Polyrotaxane-Mediated Self-Assembly of Gold Nanospheres into Fully Reversible Supercrystals

Article abstract of DOI:10.1002/anie.201406323

The use of a thiol-functionalized nonionic surfactant to stabilize spherical gold nanoparticles in water induces the spontaneous formation of polyrotaxanes at the nanoparticle surface in the presence of the macrocycle α-cyclodextrin. Whereas using an exce

Full text of DOI:10.1002/anie.201406323

Angewandte
Chemie
DOI: 10.1002/anie.201406323
Nanoparticle Self-Assembly
Polyrotaxane-Mediated Self-Assembly of Gold Nanospheres into Fully
Reversible Supercrystals**
Joao Paulo Coelho, Guillermo Gonzꢀlez-Rubio, Annette Delices, Josꢁ Osꢂo Barcina,
Cꢀstor Salgado, David ꢃvila, Ovidio PeÇa-Rodrꢂguez, Gloria Tardajos, and Andrꢁs Guerrero-
Martꢂnez*
Abstract: The use of a thiol-functionalized nonionic surfactant
to stabilize spherical gold nanoparticles in water induces the
spontaneous formation of polyrotaxanes at the nanoparticle
surface in the presence of the macrocycle a-cyclodextrin.
Whereas using an excess of surfactant an amorphous gold
nanocomposite is obtained, under controlled drying conditions
the self-assembly between the surface supramolecules provides
large and homogenous supercrystals with hexagonal close
packing of nanoparticles. Once formed, the self-assembled
supercrystals can be fully redispersed in water. The reversibility
of the crystallization process may offer an excellent reusable
material to prepare gold nanoparticle inks and optical sensors
with the potential to be recovered after use.
remains a difficult task.[5] Within this context, plasmonic
metal nanoparticles coated with rationally selected organic
molecules and biomolecules have been arranged to build up
plasmonic dimers, polymers, and supercrystals,[6] showing
interesting collective optical and sensing responses.[7]
One of the most appealing families of supramolecules
because of simplicity in their chemical synthesis are cyclo-
dextrin (CD) polyrotaxanes,[8] in which several CD macro-
cycles are entrapped by a molecular axis without bulky end
groups. From the first report describing the preparation of
a polyrotaxane-based solid complex between a-CD (six
a-d-(+)-glucopyranose units) and polyethylene glycol,[9a]
many of these architectures have been synthesized using
different CDs and polymers.[9] In all cases, both host–guest
noncovalent interactions and cooperative hydrogen bonds
between the macrocycles contribute to the stability of the
resulting products.[10] In this context, nonionic Igepal (poly-
oxyethyelene nonyl phenyl ether) surfactants have emerged
as excellent candidates to prepare solid structures of self-
assembled polyrotaxanes in the presence of CDs.[11a]
Herein, we demonstrate the use of CD polyrotaxanes to
create supercrystals of highly ordered supramolecularly
stabilized spherical gold nanocrystals, which exhibit reversi-
ble crystallization (Scheme 1). The supramolecules were
prepared in water by a-CD complexation of a new thiol-
functionalized Igepal surfactant IgeSH (Scheme 1; see Sup-
porting Information for synthesis and characterization) which
stabilizes the surface of the nanocrystals. The choice of the
Igepal as capping agent is dictated by its high tendency to
form CD polyrotaxanes,[11] the luminescent properties of such
amphiphiles and their CD complexes,[11b] and the versatility
that commercially available Igepal surfactants have shown as
stabilizers of metal nanoparticles.[12]
I
n recent years, supramolecular chemistry has focused on the
organization of molecular systems in the solid state and/or at
solid–liquid interfaces,[1] in analogy to what has been pre-
viously done at the molecular level in solution, to develop
new functional materials.[2] Of particular interest are arrange-
ments of supramolecules on the surface of inorganic nano-
particles,[3] with the aim that self-assembly should occur to
form nanostructures that are able to perform functions not
present in their individual components.[4] However, control of
supramolecular self-assembly to produce large arrays with
