A novel colorimetric and fluorescent pH sensor derived from iminocoumarin and thiophene-carboxaldehyde

Article abstract of DOI:10.1002/hc.21049

A novel colorimetric and fluorescent pH sensor derived from iminocoumarin and thiophene-carboxaldehyde was designed and synthesized. The structures of the dye and related compounds were characterized by IR, ¹H NMR, ¹³C NMR, and mass spectra. Color change from green to yellow of the new sensor solution in ethanol-water with the decrease in the pH value from 7 to 2 was observed by the naked eye. Under acidic conditions, the intensity of the maximum fluorescence emission peak of the sensor increased gradually with the decrease in the acidity of the solution (the increase in the pH value from 2 to 7) and attained to the maximum value at about pH 6. Under basic conditions, the fluorescence intensity of the emission peak of the sensor did not exhibit a distinct change at pH ≤ 11.85, and the fluorescence was quenched at pH 13.36, concomitant with the green color of the solution turning pale. The sensor can be used as a fluorescent pH probe in the presence of common metal cations and anions without interference.

Full text of DOI:10.1002/hc.21049

Heteroatom Chemistry
Volume 23, Number 6, 2012
A Novel Colorimetric and Fluorescent pH
Sensor Derived from Iminocoumarin and
Thiophene-Carboxaldehyde
Hongqi Li, Jian Guo, Xuebin Zhang, and Zhen Chen
Key Laboratory of Science & Technology of Eco-Textile, Ministry of Education, College of
Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620,
People’s Republic of China
Received 14 June 2012; revised 16 August 2012
INTRODUCTION
ABSTRACT: A novel colorimetric and fluorescent pH
sensor derived from iminocoumarin and thiophene-
carboxaldehyde was designed and synthesized. The
structures of the dye and related compounds were
The ever-increasing demand for highly selective and
sensitive detection methods in analytical chemistry,
biomedical science, biotechnology, nanotechnology,
and the environment has stimulated extensive inter-
est in the design and synthesis of novel fluorescent
molecular sensors. The distinct advantages offered
by fluorescence detection methods include high se-
lectivity and sensitivity, short response time, and
convenient local observation. Numerous chemical
and biochemical analytes can be detected by fluo-
rescent methods, and in the case of proton as the an-
alyte fluorescent pH sensors are needed [1]. By far,
the majority of the fluorescent pH sensors are de-
rived from several classes of compounds, including
coumarins [2–8], fluoresceins [9–12], 4,4-difluoro-
4-bora-3a,4a-diaza-s-indacene dyes [13–16], near-
infrared cyanine dyes [17–24], naphthalimides
[25–27], distyryl ketones [28, 29], rhodamine dyes
[30, 31], europium(III) complexes [32, 33], and 5-
aryl-2-pyridyloxazoles [34, 35]. Other compounds
such as benzothiazole derivative [36], naphthalenes
[37], fluorene derivative [38], acridizinium deriva-
tives [39], nucleobases [40], 8-hydroxyquinoline
[41], 1,4-dihydroxyphthalonitrile [42], and xanthene
dye–labeled phosphatidylethanolamines [43] have
also been used for the construction of fluores-
cent pH probes. Apart from organic compounds,
recently it was found that organic/inorganic hy-
brid mesoporous silica nanoparticles showed flu-
orescent pH-sensing capability [44]. Among these
fluorescent compounds, coumarin dyes attracted
1
characterized by IR, H NMR, 13C NMR, and mass
spectra. Color change from green to yellow of the new
sensor solution in ethanol–water with the decrease in
the pH value from 7 to 2 was observed by the naked
eye. Under acidic conditions, the intensity of the max-
imum fluorescence emission peak of the sensor in-
creased gradually with the decrease in the acidity of
the solution (the increase in the pH value from 2 to
7) and attained to the maximum value at about pH 6.
