4-(8-Quinolyl)amino-7-nitro-2,1,3-benzoxadiazole as a new selective and sensitive fluorescent and colorimetric pH probe with dual-responsive ranges in aqueous solutions

Article abstract of DOI:10.1016/j.saa.2015.09.006

Fluorescent and colorimetric pH probe possess many advantages including rapid response time, nondestructive testing, and excellent pH sensitivity. However, they usually cannot be utilized simultaneously in both acidic and basic pH ranges. In this study, a new selective and sensitive fluorescent and colorimetric pH probe, 4-(8- quinolyl)amino-7-nitro-2,1,3-benzoxadiazole (1), was designated and synthesized. The optical probe exhibited dual-responsive pH ranges to both acidic and basic aqueous solutions. When the solution pH was gradually increased from 8.5 to 13.3, the absorption spectra of 1 showed an obvious hyperchromicity, accompanied with a red shift of the absorption band at 340 nm, a blue shift of the absorption band at 482 nm, and a distinct color change from orange to violet pink to yellow. Within the pH range from 2.2 to 0.2, the fluorescent spectra of 1 showed a "turn-on" response signal to solution pH. In order to understand the response mechanism of the probe to solution pH, the probe molecule was split into two parts, 8-aminoquinoline (2) and 4-amino-7- nitro-benzofurazan (3). UV-vis absorption and fluorescent experiments of 2 and 3 indicated that both are sensitive optical pH probes. Furthermore, the NMR experiment of 1 was explored in basic and acidic conditions. The results indicated that the colorimetric responses of 1 to pH under basic condition should be attributed to the deprotonation of the imino group on the quinolyl ring, and the fluorescent recognition of 1 to pH under acidic condition was probably due to the protonation of the nitrogen atoms from the benzofurazan and quinolyl rings.

Full text of DOI:10.1016/j.saa.2015.09.006

4-(8-Quinolyl)amino-7-nitro-2,1,3-benzoxadiazole as a new selective and
sensitive fluorescent and colorimetric pH probe with dual-responsive ranges
in aqueous solutions
Xutian Li, Min Zhang, Haipeng Liang, Zhaowei Huang, Jiang Tang,
Zhi Chen, Liting Yang, Li-Jun Ma, Yuhai Wang, Baiping Xu
PII:
DOI:
Reference:
S1386-1425(15)30219-5
doi: 10.1016/j.saa.2015.09.006
SAA 14070
To appear in:
Received date:
Revised date:
Accepted date:
16 April 2015
4 September 2015
11 September 2015
Please cite this article as: Xutian Li, Min Zhang, Haipeng Liang, Zhaowei Huang,
Jiang Tang, Zhi Chen, Liting Yang, Li-Jun Ma, Yuhai Wang, Baiping Xu, 4-(8-
Quinolyl)amino-7-nitro-2,1,3-benzoxadiazole as a new selective and sensitive fluorescent
and colorimetric pH probe with dual-responsive ranges in aqueous solutions, (2015),
doi: 10.1016/j.saa.2015.09.006
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ACCEPTED MANUSCRIPT
4-(8-Quinolyl)amino-7-nitro-2,1,3-benzoxadiazole as a new selective and sensitive
fluorescent and colorimetric pH probe with dual-responsive ranges in aqueous
solutions
Xutian Lia, Min Zhanga, Haipeng Lianga, Zhaowei Huanga, Jiang Tanga, Zhi Chena,
Liting Yanga, Li-Jun Maa,b,*, Yuhai Wanga, Baiping Xuc
aSchool of Chemistry and Environment, South China Normal University, Shipai,
Guangzhou, 510631, P. R. China. Tel: +86-20-39310251; Fax: +86-20-39310187;
bGuangzhou Key Laboratory of Materials for Energy Conversion and Storage;
cTechnology Development Center for Polymer Processing Engineering of Guangdong
Colleges and Universities, Guangdong Industry Technical College, Guangzhou
510300, China;
Corresponding author email: mlj898021@aliyun.com (Li-Jun Ma)
1

ACCEPTED MANUSCRIPT
Abstract
Fluorescent and colorimetric pH probe possesses many advantages including rapid
response time, nondestructive testing, and excellent pH sensitivity. However, they
usually can not be utilized simultaneously in both acidic and basic pH ranges. In this
study, a new selective and sensitive fluorescent and colorimetric pH probe, 4-(8-
quinolyl)amino-7-nitro-2,1,3-benzoxadiazole (1), was designated and synthesized.
