Site-selective immobilization of colloids on au substrates via a noncovalent supramolecular "handcuff"

Article abstract of DOI:10.1021/la9033386

We have examined hierarchical supramolecular structure in the formation of colloidal arrays by immobilizing monodispersed naphthalene-functionalized colloids onto Au substrates bearing viologen moieties using the macrocyclic host molecule cucurbit[8]uril as a supramolecular "handcuff". Naphthalene-functionalized poly(methyl methacrylate)- and polystyrene-based colloids were synthesized by soap-free emulsion polymerization and characterized by dynamic light scattering and scanning electron microscopy to realize the colloidal arrays and to facilitate direct macroscopic imaging. The formation of host-stabilized ternary complexes on the surface of naphthalene-functionalized microspheres in colloidal suspension was verified by titration of a preformed viologen?CB[8] complex and followed by zeta potential measurements. Patterned self-assembled monolayers of a viologen derivative on Au substrates were formed by backfilling viologen-modified thiols after spontaneous chemisorption of "protective" alkylthiols by microcontact printing. After the initial complexation of CB[8] onto the viologen derivative on the Au substrates, monolayers of colloids with both 1D and 2D patterns could be formed and characterized by contact angle measurement, optical microscopy, and scanning electron microscopy. Control experiments indicated that no colloids were attached to the Au substrate after moderate washing by water if (1) CB[8] was replaced by a smaller analogue of the macrocyclic host, CB[6] or CB[7], (2) colloids without naphthalene-functionalities on the periphery were employed, or (3) alkanethiol was used entirely instead of viologenthiol to protect the Au substrate. These results suggest that the supramolecular ternary complexes were key to successfully bind the colloids onto the Au substrates with the CB[8] acts as a supramolecular "handcuff". The fundamental expertise gained from the study of these materials is believed to facilitate progress in the field of smart materials and wet nanotechnology and lead to the preparation of controlled reversible architectures on surfaces.

Full text of DOI:10.1021/la9033386

pubs.acs.org/Langmuir
© 2010 American Chemical Society
Site-Selective Immobilization of Colloids on Au Substrates via a Noncovalent
Supramolecular “Handcuff”
Feng Tian, Nan Cheng, Nicolas Nouvel, Jin Geng, and Oren A. Scherman*
Melville Laboratory for Polymer Synthesis, Department of Chemistry, University of Cambridge,
Lensfield Road, Cambridge CB2 1EW, United Kingdom
Received September 4, 2009. Revised Manuscript Received March 11, 2010
We have examined hierarchical supramolecular structure in the formation of colloidal arrays by immobilizing
monodispersed naphthalene-functionalized colloids onto Au substrates bearing viologen moieties using the macro-
cyclic host molecule cucurbit[8]uril as a supramolecular “handcuff”. Naphthalene-functionalized poly(methyl
methacrylate)- and polystyrene-based colloids were synthesized by soap-free emulsion polymerization and character-
ized by dynamic light scattering and scanning electron microscopy to realize the colloidal arrays and to facilitate direct
macroscopic imaging. The formation of host-stabilized ternary complexes on the surface of naphthalene-functionalized
microspheres in colloidal suspension was verified by titration of a preformed viologen-CB[8] complex and followed by
zeta potential measurements. Patterned self-assembled monolayers of a viologen derivative on Au substrates were
formed by backfilling viologen-modified thiols after spontaneous chemisorption of “protective” alkylthiols by
microcontact printing. After the initial complexation of CB[8] onto the viologen derivative on the Au substrates,
monolayers of colloids with both 1D and 2D patterns could be formed and characterized by contact angle measurement,
optical microscopy, and scanning electron microscopy. Control experiments indicated that no colloids were attached
to the Au substrate after moderate washing by water if (1) CB[8] was replaced by a smaller analogue of the macro-
cyclic host, CB[6] or CB[7], (2) colloids without naphthalene-functionalities on the periphery were employed, or
(3) alkanethiol was used entirely instead of viologenthiol to protect the Au substrate. These results suggest that the
supramolecular ternary complexes were key to successfully bind the colloids onto the Au substrates with the CB[8] acts
as a supramolecular “handcuff”. The fundamental expertise gained from the study of these materials is believed to
facilitate progress in the field of smart materials and wet nanotechnology and lead to the preparation of controlled
reversible architectures on surfaces.
