A chromone derivative as a colorimetric and “ON-OFF-ON” fluorescent probe for highly sensitive and selective detection of Cu2+ and S2?

Article abstract of DOI:10.1016/j.ica.2021.120280

A chromone derivative (L) which could be utilized for reversibly detecting Cu²⁺ and S^2? by means of fluorescence quenching and displacement in aqueous solution was designed and developed. Upon the addition of Cu²⁺, the strong blue-green fluorescence emission of L was quenched rapidly and then recovered in 1 min by successively adding S^2?. Furthermore, with the alternate addition of Cu²⁺ and S^2?, the reversible cycles could be repeated for at least four times, which meant that L could be identified as a renewable dual-functioning probe. What's more, due to its excellent sensing performances, this probe L could also be used to detect Cu²⁺ and S^2? with test strips conveniently.

Full text of DOI:10.1016/j.ica.2021.120280

Inorganica Chimica Acta 519 (2021) 120280
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
A chromone derivative as a colorimetric and “ON-OFF-ON” fluorescent
probe for highly sensitive and selective detection of Cu2 and S
+
2ꢀ
*
*
Cong Liu, Limei Tian, Kui Liu, Jia Xue, Long Fan, Tianrong Li , Zheng-yin Yang
College of Chemistry and Chemical Engineering, State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, PR China
A R T I C L E I N F O
A B S T R A C T
A chromone derivative (L) which could be utilized for reversibly detecting Cu2 and S by means of fluores-
+
2ꢀ
Keywords:
Fluorescent probe
Chromone derivative
2+
cence quenching and displacement in aqueous solution was designed and developed. Upon the addition of Cu
,
the strong blue-green fluorescence emission of L was quenched rapidly and then recovered in 1 min by suc-
2
+
Cu
2ꢀ
2+
2ꢀ
cessively adding S . Furthermore, with the alternate addition of Cu and S , the reversible cycles could be
2
ꢀ
S
repeated for at least four times, which meant that L could be identified as a renewable dual-functioning probe.
Reversible probe
2
+
2ꢀ
What’s more, due to its excellent sensing performances, this probe L could also be used to detect Cu and S
with test strips conveniently.
1
. Introduction
concentration effect, which induces the brain excitement at low con-
centration and inhibited brain action at high concentration [17-19].
Owing to their beneficial and detrimental influences on the envi-
2
Hydrogen sulfide (H S) is a strong inhibitor which can bind with ferric
ronmental and biological circumstances, many transition metal ions
have gained much attention among scientists [1,2]. In human body,
copper is the third most affluent indispensable trace element followed
with iron and zinc among these ions, and could act as an catalytic factor
for a series of metalloenzymes [3,4] such like copper/zinc superoxide
dismutase, cytochrome oxidase and tyrosinase because of its unique
redox activity [5,6]. Moreover, Cu2 could also exhibit a pivotal function
in many basic physiological processes, involving in blood formation,
respiration and transcriptional events [7]. Nevertheless, excessive
amount of copper in cells can cause a variety of serious diseases, espe-
cially neurodegenerative diseases like Wilson’s, Alzheimer’s, and prion
diseases [8-12]. Consequently, the World Health Organization (WHO)
has set the maximal permitted content of copper in drinking water at 2.0
ions in cytochrome oxidase, thus inhibiting the electron transport and
oxygen utilization, causing intracellular hypoxia [18,20]. At the same
time, sulfides have been developed for many crucial applications, such
as the conversion of sulfur and sulfuric acid, and the manufacture of dyes
and cosmetics [20,21]. What’s more, increasing attention has recently
been attached to the role of hydrogen sulfide (H
2
S) in organisms.
+
Hydrogen sulfide (H S) has been regarded as one of three endogenous
2
gas transporters in recent studies, which produce a range of biological
effects on various biological targets, playing a vital role in physiological
processes such as myocardial protection, neuromodulation, vasodila-
tion, anti-inflammatory, central nervous system, and endocrine system
[22-30]. Therefore, the detection of sulfide ion (S2 ) has received
extensive attention in industrial, environmental and biological assays.
