A BODIPY-based highly selective fluorescent chemosensor for Hg2+ ions and its application in living cell imaging

Article abstract of DOI:10.1002/ejoc.201101623

A new boron-dipyrromethene (BODIPY) derivative (FS1) containing two triazole units exhibits an enhanced fluorescence in the presence of Hg ²⁺ ions and a high selectivity for Hg²⁺ ions over competing metal ions in methanol: Ag⁺

Full text of DOI:10.1002/ejoc.201101623

FULL PAPER
DOI: 10.1002/ejoc.201101623
A BODIPY-Based Highly Selective Fluorescent Chemosensor for Hg2+ Ions
and Its Application in Living Cell Imaging
Mani Vedamalai[a] and Shu-Pao Wu*[a]
Keywords: Sensors / Mercury / Fluorescence / Imaging agents
A new boron–dipyrromethene (BODIPY) derivative (FS1)
containing two triazole units exhibits an enhanced fluores-
cence in the presence of Hg2+ ions and a high selectivity for
Zn2+ produced only minor changes in the fluorescence of
FS1. The apparent dissociation constant (Kd) of FS1–Hg2+
was found to be 62 μ
experiments showed that FS1 can be used as a fluorescent
Cd2+, Co2+, Cu2+, Fe2+, Fe3+, K+, Mg2+, Mn2+, Ni2+, Pb2+, and probe for detecting Hg2+ ions in living cells.
M. Moreover, fluorescence microscopy
Hg2+ ions over competing metal ions in methanol: Ag+, Ca2+
,
Because Hg2+ is known to be a fluorescence quencher,
most fluorescent chemosensors detect Hg2+ by fluorescence
Introduction
Mercury is one of the most toxic heavy metal elements.[1] quenching through spin–orbit coupling.[9] Owing to their
It exists in three different forms: Inorganic mercury, alkyl- sensitivity, fluorescent chemosensors that detect metal ions
mercury, and elemental mercury. Mercury contamination by fluorescence enhancement are more easily monitored
occurs through various processes, such as the combustion than those that operate by fluorescence quenching. This pa-
of fossil fuels, mining, and solid-waste incineration. Mer- per reports on a newly designed BODIPY-based fluorescent
cury ions show a high affinity for thiol groups in proteins, enhancement chemosensor for Hg2+ based on photoin-
leading to the malfunction of cells and consequently caus- duced electron transfer (PET). The binding of Hg2+ to the
ing many health problems in the brain, kidney, and central chemosensor blocks the PET mechanism and greatly en-
nervous system. Its accumulation in the body can contrib- hances the fluorescence of BODIPY.
ute to the development of a wide variety of diseases, such
In this work we designed a BODIPY-based fluorescent
as prenatal brain damage, serious cognitive and motion dis- chemosensor for metal-ion detection. Two parts make up
orders, and Minamata disease.[2] Owing to the extreme tox- the chemosensor FS1: A BODIPY moiety as reporter and
icity of mercury, the United States Environmental Protec- two triazole units that chelate the metal ion (Scheme 1).
tion Agency (EPA) established the standard for the maxi- FS1 exhibits weak fluorescence due to quenching by photo-
mum allowed level of mercury in dietary and environmental induced electron transfer from the lone-pair electrons on
sources to be 2 ppb (10 nm).
the nitrogen atom attached to the phenyl group. The bind-
Numerous methods[3] for the detection of mercury ions ing of metal ions to the chemosensor blocks the PET
in various samples have been proposed, including atomic mechanism and results in considerable fluorescence en-
absorption/emission spectroscopy,[4] inductively coupled hancement of BODIPY. The metal ions Ag+, Ca2+, Cd2+
plasma-mass spectroscopy (ICPMS),[5] inductively coupled Co2+, Cu2+, Fe2+, Fe3+, Hg2+, K+, Mg2+, Mn2+, Ni2+, Pb2+
,
,
plasma-atomic emission spectrometry (ICP-AES),[6] and and Zn2+were tested for metal-ion-binding selectivity with
voltammetry.[7] Most of these methods require expensive in- FS1, but Hg2+ was the only ion that caused green emission
struments and are not suitable for performing assays. Over upon binding with FS1.
the past decade more attention has been focused on the
development of fluorescent chemosensors for the detection
of Hg2+ ions.[8]
Results and Discussion
Synthesis of FS1
[a] Department of Applied Chemistry, National Chiao Tung
University,
The synthesis of FS1 is outlined in Scheme 1. Aniline
was treated with propargyl bromide in the presence of
K2CO3 to afford compound 1. Compound 2 was obtained
by reaction of compound 1 with POCl3 in the presence of
Hsinchu, Taiwan, Republic of China
Fax: +886-3-5723764
E-mail: spwu@mail.nctu.edu.tw
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101623.