Introduction
achieved by confining particles into physical templates with
different shapes;16 however, they have not yet been shown to
exhibit high selectivity. Although the “expensive” biotin-avidin
interaction also bears high specific noncovalent recognition
attributes, it becomes much weaker in organic solvents or in
high-temperature environments.
Recent efforts have been devoted to immobilization and
manipulation of colloids on substrates with a predetermined
arrangement of high accuracy. Well-defined colloidal arrays are
of great interest for the possible integration of these building
blocks into photonic devices,1 high-density patterned media,2,3
chemical and biological sensors,4 and single molecule detectors.5
A wide variety of methodologies, such as using physical confine-
ment,6 electrostatic and covalent interactions,7-9 biological inter-
actions,10,11 and supramolecular interactions including contrast
of hydrophilicity/hydrophobicity,12 hydrogen-bonding interac-
tions,13 and host-guest interactions,14,15 have been developed
to noncovalently attach colloids and nanoparticles onto surfaces
with feature patterns. Perfect colloidal patterns have been
Most recently, host-guest chemistry has been employed as an
alternative methodology to attach colloids onto substrates in a
supramolecular fashion. Recognition-directed self-assembly
serves as a more promising technique in that it can greatly
improve the precision in position control by specifically and
selectively recognizing and binding colloids to a substrate. Ad-
ditionally, employing the noncovalent nature of supramolecular
interactions provides a powerful tool to reversibly control the
binding events which can lead to the realization of more complex
functions. Fitzmaurice et al. have reported recognition-directed
noncovalent self-organization of crown ether-modified nanopar-
ticles onto cation-modified substrates by forming pseudorotax-
anes.14 More recently, Huskens et al. have employed cyclodextrin
(CD) monolayers as “molecular printboards” on substrates on
which 3D self-supported free-standing colloidal arrays were
immobilized using adamantyl-functionalized dendrimers as a
“supramolecular glue”.15,17-19
*Corresponding author. E-mail: oas23@cam.ac.uk.
(1) Hayward, R. C.; Saville, D. A.; Aksay, I. A. Nature 2000, 404, 56–59.
(2) Ross, C. Annu. Rev. Mater. Res. 2001, 31, 203–235.
(3) Antonietti, M.; Ozin, G. A. Chem.;Eur. J. 2004, 10, 29–41.
(4) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289–1292.
(5) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18–52.
(6) Xia, Y.; Yin, Y.; Lu, Y.; McLellan, J. Adv. Funct. Mater. 2003, 13, 907–918.
(7) Choi, J. H.; Adams, S. M.; Ragan, R. Nanotechnology 2009, 20, 065301.
(8) Auer, F.; Scotti, M.; Ulman, A.; Jordan, R.; Sellergren, B.; Garno, J.; Liu,
G. Y. Langmuir 2000, 16, 7554–7557.
(9) Gole, A.; Orendorff, C. J.; Murphy, C. J. Langmuir 2004, 20, 7117–7122.
(10) Katz, E.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 6042–6108.
(11) Rosi, N. L.; Mirkin, C. A. Chem. Rev. 2005, 105, 1547–1562.
(12) Masuda, Y.; Itoh, T.; Koumoto, K. Langmuir 2005, 21, 4478–4481.
(13) Zirbs, R.; Kienberger, F.; Hinterdorfer, P.; Binder, W. H. Langmuir 2005,
21, 8414–8421.
(14) Ryan, D.; Nagle, L.; Fitzmaurice, D. Nano Lett. 2004, 4, 573–575.
(15) Maury, P. A.; Reinhoudt, D. N.; Huskens, J. Curr. Opin. Colloid Interface
Sci. 2008, 13, 74–80.
(16) Yin, Y.; Lu, Y.; Gates, B.; Xia, Y. J. Am. Chem. Soc. 2001, 123, 8718–8729.
(17) Crespo-Biel, O.; Ravoo, B. J.; Reinhoudt, D. N.; Huskens, J. J. Mater.
Chem. 2006, 16, 3997–4021.
(18) Ludden, M. J. W.; Reinhoudt, D. N.; Huskens, J. Chem. Soc. Rev. 2006, 35,
1122–1134.
€
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D. N.; Vancso, G. J.; Huskens, J. Angew. Chem., Int. Ed. 2009, 48, 983–987.
