A Bis(Salamo)-Based Fluorogenic Sensor for Highly Selective and Sequential Recognition of Cu2+ and B4O72? Ions in Semi-Aqueous Medium

Article abstract of DOI:10.1007/s10895-021-02717-0

A new type of multifunctional bis(salamo)-based fluorogenic sensor H₂BS was designed and synthesized. Under the action of V_DMF: V_H2O = 9: 1, the fluorogenic sensor can identify Cu²⁺ and B₄O₇

Full text of DOI:10.1007/s10895-021-02717-0

Journal of Fluorescence (2021) 31:817–833
https://doi.org/10.1007/s10895-021-02717-0
ORIGINAL ARTICLE
A Bis(Salamo)-Based Fluorogenic Sensor for Highly Selective
and Sequential Recognition of Cu2+ and B4O72− Ions
in Semi-Aqueous Medium
Su-Xia Gao1 & Xin Xu2 & Yang Zhang2 & Wen-Kui Dong2
Received: 18 December 2020 /Accepted: 3 March 2021
/ Published online: 18 March 2021
#
The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
A new type of multifunctional bis(salamo)-based fluorogenic sensor H2BS was designed and synthesized. Under the action of VDMF
H2O = 9: 1, the fluorogenic sensor can identify Cu2+ and B4O72−, in which N and O atoms can serve as binding sites for Cu2+ and
:
V
B4O72−, the stoichiometry of the binding of the fluorogenic sensor H2BS and Cu2+ has been confirmed by titration experiment,
working curve, ESI-MS analysis and DFT calculation. The pH response experiment also confirmed that the fluorogenic sensor can
recognize Cu2+ and B4O72− in the pH range applicable to the physiological environment. The minimum detection limit of H2BS for
Cu2+ and B4O72− recognition reaches 1.12 × 10−7 and 5.56 × 10−8 M, and the fluorogenic sensor H2BS has been successfully applied
to Cu2+ detection in actual water samples, and the test strip for detecting Cu2+ and B4O72− was obtained. Meanwhile, the success of the
test strip experiment made the fluorogenic sensor H2BS to recognize Cu2+ and B4O72− widely used in daily life.
2 +
2 −
.
.
. .
PET mechanism Test strip
Keywords Bis(salamo)-based fluorogenic sensor Cu
B4O7
Introduction
detection range of these compounds for different ions in the
environment and biology has been expanded [22, 23].
Salamo-based compounds are more stable than salen-based
compounds and have high selectivity for metal cations and
anions [1–7]. At present, applications of salamo-based com-
pounds in catalytic [8], magnetic [9] and electrochemical
properties [10] have been explored. The application of com-
pounds in ion recognition needs further exploration [11–16].
By designing and introducing different functional groups to
modify the end groups, the solubility of such ligands in aque-
ous systems can be indirectly adjusted [17–21], Thus, the
The identification of copper ions by salamo-basecd com-
pounds has been reported [24, 25], but there have been few
reports on the quantitative identification of copper ions and
the use of salamo-based compounds available in the environ-
ment [26–30]. Copper ions is an important transition metal
and one of the essential trace elements in the human body
[31, 32]. The lack of copper ions in the human body reduces
the oxidative activity of ceruloplasmin in plasma, leading to
iron deficiency anemia [33–36]. However, when the human
body contains too much copper ions, it can cause acute gas-
troenteritis [37, 38]. In severe cases, it will destroy red blood
cells and cause hemolysis and anemia [39, 40]. The main way
for humans to contact copper ions is daily diet [41, 42], so
quantitative detection of copper ions in tap water is beneficial
to the natural environment and its organisms [43, 44].
Sodium borate or borax is the most important borate and is
commonly used in the manufacture of borosilicate glass [45, 46],
cleaners [47, 48], preservatives [49, 50], etc. Of course, because
borax can extend the shelf life of foods and increase the tough-
ness and crispiness of foods [51, 52], many illegal suppliers use
borax as a food additive in our daily diet [53, 54]. Borax has now
been identified as a carcinogen. Excess boron substances can
affect human reproduction, development and endocrine [55,
Highlights
1. A new bis(salamo)-based fluorogenic sensor H2BS for recognition of
Cu2+ and B4O72- was synthesized.
2. H2BS can be used to quantitatively detect Cu2+ content in tap water.
2-
3. H2BS test paper used to quickly identify Cu2+ and B4O7 has been
prepared.
* Wen-Kui Dong
dongwk@126.com
1
School of Environmental and Municipal Engineering, Lanzhou
Jiaotong University, Lanzhou 730070, Gansu, China
2
School of Chemical and Biological Engineering, Lanzhou Jiaotong
University, Lanzhou 730070, Gansu, China

