A newly synthesized visual bis(salamo)-based fluorescent chemosensor for exogenous detection of B4O72? in living cells and zebrafish

Article abstract of DOI:10.1016/j.saa.2020.119263

A newly synthesized fluorescent chemosensor H₆L was explored for detecting B₄O₇^2?, characterized by ¹H NMR spectrum, mass spectrum and fluorescence spectra. During the detection process of B₄O₇^2?, the fluorescence is significantly enhanced and naked eye recognition can be performed under 365 nm UV light without any interference by other typical anions. The limit of detection is as low as 6.97 × 10^-10 M. In addition, in order to broaden the application of salamo-based fluorescence sensors in the field of biology, except for the fluorescence imaging of HeLa cells, the first attempt of exogenous detection in zebrafish was conducted successfully.

Full text of DOI:10.1016/j.saa.2020.119263

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 249 (2021) 119263
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A newly synthesized visual bis(salamo)-based fluorescent chemosensor
for exogenous detection of B4O27À in living cells and zebrafish
b
b
b
a
c,
b,
Lu-Mei Pu a, , Lan Wang , Zhi-Li Wei , Zhuang-Zhuang Chen , Hai-Tao Long , Jia-Xi Ru , Xiu-Yan Dong
⇑
⇑
⇑
a College of Science, Gansu Agricultural University, Lanzhou, Gansu 730070, China
b School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu 730070, China
c State Key Laboratory of Veterinary Etiological Biology, Lanzhou Veterinary Research Institute, Chinese Academy of Agricultural Sciences, Lanzhou, Gansu 730046, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
H6L was explored for detecting
2À
B4O7
.
It can be used for detection in
zebrafish.
a r t i c l e i n f o
a b s t r a c t
A newly synthesized fluorescent chemosensor H6L was explored for detecting B4O27À, characterized by 1
H
Article history:
Received 24 August 2020
Received in revised form 19 November 2020
Accepted 22 November 2020
Available online 28 November 2020
NMR spectrum, mass spectrum and fluorescence spectra. During the detection process of B4O27À, the flu-
orescence is significantly enhanced and naked eye recognition can be performed under 365 nm UV light
without any interference by other typical anions. The limit of detection is as low as 6.97 Â 10-10 M. In
addition, in order to broaden the application of salamo-based fluorescence sensors in the field of biology,
except for the fluorescence imaging of HeLa cells, the first attempt of exogenous detection in zebrafish
was conducted successfully.
Keywords:
Fluorescent chemosensor
Deprotonation effect
Exogenous detection
Zebrafish cultivate
Ó 2020 Published by Elsevier B.V.
1. Introduction
element of the human body due to the slow metabolic process
in vivo. After entering the body, boron is converted into boric acid
It is well know that sodium borate makes considerable con-
tribute to modern industry, agriculture and medicine, for instance,
metallurgy, cosmetic preservative, pesticide, fertilizer and medical
disinfectant. Boron is mainly distributed in liver, kidney, brain,
bone and adipose tissue, especially in bone [1]. Although it plays
a vital role in the formation of ribonucleic acid [2], it is a limited
by the action of gastric acid and absorbed by the gastrointestinal
tract. Even if a handful of boron is ingested each time, it will cause
poisoned because of the slowly excreted. Research shows that an
excess of sodium borate can lead to a variety of symptoms [3], such
as anorexia, emesis and diarrhea. For these reason, it is highly
desired to fabricate a quick and easy method for efficient and con-
venient detecting the sodium borate in environment and bio-
systems.
⇑
Many typical ion detection method including spectrophotome-
try [4], chromatography [5], spectroscopy [6] have been applied in
Corresponding authors.
E-mail addresses: pulm@gsau.edu.cn (L.-M. Pu), rujiaxi@caas.cn (J.-X. Ru),
dxy568@163.com (X.-Y. Dong).
https://doi.org/10.1016/j.saa.2020.119263
1386-1425/Ó 2020 Published by Elsevier B.V.

