A targetable fluorescent probe for real-time monitoring of fluoride ions in mitochondria

Article abstract of DOI:10.1016/j.saa.2018.05.054

Fluorion are pivotal anions in biology because they play an important role in dental care, treating osteoporosis, preventing tooth decay and promoting the healthy growth of bone. Studies have shown that high levels of fluoride will lead to the inactivatio

Full text of DOI:10.1016/j.saa.2018.05.054

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 204 (2018) 777–782
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A targetable fluorescent probe for real-time monitoring of fluoride ions
in mitochondria
Kai Zhou, Mingguang Ren, Li Wang, Zihong Li, Weiying Lin ⁎
Institute of Fluorescent Probes for Biological Imaging, School of Chemistry and Chemical Engineering, School of Materials Science and Engineering, University of Jinan, Jinan, Shandong 250022,
PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Fluorion are pivotal anions in biology because they play an important role in dental care, treating osteoporosis,
preventing tooth decay and promoting the healthy growth of bone. Studies have shown that high levels of fluo-
ride will lead to the inactivation of the mitochondria. Therefore, it is urgent to develop a method to detect the
fluoride anions in the mitochondria. Herein, we have developed a novel mitochondrial-target fluorescent
probe for detecting F− in living cells. The probe exhibited excellent sensitivity and high selectivity for F over
the other relative species. With changing fluoride ions, the fluorescence spectrum of the probe changed signifi-
cantly with a large turn-on fluorescence signal. Cell imaging indicated that the probe can penetrate viable cell
membranes and rapidly detects and images fluorion over other anions in the mitochondria.
© 2018 Published by Elsevier B.V.
Received 28 March 2018
Received in revised form 6 May 2018
Accepted 15 May 2018
Available online xxxx
−
Keywords:
Fluorescent probe
Mitochondria
Fluoride ions
Fluorescence imaging
1
. Introduction
Fluoride ions, the smallest and most negative anions, are of particu-
applications. For example, some B-F complexing sensors are often sen-
sitive to oxygen and moisture and involve complex equilibria due to
the formation of various fluoroborate species. Furthermore, some
−
lar interest due to their important role in dental care and promoting the
healthy growth of bone [1–3]. However, excessive fluoride can cause
various diseases such as, metabolic disturbances, tooth mottling, skele-
tal fluorosis, immune damage, neurotoxicity, and kidney disease [4,5].
Additionally, NaF, as an effective activator of G protein and inhibitor
for Ser/Thr phosphatase, affects plenty of essential cell signaling ele-
ments [6]. At the cell organelles level, studies have shown that high
levels of fluoride can cause oxidative stress on mitochondria and reduce
mitochondrial respiratory chain efficiency, and leading to mitochondrial
dysfunction, which is an earliest event in most delayed neurodegenera-
tive diseases [7–10]. In order to fully elucidate the toxicity of fluoride
ions, it is very necessary to develop a sensitive, specific and accurate
method for the detection of its distribution and dynamic fluctuation in
biological systems. For traditional methods, including 19F NMR spec-
troscopy [11], ion chromatography [12] and ion-selective electrodes
desilylation F probes usually require an excess of F ions to reach satu-
ration and the response time required is often longer. Currently, fluores-
−
cent F probes based on H-bond interaction are more fascinating due to
their satisfactory response time and stable chemical property. Although
several fluoride ion fluorescence sensors based on F-induced deproton-
ation through H-bonding with protonic units have been reported
[27–31], rare of them can be used for the detection and imaging of
fluorinion levels in cellular mitochondria.
Herein, we designed and synthesized a new intramolecular
charge transfer (ICT) based, mitochondrial-targeted fluorescent probe
(Mito-FV) by linking imidazole (donor, D) and hemicyanine (acceptor,
A) through a phenyl ring. The sensor Mito-FV exhibited excellent sensi-
−
tivity and high selectivity for F over the other relative species and can
−
display strong turn-on (~204-fold) fluorescence in response to F . Fur-
thermore, Mito-FV was able to penetrate the membranes of living cells,
−
[
13], complicated pre-treatments of samples, relatively cumbersome
rapidly detect and image the F in mitochondria.
equipment, and professional operators are require ineluctably, which
hinders their application in the effective and convenient detection of
−
F . In the various analytical methods available, fluorescence sensing is
2. Experimental
regarded as an ideal technology due to its advantages of simplicity of
operation, high specificity and sensitivity [14–19]. Although significant
progresses have been made in the development of plenty fluorescent
fluoride ions probes [20–26], there are still some deficiencies in practical
2.1. Materials and Instruments
Unless otherwise noted, all reagents were obtained from commer-
cial suppliers, and used directly without further purification. Solvents
used in experiment were purified by standard methods prior to use.
The water used in all experiments refers to the twice-distilled water.
⁎
Corresponding author.
E-mail address: weiyinglin2013@163.com (W. Lin).
https://doi.org/10.1016/j.saa.2018.05.054
386-1425/© 2018 Published by Elsevier B.V.
1

