A highly copper-selective ratiometric fluorescent sensor based on BODIPY

Article abstract of DOI:10.1002/cjoc.201200260

A new ratiometric fluorescent sensor (1) for Cu²⁺ based on 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) with di(2-picolyl)amine (DPA) as ion recognition subunit has been synthesized and investigated in this work. The binding abilities of 1 towards different metal ions such as alkali and alkaline earth metal ions (Na⁺, K⁺, Mg²⁺, Ca²⁺) and other metal ions (Ba²⁺, Zn²⁺, Cd ²⁺, Fe²⁺, Fe³⁺, Pb²⁺, Ni ²⁺, Co²⁺, Hg²⁺, Ag⁺) have been examined by UV-vis and fluorescence spectroscopies. 1 displays high selectivity for Cu²⁺ among all test metal ions and a ～10-fold fluorescence enhancement in I₅₈₂/I₅₅₈ upon excitation at visible excitation wavelength. The binding mode of 1 and Cu²⁺ is a 1:1 stoichiometry determined via studies of Job plot, the nonlinear fitting of the fluorometric titration and ESI mass.

Full text of DOI:10.1002/cjoc.201200260

FULL PAPER
DOI: 10.1002/cjoc.201200260
A Highly Copper-Selective Ratiometric Fluorescent Sensor
Based on BODIPY
Huang, Jinlong(黄金龙)
Wang, Bo(王波)
Ye, Jianguo(叶建国)
Liu, Bin(刘斌)
Qiu, Huayu(邱化玉)
Yin, Shouchun*(尹守春)
College of Material Chemistry and Chemical Engineering, Hangzhou Normal University,
Hangzhou, Zhejiang 310036, China
＋
A new ratiometric fluorescent sensor (1) for Cu2 based on 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene
(BODIPY) with di(2-picolyl)amine (DPA) as ion recognition subunit has been synthesized and investigated in this
work. The binding abilities of 1 towards different metal ions such as alkali and alkaline earth metal ions (Na＋, K＋,
＋
＋
＋
＋
＋
＋
＋
＋
＋
＋
＋
Mg2 , Ca2 ) and other metal ions ( Ba2 , Zn2 , Cd2 , Fe2 , Fe3 , Pb2 , Ni2 , Co2 , Hg2 , Ag＋) have been examined
＋
by UV-vis and fluorescence spectroscopies. 1 displays high selectivity for Cu2 among all test metal ions and a
～10-fold fluorescence enhancement in I582/I558 upon excitation at visible excitation wavelength. The binding mode
＋
of 1 and Cu2 is a 1∶1 stoichiometry determined via studies of Job plot, the nonlinear fitting of the fluorometric
titration and ESI mass.
Keywords BODIPY, chemosensor, copper(II), ratiometric measurement, spectroscopic properties
Introduction
has a lot of valuable characteristics, such as high fluo-
rescence quantum yields, relatively strong and sharp
absorption and emission in the visible region, and chem-
ical stability. Furthermore, BODIPY dyes are amenable
to structural modification so that spectral shifts in the
absorption and emission bands can be generated by in-
troducing the appropriate substituent group.[35,36]
Herein, we report the synthesis and metal sensing
properties of a new ratiometric BODIPY-fluorescent
chemosensor, 5-N-(2-picolyl)amine-4,4-difluoro-3-
Because it is an essential trace element in biological
systems and an important environmental pollutant, there
has been great interest in the development of fluorescent
＋
＋
chemosensors for Cu2 in recent years.[1,2] Cu2 plays
an important role in many biological processes, however,
＋
if the level exceeds cellular needs, Cu2 can cause oxi-
dative stress and disorders associated with neurodegen-
erative diseases, such as Menkes disease, Wilson dis-
ease, and Alzheimer’s disease.[3] Most of the reported
＋
Cu2 ion fluorescent chemosensors work in a “turn-off”
methoxyl-8-(4-tolyl)-4-bora-3a,4a-diaza-s-indacene (1).
