2-Styrylquinoline-based two-photon AIEgens for dual monitoring of pH and viscosity in living cells

Article abstract of DOI:10.1039/c9tb02036e

A new class of aggregation-induced emission (AIE) fluorophores HAPHs with excellent two-photon properties is developed from styrylquinoline. Among them, the probe HAPH-1 exhibits good sensitivity towards pH and viscosity and has been successfully used for

Full text of DOI:10.1039/c9tb02036e

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2-Styrylquinoline-based two-photon AIEgens for
dual monitoring of pH and viscosity in living cells†
Cite this: J. Mater. Chem. B, 2019,
7, 7771
Yandong Dou,‡a Kenry, ‡b Jiang Liu,a Fangfang Zhang,a Chunhui Caia and
a
Qing Zhu
*
A new class of aggregation-induced emission (AIE) fluorophores HAPHs with excellent two-photon
properties is developed from styrylquinoline. Among them, the probe HAPH-1 exhibits good sensitivity
towards pH and viscosity and has been successfully used for simultaneous pH and viscosity detection in
living cells through two-photon microscopy.
Received 18th September 2019,
Accepted 5th November 2019
DOI: 10.1039/c9tb02036e
rsc.li/materials-b
The phenomenon of aggregation-induced emission (AIE) was the fluorescence of these NIR materials can be easily quenched.9
first reported by Tang’s group and has been one of the most In addition, a larger conjugated system often requires a longer
popular topics in the research of organic luminophore systems synthetic route and reduces the water solubility of the molecules,
due to their excellent fluorescent properties, including good which significantly limits their biological applications.
photostability, high brightness and large Stokes shift.1 The
Compared to a one-photon fluorophore, a two-photon (TP)
restrictions of intramolecular motions (RIM), including restric- fluorophore has many distinct advantages, including an increased
tion of intramolecular rotation (RIR) and restriction of intra- penetration depth and reduced specimen photodamage due to
molecular vibration (RIV), have been widely accepted as the its two-photon excitation at the red wavelength.10 Thus, the
mechanisms of the AIE effect.2 To date, great efforts have been development of a two-photon AIE fluorophore is attractive to
focused on the AIE molecule design, and a variety of AIE probes achieve a longer excitation wavelength. In the past few years,
based on different structures, including hexaphenylsilole (HPS) several studies in this area have been reported.11 For example, the
and tetraphenylethene (TPE), have been successfully developed.3 Ding group reported a two-photon AIE-based fluorescent probe
However, most of the reported probes exhibit complex structures, with a large Stokes shift for the selective and sensitive detection
one-photon absorption, and a short excitation wavelength, which and visualization of hypochlorite.11e These traditional structures
have strongly limited their biological applications.
usually possess P conjugation between electron-donating or
To widen their biological applications, it is highly desirable electron-accepting units and an AIE structure (Scheme 1A),
to develop AIE luminogens with a long excitation wavelength, resulting in complex structures and poor solubility in water.
simple structure and low cytoxicity.4 Thus, various near-infrared Moreover, most of these dyes have low two-photon absorption
AIE luminogens have been designed to achieve longer emission cross-section values. All these factors collectively hinder their
wavelengths by extending the conjugation system of the existing biological applications. Therefore, the development of simple
AIE molecules and/or increasing the intramolecular donor– AIEgens with excellent two-photon characteristics is essential.
receptor interaction.5,6 For example, Hua et al. described the
Herein, we report a novel class of two-photon AIE fluoro-
construction of NIR AIE-based glycosyl probes for the sensitive phores, which was developed from a simple quinoline structure
and selective detection of sugar–lectin interactions.7 Just through a one-step reaction (Scheme 1B). This class of fluoro-
recently, a novel pH-sensitive NIR AIE luminogen for long- phores was highly sensitive toward pH and viscosity. Furthermore,
term mitochondrial tracking has been reported by the Meng it could be applied for monitoring the pH changes and detecting
group.8 However, due to the strong P–P accumulation and the viscosity in HeLa cells through two-photon microscopy,
decrease in the twisted intramolecular charge transfer (TICT), demonstrating its potential application in tumour cell detection.