Up to date, fluorescent probes have drawn much attention because of
their perfect operability, high sensitivity and good biocompatibility
[31,32]. Thus, they are applied widely to the detection of various ana-
lytes in biological and environmental researches. So far, many chemical
ꢀ
ppm (~30 M) [13]. Based on these considerations, we are committed to
μ
develop sensitive and selective methods for rapid and reliable detection
and monitoring of copper ion (Cu2 ) in water environment and bio-
+
logical circumstance.
On the other side, as a traditional toxic pollutant, sulfide is widely
distributed in the environment and life [14]. As is known, hydrogen
probes detect sulfide ion (S2 ) by dragging Cu out from L-Cu due to
ꢀ 2+ 2+
ꢀ 36
a relatively low dissolution constants (Ksp = 1.27 × 10 ) of CuS
sulfide (H
2
S) at low concentration levels can stimulate the sense of
[33,34]. On this account, a large number of fluorescence probes
2
+
smell, and the olfactory nerves will be paralyzed when its concentration
exceeds a certain threshold, which is dangerous [15,16]. Additionally,
employing this strategy for sensing copper ion (Cu ) and sulfide ion
2
ꢀ
(S ) have been reported. However, most of these probes suffer from
some shortcomings, such as poor water solubility, low selectivity,
2
hydrogen sulfide (H S) could directly function on the brain and show a
*
Corresponding authors.
E-mail address: yangzy@lzu.edu.cn (Z.-y. Yang).
https://doi.org/10.1016/j.ica.2021.120280
Received 28 September 2020; Received in revised form 14 December 2020; Accepted 27 January 2021
Available online 5 February 2021
0
020-1693/© 2021 Elsevier B.V. All rights reserved.

C. Liu et al.
Inorganica Chimica Acta 519 (2021) 120280
limited reversibility and the need for a series of organic co-solvents in
the detection system, which seriously restricts their practical applica-
tions under physiological conditions [31,34-36]. Therefore, it is very
important and necessary to design and synthesize simple and effective
the synthetic route of L was shown in scheme 1. In general procedure,
compound 1 (0.109 g, 0.5 mmol) was dissolved in ethanol (10 mL) and
compound 2 (0.137 g, 0.5 mmol) was also dissolved in ethanol (10 mL).
Then the solution of compound 2 was slowly added to compound 1
solution. After the reaction mixture was stirred and refluxed for 4 h, the
light brown powder (L) was filtered and washed with water. Yield:
2
+
fluorescent probes for reversible detection of copper ion (Cu ) and
sulfide ion (S2 ) in aqueous media. Compared with other fluorophores,
coumarin displays the advantages of large absorption coefficient in the
visible region, excellent optical properties like good photostability,
easily modified structure and strong coordination ability. Considering
these factors, we have designed and synthesized a reversible fluorescent
ꢀ
1
0.124 g (52.2%), M.P.: 244–246 ℃. H NMR (400 Hz, CDCl3, TMS)
7
6
(Fig. S1): δ
H
(ppm): 11.986 (s, 1H, H ), 8.840 (s, 1H,H ), 8.815(s, 1H,
9
8
5
H ), 8.491 (s, 1H, H ), 7.604 (d, 1H, J = 2.8 Hz, H ), 7.438–7.489 (m,
2H, H1
0, 11
4
), 7.268–7.299 (m, 1H, H ), 6.682 (dd, 1H, J
12
= 6.4 Hz, J =
1
2
2
+
3
13
probe L derived from coumarin for successive recognition of Cu and
2.4 Hz, H ), 6.529 (s, 1H, H ), 4.149 (q, 2H, J = 6.8 Hz, H ), 3.379 (q,
2
ꢀ
2
14
S
in aqueous media. As shown in scheme 1, this probe L was synthe-
4H, J = 7.2 Hz, H ), 1.465 (t, 3H, J = 6.8 Hz, H ), 1.259 (t, 3H, J = 7.2
1
sized by a one-step condensation between chromonealdehyde and cou-
Hz, H ). ESI-MS (Fig. S2) peak at m/z = 498.223 could be assigned to [L
+
+
+ +
marinhydrazide, and could form a complex L-Cu by combining with
+ Na ] and peak at m/z = 514.198 could be assigned to [L + K ] .
