Novel indole based colorimetric and "turn on" fluorescent sensors for biologically important fluoride anion sensing

Article abstract of DOI:10.1016/j.jphotobiol.2011.02.021

A novel indole Hydrazone receptor 1 has been synthesized by one step of condensation, which can act as an efficient colorimetric and "turn on" fluorescent sensor for fluoride anions; Benesi-Hildebrand equation indicates that 1 associates with F^-

Full text of DOI:10.1016/j.jphotobiol.2011.02.021

Journal of Photochemistry and Photobiology B: Biology 103 (2011) 140–144
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology B: Biology
journal homepage: www.elsevier.com/locate/jphotobiol
Novel indole based colorimetric and ‘‘turn on’’ fluorescent sensors
for biologically important fluoride anion sensing
a,b
a
a
a,
⇑
Qian Li , Yong Guo , Jian Xu , Shijun Shao
a
Key Laboratory of Chemistry of Northwestern Plant Resources and Key Laboratory for Natural Medicine of Gansu Province, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, PR China
Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 January 2011
Received in revised form 24 February 2011
Accepted 28 February 2011
Available online 15 March 2011
A novel indole Hydrazone receptor 1 has been synthesized by one step of condensation, which can act as
an efficient colorimetric and ‘‘turn on’’ fluorescent sensor for fluoride anions; Benesi–Hildebrand equa-
ꢀ
1
tion indicates that 1 associates with F in a 1:1 stoichiometry; [TBA]OH and H NMR titration experi-
ments indicate that the deprotonation process involved upon addition of fluoride anions.
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Indole
Fluorescent sensor
Fluoride anions
1
. Introduction
The design and development of selective optical receptors for
2. Experiment
2.1. Materials
anions has gained considerable attentions, as anions play funda-
mental roles in a wide range of chemical and biological processes
All reagents for synthesis obtained commercially were used
without further purification. In the titration experiments, all the
anions were added in the form of tetrabutylammonium (TBA) salts,
which were purchased from Sigma–Aldrich Chemical, stored in a
vacuum desiccator.
[
1–5]. In recent years, the colorimetric sensors have attracted sig-
nificant interest due to the low cost and using inexpensive equip-
ment. In particular, considerable efforts have been devoted to
fluorescent anion sensors due to their simplicity, high degree of
specificity and low detection limit. And dual mode of colorimetric
and fluorescent sensors are favorable for the potential application
2.2. Synthesis of receptor 1
[
6–15]. So far, many fluorescent fluoride sensors have been re-
ported, but only few of them are ‘‘turn on’’ sensors. Generally,
the recognition process ‘‘turn on’’ fluorescence is advantageous
than that of ‘‘turn off’’ event in terms of the detection limit, be-
cause the signal-to-noise ratio is specific for a certain host–guest
supramolecular ensemble [13].
Indole-based anion receptors have been reported only recently
and they have exhibited high selectivity in their anions binding
capability [16]. In our previous studies, we have developed
bis(indolyl)methane, Tris(indolyl)methene and Quinonehydrazone
derivatives [17–20], which demonstrated high selectivity for an-
ions. Herein we developed a new indole based receptor 1, which
Indole-3-carbaldehyde and 4-Nitrophenylhydrazine was dis-
solved in ethanol and 3 drops of CH COOH was added to the sys-
3
tem. And which was heated to reflux for 4 h (see Scheme 1).
After cooling to room temperature, the mixture was poured into
ice–water and the precipitate was collected and dried. The product
1
was further purified by recrystalling from EtOH/H
2
O. Yield 80%. H
NMR (400 Hz, DMSO-d6), d: 11.59 (s, 1H), 11.04 (s, 1H), 8.14–8.30
1
3
(
m, 4H), 7.82 (s, 1H), 7.12–7.47 (m, 5H). C NMR (400 Hz, DMSO-
d6), d: 150.891, 140.264, 137.097, 129.826, 126.423, 124.068,
22.610, 121.641, 120.538, 111.912, 110.376. MS (ESI): M/Z 281
1
(
+
M + H ).
enable dual channel (absorption and fluorescence) detection of
ꢀ
F
in DMSO solution.
2
.3. Synthesis of receptor 2
Indole-3-carbaldehyde and Phenylhydrazine was dissolved in
⇑
Corresponding author.
E-mail address: sjshao_licpcas@hotmail.com (S. Shao).
ethanol and 3 drops of CH
which was heated to reflux for 4 h (see Scheme 1). After cooling
3
COOH was added to the system. And
1
011-1344/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotobiol.2011.02.021

