A novel fluorescent probe based on 7,8-benzochromone-3-carbaldehyde-(rhodamine B carbonyl) hydrazone for detection of trivalent cations and Zn2+ in different systems

Article abstract of DOI:10.1016/j.jphotochem.2021.113207

In this article, a new fluorescence probe 7,8-benzochromone-3-carbaldehyde- (rhodamine B carbonyl) hydrazone (L) was synthesized, which had been explored to detect Zn²⁺ and trivalent metal ions M³⁺ (Al³⁺, Cr³⁺ and Fe³⁺). When the excitation wavelength was 490 nm, L showed high selectivity and high sensitivity to M³⁺ which was superior to monovalent and divalent metal cations in methanol solution. In EtOH/H₂O (15/1, V/V), L showed selective fluorescence response to Zn²⁺, when the excitation wavelength was set to 425 nm. Furthermore, the detection limits of L for Fe³⁺, Al³⁺ and Cr³⁺ were 2.46 × 10^?9 M, 2.54 × 10^-9 M and 4.23 × 10^-8 M, respectively. And the detection limit of L for Zn²⁺ was calculated to be 1.45 × 10^-7 M. The coordination ratio between L and M³⁺ was determined from the Job's plot (fluorescent spectrum) to be 2:1 and the coordination ratio of L and Zn²⁺ was 1:1. Additionally, L might be used as a colorimetric probe of Cu²⁺ in EtOH/H₂O (15/1, V/V) solution., which could be observed with the naked eye from colorless to red in the presence of Cu²⁺.

Full text of DOI:10.1016/j.jphotochem.2021.113207

Journal of Photochemistry & Photobiology, A: Chemistry 411 (2021) 113207
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology, A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
A novel fluorescent probe based on 7,8-benzochromone-3-carbaldehy-
de-(rhodamine B carbonyl) hydrazone for detection of trivalent cations and
2
+
Zn in different systems
Jie Sun, Tian-rong 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
Keywords:
In this article, a new fluorescence probe 7,8-benzochromone-3-carbaldehyde- (rhodamine B carbonyl) hydrazone
Benzochromone
Rhodamine B
2+
3+
3+
3+
3+
(
L) was synthesized, which had been explored to detect Zn and trivalent metal ions M (Al , Cr and Fe ).
3+
When the excitation wavelength was 490 nm, L showed high selectivity and high sensitivity to M which was
Fluorescence probe
Trivalent cation
2
superior to monovalent and divalent metal cations in methanol solution. In EtOH/H O (15/1, V/V), L showed
2
+
2
+
selective fluorescence response to Zn , when the excitation wavelength was set to 425 nm. Furthermore, the
Zn
detection limits of L for Fe3 , Al and Cr were 2.46 × 10 M, 2.54 × 10 M and 4.23 × 10 M, respectively.
+
3+
3+
ꢀ 9
-9
-8
2
+
-7
And the detection limit of L for Zn was calculated to be 1.45 × 10 M. The coordination ratio between L and
3
+
+
M
was determined from the Job’s plot (fluorescent spectrum) to be 2:1 and the coordination ratio of L and
2
2+
Zn was 1:1. Additionally, L might be used as a colorimetric probe of Cu in EtOH/H
2
O (15/1, V/V) solution.,
which could be observed with the naked eye from colorless to red in the presence of Cu2
+
.