Langmuir 2010, 26(8), 5323–5328
Published on Web 03/25/2010
DOI: 10.1021/la9033386 5323

Article
Tian et al.
Scheme 1. Schematic Cartoon Illustration of Site-Selective Noncovalent Immobilization of Np-Colloids on a Micropatterned Viologen-
Terminated Au Substrate in the Presence of CB[8]a
a (a) Micro-contact printing a Au substrate by gently placing a PDMS stamp “inked” by protecting thiol for 30 s; (b) backfilling the
viologendecanethiol by immersing the partially protected Au substrate in an EtOH solution of viologendecanethiol (7) for 2 min; (c) threading the
macrocyclic host molecule as a supramolecular “handcuff” by immersing the Au substrate in a supersaturated CB[8] aqueous suspension for 5 min;
(d) immobilizing the Np-colloids onto specific sites by immersing the Au substrate into an aqueous suspension of Np-colloids for 30 s. After each
individual step, the resulting substrate was washed by a copious amount of the corresponding solvent and then dried under a stream of nitrogen if needed.
Note that the density of the surface bound species has not yet been quantified.
Table 1. Formulation of Preparation of Colloids via SFEP and
Their Hydrodynamic Diameter and Size Distribution in Aqueous
Although cucurbit[n]uril (CB[n]) exhibits extraordinary host-
guest properties20-24 that are distinctly different from those of
Environment32,33
other host molecules, little attention has been paid to the CB[n]-
mediated surface organization.25,26 Different from smaller CB
homologues, whose surface anchoring have been based on the
direct chemical functionalization,27,28 cucurbit[8]uril (CB[8]) spe-
cifically and selectively encapsulates two different guest molecules
at the same time, an electron-deficient molecule (which serves as
the first guest) and an electron-rich molecule (as the second guest),
inside the cavity to form a stable 1:1:1 ternary complex. Ternary
complex formation is driven by the hydrophobic effect and
exhibits an enhanced charge-transfer (CT) interaction between
the guest pair inside the hydrophobic cavity of CB[8].23,29
initiatorsa/
mg
Dh/
T/°C nmb PDIc
colloids monomers/g DVB/g NpMA/g
1
2
3
4
5
6
MMA/7.8
St/4
St/3.8
St/4
St/3.8
St/3.6
2
1
1
1
1
1
0.2
0.2
KPS/100
AIBA/155 60 222 0.007
AIBA/155 60 211 0.009
KPS/155
KPS/155
KPS/155
70 280 0.003
70 171 0.005
70 167 0.006
70 165 0.005
0.2
0.4
aKPS: potassium persulfate; AIBA: 2,20-azobis(isobutyramidine)
dihydrochloride. bDh: hydrodynamic diameter, as determined by DLS.
cPDI: polydispersity index, as determined by DLS.
Based on its unique attributes, CB[8] was exploited herein as a
supramolecular “handcuff” to lock second-guest functionalized
polymeric colloids onto first-guest terminated Au substrates in a
stepwise fashion by forming ternary complexes via specific
host-guest interactions. This represents the first time that CB-
[n]-based surface bound colloidal arrays with different feature
patterns can be visualized. Furthermore, colloidal arrays have
been achieved by using host-guest interactions, enhancing the
accuracy of colloid positioning on substrates, increasing stability
for nanofabrication, and introducing the opportunity to exploit
controlled, reversible colloidal binding. Moreover, CB[8]-based
ternary complexes were used instead of CD inclusion complexes
to realize a higher selectivity of colloidal immobilization on a
substrate. A scheme of site-selective noncovalent immobilization
of naphthalene-functionalized colloids (Np-colloids) on micro-
patterned viologen-terminated Au substrates via CB[8]-stabilized
host-guest interactions is shown in Scheme 1.
(20) Lee, J. W.; Samal, S.; Selvapalam, N.; Kim, H. J.; Kim, K. Acc. Chem. Res.
2003, 36, 621–630.
Results and Discussion
(21) Lagona, J.; Mukhopadhyay, P.; Chakrabarti, S.; Isaacs, L. Angew. Chem.,
Int. Ed. 2005, 44, 4844–4870.
(22) Selvapalam, N.; Ko, Y. H.; Park, K. M.; Kim, D.; Kim, J. Chem. Soc. Rev.
