An excellent BODIPY dye containing a benzo[2,1,3]thiadiazole bridge as a highly selective colorimetric and fluorescent probe for Hg2+ with naked-eye detection

Article abstract of DOI:10.1039/c0nj00850h

An excellent red-emissive BODIPY dye containing a benzo[2,1,3]thiadiazole bridge was synthesized, and its sensing ability toward metal cations was investigated in detail. It can work as a highly selective probe for Hg ²⁺ detected by the naked eye, with evident solution color and photoluminescence changes.

Full text of DOI:10.1039/c0nj00850h

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LETTER
An excellent BODIPY dye containing a benzo[2,1,3]thiadiazole bridge as
a highly selective colorimetric and fluorescent probe for Hg2+ with
naked-eye detectionwz
Hui-Bin Sun, Shu-Juan Liu, Ting-Chun Ma, Nan-Nan Song, Qiang Zhao* and
Wei Huang*
Received (in Gainesville, FL, USA) 31st October 2010, Accepted 16th March 2011
DOI: 10.1039/c0nj00850h
An excellent red-emissive BODIPY dye containing
a
fluorescence quantum yields, and robustness against light and
chemicals.6 Thus, the exploitation of excellent BODIPY dyes
for detecting Hg2+ has become a very important research
topic, especially for realizing naked-eye detection with simple
and low-cost operation.7
benzo[2,1,3]thiadiazole bridge was synthesized, and its sensing
ability toward metal cations was investigated in detail. It can
work as a highly selective probe for Hg2+ detected by the naked
eye, with evident solution color and photoluminescence changes.
For the design of an excellent fluorescent probe for Hg2+
,
Mercury is a dangerous and widespread global pollutant, and
it can cause serious environmental and health problems.1
When absorbed in the human body, mercury will cause
damage to the central nervous, DNA, mitosis and endocrine
systems.2 The concentration of this ion is therefore the
the probe should contain specific mercury-coordinating
elements (receptor) to ensure high selectivity in the presence
of other metal cations.8 Thus, the binding of Hg2+ with the
receptor can influence the energy level of molecular orbitals,
inducing changes in electronic and optical properties, such
as UV-visible absorption, fluorescent spectra and redox
potential. Based on this design principle, herein, we have
designed and synthesized a BODIPY trimer 3, containing a
object of strict regulation and should not exceed 1 mg Lꢀ1 3
.
Traditional analytical techniques for Hg2+ include atomic
absorption spectroscopy, cold vapor atomic fluorescence
spectrometry and gas chromatography. These methods, however,
require not only complicated instrumentation but also a long
measuring time. Therefore, it is urgent to develop new methods
for monitoring Hg2+ with a low limit of detection, as well as
rapid and facile detection. Methods based on fluorescent
molecular probes have attracted considerable interest because
of the high sensitivity, easy visualization and short response
time for detection of the fluorescence signal.4 Thus, considerable
efforts have been devoted to the design of fluorescent probes
benzo[2,1,3]thiadiazole (BDT) bridge as a receptor for Hg2+
,
utilizing the specific interaction between Hg2+ and the sulfur
atom in BDT, and an excellent Hg2+ probe with naked-eye
detection was realized.
The synthetic route to dye 3 is shown in Scheme 1. It was
prepared by the palladium-catalyzed Sonogashira reaction of
4,7-diethynylbenzo[2,1,3]thiadiazole with 2.5 equiv. of precursor
1. For comparison, the synthesis of reference dye 4, without
the benzo[2,1,3]thiadiazole bridge, was also synthesized from
precursor 2, as shown in Scheme 1. Crude products 3 and 4
were purified by column chromatography with yields of 42%
and 43%, respectively, and characterized by 1H NMR,
13C NMR and mass spectra. Details of the synthesis and
characterization of all the compounds are provided in the
ESI.z
for Hg2+ 5
.
