A Versatile Aggregation‐induced Emission Fluorescent Probe for Visible Detection of pH

Article abstract of DOI:10.1007/s10895-020-02669-x

By tactfully structuring a luminescent molecule as an accurate pH probe with aggregation-induced emission (AIE) feature, it is significant to overcome aggregation-caused quenching of emitted light in practice. Herein, we present a simple AIE-active fluorescence probe for pH detection on the basis of intramolecular charge transfer (ICT) with wide response range and high sensitivity reaction. The donor-acceptor-donor (D-A-D) style probe utilized a conjugated structural hybrid of the electron-withdrawing nitrile group and electron-donating hydroxyl as well as dimethylamino groups for fluorescent platform. The AIE-active probe possesses good fluorescence under water fraction up to 90% in mixed MeOH/water system. Furthermore, it can be used in profiling and visualization of pH detection in MeOH/water system at f_w = 90% under UV 365?nm lamp. What’s more, the probe can be employed to be a broad range test paper of pH detection, paving the way for low-cost practical applications.

Full text of DOI:10.1007/s10895-020-02669-x

Journal of Fluorescence (2021) 31:475–485
https://doi.org/10.1007/s10895-020-02669-x
ORIGINAL ARTICLE
A Versatile Aggregation‐induced Emission Fluorescent Probe
for Visible Detection of pH
Meihui Chen1 & Yi Ren1 & Huan Liu1 & Qian Jiang1 & Jing Zhang1 & Mingguang Zhu1
Received: 10 November 2020 /Accepted: 28 December 2020
/ Published online: 12 January 2021
#
The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature 2021
Abstract
By tactfully structuring a luminescent molecule as an accurate pH probe with aggregation-induced emission (AIE) feature, it is
significant to overcome aggregation-caused quenching of emitted light in practice. Herein, we present a simple AIE-active
fluorescence probe for pH detection on the basis of intramolecular charge transfer (ICT) with wide response range and high
sensitivity reaction. The donor-acceptor-donor (D-A-D) style probe utilized a conjugated structural hybrid of the electron-
withdrawing nitrile group and electron-donating hydroxyl as well as dimethylamino groups for fluorescent platform. The AIE-
active probe possesses good fluorescence under water fraction up to 90% in mixed MeOH/water system. Furthermore, it can be
used in profiling and visualization of pH detection in MeOH/water system at fw = 90% under UV 365 nm lamp. What’s more, the
probe can be employed to be a broad range test paper of pH detection, paving the way for low-cost practical applications.
.
.
.
.
Keywords α-cyanostilbene Aggregation‐induced emission Intramolecular charge transfer pH probe Fluorescence
Introduction
received considerable attention because of the stilbenic π-
conjugated backbone equipped with cyano-group, which can
display a lacking electron character. Affixing electron-donating
substituents at the positions of aryl groups in the α-cyanostilbene
unit, significantly influences the electron density distribution
(band gap) of π-conjugated system (i.e. the emission properties
in solid and solution states) via intramolecular charge transfer
(ICT) [46].
Accurate and reliable measurement of pH plays a great
importance in a wide range of chemical, environmental, bio-
logical and industrial applications. In recent years, although
numerous highly sensitive, flexible, real-time, reversible and
reliable pH sensors have been fabricated with self-healing
polymer [47], hydrogel [48], electrochemical reaction [49,
50], fluorescent molecular [51–53], and so forth, the biocom-
patibility and aggregation are still the main limitations in the
use of fluorescent signals. Considering that the colorimetric
fluorescent molecular for pH detection is still vastly superior
due to its high sensitivity, simple operation and dynamic mon-
itoring, especially the AIE-active pH sensors are proved to be
more advantageous than the dyes having the fluorescence
quenching effect in the aggregated or solid states [17,
54–60], Therefore, It is well accepted that development of a
versatile AIE fluorescent probe for detecting real-time chang-
es of pH in both solution and solid states has important
significance.
Since its inception coined by Tang in 2001 [1], aggregation-
induced emission (AIE) luminogens, which depend on the avail-
ability of strong fluorescence emission in the aggregated states or
solid states, have been exceedingly explored in versatile fields
such as high-performance emissive materials [2, 3], electrolumi-
nescence [4, 5], photovoltaic devices [6–9], sensors [10–12],
hydrogels [13], liquid crystals [14–16], fluorescence imaging
[17–20], pathogen identification [21], photosensitizers [22, 23],
drug delivery [24], and cancer cell ablation [25]. In view of the
manifestation and augmentation of radiative excited states for
organic molecular systems, a series of AIE-active organic
luminogens have been exploited so far, including
tetraarylethenes [5, 10, 11, 15, 26, 27], phenanthroimidazole [6,
28], carbazole [9, 29], triphenylamine [30–32], fluorenone [33],
phenanthrenequinone [34], BODIPY [35], thieno[3,2-b]thio-
phene S,S-dioxide [36], terpyridine [37], salicylaldazine [38],
triazatruxene [39], cyanostilbenes [14, 16, 20, 40–46] derivatives
and so on. Among them, α-cyanostilbene derivatives have
* Mingguang Zhu
zhumg91_njun@126.com
1
College of Chemistry and Chemical Engineering, Neijiang Normal
University, 641100 Neijiang, P. R. China

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J Fluoresc (2021) 31:475–485
In this work, we explored a colorimetric AIE-active fluo-
dissolved in ethyl alcohol (50 mL), stirred and refluxed for
6 h. Reaction progress was monitored by TLC. As the com-
pletion of the reaction, the mixture was cooled to ambient
temperature, the excess solvent was evaporated under reduced
pressure. After admixing with 30 mL of HCl solution (1 M),
the precipitate was formed in resulting mixture, filtered, and
purified with recrystallization in methyl alcohol/water (1/1,
v/v) to obtain compound 1 as a saffron yellow powder.
rescent probe for pH detection in mixed MeOH/water system.