1
. Introduction
the chemical industry, medical equipment, aircraft and ship
manufacturing industries. However, the existence of a large amount of
aluminum in nature can also cause environmental pollution and water
The synthesis of new fluorescent probes for detecting trivalent ions
M3 =Fe , Al , Cr ) is a topic worthy of research. Fluorescent probes
+
3+
3+
3+
resources damage. At the same time, the excessive content of Al in the
3+
(
have been widely used in the detection of metal cations, anions, and
biomolecules due to their simple design, short response time, high
selectivity, sensitivity and low detection limit [1–6]. The detection of
trivalent metal ions is of great value due to their own environmental
importance and biological significance. For example, Chromium is
widely used in metal processing, electroplating, tanning and other in-
dustries. Waste water and waste gas containing a large amount of
chromium produced in the industrial production process will be dis-
charged into the environment [7,8]. When there is too much chromium
in the soil, it will inhibit the nitrification of organic matter and cause
chromium to accumulate in plants. Chromium in water bodies is lethal
organism will cause the decline of liver and kidney function and inter-
fere with the metabolism of Ca2 [16–20]. In recent years, with the
widespread application of iron in industrial production, a large amount
of iron-containing waste water and waste gas are discharged into the
environment, and they will gradually accumulate and enrich in the
environment, causing serious pollution to the atmosphere, soil and
water bodies [21]. Iron is an essential trace element for the human body.
It has the function of participating in the transportation and storage of
oxygen and enhancing resistance to diseases [22–25]. However, exces-
sive intake of iron preparations may also lead to iron poisoning, causing
liver cirrhosis and diabetes [26–29]. In addition, if the iron content in
the water exceeds the standard, it will affect the sensory properties such
as water color, smell and taste. Therefore, it is necessary to monitor the
content of trivalent ions in the environment and some probes for
detecting trivalent metal ions have been reported in the previous liter-
ature [10,30,31].
+
3
+
to aquatic organisms [9]. Simultaneously, Cr is an essential element
for human nutrition. Cr3 can affect the body’s lipid metabolism and
reduce blood cholesterol and triglycerides [10,11]. However, the high
+
content of Cr3 in the organism not only negatively affects the cell
+
structure, but even increases the risk of cardiovascular and cerebro-
3
+
3+
vascular diseases and diabetes [11–15]. As we all know, Al , as the
third most abundant metal element in the earth’s crust, is widely used in
The probe synthesized in this article can not only detect M in
methanol but also detect Zn2 in EtOH/H
+
O (15/1, V/V). Zinc is one of
2
*
Corresponding authors.
E-mail addresses: litr@lzu.edu.cn (T.-r. Li), yangzy@lzu.edu.cn (Z.-y. Yang).
https://doi.org/10.1016/j.jphotochem.2021.113207
Received 29 November 2020; Received in revised form 8 February 2021; Accepted 9 February 2021
Available online 15 February 2021
1
010-6030/© 2021 Elsevier B.V. All rights reserved.

J. Sun et al.
Journal of Photochemistry & Photobiology, A: Chemistry 411 (2021) 113207
Scheme 1. Synthetic route of L.
3
+
2+
Fig. 1. The UV–vis absorption titration spectrum of L (25
μ
M) with Al
Fig. 2. The UV–vis absorption titration spectrum of L (25
(0.0–1.0 equiv) in EtOH/H O (15/1, V/V).
μM) with Zn
(
0.0–1.5 equiv) in methanol.
2
the 25 essential elements in the human body. The imbalance of zinc
content in the human body can lead to diseases such as digestive dis-
orders and low immunity [32–34]. At the same time, with the wide-
spread application of zinc in the battery, construction, and shipbuilding
industries, there is a large amount of zinc in the environment, causing
serious pollution and affecting plant growth [35,36]. Therefore, highly
TMS as an internal standard. IR spectra were obtained in KBr discs on a
Therrno Mattson FT-IR spectrometer. High Resolution Mass Spectrom-
etry (HRMS) was measured on Bruker esquire 6000 spectrometer. ESI-
MS spectrum was recorded by Bruker esquire 6000 spectrometer.
Fluorescence spectra and UV–vis absorption spectra were obtained using
Hitachi RF-4500 spectrophotometer and Shimadzu UV-240
selective detection of zinc ions is worth achieving. In EtOH/H
V/V) solution, L showed superior fluorescence selectivity to Zn2 than
other metal ions. Interestingly, in the EtOH/H O (15/1, V/V) solution,
the addition of Cu2 to L showed a red color different from that of L
2
O (15/1,
+
2
+
added with other metal ions, indicating that the colorimetry of Cu2 can
+
3
+
be detected in this solution. Herein, the optical properties of L to M
and Zn2 were studied through fluorescence spectra, IR spectra and
+
2
+
UV–vis spectra, the colorimetric properties of L to Cu through UV–vis
spectra.
