Aggregation-induced emission enhancement (AIEE)-active tetraphenylethene (TPE)-based chemosensor for Hg2+ with solvatochromism and cell imaging characteristics

Article abstract of DOI:10.1039/c9ra02119a

An aggregation-induced emission enhancement (AIEE)-active fluorescent sensor based on a tetraphenylethene (TPE) unit has been successfully designed and synthesized. Interestingly, the luminogen could detect Hg²⁺ with high selectivity in an acet

Full text of DOI:10.1039/c9ra02119a

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Aggregation-induced emission enhancement
(AIEE)-active tetraphenylethene (TPE)-based
chemosensor for Hg2+ with solvatochromism and
cell imaging characteristics†
Cite this: RSC Adv., 2019, 9, 11865
*
*
and Shouzhi Pu
Aling Tang, Zhao Chen,
Diandian Deng, Gang Liu,
Yayi Tu
An aggregation-induced emission enhancement (AIEE)-active fluorescent sensor based on
a tetraphenylethene (TPE) unit has been successfully designed and synthesized. Interestingly, the
luminogen could detect Hg2+ with high selectivity in an acetonitrile solution without interference from
other competitive metal ions, and the detection limit was 7.46 ꢀ 10ꢁ6 mol Lꢁ1. Furthermore, the
luminogen also showed interesting solvatochromic behavior and superior cell imaging performance.
Received 19th March 2019
Accepted 11th April 2019
DOI: 10.1039/c9ra02119a
rsc.li/rsc-advances
The design and synthesis of molecular sensors for the detection in a lot of elds, including bioimaging, uorescence sensors,
of metal ions, especially transition-metal ions, has attracted organic lighting emitting diode (OLED) devices and organic
much attention.1–3 Among all transition-metal ions, mercury lasers.28–33 Meanwhile, many stimuli-responsive materials have
(Hg2+) is identied as one of the most dangerous and ubiqui- been reported, including photochromism, mechanochromism,
tous heavy metals. Indeed, it is not biodegradable, and can and solvatochromism.34–46 At present, the solvatochromism
cause extreme injury to the environment as well as human materials have been generally used in chemical and biological
health.4–8 Additionally, it can be accumulated through the food systems.47 To date, many uorescent chemosensors for Hg2+ have
chain in the human body, consequently giving rise to several been reported. In contrast, the corresponding chemosensors
deleterious effects such as central nervous system defects, with AIE or AIEE effect are rare, not to mention solvatochromic
kidney damage, endocrine system disease and so on.9–12 AIE or AIEE-active uorescent sensors for Hg2+ with good cell
Although many governments around the world have adopted imaging behavior. Indeed, preparing such multifunctional
strict regulations to limit Hg2+ emission, the global Hg2+ sensors is challenging and signicative. In this paper, we re-
pollution caused by human activities is still serious.13,14 There- ported an AIEE-active tetraphenylethene (TPE)-based uorescent
fore, it is highly desirable to develop a new method for the sensor (Scheme 1) for the detection of Hg2+. Furthermore, the
detection of Hg2+ with high selectivity and sensitivity.15–17
luminogen also showed remarkable solvatochromism and good
Most traditional uorescent sensors suffer from a detrimental cell imaging characteristics.
phenomenon called aggregation-caused quenching (ACQ), which
usually results in a poor solid-state emission efficiency. Fortu-
nately, in 2001, Tang et al. reported a uorescent molecule
named 1-methyl-1,2,3,4,5-pentaphenylsilole. Interestingly, the
uorescence emission of this luminogen was induced by aggre-
gation, a phenomenon referred to as aggregation-induced emis-
sion (AIE).18 Subsequently, in 2002, Park et al. reported an
interesting phenomenon named aggregation-induced emission
enhancement (AIEE).19 In fact, both AIE and AIEE can achieve
highly efficient emission in the solid state or aggregated state.20–27
In the past years, AIE (or AIEE) phenomenon has attracted
considerable research interest owing to the potential applications
Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal
University, Nanchang 330013, PR China. E-mail: chenzhao666@126.com;
pushouzhi@tsinghua.org.cn
† Electronic supplementary information (ESI) available: Experimental section,
NMR spectra, mass spectrum, and characterization data mentioned in the
paper. See DOI: 10.1039/c9ra02119a
Scheme 1 The synthetic route of luminogen 1.
