Fluorescent chemical sensor based on double N2O2 cavities for continuous recognition of Cu2+ and Al3+

Article abstract of DOI:10.1016/j.jphotochem.2020.112829

A fluorescent chemical sensor based on a bis(salamo)-like tetraoxime H₂L_S was designed and synthesized. The chemical sensor uses double N₂O₂ cavities as sensing elements, which can be combined with specific meta

Full text of DOI:10.1016/j.jphotochem.2020.112829

Journal Pre-proof
Fluorescent chemical sensor based on double N O cavities for
2
2
continuous recognition of Cu2 and Al
+
3+
Ruo-Nan Bian, Ji-Fa Wang, Ya-Juan Li, Yang Zhang, Wen-Kui Dong
PII:
S1010-6030(20)30627-4
DOI:
https://doi.org/10.1016/j.jphotochem.2020.112829
JPC 112829
Reference:
To appear in:
Journal of Photochemistry & Photobiology, A: Chemistry
Received Date:
Revised Date:
Accepted Date:
15 June 2020
27 July 2020
3 August 2020
Please cite this article as: Bian R-Nan, Wang J-Fa, Li Y-Juan, Zhang Y, Dong W-Kui,
Fluorescent chemical sensor based on double N O cavities for continuous recognition of
2
2
2+
3+
Cu and Al , Journal of Photochemistry and amp; Photobiology, A: Chemistry (2020),
doi: https://doi.org/10.1016/j.jphotochem.2020.112829
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Fluorescent chemical sensor based on double N
2
O cavities for
2
2
+
3+
continuous recognition of Cu and Al
Ruo-Nan Bian, Ji-Fa Wang, Ya-Juan Li, Yang Zhang, Wen-Kui Dong*
School of Chemical and Biological Engineering, Lanzhou Jiaotong University,
Lanzhou, Gansu 730070, China
*
Corresponding author.
E-mail addresses: dongwk@126.com (W.-K. Dong).
Graphical Abstract
A new bis(salamo)-based chemical sensor H
2
L
S
for continuous recognition
can detect Cu2+ by
the phenomenon of fluorescence quenching, and a test strip loaded with the
2+
3+
of Cu , Al was synthesized. The chemical sensor H
L
2 S
2+
sensor is used to quickly and accurately identify Cu . Chemical sensor H
2 S
L
2+
not only can detect Cu in environmental water sample, but also continuously
3+
recognize Al , and realize the interference-free identification effect of other
3+
trivalent metal ions on Al .

Highlights

A new bis(salamo)-based chemical sensor H
2
L for continuous recognition of
S
2
+
3+
Cu , Al was synthesized.
The chemical sensor H can detect Cu by the phenomenon of fluorescence
quenching, and a test strip loaded with the sensor is used to quickly and
2
+

L
2 S
2
+
accurately identify Cu .
Chemical sensor H not only can detect Cu in environmental water sample,
but also continuously recognize Al , and realize the interference-free
2
+

L
2 S
3
+
3
+
identification effect of other trivalent metal ions on Al .
Abstract
A fluorescent chemical sensor based on a bis(salamo)-like tetraoxime H
2
L was
S
designed and synthesized. The chemical sensor uses double N
2
2
O cavities as sensing
elements, which can be combined with specific metal ions to achieve ion recognition.
2
+
The chemical sensor can detect Cu by the phenomenon of fluorescence quenching,
2
+
and a test strip loaded with the sensor is used to quickly and accurately identify Cu .
3
+
Besides, the chemical sensor also can continuously recognize Al in the system and

3
+
realize the interference-free identification effect of other trivalent metal ions on Al .
1
Through H NMR, mass spectrometry, infrared spectroscopy, and other experiments,
the identification mechanism of the fluorescent chemical sensor was verified. The
numerical values of LOD and Ka were calculated by the corresponding titration data
and Benesi-Hildebrand formula.
Keywords: bis(salamo)-like tetraoxime; fluorescent chemical sensor; Cu2+ ion; Al3+
ion; test paper strip
1
. Introduction
At present, water scarcity and metal pollution are two of the major problems of
our water resources [1-7]. Contamination of water resources by metals and other
qharmful ions not only seriously affect the ecological environment but also directly
pollute the soil, resulting in metal pollution of agricultural products, threatening
human health [8-13]. Nowadays, many agricultural products have been found to
contain metal elements, so the development of a fast, accurate, and efficient method to
detect metal ions in complex water samples is the focus and hotspot of environmental
science and biomedical research [14-18]. Fluorescence sensor molecules are widely
used in ion detection because of their facile synthesis, high sensitivity and selectivity,
fast response, and high interference resistance [19].
Salamo-like compounds have higher stability compared to salen-like compounds,
and the symmetrical double salamo-like compound has excellent flexibility and
several different cavities with high selectivity and metal ions [20-25]. Therefore, the
balanced bis(salamo)-like compounds can interact with many metal ions to form
metal complexes, which have considerable potential in ion recognition,
supramolecular construction, and biological applications. Salamo-like compounds are
derivatives of salen-like compounds by modifying the -C=N- group in salen
compounds to the -C=N-O- group, which introduces an electronegative O-atom,
4
making them 10 times more stable than salen compounds [26-29]. The design and

