Colorimetric sensing of Cu2+ using a cyclodextrin-dye rotaxane

Article abstract of DOI:10.1016/j.dyepig.2010.02.003

A dye rotaxane having 8-hydroxyquinoline as coupling component was prepared in the presence of α-cyclodextrin and its structure identified using NMR. The dye rotaxane displayed high color strength regardless of the solvent used and responded to changes in pH based on tautomeric equilibrium, without any interference by the cyclodextrin macrocycle. Compared to the non-rotaxanated forms, the dye rotaxane displayed a characteristic spectral response to the presence of Cu²⁺ which was attributed to the formation of?a polymeric, metal-dye rotaxane complex which was of high solubility because of the presence of cyclodextrin. Thus, this approach can clearly be used as a facile and efficient sensor for copper ions.

Full text of DOI:10.1016/j.dyepig.2010.02.003

Dyes and Pigments 87 (2010) 49e54
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Colorimetric sensing of Cu2þ using a cyclodextrinedye rotaxane
,
b
,
a
b
c
,
,d
Jong S. Park a , Sooyeon Jeong , Seongkook Dho , Myungwon Lee d, Chungkun Song c
*
a Laboratory of Polymer and Electronic Materials, Dong-A University, Busan 604-714, Republic of Korea
b Department of Textile Industry, Dong-A University, Busan 604-714, Republic of Korea
c Media Device Laboratory, Dong-A University, Busan 604-714, Republic of Korea
d Department of Electronic Engineering, Dong-A University, Busan 604-714, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A dye rotaxane having 8-hydroxyquinoline as coupling component was prepared in the presence of
-cyclodextrin and its structure identified using NMR. The dye rotaxane displayed high color strength
Received 19 January 2010
Accepted 19 February 2010
Available online 1 March 2010
a
regardless of the solvent used and responded to changes in pH based on tautomeric equilibrium, without
any interference by the cyclodextrin macrocycle. Compared to the non-rotaxanated forms, the dye rotaxane
displayed a characteristic spectral response to the presence of Cu2þ which was attributed to the formation
of a polymeric, metaledye rotaxane complex which was of high solubility because of the presence of
cyclodextrin. Thus, this approach can clearly be used as a facile and efficient sensor for copper ions.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Azo dye
Cyclodextrin
Rotaxane
Cu2þ
Metal sensing
Sensor
1. Introduction
[12,13]. Cyclodextrins (CDs) are conically shaped, cyclic oligosac-
charides in which the primary hydroxyl groups are situated on the
Many heavy metals are hazardous to both humans and the
environment, of which, copper is highly toxic to various organisms
and is harmful to humans at high concentration [1,2]; in recent years,
copper has been linked to organ damage in infants. For this reason,
the concentration of copper in drinking water is strictly regulated;
drinking water containing >3 mg Lꢀ1 is harmful to human health [3].
Moreover, copper ions are associated with a number of serious
diseases, such as Alzheimer's and prion diseases [4,5]. Hence, the
accurate and facile detection of copper ions is important. Classic
detection methods include atomic absorption spectrometry, induc-
tively coupled plasma mass spectroscopy, X-ray fluorescence spec-
trometry and electrochromic techniques, all of which are based on
instrumental analysis [6e9]. Although several papers describe fluo-
rescent chemosensors which offer the advantages of rapid, sensitive
and nondestructive detection methods [10,11], these sensors possess
limitations related to low signal intensity and quenching by
other compounds; moreover, they require a fluorescence measuring
device.
narrow side of torus glucose residues and the secondary hydroxyl
groups are located on the wider side. Several rotaxane dyes
have been synthesized using the hydrophobic effect of CDs to direct
rotaxane formation, in which the CDs were claimed to reduce
fluorescent quenching, enhance fluorescence and photostability
and, in addition, to increase resistance to bleaching [14e16].
The present work concerns a dye rotaxane that exhibits high
selectivity and sensitivity towards Cu2þ, this being achieved by using
8-hydroxyquinoline (8-HQ) as the blocking group. 8-HQ is a well-
known ligand with high reactivity toward metal ions [17,18]; its
complexes have been used as an important component of organic
light-emitting devices. Although azo dyes in conjunction with
8-HQs, which are collectively termed azoxines, have been reported,
none was rotaxanated [19], by means of which, the dyes are able to
display colorimetric response to copper ions. This color change can
be identified easily by the naked eye, which mitigates the need for
instrumental assessment.