2007, 36, 267–279.
(23) Ko, Y. H.; Kim, E.; Hwang, I. Chem. Commun. 2007, 1305–1315.
(24) Isaacs, L. Chem. Commun. 2009, 619–629.
Polymeric colloids were used in this study to investigate the
interactions between soft matter and hard substrate, not only for
their potential application in photonic device construction as well
as colloidal template of photolithography but also because a wide
variety of surface functionalities can be introduced as needed.
Additionally, submicrometer polymeric colloids offer the advan-
tage of direct macroscopic imaging of patterned self-organization
of colloids immobilized on substrates. Monodispersed polystyr-
ene (PS)- and poly(methyl methacrylate) (PMMA)-based Np-
colloids with different Np-functionalities and surface charges
(25) Kim, K.; Jeon, W. S.; Kang, J. K.; Lee, J. W.; Jon, S. Y.; Kim, T.; Kim, K.
Angew. Chem., Int. Ed. 2003, 42, 2293–2296.
(26) Kim, K.; Kim, D.; Lee, J. W.; Ko, Y. H. Chem. Commun. 2004, 848–849.
(27) Jon, S. Y.; Selvapalam, N.; Oh, D. H.; Kang, J. K.; Kim, S. Y.; Jeon, Y. J.;
Lee, J. W.; Kim, K. J. Am. Chem. Soc. 2003, 125, 10186–10187.
(28) Hwang, I.; Baek, K.; Jung, M.; Kim, Y.; Park, K. M.; Lee, D. W.;
Selvapalam, N.; Kim, K. J. Am. Chem. Soc. 2007, 129, 4170–4171.
(29) Rauwald, U.; Scherman, O. A. Angew. Chem., Int. Ed. 2008, 47, 3950–3953.
5324 DOI: 10.1021/la9033386
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Tian et al.
Article
Figure 1. Size and distribution of colloid 1 from SFEP as shown by (A) scanning electron microscopy (SEM) of colloidal crystalline arrays
after drying and (B) DLS in colloidal suspension.
were synthesized by soap-free emulsion polymerization (SFEP).30,31
Styrene (St) or methyl methacrylate (MMA) was used as the bulk
monomer and divinylbenzene (DVB) as the cross-linking agent.
These colloids are themselves “inert” in terms of molecular re-
cognition with CB[8] or a viologen patterned substrate. PMMA-
based colloid 1 was prepared for the immobilization on Au
substrates. PS-based colloids 2-5 were prepared for the zeta
potential (ZP) titrations, which will be discussed in detail in the
following section. Different initiators were used to obtain both
negative-charge stabilized colloids as well as positive-charge
stabilized colloids from the dissociation of potassium persulfate
(KPS) and 2,20-azobis(isobutyramidine) dihydrochloride (AIBA),
respectively. The feed ratio of the polymerization and hydrody-
namic diameter and size distribution of the colloids after purifica-
tion were characterized in an aqueous environment by dynamic
light scattering (DLS) and summarized in Table 1.
All of the colloids obtained by SFEP in this study are mono-
dispersed in water. As shown in Figure 1A, the size of colloid 1
measured by SEM is 275 ( 4.6 nm, which corresponds very well
with the DLS result (Figure 1B) that shows the average hydro-
Figure 2. Change in ZP of colloids as a function of added
MV2þ-CB[8] complex. The concentration of the titrated colloidal
suspension was 1 mg/mL with a volume is 750 μL. The concentra-
tion of the titrant MV2þ-CB[8] complex in aqueous solution was
1 mM.
dynamic diameter is around 280 nm and with a very low PDI of
0.003. Colloid 1 is stabilized by negative charges from the
suspension when one of the components is covalently bound to
a substrate.
dissociation of KPS and has a ZP of -57.9 ( 2.75 mV. Colloids
4, 5, and 6 are also negatively charged, which are chosen to follow
the ZP change upon addition of first guest and host molecule.
Colloids 2 and 3 are positively charge stabilized, from AIBA
initiation for the same titration purpose.