＋
1 shows a high selectivity toward Cu2 over all the
or “turn-on” mode with changes only in fluorescent
probes intensity.[4-30] However, the simple change of
fluorescence intensity is interfered easily by various
factors, such as probe concentration, sample environ-
ments, and variabilities in the efficiency of excitation
and emission wavelength. A ratiometric fluorescent
measurement is desirable to eliminate those effects,
which uses a ratio of the fluorescent intensities at two
different wavelengths, providing a built-in correction to
eliminate most of the ambiguities.[31-34] Thus, there is
tested metal ions and ~10-fold fluorescence ratio change
＋
after Cu2 binding upon excitation at 535 nm in
CH3CN.
Experimental
Materials
3,5-Dichloro-8-(4-tolyl)BODIPY was synthesized
according to literature procedures.[37] Sodium methoxide
di(2-picolyl)amine, and sodium hydride were purchased
from Aldrich and used without further purification. The
metal salts [AgClO4, Ba(ClO4)2, Ca(ClO4)2, Cd(ClO4)2,
Co(ClO4)2, Cu(ClO4)2 •6H2O, FeCl2 •4H2O, FeCl3,
Hg(ClO4)2, KClO4, Mg(ClO4)2, NaClO4, Ni(ClO4)2 •
6H2O, Pb(ClO4)2•3H2O and Zn(ClO4)2] were purchased
from Aldrich.
still strong demand for the development of efficient
＋
Cu2 -selective fluorescent sensor, especially ratiometric
probes that can be excited in the visible wavelength re-
gion.
BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-inda-
cene) is a popular fluorescent dye with widespread ap-
plications as fluorescent chemosensor because BODIPY
*
E-mail: yinsc@ustc.edu; Tel.: 0086-571-28867899; Fax: 0086-571-28867899
Received March 18, 2012; accepted April 22, 2012; published online XXXX, 2012.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201200260 or from the author.
Chin. J. Chem. 2012, XX, 1—5
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1

Huang et al.
FULL PAPER
Instrumentation
Synthesis of 5-chloro-4,4-difluoro-3-methoxyl-8-(4-
tolyl)-4-bora-3a,4a-diaza-s-indacene (2)
1H NMR and 13C NMR spectra were recorded at
room temperature on a Bruker Avance 400 operating at
To a solution of 3,5-dichloro-8-(4-tolyl)BODIPY
(175 mg, 0.5 mmol) in 10 mL of methanol, sodium
methoxide (54 mg, 1 mmol) was added. The reaction
mixture was stirred at room temperature for 0.5 h under
argon. After evaporating the solvent, 30 mL of water
was added and the organic layer was extracted with
CH2Cl2 (40 mL×3), dried by Mg2SO4, and evaporated
under reduced pressure. The crude product was purified
by column chromatography on silica gel, eluting with a
mixture of CH2Cl2 and petroleum ether (1∶4, V∶V) to
a frequency of 400 MHz for H and 100 MHz for 13C.
1
Melting points were taken on a Beijing Taike X-5 melt-
ing point instrument and are uncorrected. Mass spectra
were recorded on a Hewlett-Packard 5989 A mass spec-
trometer (ESI mode). High-resolution mass data were
obtained with a Kratos MS50TC instrument.
UV-vis absorption spectra were recorded on a Perkin
Elmer Lambda 40 UV-vis spectrophotometer. Corrected
steady-state excitation and emission spectra were ob-
tained using a HITACHI F-2700 Fluorescence Spectro-
photometer.