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A BODIPY Selective Fluorescent Chemosensor for Hg2+
Cation-Sensing Selectivity
The sensing ability of FS1 was tested by mixing it with
the metal ions Ag+, Ca2+, Cd2+, Co2+, Cu2+, Fe2+, Fe3+
,
Hg2+, K+, Mg2+, Mn2+, Ni2+, Pb2+, and Zn2+. Qualita-
tively, Hg2+ was the only ion that caused a change in color
(from red to yellow) of the FS1 solution and green fluores-
cence from FS1 (Figure 1). Other metal ions had no signifi-
cant effect on the fluorescence of FS1. Quantitative fluores-
cence spectra of FS1 were recorded in the presence of sev-
eral transition-metal ions. Hg2+ was the only metal ion that
caused significant green emission (Figure 2). During the ti-
tration of FS1 against Hg2+, a new emission band centered
at 520 nm was observed (Figure 3). After the addition of
4 equiv. of Hg2+, the emission intensity reached a maxi-
mum. The quantum yield of the emission band was Φ =
0.035, which is 17-fold higher than that of FS1, with Φ =
0.002. These results indicate that Hg2+ is the only metal ion
of those studied that readily binds to FS1, causing signifi-
cant fluorescence enhancement and permitting the highly
selective detection of Hg2+
.
Scheme 1. Synthesis of FS1.
DMF at 80 °C. Treatment of compound 2 with excess
pyrrole in the presence of TFA under nitrogen yielded the
corresponding dipyrromethane 3. In the next step, com-
pound 3 was oxidized with DDQ to yield the corresponding
dipyrromethene, which was transformed into the BODIPY
skeleton 4 in the presence of BF3 under N2. Treatment of
compound 4 with picolyl azide yielded FS1 under click
chemistry conditions. FS1 has an absorbance maximum at
493 nm, assigned to the S0ǞS1 transition of the BODIPY
chromophore,[10] and a molar extinction coefficient of
3.83ϫ104 m–1 cm–1. FS1 displays weak fluorescence with a
Figure 2. Fluorescence response of FS1 (30 μm) to various metal
cations (30 μm) in methanol. The excitation wavelength was
492 nm.
To study the influence of other metal ions on the binding
quantum yield of Φ = 0.002, because photoinduced electron of Hg2+ to FS1, we performed competitive experiments
transfer from the aromatic amine group to the BODIPY with Hg2+ (150 μm) and other metal ions (150 μm; Fig-
moiety takes place.
ure 4). The fluorescence enhancement observed for the mix-
Figure 1. Color (top) and fluorescence (bottom) changes in FS1 (30 μm) upon the addition of various metal ions (60 μm) in methanol.
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M. Vedamalai, S.-P. Wu
FULL PAPER
a fluorescent sensor for the analysis of Hg2+ was deter-
mined from the variation of fluorescence intensity as a
function of the concentration of Hg2+ (see Figure S12 in
the Supporting Information). It was found that FS1 has a
detection limit of 2.8 μm, which allows micromolar concen-
trations of Hg2+ to be detected.
Figure 3. Fluorescence response of FS1 (30 μm) to various equiva-
lents of Hg2+ in methanol. The excitation wavelength was 492 nm.
Figure 5. Job plot of FS1–Hg2+ complexes in methanol. The solu-
tions were monitored at a wavelength of 520 nm. The total concen-
tration of the sensor and Hg2+ ion was 250 μm.
tures of Hg2+ with most metal ions was similar to that
caused by Hg2+ alone. Reduced fluorescence enhancement
was observed when Hg2+ was mixed with Co2+ or Fe3+
.
This indicates that only Co2+ and Fe3+ compete with Hg2+
To gain a clearer understanding of the structure of the
for binding with FS1. Most of the other metal ions do not
1
FS1–Hg2+ complex, H NMR spectroscopy (Figure 6) was
interfere with the binding of FS1 to Hg2+
.
employed. Hg2+ is a heavy metal ion and can affect the
proton signals that are close to the Hg2+ binding site.[11]
The 1H NMR spectra of FS1 recorded with increasing
amounts of Hg2+ show that the proton (Hg, triazole) signal
at δ = 7.8 ppm is shifted downfield as Hg2+ is added. This
indicates that Hg2+ binds to FS1 mainly through the nitro-
gen atom in the triazole ring. The proton signals He and
Hd are shifted upfield upon addition of Hg2+. This also
indicates that Hg2+ binds through the amino group at-
tached to the phenyl ring. The proton signals from Hi, Hj,
Hk, and Hl on the pyridine ring are slightly affected by the
binding of Hg2+. These observations show that Hg2+ binds
to FS1 through an amino group, two nitrogen atoms of two
triazole units, and two pyridine nitrogen atoms.