The complex L-Cu was synthesized by dissolving L in ethanol (20
2
+
Cu , leading to fluorescence emission quenching. Nevertheless, after
adding S2 to the formed complex L-Cu, the fluorescence emission in-
tensity was restored because CuS was formed and L was released. In
particular, the corresponding “ON-OFF-ON” fluorescence emission cycle
ꢀ
3 2
mL), followed by the addition of Cu(NO ) and then the reaction mixture
was stirred for 2 days. After completing the reaction, the solvent was
removed by rotary evaporation under reduced pressure. Subsequently,
the appeared brown solid was washed with water and then dried in the
oven. Yield: 84%.
could be repeated for more than four times by adding Cu2 and S
+
2ꢀ
successively, which indicated that this probe L could be identified as an
2
+
ideal fluorescent chemical probe for the successive monitoring of Cu
2
ꢀ
and S in biological systems.
. Experimental
.1. Materials and instrumentation
2
.3. Analysis
2
All stock solutions of metal ions were obtained by the dissolution of
their nitrates or chlorides in ethanol. All stock solutions of anions were
prepared by the dissolution of their corresponding potassium or sodium
salts in distilled water. The concentration of these stock solutions were
all 0.5 mM. The probe L stock solution was prepared with DMF at a
concentration of 0.5 mM. For a typical detection process, L solution (20
2
The melting points of compounds were determined by Beijing-XT4-
00X micro-melting point apparatus. Electrospray ionization mass
1
spectrometry (ESI-MS) was obtained by Bruker esquire 6000 mass
μ
L) was injected into EtOH/HEPES solution (4/1, 10.0
μ
M HEPES, pH
1
spectrometer. Nuclear magnetic resonance spectroscopy ( H NMR) were
7
.2) (2 mL) to maintain the overall concentration at 5.0
μ
M. After adding
collected on a JNM-ECS 400 MHz nuclear magnetic spectrometer with
tetramethylsilane (TMS) as an internal standard. IR spectra were ob-
tained in KBr discs on a Therrno Mattson FT-IR spectrometer in the
corresponding chemical reagents, UV–vis absorption spectra and fluo-
rescence emission spectra were carried out directly at room
temperature.
ꢀ
1
4
000–400 cm region. UV–vis absorption spectra were measured by a
Shimadzu UV-240 UV–vis spectrophotometer using a 1 cm path of the
cuvette and the fluorescence spectra were recorded with a Hitachi RF-
3
. Results and discussion
2+
.1. Spectral studies of L with Cu
4
500 fluorescence spectrometer with a 1 cm path of the cuvette. The
3
samples were excited at 377 nm and the emission spectra were recorded
in the range of 400–600 nm. The excitation and emission slit widths
were set at 5 nm and 3 nm respectively. All chemicals and reagents
mentioned in the experiments were gained commercially and used
directly without further purification.
In order to comprehend the sensing properties of L, the sensitivity
+
and selectivity of probe L towards common metal ions including Ag ,
2+
2+
3+
2+
+
+
2+
3+
2+
2+
2+
3+
2+
2+
2+
Ca , Cr , K , Na , Zn , Mg , Ba , Mn , Fe , Cd , Pb , Hg ,
2+
Ni , Co , Cu and Al were detected by fluorescence emission re-
sponses. As investigated and shown in Fig. 1a, free L showed strong
fluorescence emission centered at 480 nm. Nevertheless, after the
2
.2. Synthesis
addition of Cu2 , the fluorescence emission quenched significantly
+
2
+
Probe L was synthesized by following the procedure in Scheme 1
which was attributed to the paramagnetic quenching effect of Cu
.
according to a modified version of the literature procedure [37,38]. 6-
Contrarily, no obvious fluorescence emission changes were observed in
the presence of other metal ions. To further investigate the practicability
of this fluorescent probe L, competition experiments were carried out. In
Fig. 1b, in the presence of these interfering metal ions, the highly
ethoxychromone-3-carbaldehyde (compound 1 M.P.: 132–133 ℃) and
7
1
-diethylaminocoumarin-3-formylhydrazide (compound
2
M.P.:
67–168 ℃) were synthesized on a basis of the reported literatures and
Scheme 1. Synthetic route of L.
