A Simple Fluorescent Probe for Sensing pH and its Application in E. coli Cells

Article abstract of DOI:10.1007/s10895-019-02368-2

We designed and synthesized a simple fluorescent probe, (E)-2-(2-(3,4,5-Trimethoxybenzylidene)hydrazinyl) benzothiazole (probe), which could be applied to the detection of strongly acidic and alkaline pH in DMSO/water (1/4, v/v) system. It could be used to quantitatively detect strong acid in the range of 2.60–3.53 with a pK_a of 2.78. Meanwhile, it also showed an excellent linear relationship between the fluorescence intensity and alkaline pH values over the range of 9.98–10.95 with a pK_a of 9.32. The probe exhibited excellent properties to pH with high selectivity and sensitivity. The mechanism studies showed that the H⁺ binding with the N atom of benzothiazole moiety and hydrazine moiety in acid solution while the deprotonation of N atom in hydrazine group in basic environment. Importantly, the probe was successfully applied for imaging the strongly acidic and alkaline in E.coil cells.

Full text of DOI:10.1007/s10895-019-02368-2

Journal of Fluorescence
https://doi.org/10.1007/s10895-019-02368-2
ORIGINAL ARTICLE
A Simple Fluorescent Probe for Sensing pH and its Application
in E. coli Cells
Jianbin Chao1 & Xiaolu Wang1,2 & Yongbin Zhang3 & Fangjun Huo3 & Caixia Yin4 & Miao Xu1,2
Received: 27 December 2018 /Accepted: 19 March 2019
#
Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
We designed and synthesized a simple fluorescent probe, (E)-2-(2-(3,4,5-Trimethoxybenzylidene)hydrazinyl) benzothiazole
(probe), which could be applied to the detection of strongly acidic and alkaline pH in DMSO/water (1/4, v/v) system. It could
be used to quantitatively detect strong acid in the range of 2.60–3.53 with a pKa of 2.78. Meanwhile, it also showed an excellent
linear relationship between the fluorescence intensity and alkaline pH values over the range of 9.98–10.95 with a pKa of 9.32. The
probe exhibited excellent properties to pH with high selectivity and sensitivity. The mechanism studies showed that the H+
binding with the N atom of benzothiazole moiety and hydrazine moiety in acid solution while the deprotonation of N atom in
hydrazine group in basic environment. Importantly, the probe was successfully applied for imaging the strongly acidic and
alkaline in E.coil cells.
.
.
.
Keywords Fluorescent probe pH-sensitive Extremely acid and alkaline Bio-imaging
Introduction
cancer, stroke and Alzheimer’s disease are caused by abnor-
mal pH values [10, 11]. So we are eager to develop a simple
and accurate way to measure pH. In fact, there are many
methods have been applied to measure the intracellular pH
values such as acid-base indicator titration [12], pH-sensitive
microelectrodes [13], 31P nuclear magnetic resonance tech-
nique [14]. However, these methods were time-consuming,
destructive and easily interfered by environment factors [15].
Compared with these ways, fluorescent probes have attracted
many successful applied in fluorescence imaging, medical
diagnosis and other fields because its high sensitivity, good
selectivity, and real-time detection [16–18].
For all we know, the pH plays a crucial role in biological
systems. It is related with cell growth and division [1, 2], ion
transport [3], endocytosis [4] and muscle contraction [5].
Therefore, it is vital to keep a appropriate pH for the normal
cellular activities. For the human body, different part usually
have different pH values, such as cytosol (pH 6.0–7.45) [6],
saliva (pH 6.7–6.9) [7], lysosomes and endosomes (pH 4.5–
6.0) [8], gastric juice (pH 2.0–3.0) [9]. Some diseases like
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s10895-019-02368-2) contains supplementary
material, which is available to authorized users.