1
give red solid 2 (150 mg, 87% yield). H NMR (400
MHz, CDCl3) δ: 7.36 (d, J＝8.0 Hz, 2H), 7.29 (d, J＝
7.6 Hz, 2H), 6.96 (d, J＝4.4 Hz, 1H), 6.61 (d, J＝4.0 Hz,
1H), 6.28 (d, J＝4.0 Hz, 1H), 6.14 (d, J＝4.8 Hz, 1H),
4.14 (3H, s, OCH3), 2.45 (3H, s, CH3); 13C NMR (100
Mz, CDCl3) δ: 169.5, 140.6, 136.9, 135.2, 132.2, 130.4,
130.0, 129.1, 115.3, 104.4, 59.2, 21.4; ESI-MS m/z:
369.5 [M＋Na]＋.
Experimental details for spectral studies
The sensor 1 was dissolved into spectroscopic grade
CH3CN. The concentration of the 1 solution for spectral
analysis is 2 µmol/L. All the test metal ions were dis-
solved into deionized water. The concentration of the
test metal ions was 3 mmol/L. For the sensitivity meas-
urement, 100 µL of the test metal ions solution was
added into 3 mL of the 1 solution (2 µmol/L).
Synthesis of 5-N-(2-picolyl)amine-4,4-difluoro-3-
methoxyl-8-(4-tolyl)-4-bora-3a,4a-diaza-s-indacene
(1)
To a solution of 2 (100 mg, 0.29 mmol) and
di(2-picolyl)amine (40 mg, 0.4 mmol) in 15 mL of ace-
tonitrile, sodium hydride (14 mg, 0.6 mmol) was added.
The reaction mixture was refluxed for 3 h under argon.
After evaporating the solvent, 30 mL of water was
added and the organic layer was extracted with CH2Cl2
(40 mL×3), dried by MgSO4, and evaporated under
reduced pressure. The crude product was purified by
column chromatography on silica gel, eluting with a
mixture of CH2Cl2 and ethyl acetate (4∶1, V∶V) to
give red solid 1 (84 mg, 57% yield). m.p. 183—185 ℃;
1H NMR (400 MHz, CDCl3) δ: 8.51 (d, J＝4.5 Hz, 2H),
7.64 (t, J＝7.8 Hz, 2H), 7.49 (d, J＝7.8 Hz, 2H), 7.33 (d,
J＝7.8 Hz, 2H), 7.22 (d, J＝7.8 Hz, 2H), 7.15 (t, J＝6.2
Hz, 2H), 6.67 (d, J＝4.8 Hz, 1H), 6.47 (d, J＝4.2 Hz,
1H), 6.06 (d, J＝4.8 Hz, 1H), 5,75 (d, J＝4.2 Hz, 1H),
5.10 (s, 4H, N(CH2)2), 4.00 (s, 3H, OCH3), 2.42 (s, 3H,
CH3); 13C NMR (100 MHz, CDCl3) δ: 162.5, 161.7,
157.4, 149.2, 139.1, 137.0, 134.8, 132.7, 132.6, 131.6,
130.5, 128.7, 126.3, 124.2, 122.4, 122.1, 111.2, 95.5,
58.3, 58.2, 21.3; ESI-MS m/z: 532.2 [M＋Na]＋. HRMS
calcd for C29H26BF2N5O 509.2198, found 509.2250.
Determination of quantum yields
For the determination of the relative fluorescence
quantum yields (Φf) in solution, only dilute solutions
with an absorbance below 0.1 at the excitation wave-
length λex were used. Cresyl violet in ethanol (Φf＝0.55,
λex＝535 nm) was used as fluorescence standards. [38] In
all cases, correction for the solvent refractive index was
applied. All spectra were recorded at room temperature
using undegassed samples.
Determination of ground-state dissociation constant
Kd
The ground-state dissociation constant Kd of the
＋
complex between 1 and Cu2
was determined in
CH3CN solution by ratiometric fluorometric titration as
＋
a function of Cu2 using the fluorescence emission
spectra. Nonlinear fitting of Eq. (1) to the steady-state
fluorimetric ratiometric data R recorded as a function of
＋
[Cu2 ] yields values of Kdξ, Rmin, Rmax, and n. Since
ξ—the ratio of the fluorescence signal of the free form
of 1 over that of the bound form at the indicated wave-
lengths—is experimentally accessible, a value for Kd
can be recovered from ratiometric excitation fluores-
cence data.