A rotaxane comprises a supramolecular assembly in which
a macrocycle is located around a dumb-bell shaped molecular axis
2. Experimental
2.1. Materials
* Corresponding author. Laboratory of Polymer and Electronic Materials, Dong-A
University, Busan 604-714, Republic of Korea. Tel.: þ82 51 200 7330; fax: þ82 51
200 7540.
The chemicals used in the synthesis and purification of the
rotaxane dye were 4,40-diaminostilbene dihydrochloride, 8-hydrox-
E-mail address: jongpark@dau.ac.kr (J.S. Park).
yquinoline, sulfanilic acid, a-cyclodextrin (a-CD), HCl (37%), NaNO2,
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.02.003

50
J.S. Park et al. / Dyes and Pigments 87 (2010) 49e54
Diazotization
-Cl +N2
HCl, NaNO2
H2N
NH2
N2+ Cl-
α-cyclodextrin
Coupling
HO
N
N
OH
N
N
N
N
OH
N
1
Scheme 1. Synthesis of dye rotaxane 1.
sodium acetate, methanol, ethanol, silica, methylethylketone,
ammonium hydroxide,1-propanol, and DMSO-d6. All chemicals used
in the syntheses were of laboratory reagent grade.
washed with water and dried in a vacuum oven. The crude product
was refluxed in methanol and ethanol for 12 h, respectively, fol-
lowed by hot filtration and washing with hot ethanol. The product
was isolated using column chromatography on silica using meth-
ylethylketone/ammonium hydroxide/1-propanol (1/1/1 vol%) as an
eluent and was subsequently dried in a vacuum oven (yield 8%). 1H
(500 MHz, DMSO-d6): 9.4 (d,1H), 9.3 (d,1H), 8.6 (m, 2H), 8.5 (d,1H),
8.0 (q, 2H), 7.8 (d, 1H), 7.7 (d, 2H), 7.6 (d, 2H), 7.4 (d, 1H), 7.3 (d, 1H),
7.1 (d, 2H), 7.0 (d, 2H), 6.2 (q, 2H), 5.3 (s, 12H), 4.7 (s, 6H), 4.3 (s, 6H),
3.8e3.2 (m, 36H).
2.2. Dye synthesis
The rotaxane dye 1 and its non-rotaxanated counterpart 2 were
prepared using the following known methods [14,15] (Scheme 1).
2.2.1. Dye rotaxane 1
4,40-Diaminostilbene dihydrochloride (1 g, 3.53 mmol) was
dissolved in water (20 mL) containing HCl (37%, 1.47 mL), to which
2.2.2. Bisazo dye 2
The synthesis method was the same as that described above but
was added,
a-CD (9.7 g, 9.96 mmol) in water (20 mL) and the
without the addition of
DMSO-d6 with NaOH): 9.1 (d, 2H), 8.5 (m, 2H), 8.0 (d, 4H), 7.6
(q, 4H), 7.4 (m, 4H), 7.2 (m, 2H), 6.4 (m, 2H).
a-CD (yield 30%). 1H NMR (500 MHz,
ensuing mixture was stirred for 12 h at room temperature. The
temperature was lowered to 0e5 ꢁC, and a 5% aq. NaNO2 solution
(7.06 mmol, 10 mL) was added dropwise. The resulting solution was
stirred for 60 min at 0e5 ꢁC to complete diazotization. 8-Hydrox-
yquinoline (1.025 g, 7.06 mmol) in 0.5 N NaOH (16 mL) was then
added dropwise over 60 min and stirring continued at room
temperature overnight, after which time, the pH was adjusted to
7.5e8.0 using dilute aq. sodium acetate solution and the precipi-
tated dye was collected by filtration and dried under vacuum.
The dye was dissolved in aq. KOH (pH 12) and the insoluble
component was removed by filtration. The pH of the filtrate was
adjusted to neutral and the resulting precipitate was collected,
2.2.3. Monoazo dye 3
Sulfanilic acid (1.221 g, 7.06 mmol) was dissolved in water
(20 mL) containing HCl (37%, 1.47 mL), the temperature of the
solution was lowered to 0e5 ꢁC, and 5% aq. NaNO2 solution
(7.06 mmol, 10 mL) was added dropwise. The ensuing solution was
stirred for 60 min at 0e5 ꢁC to complete diazotization and 8-
Hydroxyquinoline (1.025 g, 7.06 mmol) in 0.5 M aq. NaOH solution
(16 mL) was added dropwise over 60 min and stirring continued at
Fig. 1. 1H NMR spectra of dyes 1, 2, and 3.

