A quinoline-based chromogenic and ratiometric fluorescent probe for selective detection of Mg2+ ion: Design, synthesis and its application in salt lake brines and bioimaging

Article abstract of DOI:10.1016/j.dyepig.2020.108896

A fluorescent probe was rational designed and prepared to distinguish Mg²⁺ ion from Ca²⁺ ion, in which 8-hydroxyquinoline acted as not only a fluorophore but also a recognition group. Notably, this probe QB (8-hydroxyquinoline-5-benzothiazole) shows two fluorescence response modes for highly selective detection of Mg²⁺ ion, namely fluorescence ratiometric mode and turn-on mode, which can be realized by controlling the excitation wavelength at 356 nm or 425 nm. After the addition of Mg²⁺ ion, the color of the QB solution changed from colorless to yellow, which can be easily found by naked eye. All experimental results suggested that probe QB has a high selectivity toward Mg²⁺ ion in the presence of other cations. Its detection limit for Mg²⁺ ion was estimated as low as 0.142 μM, and this value was far lower than the intracellular concentration (0.5–1.2 mM). The detection mechanism was proposed further by the experiment of ¹H NMR titration and theoretical calculation. More significantly, this probe was successfully used to detect Mg²⁺ ion in brine samples as a quantitative method, and was also applied to detecting and imaging Mg²⁺ ion in living cells, indicating its great application value in practical use for the detection of Mg²⁺ ion.

Full text of DOI:10.1016/j.dyepig.2020.108896

Dyes and Pigments 185 (2021) 108896
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
A quinoline-based chromogenic and ratiometric fluorescent probe for
selective detection of Mg2+ ion: Design, synthesis and its application in salt
lake brines and bioimaging
,
b
,
d
,
,
b
,
,
,b,d
Zhen-Hai Fua
*, Jing-Can Qina b, Ya-Wen Wangc **, Yu Pengc, Yong-Ming Zhanga
,
Dong-Mei Zhaoa d, Zhi-Hong Zhanga
,
,b,d
a Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining, 810008,
People’s Republic of China
b Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province, Xining, 810008, People’s Republic of China
c School of Life Science and Engineering, Southwest Jiaotong University, Chengdu, 610031, People’s Republic of China
d Innovation Academy for Green Manufacture, Chinese Academy of Sciences, Beijing, 100049, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
A fluorescent probe was rational designed and prepared to distinguish Mg2+ ion from Ca2+ ion, in which 8-
hydroxyquinoline acted as not only a fluorophore but also a recognition group. Notably, this probe QB (8-
hydroxyquinoline-5-benzothiazole) shows two fluorescence response modes for highly selective detection of
Mg2+ ion, namely fluorescence ratiometric mode and turn-on mode, which can be realized by controlling the
excitation wavelength at 356 nm or 425 nm. After the addition of Mg2+ ion, the color of the QB solution changed
from colorless to yellow, which can be easily found by naked eye. All experimental results suggested that probe
QB has a high selectivity toward Mg2+ ion in the presence of other cations. Its detection limit for Mg2+ ion was
Keywords:
Ratiometric fluorescence
Fluorescent probe
Quinoline
Mg2+ion
Salt lake brines
estimated as low as 0.142 μM, and this value was far lower than the intracellular concentration (0.5–1.2 mM).
The detection mechanism was proposed further by the experiment of 1H NMR titration and theoretical calcu-
lation. More significantly, this probe was successfully used to detect Mg2+ ion in brine samples as a quantitative
method, and was also applied to detecting and imaging Mg2+ ion in living cells, indicating its great application
value in practical use for the detection of Mg2+ ion.