Boron-dipyrromethene (BODIPY) dyes are a class of
well-known fluorophores with widespread applications as
fluorescent probes due to their valuable characteristics, such
as sharp and intense absorption and fluorescence peaks in the
visible spectral region, high molar absorption coefficients and
The absorption spectra of dyes 3 and 4 measured in CH2Cl2
solutions display typical BODIPY features, with an intense
band at 568 and 550 nm, and a weak band at around 393 and
408 nm, respectively (see Fig. 1), corresponding to S0 - S1
(p–p*) and S0 - S2 (p–p*) transitions. Compared with
BODIPY monomer 5 (shown in Scheme 1),6a the absorption
band of trimer 3 exhibits about a 50 nm red-shift due to the
increase in the conjugated length. In addition, the introduction
of the benzo[2,1,3]thiadiazole unit into dye 3 also results in an
evident red-shift of the absorption bands compared to dye 4,
without an aromatic bridge, and dye 6, containing a benzene
Key Laboratory for Organic Electronics & Information Displays
(KLOEID) and Institute of Advanced Materials (IAM), Nanjing
University of Posts & Telecommunications (NUPT), 9 Wenyuan
Road, Nanjing 210046, China. E-mail: wei-huang@njupt.edu.cn,
iamqzhao@njupt.edu.cn; Fax: +86 25 8586 6396;
Tel: +86 25 8586 6396
w This article is part of a themed issue on Molecular Materials: from
Molecules to Materials, commissioned from the MolMat2010
conference.
z Electronic supplementary information (ESI) available: Syntheses
and characterization data of 1–4, 1H-NMR, 13C-NMR and mass
spectra, fluorescence data of 3 and 4 in the presence of different metal
ions, Job’s plots. See DOI: 10.1039/c0nj00850h
c
1194 New J. Chem., 2011, 35, 1194–1197
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Table 1 Spectroscopic and electrical properties of dyes 3 and 4
Dye lmax(abs)a/nm lmax(em)a/nm Fa t/ns Eredb/V Eoxb/V
0.45 1.00 ꢀ1.52, ꢀ1.66 0.80
3
4
568
550
603
600
0.10 ꢀ1.64 0.80
a
In CH2Cl2. Rhodamine B was used as the reference dye (F = 0.71 in
EtOH) for the measurement of F.11 b 0.1 M n-Bu4NPF6 in CH3CN, Pt
electrode, scanning rate 100 mV sꢀ1, V vs. Fc/Fc+
.
exhibits one reduction wave at ꢀ1.64 V, which can be assigned
to the reduction of the BODIPY units.
For a better understanding of the optical properties of 3,
theoretical calculations were performed using the Gaussian 03
package at the B3LYP level.12 The calculation results reveal
that both the highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) of 3 are mainly
distributed over the whole conjugated backbone (see Fig. 2).
Furthermore, the excited state of dye 3 was clarified through
time-dependent density functional theory (TDDFT) calculations,
and its lowest excited state can be assigned to the HOMO -
LUMO transition. According to the orbital distributions,
no evident charge transfer was observed. In order to better
understand the excited state properties of 3, we studied the
dependence of both the absorption and emission spectra of
dye 3 on the solvent. There was no evident change in the
absorption and emission spectra in CH2Cl2 and THF
(see ESIz), further demonstrating that the p–p* transition
instead of the charge-transfer state is responsible for the
optical properties of dye 3. In dye 3, the sulfur atom con-
tributes to the LUMO orbital. Hence, the interaction of the
sulfur atom with Hg2+ can change the orbital energy level,
realizing the optical detection.13
Scheme 1 Synthetic route to BODIPY dyes 3 and 4. Chemical
structure of dye 56a and 6.9a
The response of dye 3 to Hg2+ was investigated through
absorption and emission spectra. The absorption spectral
response of 3 in CH2Cl2 solution is shown in Fig. 3. Upon
addition of Hg2+, the absorbance of 3 at 568 nm progressively
decreases, while a new band at 501 nm appears and its
absorbance increases gradually, corresponding to a lmax(abs)
blue-shift of 67 nm, which induces an evident change of solution
color from purple to yellow. This result indicates that dye 3 can
Fig. 1 Normalized absorption and emission spectra of dye 3 and 4 in
dilute CH2Cl2 solutions.
bridge.9a Interestingly, the absorption band of dye 3 has a
peculiarly broad shape. Maybe the peak is the sum of two
distinct peaks, most likely resulting from exciton splitting of
the excited states. Similar excitonic interactions might also
exist in dye 4. However, only for dye 3 do the two peaks have
an approximately similar intensity. This phenomenon of
dipole coupling of the chromophores is quite common in
multichromophoric systems.9 Different from the absorption
bands, the emission wavelength of dye 3 exhibits a small
red-shift from 600 to 603 nm compared with that of 4.