Lu-Mei Pu, L. Wang, Zhi-Li Wei et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 249 (2021) 119263
the practical samples. Especially, the combination of fluorescent
chemosensor and visual detcetion have been booming due to they
have more remarkable advantages than the conventional ion
detection methods of economy, such as convenient operation, less
dosage, high sensitivity, straightforward, rapid equipment-free and
real-time monitoring [7]. However, it is a pity that rarely studies
have been reported on the detection of sodium borate by fluores-
cent chemosensor. Therefore it is high significant to explore a
new interference-free fluorescent chemosensor capable for detect-
ing sodium borate with outstanding efficiency and convenience.
It is known that salen-based ligand [8–10], formed by conden-
sation reaction of aldehyde and ammonia, have attracted many
attentions of researchers because of their remarkable application
on the occasions of ion recognitions [11–16], electrochemical pro-
cesses [17], catalysts [18,19], magnetic materials [20–22],
supramolecular structures [23–28], luminescence [29–32] and
the solvent through vacuum distillation. Then, the intermediate
was gained. Weighed 500 mg intermediate and dissolved it in
dichloromethane solution. Upon adding 1 mL boron tribromide,
the mixture was stirred for 4 h at ambient temperature. Washed
the crude product gained by quenching the reaction, extracting
the organic layer, removing the solvent to give the target product
H6L (390.5 mg) as a white powder. Yield: 74.8%. m.p.: 96–97 ℃.
Elemental analysis: Anal. Calc. for C26H26N4O10: C, 56.32; H, 4.73;
N, 10.10%. Found: C, 56.45; H, 4.71; N, 10.0%. 1H NMR (500 MHz,
DMSO d6): d = 4.40 (dd, J = 10.0, 5.0 Hz, 8H, –CH2), 6.67 (t,
J = 7.5 Hz, 2H, -ArH), 6.83 (dd, J = 10.0, 5.0 Hz, 2H, -ArH), 6.99
(dd, J = 10.0, 5.0, 2H, -ArH), 7.07 (s, 2H, -ArH), 8.42 (s, 2H, -
N = CH), 8.45 (s, 2H, -N = CH), 9.18 (s, 2H, –OH), 9.41 (s, 2H, –
OH), 9.60 (s, 2H, –OH).
2.3. Preparation of stock solution
biology [33]. As
a derivative of the salen-based ligand, the
Dissolving the tetrabutylammonium salt (1 Â 10-3 M) in EtOH/
H2O (v/v = 9:1) to prepare stock solution of BrÀ, the ammonium
perchlorate to prepare stock solution of ClOÀ4 and other anions
(FÀ, IÀ, ClÀ, H2PO4À, CNÀ, SiO32À, CH3COOÀ, B4O27À, HSÀ, S2À, NO3À,
CO23À, HCOÀ3 , HPOÀ4 , P2O47À and NOÀ2 ) were prepared from the corre-
sponding potassium salts or sodium salts. Simultaneously, stock
solution of sensor H6L was prepared by dissolving it in dehydrated
ethanol solution (5 Â 10-5 M).
salamo-based ligand [34–37] introduces two highly electronega-
tive oxygen atoms and forms an N2O2 coordination environment
[38–41]. Thus, the stability and compound structure diversity of
the molecule are superior to the former. In 2016, Dong et al. [42]
successfully achieved and reported at first time that salamo-
based ligands H3L as a chemosensor for recognizing the Zn2+/Cu2+
and H+/OHÀ in ethanol solution.
In this paper, we developed and synthesized a new salamo-
based chemosensor H6L which can identification B4O27À by fluores-
cence spectroscopy and visual observation in a satisfactory effi-
ciency without other typical anions interference. In addition, it
worth nothing that sensor H6L was firstly realized the imaging
exogenous in living cells and zebrafish larvae without cytotoxicity.
Therefore, we have reason to believe that the success of the zebra-
fish fluorescence imaging had a positive effect on proofing the
application of salamo-based fluorescence sensors in biological
filed.
2.4. Cytotoxic assay
The cytotoxicity of sensor H6L was determined using MTS assay
(CellTiter 96Ò AQueous One Solution Cell Proliferation Assay, Pro-
mega). Upon culturing in an atmosphere consisting of 5% CO2
and 95% air at 37 °C, Hela cells were seeded into 96-well culture
plates (1 Â 104 cells per well). After cell attachment, sensor H6L
upon different concentrations (0, 6.25, 12.5, 25, 50, 100 lM) were
added into cells and incubated for 24 h at 37 °C under 5% CO2. Then
10 L MTS stock solution was added to each well for another 3 h
2. Experimental section
l
under the same conditions. Finally, the absorbance at 490 nm
was measured using the microplate reader (ELX800 UV, BIO-TEK,
USA).
2.1. Materials and Measurements
Details of Materials and Measurements were list in Supporting
Information.
2.5. Living cells and zebrafish cultivate and imaging
2.2. Synthesis method of sensor H6L
Hela cells (CAAS, China) were grown in DMEM medium with
10% (v/v) heat-inactivated fetal bovine serum (FBS) as supplement
and were cultured at 37 °C in a 5% CO2 humidity atmosphere. Then,
Hela cells growed in log phase were divided in two groups and
both of group plated in 3.5 cm confocal petri dish and allowed to
adhere overnight. Afther treating them with 20 lM of the solution
of sensor H6L for 2 h, washing with PBS three times, imaged the
cells of first group immediately. The other group cells were intro-
1,2-Bis(aminooxy)ethane, 2-[O-(1-ethyloxyamide)]oxime-6-me
thoxyphenol and 2,3-dimethoxy-1,4-dicarbaldehyde were conve-
niently gained according to the early works [43–46]. Sensor H6L
was obtained by two-step reaction and the synthetic methods
are displayed in Scheme 1. Firstly, a dehydrated ethanol solution
containing 452.2 mg 2-[O-(1-ethyloxyamide)]oxime-6-methoxy
phenol (2 mmol) and 194.1 mg 2,3-dimethoxy-1,4-
dicarbaldehyde (1 mmol) was stirred at 60 ℃ for 8 h and removed
duced 100
lM Na2B4O7 for 0.5 h and conducted confocal fluores-
cence imaging.
Scheme 1. Synthetic methods of sensor H6L.
2

