A simple benzimidazole quinoline-conjugate fluorescent chemosensor for highly selective detection of Ag+

Article abstract of DOI:10.1016/j.tet.2016.05.020

A novel and simple fluorescent chemosensor (L), N-((1H-benzo[d]imidazol-2-yl)methyl)quinoline-2-carboxamide, based on quinoline benzimidazole scaffold was synthesized successfully, which had a ‘turn-on’ effect for Ag⁺detection in methanol/Tris buffer(1:1, v/v, pH=7.35) and exhibited a strong fluorescence emission at 357?nm (λ_ex=300?nm). The chemosensor had high sensitivity and selectivity for Ag⁺even in the presence of other metal ions with the detection limit of 4.4×10^?7?M, which reached the standard of World Health Organization (WHO) for drinking water (5.0×10^?7?M). The mechanism of the interaction between the sensor and Ag⁺was investigated in detail through NMR and MALDI Mass spectra analysis.

Full text of DOI:10.1016/j.tet.2016.05.020

Accepted Manuscript
A simple benzimidazole quinoline-conjugate fluorescent chemosensor for highly
+
selective detection of Ag
Changjun Chen, Haiyang Liu, Bin Zhang, Yanwei Wang, Kai Cai, Ying Tan, Chunmei
Gao, Hongxia Liu, Chunyan Tan, Yuyang Jiang
PII:
S0040-4020(16)30405-7
10.1016/j.tet.2016.05.020
TET 27747
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 15 March 2016
Revised Date: 29 April 2016
Accepted Date: 9 May 2016
Please cite this article as: Chen C, Liu H, Zhang B, Wang Y, Cai K, Tan Y, Gao C, Liu H, Tan C, Jiang
Y, A simple benzimidazole quinoline-conjugate fluorescent chemosensor for highly selective detection of
+
Ag , Tetrahedron (2016), doi: 10.1016/j.tet.2016.05.020.
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ACCEPTED MANUSCRIPT
A Simple Benzimidazole Quinoline-conjugate Fluorescent
Chemosensor for Highly Selective Detection of Ag+
Changjun Chena,b,† , Haiyang Liua,b,†, Bin Zhangb,c, Yanwei Wanga,b, Kai Caia,b, Ying Tanb,c*, Chunmei
Gaob,c*, Hongxia Liub,c, Chunyan Tanb,c, Yuyang Jiangb,c,d
A quinoline benzimidazole scaffold based fluorescence probe was synthesized successfully,
which had “turn on” effect for detecting Ag+ with high selectivity and good sensitivity.

ACCEPTED MANUSCRIPT
A Simple Benzimidazole Quinoline-conjugate Fluorescent
Chemosensor for Highly Selective Detection of Ag+
Changjun Chena,b,† , Haiyang Liua,b,†, Bin Zhangb,c, Yanwei Wanga,b, Kai Caia,b, Ying Tanb,c*
,
Chunmei Gaob,c*, Hongxia Liub,c, Chunyan Tanb,c, Yuyang Jiangb,c,d
a.Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China.
b.The Ministry-Province Jointly Constructed Base for State Key Lab, Shenzhen Key
Laboratory of Chemical Biology, Graduate School at Shenzhen, Tsinghua University,
Shenzhen 518055, P. R. China.
c.National & Local United Engineering Lab for Personalized anti-tumor drugs, the
Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, P. R. China
d.School of Medicine, Tsinghua University, Beijing 100084, PR China.
* Corresponding author. Tel.: +86 755 2603 6533; fax: +86 755 2603 2094;
Email address: chunmeigao@sz.tsinghua.edu.cn; tan.ying@sz.tsinghua.edu.cn.
†Authors with equal contributions.
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A Simple Benzimidazole Quinoline-conjugate Fluorescent
Chemosensor for Highly Selective Detection of Ag+
Changjun Chena,b,†, Haiyang Liua,b,†, Bin Zhangb,c, Yanwei Wanga,b, Kai Caia,b, Ying
Tanb,c*, Chunmei Gaob,c*, Hongxia Liub,c, Chunyan Tanb,c, Yuyang Jiangb,c,d
Abstract
A
novel
and
simple
fluorescent
chemosensor
(L),
N-((1H-benzo[d]imidazol-2-yl)methyl)quinoline-2-carboxamide, based on
quinoline benzimidazole scaffold was synthesized successfully, which had a
“turn-on” effect for Ag+ detection in methanol/Tris buffer(1:1, v/v, pH=7.35)
and exhibited a strong fluorescence emission at 357 nm (λex = 300 nm). The
chemosensor had high sensitivity and selectivity for Ag+ even in the presence of
other metal ions with the detection limit of 4.4 × 10-7 M, which reached the
standard of World Health Organization (WHO) for drinking water (5.0 × 10-7
M). The mechanism of the interaction between the sensor and Ag+ was
investigated in detail through NMR and MALDI Mass spectra analysis.
