Sequential self-assembly for construction of Pt(ii)-bridged [3]rotaxanes on gold nanoparticles

Article abstract of DOI:10.1039/c2cc17114g

An effective self-assembly route for construction of Pt(ii)-bridged [3]rotaxane functionalized gold nanoparticles was developed through a stepwise introduction of Pt(ii) coordination and gold-thiol binding in turn. The sequential self-assembly process can be quantitatively controlled and directly monitored by the naked eye. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc17114g

View Article Online / Journal Homepage / Table of Contents for this issue
As featured in:
Showcasing research from Yanli Zhao’s laboratory,
Nanyang Technological University, Singapore
Sequential self-assembly for construction of Pt(II)-bridged
[3]rotaxanes on gold nanoparticles
An effective self-assembly route for the construction
of Pt(II)-bridged [3]rotaxane functionalized gold nanoparticles
was developed through a stepwise introduction
of Pt(II) coordination and gold-thiol binding. The sequential
self-assembly process can be quantitatively controlled
and directly monitored by naked eye.
See Yanli Zhao et al.,
Chem. Commun., 2012, 48, 4290.
www.rsc.org/chemcomm
Registered Charity Number 207890

Dynamic ArticVleiewLiAnrktiscle Online
ChemComm
Cite this: Chem. Commun., 2012, 48, 4290–4292
www.rsc.org/chemcomm
COMMUNICATION
Sequential self-assembly for construction of Pt(II)-bridged [3]rotaxanes
on gold nanoparticlesw
Liangliang Zhu,a Hong Yan,a Kim Truc Nguyen,a He Tianb and Yanli Zhao*ac
Received 16th November 2011, Accepted 19th January 2012
DOI: 10.1039/c2cc17114g
An effective self-assembly route for construction of Pt(II)-bridged
[3]rotaxane functionalized gold nanoparticles was developed through
a stepwise introduction of Pt(II) coordination and gold–thiol binding
in turn. The sequential self-assembly process can be quantitatively
controlled and directly monitored by the naked eye.
with a substrate via sequential self-assembly is a principle for
building special ensembles,6 we designed a rod-like compound
[L][NO3] containing lipoic acid and 4,40-bipyridyl moiety as a
building block. We found that both the pyridyl nitrogen and the
1,2-dithiolane unit in [L][NO3] can be attached to the surfaces of
AuNPs forming inter-particle clusters. Ethylenediamine Pt(II)
nitrate (enPt) was employed to coordinate with two pyridyl
nitrogens in two ligand molecules by a strong orthogonal
bis-coordination mode,8 avoiding the attachment of pyridyl units
onto AuNPs. In addition, the azobenzene unit in the middle of
[L][NO3] can be effectively complexed with a-cyclodextrin (a-CD)
to form a switchable rotaxane prototype. The introduction of the
a-CD ring enhances the water-solubility of the system, which is
beneficial to the bis-coordination between enPt and [L][NO3]
conducted in water.9
Scientists have been engaged in the development of mechanically
bonded molecular systems (e.g. rotaxanes and catenanes) for years,
as these charming systems have great potential for applications
in molecular switches and machines.1 Coordination-assisted self-
assembly is becoming a promising methodology for construction
of advanced rotaxane-type structures2 since it allows for a
spontaneous, selective, and highly efficient process. An intriguing
challenge next is how to enlarge the architectures and properties
of the advanced systems in order to achieve an integral device
function. One of the supportive materials that can be relied on for
the purpose is gold nanoparticles (AuNPs), which have attracted
considerable attention on account of their many fascinating
physical, optical, and chemical properties.3 Functionalizing
switchable rotaxanes onto AuNPs will not only make the triggered
motions of the rotaxanes behave coherently relative to the surfaces,4
but also can expand the performance for designing indicators or
digital information processors at the nanoscale.5 The construction
of metal-coordinated rotaxanes on AuNPs has not been reported in
the literature, to the best of our knowledge, simply because different
self-assembly possibilities are present in the preparation steps.
Herein, we report an effective way via sequential self-assembly6 to
overcome the obstacle for fabricating Pt(II)-bridged [3]rotaxane
functionalized gold nanoparticles.
