Synthesis and fluorescence characteristics of selective ratiometric fluorescent sensors for silver ions based on 3-indolyl-4-indazolyl maleimide derivatives

Article abstract of DOI:10.1007/s11164-018-3350-z

Selective ratiometric fluorescent sensor b for detection of silver ions and its counterparts?(sensor a and c)?were efficiently synthesized based on 3-indolyl-4-indazolyl maleimide, and their photophysical properties and activities towards metal ions were?

Full text of DOI:10.1007/s11164-018-3350-z

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3350-z
Synthesis and fluorescence characteristics of selective
ratiometric fluorescent sensors for silver ions based
on 3-indolyl-4-indazolyl maleimide derivatives
1
1
2
1
•
Bo-ye Zhu
Jian-rong Gao
•
Meng Xu
1
•
Chun-xia Qian
1
•
Liang Han
1
•
Qing Ye
•
Yu-Jin Li
Received: 6 July 2017 / Accepted: 24 February 2018
Ó Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract Selective ratiometric fluorescent sensor b for detection of silver ions
and its counterparts (sensor a and c) were efficiently synthesized based on
3
-indolyl-4-indazolyl maleimide, and their photophysical properties and activities
towards metal ions were examined. The experimental results were verified by
density functional theory (DFT) and time-dependent DFT (TDDFT) calculations.
The results demonstrated that sensor b could selectively and sensitively recognize
?
Ag compared with other competing metal ions in tetrahydrofuran (THF). It
?
probably forms a 1:1 complex with Ag , displaying a 52-nm bathochromic shift of
the fluorescence emission spectrum. Moreover, the binding mechanism was
1
13
investigated using binding stoichiometry titration and H, C nuclear magnetic
resonance (NMR).
Keywords Ratiometric fluorescent sensor Á Bathochromic shift Á Maleimide
derivatives Á DFT calculations
&
&
1
Qing Ye
yeqing1975@zjut.edu.cn
Yu-Jin Li
lyjzjut@zjut.edu.cn
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014,
People’s Republic of China
2
Zhejiang Longsheng Chemical Research Co., Ltd., Shaoxing 312368, People’s Republic of
China
123

B. Zhu et al.
Introduction
Methods for monitoring heavy transition metals (HTMs) have been developed
continuously at an unabated pace for use in various chemical [1], biological [2],
environmental [3, 4], and physiological [5] application fields. Traditional techniques
for HTM detection include atomic absorption spectroscopy (AAS) [6], flame atomic
absorption spectrometry (FAAS) [7], capillary electrophoresis (CE) [8], inductively
coupled plasma–atomic emission spectroscopy (ICP-AES) [9], inductively coupled
plasma–mass spectrometry (ICP-MS) [10], inductively coupled plasma–optical
emission spectrometry (ICP-OES) [11], and flow injection analysis (FIA) [12].
Compared with such conventional instrumental techniques, which are relatively
expensive and time-consuming, fluorescent chemosensors, which can be used for
selective detection of HTM, have attracted unprecedented attention. This can be
attributed to their high sensitivity and selectivity, versatility, rapid response, low
cost, simple operation, and low detection limit, and the widespread availability of
equipment for such analysis [13–17].
Silver, one of the most common HTMs, has been extensively utilized together
with its compounds in chemistry, pharmacy, electrical industry, as well as
photographic and imaging industry [18]. Silver ions exert strong inhibitory and
bactericidal effects; moreover, they exhibit broad-spectrum antimicrobial activity
even at low concentration [19]. However, large doses of silver ions will inevitably
lead to some negative effects, such as argyria, characterized by grayish-blue
pigmentation of the skin and eye [20–22]. Silver ions can bind with sulfhydryl
terminals in food via soft–soft acid–base interaction. Ratiometric [23–29], ‘‘turn-
on’’ [30–34], and ‘‘turn-off’’ chemosensors [35–37] have been recognized as
promising methods for detection of silver ions. Therefore, fluorescent chemosensors
with high sensitivity and selectivity for detection of silver ions have become popular
in the field of academic research.
In this work, novel selective ratiometric fluorescent sensor b for detection of
silver ions and its counterparts (sensor a and c) were synthesized based on
3
-indolyl-4-indazolyl maleimide, and their structure was characterized. The results
show that the fluorescent maleimide derivatives with analogous structure exhibited
large Stokes shift [38] and aggregation-induced enhanced emission [39]. Moreover,
N and O atoms in maleimide might be potential binding sites for HTMs,
encouraging us to explore the feasibility of their use for detection of HTMs.
?
Furthermore, sensor b formed a 1:1 complex with Ag and displayed a
bathochromic shift in the fluorescence emission spectrum. The binding mechanism
?
was verified by comparing the different responses of sensors a–c to Ag using
1 13
binding stoichiometry titration and H, C NMR.
1
23