Q. Li et al. / Journal of Photochemistry and Photobiology B: Biology 103 (2011) 140–144
141
Scheme 1. Synthesis of receptors 1 and 2.
to room temperature, the mixture was poured into ice–water and
the pale yellow precipitate was collected and dried. The product
when receptor 1 was treated with the fluoride anions, the yellow
solution of receptor 1 took a purple color, but no color changes
were observed when other anions were added (Fig. 1). Which
make receptor 1 an efficient and selective colorimetric sensor
for fluoride anions. As for receptor 2, no obvious changes in color
were observed upon addition of 30 equiv. of various anions.
2
was further purified by recrystalling from EtOH/H O. Yield 82%.
1
H NMR (400 Hz, DMSO-d6), d:11.33 (s, 1H), 9.83 (s, 1H), 8.25–
.27 (m, 1H), 8.11 (s, 1H), 7.62–7.63 (d, 1H), 7.39–7.42 (m, 1H),
.02–7.23 (m, 6H), 6.65–6.69 (t, 1H). 13C NMR (400 Hz, DMSO-
8
7
d6), d:146.187, 137.007, 134.917, 129.102, 127.325, 124.234,
1
22.258, 121.655, 120.000, 117.482, 112.804, 111.682, 111.354.
3
.2. UV–vis and fluorescence emission studies in DMSO solution
þ
MSðESIÞ : M=Z 236ðM þ H Þ:
The ability of receptor 1 to complex with various anions was
studies by UV–vis absorption and fluorescence titrations, which
were carried out in DMSO solution by addition of standard solution
of corresponding anions to a solution of the receptor 1. Fig. 2
shows absorption spectral changes of receptor 1 in the course of
3
. Results and discussion
3.1. Colorimetric analysis
titration. The absorption spectrum of receptor 1 in DMSO solution
ꢀ
ꢀ
ꢀ
The interaction of receptor 1 with various anions (F , Cl , Br ,
ꢀ5
(
1.5 ꢁ 10 M) exhibited a strong absorption band with maxima at
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
I , AcO , ClO ; HSO ; H
2
PO ) was firstly investigated in DMSO
containing an estimated 0.3% v/v water) solution through
colorimetric analysis. Noticeable color changes were observed
ꢀ
4
4
4
about 425 nm, upon addition of F , the band at 425 nm progres-
sively decreases, while two new peaks at 540 and 570 nm appears
and develops (Fig. 2a). And two clear isosbestic points at about 370
and 495 nm were observed, indicating a single component was
produced during the titration process. Analogous titrations were
also carried out with other anions, no obvious changes in spectra
were observed (Fig. 2b), however.
(
The corresponding fluorescence titrations spectra of receptor 1
are shown in Fig. 3. Receptor 1 shows very weak fluorescence, with
an emission band at 410 nm when excited at 350 nm. Addition of
fluoride anions to the solution of receptor 1 induced an increasing
fluorescence emission together with a red shift (435 nm). As with
the UV–vis titrations, none of the other anions caused obvious
changes in fluorescence emission (Fig. 3b). The fluorescent quan-
ꢀ
tum yield of 1 with 30 equiv. of F
(U = 0.071) was determined
ꢀ
5
using fluorescein as a standard.
Both the absorption band and fluorescence emission band in-
duced by fluoride anions matched well with band formed in the
Fig. 1. Color changes of receptor 1 (1.5 ꢁ 10 M) in DMSO solution with the
ꢀ
addition of 10 equiv. of tetrabutylammonium anions. A = Free receptor, B = F ,
C = Cl , D = Br , E = I , F = AcO , G = ClO , H = HSO , I = H
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2
PO .
4
4
4
ꢀ
5
ꢀ
Fig. 2. Changes in the UV/vis absorption spectrum of receptor 1 (1.5 ꢁ 10 M) in DMSO upon addition of anions in TBA salts form: (a) F (0–20 equiv.); (b) 10 equiv. of
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
various anions (F , Cl , Br , I , AcO , ClO , HSO , H
2
PO ).
4
4
4

1
42
Q. Li et al. / Journal of Photochemistry and Photobiology B: Biology 103 (2011) 140–144
presence of [TBA] OH, suggesting that they can be ascribed to the
deprotonation of receptor 1.
3.3. Determination of the binding constant and stoichiometry
As for receptor 2, no obvious changes in absorption or fluores-
cence emission were observed upon addition of 30 equiv. of vari-
ous anions (see Fig. 4). The nitro-bearing receptor 1 shows high
sensitivity for fluoride anions than unsubstituted receptor 2, the
results may be attributed to the electron withdrawing nitro group,
which served to increase the NH acidity.
The binding stoichiometry of receptor 1 with fluoride anions
was calculated through the Benesi–Hildebrand equation
[12,21,22], which was given as follows:
ꢀ
ꢁ
1
1
ꢀ A
1
¼ A
þ 1
ð1Þ
ꢀ
A ꢀ A
0
1
0
K½F ꢂ
0
ꢀ
5
ꢀ
Fig. 3. Changes in the emission spectrum of receptor 1 (1.5 ꢁ 10 M) in DMSO upon addition of anions in TBA salts form (excitation at 357 nm): (a) F (0–20 equiv.); (b) 10
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
equiv. of various anions (F , Cl , Br , I , AcO , ClO , HSO , H
2
PO ).
4
4
4
Fig. 4. Spectral Changes of receptor 2 (1.5 ꢁ 10ꢀ M) in DMSO upon addition of 30 equiv. anions in TBA salts form: (a) changes in the UV/vis absorption spectrum; (b) changes
5
in the emission spectrum (excitation at 350 nm).
ꢀ
Fig. 5. Benesi–Hildebrand plot using Eq. (1), assuming 1:1 stoichiometry for association between receptor 1 and F : (a) UV/vis titration results (570 nm); (b) fluorescent
titration results (420 nm).

Q. Li et al. / Journal of Photochemistry and Photobiology B: Biology 103 (2011) 140–144
143
Fig. 6. Partial 1H NMR spectra of receptor 1 on addition of F (TBA salts) in DMSO-d
ꢀ
: (a) 0 equiv. F ; (b) 0.5 equiv. F ; (c) 1 equiv. F ; (d) 2 equiv. F ; (e) 3 equiv. F .
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
6
Scheme 2. Proposed binding mode of receptor 1 with fluoride anions.

1
44
Q. Li et al. / Journal of Photochemistry and Photobiology B: Biology 103 (2011) 140–144
A
0
is the absorbance of free 1, A
1
is the absorbance measured
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