A BODIPY derivative as a colorimetric, near-infrared and turn-on chemosensor for Cu2+

Article abstract of DOI:10.1016/j.saa.2012.04.091

A new colorimetric and near-infrared (NIR) fluorescent chemosensor (1) for Cu²⁺ based on BODIPY has been synthesized and investigated in this work. 1 Displays a high selectivity for Cu²⁺ with about 250-fold enhancement in fluorescence emission intensity and micromolar sensitivity (K_d = 2.8 ± 0.3 μM) in comparison with alkali and alkaline earth metal ions (Na⁺, K⁺, Mg²⁺, Ca ²⁺) and other metal ions (Ba²⁺, Zn²⁺, Cd ²⁺, Fe²⁺, Pb²⁺, Ni²⁺, Co ²⁺, Ag⁺) upon excitation at 635 nm in CH₃CN. Meanwhile, the distinct color changes and rapid switch-on fluorescence also provide 1 as colorimetric senor for Cu²⁺.

Full text of DOI:10.1016/j.saa.2012.04.091

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 82–88
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A BODIPY derivative as a colorimetric, near-infrared and turn-on chemosensor
for Cu2+
a
a
a
a
b
a
Shouchun Yin a, , Wenli Yuan , Jinlong Huang , Danbo Xie , Bin Liu , Kezhi Jiang , Huayu Qiu
⇑
a College of Material Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 310036, PR China
b Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University, Hangzhou, PR China
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
"
"
A new NIR colorimetric fluorescent
chemosensor 1 has been
synthesized.
1 Displays a high selectivity for Cu2+
with ꢀ250-fold fluorescence
enhancement.
The distinct color changes also
provide 1 as colorimetric senor for
Cu2+
.
a r t i c l e i n f o
a b s t r a c t
A new colorimetric and near-infrared (NIR) fluorescent chemosensor (1) for Cu2+ based on BODIPY has
been synthesized and investigated in this work. 1 Displays a high selectivity for Cu2+ with about 250-fold
Article history:
Received 2 March 2012
Received in revised form 16 April 2012
Accepted 19 April 2012
Available online 9 May 2012
enhancement in fluorescence emission intensity and micromolar sensitivity (Kd = 2.8 0.3
parison with alkali and alkaline earth metal ions (Na+, K+, Mg2+, Ca2+) and other metal ions (Ba2+, Zn2+
Cd2+, Fe2+, Pb2+, Ni2+, Co2+, Ag+) upon excitation at 635 nm in CH3CN. Meanwhile, the distinct color
changes and rapid switch-on fluorescence also provide 1 as colorimetric senor for Cu2+
lM) in com-
,
.
Keywords:
BODIPY
Ó 2012 Elsevier B.V. All rights reserved.
Fluorescent sensor
Copper (II)
Spectroscopic properties
1. Introduction
such as Menkes disease, Wilson disease, and Alzheimer disease
[3]. Thus, the determination of copper is important due to its utility
as well as toxicity. Most of the reported Cu2+ ions fluorescent
chemosensors work in a turn-off mode due to the quenching effect
of paramagnetic nature of Cu2+ [4–20]. For the practical applica-
tions, fluoroionphores showing turn-on (fluorescence enhance-
ment) as a result of metal-ion binding are more interesting.
Although Cu2+ fluorescence turn-on mode sensors are steadily
increasing [21–31], however, there are a few reports about near-
infrared (NIR) or red fluorescent turn-on sensor for Cu2+ [32–34].
Fluorescence probes for NIR or red detection are preferable for
applications in biological systems because they can reduce auto-
fluorescence and photodamage to living cells [35]. Thus, there is
Among the essential transition metal ions in the human body,
Cu2+ is the third most abundant after Fe3+ and Zn2+ because it plays
an important role in many biological processes as a catalytic cofac-
tor for a variety of metalloenzymes, including tyrosinase, superox-
ide dismutase and cytochrome oxidase [1,2]. Meanwhile, Cu2+ is
high toxic to some organisms. If unregulated, Cu2+ can lead to
the disturbance of the cellular homeostasis, causing oxidative
stress and disorders associated with neurodegenerative diseases,
⇑
Corresponding author. Tel./fax: +86 571 28867899.
E-mail address: yinsc@ustc.edu (S. Yin).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.04.091