introduction of different functional groups to modify the end groups can indirectly
regulate the solubility of these ligands in aqueous systems, thereby expanding the
range of detection of these compounds as fluorescent chemical sensors for different
ions in the environment [30-35]. Copper is one of the trace elements essential for life
in plants and animals and plays a critical role in the life process. Many of the active
sites of metallizes and metalloproteins contain dinuclear copper(II) structural units
[
36-40]. Besides, copper has a large number of coordination points and excellent
coordination properties and can form copper(II) complexes with most ligands, making
the study of copper(II) coordination more convenient. The application of copper(II)
compounds in catalytic and optoelectronic materials is gradually becoming a research
2
+
2+
focus [41-43]. The stable order of complexes generated from Mn to Zn divalent
metals with ligands containing N coordination atoms can be observed in the following
2
+
2+
2+
2+
2+
2+
order: Mn <Fe <Co <Ni <Cu >Zn , the sequence called the Irving-Williams
sequence, which roughly corresponds to the variation of the weak-field CFSE [44-45].
Based on the Irving-Williams sequence, the copper complexes are more stable in
comparison to those of other same period divalent metal ions.
H
2
L was found to recognize copper(II) ions and to recognize aluminum(III)
S
ions directionally after the formation of copper(II) complexes. Excess aluminum(III)
ion in the human body is not suitable for skeletal development and can also affect
2
+
- +
intelligence, so the complex [(L
S
+2Cu )NO
3
] (L ) has the necessary implications
C
and developmental prospects for the detection of aluminum(III) ions in organisms.
2
+
The Cu ions were first detected by using the bis(salamo)-like chemical sensor H
-
2
+
2
L
S
. The experimental results showed that sensor H
2
L can detect Cu in aqueous
S
solution by the fluorescence quenching phenomenon. The advantages of the sensor H
-
2
+
2
L
S
for detection of Cu include a low detection limit, high coordination constant,
wide pH range and fast response time. In addition, a test paper equipped with the
2
+
sensor H
quickly, accurately and qualitatively in aqueous solution. Hence, sensor H
potential applications for Cu2+ detection in real life. Besides, the sensor H
2
L was prepared, which can be used to detect the presence of Cu easily,
S
2
L
S
has
can
2 S
L
3
+
continuously identify Al in the system and achieves interference-free identification

3
+
of Al by other trivalent metal ions. Finally, test paper strip experiments were done,
and the results showed that this fluorescent chemical sensor could be applied to the
test paper to detect specific ions quickly.
2
. Experimental section
2
.1 Experimental methods and instruments
All chemicals and solvents were analytical grade reagents from Tianjin Chemical
Reagent Factory and were used without further purification. Elemental analyses were
obtained on a GmbH VarioEL V3.00 automated elemental analysis instrument.
Melting points were obtained by a microscopic melting point apparatus made in
1
Beijing Taike Instrument Limited Company and were uncorrected. H NMR spectra
were measured by German Bruker AVANCE DRX-400 spectrometer. Fluorescence
spectra were recorded on the Hitachi F-7000 spectrometers. ESI-MS spectra were
measured on the Bruker Daltonics Esquire 6000 mass spectrometer. All pH
measurements were made with a pHS-45 digital pH meter.
2
.2 Synthesis and characterization of H
L
2 S
The bis(salamo)-like tetraoxime H
2
L
S
was designed and synthesized based on
previously reported research methods and test routes in this laboratory. [46,47]
Scheme 1. Synthetic route to sensor H
L
2 S
2
,2′-(Ethylenedioxy)bis(benzaldehyde)(1) was synthesized according to a
literature procedure [48].
-[O-(1-Ethyloxyamide)]oxime-6-methoxyphenol(2) was prepared according to a
2
previously reported method [49-52]. Weighing 1,2-bis(aminooxy)ethane(d) 414.45
mg (4.5 mmol) in ethanol (20 mL), another 684.67 mg (4.5 mmol) of
3
-methoxysalicylaldehyde(c) was dissolved in ethanol (20 mL). An ethanol solution
of 3-methoxysalicylaldehyde(c) was added drop by drop to an ethanol solution of
,2-bis(aminooxy)ethane(d) in a water bath at 55°C, with a controlled drop
1