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To investigate the AIEE phenomenon of luminogen 1, we
initially studied the UV-visible absorption spectra and photo-
luminescence (PL) spectra in acetonitrile–water mixtures with
different water fractions (fw). The results indicated that the
absorption spectra exhibited level-off tails in the long wave-
length region as the water content increased (Fig. S1†). It is well-
known that such tails are usually associated with the Mie scat-
tering effect, which is the key signal of nano-aggregate forma-
tion.48,49 As presented in Fig. 1, luminogen 1 showed weak
uorescence and the luminescence quantum yield (F) was as
low as 1.35%. Interestingly, when the fw in the acetonitrile
solution was increased to 80%, an obvious emission band was
formed, and a yellow-green uorescence was observed. As the
water content was increased to 90%, the emission intensity was
further increased, and a bright yellow-green luminescence (F ¼
27.81%) with a lmax at 545 nm could be observed.
Fig. 2 Size distribution curve of luminogen 1 (2.0 ꢀ 10ꢁ5 mol Lꢁ1) in
acetonitrile–water mixtures with fw ¼ 90%.
Clearly, water is a poor solvent of luminogen 1. As a result,
the generation of the yellow-green emission can be attributed to
the aggregate formation.50–52 Moreover, as shown in Fig. 2, the
nano-aggregates obtained were veried by dynamic light scat-
terings (DLS). Therefore, 1 is a typical AIEE-active uorescent
molecule, and its AIEE behavior is caused by the restricted
intramolecular rotation. As shown in Fig. S2,† solid-state
compound 1 showed a strong green emission (F ¼ 14.50%)
with a lmax at 497 nm, and the corresponding uorescence
lifetime is 2.66 ns (Fig. S3†).
On the other hand, the luminogen 1 also displayed inter-
esting solvatochromism effect. As presented in Fig. 3, the
absorption spectra and uorescence spectra of luminogen 1 in
different solvents were investigated, respectively. Obviously, the
absorption spectra was barely affected by the polarity of
solvents. However, the photoluminescence peaks were gradu-
ally red-shied from 515 nm to 625 nm, and thus 1 exhibited
remarkable solvatochromism behavior. Obviously, the
molecular structure of luminogen 1 is distorted due to the
presence of TPE unit, and the conjugation degree of molecule 1
is different in various solvents, and the intramolecular charge
transfer (ICT) effect is possibly responsible for the sol-
vatochromism behavior of 1.
Subsequently, the changes in the uorescence of luminogen
1 induced by Hg2+ were investigated in acetonitrile (2.0 ꢀ
10ꢁ5 mol Lꢁ1) at room temperature. In the uorometric titration
experiments, as shown in Fig. 4, the emission intensity signif-
icantly decreased when the Hg2+ concentration increased from
0 to 7.0 equivalents in acetonitrile. Meanwhile, the uorescent
color changed from orange-red to colorless, and followed by
a plateau upon further titration (Fig. 5). Remarkably, the
emission intensity of luminogen 1 was almost quenched
completely. Furthermore, based on the titration experiments,
the detection limit of luminogen 1 for Hg2+ on the basis of LOD
¼ 3 ꢀ s/B (where s is the standard deviation of blank sample
and B is the slope between the uorescence intensity versus
Hg2+ concentration) was 7.46 ꢀ 10ꢁ6 M (Fig. S4†). Moreover,
Fig. 1 (a) Fluorescence spectra of the dilute solutions of 1 (2.0 ꢀ
10ꢁ5 mol Lꢁ1) in acetonitrile–water mixtures with different water Fig. 3 (a) UV-Vis absorption spectra and (b) normalized fluorescence
content (0–90%). Excitation wavelength ¼ 380 nm. (b) Changes the spectra (Excitation wavelength ¼ 380 nm) of luminogen 1 in different
emission intensity of 1 at 545 nm in acetonitrile–water mixtures with solvents (2.0 ꢀ 10ꢁ5 mol Lꢁ1). (c) Photographs of 1 under 365 nm UV
different volume fractions of water (0–90%). (c) PL images of 1 (2.0 ꢀ illumination in different solvents: tol (toluene); THF (tetrahydrofuran);
10ꢁ5 mol Lꢁ1) in acetonitrile–water mixtures with different fw values EA (ethyl acetate); BT (acetone); MeCN (acetonitrile); DMSO (dimethyl
under 365 nm UV illumination.