However, the emission wavelength of 3 exhibits an evident
red-shift compared with that of 6 (586 nm),9a and the
fluorescent quantum yields (F) are 0.45 and 0.10 for dye 3
and 4, respectively. It is also worth mentioning that the Stokes
shift of 3 is 31 nm and significantly larger than that of the
common BODIPY monomer (less than 10 nm).10
serve as a sensitive ‘‘naked-eye’’ indicator for Hg2+ 14
.
It is well known that fluorescent emission spectroscopy is
more sensitive toward small changes that affect the electronic
properties of the molecular probe. Hence, the binding ability
of 3 with Hg2+ ions was also investigated by emission spectro-
scopy (see Fig. 4). Upon addition of Hg2+, the intensity of the
In addition, cyclic voltammetry (CV) studies were con-
ducted in CH3CN solutions for 3 and 4 (see ESIz), and their
redox potentials are show in Table 1. Both dyes exhibit a
similar oxidation wave at 0.80 V. However, their reduction
waves are different. 3 exhibits two reduction waves at ꢀ1.52
and ꢀ1.66 V, respectively, which may be attributed to the
reduction of the BDT and BODIPY units. For 4, however, it
Fig. 2 The HOMO and LUMO distributions of 3.
c
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Fig. 5 (a) Change in the emission spectra of 3 in CH2Cl2 solution
with various amounts of Hg2+ ions excited at l = 530 nm. (b)
Fluorescent titration profile of I603 nm vs. the number of equivalents
of Hg2+ ions added.
Fig. 3 UV-vis absorption spectra of 3 (1.0 ꢁ 10ꢀ5 M) in CH2Cl2
dye 3 was determined to be 6.4 ꢁ 104 Mꢀ1. We also tested the
sensing performances of 3 in a THF–H2O (v/v = 3 : 1) system
and found that the presence of water decreased the sensitivity
significantly (see ESIz), which may be due to the poor solubility
of the probe in water.
solution upon addition of 0–2.3 equiv. Hg(ClO4)2. Inset: solution color
change upon addition of Hg2+
.
emission band at 603 nm decreases and several new bands at
430–550 nm appear, realizing ratiometric detection.8 After
addition of approximately 2 equiv. of Hg2+, the emission of
3 at 603 nm is completely quenched. More importantly, probe
3 can detect a Hg2+ concentration as low as 0.5 mM according
to titration experiments.
Furthermore, the influence of Hg2+ addition on the cyclic
voltammogram of 3 was studied in CH3CN solution. The
addition of 1.5 equiv. Hg2+ to a solution of 3 caused a
significant change in the cyclic voltammogram (see ESIz).
Two oxidation waves at 1.00 and 1.24 V, respectively,
appeared. Different from 3, only one reduction wave at
ꢀ1.66 V was observed, which is tentatively assigned to the
reduction of the BODIPY unit. Thus, 3 can also be used as
To shift the excitation wavelength from the UV region to
the visible range is one of the challenges in the design of
luminescent probes, particularly those aimed at bioanalysis
and practical applications. This is because background fluores-
cence cannot be excited by visible light, and visible light
excitation requires cheaper optical cells and optics.15 In view
of the strong absorption of 3 in the visible region, the variation
in the emission spectrum of 3 in response to Hg2+ excited at
l = 530 nm was investigated. The orange-red emission at l =
603 nm of 3 decreased gradually on the addition of Hg2+, thus
indicating that detection excited by visible light was also
realized (see Fig. 5a).
an electrochemical sensor for Hg2+
.
For an excellent probe, high selectivity is a matter of
necessity. Selective coordination studies of probe 3 by fluorescent
spectroscopy were then extended to related heavy, transition
and main group metal ions. The results showed that a high
selectivity for Hg2+ was observed for probe 3 (see Fig. 6).
Furthermore, achieving a high selectivity for the analyte of
interest over a complex background of potentially competing
species is a challenge in probe development. Thus, competition
experiments were also carried out by adding Hg2+ to solutions
of 3 in the presence of other metal ions. The results indicate
that the sensing of Hg2+ by 3 is hardly affected by these
commonly coexistent ions.