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J Fluoresc (2021) 31:817–833
56]. Therefore, the quantitative detection of borate becomes par-
ticularly important [57–60].
parameter hybrid method by using the Becke88exchange
functional and LYP correlation functional (B3LYP), and
selecting the 6-31G basis set for geometric optimization.
In this paper, a new bis(salamo)-based compound was de-
signed and synthesized, and characterized by 1H NMR et al. It
has been found by fluorescence spectroscopy that it has high
recognition ability for Cu2+ and is not affected by other metal
cations. Meanwhile, the fluorogenic sensor H2BS can identify
Synthesis of the Fluorogenic Chemical Sensor H2BS
2,2′-(Ethylenedioxy)bis(benzaldehyde) was synthesized ac-
cording to a literature procedure [25, 63]. A mixture of
salicylaldehyde (2 mmol), dibromoethane (1 mmol), anhydrous
K2CO3 (2 mmol) and KI (0.1 mmol) in anhydrous MeCN
(20 mL) was refluxed overnight. After completion of the reac-
tion MeCN was distilled off under vacuum. The reaction mix-
ture was then washed with H2O and extracted with CHCl3.
2-[O-(1-Ethyloxyamide)]oxime-2-naphthol was prepared ac-
cording to a previously reported method [2]. The synthetic steps
of the bis(salamo)-based ligand are shown in Scheme 1 [64–66].
A white solid powder was finally obtained with a yield of
77.35%. M.p.:140–142 °C.1H NMR (500 MHz, CDCl3): δ
10.88 (s, 1H), 9.12 (s, 1H),8.55 (s, 1H), 7.90 (d, J = 8.5 Hz,
1H), 7.80 (d, J = 7.7 Hz, 1H), 7.74 (d, J = 9.0 Hz, 2H), 7.47 (d,
J = 5.1 Hz, 1H), 7.34 (s, 2H), 7.17 (d, J = 9.0 Hz, 1H), 6.96 (t,
J = 7.5 Hz, 1H), 6.91 (d, J = 8.0 Hz, 1H), 4.53 (s, 4H), 4.33 (s,
2H) (Fig. 1). Anal. calcd. for C42H38N4O8: C, 69.41; H, 5.27; N,
7.58; O, 17.61. Found: C, 69.52; H, 5.32; N, 7.49; O, 17.56.
2−
B4O7 with high selectivity without being interfered by other
anions. In addition, this new bis(salamo)-based compound can
quantitatively identify Cu2+ in tap water and can be used for
environmental pollution control, this field has good development
prospects [61, 62].
Experimental Section
Materials and Methods
Salicylaldehyde and 2-hydroxy-1-naphthaldehyde were pur-
chased from Meryer with a purity of 99% and used without
further purification. Other reagents and solvents are analytical
reagents of Beijing Chemical Plant. All metal salts used are of
the general formula M (NO3)n· xH2O. Only FeCl3 is used for
iron ions. These metal salts are also used in cation interference
experiments, while sodium salts are used in anion competition
experiments. Melting points were obtained using a micro-
melting point instrument manufactured by Beijing Tyco
Instruments Co., Ltd. and are uncorrected. 1H NMR spectrum
was measured by a Bruker AV 500 MHz spectrometer in
Germany. The fluorescence spectrum was measured on a
Sanco 970-CRT fluorescence spectrophotometer. The fluores-
cence spectrophotometer uses a 1 cm optical path quartz cu-
vette and the excitation source is a xenon lamp. All pH mea-
surements were performed using a pH-10C digital pH meter.
The samples used in the fluorescence spectrum analysis of
this experiment, including the bis(salamo)-based fluorogenic
sensor H2BS and metal ions, were configured with a solution
of VDMF: VH2O = 9: 1. The probe molecule is [H2BS] = 1 ×
10−3 mol/L. The metal cations used are their nitrates. The
metal cations [Mn+] = 1 × 10−2 mol/L (Li+, Na+, K+, Mg2+,
Ca2+, Sr2+, Ba2+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+,
Cd2+, Ag+, Pd2+ and Al3+), all the anions are their sodium or
potassium salts, and the anion concentration is also 1 ×
Results and Discussion
Solvent Effect of the Fluorogenic Sensor H2BS
Different solvents will lead to different fluorescence intensity.
Therefore, under the conditions of excitation wavelength of
340 nm, slit width EX = 10 nm and EM = 10 nm, the influence
of different common solvents (DMF, DMSO, EA, THF, ben-
zene, Pyridine, dichloromethane and chloroform) on the fluo-
rescence intensity of the fluorogenic sensor H2BS was ex-
plored (Fig. 2). The experimental results showed that the
fluorogenic sensor has the best fluorescence intensity in
DMF solution, while the fluorescence intensity is weaker in
other common organic solvents. Then choose DMF as the
solvent used in the subsequent experiments.
Water-Containing System
10−2 mol/L (F−, Cl−, Br−, I−, ClO4 , S2−, SO32−, CO3
,
−
2−
HCO3 , CN−, OAc−, H2PO4 , HPO42−, P2O7 and B4O72−).
Dilute with a mixed solution of VDMF: VH2O = 9: 1 during the
measurement to ensure that the receptor molecules are in a
Tris-HCl buffer solution of VDMF: VH2O = 9: 1 (pH = 7.20).
The concentration is 1 × 10−5 mol/L. The emission and exci-
tation wavelengths selected are 340 and 310 nm, respectively,
and the slit width EX = 10 nm and EM = 10 nm.
In order to further study the fluorescence intensity of the
fluorogenic sensor H2BS in the aqueous system, the fluores-
cence intensity of H2BS in different water contents was tested
(Fig. 3). The experimental results showed that the fluores-
cence intensity of the fluorogenic sensor H2BS gradually de-
creases with the increase of water content in the solvent sys-
tem. In the system of VDMF: VH2O = 9: 1, the fluorescence
intensity is the strongest. When the water content exceeds
60%, white precipitation appears in the entire solvent system.
−
−
−
All quantum calculations were run using Gaussian 09,
using density functional theory (DFT), using the Becke’s three

