A new fluorescent sensor bearing three dansyl fluorophores for highly sensitive and selective detection of mercury(II) ions

Article abstract of DOI:10.1016/j.tetlet.2011.09.033

A novel fluorometric sensor bearing three dansyl moieties based on tris[2-(2-aminoethylthio)ethyl]amine was prepared by a simple approach using a conventional two-step synthesis. The sensor exhibits highly Hg ²⁺-selective ON-OFF fluorescence quenching behavior in aqueous acetonitrile solutions and is shown to discriminate various competing metal ions, particularly Cu²⁺, Ag⁺, and Pb²⁺ as well as Ca²⁺, Cd²⁺, Co²⁺, Fe³⁺, Mn ²⁺, Na⁺, Ni²⁺, and Zn²⁺, with a detection limit of 1.15 × 10^-7 M or 23 ppb.

Full text of DOI:10.1016/j.tetlet.2011.09.033

Tetrahedron Letters 52 (2011) 6133–6136
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A new fluorescent sensor bearing three dansyl fluorophores for highly
sensitive and selective detection of mercury(II) ions
Nantanit Wanichacheva a, , Ploypan Kumsorn a, Rapeepat Sangsuwan a, Anyanee Kamkaew a,
⇑
Vannajan Sanghiran Lee b,c, Kate Grudpan c,d
a Department of Chemistry, Faculty of Science, Silpakorn University, Nakorn Pathom 73000, Thailand
b Materials Science Research Center, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
c Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
d Center of Excellence for Innovation in Analytical Science and Technology, Chiang Mai University, Chiang Mai 50200, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 June 2011
Revised 28 August 2011
Accepted 7 September 2011
Available online 17 September 2011
A novel fluorometric sensor bearing three dansyl moieties based on tris[2-(2-aminoethylthio)ethyl]amine
was prepared by a simple approach using a conventional two-step synthesis. The sensor exhibits highly
Hg2+-selective ON–OFF fluorescence quenching behavior in aqueous acetonitrile solutions and is shown
to discriminate various competing metal ions, particularly Cu2+, Ag+, and Pb2+ as well as Ca2+, Cd2+, Co2+
,
Fe3+, Mn2+, Na+, Ni2+, and Zn2+, with a detection limit of 1.15 Â 10À7 M or 23 ppb.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Mercury sensor
Fluoroionophore
Hg2+-selectivity
Dansyl fluorophore
Mercury is one of the most highly toxic and widespread global
pollutants which can contaminate the environment and biota with
recognized accumulative and persistent characters.1 Mercury can
cause human health problems since it can easily pass through
the skin and respiratory and cell membranes, leading to DNA dam-
age, mitosis impairment, and permanent damage to the central
nervous system.2 In order to detect and monitor mercury concen-
trations in contaminated sources, fluorescence mercury sensors
based on synthetic ligands have received much attention since
they provide several advantages over other analytical methods in
terms of high sensitivity, high selectivity, fast response times
which can be used for real time monitoring, as well as non-
destructive detection.
nitrogen, oxygen and sulfur atoms present in the ionophores can
promote the coordination of Hg2+ 4b,5–7
.
Although many fluorescent sensors have been designed for
Hg2+-sensing, they often suffer from cross-sensitivity toward other
ions, particularly potential competitors such as copper (Cu2+), sil-
ver (Ag+), and lead (Pb2+) due to their similar chemical behavior
to Hg2+ 4–8,10
.
