Thio[2-(benzoylamino)ethylamino]-β-CD fragment modified gold nanoparticles as recycling extractors for [60]fullerene

Article abstract of DOI:10.1039/b507650a

Gold particles are modified with surface-attached bis(β-cyclodextrin)s bearing S-S bridges to give water-soluble cyclodextrin-modified gold nanoparticles, which are successfully used as recycling extractors for [60]fullerene. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b507650a

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Thio[2-(benzoylamino)ethylamino]-b-CD fragment modified gold
nanoparticles as recycling extractors for [60]fullerene{
Yu Liu,* Ying-Wei Yang and Yong Chen
Received (in Cambridge, UK) 2nd June 2005, Accepted 27th June 2005
First published as an Advance Article on the web 21st July 2005
DOI: 10.1039/b507650a
Gold particles are modified with surface-attached bis(b-
cyclodextrin)s bearing S–S bridges to give water-soluble
cyclodextrin-modified gold nanoparticles, which are success-
fully used as recycling extractors for [60]fullerene.
Fullerenes, spherical p-electron systems containing tens of
unsaturated carbons, show many interesting magnetic,1 super-
conductive,2 electrical,3 and biochemical properties.4 Thus, full-
erenes and their derivatives have attracted extensive interest in
recent years and have been successfully applied in many fields of
materials science and biological technology. Because the fullerenes
prepared via the thermal reactions of appropriate carbon sources
under various conditions5 always exist as a mixture of C60, C70 and
higher homologues, the separation of fullerenes becomes a
Scheme 1
challenge for chemists. Generally, several methods are applied to
separate C60 from fullerene mixtures,6 such as sublimation, liquid
Tb-CD fragment modified gold nanoparticle 2 was prepared
chromatography, column chromatography, capillary electrophor-
esis and recrystallization. However, these methods usually require
a large amount of organic solvents, a long time course or special
instruments and do harm to the environment. Recently, some
approaches involving the inclusion of unmodified fullerenes into
water-soluble polymers and synthetic receptors, such as
poly(N-vinylpyrrolidone),7 cyclodextrin (CD),8 or calixarene,9 to
form host–guest inclusion complexes or supramolecular assemblies
have been reported. These approaches provide the possibility to
solubilize and separate fullerenes using the supramolecular
method. On the other hand, gold nanoparticles modified with
synthetic receptors recently represent a very active topic of science
and technology,10,11 due to their extensive application in a wide
variety of areas including optoelectronic nanodevices,11,12 chemical
sensors,13 nanotechnology,14 and biological sciences.15 In the
present work, we prepared a thio[2-(benzoylamino)ethylamino]-
b-CD (Tb-CD) fragment modified gold nanoparticle (Scheme 1),
and investigated its selective association and dissociation behavior
with C60. Because b-CD, a cyclic oligosaccharide with seven
D-glucose units linked by a-1,4-glucose bonds, can only form a 2 : 1
from HAuCl4 and bis(b-CD) 1. In the preparation, HAuCl4 was
reduced to form metallic gold nuclei. Subsequently, the fast
adsorption of disulfides onto the surface of gold nuclei broke the
S–S bond in 1 to give thio[2-(benzoylamino)ethylamino]-b-CD
fragment modified gold nanoparticles through the formation of
S–Au bond.17,18 The particle formation was evidenced by the
obvious color changes of the solution from yellow to deep brown.
The resulting Tb-CD fragment modified gold nanoparticles can be
isolated as a deep brown solid and show a very good solubility in
water, where they can remain dissolved yielding a stable, clear
solution without any apparent aggregation or precipitation for
several weeks as simply monitored by UV-vis measurements. Some
spectral and microscopic data give useful information about the
structure of the Tb-CD fragment modified gold nanoparticle 2.
The FT-IR spectrum (see ESI{) of 2 strongly resembles that of the
free bis(b-CD) 1, which is consistent with previous observations in
similar systems.19 Moreover, no S–H stretching band at ca.
2560 cm21 can be found in the FT-IR spectrum of 2, indicating the
conversion of the S–S bond in bis(b-CD) 1 to the S–Au bond when
bis(b-CD) 1 chemisorbs on the surface of gold nanoparticles. In
addition, the 1H NMR spectrum of 2 also displays the
characteristic signals of the corresponding protons in 1, especially
the characteristic signals of aromatic protons and CD’s backbone
protons (C1–H, C2–H, C3–H, C4–H, C5–H, C6–H). These data
jointly afford evidence for the formation of Tb-CD fragment
modified gold nanoparticles. Particle size determinations using
transmission electron microscopy (TEM) yield an average particle
diameter of 6.6 ¡ 1.0 nm for gold nanoparticle 2 (Fig. 1a) with a
reasonable degree of monodispersity by individual measurements
on at least 200 particles (see ESI){. The surface coverage of CD
units on the nanoparticles is estimated as 65% by the elemental
analysis.19b
16
inclusion complex with C60 but does not interact with C70 and
higher fullerene analogues, this Tb-CD fragment modified gold
nanoparticle can be used as a recycling extractor for C60
(Scheme 2).
Department of Chemistry, State Key Laboratory of Elemento-Organic
Chemistry, Nankai University, Tianjin 300071, P. R. China.
E-mail: yuliu@public.tpt.tj.cn; Fax: +8622 2350 3625;
Tel: +8622 2350 3625
{ Electronic supplementary information (ESI) available: measurements;
preparation of 1 and 2; FT-IR spectra of 1 and 2; size distribution
histogram of 2; UV-vis spectra of 2, C60–2 assembly and the toluene
extract of C60–2 assembly; fluorescence spectra of 2 and C60–2 assembly.
See http://dx.doi.org/10.1039/b507650a
4208 | Chem. Commun., 2005, 4208–4210
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Scheme 2
Fig. 1 (a) A typical high-resolution TEM image of 2, which displays a representative selection of the nearly spherical nanoparticles. (b) A low-