Results and Discussion
2＋
⎡
Rmax Cu
n ＋Rmin Kd
⎤
Synthesis of chemosensor 1
⎣
⎦
R＝
(1)
2＋
n
To synthesize sensor 1, 3,5-dichloroBODIPY was
first prepared according to a previously reported proce-
dure. As reported in Wim’s work, 3,5-dichloroBODIPY
can be substituted with many different nucleophiles,
such as oxygen, nitrogen, sulfur, and carbon centred
nucleophiles, which turns out to be a very successful
approach for preparing a variety of symmetric and
asymmetric BODIPY compounds with substitution pat-
⎡
⎤
Kdξ＋ Cu
⎣
⎦
In Eq. (1), R stands for the ratiometric emission
＋
fluorescence data at [Cu2 ], whereas Rmin and Rmax de-
note the ratiometric emission fluorescence data at
＋
minimal and maximal [Cu2 ], respectively, and n is the
＋
number of Cu2 ions bound per probe molecule (i.e.,
stoichiometry of binding).[39]
2
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Chin. J. Chem. 2012, XX, 1—5

A Highly Copper-Selective Ratiometric Fluorescent Sensor Based on BODIPY
terns that are difficult to synthesize by other way.[36]
Thus, we use 3,5-dichloroBODIPY as starting material
to synthesize 1 in two steps. As outlined in Scheme 1,
one chlorine atom of 3,5-dichloroBODIPY was first
substituted by a methoxy group upon addition of two
equivalents of sodium methoxide in methanol at room
temperature to afford the monosubstituted product 2 in
good (87%) yield. The second chlorine atom of the iso-
lated 2 was subsequently substituted by di(2-picolyl)-
amine in refluxing acetonitrile with strong base (sodium
hydride) because of the lower reactivity of the second
chlorine resulting from the electron donating effect of
Figure 1 UV-vis absorption spectra of 1 (2 µmol/L) in the pres-
the oxygen atom.
＋
ence of different concentrations of Cu2 (0, 2, 4, 6, 8, 10, 12, 14,
16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36 µmol/L) in CH3CN.
Scheme 1 Synthetic route to sensor 1
＋
sorption at 573 nm was reached when 36 μmol/L Cu2
＋
H
was added, and further addition of excess Cu2 pro-
N
duced no significant changes in UV-vis spectra.
N
N
The emission spectra of 1 and its fluorescence titra-
CH3ONa
87%
＋
tion with Cu2 were recorded in CH3CN upon excita-
57%
tion at λex＝535 nm (Figure 2). The emission spectrum
N
N
N
N
B
B
of free 1 displayed a weak peak with a maximum at 558
F2
F2
Cl
H3CO
Cl
Cl
＋
nm. Interestingly, after the addition of Cu2 to chemo-
2
sensor 1, the fluorescence emission intensity at 558 nm
decreased significantly and a new red-shifted emission
＋
band at 582 nm arising from the formation of a 1-Cu2
complex showed up. Correspondingly, the ratio of the
fluorescent intensities at 582 nm and 558 nm (I582/I558
)
increased from 0.41 to 3.87 with the concentration of
＋
Cu2 changed from 0 to 36 μmol/L, which made 1
N
N
B
＋
serve as a ratiometric fluorescent sensor for Cu2 (Fig-
F2
N
H3CO
ure 2).