A pH titration of FS1 was carried out to investigate a
suitable pH range for Hg2+ sensing. As depicted in Figure 7,
the emission intensities of metal-free FS1 are very low at all
pH values. After mixing FS1 with Hg2+, the emission inten-
sity at 520 nm is markedly higher at pH = 5.0 and is a maxi-
mum in the pH range 5.0–10.0. At pH Ͼ 10, the emission
intensity decreases. This indicates poor stability of the FS1–
Hg2+ complexes at high pH values. At pH Ͻ 5, the emission
intensity is also lower due to the protonation of the amino
groups, which prevents the formation of the FS1–Hg2+
complex.
Figure 4. Fluorescence response of FS1 (30 μm) to Hg2+ (150 μm)
and other metal ions (150 μm; black bars) and to mixtures of Hg2+
(150 μm) with other metal ions (150 μm; gray bars) in methanol.
To determine the binding stoichiometry of the FS1–Hg2+
complex, the emission intensity of FS1 at 520 nm was plot-
ted as a function of the molar fraction of FS1 under a con-
stant total concentration. The resulting Job plot is shown
in Figure 5. The maximum emission intensity was reached
when the molar fraction was 0.5, which indicates a 1:1 ratio
for the FS1–Hg2+ complex, that is, one Hg2+ ion binds to
one molecule of FS1. Furthermore, the formation of a 1:1
FS1–Hg2+ complex was confirmed by ESI-MS, in which the
peak at m/z = 828.1 indicates a 1:1 stoichiometry for the
FS1–Hg2+ complex (see Figure S9 in the Supporting Infor-
Living Cell Imaging
FS1 was also used for living cell imaging. For the detec-
mation). The apparent dissociation constant was calculated tion of Hg2+ in living cells, HeLa cells were treated with
from Figure 3 by using a nonlinear regression analysis; a 20 µm Hg(BF4)2 for 30 min and washed with phosphate-
value of 62.1Ϯ5.7 μm was determined (see Figure S10 in buffered saline (PBS) three times. Then the cells were incu-
the Supporting Information). The detection limit of FS1 as bated with FS1 (20 µm) for 30 min and washed with PBS to
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A BODIPY Selective Fluorescent Chemosensor for Hg2+
1
Figure 6. H NMR spectra of FS1 (5 mm) in the presence of different concentrations of Hg2+ in CD3CN.
Figure 7. Fluorescence intensity (520 nm) of free FS1 (30 μm; ᭺)
and of FS1 after the addition of Hg2+ (150 μm; ᭿) in methanol/
water solution (9:1, v/v; 1 mm buffer) as a function of the pH. The
excitation wavelength was 492 nm. Buffer solutions: pH = 1–2,
KCl/HCl; pH = 2.5–4, KHP/HCl; pH = 4.5–6, KHP/NaOH; pH Figure 8. Hg2+-treated HeLa cell images. Top left: bright-field im-
= 6.5–12 HEPES.
age; top right: fluorescence image; bottom: merged image.
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M. Vedamalai, S.-P. Wu
FULL PAPER
13C NMR (CDCl3): δ = 146.4, 133.1, 132.8, 129.0, 117.0, 115.6,
108.1, 106.9, 79.1, 72.8, 42.9, 40.4 ppm. MS (FAB): m/z = 313.
HRMS (FAB): calcd. for C21H19N3 313.1579; found 313.1578.
remove any remaining sensor. Images of the HeLa cells
were obtained with a fluorescence microscope. Figure 8
shows the images of the HeLa cells with FS1 after treatment
of Hg2+. The overlapping of the fluorescence and bright-
field images reveals that the fluorescence signals are local-
ized in the intracellular area, which indicates a subcellular
distribution of Hg2+ and good permeability of the cell
membrane of FS1.
