A colorimetric chemosensor based on new water-soluble PODIPY dye for Hg2+ detection

Article abstract of DOI:10.1016/j.cclet.2015.07.002

The phosphorus-containing PODIPY 1 as a chemosensor can detect Hg²⁺ by a color change from pink to violet red without the use of any instrumentation. PODIPY 1 was selective to Hg²⁺ with a remarkable absorption change, and addition of

Full text of DOI:10.1016/j.cclet.2015.07.002

G Model
CCLET 3379 1–5
Chinese Chemical Letters xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
1
2
Original article
3
4
A colorimetric chemosensor based on new water-soluble PODIPY
dye for Hg2+ detection
a
a
b,
a
a
Q1 Xin-Dong Jiang a, , Hai-Feng Yu , Jiu-Li Zhao , Chang-Liang Sun , Ying Xie , Lin-Jiu Xiao
* *
5
6
7
8
a College of Applied Chemistry, Key Laboratory of Rare-Earth Chemistry and Applying Liaoning Province, Shenyang University of Chemical Technology,
Shenyang 110142, China
b Center of Physical and Chemistry Test, Shenyang University of Chemical Technology, Shenyang 110142, China
A R T I C L E I N F O
A B S T R A C T
The phosphorus-containing PODIPY 1 as a chemosensor can detect Hg2+ by a color change from pink to
violet red without the use of any instrumentation. PODIPY 1 was selective to Hg2+ with a remarkable
absorption change, and addition of other relevant metal ions caused almost no absorption change. The
new PODIPY dye 1 was sensitive to various concentrations of Hg2+. The energy gap between the HOMO
and LUMO of the metal complex 1–Hg2+ is smaller than that of chemosensor 1, which is in good
agreement with the red shift in the absorption observed upon treatment of 1 with Hg2+. The 1-based test
strips were easily fabricated and low-cost, useful in practical and efficient Hg2+ test kits.
ß 2015 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Article history:
Received 6 May 2015
Received in revised form 8 June 2015
Accepted 16 June 2015
Available online xxx
Keywords:
PODIPY
Hg2+
Colorimetric
DFT
Chemosensor
9
10
1. Introduction
food chain in the tissues of fish and marine mammals [6]. Subse- 30
quent ingestion of methylmercury by humans from seafood and 31
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Mercury pollution is a global problem, and a major source of
human exposure stems from contaminated waters [1,2]. The
terrific toxicity of mercury results from its high affinity for thiol
groups in proteins and enzymes, leading to the dysfunction of cells
and consequently causing health problems [3]. Inorganic mercury,
including Hg(0) and Hg(II), is released into the environment
through a wide variety of ways. Industrial sources of mercury
include coal and gold mining, solid waste incineration, wood
pulping, fossil fuel combustion, and chemical manufacturing. As
the results of numerous applications in industry and agriculture,
mercury has badly contaminated rivers and soil and accumulated
in agricultural products and aquatic products, which causes
serious environmental and health problems directly or indirectly
[4]. The U.S. Environmental Protection Agency (EPA) standard for
the maximum allowable level of inorganic mercury in drinking
water is 2 ppb [5]. Moreover, a significant problem stemming from
the ecological oxidation chemistry is that bacteria living in the
sediments of aqueous environments transform inorganic Hg2+ into
methylmercury, a potent neurotoxin that concentrates through the
other dietary and environmental sources is connected to serious 32
sensory, motor, and cognitive disorders. Therefore, in the interests 33
of safety the detection of Hg2+ is still of great importance.
Among the available techniques to detect and quantify Hg2+
34
35
,
colorimetric chemosensors, so-called ‘‘naked eye chemosensors’’, 36
have been developed and are attracting increasing interest 37
[7,8]. This sensing approach has clear potential application in 38
the development of commercial Hg2+ indicators, such as paper test 39
strips that can be evaluated visually without illumination 40
[7,8]. Water solubility is arguably a less important criterion for 41
colorimetric chemosensors than for fluorescent ones, which are 42
better suited for in vivo work [9,10]. As long as the colorimetric 43
indicator responds to Hg2+ in the presence of water and can be 44
absorbed onto a paper strip or into a membrane or film, it has 45
potential. Therefore, we herein communicate our studies on 46
colorimetric Hg2+ detection, based on the new water-soluble 47
PODIPY dye.