Synthesis and fluorescence characteristics of selective…
Experimental
General methods
Materials and equipments
All chemicals used in the current study were purchased from commercial vendors
and used as received without further purification, unless otherwise noted. All
solvents were purified and dried according to standard methods prior to use. Nuclear
1
13
magnetic resonance ( H and C) spectra were recorded using a Bruker AM 500
spectrometer (Bruker), with chemical shifts reported as ppm at 500 and 125 MHz,
respectively, with CDCl as internal standard. Melting points (m.p.) were recorded
3
by an Electrothermal X-4 digital melting point apparatus.
Absorbance, emission, and quantum yield
Ultraviolet–visible (UV–Vis) absorption spectra were measured using a UV-2550.
Fluorescence spectra and solid emission spectra were determined using a F-7000
fluorescence spectrophotometer. The U value in solution was determined in
f
accordance with the general method using quinine sulfate (U = 0.55 in 50 mM
H SO solution) as standard. Moreover, the compounds were diluted in organic
2
4
-5
-1
solvent prior to use at final concentration of 5 9 10 mol L . All solvents used in
photochemical measurements were spectroscopically pure. All experiments were
repeated to confirm the reproducibility of the results.
Theoretical calculations
All structures were optimized based on density functional theory (DFT) using the
B3LYP functional and 6-311G basis sets. The fluorescence emission and excitation
wavelengths of the compounds were calculated in accordance with time-dependent
density functional theory (TDDFT). All calculations were performed using
Gaussian 09W [40, 41].
Synthesis
Synthesis of 3-chloro-4-(1H-indol-3-yl)-1-phenyl-1H-pyrrole-2,5-dione (1)
To a stirred mixture of 1.8 g (75 mmol) Mg and 0.1 g (catalytic amount) iodine in
degassed anhydrous ether (15 mL), 9.8 g (90 mmol) bromoethane in anhydrous
ether (20 mL) was added dropwise under nitrogen atmosphere. After addition, the
mixture was stirred under reflux for 30 min. Then, 9.1 g (78 mmol) indole in
anhydrous benzene (10 mL) was added dropwise to the mixture. After stirring for
30 min at reflux temperature, indole Grignard reagent was obtained. To a stirred
solution of 12.2 g (50 mmol) 3,4-dichloro-1-phenyl-1H-pyrrole-2,5-dione and
anhydrous THF (20 mL), indole Grignard reagent prepared above was added
123

