A novel reaction-based fluorescent probe for sensitive and selective detection of Cu2+

Article abstract of DOI:10.1016/j.ica.2018.02.015

A novel reaction-based fluorescent probe (2) for Cu²⁺ was designed and synthesized. 2 exhibited remarkable fluorescence enhancement (65-fold) after the addition of Cu²⁺. Base on the absorption spectra, fluorescence spectra, ESI-MS and ¹H NMR spectra analysis, the fluorescence response was attributed to Cu²⁺-promoted hydrolysis of the picolinate moiety to yield a highly fluorescent product, 6-acetyl-2-hydroxynaphthalene (1). Moreover, 2 not only displayed a rapid response time (within 6 min), high sensitivity (detection limit of 8.5 nM), and high selectivity for Cu²⁺ over other metal ions, but also could detect Cu²⁺ by absorption spectra, fluorescence spectra, even an obvious fluorescence color change at room temperature.

Full text of DOI:10.1016/j.ica.2018.02.015

Accepted Manuscript
A novel reaction-based fluorescent probe for sensitive and selective detection
2
+
of Cu
Aishan Ren, Dongjian Zhu, Wei Xie, Xingcun He, Zhenhua Duan, Yanghe Luo,
Xing Zhong, Mubo Song, Xiaowei Yan
PII:
S0020-1693(17)31889-3
https://doi.org/10.1016/j.ica.2018.02.015
ICA 18125
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
13 December 2017
2 February 2018
15 February 2018
Please cite this article as: A. Ren, D. Zhu, W. Xie, X. He, Z. Duan, Y. Luo, X. Zhong, M. Song, X. Yan, A novel
2
+
reaction-based fluorescent probe for sensitive and selective detection of Cu , Inorganica Chimica Acta (2018), doi:
https://doi.org/10.1016/j.ica.2018.02.015
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2
+
Aishan Ren, Dongjian Zhu *, Wei Xie, Xingcun He, Zhenhua Duan, Yanghe Luo, Xing Zhong, Mubo Song,
Xiaowei Yan *

1
Inorganica Chimica Acta
2+
A novel reaction-based fluorescent probe for sensitive and selective detection of Cu
Aishan Ren, Dongjian Zhu , Wei Xie, Xingcun He, Zhenhua Duan, Yanghe Luo, Xing Zhong, Mubo Song,
Xiaowei Yan *
Research Institute of Food Science and Engineering Technology, College of Food and Bioengineering, Hezhou University, Hezhou 542899, PR China
ARTICLE INFO
ABSTRACT
2
+
Article history:
Received
Received in revised form
Accepted
A novel reaction-based fluorescent probe (2) for Cu was designed and synthesized. 2 exhibited
2+
remarkable fluorescence enhancement (65-fold) after the addition of Cu . Base on the
absorption spectra, fluorescence spectra, ESI-MS and H NMR spectra analysis, the fluorescence
1
2+
response was attributed to Cu -promoted hydrolysis of the picolinate moiety to yield a highly
fluorescent product, 6-acetyl-2-hydroxynaphthalene (1). Moreover, 2 not only displayed a rapid
response time (within 6 min), high sensitivity (detection limit of 8.5 nM), and high selectivity
for Cu over other metal ions, but also could detect Cu by absorption spectra, fluorescence
spectra, even an obvious fluorescence color change at room temperature.
Available online
2+
2+
Keywords:
Copper ion
2
009 Elsevier Ltd. All rights reserved.
6
-Acetyl-2-hydroxynaphthalene
Fluorescent probe
Turn-on
1
.
Introduction
Heavy and transition metal (HTM) pollution has attracted increasing attention worldwide. Among various HTM, copper ion (Cu ) is
2+
2+
of particular interest due to its physiological roles in biological systems as well as its significant environmental impact. Cu , as the
third most abundant transition metal ions, plays vital roles in various physiological processes, such as energy generation and signal
transduction [1]. However, its deficiency or overload in the human body is associated with various diseases. The deficiency of Cu can
lead anemia [2] and coronary heart disease [3], whereas the excess Cu may cause neurodegenerative diseases including Menke's [4],
Wilson's [5], Parkinson's [6], and Alzheimer's [7] disease. Furthermore, Cu also is a significant metal pollutant because of its
2+
2+
2+
2+
widespread use in current agriculture and industry [8]. Thus, the development of highly sensitive and selective methods for Cu is of
great importance for the human health and environment.
2+
At present, several methods are used for Cu detection, including atomic absorption spectrometry (AAS) [9], atomic emission
spectrometry (AES) [10], inductively coupled plasma mass spectrometry (ICP-MS) [11], electrochemical [12], etc. In spite of high
sensitivity, these methods often require expensive instruments and tedious sample preparation procedures, while unsuitability for real-
time and in vivo analysis. In contrast, fluorescent probe has attracted much attention due to its specificity, high sensitivity, real-time
2+
detection with fast response time, low cost, and simple instrumentation [13]. Hence, a wide variety of fluorescent probes for Cu have
been developed [14-24]. Among these probes, coordination-based fluorescent probes [14]usually exhibit fluorescence-quenching owing
2+
to the paramagnetic nature of Cu [15], which sometimes give false-positive results. Alternatively, reaction-based fluorescent probes
2+
[
16-20] can detect Cu through the irreversible chemical reactions, including the hydrolysis [16], oxidation [17], dethioacetalization
[
18], Heck [19], and Click [20] reaction, etc. which allow for highly sensitive and selective fluorescence enhancement signal. However,
2+
some of reaction-based probes for Cu have limitations such as high temperature [16c,18], long reaction time [15,17a], or non
physiological pH [15,16b]. Therefore, developing novel fluorescent turn-on probe that can detect Cu rapidly and selectively under
physiological conditions at room temperature is highly desired.
2+
Fig. 1. Structure of acedan and 6-acetyl-2-hydroxynaphthalene.
—
——