1. Introduction
currently the most effective way to utilize salt lake resources, and the
evaporation rate is mainly associated with the concentration of Mg2+ ion
As one of the most important divalent cations in living beings and
environments, Mg2+ ion is not only involved in numerous physiological
activities such as cell membrane stabilization, adenosine triphosphate
(ATP) utilization, stabilization of nucleotides, and cell proliferation, but
also contributes to the productivity and quality of agriculture as a
nutrient element [1–5]. Abnormal levels of Mg2+ ion can lead to various
diseases, such as hypocalcaemia, hypermagnesemia, metabolic syn-
drome, hypertension, nausea, diarrhea, Alzheimer’s disease and so on
[6–11]. Significantly, Mg2+ ion is widely distributed in seawater, rivers
and salt lakes, causing it to naturally spread throughout the environ-
ment. Specially in salt chemical industry, evaporative crystallization is
[12]. In addition, the presence of Mg2+ ion also will increase the diffi-
culty to extract lithium from the salt lake brines due to their similar ionic
radii and chemical properties [13,14]. Therefore, it is highly desirable to
detect the presence of Mg2+ ion in aqueous solution with reliable and
efficient way.
In view of its importance, the detection of Mg2+ ion has attracted
increasing attention in the past few years. A series of analytical tech-
niques for detecting it have appeared, including atomic absorption
spectrometry with flame (FAAS), inductively coupled plasma optical
emission spectroscopy (ICP-OES), titration techniques, optical tech-
niques and so on [15–17]. Among them, the method using a fluorescent
* Corresponding author. Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lakes, Chinese Academy
of Sciences, Xining, 810008, People’s Republic of China.
** Corresponding author.
E-mail addresses: fzh@isl.ac.cn (Z.-H. Fu), ywwang@swjtu.edu.cn (Y.-W. Wang).
https://doi.org/10.1016/j.dyepig.2020.108896
Received 30 August 2020; Received in revised form 25 September 2020; Accepted 27 September 2020
Available online 1 October 2020
0143-7208/© 2020 Elsevier Ltd. All rights reserved.

Z.-H. Fu et al.
Dyes and Pigments 185 (2021) 108896
probe became popular because of the advantages such as high reli-
ability, real-time application, simplicity, low detection limits, low cost
and the ability to be applied in real samples [18].
added dropwise while the reaction temperature was maintained at
75 ◦C. The reaction mixture was further stirred for 10 h at 70 ◦C, and
then cooled to room temperature. The pH value was adjusted to about
7.0 with HCl (1 M), and the khaki precipitate appeared. This solid was
filtered and purified by flash column chromatography (PET/EtOAc =
5:1) on silica gel to afford the crude 8-hydroxyquinoline-5-carbalde-
hyde, which could be recrystallized in acetone to give pure sample
So far, there were limited reports about the fluorescent probe for the
detection of Mg2+ ion compared to other cations. Even though some
fluorescent probes have been developed for detecting Mg2+ ion by using
compounds such as the derivatives of β-diketone [19–21], carboxylic
acid [22,23], schiff base [24–33], crown ether [34,35], salicylalde-hyde
[36,37], 8-hydroxyquinoline [38], porphyrin [39] and calix[4]arene
[40]. As summarized in Table S1, compounds based on schiff base or
crown ether can selectively detect Mg2+ ion in organic solvents such as
ethanol, acetonitrile, isopropyl alcohol, THF and DMSO [24–32,34,35].
And the selectivity toward Mg2+ ion is easily interfered by Ca2+ ion
[29–31,34,35]. In contrast, other derivatives of salicylaldehyde,
8-hydroxyquinoline, β-diketone or carboxylic acid can exhibit higher
selectivity toward Mg2+ ion than Ca2+ ion in aqueous solution, but their
detection limits for Mg2+ are not presented or not low enough [19–23,
37]. Moreover, it should be noted that these probes are all based on
chelation-enhanced fluorescence (CHEF) and show fluorescence turn-on
response to Mg2+ ion by using fluorescence changes at only one wave-
length, which is usually problematic for practical use. To the best of our
knowledge, there were several ratiometric fluorescent probes for Mg2+
ion [24,39,40], but they have also some disadvantages, such as the lack
of detailed information for sensing Mg2+ ion, complicated structure, no
comparison with Ca2+ ion or high detection limit. Consequently, the
development of probes with multi-output fluorescence signals special
for Mg2+ ion remains a challenge.
