A Reversible Rhodamine B Based pH Probe with Large Pseudo-Stokes Shift

Article abstract of DOI:10.1002/cplu.201900263

A reversible and sensitive pH probe DPE?Rh operates by F?rster resonance energy transfer from 1,2-diphenylethyne (DPE) to Rhodamine B (Rh). In the presence of H⁺, the spirolactam ring of the Rhodamine B unit was opened and this resulted in ca. 1000-fold enhancement of fluorescence intensity with linear change over the pH range of 2.0 to 5.5. The F?rster resonance energy transfer offered this probe an effective excitation–emission wavelength shift of around 240 nm with about 100 % quenching of the donor emission. The response of the sensor is tolerant towards a wide range of metal ions and the sensing mechanism was deduced by ¹H NMR spectrometry. This FRET-based molecule not only provides a sensitive pH probe, but also suggests an effective strategy to eliminate the interference of excitation light.

Full text of DOI:10.1002/cplu.201900263

Accepted Article
Title: A Reversible Rhodamine B Based pH Probe with Large Pseudo-
Stokes Shift
Authors: Guochen Bao, Ka-Leung Wong, and Peter A. Tanner
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Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
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Link to VoR: http://dx.doi.org/10.1002/cplu.201900263
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A Reversible Rhodamine B Based pH Probe with Large Pseudo-
Stokes Shift
Guochen Bao, Ka-Leung Wong,* Peter A. Tanner *
[
8]
Abstract: Here, we report a reversible and sensitive pH probe DPE-
Rh which is based on Förster resonance energy transfer from 1,2-
the excitation of another. The donor chromophore can harvest
higher energy photons and transfer energy to an acceptor which
then emits lower energy light. Although some efforts have been
+
diphenylethyne (DPE) to rhodamine B (Rh). In the presence of H ,
[
7a, 7c]
the spirolactam ring of the rhodamine B unit was opened and this
resulted in ~1000-fold enhancement of fluorescence intensity with
linear change, in the pH range of 2.0 to 5.5. The Förster resonance
energy transfer offered this probe an effective excitation-emission
wavelength shift of around 240 nm with ~100% quenching of the
donor emission. The response of the sensor is tolerant towards a
wide range of metal ions and the sensing mechanism was unveiled
made to develop a FRET-based pH sensor,
however, the
existence of donor emission largely interferes with the desired
acceptor emission and this limits the sensitivity of the sensor.
Herein, we report a rhodamine (Rh) and 1,2-diphenylethyne
(DPE) coupled probe, DPE-Rh, for fluorometric monitoring of
proton concentration. We chose DPE as the energy donor
because the emission spectrum of DPE overlaps with the
excitation spectra of rhodamine B. The latter works as a
1
by H NMR spectrometry. This FRET-based molecule not only
+
provides a sensitive pH probe, but also suggests an effective
strategy to eliminate the interference of excitation light.
recognition element of H and it serves as energy acceptor when
the spirolactam ring-opened form is generated. With short
distance and a good match of the pertinent energy levels, DPE-
Rh exhibits effective FRET from DPE to the rhodamine B moiety.
The emission of DPE is almost 100% quenched, giving a more
than 600-fold enhancement of rhodamine emission intensity.
Introduction
+
This probe is reversible and sensitive to H , and also tolerant to
The accurate measurement of pH is of extreme importance
because pH plays a key role in a wide range of fields, such as
chemical processes, food fermentation, medical diagnosis and
a wide range of ions.
[
1]
environmental analysis.
To date, there is still a desperate
Results and Discussion
demand for simple, low cost and reversible sensors for real time
[
2]
monitoring of pH.
Much effort has been spent on developing
DPE-Rh
(3',6'-bis(diethylamino)-2-((4-
pH probes: including electrodes and the use of nuclear magnetic
(
phenylethynyl)benzylidene)amino)spiro[isoindoline-1,9'-
[
3]
resonance and optical techniques for pH sensors. Among the
responsive probes, the fluorescence chemosensor has drawn
particular attention due to its operational simplicity, rapid
xanthen]-3-one) was synthesized in three steps with excellent
yields. To minimize the distance between DPE and the Rh unit,
which is an extremely sensitive criterion for FRET, a substituted
imine was formed by nucleophilic addition of the amine of 2
[
4]
response, high signal-to-noise ratio and non-invasiveness.
Various sensitive fluorescence organic dye-based sensors have
been reported, such as those based on rhodamine, boron
(
Scheme 1) to the aldehyde group of 1, followed by elimination
[9]
in one-pot synthesis.
