A general and efficient method to form self-assembled cucurbit[n]uril monolayers on gold surfaces

Article abstract of DOI:10.1039/b719927a

A general protocol based on spontaneous adsorption of cucurbit[n]uril (CB[n]) molecules through a strong multivalence interaction between CB[n] and gold is described, by which the formation of self-assembled CB[n] monolayers on gold surfaces can be efficiently achieved. The Royal Society of Chemistry.

Full text of DOI:10.1039/b719927a

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A general and efficient method to form self-assembled cucurbit[n]uril
monolayers on gold surfacesw
Qi An, Guangtao Li,* Chengan Tao, Yan Li, Yiguang Wu and Weixia Zhang
Received (in Cambridge, UK) 3rd January 2008, Accepted 5th February 2008
First published as an Advance Article on the web 25th February 2008
DOI: 10.1039/b719927a
A
general protocol based on spontaneous adsorption of
ligand through the interaction between carbonyl and gold.11
Since CB[n] are molecules with multiple carbonyl groups, we
wondered whether they could also interact with gold. The
performed simple experiments with AuNPs and CB[n] con-
firmed our supposition. Upon the addition of a small amount
of aqueous CB[n] into a colloidal AuNP solution, the CB[n]
molecules actually act as glue to connect AuNPs together, and
finally, the resulting larger gold particle aggregates precipi-
tated from solution, clearly evidenced by TEM observation
and UV/Vis measurements (Fig. S1 and S2 in the ESIw).
Encouraged by this preliminary result, we immersed a gold
substrate into a 0.1 mM aqueous solution of CB[7] (or
saturated aqueous CB[6] or CB[8] solution), and after 24 h
exposure, were glad to find that a self-assembled CB[n] mono-
layer was really achieved.
cucurbit[n]uril (CB[n]) molecules through a strong multivalence
interaction between CB[n] and gold is described, by which the
formation of self-assembled CB[n] monolayers on gold surfaces
can be efficiently achieved.
Cucurbit[n]urils (CB[n]) are pumpkin-shaped macrocyclic mo-
lecules which are composed of glycoluril units and each
possess a hydrophobic cavity accessible through both identical
carbonyl-fringed portals.1 Due to their unique structure and
outstanding multiple recognition properties, the CB[n] family
holds high potential in the domain of supramolecular chem-
istry and has presented many exciting research results in recent
years.1 The binding properties of CB[n] to various guests as
well as their applications for constructing sensors, drug deliv-
ery and biomimetic systems et al. have been investigated
extensively.1–7
The formation of the CB[n] SAMs was first detected by
Fourier transform IR reflection absorption spectroscopy
(FT-IRRAS). In the FT-IRRAS spectrum (Fig. 1), two char-
acteristic peaks of CB[7] at 1751 cmÀ1 and 1474 cmÀ1 corre-
sponding to CQO and C–N stretching vibrations,10
respectively, were clearly detected. Compared with the spec-
trum of CB[7] obtained using a bulk KBr pellet (Fig. S3w), the
more intense peak at 1751 cmÀ1 (amide I) suggests that CB[7]
molecules rest with their carbonyls perpendicular with respect
to the gold surface, in accordance with the FT-IRRAS surface
selective rule.12 This configuration indicates that the upright
carbonyls contact with the gold surface and may provide the
binding force by chelation.
However, most research work involving CB[n] has been
conducted in bulk solution.1 The properties of the surface-
confined CB[n] moieties, which may differ from those in bulk
solution and are essential for many applications such as the
development of chemo- or biosensor systems, have rarely been
studied. The key factor that impedes such research is the
difficulty in arranging CB[n] molecules on surfaces because
of the difficulty in introducing functional groups in their
scaffolds.8,9 Although CB[n] monolayers on solid surfaces
have been achieved either by threading CB[n] molecules on
surface-attached molecules10 or by employing substituted CB
molecules,8 there still does not exist an efficient approach to
obtain self-organized monolayers on surfaces with their cav-
ities free for recognizing guests. Thus, it would be very
attractive and highly desirable to develop a simple, convenient
and efficient approach to construct CB[n] monolayers applic-
able for all members of the CB[n] family. In this work, such an
approach is described, by which self-assembled CB[n] mono-
layers can be generally and efficiently constructed on gold
surfaces without any modification or special treatment of the
CB[n] molecules, as shown in Scheme 1.