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K. Zhou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 204 (2018) 777–782
Scheme 1. Synthesis route of probe Mito-FV.
+
+
The instruments used in this work were same as those in our previously
reported articles [32].
(ESI): m/z calculated for C35
(Scheme 2).
H
32
N
3
494.2591 [M] , found: 494.2592
2
.2. Synthesis of Mito-FV
3. Results and Discussion
The synthesis route of Mito-FV was outlined in Scheme 1. Com-
3.1. Design and Synthesis of Mito-FV
pound 1 and 2 were prepared referencing the reported literature
33,34]. The desired compound Mito-FV was readily obtained in one
simple step. Dissolved the crude product 1 (200 mg, 0.75 mmol,
.0 eq) and compound 2 (245 mg, 0.75 mmol, 1.0 eq) in 4 mL N,N-
dimethylformide (DMF), and the mixture was refluxed for 6 h at 90 °C
under a nitrogen atmosphere. After cooled to room temperature, the
mixture was poured into water and extracted with dichloromethane.
Subsequently, the organic layer was combined, washed with water
and brine, dried over sodium sulfate, filtered, and then concentrated
in vacuo. The residue was purified by column chromatography on silica
[
Imidazole derivatives were wildly applied in different areas due to
their favorable properties such as high stability, flexible photophysical
properties, high extinction coefficients, and ease synthesis [35–37].
Considering its susceptible to detect anions either by deprotonation of
potential\\NH fragment or H-bonding interaction, imidazole was se-
1
−
lected as the recognition site for F . Hemicyanine, as a strong electron
withdraw group, was widely applied to the synthesis of dyes and fluo-
rescent probes [38–41]. Moreover, hemicyanine can act as a mitochon-
drial targeting group and rapidly enter the mitochondria in cells in a
short time [42]. Thus, we constructed a mitochondra-target fluorescent
probe Mito-FV by linking carbazole and hemicyanine through the ben-
zene ring. The target sensor Mito-FV was readily synthesized in one
simple step. Treatment of compound 1 with the compound 2 in N,N-
dimethylformide afforded probe Mito-FV in good yield. The synthesis
route of Mito-FV was shown in Scheme 1, and the structure was fully
gel (petroleum ether to MeOH/DCM = 1/20, v/v) to afford the target
1
compound Mito-FV (210 mg purple powder, yield: 48%).
H
NMR (400 MHz, DMSO) δ 13.34 (s, 1H), 8.53 (d, J = 16.2 Hz, 1H), 8.42
d, J = 8.9 Hz, 2H), 8.39 (d, J = 8.8 Hz, 2H), 7.98 (dd, J = 5.2, 3.7 Hz,
(
1
7
H), 7.94–7.90 (m, 1H), 7.82 (d, J = 16.3 Hz, 1H), 7.67–7.62 (m, 2H),
.60–7.58 (m, 2H), 7.58–7.55 (m, 2H), 7.46 (t, J = 7.3 Hz, 2H), 7.41
1
13
(
4
(
d, J = 7.2 Hz, 1H), 7.34 (t, J = 7.4 Hz, 2H), 7.26 (t, J = 7.3 Hz, 1H),
.81 (q, J = 6.9 Hz, 2H), 1.84 (s, 6H), 1.48 (t, J = 7.2 Hz, 3H)·13C NMR
101 MHz, DMSO) δ 181.74 (s), 153.81 (s), 144.94 (s), 144.51 (s),
characterized by standard H NMR, C NMR and mass spectrometry
in Supporting information.
1
1
1
1
40.91 (s), 138.91 (s), 135.37 (s), 134.83 (s), 134.42 (s), 131.90 (s),
31.06 (s), 130.29 (s), 129.90 (s), 129.62 (s), 129.09 (d, J = 6.3 Hz),
28.73 (s), 128.52 (s), 127.75 (s), 127.32 (s), 126.02 (s), 123.61 (s),
15.65 (s), 112.64 (s), 52.75 (s), 42.72 (s), 26.09 (s), 14.35 (s). HRMS
3.2. Optical Response of Mito-FV to F−
With the probe Mito-FV in hand, we initially investigated its optical
−
responses to fluoride ions (TBAF as F donor) by absorption and
Scheme 2. Proposed response mechanism of Mito-FV to F−
.

