Selective fluorescent sensor for mercury ions in aqueous media using a 1,4-dihydropyridine derivative

Article abstract of DOI:10.1016/j.tet.2012.11.094

Novel 1,4-dihydropyridine (DHP) derivatives containing 3 carboxylic acid units are synthesized via cyclotrimerization of N-substituted β-aminoacrylates followed by basic hydrolysis of the triester. These DHP derivatives are readily soluble in aqueous media buffered at pH 8.0 and the solutions give blue fluorescent signals with quantum yields of 7-23%. One of these compounds, bearing a p-methoxyphenyl N-substituted group, shows specific fluorescent quenching with the mercuric ion (Hg²⁺). The fluorescent signal of the DHP derivative decays over a period of minutes to hours depending on the Hg²⁺ concentration, which implies that the sensing mechanism involves chemical reaction between the Hg²⁺ ion and the DHP compound. The ¹H NMR and MS data suggest that Hg²⁺ mediates the oxidation of the DHP ring into a pyridinium ring. The event is useful for fluorescent detection of Hg²⁺ at the micromolar level within 30 min, with a detection limit of 0.2 μM in aqueous medium.

Full text of DOI:10.1016/j.tet.2012.11.094

Tetrahedron 69 (2013) 1617e1621
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Selective fluorescent sensor for mercury ions in aqueous media
using a 1,4-dihydropyridine derivative
Daranee Homraruen a, Thirawat Sirijindalert b, Luxsana Dubas c, Mongkol Sukwattanasinitt b,
,
Anawat Ajavakom b
*
a Petrochemistry and Polymer Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
b Center for Petroleum, Petrochemicals and Advanced Materials, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
c Chromatography and Separation Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 July 2012
Received in revised form 29 October 2012
Accepted 27 November 2012
Available online 5 December 2012
Novel 1,4-dihydropyridine (DHP) derivatives containing 3 carboxylic acid units are synthesized via cy-
clotrimerization of N-substituted b-aminoacrylates followed by basic hydrolysis of the triester. These
DHP derivatives are readily soluble in aqueous media buffered at pH 8.0 and the solutions give blue
fluorescent signals with quantum yields of 7e23%. One of these compounds, bearing a p-methoxyphenyl
N-substituted group, shows specific fluorescent quenching with the mercuric ion (Hg2þ). The fluorescent
signal of the DHP derivative decays over a period of minutes to hours depending on the Hg2þ concen-
tration, which implies that the sensing mechanism involves chemical reaction between the Hg2þ ion and
the DHP compound. The 1H NMR and MS data suggest that Hg2þ mediates the oxidation of the DHP ring
into a pyridinium ring. The event is useful for fluorescent detection of Hg2þ at the micromolar level
Keywords:
1,4-Dihydropyridine
Chemodosimeter
Fluorescence sensor
Mercury(II) ion
within 30 min, with a detection limit of 0.2
m
M in aqueous medium.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
non-reversible reactions.29e38 Several reaction-based Hg2þ probes,
so called chemodosimeters, can provide high selectivity, sensitivity,
and reliable record.26 The irreversible chemical reactions between
the fluorescent molecules and mercury(II) usually change both
absorption and fluorescence spectra of the fluorophore that may
lead to high sensitivity and dual modes of detection. Among re-
ported fluorescent mercury(II) chemosensors, only a few systems
can be used in aqueous media without any presence of organic
solvents.30e34 In this contribution, we would like to report the
synthesis of novel 1,4-dihydropyridine (DHP) derivatives contain-
ing three carboxylic acid units using our recently reported cyclo-
trimerization reaction as the key step.39 These DHP derivatives are
readily soluble in aqueous media and the solutions give blue fluo-
rescent signals, which were found to be useful as a fluorescent
chemosensor of mercury(II) ion.
The design and synthesis of new fluorescent chemosensors for
the efficient detection of toxic heavy metal ions are one of the most
important research topics in environmental chemistry and biol-
ogy.1e11 Among heavy metals, mercury is considered as one of the
most highly toxic metals and is widely present as a contaminant in
both ionic and molecular compounds in all environmental spheres.