acceleration of about 10 s a drop and continuous reaction for 6 h to obtain light yellow
ethanol solution. Decompression distillation removes excess solvent and concentrates
the reaction solution to about 5 mL. The concentrated solution is a bright yellow oily
liquid, further purified by column chromatography to obtain white solid 748.44 mg,
yield: 73.51%; m.p.: 95.5-96.5°C. Anal. Calcd. for C10
H
14
N
2
O
4
: C, 53.09; H, 6.24; N,
)  3.92 (s, 3H),
1
1
3
2.38. Found: C, 53.32; H, 6.20; N, 12.16. H NMR (500 MHz, CDCl
3
.96 (t, J = 4.5 Hz, 2H), 4.38 (t, J = 4.5 Hz, 2H), 5.50 (brs, 2H), 6.82 (dd, J = 7.8, 1.6
Hz, 1H), 6.87 (t, J = 7.8 Hz, 1H), 6.90 (dd, J = 7.8, 1.6 Hz, 1H), 8.24 (s, 1H), 9.88 (s,
H).
1
An ethanol solution (10 mL) of 2,2′-(ethylenedioxy)bis(benzaldehyde)(1)
270.22 mg, 1.0 mmol) was added drop by drop to an ethanol solution (20 mL) of
-[O-(1-ethyloxyamide)]oxime-6-methoxyphenol(2) (453.08 mg, 2.0 mmol) in a
(
2
water bath at 55°C, and the reaction continued for 4h. A large amount of white
precipitation occurred in the solution. At the end of the reaction, the distillation was
decompressed to remove the excess solvent, filtered and dried. White solid: 517.29
mg. Yield: 75.32%. m.p.: 124.6-126.4°C. Anal. calcd. For C36
H
38
N
4
O10: C, 62.97; H,
1
5
.58; N, 8.16. Found: C, 63.15; H, 5.49; N, 8.09. H NMR (500 MHz, CDCl
3
) δ 9.82
(s, 2H),8.51 (s, 2H), 8.23 (s, 2H), 7.79 (d, J = 7.7 Hz, 2H), 7.34 (t, J = 8.7 Hz, 2H), 6.
9
8
7(t, J = 7.5 Hz, 2H), 6.93 (d, J = 8.4 Hz, 2H), 6.90 (d, J = 6.8 Hz, 2H), 6.84 (t, J = 7.
Hz, 2H), 6.79 (dd, J = 7.7, 1.5 Hz, 2H), 4.47-4.45 (m, 4H), 4.45-4.43 (m, 4H), 4.35 (
s, 4H), 3.89 (s, 6H) ( Figure S1).
2
2
.3 Experimental methods
.3.1 Solution preparation
The samples used in the fluorescence spectroscopy of this experiment, including
the bis(salamo)-like tetraoxime chemical sensor H
configured with a solution. The probe molecule is [H
ion solutions are derived from their metal nitrates. Metal ions [M ] = 1 × 10 mol/L
2
L
S
and the metal ions, were
-3
2
L ] = 1 × 10 mol/L. The metal
S
n+
-2
+
+
+
2+
2+
2+
2+
3+
3+
2+
2+
2+
2+
2+
+
(
Li , Na , K , Mg , Ca , Sr , Ba , Cr , Fe , Co , Ni , Cu , Zn , Cd , Ag and
3
+
Al ).

2
.3.2 Fluorescence spectroscopy experiment
The fluorescence spectroscopy experiments were performed with a reserve solution
-3
of chemical sensor H
2
L (VDMF: V Water = 9:1, c = 1×10 mol/L). The concentration of
S
the receptor molecules in the buffer of Tris-HCl in VDMF: V Water (9/1, v/v, pH = 7.20)
was diluted with a mixture of VDMF: V Water = 9:1 to ensure a concentration of 1×10-5
+
+
+
2+
2+
2+
2+
mol/L. The metal ions used were metal nitrates (Li , Na , K , Mg , Ca , Sr , Ba ,
3
+
3+
2+
2+
2+
2+
2+
+
3+
Cr , Fe , Co , Ni , Cu , Zn , Cd , Ag and Al ) and the solvent was V DMF : V
-2
Water = 9:1 solution (1×10 mol/L), the amount of metal ions added in the sample was
0 times of that of the chemical sensor H . The titration curves were determined by
2
L
2 S
the cumulative addition of Cu2+ and Al , drop by drop. All experiments were
conducted at room temperature. The testing conditions of all experiments were
consistent: EX WL: 310.0 nm, EM WL: 388.0 nm, EX slit: 10.0 nm, EM Slit: 10.0
nm.
3+
2
.3.3 Method of pH measurement
The pH value was measured at room temperature by using the pH meter of model
pHS-45. The pH value was adjusted in 1 mM Tris-HCl buffer solution with 1 M
hydrochloric acid and 1 M tetrabutylammonium hydroxide solution.
2
.3.4 Test experiment of H
L
2 S
chemical sensor on tap water sample
The fluorescence spectra of Cu2+ and Al3+ were determined by adding known
2
+
3+
amounts of Cu and Al to the tap water samples, tested five times in each sample,
2
+
3+
and the concentrations of Cu and Al were measured according to the calibration
curves obtained from the fluorescence titration experiment.
2
.3.5 Preparation of test paper
The filter paper was first soaked in 1 mol/L dilute hydrochloric acid, then dipped in
distilled water for three times and washed to neutral. After vacuum drying, the filter
paper was cut into a 5×1 cm filter paper piece, containing chemical sensor H
L
2 S
-3
(VDMF: VWater = 9:1, c = 1×10 mol/L) solution is loaded on the filter paper, and then
dried in vacuum. Under the UV light, the filter paper itself emits bright green
fluorescence. The dried filter paper was directly applied to the identification test of
2
+
-3
Cu in solution. It will also contain L (VDMF : V Water = 9:1, c = 1×10 mol/L) is
C