J Fluoresc (2021) 31:817–833
819
Scheme 1 Synthetic route to the fluorogenic sensor H2BS.
Therefore, the solvent system of VDMF: VH2O = 9: 1 was se-
lected as the solvent used in the subsequent experiments.
In the Tris-HCl buffer solution (VDMF: VH2O = 9: 1, 1 ×
10−5 mol/L), the fluorogenic sensor H2BS emits strong fluo-
rescence at 397 nm. Added 20.0 times equivalent metal cat-
ions (Li+, Na+, K+, Mg2+, Ca2+, Sr2+, Ba2+, Cr3+, Mn2+, Fe3+,
Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Ag+, Pd2+ and Al3+) to the
H2BS solution respectively, and found that only when Cu2+
is added, the maximum emission peak of the fluorogenic
sensor H2BS at 397 nm is disappeared. It is worth noting that
after added Fe3+, the fluorescence emission peak of the
fluorogenic sensor H2BS would also be reduced, but this
did not affect the recognition effect of H2BS on Cu2+, and
when other metal ions were added, the maximum emission
Recognition Performance of the Fluorogenic Sensor
H2BS for Cu2+
Recognition of Cu2+ by H2BS
Based on the above experiments, the cation recognition re-
sponse was explored in the solvent system of VDMF: VH2O
9: 1 (pH = 7.20).
=
Fig. 1 H2BS 1H NMR (500 MHz, CDCl3)