Corresponding author. Tel.: 86-774-5229756; E-mail: zhudongjian@iccas.ac.cn and yanxiaoweizb@163.com

2
Inorganica Chimica Acta
In addition, discovery and study of fluorescent probes with new molecular framework is a very active area of research. It has been
known that acedan possesses excellent photophysical properties resulting from the typical "push-pull" structure [21], and some of
acedan derivative-based fluorescent probes have been developed in recent years [22]. 6-Acetyl-2-hydroxynaphthalene (1), the electron-
donating hydroxy group instead of methylamino group at the 2-position of acedan would not change the "push-pull" structure (Fig. 1),
while 6-acetyl-2-hydroxynaphthalene-based fluorescent probe is scarce [23]. Herein, we report a novel turn-on fluorescent probe for
2+
Cu base on 6-acetyl-2-hydroxynaphthalene (Scheme 1). We reason that the hydroxy group protected by the electron-withdrawing 2-
pyridinecarbonyl group (2) would decrease the "push" ability thereby quenching the fluorescence of 6-acetyl-2-hydroxynaphthalene,
due to the disruption of the "push-pull" structure, as shown in Scheme 1. On the other hand, it is reported that the picolinate moiety is
2+
easily hydrolyzed by Cu in aqueous solution [24]. We anticipated that the fluorophore 6-acetyl-2-hydroxynaphthalene (1) would be
2+
released from 2 by Cu -promoted hydrolysis reaction of the picolinate moiety with fluorescence enhancement.
2
+
Scheme 1. Synthetic strategy for 2 and proposed reaction mechanism for Cu
.
2.
Results and discussions
As shown in Scheme 1, the probe 2 was synthesized by the coupling reaction of 6-acetyl-2-hydroxynaphthalene (1) with 2-picolinic
acid using (3-dimethylaminopropyl)ethyl-carbodiimid monohydrochloride (EDCI) as coupling agent in the presence of 4-
1
13
dimethylaminopyridine (DMAP). 2 was well characterized by H and C NMR spectroscopy, and ESI mass spectrometry.
o
Fig. 2. (a) Effect of pH on fluorescence intensity at 443 nm for 1 (red cycle) and 2 (black square) in HEPES buffer (20 mM, H
2
O/CH
3
CN, 4:1, v/v, 25 C, λex =
2
+
3
50 nm). (b) Effect of pH on fluorescence intensity at 443 nm for 2 (5 μM) in the absence (black square) and presence (red cycle) of Cu (10 μM) in HEPES
o
2 3
buffer (20 mM, H O/CH CN, 4:1, v/v, 25 C, λex = 350 nm).
o
With 2 in hand, we carried out optimizations on testing conditions including pH and solubility. At room temperature (25 C), the
2+
fluorescence intensity at 443 nm of 2 in the absence of Cu was hardly affected by the pH change from 3.0 to 10.5 (Fig. 2a). While the
fluorescence intensity at 443 nm of 1 was constant in the pH range between 3.0 and 7.4, and remarkably decreased in the pH ranging
from 8.0 to 10.5 (Fig. 2a). Moreover, by plotting the fluorescence intensity at 443 nm vs the pHs, the pK
a
of 1 was calculated to be 9.01
2+
±
0.02 (Fig. S1†). We further examined the pH-dependent response of 2 for Cu in the range of 3.0 - 8.0. As shown in Fig. 2b, in the
2+
presence of Cu , the fluorescence intensity at 443 nm rapidly increased to the maximum in the region of pH 5.5 - 7.0, and decreased
2+
slightly in the pH ranging from 7.4 to 8.0. Thus, pH 7.0 is suggested to be suitable for Cu detection. In addition, 1 and 2 exhibit good
solubility in 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer solution containing acetonitrile as a cosolvent (20 mM,
H O/CH CN, 4:1, v/v, pH 7.0) at room temperature according to the fluorescence and absorption spectra (Fig. S2†), respectively. Based
2 3
o
on the above observations, buffer solution of HEPES (20 mM, H
conditions for further spectral investigation.
2 3
O/CH CN, 4:1, v/v, pH 7.0, 25 C) was selected as the optimized
Then, we assessed the photophysical properties of 2 under the optimized conditions (20 mM HEPES, H
2
O/CH
3
CN, 4:1, v/v, pH 7.0,
o
2
5 C) by the absorption and fluorescence spectra. As shown in Fig. 3a, the solution of 2 (5 μM) displayed two absorption bands at 250
2+
and 282 nm. Upon addition of Cu (10 μM) to solution of 2, the observed two absorption bands decreased, and two new absorption
bands at 260 and 316 nm emerged with an isosbestic point at 256 nm, which resembled the spectroscopic feature of 1 (Fig. S3a†). This
2+
may be due to Cu -promoted the hydrolysis of the picolinate moiety and release of 1. Time course studies showed that the absorbance
at 316 nm reached a plateau within 6 min (Fig. 3b).