2

C. Liu et al.
Inorganica Chimica Acta 519 (2021) 120280
a
700
0.6
a
+
3+
Ag
Al
2
+
2+
6
5
4
3
2
1
00
00
00
00
00
00
0
Ba
Ca
Cd
Co
Hg
0
0
0
.5
.4
.3
2
+
+
1
.0 equiv.
K
2+
2+
2+
Mg
Mn
2
+
0
equiv.
3
+
+
Cr
Fe
Fe
Zn
Na
2
3
+
+
2+
Ni
2+
Pb
0.2
.1
0.0
2+
2+
Cu
0
3
50
400
450
500
550
4
00
450
500
550
600
Wavelength (nm)
Wavelength (nm)
b
800
b
450
other ions
2
+
400
350
300
250
200
150
100
50
0
7
6
5
4
3
2
1
00
00
00
00
00
00
00
0
other ions + Cu
0 equiv.
1
.0 equiv.
2
+
+
2+
2+
2+
2+
3+
3+
3+
2+
+
2+
2+
+
2+
2+
2+
400
450
500
550
600
u
g
a
a
d
o
r
l
g
n
g
M
a
i
n
L
K
N
C
A
B
C
C
C
C
A
Fe
H
N
Pb
Z
M
Wavelength (nm)
Fig. 1. (a) Fluorescence emission spectra of L (5.0
μ
M) upon the addition of
+
2+
3+
+
+
2+
2+
2+
various metal ions (1.0 equiv.) of Ag , Ca , Cr , K , Na , Zn , Mg , Ba
,
Fig. 2. (a) UV–vis absorption titration spectra of L (5.0
solution (4/1, 10.0 M HEPES, pH 7.2) in the presence of different concen-
μ
M) in EtOH/HEPES
2
+
3+
2+
2+
2+
2+
2+
2+
3+
Mn , Fe , Cd , Pb , Hg , Ni , Co , Cu and Al in EtOH/HEPES
solution (4/1, 10.0
M HEPES, pH 7.2) (λex = 377 nm, Slit widths: 3 nm/5 nm);
b) Fluorescence emission intensity at 480 nm of L and its complex with Cu in
μ
2
+
μ
trations of Cu (0–1.0 equiv.); (b) Fluorescence emission spectra of L (5.0
upon addition of Cu (0–1.0 equiv.) in EtOH/HEPES solution (4/1, 10.0
μ
M)
2
+
2+
(
μ
M
+
2+
2+
2+
3+
+
+
2+
2+
the presence of other various metal ions (Ag , Ca , Cr , K , Na , Zn , Mg ,
HEPES, pH 7.2).
2
+
2+
3+
2+
2+
2+
2+
3+
Ba , Mn , Fe , Cd , Pb , Hg , Ni , Co , Cu and Al ) in EtOH/
HEPES solution (4/1, 10.0 M HEPES, pH 7.2). Black bar: L (5.0 M) in the
presence of 1.0 equiv. of various metal ions. Red bar: L (5.0 M) with 1.0 equiv.
μ
μ
worth mentioning that the fluorescence emission intensity at 480 nm
μ
2+
showed a good linear relationship with the concentration of Cu
2
+
of Cu in the presence of 1.0 equiv. of other various metal ions.
2+
(
Fig. S4). Then the detection limit of probe L for sensing Cu was
ꢀ
7
calculated to be 1.39 × 10 M based on this linear relationship. Ac-
selective reaction of copper ions could be demonstrated and remarkable
quenching effects were still exhibited, which corroborated the excellent
cording to Benesi–Hildebrand equation, the association constant was
5
ꢀ 1
2
.1 × 10 M (Fig. S5) [40]. The results suggested that L could respond
selectivity of this probe L towards Cu2
+
.
2+
quantitatively to Cu with high sensitivity.