48
Our recent research interest lies in the novel BODIPY/aza- 49
BODIPY family of fluorescent dyes and their application [11–13], 50
and very recently we developed a type of PODIPY dye [14]. The new 51
PODIPY 1 (Fig. 1) can be successfully synthesized by the reaction of 52
dipyrromethene and azadipyrromethene with POCl3 in the 53
presence of Et3N. The new PODIPY had photophysical properties 54
similar to related PODIPYs/aza-PODIPYs [15–21], with a large 55
*
Corresponding authors.
E-mail addresses: xdjiang@syuct.edu.cn (X.-D. Jiang),
chemscl@126.com (C.-L. Sun).
http://dx.doi.org/10.1016/j.cclet.2015.07.002
1001-8417/ß 2015 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: X.-D. Jiang, et al., A colorimetric chemosensor based on new water-soluble PODIPY dye for Hg2+
detection, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.07.002

G Model
CCLET 3379 1–5
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X.-D. Jiang et al. / Chinese Chemical Letters xxx (2015) xxx–xxx
aluminum (CH2Cl2/n-hexane, 1:1). This solid and triethylamine
97
(2 mL) were dissolved in 50 mL of CH2Cl2 under air and the solution
was stirred at room temperature for 10 min. POCl3 (4 mL) was
added, and stirring was continued for 1 h. The reaction was slowly
quenched with crushed ice, extracted with CH2Cl2, and purified by
recrystallization fromCH2Cl2/n-hexaneto afford 1 (171.2 mg, 23.5%)
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as red solids. 1H NMR (500 MHz, CDCl3):
7.56 (d, 2H, J = 8.0 Hz), 6.24 (s, 2H), 2.56 (s, 6H), 1.39 (s, 6H). 13C NMR
(125 MHz, DMSO-d6): 156.5, 153.2, 142.6, 133.9, 131.5, 129.5,
123.8, 120.8, 99.3, 13.8, 13.7. 31P NMR (202 MHz, CDCl3):
d 8.37 (d, 2H, J = 8.0 Hz),
d
d
ꢀ50.011.
FTMS-MALDI (m/z): calcd. for C19H19N3O4P: 384.1113 [M+H]+,
found: 384.1085; calcd. for C19H19N3O2: 322.1511 [M–PO2+H]+;
found: 322.1525.
2.3. Preparation of metal ion titration solutions
110
Stock solutions (4 ꢁ 10ꢀ4 mol/L) of the salts of HgCl2, Al(NO3)3,
AgNO3, CoCl2, MnCl2, PbCl2, CuCl2, MgCl2, NiCl2, FeCl3, ZnCl2, CaCl2,
PdCl2, CrCl3, KCl, and NaCl in H2O were prepared. PODIPY 1
(1 ꢁ 10ꢀ4 mol/L) was also prepared in CH3CN. Test solutions were
111
112
113
114
115
116
117
118
Fig. 1. Structure of PODIPY dye 1.
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57
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60
61
62
63
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68
Stokes shift and moderate fluorescence quantum yield. The dye 1 is
more polar than BODIPYs, increasing its water solubility.
Moreover, the interaction between the p-orbital of the phosphorus
atom and the lone pair electrons of the nitrogen atom, and many
resonators of the limit structure of charge separation in dye 1, may
also enhance the stability in water. Based on the photophysical
properties of 1, we made further efforts for the modifications in
PODIPY as potential chemosensors and probes. Because of the low
prepared by placing 40 mL of the sensor stock solution into a test
tube, then adding an appropriate aliquot of each metal stock
(0 ꢀ 1.0 mL) and diluting the solution to 4 mL with CH3CN/H2O (1/
9, v/v).
2.4. MO calculation
119
Frontier molecular orbitals have been performed at the
Becke3LYP (B3LYP) level of the density functional theory. The
SDD basis set are used to describe Hg and P and 6-31G(d) basis set
was used for all the other atoms (see Supporting information).