Synthesis of 4,4-Difluoro-8-[4-(N,N-diprop-2-ynylamino)phenyl]-4-
bora-3a,4a-diaza-s-indacene (4): 2,3-Dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ; 318 mg, 1.4 mmol) dissolved in CH2Cl2
(50 mL) was added to a solution of compound 3 (376 mg,
1.2 mmol) in CH2Cl2 (100 mL) under nitrogen, and the mixture
was stirred for 3 h. It was then treated with Et3N (4 mL) and
BF3·OEt2 (5 mL) for 12 h. The solvent was evaporated under re-
duced pressure, and the crude product was purified by column
chromatography (ethyl acetate/hexane, 1:10) to give compound 4
as an orange solid. Yield: 258.6 mg (60%); m.p. 226–227 °C. 1H
NMR (CDCl3): δ = 7.90 (s, 2 H), 7.58 (d, J = 8.7 Hz, 2 H), 7.02–
7.05 (m, 4 H), 6.55 (d, J = 2.4 Hz, 2 H), 4.24 (d, J = 2.4 Hz, 4 H),
2.33 (t, J = 2.4 Hz, 2 H) ppm. 13C NMR: δ = 149.7, 147.8, 142.6,
134.6, 132.6, 131.1, 124.2, 118.0, 113.7, 78.4, 73.1, 40.2 ppm. MS
(FAB): m/z = 359. HRMS (FAB): calcd. for C21H16BF2N3
359.1405; found 359.1407.
Conclusions
The new fluorescence chemosensor FS1 exhibits a high
affinity and selectivity for Hg2+ ions over competing metal
ions. The fluorescence of FS1 was significantly enhanced in
the presence of Hg2+, and the addition of Ag+, Ca2+, Cd2+
,
Co2+, Cu2+, Fe2+, Fe3+, K+, Mg2+, Mn2+, Ni2+, Pb2+, or
Zn2+ barely affected the fluorescence. This BODIPY-based
Hg2+ chemosensor is also an effective method for Hg2+
sensing in living cell imaging.
Synthesis of 4,4-Difluoro-8-[4-(N,N-bis{[1-(pyridin-2-ylmethyl)-1H-
1,2,3-triazol-4-yl]methyl}amino)phenyl]-4-bora-3a,4a-diaza-s-
indacene (FS1): Picolyl azide (180 mg, 1.34 mmol), CuSO4·5H2O,
(16.8 mg, 10 mol-%), and sodium ascorbate (26.6 mg, 20 mol-%)
were added to a solution of compound 4 (240.6 mg, 0.67 mmol) in
THF/H2O (7:3, v/v; 15 mL) under nitrogen. The solution was
stirred at room temperature for 12 h. A saturated ammonium
chloride solution (20 mL) was added to the reaction mixture, and
the organic phase was extracted with dichloromethane (10 mL,
3ϫ). The combined organic extracts were dried with anhydrous
MgSO4. The solvent was evaporated under reduced pressure, and
the crude product was purified by column chromatography (dichlo-
romethane/methanol, 6:1) to give compound FS1 as a dark-red so-
Experimental Section
General: All solvents and reagents were obtained from commercial
sources and used as received without further purification. UV/Vis
spectra were recorded with an Agilent 8453 UV/Vis spectrometer.
Fluorescence spectra were recorded with a Hitachi F-4500 spec-
1
trometer. H and 13C NMR spectra were recorded with a Bruker
DRX-300 NMR spectrometer.
Synthesis of N,N-Diprop-2-ynylaniline (1): Compound 1 was ob-
tained in modest yield by treating aniline with propargyl bromide
in the presence of potassium carbonate.[12]
1
lid. Yield: 320.0 mg (76%); m.p. 145–146 °C. H NMR (CD3CN):
δ = 8.50 (d, J = 4.2 Hz, 2 H), 7.84 (s, 2 H), 7.79 (s, 2 H), 7.72 (dt,
J = 1.8, 7.5 Hz, 2 H), 7.53 (d, J = 9.0 Hz, 2 H), 7.26 (dd, J = 4.8,
7.5 Hz, 2 H), 7.18 (d, J = 7.8 Hz, 2 H), 7.09 (d, J = 8.7 Hz, 2 H),
7.03 (d, J = 3.9 Hz, 2 H), 6.59 (dd, J = 2.1, 4.1 Hz, 2 H), 5.58 (s,
4 H), 4.81 (s, 4 H) ppm. 13C NMR (CD3CN): δ = 155.7, 151.8,
150.4, 149.2, 145.2, 142.5, 138.0, 134.9, 134.0, 131.8, 124.4, 124.0,
122.9, 122.8, 118.7, 113.4, 55.8, 46.8 ppm. MS (ESI): m/z = 628.3
[M + H]+. HRMS (ESI): calcd. for C33H29BF2N11 [M + H]+
628.2669; found 628.2678.