In our preliminary studies, (E)-2-styrylquinoline (HAPH) was
a Key Laboratory of Bioorganic Synthesis of Zhejiang Province, College of
Biotechnology and Bioengineering, Zhejiang University of Technology,
Hangzhou 310014, P. R. China. E-mail: zhuq@zjut.edu.cn
b Department of Chemical and Biomolecular Engineering, National University of
Singapore, 4 Engineering Drive 4, Singapore 117585, Singapore
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9tb02036e
‡ These authors contributed equally to this work.
found to display strong yellow fluorescence in the solid state
but poor fluorescence in organic solvents, while (E)-3-styryl-
quinoline (AQ-3S) and (E)-3-styrylquinoline (AQ-6S) displayed
strong fluorescence in the wet state, suggesting that the
(E)-2-styrylquinoline compound might have AIE characteristics
(Fig. S1–S3, ESI†).12 To confirm this hypothesis, styrylquinoline
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fraction. Separately, the aggregation of the AIE fluorogens could
be detected through laser light scattering (LLS) measurements,
and the effective diameter of HAPH-1 in water is 103 Æ 1 nm
(Fig. S9, ESI†).13
To further study the AIE effect of styrylquinolines, five more
compounds with different functional groups at the C2 and C6
points of quinoline were synthesized. HAPH 1–3, with electron-
donor substitution at the styryl ring showed an obvious AIE effect.
Moreover, HAPH-5, in which quinoline was replaced with naphtha-
lene, did not exhibit any AIE characteristics, indicating that quino-
line is an essential structure for this new class of AIE molecules.
To further examine the AIE effect of HAPHs, PEG-600 was linked to
quinoline to generate a water soluble group (HAPH-6), and the
effective diameter of HAPH-6 in water reduced to 0.75 nm without
the AIE effect. (Fig. S12, ESI†). All these results demonstrated
that inorganic nano-species could induce the aggregation of the
rationally modified AIE molecules to show distinct AIE feature.
HAPHs possess conjugated structures with electron donating
and withdrawing groups, and thus are expected to have high
two-photon absorption cross-section values (d).14 The d of
HAPHs was studied using a two-photon excitation fluorescence
(TPEF) technique with a femtosecond pulsed laser source, and
the relative TPEF intensity was measured using rhodamine 6G
as the standard (Fig. S13 and Table S1, ESI†). The measured
wavelength was varied from 640 nm to 720 nm at an interval of
40 nm, and thus the d2PA values were obtained. The results are
summarized in Table S1 (ESI†). In H2O, HAPH-1 displayed the
highest maximum d value (612 GM, Ex = 640 nm), and thus
HAPH-1 with strong AIE characteristics might serve as a good
two-photon imaging fluorophore for living organisms.
Next, we were motivated to evaluate the biological applications
of HAPH-1. The changes in the intracellular pH and viscosity have
been correlated with the occurrence of various diseases, such
as cancer, diabetes, myocardial infarction, hypertension and
Alzheimer’s disease. Therefore, the quantification of pH and
viscosity in living cells has become very important.15 Due to the
fact that quinoline N favours H+ binding under acidic conditions,
we first tested the pH sensitivity of HAPH-1 in aqueous media.16
To study the optical responses of HAPH-1 toward pH, standard pH
titrations were performed and its absorption and fluorescence
spectra were monitored. As shown in Fig. 2A, as the pH was
decreased from 7.0 to 1.0, the absorbance at 320 nm reduced,
while a new peak at 410 nm increased correspondingly. The
redshift in the emission spectra confirmed that the ICT effect of
the probe was enhanced with decreasing pH, which resulted from
the H+ binding-induced enhancement of the electron-withdrawing
ability of quinoline. Moreover, as shown in Fig. 2B, when the pH
value was higher than 4.0, the solution exhibited an intense
emission band centered at 580 nm (lex = 410 nm), with a large
Stokes shift of 170 nm. Thus, the large Stokes shift could reduce
the self-quenching and measurement error of excitation and
scattered lights (Fig. 2C).17 Meanwhile, the emission intensity
showed a good linearity with pH in the range 3.0–1.25, with a
linear coefficient of 0.9831 (the inset of Fig. 2D). The calculated pKa
of HAPH-1 is 3.21, which is consistent with the above result.
Furthermore, this probe could detect pH reversibly (Fig. S13, ESI†).
Scheme 1 The traditional (A) and our current design (B) of the two-
photon AIE luminogens (AIEgens).
Scheme 2 Chemical structures of the AIEgens.
HAPH-1 was synthesized (Scheme 2), and its AIE effect was
evaluated by determining its fluorescence in different solutions,
such as DCM, THF, MeOH, acetone, water and so on. Interest-
ingly, under UV light, HAPH-1 showed the highest fluorescence
in pure water, while very weak fluorescent signals were observed
in THF and DMF, indicating that the probe HAPH-1 has good
AIE feature (Fig. 1A and B). Then, we examined its fluorescence in
the presence of different water fractions from 0 to 100% in the
THF/H2O mixture. As shown in Fig. 1C, the fluorescence intensity
of HAPH-1 increased gradually along with the increasing H2O
Fig. 1 (A) Photographs of HAPH-1 (2.5 mM) in different solvents under
365 nm UV irradiation from a hand-held UV lamp; (B) emission spectra of
HAPH-1 in different solvents (excited at 340 nm); (C) emission spectra of
HAPH-1 in THF/water mixtures with different water fractions (fw).