loaded on the filter paper, and the dried filter paper was directly applied to the
3
+
identification test of Al in the solution.
3
. Results and discussion
2
+
3
.1 Recognition of Cu by the chemical sensor
.1.1 Solvent to chemical sensor H luminescence performance study
First, the effect of the solvent system on the fluorescence intensity of the
molecular sensor should be considered. The impact of different commonly used
solvents on the H fluorescence intensity of the chemical sensor were compared at
3
L
2 S
L
2 S
an excitation wavelength of 310 nm, slit width EX = 10 nm, and EM = 10 nm (as
shown in Figure S3).
The solvent effect is the effect of the solvent on the rate, equilibrium and even the
mechanism of the reaction. In the vast majority of organic chemical reactions
occurring in solvents, the nature of the solvent is extremely important not only for the
reaction rate but also for the reaction equilibrium. The hallmark of a fluorescent
chemical sensor is the detection of ions by changes in fluorescence intensity, so the
choice of solvent for the sensor is critical. The fluorescence emission intensity of H
was measured in seven common organic solvents which could dissolve the probe
, the results show that the fluorescence intensity of H in DMF is relatively
strong. Then select DMF as the solvent for future experiments [53-59].
2
-
L
S
H
2
L
S
L
2 S
3
.1.2 Study of water content on the luminescence performance of chemical sensor H
-
2
L
S
In order to study the fluorescence intensity of the chemical sensor H
aqueous system, the fluorescence intensity of the chemical sensor H in the system
with different water ratios was detected (as shown in Figure S4). The results showed
that the fluorescence intensity of the chemical sensor H gradually decreased as the
2
S
L in the
L
2 S
L
2 S
water content of the system increased. After the water content exceeded 70%,
flocculation began to appear in the system, proving that excess water content has a

more significant effect on the solubility of the sensor molecules. The chemical sensor
fluorescence intensity is most vigorous in the VDMF: V Water = 9:1 system. The VDMF: V
Water = 9:1 solvent system was then selected as the solvent for later experiments [60].
3
.1.3 Study of the recognition performance of chemical sensor H
The fluorescence spectral characteristics of the chemical sensor H
ions were studied. The DMF/H O (9/1, v/v, pH = 7.20) Tris-HCl buffer solution
1×10 mol/L) of chemical sensor emitted a strong fluorescence at approximately 310
nm. The addition of 20-fold equivalent metal ions to the solution caused the
2
L
S
on Cu2+
2
L with metal
S
2
-5
(
2
+
maximum emission peak of H
2
L
S
to disappear only when Cu were added, while the
did not change significantly when other metal ions
maximum emission peak of H
were added (Figure 1).
L
2 S
2+
3
.1.4 Quantitation of Cu fluorescence titrations and minimum detection limits
To investigate the quantification behavior of the chemical sensor H
for Cu2+
recognition, a fluorescence titration experiment was performed (as shown in Figure
L
2 S
2
+
S5). The cumulative addition of Cu to the DMF/H
resulted in a gradual decrease of the maximum emission peak at 310 nm as the
amount of Cu2+ increased, and the maximum emission peak of the sensor H
2
O solution of the chemical sensor
H
L
2 S
L
2 S
completely disappeared when the amount of Cu2+ added was 2.0 times equivalent.
The fluorescence intensity stabilized and did not change as the Cu2+ solution
continued to be added. At the same time, it was observed that the color of the sensor
solution changed from colorless to brown, which also provided a basis for visually
2
+
detecting Cu .
The experimental results showed that the chemical sensor H
2
L
S
had good
selective recognition ability for Cu in aqueous system. This result may be due to the
cavity of a specific size formed by the chemical sensor H itself. When the complex
is formed, the chemical sensor H selectively coordinated with Cu , thus
reducing the fluorescence intensity of the chemical sensor H . The limit of
fluorescence detection (LOD) and limit of content (LOQ) of H
to Cu2+ were
2
+
L
2 S
2
+
L
C
2
L
S
L
2 S
2
L
S

calculated from the titration spectra of the sensor H
2
S
L . Based on previous literature
[
61], the following equation was used to determine the LOD and LOQ.
2
(