Under basic conditions, the fluorescence intensity of
the emission peak of the sensor did not exhibit a dis-
tinct change at pH ≤ 11.85, and the fluorescence was
quenched at pH 13.36, concomitant with the green
color of the solution turning pale. The sensor can be
used as a fluorescent pH probe in the presence of com-
mon metal cations and anions without interference.
ꢀC
2012 Wiley Periodicals, Inc. Heteroatom Chem
23:551–559, 2012; View this article online at wiley-
onlinelibrary.com. DOI 10.1002/hc.21049
Correspondence to: Hongqi Li; e-mail: hongqili@dhu.edu.cn.
2012 Wiley Periodicals, Inc.
ꢀC
551

552 Li et al.
much attention as efficient chemosensors for a
vast number of analytes because the structure of
benzopyrone has many advantages such as high
fluorescence quantum yield, large Stokes shift,
excellent light and thermal stability, and less tox-
icity [45]. Following the development of a novel
colorimetric and fluorescent sensor for copper(II)
and nickel(II) based on a coumarinyl benzoylhy-
drazone [46], we report herein the synthesis, struc-
tural characterization, and spectral property of a
novel colorimetric and fluorescent pH sensor de-
rived from iminocoumarin and 5-(5,5-dimethyl-1,3-
dioxan-2-yl)thiophene-2-carboxaldehyde.
group in thiophene-2-carboxaldehyde was pro-
tected by a reaction with neopentyl glycol in
the presence of sulfamic acid as a catalyst to
produce compound 1 in 66% yield. Formyla-
tion of compound 1 by butyl lithium, tetram-
ethylethylene diamine, and N,N-dimethylformamide
(DMF) afforded compound 2 in 60% yield [48],
which reacted with coumarin-2-imine
4 (ob-
tained from 2-cyanomethylbenzimidazole and 4-
diethylaminosalicylal in 56% yield) afforded a target
molecule 5 in 78% yield. The chemical structures of
the synthesized compounds were characterized by
1H NMR, 13C NMR, IR, and mass spectrum. All of
the data in the spectra were in good accordance with
the structures.
The molecule 5 should serve as a fluorescent
and colorimetric pH sensor based on the following
characteristics: Nitrogen atoms on the heterocycles
of 5 can be protonated and then deprotonated as
shown in Fig. 1, thus resulting in changes in the
RESULTS AND DISCUSSION
The novel fluorescent probe 5 was designed and
synthesized as shown in Scheme 1. By refer-
ring to the literature method [47], the formyl
(i) n-BuLi
TMEDA
OHC
–78oC
(ii) DMF
60%
HO
OH
O
O
O
O
S
S
H2NSO3H
66%
CHO
N
S
1
2
CHO
N
CN
Et2N
OH
N
2
78%
H
N
H
3
56%
Et2N
O
NH
4
N
N
O
S
Et2N
O
N
O
5
SCHEME 1 Synthesis of sensor 5.
N
HN
N
NH
H
O
O
S
S
Et2N
O
N
Et2N
O
N
H
O
O
5
5'
FIGURE 1 Equilibrium and color change of sensor 5 under acidic conditions (2 < pH < 7).
Heteroatom Chemistry DOI 10.1002/hc

Novel Colorimetric and Fluorescent pH Sensor Derived from Iminocoumarin and Thiophene-Carboxaldehyde 553
N
N
CF3CO2H
or HCl
N
N
O
H
S
89%
Et2N
O
N
Et2N
O
O
O
6
5
SCHEME 2 Hydrolysis of sensor 5.
FIGURE 2 Single crystal structure of coumarin 6.
fluorescent intensity (and color) of the molecule un-
der different pH values. The 1,3-dioxane moiety in
molecule 5 is stable under basic and weakly acidic
conditions but will be hydrolyzed under strongly
acidic conditions, thus enabling 5 to be a colorimet-
ric pH sensor.
The color of the solution of 5 in ethanol is green.
