(687a) Templated Synthesis of Polymer - Gold Nanocomposites with Pluronic Gels

Templated
Synthesis of Polymer - Gold Nanocomposites with Pluronic Gels

Gold nanoparticles with uniform size and shape have many applications in a
variety of fields such as biomedical, sensing and plasmonic
materials. Frequently, ordered assemblies of nanoparticles are sought to
fabricate practical devices utilizing the unique properties of these
nanoparticles. For example, assemblies of gold nanoparticles with complex
shapes can show emergent behavior such as metamaterials. Obtaining fully 3
dimensional assemblies of gold nanoparticles with macroscale ordering has been
a significant challenge. One of the methods to create such ordered structures
is the in situ formation of nanoparticles within polymer matrices, especially
block copolymer matrices which act as templates for the formation of
nanoparticles and their assembly. Nucleation and growth of the nanoparticles in
the polymer matrices can be tuned by pH, choice of reducing agent,
concentrations of reactants involved and the reaction temperature. Ordered
nanoparticle assemblies also typically require high concentration of
monodisperse nanoparticles in the assembly. In this regard, we present some
results on the formation of high concentration of monodisperse gold
nanoparticles in Pluronic gels. Pluronic block copolymers solutions in water
are known to form liquid crystal lattices at high polymer concentration at room
temperature, and the formed liquid crystals have been used to template the
formation of mesoporous inorganic oxides. In addition, Pluronic is also known
to reduce chloroauric acid to gold nanoparticles in
solutions. Here we present the formation of gold nanocomposites at high
concentration in Pluronic solutions and gels. Different conditions were tested
for the formation of uniform gold nanoparticles without the loss of gelation
behavior of the polymer at room temperature. Effect of pH and added reducing
agent on the kinetics of the redox reaction, gelation, nanoparticle yield and
morphology were studied. Basic pH was found to aid both the gelation, and fast
redox reaction. Fast reduction resulted in the formation of uniform gold
nanoparticles in the gel with diameter of 9.4 ± 1.9 nm. Under other tested
conditions, formation of gold triangles and rods was observed. Addition of trisodium citrate to the gel was found to improve the
results of the reaction considerably relative, but was found to be not as
effective as synthesis at high pH with sodium hydroxide. Our results indicate a
competition between Pluronic and added reducing agents for reducing gold ions
which can be tuned by adjusting solution pH.
Differences in observed results with pH and added salt were explained on the
basis of Hofmeister effect and the effect of pH on
the hydration of PEO blocks and the redox reaction. SAXS and cryoTEM will be used to characterize the assembly of the
particles in the gels. Using these results we aim to achieve controlled and
selective deposition of metal nanoparticles and metal films in domains of
Pluronic block copolymer gels.

Figure 1: TEM images of gold nanoparticles synthesized in 22.7 wt% Pluronic gels in 54mM sodium hydroxide with 1.6mM HAuCl_4.
The particles have a narrow size distribution (9.4 ± 1.9 nm).