3
2
+
Fig. 3. (a) Absorption spectra of 2 (5 μM) recorded every 1 min within 15 min after the addition of Cu (10 μM) in HEPES buffer (20 mM, H
2
O/CH
3
CN, 4:1,
o
v/v, pH 7.0, 25 C). (b) Absorbance at 316 nm changes as a function of time.
2
+
Fig. 4. (a) Fluorescence spectra of 2 (5 μM) recorded every 30 s within 10 min after the addition of Cu (10 μM) in HEPES buffer (20 mM, H
2
O/CH
3
CN, 4:1,
o
v/v, pH 7.0, 25 C, λex = 350 nm). Inset: The fluorescence color changes of 2 before and after the reaction in HEPES buffer under illumination with a 365 nm UV
lamp. (b) The fluorescence intensity at 443 nm changes as a function of time.
2+
Upon excitation at 350 nm, the solution of 2 (5 μM) is almost nonfluorescent (Φ
for 10 min, the solution of 2 showed significant fluorescent emission, and the fluorescence intensity at 443 nm reached a plateau within
min with a 65-fold enhancement (Φ ≈ 0.202), which is similar to that of 1 (Φ ≈ 0.214). A color change from colorless to blue can be
f
≈ 0.006) (Fig. 4a). Upon addition of Cu (10 μM)
6
f
f
visualized under illumination with a 365 nm UV lamp (inset of Fig. 4a). Interestingly, the fluorescence spectra and fluorescence color
of the reaction system quite resembled those of 1 in HEPES buffer (Fig. S3b†). The reaction product of 2 was also checked by ESI
2+
analysis to explore the reaction mechanism. The ESI spectrum of the reaction solution of 2 with Cu shows a major peak at m/z =
+
2+
1
87.1 [M+H] , which is characteristic of the mass of 1 (Fig. S4†). Moreover, we isolated the reaction product of 2 with Cu to further
1
2
+
confirm the mechanism through H NMR spectra. As shown in Fig. 5, the signal of the isolated product of 2 with Cu (b) is the same as
(a). All these results clearly demonstrate that reaction of 2 with Cu causes the hydrolysis of the picolinate moiety and release of 1,
2+
1
thus validating our proposed mechanism (Scheme 1). Time course studies showed that the fluorescence intensity at 443 nm reached a
plateau within 6 min (Fig. 4b), which agreed with the result of the above absorption spectra. From these results, it is concluded that the
2+
probe is suitable for the rapid detection of Cu . The time vs the fluorescence intensity at 443 nm was obtained by using the pseudo-first
-2
-1
order rate equation. We got the observed reaction rate of kobs = 4.279 x 10 s (Fig. S5†).
1
2+
6
Fig. 5. The H NMR spectra of 1 (a), the isolated product of 2 with Cu (b), and 2 (c) in d -DMSO.
2+
To test sensitivity, the time-dependent fluorescence spectra of 2 in the presence of Cu with concentrations from 0 μM to 20 μM
were studied (Fig. 6a). It can be seen that the fluorescence intensity at 443 nm of 2 (5 μM) increased with the increasing of the
2
+
2+
concentration of Cu from 0 μM to 10 μM and then reached a plateau, in which higher concentrations of Cu afforded a faster and
2+
more dramatic fluorescence enhancement. The fluorescence intensity at 443 nm was stable at about 10 μM of Cu (Fig. S6†).
Therefore, the concentration of Cu (10 μM) was used for the following experiments. However, no observable changes in the
fluorescence intensity at 443 nm were detected in the absence of Cu with 10 min. Moreover, a good linearity between the fluorescence
intensity at 443 nm and the Cu concentration in the range of 0 - 5 μM was observed, as depicted in Fig. 6b. The detection limit of the
probe for Cu was thus calculated to be 8.5 nM (Fig. S7†) [25], which was far lower than the WHO recommended level for Cu (31
μM) [26] and the U. S. Environmental Protection Agency set the maximum acceptable level of Cu (～20 μM) [26].
2+
2+
2+
2+
2+
2+