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J Fluoresc (2021) 31:817–833
experiment was carried out (Fig. 4c). Cu2+ ions were added to
the VDMF: VH2O = 9: 1 solution of the fluorogenic sensor H2BS,
with the increase of Cu2+, the maximum emission peak at
397 nm gradually decreases. When the amount of Cu2+ were
2.0 equivalent, the maximum emission peak of the fluorogenic
sensor H2BS disappeared completely, and continue to added
Cu2+ solution, the fluorescence intensity tended to be stable
and unchanged. Meanwhile, it was observed that the color of
the fluorogenic sensor H2BS solution changed from yellow-
green to brown (Fig. S1), which also provided a basis for color-
imetric recognition of Cu2+. The experimental results showed
that the fluorogenic sensor H2BS has better selective recognition
ability for Cu2+ in the aqueous system. This may be due to a
certain size cavity formed by H2BS itself. During the formation
of the complex, the fluorogenic sensor H2BS is selectively co-
ordinated with Cu2+, which reduced the fluorescence intensity of
H2BS by more than 10 times. In addition, LOD (4.10 × 10−7 M)
and LOQ (1.37 × 10−6 M) were calculated. According to the
Benesi–Hildebrand equation [25, 26], the binding constant K
of the fluorogenic sensor H2BS and Cu2+ is 5.08 × 1011 M−1,
which indicates that the fluorogenic sensor H2BS and Cu2+ have
a very strong bonding ability (Fig. 5).
Fig. 2 Fluorescence intensity of the fluorogenic sensor H2BS in different
solvents
peak of the fluorogenic sensor H2BS would not cause a big
change (Fig. 4a).
As an excellent fluorogenic sensor, one of the basic re-
quirements is the selectivity of the fluorogenic sensor.
Therefore, the selective recognition ability of the H2BS to
Cu2+ was tested, and the anti-interference experiment of
Benesi–Hildebrand analysis of the titration profiles based
on 2:1 binding mode resulted in a nice linearity, indicating the
2:1 binding stoichiometry of Cu2+ and H2BS. The total bind-
ing constant of H2BS complexing with Cu2+ and the detection
limit (LOD) of H2BS to Cu2+ were studied by the following
curve of fluorescence intensity ratio, which was obtained from
the fluorescence titration spectra of H2BS with Cu2+. They
were plotted according to the following equations:
Cu2+ was carried out (Fig. 4b). In the H2BS solution (VDMF
V
:
H2O = 9: 1) containing 10.0 equivalents of Cu2+, 10 equiva-
lents of other common metal ions were added, and the inter-
ference of other metal ions on the recognition of Cu2+ were
tested by fluorescence spectroscopy experiments. The results
confirmed that no other metal ions participating in the com-
petition experiment would interfere with the detection of
Cu2+. It further showed that the fluorogenic sensor H2BS
has good anti-interference ability and can recognize Cu2+.
In order to further study the recognition and quantification of
Cu2+ by the fluorogenic sensor H2BS, a fluorescence titration
Â
Ã
log ðF‐FminÞ=ðFmax‐FÞ ¼ log K þ 2 log Cu2þ
(1).
(2).
LOD ¼ 3 Â δ=S
(1) Where Fmin, Fmax and F are the emission intensities in the
absence, presence of saturated Cu2+, and addition of a
given amount of Cu2+ at a concentration, respectively.
[Cu2+] is the concentration of free Cu2+.
(2) S and F0 are the slope of the fitted equation and the
fluorescence intensity of H2BS, respectively.
Y ¼ ‐22:07418 Â X þ 929:72727 R ¼ 0:9324
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
∑ðF0−F0Þ
S ¼ 2:207418 Â 107 δ ¼
¼ 3:02 ðN ¼ 10Þ
N−1
LOD ¼ K Â δ=S ¼ 4:10 Â 10‐7M LOQ ¼ 1:37 Â 10‐6M
Fig. 3 The fluorescence intensity of the fluorogenic sensor H2BS under
different water content

