Fluorescence turn on sensor for sulfate ion in aqueous medium using tripodal-Cu2+ ensemble

Article abstract of DOI:10.1007/s10895-013-1306-7

We report the selective recognition of sulfate anion in aqueous medium at biological pH 7.2 over the other interfering anions based on naphthoic acid bearing tripodal ligand by applying fluorescence turn off-on mechanism. The carboxylic acid groups in the ligand enhance the solubility in water and enable it to form complex with copper salt. Thus formed L-Cu²⁺ ensemble quench the fluorescence of the parent ligand and in turn recognize sulfate anion via revival of fluorescence intensity. The 1:2 stoichiometry was confirmed by ESI mass spectral data and Job's plot. The average binding constant was found to be 6.2∈×∈10⁸ M^-2. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10895-013-1306-7

J Fluoresc (2014) 24:411–416
DOI 10.1007/s10895-013-1306-7
ORIGINAL PAPER
Fluorescence Turn on Sensor for Sulfate Ion in Aqueous
2+
Medium Using Tripodal-Cu Ensemble
Md. Najbul Hoque & Arghya Basu & Gopal Das
Received: 31 August 2013 /Accepted: 25 September 2013 /Published online: 5 October 2013
#
Springer Science+Business Media New York 2013
Abstract We report the selective recognition of sulfate anion
in aqueous medium at biological pH 7.2 over the other inter-
fering anions based on naphthoic acid bearing tripodal ligand
by applying fluorescence turn off-on mechanism. The car-
boxylic acid groups in the ligand enhance the solubility in
water and enable it to form complex with copper salt. Thus
sulfate anion. Thus, the design of synthetic receptors capable
of selective binding and sensing of sulfate anion in aqueous
medium has emerged as a major endeavor in recent times.
The recent trend to design synthetic receptor for anion sens-
ing are based on H-bonding capable functionality viz. amide,
urea, thiourea, imidazole, or pyrrole subunits [3–5]. However,
it may be mentioned here that a vast majority of literature
reports describe anion sensing in organic media. Thus it is of
great challenge to design synthetic anion receptor in aqueous
environment to have real life application. Hence a major
motivation in supramolecular chemistry is to selectively sense
the anions in a competitive aqueous medium. In this direc-
tion, Wu and Yang et al. developed receptors for sulfate anion
and demonstrated that the binding ability decreases with
addition of water [6–8]. Squaramide based ligand have been
derived that senses sulfate anion in 10 % water-alcohol mix-
ture and modification of those moiety resulted in enhanced
sensing of sulfate anion in pure water [9, 10]. A fluorescent
probe can be considered as a valuable sensing platform for
anions in aqueous medium. Anions are strong hydrogen
acceptor and in aqueous solution remains hydrated hence a
big competition occur for H-bond with hydrogen of water and
H-bonding site in the ligand that may be a major impediment
of anion sensing in aqueous medium. In the recent years
fluorescence turn off-on technique have been introduced for
better sensing of anions on the basis of anion binding site and
signaling unit, where in situ formed soluble coordination
complex can be an efficient ensemble for subsequent anion
sensing in aqueous medium. A fluorescence turn off-on
technique may overcome this problem as evidenced in sens-
ing of cyanide ion [11–13], sulfide ion [14–17], fluoride ion
2
+
formed L-Cu ensemble quench the fluorescence of the
parent ligand and in turn recognize sulfate anion via revival
of fluorescence intensity. The 1:2 stoichiometry was con-
firmed by ESI mass spectral data and Job’s plot. The average
8
−2
binding constant was found to be 6.2×10 M .
.
.
Keywords Fluorescenceturnoff-on sensor Sulfate sensing
.
Naphthoic acid based tripodal Aqueous medium
Introduction
The alarming increase in environmental pollutions and asso-
ciated health hazards caused by chemical productions, pesti-
cides, drug wastes and industrial wastes are of grave concern.