sulfoxide).
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Fig. 6 1H NMR (acetonitrile-d3, 400 MHz) spectra changes of lumi-
nogen 1 in the presence of Hg2+
.
Fig.
4
Fluorescence titration spectra of luminogen
1
(2.0
ꢀ
10ꢁ5 mol Lꢁ1) induced by Hg2+ (0–7.0 equiv.) in an acetonitrile solu- under the same experimental conditions. The corresponding
tion. Excitation wavelength ¼ 380 nm.
UV-Vis absorption spectra were shown in Fig. S8.† Furthermore,
as showed in Fig. 7, when these cations were added separately
into the solution containing luminogen 1, no obvious uores-
cence changes were observed. Indeed, as noticed in Fig. S9,† no
obvious interference was observed when Hg2+ (7.0 equiv.) was
added with other ions (7.0 equiv.). These results indicated that
luminogen 1 could be served as a highly selective uorescent
sensor for detection of Hg2+
.
Fluorescent probe is a powerful tool for optical imaging,
which allows direct visualization of biological analytes.53
Luminogen 1 was AIEE-active due to the restriction of intra-
molecular rotation in the aggregate state, which is benecial for
cell imaging. Indeed, the viability of HeLa cells incubated with
luminogen 1 was evaluated by the standard MTT method
(Fig. 8), and the result indicated that compound 1 exhibited low
cytotoxicity. Next, cell imaging behavior of luminogen 1 was
investigated using a confocal laser scanning microscopy
(CLSM). HeLa cells were incubated with luminogen 1 (20 mM)
for 30 min at 37 ꢂC and the uorescence images were obtained
by CLSM.
Fig. 5 The emission intensity changes of luminogen 1 at 625 nm with
different equivalents of Hg2+
.
As presented in Fig. 9, an intense yellow-green uorescence,
which was consistent with the uorescence of luminogen 1 in
a good linear relationship could be obtained (R ¼ ꢁ0.9933) and
the quenching constant of luminogen 1 with Hg2+ was 1.9 ꢀ 104
Mꢁ1 (Fig. S5†). On the other hand, the binding ratio of lumi-
nogen 1 for Hg2+ was established through Job's plot and the
results showed 1 : 1 stoichiometric complexation (Fig. S6†).
Next, the binding interactions between 1 and Hg2+ were further
investigated by NMR in acetonitrile-d3. As presented in Fig. 6,
the signal of Ha from 9.42 ppm shied to 9.62 ppm and the Hb
changed from 8.43 ppm to 9.24 ppm. Besides, the Hc or Hd was
slightly shied for 0.04 ppm or 0.06 ppm, respectively. These
consequences revealed that the N on the pyridine and the N on
the pyrazine (near the N on the pyridine) are the most probable
binding with Hg2+. Mass spectra were utilized to further
demonstrate the binding mode of luminogen 1 toward Hg2+.
The peak located at m/z ¼ 820.0 was coincided well with the
ensemble [1 + Hg2+ +2NO3 + Clꢁ]ꢁ, conrming the binding
ꢁ
ratio of luminogen 1 for Hg2+ with 1 : 1 stoichiometry (Fig. S7†).
