A highly selective and sensitive fluorescent chemosensor for Hg2+ based on a pyridine-appended π-conjugated ligand

Article abstract of DOI:10.1039/c4ra07448c

A pyridine-appended π-conjugated ligand 1,4-bis[2-(4-pyridyl)ethenyl]benzene (bpeb) has proved to be highly selective and sensitive as a chemosensor for Hg²⁺ in buffered aqueous solution. Remarkable changes in the UV-vis and fluorescence emission spectra were observed upon addition of Hg²⁺ ions to a solution of bpeb. In particular, the interaction of the lone-pair electrons on the pyridine nitrogen with Hg²⁺ ions results in a significant red-shift in the fluorescence spectrum and a large Stokes shift of up to 65 nm. DFT calculations have revealed that the energy levels of the HOMO and LUMO of bpeb are critically decreased upon coordination to Hg²⁺ ions, thereby accounting for this red-shift of the fluorescence spectrum. Furthermore, bpeb has been shown to be applicable as a fluorescent probe for imaging Hg²⁺ ions in PC3 cell lines, which may help in the understanding of relevant biological processes at the molecular level. This journal is

Full text of DOI:10.1039/c4ra07448c

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A highly selective and sensitive fluorescent
chemosensor for Hg2+ based on a pyridine-
Cite this: RSC Adv., 2014, 4, 47000
appended p-conjugated ligand†
a
a
Yaping Yang, Xin-Long Ni, Tao Sun,a Hang Conga and Gang Weib
*
A pyridine-appended p-conjugated ligand 1,4-bis[2-(4-pyridyl)ethenyl]benzene (bpeb) has proved to be highly
selective and sensitive as a chemosensor for Hg2+ in buffered aqueous solution. Remarkable changes in the UV-
vis and fluorescence emission spectra were observed upon addition of Hg2+ ions to a solution of bpeb. In
particular, the interaction of the lone-pair electrons on the pyridine nitrogen with Hg2+ ions results in a
significant red-shift in the fluorescence spectrum and a large Stokes shift of up to 65 nm. DFT calculations
have revealed that the energy levels of the HOMO and LUMO of bpeb are critically decreased upon
coordination to Hg2+ ions, thereby accounting for this red-shift of the fluorescence spectrum. Furthermore,
bpeb has been shown to be applicable as a fluorescent probe for imaging Hg2+ ions in PC3 cell lines, which
may help in the understanding of relevant biological processes at the molecular level.
Received 22nd July 2014
Accepted 9th September 2014
DOI: 10.1039/c4ra07448c
www.rsc.org/advances
In general, a uorescence enhancement (FE) response for
detecting analytes is highly preferable for practical applications
Introduction
in terms of increased sensitivity and selectivity as opposed to a
uorescence quenching response.11 However, due to the heavy
atom effect of HTM ions, the uorescence emission of sensors is
oen quenched by enhanced spin–orbital coupling or by energy12
or electron transfer13 during the probing process. On the other
hand, as uorescent chemosensors have generally been based on
hydrophobic uorophores, most of them have required a high
proportion of organic solvent in the detection medium for their
proper operation due to their low solubility in aqueous solutions.
As a result, the synthesis of uorescent chemosensors capable of
selectively and sensitively detecting HTM ions, including Hg2+, in
aqueous solution by a turn-on response is still a challenge.
Recently, various pyridine-appended p-conjugated deriva-
tives with donor–acceptor properties involving the lone-pair of
electrons on the pyridine nitrogen atom have been used as
heavy and transition metal ion sensors.14 For example, Feng and
co-workers demonstrated that pyridine-substituted carbazole
derivatives showed signicant sensing and coordinating prop-
erties towards a wide range of metal cations14d by intra-
molecular charge transfer (ICT)15 in acetonitrile solution. More
recently, the pyridine-conjugated derivative 1,4-bis[2-(4-pyridyl)
ethenyl]benzene (bpeb) has been utilized as an important unit
in the construction of photoreactive porous coordination poly-
mers.16 However, to the best of our knowledge, the use of bpeb
as a uorescent chemosensor for metal ions has not been
reported to date. Herein, we present a new chemosensor based
on bpeb for determining Hg2+ ions in buffered aqueous solu-
tion. Interestingly, it seems that the interaction of bpeb with
Hg2+ ions can tune the ICT, resulting in remarkable changes in
the UV-vis absorption and uorescence emission spectra.