After the addition of an acidic buffer solution, it can
be observed by the naked eye that the color changes
gradually from green to yellow with the increase in
the acidity of the solution (the decrease in the pH
value from 7 to 2) as shown in Fig. 1. Thus, com-
pound 5 serves as a colorimetric pH probe at acidic
conditions (2 < pH < 7). The change in color might
have originated from the equilibrium between 5 and
5’ (Fig. 1) under acidic conditions (2 < pH < 7), sim-
ilar to the result that was reported previously [8].
Under strongly acidic conditions (pH < 2), sen-
sor 5 hydrolyzed to form coumarin 6. The color
of the solution turned from green to yellow irre-
versibly. As shown in Scheme 2, when compound 5
was mixed with ethyl 2,2,2-trifluoroacetate, tetrahy-
drofuran (THF), and water or 1:6:7 (volume ratio)
of hydrochloric acid/water/THF and stirred at room
temperature overnight, coumarin 6 was obtained in
89% yield. The single crystal structure of coumarin
6 is shown in Fig. 2.
FIGURE 3 UV–vis absorption spectra of 5 under acidic con-
ditions of different pH values.
With the increase in the basicity of the solution (the
increase in the pH value from 7 to 13.4), the green
color of the solution turns pale gradually to finally
very pale green.
UV–vis absorption spectra of solution of com-
pound 5 in ethanol under acidic conditions of dif-
ferent pH values were measured and are shown in
Fig. 3. The solution showed absorption bands at
about 450 and 505 nm. With the decrease in the
acidity of the solution (the increase in the pH value
from 2 to 7), the intensity of the absorption band at
A change in color of the solution of 5 under basic
conditions can also be observed by the naked eye
but is not as distinct as that under acidic conditions.
Heteroatom Chemistry DOI 10.1002/hc

554 Li et al.
0.38
0.36
0.34
0.32
0.30
0.28
0.26
0.24
0.22
0.20
Fig. 5. It can be found that the intensity of the ab-
sorption band at 505 nm decreased gradually with
the decrease in the acidity of the solution (the in-
crease in the pH value).
UV–vis absorption spectra of solution of com-
pound 5 in ethanol under basic conditions of dif-
ferent pH values were measured and are shown in
Fig. 6. The solution showed two absorption bands
at about 450 and 480 nm. With the increase in the
basicity of the solution (the increase in the pH value
from 7.65 to 13.36), the intensity of both absorption
bands decreased.
Fluorescence emission spectra of compound 5
under acidic conditions (pH 2.05, 2.72, 3.41, 3.77,
4.15, 4.42, 4.86, 5.38, 5.92, 6.99) were measured at
an excitation wavelength of 473 nm and are shown
in Fig. 7. It could be found that sensor 5 exhibited
two fluorescence emission peaks at 511 and 539 nm.
Bathochromatic shifts of the fluorescence emission
peaks with the decrease in the acidity of the solution
(the increase in the pH value from 2.05 to 6.99) were
observed.
Changes in the fluorescence intensity of the flu-
orescence emission peak at 511 nm of 5 (in 1:1
ethanol/water) with the pH value (from 2 to 7) under
acidic conditions were measured, and the curve is
shown in Fig. 8. It can be seen that with the decrease
in the acidity of the solution (the increase in the
pH value) the intensity of the fluorescence emission
peak at 511 nm increased gradually and attained the
maximum value at about pH 6.
2
3
4
5
6
7
pH
FIGURE 4 Changes in the absorbance of the absorption
band at 450 nm of 5 with pH from 2 to 7.
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Similar changes in the fluorescence intensity of
the fluorescence emission peak at 539 nm of 5 (in 1:1
ethanol/water) with the pH value (from 2 to 7) under
acidic conditions were observed as shown in Fig. 9.
The intensity of the fluorescence emission peak at
539 nm also increased gradually with the decrease
in the acidity of the solution (the increase in the pH
value).
2
3
4
5
6
7
pH
FIGURE 5 Changes in the absorbance of the absorption
band at 505 nm of 5 with pH from 2 to 7.