(3.57 g, 20%) as a pink crystal solid. m.p.: 174.7–175.9 C. 1H NMR
◦
(DMSO‑d6, 400 MHz) δ = 11.17 (s, 1H), 10.13 (s, 1H), 9.55 (dd, J = 8.4,
1.6 Hz, 1H), 8.97 (dd, J = 4.0,1.6 Hz, 1H), 8.16 (d, J = 8.4 Hz, 1H), 7.78
(dd, J = 8.8, 4.4 Hz, 1H), 7.25 (d, J = 8.0 Hz, 1H) ppm. 13C NMR
(DMSO‑d6, 100 MHz) δ = 192.2, 159.6, 148.9, 140.3, 138.0, 133.1,
126.8, 124.6, 122.4, 110.9 ppm. ESIꢀ MS: m/z 174.1 [M + H]+.
2.2. Synthesis of probe QB (8-hydroxyquinoline-5-benzothiazole)
QB was synthesized according to a previous reported method with
minor modifications [43]. HCl (37%, 1.0 mL, 12.0 mmol) and H2O2
(30%, 2.4 mL, 24.0 mmol) were added to a solution of 2-aminothiophe-
nol (0.64 mL, 6.0 mmol) and 8-hydroxyquinoline-5-carbaldehyde (693
mg, 4.0 mmol) in EtOH (30 mL). The resulting mixture was stirred at r.t.
for 90 min, and then quenched by 300 mL H2O. The precipitate was
filtered, dried under vacuum and recrystallized from CH2Cl2-EtOH to
afford the desired product (620 mg, 56% yield). m.p.: 180.9–182.0 ◦C.
1H NMR (DMSO‑d6, 400 MHz) δ = 10.68 (s, 1H), 9.72 (d, J = 8.8 Hz,
1H), 8.98 (d, J = 4.0 Hz, 1H), 8.08–8.17 (m, 3H), 7.77 (dd, J = 8.8, 4.4
Hz, 1H), 7.57 (t, J = 8.0 Hz, 1H), 7.48 (t, J = 8.0 Hz, 1H), 7.24 (d, J =
8.0 Hz, 1H) ppm. 13C NMR (DMSO‑d6, 100 MHz) δ = 166.9, 156.3,
153.8, 148.7, 138.5, 134.6, 133.9, 132.0, 126.5, 126.2, 125.4, 123.4,
122.8, 121.9, 119.8, 111.2 ppm. ESIꢀ MS: m/z 279.1 [M + H]+.
In connection with our continuing research of chromogenic and
ratiometric fluorescent probe for the detection of different analytes [41],
herein, we report a new quinoline-based fluorescent probe QB (Fig. 1),
in which 8-hydroxyquinoline served as not only a fluorophore but also a
recognition group. By modification with a benzothiazole group, the
weak luminescence properties of 8-hydroxyquinoline caused by the PPT
(Intermolecular Photoinduced Proton Transfer) process can be effec-
tively improved. After the formation of complex between QB and Mg2+
ion, the PPT process was inhibited and the detection of Mg2+ ion with
fluorescence ratiometric changes can be realized. Finally, the rapid, high
selective, chromogenic and ratiometric detection of Mg2+ ion over Ca2+
ion was successfully achieved in an aqueous solution.
2.3. Synthesis of compound NB (4-benzothiazole-1-naphthol)
HCl (37%, 0.25 mL, 3.0 mmol) and H2O2 (30%, 0.6 mL, 6.0 mmol)
were added to a solution of 2-aminothiophenol (0.32 mL, 3.0 mmol) and
4-hydroxy-1-naphthaldehyde (344 mg, 2.0 mmol) in EtOH (15 mL), The
resulting mixture was stirred at r.t. for 90 min, then the solution was
added to 100 mL H2O, and followed by extraction with EtOAc (40 mL ×
3). The combined organic phase was dried by anhydrous Na2SO4. The
solvent was removed by evaporation, and the residue was purified by
flash column chromatography (petroleum ether/EtOAc = 10:0.5–10:1)
on silica gel to afford NB as the desired solid (391.6 mg, 70.6% yield). m.