In view of the optical response of this fluorescent probe L to copper
ion, UV–vis absorption and fluorescence emission titration experiments
2ꢀ
3
.2. Spectral studies of L-Cu with S
were carried out successively in EtOH/HEPES solution (4/1, 10.0 M
μ
HEPES, pH 7.2). As depicted in Fig. 2a, before the addition of copper ion,
L exhibited an absorption peak centered at 441 nm, which was the
typical absorption band of coumarin moiety. However, the typical ab-
sorption band of coumarin moiety was red-shifted gradually with
increasing Cu2 concentrations, accompanied by an appearance of two
new absorption peaks centered at 463 and 486 nm, which indicated that
a stable coordination complex was formed between L and Cu2 . At the
same time, the color of the solution changed from yellow to orange,
which could be explained by the shift of UV–vis absorption spectrum. As
illustrated in Fig. 2b, when this probe L was interacted with Cu2 from
2+
As described above, we concluded that L selectively binds to Cu to
form a L-Cu complex, and the spectral properties of L-Cu have changed
ꢀ 36
significantly. Since CuS (Ksp = 1.27 × 10 ) is very stable, many copper
2ꢀ
complexes have been applied to detect sulfide ion (S ) using the sub-
+
2+
stitution strategy of Cu chelating ligand. Hence, this L-Cu complex
was further identified as a potential fluorescent probe for sensing sulfide
+
2ꢀ
anion (S ). For the sake of confirming this image, the ligand metal
complex L-Cu was prepared in ethanol by mixing L (5.0
μ
M) with 1.0
2+
equiv. of Cu . Based on this strategy, the UV–vis absorption and fluo-
rescence emission response of complex L-Cu to S were investigated.
+
2ꢀ
0
to 1.0 equiv., the fluorescence emission intensity at 480 nm was
The UV–vis titration experiments of this complex L-Cu were carried out
with incremental addition of sulfur ion (S ). As depicted in Fig. 3a, this
2
+
extinguished little by little owing to the paramagnetic property of Cu
2ꢀ
[
39]. In addition, after the addition of Cu2 , the fluorescence disap-
+
complex L-Cu exhibited a maximum absorption peak centered at 441 nm
pearance of the solution was observed under a 365 nm UV lamp. It was
2ꢀ
2ꢀ
in the absence of S . Nevertheless, with the gradual addition of S , the
3

C. Liu et al.
Inorganica Chimica Acta 519 (2021) 120280
a
a
2
+
-
L
L
-Cu + Br
5
4
3
2
1
00
00
00
00
00
0
0
0
0
0
0
.4
.3
.2
.1
.0
1.0 equiv.
0 equiv.
2
+
2-
L
-Cu + S
2+
L
-Cu and other ions
3
00
400
500
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
b
b
other ions
other ions + S
5
00
00
00
00
00
0
4
00
2-
1
.0 equiv.
4
3
2
1
0
equiv.
300
2
1
00
00
0
-
1
2
+
2
2-
3
-
4
-
5
-
6
-
7
2-
8
3
-
9
10
2-
11
-
12
-
13 14 15 16 17
-
2-
-
2-
4
00
450
500
550
600
3
u
O
4
7
r
4
l
O
4
I
2
3
4
N
4
L
C
S
B
lO
C
O
F
C
O
O
O
-
B
C
C
H
PO
N
N
S
2
SC
SO
L
H
Wavelength (nm)
Fig. 4. (a) Fluorescence emission spectra of L-Cu (5.0 M) upon the addition of
μ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
–
ꢀ
2ꢀ
2ꢀ
Fig. 3. (a) UV–vis absorption titration spectra of L-Cu (5.0
solution (4/1, 10.0
μ
M) in EtOH/HEPES
various anions (1.0 equiv.) of F , Cl , Br , I , AcO , NO , ClO , HPO , SO ,
3
4
4
4
2
–
2ꢀ ꢀ ꢀ 2ꢀ
–
2–
μ
M HEPES, pH 7.2) in the presence of different concen-
CO3 , S O , NO , SCN , B O , CO and S in EtOH/HEPES solution (4/1,
2 4 2 4 7 3
2
ꢀ
trations of S (0–1.0 equiv.); (b) Fluorescence emission spectra of L-Cu (5.0
M) upon addition of S (0–1.0 equiv.) in EtOH/HEPES solution (4/1, 10.0
10.0
ex
μM HEPES, pH 7.2) (λ = 377 nm, Slit widths: 3 nm/5 nm);(b) Fluores-
2
ꢀ
μ
μ
M
cence emission intensity at 480 nm of L-Cu (5 M) in the presence of other
μ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
–
ꢀ
2ꢀ
2ꢀ
2–
2ꢀ
HEPES, pH 7.2).