Lu-Mei Pu, L. Wang, Zhi-Li Wei et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 249 (2021) 119263
The experimental zebrafish larvae were cultured in E3 medium
(5 mM NaCl, 0.17 mM KCI,0.33 mM CaCl2,0.33 mM MgSO4; pH 7.2)
at 28.5 °C and alternately given a 14 h light/10 dark cycle. For the
other anions were negligible. As shown in Fig. 1(b), neither the
solution of sensor H6L nor the mixed solution of other ions and
H6L emitted light under 365 nm UV light. Only the mixed solution
of sensor H6L and B4O27À showed bright blue fluorescence. These
results indicated that sensor H6L had a high sensitivity only for
B4O27À and could realize naked-eye recognition of B4O27À under
UV light.
imaging, the ethanol solution containing H6L (10
lM) was intro-
duced to the E3 medium of the 4-day-old zebrafish larvae for
0.5 h and washed three times with culture solution. Then incu-
bated zebrafish larvae of B4O27À (100
lM) for 0.5 h. After washing
three times, the zebrafish larvae were exposed ethyl-3-
aminobenzoate methanesulfonate and recorded the imaging using
a fluorescence microscope (OLYMPUS IX71).
Then the competition experiment was carried out upon addi-
tion of different anions to evaluate the anti-interference capacity
of sensor H6L. It could be seen in the Fig. 2(a) and 2(b) that there
were a strong emission band by adding 10 equiv. of B4O27À in the
solution of sensor H6L. When other anions were added separately
in the mixture of sensor H6L and B4O27À, the fluorescent intensity
almost unchanged. Corresponding, Fig. 2(c) unambiguously dis-
played that the addition of other anions are ineffective in inter-
rupting the visual recognition of B4O27À under 365 nm UV light.
Thus, sensor H6L could be used as a new tool for specific detection
of B4O27À under the presence of other anions.
3. Results and discussion
A salamo-based fluorescent sensor H6L with three benzene ring
groups has been designed and synthesized. The fluorescence inten-
sity of sensor molecule was very weak since the higher rigidity and
electron cloud density of the benzene ring. [47–50]. 1H NMR spec-
trum and mass spectrum of sensor H6L are depicted in Fig. S1 and
Fig. S2.
3.2. Titration experiment of B4O72À
3.1. Fluorescent selectivity of B4O72À
To further understanding the quantitative relation between
sensor H6L and B4O27À, the titration experiment was established.
As illustrated in the Fig. 3, with the increase amount of B4O27À ion
in sensor H6L, the fluorescence intensity gradually redshift and
progressive enhanced together with obvious bright blue fluores-
cence. When the amount of B4O27À reached 3 equiv., the emission
was not increase which was proving that the solution of sensor
was saturated. This result revealed the stoichiometry for sensor
H6L and B4O27À was 1:3 [51,52].
Firstly, the fluorescence spectra of sensor H6L by adding 10
equiv. of different anions separately in the solution of EtOH/H2O
(v:v = 9:1) was measured to investigate the selectivity of sensor
H6L to various anions. As exhibited in Fig. 1(a), upon the excitation
of 363 nm, sensor H6L showed very weak fluorescence intensity
centered at 429 nm. Compared to other anions, only the adding
of B4O27À could induce a remarkable 17-fold emission enhancement
at 444 nm, while the changes of fluorescent intensity caused by
Fig. 1. (a) Fluorescence responses of sensor H6L (25
HCOÀ3 , HPO4À, P2O47À and NO2À); (b) Pictures of sensor H6L (25
l ,
M) toward 10 equiv. of various anions (FÀ, IÀ, ClÀ, BrÀ, ClOÀ4 , H2PO4À, CNÀ, SiO32À, CH3COOÀ, B4O72À, HSÀ, S2À, NO3À, CO23À
lM) toward 10 equiv. of various anions under 365 nm UV irradiation.
3