4
Inorganica Chimica Acta
2
+
Fig. 6. (a) Plot of fluorescence intensity at 443 nm versus the reaction time in the presence of varied concentrations of Cu (from bottom to top): 0 (control), 0.25,
2
+
0
.5, 0.75, 1, 2.5, 5, 7.5, 10, 12.5, 15 and 20 µM. (b) The fluorescence intensity at 443 nm as a function of the concentrations of Cu in the range of 0 - 5 μM.
o
Data were acquired in HEPES buffer (20 mM, H
2
O/CH
3
CN, 4:1, v/v, pH 7.0, 25 C, λex = 350 nm).
+
+
2+
Fig. 7. Fluorescence intensity at 443 nm for 2 determined in the absence and presence of different metal ions: 1, none; 2, Na (1 mM); 3, K (1 mM); 4, Ca (1
2
+
+
2+
2+
2+
2+
2+
2+
2+
mM); 5, Mg (1 mM); 6, Ag (10 μM); 7, Zn (10 μM); 8, Cd (10 μM); 9, Hg (10 μM); 10, Ba (10 μM); 11, Co (10 μM); 12, Ni (10 μM); 13, Mn (10
2
+
3+
3+
4+
2+
μM); 14, Pb (10 μM); 15, Al (10 μM); 16, Fe (10 μM); 17, Sn (10 μM); 18, Cu (10 μM). Gray bars: free probe and probe treated with the marked metal
2
+
ions. Black bars: probe treated with the marked metal ions followed by Cu (10 μM). Data were acquired in HEPES buffer (20 mM, H
2
O/CH
3
CN, 4:1, v/v, pH
o
7
.0, 25 C, λex = 350 nm).
2+
Next, we investigated the selectivity of 2 for Cu . According to the kinetic profile experiment result, the reaction time of 6 min was
used for the following experiments. As shown in Fig. 7 and Fig. S8†, the solution of 2 showed a significant fluorescence enhancement
2
+
+
+
2+
2+
+
2+
2+
2+
2+
2+
in the presence of Cu . However, the addition of the other metal ions, including Na , K , Ca , Mg , Ag , Zn , Cd , Hg , Ba , Co ,
2+
2+
2+
3+
3+
4+
Ni , Mn , Pb , Al , Fe and Sn , induced a negligible change in the fluorescence spectra of 2. Meanwhile, competitive experiments
2
+
were performed in the presence of the metal ions mentioned above mixed with Cu . It can be seen that the existence of other potential
competitive metal ions had no obvious effect on the fluorescence response of 2 to Cu . Therefore, an excellent selectivity of 2 toward
2
+
2+
Cu was achieved.
+
+
2+
2+
+
Fig. 8. Absorption spectra of 2 (5 μM) in the absence and presence of different metal ions: Na (1 mM), K (1 mM), Ca (1 mM), Mg (1 mM), Ag (10 μM),
2
+
2+
2+
2+
2+
2+
2+
2+
3+
3+
4+
Zn (10 μM), Cd (10 μM), Hg (10 μM), Ba (10 μM), Co (10 μM), Ni (10 μM), Mn (10 μM), Pb (10 μM), Al (10 μM), Fe (10 μM), Sn (10 μM),
2
+
2+
Cu (10 μM). (a): free probe and probe treated with the marked metal ions. (b): probe treated with the marked metal ions followed by Cu (10 μM) and free
o
2 3
probe. Data were acquired in HEPES buffer (20 mM, H O/CH CN, 4:1, v/v, pH 7.0, 25 C).
2+
The selectivity of 2 for Cu detection was also investigated by absorption spectra. As shown in Fig. 8, 2 exhibited high selectivity to
2+
Cu , which was in good agreement with the result of the above fluorescence response.