It can cause severe damage to the nervous system of human be-
ings12 and long term accumulation of this metal in the body leads to
a permanent deterioration of the brain, kidneys, and developing
fetus.13e16
Presently, ICP-MS is a gold standard for quantitative analysis of
mercury(II) because of its high sensitivity.17e20 However, this
technique requires expensive instruments and a skilled operator,
which are not readily possible for onsite analysis. Sensing systems
or sensor kits for the detection of mercury(II) are thus attractive for
onsite applications. Due to their sensitivities and simple detection
procedures, fluorescent chemosensors have attracted much atten-
tion during the past two decades. Generally, there are two different
design approaches of fluorescent chemosensors for mercury(II)
detection, which are based on reversible weak interactions21e28 and
The synthesis of 1,4-dihydropyridine (DHP) derivatives was
performed according to our recently reported method based on the
cyclotrimerization of
b-amino acrylates in the presence of TiCl4
Lewis acid (Scheme 1).39 The triester derivatives of DHP (1) were
obtained in 52e82%, which were readily hydrolyzed under basic
conditions to give the corresponding DHP tricarboxylic acids (2) in
satisfactory yields (63e80%). The structures of 1 and 2 were con-
firmed by 1H NMR, 13C NMR and MS (Figs. S7eS21). The tri-
carboxylic acids 2 showed good water solubility in a pH 8.0
phosphate buffer (PB) solution and gave strong blue light emission
under black light.
* Corresponding author. E-mail addresses: anawat77@hotmail.com, thirawat.s@
student.chula.ac.th (A. Ajavakom).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.11.094

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D. Homraruen et al. / Tetrahedron 69 (2013) 1617e1621
Scheme 1. Synthesis of fluorescent sensor 2.
2. Results and discussion
Electronic absorption and emission spectra of 2 were collected in
pH 8.0 PB solution. The absorption maxima were observed in a ran-
ge of 347e358 nm (Table 1) corresponding to the characteristic
Fig. 1. Fluorescence quenching profile of 2b (1
metal ion (50
for CdSO4, FeSO4, and Fe(NO3)3.
mM), 30 min after addition of each
M) in pH 8.0 PB solution (lex¼352 nm). Acetate salts were used except
pep
* transition of the DHP ring.40e42 The molar extinction co-
m
efficients of this transition for 2aed were in
a range of
4.67ꢀ103e6.74ꢀ103 Mꢁ1 cmꢁ1, but only 4.11ꢀ102 Mꢁ1 cmꢁ1 for 2e.
The difference in the molar extinction coefficients suggests that the
presence of a phenyl substituent can facilitate the electronic tran-
sition within the DHP ring. In the case of 2a, 2b, and 2c, another
strong band of the N-substituted aryl moiety around 280e300 nm
was also observed. All of these compounds exhibited an emission
peak in a range of 432e445 nm with fluorescence quantum effi-
ciencies (Ff) of 0.07e0.23. The similarity of the absorption and
emission peaks indicates that the DHP units in these compounds
have similar HOMO and LUMO levels. Again, the lower quantum
yield of 2aed compared to 2e confirms that the benzene ring can
stabilize the DHP excited state. Hence, the lower stability of 2e ex-
cited state can lead to a faster radiative decay process, which in turn
gives the higher quantum efficiency. Due to its high extinction co-
efficient and quantum efficiency, 2b showed the highest blue
emission appearance (Table 1), and it was therefore selected for
further investigation in sensing applications.