The probe was constructed using a conjugated structural sys-
tem in which dimethylamino and hydroxyl groups appeared to
interact significantly with acid and base, respectively. With
changing the pH from 7.4 to 1.8 under acidic conditions, the
emission peak of the probe gradually declined and generated
blue shift from 547 nm to 484 nm, and the color of solution
changed from bright orange-yellow to blue under UV 365 nm
light. Under alkaline conditions, as the pH gradually in-
creased, a new fluorescence peak emerged and enhanced at
589 nm with a 42 nm red-shift, and the yellow color of the
solution significantly enhanced under UV 365 nm light.
Furthermore, the probe had high sensitivity and selectivity.
Finally, the probe was developed as a pH test strip for a broad
range of pH detection in practical application.
1
Compound 1: Yield 87%; H NMR (500 MHz, CD3OD)
δ
ppm: 7.75 (d, J = 9.0 Hz, 2H, Ar-H), 7.41 (d, J = 9.0 Hz,
2H, Ar-H), 7.37 (s, 1H, CH3), 6.78 (d, J = 9.0 Hz, 2H, Ar-
H), 6.72 (d, J = 8.5 Hz, 2H, Ar-H), 2.95 (m, 6H, N(CH3)2);
anal. Calcd for C17H16N2O: C, 77.25; H, 6.10; N, 10.60; O,
6.05. Found: C, 77.91; H, 6.02; N, 11.18; O, 6.35. MALDI-
MS (C17H16N2O) [M]+: calcd: m/z = 264.13. found: m/z =
264.04.
Fluorescence and UV-Vis Measurements
Experimental
For absorption or fluorescence measurements, probe 1 was
dissolved in CH3OH to obtain stock solutions (1.0 × 10− 3
mol/L). The stock solutions of ions (1.0 × 10− 2 mol/L) were
prepared in deionized water by dissolving a pre-determined
amount of their salts. The pH meter was adjusted by using
standard solution of potassium hydrogen phthalate (pH =
4.00) and mixed phosphate (pH = 6.86). The pH-dependent
spectral characteristics of probe 1 were adjusted by HCl and
NaOH. In the interfering experiment, the pH = 4.00 buffer
solution was prepared by dissolving potassium hydrogen
phthalate in deionized water, and the pH = 9.18 buffer solution
was prepared by dissolving sodium tetrabrate in deionized
water. The emission spectra of the samples were recorded with
Δλex = 5 nm and Δλem = 10 nm, respectively. For the absor-
bance and the fluorescence of probe in solid state, 50 mg of
probe 1 was dissolved in 10 mL methanol, and then the solu-
tion was dropwise added on quartz cuvettes, followed by nat-
ural evaporation at room temperature.
Materials and Equipments
Unless indicated otherwise, all chemicals acquired commer-
cially were used without further purification. Metal ions were
all nitrates salts, and anions were all sodium salt. Thin-layer
chromatography (TLC) was performed on glass plates pre-
coated with silica gel. Chemical structures were identified by
hydrogen nuclear magnetic resonance (1H NMR) spectra,
which were obtained with a Bruker-ARX 400 spectrometer
in CD3OD solvent, and calibrated using tetramethylsilane
(TMS, δ = 0 ppm) as internal standard. Chemical shifts are
quoted in parts per million (ppm), and coupling constants
are made out as J-values in Hz. MALDI mass spectrum was
determined on a Bruker MALDI-Mass spectrometer.
Elemental analysis (C, H and N) was performed on a
PerkinElmer 2400 CHN Analyzer. Emission and absorbance
spectra were recorded in quartz cuvettes at room temperature
on a UV-visible absorption spectrophotometer (Shimadzu,
UV-2550) and a fluorescence spectrophotometer (Shimadzu,
RF-5301PC), respectively. The fluorescence absolute quan-
tum yields (ΦF) were measured on an Edinburgh Instruments
FLS920 fluorescence spectrometer with a calibrated integrat-
ing sphere. The pH value of solution was measured and con-
trolled with a PHS-3C pH meter (Shanghai LeiCi Device
Works, China). Density functional theory (DFT) (B3LYP/6-
31G (d) level of theory) was utilized to model the structure
geometry and electronic properties of relevant molecular.
Calculation of pKa
The pKa value can be analyzed using the well-known
Henderson-Hasselbach type equation: pKa = pH log((Fmax F)/
(F Fmin)), where Fmax and Fmin are maximal and minimal fluo-
rescence intensity at detection range, respectively. F is the fluo-
rescence intensity of probe 1 under corresponding pH, whereas
the eventual pKa refers to the average estimated value for a
succession of pKa under acidic or basic pHs, respectively [61].
Synthesis of Probe 1
Preparation of Test Paper Strips
Under a nitrogen atmosphere, 4-hydroxybenzene acetonitrile
(2.663 g, 20 mmol), p-dimethylaminobenzaldehyde (2.984 g,
20 mmol) and sodium hydroxide (0.8 g, 20 mmol) were
The test paper strips were obtained by cutting filter paper with
the size of 10 × 20 mm. Prepared filter paper was placed in
methyl alcohol solution of the probe 1 (2.0 × 10− 3 mol/L), and

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477
dried in the air. Then 50 mL HCl and NaOH solutions with
different pH were dripped into the filter paper. After placing
the test paper 30 mm above the solvent of hydrochloric acid
and ethylenedi-amine respectively and keeping it for 30 s, the
color changes of the test paper strips were observed under UV
365 nm lamps.
112 nm. Compared with the spectra in MeOH solution, the
absorption and emission peaks of probe 1 in solid are severely
bathochromic shifted to 415 nm and 550 nm, respectively. A
stokes shift reaches up to 187 nm, the ΦF is of 0.34.
Additionally, the absorption and fluorescence spectra of probe
1 in both MeOH solution and solid state keep unchanged after
being left to natural light for 10 days, indicating its high
photostability.
