A highly selective TPE-based AIE fluorescent probe is developed for the detection of Ag+

Article abstract of DOI:10.1039/c8ra03591a

The detection of Ag⁺ in the environment is very important to determine the level of pollution from silver complexes, which have caused various human health problems. Herein, an aggregation-induced emission (AIE) chromophore (tetraphenylethane,

Full text of DOI:10.1039/c8ra03591a

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A highly selective TPE-based AIE fluorescent probe
+
is developed for the detection of Ag †
Cite this: RSC Adv., 2018, 8, 19701
Zhixiang Lu, Yunming Liu, Shuhan Lu, Yuan Li, Xiaolan Liu, Yu Qin and Liyan Zheng
*
+
The detection of Ag in the environment is very important to determine the level of pollution from silver
complexes, which have caused various human health problems. Herein, an aggregation-induced
emission (AIE) chromophore (tetraphenylethane, TPE) attached to a benzimidazole group (tetra-
+
benzimidazole, TBI–TPE) is synthesized and utilized to detect Ag in the environment. The strong
+
chelating effect between the benzimidazole group and Ag leads to the formation of aggregates, and
+
strong yellow fluorescence signals were observed after adding Ag into a TBI–TPE solution. The
Received 26th April 2018
Accepted 22nd May 2018
+
stoichiometry of the complex of TBI–TPE and Ag was established to be 1 : 2 using photochemical and
+
mass spectra measurements. The detection limit of the Ag assay is 90 nM with a linear range from
DOI: 10.1039/c8ra03591a
+
1
00 nM to 6 mM. This study provides a facile method to determine Ag in real environmental samples
rsc.li/rsc-advances
with satisfactory results.
+
Ag detection are based on quenching effects due to a charge
transfer process between Ag and a probe. Fluorescent probes
1
. Introduction
+
+
25–30
Silver is an important basic metal and has many applications, for Ag with “turn on” type signals are rarely reported.
including pharmaceuticals, the chemical industry, accessories,
The emergence of aggregation-induced emission (AIE) uo-
1–4
electronics, photography and so on. Metal pollution resulting rophores have provided many chances to build “signal on”
from industrial processes including leather tanning, cement probes, where uorescence is weak when dissolved, but
production, and metal plating is a serious environmental becomes signicantly more emissive in the aggregate state.
problem. In the pharmaceutical industry low concentrations Restriction of intramolecular rotation is the main AIE mecha-
of silver complexes are used for their strong antibacterial effect. nism, in which the intramolecular rotations are greatly
However, at high concentrations silver is harmful to human hampered due to limited space when aggregated. The non-
health and can cause brain damage, skin oxidation and have radiation decay channels are inhibited, and the excited states
5–8
9
–13
negative impacts on the immune system.
So it is imperative can only go back to the ground state by radiation decay, thus
31–36
to establish a highly selective and sensitive method for detect- enhancing the uorescence signicantly.
In recent years,
+
ing Ag in the environment.
uorescent materials with AIE properties have drawn extensive
+
Conventional methods for the detection of Ag include attention because they have a wide range of applications in
atomic absorption spectrometry, atomic emission spectrom- many elds, including uorescent cell imaging, organic light-
etry, potentiometric titration, inductively coupled plasma-mass emitting diodes, sensor materials, protein conformation
37–46
spectrometry (ICP-MS), electrochemistry, molecular imprinting studies, chiral recognition and so on.
Among these AIE
14–20
biosensors and colorimetry etc.
Although these methods can molecules, TPE is one of the most widely studied and used
+
be used to detect lower concentrations of Ag , they require materials owing to its excellent optical properties, high
47–51
complex instrumentation, tedious operations and time- quantum yield and photostability.
consuming sample pretreatment. Compared to these
It has been reported that Ag(I) possesses high affinity for
methods, uorescence spectroscopy has many advantages, such benzimidazole leading to stable coordination complexes.
51–57
as low cost, high sensitivity, easy operation and simple sample With these considerations in mind, we designed and synthe-
20–24
+
preparation.