Sonogashira coupling
bromobenzaldehyde.
refluxing rhodamine B with excess hydrazine hydrate in ethanol
Compound
of ethynylbenzene
Compound 2 was synthesized by
1
was obtained via
[
5]
dipyrromethene, and quinoline derivatives.
Among these,
and 4-
rhodamine derivatives are excellent candidates for their
outstanding photophysical properties which give rise to a strong
fluorescence emission upon selective spirolactam-ring opening
[10]
[9]
(
Scheme 1). All the products and intermediates were
characterized by NMR and mass spectrometry (Supporting
Information).
The UV-visible absorption spectra of DPE-Rh in buffer
solution at various pH values are displayed in Figure 1. When
the pH is 7.0, DPE-Rh is colorless, corresponding to the non-
fluorescent closed spirolactam of rhodamine B. In the absorption
spectrum at pH 7.0 (Figure 1), an absorption band at 330 nm is
[
6]
in acid media.
However, organic dyes suffer from narrow
Stokes shifts and strategies to enlarge the separation between
emission and excitation bands can be beneficial for their
fluorescence applications by reducing the interference of the
irradiating light. Förster resonance energy transfer (FRET) is a
good strategy to provide large pseudo-Stokes shifts for chemical
[
7]
sensors.
FRET is a distance-sensitive energy transfer
technique which couples the emission of one chromophore with
4
-1
-
observed with molar absorption coefficient ɛ = 3.85 x 10 cm
M
1
,
attributed to the π → π* transition of 1,2-diphenylethyne.
[
[
a]
b]
G. Bao, Prof. K. -L. Wong, Prof. P. A. Tanner
However, no absorption band of the rhodamine B moiety was
observed in the red spectral range at pH 7.0. Along with
increasing H concentration, with pH value from 7.0 to 1.5, the
color of DPE-Rh solution changed from colorless to red. The
immediate color change indicates that DPE-Rh can serve as a
colorimetric probe for pH change. When the pH decreased, an
absorption band was observed at 561 nm with increasing
intensity, attributed to a π → π* transition of the rhodamine B
unit. This transition is forbidden in the spiro-ring closure form
Department of Chemistry, Hong Kong Baptist University, Kowloon
Tong, Hong Kong S.A.R., People’s Republic of China
E-mail: klwong@hkbu.edu.hk, peter.a.tanner@gmail.com
G. Bao,
Institute for Biomedical Materials & Devices (IBMD), School of
Mathematical and Physical Sciences, Faculty of Science, University
of Technology Sydney, Sydney, New South Wales 2007, Australia
+
Supporting information for this article is given via a link at the end of
the document.
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Scheme 1. Synthesis of compound DPE-Rh.
[
6a]
Hasselbalch equation:
a
log[(Amax - A)/(A -Amin))] = pH – pK ,
where A is the absorbance and Amax and Amin indicate the
maximum and minimum signals, respectively.
In neutral media, upon excitation of the DPE moiety, no
fluorescence is observed from DPE-Rh (Figure 2). Along with
+
the increase of H concentration, with pH values from 7.0 to 1.5,
a
fluorescent band appeared and generated a 998-fold
enhancement in intensity at 582 nm. The large pseudo-Stokes
shift (582-340 = 242 nm) due to FRET from DPE to rhodamine B
eliminates the interference of irradiating light. Significantly, DPE-
Rh shows a linear enhancement of fluorescence over the
+
-6
-2
concentration range of H from 5 × 10 M to 1× 10 M (Figure
b). A similar observation was obtained when directly exciting
2
rhodamine B unit as shown in Figure S1. The fluorescence of
rhodamine B in acidic media indicates the spiro-ring opening
process is induced by H , which is consistent with the absorption
Figure 1. UV-visible absorption spectra of DPE-Rh (2 μM) at various pH
values. Inset: plot of absorbance at 561 nm.
+
change at acidic conditions.
The FRET was confirmed by the excitation spectra (Figure
S2), which showed both DPE and rhodamine excitation bands
as the pH values change from 7.0 to 2.0. The Förster resonance
energy transfer was secured by the overlap of the emission
where the two phenyl groups are separately conjugated. When
in acid media, the spiro-ring is triggered to open and the two
phenyl groups are connected and conjugated so that the π → π*
a
transition is allowed. The pK of DPE-Rh was evaluated as 2.36
by analyzing the absorbance at 561 nm using the Henderson-
Figure 2. a) The emission spectra of DPE-Rh (2 μM, λex = 340 nm) at various pH values. b) The plot of intensity of emission of DPE-Rh at 582 nm with the y axis
on a log10 scale, with a linear change in the pH range from 2 to 5.5. Inset: the emission spectra of DPE-Rh on a log10 scale.