The optical probe exhibited dual-responsive pH ranges to both acidic and basic
aqueous solution. When the solution pH was gradually increased from 8.5 to 13.3, the
absorption spectra of 1 showed an obvious hyperchromicity, accompanied with a red
shift of the absorption band at 340 nm, a blue shift of the absorption band at 482 nm,
and a distinct color change from orange to violet pink to yellow. Within the pH range
from 2.2 to 0.2, the fluorescent spectra of 1 showed a “turn-on” response signal to
solution pH. In order to understand the response mechanism of the probe to solution
pH, the probe molecule was split into two parts, 8-aminoquinoline (2) and 4-amino-7-
nitro-benzofurazan (3). UV-vis absorption and fluorescent experiments of 2 and 3
indicated that both are sensitive optical pH probes. Furthermore, the NMR experiment
of 1 was explored in basic and acidic conditions. The results indicated that the
colorimetric responses of 1 to pH under basic condition should be attributed to the
deprotonation of the imino group on the quinolyl ring, and the fluorescent recognition
of 1 to pH under acidic condition was probably due to the protonation of the nitrogen
atoms from the benzofurazan and quinolyl rings.
Keywords
Fluorescent pH probe, Colorimetric pH probe, Aminoquinoline, 2,1,3-Benzoxadiazole,
Dual-responsive range
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Introduction
The pH value plays a pivotal role in chemical reactions and biological processes
[1-4]. It not only for the physical and chemical properties and reactivity of substances
has significant effect but also has an important function on the life system [5, 6].
Many available methods have been reported to measure pH value, such as acid-base
indicator titration [7-10], microelectrodes [11], nuclear magnetic resonance (NMR)
[12], potentiometric titration [13-15], absorption and fluorescent spectroscopy[16-19],
etc. Among these methods, fluorescent and colorimetric probe is a powerful tool for
the handy, rapid, low-cost, highly selective and sensitive, and nondestructive
measurement of pH value, especially to clarify the real-time dynamics and various
biological processes in living cells. In recent years, a large variety of fluorescent and
colorimetric pH probes were fabricated successfully [18-22]. Unfortunately, most of
the reported optical probes could only be used in a single pH range, and they could
not be utilized in both acidic and basic pH ranges [23]. In this work, we developed a
new optical pH probe 4-(8-quinolyl)amino-7-nitro-2,1,3-benzoxadiazole (1) which
can monitor pH ranges from 2.2 to 0.2 with a pKa value 1.4 and from 8.5 to 13.3 with
a pKa value 10.5. The experiment results indicated that 1 was a sensitive fluorescent
and colorimetric pH probe with dual-responsive ranges, and could be used under both
acidic solution and basic solution. In addition, two components of probe 1,
8-aminoquinoline (2) and 4-amino-7-nitro-benzofurazan (3), were also sensitive
optical pH probes.
Experimental
Materials and methods
All chemicals were purchased from commercial sources and used without further
purification.
Mass spectra were carried out on a LCQDACAXPMAX mass spectrometer
1
(Finnigan). H NMR spectra were recorded on a Varian NMR Systems 400 MHz
spectrometer using TMS as the internal standard. All fluorescent emission spectra
were recorded with a Hitachi F-2500 fluorescence spectrophotometer. UV-vis
absorption spectra were determined on a Shimadzu UV-1700 spectrophotometer at
room temperature. All the pH values of aqueous solutions were measured precisely
with a PHS-3B digital pH meter.