2. Experimental
1
◦
Detailed information of chemicals and materials, spectra measure-
ment could be found in the Supplementary data.
p.: 210.3–211.8 C. H NMR (DMSO‑d6, 400 MHz) δ = 11.04 (s, 1H),
9.20 (d, J = 8.4 Hz, 1H), 8.29 (d, J = 8.0 Hz, 1H), 8.12 (t, J = 8.0 Hz,
2H), 7.95 (d, J = 8.4 Hz, 1H), 7.66–7.70 (m, 1H), 7.53–7.60 (m, 2H),
7.44–7.48 (m, 1H), 7.04 (d, J = 8.0 Hz, 1H) ppm. 13C NMR (DMSO‑d6,
100 MHz) δ = 167.9, 156.4, 153.9, 134.2, 131.6, 131.3, 128.1, 126.4,
125.6, 125.5, 125.2, 124.7, 122.7, 122.6, 121.8, 120.5, 107.9 ppm.
ESIꢀ MS: m/z 278.1 [M + H]+.
2.1. Synthesis of 8-hydroxyquinoline-5-carbaldehyde (Q-5-CHO)
This procedure was adapted from a known literature with minor
changes [42]. 8-hydroxyquinoline (14.5 g, 0.1 mol) and EtOH (70 mL)
were added to a 500 mL three-neck round-bottom flask at room tem-
perature. The resulting mixture was stirred for 10 min after the addition
of a mixture of NaOH aqueous solution (100 g, 37%). The color changed
to yellow along with the formation of some solid products, then dis-
solved the solid by heating to 110 ◦C in an oil bath. CHCl3 (20 mL) was
2.4. Cell cytotoxicity assay
The toxicity of probe QB in Hela cells was evaluated by a traditional
CCK-8 assay. HeLa cells (5 × 103) were first seeded into a 96-well plate,
Fig. 1. Our fluorescent probe for Mg2+ ion.
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Z.-H. Fu et al.
Dyes and Pigments 185 (2021) 108896
then various of probe QB (0, 2.0, 5.0, 10.0, 15.0, 20.0
μ
M) was added
3.2. UV–vis and fluorescence response of probe QB toward Mg2+ ion
and incubated at 37 ◦C in a final volume of 100
μL. Kept this condition
for 24 h, CKK-8 (10
μ
L) was added to every well and continue to be
As a ratiometric fluorescence probe, the fluorescence characteristic
of QB was initially investigated in different solvents and the results were
shown in Fig. S12. After excited at 356 nm, QB showed a strong fluo-
rescence peak at 450 nm in dimethyl sulfoxide (DMSO), isopropanol,
ethanol, N, N-dimethylformamide and methanol. The intensity of the
emission peak was different in different solvent, which should be
attributed to the disruption of intramolecular hydrogen bond in these
solvents [45]. Notably, a fluorescence peak at 534 nm was also observed
in N, N-dimethylformamide, acetonitrile and acetone, which happened
presumably due to its dimer form linked by the intramolecular hydrogen
bond N … ∙H‒O [46], which inhabited the PPT process, resulting in a
longer maximal emission wavelength. This result was also can be sup-
ported by the control substrate NB, which showed only one emission
peak at about 450 nm in all above solvents (Fig. S13). Considering the
practical application, the effect of water content on the fluorescence
intensity of QB at 450 nm was further investigated. As shown in Fig. S14,
with the increase of water, the fluorescence intensity decreased gradu-
ally. To make sure that QB can show a good ratiometric fluorescence
change for the detection of analyte, a mixture of DMSO and HEPES
(4-(2-hydroxyethyl)-1-piperazinethanesulfonic acid) buffer (9:1 (v/v))
was selected. Meanwhile, QB showed good fluorescence response to
Mg2+ ion in the pH range from 6.0 to 9.0 (Fig. S15). Therefore, the
subsequent experiments were carried out in DMSO-HEPES buffer (pH =
7.0, 9:1 (v/v)).
incubated for another 3 h at 37 ◦C. The absorbance at 450 nm was
performed on a Bio-Tek Synergy HT Multi-Mode Microplate Reader.
2.5. Cell culture and imaging
HeLa cells in the experiment were cultured in DMEM (10% FBS, 100
units/mL penicillin and 100 units/mL streptomycin) under an atmo-
sphere of 5% CO2 and 95% air at 37 ◦C. The cells were seeded at a
density of 3 × 105 and incubated at 37 ◦C for 24 h. Before the experi-
ment, the culture medium was replaced with new DMEM. First, the cells
were incubated with 0.4 mM, 4.0 mM, and 8.0 mM Mg2+ for 1h,
respectively. Then the cells were washed three times by using PBS buffer
solution to remove excess Mg2+ present in the extracellular medium,
and the culture medium was subsequently replaced with new DMEM.