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Figure 3. a) Fluorescence spectra of DPE-Rh (2 μM, λex = 340 nm) (pH = 2.0 and 7.0) with the addition of each of the interfering ions (40 μM). b) Fluorescence
2+
2+
2+
2+
intensity at 582 nm of DPE-Rh (2 μM, λex = 340 nm, pH = 2.0 and 7.0) with the concentrations of interfering ions at 40 μM. Ion: 1, Ni ; 2, Co ; 3, Cd ; 4, Ca ; 5,
+
2+
3+
2+
3+
+
2+
2+
+
+
2+
K ; 6, Cu ; 7, Al ; 8, Hg ; 9, Fe ; 10, Na ; 11, Mg ; 12, Fe ; 13, Cs ; 14, Li ; 15, Zn
.
spectrum of intermediate 1 and the absorption spectrum of the
rhodamine B part of DPE-Rh (Figure S3). The Förster distance
DPE-Rh exhibited no obvious change of fluorescence intensity
when the neutral buffer solution (pH = 7.0) was added with 40
2
+
2+
2+
R
0
(where the energy transfer efficiency is 50%) has been
μM concentration of various ions containing Ni , Co , Cd ,
2
+
+
2+
3+
2+
3+
+
2+
2+
2+
2+
+
calculated from the spectral overlap integral as 15.6 Å, by taking
Ca , K , Cu , Al , Hg , Fe , Na , Mg , Fe , Pb , Sn , Cs ,
[
11]
+
2+
into account the donor quantum yield.
The use of the DPE
Li or Zn . A negligible fluorescence change of DPE-Rh was
observed from the above metal ions under acidic solution
conditions (pH = 2.0). The interference experiment indicates that
DPE-Rh is tolerant to various metal ions for pH measurement.
unit alone showed no obvious change of absorption and
emission spectra at various pH values (Figures S4, S5). It
should be noted that the excitation at 340 nm of the Rh unit only
also enables the characteristic rhodamine emission since
rhodamine possesses a weak absorption band at around 350
nm (Figure S6). The absorption and emission spectra of Rh
illustrated no significant change with the pH change in
comparison with DPE-Rh (Figures S6, S7). The DPE-Rh moiety
exhibited a larger portion of the excitation band at ~340 nm than
Rh only (Figure S8), indicating the rhodamine unit can be
populated through DPE excitation in DPE-Ph. The 340 nm
excitation rendered a monoexponential rhodamine B emission
decay with the lifetime of 2.6 ns (Figure S9). It is interesting that
the DPE excitation offered a quantum yield value of 8.7% for
rhodamine B, while the donor compound 1 only had a quantum
yield value of 2.7% without the presence of the acceptor
rhodamine B. The integration of a rhodamine acceptor into the
unit results in a quantum yield enhancement by a factor of 322%.
We assume the reason for this increase of quantum yield may
be due to the efficient energy transfer from DPE to the acceptor
and the high quantum yield of rhodamine, which reduces the
chance of the severe nonradiative quenching of DPE. As result
of the energy transfer, the DPE emission was almost 100%
quenched (Figure S2). This phenomenon is significant since it
provides strategy to enhance the brightness of optical materials.
Actually, the use an acceptor fluorophore with high quantum
yield has been employed to enhance the luminescent
The reversibility is of importance to
a sensor when
considering its practical applications such as monitoring pH
change in real time. The spiro-ring opening process of DPE-Rh
+
-
is reversible, as verified by fluorescence titration with H and OH
-
solution (Figure 4). This experiment showed that OH could lead
to the disappearance of the color of the ring-opened DPE-Rh
+
and the quenching of its fluorescence. However, when H ions
were added to the solution again, the red color reappeared and
the fluorescence was turned on again, as expected. Therefore,
[
12]
intensity.
The influences of interfering ions upon the pH response of
Figure 4. The reversibility of DPE-Rh (2 μM, λex = 340 nm) with normalized
emission at 582 nm upon addition of HCl and NaOH aqueous solutions.
DPE-Rh were evaluated by the experiments shown in Figure 3.