Result and Discussion
Aggregation‐induced Emission
Synthesis and Characterization
Investigation of the AIE behaviour of probe 1 was carried out
in MeOH /water system through the fluorescence spectra. As
shown in Fig. 2a, the probe 1 has a weak fluorescence signal at
490 nm in pure MeOH solution. The fluorescence intensity
maintains at low level with the increase of the fw from 0 to
40% in MeOH /water system. However, on increasing fw from
40 to 90%, the fluorescence intensity begins to enhance and
reaches the summit with 7-fold enhancement at fw = 90%,
highlighting the contribution of the molecular aggregation
state, which limites the free rotation of aryl groups and reduces
the nonradiative transition. The fluorescence spectra of probe
1 exhibit a remarkably red-shift from 490 nm in pure MeOH
solution to 547 nm at fw = 90% in MeOH/water system, orig-
inating from intramolecular charge transfer (ICT) state along
with an increase of solvent polarity, which depends mainly on
the electron-withdrawing nitrile group and electron-donating
hydroxyl as well as dimethylamino groups. Meanwhile, the
ΦF of probe 1 is 0.31 at fw = 90%, which is higher than that in
pure MeOH solution (ΦF = 0.13). While the emission intensity
begin to quench when the fraction of water continue to in-
crease to fw = 95% (Fig. 2b), mainly due to the production
of more compact nanoaggregate. Since probe 1 possesses an
excellent AIE property and shows the fluorescence maximum
emission in MeOH /water system at fw = 90%, we mainly
focused on this solution system to further explore the pH
sensor.
The chemical structure and synthetic route of probe 1 is shown
in Scheme 1. Probe 1 was obtained by a simple one-step
reaction between 4-hydroxybenzene acetonitrile and
dimethylaminobenzaldehyde, having a favourable yield
(87%). The chemical structure of probe 1 was characterized
by various characterization techniques (NMR, MALDI-MS
and elemental analysis), and the results fitted well to the ex-
1
pected structure. H NMR spectrum and the details of other
characterization spectra are given in Supplementary
Information. Probe 1 exhibited good solubility in common
organic solvents such as methyl alcohol, ethyl alcohol, tetra-
hydrofuran, dichloromethane, acetonitrile, and so on, but in-
soluble in water.
Photophysical Properties of Probe 1
The photographs of probe 1 in MeOH solution (10 µM) and
solid state under natural light and UV 365 nm lamp were
presented in Fig. 1b. The probe 1 in MeOH solution is golden
colour under natural light and shows barely fluorescence un-
der UV 365 nm lamp, respectively. Moreover, the solid state
of probe 1 is pale yellow solid under natural light and presents
brick-red fluorescence under UV 365 nm lamp, respectively.
To evaluate the photophysical properties of probe 1 in a
single-molecule and agminated states, the UV–vis absorption
and fluorescence spectra of probe 1 were recorded in MeOH
solution and solid state, respectively (Fig. 1a). The corre-
sponding data was summarized in Table 1. In MeOH solution,
the well-structured probe 1 exhibits a broad absorption band in
the range of 300–500 nm due to the conjugated skeleton, a
main maximum absorption peak is located at 378 nm. Under
an excitation wavelength of 380 nm, there is a weak fluores-
cence emission peak at about 490 nm with a fluorescence
quantum yield (ΦF) of 0.13, a large stokes shift is about
Optical Response of the Probe 1 to pH
The pH-dependent sensing performances of probe 1 in the
mixed solution of MeOH/water (10 µM, 1/9, v/v) were further
explored by utilizing UV absorption and fluorescence emis-
sion spectra in detail. As observed in Fig. 3a, probe 1 shows a
strong absorption peak at 446 nm in original solution (pH =
7.4) before H+-triggered titration process. There is a drastic
NC
OH
NaOH
+
N
OH
N
CHO
EtOH
NC
1
3
2
Scheme 1 Synthesis of AIE-active pH probe 1: anhydrous EtOH, NaOH, refluxing, 6 h, yield: 87%

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Fig. 1 a the UV absorption and fluorescence emission spectra of probe 1 in MeOH solution (10 µM, λex = 380 nm) and solid (λex = 420 nm). b The
photographs of probe 1 (10 µM) in MeOH solution and solid under natural light and UV 365 nm lamp
attenuation of absorption band at 446 nm caused by the vari-
ation of pH from 7.4 to 1.8, whereas a new peak emerges and
strengthens at 358 nm with an well-defined isosbestic point at
398 nm, reflecting that the protonation of dimethylamino moi-
ety was the solely acting factor under the circumstances. The
obvious blue shift (~ 88 nm) of the absorption spectra from
446 nm to 398 nm means that the electron-donating ability of
dimethylamino group was weakened and the conjugation of N
atom and aromatic core was breaked due to the protonation of
dimethylamino moiety. Simultaneously, an intuitive colour
change of the aforementioned effect was given in
Supplementary Information (Fig. S3(a)), the solution colour
of probe 1 is from light yellow to colourless upon decreasing
pH from 7.4 to 1.8 under natural light. Additionally, the pH-
dependent changes in fluorescence emission spectra of probe
1 were demonstrated in Fig. 3b. On decreasing the solution pH
from 7.4 to 1.8, an intense emission band located at 547 nm
(ΦF = 3.15) in original solution is declined gradually and
moved to 484 nm (ΦF = 1.27) with a blue shift of 63 nm under
the solution pH of 1.8 (λex = 380 nm), indicating that the pro-
tonation of dimethylamino group can reduce the rigidity of
probe molecule, make a dent in ICT effect in molecule. As
expected, the luminescence colour of the probe 1 solution
changes from bright orange-yellow to blue as the pH value
decreases from 7.4 to 1.8 under UV365nm light irradiation (Fig.
S3(b)). What’s more, the sigmoidal fitting of the relative emis-
sion intensity ratio (I/Imax) and pH value gives a pKa (average
pKa of protonated dimethylamino unites under acidic pHs) of
4.23 (Fig. 3c), and a good linear relationship (R2 = 0.99668)
between the fluorescent intensity and pH values is obtained in
the pH range of 3.9–5.9 (Fig. 3d), the linear regression equa-
tion is y = 357.04819x-677.91114.