2
. Experimental
.1. Materials and instruments
-Naphthol was provided by Shanghai Macklin Biochemical Co., Ltd.
2
1
Rhodamine B was obtained from Tianjin Guangfu Fine Chemical
Research Institute. All solvents were purchased from Li’an Long Bohua
Tianjin Pharmaceutical Chemical Co., Ltd. All the above solvents and
1
13
chemicals could be used without further purification. H NMR and
C
3
+
NMR spectra were obtained using a Bruker 400 MHz instruments with
Fig. 3. Solvent effect on fluorescence intensity of the probe L with Al in
different solvent.
2

J. Sun et al.
Journal of Photochemistry & Photobiology, A: Chemistry 411 (2021) 113207
Fig. 5. Variation of fluorescence intensity of L (50 mM) in EtOH/H
2
O (15/1, V/
2
+
V) solution with and without Zn (1.0 equiv.) as a function of pH.
Fig. 6. Fluorescence spectra of L (25 M) after addition of 1 equiv of various
μ
+
+
2+
2+
2+
2+
2+
2+
3+
2+
+
2+
2+
2+
metal ions of Li , Ag , Ca , Co , Cu , Fe , Mg , Na , Pb , Ba , Cd ,
3
+
2+
+
3+
2+
2
Cr , Fe , K , Mn , Ni , Hg , Al and Zn in EtOH/H O (15/1, V/V).
2
+
Insert: fluorescence response of L in the presence of Zn and L under 365 nm
UV lamp.
spectrophotometer, respectively. And the melting point was measured
by Beijing XT4ꢀ 100x microscopic melting point apparatus.
2
.2. Synthesis of L
-oxo-4H-benzo[h]methylene-3-carbaldehyde [37] and rhodamine B
4
hydrazide [38] were synthesized according to previous reports. To a
solution of 4-oxo-4H-benzo[h]methylene-3-carbaldehyde (0.13 g,
0
0
.59 mmol) in ethanol was added rhodamine B hydrazide (0.27 g,
.59 mmol). The solution was refluxed and stirred at 80℃ for 18 h with
appearance of white precipitation. The mixture was cooled at room
temperature filtered under reduced pressure to obtain crude product.
Then the crude product was recrystallized in ethanol and dried in vac-
uum. After the above steps, 7,8-benzochromone-3-carbaldehyde-
(
rhodamine B carbonyl) hydrazone (L) was synthesized (Scheme 1).
1
2
+
2+
Yield :51.4 % (0.20 g), mp: 246ꢀ 247℃. H NMR (400 MHz, DMSO‑d6
,
Fig. 4. Solvent effect on fluorescence intensity of the probe L with Co , Mg
2
+
TMS) (Fig. S1): δ(ppm): 8.85 (s, 1 H), 8.38 (s, 1 H), 8.32 (d, 1H, J =8 Hz),
and Zn in solvent of different ratios of EtOH/H
2
O: a) ethanol; b) EtOH/H
2
O
7
7
6
1
.99 (d, 1H, J =8 Hz), 7.89 (d, 1H, J =8 Hz), 7.84 (m, 2 H) 7.70 (m, 2 H)
.56 (m, 2 H), 7.05 (d, 1H, J =8 Hz), 6.43 (d, 2H, J =4 Hz), 6.42 (s, 1 H),
.39 (s, 1 H), 6.31 (d, 1H, J =4 Hz), 6.29 (d, 1H, J =4 Hz), 3.25 (m, 8 H),
(
20/1, V/V); c) EtOH/H O (15/1, V/V); d) EtOH/H
2
2
O (10/1, V/V).
