A “turn on” fluorometric and colorimetric probe based on vinylphenol-BODIPY for selective detection of Au(III) ion in solution and in living cells

Article abstract of DOI:10.1016/j.dyepig.2021.109341

In this work, borondipyrromethene (BODIPY)-based fluorescent and colorimetric probe SBP has been explored as a sensor for selective detection of Au³⁺. The probe SBP containing two vinyl phenol groups at C3 and C5 position on BODIPY pyrrole rings is prepared to extend its emission wavelength into the red region (656 nm) and also to serve as a new receptor toward Au³⁺. The probe displayed colorimetric change from blue to pink and fluorometric change from red to orange (up to 40-fold at 590 nm) in aqueous medium, with the detection limit of 0.0094 μM. This colorimetric and fluorometric changes are caused by the oxidation of the double bond in the SBP facilitating by hydroxy group which is confirmed by detected intermediates from mass spectroscopy experiment. In addition, the “turn on” SBP probe is successfully applied for monitoring intracellular Au³⁺ in living RAW264.7 macrophages through fluorescence imaging.

Full text of DOI:10.1016/j.dyepig.2021.109341

Dyes and Pigments 191 (2021) 109341
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
A “turn on” fluorometric and colorimetric probe based on
vinylphenol-BODIPY for selective detection of Au(III) ion in solution and in
living cells
a
b
c
c
Suthikorn Jantra , Tanapat Palaga , Paitoon Rashatasakhon , Mongkol Sukwattanasinitt ,
c,*
Sumrit Wacharasindhu
a
Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
b
c
Department of Microbiology, Faculty of Science, Chulalongkorn University, Pathumwan, Bangkok 10330, Thailand
Thailand Nanotec-CU Center of Excellence on Food and Agriculture, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
A R T I C L E I N F O
A B S T R A C T
Keywords:
BODIPY
Gold
In this work, borondipyrromethene (BODIPY)-based fluorescent and colorimetric probe SBP has been explored as
3+
a sensor for selective detection of Au . The probe SBP containing two vinyl phenol groups at C3 and C5 position
on BODIPY pyrrole rings is prepared to extend its emission wavelength into the red region (656 nm) and also to
Sensor
3+
serve as a new receptor toward Au . The probe displayed colorimetric change from blue to pink and fluoro-
Oxidation
metric change from red to orange (up to 40-fold at 590 nm) in aqueous medium, with the detection limit of
0
.0094
μ
M. This colorimetric and fluorometric changes are caused by the oxidation of the double bond in the SBP
facilitating by hydroxy group which is confirmed by detected intermediates from mass spectroscopy experiment.
3
+
In addition, the “turn on” SBP probe is successfully applied for monitoring intracellular Au in living RAW264.7
macrophages through fluorescence imaging.
1
. Introduction
Gold is one of the least reactive metal. It is a precious metal used for
spectroscopy (ICP-MS), atomic absorption spectroscopy (AAS), atomic
emission spectroscopy (AES), and fluorescence spectroscopy [11–15].
Among these methods, fluorescence spectroscopy has several advan-
tages such as high sensitivity, excellent selectivity, real-time monitoring,
easy to process and high potential for imaging in biological system
jewellery and arts. Gold in the form of gold nanoparticle (AuNP) can be
used in drug delivery, bioimaging and as additives in cosmetics [1,2].
Ionic gold species, gold(I) and gold(III), are applied as an efficient
catalyst for various oxidation processes in many organic transformations
3
+
[16–19]. There were many reports using fluorescent probe for Au
sensing [20–39], utilizing coumarin [20], rhodamine [21–26], naph-
thalimide [27,28], quinoline [22], graphitic carbon nitride [29], naph-
[
3]. Importantly, gold ions can be used as therapeutic agents for treat-
ment of various diseases including rheumatoid arthritis, cancer, AIDS,
bronchial asthma and malaria [4]. However, gold(III) salt is reactive and
thalene
[30–32],
anthracene
[33],
thiazole
[34],
poly
(phenylenediethynylene) [35], spirobifluorene [36] nitrogen-doped
carbon dot [37] and boron dipyrromethene (BODIPY) [33,38,39] as a
signalling unit.
toxic to the biological system. Au3 strongly bind with enzymes and
+
DNA, which leads to DNA cleavage and cell toxicity [5]. Notably, only
2
00
including liver, kidneys, spleen, lungs and the peripheral nervous system
causes either blood clot or hemolysis [9,10].
μ
M AuCl
3
is harmful enough to cause damage to several organs
Among these fluorophores, BODIPY exhibited distinctive perfor-
mances such as high molar absorptivity, excellent photostability, flex-
ible modification, cell permeability and biocompatibility [40] which
makes it suitable for cellular imaging applications [41–43] and photo-
[
6–8] while HAuCl
4
3
+
Therefore, detection of small amount of Au in living cell with the aid
of a fluorescent probe is important to evaluate their role in certain
biological process.