defined geometrical arrangements of the nanoparticles
[*] J. P. Coelho, G. Gonzꢀlez-Rubio, A. Delices, Prof. G. Tardajos,
Dr. A. Guerrero-Martꢁnez
Departamento de Quꢁmica Fꢁsica I
Universidad Complutense de Madrid
Avda. Complutense s/n, 28040, Madrid (Spain)
E-mail: aguerrero@quim.ucm.es
Prof. J. O. Barcina, C. Salgado
An inclusion complex was formed between a-CD and
IgeSH by mixing both compounds in water (Figure 1a, and
see the Supporting Information). The cloudiness of the IgeSH
solution, which is related to the presence of large micelles,[13]
decreased remarkably because of the disruption of aggregates
by CD complexation. After 24 hours, a white powder is
obtained (Figure 1a). The powder was analyzed by X-ray
powder diffraction (XRPD, see Supporting Information) and
transmission electron microscopy (TEM, Figure 1b). The
amorphous nature of the solid indicates a random arrange-
ment of the complexes within the precipitate. The stoichiom-
Departamento de Quꢁmica Orgꢀnica I
Universidad Complutense de Madrid
Avda. Complutense s/n, 28040, Madrid (Spain)
Dr. D. ꢂvila
Departamento de Quꢁmica Inorgꢀnica
Universidad Complutense de Madrid
Avda. Complutense s/n, 28040, Madrid (Spain)
Dr. O. PeÇa-Rodrꢁguez
Instituto de Fusiꢃn Nuclear, Universidad Politꢄcnica de Madrid
Josꢄ Gutiꢄrrez Abascal 2, 28006, Madrid (Spain)
[**] This work has been funded by the Spanish MINECO (CTQ2010-
18564). J.P.C. acknowledges receipt of a CiÞncias sem Fronteiras
fellowship from the CNPq of Brazil. A.G.-M. acknowledges receipt of
a Ramꢃn y Cajal Fellowship from the Spanish MINECO.
1
etry of the polyrotaxane has been determined by H NMR
spectroscopy (Supporting Information), showing the forma-
tion of IgeSH:CD complexes in a 1:4 ratio. The assembly and
precipitation of such supramolecules is driven by strong
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201406323.
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tion), driven by the stronger interaction of thiol groups with
the gold surface. The UV/Vis absorption spectrum of the
surfactant-stabilized nanospheres (IgeS-AuNPs) in aqueous
solution shows two bands with maxima at l = 220 and 278 nm
which correspond to the Igepal absorption bands, and
a characteristic localized surface plasmon resonance (LSPR)
band at l = 524 nm. This LSPR band is red-shifted by 7 nm
compared to the initial cit-AuNPs (Figure 2a). This shift is in
Scheme 1. Chemical structure of the nonionic Igepal surfactant IgeSH
with hydrophobic (gray) and hydrophilic (black) regions. a) Preparation
of the amorphous nonreversible solid network of polyrotaxanes with
IgeSH and a-CD in water. b) Preparation of the reversible supercrystals
of polyrotaxane-stabilized spherical gold nanoparticles (PolyRot-
AuNPs) in water.
Figure 2. a) Normalized UV/Vis absorption spectra of gold nano-
spheres stabilized with citrate ions in aqueous solution (1), in the
presence of an excess of the IgeSH surfactant (2), after removing the
excess of IgeSH (3), and upon addition of a-CD (4). Inset: expanded
portion of the spectrum (l=500–550 nm) showing the LSPR bands.
b) Emission spectra of IgeS-AuNPs (2ꢅ10À10 m) in a-CD mixtures
(lexc =220 nm). The arrow indicates the critical concentration of a-CD
(ca. 1:500 IgeS-AuNPs:a-CD) after which point the emission intensity
of IgeS-AuNPs increases with increasing concentration of macrocycle.
Inset: relative emission intensities versus the ratio between capping
agent and a-CD (solid circles), and free IgeSH and a-CD (open
circles).
good agreement with an increase in the local refractive index
of the gold nanospheres after IgeSH binding.[14] Similarly to
the polyrotaxanes, IgeS-AuNPs in the presence of free IgeSH
precipitate upon addition of a-CD (Figure 1a). After
24 hours, the resulting dehydrated red powder was charac-
terized by XRPD (see Supporting Information) and TEM
(Figure 1c), showing an amorphous nanocomposite in which
nanocrystals are randomly assembled by CD complexation
of the capping agents within the disordered polyrotaxane
network.