Up to now, lots of fluorescent probes have been reported
for pH, however, most of them are mainly for neutral pH (6–8)
and weak acid pH (4–6) [6, 19, 20]. Only a few probes are
designed for the extremely acid conditions (pH < 4) and ex-
tremely alkaline conditions (pH > 9) [21–24]. Although the
strong acid or alkaline environment is not conducive to the
survival of most organisms, some microorganisms such as
Helicobacter pylori, eosinophilic bacteria and basophilic bac-
teria prefer living in this environment [25–27]. And, mammals
also have very acidic physiological environments, such as
gastric juice, which abnormal pH value lead to gastric dys-
function and directly cause of stomach diseases [26, 28]. For
these reasons, it is necessary to develop a fluorescence sensor
to detect the strong acid and alkaline.
* Jianbin Chao
chao@sxu.edu.cn
1
Scientific Instrument Center, Shanxi University, Taiyuan 030006,
China
2
School of Chemistry and Chemical Engineering, Shanxi University,
Taiyuan 030006, China
3
Research Institute of Applied Chemistry, Shanxi University,
Taiyuan 030006, China
4
Institute of Molecular Science, Shanxi University, Taiyuan 030006,
China

J Fluoresc
The benzothiazole group possess good fluorescence prop-
erties owing to their large range of π-conjugated systems. And
it is always regarded as fluorophore and recognition moiety
due to it contains two heteroatoms (N and S) that can be
coordinated with metal ions [29, 30]. Many fluorescent probes
based on benzothiazole have been reported in recent years.
Liu reported a fluorescent probe for Fe3+ based on 2-(2-
hydroxyphenyl)benzothiazole [31], Nguyen designed a
benzothiazole-based fluorescent probe for hypochlorous acid
[32]. Zhang synthesized a probe based on the iminocoumarin
benzothiazole for sensing hydrogen sulfide [33]. Shen de-
signed a simple fluorescent probe based on a benzothiazole
derivative that can been used to detect for copper and biothiols
[34]. In this paper, we designed a simple probe, (E)-
2-(2-(3,4,5-Trimethoxybenzylidene)hydrazinyl)
benzothiazole (probe), which was constructed via
benzothiazole and 3,4,5-trimethoxy benzene. The probe can
be used to detect the extreme acid and alkaline with high
sensitivity and good selectivity. The fluorescence intensity
showed an excellent linear relationship with pH value over
the range of 2.60–3.53 and 9.98–10.95. And the cells imaging
studies revealed that probe had a great potential to monitor pH
change in E. coli cells.
(Hitachi, Japan). Imaging in E. coli cells was conducted on a
ZEISS LSM 880 confocal laser scanning microscope.
Synthesis of (E)-2-(2-(3,4,5-Trimethoxybenzylidene)
Hydrazinyl) Benzothiazole (Probe)
2-hydrazinobenzothiazole (0.33 g, 30 mM) and 3,4,5-
trimethoxybenzaldehyde (0.40 g, 30 mM) were added into a
250 mL three-neck round bottom flask and dissolved
completely in anhydrous methanol (65.0 mL). Added 3–4
drops concentrated hydrochloric acid to the mixture and
stirred it at room temperature. After 2 h, added 2–3 drops
concentrated hydrochloric acid to the solution and kept stir-
ring it for 18 h. Then the resulting solution was heated to
reflux for 5 h. The reaction was monitored by TLC (V ethyl
acetate: V petroleum ether = 2: 1). After cooled downed to room
temperature, the solid was filtered off under reduced pressure
and washed with methanol for three times and dried in vacu-
um to afford probe (0.53 g, 76.8%, Scheme 1). The structure
1
1
of probe was characterized by H NMR and 13C NMR. H
NMR (DMSO-d6, 600 MHz, δ/ppm): 8.12 (s, 1H), 7.81 (d,
J = 7.6 Hz, 1H), 7.47 (d, J = 6.8 Hz, 1H), 7.34~7.31 (t, J =
7.5 Hz, 1H), 7.15~7.13 (t, J = 7.3 Hz, 1H), 7.06 (s, 2H), 3.85
(s, 6H), 3.71 (s, 3H), (Fig. S1). 13C NMR (DMSO-d6,
150 MHz, δ/ppm): 167.06, 153.67, 146.82, 139.77, 129.74,
127.75, 127.04, 123.12, 122.70, 117.09, 104.81, 60.61, 56.44,
49.05 (Fig. S2).
