Thermosensitive Gold Nanoparticles

Article abstract of DOI:10.1021/ja038544z

Thermosensitive gold nanoparticles were fabricated by conjugating Au with a thiol-terminated poly(N-isopropylacrylamide) or PPA; this polymer stabilizer exhibits a temperature transition while undergoing a hydrophilic to hydrophobic transformation. The introduction of PPA onto gold nanoparticles has sensitized Au nanoparticles with unique temperature dependence. At low temperature (25 °C), the solutions containing PPA-functionalized gold nanoparticles are transparent, whereas higher temperatures (30 °C) lead to opaque suspensions. The thermosensitive property of PPA-functionalized Au nanoparticles is reversible, and the clear?opaque suspensions can be repeated many times. Copyright

Full text of DOI:10.1021/ja038544z

Published on Web 02/11/2004
Thermosensitive Gold Nanoparticles
†
‡
‡
,†
Ming-Qiang Zhu, Li-Qiong Wang, Gregory J. Exarhos, and Alexander D. Q. Li*
Department of Chemistry and Center for Materials Research, Washington State UniVersity,
Pullman, Washington 99164, and Pacific Northwest National Laboratory, Richland, Washington 99352
Received September 16, 2003; E-mail: dequan@wsu.edu
Nanotechnology has experienced a rapid growth recently because
nanoparticles exhibit physical and chemical properties that are quite
Scheme 1
1
different from those of the bulk solid. Typically, an organic layer
2
3
comprised of a surfactant or a polymer is anchored at the interface
to both localize the nanoparticle and stabilize its properties. Of
particular interest is the conjugation of gold nanoparticles with
4
functional macromolecules such as saccharide and oligo deoxyri-
5
bonucleic acid (DNA). The combination of organic functionality
coupled with the dielectric properties of gold nanoparticles has
resulted in a new material that provides for sensitive colorimetric
6
detection of polynucleotides. We report here the introduction of
thermosensitive polymers onto Au nanoparticles so that the
polymer-bound gold nanoparticles now become responsive to
temperature.
Poly(N-isopropylacrylamide) (PPA) exhibits a low critical solu-
tion temperature (LCST) transition from 30 to 45 °C. The polymer
nanoparticles and T ) 33.5 °C for PPA polymer. A physical
t
7
mixture of Au nanoparticles with polymer bearing no thiol terminal
group has a transition temperature of 33.3 °C. Thermosensitive Au
nanoparticles have a sharper transparent-to-opaque transition as
indicated by the transition span; the full-width at half-maximum
of the first derivative of the transmittance-temperature curve is
narrower for thermosensitive Au nanoparticles (2.6 °C) than that
of the pure polymers (4.0 °C). The onset occurs at 25 °C and
completes at 30 °C for thermosensitive Au nanoparticles, whereas
it starts at 30 °C and completes at 45 °C with residual 10%
is hydrophilic and soluble in water below the LCST, but becomes
hydrophobic and forms a macroscopic coacervate phase above that
temperature due to the fluctuation of hydrophobic interactions and
8
hydrogen bonding. Anchoring PPA on Au nanoparticles might
enhance or amplify such phase-switching behavior unless the
interactions between the soft polymer and hard metal nanoparticle
negate the thermoresponsive properties. Conjugation with Au
nanoparticles requires that the polymer should bear a thiol group
9
at one terminus. We adopt the strategy to use reversible addition-
t
transmittance for PPA (Figure 1C). Below T , the thermosensitive
fragmentation chain transfer (RAFT) techniques to polymerize
N-isopropylacrylamide (iPA) into the well-defined thermosensitive
PPA.10 Using S-benzyl dithiobenzoate as the RAFT agent, we have
prepared a PPA homopolymer terminated with dithioester groups
gold nanoparticles have a measurable scattering mean diameter <50
nm, and the transmittance is about 80%. At high temperatures,
aggregates up to 800 nm are observed, and the solution becomes
completely opaque with transmittance <1%.
The Rayleigh scattering of polymers contributes to an order of
magnitude change in transmittance (∼100% to 10%), which was
further amplified by the strengthening and broadening of the surface
plasmon resonance band of the chemically bound gold nanoparticles
(
Scheme 1). Reduction of dithioester with sodium borohydride
provides the thiol group necessary for chemically attaching the
thermosensitive polymer on the surface of the as-synthesized Au
nanoparticle.
Gold nanoparticles (∼13 nm) synthesized in the presence of
sodium citrate11 show uniform size distribution (Figure 1A), but
they tend to aggregate into clusters or lines when dried. PPA-
tethered Au nanoparticles, however, show potential long-term
stability as indicated by good dispersion and the lack of agglomera-
tion (Figure 1B). Both citrate-stabilized and PPA-tethered Au
nanoparticles have a red color and characteristic plasmon resonance
(10% to <1%). The origin of this amplification effect comes from
the fact that one end of the polymer chain is anchored to the gold
nanoparticle. When a polymer bound to a nanoparticle undergoes
a transition from an extended hydrophilic chain to a globular
hydrophobic structure, it will collapse in only one direction, that
is, toward the nanoparticle. Moreover, a nanoparticle effectively
confines a cluster of thermosensitive polymers within a spherical
11b
band for 13-nm Au particles at ∼520 nm.
The 13-nm Au nanoparticles coated with thiol-terminated PPA
/M
1
2
space, making the hydrophilic to hydrophobic transition easier.
Below 25 °C, water is a relatively good solvent for PPA polymer
on gold nanoparticles and polymer chains are mostly extended;
therefore, the suspension of the gold nanoparticles appears as a
transparent solution. In the transition regime, water becomes a poor
solvent as polymer-water H-bonds are broken and PPA undergoes
conformation changes and exposes much of its hydrophobic surface,
including both intrachain “coil-to-globule” transitions and interchain
self-association.13 Intrachain collapse causes the individual polymer
to become hydrophobic, while interchain self-association within a
single nanoparticle drives the surface of the nanoparticle to switch
(
M
n
≈ 4600, M
w
n
≈ 1.48) show remarkable temperature
sensitivity as manifested in their optical transmittance switching
property at 600 nm. Figure 1C shows typical plots of the
temperature-dependent optical transmittance of thermosensitive Au
nanoparticles at 0.59 nM along with the mean aggregation diameter.
The transition temperature or the inflection point of the transmit-
tance-temperature curve is T ) 28.4 °C for thermosensitive
t
†
Washington State University.
Pacific Northwest National Laboratory.
‡
2656
9
J. AM. CHEM. SOC. 2004, 126, 2656-2657
10.1021/ja038544z CCC: $27.50 © 2004 American Chemical Society