Finally, these cells were cultured in the presence of probe QB (10.0 μM)
for further 1 h, then the cells were washed three times using PBS buffer
solution. The bright filed and fluorescence images were obtained using a
Leica-SP5 confocal microscope (λex = 405 nm, λem = 500–580 nm), the
temperature of cells was kept at 37 ◦C with a Tokai Hit-GSI controller.
3. Results and discussion
3.1. Design and syntheses of probes
Consequently, the absorption spectra of probe QB were carried out in
DMSO-HEPES buffer (pH = 7.0, 9:1 (v/v)). As demonstrated in Fig. 2,
QB showed two absorption peaks at about 290 nm and 356 nm,
respectively. With the increase in amount of Mg2+ ion, the intensity of
absorption peak at 290 nm decreased gradually, whereas two new ab-
sorption peaks appeared and increased simultaneously at 245 nm and
425 nm, along with the changes in appearance color from colorless to
yellow, which can be observed by naked eye (Fig. S16). These results
indicated that a new complex was formed between probe QB and Mg2+
ion.
As mentioned in our previous work [41], one characteristic of a vi-
sual and ratiometric fluorescent probe should be able to emit fluores-
cence at a different wavelength after the addition of analyte. It is well
known that 8-hydroxyquinoline are usually poor luminescent due to its
PPT (Intermolecular Photoinduced Proton Transfer) process between
the hydroxyl group and the nitrogen group [44]. To solve this problem,
aldehyde group was introduced to 8-hydroxyquinoline by using a
Reimer-Tiemann reaction, which can be further used to prepare probe
bearing benzothiazole group.
Scheme 1 shows the route for the synthesis of probe QB. 8-hydroxy-
quinoline-5-carbaldehyde was first prepared, and then the reaction of 8-
hydroxyquinoline-5-carbaldehyde with 2-aminothiophenol resulted in
the formation of probe QB. For a control experiment, we also prepared a
compound NB by the similar way, in which 4-hydroxy-1-naphthalde-
hyde was used instead of 8-hydroxyquinoline-5-carbaldehyde. The sin-
gle crystal structure of probe QB was obtained from DMSO solution
(Scheme 1). All compounds were characterized by 1H, 13C NMR, melting
point, IR and mass spectra (Figs. S1–S11, see the supporting
information).
To further investigate the interrelation between probe QB and Mg2+
ion, the fluorescence spectra response of probe QB to Mg2+ ion was
performed by using two excitation wavelengths at 356 nm and 425 nm,
respectively. As illustrated in Fig. 3a and Fig. S17, under the excitation
at 356 nm, the solution of QB exhibited blue fluorescence, and a rela-
tively stable fluorescence emission peak was observed at 450 nm, which
can be attributed to the enol form of QB. Notably, with the treatment of
Mg2+ ion, the fluorescence intensity at 450 nm disappeared, and a new
fluorescence peak was observed at 534 nm accompanied by an
appearance of yellow-green fluorescence, which ascribed to the fluo-
rescence emission of complex between probe QB and Mg2+ ion. In
Scheme 1. Syntheses of probes.
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Z.-H. Fu et al.
Dyes and Pigments 185 (2021) 108896
QB successfully served as a ratiometric fluorescent probe toward Mg2+
ion, which is superior to the reported probes with only one fluorescence
emission peak (Table S1).
Separately, probe QB was no fluorescent when excited at a long
wavelength of 425 nm, and an obvious fluorescence enhancement
appeared at about 534 nm upon the addition of Mg2+ ion. As depicted in
Fig. 3b, with the addition of Mg2+ ion to the solution of QB, the fluo-
rescence intensity at 534 nm increased gradually and reached to the
maximum value when 4.0 mM of Mg2+ ion was added. There was a good
linear relationship between the fluorescence intensity of 534 nm and the
amount of Mg2+ ion in the range from 0 to 25.0
μ
M. And the corre-
M based on the
sponding detection limit was estimated as low as 0.142
μ
equation DL = 3σ/k (S/N = 3) (Fig. S19) [47], which is superior to some
reported results (Table S1). The rapid response time also demonstrated
that probe QB is highly sensitive to Mg2+ ion (Fig. S20). The Job’s plot
revealed the formation of a 2:1 stoichiometric complex between QB and
Mg2+ ion (Fig. S21) [48]. Apparently, these changes in the fluorescence
spectra showed that probe QB can be used to detect Mg2+ ion in aqueous
solution through not only a ratiometric mode but also a fluorescence
“off-on” mode.