3
+
dynamic therapy [44]. Successful BODIPY-based sensors for Au
detection has been reported and the sensing mechanism relies on the
Several analytical methods have been developed to monitor Au3
+
binding between sulfur/nitrogen atom in receptor unit with Au
inducing hydrolysis reaction [33,38,39]. Although those probes are
3+
including electrochemical analysis, inductively coupled plasma-mass
*
Corresponding author.
E-mail address: sumrit.w@chula.ac.th (S. Wacharasindhu).
https://doi.org/10.1016/j.dyepig.2021.109341
Received 29 November 2020; Received in revised form 4 March 2021; Accepted 20 March 2021
Available online 26 March 2021
0
143-7208/© 2021 Elsevier Ltd. All rights reserved.

S. Jantra et al.
Dyes and Pigments 191 (2021) 109341
highly sensitive, they are only effective at a relatively short wavelength
within the green region or lower than 530 nm. These short-wavelength
probes usually suffer from low cellular penetration and could damage
living cells. Thus, probe that displays a longer emission wavelength is
highly desirable for gold ion monitoring in living cells. As mentioned
2.3. Synthesis of BODIPY probes (BOD, SBP, SBT and SBPT)
2.3.1. Probe BOD
BOD was synthesized from
ρ
-tolualdehyde and 2,4-dimethylpyrrole
-Tolualdehyde (631.7 mg, 5.3
according to the previous report [47].
ρ
above, Au3 can act as a catalyst specially for epoxidation on double
+
mmol) and 2,4-dimethylpyrrole (1,199.7 mg, 12.6 mmol, 2.4 equiv.)
were dissolved and stirred in round-bottom flask with dry dichloro-
3
+
bond in styrene derivative [45,46]. This oxidation ability of Au in-
spires us to create a new probe design for Au3 detection. Previously, we
+
methane (100 mL). A few drops of trifluoroacetic acid were added to the
◦
reported a BODIPY derivative, SBP (Fig. 1), was an excellent sensor for
solution at 0 C under N
2
atmosphere for 30 min. The reaction mixture
(20 mL) and washed
with deionized water (3 × 20 mL). The organic layer was dried over
Na SO . The solvent was evaporated and the residue was purified by
ꢀ
anionic ROS such as OCl [47]. In this work, we would like to report an
was quenched with saturated aqueous NaHCO
3
investigation of our BODIPY series, SBP, SBT and SBPT (Fig. 1) for
detection of cationic Au3 . We expected that the extended styryl group
+
2
4
3
+
will serve as selective reactive sites for Au via oxidation process.
silica gel column chromatography using 10% ethyl acetate in hexane as
an eluent. The oil product was dissolved in dry dichloromethane (100
mL). 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (1,193 mg, 5.3
mmol, 1 equiv.) was added to the solution. The mixture was stirred at
2
. Materials and methods
2
.1. Materials
2
room temperature under N atmosphere for 30 min. N,N-Diisopropyle-
thylamine (DIPEA) (6.5 mL, 37 mmol, 5 equiv.) and BF
3
.OEt
2
(6.5 mL,
◦
All reagents were purchased from Sigma-Aldrich, Merck, RCI Lab-
53 mmol, 10 equiv.) were added to the solution at 0 C. The mixture was
scan, TCI, Invitrogen (Grand Island, NY, USA), Fluka, Chemieliva, Carlo
Erba, Loba chemie or AnalaR and used without further purification.
Analytical thin-layer chromatography (TLC) was performed on Kieselgel
F-254 pre-coated plastic TLC plates from EM Science. Visualization was
performed under a 365 nm ultraviolet lamp (black light). Column
chromatography was carried out with Biotage Isolera One chromatog-
stirred overnight under N
quenched with saturated aqueous NaHCO
saturated aqueous NaCl (3 × 20 mL). The organic layer was dried over
Na SO . The solvent was evaporated and the residue was purified by
2
atmosphere. The reaction mixture was
3
(20 mL) and washed with
2
4
silica gel column chromatography using 10% ethyl acetate in hexane as
1
an eluent to obtain orange solid (447.7 mg, yield: 25%). H NMR (500
raphy system and pre-packed silica cartridge (40–63
Technologies, Inc.
μ
m) from Santai
MHz, CDCl
3
2H), 2.54 (s, 6H), 2.42 (s, 3H), 1.39 (s, 6H). C NMR (126 MHz, CDCl ):
3
): δ 7.27 (d, J = 7.6 Hz, 2H), 7.13 (d, J = 7.7 Hz, 2H), 5.96 (s,
13
δ 155.3, 143.3, 142.2, 138.9, 132.0, 131.7, 129.9, 127.8, 121.2, 29.8,
+
2
.2. Analytical instruments
NMR spectra were obtained from JNM-ECZ500R NMR spectrometer
21.5, 14.6. ESI-HRMS (m/z): [M] calc’d for C20
2 2
H21BF N : 338.1766,
found 338.1776.