Fig. 5b shows the metal binding titrations according to the
emission intensity at 603 nm. From the changes in the fluoro-
metric response in the presence of varying concentrations of
Hg2+, the stoichiometry of the complex system was deter-
mined to be 1 : 1, indicating the formation of a 1 : 1 complex.
To further demonstrate the binding ratio between 3 and Hg2+
,
It is well-known that Hg2+ ions (a soft acid) can preferen-
tially interact with sulfur (a soft base) according to Pearson’s
hard and soft acids and bases theory.16 Hence, we deduce that
the significant variations in the photophysical properties of
a Job’s plot analysis was also carried out (see ESIz). From the
Job’s plot analysis, a 1 : 1 binding was further confirmed.
Using the UV-vis titration data, the binding constant (K) of
Fig. 6 The photoluminescent response (I603 nm) of 3 (1.0 ꢁ 10ꢀ5 M) in
CH2Cl2 solution in the presence of Hg2+ (2 equiv.) and the interfering
ions Ag+, Cd2+, Co2+, Cr3+, Cu2+, Fe3+, Mg2+, Ni2+, Pb2+ and
Zn2+ (2 equiv.).
Fig. 4 Emission spectra of 3 (1.0 ꢁ 10ꢀ5 M) in CH2Cl2 solution upon
addition of 0–2.3 equiv. Hg(ClO4)2 excited at l = 375 nm. Inset:
fluorescent color change upon addition of Hg2+
.
c
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dye 3 are induced by coordination of Hg2+ to the lone pair
electrons on the sulfur atom of the benzo[2,1,3]thiadiazole
unit. In order to further demonstrate the importance of the
benzo[2,1,3]thiadiazole unit for the sensing ability of probe 3,
the response of dye 4, without a BDT unit, to Hg2+ was also
measured (see ESIz). Under the same experimental conditions,
4 showed no significant response to Hg2+. This further
demonstrates the interaction of Hg2+ with the sulfur atom.
We also studied the reversibility of the probe (see ESIz). In
CH3CN solution, the emission intensity of 3 is decreased after
adding Hg2+. However, it cannot be recovered after adding
excess Na2S, which has a stronger binding ability towards
Hg2+. Furthermore, the 1H NMR spectrum of 3 becomes very
complicated after binding with Hg2+ (see ESIz). At present, it
is still difficult to determine the accurate sensing mechanism.
However, according to the irreversibility of the probe, the
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absorption and emission spectral response to Hg2+ and
1
the complicated H NMR spectrum of 3 after adding Hg2+
,
we tentatively attribute the sensing mechanism to the decom-
position of dye 3 induced by its binding with Hg2+. Further
investigation of the sensing mechanism is under way.
In conclusion, we have realized an excellent colorimetric
and fluorescent probe for Hg2+ based on a BODIPY trimer
with a benzo[2,1,3]thiadiazole bridge as a binding site
utilizing the specific interaction between Hg2+ and sulfur.
It can work as a highly selective probe for Hg2+, being
detected by the naked eye with evident solution color and
photoluminescence changes. The binding of Hg2+ induces an
evident blue-shift of the absorption spectrum, resulting in a
significant change of solution color from purple to yellow. A
pronounced ON–OFF-type Hg2+-induced fluorescence
quenching behavior is also observed. Interestingly, selective
excitation at 375 and 530 nm induced different fluorescent
responses to Hg2+. The detection performances realized
by visible light excitation (530 nm) effectively minimized
background fluorescence and improved the signal-to-noise
ratio. These results demonstrate the excellent sensing ability
of probe 3 for Hg2+. Future work will focus on how to
realize the excellent sensing performance of 3 in water
through the chemical modification of this probe.
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Acknowledgements
This work was financially supported by the National Basic
Research Program of China (973 Program, 2009CB930601),
the National Natural Science Foundation of China
(50803028 and 20804019), the Natural Science Foundation
of Jiangsu Province of China (BK2009427), the Natural
Science Fund for Colleges and Universities in Jiangsu
Province (10KJB510010), the Scientific and Technological
Activities for Returned Personnel in Nanjing City (NJ209001)
and Nanjing University of Posts & Telecommunications
(NY208045 and NY210029).
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