Keywords
Fluorescence chemosensor; Silver ion; Turn-on effect; High selectivity
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1. Introduction
Sliver is comprehensively applied in electric industry, photographic,
imaging, pharmacy, and mirror. It is also widely used in the manufactory of
tableware, jewellery, water distillation equipment and wearable accessory.1,2
However, the overuse of silver and its related complexes in industrial processes
has triggered serious environmental crisis. Even worse, the accumulation of
silver ions (Ag+) in organisms results in toxic effects because they can inactive
sulfhydryl enzymes and combine with amine, imidazole and carboxyl groups of
various metabolites.3,4 Therefore, the development of rapid, high effective,
sensitive and selective methods to detect silver ions is urgent needed for the
environment and human health.
Up to now, various approaches have been developed to measure Ag+ such
as inductively coupled plasma mass spectroscopy (ICP-MS), inductively
coupled plasma atomic emission spectrometry (ICP-AES), flame atomic
absorption spectrometry (FAAS), graphite furnace atomic absorption
spectrometry (GFAAS), electrothermal atomic absorption spectroscopy, atomic
fluorescence spectrometry, X-ray fluorescence and electrochemical methods.5-8
However, most of these methods have some drawbacks including the
requirement of expensive equipment, skilled personnel and time-consuming. In
comparison, fluorescence spectroscopy with its superiority of simplicity, high
sensitivity and widespread availability has become a powerful analytical
technique and draws lots of attention. Considerable efforts have contributed to
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the silver fluorescent detection and several novel fluorescent probes for Ag+
have been developed.4,9-25 However, since Ag+ pertains to the so-called “silent
ions”, most of the reported probes are based on a fluorescence quenching
mechanism. Therefore, “turn-on” chemosensors for Ag+ remained a great
challenge.26 Up to now, only a few fluorescence “turn-on” chemosensors have
been reported.27-30 In addition, some of the reported probes have the defects
such as poor solubility in aqueous media; difficulty in synthesis methods;
insufficient selectivity and sensitivity and so on.27-29,31,32 As fluorescence
“turn-on” sensors have many advantages over “turn-off” sensors such as
increased sensitivity and reduced false-positive likability, it is of a great
importance to design “turn-on” probes with high selectivity and efficiency for
Ag+ detection.
In view of the recent literatures, the quinoline-derived and
benzimidazole-derived compounds are generally used as fluorescent
chemosensors for recognition of metal ions.32-36 Herein, we combined two units
together
and
successfully
developed
a
fluorescence
probe,
N-((1H-benzo[d]imidazol-2-yl)methyl)quinoline-2-carboxamide (
L
) (Scheme
1
), which had high sensitivity and selectivity for Ag+ detection with the
advantages of fast response (<1 min), fluorescence turn-on ability and good
detection limit.
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Scheme 1. Schematic illustration for the synthesis of chemosensor L.
2. Experimental procedures
2.1 Materials and general methods
1H and 13C NMR spectra were measured on a Bruker 400MHz
spectrometer. Fluorescence spectra were measured on a SPEX Fluorolog
3-TCSPC spectrometer with 1 cm path length cuvettes and absorption spectra
was measured on a Beckman DU 800 spectrometer. Mass spectra were
measured on a Waters Micromass Q-TOF Premier Mass Spectrometer and an
UltrafleXtreme MALDI-tandem-time-of-flight (MALDI-TOF/TOF) analyzer
(Bruker Daltonics, Bremen, Germany) equipped with a 2000 Hz, 355 nm, Nd:
YAG laser (minimum spot diameter at 20 ꢀm). Sample surface was irradiated
with laser shots in the positive and reflectron mode of the mass spectrometer.
Each spectrum was the cumulative average of 1000 laser shots. UV-vis spectra
were measured on SCINCO S-4100.