In this work, our inspiration was the design and synthesis of a
new fluoroionophore with high sensitivity and selectivity for Hg2+
over a wide range of competing ions, but with a significantly re-
duced synthetic effort. Our designed compound, 4 was modified
from the structure of tris(2-aminoethyl)amine (tren) by the addi-
tion of three sulfur atoms into the platform. Tren is an interesting
motif for chelating metal ions and has been utilized for the detec-
tion of metal ions such as Cu2+, Ag2+, and Hg2+, due to its symmet-
rical tripodal structure containing several nitrogen donor
atoms.8b,c,11 However, some of the reported tren-based Hg2+sensors
have serious drawbacks in terms of their utilization in organic sol-
vent systems11d and their lack of Hg2+-selectivity toward interfer-
ing ions,8b or in the presence of competitive ions.8c
A number of fluorescence chemosensors for Hg2+ have been pro-
posed and prepared based on synthetic or commercial ionophores,
including cyclen,3 hydroxyquinoline,4 azine,5 cyclams,6 diazatetra-
thia crown ethers,7 and calixarenes.8 However, many Hg2+-fluores-
cence chemosensors possess limitations for commercial and
practical uses due to multi-step syntheses, high costs of starting
materials or the requirements to operate in organic solvent sys-
tems.3,7,9 In addition, most of these studies have demonstrated that
Our molecular design was expected to serve as a Hg2+-sensitive
and selective fluorescence chemosensor following these consider-
ations: (1) Hg2+ can offer a strong and favorable electrostatic inter-
action with the sulfur atom which can increase the selectivity of the
sensor system,7,10b,12 (2) the symmetrical tripodal structure of the
tren-modified ligand offers good chelation ability for ‘soft acid’
⇑
Corresponding author.
E-mail address: nantanit@su.ac.th (N. Wanichacheva).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.09.033

6134
N. Wanichacheva et al. / Tetrahedron Letters 52 (2011) 6133–6136
metal ions and can provide a binding site suitable for Hg2+ complex-
ation,8b,c,11d and (3) the increase in the number of fluorophore
moieties could improve the sensitivity of the sensor system. In this
study, a dansyl fluorophore was chosen for the signalling portion of
the sensor due to its strong fluorescence, relatively long emission
wavelengths in the visible region and very large Stokes shift which
can prevent self absorption.13
Herein, fluoroionophore 4 was prepared using a conventional
two-step synthesis, according to the synthetic steps outlined in
Scheme 1. Tris[2-(2-aminoethylthio)ethyl]amine (3) was obtained
by alkylation of cysteamine hydrochloride (1) with tris(2-chloro-
ethyl)amine hydrochloride (2).14 Compound 4 was prepared by
substitution of 3 with dansyl chloride.15 Product 4 is a podant, acy-
clic host with pendant binding sites,16 containing three sulfur
atoms and four nitrogen atoms which are covalently bound to
three dansyl fluorophores.
Figure 1. Fluorescence intensity changes (516 nm) of 4 (9.2 Â 10À8 M) as a function
of water content in aqueous acetonitrile solution in the absence and presence of
Hg2+ (2.5 equiv), kex 336 nm.
In this study, the effects of water on the fluorescence emission
behavior of 4 in the presence and absence of Hg2+ ions were sys-
tematically investigated in acetonitrile solutions in order to opti-
mize the conditions for practical applications in environmental
and biological samples. The effect of water content on the fluores-
cence behavior of 4 in acetonitrile solutions is shown in Figure 1.
The fluorescence emission of 4 was found to be strongly depen-
dent on the amount of water in the acetonitrile solution. The re-
sults show that when the concentration of water was increased,
the fluorescence emission intensity of 4 decreased progressively.
It was found that, in the presence of 2.5 equiv of Hg2+, the decrease
in the response of 4 was greater in the low water concentration
region than that in the high water concentration region. Based on
this observation, we focused on the fluorescence behavior of 4 in
response to various metal ions in 90:10 acetonitrile/water solution.