1
3
.03 (t, 12H, J =8 Hz). C NMR (400 MHz, DMSO‑d6, TMS) (Fig. S14):
3

J. Sun et al.
Journal of Photochemistry & Photobiology, A: Chemistry 411 (2021) 113207
Fig. 7. Fluorescence spectra of L (25 M) after addition of 1 equiv of various
μ
+
+
2+
2+
2+
2+
2+
2+
3+
2+
+
2+
2+
2+
metal ions of Li , Ag , Ca , Co , Cu , Fe , Mg , Na , Pb , Ba , Cd ,
3
+
2+
+
2+
3+
Cr , Fe , K , Mn , Ni , Hg , Zn and Al in methanol. Insert: fluores-
3
+
cence response of L in the presence of M and L under 365 nm UV lamp.
δ(ppm): 174.77, 164.48, 153.33, 153.12, 152.00, 148.99, 140.15,
1
1
1
35.74, 134.54, 130.26, 129.27, 128.72, 128.63, 128.23, 128.02,
26.20, 124.29, 123.60, 122.30, 120.41, 120.37, 120.05, 108.52,
05.60, 97.94, 65.84, 56.50, 44.11, 19.04, 12.89. MS(ESI) (Fig. S3):
+
m/z = 663.2163 [M+H] . HRMS (Fig. S13): m/z = 685.2781
+
+
[
M + Na] , m/z = 663.2965 [M+H] .
2
.3. Measurement procedures
The 5 mM stock solution of various metal ions (Na , Mg , Ag , K ,
+
2+
+
+
3+
+
2+
2+
2+
2+
2+
2+
2+
2+
2+
3+
Li , Hg , Cu , Zn , Ca , Co , Cd , Mn , Ba , Pb , Al , Cr ,
3
+
Fe ) were prepared in EtOH. All the metal ion solutions mentioned
above were prepared from nitrate and chloride salts of metal ions. The
5
mM stock solution of L was obtained in DMF. Adding 2 mL of methanol
solution or EtOH/H O (15/1, V/V) into a cuvette, and the stock solution
of L (10 L) and the stock solutions of various metal ion (1 equiv) were
2
μ
added into it to obtain the tested solution. In methanol solution, the
excitation wavelength was set to 490 nm. The excitation and the emis-
2
sion slit were set to 3 nm and 3 nm, respectively. In EtOH/H O (15/1, V/
V), the excitation wavelength was set to 425 nm. Both the excitation and
the emission slit were set to 3 nm. All measurements were performed at
atmospheric pressure and room temperature.
Fig. 9. a. Fluorescence emission titration spectra of L (25
μ
M) upon addition of
3
+
Al (0.0–1.0 equiv) in methanol. b. Fluorescence emission titration spectra of L
3
+
(
25
μM) upon addition of Cr (0.0–1.0 equiv) in methanol. c. Fluorescence
3
+
emission titration spectra of L (25
μM) upon addition of Fe (0.0–1.0 equiv)
in methanol.
3
. Results and discuss
3
+
2+
3
.1. UV–vis studies of L towards M and L towards Zn
Fig. 8. Fluorescence emission titration spectra of L (25 M) upon addition of
μ
3
+
2
+
The UV–vis studies of L to M were conducted in methanol and the
2
Zn (0.0–1.0 equiv) in EtOH/H O (15/1, V/V).
4

J. Sun et al.
Journal of Photochemistry & Photobiology, A: Chemistry 411 (2021) 113207
Table 1
2
+
3+
Comparison of detection limit of Zn and M with other reported probes.