Amongst all known sources of pollutions, sulfur containing
compounds have drawn special attention as they are known to
be present in upper atmosphere as a contaminant, in nuclear
waste that can interfere treatment process, low solubility in
borosilicate glass during vitrification and also responsible for
permanent hardness of water [1, 2]. The most common sol-
uble sulfur containing compound in aqueous environment is
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-013-1306-7) contains supplementary material,
[
18], iodide ion [19–21], and acetate ion [22] in different
mixture of solvent. In continuation of our work on sensing
23–31] herein we report fluorescence turn off-on sensor for
which is available to authorized users.
:
:
M. N. Hoque A. Basu G. Das (*)
Department of Chemistry, Indian Institute of Technology Guwahati,
Assam 781 039, India
[
sulfate anion in aqueous medium using fluorogenic naphthoic
acid containing tripodal host.
e-mail: gdas@iitg.ernet.in

4
12
J Fluoresc (2014) 24:411–416
In most of the cases of anion sensing study anions are taken
as a tetra-alkylammonium salt of the corresponding anion and
there is concern about the solubility of the receptors in water.
The competition of H-bonding with water molecule and re-
ceptor reduces the sensing ability of the receptor for the anion
in aqueous medium. In this regard, sensing property of the
tripodal ligand for sulfate anion as a sodium salt using fluo-
rescence turn off-on techniqueis a feasible key to overcome
this issue. From that point of view our results epitomizes the
meaningful application in practical purposes.
2
Scheme 1 Synthetic scheme for Na L
Experimental
Materials and Methods
anhydrous Na SO and solvents were removed under reduced
2
4
pressure that leaves pure the brown solid product.
Commercially available 1-Hydroxy-naphthalene-2-carboxylic
acid and Triethanolamine were obtained from Sigma, USA
and used as supplied. The solvents were distilled freshly
following a standard procedure. Other chemicals were of
reagent grade and used without further purification. The FT-
IR (Fourier transform infrared) spectra were recorded on a
Perkin Elmer-Spectrum One FT-IR spectrometer with KBr
1
Yield 90 %, mp 77–79 °C. H−NMR (400 MHz, CDCl ) δ
3
(
ppm): 3.95 (s, 3H, Me), 8.40–7.19 (m, 6H, Ar) 11.97 (s, 1H,
13
OH). C-NMR (100 MHz, CDCl ) δ(ppm): 51.15, 106.09,
3
1
18.36, 123.84, 124.85, 125.22, 127.55, 129.27, 137.35,
-1
161.42, 173.77. FT-IR spectra: 1,635 cm for ester −C=O
(Fig. S1, Fig. S2, Fig. S6). Data for the compound are in
agreement with published previously [32].
−1
(
potassium bromide) disks in the range 4,000–500 cm .
The absorption spectra were recorded on a Perkin Elmer
Lambda- 25 UV-Visible Spectrometer at 298 K. While the
fluorescence measurements were carried on a Horiba
Fluoromax-4 spectrofluorimeter using 10 mm path length
quartz cuvettes with a slit width of 5 nm at 298 K by exciting
at 320 nm wavelength. The NMR (Nuclear Magnetic Reso-
nance) spectra were recorded on a Varian FT-400 MHz
Synthesis of Tripodal Ester (2) A mixture containing 1-
Hydroxy-naphthalene-2-carboxylic acid methylester (1.5 g,
7
.4 mmol) and finely powdered solid K CO (8 g) in 30 mL
2 3
acetonitrile were refluxed at 80 °C for 1 h (Scheme 1). To the
hot solution dissolved Tris(2-chloro-ethyl)amine hydrochlo-
ride salt (3.57 g, 14.7 mmol) added in portions over 15 min.
The whole reaction mixture was refluxed for another 12 h for
completion of the reaction. Then solid mass was removed by
filtration and filtrate was evaporated in vacuum to give brown
(
Megahertz) instrument. The chemical shifts were recorded
in parts per million (ppm) on the scale using tetramethylsilane
TMS) as a reference. The following abbreviations are used to
(
semi-solid product.