To realize the formation of the metal-coordinated rotaxanes
on AuNPs, two possible routes for stepwise self-assembly need to
be discussed: one beginning with the gold–thiol binding followed
by Pt(II) coordination, the other one following the reverse
sequence of the two processes. The two routes were, respectively,
monitored by surface plasmon resonance (SPR) as well as the
solution color change. Transmission electron microscopy (TEM)
and surface-enhanced Raman scattering (SERS) techniques were
also employed to investigate the sequential self-assembly.
The chemical structures of the starting materials and their
self-assembly processes on AuNPs are outlined in Fig. 1.
[L][NO3] was complexed with the a-CD ring to form a stable 1 : 1
host–guest supramolecule [L(a-CD)][NO3] in aqueous solution
with a relatively high association constant (1830 ꢀ 95 Mꢁ1). Thus,
a considerable amount of the host–guest supramolecule can be
attached onto the AuNP surfaces although there is equilibrium
between the host–guest supramolecule and its components. When
the citrate-stabilized AuNP (average diameter of 15 nm) aqueous
solution was mixed with the [L(a-CD)][NO3] aqueous solution,
black precipitates were produced immediately (State c in Fig. 2A).
The precipitates are assigned to the aggregation of [L(a-CD)][NO3]-
bridged AuNPs, evidenced by the TEM image shown in Fig. 2B. In
this case, both the pyridyl nitrogen and the 1,2-dithiolane unit on
[L(a-CD)][NO3] bound to the surfaces of AuNPs, leading to the
formation of inter-particle clusters (see the schematic representation
of Au@[L(a-CD)][NO3] in Fig. 1). An energy-minimized
molecule model shows that the length of [L(a-CD)][NO3] is
2.953 nm, well matching with the surface distance in some
adjacent AuNPs (Fig. S3 in the ESIw). Moreover, a strong SERS
The thiol group has a very high affinity to the surfaces of
AuNPs whereas the pyridyl nitrogen can coordinate to AuNPs
through weak interactions.7 On the other hand, Pt(II) is prone
to generate strong and quantitative coordination with pyridine
derivatives.8 While competitive interaction of different species
a Division of Chemistry and Biological Chemistry, School of Physical
and Mathematical Sciences, Nanyang Technological University,
21 Nanyang Link, Singapore 637371, Singapore.
E-mail: zhaoyanli@ntu.edu.sg
b Key Laboratory for Advanced Materials and Institute of Fine
Chemicals, East China University of Science & Technology,
Shanghai 200237, P.R. China
c School of Materials Science and Engineering, Nanyang
Technological University, Singapore 639798, Singapore
w Electronic supplementary information (ESI) available: Synthesis
and characterization data. See DOI: 10.1039/c2cc17114g
c
4290 Chem. Commun., 2012, 48, 4290–4292
This journal is The Royal Society of Chemistry 2012

View Article Online
[L(a-CD)][NO3] solution leads to a quantitative formation of
the complex [Pt–2L(a-CD)2][NO3]4 (Fig. S6 in the ESIw).
Protons Ha and Hb of the pyridyl unit display great downfield
shifts (from 7.75 to 8.45 ppm for Ha and from 8.63 to 8.80 ppm
for Hb) because of the Pt(II) coordination with the pyridine
nitrogen of [L(a-CD)][NO3]. A new peak at d = 2.64 ppm is
assigned to the proton signal of the symmetric methylene in
the ethylene diamine group. 1H NOESY NMR spectroscopy of
[Pt–2L(a-CD)2][NO3]4 confirms that the a-CD ring dominantly
stays at the azobenzene station (Fig. S8 in the ESIw). ESI–mass
spectrometry provides further evidence for the formation of
the bis-coordinated ensemble. In the mass spectrum of
[Pt–2L(a-CD)2][NO3]4, peaks at m/z = 1145.13 and 842.67,
corresponding to [M ꢁ 3NO3]3+ and [M ꢁ 4NO3]4+ respectively,
are observed (Fig. S10 in the ESIw).