J.S. Park et al. / Dyes and Pigments 87 (2010) 49e54
51
H
N
amount of Cu2þ was added using micro-pipette. For complete
mixing, 5 min of standing was allowed before measurement of the
spectra.
N
N
N
OH
O
N
N
A
B
-
H+
3. Results and discussion
The structures of the rotaxane dye 1 and its non-rotaxanated
counterpart 2 were confirmed by NMR (Fig. 1). Due to the presence
of both inter- and intra-molecular hydrogen bonding with 8-HQs, it
was difficult to find a suitable NMR solvent for 2; a small amount of
KOH aided solubility. Peak assignment in the aromatic region was
quite straightforward. 2 has a symmetrical structure around the
stilbene group, which is clearly apparent in the aromatic region.
Owing to the deprotonation of the OH groups in 8-HQs by the
addition of KOH, all peaks were slightly shift to the upfield region.
In comparison, 1 readily dissolved in organic solvents, including
DMSO-d6, which can be ascribed to the presence of hydroxyl groups
in the CDs. The NMR spectrum shows a nonequivalent environment
at both ends of the dye molecule due to the additional split and
complex peaks, which can be attributed to the conical shape of CD,
from which the threading of the azo chromophore within the cavity
of the CD could be confirmed.
N
-
N
-
N
O
N
O
N
N
C
Scheme 2. Tautomeric equilibrium of azo dye rotaxane 1 having 8-HQs as coupling
components: Azo tautomer A, hydrazone tautomer B, and resonance structures C at
a higher pH.
room temperature overnight. After this time, the pH was adjusted
to 7.5e8.0 using dilute aq. sodium acetate solution. The precipi-
tated dye was collected by filtration. The crude product was
recrystallized twice in water (50 mL) and then dried in a vacuum
oven (yield 35%). 1H NMR (500 MHz, DMSO-d6): 9.3 (d, 1H), 9.0 (m,
1H), 9 (d, 2H), 7.8 (d, 2H), 7.7 (q, 2H), 7.2 (d, 1H).
1 exhibited higher color strength (27,140 at 440 nm) compared
2.3. Absorbance measurement
to its unrotaxanated analogue 2 (19,040 at 410 nm, DMF:
water ¼ 80:20, pH 7.5). The same result was observed in water-rich
conditions (22,500 at 470 nm for dye 1 vs. 20,500 at 410 nm for dye
2, DMF:water ¼ 20:80, pH 7.5). Under both conditions, 2 exhibited
The UV absorbance spectra were collected using a Lambda 7
spectrometer (Perkin Elmer) in a 1 cm cuvette. For the pH adjust-
ment, HEPES and phosphate buffer solutions with 5 mmol buffer
strength were prepared for pH 7.5 and pH 11, respectively. For metal
ion screening, the concentrations of both the dye and the metal ions
were fixed at 4.0 ꢂ 10ꢀ6 mol Lꢀ1. For concentration titration, a dye
solution of 4.0 ꢂ 10ꢀ6 mol Lꢀ1 was prepared and the corresponding
a red shift due to the presence of a-CD macrocycle, as previously
reported, which hinders aggregation of the dye and promotes the
monomeric state [20,21]. 2 was also very sensitive to pH changes,
existing in two possible tautomeric forms namely the azo A and the
hydrazone B; at higher pH, deprotonation leads to its anionic form
Fig. 2. Colors of dye rotaxane 1 after an addition of equivalent metal ions under different pHs (DMF:water ¼ 80:20, wt%): (a) pH 7.5, (b) pH 11. The concentrations of both the dye
and metal ions were fixed at 4.0 ꢂ 10ꢀ6 mol Lꢀ1. The order of the metal cations added was as follows: none, Al2þ, Cr2þ, Cu2þ, Fe2þ, Liþ, Mg2þ, Pb2þ, Ca2þ, Ni2þ, Zn2þ, Cd2þ, and Hg2þ
(left w right).
Fig. 3. Absorbance spectra of dye rotaxane 1 upon an addition of different concentrations of Cu2þ under different pHs: (a) pH 7.5, (b) pH 11. Mixed solvents (DMF:water ¼ 80:20, wt%)
were used. The concentrations of all dyes were fixed at 4.0 ꢂ 10ꢀ6 mol Lꢀ1. Insets indicate the absorption changes upon the addition of Cu2þ. Note that the absorbance at 520 nm
increases regardless of pHs by adding Cu2þ
.