1
describe spin multiplicities in H NMR spectra: s = singlet; d =
doublet; t = triplet; m = multiplate. All luminescence mea-
surements were made using optically dilute solutions and
were corrected for spectral imperfections of the instrument
by reference to a standard lamp. The mass spectra were
obtained using Waters Q-ToF Premier mass spectrometer.
1
Yield 70 %, H−NMR (400 MHz, CDCl ) δ (ppm): 7.30–
3
8
4
.40 (m, 6H, Ar), 4.80 (t, 2H, OCH ), 3.80 (t, 2H, OHCH ),
.20 (s, 3H, OMe), 3.30 (t, 2H, NCH2). C-NMR (100 MHz,
2 2
13
CDCl ) δ(ppm): 42.15, 52.83, 56.91, 63.64, 105.64, 118.65,
3
1
1
23.92, 124.30, 125.32, 126.70, 129.45, 137.24, 160.94,
71.47. FT-IR spectra: 1,712 cm for ester −C=O. HRMS
−1
(
positive mode ESI): found, m/z 518.2208 (cal 518.2173) for
+
Synthesis and Characterization
[LMe +H ] (Fig. S3, Fig. S4, Fig. S7, Fig. S13).
2
Synthesis of 1-Hydroxy-Naphthalene-2-Carboxylic Acid
Hydrolysis of Ester and Synthesis of Disodium Salt (Na L)
2
Methylester (1) 1-Hydroxy-naphthalene-2-carboxylic acid
(
1
0.9 g, 5 mmol) was thoroughly mixed in methanol and
mL conc. H SO (sulfuric acid) was added slowly by
To hydrolyze the ester group the tripodal ester (2) was
dissolved in 10 mL methanol and methanolic solution of
sodium hydroxide (5 N, 40 mL) was added to it and resulting
mixture was refluxed for 12 h at 80 °C. Then the solution was
neutralized partially with dil. HCl to pH ~7.0 that yields
2
4
portions with continuous stirring (Scheme 1). Finally whole
reaction mixture was refluxed at 80 °C for 8 h. The solvent
was evaporated under reduced pressure and brown solid mass
was extracted with ethylacetate 3×20 mL. The organic layer
was washed several times with water and dried over
colorless solid powder Na L from slow evaporation of the
2
solvent.

J Fluoresc (2014) 24:411–416
413
1
Yield 70 %, mp 210–212°C H−NMR (400 MHz, D O) δ
Calculation of the Binding Constants
2
(
ppm): 7.30–8.20 (m, 6 H, Ar), 4.80 (t, 2 H, OCH ), 3.76 (t, 2 H,
2
−1
−5
HOCH ), 3.36 (t, 2 H, NCH ). FT-IR spectra: 1,592 cm for
Receptor Na L with an effective concentration of 1.0×10
M
2
2
2
carboxylate −C=O. HRMS (positive mode ESI): found, m/z.
in an aqueous Tris–HCl buffer (15 mmol, pH 7.2) was used
+
2+
2−
4
90.1747 (cal 490.1866) for [LH +H ] and 512.1824 (cal
for the emission titration studies with a Cu /SO solution.
2
4
+
+
2+
512.1685) for [LH+Na +H ] (Fig. S5, Fig. S8, Fig. S14).