To investigate the effect of pH on the changes in
the fluorescence intensity of different fluorescence
emission peaks at 511 and 539 nm, changes in the
ratio of the fluorescence intensity of the fluorescence
emission peak at 511 and 539 nm (I511/I539) of 5
with pH from 2 to 7 were measured and the curve is
shown in Fig. 10. It can be seen that the ratio I511/I539
increases with the decrease in the acidity (the in-
crease in the pH value) and attains the maximum
value of about 1.7 at pH 5.5. The value of I511/I539
is <1 at pH < 3.5 and becomes >1 when the acid-
ity decreases (pH > 3.5), indicating that the effect
of pH on the fluorescence intensity of the emission
peak at 539 nm is larger than that of the peak at
511 nm under strongly acidic conditions whereas
weak acidity conditions favor the increase in the
450 nm gradually decreased and the bathochromatic
shift of the absorption bands was observed.
Changes in the absorbance of the absorption
band at 450 nm of 5 (in 1:1 ethanol/water) with the
pH value (from 2 to 7) under acidic conditions were
measured, and the curve is shown in Fig. 4. It can
be found that with the decrease in the acidity of the
solution (the increase in the pH value) the intensity
of the absorption band at 450 nm increased and at-
tained the maximum value at about pH 6.
Changes in the absorbance of the absorption
band at about 505 nm of 5 (in 1:1 ethanol/water)
with the pH value (from 2 to 7) under acidic con-
ditions were measured, and the curve is shown in
Heteroatom Chemistry DOI 10.1002/hc

Novel Colorimetric and Fluorescent pH Sensor Derived from Iminocoumarin and Thiophene-Carboxaldehyde 555
FIGURE 6 UV–vis absorption spectra of 5 under basic conditions of different pH values.
180
160
140
120
100
80
pH
2.05
2.72
3.41
3.77
4.15
4.42
4.86
5.38
5.92
6.99
60
40
20
0
480
500
520
540
560
580
600
Wavelength (nm)
FIGURE 7 Fluorescence emission (λex = 473 nm) spectra of 5 under acidic conditions of different pH values.
solutions containing 5 in ethanol (10−5 mol L−1
,
fluorescence intensity of the fluorescence emission
peak at 511 nm.
10 mL) and aqueous solution of metal cation or
anion (2 × 10−4 mol L−1, 1 mL) were measured
and are shown in Fig. 12 to investigate the influ-
Fluorescence emission spectra of compound 5 at
basic conditions (pH 7.65, 8.63, 10.61, 11.85, 13.36)
were measured and are shown in Fig. 11. It could be
found that the fluorescence intensity of the emission
peak at 511 nm did not exhibit a distinct change at
pH ≤ 11.85. However, under strongly basic condi-
tion (pH 11.36) the fluorescence was quenched, con-
comitant with the green color of the solution turning
pale.
ence of common metal cations (Mg2+, Zn2+, Ca2+
,
Cu2+, Ni2+, Hg2+, Co2+, Fe3+, Al3+, K+) and anions
(NO2−, F−, Cl−, HSO3−, HSO4−, CH3COO−, NO3
)
−
on the fluorescence intensity of the emission peak
of 5. It can be seen that the addition of common
metal cations and anions does not show substan-
tial influence on the fluorescence intensity of the
emission peak of 5. Therefore, compound 5 can
be used as a fluorescent pH probe in the presence
Fluorescence intensities of the emission peak at
507 nm (at an excitation wavelength of 473 nm) of
Heteroatom Chemistry DOI 10.1002/hc

556 Li et al.
180
160
140
120
100
80
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
60
40
20
0
2
3
4
5
6
7
2
3
4
5
6
7
pH
pH
FIGURE 10 Changes in the ratio of the fluorescence inten-
sity of the fluorescence emission peak at 511 and 539 nm
(I511/I539) of 5 with pH from 2 to 7.
