Highly selective colorimetric fluorescent sensor for Pb2+

Article abstract of DOI:10.1002/ejoc.201000142

Phenanthroline-based colorimetric sensors 1 and 2 have been designed, synthesized, and compared with phenanthrene-based receptor 3 for sensing of Pb²⁺ by color change. Receptor 1 imparts color change (from yellow to red) selectively with Pb²⁺ in acetonitrile/water (9:1) as well as in methanol/water (9:1) when in the presence of other metal ions studied (Li ⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺, Ba²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu ²⁺, Zn²⁺, Cd²⁺, Hg²⁺, and Mn ²⁺ as their Perchlorate salts). Receptor 1 also shows fluorescence enhancement upon addition of lead Perchlorate in acetonitrile/water (9:1) solvent possibly due to the chelation enhanced fluorescence (CHEF) effect. However, the binding behavior of 2 with Pb²⁺ is found to be less effective compared to that of receptor 1.

Full text of DOI:10.1002/ejoc.201000142

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201000142
Highly Selective Colorimetric Fluorescent Sensor for Pb2+
Shyamaprosad Goswami*[a] and Rinku Chakrabarty[a]
Keywords: Colorimetry / Lead / Sensors / Fluorescent probes / Chelates
Phenanthroline-based colorimetric sensors 1 and 2 have
been designed, synthesized, and compared with phen-
anthrene-based receptor 3 for sensing of Pb2+ by color
change. Receptor 1 imparts color change (from yellow to red)
selectively with Pb2+ in acetonitrile/water (9:1) as well as in
methanol/water (9:1) when in the presence of other metal
Cu2+, Zn2+, Cd2+, Hg2+, and Mn2+ as their perchlorate salts).
Receptor 1 also shows fluorescence enhancement upon ad-
dition of lead perchlorate in acetonitrile/water (9:1) solvent
possibly due to the chelation enhanced fluorescence (CHEF)
effect. However, the binding behavior of 2 with Pb2+ is found
to be less effective compared to that of receptor 1.
ions studied (Li+, Na+, K+, Ca2+, Mg2+, Ba2+, Fe3+, Co2+, Ni2+
,
Introduction
Results and Discussion
We report here phenanthroline-based receptor 1 (Fig-
ure 1), which selectively binds Pb2+ by changing its color
from yellow to red in mixtures of acetonitrile/water (9:1) as
well as in methanol/water (9:1). Chromogenic sensor 1 is
basically a hydrazone-based receptor, and it displays ex-
treme sensitivity and selectivity for Pb2+ even at the micro-
molar level. Receptor 1 was synthesized by the reaction of
phenanthroline dione (A) and hydrazide (C; Scheme 1) in
methanol at room temperature.[6] The structures of recep-
In recent years the design and synthesis of sensors that
can selectively bind a particular metal ion has received con-
siderable attention.[1] These sensors have various applica-
tions in clinical toxicology, bioremediation, waste manage-
ment, and so on.[2] Lead is a very useful metal ion because
of its widespread use in our daily lives. For example, lead is
found in insulation, coating electronics, leaded crystal, and
storage batteries. It is also used in coating wicks of some
candles and in pool paint, especially those painted by lead-
based epoxy paints.[3] We are exposed to trace amounts of
lead every day. The main concern is lead exposure with
small children, because lead can cause developmental prob-
lems. Lead is useful and necessary, but the reality is, any
amount of lead is too much. When the body is exposed to
lead by inhalation, swallowing, or in a small number of
cases, absorption through the skin, it can act as a poison.
Although many methods including atomic absorption spec-
troscopy (AAS) and inductively coupled plasma atomic
emission spectroscopy (ICP-AES) have been applied for the
quantitative detection of lead ions, all of these processes are
highly costly. Therefore, research has been devoted to the
detection of lead ions. Among the sensors targeted to deter-
mine the lead ion,[4] colorimetric sensors have received con-
siderable attention.[5]
1
tors 1 and 2 were confirmed by H and 13C NMR spec-
troscopy and high-resolution mass spectrometry.