LOD/M
Solvent (V/V)
Reference
3
+
3+
3+
2+
Zn
Al
Cr
Fe
ꢀ
ꢀ
ꢀ
9
5
7
ꢀ 6
ꢀ 9
ꢀ 5
ꢀ 6
ꢀ 5
ꢀ 5
CH
CH
3
OH/H
2
O (6/4)
1.74 × 10
2.36 × 10
2.90 × 10
/
/
/
[45]
3
CN/HEPES buffer solution (40/60)
2.30 × 10
2.50 × 10
2.00 × 10
[46]
methanol
O/DMSO (9/1)
DMF/H O (1/1)
6.90 × 10
6.80 × 10
6.50 × 10
[47]
ꢀ
ꢀ
ꢀ
7
7
7
H
2
/
/
/
1.30 × 10
1.27 × 10
5.03 × 10
/
[48]
2
/
/
/
[49]
EtOH/HEPES buffer (95/5)
methanol
/
/
/
[50]
ꢀ
9
ꢀ 8
ꢀ 9
2.54 × 10
4.23 × 10
2.46 × 10
This work
ꢀ
7
EtOH/H
2
O (15/1)
/
/
/
1.45 × 10
metal ions by adjusting the polarity of the solvent [39]. We first adjusted
the ratio of ethanol to water as 20:1, as shown in Fig. 4b, although the
2
+
2+
fluorescence intensity of L after adding Mg or Cd was reduced, it
still interfered with the detection of Zn2 . We continued to increase the
+
proportion of water and set the ratio of ethanol to water as 10:1 (Fig. 4c).
At this time, not only the fluorescence of L-Mg2 and L-Cd were
+
2+
2
+
significantly quenched, but the fluorescence intensity of L-Zn was also
reduced obviously. Therefore, we adjusted the EtOH/H O as 15:1. In
this solution, Mg and Cd have almost no interference in detecting
2
2
+
2+
2
+
2+
Zn . This indicated that L had good selectivity for the detection of Zn
in EtOH/H O (15/1, V/V).
2
3
.3. Effect of pH
In order to detect metal ions in various environments, it is important
to determine the pH range where the probe emits the best fluorescence.
Since the pH could not be adjusted in pure methanol solvent, we studied
2
+
2+
Fig. 10. Fluorescence response of L and L+Zn upon the addition of various
whether the pH value affected the fluorescence emission of L and L-Zn
metal.
complexes in HEPES buffer. The ideal pH condition for the best fluo-
2
+
2+
3+
+
2+
3+
+
2+
2+
2+
2+
+
ions (1=Zn , 2=Fe , 3=Fe , 4=Ag , 5=Al , 6=Ba , 7=Co , 8=Ca
,
rescence emission was determined when the probe L was combined with
2
+
3+
2+
+
9
1
=Cd , 10=Cr , 11=Cu , 12=Hg , 13=K ,14=Li , 15=Mn , 16=Na ,
2+
2+
Zn . The effect of pH on the emission intensity of L and L-Zn com-
plexes were monitored in the pH range of 1–14 (Fig. 5). The fluorescence
emission intensity of probe L was basically not affected by the pH value,
+
2+
2+
7=Ni , 18= Pb , 19=Mg ) in EtOH/H
2
O (15/1, V/V).
studies of L to Zn2 were conducted in EtOH/H
+
O (15/1, V/V). The
2
+
2
and the fluorescence intensity of L-Zn was the strongest only when the
pH was 7. The fluorescence intensity of L-Zn2 at other pH values was
basically the same as that of L. From this, we could infer that L did not
coordinate with Zn2 in other pH environments. The results of the pH
range demonstrated that the probe L was suitable for the detection of
3
+
3+
+
UV–vis titration of L to M took Al as an example. As shown in Fig. 1,
the single probe L had absorption at 287 nm and 325 nm. After Al was
3
+
+
added, the absorption appeared at 505 nm. And with the gradual addi-
tion of Al3 , the absorption peak at 505 nm gradually increased, and the
absorption peak at 325 nm gradually decreased. And it could be
observed that an isosbestic point appeared at 345 nm. These changes
+
2
+
Zn in a pH = 7 environment.