In order to verify whether the ternary complexes could form on
the periphery of Np-functionalized colloids in an aqueous suspen-
sion, a solution containing 4,40-methylviologen (MV2þ)-CB[8]
1:1 preformed inclusion complex was added to a variety of
aqueous suspensions of Np-colloids. Subsequently, ZP measure-
ments were used to follow the change of surface charge on the
Np-colloids upon addition of the MV2þ-CB[8] complex. As
shown in Figure 2, the ZP of colloid 5 in an aqueous suspen-
sion (1 mg/mL) increased gradually from -47.6 to -32.9 mV
upon the addition of the MV2þ-CB[8] complex. When colloid
4 without any Np-functionality was tested as a control, only a
very slight change in ZP was detected, suggesting that the increase
in ZP of colloid 5 indeed comes from the MV2þ-CB[8] comp-
lex recognizing Np-functionality and formation of the ternary
complex. Upon titrating colloid 6, which has twice the amount
of the surface Np-functionality as colloid 5, the increase in ZP is
even more obvious from -46.9 to -19.4 mV, showing that the
presence of Np-functionality is indeed key to the formation of
the ternary complex on the colloid surface in an aqueous
environment.
Viologen derivatives (4,40-bipyridium dications) are well-
known as electron-deficient species and can readily form 1:1
inclusion complexes with CB[8] with a binding constant (Ka) of
1.1 ꢀ 105 M-1 in buffered water (0.1 M phosphate buffer). The
viologen-CB[8] 1:1 inclusion complex can form a 1:1:1 ternary
complex instantaneously upon addition of 1 equiv of a second,
electron-rich species, which in our case are the Np-moieties. This
ternary complex, which can be formed from both small mole-
cules23 and polymeric structures,29 has been well-characterized in
an aqueous environment with an overall binding constant up to
1010-1011 M-2. However, no previous report has shown that the
ternary complexes are still capable of forming in a colloidal
(30) Ni, H.; Du, Y.; Ma, G.; Nagai, M.; Omi, S. Macromolecules 2001, 34, 6577–
6585.
(31) Egen, M.; Zentel, R. Macromol. Chem. Phys. 2004, 205, 1479–1488.
(32) Liu, H. B.; Wang, C. C. Acta Chim. Sin. 2008, 66, 1269–1273.
(33) Yamamoto, T.; Higashitani, K. Asia-Pac. J. Chem. Eng. 2008, 3, 239–249.
Langmuir 2010, 26(8), 5323–5328
DOI: 10.1021/la9033386 5325

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Tian et al.
Figure 3. A water drop on (A) a bare Au substrate, (B) a viologendecanethiol-modified Au substrate, and (C) CB[8]-complexed
viologendecanethiol-modified Au substrate, displaying enhanced hydrophilicity. The bare Au substrate was immersed in an ethanol solution
(2 mM) for 30 s under nitrogen before drying. Then the resulting Au substrate was immersed in the CB[8] supersaturated aqueous solution for
5 min.
To eliminate the possibility of any electrostatic effect on the
change of ZP of the negatively charged Np-colloids upon addition
of the positively charged titrant, ZP measurements were also
carried out on the positively charged Np-colloids 2 and 3. No
change in ZP was detected upon addition of the MV2þ-CB[8]
complex to the control colloid 2 due to the absence of Np-
functionalities. However, the ZP of colloid 3 increased from
49.5 to 56.8 mV upon addition of the MV2þ-CB[8] complex.
This increase in ZP is more convincing, suggesting that the
MV2þ-CB[8] complex can overcome the electrostatic repulsion
and form the ternary complexes on the surface of positively
charged Np-colloids, thereby contributing to the increase of the
surface charge.
The immobilization of colloids onto both nonpatterned and
patterned assemblies was carried out in a stepwise fashion as the
first guest was immobilized onto the Au substrate and the second
guests onto the colloids. For the nonpatterned assembly, a
viologen-terminated self-assembled monolayer (SAM) was first
prepared by immersing a bare Au substrate into an ethanol
solution (2 mM) of 7 for 30 s. After washing with ethanol and
drying with a flow of nitrogen, the contact angle of a bare Au
substrate changed from 93.8° (Figure 3A) to 55.5° (Figure 3B) on
a viologen-terminated Au substrate. Then CB[8] was assembled
Figure 4. SEM images (A, B, and D) and optical microscopy
image (C) of Np-colloids on viologen-terminated Au substrates
onto the viologen motifs by immersing the viologen-terminated
Au substrate into a supersaturated CB[8] aqueous suspension for
5 min. After washing with water and again drying under a flow of
nitrogen, the contact angle of viologen-CB[8]-terminated Au
in the presence of CB[8]: (A) nonpatterned monolayer colloidal
immobilization; (B) 20 μm (line width) ꢀ 10 μm (interval width)
linear colloidal arrays; (C) 50 μm (square length) ꢀ 50 μm (interval
length) squared colloidal arrays; (D) 20 μm (line width) ꢀ 20 μm
(interval width) linear colloidal arrays. No “protecting” thiol was
used for (A). Dodecanethiol was used as the “protecting” species
for the μCP of (B) and (C) and 2-aminoethanethiol hydrochloride
for (D). Colloid 1 was used for (A), (B), and (C) while colloid 3 was
used for (D).
