Supramolecular cyclodextrin-dye complex exhibiting selective and efficient quenching by lead ions

Article abstract of DOI:10.1016/j.dyepig.2011.08.011

In this article, we describe a supramolecular complex made of cyclodextrin and a chromophoric dye, in which anistropy occurred by the formation of the hexagonal packing of cylindrical cyclodextrin channels. The obtained complex exhibited effective fluores

Full text of DOI:10.1016/j.dyepig.2011.08.011

Dyes and Pigments 93 (2012) 1544e1548
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Short communication
Supramolecular cyclodextrin-dye complex exhibiting selective and efficient
quenching by lead ions
,c,
Sooyeon Jeonga, Won Young Kanga, Chung Kun Songb, Jong S. Parka
*
a Laboratory of Polymer and Electronic Materials, Department of Textile Industry, Dong-A University, Busan 604-714, South Korea
b Media Device Center, Department of Electronic Engineering, Dong-A University, Busan 604-714, South Korea
c Department of Nano Engineering, Dong-A University, Busan 604-714, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
In this article, we describe a supramolecular complex made of cyclodextrin and a chromophoric dye, in
which anistropy occurred by the formation of the hexagonal packing of cylindrical cyclodextrin channels.
The obtained complex exhibited effective fluorescence quenching in the presence of lead ions based on
supramolecular wire assembly and dissociation. Selective and effective fluorescence quenching of the
complex is attributed to the break-ups of the supramolecular wire and the unthreading of included dye
molecules by lead ions. Though we are not yet sure about the origin of such a high selectivity, a topic that
will be elaborated in subsequent investigations, these CD-templated supramolecular properties will be
useful in analytical applications as a novel platform for metal ion sensing.
Received 15 June 2011
Received in revised form
15 August 2011
Accepted 18 August 2011
Available online 31 August 2011
Keywords:
Cyclodextrin
Supramolecular complex
Inclusion complex
Metal sensing
Ó 2011 Elsevier Ltd. All rights reserved.
SterneVolmer equation
Fluorescence quenching
1. Introduction
and many organic molecules have been reported to form inclu-
sion complexes with them by non-covalent interactions [6e8].
Supramolecular assemblies have drawn increasing attention
due to their unusual, intrinsic properties that are useful in various
fields, including drug delivery, chemo- and bio-sensors, and elec-
tronic devices [1e3]. Self-assembly is considered to be the most
practical method for the formation of these assemblies, and there
have been numerous reports on accomplishing well-defined
nanostructures and the precise control of the supramolecular
response at the molecular level. In particular, rod-like assemblies
are of great interest because of their biomimetic characteristics and
liquid crystalline behavior [4,5]. Self-assembly of such synthetic
building blocks by non-covalent interactions is attractive especially
for sensing applications if these supramolecular structures can be
assembled or dissembled depending on the presence of a specific
analyte.
Among them, g-cyclodextrin (g-CD) has a large cavity with
a diameter of up to 9 Å. The large cavity size allows the inclusion
of more than one molecule and the proximity of included mole-
cules facilitates electronic interaction. Some interesting examples
include 2,5-diphenyl-1,3,4-oxadiazole (PPD) and 1,6-diphenyl-
1,3,5-hexatriene (DPH), which self-assemble to form extended
aggregates that are supported together when included inside CD
channels [9,10]. These studies have successfully demonstrated the
formation of chromophore-anchored supramolecular aggregates
and the role of CDs for nanotubular channels under appropriate
conditions.
Numerous CD-based inclusion complexes have been made
which exhibit stimuli-responsive properties to external sources,
such as heat, light, pH, and chemicals [11e13]. As far as we are
aware, however, few studies have dealt with a system that is
specific to the presence of lead ions. Lead is a well-known heavy
metal that plays a critical role in environmental politics due to its
high neurotoxicity, and its long-term harmful effects on humans
and the environmental system are significant, even upon exposure
to a trace amount [14,15]. Selective detection and determination of
this metal ion, therefore, is an important task, and in spite of
impressive results to date, there is still increasing pressure to
develop promising novel systems.
Cyclodextrins (CDs) are cyclic oligosaccharides, most
commonly consisting of 6-, 7- or 8-glucose units, which are called
a
-, b- or g-CDs, respectively. CDs are well-known host materials,
* Corresponding author. Laboratory of Polymer and Electronic Materials,
Department of Textile Industry, Dong-A University, Busan 604-714, South Korea.
Tel.: þ82 51 200 7330; fax: þ82 51 200 7540.
E-mail address: jongpark@dau.ac.kr (J.S. Park).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.08.011