FIGURE 8 Changes in the fluorescence intensity of the
fluorescence emission peak at 511 nm of 5 with pH from
2 to 7.
100
90
80
70
60
50
40
30
20
FIGURE 11 Fluorescence emission spectra of 5 under basic
conditions of different pH values.
2
3
4
5
6
7
pH
FIGURE 9 Changes in the fluorescence intensity of the
fluorescence emission peak at 539 nm of 5 with pH from
2 to 7.
chemical shifts were recorded internally referenced
to Me4Si (0 ppm). IR spectra were obtained on a
Thermo Electron Corporation (USA) Nicolet 380
FT-IR spectrophotometer. Mass spectra were ob-
tained on an Agilent 6000 LC-MS instrument (ESI,
pos mode, 70–1000 amu). Fluorescence spectra were
recorded on a Hitachi F-4500 spectrofluorometer
(Tokyo, Japan). UV–vis spectra were recorded on a
JASCO V530 spectrometer (Tokyo, Japan). X-Ray
crystal structure was measured on a Bruker Smart
CCD diffractometer by using Mo Kα radiation at
293 K.
5-(5,5-Dimethyl-1,3-dioxan-2-yl)thiophene-2-
carboxaldehyde (compound 2) was obtained as
an oil by using the method reported in the lit-
erature [48]. 2-Cyanomethylbenzimidazole (3)
was prepared from o-phenylenediamine and ethyl
of common metal cations and anions without
interference.
EXPERIMENTAL
The reagents and solvents employed in this study
were of analytical grade and were purchased
from commercial sources and used as received.
Melting points were determined on a WRS-2A
capillary melting apparatus (Shanghai Optical
Instruments Company, Shanghai, China), and the
quoted temperatures were uncorrected. 1H NMR
and 13C NMR spectra were recorded on a Bruker AM
400 spectrometer. CDCl3 was used as a solvent, and
Heteroatom Chemistry DOI 10.1002/hc

Novel Colorimetric and Fluorescent pH Sensor Derived from Iminocoumarin and Thiophene-Carboxaldehyde 557
piperidine (5 drops) in EtOH (30 mL) were added
and the mixture was refluxed at 79◦C for 2 h. After
the reaction, the mixture was cooled to room tem-
perature and filtered. The solid product was washed
with ethanol and recrystallized from DMF to afford
compound 4 (1.86 g, 56%) as orange-yellow powder,
mp 239◦C (lit. value: 239◦C) [51].
Synthesis of 5,5-Dimethyl-2-{5-(9-diethylamino-
5H-7-oxa-4b,6,13-triazaindeno[2,1-a]anthra-
cen-5-yl)thiophen-2-yl}-1,3-dioxane (5)
To a flask, compound 4 (1.32 g, 4 mmol), compound
2 (1.36 g, 4 mmol), ammonium acetate (0.92 g,
12 mmol), piperidine (10 drops), and ethanol
(60 mL) were added. The mixture was refluxed at
79◦C for 20 h then cooled to room temperature. After
FIGURE 12 Influence of metal cations or anions on the flu-
orescence intensity of the emission peak at 507 nm (λex
=
filtration, the solid product was washed with ethanol
and recrystallized from a mixed solvent consisting of
DMF and butanol to give 5 (1.56 g, 78%) as orange-
yellow powder, mp 242–244◦C. IR (KBr): 3450, 3052,
2949, 2852, 1665, 1611, 1418, 1265, 1212, 1090, 1018,
473 nm) of 5 (1: blank; 2: Mg2+; 3: Zn2+; 4: Ca2+; 5: Cu2+; −6:
Ni2+; 7: Hg2+; 8: Co2+; 9: Fe3+; 10: Al3+; 11: K+; 12: NO2
;
13: F−; 14: Cl−; 15: HSO3−; 16: HSO4−; 17: CH3COO−; 18:
NO3−).
747 cm−1
.
