Supramolecular gold nanoparticle-polymer composites formed in water with cucurbit[8]uril

Article abstract of DOI:10.1039/c0cc03250f

A gold nanoparticle-polymer composite material has been prepared in water using cucurbit[8]uril as a supramolecular "handcuff" to hold together viologen-functionalised gold nanoparticles and a naphthol-functionalised acrylamide copolymer.

Full text of DOI:10.1039/c0cc03250f

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Supramolecular gold nanoparticle–polymer composites formed in water
with cucurbit[8]urilwz
Roger J. Coulston, Samuel T. Jones, Tung-Chun Lee, Eric A. Appel and Oren A. Scherman*
Received 13th August 2010, Accepted 23rd August 2010
DOI: 10.1039/c0cc03250f
A gold nanoparticle–polymer composite material has been
prepared in water using cucurbit[8]uril as a supramolecular
‘‘handcuff’’ to hold together viologen-functionalised gold nano-
particles and a naphthol-functionalised acrylamide copolymer.
of dendrites,14 block copolymers,15 as well as peptides and
proteins.16,17 Moreover, CB[8] has been used to affix both
small and large molecules including Np-functionalised colloids
onto an Au surface patterned with viologen-functionalised
thiols.18 Utilising this supramolecular binding approach on
NP surfaces dispersed within an aqueous environment would
result in a versatile platform for appending a wide variety of
motifs. Achieving this through a straightforward self-assembly
process would have implications for the development of
materials which could be used in biological9,19 and sensing
applications20 as well as in polymer based optoelectronic
devices.3
Gold nanoparticles (AuNPs) have received a great deal of
interest on account of their unique optical, electrical and
chemical properties.1 Incorporating nanoparticles into polymeric
materials has potential for accessing a range of new composites2
with interesting properties for applications ranging from
polymeric photovoltaic devices to the control over polymer
viscosity with nanoparticle crosslinks,3 however, controlling
the formation of such dispersions remains a challenge.4 The
ability to direct the self-assembly of AuNPs into networks and
higher-order self-assembled structures5–7 allows them to be
used as building blocks for the ‘bottom-up’ design8 of complex
macroscopic materials. The surface modification of AuNPs
with self-assembled monolayers (SAMs) consisting of organic
ligands has allowed the chemical properties of such systems to
be tailored.1 Moreover, the size and shape of the organically
modified AuNPs as well as the nature of the surface functionality
play an important role in the binding interactions of these
NPs.9 Currently, the SAM composition and subsequent
properties of the AuNPs are determined by the covalently
bound ligands, alternatively, AuNP surface functionality
could be achieved from a SAM capable of ‘dynamic’
noncovalent chemistry thus gaining external control over the
AuNP’s interactions with the surroundings.
As water solubility is a prerequisite for CB[8] ternary
complexation, the AuNPs must be functionalised by a
water-soluble SAM yet remain accessible for CB[8] host–guest
binding. Therefore,
a mixed self-assembled monolayer
(mSAM) approach was employed to prepare water soluble
functionalised-AuNP 3 with a neutral (major) ligand tri(ethylene
glycol)-1-butanethiol (EG3-C4-SH; 1) and a viologen-containing
(minor) ligand, 1-methyl-4,40-bipyridinium-dodecanethiol
bisbromide ([MV2+-C12-SH]ꢁ2Brꢀ; 2). The functional ligand
2 was designed to extend the MV2+ recognition motif away
from the secondary surface of the NP to allow for the
formation of CB[8] ternary complexes. AuNPs with a diameter
of roughly 5 nm were prepared following a modified literature
procedure,21 and were functionalised with varying ligand
mixtures of 1 and 2 leading to the AuNP 3 as depicted in
Fig. 1a.
A NP control was prepared in the same manner with a SAM
consisting of solely EG3 (EG3-AuNP 4). All AuNPs were
characterised using dynamic light scattering (DLS). When no
MV functionality was present, the average hydrodynamic
diameter (Dh) was 9 ꢂ 2 nm while the Dh for all three
MV2+-AuNPs was 11 ꢂ 2 nm. Moreover, zeta potential
(ZP) measurements indicated that the surface of the
MV2+-AuNPs 3 had a positive charge of 38 ꢂ 13mV, while
the ZP for the EG3-AuNP 4 was below the minimum detection
as expected for a neutral AuNP. This observation is consistent
with the presence of MV2+ ligands on the surface of the
AuNP 3.
A supramolecular approach could be achieved through the
use of cucurbit[n]urils (CB[n], n = 5–8 of glycouril units)
which are a class of barrel-shaped macrocyclic hosts with
symmetric carbonyl-lined portals.10 CB[n]s are capable of
forming inclusion complexes with appropriately sized guest
compounds in water with high affinity (Ka > 105 Mꢀ1).10–12
In particular, CB[8] can form stable 1 : 1 : 1 ternary
complexes with an electron deficient and electron rich guest
pair, such as a methyl viologen dication (MV2+) and a
naphthol (Np) derivative, leading to an overall binding
constant (Ka) up to 1012 Mꢀ2 in aqueous buffer.13 This has
been previously exploited for the noncovalent ‘‘handcuffing’’
While the DLS data provided a clear indication of varying
surface MV-functionality and density, it was important to
demonstrate that CB[8]-based ternary complexation remained
viable on the NP surface. Unfortunately, optical spectroscopic
techniques such as UV/vis and fluorescence were unable to
provide any evidence for supramolecular ‘handcuff’ formation
inside the CB[8]. Ternary complexation inside CB[8] typically