S. Jeong et al. / Dyes and Pigments 93 (2012) 1544e1548
1545
Scheme 1. Synthesis of the acetylene dye.
2. Experimental
benzoic acid (739 mg, 2.8 mmol) were dissolved in THF (4 mL) in the
presence of piperidine (2 mL), (Ph3P)2PdCl2 (49 mg, 0.01 mmol), and
CuI (9.5 mg, 0.05 mmol). The mixture was reacted overnight under
nitrogen at room temperature. After the reaction, the mixture was
diluted with aqueous KOH solution (1 M, 30 mL), filtered, and then
acidified to below pH 1 with dilute HCl. The precipitate was collected
by filtration, washed profusely with water, and dried in air overnight
at room temperature. The product was isolated by column chroma-
tography on silica using methylethylketone/ammonium hydroxide/
1-propanol (0.5/1.0/1.0 vol%) as an eluent, followed by drying under
vacuo (0.352 g, yield 57%). 1H NMR (600 MHz, DMSO-d6): 7.8 (d, 2H),
7.7 (d, 4H), 7.5 (d, 4H), 7.4 (q, 2H), 6.6 (d, 2H); 13C NMR (600 MHz,
DMSO-d6): 172.0, 159.5, 139.0, 136.3, 135.1, 131.6, 127.0, 121.1, 119.0,
116.4, 115.0, 90.6, 89.2; m/z (negative MALDI-TOF) 473.1 (Mꢁ).
2.1. Materials
All chemicals were purchased from Aldrich (SigmaeAldrich
Korea, Korea) and used without further purification.
obtained from Tokyo Chemical Industry (TCI Korea, Korea), and
recrystallized twice from water.
g-CD was
2.2. Experimental methods
X-ray measurements were performed on a multi-purpose X-ray
diffractometer (X’pert Pro MPD, PANalytical, Korea Basic Science
Institute) with Cu Ka radiation (
l
¼ 1.54 Å, 3 kW) at 25 ꢀC. The UV
absorbance spectra were collected using a Lambda 7 spectrometer
(Perkin Elmer) in a 1 cm cuvette. The steady-state fluorescent
spectra were collected using a Perkin Elmer LS-45 spectro-
fluorophotometer using a cuvette of (path-length: 1 cm). The 2D-
ROESY spectrum was measured on a Bruker Avance II 900
(900 MHz, Korea Basic Science Institute), with a relaxation delay of
2 s and a ROESY spinlock time of 250 ms.
3. Results and discussion
The chromophoric moiety of the acetylene dye used in the
current work is identical to the monomer unit of poly(p-phenyl-
eneethynylene)s (PPEs) in its chemical structure. Thus, it shows high
fluorescence emission in most polar solvents [17e19], but due to its
inherent rigid and linear structure, it inevitably exhibits limited
solubility at a high molar concentration, which leads to a consider-
able decrease in emission intensity. However, in a mixture with
2.3. Synthesis
The acetylene dye, 5,50-(biphenyl-4,40-diylbis(ethyne-2,1-diyl))
bis(2-hydroxybenzoic acid), was prepared by the Pd-catalyzed Heck-
Cassar-Sonogashira-Hagihara reaction following the previous report
(Scheme 1) [16]. The detailed synthetic precedure is as follows: 4,40-
diethynylbiphenyl (270 mg, 1.3 mmol) and 2-hydroxy-5-iodo-
g-CD, its fluorescence emission is much enhanced mainly due to
a remarkable increase in the dye’s solubility. As the concentration of
the complex increases, an anisotropic architecture is easily observed
with the naked eye. At an elevated temperature, this anisotropic
phase disappears, leading to a clear isotropic solution, but the
Fig. 1. (a) Chemical structures of g-CD and acetylene dye, (b) X-ray powder diffraction patterns (WAXD) of the vacuum-dried complex, and (c) small-angle X-ray scattering (SAXS)
data of the complex in regard to the q vector. The complex consists of dye (1.0 ꢂ 10ꢁ2 mol Lꢁ1) and
g
-CD (2.0 ꢂ 10ꢁ2 mol Lꢁ1).