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123
fluorescence quantum yield (Ff = 0.17) of PODIPY 1 [14] and the
additional fluorescence quenching by detecting Hg2+, the investi-
gation of emission spectra becomes meaningless and was
abandoned. Delightedly, we found that PODIPY 1 can be used as
a chemosensor for the colorimetric Hg2+ detection.
3. Results and discussion
124
69
70
2. Experimental
The sensitivity of phosphorus-containing PODIPY
abs = 507 nm) was first studied by the UV–visible absorption
response towardvarious concentrations of Hg2+ in CH3CN/H2O at pH
7.2 (Fig. 2) [22]. As can be observed, a distinct response of 1 mol/L
PODIPY 1 to Hg2+ in the concentration range of 0–20
mol/L was
discovered. When 10 equiv. of Hg2+ (10
mol/L) was added, the
reduction of the absorption intensity ( abs = 550 nm) was minimal,
and a further increase in of Hg2+ concentration did not provide
1
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132
(l
2.1. General methods
m
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85
1H NMR spectra were recorded on a Bruker AVANCE III 500 MHz
m
spectrometer. 1H NMR chemical shifts (
d
) are given in ppm
downfield from Me4Si, determined by chloroform ( = 7.26 ppm).
13C NMR spectra were recorded on a Bruker AVANCE III 125 MHz
spectrometer. 13C NMR chemical shifts (
) are reported in ppm
with the internal CDCl3 at 77.0 ppm as standard. ESI was
m
d
l
d
d
measured by LCQ Deca XP. Tetrahydrofuran (THF) was freshly
distilled from Na/benzophenone, n-hexane was distilled over Na,
and other solvents were distilled over CaH2. Merck silica gel 60 was
used for the column chromatography. Fluorescence spectra were
recorded on an F-4600 spectrophotometer. UV/Vis spectra were
recorded on a UV-2550 spectrophotometer at room temperature.
The pH measurement was performed with a PHS-3E pH meter. The
refractive index of the medium was measured by 2 W Abbe’s
refractometer at 20 8C.
86
2.2. Synthesis of PODIPY 1
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93
94
95
96
2.4-Dimethylpyrrole (0.51 mL, 4.9 mmol) and paranitrobenzal-
dehyde (300 mg, 1.9 mmol) were dissolved in 20 mL of absolute
CH2Cl2 undera N2 atmosphere. One drop of trifluoroacetic acid (TFA)
was added and the solution was stirred at room temperature
overnight. When TLC monitoring (silica; CH2Cl2) showed complete
consumption of the benzaldehyde, a solution of 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ) (900 mg) in CH2Cl2 (10 ml) was
added, and stirring was continued for 1 h. The reaction mixture was
washed with water, dried over MgSO4, filtered, and evaporated.
The crude compound was purified by column chromatography on
Fig. 2. Absorption change of 1
amounts of Hg2+ (0, 0.1, 0.25, 0.5, 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 18 and 20
CH3CN/H2O (1:9, v/v) at room temperature. The inner panel displays the absorption
intensity ( abs = 507 nm) of 1
mol/L PODIPY 1 toward Hg2+ at 0, 0.1, 0.25, 0.5, 1, 2,
3, 4, 5, 6, 8, 10, 12, 15,18 and 20 mol/L.
m
mol/L PODIPY 1 after the addition of increasing
m
mol/L) in
l
m
m
Please cite this article in press as: X.-D. Jiang, et al., A colorimetric chemosensor based on new water-soluble PODIPY dye for Hg2+
detection, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.07.002

G Model
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X.-D. Jiang et al. / Chinese Chemical Letters xxx (2015) xxx–xxx
3
Fig. 3. Digital picture of PODIPY 1 (1
mmol/L) in CH3CN/H2O (1:9, v/v). From left to
right: Na+, K+, Mg2+, Al3+, Cu2+, Fe3+, Zn2+, Ba2+; no-metal, Hg2+, Ag+, Mn2+, Ni2+, Co2+
,
Cr3+
,
Pb2+
,
equivalent amounts of all the cations and Hg2+
mol/L.