Figure 1. Structures of receptors 1–3.
Synthesis
The NH proton of receptor 1 appears considerably
downfield (δ = 15.52 ppm) as a broad singlet because this
compound adopts a six-membered ring conformation in
which a strong hydrogen bond between the NH proton and
the carbonyl group of the phenanthroline moiety exists
(Figure S4, Supporting Information).[6] Again, strong elec-
tron-withdrawing resonance of such conjugated carbonyl
and adjacent amide carbonyl moieties are also responsible.
[a] Department of Chemistry, Bengal Engineering and Science
University,
Shibpur, Howrah 711103, India
Fax: +91-33-2668-2916
E-mail: spgoswamical@yahoo.com
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000142.
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. Goswami, R. Chakrabarty
SHORT COMMUNICATION
Binding Studies
Binding Studies by UV/Vis Method
The binding behavior of receptor 1 with metal ions as
their perchlorate salts was studied by UV/Vis. The UV/Vis
titration of receptor 1 was carried out in an aqueous aceto-
nitrile medium in the presence of HEPES buffer solution
(pH 7.4, 10 m). The UV/Vis spectra of receptor 1 reveal
an absorption maximum at 352 nm. Addition of Pb2+ ions
very strongly influences the absorption maximum. It is
worth noting that a new peak appears at 483 nm (∆λ =
131 nm), which increases with an increase in the concentra-
tion of Pb2+. The isosbestic point at 420 nm indicates the
formation of a new complex between 1 and Pb2+. The cor-
responding spectrum is depicted in Figure 3b. The binding
constant (Ka)[6] values for receptor 1 were calculated by
using the UV/Vis titration experiment. The Ka value deter-
mined by this method, that is, the Ka value of receptor 1
with Pb2+, is 4.56ϫ105 –1. The binding constant values of
receptor 1 with alkali and alkaline earth metal ions are
lower compared to those of other transition metal ions, for
example, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, and so on (Fig-
ure 3c).[6]
Scheme 1. Reagents and conditions: (i) H2SO4, HNO3, 80 °C;
(ii) dry ethanol, reflux, 12 h; (iii) hydrazine hydrate, ethanol, reflux,
2 h; (iv) dry methanol, room temp.
Titrations were also carried out with other metal ions
used as their perchlorate salts like Li+, Na+, K+, Ca2+
,
Mg2+, Ba2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, and
Mn2+. For the cases of other metal ions stated earlier, the
changes in the spectra of receptor 1 are almost negligible
(Figure 3a). The binding constant for receptor 2 and Pb2+
is found to be 1.22ϫ103 –1, which is less than receptor 1
(for spectra, see Figure S8, Supporting Information).[6]
The stoichiometry of complexation was determined from
the Job plot (Figure S2, Supporting Information)[6] by UV/
Vis experiment between receptor 1 and Pb2+. A continuous
variation method was applied to determine the stoichiome-
try in the Job plot. The break at 0.5 indicates the 1:1 stoi-
Naked-Eye Detection
Naked-eye detection experiments were carried out ini-
tially in acetonitrile/water (9:1) by addition of the corre-
sponding cation solutions in an acetonitrile/water mixture
as their perchlorate salts. The addition of the solution of
lead perchlorate into the solution of receptor 1 resulted in
a color change from yellow to red (Figure 2) as a result of
the appearance of a band at 483 nm. Thus, the resulting
species is stabilized through aromaticity. The azo[7] form of
the receptor generated from the hydrazo moiety is probably
responsible for the formation of the color (Figure S5, Sup-
porting Information).[6] To realize the necessity of the 4-
nitrophenyl group in receptor 1, we synthesized receptor 2
lacking a nitro substituent. Receptor 2 also displays a red
color with Pb2+, but at a much higher concentration com-
pared to that of 1. The detection limit[5c] of PbII to the
naked eye is 1.6ϫ10–7  with receptor 1.
chiometry of the receptor with Pb2+
.
