CSEM Scientific and Technical Report 2008

Gold Nanorings from Block Copolymer Self-assembly
L. Wang, P. Hoffmann • , R. Pugin, F. Montagne
CSEM presents a simple approach for the synthesis of long range ordered arrays of gold nanorings by exploiting the stimuli-responsive properties
of amphiphilic block copolymer micelles. The size and periodicity of the nanoring array can be tuned by adjusting the dimensions of the parent
micelles. A mechanism for the nanoring formation based on a pH-dependent kinetic competition between micelle phase inversion and metal loading
has been proposed. Preliminary surface plasmon resonance (SPR) indicated significant change in the optical properties compared to solid gold
nanoparticles, opening the door to interesting applications in sensing. This work demonstrates the ability to achieve precise positioning of metallic
nanostructures on semiconductor surfaces using self-assembly processes.
Surface patterning at the nanometer scale is attracting
considerable attention due to its enormous potential in the
design of surfaces exhibiting radically new functionalities. For
instance, the possibility to form regular arrays of metallic
nanostructures on semiconductor surfaces opens a wide
spectrum of applications in electronics, photonics and
biosensing. Advanced top-down nanolithography techniques
(e.g. deep UV, e-beam, focused ion beam) allow to produce
feature sizes of less than 100 nm, but become increasingly
costly as the dimensions move further down towards the
10 nm mark.
In this context, the synthesis of metallic nanostructures and
the control of their spatial arrangement at the nanoscale by
means of block copolymer (BCP) self-assembly are a very
attractive and powerful alternative. Here, for the first time,
CSEM exploits the responsive properties of BCP micelles to
form long range and tunable arrays of ring-shaped gold
nanostructures having feature sizes below 100 nm.
To produce the gold nanorings, a silicon substrate coated with
reverse polystyrene-block-poly (2-vinylpyridine) (PS-b-P2VP)
micelles is immersed in an aqueous tetrachloroauric solution
(1 mM HAuCl4/0.9 wt% HCl (aq.)) for several minutes. It is
then rinsed with deionised water, dried under a nitrogen
stream and exposed to oxygen plasma in order to
simultaneously remove the surrounding polymer and reduce
the metal salt to Au(0). Figure 1 presents two SEM images (at
different magnifications) of gold nanorings having sizes (outer
diameter) around 50 nm.
Figure 1: SEM images of continuous gold nanorings obtained by
immersion of the reverse micelles in 1 mM HAuCl4/0.9 wt% HCl (aq.)
solution for 10 min, followed by oxygen plasma exposure.
Interestingly, only observed the formation of the gold
nanorings at low pH was observed. Indeed, in aqueous acidic
conditions, and ideally at a pH value ≤ pKa - 2 (pKa of
pyridine is ∼5.2), the P2VP blocks are rapidly swollen and
protonated and thus carry a net positive charge which allow
them to strongly bind anionic metal species through the
establishment of electrostatic interactions. Since at low pH
values, the phase inversion is faster than metal coordination,
[AuCl4] - will interact with the cationic polymeric micelles.
Subsequent treatment under oxygen plasma will then reduce
the metal salt and remove the block copolymer leading to the
formation of the gold nanoring array. Obviously, this
mechanism excludes the synthesis of nanorings from cationic
metallic species (eg. Ag + ) since repulsive interaction with the
protonated pyridine group would prevent metal binding.
The localized SPR properties of the gold nanorings were
studied by spectrophotometry and compared with plain gold
nanoparticles of the same size (Figure 2). All the samples
were prepared on planar quartz substrates. It appears that
both spectra have the same absorbance peak at
approximately 520 nm, even though it is significantly weaker
for the gold nanorings. The rise of a new peak in the nearinfrared
which may be attributed to the nanoring morphology
generating localized surface plasmons in the ring cavity were
also observed. Further work is currently in progress to
improve ring shape control, to better understand their optical
properties and to evaluate their potential applications in the
field of biosensing.
Figure 2: Experimental SPR spectra of gold nanorings and plain gold
nanoparticles
This work is partly supported by the Swiss Nanoscience
Institute (SNI) of the National Center of Competences in
Research (NCCR “Nanoscale Science”). CSEM thanks them
for their support.
• P. Hoffmann is from Ecole Polytechnique Fédérale de Lausanne
(EPFL), and he is leading the Nanostructuring Research Group
(NRG lab) in EPFL
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