1H NMR (400 MHz, DMSO-d6): δ =
cyanoacetate according to the literature procedures,
mp 210–211◦C (lit. value 209.7–210.7◦C) [49].
0.67 (s, 3H, CH3), 1.04 (s, 3H, CH3), 1.14 (t,
6H, J = 7.0 Hz, 2 × CH3), 3.44 (q, 4H, J =
7.0 Hz, 2 × NCH2), 3.53 (d, 4H, J = 9.9 Hz,
2 × OCH2), 5.54 (s, 1H, N2CH), 6.51 (d, 1H, J =
2.4 Hz, Ph-H), 6.66 (d, 1H, J = 8.8, 2.4 Hz, Ph-H),
6.95–7.23 (m, 4H, Ph-H), 7.41 (d, 1H, J = 8.0 Hz,
Ph-H), 7.51 (d, 1H, J = 8.8 Hz, Ph-H), 7.63 (d, 1H,
J = 8.0 Hz, Ph-H), 7.69 (s, 1H, O2CH), 8.19 (s, 1H,
CH). MS (ESI): m/z 541.2 (M+ + 1).
Synthesis of 5,5-Dimethyl-2-(2’-thiophenyl)-1,3-
dioxane (1)
A mixture consisting of thiophene-2-carboxaldehyde
(2.24 g, 20 mmol), neopentyl glycol (4.16 g,
40 mmol), sulfamic acid (0.29 g, 3 mmol), and
toluene (20 mL) was heated to 110◦C and refluxed
for 24 h. After the reaction, the mixture was cooled
to room temperature and evaporated. The residue
was purified by column chromatography on silica gel
(petroleum ether/ethyl acetate, 100:1, v/v) to give 1
(2.83 g, 66%) as white crystals, mp 71–73◦C (lit. value
70–71◦C) [50]. Rf = 0.50 (petroleum ether/ethyl ac-
etate, 100:1, v/v). IR (KBr): 2956, 2852, 1576, 1448,
Hydrolysis of 5 to Form 7-Diethylamino-3-(ben-
zimidazol-2-yl)coumarin (6)
A mixture consisting of compound 5 (0.15 g,
0.277 mmol), ethyl 2,2,2-trifluoroacetate (3 mL),
THF (60 mL), and water (15 mL) was stirred at room
temperature overnight. The reaction was quenched
with saturated sodium bicarbonate. The solution
was extracted with methylene dichloride (30 mL ×
3). The organic phase was washed with 2% of sodium
bicarbonate solution (30 mL) and dried over anhy-
drous magnesium sulfate. After filtration, the sol-
vent was evaporated and the residue was purified
by column chromatography on silica gel (methylene
dichloride/ethyl acetate, 20:1, v/v) to give 6 (0.11 g,
89%) as a yellow solid, mp 232–234◦C (lit. value
233◦C) [52]. Rf = 0.50 (methylene dichloride/ethyl
acetate, 20:1, v/v). IR (KBr): 3340, 3044, 2968, 1691,
1619, 1584, 1526, 1401, 1349, 1308, 1248, 1188, 1128,
1
1384, 1210, 1099, 1022 cm−1. H NMR (400 MHz,
CDCl3): δ = 0.82 [s, 3H, 5-CH3 (eq)], 1.31 [s, 3H, 5-
CH3 (ax)], 3.67 (dd, J = 11.2, 0.8 Hz, 2H, 4,6-Hax),
3.79 (dd, J = 11.2, 1.2 Hz, 2H, 4,6-Heq), 5.68 (s, 1H,
2-H), 7.02 (dd, J = 5.0, 3.6 Hz, 1H, 4’-H), 7.17 (ddd,
J = 3.6, 1.2, 0.8 Hz, 1H, 3’-H), 7.33 (dd, J = 5.0, 1.2
Hz, 1H, 5’-H). 13C NMR (101 MHz, CDCl3): δ = 21.85
[CH3 (eq)], 23.00 [CH3 (ax)], 30.22 (5-C), 77.57 (4,6-
C), 98.31 (2-C), 125.09 (4’-C), 125.69 (3’-C), 126.47
(5’-C), 141.37 (2’-C). MS (ESI): m/z 199.1 (M+ + 1).