J Fluoresc (2021) 31:817–833
821
Fig. 4 a Fluorescence spectra of
H2BS solution (VDMF: VH2O = 9:
1) in the absence and presence of
various metal cations (Li+, Na+,
K+, Mg2+, Ca2+, Sr2+, Ba2+, Cr3+
,
Mn2+, Fe3+, Co2+, Ni2+, Cu2+
,
Zn2+, Cd2+, Ag+, Pd2+ and Al3+).
b Fluorescence response of the
H2BS + Cu2+ solution to various
metal ions (value at 397 nm, the
black column is the fluorescence
intensity of the fluorogenic
chemical sensor H2BS, the red
column is the fluorescence
intensity when common metal
ions are added to the sensor
H2BS, and the blue column is the
fluorescence intensity of Cu2+
after common metal ions are
added to the sensor H2BS). c
Fluorometric titration of Cu2+
with the fluorogenic chemical
sensor H2BS

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Fig. 5 Fluorescence intensity
ratio (log (F-Fmin)/(Fmax-F)) of
H2BS versus increasing
concentration of log [Cu2+]. K =
5.08 × 1011 M−1
Cycling Performance of Cu2+ Recognition by H2BS
Recognition Mechanism of Cu2+ by the Fluorogenic Sensor
H2BS
The cycle performance is a good performance of fluorescence
sensor [2]. In the process of identifying Cu2+, EDTA with
stronger binding ability to Cu2+ was selected as the
complexing agent for experiments (Fig. 6). When Cu2+ was
added to H2BS solution, the fluorescence intensity of the so-
lution was significantly reduced, then EDTA solution was
added to the solution, and the fluorescence intensity of the
solution was found to be similar to that of the original H2BS
solution. Recently, it was found that the fluorescence intensity
of the solution decreases again after added Cu2+. After repeat-
ing the above experiment three times, it was found that the
fluorescence loss of the solution is very small. This “off-on-
off” mode reflects the recycling performance of H2BS in the
recognition of Cu2+.
Density functional theory (DFT) is a method of using quan-
tum mechanics to study the electronic structure of multi-
electron systems [17, 20]. DFT can be used to calculate and
analyze the HOMO and LUMO orbital energy levels of the
fluorogenic sensors, further confirming the recognition mech-
anism of the fluorogenic sensors to Cu2+. The optimized struc-
ture of H2BS and H2BS + Cu2+ is shown in Fig. 7a. The
HOMO and LUMO orbitals of H2BS and H2BS + Cu2+ after
structural optimization were shown in Fig. 7b. In the orbital
energy level diagram of H2BS, HOMO orbitals are mainly
concentrated on naphthalene ring, while LUMO orbitals are
mainly concentrated around benzene ring, and the orbital en-
ergy level difference is ΔE = 2.26 eV. When Cu2+ is added,
due to the formation of a new complex, the orbital energy
level difference is 0.53 eV, which is much smaller than the
energy level difference of H2BS, which indicated that H2BS
and Cu2+ can form stable Cu2+ complexes. For H2BS + Cu2+
with high symmetry, its HOMO or LUMO is likely to be
degenerate. Therefore, the HOMO-1 and LUMO+1 of
H2BS + Cu2+ were analyzed. It is found that the energy dif-
ference is 1.58 eV, which is also lower than the level of H2BS.
This further proves the stability of H2BS + Cu2+.
To further prove the coordination ratio between the
fluorogenic sensor H2BS and Cu2+, the working curve of the
fluorogenic sensor H2BS and Cu2+ was measured and plotted.
As shown in Fig. S2, the inflection point of the fluorescence
intensity of H2BS at 397 nm appears when the content fraction
of Cu2+ is 0.66. It shows that the fluorescence sensor H2BS
and Cu2+ appear in the stoichiometric ratio of 1:2.
Fig. 6 Cycling performance of Cu2+ recognition by H2BS