Selective and sensitive detection of toxic heavy and transition
metal (HTM) ions has been of great interest because of their
related environmental and health issues.1 Mercury is consid-
ered as a prevalent toxic metal in the environment because
both elemental and ionic mercury can be converted into toxic
methylmercury by bacterial and chemical actions.2 When
absorbed in the human body, mercury causes damage to the
central nervous system, DNA, mitosis, and the endocrine
system.3 The U.S. Environmental Protection Agency (EPA)
standard for the maximum permissible level of inorganic
Hg(II) in drinking water is 2 ppb.4 Several methods, such as
high-performance liquid chromatography, mass spectrometry,
atomic absorption spectroscopy, inductively coupled plasma
atomic emission spectrometry, and electrochemical sensing,
have been used to monitor low levels of target metal ions.5
However, these methods require extensive and time-
consuming procedures that involve the use of sophisticated
instrumentation. Fluorogenic methods in conjunction with
suitable probes are preferable approaches for the determina-
tion of these analytes because of their high sensitivity, simple
application, aptitude for high-throughput screening, and low
cost.6 Therefore, great efforts have been directed towards the
design and construction of small uorescent chemical sensors
for Hg2+ ions.7–10
aKey Laboratory of Macrocyclic and Supramolecular Chemistry of Guizhou Province,
Guizhou University, Guiyang 550025, China. E-mail: longni333@163.com
bCSIRO Manufacturing Flagship, PO Box 218, Lindeld, NSW 2070, Australia
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra07448c
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Results and discussion
As shown in Scheme 1, bpeb was readily synthesized in 48%
yield by condensation of g-picoline and terephthalaldehyde in a
mixed solvent of acetic anhydride and acetic acid according to a
previous literature method.17 The selective binding behavior of
bpeb towards different cations (Li+, Na+, K+, Ca2+, Mg2+, Ba2+,
Al3+, Ag+, Cu2+, Hg2+, Pb2+, Zn2+, Ni2+, Co2+, Cd2+, Cr3+) as their
perchlorate salts was studied by UV-vis and uorescence spec-
troscopies. All of the titration experiments were carried out in
buffered aqueous solution (10 mM HEPES, pH ¼ 7.4) contain-
ing 0.5% CH3OH by adding the respective metal ions. Since
uorescence response is usually more sensitive than UV-vis
absorption, the binding and sensing properties of bpeb were
rst investigated by uorescence titrations with various metal
cations in buffered solution.
Fig. 1 Fluorescence spectral changes of bpeb (10 mM) upon addition
of various metal ions (50 mM) in HEPES buffer solution (10 mM, pH 7.4)
at 298 K. lex ¼ 360 nm.
Fig. 1 shows the changes in the uorescence spectrum of
bpeb upon addition of the various tested metal ions. As can be
seen, the uorescence spectrum of bpeb (10 mM) in aqueous
solution features a weak emission at 475 nm upon excitation at
360 nm. Following the addition of 5.0 equiv. of various metal
ions to the solution of bpeb, among the metal ions tested, very
small or no signicant spectral changes were observed in the
case of Li+, Na+, K+, Ca2+, Mg2+, Ba2+, Al3+, Zn2+, Ni2+, or Cd2+.
A small enhancement in uorescence intensity at 475 nm was
detected upon the addition of Co2+, while a slight uorescence
quenching was seen upon the addition of Cu2+, Ag+, Pb2+, or Cr3+.
However, a much larger uorescence enhancement and a red-
shi of about 65 nm were observed for bpeb upon addition of
Hg2+. Furthermore, as shown in Fig. 2, upon addition of
increasing concentrations of Hg2+ ions to a buffered aqueous
solution of bpeb, the emission band centered at 475 nm was
quenched and a signicant uorescence enhancement at 540 nm
was elicited. Moreover, the uorescence emission intensity at 540
nm was supported by the quantum yield (ff),18 which increased
from 0.113 to 0.242. This result implies that the new species is a
stronger uorescent chromophore than the neutral one. The
uorescence behavior of bpeb in the presence of Hg2+ ions can be
attributed to alteration of the electronic properties of the ligand,
such as increased ICT upon metal ion complexation. The
nitrogen atoms of the pyridine moieties are involved in coordi-
nation with the Hg2+ ions, which increases their electron-
withdrawing ability and enhances the intramolecular charge-
transfer process, thus resulting in a larger red-shi.
Fig. 2 Fluorescence spectral changes of bpeb (10 mM) upon addition
of increasing concentrations of Hg(ClO4)2 in HEPES buffer solution
(10 mM, pH 7.4) at 298 K. lex ¼ 360 nm. Inset shows the fluorescence
before and after the addition of Hg2+ ions (50 mM).