Lu-Mei Pu, L. Wang, Zhi-Li Wei et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 249 (2021) 119263
Fig. 2. Fluorescence intensity of sensor H6L solution (25
l
M) with various competing anions (10 equiv.) in the absent and present B4O27À. Black columns represent the
fluorescence intensity of sensor H6L and other ions without B4O27À; Red columns represent the fluorescence intensity of sensor H6L at increase of different anions and B4O27À
.
calculated to be 98% and 1.5, respectively. These results empha-
sized the high sensitivity of sensor H6L to B4O27À, which provided
a platform for the practical application of the sensor.
3.3. Time respond and pH effect on the recognition
To further study whether sensor H6L was a qualified sensor to
fulfill the applications in the environment and biology, we evalu-
ated the pH effect on the recognition and time respond tests. As
depicted in Fig. S5, the emission intensity of sensor H6L remained
in a stable state when the pH range of 5–10, while the strong flu-
orescence intensity of the solution could be clearly observed at pH
range of 5–11 upon addition of B4O27À. Therefore the suitable pH
range of the solution for the recognition was 5–10, which provided
the possible for the application in the biological system.
On the other hand, time respond of the recognition at ambient
temperature is displayed in Fig. S6. When added 10 equivalents of
B4O27À ions, after increased dramatically and get to the peak value
during 111.0 s, the fluorescence intensity slowly weakened until
the time reached 10 min and then remained stable. This phe-
Fig. 3. Fluorescent spectra Changes of sensor H6L (25 lM) upon gradually increase
B4O27À (0–3.0 equiv.). Inset: Color changes of the solution before and after increase of
CNÀ under 365 nm UV irradiation.
nomenon presented that sensor H6L has a rapid respond for B4O72À
.
3.4. Detection mechanism to B4O72À
Based on the above results, the binding constant and limit of
detection (LOD) toward target anion have been measured. The
binding constant was calculated to be 2.12 Â 1014 MÀ1 [53] accord-
ing to the linear fitting in Fig. S3. Furthermore, based on the rela-
tionship between fluorescence intensity and B4O27À concentration
showed in Fig. S4, the LOD value [54] was estimated to be
6.97 Â 10-10 M with a good linear relation (R = 0.984) in the con-
In order to explore the recognize mechanism between the sen-
sor and B4O27À, nuclear magnetic titration and mass spectrometry
were analyzed. As shown in Fig. 4(a), six hydrogen atoms on the
hydroxyl groups of sensor H6L totally disappeared with the 3
equivalents addition of B4O27À. Beyond that, upon alternately add-
ing H+ and OHÀ to the system of H6L + B4O27À, the fluorescence
intensity alternately enhanced or decreased (Fig. S7), proving that
centration range of 2-14
lM . The average recovery and R.S.D. were
4