The presence of CB[7] molecules on the gold surface as well
as the interaction between the carbonyl groups and gold was
further confirmed by comparing the XPS spectra (Fig. 2b1–b3)
of a CB[7]-treated gold surface with those (Fig. 2a1–a3) of
The new approach described here is based on our previous
finding that acetone molecules can bind to the surface of
citrate-coated gold nanoparticles (AuNPs) by replacing the
Key Lab of Organic Optoelectronics and Molecular Engineering,
Department of Chemistry, Tsinghua University, 100084 Beijing,
China. E-mail: LGT@mail.tsinghua.edu.cn; Fax: +86-10-62792905;
Tel: +86-10-62792905
w Electronic supplementary information (ESI) available: Details of
synthesis and characterization. See DOI: 10.1039/b719927a
Scheme 1 Schematic illustration of the construction of a self-as-
sembled CB[n] monolayer on a gold surface (a), and inclusion com-
plexes (b, c).
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Fig. 2 High-resolution XPS spectra for C 1s, N 1s and O 1s. (a1–a3):
CB[7] deposited on Si substrate; (b1–b3): self-assembled CB[7] on
gold.
Fig. 1 FT-IRRAS spectrum of the CB[7] monolayer on a gold
rowest due to its smallest molecular scale among these three
kinds of CB[n]. More importantly, the hydrophobic cavities of
the attached CB[n] are accessible and their unique recognition
capability is maintained. When a CB[7]-modified electrode was
exposed to a 5 mM acetonitrile solution of ferrocene for 6 h,
the typical and well-defined redox signal of trapped ferrocene
was detected after thoroughly washing (Fig. 4b). The linear
relationship between scan rates and peak currents indicates
that the ferrocene molecules were confined to the electrode
surface. In contrast, due to the lack of a specific binding force,1
CB[6]-modified electrodes following the same incubation pro-
cess could not exhibit the similar response to ferrocene mole-
cule, as expected (Fig. S4w). In the case of CB[8], however, a
trace signal of ferrocene was detected after the same washing
process (Fig. S4w), indicating that the trapped ferrocene
molecules in CB[8] are easily removable, due to the larger
cavity of CB[8].
surface.
CB[7] deposited on an inert silicon surface, on which no
interaction should exist. Apparently, the peak corresponding
to O 1s broadens and moves to a higher binding energy region
in the sample of CB[7]-treated gold. This result fits well with
our argument that the O atoms of one carbonyl-fringed portal
should interact with gold and bear a higher binding energy.
The overlap of the spectrum of interacting O atoms (532.5 eV)
upon that of the regular ones (531.9 eV) of another carbonyl-
fringed portal broadens the peak (Fig. 2b3). The peak corre-
sponding to the C 1s region was also influenced. The regular C
1s spectrum comprises three distinct peaks (Fig. 2a1), corre-
sponding to contamination (285.0 eV), C in C–N (287.5 eV)
and C in the CQO bond (289.1 eV), respectively. While in the
spectrum of surface-attached CB[7], the figure of the peaks is
blurred, which may be due to the change of oxidation state of
the C atom in the surface-attached CQO bond. Detailed
explanation of this change demands further investigation.
The peak corresponding to the N 1s region stays identical
with that of the regular ones. Based on these results, it is
reasonable to assume that the CB[7] molecules orient them-
selves normal relative to the surface, as demonstrated in
Scheme 1, and the strong interaction of one of the carbonyl
ports of CB with the gold surface induces the additional
carbon and oxygen species, as shown in the C 1s and O 1s
spectra.