various anions (F , Cl , Br , I , AcO , NO , ClO , HPO , SO , CO , S O ,
3
4
4
4
3
2 4
–
ꢀ
ꢀ
2–
2ꢀ
NO
2
, SCN , B
4
O
7
, CO
3
andS ) in EtOH/HEPES solution (4/1, 10.0
μ
M
HEPES, pH 7.2). Black bar: L (5.0
μ
M) in the presence of 1.0 equiv. of various
absorbance at 486 nm decreased distinctly, accompanied by a gradual
increase of two absorption bands centered at 322 and 441 nm. More-
over, the complex solution showed a clear and visible color change from
2
ꢀ
anions. Red bar: L (5.0 M) with 1.0 equiv. of S in the presence of 1.0 equiv. of
μ
other various anions.
2
ꢀ
orange to yellow with addition of S , suggesting that this complex L-Cu
However, the addition of S2 resulted in the generation of CuS and then
L could be released to regain the fluorescence emission while the in-
fluences of other anions on the fluorescence emission of L-Cu were
neglected. Furthermore, by recording the fluorescence emission in-
ꢀ
was suitable for detecting S2 with the naked eye. In the fluorescence
ꢀ
2
ꢀ
emission titration experiments, after adding S gradually, the fluores-
cence emission intensity at 480 nm was recovered (Fig. 3b). Addition-
ally, the fluorescence emission intensity at 480 nm also showed a good
tensity at 480 nm of L-Cu-S2 in the presence of background competing
ꢀ
linear relationship with S2 concentration. The detection limit of this
ꢀ
complex L-Cu for sensing S2 was calculated to be 6.437 × 10 M,
ꢀ
ꢀ 9
anions in the solution, the competitive studies of L-Cu were carried out
(Fig. 4b). In the absence of S2 , there were few fluorescence emission in
ꢀ
2
ꢀ
exhibiting high sensitivity of L-Cu towards S (Fig. S6).
the background solutions, while all the fluorescence emission intensity
To evaluate the selectivity of this complex L-Cu towards various
2
ꢀ
.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
–
ꢀ 2ꢀ 2ꢀ 2–
at 480 nm of the solutions were recovered after the addition of S
anions such as F , Cl , Br , I , AcO , NO
3
, ClO
4 4 4 3
, HPO , SO , CO ,
2
ꢀ – ꢀ ꢀ 2–
2ꢀ
These results indicated that L-Cu exhibited high selectivity for the
S O , NO , SCN , B O , CO
2 4 2 4 7 3
and S , UV–vis absorption and fluo-
detection of S2 in the presence of background competitive anions.
ꢀ
rescence emission spectroscopy were studied in parallel under the
identical conditions. The absorption spectra of L-Cu in the absence and
presence of various anions were illustrated in Fig. S7. The absorption
The effect of response time on the fluorescence emission intensity at
4
80 nm of L-Cu-S2 was studied by adding S
ꢀ
2ꢀ
(1.0 equiv.) to the
2
ꢀ
peak centered at 486 nm disappeared only when S2ꢀ was added to L-Cu
complex L-Cu solution at room temperature. In Fig. 5, when S was
added, the fluorescence emission intensity at 480 nm increased
dramatically in 0.5 min. It reached the maximum in 1 min and became
saturated for more time. Thus, the results showed that this complex L-Cu
solution. However, when other anions were separately introduced, no
significant changes in absorption peak centered at 486 nm were
observed, thus suggesting the high selectivity of L-Cu towards S2
ꢀ
.
could be applied for real-time detection of S2
ꢀ
.
Additionally, as seen from Fig. 4a, in the absence of any anion, L-Cu
displayed no fluorescence emission in EtOH/HEPES solution (4/1, 10.0
2
+
μ
M HEPES, pH 7.2) due to the paramagnetic quenching effect of Cu
.