Under alkaline conditions, the absorption spectra of probe 1
displayed in Fig. 4a have a gradual enhancement and the relevant
absorption bands become stronger when the pH increase from
7.4 to 12.0, implying that the structure of probe 1 was altered at a
higher pH. The peak shifted from 446 nm to a higher wavelength
around 487 nm at pH = 12.0, generating a conspicuous red-shift
(41 nm) as compared to that at pH = 7.4. Moreover, the fluores-
cence titration reveals a tendency similar to that of the UV spectra
(Fig. 4b). As the pH jump from 7.4 to 12.0, a new fluorescence
peak emerges and heightens at 589 nm (ΦF = 3.49), along with a
42 nm red-shift. Because the deprotonation of the phenolic hy-
droxyl group of probe 1 is taken into account in the OH−-trig-
gered process, the above observations could be explained by the
electron-donating and p-π conjugate effects of O atom with a
negative charge under alkaline conditions. What’s more, the sig-
moidal fitting of the relative emission intensity ratio (I/Imax) and
pH value affords a pKa (average pKa of deprotonated phenolic
hydroxyl groups under basic pHs) of 10.87 (Fig. S4(a)). A good
linear relationship (R2 = 0.99196) between the fluorescent inten-
sity and pH values is achieved in range of pH values of 10.0-11.3
(Fig. S4(b)), and the linear regression equation is y =
293.36032x-1314.22267. Of note, this OH−-triggered titration
process makes an apparent effect on luminescence colour of
the probe 1 solution, which changes from bright orange-yellow
to yellow as the pH jump from 7.4 to 12.0 under UV365nm light
irradiation (Fig. S5). As a result, probe 1 is capable of sensing
Table 1 The photophysical data
of probe 1
Compound
states
λabs (nm)
λem (nm)
Stokes shift (nm)
Φ F
1
in MeOH solution
in solid
378
415
490[a]
550[b]
112
135
0.13
0.34
[a] λex = 380 nm;
[b] λex = 420 nm

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Fig. 2 a Fluorescence emission spectra of probe 1 in MeOH/water system with different water fractions (10 µM, λex = 380 nm). b Plot of the
fluorescence intensities of probe 1 in MeOH/water system with different water fractions (Inset: fluorescence images)
extreme pH values over two wide pH windows (pH = 3.9–5.9
and 10.0-11.4). This result indicates that this probe has potential
application value for the quantitative determination of acidic pH
values by the fluorescence methods.
monitored repeatedly through regulating and controlling pH
values between 1.8 and 7.4, 7.4 and 12.0 several times in the
mixed solution of MeOH/water (10 µM, 1/9, v/v). As shown
in Fig. 5a and b, the probe enables to restore the fluorescence
intensity at pH = 1.8 and pH = 12.00 after six cycles, hinting
the probe with a favourable reversibility.
Reversibility, Interference and Time Course Test
In addition, an estimation of the fluorescent time-response
of probe 1 (10 µM) to the solution of pH = 1.8 and pH = 12.00
was carried out, respectively. The sensing process is very
A good reversibility of the probe is the prerequisite for prac-
tical application, the emission intensities of probe 1 were
Fig. 3 a The absorption spectra of probe 1 (10 µM) in MeOH/water
system with decreasing pH from 7.4 to 1.8. b The fluorescent emission
spectra of probe 1 in MeOH/water system with decreasing pH from 7.4 to
1.8 (10 µM, λex = 380 nm). c The sigmoidal fitting of fluorescence
intensity ratios I/Imax of probe l in MeOH/water system at various pH
values. d A good linear plot of fluorescence intensity Imax in the pH range
of 3.9–5.9

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Fig. 4 a The absorption spectra of probe 1 (10 µM) in MeOH/water system with decreasing pH from 7.4 to 12.0. b The fluorescent emission spectra of
probe 1 in MeOH/water system with decreasing pH from 7.4 to 12.0 (10 µM, λex = 380 nm)
quick after probe 1 mixed with pH solution for 1 min, and the
fluorescence intensity remains stable over a period of 2 h
(Fig. 5c). That means the probe 1 can be used as a high-
efficiency fluorescence sensor for pH detection.
In fact, there may be a deal of ion species to disturb the
sensing process in actual application. To evaluate the high
selectivity of probe 1 towards H+ and OH− over other potential
competitive ion species, the interference experiments of probe
1 (10 µM) for pH detection were measured the in the media
(pH = 5.9 and pH = 10.5) mixed with other ion species (5.0
equiv.), including K+, Na+, Ca2+, Mg2+, Zn2+, Fe2+, Fe3+,
Cu2+, Ba2+, Al3+, Co2+, Ni2+, Cd2+, Cr2+, Pb2+, Mn2+, F−,
Cl−, Br−, I−, CO32−, NO3−, NO2−, SO42−, SO32−, S2−
,
CH3COO− ClO4 , H2PO4 , HPO42−. As shown in Fig. 5d,
−
−
,
the fluorescence intensities examined for other metal ions in
the solution of pH = 5.9 is similar to that induced by H+ alone.
Fig. 5 The reversible fluorescence emission intensities of probe 1 at
484 nm between pH 1.8 and 7.4 (a) and at 589 nm between pH 7.4 and
12.0 (b) in the mixed solution of MeOH/water (λex = 380 nm, 10 µM, 1/9,
v/v). (c) Time-dependent fluorescence intensity changes of probe 1 at
pH = 1.8 (λem = 484 nm) and at pH = 12.0 (λem = 589 nm). (d) Change
in emission intensity of probe 1 in the mixed solution of MeOH/water
(λex = 380 nm, 10 µM, 1/9, v/v) at pH = 5.9 (λem = 546 nm) and at pH =
10.5 (λem = 585 nm) upon addition of various ions (5.0 equiv.), respec-
tively. 1, blank; 2, K+; 3, Na+; 4, Ca2+; 5, Mg2+; 6, Zn2+; 7, Fe2+; 8, Fe3+
;
9, Cu2+; 10, Ba2+; 11, Al3+; 12. Co2+; 13, Ni2+; 14, Cd2+; 15, Cr2+; 16,
Pb2+; 17, Mn2+; 18, F−; 19, Cl−; 20, Br−; 21, I−; 22, CO32−; 23, NO3−; 24,
NO2−; 25, SO42−; 26, SO32−; 27, S2−; 28, CH3COO−; 29, ClO4−; 30,
H2PO4−; 31, HPO42−; black bar: pH = 5.9; red bar: pH = 10.5

J Fluoresc (2021) 31:475–485
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In the solution with pH = 10.5, most of the competitive ions
have negligible impact on the pH detection, whereas the pres-
ence of Cu2+ leads to minor fluorescence enhancement by
about 15%. These results suggest that the probe 1 has excel-
lent anti-interference ability against the aforementioned addi-
tives for fluorescent pH detection.