52
J.S. Park et al. / Dyes and Pigments 87 (2010) 49e54
Fig. 4. Colors after an addition of equivalent metal ions in a mixed solvent of water and DMF (80:20, wt%); (a) dye rotaxane 1 (pH 7.5), (b) dye rotaxane 1 (pH 11), (c) bisazo dye 2
(pH 7.5), (d) bisazo dye 2 (pH 11). The concentrations of both the dye and the metal ions were fixed at 4.0 ꢂ 10ꢀ6 mol Lꢀ1. The order of the metal cations added was as follows: none,
Al2þ, Cr2þ, Cu2þ, Fe2þ, Liþ, Mg2þ, Pb2þ, Ca2þ, Ni2þ, Zn2þ, Cd2þ, Hg2þ (left w right).
C in resonance (Scheme 2) [22]. As a result, the absorption
maximum shifts to longer wavelength, this color change being
completely reversible. This indicates that the CD macrocycle
does not interfere with the tautomeric equilibrium of the threaded
chromophore.
responsive only in DMF-rich conditions (Fig. 4). In water-rich
conditions, the dye did not show spectral changes but rather, its
absorbance decreased gradually with the addition of metal ions due
to its limited aqueous solubility. The structure of 2 resembles that
of a disperse dye whereas, in contrast, the rotaxane 1 exhibits an
identical colorimetric shift that is independent of the solvent.
Even under water-rich conditions, 1 remains stable because of the
hydroxyl groups in CD, despite the fact that there is no solubilizing
group in its molecular structure. It appears that the OH groups in
CD provide a sufficiently hydrophilic environment for the thread
chromophore. As a result, a strong spectral shift with Cu2þ was
observed, even under water-rich conditions.
The influence of metal ions is easily recognized from inspection
of the dye solutions. Eleven different metal ions were prepared
and for simplicity, 1:1 solutions of dye 1 and various metals were
prepared. The rotaxane dye exhibited a color change upon the
addition of metal ions (Fig. 2) of which, its response towards Cu2þ is
characteristic, showing that the color shift depended upon pH
namely, a red shift, from orange to red (pH 7.5) as well as a blue shift
from blue to red (pH 11). Other metal ions, such as Ni2þ and Zn2þ
,
The sensitivity of 1 to Cu2þ was very high; indeed, color was
imparted similar, but comparatively weak, red shifts at pH 7.5;
however, none of these metals imparted a blue shift at pH 11. Under
acidic conditions (pH 5), an identical spectral response to that of pH
7.5 was observed. Hence, the azo tautomer is the likely dominant
species at neutral to acidic pH, leading to identical response to the
different metal ions.
generated upon the addition of <1
alent to 0.17
g mLꢀ1, which is 10 times more sensitive than EPA
regulations regarding the copper content of drinking water. The
large -conjugated structure and presence of two 8-HQs in 1 are
m
mol of Cu2þ, this being equiv-
m
p
responsible for its ability to chelate metal ions and form the cor-
responding, polymeric metal complexes. This extended conjugation
reportedly decreases the loss of energy by thermal vibrational
decay while exhibiting a red shift [23,24]. Moreover, CDs provide
Fig. 3 shows the absorbance spectra of 1 in the presence of various
concentrations of Cu2þ. Upon the addition of Cu2þ, at pH 7.5e80 nm
red shift was observed, from lmax 440e520 nm. Until 1 equivalent
Cu2þ was added to the dye solution, the absorbance at 440 nm
decreased gradually with the appearance of a new absorption peak at
the longer wavelength (520 nm); adding more than 1 equivalent lead
to a slight decrease in absorbance at 520 nm. It is known that the N, O
trans geometry that results from quinoline N together with carbonyl
O, effectively binds metal ions. In such a case, the De
of 8-HQ-containing the azo dye transforms to the Die
p
p
eD0 structure
eA structure
[23,24], which increases the absorbance at 520 nm, this being
responsible for the observed red shift. Meanwhile, dye 1 underwent
a w40 nm blue shift at pH 11, from lmax 560e520 nm in the presence
of Cu2þ. The binding of Cu2þ prevents resonance stabilization of the
anion C, resulting in the observed spectral shift. In addition, at
high pH, metal uptake will increase significantly. The absorbance of
1 at 520 nm continuously increases with increasing [Cu2þ] until 2
equivalents of Cu2þ are added, which is related to the formation of
metal hydroxides [25]. At higher pH, the electron pair of the nitrogen
of 8-HQs is more likely to interact with metal ions and the aromatic
eOH group also presents greater metal ion attraction, by means of
which, dyeemetal complexes are formed. Hence, more Cu2þ is
consumed under alkaline conditions than at neutral pH.