magnification TEM image of C60-induced assembly of 2. (c) A representative high-resolution TEM image of C60-induced assembly of 2 after adding
2-adamantanol.
By examining the structure of 2, we can find that there are a
number of free CD cavities attached on the surface of gold
nanoparticles, all of which have the capability to bind C60 to form
2 : 1 inclusion complexes. Therefore, we can use these Tb-CD
fragment modified gold nanoparticles as captors for C60.
Commercially available C60 and C70 are used to test the selective
capturing ability of 2 toward fullerenes. In a typical experiment,
50 mg of 2 is dissolved in 80 mL of water, and then 10 mg of C60 is
added to the solution under stirring. After 3 days, the unreacted
solid is removed by filtration, and the filtrate is evaporated under
reduced pressure to dryness. The residue is dried under vacuum at
50 uC to give a deep brown solid (55 mg).
The association of C60 with 2 is proved by UV-vis and TEM.
The UV spectrum (See ESI{) of 2 (0.1 mg mL21) shows a
characteristic band for the surface plasmon resonance (SPR)
absorption of gold at 550 nm. Moreover, a shoulder peak at ca.
260 nm, which is assigned to the absorption of the phenyl chromo-
phore, is also observed. After reaction with C60 for about 1 h, the
SPR band of 2 red shifts to 594 nm, accompanied with an increase
of the scattering envelope. This red shift is attributed to the
coupled plasmon absorption of gold nanoparticles in close contact,
demonstrating the formation of particle aggregates.17 In addition,
besides the shoulder peak at 260 nm, a shoulder at ca. 340 nm,
which is assigned to the characteristic absorption band of C60, also
appears in the UV spectrum of the C60–2 assembly. Moreover, the
toluene extract of the C60–2 assembly gives a UV-vis spectrum that
presents the typical bands of C60, indicating the partial solubiliza-
tion of the C60 sample by Tb-CD fragment modified gold
nanoparticles in aqueous solution arising from the inclusion
complexation of attached b-CDs with C60. Further evidence comes
from the fluorescence behavior. In a control experiment, the
unmodified gold nanoparticles are nonfluorescent but the Tb-CD
fragment modified gold nanoparticle 2 emits fluorescence with the
emission maximum at ca. 410 nm in aqueous solution. After
reaction with C60, the C60–2 assembly emits stronger fluorescence
benefiting from the good photophysical property of C60 linkers
(see ESI{). Furthermore, the TEM image also validates the
aggregation of 2 in the presence of C60. Fig. 1b gives a rough
insight into the size and shape of the C60–2 assembly, which are in
sharp contrast to the well-isolated homogeneous nanoparticles of 2
in the absence of C60 (Fig. 1a). Therefore, we deduce a possible
structure (see ESI{) of the C60–2 assembly, where C60 molecules
actively behave as noncovalent inter-particle linkers to assemble the
discrete Tb-CD fragment modified gold nanoparticles into large
supramolecular architectures attributed to the stoichiometric 1 : 2
inclusion complexation between C60 and b-CD cavities.
Similar experiments were performed using C70 instead of C60,
but no inclusion and aggregation phenomena were found even
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after a longer time course. This can be explained from the size-fit
concept of host–guest complexation. CPK molecular model
experiments demonstrate that only C60 can form 1 : 2 inclusion
complexes with b-CDs, while C70 is too large to be accommodated
in the b-CD cavity.
This work was supported financially by NNSFC (No. 90306009,
20272028, 20402008, and 20421202) and the Tianjin Natural
Science Foundation (043604411).
Notes and references
After validating the effective capturing of the Tb-CD fragment
modified gold nanoparticle 2 towards C60, we subsequently
examined the controlled release of C60 from the assembly.
2-Adamentanol is reported to have a very high binding ability
with the CD cavity.20 Therefore, the guest molecule that is
included in the CD cavity should be driven out when
2-adamentanol is added to the system. In a typical experiment,
an excess amount of 2-adamantanol was added to an aqueous
solution of the C60–2 assembly, and the mixture was kept under
ultrasonic shake for 10 minutes. Then, the insoluble solid was
collected by filtration and washed with ethanol to remove the
unreacted 2-adamentanol, and the residue was characterized to be
C60. Moreover, by weighing the residue, we can deduce that 50 mg
of 2 could capture ca. 5 mg of C60. In addition, the TEM image
also confirms the effective release of C60. As seen from Fig. 1c, the
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1
free 2 as a precipitate. H NMR demonstrates that the included
2-adamantanol is negligible after treating the filtrate by ethanol.
In another experiment, an equimolar mixture of C60 and C70
was added to a solution of 2. After stirring the reaction mixture for
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amount of 2-adamantanol was added to the filtrate. The mixture
was kept under ultrasonic shake for 10 minutes. Then, the
insoluble solid was collected by filtration and washed with ethanol
to remove the unreacted 2-adamentanol, and the residue was
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mixed fullerene system.
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In conclusion, we succeed in achieving water-soluble Tb-CD
fragment modified gold nanoparticles through the association of
bis(b-CD)s possessing S–S bonds with gold particles. These nano-
particles can selectively capture C60 to form large supramolecular
assemblies, and the captured C60 can be sufficiently released by
adding 2-adamantanol. More importantly, this capture-release
process can be easily recycled under the appropriate conditions.
These results may not only have important applications in the
separation techniques of fullerenes, but may also open the door to
the design of functional hybrid materials based on supramolecular
systems.
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4210 | Chem. Commun., 2005, 4208–4210
This journal is ß The Royal Society of Chemistry 2005