3
+
.4. Selectivity of L to Zn2 and M
+
3+
indicated that an interaction took place between L and Al . The UV–vis
3
titration spectra of L to Fe3 and L to Cr were shown in Figs. S9 and
+
3+
2
+
S10, respectively. The UV–vis titration of L to Zn was expressed in
Fig. 2. The absorption peak of L alone had absorption at 287 nm and
Fluorescence spectra analysis experiments were carried out to
research the selective recognition of metal ions by the probe L. As
depicted in Fig. 6, in EtOH/H O (15/1, V/V) solution, L alone was no
2
+
3
4
25 nm. After Zn was added, the absorption appeared at 405 nm,
25 nm and 455 nm. And with the gradually adding of Zn2 , the ab-
2
+
obvious fluorescence emission at 498 nm when the excitation wave-
length was set at 425 nm. However, after addition of Zn2 , the signifi-
cant fluorescence emission at 498 nm was observed. In addition, after
adding other metal ions, there was only weak or almost no fluorescence
emission. It was worth mentioning that when Cu2 was added, the
+
sorption peaks at 405 nm, 425 nm, and 455 nm gradually increased, and
the absorption peaks at 325 nm gradually decreased. And an isosbestic
point could be observed at 355 nm. Similarly, these changes indicated
that an interaction took place between L and Zn2
+
+
.
EtOH/H
2
O (15/1, V/V) solution of L showed a red color which was
3
.2. Effect of solvent
different from other metal ions. In addition, after Al3 , Fe , and Cr
+
3+
3+
were added to L, they appeared pink color with faint pink fluorescence
3
+
To explore the effect of solvent on the selectivity of L to M and L to
at 588 nm.
Zn2 , fluorescence experiments of L to M and L to Zn in different
+
3+
2+
The selective fluorescence response experiment of L to M was
3+
solvents were carried out. Firstly, when the excitation wavelength was
carried out in methanol solution. Fig. 7 showed the fluorescence
set to 490 nm, the L after addition of Al3 was subjected to fluorescence
+
response spectrum of probe L to M when the excitation wavelength
3+
detection in ethanol, methanol, acetonitrile, ethyl acetate, dichloro-
was set to 490 nm. L alone showed no fluorescence and the color of the
methane, DMSO and DMF. As shown in Fig. 3, the L-Al3 system showed
+
solution was colorless. When M was added to the methanol solution of
3+
the strongest fluorescence in methanol solvent. However, under the
L, the solution appeared pink, at the same time, the methanol solution of
2
+
3+
condition of an excitation wavelength of 425 nm, L after adding Cd
,
L with M showed obvious fluorescence emission at 588 nm. Except for
2
+
2+
2+
2+
Zn , Mg
respectively exhibited fluorescence in ethanol solution
the L solution containing Zn and Co , which performed weak fluo-
rescence, all other metal ions had no apparent fluorescence. Therefore, L
(
Fig. 4a). We tried to change the fluorescence intensity of L after adding
5

J. Sun et al.
Journal of Photochemistry & Photobiology, A: Chemistry 411 (2021) 113207
Fig. 12. The UV–vis absorption spectra of L after addition of 1 equiv of various
2
metal ions in EtOH/H O (15/1, V/V). Insert: Color changes of probe L and L in
2
+
2+
3+
the presence of Cu , Zn and M under sunlight.
showed more selective to M3 (excited at 490 nm) than other mono-
+
valent and divalent metal cations in methanol.
3
.5. Titration experiment of L to M3 and Zn
+ 2+
The fluorescence titration experiments were conducted to explore
the relationship between fluorescence intensity and metal ion concen-
tration. Fig. 8 was the fluorescence titration spectrum of L in EtOH/H
2
O
2
+
(
15/1, V/V) solution with continuous dropwise addition of Zn
0.0–1.1 equiv). It could be seen intuitively from the spectrum that as the
(
2
+
concentration of Zn increased, the fluorescence intensity at 498 nm
enhanced gradually. When Zn2 was added to 1.0 equiv, the fluorescence
+
intensity was basically stable, indicating that the L-Zn2 compound with
+
a coordination ratio of 1:1 had been formed. Besides, the fluorescence
intensity had a linear relationship with Zn2 (Fig. S4). According to the
+
+
formula LOD = 3
σ
/k, the detection limit of L for Zn2 was calculated to
ꢀ
7
2+
be 1.45 × 10 M. The binding constant (Ka) of L and Zn calculated by
4
-1
the Benesi–Hildebrand equation was 6.75 × 10 M (Fig. S5).