Synthesis of 4-(Diprop-2-ynylamino)benzaldehyde (2): Phosphorus
oxychloride (306.7 mg, 2 mmol) was added dropwise to a solution
of compound 1 (338.2 mg, 2 mmol) in DMF (2 mL), and the mix-
ture was then heated at 90 °C for 3 h. After cooling to ambient
temperature, the reaction mixture was dissolved in CH2Cl2
(100 mL) and washed with a dilute sodium hydrogen carbonate
solution. The solvent was evaporated under reduced pressure, and
the crude product was purified by column chromatography (ethyl
acetate/hexane, 1:10) to give compound 2 as a white solid. Yield:
Determination of the Binding Stoichiometry and the Apparent Disso-
ciation Constants for the Binding of HgII to FS1: The binding stoi-
chiometry of the FS1–Hg2+ complex was determined from a Job
plot. The fluorescence intensity at 520 nm was plotted against the
molar fraction of FS1 with a total concentration of the sensor and
Hg2+ ion of 250 μm. The molar fraction at maximum emission in-
tensity represents the binding stoichiometry of the FS1–Hg2+ com-
plex. The maximum emission intensity was reached at a molar frac-
tion of 0.5 (Figure 4). This result indicates that chemosensor FS1
forms a 1:1 complex with Hg2+. The apparent dissociation constant
(Kd) was calculated by nonlinear regression analysis.[13] The plot
was fitted with normalized fluorescence emission intensity against
the concentration of the Hg2+ ion according to Equation (1) in
which F is the fluorescence intensity at 520 nm at any given Hg2+
concentration, Fmin is the fluorescence intensity at 520 nm in the
absence of Hg2+, Fmax is the maximum fluorescence intensity at
520 nm in the presence of Hg2+, n is the number of Hg2+ ions
bound per probe molecule, and Kd is the dissociation constant: n
= 1 according to the Job plot.
1
354.7 mg (90%); m.p. 54–55 °C. H NMR (CDCl3): δ = 9.82 (s, 1
H), 7.81 (d, J = 8.7 Hz, 2 H), 6.96 (d, J = 8.7 Hz, 2 H), 4.22 (d, J
= 2.4 Hz, 4 H), 2.30 (t, J = 2.4 Hz, 2 H) ppm. 13C NMR (CDCl3):
δ = 191.0, 152.2, 132.2, 127.9, 113.7, 78.7, 73.6, 40.5 ppm. MS (EI):
m/z (%) = 197 (100.0), 172 (13.86), 167 (58.54), 132 (19.59). HRMS
(EI): calcd. for C13H11NO 197.0841; found 197.0838.
Synthesis of 4-[Bis(1H-pyrrol-2-yl)methyl]-N,N-diprop-2-ynylaniline
(3): Trifluoroacetic acid (TFA, 0.1 mL) was added to a solution of
compound 2 (296 mg, 1.5 mmol) in pyrrole (2 mL). The solution
was stirred under nitrogen at room temperature for 4 h, and the
reaction was then quenched with 0.1 m sodium hydroxide. The or-
ganic phase was extracted with ethyl acetate and dried with anhy-
drous MgSO4. The solvent was evaporated under reduced pressure,
and the crude product was purified by column chromatography
(ethyl acetate/hexane, 3:10) to give compound 3 as a yellowish vis-
cous liquid. Yield: 389.9 mg (83%). 1H NMR (CDCl3): δ = 7.91 (s,
2 H), 7.13 (d, J = 8.7 Hz, 2 H), 6.91 (d, J = 8.7 Hz, 2 H), 6.67–
6.69 (m, 2 H), 6.15 (q, J = 2.7 Hz, 2 H), 5.91–5.94 (m, 2 H), 5.42
(s, 1 H), 4.11 (d, J = 2.4 Hz, 4 H), 2.25 (t, J = 2.4 Hz,2 H) ppm.
F = (Fmax[Hg2+]n + FminKd)/(Kd + [Hg2+]n)
(1)
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A BODIPY Selective Fluorescent Chemosensor for Hg2+
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Supporting Information (see footnote on the first page of this arti-
cle): 1H and 13C NMR spectra of compounds 2, 3, 4, and FS1;
ESI-MS of FS1–Hg2+
.
Acknowledgments
We gratefully acknowledge the financial support of the National
Science Council (ROC) and National Chiao Tung University.
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Received: November 10, 2011
Published Online: January 4, 2012
Eur. J. Org. Chem. 2012, 1158–1163
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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