1546
S. Jeong et al. / Dyes and Pigments 93 (2012) 1544e1548
c
b
a
8
7
6
5
4
3
ppm
Fig. 4. 1H NMR spectra of (a) a dye only, (b) a mixture made of dye and
g-CD, and (c)
the mixture with equimolar Pb2þ
. In all cases, the concentrations of dye
(1.0 ꢂ 10ꢁ2 mol Lꢁ1) and
g
-CD (2.0 ꢂ 10ꢁ2 mol Lꢁ1) are fixed. All measurements were
Fig. 2. (a) Photographic images of the complex taken under UV light
made using D2O as a solvent at 25 ꢀC.
(
lexcitation ¼ 365 nm) after it was standing for 24 h with the addition of metal ions. The
complex is prepared with dye (1.0 ꢂ 10ꢁ4 mol Lꢁ1) and
g
-CD (2.0 ꢂ 10ꢁ4 mol Lꢁ1) for
an increased visual clarification. The order of the added metal ions was as follows:
none, Al3þ, Cr3þ, Cu2þ, Fe3þ, Liþ, Mg2þ, Pb2þ, Ca2þ, Ni2þ, Zn2þ, Cd2þ, and Hg2þ (left-
right). All solutions were prepared in a HEPES buffer solution (pH 7.5). Cu2þ shows
a similar effect to that of Pb2þ, but it is rather weak and slow. (b) Photographic images
of the complex under normal light (left) and under UV light (right). The complex was
metals featuring a guest chromophore containing salicylic acid. In
our preliminary experiment, Pb2þ was distinguished from all other
metals: the fluorescence of the supramolecular complex was
completely quenched in its presence (Fig. 2a). No other tested metal
ions exhibited such an effective quenching. The response to Pb2þ
occurred exclusively in the CD-dye complex, but not in the chro-
morphore itself, and such behavior occurred only in the presence of
prepared at increased concentrations of dye (1.0
ꢂ
10ꢁ2 mol Lꢁ1
) and g-CD
(2.0 ꢂ 10ꢁ2 mol Lꢁ1). The order of the added metal ions was as follows: none, Cr3þ, and
Pb2þ (left-right).
g-CD, not in either a-CD or b-CD. At higher concentrations, the
anisotropic texture disappeared because of the metal ion, followed
by precipitation of the chromophore from the solution after it was
left standing for a day (Fig. 2b). As a comparison, the addition of
Cr3þ caused the complex to become turbid with slight changes in
both absorption and emission of the complex.
anisotropy reappears when cooled down. X-ray scattering
measurements (
l
¼ 1.54 Å) reveal a strong peak at 2
q
¼ 5.9ꢀ (Fig.1a).
This peak is attributable to the characteristic reflection at the (200)
plane of the hexagonal CD packing [20]. While scanning at a scat-
tering vector q, we observed several peaks at 0.062, 0.172, and
0.282 Åꢁ1 (Fig. 1b), which indicate the existence of a sublayer with
a regular spacing of 10.2 nm. This corresponds to a stacking value of
The SterneVolmer equation is a useful tool for quantitatively
measuring fluorescence quenching: [Q] is the quencher concen-
tration, KSV is the SterneVolmer constant, F0 is the fluorescence
intensity measured without an additional quencher, and F[Q] is the
fluorescence intensity with the quencher at a given [Q] [21,22]. The
slope of the graph (F0/F[Q]) versus [Q] yields KSV according to the
equation F0=F½Qꢃ ¼ 1 þ KSV ½Qꢃ. The more sensitive a system is to
a specific quencher, the steeper the SterneVolmer plot, which
results in a higher KSV value. We observed linear SterneVolmer
plots with a large KSV value for lead ion as a quencher. The complex
exhibited significant fluorescence quenching for lead salts as
21.8
-CD units to stack on top of each other to form cylindrical CD
channels, which are reinforced by hydrogen bonding between the
included chromophore molecules and neighboring -CD molecules.
g-CD (0.79 nm ꢂ 12.9 ¼ 10.2 nm). Hexagonal packing allows
g
g
Consequently, the tightly-packed CD crystal constructs anisotropic
textures with the aid of included chromophoric threads inside CDs,
which strengthens the extended hexagonal channels.
After obtaining the above results, we turned our attention to the
influence of metal ions on the current supramolecular complex. The
objective here was to investigate the response of the complex to
quenchers, with KSV
¼
1.3 ꢂ 105 (Fig. 3a). An anisotropic
Fig. 3. (a) Fluorescence emission spectra of the complex with the addition of Pb2þ. The arrow indicates an increasing amount of Pb2þ, and the inset represents relative intensities
with the concentration of Pb2þ. The complex consists of dye (1.0 ꢂ 10ꢁ5 mol Lꢁ1) and
g
-CD (2.0 ꢂ 10ꢁ5 mol Lꢁ1). (b) Schematic representation of the complex break-up induced by
lead ion, which provides apparent advantages over small molecules for sensing applications.

S. Jeong et al. / Dyes and Pigments 93 (2012) 1544e1548
1547
Fig. 5. ROESY spectrum of the complex (a) before and (b) after adding equimolar Pb2þ. Magnified inset in (a) shows strong NOE interactions between the dye and
g
-CD. The
concentrations of the dye and
g
-CD are 1.0 ꢂ 10ꢁ2 mol Lꢁ1 and 2.0 ꢂ 10ꢁ2 mol Lꢁ1, respectively. Measurements were made using D2O as a solvent at 25 ꢀC.
supramolecule, as mentioned earlier, is constructed by multiple
hydrogen bonding between adjacent CD stacks. Consequently, it is
more susceptible to the attack of lead metal ions, which is
considered responsible for the quenching enhancements. Such
behavior is similar to the molecular wire effect of conjugated
polymers associated with electronic communication along the
polymer backbone [23,24]. From the above results, a possible
quenching mechanism may be proposed, in which the break-up of
the supramolecular chain is induced by lead ions, which provides
apparent advantages over small molecules in sensing applications
(Fig. 3b). Meanwhile, with Cr3þ, the complex was modestly
quenched with a KSV value of w103; hence the complex still
exhibited fluorescence even in its presence.
not yetsurewhat causes such a high selectivityof the supramolecular
complex, a topic that will be elaborated in subsequent investigations,
these CD-templated supramolecular properties may be useful in
analytical applications as a novel platform for metal ion sensing.
Acknowledgments
This research was supported by the Basic Science Research
Program through the National Research Foundation of Korea fun-
ded by the Ministry of Education, Science and Technology (Grant
No. 2010-0007145).
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