1
13
(
JEOL company) operating at 500 MHz for H NMR and 126 MHz for
C
2.3.2. Probe SBP
NMR. Mass spectra were obtained from JMS-S3000 MALDI-TOF mass
spectrometer (JEOL company) and High-resolution mass spectra
BOD (52.0 mg, 0.15 mmol), 4-hydroxybenzaldehyde (112.1 mg,
0.92 mmol, 6 equiv.), piperidine (0.20 mL) and acetic acid (0.15 mL)
were dissolved and stirred in sealed tube with dry toluene (0.50 mL).
(
HRMS) were recorded using electron spray ionization (ESI) with a
◦
MicroTOF Bruker mass spectrometer. Absorption and emission spectra
were obtained from Cary100Bio UV–Vis spectrophotometer and Carry
Eclipse fluorescence spectrophotometer (Agilent Technologies) using
The reaction mixture was heated by microwave reactor at 115 C for 60
min. After cooling to room temperature, deionized water (10 mL) was
added to the reaction mixture. The mixture was extracted with
dichloromethane (3 x 10 mL). The organic layers were combined and
1
0 × 10 mm quartz cuvette (Starna Scientific). The pH of each sample
solution was recorded by Ohaus Starter2001 pH meter. Confocal
microscopic images were taken by Nikon Eclipse Ti confocal laser
scanning microscope.
2 4
dried over Na SO . The solvent was evaporated and the residue was
purified by silica gel column chromatography using 30% ethyl acetate in
1
hexane as an eluent to obtain dark blue solid (14.7 mg, yield: 17%). H
NMR (500 MHz, acetone-d
6
): δ 7.53 (dd, J = 17.7, 12.5 Hz, 6H),
.46–7.36 (m, 4H), 7.29 (d, J = 7.9 Hz, 2H), 6.92 (d, J = 8.5 Hz, 4H),
.79 (s, 2H), 2.43 (s, 3H), 1.45 (s, 6H). 13C NMR (126 MHz, acetone-d
):
7
6
6
δ 158.9, 152.8, 141.8, 139.0, 138.7, 136.3, 133.1, 132.2, 129.8, 129.0,
1
28.5, 128.5, 117.6, 116.2, 116.0, 20.6, 14.0. ESI-HRMS (m/z):
[
M+H+]+ calc’d for C34
2 2 2
H30BF N O : 547.2363, found 547.2378.
2
.3.3. Probe SBT
BOD (43.3 mg, 0.13 mmol),
ρ
-tolualdehyde (98.0 mg, 0.82 mmol, 6
equiv.), piperidine (0.20 mL) and acetic acid (0.15 mL) were dissolved
and stirred in sealed tube with dry toluene (0.50 mL). The reaction
◦
mixture was heated by microwave reactor at 115 C for 10 min. After
cooling to room temperature, deionized water (10 mL) was added to the
reaction mixture. The mixture was extracted with dichloromethane (3 x
1
2 4
0 mL). The organic layers were combined and dried over Na SO . The
solvent was evaporated, and the residue was purified by silica gel col-
umn chromatography using 5% dichloromethane in hexane as an eluent
to obtain dark blue solid (19.7 mg, yield: 28%). 1H NMR (500 MHz,
CDCl
J = 17.8, 16.7, 8.0 Hz, 10H), 6.62 (s, 2H), 2.44 (s, 3H), 2.38 (s, 6H), 1.45
s, 6H). 13C NMR (126 MHz, CDCl
3
): δ 7.69 (d, J = 16.3 Hz, 2H), 7.52 (d, J = 8.0 Hz, 4H), 7.22 (ddd,
(
3
): δ 152.6, 142.1, 139.2, 138.9, 136.1,
34.0, 133.5, 132.2, 129.8, 129.6, 128.3, 127.8, 127.6, 118.5, 117.7,
1
2
+
+
9.8, 21.6, 14.8. ESI-HRMS (m/z): [M+H ] calc’d for C36
2 2
H34BF N :
Fig. 1. Chemical structure of the probes SBP, SBT and SBPT.
543.2778, found 543.2792.