5
2
+
Fig. 9. The fluorescence color changes observed before (a) and after (b) the addition of Cu (10 μM) to the solution of 2 (5 μM) in HEPES buffer (20 mM,
o
+
+
2+
2 3
H O/CH CN, 4:1, v/v, pH 7.0, 25 C) under illumination with a 365 nm UV lamp in the presence of different metal ions: Na (1 mM), K (1 mM), Ca (1 mM),
2
+
+
2+
2+
2+
2+
2+
2+
2+
2+
3+
Mg (1 mM), Ag (10 μM), Zn (10 μM), Cd (10 μM), Hg (10 μM), Ba (10 μM), Co (10 μM), Ni (10 μM), Mn (10 μM), Pb (10 μM), Al (10 μM),
3
+
4+
2+
Fe (10 μM), Sn (10 μM), Cu (10 μM).
2
+
2+
In addition, 2 can detect Cu by fluorescence color change. As shown in Fig. 9, Cu , whether present alone or in the presence of
+
+
mixed metal ions, displayed blue fluorescence under illumination with a 365 nm UV lamp, but the other metal ions, such as Na , K ,
Ca , Mg , Ag , Zn , Cd , Hg , Ba , Co , Ni , Mn , Pb , Al , Fe and Sn , exhibited colorless, indicating that 2 showed
excellent selectivity to Cu over other metal ions.
2+
2+
+
2+
2+
2+
2+
2+
2+
2+
2+
3+
3+
4+
2+
The above results of fluorescence response, absorption spectra, and fluorescence color change revealed that 2 had high selectivity for
2+
2+
Cu over the other metal ions, which could be ascribed to the specific hydrolysis reaction of the picolinate moiety promoted by Cu
28].
[
3.
Conclusions
2+
In conclusion, we designed, synthesized and characterized a novel reaction-based fluorescent probe 2 for Cu . 2 displayed a
remarkable fluorescence enhancement (65-fold), rapid response time (within 6 min), high sensitivity (detection limit of 8.5 nM) and
2+
2+
high selectivity for Cu over other metal ions. Moreover, 2 can detect Cu by fluorescence color change which was observed easily by
naked-eye under illumination with a 365 nm UV lamp.
4.
Experiment section
4.1. Reagents and Apparatus
All the chemicals and solvents were purchased from Energy Chemical or Aladdin Industrial Corporation, and used as received with
the following exceptions. Dichloromethane (DCM) was distilled from calcium hydride. Pure water (18.25 Ω) was used to prepare all
1
1
aqueous solutions. H NMR spectra were measured in d
6
-DMSO by JEOL 600 MHz spectrometer and referenced to solvent signals. H
on Bruker Avance-400 400 MHz NMR spectrometer and referenced to solvent signals.
Electrospray ionization mass spectra (ESI-MS) were measured on an LC-MS 6120 equipped with Single Quadrupole LC/MS system
Agilent, America) instrument. UV-visible spectra were recorded on ThermoFisher Evolution 300 UV-vis spectrometer. Fluorescence
13
and C NMR spectra were measured in CDCl
3
(
spectra were recorded using a HITACHI F-4600 spectrometer. The PMT voltage was 700 V, excitation slit and emission slit were 5/5
nm, respectively. The path length was 1 cm with cell volume of 3.0 mL. Fluorescence quantum yield was determined in the reference of
+
quinine sulfate in 0.1 N H
2
SO
4
(Φ
f
= 0.54)[28]. The stock solution of 1 and 2 were prepared in DMSO (2 mM). The solutions of Na ,
+
2+
2+
2+
2+
2+
2+
2+
3+
4+
2+
+
2+
K , Ca , Mg , Zn , Cd , Hg , Ba , Co , Fe , Sn and Cu were prepared from their chloride salts; the solutions of Ag , Pb and