Experimental Section
Materials and Instrumentation
All reagents and solvents were commercially available
(Sinopharm Chemical Reagent Beijing Co., Ltd.). The sol-
vents and chemicals were analytical grade and used without
further purification unless for special needs. Distilled water
was used throughout the process of UV-vis and fluorescence
measurements.
Sample Preparation and Spectroscopic Measurements
Dissolved the probe in DMSO as the stock solutions
(1.0 mM). We diluted the stock solution to 10.0 μM with
DMSO/water (1/4, V/V) for fluorescence and UV-vis mea-
surement. Absorption and fluorescence spectra were mea-
sured with 1.0-cm quartz cells. A series of ions solutions
(K+, Na+, Ca2+, Zn2+, Mg2+, Al3+, Co2+, Cr3+, Ni2+, Bi3+,
Cu2+, Hg2+, Fe2+, Fe3+, Pb2+ and Mn2+) for anti-interference
tests were prepared from the corresponding hydrochloride
salts. The solutions of common amino acids (Phe, Asp, Ala,
Leu, Lys, Val, Ser, Gly) prepared in redistilled water. The
excitation wavelength was set at 355 nm (slit: 5 nm /10 nm)
for the fluorescence measurements.
1H NMR and 13C NMR experiments were taken on a
Bruker AVANCE-600 MHz and 150 MHz NMR spectrome-
ter, respectively. HRMS data were achieved with a Thermo
Scientific Q Exactive LC-MS/MS system. All pH was con-
trolled by a PHS-3C digital pH-meter (YouKe, China).
Absorption spectra were obtained by a UV-2450 spectropho-
tometer (Shimadzu, Japan). Fluorescence measurements were
recorded on a F-7000 fluorescence spectrophotometer
CHO
S
OCH3
S
NH
CH3OH, HCL
NH
+
N
N
OCH3
N
NH2
OCH3
H3CO
OCH3
OCH3
Scheme 1 Synthetic routes of probe

J Fluoresc
Fig. 1 a Absorption and b fluorescence spectra (λex = 355 nm, slit = 5/
10 nm) of 10.0 μM probe in DMSO/water (1/4, V/V) at different pH value
(from 7.40 to 1.68); c plot of the emission fluorescence intensity of probe
at 425 nm at various pH values. The inset showed the linear relationship
of fluorescence intensity at 425 nm and pH values from 2.60 to 3.53
Fig. 2 a UV-vis and b fluorescence spectra (λex = 355 nm, slit = 5/10 nm)
of 10.0 μM probe in DMSO/water (1/4, V/V) at different pH value (from
7.40 to 12.45); c plot of the emission fluorescence intensity of probe at
451 nm at various pH values. The inset showed the linear relationship of
fluorescence intensity at 451 nm and pH values from 9.98 to 10.95

J Fluoresc
Bacteria Culture and Imaging
The Selectivity of Probe
E. coli strains were inoculated into Luria-Bertani (LB)
culture medium (NaCl 10 g/L, Trptone 10 g/L and yeast
extract 5 g/L) and incubatednn at 37 °C in a table con-
centrator at 180 rpm for 15 h. Next, the bacteria were
equally divided into nine centrifuge tubes and collected
by centrifugation at 4300 rpm for 5 min. The sediment
was washed three times and resuspended with various pH
solutions (11.00, 10.60, 10.30, 9.00, 7.40, 5.50, 3.00,
2.50, 1.70), respectively. After 10 min, every tube was
added with probe to make the final concentration of probe
to be 20 μM and incubated in a table concentrator for 2 h
at 37 °C. Before imaging, the E. coli cells were washed
twice with distilled water.
Taking into account the intracellular environment was com-
plex, we evaluated the anti-interference capacity of 10.0 μM
probe to pH at pH 8.20, 7.40 and 2.75 by an anti-interference
test, respectively. The influence of various metal cations (K+,
Na+, Ca2+, Zn2+, Mg2+, Al3+, Co2+, Cr3+, Ni2+, Bi3+, Cu2+,
Hg2+, Fe2+, Fe3+, Pb2+ and Mn2+) and some common amino
acids (Phe, Asp, Ala, Leu, Lys, Val, Ser, Gly) on the fluores-
cence intensity of probe were shown in Fig. 3 and Fig. S3.