Next, in order to study the selectivity behavior of luminogen
1 as a uorescent sensor for Hg2+, other metal ions, such as
Zn2+, Cd2+, Ba2+, Sr2+, Mn2+, Mg2+, Ca2+, Pb2+, Ni2+, Co2+, Cu2+,
Al3+, Fe3+, Cr3+, Ag+ and K+ were also measured in acetonitrile
Fig. 7 (a) Fluorescence spectra of luminogen 1 (2.0 ꢀ 10ꢁ5 mol Lꢁ1
)
,
towards various cations including Zn2+, Cd2+, Hg2+, Ba2+, Sr2+, Mn2+
Mg2+, Ca2+, Pb2+, Ni2+, Co2+, Cu2+, Al3+, Fe3+, Cr3+, Ag+ and K+.
Excitation wavelength ¼ 380 nm. (b) Fluorescence photographs of
luminogen 1 after addition of various metal ions (7.0 equiv.) under
365 nm light.
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Foundation of Jiangxi Province (20171BAB213004), the Young
Talents Project of Jiangxi Science and Technology Normal
University (2017QNBJRC005), and the Project of the Science
Fund of Jiangxi Education Office (GJJ180593).
Notes and references
1 C. Zhang, S. Pu, Z. Sun, C. Fan and G. Liu, J. Phys. Chem. B,
2015, 119, 4673–4682.
2 H. N. Kim, W. X. Ren, J. S. Kim and J. Yoon, Chem. Soc. Rev.,
2012, 41, 3210–3244.
3 S. Xia, G. Liu and S. Pu, J. Mater. Chem. C, 2015, 3, 4023–4029.
4 J. D. Demers, J. D. Blum, S. C. Brooks, P. M. Donovan,
A. L. Riscassi, C. L. Miller, W. Zheng and B. Gu, Environ.
Sci.: Processes Impacts, 2018, 20, 686–707.
Fig. 8 The MTT assay of compound 1 for measuring cell viability.
5 M. F. Wolfe, S. Schwarzbach and R. A. Sulalman, Environ.
Toxicol. Chem., 1998, 17, 146–160.
6 X. Liu, X. Liu, M. Tao and W. Zhang, J. Mater. Chem. A, 2015,
3, 13254–13262.
7 W. Luo, H. Jiang, K. Zhang, W. Liu, X. Tang, W. Dou, Z. Ju,
Z. Li and W. Liu, J. Mater. Chem. B, 2015, 3, 3459–3464.
8 S. Gupta and M. D. Milton, New J. Chem., 2018, 42, 2838–
2849.
9 W. Fang, G. Zhang, J. Chen, L. Kong, L. Yang, H. Bi and
J. Yang, Sens. Actuators, B, 2016, 229, 338–346.
10 C. Li and S. Liu, J. Mater. Chem., 2010, 20, 10716–10723.
11 O. Abollino, A. Giacomino, M. Malandrino, G. Piscionieri
and E. Mentasti, Electroanalysis, 2008, 20, 75–83.
12 L. Yang, Y. Su, Y. Geng, H. Xiong, J. Han, Q. Fang and
X. Song, Org. Biomol. Chem., 2018, 16, 5036–5042.
13 Z. Ruan, Y. Shan, Y. Gong, C. Wang, F. Ye, Y. Qiu, Z. Liang
and Z. Li, J. Mater. Chem. C, 2018, 6, 773–780.
14 K. Wang, J. Li, S. Ji, L. Li, Z. Qiu, C. Pan, J. Zhang and Y. Huo,
New J. Chem., 2018, 42, 13836–13846.
15 S.-L. Pan, K. Li, L.-L. Li, M.-Y. Li, L. Shi, Y.-H. Liu and
X.-Q. Yu, Chem. Commun., 2018, 54, 4955–4958.
16 S. Chen, W. Wang, M. Yan, Q. Tu, S.-W. Chen, T. Li,
M.-S. Yuan and J. Wang, Sens. Actuators, B, 2018, 255,
2086–2094.