N
N
1
＋
Spectral characteristics with and without Cu2
The UV-vis absorption and fluorescence spectro-
scopic measurements and titration studies in the absence
＋
and presence of Cu2 in CH3CN solution were investi-
gated after full characterization of the chemical struc-
1
ture by H and 13C NMR and MS spectroscopy. The
UV-vis absorption spectrum of 1 in CH3CN exhibited a
main peak with a maximum at 535 nm assigned to the
S0→S1 transition of the BODIPY chromophore (Figure
1). In addition, a weak absorption band at 502 nm cor-
Figure 2 Fluorescent emission spectra of 1 (2 µmol/L) upon
excitation at 535 nm in the presence of different concentrations of
responding to the S0→S2 transition of the BODIPY
＋
Cu2 (0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32,
＋
chromophore was also observed. When Cu2 was
34, 36 µmol/L) in CH3CN.
added to 1 in CH3CN solution, the intensity of the main
absorbance peak at 535 nm gradually decreased while a
new low energy broad band around 580 nm appeared
and increased progressively with two isosbestic points
＋
The Фf values of free 1 and 1-Cu2 complex were
0.008 and 0.17, respectively. The low Фf value (0.008)
for uncomplexed 1 could be attributed to an efficient
quenching via an excited-stated intramolecular charge
transfer (ICT) process from the nitrogen atom of the
at 491, 565 nm. After addition of solutions up to 20
＋
μmol/L Cu2 , the initial strong band at 535 nm almost
di(2-picolyl)amine moiety to the strongly electron-
disappeared, and the intensity of the low energy broad
band at 573 nm increased very sharply with a new iso-
sbestic point at 506 nm. The maximum intensity of ab-
＋
deficient BODIPY acceptor. Upon binding of Cu2 , the
electron-donating properties of the amine were reduced,
Chin. J. Chem. 2012, XX, 1—5
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3

Huang et al.
FULL PAPER
which resulted in an enhancement in the fluorescence
rescent spectra of 1 response to other metal ions (Na＋,
＋
＋
＋
＋
＋
＋
＋
＋
intensity.[39]
K＋, Mg2 , Ca2 , Ba2 , Zn2 , Cd2 , Fe2 , Fe3 , Hg2 ,
Pb2 , Ni2 , Co2 , Ag＋) that probably affected the fluo-
rescence intensity were examined (Figures 4 and 5). As
shown in Figure 4, titration of alkali and alkaline earth
metal perchlorates did not show much change to the
fluorescence intensities of 1. The emission of 1 at 558
＋
＋
＋
＋
Binding stoichiometry of 1 and Cu2
To determine the binding stoichiometry of 1 and
＋
Cu2 , Job’s method for the emission was employed. The
＋
total concentration of 1 and Cu2 was kept at a constant
＋
＋
＋
＋
nm was partly quenched by Co2 , Ni2 , Hg2 , and Pb2 ,
4 μmol/L, with a continuous variation of the molar frac-
＋
tion of Cu2 . The change of the fluorescence intensity at
＋
whereas the titration of Cu2 resulted in a remarkable
＋
582 nm with the concentration ratio of 1 to Cu2 was
red-shift and a great enhancement of the emission at 582
nm. However, variation of the fluorescence intensity
＋
shown in Figure 3. When the molecular fraction of Cu2
＋
was about 0.5, the complex of 1 and Cu2 exhibited a
ratio (I582/I558) was rather small when other selected
＋
metal perchlorates were titrated compared to Cu2 as
maximum fluorescence emission at 582 nm. This indi-
＋
＋
＋
shown in white bars of Figure 5. Even Co2 , Ni2 , Hg2 ,
cated that a 1∶1 stoichiometry is most possible for the
＋
binding mode of 1 and Cu2 .
＋
or Pb2 only induced a negligible effect on the fluores-
cence intensity ratio (I582/I558). These results clearly in-
dicate that 1 shows good selectivity and sensitivity to-
＋
ward Cu2 . The competition experiments were further
＋
conducted in the presence of Cu2 at 30 μmol/L, fol-
lowed by addition of 200 μmol/L of other metal ions,
respectively. The competition experiments revealed that
＋
Figure 3 Job’s plot of 1 and Cu2 . The total concentrations of 1
＋
and Cu2 were kept at a constant 4 μmol/L in CH3CN. Excitation
was at 535 nm and emission intensity was measured at 582 nm.