Fig. 5. Absorbance spectra of monoazo dye 3 upon the addition of different concentrations
of Cu2þ (pH 7.5, DMF:water ¼ 8:2). The concentration of the dye was 2.0 ꢂ 10ꢀ5 mol Lꢀ1
.
In order to understand the role of the CDs, azo dyes without CD
were examined. 2 responded to the presence of Cu2þ, but it was
The concentration of Cu2þ was increased from 0 to 20
mmol, as indicated.

J.S. Park et al. / Dyes and Pigments 87 (2010) 49e54
53
Fig. 6. 1H NMR spectra of
a
-CD in (a) dye rotaxane 1, and (b) upon the addition of an equivalent amount of Cu2þ. Subscript number indicates the position of carbon atom in glucose unit.
Acknowledgements
Table 1
Elemental analysis of dye 1 and its complex with Cu2þ and comparison with possible
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea funded by the
Ministry of Education, Science and Technology (Grant No. 2009-
0070801).
empirical formulas.
Empirical formula
Measured (calculated) (%)
C
H
N
Dye 1
48.48
5.96
6.82
(48.29)
(5.89)
(6.83)
References
Complex of dye 1 and Cu2þ
46.65
5.62
6.58
(46.62)
(5.69)
(6.58)
[1] Frausto da Silva JJR, Williams RJP. The biological chemistry of elements:
the inorganic chemistry of life. Oxford: Clrendon Press; 1993.
[2] Harris HH, Pickering IJ, George GN. The chemical form of mercury in fish e
response. Science 2004;303(5659):764e6.
[3] Board on Environmental Studies and Toxicology. Copper in drinking water.
Washington DC: The National Academies Press; 2000.
[4] Jackson GS, Murray I, Hosszu LLP, Gibbs N, Waltho JP, Clarke AR, et al. Location
and properties of metal-binding sites on the human prion protein. Proceed-
ings of the National Academy of Sciences of the United States of America
2001;98(15):8531e5.
increased solubility of the polymeric forms. Thus, high sensitivity
toward copper ions is exhibited regardless of the solvent used.
The sensitivity of 1 was compared to that of 3, which possessd
one 8-HQ and thus cannot form a polymeric structure. 3 showed
lower response levels towards Cu2þ and its absorbance decreased
markedly upon the addition of the metal ion (Fig. 5).
[5] Gaggelli E, Kozlowski H, Valensin D, Valensin G. Copper homeostasis and
neurodegenerative disorders (Alzheimer's, Prion, and Parkinson's diseases and
amyotrophic lateral sclerosis). Chemical Reviews 2006;106(6):1995e2004.
[6] Wang ZK, Fu Z, Ye CL. Journal of Hazardous Materials 2009;170(2e3):705e10.
[7] Kolokassidou K, Szymczak W, Wolf M, Obermeier C, Buckau G, Pashalidis I.
Hydrophilic olive cake extracts: characterization by physicochemical proper-
ties and Cu(II) complexation. Journal of Hazardous Materials 2009;164
(2e3):442e7.
[8] Pancras JP, Ondov JM, Zeisler R. Multi-element electrothermal AAS determi-
nation of 11 marker elements in fine ambient aerosol slurry samples collected
with SEAS-II. Analytica Chimica Acta 2005;538(1e2):303e12.
[9] Gao GG, Xu L, Wang WJ, An WJ, Qiu YF, Wang ZQ, et al. Electrochromic
multilayer films of tunable color by combination of copper or iron complex
and monolacunary Dawson-type polyoxometalate. Journal of Physical Chem-
istry B 2005;109(18):8948e53.
[10] Jiang W, Wang W. A selective and sensitive "turn-on" fluorescent che-
modosimeter for Hg2þ in aqueous media via Hg2þ promoted facile
desulfurization-lactonization reaction. Chemical Communications 2009;
26:3913e5.