1H NMR titrations of the probe 1 in acid and alkali conditions
were performed in CD3OD/D2O (1/1, v/v). As depicted in
Fig. 6c, the aromatic resonance signals of phenolic ring exhibit
two double peaks at 6.72 ppm (a) and 7.41 ppm (b), one cuspidal
singlet located at 7.37 ppm (c) is characteristic signal of CH
group, and the aromatic protons of dimethylaniline ring have
two double peaks at 7.75 ppm (d) and 6.78 ppm (e) in neutral
media. However, the chemical shifts of resonance signals (a ~ e)
of probe 1 display an obvious downfield shift by Δδ 0.13 ~
0.79 ppm under acidic condition, which derives from the en-
hanced electron-withdrawing effect and the decreased electron
density of the conjugation system due to the protonation of the
Proposed Mechanism and Theoretical Calculations
In the light of the above, the pH-dependent photophysical prop-
erties of probe 1 with proposed change and adjustment of the
structure are demonstrated in Fig. 6a. To support this mechanism,
Fig. 6 (a) The proposed action mechanism of probe 1 for the pH-dependent sensing under UV365nm light irradiation. 1H NMR titrations of the probe 1 in
(b) acid, (c) neutral and (d) alkali conditions, respectively

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Fig. 7 The pH sensor trips
containing probe 1 under UV
365 nm light
dimethylamino group (Fig. 6b). Under alkali condition, the de-
protonation of the phenolic hydroxyl group of probe 1 improve
the electron-donating and p-π conjugate effect, which would
impact considerably on the electron cloud distribution in mole-
cule. The proton signals of phenolic ring and CH group produce
a slight upfield shift at 6.71 ppm (a’’), 7.37 ppm (b’’) and
7.34 ppm (c’’), respectively. While the protons of
dimethylaniline ring are shifted downfield by Δδ 0.06 ppm
(Fig. 6d). To gain insight into the electronic structures of probe
1 under different conditions, the density functional theory (DFT)
calculations were carried out at the B3LYP/6-31G* level as basic
set with the Gaussian 09 program, and viewed from Fig. S6. In
water (neutral media), the electron cloud of LUMO and HOMO
is delocalized on the whole molecular, which indicates that the
complete charge separation could transport on the whole mole-
cule. Except for HOMO electron cloud in acid condition, others
in acid and alkali conditions possess the similar electron cloud
distributions to that of probe 1 in water. It means that the proton-
ation of the dimethylamino group eliminates the p-π conjugate
system of N atom and aromatic core. The energy band gaps of
probe 1 in water, acid and alkali conditions were calculated as
3.32 eV, 3.53 eV and 2.67 eV, respectively. It is seen that the
energy gap of probe 1 in alkali media is lower than that in neutral
media, contributing to the additional π-electron conjugated struc-
ture from the deprotonation of the phenolic hydroxyl group.
Additionally, a trend of decreasing band gaps (3.53 eV,
3.32 eV and 2.67 eV) suggests that the optical behaviour of probe
1 had a gradual red-shift on increasing the pH values. The anal-
ysis of 1H NMR experiment results and theoretical calculations
confirms that the proposed mechanism agrees well with the elec-
tronic structure and optical properties.
probe 1 were treated by the acidic solution. After immersing the
strips into the alkaline solutions containing probe 1, the paper
strips turns to yellow color under UV365nm light irradiation. With
regards to the paper strips soaked by the solutions with increased
pH, the color of the paper strips shows the deepened yellow
fluorescence under UV365nm light irradiation. Besides, the test
paper strips could be used to detect volatilized solvent such as
hydrochloric acid and ethylenedi-amine. Due to the gas from the
solvent interacted with probe 1, the color of test paper above
hydrochloric acid and ethylenediamine changed to cyan and yel-
low under UV 365 nm lamp respectively.
Conclusions
In summary, we have described a conjugated structural AIE-
active pH probe 1, which could respond to pH via intramolecular
charge transfer. Probe 1 could respond to a broad pH range from
1.8 to 12.0, exhibting different fluorescence emissions (blue/
orange yellow at pH 1.8 ~ 7.4, and orange yellow/yellow at pH
7.4 ~ 12.0). Probe 1 was not susceptible to common potentially
competing ions in the pH sensing process. In particular, the pro-
tonation and deprotonation mechanism of probe 1 was confirmed
by 1H NMR titrations and the density functional theory (DFT)
calculations in the pH sensing process. What’s more, the appli-
cation of probe 1 as a pH test strip for a broad range of pH
detection was successfully achieved, providing a promising can-
didate for pH detection in other fields.
Supplementary Information The online version contains supplementary
material available at https://doi.org/10.1007/s10895-020-02669-x.
Acknowledgements We are grateful for the financial support from the
Undergraduate Innovation Program in Neijiang Normal University (No.
X2020059).
Test Paper Strips of Probe 1 for pH Detection
In consideration of visible pH sensing performance of probe 1 at
different wavelength with the same excitation wavelength, test
paper strips of probe 1 for pH detection were developed and
demonstrated in Fig. 7. It is obvious that the color of the strips
is nattier blue under UV365nm light irradiation while the strips of
Author Contributions M. C.: experiments and data treatment; Y. R.: syn-
thesis of the pH probe; H. L.: structural characterization of the pH probe;
Q. J.: calculation of density functional theory (DFT); J. Z.: some mea-
surements of fluorescence; M. Z.: study design and writing of the
manuscript.

J Fluoresc (2021) 31:475–485
483
Funding The study was supported by the Undergraduate Innovation
Program in Neijiang Normal University (No. X2020059).
11. Hu Q, Huang Q, Liang K, Wang Y, Mao Y, Yin Q, Wang H (2020)
An AIE + TICT activated colorimetric and ratiometric fluorescent
sensor for portable, rapid, and selective detection of phosgene. Dyes
Pigments 176:108229. https://doi.org/10.1016/j.dyepig.2020.
108229
Data Availability All the data and materials from this manuscript will be
made available on request.