The complexation process between IgeS-AuNPs and
a-CD has been investigated by fluorescence measurements,
upon addition of the macrocycle to a highly diluted solution of
nanoparticles in which the excess of surfactant was previously
removed by centrifugation (Figure 2b). The concentration of
capping molecules around a nanocrystal has been estimated
at approximately 5 ꢀ 103 molecules (see Supporting Informa-
tion). The emission spectra exhibit a wide emission band
centered at l = 300 nm[11b] that is not quenched by the metal
core (Figure 2b). Although no significant changes of the IgeS-
AuNPs LSPR band have been detected upon macrocycle
addition (inset in Figure 2a), an enhancement of the fluores-
Figure 1. a) Evolution of a cloudy solution of IgeSH (1 mm) in the
absence (1) and the presence (2) of a-CD (10 mm) after 24 hours.
Evolution of the same solutions of IgeSH with added IgeS-AuNPs
(4ꢅ10À9 m), in the absence (3) and the presence (4) of a-CD. b) TEM
micrograph of the amorphous polyrotaxane solid obtained in (2).
c) TEM micrograph of the gold nanocrystalline nanocomposite
obtained in (4). Scale bar in (b) and (c)=100 nm.
intermolecular interactions because of the spatial anisotropy
of their dielectric properties, reducing the hydration states of
host and guest molecules.[10]
To test the ability of the polyrotaxanes to direct the self-
assembly of gold nanocrystals, we synthesized monodisperse
citrate-stabilized spherical gold nanoparticles (cit-AuNPs) of
16.0 Æ 1.2 nm in diameter. The citrate ions were subsequently
replaced in water with the IgeSH (see Supporting Informa-
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cence intensity of IgeS-AuNPs has been detected (Figure 2b).
This evidence suggests a stabilization of the phenyl group
within the hydrophobic CD cavity, as a consequence of the
dehydration of the aromatic moiety during the polyrotaxane
formation. Interestingly, a critical concentration of a-CD was
needed to initiate such enhancement (inset in Figure 2b; see
also the Supporting Information), showing that the macro-
cycles are cooperatively incorporated through the oxy-
ethylene chains until they reach the aromatic moiety. There-
fore, thousands of polyrotaxanes may stabilize a unique gold
nanocrystal in water (PolyRot-AuNPs, Scheme 1).
Although no crystallization has been registered between
PolyRot-AuNPs in solution, even at relatively high concen-
trations of nanoparticles (ca. 10À6 m) and a-CD (solubility ca.
1.5 ꢀ 10À1m) and with a preparation time of one month,
a spontaneous formation of self-assembled layers of nano-
particles has been obtained by simple drop-casting on carbon-
coated TEM grids (Figure 3; see also the Supporting Infor-
mation). Whereas relatively low-ordered assemblies of
IgeS-AuNPs were detected in the absence of a-CD (Figur-
es 3a,c), well-defined islands of perfectly ordered layers of
nanoparticles were formed in the presence of the macrocycle
(Figure 3b). Figure 3d shows a representative TEM image of
a highly ordered, bilayer array of PolyRot-AuNPs with
hexagonal packing, where the octahedral and tetrahedral
holes can be perfectly distinguished. The fast Fourier trans-
form (FFT) of the image confirms a perfect hexagonal
geometry, with an average distance between the centers of
neighboring PolyRot-AuNPs of 19.2 nm, and a minimum
distance of 3.2 nm between PolyRot-AuNPs. In the polyrot-
axane structure, both the hydrophobic and hydrophilic
regions of the surfactant, when completely extended with
respect to the aromatic group (3.3 nm in length), are threaded
by four head-to-head a-CDs (0.8 nm in height each).[15] With
this structure in mind, we propose a simple molecular model
in which the supramolecules self-assemble into monolayers at
the surface of the nanocrystals during formation of the
hexagonal superlattices (Scheme 1).