2.2 Synthesis of L
As shown in scheme 1,
L
was synthesized via a simple
procedure.(1H-benzo[d]imidazol-2-yl)methanamine was obtained via the
method reported in our previous paper. 37 2-Quinolinecarboxylic acid (0.38 g, 2
mmol) was refluxed in oxalyl dichloride (20 mL) for 2 h, and then the solvent
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was removed. The crude quinoline-2-carbonyl chloride was obtained, which
was added into (1H-benzo[d]imidazol-2-yl)methanamine (0.29 g, 2 mmol)
o
dichloromethane solution containing 0.3 mL trimethylamine at 0 C in a 0.5 h
timescale. The reaction mixture was gradually returned to room temperature
and maintained for 4 h, which was evaporated and separated via column
chromatography. Pale yellow solid was obtained. Yield: 0.42 g, 70%. m.p., 212
1
oC; HR-MS (ESI): Calcd for [M+H]+ 303.1246; found: 303.1236. H NMR
(400 MHz, CD3OD, ppm): 8.49 (d, 1H,
J=8.40 Hz), 8.20-8.25 (m, 2H), 8.02 (d,
1H, =8.40 Hz), 7.85 (m, 1H), 7.71 (m, 1H), 7.53 (m, 2H), 7.21-7.23 (m, 2H),
J
13
4.97(s, 2H). C NMR (100 MHz, CD3OD, ppm): 166.08, 158.58, 151.92,
149.36, 146.68, 137.55, 130.11, 129.50, 129.45, 129.39, 128.01, 127.62,
122.13, 118.33, 115.12. anal. calcd for C18H14N4O: C, 71.51; H, 4.69; N, 18.53;
Found: C, 71.39; H, 4.75; N, 18.19.
2.3 Analysis
The nitrate and chloride salts of the metal including Na+, K+, Fe2+, Cd2+,
Ca2+, Mg2+, Cu2+, Mn2+, Zn2+, Pb2+, Hg2+, Fe3+, Cr3+, Al3+, Ag+ were dissolved
in deionized water to prepare stock solution (5 mM). Stock solution of
L was
prepared in DMSO (10 mM). The stock solutions were further diluted as
needed with methanol-water (v/v=1:1 pH=7.35 10mM Tris-HCl). Therefore ion
solutions were added into the buffer solution by using a pipette for UV-vis
absorption spectrum and fluorescence spectrum. Both the excitation and
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emission slit widths were 5.0 nm. The PH of Tris-HCl buffer was titrated via
0.1 M HCl and 0.1 M NaOH.
3. Results and discussion
3.1 Absorption spectroscopic studies
The binding properties of
L (10 µM) with various metal ions including
Na+, K+, Cd2+, Ca2+, Mg2+, Cu2+, Mn2+, Zn2+, Pb2+, Fe3+, Cr3+ and Ag+ were
firstly investigated via UV-vis spectrum in methanol-water (v/v=1:1, pH=7.35,
10 mM Tris-HCl). As shown in Fig. S4. (Supporting Information), the
absorption spectrum of
L appeared two strong band at 272 nm and 280 nm. The
two absorption band may contribute to the benzimidazole and quinolone
respectively. Only upon addition of Cu2+, Fe3+ and Ag+, the
segments of
L
spectra of
L
increased apparently, among which,
L
exhibited the most sensitive
with Ag+ was
binding ability with Ag+. Therefore, the binding capability of
L
further studied. As shown in Fig. 1, the absorbance of
L increased gradually
with the amount of Ag+ increased. The result of UV-vis spectra indicated that
formed a complex with Ag+.
L
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Fig. 1. UV-vis spectra of L (10 µM) with different Ag+ in methanol-water (v/v=1:1, pH=7.35, 10
mM Tris-HCl). The concentrations of Ag+ values ranged from 0 to 100 µM.
3.2 pH effect to the sensing system
In order to investigate the influence of pH on Ag+ detection and evaluate
the suitable pH span, the acid titration experiments were carried out. As shown
in Fig. S5, the fluorescence titration curve of free
L
showed that pH had slight
with Ag+
influence on fluorescence emission spectra. However, the spectra of
L
showed the highest detection intensity from pH 6.0 to 9.0, which were the most
common pH environment in nature and daily life. The decrease intensity might
be caused by the increasing amount of hydroxyl ions and chloride ions of
system which could form Ag(OH) and AgCl precipitations. The wide range of
pH in detecting Ag+ proved that it could be used in various area. Considering
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the chemosensor applied to environmental detection, all the further
fluorescence measurement were conducted in methanol/water buffer
(pH=7.35).