4

C. Liu et al.
Inorganica Chimica Acta 519 (2021) 120280
5
00
00
00
00
00
0
4
3
2
1
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Time (min)
Fig. 5. Time-course fluorescence emission intensity at 480 nm of L-Cu (5.0 M)
μ
2
ꢀ
in the presence of S (1.0 equiv.) in EtOH/HEPES solution (4/1, 10.0
μ
M
HEPES, pH 7.2).
b
500
2
+
+
Cu
.3. Reversibility of L for Cu2+ and S2ꢀ
2-
3
4
3
2
1
00 ON
+ S
In order to test the practical applicable performance of this probe L,
we further studied the recyclability and reversibility of L for sensing
00
00
00
0
2
+
2ꢀ
Cu and S . As shown in Fig. 6a (1), the color of the solution imme-
2
+
diately changed from light yellow to yellow after adding Cu and then
went back with successive addition of S2 , which could be repeated for
many times. Moreover, under the irradiation of 365 nm ultraviolet lamp,
the blue fluorescence of L could be observed (Fig. 6a (2)), but the
ꢀ
2
+
fluorescence was quenched upon the addition of Cu , followed by
fluorescent recovery with the addition of sulfide ion (S2ꢀ ), which could
OFF
be cycled severally. Fig. 6b illuminated that the alternate addition of a
0
1
2
3
4
certain concentration of Cu2 and S into L solution resulted in a
convertible change of fluorescence emission intensity at 480 nm, and the
reversible cycles could be repeated for at least four times by the alternate
+
2ꢀ
Cycles
addition of Cu2 and S , which expressed that L had a prospect for
+
2ꢀ
Fig. 6. (a) Color changes (1) and fluorescence changes (2) of L (5.0
μ
M) upon
2
+
the alternative addition of Cu (1.0 equiv.) and Na
HEPES solution (4/1, 10.0 M HEPES, pH 7.2); (b) Fluorescence emission in-
tensity at 480 nm of compound L (5.0 M) upon the alternate addition of
2
S (1.0 equiv.) in EtOH/
acting as a reversible fluorescence ON-OFF-ON probe and an ideal tool
to detect Cu2 and S simultaneously.
+
2ꢀ
μ
μ
2
+
Cu and Na
2
S in several cycles in EtOH/HEPES solution (4/1, 10.0
μ
M HEPES,
pH 7.2) (λex = 377 nm).
+
2ꢀ
3
.4. Reaction mechanism of the probe L for sensing Cu2 and S
So as to further reasonably verify the mechanism of probe L for
4
3
2
00
00
00
sensing Cu2 and S , UV–vis absorption titration experiments (Fig. 2a),
ESI-MS spectrometry analysis (Fig. S3), Job’s plot method (Fig. 7), IR
spectra (Fig. S8) and reversibility experiment (Fig. 6) were conducted.
As shown in the Job’s plot (Fig. 7), it could be seen that when the value
+
2ꢀ
2
+
of [L]/([L] + [Cu ]) was 0.5, the fluorescence emission intensity at
+
4
80 nm decreased the most, which confirmed that this probe L and Cu2
formed a 1:1 stoichiometry complex. Subsequently, the ESI-MS spec-
trometry of the forming complex L-Cu2 was determined (Fig. S3), in
+
2
+
which the peak at m/z = 610.173 was assigned to [L + Cu + DMF –
+
+
2+
H ] and the peak at m/z = 656.218 was attributed to [L + Cu + DMF
100
0
+
+
2+
+
C
2
H
5
OH – H ] , proving a 1:1 coordination ratio of L with Cu
–
–
again. In IR spectra, the C
O of hydrazine moiety frequency decreases
ꢀ 1
ꢀ 1
–
–
from 1641 cm to 1620 cm , the C O of chromone moiety frequency
decreases from 1618 cm to 1592 cm and the imine C
ꢀ
ꢀ
1
ꢀ 1
ꢀ 1
–
–
N frequency
0
.0
0.2
0.4
0.6
0.8
+
1.0
1
2+
decrease from 1581 cm to 1568 cm due to the addition of Cu
2
2
+
Molar fraction of Cu
(
Fig. S8). By examining these results, we inferred that L and Cu formed
a new stable complex L-Cu, resulting in color and spectral changes.