Lu-Mei Pu, L. Wang, Zhi-Li Wei et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 249 (2021) 119263
Fig. 4. Detection mechanism of B4O27À by sensor H6L. (a) 1H NMR spectra at absent and present B4O27À; (b) The recognize mechanism of H6L to B4O27À
.
the recognition of B4O27À by H6L was reversible. According to the
experimental results and references [55,56], the recognize mecha-
nism is proposed and displayed in Fig. 4(b). Due to the PET effect,
the fluorescence of H6L was very weak. After adding borax, B4O72À
hydrolyzed in aqueous system and produced boric acid. Then boric
acid coordinated with the phenol oxygen atoms of H6L, which
inhibits the effect of PET, enhances the rigidity of the molecule,
and increases the fluorescence. In the mass spectrum of the
H6L + B4O27À system (Figure S8), a strong signal peak appeared at
m/z = 682.1322, which was corresponding to [H6L + 3B(OH)2-H+].
This result confirmed the formation of complex between H6L and
boric acid.
3.5. Exogenous detection of B4O27À in living cells and zebrafish
Based on the above result, we further performed the exoge-
nous detection for B4O27À in the in living cells and zebrafish
through fluorescence bio-imaging experiments. Initially, the cyto-
toxicity of sensor H6L was evaluated by the MTS assay and the
result indicated that cell viability hardly caused any effect even
5

Lu-Mei Pu, L. Wang, Zhi-Li Wei et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 249 (2021) 119263
at higher concentration (100
lM) of sensor (Fig. 5). These results
confirmed that sensor H6L could be used to exogenous detection
for B4O27À in biological system. The results of fluorescence confo-
cal imaging of HeLa cells are shown in Fig. 6. Cells cultured with
sensor H6L solution for 2 h at 37℃ exhibited no fluorescence and
continuously treated with B4O27À ion for 0.5 h, fluorescence turned
on obviously.
In view of developing the application in biological field and the
good fluorescence property in HeLa cells, we accomplished the flu-
orescence imaging in the 4-day-old zebrafish larvae. As displayed
in Fig. 7, zebrafish larvae reared with sensor H6L only upon the
excitation of 363 nm were non-fluorescent. Subsequently, incu-
bated the zebrafish with B4O27À for 0.5 h, a noteworthy bright blue
fluorescence along the head and entire spine of the zebrafish was
observed. These findings evidently demonstrated that was an
appropriate biological sensor with excellent cell membrane
permeability.
Fig. 5. Cytotoxic assay of sensor H6L.
Fig. 6. Fluorescent imaging of exogenous B4O27À in the living HeLa cells. (a) Fluorescence, bright-field and overlay imaging in HeLa cells incubated with sensor H6L (20
2 h; (b) Fluorescence, bright-field and overlay imaging in HeLa cells treated with sensor H6L (20.0 M) for 30 min.
M) and B4O27À (100
lM) for
l
l
Fig. 7. Fluorescent imaging of exogenous B4O27À in the zebrafish larvae. (a) Fluorescence, bright-field and overlay imaging in living zebrafish larvae incubated with sensor H6L
(10
l
M) for 4 h; (b) Fluorescence, bright-field and overlay imaging in living zebrafish larvae treated with sensor H6L (10.0 l lM) for 30 min.
M) and B4O27À (100
6

Lu-Mei Pu, L. Wang, Zhi-Li Wei et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 249 (2021) 119263
Table 1
Comparison of detection methods that recognize B4O72À.
No.
Sensor
Solvent
LOD (M)
Cell imaging
Zebrafish imaging
Reference
1
2
carbon dots (CDs)
aqueous solution
Tris-HCl buffer (DMF/H2O = 9:1, v/v, pH = 7)
1.5 Â 10-6
No
Yes
No
No
[6]
[56]
8.61 Â 10-7
3
EtOH/H2O (9:1 v/v)
6.97 Â 10-10
Yes
Yes
This work
4. Conclusions
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CRediT authorship contribution statement
Lu-Mei Pu: Project administration, Funding acquisition. Lan
Wang: Data curation, Formal analysis, Investigation, Methodology,
Project administration, Writing - original draft. Zhi-Li Wei: Data
curation, Formal analysis, Investigation, Methodology, Project
administration, Software. Zhuang-Zhuang Chen: Formal analysis,
Investigation. Hai-Tao Long: Software, Formal analysis. Jia-Xi Ru:
Supervision. Xiu-Yan Dong: Conceptualization, Funding acquisi-
tion, Resources, Supervision.
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The authors declare that they have no known competing finan-
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to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foun-
dation 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),
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Appendix A. Supplementary data
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