Nevertheless, it is well known that Ag is sized a novel uorophore containing TPE and a benzimidazole
commonly used as a uorescent quencher, and most reports on group for the rst time. The TBI–TPE is rich in N atoms, which
+
offers good coupling ability with Ag . Based on the strong
+
coordination, Ag and TBI–TPE formed an AIE aggregate
Key Laboratory of Medicinal Chemistry for Natural Resource, Ministry of Education,
inducing signicant uorescence enhancement, hence a new
Functional Molecules Analysis and Biotransformation Key Laboratory of Universities
+
in Yunnan Province, School of Chemical Science and Technology, Yunnan University, uorescence “turn on” sensor for the detection of Ag is devel-
Kunming, Yunnan 650091, China. E-mail: zhengliyan@ynu.edu.cn
oped. This AIE probe exhibits high chemical selectivity and
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ra03591a
1
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ꢁ
a fast response time and has been successfully utilized in the ꢀ5 C, and then TiCl
4
(1.1 mL, 10 mmol) and pyridine (0.05 mL,
+
determination of Ag in real environmental samples.
0.6 mmol) were added dropwise. The mixture was reuxed at
ꢁ
0
7
5 C for 2 h and 20 mL THF solution of 4,4 -dimethylbenzo-
phenone (1.05 g, 5 mmol) was added. The solution was further
reuxed at 75 C for 12 h. Upon completion of the reaction, the
solution was cooled to room temperature and poured into
2
. Experimental section
ꢁ
2.1 Materials
0
4
,4 -Dimethylbenzophenone, titanium tetrachloride (TiCl ), a 40 mL 30% water solution of K
2
CO
3
, which was stirred for
4
pyridine, N-bromobutanimide (NBS), dibenzoyl peroxide (BPO), 5 min and then ltered. The ltrate was extracted with ethyl
and 1,2-diaminobenzene were purchased from Aladdin. Anhy- acetate, successively washed with water, brine, and dried with
drous tetrahydrofuran, silver nitrate, sodium acetate trihydrate, Na
SO . A light-yellow solid was obtained by rotary evaporation,
4
2
and chloroform-d, 99.8 atom% D were purchased from Energy and puried by column chromatography (silica gel, petroleum
1
Chemical. Dimethyl sulfoxide-d
Aldrich. All other reagents were of analytical grade and used and C NMR spectra of product 1 (400 MHz, CDCl
as received. Ultrapure water was prepared using a Milli-Q water d (TMS, ppm) are shown in Fig. S1 and S2,† respectively. High
6
was purchased from Sigma- ether) to result in a white product 1 (yield, 85%). The H NMR
1
3
ꢁ
, 25 C),
3
purication system.
resolution MS of product 1 is shown in Fig. S3,† (m/z): calculated
+
+
for C30
H
28 [M + H] 389.2225, found 389.2228, [M + Na]
+
411.2083, found 411.2080, [M + K] 427.1822, found 427.1825
2.2 Apparatus
(
high resolution).
Fluorescence spectra were recorded on a Hitachi High-
Technologies Corporation Tokyo Japan 5J2-0004 model F-7000
FL spectrouorometer. UV-vis absorption was characterized by
a UV-vis/NIR spectrophotometer (Shimadzu, Japan). The NMR
spectra were recorded with a AVANCEDRX400 NMR spectrom-
eter (Bruker, Germany) operated at 400/600 MHz. Electrospray
ionization mass spectra were obtained with a High Performance
2.3.2 Compound
2
(1,1,2,2-tetrakis(4-(bromomethyl)
phenyl)-ethene). 1,1,2,2-Tetra-p-tolylethene 770 mg (2 mmol)
obtained in the previous step was added into a 100 mL round
bottom ask, and then NBS (1424 mg, 8 mmol), BPO (64 mg)
and CCl
4
(30 mL) were added. Under an N
2
atmosphere, the
ꢁ
mixture reacted at 80 C for 2 h. Then, adding in NBS (1424 mg,
8
a further 2 h. Aer 2 h, NBS (1424 mg, 8 mmol), and BPO (64
mg) were added, and the mixture was le for 24 h. It was then
ꢁ
mmol) and BPO (64 mg), the mixture reacted at 80 C for
1100 Liquid Chromatography-Mass Spectrometer (Agilent
Technologies, USA). The morphology and size of aggregates
were characterized with Scanning Electron Microscopy (SEM)

ltered quickly while the solution was hot, and the solid was
(Quanta, FEI) and a JEM 2100 high resolution transmission
washed with ethyl acetate, and the ltrate collected. The organic
phase is obtained by rotary evaporation of the yellow solid,
without further purication we then proceeded directly to the
next step.
electron microscope (HRTEM, JEOL, Japan).