F  F )
0
0

N 1
LOD = K × δ/S; LOQ =10 × δ/S;
Where N, S, and F
the blank solution of sensor H
(N = 20); K = 3.
0
are the measured values, slope, and fluorescence intensity of
2
L
S
, respectively. The minimum detection limit (LOD)
2+
-7
of sensor H
2
L
S
to Cu was calculated to be 6.62 × 10 M.
3
.1.5 Anti-interference experimental determination of the fluorescent chemical sensor
toward Cu2+
The basic requirement of fluorescent chemical sensor is that it can selectively
H
L
2 S
2
+
recognize ions. Therefore, the anti-interference experiment of Cu was carried out on
the sensor H . Subsequently, other
. First, Cu2+ was added to the sensor H
common metal ions were added, stirred and left for 10 min to measure the
2
L
S
L
2 S
3
+
fluorescence intensity. The experimental results show that except for Al , other
2
+
common metal ions have no obvious interference on the recognition of Cu by H
L
2 S
(
Figure 2). It is illustrated that the chemical sensor H has better anti-interference
2 S
L
2
+
resistance to the recognition of Cu .
3
.1.6 The pH response of the fluorescent chemical sensor H
To further investigate the applicable pH range of the fluorescence chemical
sensor H , the change of the fluorescence intensity of the fluorescence chemical
sensor H
and 13.0. As shown in Figure S6, the change in fluorescence intensity of fluorescent
chemosensor H between pH = 3 and 11 was insensitive, whereas, in the pH range
between 3 and 8, the addition of Cu significantly reduced the fluorescence intensity
of fluorescent chemosensor H . The results showed that the fluorescent chemical
sensor H has a better ability to recognize Cu in the physiological pH range.
2
L
S
to Cu2+
2 S
L
2
+
2
L interacting with Cu was studied when the pH value was between 2.0
S
L
2 S
2
+
2
L
S
2
+
2
L
S
3
.1.7 Time response of the fluorescent chemical sensor H
2
L
to Cu2+
S

In order to investigate the response time of the fluorescent chemical sensor H
L
2 S
2
+
L
to Cu2+ was
to Cu , the response time of the fluorescent chemical sensor H
2
S
further studied. As shown in Figure S7, the experimental results showed that the
2+
response time of sensor receptor molecule H
2
L to Cu was about 10s. This result
S
2
+
suggested that sensor H can quickly recognize Cu in aqueous systems.
2 S
L
3
.1.8 Determination of the Job curve of the fluorescent chemical sensor H
2
L with
S
Cu2+
To further determine the optimal ratio between the H
2
L
S
and Cu2+ of the
and Cu2+ of the
fluorescence chemical sensor, the working curve of the H
2
L
S
fluorescence chemical sensor was determined and plotted. As shown in Figure S8, the
fluorescence intensity at approximately 397 nm of the sensor molecule showed an
2
+
inflection point at a quantity fraction of 0.37 of the Cu ion's substance, indicating
that the coordination between the fluorescent chemical sensor H
and Cu2+ has
occured according to a 1:2 chemometric ratio. The molecular ion peaks of m/z =
2
L
S
+2Cu )NO
-]+ can be seen from
2
+
8
72.0987 (as shown in Figure S9) attributed to [(L
S
3
the ESI-MS spectrogram, further confirming that the fluorescent chemical sensor H
2
-
2
+
L is ligated to Cu according to a 1:2 coordination ratio.
S
2
+
3
.1.9 Calculation of the ligand constants of H
sensor
The fluorescence titration experiments of the fluorescent chemosensor H
2
L and Cu in a fluorescent chemical
S
2
L
S
with
Cu , combined with the results of the titration work curve, and the ligand constant of
the fluorescent chemosensor H
with Cu2+ calculated by Benesi-Hildebrand
equation (1) is 3.50×10 M , which indicated that the fluorescent chemosensor H
2
+
L
2 S
1
0
-1
L
2 S
2
+
had a powerful bonding ability to Cu (shown in Figure S10).
m+
n
1
/ (F - F
0
) = 1/ (Fmax - F
0
) {(Ka [M ] ) + 1}
(1)
F is the intensity of the fluorescence intensity corresponding to the Cu2+
2
+
concentration at 388 nm, F
0
is the intensity at 397 nm when the Cu concentration is
m+
zero, and Fmax is the maximum fluorescence intensity at 388 nm. [M ] is the