S. Yin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 82–88
83
still strong demand for new Cu2+ sensors, especially that can be ex-
were dissolved in 15 mL of THF and 50
lL of Et3N under argon.
cited in NIR or red region.
To this solution 14 mg (0.02 mmol) PdCl2(PPh3)2 and 4 mg
(0.02 mmol) CuI were added. The reaction mixture was heated at
60 °C for 12 h. After evaporating the solvent, the crude product
was extracted with CH2Cl2 (3 ꢂ 40 mL), dried over MgSO4, and
the solvent was evaporated under reduced pressure. Purification
was performed by chromatography on silica gel with CH2Cl2/ace-
tone (15:1, v/v) as eluent to give purple solid 2 (147 mg, 87% yield).
1H NMR (400 MHz, CDCl3) d (ppm): 7.61 (2H, d, J = 8.8 Hz), 7.41
(2H, t, J = 8.0 Hz), 7.32 (2H, d), 6.93 (2H, d), 6.78 (1H, d,
J = 4.4 Hz), 6.85 (1H, d), 6.69 (1H, d, J = 4.2 Hz), 6.42 (1H, d), 4.18
(2H, t, J = 4.6 Hz), 3.89 (2H, t, J = 4.4 Hz), 3.73 (6H, m), 3.63 (2H, t,
J = 4.0 Hz, OH), 2.47 (3H, s, Ar–CH3).
In recent year, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes
(abbreviated as BODIPYs) have gained a great deal of attention
due to their outstanding properties, such as large molar absorption
coefficients, narrow absorption and emission bands located in the
visible region, high fluorescence quantum yields and excellent sta-
bility. Moreover, their spectroscopic and photophysical properties
can be fine-tuned by structural modification at the appropriate
positions of the difluoroboron dipyrromethene core. The attach-
ment of aryl group at the 2,3,5,6 positions of the difluoroboron dip-
yrromethene core can greatly red-shift the absorption and
emission bands of BODIPY dyes to NIR or red region [36–38].
Herein, we report the synthesis and metal ion recognition prop-
erties of a new BODIPY based sensor, 5-N-(2-picolyl)amine-4,4-di-
fluoro-3-(4-(hydroxy(ethyloxy(ethyloxy(ethyloxy)))) phenyleth-
ynyl)-8-(4-tolyl)-4-bora-3a,4a-diaza-s-indacene (1). 1 displays a
high selectivity for Cu2+ with about 250-fold enhancement in fluo-
rescence emission intensity in comparison with all other test metal
ions upon excitation at red region (kex = 635 nm) in CH3CN.
2.5. Synthesis of 5-N-(2-picolyl)amine-4,4-difluoro-3-(4-
(hydroxy(ethyloxy(ethyloxy(ethyloxy)))) phenylethynyl) -8-(4-tolyl)-
4-bora-3a,4a-diaza-s-indacene (1)
To a solution of 2 (113 mg, 0.2 mmol) and di(2-picolyl)amine
(20 mg, 0.2 mmol) in 20 mL of acetonitrile, sodium hydride
(5 mg, 0.2 mmol) was added. The reaction mixture was stirred at
room temperature for 12 h under argon. After evaporating the sol-
vent, 20 mL of water was added and the organic layer was ex-
tracted with CH2Cl2 (3 ꢂ 20 mL), 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/
methanol (20:1, v/v) as eluent to give purple solid 1 (83 mg, 57%
yield). M.P. 178–179 °C. 1H NMR (400 MHz, CDCl3) d (ppm): 8.55
(2H, d, J = 3.6 Hz),7.41 (2H, d, J = 6.8 Hz), 7.34 (2H, d, J = 6.4 Hz),
7.22 (4H, m), 6.84 (2H, d, J = 6.8 Hz), 6.81 (1H, d, J = 4.0 Hz), 6.55
(1H, d, J = 3.2 Hz), 6.26 (1H, d), 5.28 (4H, s), 4.14 (2H, t,
J = 5.4 Hz), 3.86 (2H, t, J = 3.8 Hz), 3.62 (2H, t, J = 3.6 Hz), 3.71
(6H, m), 3.38 (1H, t, J = 5.6 Hz, OH), 2.42 (3H, s, Ar–CH3). ESI–MS:
m/z 750.3 [M+Na]+. HRMS (MH+): calcd. for C42H41BF2N5O4
728.3220, found 728.3210.
2. Experimental
2.1. Apparatus
1H and 13C NMR spectra were recorded at room temperature on
a Bruker Avance 400 operating at a frequency of 400 MHz for 1H
and 100 MHz for 13C. Melting points were taken on a Beijing Taike
X-5 melting point instrument and are uncorrected. Mass spectra
were recorded on a Hewlett–Packard 5989 A mass spectrometer
(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 obtained using
a HITACHI F-2700 Fluorescence Spectrophotometer.
2.2. Reagents
3. Results and discussion
3,5-Dichloro-8-(4-tolyl)BODIPY [39] and 2-(2-(2-(4-ethynyl-
phenoxy)ethoxy)ethoxy)ethanol [40] was synthesized according
to literature procedures. Other reagents were purchased from Ac-
ros and used without further purification.
3.1. Synthesis
1 was readily synthesized in two steps from 3,5-dichloroBO-
DIPY as starting material. As outlined in Scheme 1, one chlorine
atom of 3,5-dichloroBODIPY was first reacted with 3 by palla-
dium-catalyzed reaction to afford the monosubstituted product 2
in good (87%) yield. Here, the feed ratio of 3,5-dichloroBODIPY
and 3 was 3:1 to make sure that 3 can be reacted completely, so
that the separation of the desired product was easy. After being
purified by chromatography several times, the purity of 2 was en-
ough for NMR spectroscopic characterization. Unlike the reaction
of 3,5-dichloroBODIPY with phenylacetylene reported by Wim
[42], in this Sonogashira reaction no disubstituted side-product
could be found because of the lower reactivity of the second chlo-
rine resulting from the electron donating effect of the oxygen atom.
The second chlorine atom of the isolated 2 was then substituted by
di(2-picolyl)amine in room temperature with sodium hydride as
base to get 1 in modest (57%) yield.
2.3. Determination of ground-state dissociation constant Kd
The ground-state dissociation constant Kd of the complex be-
tween 1 and Cu2+ was determined in CH3CN solution at 25 °C by di-
rect fluorometric titration as
a
function of Cu2+ using the
fluorescence emission spectra. Nonlinear fitting of Eq. (1) to the
steady-state fluorescence data F recorded as a function of [Cu2+
yields values of Kd, Fmin, Fmax, and n.
]
n
Fmax½Cu2þꢁ þ FminKd
F ¼
ð1Þ
n
Kd þ ½Cu2þꢁ
In Eq. (1), F stands for the fluorescence signal at [Cu2+], whereas Fmin
and Fmax denote the fluorescence signals at minimal and maximal
[Cu2+], respectively, and n is the number of Cu2+ ions bound per
probe molecule (i.e., stoichiometry of binding) [41].
3.2. Spectral characteristics
2.4. Synthesis of 5-Chloro-4,4-difluoro-3-(4-(hydroxy(ethyloxy
(ethyloxy(ethyloxy))))phenylethynyl)-8-(4-tolyl)-4-bora-3a,4a-diaza-
s-indacene (2)
After full characterization of the chemical structure by NMR and
MS spectroscopy, the photophysical properties of 1 upon addition
of several metal cations (Na+, K+, Mg2+, Ca2+, Ba2+, Cu2+, Zn2+
,
Cd2+, Fe2+, Pb2+, Ni2+, Co2+, Ag+) in CH3CN were investigated by
UV–Vis absorption and fluorescence spectroscopic measurements
and titration studies. Fig. 1 showed the UV–Vis absorption spectra
315 mg (0.9 mmol) 3,5-dichloro-8-(4-tolyl) BODIPY and 75 mg
(0.3 mmol) 2-(2-(2-(4-ethynylphenoxy)ethoxy)ethoxy)ethanol (3)