Synthesis and characterization of probe 1
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Sensor 1 was synthesized through a one-step reaction using 4-chloro-7-
nitrobenzofurazan (NBD-Cl) and 8-aminoquinoline (2) (refer to Scheme 1). 2 (0.053 g,
0.37 mmol) was dispersed in 10.0 mL NEt3. Then NBD-Cl (0.073 g, 0.37 mmol)
dispersed in 5.0 mL NEt3 was added to the above solution dropwise, and the resulting
mixture was stirred for 30 min at room temperature, followed by reflux under 80 °C
for 3 h [24]. After removing the solvent, the resulting 1 was further purified using
column chromatography on silicagel (elution with dichloromethane), with purified 1
in 63.5% yield. 1H NMR (400 MHz, DMSO-d6, 298 K, ppm): 11.01(s, 1H), 8.93(d，
13
1H), 8.51(m，2H), 7.96(m，2H), 7.69(m，3H), 6.72(d，1H). C NMR (100 MHz,
DMSO-d6) δ: 103.72, 121.61, 122.59, 123.84, 125.91, 126.83, 128.74, 134.30, 136.82,
137.52, 140.90, 141.71, 143.92, 145.21, 150.20. ESI-MS (m/z): 306.24; calcd for
[1-H]–, 306.26.
Synthesis and characterization of probe 3
Under nitrogen atmosphere, the mixture of NBD-Cl (0.203 g, 1.0 mmol),
ammonium hydroxide (4.0 ml, 30 Wt%), and methanol (20.0 mL) was stirred at room
temperature for 24 h. Then, the solvent was removed, and the resulting crude product
of 3 (see Scheme 1) was further purified using column chromatography on silicagel
1
(elution with n-hexane: ethyl acetate = 1:1), achieving purified 3 in 60.03% yield. H
NMR (400 MHz, CD3OD, 298 K, ppm): 8.49(d, 1H)，6.39(d, 1H). ESI-MS (m/z):
180.12; calcd for [3+H]+, 180.11.
O2N
N
O
N
NO2
NO2
8
N
N
N
N
N(CH2CH3)3, 80 ºC , 3h
NH3·H2O
r.t. 24h
9
O
O
7
H
NH2
N
N
N
1
2
6
5
Cl
NBD-Cl
NH2
probe 3
probe 2
3
4
probe 1
Scheme 1. Synthetic routes for probes 1 and 3.
Measurements of UV-vis absorption and fluorescent spectra
All UV-vis absorption and fluorescent experiments were carried out in
buffer-DMSO (98:2, v/v) solution at 25 °C. And the buffered solutions with various
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pH values were modulated by mixing 3.0 mM homologous sodium salt: acetate
(NaAc-HAc, pH=3.6-5.8), phosphate (Na2HPO4-NaH2PO4, pH=5.8-8.0), borate
(Na2B4O7-H2BO3, pH=8.0-9.0) and carbonate (Na2CO3-NaHCO3, pH=9.0-10.8),
respectively. Other pH value of the solution was adjusted by adding small amounts of
1.0 M NaOH solution or 1.0 M HCl solution. Prior to UV-vis absorption and
fluorescent measurement, solutions were kept at room temperature for 24 h. A fixed
excitation wavelength at 450 nm was used as the emission spectra. The concentration
of 1, 2 and 3 in all the fluorescent and UV-vis experiments is 2.0×10-5 mol L-1 in
buffered solutions.
Results and discussion
UV-vis response of 1 to pH changes
As shown in Fig. 1, when pH value is 8.5, the absorption spectra of 1 exhibited two
weak absorption bands at 340 nm and 482 nm in aqueous solution. As the pH value of
1 solutions increased from 8.5 to 13.3, the absorption spectra of 1 showed an obvious
hyperchromicity, accompanied with a red shift of the absorption band at 340 nm, and
a blue shift of the absorption band at 482 nm. When the pH value rose to 13.3, the
intensity of absorption bands of 1 showed no further change. Meanwhile, the position
of two absorption bands shifted from 340 nm and 482 nm to 390 nm and 450 nm, and
color changed from orange to violet pink to yellow (see inset in Fig 1A). Especially,
the absorption spectra could be recovered when the pH was re-adjusted back from
13.3 to 8.5. The absorption bands at 340 nm and 482 nm were mainly attributed to the
characteristic absorption bands of 2,1,3-benzoxadiazole [24,25]. Thus, the remarkable
changes of UV–vis absorption spectra should be attributed to the deprotonation of the
imino group on 2,1,3-benzoxadiazole of 1, which can cause a significant influence on
the charge density of the benzoxadiazole aromatic nucleus. Consequently, changing of
the charge–transfer interactions between electron-rich and electron-deficient moieties
resulted in a clear absorption band shift [26].