To elucidate the quantitative binding affinity of 4, fluorescence
titrations of 4 with Hg2+ were performed. The absorption and emis-
sion spectra of 4 illustrated a very large Stokes shift, approximately
180 nm (Supplementary data, Fig. S1). Figure 2 shows the fluores-
cence spectra obtained for 4 in 90:10 acetonitrile/water in the pres-
ence and absence of Hg2+ions. The fluorescence behavior of 4 clearly
demonstrated the ON–OFF switching mechanism which occurred
in response to Hg2+ ion complexation. In the absence of Hg2+ ions,
the fluorescence response was at a maximum and fluorescence
‘turn-off’ took place as the Hg2+ concentration increased. More than
Figure 2. Fluorescence emission spectra (kex 336 nm) of 4 (0.12
CH3CN:H2O as a function of [Hg2+]. (a) 0
M, (b) 0.13 M, (c) 0.17 M, (d) 0.20
(e) 0.23 M, (f) 0.30 M, (g) 0.47 M, (h) 3.33 M, (i) 4.16 M.
l
M) in 90:10
l
l
l
lM,
l
l
l
l
l
85% of the initial fluorescence intensity of 4 was quenched by
adding mercury acetate (more than 35 times higher than the
S
S
H2N
N
NH2
Cl
Cl
NH
1) NaOMe, MeOH
reflux , 18 h
Cl
S
HS
NH3
Cl
Cl
2 ) NaOH , 25 oC, 18 h
H2N
1
2
3
H3C
CH3
N
N
Cl
H3C
O
S
O
O
S
O
CH3
S
S
O
S O
N
H
N
N
H
S
N
H3C CH3
HN
O
S O
K2CO3 , THF
25 oC, 24 h
N
H3C
CH3
4
Scheme 1. Preparation of 4.

N. Wanichacheva et al. / Tetrahedron Letters 52 (2011) 6133–6136
6135
concentration of 4) and the fluorescence response reached a mini-
mum point. A good linear correlation between the emission
response and mercury concentration was found over a range of
26–72 ppb. The detection limit of 4 as a fluorescent sensor for the
analysis of Hg2+ was determined from the plot of the fluorescence
intensity as a function of the Hg2+ concentration.17 It was found that
4 had a detection limit of 1.15 Â 10À7 M or 23 ppb for Hg2+ ions,
which is sufficient for the detection of sub-micromolar concentra-
tions of Hg2+ ions found in many biological and environmental sys-
tems such as edible fish.18
The fluorescence quantum yield (Uf) of 4 in acetonitrile was
found to be 0.31, using quinine sulfate as a reference.19 However,
the fluorescence quantum yield (Uf) of 4 with Hg2+ was deter-
mined to be 0.09, suggested that the photoinduced electron trans-
fer mechanism was operating. The association constant, Kassoc, was
determined by nonlinear curve fitting of the signal changes in the
fluorescence titration results.4b,6b,20 It was found to be
4.23 Â 106 MÀ1 and suggesting 1:1 complex formation between 4
and Hg2+. The 1:1 complex formation was consistent with the Job
plot analysis and molecular modeling experiments (Supplemen-
tary data, Fig. S2).
Molecular modeling was performed using the Discovery Studio
2.5 program package in order to clarify the coordination geometry
of 4 and Hg2+ upon binding. The optimized structure of 4 and the
final structure of the 1:1 complex are shown in Figure 3. The
dynamic molecular modeling observations suggested that ion rec-
ognition through the sensor originated from self-assembly of 4
and Hg2+ by favorable electrostatic interactions of Hg2+ coordinated
with S and N atoms. The binding distances are illustrated in Figure
3b. From the optimization using DFT calculations, Hg2+ was coordi-
nated between the two sulfur atoms and one nitrogen atom with
distances of 2.48, 2.29, and 1.66 Å, respectively.
Figure 4. (a) Fluorescence spectra (kex = 336 nm) of 4 (0.12
l
M) on addition of
acetate salts of Hg2+, Ag+, Cu2+, Pb2+, Ca2+, Cd2+, Co2+, Fe3+, Mn2+, Na+, Ni2+, and Zn2+
(0.74
lM), (b) Normalized emission intensity (516 nm) of 4 (0.12 lM) versus the
concentrations of various metal ions in 90:10 CH3CN:H2O.