When the citrate-stabilized AuNP aqueous solution was
added to the aqueous solution of [Pt–2L(a-CD)2][NO3]4, a
totally different phenomenon was observed. The color of the
[Pt–2L(a-CD)2][NO3]4 solution turns from yellow to reddish
purple when mixed with the AuNP solution. The mixture solution
is rather stable and there is no precipitate generated even after
keeping it for several days (State e in Fig. 2A). The TEM image
(Fig. 2C) shows that AuNPs are mainly mono-dispersed and a
few AuNP oligomerization observed should be originated
from the inter-particle attachment of [Pt–2L(a-CD)2][NO3]4.
The thickness of the outer layer around a nanoparticle is
basically consistent with the height (3.262 nm) of the energy-
minimized molecule model [Pt–2L(a-CD)2][NO3]4 (Fig. S11 in the
ESIw). Furthermore, a relatively strong SERS signal (curve e,
Fig. S4 in the ESIw) was observed in this state as compared to that
of the free ligand solution, indicating that [Pt–2L(a-CD)2][NO3]4 is
attached on the surfaces of AuNPs and encapsulates AuNPs.10
The different observations between States d and e demonstrate
that the orthogonal bis-coordination between one Pt(II) center and
two [L(a-CD)][NO3] enables the two 1,2-dithiolane units to attach
onto the same nanoparticle (see the schematic representation of
Au@[Pt–2L(a-CD)2][NO3]4 in Fig. 1).
As seen from Fig. 2A, only the solution color of State e is
reddish purple, the colors of the rest four solutions remain
yellow, while the AuNP precipitates were formed in States
c and d. Thus, we can visually tell State e from other states by
the naked eye. In addition, the extinction spectra provide more solid
information to distinguish these states. The absorption band of the
azobenzene unit in the ligand is around 350 nm, while the one of the
pyridyl unit is below 300 nm. Therefore, the ligand compounds
have no extinction over a wavelength of 500 nm (see curves a and b
in Fig. 3). The AuNP aggregates should have a shifted SPR band at
the long-wavelength region (more than 650 nm). However, this
band cannot be stably recorded from the supernatants of States
c and d because of the formation of the AuNP precipitates
(see curves c and d in Fig. 3). Only State e reveals a characteristic
strong SPR peak around 530 nm (see curve e in Fig. 3), which is
originated from the [Pt–2L(a-CD)2][NO3]4-capped AuNPs. This
extinction signal is stable and long-standing.
Fig. 1 Illustration of the self-assembly routes for AuNP–Pt(II)–
rotaxane hybrids. Step A: bis-coordination by adding 0.5 equiv. of
enPt; Step B: gold–thiol binding by adding 14.25 nM of AuNPs.
Fig. 2 (A) [L(a-CD)][NO3] aqueous solution added with (a) H2O,
(b) enPt, (c) AuNPs, (d) AuNPs followed by enPt, and (e) enPt followed
by AuNPs. 0.5 equiv. of enPt was added into the [L(a-CD)][NO3] solution.
The final concentrations of [L(a-CD)][NO3] and AuNPs were ca. 3.3 mM
and 4.8 nM, respectively. (B) TEM image of State c. (C) TEM image of
State e.
signal10 was observed for the [L(a-CD)][NO3]-bridged AuNPs.
Since [L(a-CD)][NO3] has almost no Raman signal under the
same conditions, the strong SERS signal is attributed to the
[L(a-CD)][NO3]-bridged AuNP assembly (Fig. S4 in the ESIw).
Once the precipitates are formed, it is difficult to re-disperse
them even by sonication. When enPt (0.5 equiv. to the original
[L(a-CD)][NO3]) is introduced, the AuNP precipitates still
cannot be re-dispersed and the resulting State d (Fig. 2A)
looks the same as State c when monitored by the naked eye
and determined by UV-Vis and Raman spectroscopies. The
phenomena indicate that enPt cannot effectively penetrate into
the precipitates to dissociate the AuNP clusters. Therefore,
Pt(II)-bridged [3]rotaxanes on AuNPs cannot be generated via
such a self-assembly sequence.