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0.5
0.4
0.3
0.2
0.1
0.0
30
(A)
13.3
(B)
25
20
15
10
(a) (b)
8.5
5
0
8
9
10
11
12
13
14
300
400
500
600
pH
Wavelength/nm
Fig. 1. (A) Absorption spectra of 1 (pH 8.5-13.3), Inset: photographs of probe 1 at pH
value is (a) 8.5 and (b) 13.3. (B) The change amount of absorption band intensity of 1
at 450 nm, (A-A0)/A0, as a function of pH values.
Meanwhile, the variational ratios of absorption intensity at 450 nm, (A-A0)/A0, for
1 with varying pH were showed in Fig. 1B, with a 26.7-fold gain at pH 13.3 when
compared to that observed at pH 8.5. The result showed a “turn-on” response in
UV–vis absorption spectra for 1 to pH in basic aqueous solution. The pKa value of
probe 1 was calculated by Henderson-Hasselbach-type equation [27, 28], which gave
a pKa value of 10.5.
When the pH value gradually changed from alkaline to weakly acidic, only slight
undulation of UV-vis absorption spectra was induced. However, when pH value was
less than 3.0, obvious changes in the absorption spectra of 1 were observed, as shown
in Fig. 2A. The absorption bands at 338 nm and 476 nm showed a hypochromicity
with the gradual decrease of pH value from 3.1 to 1.3. Then, when pH value
decreased from 1.3 to 0.2, the spectra showed the red shifts of the absorption bands
from 338/476 nm to 315/457 nm with an obvious hyperchromicity. When the pH was
re-adjusted from 0.2 to 3.1, the absorption spectra could be recovered. The experiment
phenomenon probably induced by the protonation of the nitrogen atoms on the
benzofurazan and quinolyl ring of 1 at strong acid condition, which caused a decrease
of the charge density of the aromatic nucleus in 1.[26]
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0.4
0.3
0.2
0.1
0.0
300
240
(B)
(A)
200
250
0.2
pH
3.1
0.2
pKa = 1.4
160
120
80
200
150
100
50
pH
2.2
pH
1.3
1.3
0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7
Wavelength/nm
0
300
400
500
600
700
500
550
600
650
700
750
Wavelength/nm
Wavelength/nm
Fig. 2. (A) Absorption spectra of 1 (pH 3.1-0.2). (B) Fluorescent spectra of 1 at
various pH values (pH 2.2-0.2, em=450 nm); Inset: the variation of fluorescent
emission intensity of 1 at 531 nm with pH (pH 2.6-0.2).
Fluorescent response of 1 to strong acid
The fluorescent spectra of 1 were measured under the various pH values. The
experiment results under alkaline and weak acidic conditions indicated that the probe
1 only shows very weak fluorescent emission, which was potentially due to blockage
of intramolecular charge transfer (ICT) process for benzofurazan ring with two
electron-deficient group (nitro group and quinoline ring) [29]. However, as shown in
Fig. 2B, with the gradual decrease of pH value from 2.2 to 0.2, obvious enhancement
of the fluorescent emission of 1 was observed, resulting in a 2.5-fold gain at 531 nm
at pH 0.2 when compared to that observed at pH 2.2. Meanwhile, the fluorescent band
of 1 at 590 nm was quenched. The fluorescent band at 590 nm was assigned to a
characteristic emission of benzofurazan group, which may overlap the characteristic
emission of the quinoline group whose fluorescent emission was very weak.