Based on the number of surface-confined ferrocene mole-
cules, which is indicated by the area of the redox peak, the
surface density of CB[7] molecules on the gold electrode was
estimated to be at least 2.41 Â 1013 molecules cmÀ2. Taking the
large molecular size of CB[7] into consideration, the surface
density is satisfied by comparison with that of a monolayer
formed from alkanethiol (ca. 4.5 Â 1014 molecules cmÀ2),
especially thiolated cyclodextrin molecules on a gold sur-
face.13,15 From the surface coverage, the degree of packing
Electrochemical data further support the efficient formation
of a CB[n] monolayer on gold. A CB[n]-modified gold elec-
trode was obtained by immersing a clean gold electrode in a
1 mM aqueous solution of CB[7] (or a saturated aqueous
solution of CB[6] or CB[8]) for 24 h. After the incubation
process, the electrode was washed with deionized water and
dried with nitrogen. Fig. 3 gives the capacitances of gold
electrodes before and after CB[n] modification measured by
cyclic voltammetry (CV)13 in the following order: bare elec-
trode 4 CB[8]-modified 4 CB[7]-modified 4 CB[6]-modified
electrode, indicative of the attachment of CB[n] molecules on
the gold surface. It is reasonable that the CB[6]-modified
electrode bears the largest capacitance drop, since CB[6]
molecules have the smallest cavities which serve as pinholes
on the surface. What is more, the uncovered space of the
electrode surface among CB[6] molecules would be the nar-
Fig. 3 Capacitances of gold electrodes before and after modification
using CB[6], CB[7] or CB[8].
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first time that such a kind of interaction between CB[n]
molecules and gold has been discovered and applied. We
believe that this finding might extend the understanding of
CB[n] chemistry and promises great potential in the construc-
tion of CB[n]-based supramolecular structures or assemblies
on solid substrates.
The authors thank the financial support from the NSF
China (20473044, 20533050 and 50673048) and 973 Program
(2006 CB806200). We thank Prof. X. Zhang and his co-
workers for help with recording the FT-IRRAS spectra and
AFM.
Fig. 4 (a) CVs of a bar Au electrode (below), CB[7] modified
Notes and references
electrode (middle) and Fc@CB[7] modified electrode (top) at 1.0 V
À1 in 0.1 M aqueous solution; (b) CV of Fc@CB[7] modified electrode
1 (a) J. W. Lee, S. Samal, N. Selvapalam and K. Kim, Acc. Chem.
Res., 2003, 36, 621 and references therein; (b) J. Lagona, P.
Mukhopadhyay, S. Chakrabarti and L. Isaacs, Angew. Chem.,
Int. Ed., 2005, 44, 4844 and references therein; (c) K. Kim, N.
Selvapalam, Y. H. Ko, K. M. Park and J. Kim, Chem. Soc. Rev.,
2007, 36, 267 and references therein.
s
at 1.0 V sÀ1. Inset: anodic peak current vs. scan rate plot for the
modified electrode. Electrolyte: 0.1 M Na2SO4 aqueous solution with-
out any electroactive species.
2 (a) W. L. Mock and N. Y. Shih, J. Am. Chem. Soc., 1988, 110,
4706; (b) W. L. Mock and N. Y. Shih, J. Am. Chem. Soc., 1989,
111, 2697; (c) W. L. Mock, Top. Curr. Chem., 1995, 175, 1.
3 (a) C. Marquez, R. R. Hudgins and W. M. Nau, J. Am. Chem.
Soc., 2004, 126, 5806; (b) A. C. Bhasikuttan, J. Mohanty, W. M.
Nau and H. Pal, Angew. Chem., Int. Ed., 2007, 46, 4120.
4 (a) W. Ong and A. E. Kaifer, Angew. Chem., Int. Ed., 2003, 42,
2214; (b) W. S. Jeon, K. Moon, S. H. Park, H. Chun, Y. H. Ko, J.
Y. Lee, E. S. Lee, S. Samal, N. Selvapalam, M. V. Rekharsky, V.
Sindelar, D. Sobransingh, Y. Inoue, A. E. Kaifer and K. Kim, J.
Am. Chem. Soc., 2005, 127, 12984; (c) D. Sobransingh and A. E.
Kaifer, Chem. Commun., 2005, 5071.
5 R. J. Blanch, A. J. Sleeman, T. J. White, A. P. Arnold and A. I.
Day, Nano Lett., 2002, 2, 147.
6 (a) P. Mukhopadhyay, P. Y. Zavalij and L. Isaacs, J. Am. Chem.
Soc., 2006, 128, 14093; (b) J. Lagona, J. C. Fettinger and L. Isaacs,
Org. Lett., 2003, 5, 3745.