The nitrogen atoms of the phenanthroline core probably
remain innocent towards the binding process. The explana-
tion may be found from the azo form of the Pb2+-bound
protonated intermediate[8] (Figure S6, Supporting Infor-
mation). As a result of the protonation of the phen-
anthroline ring nitrogen atoms, the azo form with the H
atom attached is stabilized, and therefore, these ring nitro-
gen atoms do not take part in the binding process. There-
fore, the nitrogen atoms of the phenanthroline core play an
important role in the binding process. For comparison, we
also synthesized receptor 3 (Scheme 1), which shows red
coloration on prolonged exposure to Pb2+. Thus, even the
phenanthroline system is a more effective sensor for Pb2+
,
receptor 3 may also be used for this sensing purpose.
From the experimental read out, it can be concluded that
receptor 1 possesses high selectivity and sensitivity towards
Pb2+ in acetonitrile/water (Figure 3c). The other metal per-
Figure 2. Change in the color of receptor 1 (1.1ϫ10–5 ) after ad-
dition of different metal ions [from left: receptor 1, 1+Pb2+ (red
chlorates, for example, Li+, Na+, K+, Ca2+, Mg2+, Ba2+
,
Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, and Mn2+, had
solution), 1+Li+, 1+Na+, 1+K+, 1+Ca2+, 1+Mg2+, 1+Ba2+
,
1+Fe3+, 1+Co2+, 1+Ni2+, 1+Cu2+, 1+Zn2+, 1+Cd2+, 1+Hg2+
as their perchlorate salts (49 µ, 1 equiv.) in CH3CN/H2O (9:1).
]
no significant influence. Thus, receptor 1 is selective for the
recognition of Pb2+
.
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Highly Selective Colorimetric Fluorescent Sensor for Pb2+
Figure 3. (a) UV/Vis spectra of receptor 1 with the addition of different metal ions; (b) change in the UV/Vis spectra of receptor 1 (c ≈
1ϫ10–5 ) after addition of increasing amounts of Pb2+ (c ≈ 1ϫ10–4 ) in acetonitrile/water (9:1) [0, 0.5, 1, 1.5, 2, 3, 5, and 7 equiv. of
Pb2+]; (c) the response of receptor 1 (at 483 nm) in UV/Vis towards metal ions after the addition of 1 equiv. of metal ions, which shows
the response toward the Pb2+ of 10–4  plus coexisting metal ions at 10–4 . “All” means all the tested interfering metal ions are present
but at a concentration of 10–5 . [Receptor 1] = 10–5 .
Binding Studies by Fluorescence Method
The binding behavior of receptor 1 with different metal
perchlorates (Li+, Na+, K+, Ca2+, Mg2+, Pb2+, Ba2+, Fe3+
,
Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, and Mn2+) was also
studied by fluorescence spectroscopy. All the titrations were
carried out by using a 1ϫ10–5  concentration of 1. By
using the metal ions mentioned earlier, 1 possibly showed
the CHEF (chelation-enhanced fluorescence)[4a,5a,5b] effect
only with Pb2+ [other metal ions show negligible change
after addition of a large excess of the metal ions
(100 equiv.)], resulting in an increase in intensity in the ob-
served fluorescence spectra (Figure 4).
Receptor 1 showed emission maximum at 395 nm at an
excitation wavelength of 352 nm. As a result of the addition
of Pb2+ ions, the emission maximum is shifted towards
slightly longer wavelengths and appears at 401 nm. The
value of Ka calculated by the fluorescence method was
found to be 4.67ϫ105 –1. The stoichiometry for the for-
mation of the complex of receptor 1 was again determined
by a Job plot in the fluorescence method (Figure S3, Sup-
porting Information).[6]
Figure 4. Fluorescence spectra of receptor 1 (c ≈ 1ϫ10–5 ) with
the addition of increasing amounts of Pb2+ perchlorate (c ≈
1ϫ10–4 ) [0, 0.5, 1, 1.5, 2, 3, 5, and 7 equiv.].