Table 1
Photophysical properties of tricarboxylic DHP (2aee) in pH 8.0 PB solution
DHP
R
Absorption
Emission
Appearance (1 mM)
under black light
lmax
ε (Mꢁ1 cmꢁ1
)
lmax
Ffa
(nm)
(nm)
2a
Ph
288
347
1.85ꢀ104
6.03ꢀ103
435
0.08
2b
2c
p-MeO-Ph 281
2.24ꢀ104
6.74ꢀ103
443
432
0.14
0.07
352
Fig. 2. a) Fluorescence response (I0/I)-1of 2b (1
m
M) and the photograph under black
M and 1 mM, respectively),
M]) system with the interfering
M in pH 8.0 PB solution (lex¼352 nm).
p-I-Ph
300
353
1.73ꢀ104
4.67ꢀ103
light of 2b (0.1 mM) 30 min after addition of metal ion (50
m
b) inference study in the [2b]$[Hg2þ] ([1
mM]$[10 m
metal ion [Mnþ]¼50
m
2d
2e
Bn
357
358
6.36ꢀ103
4.11ꢀ102
439
445
0.09
0.23
quenching responses were within 90e105% of the original
quenching by only Hg2þ (Fig. 2b). However, with higher concen-
tration of the interfering metal ions (10ꢀ), extra quenching re-
n-Bu
sponses were observed for Fe2þ and Fe3þ
.
During the preliminary sensing study, we found that the fluo-
rescence quenching by Hg2þ proceeded gradually, therefore, the
fluorescence intensity was monitored for 60 min (Fig. S2). The ki-
netic results showed that the intensity decreased and reached the
minimum after 30 min. Since the quenching process was relatively
slow, we hypothesized that it was a result of a chemical reaction
rather than the fast dynamic or static quenching mechanism. Fur-
thermore, the addition of a strong chelating reagent, such as EDTA,
did not regain the fluorescence signal (Fig. S3). The result confirmed
that the quenching was not caused by the complexation between
a
Ref. ¼ Quinine sulfate in 0.1 M H2SO4 Ff ¼ 0.54) was the reference.
(
The fluorescent responses of 2b (1
mM) toward 14 metal ions
(Naþ, Ca2þ, Zn2þ, Hg2þ, Co2þ, Ni2þ, Cu2þ, Agþ, Fe2þ, Fe3þ, Cd2þ, Al3þ
,
Mn2þ, and Pb2þ) were evaluated in aqueous PB solutions. Upon the
addition of the metal ion (50 equiv) into a solution of 2b, only Hg2þ
caused a significant fluorescence quenching effect after 30 min
(Fig. 1). This highly selective fluorescence quenching was also ob-
servable by the naked-eye (Fig. 2a). In the presence of interfering
metal ions at the concentration of 5ꢀ [Hg2þ], the fluorescence

D. Homraruen et al. / Tetrahedron 69 (2013) 1617e1621
1619
2b and Hg2þ ion but by the chemodosimeter type Hg2þ induced
structural transformation.
spectrum of the product also showed strong signal at m/z¼332
(Fig. S4) in good agreement with the molecular weight of the
proposed pyridinium product 3. However, it is not clear to us at this
moment from the observation of two 1H NMR signals at 8.9 and
6.8 ppm for the two supposedly equivalent protons of pyridinium
ring. The two observed proton signals theoretically fit better with
the hydroxy pyridinium compound 4, another possible product
resulting from the sequence of hydrolysis and oxidation of 3.
However, only a small peak in the region corresponding to the
formula weight of 4 (m/z¼347) appeared in the MS spectrum
(Fig. S4). Unfortunately, our attempt to isolate the product for better
defined characterization has not yet been successful due to the
instability of this product.
In order to gain insightful information about the quenching
mechanism, time tracking UVevis spectroscopy of 2b with 50 equiv
of Hg2þ was carried out from 1 to 45 min. As a result, the absor-
bance spectrum of 2b was altered significantly with time. The ar-
omatic band at 281 nm gradually dropped, while the DHP band at
352 nm blue shifts by 17 nm confirming the possibility of structural
change in the DHP ring (Fig. 3). This hypsochromic shift is probably
caused by the aromatization of the DHP ring to a pyridine ring, as
the new band appears in the same area as the characteristic band of
a pyridinium ring (around 325e342 nm).43e45
A Job’s plot of the fluorescence intensity against Hg2þ molar
fraction according to the Valeur method46 revealed the maximum
value at 0.5 indicating a 1:1 stoichiometrical ratio between 2b and
Hg2þ (Fig. S5). Titrations of 2b (1 M) with Hg2þ exhibited a gradual
m
decrease of fluorescence intensity with the increase of Hg2þ con-
centration that gave a SterneVolmer like linear relationship be-
tween I0/I and Hg2þ concentration with a slope of 78,300 Mꢁ1 and
a detection limit of 0.2 m
M of Hg2þ (Fig. 4). The results indicate that
2b is a sensitive sensor and can be used for quantitative analysis of
Hg2þ in an aqueous medium.