2
+
concentration of Cu , and n is the chemometric ratio of the chemical sensor H
Cu2+ [62]. Compared with other references [63-64], it was found that the binding
constant of H
for Cu2+ recognition is larger, which indicates that the copper(II)
complex of H is more stable. In addition, the LOD of the chemical sensor H is
smaller than that of other Cu2+ probe in the literature, indicating that the chemical
2
L to
S
2
L
S
2
L
S
L
2 S
2
+
sensor is more sensitive to recognize Cu . Binding constants and detection lines
between the related chemosensors mentioned in literatures were compared, as shown
in Table 1.
Table 1 Comparison of binding constants and detection lines between the
chemosensors.
2
+
3
.1.10 Exploration of the mechanism of Cu recognition by the fluorescent chemical
sensor H
Based on the above experimental results, a mechanism for the Cu2+ ions
recognition by the fluorescent chemical sensor H was proposed, as shown in
Figure 3. The fluorescent chemical sensor H itself gives green fluorescence. The
outer electron configuration of Cu is d , which is paramagnetic and the formation of
the complex L leads to fluorescence quenching of chemical sensor H . According
L
2 S
L
2 S
2
L
S
2
+
9
C
L
2 S
to Jahn-Teller effect, there are three electrons in the two degenerate d orbitals (Eg) of
divalent copper(II) ions in octahedral complexes due to the splitting of crystal field.
The number of electrons occupied in these two orbitals is different, and the complex
is a nonlinear molecule, which leads to the splitting of Eg [65]. The results of splitting
lead to Jahn-Teller distortion, and the Irving-Williams (CFSE increase) order obtained
from the crystal field splitting energy is abnormal, which leads to the copper(II)
complexes being more stable than other bivalent metal(II) complexes in the same
period. After Cu2 + combines with H
has lower energy than H
L , the copper(II) complex is more stable and
2
S
2
S
L .
The effect of the complexes in different oxidation states on the charge density is
linear, indicating that there is a strong relationship between the charge density around

2
+
the metal and the oxidation state [66-67]. In the presence of Cu , the fluorescence
2
+
intensity of the system is quenched. This indicates that Cu interacts with the system
effectively, which reduces the fluorescence intensity.
3
.1.11 Experiments on tap water samples with Cu2+
2
+
It was detecting Cu in tap water with H
2
L
S
to explore the utility of chemical
2+
sensor H . A test sample was first made by adding a known exact amount of Cu to
2
L
S
tap water. Quantitative analysis of the test samples using the standard addition method
using fluorescence spectroscopy was repeated. It can be seen from Table S1 that the
average recoveries ranged from 97.6% to 104.2% with relative standard deviations
within 3%. The chemical sensor H
2
S
L was shown to be suitable for the quantitative
2
+
detection of Cu in tap water.
3
.1.12 Application of test paper
Based on the fluorescence selectivity of the chemical sensor to Cu ,
fluorescence detection test papers loaded with the sensor H were fabricated. The
dilute hydrochloric acid-treated filter paper was immersed in a DMF/H O solution of
at a concentration of 1.0×10 mol/L for loading. After the load was
uniform, the test paper was placed in a vacuum drying chamber and dried at low
2+
2
L
S
2
-4
sensor H
2
L
S
2
+
temperature, and then dried paper was used for Cu determination. Before the test,
2
+
the test paper was green under UV light. After dropping the Cu solution on the strip,
the color of the test paper became colorless under the UV light, as shown in Figure 4.
3
.2 Study of the recognition performance of the chemical sensor L
.2.1 Fluorescence recognition study of metal ions by L
Through the study and exploration of the preliminary experiments, it was found
that H
had a good recognition effect on Cu2+ in aqueous system. However, in
anti-interference experiments, the presence of Al3+ in the system affected the
C
on Al3+
3
C
2
L
S
2
+
2+
recognition of Cu . Therefore, it was envisaged that after the Cu was recognized,
3
+
the sensor will continuously recognize Al . With this scenario in mind, the