K. Zhou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 204 (2018) 777–782
779
Fig. 1. (A) Fluorescence spectra of Mito-FV (10 μM) upon the addition of TBAF (0–40 eq) in CH
3
CN after 30 min at room temperature. (B) Fluorescence intensity at 495 nm as a function of
−
F
concentration. Inset: Linearity between the fluorescence intensity at 495 nm and TBAF concentration in the range of 0–100 μM. (λex = 370 nm; EX Slit: 5.0 nm; EM Slit: 5.0 nm; PMT
Voltage: 400 V).
emission spectroscopy. Firstly, we studied the absorption spectra of the
probe Mito-FV with changing the concentration of fluoride ions. As
(Fig. 1B), indicating that the probe Mito-FV can be utilized for the quan-
titative determination of fluoride ions.
shown in Fig. S1, the free sensor Mito-FV displayed two major absorp-
As there is a decrease at 554 nm in the absorption spectrum, so we
chose a long wavelength (530 nm) to excite the probe, induced a
weak fluorescence at 717 nm and found a little decrease of the intensity
−
tion band at 554 nm and 313 nm. However, with the addition of F
,
the absorption at 554 nm was decreased and the band at 313 nm was
increased with a 19 nm red-shift while there is a distinct isosbestic
point at 375 nm. An evident naked-eye color changes for the Mito-FV
solution treated with TBAF was observed from light brown to colorless
under ambient light and the fluorescence emission was changed from
colorless to bright blue under a UV lamp at 365 nm (Fig. S2). These re-
sults mentioned above indicated that Mito-FV can serve as a colorimet-
ric detector for the visual detection of fluoride ions.
−
in the presence of a small amount of F . However, the fluorescence in-
−
tensity was plotted as a function of the F concentration with a good
fitting coefficient for 0.98 (Fig. S4).
On account of the complexity of the intracellular environment, we
chose some of the tetrabutylammonium (TBA) salts as the reference
to carry out a controlled trial in acetonitrile. The probe Mito-FV was
−
−
−
−
−
treated with various analytes such as NO
3
, H
2 4 4
PO , OAc , HSO , I ,
−
−
Subsequently, we studied the emission spectral of Mito-FV with the
addition of TBAF. As shown in Fig. 1A, Mito-FV exhibits weak emission
Br and Cl from TBA salts respectively. As shown in Fig. 2, none but
can efficiently exhibit fluorescence emission and the fluorescence in-
tensity of Mito-FV in the presence of F achieved about 26.5 times than
−
F
−
(
φ = 0.0043) when excited at 370 nm in CH
3
CN. However, upon grad-
−
−
ual addition of F , the corresponding emission intensity of Mito-FV was
that of other anions. Mito-FV showed a strong response toward F and
enhanced accompanied a red-shift and the maximum emission wave-
length was shift to 495 nm with the concentration of F increased to
negligible responses toward other anions. As seen in Fig. 2A, the spec-
trum of Mito-FV was more susceptible to F than these tetrabutyl
−
−
1
0 μM. With the continued addition of fluoride, the corresponding emis-
salts. Therefore, Mito-FV seems to be useful for the selective detection
of F even with these relevant analytes.
−
sion intensity of Mito-FV enhanced significantly and showed a quan-
tum yield of 0.2797, approximately 65-fold enhancement of the
quantum yield with 40 eq of fluoride added, meanwhile, the fluores-
cence intensity at 495 nm was increased about 204-fold. A good linear
correlation (R = 0.99) existed between the fluorescence intensity
and the concentration of TBAF within the range from 0 to 100 μM
The kinetic profiles of the probe Mito-FV (10 μM) treated with the
−
varied concentrations of F (0, 10 eq, 20 eq, 40 eq) were shown in
−
Fig. 3. The probe Mito-FV untreated with F had almost no change in
2
the fluorescence intensity at 495 nm. However, a fluorescence enhance-
−
ment was observed when incubated with F , and the signal reached an
3
Fig. 2. (A) Fluorescence spectra and (B) fluorescence intensity at 495 nm of Mito-FV (10 μM) treated with various species in CH CN solution after 30 min at room temperature (Concen-
tration: TBAF, 100 μΜ; the other species: 200 μM).