They all didn’t cause visible effect on the pH response of
probe. From the experimental results, we could conclude that
Results and Discussion
Spectroscopic Properties of Probe
The UV-vis absorption spectra of 10.0 μM probe were
discussed in DMSO/water (1/4, V/V) at various pH
values. At neutral condition, probe exhibited a maximum
absorption at 341 nm (ε = 2.89 × 104), however, the ab-
sorption peak enhanced and blue-shifted to 336 nm (ε =
3.21 × 104) upon adding the environmental acidity from
pH 7.40 to 1.68 (Fig. 1a). When the alkalinity of solu-
tion increased from pH 7.40 to 12.45, the peak at
341 nm significantly decreased accompanied by the ap-
pearance of two new peaks centred at 265 nm and
394 nm (Fig. 2a).
Next, the fluorescence changes of probe (10 μM) in
different pH environment were studied. As seen from the
Fig. 1b, the probe fluorescence intensity at 451 nm in-
creased and blue-shifted to 425 nm with the lowering the
pH from 7.40 to 1.68. The fluorescence emission inten-
sity at 425 nm (F425 nm) versus pH showed BS^ shaped
calibration graph with a pKa of 2.78 (Fig. 1c). Besides,
there was a great linear relationship between the F425 nm
and pH values from 2.60 to 3.53 with the linear equation
F425 nm = −592.8*pH + 3353.8 (inset of Fig. 1c, R2 =
0.9833). With increasing the environmental basicity from
7.40 to 12.45, the fluorescence intensity at 451 nm (F451
nm) decreased remarkably (Fig. 2b). From the sigmoidal
plot of the F451 nm with pH value, a pKa value of 9.32
was calculated which implied the probe was sensitive
around this pH values (Fig. 2c). Concomitantly, the
Fig. 3 Fluorescence changes of 10.0 μM probe in DMSO/water (1/4,
V/V) toward various metal ions and common amino acids (λex =
355 nm, slits = 5/10 nm). a pH 2.75, λem = 425 nm; b pH 8.20, λem =
451 nm. 1. Blank; 2. K+ (25 mM); 3. Na+ (25 mM);4. Ca2+ (5 mM); 5.
Zn2+; 6. Mg2+; 7. Al3+; 8. Co2+; 9. Cr2+; 10. Ni2+; 11. Bi3+; 12. Cu2+
F451
displayed a good linear correction with pH in
nm
the range of 9.98–10.95 with the function of F451 nm
=
(0.1 mM); 13. Hg2+; 14. Fe2+ (0.1 mM); 15. Fe3+ (0.05 mM); 16. Pb2+
;
−538.6*pH + 6339.9, R2 = 0.9733 (inset of Fig. 2c).
These linear curves make it easier to quantitative deter-
mination of pH over this pH range.
17. Mn2+; 18. Phe (5 μM); 19. Asp (5 μM); 20. Ala (5 μM); 21. Leu
(5 μM); 22. Lys (5 μM); 23. Val (5 μM); 24. Ser (5 μM); 25. Gly (5 μM);
other unlabeled ions: 0.2 mM

J Fluoresc
probe had the ability to detect pH selectivity in complex in-
tracellular environment.
finished within 5 min. And the fluorescence intensity kept
unchanged during 2 h, indicating probe had good
photostability and was appropriate for real-time monitoring
pH. The reversibility was another very vital character for fluo-
rescent probes. Thus, the reversibility experiment of probe
towards pH was carried out. The fluorescence intensities of
probe were recorded when the pH of solution was adjusted
back and forth between 7.40 and 12.45 four times. As shown
Photostability and Reversibility
The time courses of the fluorescence intensity of probe
(10.0 μM) at pH 1.68, 2.75, 10.51 and 13.04 were studied.