substrates decreased further to 23.0° (Figure 3C). Figure 4A
shows the SEM image of Np-colloids bound to a viologen-CB[8]-
terminated Au substrate after dipping the viologen-CB[8]-termi-
nated Au substrate into a colloidal suspension for 30 s. These
surface-bound colloidal arrays are stable under slight sonication
and maintain this arrangement in aerobic conditions even after 9
months.
Subsequently, patterned colloidal arrays were also prepared to
illustrate that the colloids were only bound to areas of the surface
capable of ternary complexation. Introduction of a feature
pattern was accomplished by first protecting a portion of the
Au substrate with an inert dodecanethiol via μCP and then
backfilling the “bare” areas with 7, thus forming a patterned
SAM of viologen derivative on a Au substrate. After allowing
CB[8] to form 1:1 complexes on the mixed thiol layer, Np-colloids
were only found bound in the appropriate locations as evidenced
by the linear patterns shown in Figure 4B. Furthermore, 2D
square colloidal patterns were prepared through the same two-
step thiol protection method by using a PDMS stamp with
indented square areas for the μCP step and further confirms that
the Np-colloids were bound onto the areas precoated by the
viologen-CB[8] complex only rather than sitting on the protec-
tive layers. Finally, in an effort to rule out any possibility of simple
electrostatic interactions between the colloids and the viologen
covered Au substrates, positively charged colloid 3 was used to
form a linear array (Figure 4D) on a Au substrate which had been
first protected with 2-aminoethanethiol hydrochloride and then
backfilled with 7. Thus, the ternary complex “handcuff” structure
serves as the driving force to immobilize the colloids onto the Au
substrates, which must overcome electrostatic repulsion from like
charges (accounting for the “negative” defects in the lines). The
“positive” defects in between two viologen arrays are probably
due to portal binding of CB[8] with the protonated amines of the
protective layer used, which held the colloids by accommodating
Np-functionality inside the cavity of CB[8].
A series of control experiments were further performed to
reinforce the role of the CB[8]-stabilized ternary complexes in
binding the colloids onto the substrates. When CB[8] was either
5326 DOI: 10.1021/la9033386
Langmuir 2010, 26(8), 5323–5328

Tian et al.
Article
removed completely from the procedure or replaced by smaller
homologues, CB[6] and CB[7], there were no colloids remaining
on the substrate after washing, as verified by both optical
microscopy and SEM. Even if the extremely low binding constant
of CB[6] with viologen (Ka = 21 M-1) could lead to a very small
amount of CB[6]-viologen inclusion complexes on the sub-
strate,34 the cavity of CB[6] is incapable of binding a pair of
molecules simultaneously.21 When CB[7] was used, again access
of the Np-functionalized colloids to substrate was not possible
because the cavity of CB[7] is still not big enough to accommodate
a pair of guest molecules.35 Additionally, when colloid 4 (without
any Np-moieties) was employed with CB[8], no colloids were
detected after washing under a moderate flow of water. Further-
more, when the Au substrate was entirely covered by the “inert”
alkanethiol afterimmersedthe Au substrate in its ethanol solution
(2 mM) for 72 h, no colloids were detected after moderate
washing. It is worth noting that all successful immobilizations
and patterns of Np-colloidal monolayers onto the Au substrates
could be immediately identified by the naked eye as different
colors could be seen by simply manipulating the substrates in a
variety of angles upon reflection of sunlight by the colloidal
arrays.