Synthesis of 7-Dethylamino-3-(benzimidazol-2-
yl)coumarin-2-imine(4)
1074, 735 cm−1 1H NMR (400 MHz, DMSO-d6):
.
To a flask, compound 3 (1.57 g, 10 mmol), 4-
diethylaminosalicylaldehyde (1.93 g, 10 mmol), and
δ = 1.15 (t, 6H, J = 7.0 Hz, 2 × CH3), 3.48 (q,
4H, J = 6.9 Hz, 2 × CH2), 6.66 (d, 1H, J = 2.2 Hz,
Heteroatom Chemistry DOI 10.1002/hc

558 Li et al.
8-H), 6.81 (dd, 1H, J = 9.2, 2.4 Hz, 6 - H), 7.17–7.29
(m, 2H, Ph-H), 7.58–7.67 (m, 2H, Ph-H), 7.69 (d, 1H,
J = 9.2 Hz, 5-H), 8.90 (s, 1H, CH). 13C NMR (101
MHz, CDCl3): δ = 12.38 (2×CH3), 44.98 (2×CH2),
96.82, 107.87, 108.70, 109.98, 111.26, 122.55, 130.31,
142.59, 147.89, 151.81, 156.71, 161.98. MS (ESI): m/z
334.1 (M+ + 1).
pH probe in the presence of common metal cations
and anions without interference.
ACKNOWLEDGMENT
The authors acknowledge the support of the Funda-
mental Research Funds for the Central Universities.
Measurement of UV–Vis and Fluorescence
Spectra
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A solution of compound 5 in ethanol (10−5 mol
L
−1) was used for measurement of UV–vis absorp-
tion and fluorescence emission spectra. For the
measurement under acidic conditions, buffer solu-
tions of different pH values were prepared from
0.1 mol L−1 of citric acid aqueous solution and
0.2 mol L−1 of Na2HPO4 aqueous solution. The
solution of 5 in ethanol (5 mL) and the buffer so-
lution (5 mL) of different pH values were mixed be-
fore measurement. For the measurement under ba-
sic conditions, basic aqueous solutions of different
pH values were prepared from 0.1 mol L−1 of aque-
ous hydrochloric acid solution and 0.1 mol L−1 of
aqueous sodium hydroxide solution. The solution of
5 in ethanol (5 mL) and the basic aqueous solution
(5 mL) of different pH values were mixed before
measurement.
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L.-G. J Fluoresesc 2005, 15, 829–833.
CONCLUSIONS
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A new compound derived from iminocoumarin and
thiophene-carboxaldehyde was designed and synthe-
sized as a colorimetric and fluorescent pH sensor.
When the pH value of the solution of the sensor in
ethanol–water decreased from 7 to 2, a distinct color
change from green to yellow was observed with the
naked eye. Under acidic conditions, the intensity of
the maximum fluorescence emission peak of the sen-
sor increased gradually with the decrease in the acid-
ity of the solution (the increase in the pH value from
2 to 7) and attained the maximum value at about pH
6. Under basic conditions, the fluorescence inten-
sity of the emission peak of the sensor was almost
constant at pH ≤ 11.85 and the fluorescence was
quenched at pH 13.36, concomitant with the green
color of the solution turning pale. Investigation on
the influence of common metal cations (Mg2+, Zn2+
,
Ca2+, Cu2+, Ni2+, Hg2+, Co2+, Fe3+, Al3+, K+) and
anions (NO2−, F−, Cl−, HSO3−, HSO4−, CH3COO−,
NO3−) on the fluorescence intensity of the emission
peak of the sensor indicated that the addition of com-
mon metal cations and anions did not affect the fluo-
rescence intensity of the emission peak of the sensor
substantially. The sensor can be used as a fluorescent
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