A stock solution of Cu (as chloride salt), having a concen-
−4
tration of 2.0×10 M in an aqueous Tris–HCl buffer (pH 7.2)
solution was used. The binding constant K of the metal
complex was determined by equation (2), assuming the con-
centration of free metal is about equal to its total concentration
General Methodology Adopted for Spectroscopic Studies
Solutions of the sodium or potassium salts of the respective
anions were used for the studies. For checking the relative, but
qualitative, binding affinity of individual anions toward the L-
([M] ≅ [M] ),
t
n
2
+
2+
ðF−F0Þ=ðFm−FÞ ¼ ½CŠ=½DŠ ¼ K½MŠ
ð2Þ
Cu (complex of L and Cu salt), the effective anion con-
centration aqueous Tris–HCl buffer was maintained at
Where F , F, and F are the corrected fluorescence emis-
1
5 mmol, while that for the receptor Na L was maintained
o
m
2
−6
sion intensity of the complex at initial, interval t, and the final
state at which the complex was fully formed upon addition of
metal ion, respectively. The binding constant K was deter-
at 10.0×10 M. UV-visible study could not give satisfactory
2
+
result of effect of anion towards L-Cu ensemble. So we
mainly focused on fluorescence spectroscopic study. A solu-
tion of the sodium or potassium salts of the respective ions in a
spectroscopic-grade aqueous Tris–HCl buffer (15 mmol,
pH 7.2) was used for the studies, while maintaining an effec-
mined from the plot of the linear regression of log [(F−F0)/
(
Fm−F)] vs. log [M] in equation (3), derived from equation (2),
to obtain the intercept as log K and the slope as n. [34]
−5
tive concentration of the respective anions at 5×10 M. A
logðF−F0Þ=ðFm−FÞ ¼ ½CŠ=½DŠ ¼ log K þ n log ½MŠð3Þ
solution of the receptor Na L with an effective concentration
2
of 1.0×10− M was prepared in aqueous medium and was
used for checking the relative sensing affinity with different
anions. Relative Fluorescence Quantum Yield (Φ, It is defined
as the ratio of the number of photons emitted to the number of
photons absorbed.) was measured following the reported lit-
erature (fluorescein in 0.1 N NaOH as a reference) [33].
5
Evaluation of the Detection Limit
The detection limit was calculated based on the fluorescence
2
+
titration. The fluorescence emission spectrum of L-Cu was
measured for ten times and the standard deviation of blank
measurement was achieved. To gain the slope, the ratio of the
fluorescence intensity at 412 nm was plotted as a concentra-
Determination of Stoichiometry Number of the Complex
2−
tion of SO4 . So the detection limit was calculated with the
following equation (4):
The basic equation (1) for determination of the ligand-metal
complexation is:
Detection limit ¼ 3σ=k
ð4Þ
D þ nM ¼ C
ð1Þ
where D is the ligand; M is the metal ion, and C is the
metal-complex.
Where σ is the standard deviation of blank measurement, k
is the slope between the ratio of Fluorescence versus [SO4 ].
2−
Fig. 1 a Fluorescence spectra of
2+
2
Na L in presence of Cu ion (10
equiv) in 15 mmol Tris–HCl
buffer, pH=7.2. λex=320 nm
(
slit=5/5). b Fluorescence
emission spectra of Na
10 μM) with increasing amount
2
L
(
2+
Cu ion (0–2 equiv) in 15 mmol
Tris–HCl buffer, pH=7.2).
INSET: Job’s plot showing
2+
formation of L-Cu ensemble in
:2 ratio
1

4
14
J Fluoresc (2014) 24:411–416
Fig. 2 a Effect of various anions
on the Fluorescence emission
2+
spectra of L-Cu ensemble. b
Fluorescence emission spectra of
2+
L-Cu ensemble (10 μM) with
2
-
increasing amount SO
4
ion
(0–2 equiv) in 15 mM Tris-HCl
buffer, pH=7.2. INSET. Change
in fluorescence intensity of
2+
L-Cu ensemble with respect to
2
-
equivalent of added SO
4
ion
Results and Discussion
fluorescence intensity is essentially completely quenched by
2
+
the addition of Cu ion in aqueous medium. Coordination of
2
+
Owing to the concern of sensing property of the ligand in
water development of sensor for anions in aqueous medium is
a great challenge and we have chosen fluorescence turn off-on
technique, hence we have focused to design a molecule which
is sufficiently soluble in water and have some binding site for
metal ion. Hence we have chosen naphthoic acid based acyclic
nitrogen bridged tripodal ligand which satisfies our assump-
tions. On the other hand the flexibility of the tripodal receptor
provides added advantage in changing its conformation to
accommodate guest species in solution.
the Cu ion facilitates the metal to ligand charge transfer
(MLCT) that is concomitant with the quenching of fluores-
cence intensity of the ligand.
Fluorescence emission of the disodium complex Na Lwas
2
2
+
almost completely quenched by Cu ion in Tris–HCl buffer
at pH 7.2. The effect of different oxy anions and halides (viz.