3.3 Fluorescence spectroscopic studies
Fluorescence response properties of
L
(10 µM) with Ag+ were also
investigated in methanol-water (v/v=1:1, pH=7.35, 10 mM Tris-HCl) with the
excitation at 300 nm. There were three emission bands which attributed to the
benzimidazole and quinolone segments of
L
respectively.38,39 As shown in Fig.
2, upon the addition of an increasing amount of Ag+ (0-300 µM), the
fluorescence emission intensity of
L increased gradually, accompanied with a
7-fold improvement at 357 nm, which could be assigned to a
chelation-enhanced fluorescence effect between Ag+ and L.33 Upon the addition
of more than 30 equiv. of Ag+, the complex of
L
and Ag+ will be precipitated.
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Fig. 2. The fluorescence spectra of L (10 µM) upon addition of Ag+ (0, 10, 20, 30, 40, 50, 75,
100, 150, 200, 225, 250, 275, 300 µM) in methanol-water (v/v=1:1, pH=7.35, 10 mM Tris-HCl).
(Inset) The fluorescence intensity I/I0 upon the addition of Ag+ (0, 10, 20, 30, 40, 50, 75, 100, 150,
200, 225, 250, 275, 300 µM), which I0 was the fluorescence intensity of L without Ag+. λex = 300
nm, λem = 357 nm.
The fluorescence binding efficient was acquired via the Stern-Volmer
equation, I/I0 = Ksv [Ag+] + 1, where I represented the fluorescence intensity of
L
with different concentration of Ag+, I0 represented the intensity of
L without
Ag+, [Ag+] represented the concentration of silver ions and Ksv represented the
Stern-Volmer constant.
According to Stern-Volmer equation, a linear Stern-Volmer plot was
obtained with the Ksv value of 3.2 × 104 M-1, as shown in Fig. 3.
Fig. 3. Stern-Volmer plot for emission intensity enhancement of L in methanol-water (v/v=1:1,
pH=7.35, 10 mM Tris-HCl) upon addition of Ag+ (0-100 µM). λex = 300 nm, λem = 357 nm.
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The detection limit of Ag+ was calculated through the following equation:
detection limit=3k/σ, where σ was the standard deviation of blank
measurements, and k was the slope between the intensity versus Ag+
concentration.40-42 According to the equation, the detection limit of Ag+ reached
4.4 × 10-7 M (Fig. S6). According to the drinking water standard given by
WHO,
L was capable to distinguish between safety and toxic levels of silver
ion in drinking water. Furthermore, the comparison between our probe and the
reports has been added in the supporting information (Table S1), which
indicated the detection limit of L is comparable or better than some similar
fluorescent probes literature reported.
Furthermore, the selectivity of the target ion from the competition system
is one of the vital features for a practical chemosensor. Therefore, the
competition experiments were carried out by recording the changes of the
fluorescence intensity before and after adding Ag+ into the probe solution with
interferants (10 ꢀM
L in methanol/water, λex = 300 nm, λem = 357 nm). Addition
of 500 ꢀM of aqueous solution of Na+, K+, Fe2+, Cd2+, Ca2+, Mg2+, Cu2+, Mn2+,
Zn2+, Pb2+, Hg2+, Al3+, Fe3+ and Cr3+, no significant spectral changes were
observed except Hg+, (black bars in Fig. 4). Upon the addition of Ag+,
immediate increase in fluorescence emission were observed, (red bars in Fig. 4).
The result showed that only Hg2+ caused minor increase on fluorescence
intensity, other ions almost did not interfere the detection of Ag+.
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Fig. 4. Fluorescence intensity at 357 nm of L (10 µM) in the presence of Ag+ (100 µM) containing
500 µM of various metal ions in methanol-water (v/v=1:1, pH=7.35, 10 mM Tris-HCl). Black bars
represented the fluorescence intensity of L (10 µM) upon the addition of various metal ions (500
µM).Red bars represented the fluorescence intensity of the mixture of L with various ions
followed by addition of Ag+ (100 µM)) respectively.
Although it was shown that
of Fe3+ in the UV-vis spectra (Fig. S4), fluorescence selectivity studies showed
that Fe3+ only slightly quenched the fluorescence of
effect on fluorescence detection upon addition of Ag+. The results proved that
L had a strong absorbance upon the addition
L
and had very minor
L
can be a potential chemosensor for Ag+ with promising selectivity under the
assay condition in the presence of competitive ions.