Fig. 3. UVevis spectra of 2b in pH 8.0 PB solution (24
in the period of 1e45 min.
m
M) after adding Hg2þ 50 equiv
The 1H NMR experiment was conducted to seek more evidence
to support the formation of a pyridine ring from the DHP ring. Due
to the solubility limitation, only 1 equiv of Hg2þ could be used, and
there was no significant change in the 1H NMR spectrum within
30 min. After 2 days, the signals of the DHP olefinic and allylic
protons at 7.4 and 4.0 ppm disappeared (Fig. 5), signifying the loss
of the DHP structure. The signal of methylene protons at 2.2 ppm
and the aromatic signal at 7.0 ppm down field shifted to 3.7 and
7.6 ppm, respectively, suggesting that the DHP ring had turned into
an electron withdrawing moiety. We thus believe that the DHP ring
was oxidized into a pyridinium ring by Hg2þ (Scheme 2). The MS
Fig. 4. Fluorescence change of 2b (1 m
M) with the addition of Hg2þ (0e100 equiv) in
pH 8.0 PB solution at 30 min (lex¼352 nm), linear graph plotted from I0/I and Hg2þ
concentration (inset).
3. Conclusion
In summary, we have reported
a new fluorescent 1,4-
dihydropyridine (DHP), which can be used as a selective chemo-
dosimeter for Hg2þ in aqueous solution. The decrease of fluores-
cence signal was proportional to Hg2þ concentration with high
quenching efficiency (Ksv¼78,300) providing a detection limit of
Scheme 2. Proposed mechanism for the formation of 3 and 4.
Fig. 5. 1H NMR spectra of 2b in D2O, 30 min to 2 days after adding of 1 equiv Hg2þ
.

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D. Homraruen et al. / Tetrahedron 69 (2013) 1617e1621
0.2
m
M. The quenching process involves an oxidation of the DHP
according to above general procedure using THF/H2O as solvent as
a yellow solid (109 mg, 63%): mp 169.9 ꢂC; 1H NMR (400 MHz,
into a pyridinium ring specifically induced by Hg2þ that brought
about its remarkable selectivity over other metal ions.
DMSO-d6) d 12.00 (3H, s, CH2CO2H and CH]CCO2H), 7.36 (2H, s,
CH]C), 7.21 (2H, d, J 8.9 Hz, ArH), 6.97 (2H, d, J 8.9 Hz, ArH), 3.93
(1H, t, J 4.5 Hz, CHCH2CO2H), 3.74 (3H, s, OMe), 2.30 (2H, d, J 4.5 Hz,
4. Experimental
4.1. General
CH2CO2H); 13C NMR (100 MHz, DMSO-d6)
d 172.1, 166.8, 157.0, 137.2,
135.7, 122.0, 114.3, 106.7, 54.5, 39.4, 28.2; IR (KBr) 3670e2500, 3068,
2914, 2848, 1697, 1685, 1572, 1514, 1431, 1281, 1213 cmꢁ1; HRMS
(ESI) m/z 332.0796 (MꢁHþ, C16H14NO7ꢁ, requires 372.0770).
4.1.1. Chemicals and materials. Aniline, p-methoxy aniline, p-iodo
aniline, benzylamine, butylamine, ethyl propiolate, TiCl4, and po-
tassium hydroxide (KOH) were purchased from SigmaeAldrich and
Fluka. Dichloromethane (CH2Cl2) was dried over CaH2 and distilled
prior to use. Ethanol (EtOH) was analytical grade solvent purchased
from Merck. Deionized water was used throughout in the pre-
cipitation and all extraction procedures. Thin layer chromatography
(TLC) was carried out using Merck 60 F254 plates with a thickness
of 0.25 mm. Column chromatography was performed on Merck
silica gel 60 (70e230 mesh).