B. Zhu et al.
dropwise at room temperature. After stirring at room temperature for 4 h, the
reaction mixture was poured into solution of HCl in water (1 M, 100 mL), extracted
using ethyl acetate (3 9 100 mL), washed with saturated NaCl solution
(
recrystallized from ethyl acetate to yield 11.8 g (73%) compound 1 as red solid.
3 9 300 mL), concentrated to give crude product under reduced pressure, and
1
M.p. 203–204 °C. H NMR (500 MHz, DMSO-d ) d (ppm): 7.19–7.26 (2H, m),
6
7
.45–7.54 (6H, m), 7.97 (d, J = 8.0 Hz, 1H), 8.13 (1H, s), 12.18 (1H, s).
Synthesis of 3-(1H-indol-3-yl)-4-(1-methyl-1H-indazol-3-yl)-1-phenyl-1H-pyrrole-
,5-dione (2)
2
A mixture of 1.27 g (3.5 mmol) 1, 1.47 g (3.5 mmol) 1-methyl-3-(tributylstannyl)-
H-indazole, 0.1 g (catalytic amount) anhydrous LiCl, and 0.1 g (catalytic amount)
Pd(Ph ) Cl in anhydrous toluene (100 mL) was stirred for 1.5 h at reflux temperature
1
3
2
2
under nitrogen atmosphere. After cooling to room temperature, the reaction mixture
was poured into water (100 mL) and extracted using ethyl acetate (3 9 100 mL). The
combined organic phase was washed with saturated NaCl solution (3 9 300 mL) and
concentrated to give crude product under reduced pressure, which was purified by
silica gel chromatography elution (silica gel, ethyl acetate:petroleum ether = 1:4, v/v)
1
to afford 1.28 g (77%) 2 as red solid. M.p.[ 250 °C. H NMR (500 MHz, CDCl ) d
3
(
ppm): 9.12 (s, 1H), 8.06 (d, J = 2.9 Hz, 1H), 7.76 (d, J = 8.3 Hz, 1H), 7.60–7.50
m, 4H), 7.47–7.37 (m, 3H), 7.35–7.25 (m, 1H), 7.14–7.08 (m, 2H), 6.74 (t,
(
J = 7.5 Hz, 1H), 6.37 (d, J = 8.1 Hz, 1H), 4.02 (s, 3H). C NMR (125 MHz,
1
3
CDCl ) d (ppm): 169.1 (1C), 168.7 (1C), 139.4 (1C), 135.6 (1C), 133.5 (1C), 133.2
3
(
(
(
1C), 131.2 (1C), 131.0 (1C), 127.7 (2C), 126.4 (1C), 125.4 (2C), 125.3 (1C), 123.8
1C), 122.3 (1C), 121.1 (1C), 120.8 (1C), 120.4 (1C), 120.2 (1C), 119.9 (1C), 118.9
1C), 110.9 (1C), 108.2 (1C), 104.5 (1C), 34.8 (1C). HRMS (ESI) m/z calcd. for
?
?
C H N O (M ? H) 419.15025, found 419.15057.
26 19 4 2
Synthesis of 3-(1-methyl-1H-indol-3-yl)-4-(1-methyl-1H-indazol-3-yl)-1-phenyl-1H-
pyrrole-2,5-dione (sensor a)
To solution of 105 mg (0.25 mmol) 2 in anhydrous DMF (20 mL), 10.5 mg (60%,
0
temperature for 1 h. Then, 71 mg (0.5 mmol) CH I in anhydrous DMF (5 mL) was
.26 mmol) NaH was added at 0 °C. After addition, the mixture was stirred at room
3
added dropwise at 0 °C; after stirring at room temperature for 4 h, the reaction mixture
was poured into water (100 mL) and extracted using ethyl acetate (3 9 100 mL). The
combined organic phase was washed with saturated NaCl solution (3 9 100 mL) and
concentrated to give crude product under reduced pressure. The crude product was
purified by silica gel chromatography elution (silica gel, ethyl acetate:petroleum
1
ether = 1:5, v/v) to afford 79.6 mg (74%) 3 as red solid. M.p. [250 °C. H NMR
(
500 MHz, CDCl ) d (ppm): 8.17 (s, 1H), 7.79 (d, J = 8.3 Hz, 1H), 7.58–7.50 (m,
3
4
H), 7.47–7.37 (m, 3H), 7.29 (d, J = 7.4 Hz, 1H), 7.18–7.10 (m, 2H), 6.77–6.71 (m,
1
H), 6.31 (d, J = 8.1 Hz, 1H), 4.00 (s, 3H), 3.90 (s, 3H). C NMR (125 MHz, CDCl )
3
1
3
d (ppm): 169.2 (1C), 168.0 (1C), 139.3 (1C), 135.8 (1C), 133.7 (1C), 133.4 (1C), 132.0
1C), 131.9 (1C), 127.8 (2C), 126.4 (1C), 125.5 (2C), 125.2 (1C), 123.6 (1C), 122.4
(
1
23