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Fig. 2 (A) Evolution of UV-vis absorption; (B) fluorescence emission
spectra of HAPH-1 (2.5 mM) in water at various pH values (pH = 1–7).
Ex = 360 nm and Em = 400–700 nm; (C) fluorescence intensity at 580 nm,
as a function of pH in the range of 1.00–7.00. Ex = 410 nm and Em =
450–720 nm; (D) a plot of fluorescence intensity at 580 nm, Ex = 410 nm
and Em = 450–720 nm. Inset: Fluorescence intensity as a function of pH
in the range of 1.25–3.00, R2 = 0.9831.
Fig. 3 (A) Evolution of UV-vis absorption at different viscosities in the
water/glycerol system (pH = 7.0); (B) fluorescence emission spectra of
HAPH-1 (2.5 mM) at various viscosity values (pH = 7.0), Ex = 360 nm and
Em = 400–700 nm; (C) the fluorescence intensity at 550 nm Ex = 360 nm
and Em = 450–680 nm, inset: the linear relationship between log(I/I0) and
log(viscosity) at Em = 550 nm, R2 = 0.998; (D) fluorescence emission
spectra of HAPH-1 (2.5 mM) at various viscosity values (pH
= 1.0).
Ex = 360 nm and Em = 400–700 nm; inset: the fluorescence intensity
of HAPH-1 in different acidic buffer solutions at various viscosities.
It is noteworthy that the probe showed no obvious AIE properties
when the pH value was lower than 4.0 due to the increased
solubility of its protonated form (Fig. S15, ESI†).
To further investigate the pH sensing mechanism, the glycerol, the probe immediately displayed a dramatic fluorescence
proton-binding behaviours of HAPH-1 and its protonated form, response at 470 nm, while a new increasing emission at 550 nm
HAPH-1-HCl, were characterized by 1H NMR spectroscopy with a new absorption band appeared at 360 nm, providing a
(Fig. S16, ESI†). Upon protonation of the probe HAPH-1, new way for the detection of viscosity. Moreover, a good linear
quinolinic protons (H1 and H5) displayed obvious downfield relationship existed between log(I/I0) and log(viscosity) at 550 nm,
shifts of 1.05 and 0.72 ppm, respectively. The data indicate that with a correlation coefficient of 0.998 (Fig. 3C), indicating that the
the protonation occurred at the N atom of the quinoline ring. probe HAPH-1 could be used as a sensor to quantitatively detect
These results coincide with previous reports and identify the solution viscosity.
the existence of resonance H+ binding with quinoline N in
the probe HAPH-1, leading to significant optical response to viscosity at different pH values. Strong fluorescence at 580 nm
acidic pH. was observed at pH below 4 and its intensity increased with the
We further characterized the response of the probe towards
In order to understand the spectral behaviour of HAPH-1 increasing viscosity due to the enhancement of the ICT effect
and HAPH-1-HCl, density functional theory (DFT) computa- (Fig. 3D). Meanwhile, Fig. 3B shows that both fluorescent
tions were also conducted. Fig. S17 (ESI†) shows that the signals at 470 nm and 580 nm increased with the addition of
highest occupied molecular orbital (HOMO) of HAPH-1 was glycerol. This phenomenon could be due to the better solubility
distributed over the entire molecule, suggesting the existence of of the protonated HAPH-1 than that of HAPH-1 at low pH,
P-conjugation and ICT transition in HAPH-1. Meanwhile, the resulting in a lower probability for the protonated HAPH-1
HOMO of HAPH-1-HCl was mainly distributed over the electron to form large aggregates, and thus its weaker AIE effect
donor structure, and the lowest unoccupied molecular orbital (Em = 470 nm). In addition, when the probe was subjected to
(LUMO) was distributed over the acceptors. The energy gap (DE) low pH and high viscosity conditions, the emission intensity
between the HOMO and the LUMO in HAPH-1 was found to be of HAPH-1 in a viscous glycerol/DMF mixture (9 : 1, v/v) was
1.921 eV, and the energy gap of the probe after protonation 20-fold higher than that in pure DMSO (Fig. S18, ESI†). Thus,
was increased to 2.892 eV, which further confirmed that the HAPH-1 was also AIE-active. This phenomenon occurred
protonated HAPH-1 could enhance the ICT effect. These DFT because high viscosity could hamper the intramolecular rota-
calculations agreed well with the experimental observations.