Fig. 3, the typical UV-vis absorption peak of bpeb (10 mM) was
observed at 320 nm, attributable to the p / p* transition of the
p-conjugated core. The addition of increasing amounts of Hg2+
ions from 1 to 50 mM resulted in a decrease in the absorption at
320 nm and the formation of a broad red-shied band at
around 400 nm. Meanwhile, a discernible isoemissive point was
observed at 338 nm. The formation of new band at 400 nm
further conrmed the interaction of Hg2+ ions with the lone-
pair electrons on the pyridine nitrogen, leading to an ICT
phenomenon from the p-conjugated core to the coordinating
nitrogen atom.15
The binding behavior of bpeb towards Hg2+ ions was also
investigated by UV-vis absorption spectroscopy. As shown in
On the other hand, as mentioned previously, Feng et al.
reported that the affinity of the pyridine nitrogen lone pair
decreases with increasing conjugation length and molecular
dimensions.14d Therefore, in order to obtain more detailed
information on the cation-selective ability of bpeb, 1,2-
di(4-pyridyl)ethylene (R), having a shorter p-conjugated moiety
Scheme 1 Chemical structure of bpeb.
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with Hg2+ at 50 mM, which mixed with Li+, Na+, K+, Ca2+, Mg2+,
Ba2+, Al3+, Ag+, Cu2+, Pb2+, Zn2+, Ni2+, Co2+, Cꢀd2+, or Cr32+ꢀ or
anions such as Fꢀ, Clꢀ, Brꢀ, NO3ꢀ, Acꢀ, H2PO4 and SO4 at
100 mM. As shown in Fig. 5 and S1,† no signicant interference
in the detection of Hg2+ ion was observed in the presence of
these other competitive cations and counter anions. A detection
limit of 4.07 ꢁ 10ꢀ7 mol Lꢀ1 for Hg2+ ions was found for bpeb,
which is sufficiently low for the detection of Hg2+ ions at the
submillimolar concentration range found in many chemical
systems. Additionally, the reversibility of the coordination-
based probe bpeb–Hg2+ was further evaluated by the addition
of Cys and EDTA. We found that the uorescence emission of
the complex at 540 nm was decreased slightly when upon
addition of EDTA, but no obvious uorescence intensity change
was observed for the addition of amino acid of Cys (Fig. S2 and
S3†). As a result, bpeb may be an ideal ratiometric selective
chemosensor for Hg2+ ions.
Fig. 3 UV-vis spectra changes of bpeb (10 mM) in aqueous solution
buffered with HEPES (10 mM, pH 7.4) in the presence of increasing
concentrations of Hg(ClO4)2 at 298 K. Inset shows the change in color
before and after the addition of Hg2+ ions (50 mM).
To obtain information about the binding mode of bpeb with
Hg2+ ions, the binding stoichiometry was investigated by
isothermal titration calorimetry (ITC) and Job plot (Fig. S4†).
For example, a constant volume (10 mL per injection) of guest
(Hg2+ ions) solution in a 0.25 mL syringe was injected into a
reaction cell (1.42 mL) charged with an aqueous solution of the
host molecule (bpeb). A representative titration curve is shown
in Fig. 6a; an abrupt transition point appeared when the molar
ratio of Hg2+ : bpeb reached 0.501, indicating the formation of a
1 : 2 complex of Hg2+ ions with the ligand. The binding constant
was calculated to be Ka ¼ (4.71 ꢂ 0.66) ꢁ 104 Mꢀ1. Attempts to
obtain more detailed information on the binding properties of
bpeb with Hg2+ ion by 1H NMR titration were not successful due
to rapid precipitation of bpeb upon addition of Hg2+ ions in D2O
solution. However, it was noted that the color of solid bpeb
changed from light-yellow to pale-red aer the addition of Hg2+
ions (Fig. 6b). Matrix-assisted laser desorption ionization time-
of-ight mass spectrometry (MALDI-TOF) was also used to
investigate the binding stoichiometry (Fig. 6c). When Hg2+ ion
was added to a solution of bpeb, a new peak appeared at m/z
888.865, corresponding to [2bpeb + Hg2+ + ClO4ꢀ + H2O + H+]+.
compared to bpeb, was selected as a reference uorescent probe
(Fig. 4). Interestingly, we found that this probe displayed high
affinity for Hg2+, Ag+, Pb2+, and Cd2+ ions under the same
experimental conditions as used for bpeb, showing enhance-
ment of the uorescence intensity at 375 nm (Fig. 4) and no
signicant emission peak shi. This result indicated that the
longer p-conjugated group of bpeb would seem to play an
important role in the selective ability and ICT effect of this
ligand towards metal ions.