Synthesis and fluorescence characteristics of selective…
(
(
1C), 121.1 (1C), 120.6 (1C), 120.4 (1C), 120.3 (1C), 119.8 (1C), 118.1 (1C), 110.7
1C), 108.4 (1C), 104.6 (1C), 39.6 (1C), 34.6 (1C). HRMS (ESI) m/z calcd. for
?
?
C H N O (M ? H) 433.16590, found 433.16577.
27 21 4 2
Synthesis of 3-(1-methyl-1H-indazol-3-yl)-4-(1-methyl-1H-indol-3-yl)-1H-pyrrole-
,5-dione (sensor b)
2
A mixture of 43 mg (0.1 mmol) sensor a and 2 g (overdose) NH OAc was heated
4
for 6 h at 140 °C under nitrogen environment. After cooling, the mixture was
poured into water (100 mL), adjusted to weak alkalinity with Na CO , and extracted
2
3
using ethyl acetate (3 9 100 mL). The organic layer was washed with saturated
NaCl solution (3 9 300 mL), dried over Na SO , and concentrated in vacuum.
Silica gel chromatography elution was used to wash the crude product [silica gel,
2
4
dichloromethane (DCM):methanol = 200:1, v/v] to afford 23.7 mg sensor b (yield,
1
6
7%) as red solid. M.p. 202–203 °C. H NMR (500 MHz, CDCl ) d (ppm): 8.10 (s,
3
1
H), 7.70–7.53 (m, 1H), 7.60 (s, 1H), 7.48–7.36 (m, 2H), 7.31–7.23 (m, 1H), 7.11
d, J = 8.2 Hz, 2H), 6.71 (t, J = 7.6 Hz, 1H), 6.24 (d, J = 8.1 Hz, 1H), 4.01 (s,
(
1
3
3
1
1
1
H), 3.89 (s, 3H). C NMR (125 MHz, CDCl ) d (ppm): 172.1 (1C), 171.9 (1C),
3
40.1 (1C), 136.7 (1C), 134.7 (1C), 134.5 (1C), 134.0 (1C), 126.0 (1C), 125.1 (1C),
23.0 (1C), 122.0 (1C), 121.9 (1C), 121.6 (1C), 121.4 (1C), 120.6 (1C), 119.7 (1C),
09.5 (1C), 108.9 (1C), 104.4 (1C), 35.5 (1C), 33.0 (1C). HRMS (ESI) m/z calcd.
?
?
for C H N O (M ? H) 357.13460, found 357.13455.
21 17 4 2
Synthesis of 1-methyl-3-(1-methyl-1H-indazol-3-yl)-4-(1-methyl-1H-indol-3-yl)-1H-
pyrrole-2,5-dione (sensor c)
A mixture of 43 mg (0.1 mmol) sensor a and 6 mL 32% CH NH in ethanol
2
3
solution was stirred for 6 h at room temperature. The solvent was removed in
vacuum, and silica gel chromatography elution was used to wash the crude product
(
silica gel, DCM:methanol = 200:1, v/v) to afford 25.2 mg sensor c (yield, 68%) as
1
red solid. M.p. 241–242 °C. H NMR (500 MHz, CDCl ) d (ppm): 8.10 (s, 1H),
3
7
.68 (d, J = 8.2 Hz, 1H), 7.45–7.34 (m, 2H), 7.29 (d, J = 8.6 Hz, 1H), 7.18–7.03
m, 2H), 6.71 (t, J = 7.6 Hz, 1H), 6.26 (d, J = 8.1 Hz, 1H), 4.00 (s, 3H), 3.87 (s,
(
1
3
3
H), 3.23 (s, 3H). C NMR (125 MHz, CDCl ) d (ppm): 171.9 (1C), 171.5 (1C),
3
1
40.8 (1C), 137.4 (1C), 135.2 (1C), 135.1 (1C), 133.8 (1C), 126.51 (1C), 125.6
(
(
(
1C), 123.7 (1C), 122.5 (1C), 122.4 (1C), 122.3 (1C), 121.8 (1C), 121.2 (1C), 120.3
1C), 109.6 (1C), 109.0 (1C), 105.3 (1C), 35.9 (1C), 33.4 (1C), 24.2 (1C). HRMS
?
ESI) m/z calcd. for C H N O (M ? H) 371.15025, found 371.15012.
?
2
2 19 4 2
Results and discussion
Synthesis
As shown in Scheme 1, according to reported literature [39–43], efficient reactions
were used to synthesize the target compounds sensor a–c based on 3-indolyl-4-
123