leads to visible charge transfer (CT) bands in the UV/vis
spectrum,15,22 however, all measurements carried out on the
functionalised AuNPs masked such CT bands on account of
Melville Laboratory for Polymer Synthesis, Department of Chemistry,
University of Cambridge, Lensfield Road, Cambridge, UK CB2 1EW.
E-mail: oas23@cam.ac.uk; Fax: +44 (0)1223 334866;
Tel: +44 (0)1223 334372
w This article is part of the ‘Emerging Investigators’ themed issue for
ChemComm.
z Electronic supplementary information (ESI) available: Experimental
details and full characterisation including the synthesis of all ligands
and functionalised AuNPs as well as UV/vis and DLS data. See DOI:
10.1039/c0cc03250f
c
164 Chem. Commun., 2011, 47, 164–166
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or in the presence of CB[7] (Fig. 2e), which is unable to form
ternary complexes.11
To demonstrate the versatility of the MV2+-AuNP 3
towards supramolecular self-assembly in aqueous solution, a
functional copolymer, poly(2-hydroxyethyl acrylamide)-
co-(naphtholtriazole acrylamide) (P(HEAm-co-NpTAM)), 5,
bearing pendant Np second guest side chains was prepared.
Addition of the multivalent Np-copolymer 5 to an aqueous
solution of MV2+-AuNP 3 with CB[8] indicated a slight red
shift in lmax relative to the controls (3 + CB[7] + 5, 3 + 5,
and 4 + CB[8] + 5) (for additional information see Fig. S5z),
which taken alone does not provide conclusive evidence of
aggregation in solution. We therefore turned to centrifugation
in the presence of CB[8] to demonstrate the multivalent inter-
actions between functionalised AuNPs and Np-copolymer 5.
The small 5 nm size of the AuNPs prevents them from
undergoing aggregation and precipitation from aqueous
solution upon exposure to conventional centrifugal forces.
However, precipitation was observed after centrifugation of
the MV2+-AuNP 3 for 2 h at 12 100 g in the presence of CB[8]
and 5 while nothing was observed when the EG3-AuNP 4
was used.
Fig. 1 Schematic representation of (a) preparation of MV2+-AuNP 3
and EG3-AuNP 4, (b) formation of a 2 : 1 (MV+ꢃ)2CCB[8] inclusion
complex upon reduction and (c) the noncovalent functionalisation of
MV2+-AuNP 3 with CB[8] and multivalent Np-copolymers 5.
In order to investigate this further, the AuNPs were treated
with ultracentrifugation (290 000 g for 5 min) in a series of
copolymer–AuNP mixtures, which included a control with the
smaller homologue CB[7]. From this, only the samples which
contained all three necessary components for formation of the
ternary complexes: the MV2+-AuNP 3, CB[8] and the
Np-copolymer 5, formed a pellet as shown in Fig. 3b (for 3).
When either the smaller CB[7] homologue or no CB[n] was
used with the AuNP 3 or when the AuNP 4 was employed, the
concentrated regime was observed instead of a pellet. Both the
conventional and ultracentrifugation experiments show that
copolymer induced aggregation of the AuNPs was occurring
in solution as a direct result of the supramolecular CB[8]
ternary complexes (Fig. 1c).
the surface plasmon resonance from the AuNPs which absorbs
over a wide optical range (450–600 nm).
Nevertheless, viologen moieties can readily undergo
one-electron reduction in the presence of either chemical or
electrochemical stimuli yielding the radical cation form;23 in
the presence of CB[8], this leads to rapid generation of a 2 : 1
(MV+ꢃ)2CCB[8] inclusion complex (Fig. 1b). This redox
control of the host–guest binding stoichiometry provided an
opportunity to demonstrate the presence of MV functionality
on the AuNP surface by interparticle aggregation of the
MV2+-AuNPs in the presence of CB[8] upon addition of a
suitable reductant. Addition of sodium dithionite (Na2S2O4)
to a solution of the MV2+-AuNP 3 with CB[8] in the
absence of O2 led to the complete precipitation of AuNPs
from solution after 1 h as shown in Fig. 2c. On the other
hand, aggregation was not observed for the reduced
MV+ꢃ-AuNPs in either the absence of any CB[n] (Fig. 2b)
To observe the level of control gained through the supra-
molecular aggregation of the AuNPs, transmission electron
microscopic (TEM) images were taken on the composite
formed from MV2+-AuNP 3, multivalent Np-copolymer 5
and CB[8] (Fig. 4a). Interestingly, the AuNPs were observed to
form a single close packed layer and were completely dispersed
within the thin film of the composite material that formed.
A close up of the thin film boundary is shown in Fig. 4c, which
Fig. 2 Vials containing MV2+-AuNP 3 1 h after the addition of
(a) H2O, (b) Na2S2O4, (c) CB[8] + Na2S2O4, (d) CB[8] and (e) CB[7] +
Na2S2O4.
Fig. 3 Centrifuge tubes after 5 min at 290 000 g, containing 20 eq. of
5 and (a) CB[8] and EG3-AuNP 4, (b) CB[8] and MV2+-AuNP 3,
(c) CB[7] and MV2+-AuNP 3, and (d) MV2+-AuNP 3.
c
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Notes and references
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a
mSAM bearing MV2+ binding motifs and water soluble, non-
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(MV+ꢃ)2CCB[8] inclusion complexation occurs, leading to
interparticle aggregation in water and precipitation. Importantly,
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Np-functionalised multivalent copolymer to form well-defined
nanoparticle–polymer composites in the bulk. This work
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chemistry with gold nanoparticle science using CB[8] host–
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166 Chem. Commun., 2011, 47, 164–166
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