Fig. 2. UV–vis spectra of probe QB (10.0
μ
M) with Mg2+ ion (0–4.0 mM) in
DMSO-HEPES buffer (pH = 7.0, 9:1 (v/v)).
3.3. Fluorescence selectivity of probe QB
To further understand the selectivity of probe QB toward Mg2+ ion,
the fluorescence responses of QB (10.0 μM) toward different metal ions
were performed accordingly. As illustrated in Fig. 4, with the addition of
metal cations such as Li+, Na+, K+, Rb+, Cs+, Mg2+, Ca2+, Ba2+, Sr2+
,
Ni2+ and Mn2+, the fluorescence intensity at 450 nm decreased and a
new emission peak appeared at 534 nm only upon the addition of Mg2+
ion when excited at 356 nm. While the excitation wavelength was
changed to 425 nm, the significant fluorescence enhancement was
observed only in the presence of Mg2+ ion. However, the fluorescence
change was also observed in the presence of some ions such as Al3+
,
Zn2+and Cd2+, which indicated that the detection of Mg2+ by probe QB
should be performed in the absence of these ions (Fig. S22). Specifically,
probe QB almost showed no response toward Ca2+ ion.
Next, the competitive experiments in the presence of other tested
metal ions were carried out to reveal the influence of other metal ions on
the interaction between probe QB and Mg2+ ion. As shown in Fig. 4c and
d, upon the addition of Mg2+ ion into the solution of QB and other
interfering metal ions such as Li+, Na+, K+, Rb+, Cs2+, Ca2+, Ba2+, Sr2+
,
Ni2+ and Mn2+, negligible disturbance was observed when excited by
356 nm or 425 nm, which indicated that these coexistent metal ions had
no remarkable influence in the detection of Mg2+ ion. And QB could act
as a good ratiometric and turn on fluorescent probe for the detection of
Mg2+ ion in aqueous solution. Meanwhile, probe QB is highly selective
toward Mg2+ ion over the above metal ions, especially Ca2+ ion.
3.4. Proposed sensing mechanism
1
In order to confirm the binding modes between QB and Mg2+, H
NMR spectra were employed to explain the behavior of QB in the
presence of Mg2+ ion. As shown in Fig. 5, Ha and H1 are the proton
signals of moiety in QB. With the addition of Mg2+ ion to the system,
their signals shifted downfield clearly. And Ha disappeared in the pres-
ence of adequate Mg2+ ion, which indicated that the hydroxyl group is
probably involved in the formation of complex between QB and Mg2+
ion. Moreover, the control substrate NB is insensitive to Mg2+ ion in
fluorescence spectra (Fig. S23). By comparing their molecular struc-
tures, Mg2+ ion should coordinate with ‒OH and ‒N groups in QB.
Fig. 3. Fluorescence spectral changes of probe QB (10.0 μM) with the addition
of Mg2+ ion (0–24.0 mM) in DMSO-HEPES buffer (pH = 7.0, 9:1 (v/v)). (a)
Inset: Fluorescence intensity of I534 and I450 changes upon the addition of Mg2+
ion (λex = 356 nm). (b) Inset: Fluorescence intensity changes at 534 nm upon
the addition of Mg2+ ion (λex = 425 nm).
3.5. Theoretical calculations
addition, there was a good linear relationship between the ratio of
fluorescence intensity at 534 nm/450 nm and Mg2+ concentration
For a better understanding of the behavior of QB toward Mg2+ ion,
DFT (density functional theory) studies were performed for optimizing
the structures by using the Gaussian 09 program with B3LYP/6-311G(d)
(0–100.0
μM), the detection limit was calculated to be 0.439 μM based
on the equation DL = 3σ/k (S/N = 3) (Fig. S18) [47]. Therefore, probe
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Z.-H. Fu et al.