4.2.3. 4-(Carboxymethyl)-1-(4-iodophenyl)-1,4-dihydropyridine-3,5-
dicarboxylic acid (2c). Synthesized from 1c (220 mg, 0.43 mmol)
according to above general procedure using THF/H2O as solvent as
a light yellow solid (132 mg, 72%): mp 186.7 ꢂC; 1H NMR (400 MHz,
DMSO-d6)
d 12.08 (3H, s, CH2CO2H and CH]CCO2H), 7.76 (2H, d, J
7.1 Hz, ArH), 7.47 (2H, s, CH]C), 7.14 (2H, d, ArH), 3.96 (1H, t, J
4.0 Hz, CHCH2CO2H), 2.34 (2H, d, J 4.0 Hz, CH2CO2H); 13C NMR
(100 MHz, DMSO-d6)
d 171.9, 167.2, 141.6, 138.4, 136.3, 121.9, 108.6,
90.2, 40.3, 29.4; IR (KBr) 3670e2500, 2914, 2846, 1693, 1581, 1489,
1215 cmꢁ1; HRMS (ESI) m/z 427.9664 (MꢁHþ, C15H12INO6ꢁ, re-
quires 427.9631).
4.1.2. Analytical instruments. Absorption spectra were measured by
a Varian Cary 50 UVevis spectrophotometer. Fluorescence spectra
were performed on a Varian Cary Eclipse spectrofluorometer. The
1H NMR spectra were collected on a 400 MHz NMR spectrometer
(Mercury 400, Varian) and at 100 MHz for 13C. The HRMS spectra
were measured on an electrospray ionization mass spectrometer
(micrOTOF-QII, Bruker Daltomics) with negative mode. Elemental
(C, H, N) analysis was performed on a PE 2400 Series II elemental
analyzer (PerkineElmer, USA). Melting points were performed on
a differential scanning calorimeter (NETZSCH DSC 204 F1).
4.2.4. 1-Benzyl-4-(carboxymethyl)-1,4-dihydropyridine-3,5-
dicarboxylic acid (2d). Synthesized from 1d (295 mg, 0.73 mmol)
according to above general procedure using EtOH/H2O as solvent as
a light yellow solid (152 mg, 65%): mp 163.1 ꢂC; 1H NMR (400 MHz,
DMSO-d6) d 11.79 (3H, s, CH2CO2H and CH]CCO2H), 7.40e7.25 (5H,
m, Ph), 7.25 (2H, s, CH]C), 4.67 (2H, s, NCH2), 3.94 (1H, t, J 4.7 Hz,
CHCH2CO2H), 2.22 (2H, d, J 4.7 Hz, CH2CO2H); 13C NMR (100 MHz,
DMSO-d6)
d 172.4, 167.7, 139.8, 137.4, 128.8, 127.6, 127.3, 105.3, 56.3,
4.1.3. Synthetic procedures. Compound 2 was prepared by a simple
hydrolysis reaction of 1,4-dihydropyridine triester (1) in KOH, H2O,
and ethanol or THF at 70 ꢂC. A mixture of compound 2, excess KOH
pellets in THF or ethanol (10 mL) and water (10 mL) was stirred
overnight at 70 ꢂC. The solution was evaporated to gain yellow
crude, which was then redissolved in water (15 mL). After addition
of approximately 20 g of ice the aqueous solution residue was
acidified with 0.1 M HCl and kept in a refrigerator overnight. The
product was precipitated and filtered to afford compounds 2aee.
40.3, 29.0; IR (KBr) 3670e2500, 3008, 2885, 1685, 1647, 1564, 139ꢁ6,
1284, 1190 cmꢁ1; HRMS (ESI) m/z 316.0845 (MꢁHþ, C16H15NO6
,
requires 316.0821).