B. Zhu et al.
Scheme 1 Synthesis of compounds sensor a–c
indazolyl maleimide. Using 3,4-dichloro-1-phenyl maleimide as raw material,
sensor a was synthesized by Grignard reaction (73% yield), Stille coupling reaction
(
from sensor a via ammonolysis reaction in 67 and 68% yield, respectively. The
77% yield), and N-methyl reaction (74% yield). Sensor b and c were synthesized
1
structures of compounds sensor a–c were further adequately characterized by H
1
3
NMR, C NMR, and HRMS.
UV–Vis absorption and fluorescence emission spectra
The fluorescence properties of sensor a–c were examined in different organic
solvents. The photophysical properties of sensor a in different solutions are
presented in Fig. 1 and Table 1, while those of sensor a–c in toluene are presented
in Fig. 2 and Table 1.
UV–Vis and fluorescence in solution
The final products sensor a–c exhibited favorable stability and solubility in solvents
including EtOH, DCM, DMF, THF, toluene, and chloroform. The impact of
solvents on the absorption and fluorescence properties of sensor a was investigated
and is presented in Fig. 1 and Table 1. A sharp absorption peak can be observed for
sensor a at 454 nm. Besides, it was also seen that the polarity of the solvent had the
least effect on the maximum absorption wavelength (kmax) of sensor a, suggesting
that the dipole moment of the molecules was more or less similar in both excited
and ground state [44]. Sensor a showed weak fluorescence in protonic solvents. The
molar absorption coefficient (e) of sensor a depended on the solvent used, with the
most impressive values seen in toluene and THF. Additionally, the fluorescence
quantum yield (U ) of sensor a reached its highest value of 0.38 in toluene.
f
1
23

Synthesis and fluorescence characteristics of selective…
-
Fig. 1 Absorption spectra (a) and fluorescence emission spectra (b) of sensor a (1 9 10 mol L ) in
5
-1
different solvents (UV-2550, PMT voltage: 700 V)
Table 1 Photophysical properties for compounds sensor a–c
-
1
-1
)
Compound
Solvent
DCM
e (M cm
k
abs (nm)
k
em (nm)
Stokes shift (nm)
U
f
Sensor a
7400
6000
8700
5900
6900
6100
4800
5900
6800
4000
3200
454
462
447
459
443
456
450
447
459
448
450
580
584
562
589
552
562
586
581
566
543
552
127
122
116
130
108
106
136
133
107
95
0.22
0.30
0.12
0.00
0.28
0.25
0.03
0.02
0.38
0.27
0.42
CHCl
EA
3
EtOH
2
Et O
THF
DMF
3
CH CN
Toluene
Toluene
Toluene
Sensor b
Sensor c
102
f 2 4
Fluorescence quantum yields measured with quinine sulfate (U = 54.6% in 0.05 M H SO ) as reference
Therefore, toluene was conjectured to be the best solvent when mixed with
3
-indolyl-4-indazolyl maleimide.
Various substituents were integrated with the N atom in the maleimide ring of
sensor a–c to study the effect of the substituent group. As seen from Fig. 2 and
Table 1, compared with sensor b and c, sensor a displayed a conspicuous
bathochromic shift of around 10 nm, seen in both the absorption and fluorescence
emission spectra. Such a phenomenon was recorded when applying the particular
substituent N-phenyl on the maleimide ring, thus demonstrating that the large
conjugated framework contributed to the bathochromic shift of 3-indolyl-4-
indazolyl maleimide compounds. Solutions of sensor a–c exhibited large Stokes
shift of 107, 95, and 102 nm, respectively. Moreover, N-phenyl-substituted sensor a
manifested greater Stokes shift on the maleimide ring compared with sensor b and
c. Table 1 shows the greatest Stokes shift of sensor a at wavelength of 107 nm,
with excitation and emission wavelength of 459 and 566 nm, respectively. These
123