2

S. Jantra et al.
Dyes and Pigments 191 (2021) 109341
2
.3.4. Probe SBPT
(NO
Cd(NO
3 6
[Co(III)(NH )
3
)
3
, Co(NO
, Bi(NO
]Cl
3
)
2
, Ni(NO
3
)
2
, Cu(NO
, AgNO , HAuCl
(10 mM each in DI water). Mixture of methanol, DI
3
)
2
, Zn(NO
3
)
2
, Pb(NO
3
)
2
, Hg(NO
3 2
) ,
BOD (100.8 mg, 0.30 mmol), 4-hydroxybenzaldehyde (36.9 mg,
.30 mmol, 1 equiv.), piperidine (0.20 mL) and acetic acid (0.15 mL)
3
)
2
3
3
)
2
3
4
Au(I)Cl, Pb(IV)(CH
3
COO) and
4
0
were dissolved and stirred in sealed tube with dry toluene (0.50 mL).
water, SBP stock solution and metal ion stock solutions were prepared to
obtain final concentration of 5 M dye and 10 M (2 equiv.) metal ions
◦
The reaction mixture was heated by microwave reactor at 115 C for 60
μ
μ
min. After cooling to room temperature, deionized water (10 mL) was
added to the reaction mixture. The mixture was extracted with
dichloromethane (3 x 10 mL). The organic layers were combined and
in 80% methanol/water. After 60 min, each mixture was measured by
UV–vis and fluorescence spectrophotometers to obtain absorption and
emission spectra. In case of sensitivity test, various concentrations of
3
+
dried over Na
2 4
SO . The solvent was evaporated and the residue was
Au (0–50
μ
M) were used in the mixture instead of just 10 M.
μ
purified by silica gel column chromatography using 20% ethyl acetate in
hexane as an eluent to obtain dark violet solid hSBP (44.3 mg, yield:
2.4.3. Time dependent study
Mixture of methanol, DI water, SBP stock solution and Au stock
1
3+
3
4%). H NMR (500 MHz, CDCl
3
): δ 7.51 (d, J = 17.7 Hz, 1H), 7.44 (d, J
=
8.5 Hz, 2H), 7.28 (d, J = 8.3 Hz, 2H), 7.16 (t, J = 7.1 Hz, 3H), 6.82 (d,
J = 8.5 Hz, 2H), 6.56 (s, 1H), 5.98 (s, 1H), 2.58 (s, 3H), 2.43 (s, 3H), 1.44
s, 3H), 1.40 (s, 3H). 13C NMR (126 MHz, CDCl
solutions were prepared to obtain final concentration of 5
μ
M SBP and
+
10
μ
M (2 equiv.) Au3 in 80% methanol/water. Emission spectra were
(
3
): δ 156.8, 154.6, 153.3,
42.8, 142.6, 140.6, 138.9, 136.1, 134.3, 133.1, 132.1, 129.8, 129.5,
29.3, 128.1, 121.1, 117.5, 117.0, 115.9, 29.8, 21.5, 14.8, 14.5. ESI-
measured every 5 min for 2 h.
1
1
2.4.4. pH dependent study
+
+
HRMS (m/z): [M+H ] calc’d for C27
H
26BF
43.2271. Then hSBP (44.3 mg, 0.10 mmol),
.62 mmol, 6 equiv.), piperidine (0.20 mL) and acetic acid (0.15 mL)
N
2 2
O: 443.2101, found
Buffer solutions were prepared (acetate buffer for pH 4–5, phosphate
buffer for pH 6–8, glycine buffer for pH 9 and carbonate buffer for pH
10). Mixture of methanol, DI water, buffer solutions, SBP stock solution
4
0
ρ-tolualdehyde (74.0 mg,
and/or Au3 stock solutions were prepared to obtain final concentration
+
were dissolved and stirred in sealed tube with dry toluene (0.50 mL).
◦
3+
The reaction mixture was heated by microwave reactor at 115 C for 30
of 5
μ
M SBP, 10 mM buffer solution and/or 10
μ
M (2 equiv.) Au in
min. After cooling to room temperature, deionized water (10 mL) was
added to the reaction mixture. The mixture was extracted with
dichloromethane (3 x 10 mL). The organic layers were combined and
80% methanol/water. Emission spectra were measured after 60 min.
2.4.5. Real water sample test
dried over Na
2
SO
4
. The solvent was evaporated and the residue was
Tap water from our laboratory was used for real water sample test.
3
+
purified by silica gel column chromatography using 60% dichloro-
methane in hexane as an eluent to obtain dark blue solid (29.3 mg, yield:
Mixture of methanol, tap water, SBP stock solution and Au stock so-
lutions were prepared to obtain final concentration of 5
μ
M SBP, 10 mM
1%). 1H NMR (500 MHz, CD
Cl2/): δ 7.62 (d, J = 16.3 Hz, 1H),
.56–7.47 (m, 5H), 7.30 (d, J = 7.7 Hz, 2H), 7.21 (ddd, J = 11.2, 8.0, 4.6
buffer solution and 0.49 or 0.74 ppm Au in 80% methanol/water.
Emission spectra were measured after 60 min.