Next, we exchanged the self-assembly sequence in the
preparation process. [L(a-CD)][NO3] quickly coordinated with
enPt in aqueous solution at room temperature in a few minutes.9
The 1H NMR titration experiments show that bis-coordination is
mainly ratio-dependent, and adding 0.5 equiv. of enPt into the
The [Pt–2L(a-CD)2][NO3]4-capped AuNPs can be facilely
isolated from the ligand solution. The average molar ratio
between the bis-coordinated ligand [Pt–2L(a-CD)2][NO3]4 and
AuNPs is 210 : 1 (see the calculation procedures in the ESIw),
hence the absorption peak around 350 nm in the extinction
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4290–4292 4291

View Article Online
process can be quantitatively controlled and is accompanied
by apparent surface plasmon resonance as well as the solution
color change. Thanks to the photo-switching property of the
[3]rotaxanes on AuNPs, the present hybrid system may find its
applications in nanoscale digital information processing.
We thank the Singapore National Research Foundation
Fellowship (NRF2009NRF-RF001-015) and Nanyang Techno-
logical University for the financial support.
Notes and references
1 (a) V. Balzani, A. Credi and M. Venturi, Chem. Soc. Rev., 2009,
38, 1542; (b) L. Fang, M. A. Olson, D. Benıtez, E. Tkatchouk,
´
W. A. Goddard III and J. F. Stoddart, Chem. Soc. Rev., 2010, 39,
17–29; (c) S. Durot, F. Reviriego and J.-P. Sauvage, Dalton Trans.,
2010, 39, 10557; (d) D. H. Qu and H. Tian, Chem. Sci., 2011,
2, 1011; (e) J. E. Beves, B. A. Blight, C. J. Campbell, D. A. Leigh
and R. T. McBurney, Angew. Chem., Int. Ed., 2011, 50, 9260.
2 (a) L. L. Zhu, M. Q. Lu, D. H. Qu, Q. C. Wang and H. Tian, Org.
Biomol. Chem., 2011, 9, 4226; (b) T. Taira, Y. Suzaki and
K. Osakada, Chem.–Asian J., 2008, 3, 895; (c) G. Koshkakaryan,
K. Parimal, J. M. He, X. Y. Zhang, Z. Abliz, A. H. Flood and
Y. Liu, Chem.–Eur. J., 2008, 14, 10211.
3 (a) A. Coskun, P. J. Wesson, R. Klajn, A. Trabolsi, L. Fang,
M. A. Olson, S. K. Dey, B. A. Grzybowski and J. F. Stoddart,
J. Am. Chem. Soc., 2010, 132, 4310; (b) A. Housni, Y. Zhao and
Y. Zhao, Langmuir, 2010, 26, 12366; (c) Y.-L. Zhao, Y. Chen,
M. Wang and Y. Liu, Org. Lett., 2006, 8, 1267.
4 (a) J. J. Davis, G. A. Orlowski, H. Rahman and P. D. Beer, Chem.
Commun., 2010, 46, 54; (b) R. A. van Delden, M. K. J. ter Wiel,
M. M. Pollard, J. Vicario, N. Koumura and B. L. Feringa, Nature,
2005, 437, 1337; (c) Y. Duo, S. Jacob and W. Abraham, Org.
Biomol. Chem., 2011, 9, 3549.
5 (a) J. S. Lee, M. S. Han and C. A. Mirkin, Angew. Chem., Int. Ed.,
2007, 46, 4093; (b) D. Li, A. Wieckowska and I. Willner, Angew.
Chem., Int. Ed., 2008, 47, 3927; (c) D. B. Liu, W. W. Chen, K. Sun,
K. Deng, W. Zhang, Z. Wang and X. Y. Jiang, Angew. Chem., Int.
Ed., 2011, 50, 4103.
6 (a) F. A. Aldaye and H. F. Sleiman, Angew. Chem., Int. Ed., 2006,
45, 2204; (b) J. G. Son, A. F. Hannon, K. W. Gotrik,
A. Alexander-Katz and C. A. Ross, Adv. Mater., 2011, 23, 634.
7 (a) R. A. Sperling and W. J. Parak, Philos. Trans. R. Soc. London,
Ser. A, 2010, 368, 1333; (b) X. R. He, H. B. Liu, Y. L. Li, S. Wang,
Y. J. Li, N. Wang, J. C. Xiao, X. H. Xu and D. B. Zhu, Adv.
Mater., 2005, 17, 2811.