. The cation total
Fig. 5. The relative absorption of PODIPY 1 and its complexation with Hg2+ in the
presence of various metal ions. PODIPY 1 + Hg2+; PODIPY 1 + Hg2+ + Mn+, where
concentration was 10
m
M
n+ = Zn2+, Ag+, Fe3+, Ni2+, Cr3+, and Pb2+ ions. Conditions: 0.3
mol/L of Hg2+ in the presence of 5
m
mol/L of PODIPY 1,
5
m
mmol/L of other metal ions.
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further reduction (Fig. 2). Based on a linear range for Hg2+ covering
from 0.1 mol/L to 3 mol/L (inner panel of Fig. 2), the detection
limit (3 /k) was found to be 0.5 mol/L [23].
In fact, the presence of Hg2+, can be detected without the use of
any instrumentation as illustrated in Fig. 3, with the color change
from pink to violet red. The selectivity toward mercury has been
demonstrated even in the presence of similar amounts of other
metal ions, such as Na+, K+, Mg2+, Al3+, Cu2+, Fe3+, Zn2+, Ba2+, Ag+,
Mn2+, Ni2+, Co2+, Cr3+, Pb2+ and no metal (Fig. 3).
Furthermore, the selectivity of the chemosensor 1 toward other
metal ions was investigated with UV–visible spectroscopy. As
shown in Fig. 4, PODIPY 1 was highly selective to Hg2+ with a
remarkable absorption change. In contrast, addition of other
m
m
s
m
absorption spectra were almost identical to those obtained in the 156
presence of Hg2+ alone.
157
The recognition mechanism of the chemosensor 1 with Hg2+ 158
was investigated by FT-IR spectra (Fig. 6a). 1 equiv. of Hg2+ with 159
PODIPY 1 (red curve) intensified and widened the peak at the 160
characteristic stretching frequency (1346.2/cm) [26] of the P55O 161
bond of POPDIPY 1 alone (black curve). This indicated a strong 162
polarization of the P55O bond upon efficient binding to the Hg2+ ion 163
(Fig. 6b).
To gain insight into the response of chemosensor 1 to Hg2+
164
165
,
chemosensor 1 and the corresponding metal complex species 1– 166
Hg2+ were examined by density function theory (DFT) at the 167
Becke3LYP (B3LYP) level of the Gaussian 03 program. The SDD 168
relevant metal ions, including Na+, K+, Mg2+, Al3+, Cu2+, Fe3+, Zn2+
,
Ba2+, Hg2+, Ag+, Mn2+, Ni2+, Co2+, Cr3+, and Pb2+, caused almost no
absorption change.
Some ions, for example, Zn2+, Ag+, Fe3+, Ni2+, Cr3+, or Pb2+, often
interfere with the selectivity of Hg2+ [24,25]. Therefore, to
investigate the selectivity for Hg2+ in a complex background of
potentially competing species, the absorption change of PODIPY 1
with Hg2+ was examined in the presence of other metal ions. From
Fig. 5, we observed that there was no interference in the detection
of Hg2+ in the presence of Zn2+, Ag+, Fe3+, Ni2+, Cr3+, or Pb2+, and the
Fig. 6. (a) IR spectra of compound PODIPY 1 (black curve) and PODIPY 1–Hg2+
Q4
Fig. 4. Absorption spectra of PODIPY 1 (0.5
metal salts (10
mol/L) of Na+, K+, Mg2+, Al3+, Cu2+, Fe3+, Zn2+, Ba2+, Hg2+, Ag+, Mn2+
Ni2+, Co2+, Cr3+, Pb2+ in CH3CN/H2O (1:9, v/v).
m
mol/L) before and after the addition of
complex (red curve) in KBr disks. (b) Structure of 1–Hg2+. (For interpretation of the
m
,
references to color in this figure legend, the reader is referred to the web version of
this article.)
Please cite this article in press as: X.-D. Jiang, et al., A colorimetric chemosensor based on new water-soluble PODIPY dye for Hg2+
detection, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.07.002

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X.-D. Jiang et al. / Chinese Chemical Letters xxx (2015) xxx–xxx
Fig. 7. Frontier molecular orbitals of 1 and 1–Hg2+ at the Becke3LYP (B3LYP) level of the density functional theory with Gaussian 03. The SDD basis set is used to describe Hg
and P, and 6-31G(d) basis set was used for all the other atoms. HOMO/LUMO (eV) = ꢀ5.96/ꢀ3.78 for 1; HOMO/LUMO (eV) = ꢀ5.84/ꢀ3.80 for 1–Hg2+
.