Further evidences for proving a 1∶1 stoichiometry
＋
for the 1-Cu2 complex were the results of the nonlin-
ear fitting of the fluorometric titration and ESI. Figure
S1 showed the dependence of the intensity ratios of
Figure 4 Fluorescence emission spectra of 1 (2 µmol/L) upon
addition of various metal ions (100 µmol/L) in CH3CN (λex＝535
nm).
emission at 582 nm to that at 558 nm (I582/I558) on the
＋
concentrations of Cu2 getting the data from Figure 2.
＋
With the increase of the concentration of Cu2 , the in-
tensity ratios of I582/I558 were increasing, which reached
＋
the maximum values when 30 μmol/L Cu2 was added.
By the nonlinear fitting of the fluorometric titration data
using equation 1, the stoichiometry of the complex of 1
＋
and Cu2 was obtained as 1∶1 and the disassociation
constant (Kd) was 7.8±0.4 µmol/L. Figure S2 shows
＋
the ESI mass spectra of 1 and the 1-Cu2 complex. As
shown in Figure S2, 1 showed two peaks at m/z＝510.3
and 532.2 corresponding to [1＋H]＋ and [1＋Na]＋,
＋
respectively. After 15 equiv. of Cu2 was added to 1 in
CH3CN, the peaks mentioned above decreased and a
unique peak at m/z＝572.2 corresponding to [1＋Cu－
H] ＋ was clearly observed, which reveals a 1 ∶1
Figure 5 Metal ion selectivity profiles of 1 (2 µmol/L): white
bars, fluorescence of 1 in the absence and the presence of the
selected metal ions (100 µmol/L); black bars, fluorescence of 1 in
＋
stoichiometry for the 1-Cu2 complex.
＋
Selectivity and tolerance of 1 to Cu2 over other
＋
the presence of 30 µmol/L Cu2 , followed by the selected metal
＋
ions (200 µmol/L) except Cu2 . All samples were measured in
metal ions
＋
To further explore the selectivity of 1 to Cu2 , fluo-
CH3CN and excitation was at 535 nm.
4
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Chin. J. Chem. 2012, XX, 1—5

A Highly Copper-Selective Ratiometric Fluorescent Sensor Based on BODIPY
＋
the Cu2 -induced ratiometric fluorescence response was
unaffected in the presence of all the tested metal cations
14, 6892.
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＋
Cu2 -selective binding ratiometric response to 1 can
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take place in the coexistance of the other competitive
metal ions.
We have also investigated the effect of water content
＋
on the fluorescence measurement of 1-Cu2 . It has been
＋
found that the fluorescence signal of 1-Cu2 had al-
＋
ready quenched and sensor 1 has no sensitivity to Cu2
if the water content exceeded 10%.
To investigate practical application, the detection
limit of this new fluorescent chemosensor 1 was also
evaluated. The fluorescence emission changes of 1 (2
＋
µmol/L) upon addition of Cu2 ions by 0.2 µmol/L in
CH3CN were shown in Figure S3. From Figure S3, it
＋
can be seen that the detection limit of sensor 1 for Cu2
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7
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Conclusions
In conclusion, we have presented a new ratiometric
Cu2＋-specific fluorescent chemosensor 1. 1 displays
＋
high selectivity for Cu2 over other metal ions and
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a ～10-fold fluorescence ratio change upon excitation
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Acknowledgement
This work was financially supported by the National
Natural Science Foundation of China (Nos. 91127032,
21174035), Zhejiang Provincial Natural Science Foun-
dation of China (No. Y4100287), Program for Excellent
Young Teachers in Hangzhou Normal University (No.
HNUEYT 2011-01-019) and the Opening Foundation of
Zhejing Provincial Top Key Discipline (No. 20110943).
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