−
−
2−
3−
−
−
−
−
AcO , NO , SO , PO4 , ClO , F , Br , and I of con-
3
4
4
−5
2+
centration 5.0×10 M) on L-Cu ensemble were examined
in Tris–HCl buffer at pH=7.2. As evident in Fig. 2a, selective
recognition of sulphate anion was achieved by a significant
enhancement of the fluorescence intensity of L-Cu ensem-
ble (Φ=0.52). Addition of other anions could not recover the
diminished fluorescence intensity of L-Cu .
It was further conceived that reacting the L-Cu complex
with anions may replace Cu ion from the ligand environ-
ment leading to recovery of the fluorescence of the free ligand
in solution. It is known that binding affinity of the sulfate
2
+
Absorption spectra of the ligand do not provide any signif-
icant outcome while interacting with metal ions and/or anions
2
+
(
Fig. S9). This suggests that ligand-metal interaction is unable
2
+
to change any ground state interaction efficiently. Hence we
mainly focused on fluorescence study to explore sensing
property of the ligand. Fluorescence emission spectra of the
2
+
receptor Na L (10 μM) in 100 % aqueous solution of Tris–
2
2
+
HCl buffer, pH=7.2 was recorded at room temperature (RT).
Upon excitation at 320 nm it exhibits a relatively strong
fluorescence emission at 412 nm (Quantum yield, Φ=0.58).
anion to Cu is stronger than the anionic ligand which in turn
2
+
On addition of Cu ions to Na L causes almost 95 %
2
quenching of the emission intensity at 412 nm (Φ=0.03)
(
Fig. 1a). Titration of the ligand with copper salt resulted in
gradual decrease in the fluorescence intensity of the ligand
Fig. 1b). Two equivalent of copper salt led to 16-fold
quenching of fluorescence intensity and the Job’s plot showed
Fig. 1b INSET) the complex formation in 1:2 binding mode
(
(
between ligand and metal ion. The average binding constant
determined on the basis of fluorescence titration data was
8
−2
found to be 6.2×10 M for a considering 1:2 binding model
(
(
Fig. S10). The chelation enhanced fluorescence quenching
2+
Fig. 3 Normalized fluorescence responses of L-Cu (10 μM) to various
2
+
CHEQ) behavior of Na L in presence of Cu could be
anions in a 100 % aqueous Tris-HCl buffer solution (pH=7.2). The blue
bars represent the emission intensities of L-Cu in the presence of anions
of interest (50 μM.). The red bars represent the change of the emission
that occurs upon the subsequent addition of (30 μM) of SO
above solution. The intensities were recorded at 412 nm. Excitation wave
length 320 nm, slit 5/5
2
2
+
2+
attributed to the effective binding of Cu in the tripodal
pseudo-cavity. Carboxylate group of the tripodal anionic li-
gand forms coordination complex with Cu in a fashion as
2-
4
to the
2
+
known for metal-carboxylate complexes [35–38]. Hence the

J Fluoresc (2014) 24:411–416
415
Scheme 2 Probable displacement
mechanism of turn off-on
fluorescence sensing of sulfate
anion as a sodium salt in aqueous
medium
yields water soluble copper sulfate in aqueous medium. Only
the addition of sulfate anion causes increase in the fluores-
spectra of free ligand gave two fundamental peaks at m/z
+
490.1747 (calc. 490.1866) for [LH +H ] and 512.1824 (calc.
2
2
+
+
+
cence intensity of L-Cu complex. The fluorescence titration
512.1685) for [LH+Na +H ]. Mass spectrometry data of the
L-Cu complex after addition of sulfate anion (Fig. S16)
2
+
2+
of L-Cu complex by sulfate ion showed almost complete
revival of fluorescence by 2 equivalent of sulfate anion
essentially matches with fundamental peak of the free ligand
(
Fig. 2b, INSET). A linear relationship was observed from
(Fig. S14). Based on the fluorescence and mass spectral data
2
+
2−
2+
the plot of fluorescence intensity of L-Cu with SO4 (at
λmax 412 nm) with respect to the concentration of sulfate and
the detection limit was calculated to 514 nM, suggesting the
(685.0512 corresponding to [L+2Cu +4H O]) plausible
2
mechanism of sulfate ion sensing can be depicted as indicated
in Scheme 2.