12. Zhao R, Zhang M, Liu Y, Zhang X, Duan Y, Han T (2020)
Fabricating D-A type AIE luminogen into film sensor for turn-on
detection of methanol vapour. Sensors Actuators B Chem 319:
128323. https://doi.org/10.1016/j.snb.2020.128323
Compliance with Ethical Standards
Competing Interests No potential conflict of interest was reported by
the authors.
13. Ji X, Li Z, Liu X, Peng HQ, Song F, Qi J, Lam JWY, Long L,
Sessler JL, Tang BZ (2019) A functioning macroscopic “Rubik’s
Cube” assembled via controllable dynamic covalent interactions.
Adv Mater 31:1902365. https://doi.org/10.1002/adma.201902365
14. Zhu M, Chen Y, Zhang X, Chen M, Guo H, Yang F (2018)
Perylene bisimide with diphenylacrylonitrile on side-chain: strong-
ly fluorescent liquid crystal with large pseudo Stokes shift based on
AIE and FRET effect. Soft Matter 14:6737–6744. https://doi.org/
10.1039/C8SM01183D
Ethical Approval Not applicable.
References
1. Luo J, Xie Z, Lam JWY, Cheng L, Chen H, Qiu C, Kwok HS, Zhan
X, Liu Y, Zhu D, Tang BZ (2001) Aggregation-induced emission
of 1-methyl-1,2,3,4,5-pentaphenylsilole. Chem Commun 18:1740–
1741. https://doi.org/10.1039/B105159H
15. Jiang S, Qiu J, Lin L, Guo H, Yang F (2019) Circularly polarized
luminescence based on columnar self-assembly of tetraphenylethylene
with multiple cholesterol units. Dyes Pigments 163:363–370. https://
doi.org/10.1016/j.dyepig.2018.12.021
2. Fan YQ, Liu J, Chen YY, Guan XW, Wang J, Yao H, Zhang YM,
Wei TB, Lin Q (2018) An easy-to-make strong white AIE supra-
molecular polymer as a colour tunable photoluminescence material.
J Mater Chem C 6:13331–13335. https://doi.org/10.1039/
C8TC04540B
16. Zhu M, Zhuo Y, Cai K, Guo H, Yang F (2017) Novel fluorescent
perylene liquid crystal with diphenylacrylonitrile groups:
Observation of a large pseudo stokes shift based on AIE and
FRET effects. Dyes Pigments 147:343–349. https://doi.org/10.
1016/j.dyepig.2017.08.027
3. Chen C, Li RH, Zhu BS, Wang KH, Yao JS, Yin YC, Yao MM,
Yao HB, Yu SH (2018) Highly luminescent inks: Aggregation-
induced emission of copper–iodine hybrid clusters. Angew Chem
Int Ed 57:7107–7110. https://doi.org/10.1002/anie.201802932
4. Song F, Xu Z, Zhang Q, Zhao Z, Zhang H, Zhao W, Qiu Z, Qi C,
Zhang H, Sung HHY, Williams ID, Lam JWY, Zhao Z, Qin A, Ma
D, Tang BZ (2018) Highly efficient circularly polarized electrolu-
minescence from aggregation-induced emission luminogens with
amplified chirality and delayed fluorescence. Adv Funct Mater
28:1800051. https://doi.org/10.1002/adfm.201800051
17. Qiu J, Jiang S, Guo H, Yang F (2018) An AIE and FRET-based
BODIPY sensor with large Stoke shift: Novel pH probe exhibiting
application in CO32– detection and living cell imaging. Dyes
Pigments 157:351–358. https://doi.org/10.1016/j.dyepig.2018.05.013
18. Qi J, Sun C, Li D, Zhang H, Yu W, Zebibula A, Lam JWY, Xi W,
Zhu L, Cai F, Wei P, Zhu C, Kwok RTK, Streich LL, Prevedel R,
Qian J, Tang BZ (2018) Aggregation-induced emission luminogen
with near-infrared-II excitation and near-infrared-I emission for
ultradeep intravital two-photon microscopy. ACS Nano 12:7936–
7945. https://doi.org/10.1021/acsnano.8b02452
19. Niu G, Zhang R, Kwong JPC, Lam JWY, Chen C, Wang J, Chen Y,
Feng X, Kwok RTK, Sung HHY, Williams ID, Elsegood MRJ, Qu
J, Ma C, Wong KS, Yu X, Tang BZ (2018) Specific two-photon
imaging of live cellular and deep-tissue lipid droplets by lipophilic
AIEgens at ultralow concentration. Chem Mater 30:4778–4787.
https://doi.org/10.1021/acs.chemmater.8b01943
20. Lim NY, Ahn J, Won M, Choi W, Kim JS, Jung JH (2019) Novel
cyanostilbene-based fluorescent chemoprobe for hydroxyl radicals
and its two-photon bioimaging in living cells. ACS Appl Bio Mater
2:936–942. https://doi.org/10.1021/acsabm.8b00796
21. Zhou C, Xu W, Zhang P et al (2019) Engineering sensor arrays
using aggregation-induced emission luminogens for pathogen iden-
tification. Adv Funct Mater 29:1805986. https://doi.org/10.1002/
adfm.201805986
22. Hu F, Xu S, Liu B (2018) Photosensitizers with aggregation-
induced emission: Materials and biomedical applications. Adv
Mater 30:1801350. https://doi.org/10.1002/adma.201801350
23. Wu W, Mao D, Xu S et al (2019) Precise molecular engineering of
photosensitizers with aggregation-induced emission over 800 nm
for photodynamic therapy. Adv Funct Mater 29:1901791. https://
doi.org/10.1002/adfm.201901791
5. Zhan Y, Yang Z, Tan J, Qiu Z, Mao Y, He J, Yang Q, Ji S, Cai N,
Huo Y (2020) Synthesis, aggregation-induced emission (AIE) and
electroluminescence of carbazole-benzoyl substituted
tetraphenylethylene derivatives. Dyes Pigments 173:107898.
https://doi.org/10.1016/j.dyepig.2019.107898
6. Wan Q, Tong J, Zhang B, Li Y, Wang Z, Tang BZ (2020)
Exploration of high efficiency AIE-active deep/near-infrared red
emitters in OLEDs with high-radiance. Adv Optical Mater 8:
1901520. https://doi.org/10.1002/adom.201901520
7. Liu B, Nie H, Zhou X, Hu S, Luo D, Gao D, Zou J, Xu M, Wang L,
Zhao Z, Qin A, Peng J, Ning H, Cao Y, Tang BZ (2016)
Manipulation of charge and exciton distribution based on blue
aggregation-induced emission fluorophors: A novel concept to
achieve high-performance hybrid white organic light-emitting di-
odes. Adv Funct Mater 26:776–783. https://doi.org/10.1002/adfm.