To check the practical ability of bpeb as an Hg2+-selective
uorescent sensor, competitive experiments were carried out
Fig. 5 Ratiometric fluorescence response of bpeb (10 mM) in aqueous
Fig. 4 Fluorescence spectral changes of R (10 mM) upon addition of solution buffered with HEPES (pH 7.4) to 50 mM different tested metal
various metal ions (50 mM) in HEPES buffer solution (10 mM, pH 7.4) at ions (blue bars) and to mixtures of 100 mM tested metal ions with 50 mM
298 K. lex ¼ 320 nm.
Hg2+ ion (red bars) at 298 K. lex ¼ 360 nm.
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Fig. 8 Fluorescence images obtained from PC3 cells upon treatment
in PBS at pH 7.4: (a) brightfield image of cells treated with bpeb (10 mM);
(b) brightfield image of bpeb (10 mM) in the presence of 30 mM of Hg2+
(c) fluorescence image corresponding to that in (a); (d) fluorescence
image corresponding to that in (b). lex ¼ 370 nm.
;
Fig. 6 (a) ITC data for the titration of bpeb with Hg2+, the “molar ratio”
is defined as Hg2+ : bpeb; (b) color of solid bpeb before and after the
addition of Hg2+ ions; (c) partial MALDI-TOF mass spectrum of bpeb
with Hg2+ ions.
pyridine moiety. This therefore leads to a decrease in the
HOMO–LUMO gap (0.124 eV / 0.072 eV) and results in a red-
shi of the uorescence spectrum upon coordination with
Hg2+ ion.6c
In order to clarify the change in the electronic properties of
bpeb upon coordination with Hg2+ ion, theoretical calculations
of the HOMO and LUMO were carried out by density functional
theory (DFT) at the B3LYP/6-311G (d,p) level using Gaussian 09.
As shown in Fig. 7, upon coordination of Hg2+ ion with the
pyridine group, the energy levels of both the HOMO and LUMO
decreased relative to those of free bpeb. The decrease in the
LUMO level (0.185 eV) was more signicant than that in the
HOMO level (0.133 eV), indicating that the LUMO was more
stabilized than the HOMO. Upon complexation of Hg2+, the
Hg2+ plus the coordinating pyridine moiety became the
accepting part and p electrons in the LUMO were thus distrib-
uted over the pyridine group, whereas in free bpeb the p elec-
trons are localized on the atoms of the whole molecule. This
result further suggested that the coordination of Hg2+ with the
pyridine moiety promoted ICT through increasing the electron-
withdrawing ability of the coordinating nitrogen atom of the
Additionally, to demonstrate the potential application of
bpeb for detection of Hg2+ in biological media, uorescence
microscopy studies were carried out using prostate cancer (PC3)
cell lines. PC3 cells were incubated in phosphate buffered saline
(PBS; pH 7.4) containing 10 mM p-conjugated (bpeb) for 20 min
at 37 ꢃC, then washed with the same buffer to remove the excess
bpeb. As shown in Fig. 8c, the treated PC3 cells exhibited very
weak uorescence. These cells were then treated with mercury
perchlorate (30 mM) in RPMI-1640 medium and further incu-
bated for 20 min at 37 ꢃC. Aer washing with PBS, the cells, and
in particular their nuclei, exhibited intense yellow uorescence
(Fig. 8d). A MTT assay subsequently performed revealed that
bpeb is not toxic to the cell line used (Fig. S5†) Therefore, these
cellular studies clearly indicated that the p-conjugate bpeb
exhibited good cell permeability and effective intracellular
uorescence emission in the presence of Hg2+ ions.
Conclusions
In summary, we have developed a new ratiometric uorescent
chemosensor for detecting Hg2+ ion in neutral aqueous solu-
tions by a turn-on response based on p-conjugated bpeb. The
interaction of the lone-pair electrons on the pyridine nitrogen
with Hg2+ ions signicantly affected the ICT state, leading to
remarkable changes in the UV-vis and uorescence emission
spectra. DFT calculations have revealed that the energy levels of
the HOMO and LUMO of bpeb are critically changed by the
coordination of Hg2+ ions, resulting in a red-shi of the uo-
rescence spectrum. Furthermore, bpeb has been shown to be
applicable as a uorescent probe for imaging Hg2+ ions in PC3
cell lines, which may help in the understanding of relevant
biological processes at the molecular level. As a result, the
chemosensor applied here, based on the ICT effect between
Fig. 7 Frontier molecular orbitals of bpeb and 2bpeb$Hg2+ at the DFT
B3LYP/6-311G (d,p) level.
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metal binding sites and p-conjugated pyridine nitrogen centers
as electron donor and acceptor moieties, may be a candidate for
HTM ion detection both in vitro and in vivo.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (no. 21361006), the International Collab-
orative Project Fund of Chinese Ministry of Education and
the Science and Technology Fund of Guizhou province
(no. 20137005 and 20132107).
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