J Fluoresc (2021) 31:817–833
823
Fig. 7 a The optimized structure of H2BS and H2BS + Cu2+. b Energy level diagrams of HOMO and LUMO orbitals of H2BS and H2BS + Cu2+
It can be seen from the ESI-MS spectrum that the molecular
ion peak is m / z = 872.0987 (Fig. S3), which belongs to
[H2BS + 2Cu2+ + H2O]. It was further verified that the coor-
dination ratio of the fluorogenic chemical sensor H2BS and
Cu2+ is 1:2 (Fig. 8).
(1 × 10−5 mol/L) of VDMF: VH2O = 9: 1, H2BS emits strong
fluorescence at 397 nm, added 20.0 times equivalent of anions
(F−, Cl−, Br−, I−, ClO4 , S2−, SO32−, CO32−, HCO3 , CN−,
−
−
OAc−, H2PO4 , HPO42−, P2O7 , PO43− and B4O72−) solution,
as shown in Fig. 9a. Only when B4O72− was added, the max-
imum emission peak of H2BS disappeared at 397 nm and a
new emission peak appeared at 468 nm. It should be noted that
the new emission peak at 468 nm also indicated the interaction
between H2BS and B4O72−. When other anions were added,
the maximum emission peak of H2BS will not cause much
change.
−
−
Recognition Performance of the Fluorogenic Sensor
2−
H2BS for B4O7
Recognition of B4O72− by H2BS
An excellent fluorescence chemical sensor requires specific
selectivity. Therefore, the ability of H2BS to selectively
The effect of the fluorogenic sensor H2BS on the recognition
of anions was further studied. In the Tris-HCl buffer solution

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J Fluoresc (2021) 31:817–833
Fig. 8 The mechanism of the
fluorogenic sensor H2BS
identifying Cu2+
recognize B4O72− was tested, and conducted anti-interference
experiments with B4O72−. Ten equivalents of other common
anions were added to the VDMF: VH2O = 9: 1 solution of H2BS
to detect the interference of other anions with the recognition
of B4O72−, respectively. As shown in Fig. 9b, the results
showed that other anions have little effect on the recognition
They were plotted according to the following equations:
(1)
(2)
Â
Ã
2‐
log ðF‐FminÞ=ðFmax‐FÞ ¼ log K þ 2 log B4O7
2−
of B4O7 by H2BS, which further demonstrates that the
fluorogenic sensor H2BS has good anti-interference ability
LOD ¼ 3 Â δ=S
2−
and can be used for single recognition of B4O7
.
In order to further study the recognition and quantitative be-
havior of B4O72− by H2BS, fluorescence titration experiment was
carried out (Fig. 9c). In the pre-prepared H2BS solution, cumu-
(1) Where Fmin, Fmax and F are the emission intensities in the
absence, presence of saturated B4O72−, and addition of a
given amount of B4O72− at a concentration, respectively.
2−
lative addition of B4O72−, with the increase of B4O7 content,
2−
[B4O72−] is the concentration of free B4O7
.
the maximum emission peak at 397 nm gradually decreased, and
a new emission peak appeared at 468 nm. When the amount of
B4O72− is 2.0 equivalent, the maximum emission peak of H2BS
at 397 nm disappears completely, and the emission peak at
(2) S and F0 are the slope of the fitted equation and the
fluorescence intensity of H2BS, respectively.
2−
468 nm reaches the maximum. And continue to add B4O7
solution, the fluorescence intensity tends to be stable and un-
changed. At the same time, it was observed that the color of the
Y ¼ 11:33552 Â X þ 74:53221 R ¼ 0:99135
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u
u
t
ꢀ
ꢁ
2
H2BS solution changes from yellow-green to colorless, which
∑ F −F
0
0
2−
S ¼ 1:133552 Â 107 δ ¼
¼ 1:355892 ðN ¼ 20Þ
also provides a basis for colorimetric identification of B4O7
.
N−1
From the titration spectrum of the fluorogenic sensor H2BS
LOD ¼ K Â δ=S ¼ 3:59 Â 10‐7M
LOQ ¼ K Â δ=S ¼ 1:20 Â 10‐6M
containing B4O72−, the fluorescence detection limit (LOD)
and content limit (LOQ) of B4O7 are 3.59 × 10−7 and
2−
Finally, the previously reported literatures were compared
with the binding constants and detection lines of Cu2+ and
1.20 × 10−6 M, respectively (Fig. 10a). According to the
Benesi–Hildebrand equation (Fig. 10b) [27, 34], the coordi-
nation constant of the fluorogenic sensor H2BS and B4O72− is
2−
B4O7 identified in this work. It can be concluded from
Table 1 that the fluorogenic sensor H2BS is superior to them
in terms of detection line and binding ability [26, 33, 37, 38].
2.87 × 1012 M−1, which indicated that the H2BS has the ability
2−
to bind firmly to the B4O7
.