Figure S4 exhibited the reaction of probe to pH could be
Fig. 4 a Partial 1H NMR spectra
of probe and probe + H+ in
DMSO-d6. b Partial 1H NMR
spectra of probe and probe + OH−
in DMSO-d6

J Fluoresc
Fig. 5 Imaging acidity in E. coli cells with probe (20.0 μM). A: pH 7.4;
B: pH 5.5; C: pH 3.0; D: pH 2.50; E: pH 1.7. First column: blue channel
(410–500 nm, λex = 405 nm); second column: bright field; third column:
overlapped of blue channel and bright field. F: The relative fluorescence
intensity of E. coli cells incubated in different pH buffer
Fig. 6 Imaging alkalinity in E. coli cells of probe (20.0 μM). A: pH 7.40;
B: pH 9.0; C: pH 10.3; D: pH 10.6; E: pH 11.0. First column: blue channel
(410–500 nm, λex = 405 nm); second column: bright field; third column:
overlapped of blue channel and bright field. G: The relative fluorescence
intensity of E. coli cells incubated at different pH buffer

J Fluoresc
in Fig. S5, the probe was fully reversible in basic circum-
stance, and the response and recovery times are rapid within
seconds. Therefore, the probe could detect the base in real
time.
Conclusion
In summary, we reported a simple probe that could be used to
detect the extreme acidic and alkaline. The probe displayed a
Bturn on^ fluorescence respond in acid solutions and had a
linearly relationship with pH value in the range of 2.60–3.53
with a pKa 2.78. However, it exhibited a Bturn off^ response to
basic at the λex = 355 nm and the linear range was 9.98–10.95.
The probe possesses excellent photostability, good selectivity
and cell membrane permeability. And the probe exhibited re-
versible in basic environment. The mechanism was verified to
Proposed Mechanism of the pH Response of Probe
We speculated that the fluorescence and absorption changes of
probe could owing to the intramolecular charge transfer (ICT)
mechanism from the benzothiazole (electron-donor) to the
methoxy benzene moiety (electron-acceptor). To further pro-
vide direct evidence for the sensing mechanism between the
1
protonation and deprotonation of N atom by the H NMR
spectra. Most importantly, the application of probe to image
pH in bacteria was realized successfully. Based on the great
properties of probe, we confirmed that the probe would be an
efficient tool for imaging pH distribution and change tracking.
1
probe and pH, H NMR experiment was conducted in d6-
DMSO (Fig. 4). In the presence of HCl, the chemical shifts
of the benzothiazole ring protons (H-1, H-2, H-3 and H-4)
were downfiled shift, which indicated that H+ have been bind-
ing with nitrogen atom of the benzothiazol. At the same time,
the protons of methoxy benzene moiety (H-6) and H-5
showed downfiled shift, which implied that the protonation
of hydrazine nitrogen atom. The reason why these protons
down-shifted was that the binding of H+ and N atom led to
the decrease in the charge density around these protons. At
the same time, the protonation of nitrogen atom led to a
decrease of the ability of benzothiazole giving electrons.
Upon the addition of NaOH, the chemical shifts of all pro-
tons were up-field shifted as shown in Fig. 4b, which in-
dicated that the deprotonation of N atom in hydrazine
group.
Acknowledgements The work was supported by the National Natural
Science Foundation of China (No. 21472118, 21672131), the Program
for the Top Young and Middle-aged Innovative Talents of Higher
Learning Institutions of Shanxi (No. 2013802), Talents Support
Program of Shanxi Province (No. 2014401), Shanxi Province
Outstanding Youth Fund (No. 2014021002), Natural Science
Foundation of Shanxi Province of China (No. 201701D121018).
References
1. Gottlieb RA, Nordberg J, Skowronski E, Babior BM (1996)
Apoptosis induced in Jurkat cells by several agents is preceded by
intracellular acidification. Proc Natl Acad Sci U S A 93:654–658
2. Gottlieb RA, Giesing HA, Zhu JY, Engler RL, Babior BM (1995)
Cell acidification in apoptosis: granulocyte colony-stimulating fac-
tor delays programmed cell death in neutrophils by up-regulating
the vacuolar H(+)-ATPase. Proc Natl Acad Sci U S A 92:5965–
5968
3. Hoyt KR, Reynolds IJ (1998) Alkalinization prolongs recovery
from glutamate-induced increases in intracellular Ca2+ concentra-
tion by enhancing Ca2+ efflux through the mitochondrial Na+/Ca2+
exchanger in cultured rat forebrain neurons. J Neurochem 71:1051–
1058
4. Blott EJ, Griffiths GM (2002) Secretory lysosomes. Nat Rev Mol
Cell Biol 3:122–131
5. Edmonds BT, Murray J, Condeelis J (1995) pH regulation of the F-
actin binding properties of Dictyostelium elongation factor 1 alpha.