1
1
1
= K
(Fm ax ꢀ Fm in)[Zn2+]
+
F
m ax ꢀ Fm in
F ꢀ Fm in
a
3
+
Fig. 11. a. Fluorescence response of L and L+Al upon the addition of various
3
+
+
2+
2+
2+
+
2+
2+
2+
2+
2+
2+
3+
metal ions (1=Al , 2=Ag , 3=Ba , 4=Ca , 5=Cd , 6=Co , 7=Hg
,
,
+
+
2+
8
1
=K , 9=Li , 10=Mg , 11=Mn , 12=Na , 13=Pb ,14=Zn , 15=Fe
2
+
2+
6=Ni , 17=Cu ) in methanol. b. Fluorescence response of L and L+Cr
2+
,
3
+
+
2+
2+
upon the addition of various metal ions (1=Cr , 2=Ag , 3=Ba , 4=Ca
2
+
+
2+
2+
+
+
2+
+
5
1
=Cd , 6=Co , 7=Hg , 8=K , 9=Li , 10=Mg , 11=Mn , 12=Na ,
2 2+ 2+ 2+ 2+
3=Pb ,14=Zn , 15=Fe , 16=Ni , 17=Cu ) in methanol. c. Fluorescence
3
+
3+
2+
response of L and L+Fe upon the addition of various metal ions (1=Fe
,
,
+
2+
2+
2+
2+
2+
2+
2+
+
+
2
1
=Ag , 3=Ba , 4=Ca , 5=Cd , 6=Co , 7=Hg , 8=K , 9=Li , 10=Mg
2
+
+
2+
2+
2+
1=Mn
,
12=Na , 13=Pb ,14=Zn
,
15=Fe
,
16=Ni
,
17=Cu
)
in methanol.
2
+
Fig. 13. Job’s plot for determining the stoichiometry between L and Zn in
EtOH/H O (15/1, V/V).
2
6

J. Sun et al.
Journal of Photochemistry & Photobiology, A: Chemistry 411 (2021) 113207
M3 was gradually added, the wavelength was gradually redshifted from
+
5
80 nm to 588 nm and the fluorescence intensity at 588 nm increased
3
+
gradually. When M was dropped to 0.5 equiv, the fluorescence in-
3
+
tensity remained basically unchanged, which indicated that the L-M
complex had been formed this moment. The linear relationship between
metal ion concentration and fluorescence emission intensity was also
3
+
3+
3+
fitted (Fig. S6). The detection limits of L for Fe , Al and Cr were
ꢀ 9
-9
-8
2
.46 × 10 M, 2.54 × 10 M and 4.23 × 10 M, respectively. Accord-
ing to the Benesi–Hildebrand equation, the binding constants between L
3
+
3+
3+
5
and Fe , between L and Al , and between L and Cr were 4.23 × 10
-
1
5
-1
5
-1
M , 2.41 × 10
M
and 3.93 × 10 M , respectively (Fig. S7). We
3
+
2+
compared the probe we synthesized with the M and Zn probe pre-
viously reported (Table 1), the probe in our work had a lower detection
limit and could be used as a dual detection probe for M3 and Zn
+
2+
.
3
.6. Competitive experiment of L to Zn2 and L to M
+ 3+
2
+
To further explore the selectivity of L to Zn in the presence of other
+
2+
2+
3+
+
3+
+
+
2+
2+
2+
2+
metal ions such as Na , Mg , Ag , K , Li , Hg , Cu , Ca , Co ,
2
+
2+
2+
3+
Cd , Mn , Ba , Pb , Al , Cr and Fe , fluorescence analysis
experiments were carried out. The results were illustrated in Fig. 10,
2
+
when Zn was added to L with addition of other metal ions, except for
2
+
Cu , other metal ions showed fluorescence emission, which indicted
2
+
those metal ions would not interfere the coordination of L and Zn . This
might be due to the paramagnetism of Cu2 , which led to the transfer of
+
2
+
electrons and energy from the ligand to the metal ion. When Zn was
added to L-Cu2 , fluorescence quenching would occur. This result
+
indicated that other metal ions would not interfere with the selectivity of
L to Zn2 . We used the same method to detect whether other metal ions
+
would affect the coordination of L-M3 . The spectrum was shown in
+
2
+
Fig. 11, the result was similar to the above, except for Cu , other metal
ions would not interfere with L-M3 . Similarly, other metal ions would
+
not affect the selectivity of L to M3
+
.