fluorescence recognition properties of the complex L
C
as a sensor L on various
C
metal ions were investigated.
As shown in Figure S11, in the solution of the sensor L
intensity of the sensor L changed almost nothing with the addition of other metal
ions, only the fluorescence was enhanced with the addition of Al . The sensor L
C
the fluorescence
C
3
+
C
has
3
+
single recognition for Al .
3
+
3
.2.2 Anti-interference experimental study of L
C
on Al recognition
As can be seen from the competition experiment (Figure S12), the recognition of
3
+
3+
L
C
to Al was not affected when other metal ions were present. Only the Al can
2
+
replace the Cu in the complex L
C
to form a new complex L
C
. Thus, the sensor L
C
3
+
had high selectivity and single recognition as a chemical sensor for identifying Al .
3
.2.3 Fluorescence titration of L
Fluorescence titration experiments were performed, and the minimum detection
limit of L
for Al3+ was calculated, as shown in Figure S13. The intensity of the
emission peak of L
spectrum does not change with the addition of 3.0 equivalents of Al . The results
C
against Al3+
C
3+
C
increased with the constant addition of Al until the emission
3
+
2
+
3+
showed that the Cu in the complex L
C
were utterly replaced by Al and formed L
C
3
+
-
Al . Thus, the fluorescence intensity gradually increased.
11
-1
At the same time, the binding equilibrium constant (K = 3.05 × 10 M ) of L
2
C
3
+
to Al was calculated based on linear fitting (Figure S18), and it was found that the
3
+
binding equilibrium constant of H
2
L
S
to Al was ten times higher than that of H
L
2 S
2
+
for Al3+ is 5.33 × 10 M.
-7
to Cu . Besides, the minimum detection limit of L
C
Reference to other literature [68-69], comparison of binding constants and detection
lines between the chemosensors as shown in Table 1.
3
.2.4 The pH response of the fluorescent chemical sensor L
To further investigate the applicable pH range of the fluorescence chemical
sensor L , the change of the fluorescence intensity of the fluorescence chemical
C
to Al3+
C

3
+
sensor L
As shown in Figure S14, the change in fluorescence intensity of fluorescent
chemosensor L between pH = 4 and 11 was insensitive, whereas, in the pH range
between 5 and 8, the addition of Al significantly enhanced the fluorescence intensity
of fluorescent chemosensor L . The results show that the fluorescent chemical sensor
has a better ability to recognize Al in the neutral pH range.
C
interacting with Al was studied when the pH value was between 2 and 13.
C
3
+
C
3
+
L
C
3
.2.5 Time response of the fluorescent chemical sensor L
C
to Al3+
In order to investigate the response time of the fluorescent chemical sensor L
C
to
to Al3+ was further
studied. As shown in Figure S15, the experimental results show that the response
3
+
Al , the response time of the fluorescent chemical sensor L
C
3
+
time of sensor receptor molecule L to Al is about 20 s. This result suggests that
C
3
+
sensor L
C
can quickly recognize Al in systems.
3
.2.6 Determination of Job curves of L
C
versus Al3+
Working curves of L
C
and Al3+ were plotted to determine further the optimal
3
+
ratio of L and Al . As shown in Figure S16, the fluorescence intensity of the sensor
C
3
+
molecule appeared a turning point at the mass fraction of 0.75 for Al . This result
3
+
indicated that the fluorescence chemical sensor L
C
was coordinated with the Al ,
according to the stoichiometry of 1:3. The molecular ion peaks of m/z = 819.2246 (as
3
+
7+
shown in Figure S17) attributed to [(L
S
+3Al )·H O·DMF] can be seen from the
2
3
+
ESI-MS spectrogram, further verifying that the L and Al were ligated according to
C
a 1:3 coordination ratio.
3
+
3
.2.7 Exploration of the mechanism of Al ion recognition by L
C
The mechanism of L
C
recognition of Al3+ is proposed based on the above
3
+
experimental results, as shown in Figure 5. The outer electron configuration of Al is
p6, and the p orbital is full, which forms a one-dimensional plane
3
+
O·DMF]7+ due to the addition of Al , and the chelation-enhanced
3+
[(L
S
+3Al )·H
2
fluorescence (CHEF) is the main reason for the enhanced fluorescence [70-71]. In the

3
+
presence of Al , the fluorescence intensity of the system is enhanced, which means
that the copper(II) complex of H as a quenched fluorescent group is activated at
the presence of Al in water system, which improves the fluorescence intensity of L
2
L
S
3
+
C
.
In addition, the planarity of the complex structure is also enhanced, leading to a strong
PET (photoinduced electron tranfer) from the receptor, which further enhances the
fluorescence emission [72-73].
3
.2.8 Application of test paper
3
+
Based on the fluorescence selectivity of the chemical sensor L
C
to Al ,
fluorescence detection test papers loaded with the sensor L
C
were fabricated. The
dilute hydrochloric acid-treated filter paper was immersed in a DMF/H
2
O solution of
-4
the copper(II) complex of H
2
L at a concentration of 1.0×10 mol/L for loading. After
S
the load was uniform, the test paper was placed in a vacuum drying chamber and
dried at low temperature, and then dried paper was used for Al3+ determination.
Before the test, the test paper was colorless under UV light. After dropping the Al3+
solution on the strip, the color of the test paper became green under the UV light, as
shown in Figure 4.
4
. Conclusion
In this paper, Cu2+ was first detected by using a bis(salamo)-like tetraoxime
chemical sensor H
2
L
S
. The experimental results showed that the sensor H
2
L can
S
detect Cu2+ in aqueous solution by the fluorescence quenching phenomenon. The
2
+
advantages of the sensor H
coordination constant, a wide pH range (3.0~8.0), and fast response time (about 10
seconds). A test paper loaded with the sensor H has been prepared for the
2
S
L for detection of Cu are a low detection limit, high
2
L
S
detection of Cu2+ in aqueous solution easily, quickly, accurately and qualitatively.
Therefore, this fluorescent chemical sensor has potential applications for detecting
2
+
3+
Cu in real life. Besides, the sensor L
C
can identify Al in the system and achieves
3
+
interference-free identification of Al by other trivalent metals.