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1
3 3 2
Figure 5. a) Proposed mechanism and b) The H NMR spectra of DPE-Rh in CDCl upon addition of CF COOH and NaOH in D O.
our sensor is recoverable and four cycles of reversible
responses are recorded in Figure 4. This feature suggests DPE-
Rh is feasible for real time pH monitoring.
rhodamine B, Rh. The sensor provides an outstanding
enhancement of fluorescence intensity in acidic conditions upon
UV excitation of DPE. The considerable pseudo-Stokes shift
(~240 nm) can eliminate the interference of irradiating light. The
current excitation at around 340 nm lies in the short wavelength
range. A further exploration to shift the excitation band to longer
wavelengths is highly desired in future studies for applications in
biosystems, such as deep tissue imaging. The sensor is
reversible and tolerant towards a wide range of metal ions,
which makes it more practical in real time pH monitoring. The
The mechanism of spiro-ring opening and recovery of DPE-
1
Rh was briefly examined by H NMR spectrometry in CDCl
3
(
3
Figure 5). Addition of CF COOH led to a large downfield shift of
signal from 8.63 ppm to 9.20, which is assigned to proton of the
linker, -CH=N-. The two protons on the phenyl group adjacent to
the spiro-ring, with double-peak at 7.99 – 8.01 ppm (proton close
to carbonyl group) and double peak at 7.12 – 7.14 ppm (proton
close to spiro-carbon) showed slight downfield shifts, with the
shift values of 0.02 and 0.04 ppm, respectively. The protons on
the rhodamine skeleton, with signals at 6.44 – 6.45 ppm, 6.23 –
1
ring opening and recovery mechanism has been illustrated by H
NMR spectrometry. The low pK limits the probe in the acid
a
range; a future improvement with a larger applicable pH range
would be useful for a wider scope of applications. The FRET
offers a quantitative quenching of donor emission and the
elimination of the interference to the acceptor’s luminescence
signal. The effective FRET process also enabled an almost 100%
quenching of the donor emission, reducing the interference
noise from the donor. This work not only provides a sensitive
chemosensor for pH monitoring, but it also offers an effective
strategy for eliminating the interference of excitation light.
6.26 ppm and 6.51 – 6.53 ppm, exhibited apparent downfield
shifts, with values of 0.55 ppm, 0.37 ppm and 0.18 ppm,
respectively. Other slight downfield shifts of protons on DPE and
on the secondary carbon of the methylene group were observed,
3
while the –CH protons had almost no change. The downfield
shifts of these protons arise because of the withdrawing effect of
the obtained charge in the ring-opening form, which delocalizes
on the conjugated system formed by the ring-opening process.
1
The reversibility is also confirmed by H NMR spectrometry,
when the signals recovered their initial positions upon addition of
NaOH into the solution. This is consistent with the change in Experimental Section
+
-
fluorescence of DPE-Rh upon addition of H and OH solutions.
Synthetic procedures
Conclusions
Synthesis of 4-(phenylethynyl)benzaldehyde (1). 4-bromobenzaldehyde
(
2.174 g, 11.750 mmol) was added into 100 mL of dry THF. The reaction
mixture was bubbled with nitrogen gas for 0.5 h. DIPEA (18.056 mL,
7.914 mmol), Pd(PPh (0.226 g, 0.196 mmol), CuI (0.075 g, 0.392
To conclude, a FRET based pH-sensitive chemosensor DPE-Rh
has been demonstrated, with the combination of DPE and
9
3 4
)
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mmol), ethynylbenzene (1.944 mL, 17.624 mmol) were added to the
solution mixture. The solution was stirred for 36 h at room temperature.
Then 100 mL of water was added and the mixture was extracted with
using a buffer solution DCM/MeOH/PBS (20%/60%/20%, v/v/v) with
various pH values.
2 4
DCM (100 mL × 3) and the extractant was dried over anhydrous Na SO .
The solvent of the organic phase was removed and the residue was
Acknowledgements
chromatographed on silica gel, eluting with hexane/EA (40:1 v/v) to give
1
white solid as product (2.01 g, yield 82.6%). H NMR (400 MHz, CDCl
3
) δ
=
7
1
9
2
10.02 (s, 1H), 7.87 (d, J = 8.5 Hz, 2H), 7.68 (d, J = 8.2 Hz, 2H), 7.60 –
.52 (m, 2H), 7.43 – 7.33 (m, 3H). C NMR (101 MHz, CDCl ) δ =
3
This work was funded by grants from The Hong Kong Research
Grants Council (HKBU 22301615), and Hong Kong Baptist
University (FRG 2/17-18/007).
13
91.43, 135.35, 132.08, 131.76, 129.58, 129.57, 128.95, 128.46, 122.44,
+
3.43, 88.49. MALDI-TOF MS: calcd. for [M + H] = 207.0804. found:
07.2024.