2
.3 Synthesis
The synthetic strategies adopted are outlined in Scheme 1.
.3.1 Compound 1 (1,1,2,2-tetra-p-tolylethene). Compound benzaldehyde). The solid from the previous step (2.0 g), sodium
0
00 000
2.3.3 Compound 3 (4,4 ,4 ,4 -(ethene-1,1,2,2-tetrayl)tetra-
2
58
1
was synthesized as previously reported. Zinc powder (1.0 g, acetate trihydrate (1500 mg, 18 mmol), silver nitrate (3400 mg,
1
5.3 mmol) was added into a 100 mL double-neck ask. Under 20 mmol), THF (25 mL) and H O (5 mL) were mixed and further
2
ꢁ
an N
2
atmosphere, 25 mL anhydrous THF was added, cooled to reuxed at 75 C for 24 h, the solution was cooled to room
temperature and ltered. The ltrate was extracted with ethyl
acetate, washed successively with water, brine, and dried with
Na SO . A light yellow viscous liquid was obtained by rotary
2 4
evaporation, and puried by column chromatography (silica
gel, petroleum ether/ethyl acetate ¼ 5 : 1) to result in a yellow-
5
9–61
1
13
green product 3 (yield: 25%).
H NMR and C NMR
ꢁ
spectra of product 3 (400 MHz, CDCl , 25 C), d (TMS, ppm) are
3
shown in Fig. S4 and S5,† respectively. As shown in Fig. S6,†
high resolution MS (m/z) of product 3: calculated for C30H O
20 4
+
+
[
4
M + H] 445.1434, found 445.1433, [M + Na] 467.1253, found
67.1259 (high resolution).
.3.4 Compound
imidazol-2-yl)phenyl)ethene) TBI–TPE. 4,4 ,4 ,4 -(Ethene-
,1,2,2-tetrayl)tetrabenzaldehyde (46 mg, 0.1 mmol), 1,2-dia-
2
4
(1,1,2,2-tetrakis(4-(1H-benzo[d]
0
00 000
1
minobenzene (130 mg, 1.2 mmol), ammonium acetate (92 mg,
ꢁ
1
.2 mmol), EtOH (2 mL) were further reuxed at 75 C for 5 h.
The precipitate was collected by centrifugation and washed with
ꢁ
ethanol. The product was dried in vacuum at 60 C overnight to
6
1–64
1
13
give a yellow solid (yield, 70%).
H NMR and C NMR
Scheme 1 Synthetic route to new 1,1,2,2-tetrakis(4-(1H-benzo[d]
imidazole-2-yl)phenyl)ethene molecules, TBI–TPE.
ꢁ
spectra (600 MHz, DMSO-d
6
, 25 C), d (TMS, ppm) of compound
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4
are shown in Fig. S7 and S8,† respectively. As shown in 90%, which is 13 times stronger than the one at f
w
¼ 0%. The
Fig. S9,† high resolution MS (m/z) of compound 4: calculated for compound is almost non-emissive when it is dissolved in pure
C H N [M
resolution).
2
+
+
2H]
399.1612, found 399.1604 (high organic solution (f ¼ 0%) and it gradually becomes emissive as
5
4
36
8
w
water is added, proving its prominent AIE activity. The increase
in uorescence intensity indicated TBI–TPE tended to aggregate
2.4 Analysis of real sample
w
when the f value increased. Upon aggregation, the free intra-
molecular rotations of the excited molecules are restricted. This
blocks the non-radiative relaxation pathway and favors the
radiative decay.
A waste liquor sample was taken from a factory in Kunming city.
The sample was ltered through a 0.22 mm membrane (Milli-
pore) prior to detection. One hundred microliters of the above
waste sample was added to 900 mL of methyl alcohol solution
containing TBI–TPE (2 mM) and then analyzed using the devel-
oped detection method.