4, the fluorescence was quenched selectively with Hg2+ ions. The
values in the plot in Figure 4b are normalized to the fluorescence
intensity (516 nm) in the absence of any cations and demonstrated
that the fluorescence intensity of 4 decreased as a function of added
Hg2+ until it reached a minimum point. In contrast, the fluorescence
response of 4 promoted only a small change after the addition of
other foreign ions under identical conditions. It should be noted
here that sensor 4 showed very high selectivity for Hg2+ over other
‘soft acid’ metal ions such as Cu2+, Pb2+, and Ag+, which are potential
competitors and provided some limitations in many reports.4–9
To explore further the utility of 4 as a Hg2+-selective fluores-
cence sensor, competition experiments were performed (Fig. 5)
by observing the fluorescence spectra of solutions of 4 in the
Selectivity studies on 4 were performed in 90:10 acetonitrile/
water solutions by observing the fluorescence spectra of the solu-
tions of 4 after addition of each representative metal ion. Herein,
the selectivity studies of 4 were carried out by a similar method
to the Separate Solution Method (SSM) used in ion-selective elec-
trode applications.21 This method involves the measurement of a
series of separate solutions, each containing only a salt of the stud-
ied ion.21 Figure 4 shows the fluorescence intensity of 4 in the pres-
ence and absence of cations of Hg2+, and transition metal, heavy
metal, alkali earth, and alkali ions including Ag+, Cu2+, Pb2+, Ca2+
,
Cd2+, Co2+, Fe3+, Mn2+, Na+, Ni2+, and Zn2+
.
The selectivity studies clearly demonstrated the high selectivity
of 4 for Hg2+ in comparison with the other cations. Figure 4a shows
that with the addition of Hg2+, Ag+, Cu2+, Pb2+, Ca2+, Cd2+, Co2+, Fe3+
,
presence of Hg2+ at 0.17
l
M mixed with Ag+, Ca2+, Cd2+, Co2+
M.
,
Mn2+, Na+, Ni2+, and Zn2+, approximately up to 6 equiv compared to
Cu2+, Fe3+, Mn2+, Na+, Ni2+, Pb2+, and Zn2+ at 1.7
l
Figure 3. Optimized structure using the CHARMm force field in 90:10 CH3CN:H2O and implicit distance-dependent dielectrics of 39.41 (a) compound 4 and (b) 1:1 complex
of 4:Hg2+ with the lowest interaction energy at a time step of 242 ps.

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N. Wanichacheva et al. / Tetrahedron Letters 52 (2011) 6133–6136
References and notes
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Figure 5. Competition experiments in the 4-Hg2+ system with common foreign
metal ions: [4] = 0.12
l
M, [Hg2+] = 0.17
l
M, and [Mn+] = 1.7
lM
in 90:10
CH3CN:H2O solution (kex = 336 nm).
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14. Synthesis of tris[2-(2-aminoethylthio)ethyl]amine (3): In a round bottom flask,
NaOMe (0.270 g, 5 mmol) and cysteamine hydrochloride (1) (0.424 g,
3.74 mmol) were dissolved in dry MeOH (3 mL) under an argon atmosphere
and the mixture was stirred for 30 min. This solution was added to tris(2-
chloroethyl)amine hydrochloride (2) (0.200 g, 0.83 mmol) and the mixture was
refluxed for 18 h under an argon atmosphere. The resulting insoluble
precipitate was removed by filtration. The solvent was removed on a rotary
evaporator. Aqueous NaOH solution (30% w/v, 10 mL) was added to the residue
and the resulting solution stirred slowly overnight. The mixture was extracted
with CH2Cl2 (3 Â 20 mL). The organic phase was collected and washed with
distilled H2O (30 mL) and then dried over anhydrous Na2SO4. The solvent was
removed under vacuum to give the product as a yellow oil which was used
without further purification. 1H NMR (300 MHz, CDCl3) d 2.06 (s, 6H), 2.57–
2.79 (m, 18H), 2.87 (t, J = 6.3, 6H). 13C NMR (75 MHz, CDCl3) d 28.9 (3CH2), 35.6
(3CH2), 40.3 (3CH2), 52.0 (3CH2). HRMS calcd for C12H31N4S3 (M+H)+ 327.1705,
found 327.1637.