3+
4
2
7
Hz, 6H), 6.86 (d, J = 8.5 Hz), 6.63 (d, J = 4.3 Hz, 2H), 2.42 (s, 3H), 2.36
s, 3H), 1.46 (s, 6H). 13C NMR (126 MHz, CD
Cl
42.7, 142.2, 139.4, 139.3, 139.1, 136.0, 135.8, 133.9, 133.5, 133.3,
32.0, 129.8, 129.6, 129.4, 129.2, 128.3, 127.3, 118.0, 117.5, 117.4,
(
2
2
): δ 157.0, 152.8, 151.9,
2.5. Cell culture
1
1
1
The murine macrophage cell line RAW264.7 (ATCC number TIB-71)
was purchased from ATCC (Bethesda, MD, USA). Cells were cultured in
Dulbecco’s Modified Eagle Medium (DMEM) supplemented with 10%
+
+
16.8, 115.8, 21.2, 14.6, 14.5. ESI-HRMS (m/z): [M+H ] calc’d for
32BF O: 545.2570, found 545.2595.
C
35
H
2 2
N
heat-inactivated Fetal Bovine Serum (FBS), penicillin (100 units/mL)
◦
and streptomycin (100
μ
g/mL) at 37 C in a humidified atmosphere of
2
2
.4. Sample preparation for UV–vis and fluorescent spectroscopy
5% CO /air.
2
.4.1. Photophysical properties
2.6. Cells imaging
Prepare probe stock solutions (~360 M each in DMSO) and store
μ
them in refrigerator. The mixture of methanol, deionized (DI) water and
probe stock solutions were prepared to obtain final concentration of 5
After cells were cultured in 8-well chambers for at least one day, the
cells in DMEM (400 L) were treated with SBP in DMSO (final con-
M SBP, 1% DMSO in DMEM) and incubated for 30 min at
μ
μ
M probe in 80% methanol/water. Each mixture was measured by
UV–vis and fluorescent spectrometers to obtain absorption and emission
spectra. Molar absorptivity ( ) were evaluated
centration: 5
μ
◦
37 C. The treated cells were washed with phosphate buffer saline (0.1
ε
) and quantum yield (Φ
F
M, 3 × 200
μ
L) to remove remaining SBP. Various concentrations of
3
+
using the mixture of methanol and probe stock solutions at various
concentration. Molar absorptivity was calculated at maximum absorp-
Au in water were treated in the same method as SBP. All samples were
◦
treated with 4%
ρ
-formaldehyde solution for 10 min at 37 C. Then the
tion wavelength. Zinc phthalocyanine (Φ
toluene) was used as standard for quantum yield calculation from
equation (1).
F
= 0.30 in 1% pyridine/
solution was removed. Mowiol was used as a contrast agent by dropping
on each sample. The samples were covered with cover slide. Fluorescent
images were taken by confocal laser microscope with excitation at 561
nm and emission at 595 nm.
2
Grad
X
η
X
Φ
X
= ΦST
(
)(
)
(1)
2
GradST
η
ST
3
. Results and discussion
Φ
X
= quantum yield of the probe, ΦST = quantum yield of the
standard, Grad
X
= slope of the plot graph between absorbance and in-
3.1. Synthesis and photophysical properties of SBP, SBT and SBPT
To prepare BODIPY probes used in this works, SBP, SBT and SBPT,
tegrated fluorescence intensity of the probe solution, GradST = slope of
the plot graph between absorbance and integrated fluorescence intensity
of the standard solution,
η
X
= refractive index of solvent used for the
starting material BOD was first synthesized from -tolualdehyde and
ρ
probe solution,
solution.
η
ST = refractive index of solvent used for the standard
2,4-dimethylpyrrole according to the previous report [47]. Conse-
quently, target probes SBP, SBT and SBPT were synthesized from BOD
via Knoevenagel condensation with the corresponding benzaldehydes
under the microwave irradiation (Scheme 1). The use of excess amount
2
.4.2. Selectivity and sensitivity test
Prepare metal ion stock solutions containing LiNO
Mg(NO , Ca(NO , Ba(NO , Al(NO , Cr(NO
3 3
, NaNO , KNO
3
,
of 4-hydroxybenzaldehyde and
ρ
-tolualdehyde provided symmetrical
3
)
2
3
)
2
3
)
2
3
)
3
3
)
, Fe(NO )
3 3 2
, Fe
probes SBP and SBT in 17% and 28% yield, respectively. On the other
3

S. Jantra et al.
Dyes and Pigments 191 (2021) 109341
Scheme 1. Synthesis of SBP, SBPT and SBT.