J Biol Chem 270:15222–15230
6. Sun KM, McLaughlin CK, Lantero DR, Manderville RA (2007)
Biomarkers for phenol carcinogen exposure act as pH-sensing fluo-
rescent probes. J Am Chem Soc 129:1894–1895
7. Jr LR, Bentley CD, Haywood VB (1994) Salivary pH changes
during 10% carbamide peroxide bleaching. Quintessence Int 25:
547–550
Fluorescence Imaging pH in E. coil Cells
In order to access the potential application of probe for pH
detecting in living sample, E. coil cells were employed to
image the pH change. To create acid and alkaline surround-
ings for bacteria, we used buffer with pH 11.0, 10.6, 10.3,
9.00, 7.40, 5.50, 3.00, 2.50 and 1.70, respectively, to incubate
E. coil. As seen from the Fig. 5, the E. coli cells exhibited
weak blue fluorescence (410–500 nm, λex = 405 nm) at
pH 7.40, but it enhanced gradually with reduction of the H+
concentration from 7.40 to 1.70. When the environmental al-
kaline changed from 7.40 to 11.00, the fluorescence in blue
channel enhanced obviously (Fig. 6). The variation of relative
fluorescence intensity in blue channel under different pH val-
ue were shown in Figs. 5f and 6g. These results were in agree-
ment with the changes of the fluorescence at λex = 405 nm
that showed in Fig. S6. Meanwhile, we affirmed that E. coli
cells could survive in such extremely acidic or alkaline cir-
cumstance, and the probe was suitable for monitoring such
alkaline and acidic extracellular pH changes.
8. Galindo F, Burguete MI, Vigara L, Luis SV, Kabir N, Gavrilovic J,
Russell DA (2005) Synthetic macrocyclic peptidomimetics as tun-
able pH probes for the fluorescence imaging of acidic organelles in
live cells. Angew Chem 117:6662–6666

J Fluoresc
9. Zhang WJ, Fan L, Li ZB, Ou T, Zhai HJ, Yang J, Dong C, Shuang
SM (2016) Thiazole-based ratiometric fluorescence pH probe with
large stokes shift for intracellular imaging. Sensors Actuators B
Chem 233:566–573
10. Davies TA, Fine RE, Johnson RJ, Levesque CA, Rathbun WH,
Seetoo KF, Smith SJ, Strohmeier G, Volicer L, Delva L (1993)
Non-age related differences in thrombin responses by platelets from
male patients with advanced Alzheimer’s disease. Biochem
Biophys Res Commun 194:537–543
11. Izumi H, Torigoe T, Ishiguchi H, Uramoto H, Yoshida Y, Tanabe M,
Ise T, Murakami T, Yoshida T, Nomoto M (2003) Cellular pH
regulators: potentially promising molecular targets for cancer che-
motherapy. Cancer Treat Rev 29:541–549
12. Spanswick RM, Miller AG (1977) Measurement of the cyto-
plasmic pH in nitella translucens. Comparison of values obtain-
ed by microelectrode and weak acid methods. Plant Physiol 59:
664–666
13. Antoneko YN, Bulychev AA (1991) Measurements of local pH
changes near bilayer lipid membrane by means of a pH microelec-
trode and a protonophore-dependent membrane potential.