The bars represent the final fluorescence emission response (IF)
over the initial fluorescence emission response (I0) at 516 nm. IF val-
ues are the fluorescence responses of 4 in the presence of represen-
tative competitive background cations. IF/I0 (where IF is the
fluorescence emission intensity of 4 in the presence of Hg2+ only)
was used as a reference and is equal to 0.77. The IF/I0 values were
found to lie between 0.75–0.80, which indicated that a relatively
consistent Hg2+-induced fluorescence quenching was observed in
background competing ions. In addition, only small variations in
the fluorescence intensity were found by comparison with and with-
out other competitive ions besides Hg2+. It is clear that 4 can be
utilized as an Hg2+-selective fluorescence sensor providing sensitiv-
ity for Hg2+ even in the presence of 10 equiv of background Cu2+
,
Pb2+, and Ag+ in which are known to be important competitors.
The observed selectivity of 4 for Hg2+ is notable compared to many
multidentate sulfide-containing ligands, cyclam, cyclen, and tren
moieties in previous reports.4–9
In conclusion, we have successfully prepared a new tripodal sul-
fur-rich ligand, tris[2-(2-aminoethylthio)ethyl]amine, connected to
three dansyl fluorophores and utilized it as a sensitive fluorescence
chemosensor, which shows high selectivity toward Hg2+ in the pres-
ence of various background competitive cations in aqueous–organic
solutions. The readily accessible synthetic sensor presented here is
advantageousin terms of synthetic simplicity, its low detection limit
for Hg2+ and its high selectivity in the presence of potential compet-
itors such as Cu2+, Pb2+, and Ag+. The molecular design presented
here could serve as a new potential platform for commercial uses
and future developments for sensor systems.
15. Synthesis of 4: In a round bottom flask, tris[2-(2-aminoethylthio)ethyl] amine
(3) (0.100 g, 0.31 mmol) and K2CO3 (0.170 g, 1.22 mmol) were stirred in dry
THF for 30 min under an argon atmosphere. Dansyl chloride (0.331 g,
1.23 mmol) was added and the mixture was stirred overnight at room
temperature. The resulting insoluble precipitate was removed by filtration.
The solvent was removed under vacuum. The crude product was purified by
preparative thin layer chromatography, run in the dark (5% MeOH in CH2Cl2) to
yield 24 mg of a yellow oil, yield 8%). 1H NMR (300 MHz, CDCl3) d 1.66 (s, 3H),
2.43 (t, J = 6.6, 6H), 2.51–2.68 (m, 12H), 2.90 (s, 18H), 3.04–3.10 (m, 6H), 7.19
(d, J = 7.5, 3H), 7.50–7.59 (m, 6H), 8.24–8.32 (m, 6H), 8.55 (d, J = 8.4, 3H), 13C
NMR (300 MHz, CDCl3) d 31.4 (3CH2), 41.3 (3CH2), 44.4 (6CH3), 52.2 (3CH2),
54.1 (3CH2), 114.2 (3CH), 114.3 (3C), 117.8 (3CH), 122.2 (3CH), 127.5 (3CH),
128.5 (3CH), 128.5 (3C), 128.9 (3C), 129.5 (3CH), 151.0 (3CH), HRMS calcd for
Acknowledgments
This work was supported by the Grant MRG 5380093 from the
Thailand Research Fund, the Center for Innovation in Chemistry
(PERCH-CIC), and National Research University Project under Thai-
land’s Office of the Higher Education Commission. The authors
would like to express their gratitude to the Computational Nano-
science Consortium (CNC), Nanotechnology (NANOTEC), Thailand
for access to the Discovery Studio 2.5 program package.
C
48H64N7O6S6 (M+H)+ 1026.3237, found 1026.2985.
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York, 2000.
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.09.033.
21. Bakker, E.; Buhlmann, P.; Pretsch, E. Chem. Rev. 1997, 97, 3083.