Dyes and Pigments 185 (2021) 108896
Fig. 4. Fluorescence responses of probe QB (10.0
μ
M) toward various metal ions: (a) λex = 356 nm; (b)
λex = 425 nm. Selectivity of probe QB (10.0 μM): (c)
λex = 356 nm; (d) λex = 425 nm. The black bars
represent the emission intensity of probe QB in the
presence of other ions; the red bars represent the
emission intensity that occurs upon the subsequent
addition of 4.0 mM Mg2+ ion to the above solution.
From 1 to 11: only QB, 4.0 mM for Li+, Na+, K+, Rb+,
Cs+, Ca2+, Ba2+, Sr2+, 10.0
μ
M for Ni2+ and Mn2+
.
(For interpretation of the references to color in this
figure legend, the reader is referred to the Web
version of this article.)
Fig. 5. Partial 1H NMR spectra (400 MHz) of QB with Mg2+ ion in DMSO‑d6.
basis set [49]. As demonstrated in Fig. 6, for free QB, it is electron rich
and the electron densities of both HOMOs and LUMOs are mainly
distributed at the whole skeleton, which indicated that probe itself can
exhibit fluorescence. However, after the formation of a 2:1 stoichio-
metric complex between QB and Mg2+ ion, the electron density on the
LUMO transferred from the benzothiazole moiety to quinoline of the
complex, and mainly concentrated on around Mg2+ ion, which was
attributed to a LMCT process. Consequently, the complex showed a
strong fluorescence at a longer excited wavelength due to the smaller
energy gap of 2QB + Mg2+ system.
3.6. Analysis of Mg2+ ion in brine samples
Since QB exhibited good sensibility and selectivity toward Mg2+ ion
in aqueous solution, it was used as probe to evaluate Mg2+ ion in real
samples. It is well known that the salt lake brine is mainly composed of
alkali metal and alkaline earth metal ions. Consequently, the determi-
nation of Mg2+ ion by using QB was carried out in different source of
brines, such as laguocuo salt lake, da chaidam salt lake and west taijinar
salt lake. As shown in Table 1, there were three methods for detecting
Mg2+ ion, namely titration with EDTA, ICP-OES and our method,
respectively. Method 1 and 2 are currently mainly used in practical
application [50]. By comparing the measured concentration of Mg2+
ion, our method is in good agreement with that of other two methods.
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Dyes and Pigments 185 (2021) 108896
3.7. Fluorescence imaging of Mg2+ in HeLa cells
To further show its ability to be applied in biology, the detection of
Mg2+ using probe QB was carried out in HeLa cells. Initially, a CKK-8
assay was employed to evaluate the cytotoxicity of probe QB in HeLa
cells, the cells was cultured in the presence of 0, 2.0, 5.0, 10.0, 15.0,
20.0 μM probe QB for 24 h. As demonstrated in Fig. S24, QB showed low
cytotoxicity to these cells and it could be used in biological fluorescent
imaging. First, HeLa cells was incubated with only QB for 1 h, and no
obvious fluorescence was observed due to the lack of DMSO in cell
culture medium, which also indicated that the effect of small amount of
Mg2+ ion in culture medium was negligible (Fig. S25). Next, HeLa cells
was incubated with 0.4 mM, 4.0 mM, and 8.0 mM of Mg2+ ion for 1 h,
respectively. As shown in Fig. 7, these cells almost showed no fluores-
cence, then probe QB (10.0 μM) was added and incubated for another 1
h, the yellow fluorescence was observed compared to the cells without
QB, and the fluorescence intensity increased with increasing concen-
tration of Mg2+ ion. Therefore, these results further suggested that probe
QB is permeable and can be applied in bioimaging for Mg2+ ion in living
cells.
4. Conclusions
In summary, a quinoline-based chromogenic and ratiometric probe
QB was rationally designed for the detection of Mg2+ ion. In the pres-
ence of tested metal ions, it exhibited high selectivity toward Mg2+ ion
over other tested cations. With the treatment of Mg2+ ion, fluorescence
ratiometric and turn-on responses were observed by using two different
excited wavelengths. And the detection limit was estimated as low as
Fig. 6. Optimized structures, HOMOꢀ LUMO energy levels, and the molecular
orbital plots of QB and 2QB + Mg2+
.