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S. Yin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 82–88
3 2
2
3
2
2
2
2
3
2
2
3
2
2
2
3
Scheme 1. Synthetic route to sensor 1.
0.18
0.15
0.12
0.09
0.06
0.03
12 μM
0 μM
0.00
300
400
500
Wavelength / nm
600
700
Fig. 1. UV–Vis absorption spectra of 1 in CH3CN upon addition of various metal
ions. The concentration of 1 was 2 M and that of the selected metal ion was 50 M.
Fig. 2. UV–Vis absorption spectra of
concentrations of Cu2+ (0, 0.125, 0.25, 0.375, 0.5, 0.75, 1, 1.25, 1.5, 2, 2.5, 3, 4, 6,
8, 10, 12 M) in CH3CN.
1 (2 lM) in the presence of different
l
l
l
of 1 with excess amount of the different test melt ions in CH3CN.
When Na+, K+, Mg2+, Ca2+, Ba2+, Cd2+, Fe2+, Ni2+, Co2+, Ag+ were
added to the CH3CN solution of 1, respectively, the UV–Vis absorp-
tion spectrum had almost no change and exhibited a main peak at
525 nm assigned to the S0 ? S1 transition of the BODIPY chromo-
phore. Zn2+ and Pb2+ resulted in a new, weak and red absorption
band. However, when Cu2+ was added, a new, strong and red
absorption peak at 636 nm appeared and the main peak at
525 nm disappeared. Fig. 2 showed the UV–Vis absorption titration
of 1 with the different Cu2+ concentrations in CH3CN. As shown as
in Fig. 2, with the increase of the concentration of Cu2+ ions, the
intensity of the main absorption peak at 525 nm decreased gradu-
ally and red-shifted to 536 nm, while the intensity of a new band at
636 nm increased continuously. The disappearance of the absorp-
tion peak at 525 nm and the appearance of a new peak at
636 nm upon addition of Cu2+ resulted in a naked eye color change
from pink to purple. The color of 1 in CH3CN with Zn2+ or Pb2+ had
almost no changes although Zn2+ or Pb2+ could result in a new,
weak, red absorption band (as shown in lower row of Fig. 3). Those
results indicated that 1 can be used as a colorimetric probe for
Cu2+. The presence of three well-defined isosbestic points at