Conflicts of Interest
The authors declare no competing financial interests.
Acknowledgments
This work was supported by the National Natural Science Foundation of
China (21761018), Science and Technology Program of Gansu Province
(18YF1GA054), and the Program for Excellent Team of Scientific Research in
Lanzhou Jiaotong University (201706), three of which are gratefully acknowledged.
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Author Biographies
Ruo-Nan Bian is currently pursuing her master’s degree in the School of Chemical
and Biological Engineering at Lanzhou Jiaotong University, China. She obtained her
B.S. in the School of Chemical and Biological Engineering at Lanzhou Jiaotong
University, China, in 2017.
Ji-Fa Wang is currently pursuing his master’s degree in the School of Chemical and
Biological Engineering at Lanzhou Jiaotong University, China. He obtained his B.S.
in the School of Materials Science and Engineering at Qilu University of Technology,
China, in 2018.
Ya-Juan Li is currently pursuing her master’s degree in the School of Chemical and
Biological Engineering at Lanzhou University, China. She obtained her B.S. in the
School of chemical engineering and technology, at Tianshui Normal University ,
China, in 2018.
Yang Zhang received his PhD degree in 2019 from Lanzhou Jiaotong University,
China. He is a lecturer in the school of chemistry and bioengineering of Lanzhou
Jiaotong University now.
Wen-Kui Dong received his PhD degree in 1998 from Lanzhou University, China. He
is a professor in the School of Chemical and Biological Engineering at Lanzhou
Jiaotong University. His current research interests are focusing on the chemistry of
functional supramolecular complexes, materials and new products, as well as
processes and applications of environmental chemical engineering.

Table 1 Comparison of binding constants and detection lines between the
chemosensors.
Binding
constant
Detection
limit
Identification
substance
No.
sensor
pH range Reference
-1
(
M )
(M)
1
6.42 × 104
4.7 × 10-6
Cu2+
4~8
[63]
2
2.4 × 102
1.7 × 10-6
Cu2+
4~12
[64]
3
2.6 × 104
3.4 × 10−7
Al3+
4.5~9.5
[68]
4
4.95 × 104
8.65 × 10−8
Al3+
5~8
[69]
3.5 × 1010 6.62 × 10-7
Cu2+
5
3~11
5~8
This work
This work
.05× 1011 5.33 × 10-7
Al3+
3

Scheme 1. Synthetic route to sensor H
L
2 S
Scheme 1

Figure 1 Fluorescence spectra of chemical sensor H
2
L
S
and metal ions (20 equiv.) in
-3
DMF/H
2
O buffer solution (H
2 S
L
, 1×10 mol/L; Tris-HCl buffer solution, 9/1, v/v, pH
=
7.20).
Figure 2 The sensor H
2
L
S
(1×10-3 mol/L) in DMF/H
solution for Cu recognition anti-interference experiment. (Emission peak at 388 nm,
the black bar is the fluorescence intensity of the chemical sensor H , the red bar is
the fluorescence intensity of the sensor H
with Cu2+ added, the blue bar is the
+Cu after adding common metal ion. H
, 1×10-3
mol/L; Tris-HCl buffer solution, 9/1, v/v, pH = 7.20).
2
O buffer solution buffer
2
+
2
L
S
L
2 S
2
+
fluorescence intensity of H
2
L
S
L
2 S
2
+
Figure 3 Proposed recognition mechanism of Cu by fluorescence chemical sensor
H
L
2 S
Figure 4 The photos of chemical sensor H
2
L
S
using test paper (a) H
+ other metal ions; (e) L
ions+Al ( all taken under 365 nm ultraviolet lamp).
2
L
S
; (b) L
C
; (c)
3
+
L
C
+Al ; (d) H
2
L
S
+other metal ions; (e) L
C
C
+ other metal
3
+
3
+
Figure 5 Proposed recognition mechanism of Al by fluorescent chemical sensor L .
C

Figure 1