Table 1
Comparison of different methods for Mg2+ ion detection in brine samples (g/L).
0.142 μM. The color of QB solution changed from colorless to yellow,
which can be easily observed by naked eye. 1H NMR spectra and
computational studies further explained the origin of the fluorescence
changes. Finally, probe QB was successfully applied to detect Mg2+ ion
in brine samples as a quantitative method, and was also successfully
applied to detect and image Mg2+ ion in HeLa cells. We think this
strategy would be favorable for developing fluorescence probes for other
analytes.
Sample
Method 1
Method 2
Method 3
Sample 1
Sample 2
Sample 3
0.70
0.67
0.71
10.81
23.30
10.07
23.11
10.49
23.28
Samples: 1. Laguocuo salt lake brine; 2. Da chaidam salt lake brine; 3. West
taijinar salt lake brine.
Methods: 1. The concentration of Mg2+ ion was determined with a precision of
within ±0.3% in mass fraction by titration with an EDTA standard solution in
the presence of the indicator Eriochrome Black-T; 2. ICP-OES; 3. Our method.
CRediT authorship contribution statement
Therefore, the analysis results suggested that QB can be an excellent tool
to quantitatively detect Mg2+ ion content and have a good potential
application in practice.
Zhen-Hai Fu: Investigation, Methodology, Data curation, Writing -
original draft. Jing-Can Qin: Investigation, Software, Writing - review
& editing. Ya-Wen Wang: Supervision, Writing - review & editing. Yu
Peng: Supervision, Writing - review & editing. Yong-Ming Zhang: Su-
pervision, Project administration. Dong-Mei Zhao: Supervision, Project
administration. Zhi-Hong Zhang: Project administration.
Fig. 7. HeLa cell images: (a) bright field and (g) fluorescence images of HeLa cells with Mg2+ (0.4 mM) for 1h; (b) bright field and (h) fluorescence images of HeLa
cells incubated with Mg2+ (0.4 mM) for 1 h, and then incubated with probe QB (10.0
μ
M) for 1 h; (c) bright field and (i) fluorescence images of HeLa cells with Mg2+
(4.0 mM) for 1h; (d) bright field and (j) fluorescence images of HeLa cells incubated with Mg2+ (4.0 mM) for 1h, and then incubated with probe QB (10.0
μM) for 1 h;
(e) bright field and (k) fluorescence images of HeLa cells with Mg2+ (8.0 mM) for 1h; (f) bright field and (l) fluorescence images of HeLa cells incubated with Mg2+
(8.0 mM) for 1 h, and then incubated with probe QB (10.0
μ
M) for 1 h. Scale bar = 75 μm.
6

Z.-H. Fu et al.
Dyes and Pigments 185 (2021) 108896
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Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
[22] Matsui Y, Funato Y, Imamura H, Miki H, Mizukami S, Kikuchi K. Visualization of
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probe. Chem Sci 2017;8:8255–64.
[23] Komatsu H, Miki T, Citterio D, Kubota T, Shindo Y, Kitamura Y, Oka K, Suzuki K.
Single molecular multianalyte (Ca2+, Mg2+) fluorescent probe and applications to
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Acknowledgment
[24] Marimuthu P, Ramu A. A ratiometric fluorescence chemosensor for Mg2+ ion and
its live cell imaging. Sens Actuators, B 2018;266:384–91.
We thank the Natural Science Foundation of Qinghai Province (Grant
Nos. 2019-ZJ-954Q), the Youth Innovation Promotion Association CAS
(2018466, Y810081023) and the CAS “Light of West China” Program
(Y810041019).
[25] Xu J, Zheng W, Huang X, Cheng Y, Shen P. Selective fluorescent probe based on
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Appendix A. Supplementary data
[27] Wang G-Q, Qin J-C, Fan L, Li C-R, Yang Z-Y. A turn-on fluorescent sensor for highly
selective recognition of Mg2+ based on new Schiff’s base derivative. J Photochem
Photobiol A 2016;314:29–34.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2020.108896.
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