Moreover, the fluorescent enhancing could be recovered when the pH was re-adjusted
from 0.2 to 2.2. The result indicated that 1 was a sensitive fluorescent switch to pH/H+
within the pH range from 2.2 to 0.2. The fluorescent enhancement response was
consistent with the results from the UV-vis absorption spectra, where the protonation
of the nitrogen atoms in the benzofurazan and quinolyl rings of 1 was bring the ICT
between electron-rich and electron-deficient moieties on benzofurazan group. Thus,
the fluorescent emission was increased at strong acid condition. In addition, the
analysis of fluorescent intensity changes of 1 as a function of pH by the
Henderson–Hasselbach-type mass action equation [28] yielded a pKa of 1.4 for 1 (298
K), which indicated its usefulness for the investigation in strong acidic condition (see
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inset of Fig. 2B).
Interference experiment
To test the practical application of 1 in the fluorescent detection for pH,
interference experiments were performed to estimate the influence caused by other
ions which may be present in the systems being analyzed. The relative fluorescent
intensities of 1 in the absence or presence of an excess of Cr3+, Mg2+, Cd2+, Co2+, Ca2+,
Mn2+, Na+, Zn2+, Ni2+ and Hg 2+ (100.0 µM) at pH 0.7 were shown in Fig. 3. The
results indicated that the effect of these metals on pH measurement should be
negligible. Therefore, 1 can be used as a potential probe for the practical detection of
pH changes in strong acidic aqueous solution.
250
200
150
100
50
0
2+
2+
2+
2+
g
2+
2+
+
2+
n
3+
2+
y
Z
H
onl
+ Cr
+ Mg
+ Co
+ Ca + Mn
+ Na
+
+ Ni
+ Cd
+
Fig. 3. Fluorescent intensity at 531 nm of 1 in the absence or presence of Cr3+, Mg2+,
Cd2+, Co2+, Ca2+, Mn2+, Na+, Zn2+, Ni2+ and Hg2+ (100.0 µM), respectively, at pH 0.7.
The response mechanism of 1 to pH changes
In order to understand the response mechanism of the probe to pH value, the probe
molecule was split into two parts, 8-aminoquinoline (2) and 4-amino-7-nitro-
benzofurazan (3), and their UV absorption and fluorescent spectra under the various
pH values were examined.
UV-vis response of 2 to pH changes
As shown in Fig. 4A, the UV-vis absorption spectra of 2 showed an absorption
band at 332 nm in neutral or alkaline condition. However, with gradual decrease of
pH value of the solution from 6.0 to 1.0, the absorption band at 332 nm was weakened
gradually, while a new absorption band at 386 nm increased gradually, and the color
8

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of the solutions of 2 changed from the original colorless to yellow (see inset of Fig.
4A). The results were due to the protonation of imino group and nitrogen atom on the
quinoline ring of 2, which changed the charge density on the aromatic nucleus of 2
and resulted in the red shift of absorption band. The fluorescent spectra of 2 also were
tested under different pH values conditions, and the results showed that the probe 2
scarcely shows fluorescent emission. These results showed that 2 is a colorimetric pH
probe in acidic condition. The pKa value of probe 2 has been calculated by
Henderson-Hasselbach-type equation [27, 28], which gave a pKa value of 3.9 [30].
Compared to the above UV-vis and fluorescent data of probe 1, these results indicated
that the colorimetric response under alkaline condition and the fluorescent recognition
of 1 to pH under acidic condition were contributed by the benzofurazan moiety rather
than the 8-aminoquinoline moiety. Meanwhile, the ratios of absorption intensity at
386 nm and 332 nm, A386/A332, for 2 with pH were showed in Fig. 4B. The result
showed a ratiometric response in UV–vis absorption spectra for 2 to pH in acid
aqueous solution.
2.0
1.5
1.0
0.5
0.0
0.20
0.15
0.10
0.05
0.00
(A)
(B)
6.0
1.0
pH
(a) (b)
300
350
400
450
500
1
2
3
4
5
6
pH
Wavelength/nm
Fig. 4. (A) UV-vis absorption spectra of 2 (pH 6.0-1.0), Inset: Photographs of probe 2
in the condition of pH value is (a) 6.0 and (b) 1.0. (B) The ratio of absorption peak
intensity of 2 at 386 nm and 332 nm in different pH values.