201503368
8. Yu M, Huang R, Guo J, Zhao Z, Tang BZ (2020) Promising appli-
cations of aggregation-induced emission luminogens in organic op-
toelectronic devices. PhotoniX1 11:598. https://doi.org/10.1186/
s43074-020-00012-y
9. Wang T, Su X, Zhang X, Nie X, Huang L, Zhang X, Sun X, Luo Y,
Zhang G (2019) Aggregation-induced dual-phosphorescence from
organic molecules for nondoped light-emitting diodes. Adv Mater
31:1904273. https://doi.org/10.1002/adma.201904273
10. Li Q, Yang Z, Ren Z, Yan S (2016) Polysiloxane-modified
tetraphenylethene: Synthesis, AIE properties, and sensor for detect-
ing explosives. Macromol Rapid Commun 37:1772–1779. https://
doi.org/10.1002/marc.201600378
24. Dong Z, Bi Y, Cui H et al (2019) AIE supramolecular assembly
with FRET effect for visualizing drug delivery. ACS Appl Mater
Interfaces 11:23840–23847. https://doi.org/10.1021/acsami.
9b04938
25. Li Q, Li Y, Min T et al (2020) Time-dependent photodynamic
therapy for multiple targets: A highly efficient AIE‐active photo-
sensitizer for selective bacterial elimination and cancer cell ablation.

484
J Fluoresc (2021) 31:475–485
Angew Chem Int Ed 59:9470–9477. https://doi.org/10.1002/anie.
201909706
aggregation-induced emission. Chin Chem Lett 27:1592–1596.
https://doi.org/10.1016/j.cclet.2016.04.020
26. Huang G, Jiang Y, Yang S, Li BS, Tang BZ (2019) Multistimuli
response and polymorphism of a novel tetraphenylethylene deriva-
tive. Adv Funct Mater 29:1900516. https://doi.org/10.1002/adfm.
201900516
42. Vasu AK, Radhakrishna M, Kanvah S (2017) Self-assembly tuning
of α-cyanostilbene fluorogens: Aggregates to nanostructures. J
Phys Chem C 121:22478–22486. https://doi.org/10.1021/acs.jpcc.
7b06225
27. Li C, Xiong K, Chen Y, Fan C, Wang YL, Ye H, Zhu MQ (2020)
Visible-light-driven photoswitching of aggregated-induced emis-
sion-active diarylethenes for super-resolution imaging. ACS Appl
Mater Interfaces. https://doi.org/10.1021/acsami.0c03122
28. Zhang H, Zeng J, Luo W et al (2019) Synergistic tuning of the
optical and electrical performance of AIEgens with a hybridized
local and charge-transfer excited state. J Mater Chem C 7:6359–
6368. https://doi.org/10.1039/C9TC01453E
29. Zhao F, Chen Z, Fan C, Liu G, Pu S (2019) Aggregation-induced
emission (AIE)-active highly emissive novel carbazole-based dyes
with various solid-state fluorescence and reversible
mechanofluorochromism characteristics. Dyes Pigments 164:390–
397. https://doi.org/10.1016/j.dyepig.2019.01.057
30. Wang X, Ding G, Duan Y et al (2020) A novel triphenylamine-
based bis-Schiff bases fluorophores with AIE-Activity as the hy-
drazine fluorescence turn-off probes and cell imaging in live cells.
Talanta 217:121029. https://doi.org/10.1016/j.talanta.2020.121029
31. Yuan Y, Yin P, Wang T et al (2020) Synthesis of triphenylamine
(TPA) dimers and applications in cell imaging. Dyes Pigments 174:
108009. https://doi.org/10.1016/j.dyepig.2019.108009
32. Lin HT, Huang CL, Liou GS (2019) Design, synthesis, and
electrofluorochromism of new triphenylamine derivatives with
AIE-active pendent groups. ACS Appl Mater Interfaces 11:
11684–11690. https://doi.org/10.1021/acsami.9b00659
33. Xu F, Wang H, Du X et al (2016) Structure, property and mecha-
nism study of fluorenone-based AIE dyes. Dyes Pigments 129:
121–128. https://doi.org/10.1016/j.dyepig.2016.02.023
34. Yan X, Zhu P, Li Y, Yuan S, Lan H, Xiao S (2019) Aggregation-
induced emission (AIE)-active phenanthrenequinone hydrazone-
based dyes with reversible mechanofluochromism. Mater Today
Commun 20:100565. https://doi.org/10.1016/j.mtcomm.2019.
100565
35. Zhou J, Xu S, Yu Z, Ye X, Dong X, Zhao W (2019) Two-channel
responsive fluorescent probe of meso carboxylate of BODIPY with
AIE characteristics for fast detection of palladium. Dyes Pigments
170:107656. https://doi.org/10.1016/j.dyepig.2019.107656
36. Chen B, Zeng J, Xiong Y, Nie H, Luo W, Zhao Z, Tang BZ (2018)
Synthesis, aggregation-induced emission and electroluminescence
of new luminogens based on thieno[3,2-b]thiophene S,S-dioxide.
Dyes Pigments 159:275–282. https://doi.org/10.1016/j.dyepig.
2018.04.069
37. Liu C, Zhang K, Sun Q, Li W (2020) Bile acid-terpyridine conju-
gates: Steroidal skeleton controlled AIE effect and metal-tunable
fluorescence and supramolecular assembly properties.