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basis set are used to describe Hg and P, and 6-31G(d) basis set was
used for all the other atoms. As shown in Fig. 7, for chemosensor 1,
the HOMO are distributed at the PODIPY core, and the LUMO are
localizedonthewholePODIPYstructureincludingthep-nitrophenyl
group in the neutral compounds. In addition, the planar BODIPY
fragment is almost coplanar to the adjacent p-nitrophenyl part with
a small dihedral angle (48). While for the metal complex species 1–
Hg2+, the HOMO is distributed in the left BODIPY core, and the LUMO
is mostly located at the right BODIPY unit. Furthermore, the energy
gap between the HOMO and LUMO of the metal complex species 1–
Hg2+ (2.04 eV) is smaller than that of chemosensor 1 (2.18 eV)
(Fig. 7), which is in good agreement with the red shift in the
in air. When each drop of 5.0 ꢁ 10ꢀ5 mol/L different metal ions was
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dripped into the 1-based test strips, a clear color change from pink
to brown was observed in the presence of Hg2+, comparing to
noncolor change for the other metal ions, such as Al3+, Cr3+ Ba2+
,
Pb2+ Ni2+ Mn2+,Cu2+, Ag+ and free (Fig. 8). Therefore, the 1-based
test strips can conveniently detect Hg2+ in solutions without any
additional equipment. The 1-based test strips were easily
fabricated and low-cost, useful in practical and efficient Hg2+ test
kits.
4. Conclusion
194
absorption observed upon treatment of chemosensor 1 with Hg2+
.
PODIPY 1 can detect Hg2+ by the color change from pink to
violet red without the use of any instrumentation. PODIPY 1 was
selective to Hg2+ with remarkable absorption change, and the
addition of other relevant metal ions caused almost no absorption
change. The phosphorus-containing PODIPY 1 was sensitive to
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Motivated by the obvious color change of the system in metal
ions, test strips were prepared by immersing filter papers in the
CH3CN/H2O solution of 1 (1.0 ꢁ 10ꢀ5 mol/L) and then drying them
various concentrations of Hg2+. From the changes in Hg2+
dependent absorption intensity, the detection limit was found
to be 0.5 mol/L. Using PODIPY 1 there was no interference in the
-
m
detection of Hg2+ in the presence of other ions. FT-IR indicated a
strong polarization of the P55O bond in PODIPY 1 upon efficient
binding to Hg2+. The energy gap between the HOMO and LUMO of
the metal complex 1–Hg2+ (2.04 eV) is smaller than that of
chemosensor 1 (2.18 eV), which is in good agreement with the red
shift in the absorption observed upon treatment of sensor 1 with
Hg2+. The 1-based test strips were easily fabricated and low-cost,
useful in practical and efficient Hg2+ test kits. Further efforts for
development of probes based on PODIPY/aza-PODIPY in biotech-
nology are ongoing in our lab.
Acknowledgments
213
This work was supported by the Public Research Foundation of Q2 214
Liaoning Province for the Cause of Science (No. 2014003009),
Science and Technology Key Project of Liaoning Province (No.
2013304007), the Foundation of the Education Department of
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Fig. 8. Photographs of test strips 1 to one drop of different metal ions. Test strips
were prepared by immersing filter papers in the CH3CN/H2O (1/1, v/v) solution of 1
(1.0 ꢁ 10ꢀ5 mol/L) and then drying them in air.
Please cite this article in press as: X.-D. Jiang, et al., A colorimetric chemosensor based on new water-soluble PODIPY dye for Hg2+
detection, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.07.002

G Model
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X.-D. Jiang et al. / Chinese Chemical Letters xxx (2015) xxx–xxx
5
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Henan Province for Science and Technology Research Projects (No.
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Please cite this article in press as: X.-D. Jiang, et al., A colorimetric chemosensor based on new water-soluble PODIPY dye for Hg2+
detection, Chin. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.cclet.2015.07.002