Optical response of 3 to pH changes
When the pH of the solution is 6.1, the UV-vis absorption spectra of 3 had two
characteristic absorption band of benzofurazan ring at 340 nm and 467 nm, as shown
in Fig. 5A. With the increase of the pH values from 6.1 to 12.3, the absorption band at
467 nm decreased gradually, and a new absorption band at 382 nm increased
gradually. Meanwhile, a clear isosbestic point at 434 nm in the absorption spectra was
observed, which suggested the formation of a new complex. Under the same
9

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experimental condition, the fluorescent response of 3 to pH changes was explored. As
show in Fig. 5B, the fluorescent spectra of 3 have a strong fluorescent emission peak
at 543 nm. With increasing of the solution pH, the fluorescent emission intensity was
gradually reduced, resulting in a 92.1% quenching from a pH of 6.1 to a pH of 12.5.
Under the irradiation of 365 nm UV light, the fluorescent color of the solution turned
from bright yellow to colorless. The results should be attributed to the deprotonation
of amino group in probe 3, which weakened the charge–transfer interactions between
electron-rich (amino group) and electron-deficient (nitro group) moieties, inducing the
clear absorption band shift and the fluorescent quenching. The UV-vis and fluorescent
spectra of 3 was also explored in strong acidic condition from 3.1 to 0.2. UV-vis
spectra of 3 did not show obvious changes, and its fluorescent spectra only showed
weak quenching. The result showed that 3 is a fluorescent probe for pH under basic
condition. Combining with the above experiment results, the UV-vis response of 1 to
pH may be attributed to the deprotonation of imino group of 1 in basic condition, and
fluorescent recognition of 1 to pH was contributed by the cooperative interaction
between the quinoline and benzofurazan rings.
0.6
2000
(A)
(B)
1600
1200
800
400
0
6.1
pH
12.5
6.1
pH
0.4
0.2
0.0
12.5
(a) (b)
300
350
400
450
500
550
450
500
550
600
650
700
Wavelength/nm
Wavelength/nm
Fig. 5. (A) UV-absorption and (B) fluorescent spectra of 3 (pH 6.1-12.5, em = 380
nm). Inset (B): fluorescent photographs of probe 3 at pH value of (a) 6.1 and (b) 12.5
under illumination with 365 nm light.
1H NMR experiments of 1 in acidic and alkaline condition
1
To further explore the mechanism of 1 for determining the pH value, H NMR
experiments of 1 in acidic and alkaline conditions were tested in the solutions of
DMSO-d6: D2O (10:1, v/v). As shown in Fig. 6A, with the addition of NaOH, the
chemical shift of several protons on 1 was significant changed. The disappearance of
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H(9) and the remarkable upfield shifts of H(1) (δH1 = -1.25 ppm) and H(7) (δH7
=
-1.39 ppm) implied deprotonation of the imino group in probe 1, in consistent with the
UV-vis absorption spectra of 1 to pH in Fig. 1A. Moreover, after the addition of HCl,
the chemical shifts for all hydrogen protons of probe 1 showed obvious changes in
chemical shift, as shown in Fig. 6B. With the continuous addition of HCl, the
chemical shifts of H(6), H(5) and H(4) in probe 1 showed the large changes (δH6
=
0.15 ppm, δH5 = 0.09 ppm and δH4 = 0.09 ppm), which revealed the protonation of
nitrogen atom on the quinolyl ring. Meanwhile, the obvious upfield shifts of H(7)
(δH7 = -0.12 ppm) and H(8) (δH8 = -0.04 ppm) were observed, which indicated the
increase for electron density of benzofurazan ring. The results should cause by the
protonation of three nitrogen atoms on the quinoline and benzofurazan rings under
acidic conditions, where the attracting for the electron of the benzofurazan ring with
two protonated nitrogen atoms was stronger than the quinoline ring with one
protonated nitrogen atom. ICT between electron-rich (imino group) and electron-
deficient (nitro group) moieties on benzofurazan group and the fluorescent enhancing
signal were generated. Thus, the colorimetric and fluorescent responses of 1 to pH
should attribute to the deprotonation of the imino group of 1 and the protonation of
three nitrogen atoms on the benzofurazan and quinoline rings. Based on these results,
a potential response model of 1 to pH was proposed in Scheme 2.