Tetrahedron 15:131283. https://doi.org/10.1016/j.tet.2020.131283
38. Zhang R, Gao M, Bai S, Liu B (2015) A fluorescent light-up plat-
form with “AIE + ESIPT” characteristics for multi-target detection
both in solution and on paper strip. J Mater Chem B 3:1590–1596.
https://doi.org/10.1039/C4TB01937G
39. Chen Y, Wang S, Wu X et al (2018) Triazatruxene-based small
molecules with thermally activated delayed fluorescence,
aggregation-induced emission and mechanochromic luminescence
properties for solution-processable nondoped OLEDs. J Mater
Chem C 6:12503–12508. https://doi.org/10.1039/C8TC04721A
40. Duan P, Yanai N, Kurashige Y, Kimizuka N (2015) Aggregation-
induced photon upconversion through control of the triplet energy
landscapes of the solution and solid states. Angew Chem Int Ed 54:
7544–7549. https://doi.org/10.1002/ange.201501449
43. Hang C, Wu HW, Zhu LL (2016) π-Conjugated cyanostilbene-
based optoelectric functional materials. Chin Chem Lett 27:1155–
1165. https://doi.org/10.1016/j.cclet.2016.04.003
44. Dhoun S, Kaur S, Kaur P, Singh K (2017) A cyanostilbene-
boronate based AIEE probe for hydrogen peroxide—Application
in chemical processing. Sensors Actuators B Chem 245:95–103.
https://doi.org/10.1016/j.snb.2017.01.143
45. Kumari B, Paramasivam M, Dutta A, Kanvah S (2018) Emission
and color tuning of cyanostilbenes and white light emission. ACS
Omega 3:17376-17385. https://doi.org/10.1021/acsomega.
8b02775
46. Paramasivam M, Kanvah S (2016) Rational tuning of AIEE active
coumarin based α-cyanostilbenes toward far-red/NIR region using
different π-spacer and acceptor units. J Phys Chem C 120:10757–
10769. https://doi.org/10.1021/acs.jpcc.6b01334
47. Yoon JH, Kim SM, Park HJ, Kim YK (2020) Highly self-healable
and flexible cable-type pH sensors for real-time monitoring of hu-
man fluids. Biosens Bioelectron 150:111946. https://doi.org/10.
1016/j.bios.2019.111946
48. Gong J, Tanner MG, Venkateswaran S, Stone JM (2020) A
hydrogel-based optical fibre fluorescent pH sensor for observing
lung tumor tissue acidity. Chin Chem Lett 1134:136–143. https://
doi.org/10.1016/j.aca.2020.07.063
49. Vivaldi F, Santalucia D, Poma N, Bonini A (2020) A voltammetric
pH sensor for food and biological matrices. Sensors Actuators B
Chem 322:128650. https://doi.org/10.1016/j.snb.2020.128650
50. Mazzaracchio V, Fiore L, Nappi S, Marrocco G (2021) Medium-
distance affordable, flexible and wireless epidermal sensor for pH
monitoring in sweat. Talanta 222:121502. https://doi.org/10.1016/j.
talanta.2020.121502
51. Tariq A, Baydoun J, Remy C, Ghasemi R (2021) Fluorescent mo-
lecular probe based optical fiber sensor dedicated to pH measure-
ment of concrete. Sensors Actuators B Chem 327:128906. https://
doi.org/10.1016/j.snb.2020.128906
52. More KN, Mun SK, Kang J, Kim JJ (2021) Molecular design of
fluorescent pH sensors based on reduced rhodol by structure-pKa
relationship for imaging of lysosome. Dyes Pigments 184:108785.
https://doi.org/10.1016/j.dyepig.2020.108785
53. Mishra VR, Ghanavatkar CW, Sekar N (2019) ESIPT clubbed azo
dyes as deep red emitting fluorescent molecular rotors:
Photophysical properties, pH study, viscosity sensitivity, and DFT
studies. J Lumin 215:116689. https://doi.org/10.1016/j.jlumin.
2019.116689
54. Wang X, Wang H, Niu Y, Wang Y (2020) A facile AIE fluorescent
probe for broad range of pH detection. Spectrochim Acta Part A
Mol Biomol Spectrosc 226:117650. https://doi.org/10.1016/j.saa.
2019.117650
55. Li J, Liu Y, Li H, Shi W (2018) pH-Sensitive micelles with
mitochondria-targeted and aggregation-induced emission charac-
terization: synthesis, cytotoxicity and biological applications.
Biomater Sci 6:2998–3008. https://doi.org/10.1039/C8BM00889B
56. Wang Z, Ye J, Li J, Bai Y (2015) A novel triple-mode fluorescent
pH probe from monomer emission to aggregation-induced emis-
sion. RSC Adv 5:8912–8917. https://doi.org/10.1039/
C4RA15240A
57. Ma X, Cheng J, Liu J, Zhou X (2015) Ratiometric fluorescent pH
probes based on aggregation-induced emission-active
salicylaldehyde azines. New J Chem 39:492–500. https://doi.org/
10.1039/C4NJ01908C
41. Wang F, Li X, Wang S et al (2016) New π-conjugated
cyanostilbene derivatives: Synthesis, characterization and

J Fluoresc (2021) 31:475–485
485
58. Zhang M, Yang W, Gong T, Zhou W (2017) Tunable AIEE fluo-
rescence constructed from triphenylamine luminogen containing
quinoline —application to reversible and tunable pH sensor. Phys
Chem Chem Phys 19:21672–21682. https://doi.org/10.1039/
C7CP03234J
59. Zhou Z, Gu F, Peng L, Hu Y (2015) Spectroscopic analysis and
in vitro imaging applications of a pH responsive AIE sensor with a
two-input inhibit function. Chem Commun 51:12060–12063.
https://doi.org/10.1039/C5CC03788C
sensing. Chem Commun 52:3123–3126. https://doi.org/10.1039/
C5CC10423H
61. Mazi W, Adhikari R, Zhang Y, Xia S (2019) Fluorescent probes
with high pKa values based on traditional, near-infrared rhodamine,
and hemicyanine fluorophores for sensitive detection of lysosomal
pH variations. Methods 168:40–50. https://doi.org/10.1016/j.
ymeth.2019.07.012
Publisher’s Note Springer Nature remains neutral with regard to jurisdic-
tional claims in published maps and institutional affiliations.
60. Feng Q, Li Y, Wang L, Li C (2016) Multiple-color aggregation-
induced emission (AIE) molecules as chemodosimeters for pH