tion, leading to the closure of the nonradiative decay channel
Previous studies have reported that TICT fluorophores could which enhanced the radiative emission.19
be employed for detecting viscosity.18 Thus, the sensitivity of
To verify the selectivity of HAPH-1 in complex biological
the compound HAPH-1 toward viscosity was also investigated environments, different reactive nitrogen species, reactive
in different H2O with increasing glycerol fractions. The viscosity of oxygen species (ROS), metal ions and small molecule thiols
the solution increased from 1.01 cP of pure water to 950.17 cP were chosen as negative controls, and the selectivity of the
of 90% glycerol. As shown in Fig. 3A–C, upon the addition of probe towards these species is shown in Fig. S19 (ESI†). All these
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Fig. 4 (A) Two-photon confocal fluorescence microscopy images of
HeLa cells incubated with the probe HAPH-1 (2.5 mM) and further treated
with buffer at pH 7.4, 4.5, 3.7 and 3.5; (B) average RFU at different pH
values; (C) ratio images of Forange/Fblue (the blue channel: Ex = 640 nm and
Em = 420–490 nm, the yellow channel: Ex = 820 nm and 540–620 nm.
Scale bar: 20 mm).
Fig. 5 Confocal two-photon fluorescence images of HAPH-1 (2.5 mM)
(the blue channel: Ex = 640 nm and Em = 420–500 nm) and the yellow
channel (Ex = 720 nm and Em = 520–580 nm) of HAPH-1 in HeLa cells
incubated at different temperatures 37 1C, 25 1C and 4 1C and Nystatin.
Scale bar: 20 mm.
species, such as NO2À, ONOOÀ, H2O2, TBHP (tert-butyl hydro-
peroxide), Co2+, Fe2+, Hg2+, Ag+, Ni2+, GSH, Cys, and Hcy, induced
negligible perturbation. Overall, the results indicated that the
probe has high sensitivity and good selectivity toward viscosity
in complex environments.
Among them, HAPH-1 was found to have good selectivity and
sensitivity toward viscosity and pH even in the presence of
other relevant analytes, including ROS, ions and biothiols,
under physiological conditions. Additionally, the probe was
successfully applied for imaging the viscosity and pH in living
cells with two-photon microscopy, indicating that it could be
potentially used for the analysis of pathological and biological
functions. We believe that this class of two-photon fluoro-
phores could become a useful platform for the construction
of other AIE-based fluorescent probes with low cytotoxicity and
high sensitivity for a wide range of biological applications.
We further examined the application of HAPH-1 for moni-
toring pH fluctuation in living cells. MTT assay was first
performed and the low cell toxicity of the probe was noted
(Fig. S20, ESI†). Next, HeLa cells were treated with HAPH-1
(2.5 mM) for 30 min at 37 1C, and blue fluorescence was
observed (Fig. 4A), while no yellow emission was observed.
However, when the pH was increased up to 4.5, the fluores-
cence intensity from the yellow channel was significantly
enhanced along with the decreasing intensity of the blue channel.
Interestingly, the fluorescence intensity ratio (Fyellow/Fblue) was
observed to increase dramatically to indicate the lower pH in
HeLa cells. These results have clearly suggested that HAPH-1
could be used for cellular imaging and ratiometrically detecting
the intracellular pH fluctuation, especially in the cytoplasm.
The ability of the probe HAPH-1 for detecting changes in the
intracellular viscosity was further evaluated by fluorescence
imaging. HeLa cells were first incubated with HAPH-1 at
37 1C, 25 1C and 4 1C for 30 min. Previous studies have showed
that a decrease in the culture temperature could result in an
enhancement of the intracellular viscosity.20 As shown in Fig. 5,
an enhanced fluorescence signal at the yellow channel was
observed in the HeLa cell with the decreasing incubation
temperature. Furthermore, we used nystatin, a well-known
ionophore that can induce mitochondrial structural changes
or swelling through interruption of the ionic balance, as an
inductive agent to regulate the viscosity in cells. As a result, the
cells displayed a brighter yellow signal (Fig. 5).21 These results
demonstrated that HAPH-1 could be employed for monitoring
the changes in intracellular viscosity.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors are grateful to the National Natural Science Foun-
dation of China (No. 21472172 and 21272212) and the Natural
Science Foundation of Zhejiang Province (No. LY17B060009).
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