In this report, we have examined the utility of a supramolecular
structure to immobilize and pattern functionalized monodis-
persed colloids onto viologen-functionalized Au substrates using
CB[8] as a supramolecular “handcuff”. Monodispersed func-
tional colloids were prepared by soap-free emulsion polymeriza-
tionto facilitate the direct imaging of the colloidal patterns. Stable
and robust ternary complexes were formed by anchoring the
viologen-CB[8] complexes onto an Au surface followed by
addition of Np-functionalized colloids and were confirmed by
ZP titrations as well as by SEM and optical microscopy. An
efficient and convenient “dipping” methodology was developed
to quickly immobilize and detect the patterned colloidal arrays on
the substrates. Both 1D and 2D patterned colloidal arrays as well
as control experiments demonstrated that the specific supramo-
lecular ternary architecture was required for colloidal immobili-
zation. The materials serve as a new class of building blocks to
generate hierarchically self-assembled surface bound structures
that are expected to exhibit interesting features valuable to areas
ranging from condensed matter physics to photonics.
performed on Leo 1530 VP FE-SEM. Contact angle measure-
ments were performed on KSV Instruments KSV CAM 200
goniometer.
Synthesis of 2-Naphthyl Methacrylate (NpMA). NpMA
was prepared by the condensation reaction of 2-naphthol (Np)
and methacryloyl chloride in Et2O in the presence of TEA to trap
HCl. Np (10 g, 69.4 mmol) was dissolved in 150 mL of Et2O in a
500 mL flask, to which TEA (14.5 mL, 104 mmol) was added.
Methacryloyl chloride (8.056 mL, 83.2 mmol) in 100 mL of Et2O
was added dropwise to the solution for 1 h via a dropping funnel
while the solution was kept at 0 °C. The reaction mixture was
stirred for 24 h under room temperature. The resulting precipitate
was filtered off, and the solvent was evaporated to give a white
solid, which was dissolved in chloroform again. The residue was
purified by column chromatography on silica gel in petroleum
ether/EtOAc (30:1) as the solvent. A yield of 85% was obtained
1
after purification. H NMR (CDCl3): δ 7.79-7.88 (m, 3H, ar),
7.60-7.61 (d, 1H, ar), 7.44-7.51 (m, 2H, ar), 7.25-7.28 (m, 1H,
ar), 6.41 (d, 1H, CH2-C-CH3), 5.79 (t, 1H, CH2-C-CH3), 2.10
(t, 1H, CH2-C-CH3). 13C NMR (d6-DMSO): δ 129.3, 127.7,
127.6, 127.2, 126.4, 125.6, 121.1, 118.5, 18.4.
Soap-Free Emulsion Polymerization (SFEP). Polymeric
colloids were synthesized by the SFEP technique. A typical
procedure was as follows: MMA (or St), NpMA, and DVB were
dispersed in water (100 g). The solution was placed into a 250 mL
four-neck RBF with a mechanical stirrer, a nitrogen inlet, a
condenser, and a thermometer. Nitrogen was bubbled through
the mixture of reagents for 1 h before elevating the temperature,
and the nitrogen blanket was maintained throughout the polym-
erization. After stabilizing at 70 °C (for AIBA initiation, the
temperature was set to 60 °C) for at least 30 min, polymerization
was initiated by addition of a degassed aqueous solution of KPS
(or AIBA). After 8 h polymerization, the product was washed by
three cycles of centrifuge/redispersion using deionized water.
Synthesis of N-Methyl-4,40-bipyridinium Iodide (MVþ).
To a solution of 4,40-bipyridine (1.0 g, 6.4 mmol) in 15 mL of
DCM, methyl iodide (0.5 mL, 8.1 mmol) in DCM (5 mL) was
added dropwise to the stirred solution. The mixture was left at
room temperature for overnight. The yellow product was filtered
off and purified by recrystallization from MeOH (1.544 g, 81%).
1H NMR (d3-MeCN): δ = 8.84 (d, 2H), 8.79 (d, 2H), 8.32(d, 2H),
7.80 (d, 2H), 4.35 ppm (s, 3H). 13C NMR (d6-DMSO): δ = 152.3,
151.5, 146.6, 141.3, 125.4, 122.3, 48.0 ppm.