B. Zhu et al.
Fig. 2 Absorption spectra (a) and fluorescence emission spectra (b) of compounds sensor a–c
(
-
1 9 10 mol L ) in toluene (UV-2550, PMT voltage: 700 V)
5
-1
results demonstrate that compounds with relatively large conjugated framework
were associated with larger Stokes shifts. Meanwhile, the U value of sensor a–c
was analyzed, being calculated to be 38, 27, and 42%, respectively.
f
DFT calculations to rationalize the excitation and emission spectra
Density functional theory (DFT) quantum calculations were carried out using the
optimized geometry of each molecule. The ground-state geometry was optimized in
gas phase at B3LYP/6-31G level of theory [40, 41]. According to the results of the
DFT calculations, the molecular system of sensor c showed nearly planar structure
(
Fig. 3), enhancing the extent of conjugation. As a result, sensor c exhibited the
highest fluorescence quantum yield among all the compounds. The absorption and
emission spectra were calculated using time-dependent DFT (TD-DFT). Note that
the values of the experimental absorption and emission spectra were close to the
calculation results (Table 2).
1
Fluorescence spectral studies of sensors a–c with Ag
The fluorescence characteristics of sensor a–c were investigated in THF after
3?
?
?
2?
2?
2?
2?
3?
addition of several metal cations (Ag , Li , Cu , Zn , Hg , Ni , Al , Fe
,
and Cr ) by fluorescence titration. The fluorescence response of sensor a–c
3
?
-
5
1.0 9 10 mol/L) to the above-mentioned metal cations (10 eq) is illustrated in
(
Fig. 4. Notably, the fluorescence emission spectrum (kem = 540 nm) of sensor b
displayed a bathochromic shift from 540 to 592 nm in presence of Ag , but did not
show any obvious changes in presence of other metal cations. This can be attributed
?
?
to formation of metal–organic coordinating clathrate between sensor b and Ag .
However, no significant changes in the fluorescence emission spectrum of sensor a
and c was seen (substituted with N-alkyl and N-phenyl on maleimide ring) even in
?
the presence of any metal cation. Therefore, the coordinating position between Ag
1
23

Synthesis and fluorescence characteristics of selective…
Fig. 3 Optimized geometry of compounds sensor a–c in different orientations
Table 2 Calculation results for sensors a–c
a
b
c
abs
a
em (nm, cal)
b
em (nm, exp)
c
f
em
Compound
k
abs (nm, cal)
k
abs (nm, exp)
f
k
k
Sensor a
Sensor b
Sensor c
a
468
458
436
454
437
444
0.1250 570
0.0675 547
0.0774 535
566
543
553
0.1511
0.2541
0.0918
Theoretical calculations using B3LYP/6-31G basis set without solution
b
In toluene solvent
c
Oscillator strength coefficients
and sensor b could be located in the imidogen radical of maleimide ring of
sensor b.
?
Moreover, the possible mode of interaction between sensor b and Ag was
determined from the experimental results, as confirmed by the ion titration method
1
and H NMR spectroscopy. These observations indicated that sensor b could be
?
used as a chemosensor for Ag .
1
Stoichiometry and sensing mechanism of Ag
Figure 5 shows a peak in the absorption spectrum of sensor b at wavelength of
about 445 nm. Furthermore, a 15-nm bathochromic shift in the absorption peak
?
-6
-5
from 445 to 461 nm was observed when Ag (2.0 9 10 –2.0 9 10 mol/L) was
-
5
gradually added to the solution of sensor b (1 9 10 mol/L). The fluorescence
spectrum of sensor b showed a maximum emission peak at nearly 540 nm,
?
associated with a 52-nm bathochromic shift from 540 to 592 nm as Ag
-
2.0 9 10 –2.0 9 10 mol/L) was gradually added. Both the maximum absorp-
6
-5
(
tion wavelength and fluorescence emission wavelength displayed negligible changes
123

B. Zhu et al.
-
5
Fig. 4 Fluorescence spectra of sensor a (a), sensor b (b), and sensor c (c) (1.0 9 10 mol/L) in
presence of different metal ions (10 eq) in THF/H O (200:1, v/v) (PMT voltage: 700 V), and d sensor b
2
-
5
(
(
2
1.0 9 10 mol/L) in presence of different metal ions (10 eq) in THF/H O (200:1, v/v) under UV lamp
365 nm)
-5
?
Fig. 5 Absorption (a) and fluorescence (b) response of sensor b (1 9 10 mol/L) exposed to Ag at
-
6
-6
-6
-6
-5
various concentrations: 2.0 9 10 , 4.0 9 10 , 6.0 9 10 , 8.0 9 10 , 1.0 9 10 , 1.2 9 10
-5
,
.4 9 10 , 1.6 9 10 , 1.8 9 10 , and 2.0 9 10 mol/L (from left to right). Spectra measured in
-5
-5
-5
-5
1
THF/H O (200:1, v/v) solution (PMT voltage: 700 V)
2
1
23