3.3 Silver ion sensing studies
+
3
.3.1 The action and mechanism of TBI–TPE and Ag . To
+
examine the uorescence response of TBI–TPE towards Ag , we
3. Results and discussion
+
added Ag into a CH OH solution of TBI–TPE. As shown in
3
3.1 Design and synthesis of (1,1,2,2-tetrakis(4-(1H-benzo[d]
Fig. 2A, S11,† the uorescence spectra indicated that the addi-
imidazol-2-yl)phenyl)ethene) TBI–TPE
+
tion of Ag obviously increased the uorescence intensity of
The four-step synthesis adopted for preparing TBI–TPE is composites at 525 nm (Fig. 2A, S11†). Whereas, the UV
+
3
shown in Scheme 1. Compound 1 (1,1,2,2-tetra-p-tolylethene) absorption spectra of TBI–TPE in CH OH in the presence of Ag
0
was prepared from 4,4 -dimethylbenzophenone by McMurry did not change (Fig. 2B).
coupling. Compound 2 was obtained by NBS under the induc-
In Fig. 3A the Tyndall effect can be observed in a Ag–TBI–TPE
at 80 C. Compound solution by laser irradiation, indicating the formation of
is oxidized to compound 3 using silver as the catalyst. TBI–TPE aggregates. The formation of Ag–TBI–TPE aggregates was
ꢁ
tion of a free radical initiator (BPO) in CCl
4
2
ꢁ
is produced by reuxing 3 at 75 C.
To verify the results of our synthesis, these compounds were S12†). Nanoparticles with diameter of 20 nm were observed in
checked by NMR and MS (Fig. S1–S9†).
these images. The above results reveal that aggregates of Ag–
TBI–TPE were formed in CH OH solution, resulting in
further conrmed by HRTEM and SEM measurements (Fig. 3B,
3
enhanced uorescence, which is similar to the AIE mechanism
of assay.
3
.2 AIE properties
Aer structure conrmation, the AIE properties of TBI–TPE
were investigated. To further illustrate the AIE feature, the study
of the emission characteristics of TBI–TPE in mixtures of
CH OH and water with different water fractions was conducted.
As shown in Fig. 1 and S10,† the uorescence of TBI–TPE is
Next, we studied the stoichiometric coordination of TBI–TPE
+
with Ag . As shown in Fig. 3C, Job’s plot established that the
+
stoichiometry ratio of the TBI–TPE and Ag strongly uorescent
3
complex is 1 : 2. Furthermore, the mass of Ag–TBI–TPE complex
was determined using MALDI-TOF MS. A peak at 2021.398 was
obtained (Fig. 4A), suggesting the Ag–TBI–TPE complex formed
a yellowish green emission at 525 nm and is strongest at f
w
¼
2 4
is (TBI–TPE) Ag , which corresponds to the theoretical calcu-
lation (2021.312, Fig. 4B). These results are consistent with the
Job’s plot. According to these results, we speculate that the N
atoms of imidazole are good electron donors, which display
+
good ability to coordinate positive Ag , forming a stable and
strong uorescent complex. The schematic diagram of this
phenomenon is shown in Fig. 3D.
Furthermore, it is well known that pH changes according to
+
the concentration of Ag , which will greatly effect the interac-
+
tion between Ag and the probe. The effect of pH on the
Fig. 1 (A) Fluorescence intensity spectra. (B) Plots of fluorescence
intensity versus water fraction of TBI–TPE in mixtures of CH OH/water
with different water fractions (from 0 to 95%). (C) Images of TBI–TPE in
CH OH with the increasing water fraction taken under daily light (top),
and UV light illumination (bottom) (lex 365 nm). TBI–TPE Fig. 2 (A) Fluorescence spectra. (B) UV absorption spectra of TBI–TPE
concentration ¼ 10 mM, E ¼ 335 nm.
3
3
¼
+
x
(4 mM) in CH
3
OH in the presence of Ag (4 mM), E
x
¼ 370 nm.
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+
probe and Ag would be effected by the deprotonation process.
The best uorescent performance of Ag–TBI–TPE at neutral pH
+
is benecial for the detection of Ag in environmental samples.
3.3.2 Selectivity of assay. The N atoms of the imidazole
moiety have a unique chelating ability to transition metal ions.