Synthesis of N-(10-Mercaptodecyl)-N0-methyl-4,40-bi-
pyridinium Chloride (7). MVþ (2 g, 6.7 mmol) and 1,10-
dibromodecane (1.76 g, 8.7 mmol) were refluxed in MeCN for 3
days. The precipitant 9 was filtered and washed with warm MeCN
and dried under vacuum. Then 9 (4.4 g, 7.4 mmol) and potassium
thioacetate (1.3 g, 11.1 mmol) were taken into EtOH (100 mL)
and refluxed for several hours. The solution was added concen-
trated H2SO4 to obtain a pH = 1 solution under reflux for a
further 24 h. The black precipitant was filtered off. To the water
filtrate was added KPF6 (2.6 g, 16 mmol), and the resulting white/
yellow precipitate 8 was filtered off and washed with water and
dried under vacuum. Then 8 (0.58 g, 0.91 mmol) was dissolved in
15 mL of acetone. To this solution, n-Bu4NCl (1.06 g, 3.8 mmol)
in 10 mL of acetone was added; the yellow/white precipitate (7,
299 mg, 79%) was collected bycentrifuge, washedbyacetone, and
dried under vacuum. 1H NMR (d4-MeOD): δ = 8.42-9.28
(aromatic, 8H), 4.74 (t, 2H), 4.42 (s, 3H), 2.66 (t, 2H), 2.58 (t,
2H), 1.28-1.65 ppm (t, 14H).
Experimental Section
Materials. All starting materials were purchased from Alfa
Aesar and Sigma-Aldrich and used as received unless stated
otherwise. CB[8] was prepared according to literature proce-
dure.36 Methyl methacrylate (MMA), styrene (St), and divinyl-
benzene (DVB) were passed through a basic alumina column to
˚
remove the inhibitor and stored over 4 A molecular sieves,
respectively. Triethylamine (TEA) was dried with potassium
hydroxide and distilled before use. Potassium persulfate (KPS)
was recrystallized twice from methanol.
Instrumentation. 1H NMR (400 MHz) spectra was recorded
using a Bruker Avance QNP 400. 13C NMR (125 MHz) spectra
was recorded using a Bruker Avance Cryobrobe ATM TCI DRX
500 or a Bruker Avance 500 BB-ATM. Optical microscopy was
performed on Nikon Eclipse ME600L microscope using the
DN100 capture system. Dynamic light scattering (DLS) and zeta
potential (ZP) measurements were performed on Malvern Zeta-
sizer NS90 instrument. Scanning electron microscopy (SEM) was
Immobilization of Np-Colloids on Viologen-Terminated
Au Substrate in the Presence of CB[8]. For μCP, PDMS
stamps were wetted by a solution of dodecanethiol or 2-ami-
noethanethiol hydrochloride in EtOH (2 mM) and then placed
onto a Au substrate for 30 s. After peeling away the PDMS stamp,
the substrate was washed by EtOH and then immersed into a
solution of 7 in EtOH (2 mM) for 5 min under a nitrogen
atmosphere. The substrate was washed by EtOH and dried under
(34) Ong, W.; Gomez-Kaifer, M.; Kaifer, A. E. Org. Lett. 2002, 4, 1791–1794.
(35) Kim, H. J.; Jeon, W. S.; Ko, Y. H.; Kim, K. Proc. Natl. Acad. Sci. U.S.A.
2002, 99, 5007–5011.
(36) Kim, J.; In-Sun, J.; Soo-Young, K.; Lee, E.; Jin-Koo, K.; Sakamoto, S.;
Yamaguchi, K.; Kim, K. J. Am. Chem. Soc. 2000, 122, 540–541.
Langmuir 2010, 26(8), 5323–5328
DOI: 10.1021/la9033386 5327

Article
Tian et al.
nitrogen and immersed into a supersaturated CB[8] aqueous
suspension, which was put onto a shaker with 200 rpm for
5 min. The resulting substrate was washed by a copious amount
of water and dipped into the aqueous suspension of Np-colloids
(0.5% w/v) for 30 s, washed by a copious amount of water, and
dried under a stream of nitrogen.
Acknowledgment. The authors thank Z. Rong for contact angle
measurements, F. Biedermann for the viologendecanethiol, Y. Bai
for the PDMS stamps, and Professor U. Steiner and M. Kolle for
the helpful discussions. F.T. is thankful for the CSC Cambridge
Scholarship. This work was also supported by the Next Generation
Fellowship provided by the Walters-Kundert Foundation.
5328 DOI: 10.1021/la9033386
Langmuir 2010, 26(8), 5323–5328