2
+
2
ꢀ
Fig. 7. Job’s plot for determining the stoichiometry between probe L and Cu
However, when sulfide ion (S ) was introduced into L-Cu solution,
2
+
2+
in EtOH/HEPES solution (4/1, 10.0
μ
M HEPES, pH 7.2). XCu = [Cu ]/([Cu
]
2
+
2ꢀ
Cu ion was caught by sulfide ion (S ), causing the fluorescence
emission recovery and the color regeneration. Based on the above
findings, the possible displacement mechanism and combination mode
2+
+
[L]), the total concentration of L and Cu was 5
μ
M.
2
ꢀ
2+
of probe L for sensing S and Cu were proposed and depicted in
5

C. Liu et al.
Inorganica Chimica Acta 519 (2021) 120280
Scheme 2. Proposed displacement mechanism for fluorescence “ON-OFF-ON” sensing of Cu + and S
2
2ꢀ
.
scheme 2.
other interfering anions. These results indicated that this probe L also
showed perceptible ON-OFF-ON fluorescence emission switch perfor-
2
+
3
.5. Applications
Inspired by the properties of this probe L towards Cu2 and S , the
mance in solid state, and could be applied for the detection of Cu and
2
ꢀ
in real samples.
S
+
2ꢀ
practical application of using L as a solid-state probe was further stud-
2+ 2ꢀ
.6. Comparisons with some of the previously reported Cu /S
3
ied. For this purpose, several pieces of the filter papers were immersed in
fluorescent probes
EtOH/HEPES solution (4/1, 10.0 M HEPES, pH 7.2) of L (5 mM) for one
μ
night and then dried naturally. After drying, the fluorescence of these
filter papers were observed under the ultraviolet lamp (365 nm). Sub-
sequently, we dropped different metal ions solutions (5 mM) onto the
air-dried filter papers, and allowed to dry them in the air. As shown in
We compared the designed fluorescent probes with previous fluo-
2+
2ꢀ
rescent probes detecting Cu and S . As shown in Table 1, the solvent
in this paper is environmental friendly and L has a low detection limit in
Fig. 8a, only the filter paper dropped with Cu2 solution exhibited sig-
nificant fluorescence quenching, while other metal ions caused negli-
gible fluorescence changes in these filter papers. Therefore, the
+
Table 1
2
+
2ꢀ
Comparisons with some of the previously reported Cu /S fluorescent probes.
detection of Cu2+ by L could be realized by means of filter papers easily
Solvent
Detected ions and LOD (M)
Reference
2
+
2ꢀ
without the aid of other analytical instruments. Additionally, the above
mentioned filter papers adsorbed L and dropped with Cu2 were also
prepared, and then the solutions of multifarious anions (5 mM) were
dropped onto these test strips, followed by the illumination under a 365
nm UV lamp which were shown in Fig. 8b. It could be perceived that the
fluorescence of the coated test paper returned to light blue upon the
HEPES
Cu :/, S :/
[41]
+
Cu : 1.6 × 10 , S2ꢀ : 1.7 × 10ꢀ 7
2+
ꢀ 9
ꢀ 7
ꢀ 7
ꢀ 8 2ꢀ
ꢀ 7 2ꢀ
ꢀ 7 2ꢀ
HEPES
[42]
ꢀ
HEPES/DMSO
PBS/DMF (4:1)
HS : 1.67 × 10
[43]
2
ꢀ
2+
S
: 1.1 × 10
[44]
ꢀ 8
ꢀ 6
ꢀ 9
3
CH OH-PBS (3:7)
Cu : 1.52 × 10 , S : 1.79 × 10
[45]
2
+
+
HEPES
Cu : 3.5 × 10 , S : 1.35 × 10
[21]
2
EtOH/HEPES (4:1)
Cu : 1.39 × 10 , S : 6.437 × 10
This study
addition of S2 , which could be clearly distinguished from the cases of
ꢀ
Fig. 8. (a) The photographs of coated test papers of L after addition of different metal ions under illumination using a 365 nm UV lamp; (b) The photographs of
coated test papers of L-Cu after addition of different anions under illumination using a 365 nm UV lamp.
6

C. Liu et al.
Inorganica Chimica Acta 519 (2021) 120280
2
+
2ꢀ
the detection of Cu and S . In addition, L accomplished the detection
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7