Comparison of the methods. Biochim Biophys Acta Biomembr
1070:279–282
14. Seo Y, Murakami M, Watari H, Imai Y, Yoshizaki K, Nishikawa H,
Morimoto T (1983) Intracellular pH determination by a 31P-NMR
technique. The second dissociation constant of phosphoric acid in a
biological system. Magn Reson Imaging 2:147–147
15. Slavik J (1982) Intracellular pH of yeast cells measured with fluo-
rescent probes. FEBS Lett 140:22–26
16. Shi W, Li X, Ma H (2014) Fluorescent probes and nanoparticles for
intracellular sensing of pH values. Methods Appl Fluoresc 2:
042001
17. Wang J, Vernier PT, Sun Y, Marcu L (2005) A fluorescence micros-
copy study of quantum dots as fluorescent probes for brain tumor
diagnosis. Proc SPIE 5703:127–134
18. Winkler AM, Rice PFS, Barton JK (2009) In vivo imaging using a
VEGF-based near-infrared fluorescent probe for early cancer diag-
nosis in the AOM-treated mouse model. Proc SPIE 7190:71900M–
71900M-9
19. Cui DW, Qian XH, Liu FY, Zhang R (2004) Novel fluorescent pH
sensors based on intramolecularhydrogen bonding ability of
naphthalimide. Org Lett 6:2757–2760
20. Wan Q, Chen S, Shi W, Li L, Ma H (2015) Lysosomal pH rise
during heat shock monitored by a lysosome-targeting near-infrared
ratiometric fluorescent probe. Angew Chem 126:11096–11100
21. Chao JB, Wang HJ, Zhang YB, Yin CX, Huo FJ, Song KL, Li ZQ,
Zhang T, Zhao Y (2017) A novel Bdonor-π-acceptor^ type fluores-
cence probe for sensing pH: mechanism and application in vivo.
Talanta 174:468–476
22. Xu Y, Jiang Z, Xiao Y, Bi FZ, Miao JY, Zhao BX (2014) A new
fluorescent pH probe for extremely acidic conditions. Anal Chim
Acta 820:146–151
23. Zhang X, Jing SY, Huang SY, Zhou XW, Bai JM, Zhao BX (2015)
New fluorescent pH probes for acid conditions. Sensors Actuators
B Chem 206:663–670
24. Niu W, Fan L, Nan M, Wong MS, Shuang SM, Dong C (2016) A
novel fluorescent probe for sensing and imaging extreme acidity.
Sensors Actuators B Chem 234:534–540
25. Krulwich TA, Sachs G, Padan E (2011) Molecular aspects of bac-
terial pH sensing and homeostasis. Nat Rev Microbiol 9:330–343
26. Merrell DS, Camilli A (2002) Acid tolerance of gastrointestinal
pathogens. Curr Opin Microbiol 5:51–55
27. Ferguson BJ, Seethala R, Wood WA (2007) Eosinophilic bacterial
chronic rhinosinusitis. Laryngoscope 117:2036–2040
28. HE MS, Li Z (2012) Study on the influence of folic acid for the
gastric juice pH value of helicobacter pylori positive patients
with chronic atrophic gastritis. National Medical Frontiers of
China 7:24–25
29. Nas A, Dilber G, Durmus M, Kantekin H (2015) The influence of
the various central metals on photophysical and photochemical
properties of benzothiazole-substituted phthalocyanines.
Spectrochim Acta A 135:55–62
30. Wagh YB, Kuwar A, Sahoo SK, Gallucci J, Dalal DS (2015)
Highly selective fluorimetric sensor for Cu2+ and Hg2+ using a
benzothiazole-based receptor in semi-aqueous media and molecular
docking studies. RSC Adv 5:45528–45534
31. Liu SD, Zhang LW, Liu X (2013) A highly sensitive and selective
f l u o r e s c e n t p r o b e f o r F e 3 + b a s e d o n 2 - ( 2 -
hydroxyphenyl)benzothiazole. New J Chem 37:821–826
32. Nguyen KH, Hao Y, Zeng K, Fan S, Li F, Yuan S, Ding X, Xu M,
Liu YN (2018) A benzothiazole-based fluorescent probe for
hypochlorous acid detection and imaging in living cells.
Spectrochim Acta A 199:189–193
33. Zhang H, Xie Y, Wang P, Chen G, Liu R, Lam YW, Hu Y, Zhu Q,
Sun H (2015) An iminocoumarin benzothiazole-based fluorescent
probe for imaging hydrogen sulfide in living cells. Talanta 135:
149–154
34. Shen Y, Zhang X, Zhang C, Zhang Y, Jin J, Li H (2018) A simple
fluorescent probe for the fast sequential detection of copper and
biothiols based on a benzothiazole derivative. Spectrochim Acta
A 191:427–434
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