hand, the unsymmetrical probe SBPT were prepared via a two-step
synthesis process. Firstly, Knoevenagel condensation of BOD were ob-
tained from the use of 1 equivalent of 4-hydroxybenzaldehyde,
providing monostyryl-BODIPY, hSBP in 34% yield. Secondly, conden-
sation with 6 equivalents of tolualdehyde gave desired SBPT in 41%
yield. The structures of these synthesized products were fully confirmed
by NMR and mass spectroscopy (Figs. S1–15). The UV–vis and fluores-
cent spectra of these three probes were measured in MeOH/water
mixture (80% v/v, Fig. S16). As expected, all prepared styryl de-
rivatives, including SBP, SBT and SBPT showed obvious red shift having
maximum absorptions (λab) in the range between 624 and 638 nm along
from styryl groups. Under the black light, SBP, SBT and SBPT displayed
strong red emission with emission maxima (λem) at 656, 636 nm and
649 nm respectively. A large Stokes shift observed in case of SBP and
SBPT perhaps resulted from the intramolecular charge transfer (ICT)
process in donor–acceptor systems between hydroxy group(s) and the
BODIPY pyrrole rings. The fluorescent quantum yields of SBT, SBPT and
SBP in methanol solution, using zinc phthalocyanine (Ф = 0.3 in 1%
F
pyridine/toluene) as the standard, were 0.86, 0.56 and 0.52, respec-
tively. The lower quantum yield in case of SBPT and SBP was caused by
photoinduced electron transfer (PET) process from hydroxy group which
makes those probes suitable for turn-on fluorescent sensor.
with light blue colour resulting from the extended -conjugation system
π
Fig. 2. Absorption (blue line) and emission (red line) spectra of 5 M SBT (a), SBPT (b) and SBP (c) before (bold line) and after (dashed line) addition of 1 equiv.
μ
3
+
Au for 60 min (excitation wavelength = 540 nm). Inset: Images of the solution under room light and black light. Fluorescent enhancement ratio (d) of SBT, SBPT
and SBP at emission wavelength = 577, 577 and 590 nm, respectively (error bar: n = 3). All samples were measured in solution (80% MeOH/H
2
O) after 60 min
of mixing.
4

S. Jantra et al.
Dyes and Pigments 191 (2021) 109341
+
3
.2. Response of SBT, SBPT and SBP toward Au3
under black light from red to orange (Fig. 3b). In the UV–vis spectra, SBP
displayed no noticeable decreasing and increasing in absorption band at
3
+
To evaluate all prepared probes as both colorimetric and fluoro-
538 and 573 nm, respectively, toward other ions except only for Au
.
metric Au3 sensors, the addition of Au to SBT, SBPT and SBP were
performed in 80% MeOH/water and recorded by UV–vis absorption and
fluorescence spectrophotometer (Fig. 2). There were no significant
changes in both absorption and emission spectra of SBT and SBPT before
+
3+
Upon excitation at 540 nm, a remarkable increased in emission band
around 590 nm were observed only for Au3 . These results demon-
strated that SBP had selective colorimetric and fluorometric responses
+
3
+
towards Au (blue bar, Fig. 3c). Interference test using 2 equivalents of
and after the addition of Au3 (Fig. 2a and b). On the other hand, the
+
Au and 22 other metal ions showed an insignificant interference from
3+
+
2+
+
2+
3+
absorption band of SBP at 638 nm decreased with a new peak at 573 nm
upon the addition of Au3 (Fig. 2c). These results corresponded to the
common metal ions such as Na , Mg , K , Ca and Al but there
were negative interferences from some less common transition metal
+
ions and positive interference from Pb4 (orange bar, Fig. 3c).
+
observation by naked eye displaying a light blue to pink colour change
upon addition of Au3 to the solution (Fig. 2c inset). For fluorescence
+
responses, SBP showed significantly enhancement of emission band at
3.4. Sensitivity and optimization of SBP toward Au3
+
5
90 nm when excited at 540 nm (Fig. 2c) while there was no fluores-
cence response for both SBT and SBPT under the same condition (Fig. 2a
and b). Moreover, under black light, only SBP probe displayed the vivid
colour change from red to orange (Fig. 2c inset). Strong colorimetric
change and large fluorescence enhancement (25-fold, Fig. 2d) of SBP
clearly suggested that it was the most sensitive probe and could be used
Next, we examined the sensitivity of the SBP probe by performing
3
+
the titration of Au from 0 to 10 equivalents (Fig. 4). Upon the addition
of Au3 up to 2 equivalents, a new peak appeared at 573 nm together
with a decrease in the original absorption peak at 638 nm (Fig. 4a).
Then, the peak at 573 nm gradually dropped concomitantly with an
+
in colorimetric or fluorometric detection of Au3 . Based on these find-
ings, we concluded that both hydroxy groups and double bonds on
vinylphenol groups anchored on BODIPY are critical in colorimetric
+
increase in the new peak at 505 nm upon addition of Au3 from 2 to 10
equivalents. These results corresponded to the two-step colour change
from blue to pink and then to colourless under room light observation
+
change and fluorescence enhancement of SBP toward Au3
+
.