Fig. 9 Fluorescence images of HeLa cells incubated with luminogen 1
(20 mM) for 30 min at 37 ꢂC: (a) bright field image; (b) fluorescence
image; (c) merge image (a and b).
acetonitrile–water mixture with high water content (80% or
90%), was displayed inside the cells. This result indicated that
luminogen 1 tended to aggregate inside the cells, and thus the
bright yellow-green luminescence was clearly observed.
Furthermore, the merged picture c demonstrated that picture
a and picture b overlapped very well. These results indicated
that luminogen 1 showed superior cell imaging performance.
In summary, a TPE-based uorescent molecule was designed
and synthesized. The luminogen exhibited obvious AIEE
phenomenon. Moreover, luminogen 1 could be used as a highly
selective uorescence turn-off chemosensor for Hg2+, and the
detection limit was 7.46 ꢀ 10ꢁ6 mol Lꢁ1. Furthermore, 1 also
displayed interesting solvatochromic behavior and superior cell
imaging performance. Further studies on the design of new AIE
or AIEE-active uorescent chemosensors are in progress.
17 Y. Chen, W. Zhang, Y. Cai, R. T. K. Kwok, Y. Hu, J. W. Y. Lam,
X. Gu, Z. He, Z. Zhao, X. Zheng, B. Chen, C. Gui and
B. Z. Tang, Chem. Sci., 2017, 8, 2047–2055.
18 J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu,
H. S. Kwok, X. Zhan, Y. Liu, D. Zhu and B. Z. Tang, Chem.
Commun., 2001, 1740–1741.
19 B.-K. An, S.-K. Kwon, S.-D. Jung and S. Y. Park, J. Am. Chem.
Soc., 2002, 124, 14410–14415.
20 Z. Chen, J. Zhang, M. Song, J. Yin, G.-A. Yu and S. H. Liu,
Chem. Commun., 2015, 51, 326–329.
Conflicts of interest
There are no conicts to declare.
21 Z. Chen, Z. Li, F. Hu, G.-A. Yu, J. Yin and S. H. Liu, Dyes Pigm.,
2016, 125, 169–178.
Acknowledgements
The authors are grateful for the nancial support from the 22 F. Zhao, Z. Chen, G. Liu, C. Fan and S. Pu, Tetrahedron Lett.,
National Natural Science Foundation of China (21702079 and 2018, 59, 836–840.
41867053), the “5511” Science and Technology Innovation 23 Y. Yin, F. Zhao, Z. Chen, G. Liu and S. Pu, Tetrahedron Lett.,
Talent Project of Jiangxi Province (20165BCB18015), the key 2018, 59, 4416–4419.
project of Natural Science Foundation of Jiangxi Province 24 Z. Chen, G. Liu, S. Pu and S. H. Liu, Dyes Pigm., 2017, 143,
(20171ACB20025), the youth project of Natural Science
409–415.
11868 | RSC Adv., 2019, 9, 11865–11869
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
25 L. Yu, Z. Wu, G. Xie, C. Zhong, Z. Zhu, D. Ma and C. Yang, 40 J. Liang, Z. Chen, J. Yin, G.-A. Yu and S. H. Liu, Chem.
Chem. Commun., 2018, 54, 1379–1382. Commun., 2013, 49, 3567–3569.
26 X. Ma, L. Hu, X. Han and J. Yin, Chin. Chem. Lett., 2018, 29, 41 C. Gstrein, B. Zhang, M. A. Abdel-Rahman, O. Bertran,
´
¨
1489–1492.
C. Aleman, G. Wegner and A. D. Schluter, Chem. Sci., 2016,
27 X. Han, Y. Liu, G. Liu, J. Luo, S. H. Liu, W. Zhao and J. Yin,
Chem.–Asian J., 2019, 14, 890–895.
28 F. Hua and B. Liu, Org. Biomol. Chem., 2016, 14, 9931–9944.
7, 4644–4652.
42 R. Tan, S. Wang, H. Lan and S. Xiao, Curr. Org. Chem., 2017,
21, 236–248.