J Fluoresc (2021) 31:817–833
825
Fig. 9 a Fluorescence spectra of
H2BS solution (VDMF: VH2O = 9:
1) in the absence and presence of
various anions. b Fluorescence
2−
response of the H2BS-B4O7
solution to various anions (value
at 468 nm). c Fluorometric
titration of B4O72− with the
fluorogenic sensor H2BS

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Fig. 10 a Fluorescence change of
H2BS solution after adding
different amounts of B4O72− (0 ~
2.0 equivalents). λem = 468 nm. b
The increase in the fluorescence
intensity ratio (log (F-Fmin) /
(Fmax-F)) of H2BS relative to the
concentration of log [B4O72−].
K = 2.87 × 1012 M−1
Recognition Mechanism of B4O72− Ions by the Fluorogenic
Sensor H2BS
The prone to hydrolysis of borate is the main cause of fluo-
rescence quenching:

J Fluoresc (2021) 31:817–833
827
Table 1 Comparison of binding constants and detection lines between chemosensors
Binding
constant
(M-1)
Detection
limit
Identification
substance
No.
sensor
pH range Reference
(M)
1
4.5 × 103
4.4 × 10-7
Cu2+
None
[33]
2
3
None
1.19 × 10-6
Cu2+
4~8
[37]
[38]
9.82 × 103
None
Cu2+
None
1.36 × 109 1.99 × 10-7
4
Cu2+
4~10
[26]
[26]
2-
2.17 × 109 7.57 × 10-7
B4O7
4~8
5.08 × 1011 4.10 × 10-7
2.87 × 1012 3.59 × 10-7
5
Cu2+
4~8
4~8
This work
This work
2-
B4O7

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J Fluoresc (2021) 31:817–833
Fig. 11 The mechanism of the
fluorogenic sensor H2BS
2−
identifying B4O7
Because the fluorogenic sensor H2BS has electron donor
characteristics, and the special configuration of boric acid will
enter the H2BS conjugate system [26], changing the optical
characteristics of the system and quenching the fluorescence
(Fig. 11), leading to the occurrence of photoinduced electron
11, the change of fluorescence intensity is not sensitive, in the
range of pH = 3.0 ~ 8.0, added Cu2+ ions will significantly
reduce the fluorescence intensity of the fluorogenic sensor
H2BS. In the same way, the applicable conditions for H2BS
to recognize B4O72− were explored. In the range of pH = 3.0 ~
8.0, after added B4O72−, the fluorescence intensity at 397 nm
was significantly reduced. Experimental results showed that
the fluorogenic sensor H2BS has a better ability to recognized
Cu2+ and B4O72− in the physiological pH range.
1
transfer (PET). The H NMR titration experiment has also
confirmed this idea, as shown in Fig. S7, it can be seen that
the phenol oxygen bond of H2BS has disappeared obviously,
while the other hydrogen atoms on the fluorogenic sensor
have no obvious chemical shifts, which indicates that the phe-
nol oxygen bond of H2BS is broken, and H2BS interacts with
deprotonated H2BS, thus changing the fluorescence intensity
of the system. On the other hand, boronic acid is a C3h point
group, and the C3h point group is conducive to the expansion
of the intermolecular hydrogen bond system and can form a
periodic planar structure in two dimensions. This causes the
boric acid molecules to be in a state of aggregation in the
solution, causing aggregation-induced quenching (ACQ) to
occur.
The Influence of pH on the Fluorogenic Sensor H2BS
In order to further study the applicable pH range of the
fluorogenic sensor H2BS [25], when the pH is 2.0 to 13.0,
the change in fluorescence intensity of the fluorogenic sensor
H2BS recognized the changes in the fluorescence intensity of
Cu2+ and B4O72−, as shown in Fig. 12. Between pH = 3.0 ~
Fig. 12 The influence of pH on the fluorogenic sensor H2BS