So to verify the uorescence response of TBI–TPE towards other
metal ions, we examined the emission characteristics of TBI–
3
TPE in the presence of various metal ions in CH OH solution. As
shown in the inset of Fig. S14,† the uorescence emission of
TBI–TPE in the presence of other metal ions is weak. The cor-
responding uorescence spectra (Fig. 5A) indicates that the
2
+
2+
2+
2+
2+
2+
addition of other metal ions (Cu , Ca , Cd , Co , Cr , Mg ,
3
+
3+
2+
3+
+
2+
+
2+
2+
2+
3+
Er , Eu , Fe , Fe , K , Zn , Na , Ni , Pd , Sn , Tb and
Zr ) into TBI–TPE solution causes no change, weak uores-
cence is retained. This result indicates that the uorescence
4
+
+
Fig. 3 (A) Photographs of TBI–TPE (100 mM) in CH
3
OH exposed to
response of TBI–TPE is highly selective towards Ag over other
+
daylight (left), and TBI–TPE in the presence of Ag (300 mM) exposed to
a laser pen (right). (B) TEM image of the aggregates. (C) Job’s plot
metal ions.
+
3.3.3 Response time. Fig. 5B depicts the typical time-
showing 1 : 2 binding of TBI–TPE with Ag . (D) Illustration of the self-
dependent uorescent spectra of the CH OH solution contain-
ing TBI–TPE aer the addition of Ag into the TBI–TPE solution.
assembly aggregation.
3
+
Fig. 5 (A) Bars denote the fluorescence intensity difference (F ꢀ F
in the presence of a series of metal ions. F is the fluorescence intensity
of TBI–TPE (10 mM) only, and F is the fluorescence intensity of TBI–TPE
Fig. 4 Experimental (A), and calculated (B), MALDI-TOF of the Ag– in the presence of metal ions (20 mM). (B) Time-dependent fluores-
0 0
)/F
0
+
TBI–TPE complex.
cence response of the 2 mM TBI–TPE to 4 mM Ag in CH
Fluorescence spectra of TBI–TPE (2 mM) in CH OH in the presence of
increasing concentrations of Ag (from 0 to 6 mM); E
3
OH. (C)
3
+
x
¼ 370 nm. (D)
Variation of the fluorescence intensity at 525 nm of TBI–TPE with
a gradual increase in the concentration of Ag . (E) Variation of the
+
+
interaction between Ag and the probe were investigated by

uorescence measurement. As shown in Fig. S13,† the uores- intensity ratio of TBI–TPE (2 mM) with an increase in the concentration
+
of Ag for the determination of the detection limit, where F
intensity at 525 nm when the metal ion concentration is 0, and F is the
intensity at each metal ion concentration tested during titration. E
70 nm.
0
is the
cence intensity at 525 nm increased with increasing pH from 4.0
to 7.0, and the uorescence signal decreased with increasing pH
value. This tendency revealed that the interaction between the
x
¼
3
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+
Upon addition of Ag , the uorescent spectrum of TBI–TPE was
recorded immediately and then every 30 s. The uorescent
intensity of TBI–TPE was increased signicantly and reached
a plateau aer 1 min. Hence, this result indicates that TBI–TPE
can serve as a uorescent chemical probe for the detection of
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+
Ag with fast response.
3
.3.4 Sensitivity of assay. The uorescence response of
+
TBI–TPE to Ag at different concentrations is shown in Fig. 5C
and D. The uorescence at 525 nm was increased gradually as
+
Ag concentration increased 0 to 4 mM. When the concentration
reaches 4 mM, the intensity of uorescence remains constant.
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tionship with Ag when the uorescence intensity reaches
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)/F
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] and log[Ag ] in the range from 0.1 to 4 mM
(
Fig. 5E) where, F
0
is the uorescence intensity of TBI–TPE (2
mM) only, and F is the uorescence intensity of TBI–TPE in the
presence of metal ions. The detection limit is 90 nM on the
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+
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In conclusion, we have designed and synthesized an AIE uo-
rescence probe containing benzimidazole group, which
exhibited highly sensitive and selective “turn-on” uorescence
for the determination of Ag with a detection limit of 0.090 mM.
The stoichiometry of TBI–TPE and Ag in the complex was
established to be 1 : 2 using Job’s plot and MS studies. Thanks
to the unique AIE property of TPE, a facile strategy for the
construction of high performance “signal on” uorescent
probes has been presented and could be developed for other
potential applications.
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Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (NSFC, 21765024).
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