Synthesis and fluorescence characteristics of selective…
?
2
Fig. 6 Job’s plot for determining the stoichiometry for sensor b (kabs = 457 nm) and Ag in THF/H O
-
4
(
200:1, v/v). Total concentration = 1.0 9 10 mol/L
?
when more Ag was titrated. Therefore, sensor b could be an effective ratiometric
?
fluorescent sensor for Ag .
A Job’s plot was constructed by fluorescence titration to determine the binding
?
stoichiometric ratio between sensor b and Ag and ascertain the sensing
mechanism. As seen in Fig. 6, the Job’s plot (with the vertical axis representing
fluorescence intensity at 592 nm) showed an inflection point at 0.5 mol fraction of
sensor b, indicating formation of a 1:1 metal–ligand complex between sensor b
?
?
and Ag . As a result, it can be speculated that the coordination between Ag and
sensor b resulted from the imidogen and the carbonyl of maleimide ring, suggesting
that N and O atoms were the binding sites (Scheme 2).
?
1
13
The Ag sensing mechanism was further investigated using H and C NMR
1
?
titration. Figure 7 compares the H NMR spectra of sensor b with and without Ag .
The results show that sensor b in mixed solvent [dimethylsulfoxide (DMSO)-d6/
H O = 200:1] displayed the proton signals of imidogen radical at 11.20 ppm,
2
?
13
which disappeared in presence of Ag (2 eq). Figure 8 compares the C NMR
?
spectra of sensor b with and without Ag . The chemical shifts of the carbon atom
on carbonyl were approximately 172 ppm, while that of the carbon atom on
?
nitrogen methyl was approximately 35 ppm. However, in presence of Ag , the
?
Scheme 2 Response mechanism of sensor b to Ag
123

B. Zhu et al.
1
?
6 2
Fig. 7 H NMR spectra of sensor b and sensor b with Ag (2 eq) in DMSO-d /H O (200:1, v/v)
chemical shift of the carbon atom on carbonyl changed to about 110 ppm, while that
of the carbon atom on nitrogen methyl changed to about 20 ppm. The change in the
chemical shifts of the carbon atom on carbonyl was greater than that of the carbon
?
atom on nitrogen methyl, demonstrating that Ag exerted a prominent influence on
the carbonyl of sensor b. These observations indicate ionization of the imidogen
radical, generally supporting the sensing mechanism proposed above.
Selectivity to analyte compared with competing metal ions
High selectivity to the analyte compared with competing metal ions is a key factor
influencing the utilization of a fluorescent sensor for a certain metal ion. Therefore,
the fluorescence intensity ratio (I592 nm/I540 nm) of sensor b (k = 457 nm) was
abs
?
2?
2?
determined in presence of various metal ions, including Li , Cu , Zn , Hg
2?
,
Ni , Al , Fe , and Cr and Ag (2 eq, respectively). Dramatic growth in the
2
?
3?
3?
3?
?
?
bars can be seen when Ag was introduced into sensor b in THF (Fig. 9). These
results clearly demonstrate that such a chemosensor could be applied for selective
?
detection of Ag without interference from other metal ions.
1
23

Synthesis and fluorescence characteristics of selective…
1
3
?
6 2
Fig. 8 C NMR spectra of sensor b and sensor b with Ag (2 eq) in DMSO-d /H O (200:1, v/v)
Fig. 9 Fluorescence intensity
ratio (I592 nm/I540 nm) of
-5
sensor b (1 9 10 mol/L) in
THF. Red bars indicate the ratio
I592 nm/I540 nm of sensor b in
presence of different metal ions
(2 eq), while blue bars represent
the ratio I592 nm/I540 nm of
sensor b in presence of different
metal ions and Ag (2 eq) (with
?
mean and standard deviation
computed using data from three
parallel experiments)
123

B. Zhu et al.
Conclusions
A novel, selective ratiometric fluorescent sensor b for detection of silver ions and
its counterparts were developed based on 3-indolyl-4-indazolyl maleimide. Photo-
physical measurements on the maleimide compounds showed that they exhibit
conspicuous fluorescence in solution. Sensor b showed excellent selectivity and
?
sensitivity for Ag compared with other competing metal ions. An obvious
bathochromic shift (52 nm) of the fluorescence emission was observed on addition
of silver ion. Therefore, sensor b is appropriate for application in biomedical and
environmental detection.
Acknowledgements Wegratefully acknowledgefinancialsupportfromthePostdoctoral ScienceFoundation
ofChina(2014M550256),NationalNatural ScienceFoundation of Zhejiang(LY14H300003), KeyInnovation
Team of Science and Technology in Zhejiang Province (2010R50018), and Drug Research (SIMM1403KF-
02).
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