7 (a) D. Tuncel and J. H. G. Steinke, Chem. Commun., 2001, 253; (b)
D. Tuncel and J. H. G. Steinke, Macromolecules, 2004, 37, 288.
8 S. Y. Jon, N. Selvapalam, D. H. Oh, J.-K. Kang, S.-Y. Kim, Y. J.
Jeon, J. W. Lee and K. Kim, J. Am. Chem. Soc., 2003, 125, 10186.
9 (a) J. Zhao, H.-J. Kim, J. Oh, S. Kim, J. W. Lee, S. Sakamoto, K.
Yamaguchi and K. Kim, Angew. Chem., Int. Ed., 2001, 40, 4233;
(b) H. Isobe, S. Sato and E. Nakamura, Org. Lett., 2002, 4, 1287.
10 (a) K. Kim, W. S. Jeon, J.-K. Kang, J. W. Lee, S. Y. Jon, T. Kim
and K. Kim, Angew. Chem., Int. Ed., 2003, 42, 2293; (b) K. Kim,
D. Kim, J. W. Lee, Y. H. Ko and K. Kim, Chem. Commun., 2004,
848.
11 G. Li, M. Lauer, A. Schulz, C. Boettcher, F. Li and J.-H. Fuhrhop,
Langmuir, 2003, 19, 6483.
12 (a) X. Wen, R. W. Linton, F. Formaggio, C. Toniolo and E. T.
Samulski, J. Phys. Chem. A, 2004, 108, 9673; (b) Y. Jiang, Z.
Wang, H. Xu, H. Chen, X. Zhang, M. Smet, W. Dehaen, Y.
Hirano and Y. Ozaki, Langmuir, 2006, 22, 3715.
of the CB[7] SAM was estimated. It was found that 48% of the
gold electrode surface was covered by CB[7] molecules, in-
dicating an imperfect monolayer. However, we believe our
data cannot reflect the molecular surface density accurately.
Since the inclusion of ferrocene into attached CB[7] is a
dynamic process,14 association and dissociation take place at
all times. It is reasonable to believe that there exists a fraction
of CB[7] molecules whose cavities are not occupied by ferro-
cene molecules. Thus, the real surface density of self-as-
sembled CB[7] molecules as well as the degree of packing
should be larger than the value calculated. This promising
result prompts us to employ the attached CB[n] molecules as
anchor sites for modification of electrode surfaces. In this
respect, carboxyl group-substituted naphthalene was bound to
surface-confined CB[7] (Scheme 1), and detected by impedance
spectroscopy (Fig. S5w). It was also found that, probably due
to cooperative interaction of multiple carbonyl groups with
gold, the formed CB[n] monolayers are stable enough for
recognition processes. Upon washing with different solvents,
desorption of CB[n] from the gold surface was not detected.
In summary, we report the first facile and efficient method
for forming self-assembled CB[n] monolayers on gold surfaces.
This method is based on the spontaneous adsorption of CB[n]
molecules through the discovered interaction between CB[n]
and gold, does not require any prior modification or special
treatment of CB[n] molecules, and is applicable for all mem-
bers of the CB[n] family, at least CB[6–8]. The surface-
attached CB[n] molecules are uniform in orientation and hold
their carbonyls perpendicular with respect to the gold surface
and their cavities open to the outer atmosphere or solution.
This arrangement maintains the recognition properties of
CB[n] and facilitates guest molecules to approach the cavities
and be recognized. To the best of our knowledge, this is the
13 M. T. Rojas, R. Koniger, J. F. Stoddart and A. E. Kaifer, J. Am.
¨
Chem. Soc., 1995, 117, 336.
14 W. S. Jeon, K. Moon, S. H. Park, H. Chun, Y. H. Ko, J. Y. Lee, E.
S. Lee, S. Samal, N. Selvapalam, M. V. Rekharsky, V. Sindelar, D.
Sobransingh, Y. Inoue, A. E. Kaifer and K. Kim, J. Am. Chem.
Soc., 2005, 127, 12984.
15 A. Ulman, Chem. Rev., 1996, 96, 1533; J. C. Love, J. K. Kriebel, R.
G. Nuzzo and G. M. Whitesides, Chem. Rev., 2005, 105, 1103.
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This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 1989–1991 | 1991