[11] Zhang H, Han LF, Zachariasse KA, Jiang YB. 8-Hydroxyquinoline benzoates as
highly sensitive fluorescent chemosensors for transition metal ions. Organic
Letters 2005;7(19):4217e20.
The 1H NMR spectra of the CD components of 1 both before and
after the addition of Cu2þ were compared (Fig. 6). The peaks of the
CDs broadened when complexed with Cu2þ. It was assumed that,
upon metal coordination, the conformational flexibility of the CDs
would be reduced due to the formation of a polymeric structure
[26]. From this, the formation of an extended structure was also
recognized. Elemental analysis of dye 1 and its copper complex is
given in Table 1, from which the proposed formula of the complex
is obtained. These data agree well to an empirical formula of
[dye1eCu2þ$6H2O]; one dye molecule coordinates with one metal
ion at one end. The stoichiometry of the metal complex is, there-
fore, estimated to be 1:1 (dye1:Cu). Water molecules, 6H2O, are
considered to be a hydrated form of
a-CD, and still exist both in dye
rotaxane and in its Cu2þ complex.
[12] Nepogodiev SA, Stoddart JF. Cyclodextrin-based catenanes and rotaxanes.
Chemical Reviews 1998;98(5):1959e76.
4. Conclusions
[13] Harada A. Cyclodextrin-based molecular machines. Accounts of Chemical
Research 2001;34(6):456e64.
[14] Craig MR, Hutchings MG, Claridge TDW, Anderson HL. Angewandte Chemie
International Edition 2001;40(6):1071e4.
[15] Craig MR, Claridge TDW, Hutchings MG, Anderson HL. Synthesis of a cyclo-
dextrin azo dye [3]rotaxane as a single isomer. Chemical Communications
1999;16:1537e8.
[16] Park JS, Wilson JN, Hardcastle KI, Bunz UHF, Srinivasarao M. Reduced fluo-
rescence quenching of cyclodextrin-acetylene dye rotaxanes. Journal of the
American Chemical Society 2006;128(24):7714e5.
In summary, an azo dye rotaxane 1 was prepared using 8-HQ as
a coupling component. Compared to unrotaxanated forms, it shows
a characteristic spectral shift in the presence of Cu2þ with a high
sensitivity. This is attributed to formation of a polymeric structure
of metaledye rotaxane with increased solubility provided by CD
macrocycles. This approach can clearly be used as a facile and
efficient sensor for metal ions.

54
J.S. Park et al. / Dyes and Pigments 87 (2010) 49e54
[17] Wang SN. Luminescence and electroluminescence of Al(III), B(III), Be(II) and Zn
(II) complexes with nitrogen donors. Coordination Chemistry Reviews 2001;
215:79e98.
[23] Cheng YF, Zhang M, Yang H, Li FY, Yi T, Huang CH. Azo dyes based on
8-hydroxyquinoline benzoates: synthesis and application as colorimetric
Hg2þ-selective chemosensors. Dyes and Pigments 2008;76(3):775e83.
[24] Huang H, Zhong C, Zhou Y. Synthesis and luminescent properties of polymeric
metal complexes containing bis(8-hydroxyquinoline) group. European Poly-
mer Journal 2008;44(9):2944e50.
[25] Sankar R, Vijayalakshmi S, Subramanian S, Rajagopan S, Kliyappan T. Synthesis and
chelation properties of new polymeric ligand derived from 8-hydroxy-5-azoqui-
noline hydroxy benzene. European Polymer Journal 2007;43(11):4639e46.
[26] Zhao T, Beckham HW. Direct synthesis of cyclodextrin-rotaxanated poly
(ethylene glycol)s and their self-diffusion behavior in dilute solution. Macro-
molecules 2003;36(26):9859e65.
[18] Chen CH, Shi JM. Metal chelates as emitting materials for organic electrolu-
minescence. Coordination Chemistry Reviews 1998;171:161e74.
[19] Hollingshead RGW. Oxine and its derivatives. London: Butterworths; 1954.
[20] Karakstis KK, Perelman LA, Wong WK. Spectroscopic characterization of azo
dye aggregation on dendrimer surfaces. Langmuir 2002;18(26):10363e71.
[21] Park JS, Koh J. The synthesis and spectral properties of an encapsulated
aminoazobenzene dye. Dyes and Pigments 2009;82(3):347e52.
[22] Gordon PF, Gregory P. Organic chemistry in colour. Berlin Herdleberg: Spinger-
Verlag; 1987.