of the RB fluorophore (Fig. 2b). It is noteworthy
that the polarization microscopy cannot visualize nano-sized
particles and the observed are mostly particle aggregates.
We designed a flexible PNIPAM spacer between the fluoro-
phores and AgNPs knowing that PNIPAM is a thermosensitive
polymer with a LCST in water of ca. 32 1C. The critical transition
can be exploited to vary the distance between AgNP and RB,
leading to a variable MEF. As shown in Fig. 3 the nanohybrids
fabricated with a PNIPAM spacer (having a degree of polymeriza-
tion (DP) 300) exhibited enhanced fluorescence (with a MEF factor
of 3.7) at temperatures below the LCST. Furthermore, a reversible
MEF phenomenon was observed when the solution temperature
was cycled below and above the LCST (Fig. 3 inset). We also
investigated the influence of molecular weight of PNIPAM on the
MEF while maintaining other experimental conditions constant.
As shown in Fig. 4 when the PNIPAM was designed with a DP
of 300, the highest MEF factor was achieved. The fluorescent
nanohybrids tailored with PNIPAM spacers with either higher
or lower DP exhibited sub-optimal MEF values.
Fig. 4 The MEF factors (ratio of the fluorescence at 20 1C to that
at 40 1C) obtained with fluorescent nanohybrids with PNIPAM spacers
of different molecular weight (expressed as degree of polymerization on
the x-axis).
nanohybrids (with PNIPAM of DP 300) in aqueous suspensions
below and above the LCST. It is evident that the LCST affects the
spacer length thereby inducing MEF variation. If we hypothesize
that the theoretical length of PNIPAM (DP = 300) is ca. 75 nm
the conformation of PNIPAM chains in an aqueous medium is
consistent with a coiled morphology (as observed by others).17,19
We are currently investigating the dynamic chain conformations
of PNIPAM using fluorescent probes.
We thank the NSFC (51173087), Taishan Scholars Program
and NSF of Shandong Province for financial support.
Notes and references
1 P. Alivisatos, Nat. Biotechnol., 2004, 22, 47; W. C. W. Chan and
S. Nie, Science, 1998, 281, 2016; M. E. Akerman, W. C. W. Chan,
P. Laakkonen, S. N. Bhatia and E. Ruoslahti, Proc. Natl. Acad. Sci.
U. S. A., 2002, 99, 12617; Y. He, Z. H. Kang, Q. S. Li, C. H. A.
Tsang, C. H. Fan and S. T. Lee, Angew. Chem., 2009, 121, 134;
J. R. Lakowicz, Anal. Biochem., 2001, 298, 1; F. Ercole, T. P. Davis
and R. A. Evans, Polym. Chem., 2010, 1, 37.
2 J. Kummerlen, A. Leitner, H. Brunner, F. R. Aussenegg and
¨
A. Wokaun, Mol. Phys., 1993, 80, 1031.
3 W. H. Weber and C. F. Eagen, Opt. Lett., 1979, 4, 236; K. Aslan,
Z. Leonenko, J. R. Lakowicz and C. D. Geddes, J. Phys. Chem. B,
2005, 109, 3157.
It is well-known that polymeric chains are flexible in solution
and a geometrical length is difficult to define. Dynamic light
scattering can be used to analyze particle sizes and was therefore
exploited to analyze the fluorescent nanohybrids. As shown in
Fig. S3 (ESIw) a diameter difference of 30 nm was revealed for the
4 K. Kim, Y. M. Lee, J. W. Lee and K. S. Shin, Langmuir, 2009,
25, 2641.
5 Y. Zhang, K. Aslan, M. J. R. Previte and C. D. Geddes, Appl.
Phys. Lett., 2007, 90, 173116.
6 S. Nathali, M. Alexandre, M. Sergey, J. Jean-Claude and N. Igor,
Biopolymers, 2000, 57, 325.
7 K. Ray, M. H. Chowdhury and J. R. Lakowicz, Anal. Chem., 2007,
79, 6480.
8 K. Aslan, M. J. R. Previte, Y. Zhang and C. D. Geddes, J. Phys.
Chem. C, 2008, 112, 18368.
9 R. Pribik, K. Aslan, Y. Zhang and C. D. Geddes, J. Phys. Chem.
C, 2008, 112, 17969.
10 K. Aslan, M. Wu, J. R. Lakowicz and C. D. Geddes, J. Am. Chem.
Soc., 2007, 129, 1524; M. Lessard-Viger, M. Rioux, L. Rainville
and D. Boudreau, Nano Lett., 2009, 9, 3066; L. Guo, A. Guan,
X. Lin, C. Zhang and G. Chen, Talanta, 2010, 82, 1696; D. Cheng
and Q. H. Xu, Chem. Commun., 2007, 248.
11 K. Aslan, J. Huang, G. M. Wilson and C. D. Geddes, J. Am.
Chem. Soc., 2006, 128, 4206.
12 K. Aslan, J. R. Lakowicz, H. Szmacinski and C. D. Geddes,
J. Fluoresc., 2004, 14, 677.
Fig. 3 Fluorescent emission spectra of the rhodamine B modified
AgNPs (DP = 300) at 20 and 40 1C in an aqueous medium. The inset
shows switchable fluorescence behavior upon temperature cycling.
c
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13 C. J. Sandroff, S. Garoff and K. P. Leung, Phys. Lett., 1983,
22, 547.
14 J. A. Creighton, Surf. Sci., 1983, 124, 209.
15 M. Moskovits and J. S. Suh, J. Am. Chem. Soc., 1985, 107,
6826.
J. Liu, E. Setijadi, Y. Liu, M. R. Whittaker, C. Boyer and
T. P. Davis, Aust. J. Chem., 2010, 63, 1245.
17 F. M. Winnik, Polymer, 1990, 31, 2125.
18 C. Boyer, V. Bulmus, T. P. Davis, V. Ladmiral, J. Liu and
S. Perrier, Chem. Rev., 2009, 109, 5402.
16 A.-S. Duwez, P. Guillet, C. Colard, J.-F. Gohy and C.-A. Fustin,
Macromolecules, 2006, 39, 2729; C. Boyer, M. R. Whittaker,
C. Nouvel and T. P. Davis, Macromolecules, 2010, 43, 1792;
19 C. Boyer, M. H. Stenzel and T. P. Davis, J. Polym. Sci., Part A: Polym.
Chem., 2010, 49, 551; H. Mao, C. Li, Y. Zhang, D. E. Bergbreiter and
P. S. Cremer, J. Am. Chem. Soc., 2003, 125, 2850.
c
4682 Chem. Commun., 2012, 48, 4680–4682
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