(
Fig. 4d inset top). Based on the UV–vis absorption results, fluorescence
titration was studied at two excitation wavelengths at 590 and 480 nm.
When excited at 540 nm, significant fluorescent enhancement was
observed at the peak of 590 nm and reached its maximum upon addition
3
.3. Selectivity of SBP toward metal ions
To evaluate the selectivity of all prepared probe, 23 metal ions (Li ,
+
2+
3+
3+
of 2 equivalents of Au (Fig. 4b). Further addition of Au resulted in
the decrease in emission intensity (λem = 590 nm). Switching the exci-
tation wavelength from 540 nm to 480 nm provided a similar trend
(Fig. 4c). At the concentration of Au3 below 2 equivalents, the
enhancement of emission peak centred at 573 nm were observed. On the
+
+
2+
2+
2+
3+
3+
2+
3+
2+
2+
Na , K , Mg , Ca , Ba , Al , Cr , Fe , Fe , Co , Ni , Cu ,
2
+
2+
2+
2+
2+
+
3+
+
4+
3+
Zn , Pb , Hg , Cd , Bi , Ag , Au , Au , Pb , Co ) were tested
+
with SBP and monitored by UV–vis absorption and fluorescence spec-
trophotometer (Fig. 3a and 3b). Clearly, only Au3 induced the color
+
+
changes from blue to pink (Fig. 3a) along with fluorometric change
other hands, when amount of Au3 was over 2 equivalents, we observed
Fig. 3. Absorption (a) and emission (b) spectra of SBP (5
μ
M) in 80% MeOH/H
2
O upon addition of 2 equiv. metal ions for 60 min (excitation wavelength = 540 nm).
3
+
Enhancement ratio (c) of fluorescent intensity at 590 nm of SBP upon addition of metal ions with (orange bar) and without (blue bar) Au (error bar: n = 3).
5

S. Jantra et al.
Dyes and Pigments 191 (2021) 109341
Fig. 4. Absorption (a) and emission spectra under excitation wavelength of 540 (b) and 480 (c) nm of SBP upon addition of 0, 0.25, 0.5, 0.75, 1, 1.5, 2, 2.5, 3, 5 and
3
+
1
0 equiv. Au . Enhancement ratio (d) at 590 and 515 nm under excitation wavelength of 540 (orange line) and 480 (green line) nm, respectively (error bar: n = 3).
Inset: Images of solutions under room light and black light. All samples were measured from 5
μ
M of SBP in 80% MeOH/H
2
O after 60 min of mixing.
+
3+
the decline of the emission peak at 573 nm together with the emerging of
toward Au3 . At 10
μ
M (2 equivalents) of Au , fluorescence enhance-
new peak at 515 nm upon the addition of Au3 (Fig. 4c). This change can
also be observed under black light displaying the colour change from red
into orange and then green (Fig. 4d inset bottom). The results indicated
that there are at least two intermediates responsible for different pho-
tophysical properties. These results will be further explained later in
mechanistic investigation part. Importantly, the enhancement ratio (I/
+
ment reached saturation point at 60 min indicating the time required for
reaching the equilibrium (Fig. 5a). The solvent effect on the sensitivity
of SBP toward Au3 in various ratios of aqueous solvents including
methanol, ethanol, acetonitrile, dimethyl sulfoxide and tetrahydrofuran
were assessed (Fig. S17). Among them we founded that both aqueous
methanol and acetonitrile provided good fluorescence enhancement
+
+
I
0
) from emission peaks at 590 (excited at 540 nm) higher than that of
15 nm (excited at 480 nm) at low concentration (up to 40-fold at 2
toward Au3 . Next, we carefully analyzed the effect of pH on SBP in
buffer condition from both solvent systems (MeOH/10 mM buffer, 4/1
v/v (Fig. 5b) and MeCN/10 mM buffer, 3/2 v/v (Fig. S18)). Within pH
4–10, fluorescent emission intensity of SBP at 590 nm upon the addition
5
equivalents) suggested that monitoring of the peak at 590 nm is more
suitable for detection of Au3 at low concentration, prompting us to
+
further explore SBP probe for fluorometric analysis of Au3 at long
+
of Au in methanol/water (4/1 v/v) were more stable in comparison to
3+
wavelength for live cell imaging (Fig. 4d).
the emission intensity in the acetonitrile system. This result suggested
Next, we investigated the time dependence of the emission of SBP
that our SBP probe can be used within physiological pH range and could
3
+
Fig. 5. Time dependent (a) and pH dependent studies (b) of 5 M SBP in 80% MeOH/H O upon addition of 2 equiv. Au Inset: Images of solutions before and after
μ
2
3
+
μ
M (0.5–1.5 equiv.) Au3+ (excitation wavelength = 540 nm). Error bar: n = 3.
addition of Au . Determination of LOD (c) upon addition of 2.5–7.5
6

S. Jantra et al.
Dyes and Pigments 191 (2021) 109341
be applied in cellular imaging. The limit of detection (LOD) of our SBP
diluted by MeOH to obtain final concentration at 0.49 and 0.74 ppm
(Table 1). The sample was measured and Au3 concentration was
determined by linear equation (Fig. 5c). The result showed that this
probe could determine Au3 concentration with accuracy of 90–99%.