3+
2+
2+
Al were prepared from their nitrate salts; the solutions of Mn and Ni were prepared from their sulphate salts.
4
.2. Synthesis of compound 2
-Acetyl-2-hydroxynaphthalene (1) was synthesized according to the literature procedure [29]. A solution of compound 1 (186.2 mg,
6
1.0 mmol) in anhydrous DCM (10 mL) was treated with 2-picolinic acid (183.2 mg, 1.5 mmol), EDCI (287.6 mg, 1.5 mmol), and
DMAP (73.3 mg, 0.6 mmol). The resultant solution was stirred at room temperature for 2 h under argon atmosphere. After completion
of the reaction, the reaction was diluted with water. The organic phase was separated, and the aqueous phase was extracted with DCM
(2 x 20 mL). The organic phases were combined and washed with 0.1 N HCl, saturated brine, dried over anhydrous sodium sulfate. The
solvents were evaporated to give crude solid, which was purified by silica gel column chromatography using petroleum ether/25-50%
1
ethyl acetate as eluent to afford the desired product as a white solid (227.5 mg, 78%). H NMR (400 MHz, CDCl
3
, ppm) δ 8.86 (d, J =
4
.0 Hz, 1H), 8.47 (s, 1H), 8.31 (d, J = 7.6 Hz, 1H), 8.05 (t, J = 9.2 Hz, 2H), 7.94 (t, J = 7.2 Hz, 1H), 7.88 (d, J = 8.8 Hz, 1H), 7.76 (s,
13
1H), 7.58 (dd, J
1 2 1 2 3
= 5.2 Hz, J = 7.2 Hz, 1H), 7.48 (dd, J = 7.2 Hz, J = 8.8 Hz, 1H), 2.71 (s, 3H). C NMR (100 MHz, CDCl , ppm) δ
1
97.88, 163.90, 150.67, 150.30, 137.38, 136.21, 134.62, 131.32, 130.77, 130.03, 128.35, 127.72, 126.06, 124.82, 122.26, 118.97, 26.77.
+
+
3
ESI-MS m/z for C18H14NO ([M+H] ): calcd: 292.1, found: 292.1.
Acknowledgments
We thank the National Natural Science Foundation of China (21765008, 21603064, 21367012, 21365011), Natural Science
Foundation of Guangxi (2017GXNSFBA198178, 2016GXNSFBA380002, 2015GXNSFBA139040), the Doctor's Scientific Research
Foundation of Hezhou University (HZUBS201509), Special Fund for Science and Technology Base and Talent of Guangxi
(GKAD17195088), Guangxi Preservation and Deep Processing Research in Fruit and Vegetables Talent Highland Project and Special
Fund for Distinguished Experts in Guangxi for financial supports.
Supplementary Material
Supplementary materials including additional spectroscopic data.
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7
Highlights
2
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A novel reaction-based fluorescent probe 2 for Cu was developed.
2
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The detection limit of 2 for Cu was found to be 8.5 nM.
2
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2 exhibited a rapid response of Cu within 6 min.
2
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2 could detect Cu with an obvious fluorescence color change at room temperature.

8
Inorganica Chimica Acta
The probe 2 not only displayed a remarkable fluorescence enhancement (65-fold), rapid response time
2
+
(
within 6 min), very high sensitivity (detection limit of 8.5 nM), and high selectivity for Cu over other
2
+
metal ions, but also could detect Cu by absorption spectra, fluorescence spectra, even an obvious
fluorescence color change at room temperature.