2
+
3
.7. UV–vis spectroscopy experiment of L on Cu
O (15/1, V/V), it was found that L after adding Cu2
+
In EtOH/H
2
showed a red color different from L after adding others. Therefore, the
UV–vis spectrum analysis method was used to study the selectivity of L
to Cu2 . The UV–vis spectrum was shown in Fig. 12, L added with Cu
+
2+
showed absorption band at 409 nm, 430 nm, 455 nm and 553 nm, while
3
+
L after adding other metal ions were no absorption band except M
2
+
2+
2+
2+
2+
3+
Ni , Zn , Co , Cd and Pb . Although L after adding Fe showed a
clear pink color and exhibited obvious absorption band at 553 nm, there
was no absorption band at 409 nm, 430 nm, 455 nm, which would not
2
+
affect the detection of Cu . Similarly, the yellow-colored L with the
addition of Ni2 , Zn , Co , Cd and Pb respectively had absorption
+
2+
2+
2+
2+
bands at 409 nm, 430 nm, and 455 nm, but there was no absorption at
2
+
.
5
53 nm, and it would also not interfere with the selectivity of L to Cu
2
+
This suggested that L had favorable recognition of Cu . The change in
wavelength was caused by the change in the color of the solution. The
color change from colorless to red, which could be observed with the
naked eye. Moreover, the competitive experiment of L-Cu2 was carried
+
out, and other metal ions would not interfere with the coordination of L
and Cu2 (Fig. S8).
+
Fig. 14. a. Job’s plot for determining the stoichiometry between L and Al3+ 3in
3
.8. Coordination studies
+
methanol. b. Job’s plot for determining the stoichiometry between L and Cr
3
+
in methanol. c. Job’s plot for determining the stoichiometry between L and Fe
2
+
The Job’s plot of L-Zn was researched based on the fluorescence
spectrum. The total concentration of L and specific metal ions kept
in methanol.
constant (25
μ
M), and the molar fraction ratio was continuously variable
n+
n+
Herein, Fmax, Fmin and F respectively indicated the fluorescence intensity
emission wavelength was 498 nm) of the maximum concentration of
([M ]/[L]+[M ]). As shown in Fig. 13, as the mole fraction was 0.5,
(
the fluorescence intensity reached the maximum, which signified that
2
+
2+
2+
Zn , L alone, and the various concentration of Zn
.
the coordination ratio of L and Zn was 1:1. Differently, in the Job’s
3
+
3+
Similarly, the fluorescence titration experiments of L to M were
plot of L-M , when the mole fraction was 0.3, the fluorescence intensity
3
+
carried out according to the above method. As demonstrated in Fig. 9, as
reached the maximum. The Job’s plot of L-M
indicated that the
7

J. Sun et al.
Journal of Photochemistry & Photobiology, A: Chemistry 411 (2021) 113207
2
+
3+
Scheme 2. The proposed sensing mechanism of L with Zn and M .
coordination ratio of L and M3 was 2:1 (Fig. 14a–c).
+
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jphotochem.2021.
3
.9. The sensing mechanism of L
1
13207.
When L is present alone, the nitrogen atom of the hydrazide moiety
of Rhodamine B transfers electrons to the moiety of 4-oxo-4H-benzo[h]
methylene-3-carbaldehyde, causing the individual L to not show Fluo-
rescence, which is due to the light-induced electron transfer process
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