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K. Zhou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 204 (2018) 777–782
acetonitrile solution, the probe Mito-FV seems to be useful for detecting
fluoride ions in real biological samples. The standard MTT assays was
carried out firstly, the result indicates that the probe Mito-FV at low mi-
cromolar concentrations have no marked cytotoxicity to the cells after a
long period (N85% HeLa cells survived after 24 h incubation with 10 μM
probe) (Fig. S6), implying that the new probe Mito-FV is less toxic to
cells and can be used for cell imaging.
We evaluated the probe Mito-FV imaging fluoride ions in live
cells, and the fluorescence imaging experiments were performed in
living cells (HeLa cells) on confocal laser scanning microscopy. We
incubated the HeLa cells with probe Mito-FV (10 μM) for 10 min at
3
2
7 °C under 5% CO atmosphere, then washed the cells with a phos-
phate buffered saline (PBS) solution two times, and treated the cells
with or without fluoride (100 μM) for 15 min and again washed
twice with the PBS solution. When HeLa cells incubated only with
Mito-FV for 15 min, no detectable fluorescence was observed. Con-
trastively, when pre-treated with Mito-FV for 10 min followed by
the incubation of TBAF (100 μM) solution for another 15 min, strong
Fig. 3. The fluorescence intensities at 495 nm of Mito-FV (10 μΜ) as a function of time
upon addition of different concentration of TBAF (20, 100, 200, 400 μM) at room temper-
ature for continuously monitored in CH CN with the excitation at 370 nm.
3
fluorescence was observed in the blue channel (Fig. 4e; λex
=
4
05 nm; λem = 425–475 nm) under the same test conditions,
confirming that Mito-FV possesses good membrane permeability
and could image fluoride ions in cellular environment.
equilibrium state after about 5–10 min. This superior performance of
Mito-FV indicated that it may be used as a real-time indicator for F
detection.
Photostability is one of the most important parameters of fluores-
cent dyes and probes. Hence, we investigated the photostability of the
compound Mito-FV under UV light conditions (Fig. S5). Photostability
studies suggested that the probe was highly stable, even under excita-
tion using a short wavelength of UV light for 60 min.
To further examine the subcellular localization of Mito-FV, Mito
Tracker Red, the commercially available mitochondrial dye, was
employed for the colocalization studies. HeLa cells were co-stained
with the mitochondral tracker and Mito-FV in the presence of TBAF
(30 μM) to performed the colocalization experiments. The merged
image (Fig. 5e) showed that the staining of Mito-FV fits well with that
of Mito Tracker Red. Moreover, the intensity profile of the linear regions
of interest across HeLa cells costained with Mito-FV and Mito Tracker
Red in the two channels varies in close synchrony (Fig. 5f). Additionally,
a high overlap Pearson's colocalization coefficient (0.91) was calculated
from the intensity correlation plots (Fig. 5g), indicating that the probe
Mito-FV was localized mainly in the mitochondria and can specifically
imaging the mitochondrial fluoride ions in living cells.
−
3
.3. Fluorescence Imaging in Living Cells
Encouraged by the above superior results of the spectral data estab-
lishing that the probe Mito-FV can respond to fluoride ions in
Fig. 4. Imaging of F− in HeLa cells stained with Mito-FV (10 μM) (a) brightfield image of HeLa cells costained only with Mito-FV; (b) fluorescence images of (a) from green channel;
c) overlay of (a) and (b); (d) brightfield image of HeLa cells costained with Mito-FV and treated with TBAF; (e) fluorescence images of (d) from green channel; (f) overlay of (d) and (e)
(

K. Zhou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 204 (2018) 777–782
781
Fig. 5. Mitochondrial multicolor colocalization in HeLa cells with the probe Mito-FV and Mito Tracker Red. a) brightfield image; b) from blue channel (Mito-FV imaging of F−); c) from the
red channel (mitochondria staining); d) overlay of brighfield, blue, and red channels; e) overlay of blue and red channels; f) intensity profile of linear region of interest across the HeLa cell
costained with Mito Tracker Red and blue channel of Mito-FV; g) intensity scatter plot of blue and red channels.
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. Conclusions
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Acknowledgement
This work was financially supported by NSFC (21472067, 21502067,
1672083), the Natural Science Foundation of Shandong Province,
China (ZR2014BP001), Taishan Scholar Foundation (TS 201511041),
and the startup fund of University of Jinan (309-10004, 160082101).
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