Conflict of interest
Synthesis
of
2-amino-3',6'-bis(diethylamino)spiro[isoindoline-1,9'-
xanthen]-3-one (2). 5 mL of hydrazine hydrate (86 mmol) was added to a
solution of rhodamine B (2.0 g, 4.175 mmol) in 24 mL of EtOH. The
reaction mixture was refluxed for 2 h and 80 mL of water was added. The
solution was extracted with ethyl acetate (80 mL × 3) and dried over
anhydrous Na SO . The organic phase was combined and the solvent
2 4
was removed. The residue was purified by chromatography on silica gel
The authors declare that they have no conflict of interest.
Keywords: fluorescence • FRET • pH sensors • rhodamine
•spirolactams
eluting with hexane/ethyl acetate (40:1 v/v), giving the product as a light
1
pinky-white solid (1.8 g, 94.4% yield). H NMR (400 MHz, CDCl
–
3
) δ 7.97
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Hz, 2H), 6.42 (d, J = 2.6 Hz, 2H), 6.29 (dd, J = 8.9, 2.6 Hz, 2H), 3.61 (s,
H), 3.34 (q, J = 7.1 Hz, 8H), 1.17 (t, J = 7.1 Hz, 12H). C NMR (101
MHz, CDCl ) δ = 166.12, 153.80, 151.51, 148.83, 132.49, 129.98, 128.08,
23.80, 122.97, 107.97, 104.46, 97.89, 77.21, 65.90, 44.35, 12.60.
13
2
3
1
+
MALDI-TOF MS: calcd. for [M + H] = 457.2598. found: 457.2580.
Synthesis
of
phenylethynyl)benzylidene)amino)spiro[isoindoline-1,9'-xanthen]-3-one
DPE-Rh). 4-(phenylethynyl)benzaldehyde (1) (124 mg, 0.6 mmol) and a
drop of glacial acetic acid were added to a solution of 2-amino-3',6'-
bis(diethylamino)spiro[isoindoline-1,9'-xanthen]-3-one (2) (228 mg, 0.5
mmol) in 15 mL of EtOH under nitrogen gas atmosphere. The reaction
mixture was refluxed for 24 h and a white solid precipitated. The solid
was filtered and washed with ethanol, giving the product as a white solid
301 mg, yield 93.5%). H NMR (400 MHz, CDCl
d, J = 6.7 Hz, 1H), 7.55 – 7.40 (m, 8H), 7.33 (m, 3H), 7.13 (d, J = 7.3 Hz,
H), 6.52 (d, J = 8.8 Hz, 2H), 6.44 (d, J = 2.2 Hz, 2H), 6.25 (dd, J = 8.9,
.2 Hz, 2H), 3.33 (q, J = 7.0 Hz, 8H), 1.16 (t, J = 7.0 Hz, 12H). C NMR
) δ = 165.02, 153.14, 151.78, 148.95, 146.16, 135.24,
33.42, 131.62, 131.47, 129.25, 128.34, 128.32, 128.03, 127.37, 124.25,
23.89, 123.40, 123.13, 108.02, 105.97, 97.85, 90.83, 89.47, 77.22,
6.06, 44.33, 12.62. MALDI-TOF MS: calcd. for [M + H] = 645.3224.
3',6'-bis(diethylamino)-2-((4-
50-58.
(
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) δ = 8.63 (s, 1H), 8.00
(
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(101 MHz, CDCl
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purification. NMR spectra were recorded on a Bruker400 (400 Hz)
nuclear magnetic resonance spectrometer with chemical shifts expressed
as parts per million (ppm) and coupling constants J as Hertz (Hz). High-
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(MALDI-TOF)
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UV-Vis spectrophotometer. The measurements of emission and
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excitation spectra were performed using
a
Horiba Fluorolog-3
spectrofluorometer with a xenon lamp as excitation source or using a
Shimadzu Spectrofluorometry (RF6000). The pH values were recorded
with a Hach H170 pH meter. Quantum yields were measured using a
Horiba Fluorolog with the accessory of a Horiba integrating sphere,
Quanta–φ. The emission lifetime was determined by the Edinburgh
Instrument Mini-tau. The photophysical measurements were recorded
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+
H activates the effective FRET and turns on the rhodamine emission, providing
around 1000-fold enhancement of rhodamine emission with linear intensity change,
in the pH range of 2.0 to 5.5. The Förster resonance energy transfer offered this
probe an effective excitation-emission wavelength shift of around 240 nm with
~100% quenching of the donor emission. This pH probe is reversible and tolerant
towards a wide range of metal ions.
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