C O MMU N I C A T I O N S
Interestingly, the opaque suspension does not precipitate, and
this temperature-dependent clear-opaque transition of the ther-
mosensitive gold nanoparticles is completely reversible (Figure 1D).
The suspension is transparent at temperatures below 25 °C and
becomes opaque at temperatures above 30 °C. In the transition
regime, the optical transparency is a function of temperature. Figure
1D shows the response of optical transparency as the thermosen-
sitive gold nanoparticle suspension undergoes several heat-cool
cycles between 35 and 25 °C. The optical transparency changes
reversibly between ∼75% and ∼0% transmittance.
In conclusion, we have synthesized thermosensitive gold nano-
particles, exhibiting a sharp, reversible, clear-opaque transition in
solution between 25 and 30 °C. Such thermosensitive gold
nanoparticles can be potentially fabricated into “smart” liquid cell
windows that block the solar heat by turning transparent windows
opaque at high temperature. Thus, thermosensitive Au nanoparticles
may have potential use for stimuli-responsive applications.
Acknowledgment. We acknowledge the support of the U.S.
Department of Energy (DOE), Office of Basic Energy Sciences:
Division of Materials Science and Engineering. A.D.Q.L. is a
Beckman Young Investigator (BYI). We thank Prof. G. J. Liu (U.
of Calgary) for GPC measurements.
Supporting Information Available: Synthesis, methods, and
controls (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
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