2
+
L-Cu ensemble is efficiently sensitive towards sulfate anion
2
+
(
Fig. S11). Sensing capability of the L-Cu complex was
tested in aqueous medium in presence of other competitive
Conclusions
−
−
3−
−
−
−
−
anions like AcO , NO , PO , ClO , F , Br , and I in
3
4
4
−5
much higher concentration (5.0×10 M) than the sulfate ion.
However, no noticeable fluorescence enhancement was ob-
served with these interfering anions. However, addition of
We have reported selective sensing of sulfate anion in 100 %
aqueous medium by using fluorescence turn off-on mechanism.
Our sensing component is a water soluble fluorophoric
naphthoic acid based N-bridged tripodal ligand. Fluorescence
of the ligand was quenched completely by adding copper(II)
salt, which was retrieved selectively by sulfate anion in aqueous
environment through de-metalation in a highly competitive
environment with other interfering anions. The fluorescence
turn off-on sensing property is well applicable in a physiological
range of pH. We have also deduced the probable binding mode
for copper from ESI mass and average binding constant from
fluorescence titration data. Supporting characterization data also
established copper mediated sensing of sulfate anion in solution.
2−
SO4 anion to this mixture caused a dramatic increase in
2
+
fluorescence intensity of L-Cu complex (Fig. 3). Hence,
2
+
the L-Cu ensemble is highly selective towards the sensing
of sulfate anion even in the presence of other competitive
anions in aqueous condition. We have also carried out pH
dependent study of the turn off-on sensing of sulfate anion in
Tris–HCl buffer solution (Fig. S12), which indicates the ra-
tional use of the ensemble in a wide pH range.
These experiments showed good sensing property for sul-
2
+
fate anion over a pH range 6.5–8.5 where intensity of L-Cu
2−
at 412 nm was fully recovered with addition of SO ion. The
Cu induced quenching of fluorescence of Na L and subse-
4
2
+
2
Acknowledgments G. D. gratefully acknowledges Council of Scien-
tific and Industrial Research (01/2727/13/EMR-II) and Department of
Science and Technology (DST), New Delhi, India, for financial support.
NH and AB acknowledge Indian Institute of Technology Guwahati,
India, for fellowship.
quent sulfate selective recovery of fluorescence is due to the
2
+
formation of Cu complex and subsequent de-complexation
2
−
by SO4 ion in solution. This sequence of event is also
confirmed by a detailed positive ion ESI mass spectral analy-
2
+
sis. Mass spectra analysis (Fig. S15) of the L-Cu suggested
a 1:2 binding of metal with ligand having and a mass of m/z
2
+
6
4
85.0512 (calc. 685.0646) corresponding to [L+2Cu +
References
H O] which was in agreement with Jobs plot obtained from
2
fluorescence titration experiment. This also supports solution
state coordination/chelation induced quenching of fluores-
cence. The sensing of sulfate anion was accomplished through
the displacement of metal ion from ligand coordination envi-
ronment with revival of fluorescence of free ligand, which