NMR Studies
1H NMR experiments (Figure S7, Supporting Infor-
mation)[6] were performed in CD3CN to understand the co-
The quantum yield (ΦF) of the system (receptor 1) is ordination mechanism of receptor 1 and Pb2+ (Figure 5).
0.0039 in acetonitrile/water at 298 K with the use of trypto- As a result of the complexation process, the NH proton
phan as a standard. After the addition of Pb2+ the value of of hydrazone undergoes an upfield shift from δ = 15.55 to
ΦF increases.
15.38 ppm. Again noticeable upfield chemical shifts have
Eur. J. Org. Chem. 2010, 3791–3795
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S. Goswami, R. Chakrabarty
SHORT COMMUNICATION
been induced for the protons of the benzene ring as well as the mixture was shaken well. The water/chloroform inter-
those of the phenanthroline rings. This indicates that the face became red, and the yellow-colored chloroform layer
azo form, which was the minor form in solution before the also became reddish (due to the lower solubility of the re-
addition of Pb2+, probably predominates[7a] in the presence ceptor 1–Pb2+ complex in chloroform). Thus, receptor 1
of Pb2+ in the case of receptor 1.
shows potential practical value in the extraction of Pb2+
from water. Further improvements and modification to the
receptor to increase the solubility of the receptor–Pb2+
complex in chloroform is in progress in our laboratory. The
IR spectra of receptor 1 and different metal ions in acetoni-
trile also support the strong binding of 1 and Pb2+ (Fig-
ure S1, Supporting Information).[6]
Conclusion
In conclusion, designed receptor 1 was demonstrated to
be a selective colorimetric as well as a fluorescent sensor
for the recognition of Pb2+. Receptor 1 shows high selectiv-
ity and sensitivity towards Pb2+ by its color change from
yellow to red in acetonitrile/water (9:1). Receptor 1 also
shows fluorescence enhancement due to the addition of lead
perchlorate because of the chelation enhanced fluorescence
enhancement (CHEF) effect.
1
Figure 5. Evolution of the H NMR spectra of 1 in CD3CN upon
the addition of (a) 0, (b) 0.3, (c) 1.0, and (d) 2.0 equiv. of Pb2+
.
Supporting Information (see footnote on the first page of this arti-
We acquired the 1H NMR spectra of receptor 1 and Pb2+
1
cle): H and 13C NMR and mass spectra (HRMS) of receptors 1–
in the presence of 1 equiv. of Pb2+ and either Zn2+ or Na2+
.
3; general procedure for their synthesis; table of binding constants.
In both cases, the H NMR spectra of the mixture of Pb2+
and Zn2+ or Pb2+ and Na2+ were the same. Therefore, we
can conclude that the interferences of other metal cations
to the binding process of receptor 1 and Pb2+ in CH3CN is
negligible.
1
Acknowledgments
We wish to express our appreciation to the Council of Scientific
and Industrial Research (CSIR), Govt. of India, for financial sup-
port. R. C. thanks the CSIR, Govt. of India, for a Senior Research
Fellowship.
The 13C NMR spectrum (Figure 6) also reveals a strong
interaction between receptor 1 and Pb2+. Complexation in-
duces downfield shifts of the carbonyl carbon atom (∆δ =
0.31 ppm) as well as the benzene and phenanthroline ring
carbon atoms.[6] The reversibility of the binding process can
be proved by the fact that after the addition of an excess
amount of ethylenediamine[4b] to the red-colored solution
of receptor 1 and Pb2+, the solution became yellow (color
of 1 itself).
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Received: February 2, 2010
Published Online: June 9, 2010
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