Figure 3. TEM micrographs of a) IgeS-AuNPs and b) PolyRot-AuNPs
assemblies (NP concentration=4ꢅ10À7 m). c) Micrograph showing
the packing of IgeS-AuNPs. d) Micrograph showing a hexagonally
packed bilayer of PolyRot-AuNPs and the corresponding FFT of the
image (inset). Scanning electron microscopy (SEM) micrographs of
e) IgeS-AuNPs and f) PolyRot-AuNPs assemblies (NP concentra-
tion=4ꢅ10À7 m). g) Expanded portion of image (e) showing the
packing of IgeS-AuNPs. h) Expanded portion of image (f) showing
a hexagonally packed multilayer of PolyRot-AuNPs.
In good agreement with previous studies of AuNPs
supercrystal preparation,[16] the drop-casting of cit-AuNPs
and IgeS-AuNPs solutions onto quartz wafers under opti-
mized temperature and humidity conditions (see Supporting
Information), typically resulted in the formation of coffee-
ring deposits of approximately 0.5 mm width and 6 mm
external diameter (inset, Figure 4a), with a significant
amount of inner cracks (Figures 3e,g). In contrast, a macro-
scopically homogenous pattern of PolyRot-AuNPs has been
obtained under the same drop-casting conditions (ca. 4 mm
wide, 12.6 mm2 area; see Supporting Information). The
absence of a coffee-ring deposit and the lower dimension of
the PolyRot-AuNPs pattern point to strong hydrophobic
interactions between nanocrystals that avoid Marangoni
effects during the drying process.[17] As shown in Figures 3 f
and h, PolyRot-AuNPs self-assembled into supercrystals
spanning approximately 4 mm in width (inset, Figure 4a)
with an extraordinary long-range order. Figure 4a shows the
UV/Vis absorption spectra of the different AuNP deposits
upon excitation by nonpolarized irradiation perpendicularly
oriented to the substrates (Supporting Information). Whereas
large broadenings of the LSPR bands occur in the case of
cit-AuNPs and IgeS-AuNPs indicating low nanocrystal order-
ing, a narrow and intense LSPR band is obtained in the case of
the PolyRot-AuNPs deposit, as a consequence of plasmon
coupling between the highly ordered close-packed nano-
particles at the supercrystals (see simulated optical properties
in the Supporting Information).[18] Additionally, the PolyRot-
AuNPs deposit leads to highly reproducible LSPR bands
within the long-range area of the substrate (Figure 4a), unlike
the significant differences in the LSPR band which have been
detected at the ring and center of the drop patterns of
cit-AuNPs and IgeS-AuNPs. The high homogeneity of the
PolyRot-AuNPs deposit may open up applications in the
difficult task of imprinting metal nanoparticles from aqueous
solutions.[19]
Taking into account the reversibility of supramolecular
processes in solution,[20] we investigated the origin of the
interactions between the self-assembled nanocrystals within
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focusing on the preparation of new longer thiol-functional-
ized Igepal surfactants, which could stabilize nanocrystals
with different morphologies and sizes (> 20 nm) upon addi-
tion of a-CD.
Received: June 17, 2014
Revised: September 3, 2014
Published online: && &&, &&&&
Keywords: cyclodextrins · gold · nanoparticles · rotaxanes ·
.
self-assembly
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Communications
Nanoparticle Self-Assembly
J. P. Coelho, G. Gonzꢁlez-Rubio,
A. Delices, J. O. Barcina, C. Salgado,
D. ꢂvila, O. PeÇa-Rodrꢃguez, G. Tardajos,
A. Guerrero-Martꢃnez*
&&&& — &&&&
Polyrotaxane-Mediated Self-Assembly of
Gold Nanospheres into Fully Reversible
Supercrystals
A golden get-together: A thiol-functional-
ized nonionic surfactant and a-cyclodex-
trin undergo complexation at the surface
of gold nanoparticles to form polyrot-
axane-stabilized nanocrystals. Under
controlled drying conditions, the nano-
crystals can reversibly self-assemble into
homogenous hexagonal closed-packed
supercrystals.
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