29 Z. Chen, J. Liang, Y. Nie, X. Xu, G.-A. Yu, J. Yin and S. H. Liu, 43 R. Wang, Y. Sun, F. Zhang, M. Song, D. Tian and H. Li,
Dalton Trans., 2015, 44, 17473–17477. Angew. Chem., Int. Ed., 2017, 56, 5294–5298.
30 X. Sun, A. Zebibula, X. Dong, G. Zhang, D. Zhang, J. Qian and 44 W. Chen, Y. Pan, J. Chen, F. Ye, S. H. Liu and J. Yin, Chin.
S. He, ACS Appl. Mater. Interfaces, 2018, 10, 25037–25046. Chem. Lett., 2018, 29, 1429–1435.
31 D. Wu, A. C. Sedgwick, T. Gunnlaugsson, E. U. Akkaya, 45 J. Chen, D. Li, W. Chi, G. Liu, S. H. Liu, X. Liu, C. Zhang and
J. Yoon and T. D. James, Chem. Soc. Rev., 2017, 46, 7105–
7123.
32 J. Huang, N. Sun, P. Chen, R. Tang, Q. Li, D. Ma and Z. Li,
Chem. Commun., 2014, 50, 2136–2138.
J. Yin, Chem.–Eur. J., 2018, 24, 3671–3676.
46 J. Chen, F. Ye, Y. Lin, Z. Chen, S. Liu and J. Yin, Sci. China:
Chem., 2019, 62, 440–450.
47 C. Karthik, V. Manjuladevi, R. K. Gupta and S. Kumar, RSC
Adv., 2015, 5, 84592–84600.
33 F. Liu, J. Tu, X. Wang, J. Wang, Y. Gong, M. Han, X. Dang,
Q. Liao, Q. Peng, Q. Li and Z. Li, Chem. Commun., 2018, 48 C. W. T. Leung, Y. Hong, S. Chen, E. Zhao, J. W. Y. Lam and
54, 5598–5601. B. Z. Tang, J. Am. Chem. Soc., 2013, 135, 62–65.
34 S. Guo, C. Fan, G. Liu and S. Pu, RSC Adv., 2018, 8, 39854– 49 Z. Chen, D. Wu, X. Han, J. Liang, J. Yin, G.-A. Yu and
39864.
S. H. Liu, Chem. Commun., 2014, 50, 11033–11035.
50 Z. Chen, X. Han, J. Zhang, D. Wu, G.-A. Yu, J. Yin and
S. H. Liu, RSC Adv., 2015, 5, 15341–15349.
51 Z. Chen, G. Liu, S. Pu and S. H. Liu, Dyes Pigm., 2018, 152, 54–
59.
52 J. Mei, N. L. C. Leung, R. T. K. Kwok, J. W. Y. Lam and
B. Z. Tang, Chem. Rev., 2015, 115, 11718–11940.
53 Z. Xu, X. Huang, X. Han, D. Wu, B. Zhang, Y. Tan, M. Cao,
S. H. Liu, J. Yin and J. Yoon, Chem, 2018, 4, 1609–1628.
35 B. Wang and C. Wei, RSC Adv., 2018, 8, 22806–22812.
36 Z. Chi, X. Zhang, B. Xu, X. Zhou, C. Ma, Y. Zhang, S. Liu and
J. Xu, Chem. Soc. Rev., 2012, 41, 3878–3896.
37 F. Chen, W. Zhang, Z. Liu, L. Meng, B. Bai, H. Wang and
M. Li, RSC Adv., 2019, 9, 1–10.
38 J. Liang, Z. Chen, L. Xu, J. Wang, J. Yin, G.-A. Yu, Z.-N. Chen
and S. H. Liu, J. Mater. Chem. C, 2014, 2, 2243–2250.
39 Z. Chen, Z. Li, L. Yang, J. Liang, J. Yin, G.-A. Yu and S. H. Liu,
Dyes Pigm., 2015, 121, 170–177.
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