+
probe was 0.0094
μ
M using 3 /slope method (Fig. 5c). It is one of the
σ
3
+
most sensitive probes compared to reported Au probes in Table S1.
+
+
3
.5. Proposed reaction mechanism between SBP and Au3
.7. Imaging of intracellular Au3 using SBP
+
3
3
+
Based on the previous reports on the oxidation of alkene by Au [48,
9], we would like to propose the similar mechanism for sensing of Au3
+
For cells imaging application, we used RAW264.7 macrophages
4
(
white blood cells from mouse) as cells sample and monitored fluores-
3
by our SBP probe (Scheme 2). First, electrophilic AuCl could be added
cent signal at 595 nm under excitation at 561 nm by using confocal laser
microscope (Fig. 6). The cells showed no significant fluorescent signal
into double bond in SBP probe due to the high electron density of the
double bond promoted from hydroxy group to afford compound 1. This
is explained the sensitivity of SBP over SBT probe which have no hy-
droxy substituent. Then rearomatization occurred via attacking of water
molecule to afford compound 2 followed by epoxidation to provide in-
termediate 3. Then 3 could further react with solvent such as water and
methanol to provide intermediates 4 or 5, respectively. The diol moiety
on 4 then could undergo further oxidation to generate compound 6. We
both before and after incubation with 5
μM of SBP probe (Fig. 6a–b). The
observation of cell morphology suggested that the cell is not damaged in
+
this process. After incubation with 5 and 100
μ
M of Au3 (Fig. 6c–d), red
fluorescent signal was enhanced. We would like to note here that, unlike
the in vitro, we did not observe the reduction of fluorescent emission
even when the large excess of Au3 is presented. This result indicates
that live cell may sluggish the second oxidation process at the second
double bond of SBP as described in the proposed mechanism. Never-
+
hypothesized that these compounds (1–6) possess a shorter
π
-conjugated
system and are responsible for orange emission (λem = 590 nm) observed
theless, the ability of SBP to detect Au3 across a wide concentration
+
upon addition of Au3 at low concentration below 2.0 equivalents. In
+
the present of excess Au3+, the unreacted double bond on 6 could react
range suggested that it can potentially be used for in vivo imaging of
3
+
Au in living cell.
further through the same process generating compound 7 which is
accountable to green emission (λem = 515 nm). To confirm this sensing
3
+
4. Conclusions
mechanism, SBP were mixed with 2 equivalents of Au for 1 h and mass
spectrum was acquired (Fig. S19a). We were able to detect peaks at m/z
In summary, we have demonstrated the application of SBP probe as
colorimetric and “turn on” fluorescent sensor for selective detection of
=
585.22 and 595.27 corresponding to the molecular ion of interme-
3
+
diate 3 and 5. As expected, upon the addition of large excess of Au (10
equivalents), we observed only main peak at m/z = 590.90 which could
be attributed to intermediate 7 (Fig. S19b).
Table 1
3
+
Analytical results of Au in tap water (n = 3).
3
+
3+
Au added (ppm)
Au found (ppm)
Recovery (%)
3
.6. Application on real water samples
To determine efficiency of the probe SBP for detection of Au3 in real
0
.49
0.49 ± 0.01
0.67 ± 0.03
99 ± 2
90 ± 4
+
0.74
sample, small amount of Au3 stock solution was spiked in tap water and
+
3
+
.
Scheme 2. Proposed reaction mechanism and intermediates between SBP and Au
7

S. Jantra et al.
Dyes and Pigments 191 (2021) 109341
Fig. 6. Confocal microscopic images (excitation at 561 nm and detection at 595 nm) of RAW264.7 macrophage cells (a). Cells incubated with 5 M SBP for 30 min
μ
3
+
(
b), followed by incubation with Au at 5
μ
M (c) and 100 M (d) for 30 min.
μ
Au3 . The design is based on incorporation of vinylphenol group into
BODIPY core leading to the shifting in emission wavelength into red
region and also provide a reactive double bond as a reaction site for
+
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2021.109341.
3
+
Au oxidation. SBP probe displayed high selectivity and sensitivity
toward Au3 in MeOH/H
+
O (4:1, v/v) with the significant enhancement
2
in absorption (λab = 573 nm) and emission (λem = 590 nm) intensity (up
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