S. Yin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 82–88
85
Fig. 3. Photographs of solutions of 1 in CH3CN upon addition of various metal ions. The concentration of 1 was 2
lM and that of the selected metal ion was 50
lM. Upper row:
fluorescence observed upon excitation at 365 nm. Lower row: optical.
40
60
50
Cu2+
30
40
Pb2+, Zn2+
20
10
0
30
20
10
0
none, Ag+, Ba2+, Ca2+, Cd2+,
Co2+, K+, Na+, Mg2+, Ni2+
650
700
750
800
0.0
0.2
0.4
0.6
0.8
1.0
XCu2+
Wavelength / nm
Fig. 4. Fluorescence emission spectra of 1 (2
ions (50 M) in CH3CN (kex = 635 nm).
lM) upon addition of various metal
Fig. 6. Job’s plot for 1 and Cu2+. The total concentration of 1 and Cu2+ was kept at a
fixed 4 M. The fluorescence intensity was measured at 652 nm.
l
l
40
30
20
10
0
40
30
20
10
0
12 μΜ
0 μΜ
640
680
720
Wavelength / nm
760
800
0
3
6
9
12
Cu2+ / μΜ
Fig. 5. Fluorescent emission spectra of 1 (2
presence of different concentrations of Cu2+ (0, 0.125, 0.25, 0.375, 0.5, 0.75, 1, 1.25,
1.5, 2, 2.5, 3, 4, 6, 8, 10, 12 M) in CH3CN.
lM) upon excitation at 635 nm in the
Fig. 7. Fluorescence emission intensity of 1 (2 lM) in CH3CN monitored at 652 nm
as a function of Cu2+ concentration (kex = 635 nm). The red line represents the best
fit to the data (filled squares) according to Eq. (1). (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
l
k = 335, 453 and 567 nm demonstrated the presence of an equilib-
rium process corresponding to apo 1 and copper complex 1–Cu2+
Further addition of excess Cu2+ produces no significant changed
in UV–Vis spectra when the concentration of Cu2+ reached 12
M.
.
l
induce a new red emission when excitation at the maximum
Since a new, strong and red absorption peak can be produced
absorption of 1–Cu2+. Fig. 4 displayed the fluorescence emission
when Cu2+ was added to 1 in CH3CN, we wondered if Cu2+ can
spectra of 1 (2 lM) with the above mentioned metal cations (25