4.2.5. 1-Butyl-4-(carboxymethyl)-1,4-dihydropyridine-3,5-
dicarboxylic acid (2e). Synthesized from 1e (300 mg, 0.94 mmol)
according to above general procedure using EtOH/H2O as solvent as
a yellow solid (210 mg, 75%): mp 139 ꢂC; 1H NMR (400 MHz, DMSO-
d6)
d 11.71 (3H, s, CH2CO2H and CH]CCO2H), 7.18 (2H, s, CH]C),
3.91 (1H, t, J 4.7 Hz, CHCH2CO2H), 3.40 (2H, t, J 6.8 Hz, NCH2), 2.18
(2H, d, J 4.7 Hz, CH2CO2H), 1.51e1.44 (2H, m, NCH2CH2CH2),
4.1.4. 1H NMR experiment. Compound 2b (26 mg, 0.08 mmol) was
dissolved in a pH 8.0 PB solution (0.4 mL). Into this aqueous solu-
tion Hg(OAc)2 (24 mg, 1 equiv) was added and the solution was
shaken thoroughly. 1H NMR experiment was then conducted im-
mediately with a NMR spectrometer (Bruker, 400 MHz). The
spectra of this experiment at the times of 0 min, 30 min, and 2 days
were illustrated for the comparison.
1.31e1.23 (2H, m, CH2CH2CH3), 0.89 (3H, t, J 7.3 Hz, CH2CH2CH3); 13
C
NMR (100 MHz, DMSO-d6)
d 172.1, 167.5, 139.4, 104.2, 52.8, 38.5,
31.7, 28.6, 18.5, 13.2; IR (KBr) 3680e2990, 2940, 2914, 2858, 162ꢁ0,
1363, 1147 cmꢁ1; HRMS (ESI) m/z 282.0998 (MꢁHþ, C13H17NO6
,
requires 282.0978).
Acknowledgements
4.2. Preparation of 1,4-dihydropyridine tricarboxylic acid
The authors would like to acknowledge the financial support
from the Research, Development and Engineering Fund through
National Nanotechnology Center (NANOTEC), National Science and
Technology Development Agency (Project NN-B-22-FN9-10-52-
06). We would also like to thank the Integration Project: In-
novations for the improvement of Food Safety and Food Quality for
New World Economy. This work is part of the Project for Estab-
lishment of Comprehensive Center for Innovative Food, Health
Products and Agriculture supported by the Thai government
stimulus package 2 (TKK2555, SP2). The student scholarship was
funded by the 90th Anniversary of Chulalongkorn University Fund
(Ratchadaphiseksomphot Endowment Fund).
4.2.1. 4-(Carboxymethyl)-1-phenyl-1,4-dihydropyridine-3,5-dicarb-
oxylic acid (2a). Synthesized from 1a (205 mg, 0.53 mmol)
according to the above general procedure, using THF/H2O as
a solvent, to afford a yellow solid (160 mg, 80%): mp 185.6 ꢂC; 1H
NMR (400 MHz, DMSO-d6)
d 12.07 (3H, s, CH2CO2H and CH]
CCO2H), 7.52 (2H, s, CH]C), 7.46 (2H, t, J 7.8 Hz, ArH), 7.32e7.28
(3H, m, ArH), 3.97 (1H, t, J 4.4 Hz, CHCH2CO2H), 2.35 (2H, d, J
4.4 Hz, CH2CO2H); 13C NMR (100 MHz, DMSO-d6)
d 172.4, 167.5,
142.5, 137.0, 129.8, 126.0, 120.2, 108.1, 39.5, 29.1; IR (KBr)
3670e2500, 3070, 2900, 1734, 1687, 1666, 1572, 1495, 1431,
1211 cmꢁ1; Elem. Anal. Found; C, 59.35%; H, 4.35%; N, 4.74% (calcd
for C15H13NO6, 59.41%; H, 4.32%; N, 4.62%); HRMS (ESI) m/z
302.0688 (MꢁHþ, C15H12NO6ꢁ, requires 302.0665).
Supplementary data
4.2.2. 4-(Carboxymethyl)-1-(4-methoxyphenyl)-1,4-dihydropyridine-
3,5-dicarboxylic acid (2b). Synthesized from 1b (216 mg, 0.51 mmol)
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2012.11.094.

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