J Fluoresc (2021) 31:817–833
829
Table. 2 The fluorescent sensor H2BS to detect tap water samples
The Influence of Time on the Fluorogenic Sensor H2BS
Added Cu2+ (μmol·L−1
)
Found
Recovery(%) R.S.D.(n=3)(%)
(μmol·L−1
)
In order to study the response speed of the fluorogenic sensor
H2BS to Cu2+, the response time of H2BS to Cu2+ were fur-
ther studied [9, 67]. As shown in Fig. 13a, after added Cu2+,
the fluorescence intensity of H2BS decreased to the end point
of the titration within 10 s. Using the same method (Fig. 13b),
the recognition speed of H2BS to B4O72− was explored, and it
was also found that the fluorescence intensity of H2BS at
397 nm decreases to the lowest value within 10 s, while the
fluorescence at 468 nm increases. The above experimental
0
Not detected
0.462
0.5
1.0
2.0
4.0
96.30
10.29
97.79
98.93
3.4
1.1
4.3
2.6
1.029
1.968
3.992
results showed that the fluorogenic sensor H2BS can quickly
A test sample was made by added a known exact
amount of Cu2+ to tap water. At first, the water samples
were pretreated with organic solvents to ensure that the
detection process is not interfered by water, the standard
addition method was used to quantitatively analyze the
samples by fluorescence spectra, it can be seen from
Table 2 that the average recovery is 97.7% to 104.2%
and the relative standard deviation is within 3%.
Experiments showed that the fluorogenic sensor H2BS
can be used for the quantitative detection of Cu2+ in tap
water.
2−
identify Cu2+ and B4O7
.
Practical Application
Detection of Cu2+ Content in Water Samples
In order to explore the practical application value of
H2BS, it can be used to detect Cu2+ in tap water [1].
Test Strip
Based on the fluorescence selectivity of the fluorogenic
sensor H2BS for Cu2+ and B4O72−, fluorogenic test strip
was made [25, 26]. In order to facilitate the configuration
of the solution and ensure the obvious recognition effect,
the filter paper treated with dilute hydrochloric acid was
immersed in the VDMF: VH2O = 9: 1 solution of the
fluorogenic sensor H2BS at a concentration of 1.0 ×
10−4 mol/L for loading. After the load is uniform, the
test paper is placed in a vacuum drying box for low-
temperature drying. The dried test paper is used for the
detection of Cu2+ content. This application test method is
similar to the application of pH test paper. Among them,
the UV absorption and light emission by H2BS is limited
to the application of naked eye identification. Before the
experiment, the test paper is blue under UV light, and
the color of the test paper becomes black when Cu2+ was
added into the solution. When other competitive metal
ions were added to the test paper, the test paper was still
blue under UV light (Fig. 14a).
2−
Using the same method, the B4O7 solution was
dropped on the test strip, and the color of the test paper
turned black under the UV light. When other competitive
metal ions drop on the test paper and the solution dis-
solves, the test paper is still blue-green under UV light
(Fig. 14b). The experimental results showed that the test
strip can be used for qualitative analyses of Cu2+ and
Fig. 13 a Time response of the fluorogenic sensor H2BS to recognize Cu2+.
b Time response of the fluorogenic sensor H2BS to recognize B4O7
2−
2−
B4O7 (Fig. 14c).

830
J Fluoresc (2021) 31:817–833
Fig. 14 Test strip color change (a) with various metal ions (b) anions and (c) Cu2+ and B4O72− response. (Under 365 nm UV light)
Funding This work was supported by the National Natural Science
Foundation of China (21761018), which is gratefully acknowledged.
Conclusions
A new bis(salamo)-based fluorogenic sensor H2BS based
Data Availability The data that supports the findings of this study are
on 2-hydroxy-1-naphthaldehyde was designed and synthe-
available in the supplementary material of this article.
sized. The fluorescence chemical sensor can form a 1:2
complex with copper ions, thereby, causing fluorescence
Declarations
quenching and quantitative identification of Cu2+. Further
exploration of the anion showed that the fluorogenic sen-
sor H2BS can recognize B4O72−, and due to the occur-
rence of the PET effect resulting in the deprotonation of
the phenolic hydroxyl group, the H3BO3 generated by the
Competing Interests The authors declare no competing financial
interests.
2−
hydrolysis of B4O7 will also aggregate and cause aggre-
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rescence change of H2BS can be used to quantitatively
detect Cu2+ in tap water. In order to promote practicality
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