8 (a) M. Fujita, F. Ibukuro, H. Hagihara and K. Ogura, Nature,
1994, 367, 720; (b) C. Peinador, E. Pıa, V. Blanco, M. D. Garcıa
´ ´
and J. M. Quintela, Org. Lett., 2010, 12, 1380; (c) L. L. Zhu,
M. Q. Lu, Q. W. Zhang, D. H. Qu and H. Tian, Macromolecules,
2011, 44, 4092.
Fig. 3 The corresponding extinction spectra of the five states described
in Fig. 2A.
spectra (curve e1 of Fig. 4) is originated from the trans-
azobenzene unit of the capped ligand. The a-CD ring located
on the trans-azobenzene unit is certified by a positive signal at
350 nm in the induced circular dichroism (ICD) spectrum of
the ligand (Fig. S12 in the ESIw).11 Photoirradiation using
365 nm UV light causes the trans-to-cis photoisomerization of the
azobenzene unit, transforming Au@[Pt–2L(a-CD)2][NO3]4 to its
photostationary state e2. The a-CD ring moves to the rim of the
azobenzene unit, driven by the trans-to-cis photoisomerization.12
The movement is evidenced by a reduction of the positive ICD
signal and an enhancement of the negative one (Fig. S12 in the
ESIw).11 The photo-switchable process of the a-CD-based
[3]rotaxane in Au@[Pt–2L(a-CD)2][NO3]4 is reversible and
repeatable by alternating the photoirradiation with 365 nm
UV light and visible light (inset figure of Fig. 4).
In summary, we have developed a unique protocol for the
preparation of Pt(II)-bridged [3]rotaxane capped AuNPs
through a stepwise introduction of the Pt(II) coordination
and the gold–thiol binding. The key step for this methodology
is the successful preparation of a bis-coordinated complex as
the intermediate. The strong orthogonal bis-coordination
mode avoids the attachment of pyridyl units onto AuNPs so
that it ensures the thiol group binding to the gold surface to
form the desired hybrid. Such a sequential self-assembly
9 The bis-coordinated reaction between enPt and [L][NO3] without
a-CD in DMSO is relatively slow, lasting more than 10 h. The
resulting complex [Pt-2L][NO3]4 can be attached onto AuNPs in a
DMSO–H2O mixture solution using a similar procedure.
10 SERS can be found on a Raman label grafted to the surfaces of
AuNPs, and it will be enhanced more when this label is linked between
AuNPs. (a) H. Yan, S. I. Lim, Y.-J. Zhang, Q. Chen, D. Mott,
W.-T. Wu, D.-L. An, S. Q. Zhou and C.-J. Zhong, Chem. Commun.,
2010, 46, 2218; (b) C. A. Otter, P. J. Patty, M. A. K. Williams,
M. R. Waterland and S. G. Telfer, Nanoscale, 2011, 3, 941.
11 The positive ICD signal can be attributed to the electronic transi-
tion p–p* of the azobenzene unit, when the azobenzene unit locates
in the cavity of the a-CD ring and aligns parallel to the axis of the
chiral host, whereas the negative one belongs to the transition n–p*
of the azobenzene unit aligned vertical to the axis of the a-CD ring.
(a) M. Kodaka, J. Phys. Chem. A, 1998, 102, 8101; (b) Y. Liu,
Y.-L. Zhao, H. Y. Zhang, Z. Fan, G. D. Wen and F. Ding, J. Phys.
Chem. B, 2004, 108, 8836; (c) L. L. Zhu, X. Ma, F. Y. Ji,
Q. C. Wang and H. Tian, Chem.–Eur. J., 2007, 13, 9216.
12 (a) I. Tomatsu, A. Hashidzume and A. Harada, J. Am. Chem. Soc.,
2006, 128, 2226; (b) Y.-L. Zhao and J. F. Stoddart, Langmuir,
2009, 25, 8442.
Fig. 4 The extinction spectra of the AuNP–Pt(II)–rotaxane hybrid
Au@[Pt–2L(a-CD)2][NO3]4 before (e1) and after (e2) photoirradiation
under 365 nm UV light. Inset: changes in the extinction signal at 350 nm
for several cycles. In one cycle, photoirradiations of 365 nm UV light and
visible light were alternately employed to isomerize the azobenzene unit.
c
4292 Chem. Commun., 2012, 48, 4290–4292
This journal is The Royal Society of Chemistry 2012