As the probe is used to detect Ag+ ions in environment,
L (10 ꢀM) was
applied to detect Ag+ in tap water/methanol (v/v=1:1). Ag+ was deliberately
introduced to simulate contaminated tap water. As shown in Fig. S7, the
fluorescence intensity gradually increased upon the addition of Ag+ (0-50 ꢀM).
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The result indicated that this chemosensor was suitable for detecting Ag+ in
nature water sample.
3.4 Sensing mechanism of L with Ag+
In order to gain more information for the complexation between
L and
1
Ag+, the H NMR spectra of
L in CD3OD solution were studied in the absence
and presence of different concentration of Ag+ (Fig. 5). With the increase of
Ag+ concentration (from 0.2 to 1 equiv.), the signals of all the protons of
benzimidazole and quinolone moieties were shifted significantly upfield, which
proved
may be illustrated as followed: when Ag+ combined with
Ag+ were shared with
L
formed a firmed binding with Ag+. The reasons of chemical shifts
L
, the electrons of
L
and thickened the electron cloud of L, therefore the
signals shifted upfield. When the Ag+ concentration increased to 2 equiv., the
chemical shifts displayed a little difference compared with 1 equiv. Ag+, which
indicated that the binding mode of Ag+ and
L changed.
Fig. 5. 1H NMR spectra of L (10 µM) with addition of Ag+ from (0-2 eq.) respectively.
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To prove the hypothesis, further studied was measured on MALDI mass
spectra. As shown in Fig. 6a, the MS spectrum of
which was [M+H]+. When Ag+ was added to the
-Ag+ complexes was identified (Fig. 6b). The mixture of 1:1 and
-Ag+ complexes occurred. As the concentration of Ag+ increased to 2
L revealed one main ion,
L
solution in a 1:1 molar ratio,
a mixture of
L
2:1
L
equiv., the peaks of complexes increased and almost all complexes turned to [
L
1
+ Ag+] (Fig. 6c), which was in accordance with the H NMR data. All the
results confirmed that
L
could formed a stable complex with Ag+ and the
was dependent on the concentration of Ag+.
binding mode of Ag+ and
L
Fig. 6. Mass spectra of L with addition of Ag+ from 0-2 equiv. (a) Mass spectra of L; (b) Mass
spectra of L with addition of 1 equiv. of Ag+.; (c) Mass spectra of L with addition of 2 equiv. of
Ag+.
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According to the results above and literature25,27,43-46, a possible interaction
mode of sensor
L
with Ag+ was proposed in Scheme 1. Ag+ was supposed to
via coordination bond. In conclusion, Ag+
formed alternative complex via chelation method and as Ag+ increased, most of
bind to the nitrogen atom of
L
complex turned to [L
+ Ag+].
Scheme 2. Proposed binding mode of L with Ag+.
4. Conclusions
We synthesized a novel fluorescence chemosensor
L with quinoline and
benzimidazole scaffold, which exhibited high selectivity for Ag+ over other
metal ions. The probe could be easily synthesized and showed good sensitivity
for Ag+. The limit detection for Ag+ was down to 4.4 × 10-7 M in aqueous
solution, which reached the standard of WHO for drinking water. Considering
its application area, we tested Ag+ in tap water and the result proved that the
probe can work in nature water sample as well. The complex formation and
1
binding mode were determined by H NMR studies and mass spectroscopy. It
was found that as the concentration of Ag+ increased, the
L
-Ag+ complex
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+ Ag+] to [ + Ag+]. In conclusion, the ligand
L can be a
changed from [2
L
L
potential fluorescence “turn-on” probe for Ag+ in aqueous system with
promising selectivity and sensitivity.
Acknowledgements
The authors would like to thank the financial supports from Shenzhen
Municipal Development and Reform Commission (Disciplinary Development
Program for Chemical Biology), the Chinese National Natural Science
Foundation (21272134, 21302108 and 21372141), and Shenzhen Sci& Tech
Bureau (JCYJ20150331151358131, CXZZ20150529165045064, and CXB2011
04210014A).
Supplementary data
1
13
See the supporting information for the H NMR, C NMR spectra and
high resolution mass spectrometry of
L, UV-vis spectra for L with various
mental ions, and linear concentration of Ag+ in methanol-water containing 10
ꢀM L.
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