(A)
(B)
(e)
(d)
(c)
(d)
(c)
3 5
2
4
1
6
8
(b)
(b)
9
9
6
4 1 8
7
7
(a)
(a)
11
10
9
8
7
6
11
10
9
8
7
6
5
ppm
ppm
1
Fig. 6. (A) H NMR spectra of (a) 1 (10.0 mM), (b) 1–NaOH(10:1, mol/mol), (c)
1–NaOH (5:1, mol/mol), (d) 1–NaOH (2:1, mol/mol), (e) 1–NaOH (1:1, mol/mol); (B)
1H NMR spectra of (a) 1, (b) 1–HCl (10:1, mol/mol), (c) 1–HCl (5:1, mol/mol), (d)
1–HCl (2:1, mol/mol) in DMSO-d6 : D2O (10:1, v/v).
11

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“electron-pull”
“electron-pull”
H
+
O2N
O2N
O2N
N
N
N
O
O
N
O
N
N
H
+
H
H
N-
H
N
N
+
N
N
N
“electron-push”
“electron-pull”
Fluorescent enhancement
Very weak fluorescent emission
8.5 > pH > 2.2
Colorimetric probe
pH ≤ 2.2
pH ≥ 8.5
Scheme 2. Proposed response model of 1 to pH.
Conclusions
In this study, an optical pH probe 1 with dual-responsive ranges in acidic and
alkaline condition was synthesized. Under alkaline condition, 1 with a pKa value 10.5
could be applied as a colorimetric probe to pH change. And 1 with a pKa value 1.4
could be used as a fluorescent “turn-on” probe under acidic condition. The
investigation for the recognition mechanism of 1 to pH variation indicated that the
colorimetric responses of 1 to pH was attributed to the deprotonation of imino group
of 1, and the fluorescent recognition of 1 to pH was potentially due to the protonation
of three nitrogen atoms on the benzofurazan and quinoline rings. The present work
was not only a supplement to the detection method of pH value, but also a
development for new optical pH probes with the dual-responsive ranges.
Acknowledgments
The authors are grateful to the project of Science & Technology New Star of Zhu
Jiang in Guangzhou city (No. 2012J2200026), Guangdong Natural Science
Foundation, China (Development and recognition property of new fluorescence
probes for heavy metal ions, 2015), and Guangdong Province Higher Vocational
Colleges & Schools Pearl River Scholar Funded Scheme (2012).
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ACCEPTED MANUSCRIPT
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ACCEPTED MANUSCRIPT
300
250
200
150
100
50
0.5
0.4
0.2
2.2
13.3
0.3
pH
pH
8.5
In acidic solution
In basic solution
0.2
0.1
0.0
pKa=1.4
pKa'=10.5
0
500
550
600
650
700
750
300
400
500
600
Probe 1
Wavelength/nm
Wavelength/nm
Graphical abstract
and colorimetric
A
fluorescent
pH
probe,
4-(8-Quinolyl)amino-7-nitro-2,1,3-benzoxadiazole (1), was synthesized. Under
alkaline condition, 1 with a pKa value 10.5 could be applied as a colorimetric probe to
pH change from 8.5 to 13.3. And 1 with a pKa value 1.4 could be used as a
fluorescent “turn-on” pH probe under strong acidic condition (pH value is from 2.2 to
0.2).
15

ACCEPTED MANUSCRIPT
Highlights
• A new fluorescent and colorimetric pH probe (1) was synthesized.
• The probe could be applied as a colorimetric probe to pH change from 8.5 to 13.3.
• The probe could be used as a fluorescent “turn-on” pH probe with pH range from
2.2 to 0.2.
16