also could be established from mass spectroscopy. The mass
1
2
. Katayev EA, Ustynyuk YA, Sessler JL (2006) Coord Chem Rev 250:
3004–3037
. Weingärtner H, Franck EU, Weigand G, Dahmen N, Schwedt G,
Frimmel FH, Gordalla BC, Johannsen K, Summers RS, Höll W, Jekel
M, Gimbel R, Rautenbach R, Glaze HW (2000) Ullman’s Encyclo-
paedia of Industrial Chemistry. Wiley-VCH Verlag GmbH & Co. doi:
10.1002/14356007.a28_001

4
16
J Fluoresc (2014) 24:411–416
3
4
5
. Caltagirone C, Gale PA (2009) Chem Soc Rev 38:520–563
19. Joseph R, Chinta JP, Rao CP (2010) Inorg Chim Acta 363:2833–2839
20. Wang H, Xue L, Jiang H (2011) Org Lett 13:3844–3847
21. Ma B, Zeng F, Zheng F, Wu S (2011) Chem Eur J 17:14844–14850
22. Hu ZQ, Wang XM, Feng YC, Ding L, Li M, Lin CS (2011) Chem
Commun 47:1622–1624
23. Kar C, Das G (2013) J Photochem Photobiol A 251:128–133
24. Dey SK, Das G (2011) Chem Commun 47:4983–4985
25. Pramanik A, Das G (2009) Tetrahedron 65:2196–2200
26. Basu A, Das G (2011) Dalton Trans 40:2837–2843
27. Basu A, Das G (2012) Inorg Chem 51:1727–1738
28. Kar C, Basu A, Das G (2012) Tettrahedron Lett 53:4754–4757
29. Dutta BK, Kar C, Basu A, Das G (2012) Tettrahedron Lett 54:771–
774
. Gale PA (2006) Preface Coord Chem Rev 250:2917
. Oton F, Tarraga A, Espinosa A, Velasco MD, Molina P (2006) J Org
Chem 71:4590–4598
6
7
. Jia C, Wu B, Li S, Huang X, Yang XJ (2010) Org Lett 12:5612–5615
. Jia C, Wu B, Li S, Yang Z, Zhao Q, Liang J, Li QS, Yang XJ (2010)
Chem Commun 46:5376–5378
8
. Jia C, Wu B, Li S, Huang XJ, Zhao Q, Li QS, Yang XJ (2011) Angew
Chem Int Ed 50:486–490
9
. Prohens R, Martorell G, Ballester P, Costa A (2001) Chem Commun
1
6:1456–1457
1
0. Delgado-Pinar E, Rotger C, Costa A, Piña MN, Jiménez HR, Alarcón
J, García-España E (2012) Chem Commun 48:2609–2611
1
1
1
1
1
1
1. Zeng Q, Cai P, Li Z, Qin J, Tang BZ (2008) Chem Commun 9:1094–
096
2. Li Z, Lou X, Yu H, Li Z, Qin J (2008) Macromolecules 41:7433–
439
3. Chung SY, Nam SW, Lim J, Park S, Yoon J (2009) Chem Commun
0:2866–2868
4. Zhang L, Lou X, Yu Y, Qin J, Li Z (2011) Macromolecules 44:5186–
193
5. Lou X, Mu H, Gong R, Fu E, Qin J, Li Z (2011) Analyst 136:684–
87
6. Choi MG, Cha S, Lee H, Jeon HL, Chang SK (2009) Chem Commun
7:7390–7392
30. Kar C, Adhikari MD, Ramesh A, Das G (2013) Inorg Chem 52:742–
752
31. Kar C, Adhikari MD, Ramesh A, Das G (2012) RSC Adv 2:9201–
9206
32. Cikotiene I, Buksnaitiene R, Sazinas R (2011) Tetrahedron 67:706–
717
33. Paeker A, Rees WT (1960) Analyst 85:587–600
34. Nguyen DM, Wang X, Ahn HY, Rodriguez L, Bondar MV, Belfield
KD (2010) Appl Mater Interfaces 2:2978–2981
1
7
2
5
6
35. Neogi S, Bharadwaj PK (2006) Cryst Growth Des 6:433–438
36. Luan J, Sui FF, Lin HY, Lui GC, Xu C, Wang XL (2013) Cryst
Growth Des 13:3561–3576
4
1
1
7. Cao X, Lin W, He L (2011) Org Lett 13:4716–4719
8. Lu W, Jiang H, Hu F, Jiang L, Shen Z (2011) Tetrahedron 67:7909–
37. Chen Z, Gao DL, Diao CH, Liu Y, Ren J, Chen J, Zhao B, Shi W,
Cheng P (2012) Cryst Growth Des 12:1201–1211
38. Liu GZ, Li SH, Wang LY (2012) CrystEngComm 14:880–889
7
912