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S. Yin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 82–88
ESI-yinshouchun111013-yin5_01 #4-8 RT: 0.04-0.08 AV: 5 NL: 3.33E7
T: + c ESI Full ms [ 320.00-800.00]
100
750.3
90
80
70
60
50
40
30
20
10
751.4
728.3
727.3
726.2
752.4
766.3
613.3
786.2
800
532.4
463.6
450
507.6
500
629.3
659.4
336.3
419.5
551.7
350.4
596.5
600
687.5
700
0
350
400
550
m/z
650
750
ESI-yinshouchun111013-yin5-Cu_01 #141-173 RT: 2.33-2.86 AV: 33 NL: 4.58E7
T: + c ESI Full ms [ 155.00-1200.00]
742.3
100
90
80
70
60
50
40
30
20
790.3
750.3
741.3
789.2
361.8
757.2
758.2
728.2
723.5
10
590.5
572.2
633.4
362.6
507.5
500
692.7
419.3
463.5
450
551.6
674.5
650
341.5
350
0
400
550
600
700
750
800
m/z
Fig. 8. ESI ms spectra of 1 (top) and 1–Cu2+ (low).
equiv) measured in CH3CN upon excitation at kex = 635 nm. As
shown in Fig. 4, 1 emitted almost no fluorescence upon excitation
at 635 nm because 1 had almost no absorbance at this wavelength.
However, when Cu2+ ions were added to 1 in CH3CN, a new strong
fluorescence emission with maximum at 652 nm appeared. Under
the same conditions, no obvious fluorescence changes were ob-
served for other tested metal ions including Zn2+ and Pb2+, which
only induced negligible fluorescence enhancement at red range
compared with Cu2+. Those observations indicated that 1 has a very
high selectivity for Cu2+. 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 already
quenched if the water content exceeded 10%. Thus, we will focus
our investigation on the metal ion sensing properties of 1 in pure
CH3CN in the following discussion.
To investigate the sensitivity of 1 for Cu2+, the fluorescence
titration of 1 in the presence of different Cu2+ concentrations was
then performed. As shown in Fig. 5, upon excited at 635 nm, with
the increase of the concentration of Cu2+ the fluorescence intensity
increased continuously from the background level of free 1. When
the concentration of Cu2+ increased to 12
lM, the fluorescence
enhancement reached the maximum value (about 250-fold

S. Yin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 96 (2012) 82–88
87
binding and turn-on response could take place in the coexistence
of the competitive metal ions in the absence of Ag+.
1.0
0.8
0.6
0.4
0.2
0.0
4. Conclusions
In conclusion, we have reported a colorimetric, NIR and turn-on
fluorescent sensor 1 based on BODIPY that can sensitively and
selectively detect Cu2+ in CH3CN. 1 displays a new strong red
absorption peak and a significant fluorescence enhancement in
the presence of Cu2+ using
a red excitation wavelength
(kex = 635 nm) in CH3CN. The obvious changes in the color and
fluorescence induced by Cu2+ make 1 be as colorimetric sensor.
The mechanism of signal change in absorption and emission when
Cu2+ is bound are not known at this moment and this will be the
subject of further study. An answer will hopefully be given in a
forthcoming paper.
Mg2+
K+
Fe2+
Ag+
Cu2+
Ba2+ Ca2+ Cd2+
Na+
Pb2+
Zn2+
Co2+
Ni2+
Fig. 9. Fluorescence response of 1 (2
lM) containing 12
l
M Cu2+ to the selected
metal ions (50 lM). F1+Cu and F1+Cu+M denote the fluorescence signals of 1 in the
Acknowledgements
presence of Cu2+ only and in the presence of Cu2+ as well as the competing ion,
respectively. Excitation was at 635 nm and emission was at 652 nm.
We thank 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) for financial supports.
enhancement) and the fluorescence quantum yield increased to
0.22 for 1–Cu2+, correspondingly, which indicated that 1 exhibits
a very high sensitivity for Cu2+
.
3.3. Binding stoichiometry of 1 and Cu2+
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method for the emission was employed. The total concentration
of 1 and Cu2+ was kept at a constant 4
lM, with a continuous var-
iable the molar fraction of Cu2+. The change of the fluorescence
intensity at 652 nm with the concentration ratio of 1 to Cu2+ was
shown in Fig. 6. When the molecular fraction of Cu2+ was about
0.5, the complex of 1 and Cu2+ exhibited a maximum fluorescence
emission, which indicated that a 1:1 stoichiometry is most possible
for the binding mode of 1 and Cu2+
.
Further evidences for proving a 1:1 stoichiometry for the
1–Cu2+ complex were the result of the nonlinear fitting of the fluo-
rometric titration and ESI mass. Fig. 7 showed the dependence of
the emission intensity at 652 nm on the concentration of Cu2+ get-
ting the data from Fig. 2. By the nonlinear fitting of the fluoromet-
ric titration data using Eq. (1), the stoichiometry of the complex of
1 and Cu2+ was obtained as 1:1 and the disassociation constant (Kd)
was 2.8 0.3 lM. The ESI mass spectrum of 1 showed two peaks at
m/z = 728.3 and 750.3 corresponding to [1+H]+ and [1+Na]+,
respectively (seen in Fig. 8), while the complex of 1 and Cu2+ exhib-
ited a unique peak at m/z = 790.3 corresponding to [1+Cu–H]+,
which reveals a 1:1 stoichiometry for the 1–Cu2+ complex.
3.4. Selectivity and tolerance of 1 to Cu2+ over other metal ions
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M Cu2+. As shown in Fig. 9, the competing me-
lM) were added to
l
l
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88
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